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Dipartimento di Chimica “G. Ciamician” 

Università di Bologna 

Via Selmi 2 

40126 Bologna 

Italy 

vincenzo.balzani@unibo.it 

Editorial Board 

Prof. Vincenzo Balzani 

Dipartimento di Chimica „G. Ciamician“ 

University of Bologna 

via Selmi 2 

40126 Bologna, Italy 


Prof. Dr. Armin de Meijere 

Institut für Organische Chemie 

der Georg-August-Universität 

Tammanstr. 2 

37077 Göttingen, Germany 

ameijer1@uni-goettingen.de 

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University of California 

Department of Chemistry and 

Biochemistry 

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houk@chem.ucla.edu 

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TU München 

Lichtenbergstraße 4 

86747 Garching, Germany 

kessler@ch.tum.de 

Prof. Sebastiano Campagna 

Dipartimento di Chimica Inorganica 

Chimica Analitica e Chimica Fisica 

University of Messina 

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Italy 

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ISIS 

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Harvard University 

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Universität Hamburg 

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Kekulé-Institut für Organische Chemie 

und Biochemie 

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Preface 

Photochemistry (a term that broadly speaking includes photophysics) is 

a branch of modern science that deals with the interaction of light with matter 

and lies at the crossroads of chemistry, physics, and biology. However, before 

being a branch of modern science, photochemistry was (and still is today), 

an extremely important natural phenomenon. When God said: “Let there be 

light”, photochemistry began to operate, helping God to create the world as 

we now know it. It is likely that photochemistry was the spark for the origin of 

life on Earth and played a fundamental role in the evolution of life. Through 

the photosynthetic process that takes place in green plants, photochemistry 

is responsible for the maintenance of all living organisms. In the geological 

past photochemistry caused the accumulation of the deposits of coal, oil, and 

natural gas that we now use as fuels. Photochemistry is involved in the control 

of ozone in the stratosphere and in a great number of environmental processes 

that occur in the atmosphere, in the sea, and on the soil. Photochemistry is the 

essence of the process of vision and causes a variety of behavioral responses in 

living organisms. 

Photochemistry as a science is quite young; we only need to go back less 

than one century to find its early pioneer [1]. The concept of coordination 

compounds is also relatively young; it was established in 1892, when Alfred 

Werner conceived his theory of metal complexes [2]. Since then, the terms 

coordination compound and metal complex have been used as synonyms, 

even if in the last 30 years, coordination chemistry has extended its scope to 

the binding of all kinds of substrates [3, 4]. 

The photosensitivity of metal complexes has been recognized for a long 

time, but the photochemistry and photophysics of coordination compounds 

as a science only emerged in the second half of the last century. The first attempt 

to systematize the photochemical reactions of coordination compounds 

was carried out in an exhaustive monograph published in 1970 [5], followed by 

an authoritative multi-authored volume in 1975 [6]. These two books gained 

the attention of the scientific community and certainly helped several inorganic 

and physical chemists to enter the field and to enrich and diversify their 

research activities. Interestingly, 1974 marked the beginning of the series of 

International Symposia on the Photochemistry and Photophysics of Coordi-

X Preface 

nation Compounds. The venue of the 17th symposium of this series is Dublin 

and will be held in June 2007. 

Up until about 1975, most activity was focused on intramolecular photochemical 

reactions. Subsequently, due partly also to the more diffuse availability 

of flash techniques, the interest of several groups moved to investigations 

of luminescence and bimolecular energy and electron transfer processes. In 

the last decade of the past century, with the development of supramolecular 

chemistry, it was clear that photochemistry would play a very important role in 

the achievement of valuable functions, such as charge separation, energy migration 

and conformational changes [7], related to applications spanning from 

solar energy conversion to signal processing and molecular machines [8, 9]. In 

the last few years, an increasing number of scientists have become involved in 

these fields. Because of their unique ground and excited state properties, metal 

complexeshavebecomeinvaluablecomponentsofmoleculardevicesandmachines 

exploiting light (often sunlight) to perform useful functions [8, 9, 10]. 

The photochemistry of coordination compounds can also contribute to solving 

the energy crisis by converting sunlight into electricity or fuel [11]. In the 

meantime, the basic knowledge of the excited state properties of coordination 

compounds of several metal ions has increased considerably. However, this 

has resulted in an unavoidable loss of general knowledge and an increase in 

specialization. Currently, all scientists working in the field of the photochemistry 

and photophysics of coordination compounds have their own preferred 

metal. There is, therefore, an urgent need to spread the most recent developments 

in the field among the photochemical community. To write an exhaustive 

monograph like [5], however, would now be an impossible enterprise. For this 

reason, we decided to ask experts to write separate chapters, each one dealing 

with a specific metal whose complexes are currently at the frontier of research. 

It has been a delight as well as a privilege to work with an outstanding group 

of contributing authors and we thank them for all their efforts. We would also 

like to thank all the members of our research groups for their support. 

Bologna and Messina, March 2007 Vincenzo Balzani 

Sebastiano Campagna 

References 

1. Ciamician G (1912) Science 36:385 

2. Werner A (1893) Zeit Anorg Chem 3:267 

3. LehnJ-M(1992)Fromcoordinationchemistrytosupramolecular chemistry.In:Williams 

AF, Floriani C, Merbach AE (eds) Perspectives in coordination chemistry. VCH, Weinheim,p.447 

4. Balzani V, Credi A, Venturi M (1998) Coord Chem Rev 171:3 

5. Balzani V, Carassiti V (1970) Photochemistry of Coordination Compounds. Academic 

Press, London

Preface XI 

6. Adamson AW, Fleischauer PD (eds) (1975) Concepts in inorganic photochemistry. 

Wiley-Interscience, New York 

7. Balzani V, Scandola F (1991) Supramolecular photochemistry. Horwood, Chichester 

8. Balzani V, Credi A, Venturi M (2003) Molecular devices and machines. Wiley-VCH, 

Weinheim 

9. Kelly TR (ed) (2005) Molecular machines, Topics Current Chem 262 

10. Kay EU, Leigh DA, Zerbetto F (2007) Angew Chem Int Ed 46:72 

11. Armaroli N, Balzani V (2007) Angew Chem Int Ed 46:52

Contents 

Photochemistry and Photophysics of Coordination Compounds: 

Overview and General Concepts 

V.Balzani·G.Bergamini·S.Campagna·F.Puntoriero ......... 1 


Chromium 

N.A.P.Kane-Maguire ........................... 37 


Copper 

N.Armaroli·G.Accorsi·F.Cardinali·A.Listorti ............ 69 


Ruthenium 

S. Campagna · F. Puntoriero · F. Nastasi · G. Bergamini · V. Balzani . . . 117 


Rhodium 

M.T.Indelli·C.Chiorboli·F.Scandola.................. 215 

Author Index Volumes 251–280 ...................... 257 

Subject Index ................................ 271

Contents of Volume 281 

Photochemistry and Photophysics of Coordination 

Compounds II 

Volume Editors: Balzani, S., Campagna, V. 

ISBN: 978-3-540-73348-5 


Lanthanides 

J. P. Leonard · C. B. Nolan · F. Stomeo · T. Gunnlaugsson 


Rhenium 

R. A. Kirgan · B. P. Sullivan · D. P. Rillema 


Osmium 

D. Kumaresan · K. Shankar · S. Vaidya · R. H. Schmehl 


Iridium 

L.Flamigni·A.Barbieri·C.Sabatini·B.Ventura·F.Barigelletti 


Platinum 

J. A. G. Williams 


Gold 

V.W.-W.Yam·E.C.-C.Cheng

Top Curr Chem (2007) 280: 1–36 

DOI 10.1007/128_2007_132 

© Springer-Verlag Berlin Heidelberg 

Published online: 23 June 2007 

Photochemistry and Photophysics 

of Coordination Compounds: 

Overview and General Concepts 

Vincenzo Balzani 1 (✉)·GiacomoBergamini 1 · Sebastiano Campagna 2 · 

Fausto Puntoriero 1 

1 Dipartimento di Chimica “G. Ciamician”, Università di Bologna, 40100 Bologna, Italy 


2 Dipartimento di Chimica Inorganica, Chimica Analitica, e Chimica Fisica, 

Università di Messina, 98166 Messina, Italy 

1 Early History 

And God said: “Let there be light”; 

And there was light. 

And God saw that the light was good. 

(Genesis, 1, 3–4) 

.................................. 2 

2 Molecular Photochemistry ........................... 3 

2.1OrganicMolecules................................ 3 

2.2MetalComplexes ................................ 5 

2.3LightAbsorptionandIntramolecularExcited-StateDecay.......... 8 

3 Bimolecular Processes ............................. 10 

3.1GeneralFeatures................................. 10 

3.2BimolecularProcessesInvolvingMetalComplexes.............. 11 

4 Supramolecular Photochemistry ........................ 12 

4.1OperationalDefinitionofSupramolecularSpecies .............. 12 

4.2PhotoinducedProcessesinSupramolecularSystems ............. 15 

4.3ElectronTransfer ................................ 16 

4.3.1MarcusTheory.................................. 16 

4.3.2QuantumMechanicalTheory.......................... 19 

4.3.3OpticalElectronTransfer............................ 20 

4.4EnergyTransfer ................................. 21 

4.4.1CoulombicMechanism ............................. 22 

4.4.2ExchangeMechanism.............................. 23 

5 Coordination Compounds as Components 

of Photochemical Molecular Devices and Machines ............. 24 

5.1AMolecularWire ................................ 24 

5.2AnAntennaSystem............................... 26 

5.3AnExtensionCable............................... 28 

5.4AnXORLogicGatewithanIntrinsicThresholdMechanism ........ 29 

5.5ASunlight-PoweredNanomotor ........................ 30 

6 Conclusions ................................... 33 

References ....................................... 33

2 V. Balzani et al. 

Abstract Investigations in the field of the photochemistry and photophysics of coordination 

compounds have proceeded along several steps of increasing complexity in the last 

50 years. Early studies on ligand photosubstitution and photoredox decomposition reactions 

of metal complexes of simple inorganic ligands (e.g., NH3, CN – )werefollowed 

by accurate investigations on the photophysical behavior (luminescence quantum yields 

and lifetimes) and use of metal complexes in bimolecular processes (energy and electron 

transfer). The most significant differences between Jablonski diagrams for organic 

molecules and coordination compounds are illustrated. A large number of complexes 

stable toward photodecomposition, but capable of undergoing excited-state redox processes, 

have been used for interconverting light and chemical energy. The rate constants 

of a great number of photoinduced energy- and electron-transfer processes involving coordination 

compounds have been measured in order to prove the validity and/or extend 

the scope of modern kinetic theories. More recently, the combination of supramolecular 

chemistry and photochemistry has led to the design and construction of supramolecular 

systems capable of performing light- induced functions. In this field, luminescent 

and/or photoredox reactive metal complexes are presently used as essential components 

for a bottom-up approach to the construction of molecular devices and machines. A few 

examples of molecular devices for processing light signals and of molecular machines 

powered by light energy, based on coordination compounds, are briefly illustrated. 

Keywords Coordination compounds · Electron transfer · Energy transfer · 

Excited-state properties · Photochemistry · Supramolecular photochemistry 

1 

Early History 

The photosensitivity of metal complexes has been known for a long time. 

The first paper exhibiting some scientific character was that of Scheele (1772) 

on the effect of light on AgCl, and photography was becoming established in 

several countries in the 1830s [1]. The light sensitivity of other metal complexes 

(particularly Na4[Fe(CN)6]) was also observed very early [2]. At the 

beginning of the last century the importance of photochemistry became more 

widely recognized, mainly due to the work and the ideas of Giacomo Ciamician 

[3], Professor of Chemistry at the University of Bologna. In the same 

period (1912–1913), modern physics introduced the concept that light absorption 

corresponds to the capture of a photon by a molecule. This concept, 

and the distinction (sometimes difficult) between primary and secondary 

photoprocesses, led to the definition of quantum yield. In the following years, 

investigations on Fe 3+ and UO2 2+ complexes were performed in looking for 

useful chemical actinometers (see, e.g., [4]). Several quantitative works also 

appeared on the photochemical behavior of [Fe(CN)6] 4– and Co(III)–amine 

complexes in aqueous solution [2]. The lack of a theory on the absorption 

spectra and on the nature of the excited states, however, prevented any mechanistic 

interpretation of the observed photoreactions as well as of the few 

scattered reports on luminescent complexes.

Photochemistry and Photophysics of Coordination Compounds 3 

After the Second World War, the interpretation of the absorption spectra 

started thanks to the development of the ligand field theory [5, 6] and the first 

attempts to rationalize the charge-transfer bands [7, 8]. Following these developments, 

the photochemistry of coordination compounds could take its first 

steps as a modern science and in a time span of 2 years four important laboratories 

published their first photochemical paper [9–12]. Much of the attention 

was focused on Cr(III) complexes, whose luminescence was also investigated 

in some detail [13]. Later, Co(III) complexes attracted a great deal of attention 

since their photochemical behavior was found to change drastically with 

excitation wavelength [14, 15]. A few, isolated flash photolysis investigations 

began to appear, but this technique remained unavailable to most inorganic 

photochemists for several years. 

Since the late 1960s, the great development of photochemical and luminescence 

investigations on organic compounds led to the publication of 

books [16–19] illustrating fundamental photochemical concepts that were 

also quickly exploited for coordination compounds [2]. From that period, it 

became common to discuss the photochemical and photophysical behavior of 

a species (be it an organic molecule or a charged metal complex) on the basis 

of electronic configurations, selection rules, and energy level diagram, as we 

do today. 

2 

Molecular Photochemistry 

Molecules are multielectron systems. Approximate electronic wavefunctions 

of a molecule can be written as products of one-electron wavefunctions, each 

consisting of an orbital and a spin part: 

Ψ = ΦS = Πiϕisi . (1) 

The ϕis are appropriate molecular orbitals (MOs) and si is one of the two 

possible spin eigenfunctions, α or β. The orbital part of this multielectron 

wavefunction defines the electronic configuration. 

We illustrate now the procedure to construct energy level diagrams, using 

as examples an organic molecule and a few coordination compounds. 

2.1 

Organic Molecules 

The MO diagram for formaldehyde is shown in Fig. 1 [20]. It consists of three 

low-lying σ-bonding orbitals, a π-bonding orbital of the CO group, a nonbonding 

orbital n of the oxygen atom (highest occupied molecular orbital, 

HOMO), a π-antibonding orbital of the CO group (lowest unoccupied molecular 

orbital, LUMO), and three high-energy σ-antibonding orbitals. The


Fig. 1 Molecular orbital diagram for formaldehyde. The arrows indicate the n → π ∗ and 

π → π ∗ transitions 

lowest-energy electronic configuration is (neglecting the filled low-energy orbitals) 

π 2 n 2 . Excited configurations can be obtained from the ground configuration 

by promoting one electron from occupied to vacant MOs. At relatively 

low energies, one expects to find n → π ∗ and π → π ∗ electronic transitions 

(Fig. 1), leading to π 2 nπ ∗ and πn 2 π ∗ excited configurations (Fig. 2a). 

In a very crude zero-order description, the energy associated with a particular 

electronic configuration would be given by the sum of the energies 

of the occupied MOs. In order to obtain a more realistic description of the 

energy states of the molecule, two features should be added to the simple 

configuration picture: (1) spin functions must be attached to the orbital 

functions describing the electronic configurations, and (2) interelectronic repulsion 

must be taken into account. These two closely interlocked points have 

important consequences, since they may lead to the splitting of an electronic 

configuration into several states. 

In the case of formaldehyde, the inclusion of spin and electronic repulsion 

leads to the schematic energy level diagram shown in Fig. 2b: each excited 

electronic configuration is split into a pair of triplet and singlet states, with 

the latter at higher energy because electronic repulsion is higher for spinpaired 

electrons. It can be noticed that the singlet–triplet splitting for the 

states arising from the ππ ∗ configuration is larger than that of the states cor-


Fig. 2 Configurations (a) andstates(b) diagrams for formaldehyde 

responding to the nπ ∗ configuration. This result arises from the dependence 

of the interelectronic repulsions on the amount of spatial overlap between the 

MOs containing the two electrons, and this overlap is greater in the first than 

in the second case (see the MO shapes in Fig. 1). The electronic states can 

be designated by symbols that specify the symmetry of the wavefunction in 

thesymmetrygroupofthemolecule(e.g.,A1, A2, etc.intheC2v group of 

formaldehyde) and the spin multiplicity (number of unpaired electrons + 1) 

as a left superscript. In organic photochemistry, it is customary to label the 

singlet and triplet states as Sn and Tn, respectively, with n =0forthesinglet 

ground state and n =1,2,etc.forstatesarisingfromthevariousexcited 

configurations (often indicated in parentheses). Both notations are shown for 

formaldehyde in Fig. 2b. The situation sketched above (i.e., singlet ground 

state, pairs of singlet and triplet excited states arising from each excited configuration, 

lowest excited state of multiplicity higher than the ground state) is 

quite general for organic molecules that usually exhibit a closed-shell groundstate 

configuration. 

State energy diagrams of this type, usually called “Jablonski diagrams”, 

are used for the description of light absorption and of the photophysical processes 

that follow light excitation (vide infra). 

2.2 

Metal Complexes 

For metal complexes, the construction of Jablonski diagrams via electronic 

configurations from the MO description follows the same general lines described 

above for organic molecules [2]. A schematic MO diagram for an 

octahedral transition metal complex is shown in Fig. 3. The various MOs 

can be conveniently classified according to their predominant atomic orbital


Fig. 3 Molecular orbital diagram for an octahedral complex of a transition metal. The 

arrows indicate the four types of transitions based on localized MO configurations. For 

more details, see text 

contributions as: (1) strongly bonding, predominantly ligand centered σL orbitals; 

(2) bonding, predominantly ligand-centered πL orbitals; (3) essentially 

nonbonding, metal-centered πM orbitals of t2g symmetry; (4) antibonding, 

predominantly metal-centered σ ∗ M orbitals of eg symmetry; (5) antibonding, 

predominantly ligand-centered π∗ L orbitals; and (6) strongly antibonding, 

predominantly metal-centered σ ∗ M 

orbitals. In the ground electronic configu- 

ration of an octahedral complex of a d n metalion,orbitalsoftypes1and2are 

completely filled, while n electrons reside in the orbitals of types 3 and 4. 

As for organic molecules, excited configurations can be obtained from the 

ground configuration by promoting one electron from occupied to vacant 

MOs. At relatively low energies, one expects to find electronic transitions of 

the following types (Fig. 3): metal-centered (MC) transitions from orbitals of 

type 3 to orbitals of type 4; ligand-centered (LC) transitions of type 2 →5; 

ligand-to-metal charge-transfer (LMCT) transitions, e.g., of type 2 →4; and 

metal-to-ligand charge-transfer (MLCT) transitions, e.g., of type 3 →5. The 

relative energy ordering of the resulting excited electronic configurations depends 

on the nature of metal and ligands in more or less predictable ways. 

Low-energy metal-centered transitions are expected for metals of the first 

transition row, low-energy ligand-to-metal charge-transfer transitions are expected 

when at least one of the ligands is easy to oxidize and the metal is 

easy to reduce, low-energy metal-to-ligand charge-transfer transitions are expected 

when the metal is easy to oxidize and a ligand is easy to reduce,


and low-energy ligand-centered transitions are expected for aromatic ligands 

with extended π and π ∗ orbitals (vide infra). 

The step from configurations to states is conceptually less simple than for 

organic molecules because coordination compounds may have high symmetry 

(i.e., degenerate MOs) and open-shell ground configurations (i.e., partially 

occupied HOMOs). 

For octahedral complexes of Co(III), Ru(II), and the other d 6 metal ions, 

the σL and πL orbitals are fully occupied and the ground-state configuration 

is closed-shell since the HOMO, πM(t2g) 6 , is also completely occupied. 

The ground state is therefore a singlet, and the excited states are either singlets 

or triplets, as in the case of formaldehyde. In octahedral symmetry, 

the ground-state configuration gives rise to the state 1 A1g. Inthecaseof 

[M(NH3)6] n+ complexes (e.g., M = Co or Ru), whose ligands do not possess 

orbitals, the lowest-energy transition is metal centered and the re- 

πL and π∗ L 

sulting πM(t2g) 5σ ∗ M (eg) configuration gives rise to the singlet states 1S1g and 

1S2g and the corresponding triplets 3T1g and 3T2g. The energy level diagram 

(at low energies) for [Ru(NH3)6] 2+ is shown in Fig. 4 (the triplet-state energy 

has been obtained by comparison with the analogous Ir(III) complex [21]). 

In the case of [M(bpy)3] 2+ (M = Ru or Os), however, since the M(II) metal 

is easy to oxidize and the 2,2 ′ -bipyridine ligands are easy to reduce, the lowest 

triplet and singlet excited states are metal-to-ligand charge-transfer in 

character (Fig. 4). For the corresponding [M(bpy)3] 3+ complexes, the lowest 

triplet and singlet excited states are ligand-to-metal charge-transfer in character 

[22], since the M(III) metal can be easily reduced and the 2,2 ′ -bipyridine 

ligands are not too difficult to oxidize (Fig. 4). 

In Cr(III) complexes (d3 metal ion), there are three electrons in the HOMO 

πM(t2g) orbitals. Therefore, these complexes exhibit an open-shell groundstate 

configuration, πM(t2g) 3 , that splits into quartet and doublet states 

Fig. 4 Schematic energy level diagrams for [Ru(NH3)6] 2+ , [Ru(bpy)3] 2+ ,and[Ru(bpy)3] 3+


(Fig. 5). For most Cr(III) complexes, e.g., for [Cr(NH3)6] 3+ ,thelowest-energy 

transition is metal centered and the resulting πM(t2g) 2 σ ∗ M (eg) configuration 

gives rise to 4 T2g and 4 T1g excited states (Fig. 5). Several other coordination 

compounds, including the complexes of the lanthanide ions, have an openshell 

ground-state configuration and, as a consequence, a ground state with 

high-multiplicity and low-energy intraconfigurational metal-centered excited 

states. 

Fig. 5 Configurations (a) andstate(b) diagrams for an octahedral Cr(III) complex. Only 

the lower-lying excited states of each configuration are shown [20] 

In conclusion, metal complexes tend to have more complex and specific 

Jablonski diagrams than organic molecules. Points to be noticed are: (1) spin 

multiplicity other than singlet and triplet can occur, but for each electronic 

configuration the state with highest multiplicity remains the lowest one; 

(2) excited states can exist that belong to the same configuration of the ground 

state (this implies that the ground state has the highest multiplicity); and 

(3) more than one pair of states of different multiplicity can arise from a single 

electron configuration. In the following, in order to discuss some general 

concept of molecular photochemistry we will make use of a generic Jablonski 

diagram based on singlet and triplet states. 

2.3 

Light Absorption and Intramolecular Excited-State Decay 

Figure 6 shows a schematic energy level diagram for a generic molecule [23]. 

In principle, transitions between states having the same multiplicity are allowed, 

whereas those between states of different multiplicity are forbidden. 

Therefore, the electronic absorption bands observed in the UV–visible spec-


Fig. 6 Schematic energy level diagram for a generic molecule 

trum of such a generic molecule would display bands corresponding to the 

S0 → Sn transitions of the diagram. For metal complexes, which usually are 

highly symmetric species, symmetry selection rules can also play a role in determining 

the intensity of the absorption bands. Furthermore, the presence of 

a heavy atom (namely, the metal) relaxes the spin-conservation rule. 

The excited states are unstable species that decay not only by intramolecular 

chemical reactions (e.g., dissociation, isomerization) but also (actually, 

more often) by intramolecular radiative and nonradiative deactivations. 

When a species is excited to upper spin-allowed excited states, it usually 

undergoes a fast and 100% efficient radiationless deactivation (internal conversion, 

ic) to the lowest spin-allowed excited (S1 in Fig. 6). Setting aside the 

intramolecular photochemical processes, such an excited state undergoes deactivation 

via three competing first-order processes: nonradiative decay to the 

ground state (internal conversion, rate constant kic); radiative decay to the 

ground state (fluorescence, kfl); and intersystem crossing (isc) to the lowest 

triplet state T1 (kisc). In its turn, T1 can undergo deactivation via nonradiative 

(intersystem crossing, k ′ isc ) or radiative (phosphorescence, kph) decayto 

the ground state S0. When the species contains heavy atoms, as in the case 

of metal complexes, the formally forbidden intersystem crossing and phosphorescence 

processes become faster. The lifetime (τ) of an excited state, i.e., 

the time needed to reduce the excited-state concentration by 2.718, is given


by the reciprocal of the summation of the deactivation rate constants. For the 

molecule of Fig. 6, 

τ(S1)= 

1 

(kic + kfl + kisc) 

1 

τ(T1)= 

(k ′ . (3) 

isc + kph) 

The lifetimes of the lowest spin-allowed and spin-forbidden excited state 

(τ(S1) andτ(T1) in the example of Fig. 6) are approximately 10 –9 –10 –7 sand 

10 –3 –100 s, respectively, for organic molecules, but they become shorter by 

several orders of magnitude for metal complexes. For example, the lifetime of 

the lowest spin-forbidden excited state of naphthalene is around 2 s, whereas 

that of [Ru(bpy)3] 2+ , because of the presence of the heavy Ru ion, is about 

1 µs [24]. 

The quantum yields of fluorescence (ratio between the number of photons 

emitted by the lowest spin-allowed excited state, S1 in Fig. 6, and the number 

of absorbed photons) and phosphorescence (ratio between the number of 

photons emitted by the lowest spin-forbidden excited state, T1 in Fig. 6, and 

the number of absorbed photons) can range between 0 and 1 and are given 

by the following expressions: 

Φ fl = 

k fl 

(kic + k fl + kisc) 

kph × kisc 

Φph = 

(k ′ isc + kph) 

. (5) 

× (kic + kfl + kisc) 

The excited-state lifetimes and fluorescence and phosphorescence quantum 

yields of a great number of organic molecules and metal complexes are 

known [24]. 

3 

Bimolecular Processes 

3.1 

General Features 

In fluid solution, when the intramolecular deactivation processes are not too 

fast, i.e., when the lifetime of the excited state is sufficiently long, an excited 

molecule ∗ A may have a chance to encounter a molecule of another solute B. 

In such a case, some specific interaction can occur leading to the deactivation 

of the excited state by second-order kinetic processes. The two most important 

types of interactions in an encounter are those leading to electron or 

(2) 

(4)


energy transfer: 

∗ A+B→ A + +B – 

∗ A+B→ A – +B + 

oxidative electron transfer (6) 

reductive electron transfer (7) 

∗ ∗ 

A+B→ A+ B energy transfer . (8) 

Bimolecular electron- and energy-transfer processes are important because 

they can be used (1) to quench an electronically excited state, i.e., to prevent 

its luminescence and/or intramolecular reactivity, and (2) to sensitize other 

species, for example, to cause chemical changes of, or luminescence from, 

species that do not absorb light. 

Simple kinetic arguments show that only the excited states that live longer 

than ca. 10 –9 s may have a chance to be involved in encounters with other solute 

molecules. Usually, in the case of metal complexes only the lowest excited 

state satisfies this requirement. The kinetic aspects of energy- and electrontransfer 

processes are discussed in detail elsewhere [17, 20, 23]. A point that 

must be stressed is that an electronically excited state is a species with quite 

different properties compared with those of the ground-state molecule. In 

particular, because of its higher energy content, an excited state is both 

a stronger reductant and a stronger oxidant than the corresponding ground 

state. To a first approximation, the redox potentials of the excited-state couples 

may be calculated from the potentials of the ground-state couples and the 

one-electron potential corresponding to the zero–zero excited-state energy, 

E0–0 , as shown by Eqs. 9 and 10 [25]: 

E � A + / ∗ A � ≈ E � A + /A � – E 0–0 

E � ∗ A/A – � ≈ E � A/A –� + E 0–0 . (10) 

3.2 

Bimolecular Processes Involving Metal Complexes 

From an exhaustive monograph that appeared in 1970 [2] and a multiauthored 

volume of 1975 [26], it clearly appears that most of the interest was 

then focused on ligand photosubstitution reactions, photoredox decomposition, 

and photoisomerization reactions, while bimolecular processes were 

barely investigated. This picture, however, changed profoundly in a few years 

following the extensive work carried out by several research groups on the luminescence 

of coordination compounds [27–29] and the discovery that the 

lowest excited state of a number of Cr(III), Ru(II), and Os(II) complexes exhibits 

a sufficiently long lifetime in fluid solution to be able to participate 

as a reactant in bimolecular reactions [25, 30]. A further advantage offered 

by Ru(II) and Os(II) bipyridine-type complexes is that they can undergo reversible 

redox reactions both in the ground and excited state, so they were 

soon used as reactants and, even more interesting, as mediators, in light- 

(9)


induced [30–34] and light-generating [35, 36] electron-transfer processes. 

Such studies were further boosted by the fact that, after the energy crisis of 

the early 1970s, several photochemists became involved in the problem of solar 

energy conversion. Particular interest arose around photosensitized water 

splitting [37–41] and it was soon realized [42] that [Ru(bpy)3] 2+ and related 

complexes, because of their excited-state redox properties, might function as 

photocatalysts for such a process. 

As a matter of fact, in the period 1975–1985 a real revolution occurred in 

the field of the photochemistry of coordination compounds. The study of intramolecular 

ligand photosubstitution, photoredox decomposition, and photoisomerization 

reactions was almost completely set apart, about 300 Ru(II) 

bipyridine-type complexes were synthesized and investigated in an attempt 

(mostly vain) to improve the already outstanding excited-state properties 

of [Ru(bpy)3] 2+ [43], and, thanks to an extensive use of pulsed techniques, 

huge amounts of data were collected on the rate constants of bimolecular 

processes [44]. The high exergonicity of the excited-state electron-transfer 

reactions (and/or of their back reactions) offered the opportunity for the 

first time to investigate some fundamental aspects of electron-transfer theories 

[45], with particular attention to the so-called Marcus inverted region. 

4 

Supramolecular Photochemistry 

4.1 

Operational Definition of Supramolecular Species 

In the late 1980s, following the award of the 1987 Nobel prize to Pedersen, 

Cram, and Lehn, there was a sudden increase of interest in supramolecular 

chemistry, a highly interdisciplinary field based on concepts such as molecular 

recognition, preorganization, and self-assembling. 

The classical definition of supramolecular chemistry is that given by 

J.-M. Lehn, namely “the chemistry beyond the molecule, bearing on organized 

entities of higher complexity that result from the association of two 

or more chemical species held together by intermolecular forces” [46]. There 

is, however, a problem with this definition. With supramolecular chemistry 

there has been a change in focus from molecules to molecular assemblies 

or multicomponent systems. According to the original definition, however, 

when the components of a chemical system are linked by covalent bonds, the 

system should not be considered a supramolecular species, but a molecule. 

This point is particularly important in dealing with light-powered molecular 

devices and machines (vide infra), which are usually multicomponent systems 

in which the components can be linked by chemical bonds of various 

natures.


To better understand this point, consider, for example, the three systems 

[47] shown in Fig. 7, which play the role of photoinduced chargeseparation 

molecular devices [48]. In each one of them, two components, 

a Zn(II) porphyrin and an Fe(III) porphyrin, can be immediately singled 

out. In 1, these two components are linked by a hydrogen-bonded bridge, 

i.e., by intermolecular forces, whereas in 2 and 3 they are linked by covalent 

bonds. According to the above-reported classical definition of supramolecular 

chemistry, 1 is a supramolecular species, whereas 2 and 3 are (large) 

molecules. In each one of the three systems, the two components substantially 

maintain their intrinsic properties and, upon light excitation, electron 

transfer takes place from the Zn(II) porphyrin unit to the Fe(III) porphyrin 

one. The values of the rate constants for photoinduced electron transfer 

(k el = 8.1 × 10 9 , 8.8 × 10 9 ,and4.3 × 10 9 s –1 for 1, 2, and3, respectively) show 

that the electronic interaction between the two components in 1 is comparable 

to that in 2, and is slightly stronger than that in 3. Clearly, as far as 

photoinduced electron transfer is concerned, it would sound strange to say 

that 1 is a supramolecular species, and 2 and 3 are molecules. 

Fig. 7 Three dyads possessing Zn(II) porphyrin and Fe(III) porphyrin units linked by an 

H-bonded bridge (1), a partially unsaturated bridge (2), and a saturated bridge (3) [47]


The example discussed above shows that, although the classical definition 

of supramolecular chemistry as “the chemistry beyond the molecule” [46] is 

quite useful in general, from a functional viewpoint the distinction between 

what is molecular and what is supramolecular can be better based on the degree 

of intercomponent electronic interactions [20, 49–52]. This concept is 

illustrated, for example, in Fig. 8 [53]. In the case of photon stimulation, a system 

A∼B, consisting of two units (∼ indicates any type of “bond” that keeps 

the units together), can be defined as a supramolecular species if light absorption 

leads to excited states that are substantially localized on either A or B, or 

causes an electron transfer from A to B (or vice versa). By contrast, when the 

excited states are substantially delocalized on the entire system, the species 

can be better considered as a large molecule. Similarly (Fig. 8), oxidation and 

reduction of a supramolecular species can substantially be described as oxidation 

and reduction of specific units, whereas oxidation and reduction of 

a large molecule leads to species where the hole or the electron are delocalized 

on the entire system. In more general terms, when the interaction energy 

between units is small compared to the other relevant energy parameters, 

a system can be considered a supramolecular species, regardless of the nature 

of the bonds that link the units. Species made of covalently linked (but 

weakly interacting) components, e.g., 2 and 3 showninFig.7,cantherefore 

be regarded as belonging to the supramolecular domain when they are stimulated 

by photons or electrons. It should be noted that the properties of each 

component of a supramolecular species, i.e., of an assembly of weakly interacting 

molecular components, can be known from the study of the isolated 

components or of suitable model molecules. 

Fig. 8 Schematic representation of the difference between a supramolecular system and 

alargemoleculebasedontheeffectscausedbyaphotonoranelectroninput.Formore 

details, see text


4.2 

Photoinduced Processes in Supramolecular Systems 

Clearly, preorganization is a most valuable property from a photochemical 

viewpoint since a supramolecular system, for example, can be preorganized 

so as to favor the occurrence of energy- and electron-transfer processes [20]. 

Consider, for example, an A–L–B supramolecular system, where A is the 

light-absorbing molecular unit (Eq. 11), B is the other molecular unit to be involved 

in the light-induced processes, and L is a connecting unit (often called 

bridge). In such a system, after light excitation of A there is no need to wait 

for a diffusion-controlled encounter between ∗ A and B, as in molecular photochemistry 

(vide supra), since the two reaction partners can already be at an 

interaction distance suitable for electron and energy transfer: 

A–L–B + hν → ∗ A–L–B photoexcitation (11) 

∗ A–L–B → A + –L–B – 

∗ A–L–B → A – –L–B + 

oxidative electron transfer (12) 

reductive electron transfer (13) 

∗ A–L–B → A–L– ∗ B electronic energy transfer . (14) 

In the absence of chemical complications (e.g., fast decomposition of the 

oxidized and/or reduced species), photoinduced electron-transfer processes 

are followed by spontaneous back electron-transfer reactions that regenerate 

the starting ground-state system (Eqs. 15 and 16), and photoinduced energy 

transfer is followed by radiative and/or nonradiative deactivation of the excited 

acceptor (Eq. 17): 

A + –L–B – → A–L–B back oxidative electron transfer (15) 

A – –L–B + → A–L–B back reductive electron transfer (16) 

A–L– ∗ B → A–L–B excited-state decay . (17) 

In supramolecular systems, electron- and energy-transfer processes take 

place by first-order kinetics. As a consequence, in suitably designed supramolecular 

systems these processes can involve even very short lived excited 

states. 

In most cases, the interaction between excited and ground-state components 

in a supramolecular system is weak. When the interaction is strong, 

new chemical species are formed, which are called excimers (from excited 

dimers) or exciplexes (from excited complexes), depending on whether the 

two interacting units have the same or different chemical nature (Fig. 9). It 

is important to notice that excimer and exciplex formation is a reversible 

process and that both excimers and exciplexes are sometimes luminescent.


Fig. 9 Schematic representation of excimer and exciplex formation in a supramolecular 

system 

Compared with the “monomer” emission, the emission of an excimer or exciplex 

is always displaced to lower energy (longer wavelengths) and usually 

corresponds to a broad and rather weak band. 

Excimers are usually obtained when an excited state of an aromatic 

molecule interacts with the ground state of a molecule of the same type. For 

example, between the excited and ground states of anthracene units. Exciplexes 

are obtained when an electron donor (acceptor) excited state interacts 

with an electron acceptor (donor) ground-state molecule; for example, between 

excited states of aromatic molecules (electron acceptors) and amines 

(electron donors). Excited states of coordination compounds are seldom involved 

in excimers or exciplexes, since their components (metal and ligands) 

have already used their electron donor or acceptor properties in forming 

the complex. Furthermore, the three-dimensional structure of coordination 

compounds usually prevents strong electronic interaction with other species. 

However, for some square planar complexes excimer emission has long been 

reported [54] and can indeed be found for some families of Au and Pt complexes, 

as discussed in other chapters of this volume. 

The working mechanisms of a number of biological and artificial molecular 

devices and machines are based on photoinduced electron- and energytransfer 

processes [20, 48, 55]. Since these processes have to compete with 

the intrinsic decays of the relevant excited states, a key problem is that of 

maximizing their rates. It is therefore appropriate to summarize some basic 

principles of electron- and energy-transfer kinetics. [56]. 

4.3 

Electron Transfer 

4.3.1 

Marcus Theory 

Electron-transfer processes involving excited-state and/or ground-state molecules 

can be dealt with in the frame of the Marcus theory [57] and of the


successive, more sophisticated theoretical models [58, 59]. Of course, when 

excited states are involved, the redox potential of the excited-state couple has 

to be used (Eqs. 9 and 10). 

According to the Marcus theory [57], the rate constant for an electrontransfer 

process can be expressed as 

κel = νNκel exp 

� 

– ∆G‡ 

RT 

� 

, (18) 

where νN is the average nuclear frequency factor, κel is the electronic transmission 

coefficient, and ∆G ‡ is the free energy of activation. This last term 

can be expressed by the Marcus quadratic relationship 

∆G ‡ = λ 

� 

1+ 

4 

∆G0 

�2 

, (19) 

λ 

where ∆G 0 is the standard free energy change of the reaction and λ is the 

nuclear reorganizational energy (Fig. 10). This equation predicts that for 

a homogeneous series of reactions (i.e., for reactions having the same λ and 

κ el values), a ln k el vs ∆G 0 plot is a bell-shaped curve (Fig. 11) involving 

(1) a “normal” region for endoergonic and slightly exoergonic reactions, in 

which ln k el increases with increasing driving force; (2) an activationless maximum 

for λ ≈ – ∆G 0 ; and (3) an “inverted” region for strongly exoergonic 

reactions, in which ln kel decreases with increasing driving force. 

Fig. 10 Profile of the potential energy curves of an electron-transfer reaction: i and f indicate 

the initial and final states of the system. The dashed curve indicates the final state 

for a self-exchange (isoergonic) process. For more details, see text


Fig. 11 Free energy dependence of electron-transfer rate (i, initialstate;f ,finalstate)according 

to the Marcus (a) and quantum mechanical (b) treatments.Thethreekinetic 

regimes (normal, activationless, and “inverted”) are shown schematically in terms of 

Marcus parabolas 

The reorganizational energy λ can be expressed as the sum of two independent 

contributions corresponding to the reorganization of the “inner” (bond 

lengths and angles within the two reaction partners) and “outer” (solvent 

reorientation around the reacting pair) nuclear modes: 

λ = λi + λo . (20) 

The electronic transmission coefficient κel is related to the probability of 

crossing at the intersection region (Fig. 10). It can be expressed by the equation 

κel = 2 � 1–exp � �� 

– νel/2νN 2–exp � � , (21) 

– νel/2νN 

where 

νel = 

� 

2 Hel� 2 

h 

� π 3 

λRT 

�1/2 

, (22) 

and Hel is the matrix element for electronic interaction (Fig. 10). 

If Hel is large, νel ≫ νN, κel =1and 

� 

– ∆G ‡ � 

kel = νN exp 

(adiabatic limit) . (23) 

RT


If Hel is small, νel ≪ νN, κel = νel/νN and 

� 

– ∆G ‡ � 

kel = νel exp 

(nonadiabatic limit) . (24) 

RT 

Under the latter condition, kel is proportional to (H el ) 2 .ThevalueofH el depends 

on the overlap between the electronic wavefunctions of the donor and 

acceptor groups, which should decrease exponentially with increasing donor– 

acceptor distance. 

It should be noticed that the amount of electronic interaction required to 

promote photoinduced electron transfer is very small. By substituting reasonable 

numbers for the parameters in Eq. 24, for an activationless reaction H el 

values of a few wavenumbers are sufficient to give rates in the sub-nanosecond 

timescale, while a few hundred wavenumbers may be sufficient to reach the 

limiting adiabatic regime (Eq. 23). 

4.3.2 

Quantum Mechanical Theory 

From a quantum mechanical viewpoint, both the photoinduced and back 

electron-transfer processes can be viewed as radiationless transitions between 

different, weakly interacting electronic states of the A–L–B supermolecule 

(Fig. 12). The rate constant of such processes is given by an appropriate 

Fermi “golden rule” expression: 

k el = 4π2 

h 

� 

H el� 2 

FC el , (25) 

Fig. 12 Electron-transfer processes in a supramolecular system: 1 photoexcitation; 2 photoinduced 

electron transfer; 3 thermal back electron transfer; 4 optical electron transfer


where H el and FC el are the electronic coupling and the Franck–Condon density 

of states, respectively. 

In the absence of any intervening medium (through-space mechanism), 

the electronic factor decreases exponentially with increasing distance: 

H el = H el � 

(0) exp – βel 

2 

� � 

rAB – r0 

� 

, (26) 

where rAB is the donor–acceptor distance, H el (0) is the interaction at the “contact” 

distance r0,andβ el is an appropriate attenuation parameter. 

For donor–acceptor components separated by vacuum, β el is estimated to 

be in the range 2–5 ˚A –1 . When donor and acceptor are separated by “matter” 

(e.g., a bridge L), the electronic coupling can be mediated by mixing of 

the initial and final states of the system with virtual, high-energy electrontransfer 

states involving the intervening medium (superexchange mechanism) 

[60, 61]. 

The FC el term of Eq. 25 is a thermally averaged Franck–Condon factor connecting 

the initial and final states. In the high temperature limit (hν


The Hush theory [62] correlates the parameters that are involved in the 

corresponding thermal electron-transfer process by means of Eqs. 28–30: 

(28) 

∆ν1/2 = 48.06 � Eop – ∆G 0�1/2 (29) 

� 

εmax∆ν1/2 = H el� 2 r2 4.20 × 10 –4 , 

Eop 

(30) 

Eop = λ + ∆G 0 

where Eop, ∆ν1/2 (both in cm –1 ), and εmax are the energy, halfwidth, and 

maximum intensity of the electron-transfer band, respectively, and r is the 

center-to-center distance. As shown by Eqs. 28–30, the energy depends on 

both reorganizational energy and thermodynamics, the halfwidth reflects the 

reorganizational energy, and the intensity of the transition is mainly related 

to the magnitude of the electronic coupling between the two redox centers. 

In principle, therefore, important kinetic information on a thermal 

electron-transfer process could be obtained from the study of the corresponding 

optical transition. In practice, it can be shown that weakly coupled 

systems may undergo relatively fast electron-transfer processes without exhibiting 

appreciably intense optical electron-transfer bands. More details on 

optical electron transfer and related topics (i.e., mixed valence metal complexes) 

can be found in the literature [63–65]. 

4.4 

Energy Transfer 

The thermodynamic ability of an excited state to intervene in energy-transfer 

processes is related to its zero–zero spectroscopic energy, E0–0 .Bimolecular 

energy-transfer processes involving encounters can formally be treated using 

a Marcus-type approach with ∆G0 = E0–0 A – E0–0 

B and λ ∼ λi [66]. 

Energy transfer in a supramolecular system can be viewed as a radiationless 

transition between two “localized” electronically excited states. Therefore, 

the rate constant can again be obtained by an appropriate “golden rule” 

expression, similar to that seen above for electron transfer: 

ken = 4π2 � en 

H 

h 

�2 en 

FC , (31) 

where Hen is the electronic coupling between the two excited states interconverted 

by the energy-transfer process and FCen is an appropriate Franck– 

Condon factor. As for electron transfer, the Franck–Condon factor can be 

cast either in classical [67] or quantum mechanical [68–70] terms. Classically, 

it accounts for the combined effects of energy gradient and nuclear 

reorganization on the rate constant. In quantum mechanics terms, the FC 

factor is a thermally averaged sum of vibrational overlap integrals. Experimental 

information on this term can be obtained from the overlap integral


between the emission spectrum of the donor and the absorption spectrum of 

the acceptor. 

The electronic factor H en is a two-electron matrix element involving the 

HOMOs and LUMOs of the energy–donor and energy–acceptor components. 

By following standard arguments [20, 23], this factor can be split into two 

additive terms, a coulombic term and an exchange term. The two terms depend 

differently on the parameters of the system (spin of ground and excited 

states, donor–acceptor distance, etc.) and each of them can become predominant 

depending on the specific system and experimental conditions. 

The orbital aspects of the two mechanisms are schematically represented in 

Fig. 13. 

Fig. 13 Pictorial representation of the coulombic and exchange energy-transfer mechanisms 

4.4.1 

Coulombic Mechanism 

The coulombic (also called resonance, Förster-type [71], or through-space) 

mechanism is a long-range mechanism that does not require physical contact 

between donor and acceptor. It can be shown that the most important 

term within the coulombic interaction is the dipole–dipole term, which obeys 

the same selection rules as the corresponding electric dipole transitions of 

the two partners ( ∗ A → AandB→ ∗ B, Fig. 13). Therefore, coulombic energy 

transfer is expected to be efficient in systems in which the radiative transitions 

connecting the ground and the excited states of each partner have high 

oscillator strength. The rate constant for the dipole–dipole coulombic energy


transfer can be expressed as a function of the spectroscopic and photophysical 

properties of the two molecular components: 

k F en = 8.8 × 10–25 K2 Φ 

JF = 

n4r6 JF 

(32) 

ABτ � F(ν)ε(ν)/ν 4 dν 

� F(ν)dν 

, (33) 

where K is an orientation factor which accounts for the directional nature 

of the dipole–dipole interaction (K 2 =2/3 for random orientation), Φ and τ 

are the luminescence quantum yield and lifetime of the donor, respectively, 

n is the solvent refractive index, rAB is the distance (in ˚A) between donor 

and acceptor, and JF is the Förster overlap integral between the luminescence 

spectrum of the donor, F � ν � , and the absorption spectrum of the acceptor, 

ε � ν � ,onanenergyscale(cm –1 ). With a good spectral overlap integral and 

appropriate photophysical properties, the 1/r6 AB distance dependence allows 

energy transfer to occur efficiently over distances largely exceeding the molecular 

diameters. The typical example of an efficient coulombic mechanism 

is that of singlet–singlet energy transfer between large aromatic molecules, 

a process used by nature in the “antenna” systems of the photosynthetic apparatus 

[72]: 

∗ 

A(S1)–L–B(S0) → A(S0)–L– ∗ B(S1). (34) 

4.4.2 

Exchange Mechanism 

The exchange (also called Dexter-type [73]) mechanism requires orbital overlap 

between donor and acceptor, either directly or mediated by the bridge 

(through-bond), and its rate constant, therefore, decreases with increasing 

distance: 

k D 4π2 � en 

en = H 

h 

�2 JD , (35) 

where 

H en = H en � 

(0) exp – βen � � 

rAB – r0 

2 

� 

(36) 

� � � � � 

F ν ε ν dν 

JD = � � � � � � . (37) 

F ν dν ε ν dν 

The exchange interaction can be regarded (Fig. 13) as a double electrontransfer 

process, one electron moving from the LUMO of the excited donor 

to the LUMO of the acceptor, and the other from the acceptor HOMO to 

the donor HOMO. Therefore, the attenuation factor β en for exchange energy


transfer should be approximately equal to the sum of the attenuation factors 

for two separated electron-transfer processes, i.e., βel for electron transfer between 

the LUMOs of the donor and acceptor, and βht for the electron transfer 

between the HOMOs (superscript ht is for hole transfer from the donor to the 

acceptor). This prediction has been confirmed by experiments [74]. 

The spin selection rules for this type of mechanism arise from the need 

to obey spin conservation in the reacting pair as a whole. This allows the 

exchange mechanism to be operative in many cases in which the excited 

states involved are spin-forbidden in the usual spectroscopic sense. Thus, the 

typical example of an efficient exchange mechanism is that of triplet–triplet 

energy transfer: 

∗ 

A(T1)–L – B(S0) → A(S0)–L – ∗ B(T1) . (38) 

Exchange energy transfer from the lowest spin-forbidden excited state is expected 

to be the rule for metal complexes [61, 75]. 

Although the exchange mechanism was originally formulated in terms of 

direct overlap between donor and acceptor orbitals, it is clear that it can be 

extended to cover the case in which coupling is mediated by the intervening 

medium (i.e., the connecting bridge), as discussed above for electron-transfer 

processes (superexchange mechanism) [61]. 

5 

Coordination Compounds as Components 

of Photochemical Molecular Devices and Machines 

In the last few years, a combination of supramolecular chemistry and photochemistry 

has led to the design and construction of supramolecular systems 

capable of performing interesting light-induced functions. Photoinduced energy 

and electron transfer are indeed basic processes for connecting light 

energy inputs with a variety of optical, electrical, and mechanical functions, 

i.e., to obtain molecular-level devices and machines [48, 55]. We will now describe 

a few classical examples of molecular devices and machines in which 

coordination compounds are used to process light signals or to exploit light 

energy. Other examples are, of course, described in the chapters dealing with 

the complexes of the various metals. 

5.1 

A Molecular Wire 

An important function at the molecular level is photoinduced energy and 

electron transfer over long distances and/or along predetermined directions. 

This function can be obtained by linking donor and acceptor components by 

a rigid spacer, as illustrated in Fig. 14.


Fig. 14 Photoinduced energy (a) and electron (b) transfer processes in a molecular wire 

based on coordination compounds [76] 

An example [76] is given by the [Ru(bpy)3] 2+ -(ph)n-[Os(bpy)3] 2+ compounds 

(bpy=2,2 ′ -bipyridine; ph = 1,4-phenylene; n =3,5,7),inwhichexcitation 

of the [Ru(bpy)3] 2+ unit is followed by electronic energy transfer 

to the ground state [Os(bpy)3] 2+ unit, as shown by the sensitized emission 

of the latter. For the compound with n = 7 (Fig. 14a), the rate constant for 

energy transfer over the 4.2-nm metal-to-metal distance is 1.3 × 10 6 s –1 .In 

the [Ru(bpy)3] 2+ -(ph)n-[Os(bpy)3] 3+ compounds, obtained by chemical oxidation 

of the Os-based moiety, photoexcitation of the [Ru(bpy)3] 2+ unit 

causesthetransferofanelectrontotheOs-basedonewitharateconstant 

of 3.4 × 10 7 s –1 for n = 7 (Fig. 14b). Unless the electron added to the 

[Os(bpy)3] 3+ unit is rapidly removed, a back electron-transfer reaction (rate 

constant 2.7 × 10 5 s –1 for n = 7) takes place from the [Os(bpy)3] 2+ unit to the 

[Ru(bpy)3] 3+ one. 

Spacers with energy levels or redox states in between those of the donor 

and acceptor may help energy or electron transfer (hopping mechanism). 

Spacers whose energy or redox levels can be manipulated by an external 

stimulus can play the role of switches for the energy- or electron-transfer processes 

[48]. For a more thorough discussion of photoinduced energy- and 

electron-transfer processes in systems involving metal complexes, see [61].


5.2 

An Antenna System 

In suitably designed dendrimers, electronic energy transfer can be channeled 

toward a specific position of the array. Compounds of this kind play the role 

of antennas for light harvesting. We briefly illustrate an example involving 

luminescent lanthanide ions. For a more extended discussion of dendritic antenna 

systems, see [77]. 

Lanthanide ions are known to show a very long-lived luminescence which 

is a potentially useful property. Because of the forbidden nature of their electronic 

transitions, however, lanthanide ions exhibit very weak absorption 

bands, which is a severe drawback for applications based on luminescence. 

In order to overcome this difficulty, lanthanide ions are usually coordinated 

to ligands containing organic chromophores whose excitation, followed by 

energy transfer, causes the sensitized luminescence of the metal ion (antenna 

effect). Such a process can involve either direct energy transfer from 

the singlet excited state of the chromophoric group with quenching of the 

chromophore fluorescence [78], or, most frequently, via S1 → T1 intersystem 

crossing followed by energy transfer from the T1 excited state of the chromophoric 

unit to the lanthanide ion [79, 80]. 

Amide groups are known to be good ligands for lanthanide ions. The 

dendrimer shown in Fig. 15 is based on a benzene core branched in the 

1, 3, and 5 positions, and it contains 18 amide groups in its branches and 

24 chromophoric dansyl units in the periphery [81]. The dansyl units show 

strong absorption bands in the near-UV spectral region and an intense fluorescence 

band in the visible region. In acetonitrile/dichloromethane (5 : 1 

v/v) solutions, the absorption spectrum and the fluorescence properties of 

the dendrimer are those expected for a species containing 24 noninteracting 

dansyl units. Upon addition of lanthanide ions to dendrimer solutions 

the following effects were observed [81]: (a) the fluorescence of the dansyl 

units is quenched; (b) the quenching effect is very large for Nd 3+ and 

Eu 3+ ,moderateforYb 3+ ,smallforTb 3+ ,andverysmallforGd 3+ ;and 

(c) in the case of Nd 3+ ,Er 3+ ,andYb 3+ the quenching of the dansyl fluorescence 

is accompanied by the sensitized near-infrared emission of the 

lanthanide ion. Interpretation of the results obtained on the basis of the energy 

levels and redox potentials of the dendrimer and of the metal ions 

has led to the following conclusions: (1) at low metal ion concentrations, 

each dendrimer hosts only one metal ion; (2) when the hosted metal ion 

is Nd 3+ or Eu 3+ , all 24 dansyl units of the dendrimer are quenched with 

unitary efficiency; (3) quenching by Nd 3+ takes place by direct energy transfer 

from the fluorescent (S1) excitedstateofdansyltoamanifoldofNd 3+ 

energy levels, followed by sensitized near-infrared emission from the metal 

ion (λmax = 1064 nm for Nd 3+ ); (4) quenching by Eu 3+ does not lead to 

any sensitized emission since a ligand-to-metal charge-transfer level lies be-


Fig. 15 Dendrimer based on a benzene core branched in the 1, 3, and 5 positions, which 

contains 18 amide groups in its branches and 24 chromophoric dansyl units in the periphery 

[81] 

low the luminescent Eu 3+ excited state; (5) in the case of Yb 3+ ,thesensitization 

of the near-infrared metal-centered emission occurs via the intermediate 

formation of an upper lying charge-transfer excited state; (6) the 

small quenching effect observed for Tb 3+ is partly caused by a direct energy 

transfer from the fluorescent (S1) excitedstateofdansyl;and(7)thevery 

small quenching effect observed for Gd 3+ is assigned to either induced intersystem 

crossing or, more likely, to charge perturbation of the S1 dansyl 

excited state.


5.3 

An Extension Cable 

In the attempt of constructing a molecular-level extension cable, the [3]pseudorotaxane 

shown in Fig. 16, made of the three components A 2+ ,[BH] 3+ ,and 

C, has been synthesized and studied [82]. Component A 2+ consists of two 

moieties: a [Ru(bpy)3] 2+ unit, which plays the role of electron donor under 

light excitation, and a crown ether, which plays the role of a first socket. 

The ammonium center of [BH] 3+ , driven by hydrogen-bonding interactions, 

threads as a plug into the first socket, whereas the bipyridinium unit, owing 

to charge-transfer (CT) interactions, threads as a plug into the third component, 

C, which plays the role of a second socket. In CH2Cl2/CH3CN (98 : 2 v/v) 

solution, reversible connection/disconnection of the two plug/socket functions 

can be controlled independently by acid–base and redox stimulation, 

respectively. In the fully connected triad, light excitation of the [Ru(bpy)3] 2+ 

unit of component A 2+ is followed by electron transfer to the bipyridinium 

unit of component [BH] 3+ , which is plugged into component C. Although 

the transferred electron does not reach the final component of the assembly, 

the intercomponent connections employed fulfill an important requirement, 

namely, they can be controlled reversibly and independently. An improved 

example of a molecular extension cable based on [Ru(bpy)3] 2+ has been reported 

more recently [83]. 

Fig. 16 Schematic representation of a supramolecular system that behaves as a molecularlevel 

extension cable [82]


5.4 

An XOR Logic Gate with an Intrinsic Threshold Mechanism 

A system based on the photochemistry of a metal complex has been reported 

to mimic some elementary properties of neurons [84]. Such a system consists 

of an aqueous solution containing the trans-chalcone form (Ct, Fig. 17a) 

of the 4 ′ -methoxyflavylium ion (AH + ), and the [Co(CN)6] 3– complex ion (as 

a potassium salt). Excitation by 365-nm light of Ct, which is the thermodynamically 

stable form of the flavylium species in the pH range 3–7, causes 

a trans → cis photoisomerization reaction (Φ = 0.04). If the solution is sufficiently 

acid (pH


in acid or neutral aqueous solution causes the dissociation of a CN – ligand 

from the metal coordination sphere (Φ = 0.31), with a consequent increase in 

pH (Fig. 17b). 

When an acid solution (pH=3.6) containing 2.5 × 10 –5 mol L –1 Ct and 

2.0 × 10 –2 mol L –1 [Co(CN)6] 3– is irradiated at 365 nm most of the incident 

light is absorbed by Ct, which undergoes photoisomerization to Cc.Sincethe 

pH of the solution is sufficiently acid, Cc is rapidly protonated (Fig. 17a), with 

the consequent appearance of the absorption band with maximum at 434 nm 

and of the emission band with maximum at 530 nm characteristic of the AH + 

species. On continuing irradiation, it can be observed that such absorption 

and emission bands increase in intensity, reach a maximum value, and then 

decrease up to complete disappearance. In other words, AH + first forms and 

then disappears with increasing irradiation time. The reason for the off–on– 

offbehaviorofAH + under continuous light excitation is related to the effect of 

the [Co(CN)6] 3– photoreaction (Fig. 17b) on the Ct photoreaction (Fig. 17a). 

As Ct is consumed with formation of AH + , an increasing fraction of the incident 

light is absorbed by [Co(CN)6] 3– , whose photoreaction causes an increase 

in the pH of the solution. This change in pH not only prevents further formation 

of AH + , which would imply protonation of the Cc molecules that continue 

to be formed by light excitation of Ct, but also causes the back reaction to Cc 

(and, then, to Ct) of the previously formed AH + molecules. Clearly, the examined 

solution performs like a threshold device as far as the input (light)/output 

(spectroscopic properties of AH + ) relationship is concerned. 

Instead of a continuous light source, pulsed (flash) irradiation can be 

used [83]. Under the input of only one flash, a strong change in absorbance at 

434 nm is observed, due to the formation of AH + . After two flashes, however, 

the change in absorbance practically disappears. In other words, an output 

(434-nm absorption) can be obtained only when either input 1 (flash 1) or input 

2 (flash 2) are used, whereas there is no output under the action of none 

or both inputs. This finding shows that the above-described system behaves 

according to XOR logic, under control of an intrinsic threshold mechanism 

(Fig. 17c). 

Two important features of the above system should be emphasized: 

(1) intermolecular communication takes place in the form of H + ions, and 

(2) the input and output signals have the same nature (light) and the fluorescence 

output can be fed, in principle, into another device [85]. 

5.5 

A Sunlight-Powered Nanomotor 

In the last few years, a great number of light-driven molecular machines have 

been developed and the field has been extensively reviewed [48, 55, 86–92]. In 

several cases, the working principle of such machines exploits photoinduced 

electron transfer by [Ru(bpy)3] 2+ or related coordination compounds.


Fig. 18 Chemical formula and cartoon representation of the rotaxane D 6+ ,showingits 

modular structure [94] 

In order to achieve photoinduced ring switching in rotaxanes containing 

two different recognition sites in the dumbbell-shaped component, the thoroughly 

designed rotaxane (D6+ ) shown in Fig. 18 was synthesized [93–95]. 

This compound is made of the electron-donor macrocycle R,andadumbbellshaped 

component which contains (1) [Ru(bpy)3] 2+ (P)asoneofitsstoppers, 

(2) a 4,4 ′ -bipyridinium unit (A1) anda3,3 ′ -dimethyl-4,4 ′ -bipyridinium unit 

(A2) aselectron-acceptingstations,(3)ap-terphenyl-type ring system as 

arigidspacer(S), and (4) a tetraarylmethane group as the second stopper 

(T). The stable translational isomer is the one in which the R component 

encircles the A1 unit, in keeping with the fact that this station is a better 

electron acceptor than the other one. The strategy devised in order to obtain 

the photoinduced abacus-like movement of the R macrocycle, between the 

two stations A1 and A2 illustrated in Fig. 19, is based on the following four 

operations: 

(a) Destabilization of the stable translational isomer: light excitation of the 

photoactive unit P (step 1) is followed by the transfer of an electron from 

the excited state to the A1 station, which is encircled by the ring R (step 2), 

with the consequent “deactivation” of this station; such a photoinduced 

electron-transfer process has to compete with the intrinsic excited-state 

decay (step 3). 

(b) Ring displacement: the ring moves by Brownian motion from the reduced 

station A – 1 to A2 (step 4), a step that has to compete with the back electron- 

transfer process from A – 1 (still encircled by R) to the oxidized photoactive 

unit P + (step 5).


Fig. 19 Schematic representation of the operation of rotaxane D 6+ as a four-stroke linear 

nanomotor powered by sunlight [94] 

(c) Electronic reset: a back electron-transfer process from the “free” reduced 

station A – 1 to P+ (step 6) restores the electron-acceptor power to the A1 

station. 

(d) Nuclear reset: as a consequence of the electronic reset, back movement of 

the ring from A2 to A1 takes place (step 7). 

The crucial point is the competion between ring displacement (step 4) and 

back electron transfer (step 5). The results revealed that in acetonitrile solution 

at room temperature the ring shuttling rate is one order of magnitude 

slower than the back electron transfer. Hence, the absorption of 

light can cause the occurrence of a forward and back ring movement (i.e., 

afullcycle)witha2% quantum yield. The low efficiency is compensated 

by the following features: (1) the operation of the system relies exclusively 

on intramolecular processes, without generation of any waste product; and 

(2) since [Ru(bpy)3] 2+ shows an intense absorption band in the visible region, 

the system works simply upon exposure to sunlight. A much higher efficiency


can be obtained by using an electron relay, again consuming only sunlight 

without generation of waste products [95]. 

6 

Conclusions 

Research on the photochemistry and photophysics of coordination compounds 

has shown an extraordinary quantitative development as well as profound 

qualitative changes over the years. Studies on intramolecular photoreactions 

and luminescence properties of coordination compounds of simple 

ligands have been followed by investigations on compounds containing complex 

synthetic ligands. Characterization of excited-state properties has been 

followed by extensive use of metal complexes in bimolecular processes. With 

the advent of supramolecular chemistry, luminescent and/or photoredox reactive 

metal complexes have been used as essential components in a bottomup 

approach to the construction of molecular devices and machines. In the 

next few years research on the photochemistry and photophysics of coordination 

compounds will largely be concentrated on the development of supramolecular 

systems for solar energy conversion and information processes. In this 

regard, it should be noted that the photoactive components presently used are 

very limited in number. Therefore, there is a need to extend basic research 

in order to discover novel mononuclear coordination compounds capable of 

exhibiting long excited-state lifetimes, reversible redox behavior, and stability 

toward photodecomposition. The large number of metals that can be used 

and the endless number of ligands that can be designed and synthesized open 

an ample horizon to these studies, as illustrated in the other chapters in this 

volume. 

Acknowledgements We acknowledge MIUR (PRIN projects no. 2006034123 & 2006030320), 

the University of Bologna and the University of Messina for financial support. 

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DOI 10.1007/128_2007_141 


Published online: 11 July 2007 


of Coordination Compounds: Chromium 

Noel A. P. Kane-Maguire 

Department of Chemistry, Furman University, 3300 Poinsett Highway, 

Greenville, SC 29613, USA 

noel.kane-maguire@furman.edu 

1 Introduction ................................... 38 

2 State Energy Levels and General Excited State Behavior ........... 38 

3 Ultrafast Dynamics of Cr(III) Ligand Field Excited States ......... 41 

3.1 Ultrafast Dynamics of 2 Eg State Formation in Cr(acac)3 ........... 42 

4 Photosubstitution Studies ........................... 43 

4.1 [Cr(phen)3] 3+ Photoracemization/Hydrolysis................. 43 

4.2 Axial Ligand Photodissociation in Cr(III) Porphyrins ............ 45 

5 Thermally Activated 2 Eg Excited State Relaxation Studies 

of Sterically Constrained Systems ....................... 47 

5.1 [Cr(sen)] 3+ and [Cr[18]aneN6] 3+ ....................... 49 

5.1.1 [Cr(sen)3] 3+ ................................... 49 

5.1.2 [Cr[18]aneN6] 3+ ................................. 50 

5.2 trans-[Cr(N4)(CN)2] + 

(where N4 = cyclam, 1,11-C3-cyclam, and 1,4-C2-cyclam).......... 51 

6 Energy Transfer Studies ............................ 52 

6.1 Self-Exchange Energy Transfer Between Identical Chromophores . . . . . . 53 

7 Photoredox Behavior of [Cr(diimine)3] 3+ Systems .............. 54 

7.1 DNAInteractions ................................ 56 

8 Photoredox Involving Coordinated Ligands ................. 59 

8.1 Photolabilization of NO from Cr(III)-Coordinated Nitrite . . . ....... 59 

8.2 Photogeneration of Nitrido Complexes from Cr(III) Coordinated Azide . . 61 

9 Final Comments ................................. 62 

References ....................................... 63 

Abstract The study of the photochemistry and photophysics of octahedral and pseudooctahedral 

Cr(III) complexes has a rich history. An initial discussion is devoted to 

a general appraisal of the state of these two subjects up to December 1998, after providing 

a framework of state energy levels and radiative and non-radiative relaxation processes 

relevant to Cr(III) systems. The remaining sections cover some of the more active areas 

in the Cr(III) field, such as ultrafast dynamics, photosubstitution, thermally activated

38 N.A.P. Kane-Maguire 

excited state relaxation, energy transfer, and photoactivated redox processes (both intermolecular 

and intramolecular). Each of these sections begins with an overview of the 

subject area, and then one or more representative papers from the recent literature are 

selected for more detailed discussion. 

Keywords Energy transfer · Photoredox · Photosubstitution · 

Thermal excited state relaxation · Ultrafast dynamics 

1 

Introduction 

Investigations of the photobehavior of octahedral (O h) or pseudo-octahedral 

chromium(III) complexes have played a pivotal role in the development of 

transition metal photochemistry as a vital scientific discipline. Except for 

the case of ruthenium(II), the photochemistry and photophysics of Cr(III) 

systems have been explored more fully than those of any other transition 

metal ion. Their photoactivity has been extensively reviewed previously, and 

readers are directed to the coverage in three texts [1–3], and the excellent discussion 

in the most recent comprehensive review of the topic by Kirk (which 

covered the literature up to December 1998) [4]. Several shorter Cr(III) reviews 

have since appeared, which have focused on a diverse range of more 

specific topics such as the excited state chemistry of pentacyanochromate(III) 

anions [5], intermediates in Cr(III) photochemistry [6], emission properties 

of hexam(m)ine Cr(III) systems [7], the interaction of [M(diimine)3] n+ 

complexes of Ru(II) and Cr(III) with DNA [8], and thermal excited state relaxation 

[9]. 

The Cr(III) field remains an active one, and the objective of the present 

chapter is to provide an overview of some of the interesting developments in 

the area from 1999 to December 2006. 

2 

State Energy Levels and General Excited State Behavior 

The electronic configuration of the Cr 3+ ion is [Ar]3d 3 . In its octahedral (Oh) 

complexes the degeneracy of the Cr d orbitals is lifted, resulting in two orbital 

subsets of t2g and eg symmetry. The 4 A2g (quartet) ground state has the 

electronic configuration (t2g) 3 ,withthed orbitals filled according to Hund’s 

Rule. Two excited states with (t2g) 2 (eg) 1 electronic configuration result from 

promotion of a t2g electron to an eg orbital while preserving electronic spin. 

Six such spin-allowed promotions are possible, which in O h symmetry are 

divided into two sets differing in the magnitude of the interelectronic repulsion 

terms. The associated quartet excited states generated are labeled 4 T2g

Photochemistry and Photophysics of Coordination Compounds: Chromium 39 

and 4 T1g , respectively, with the former level being the more stable. For almost 

all octahedral Cr(III) complexes studied, the two lowest-lying excited states 

are the 2 T1g and 2 Eg (doublet) levels of (t2g) 3 electronic configuration, where 

spin-pairing has occurred within the t2g subshell. A qualitative orbital energy 

level diagram depicting the electron occupation of the pertinent electronic 

ground and excited states discussed above is provided in Fig. 1. 

A representative state energy level diagram for octahedral Cr(III) compounds 

was presented in Chap. 1 of this volume (Fig. 5b). Consistent with 

this diagram, for the preponderance of Cr(III) complexes the 2 Eg state has 

the lower energy of the two doublet excited states [1, 2]. However, exceptions 

occur occasionally for mixed ligand systems such as trans-[CrN4F2] + of D4h 

symmetry, where splitting of the 2 T1g (Oh)excitedstate(into 2 A2g and 2 Eg in 

D4h symmetry) results in its 2 Eg (D4h) component lying lower in energy than 

the components of the 2 Eg (O h) level [10, 11]. The generic Jablonski diagram 

shown in Chap. 1 of this volume (Fig. 6) has been modified in Fig. 2 of this 

chapter for the specific case of octahedral Cr(III) complexes. 

Fig. 1 The d orbital diagram for the electronic ground and excited states for d 3 Cr(III) 

complexes assuming Oh symmetry 

Fig. 2 Jablonski state energy level diagram for Oh Cr(III) complexes, showing the principal 

processes that may occur subsequent to vertical excitation to a 4 T2g or 4 T1g 

Franck–Condon excited state. Full arrows represent radiative processes (absorption or 

emission), whereas wavy arrows designate radiationless deactivation processes


The dominant ligand field bands in the UV-visible absorption spectra are 

associated with the 4 A2g → 4 T2g and 4 A2g → 4 T1g transitions, since the corresponding 

absorptions generating the two doublet excited states are both 

Laporte and spin multiplicity forbidden. Photochemical and photophysical 

studies of Cr(III) species are, therefore, usually restricted to initial excitation 

into one of the quartet excited states. The spin-allowed absorption bands for 

the 4 A2g → 4 T2g and 4 A2g → 4 T1g transitions are broad. This is a consequence 

of the 4 T2g and 4 T1g excited states both having an electron residing 

in an eg antibonding σ ∗ orbital (Fig. 1), which results in a large nuclear displacement 

relative to the ground state. For excitation into the higher lying 

4 T1g level, very fast internal conversion (IC) occurs to the 4 T2g state with 

near unit efficiency. Under normal photochemical conditions (i.e., in solution 

near room temperature) 4 T2g → 4 A2g fluorescence is rarely observed [12, 13], 

due to 4 T2g → 2 Eg intersystem crossing (ISC) being an unusually rapid process 

[4, 13–15] and often occurring with high efficiency (see Table 4 in [4]). 

With only occasional exceptions, Cr(III) complexes in rigid low temperature 

media exhibit intense, long-lived phosphorescence from the 2 Eg level generated 

by ISC. Very little geometric change is expected between the 4 A2g ground 

state and 2 Eg excited state, due their common (t2g) 3 orbital parentage. As 

a result, low temperature 2 Eg → 4 A2g phosphorescence spectra often display 

sharp, highly resolved fine structure, which has led to a very extensive 

literature (including medium and temperature effects) on the photophysical 

properties of these systems [2, 4, 9, 11, 16]. 

In room temperature (rt) solution, 4 A2g → 4 T2g (or 4 A2g → 4 T1g )excitation 

often leads to facile substitution of one or more bound ligands by solvent 

or an added nucleophile [1–4]. This observation does not, however, preclude 

the possibility of reaction out of the lower lying 2 Eg level, since this state is 

subsequently populated by rapid and efficient 4 T2g → 2 Eg ISC. An enormous 

effort has been expended over the last 30 years in an attempt to determine 

the relative photochemical roles of the 4 T2g and 2 Eg excited states. Importantly, 

a significant number of Cr(III) complexes exhibit relatively long-lived 

(≥ 100 ns) phosphorescence in solution near rt, and this emission can be bimolecularly 

quenched by added reagents. Comparing photoreaction data for 

experiments carried out in the presence and absence of these added reagents 

(an approach first pioneered by Chen and Porter [17]) has in many instances 

proved very informative. 

The most definitive quenching studies have been for the strong field hexacyano 

complex [Cr(CN)6] 3– [18] and the pentacyano species [Cr(CN)5(X)] n– , 

where X = NH3 [19], pyridine (py) [20], and NCS – [5]. Under experimental 

conditions of total emission quenching, no reaction quenching was detected. 

Such data provide compelling evidence for the reaction proceeding exclusively 

out of the 4 T2g level (a similar conclusion was reached for [Cr(CN)6] 3– 

based on sensitization studies [21, 22]). For most Cr(III) systems, however,


total phosphorescence quenching is accompanied by significant but less than 

total reaction quenching. Definitively establishing the precise role of the 

doublet level in this quenched reaction component has proven an elusive 

goal, with competing options including direct 2 Eg reaction, 2 Eg tunneling to 

a ground state intermediate (GSI) surface, or “delayed” quartet excited state 

reaction via thermally activated 2 Eg → 4 T2g back-intersystem crossing, BISC 

(Fig. 2). 

This debate has been exhaustively discussed elsewhere [4, 6, 23, 24], and 

will not be a focus of this review. From the outset, it was appreciated that 

the 4 T2g level of (t2g) 2 (eg) 1 orbital parentage was an attractive candidate for 

substitution chemistry, based on the occupation of an eg orbital which is σ ∗ 

antibonding with respect to the metal–ligand (M – L) bond. At present, the 

most widely employed theoretical model for rationalizing Cr(III) photosubstitution 

behavior, assuming quartet reactivity, is the semi-empirical symmetry 

restricted angular overlap model (AOM) developed by Vanquickenborne 

and Ceulemans [25–27]. For mixed ligand systems it has had considerable 

success in predicting relative ligand labilities based on identifying the plane 

of labilization and assuming that the ligand with the smallest excited state 

M – L bond strength is preferentially substituted. A further strength of the 

model is the rationalization it provides for the stereochemical change that 

is a common feature of Cr(III) photochemistry (in contrast to their corresponding 

thermal behavior), especially for cases of axial ligand loss in mixed 

ligand systems of D4h or C4v symmetry [4, 28]. A more recent ab initio study 

of the photochemistry of Cr(III) ammine systems yielded results in good 

agreement with the earlier AOM calculations [29]. Tris-polypyridyl Cr(III) 

complexes may prove to be an exception to this apparent preference for quartet 

excited state reactivity. Early quenching studies on [Cr(phen)3] 3+ (where 

phen = 1,10-phenanthroline) revealed up to 95% reaction quenching in the 

presence of doublet quenchers such as I – and NCS – [30, 31], and the data from 

subsequent and more detailed investigations were most readily interpreted in 

terms of a direct doublet excited state reaction pathway [32–34]. 

The remaining sections of this chapter are devoted to a discussion of developments 

since December 1998 in a range of different focus areas of Cr(III) 

photochemistry and photophysics. For convenience, the state term symbols 

for Oh symmetry shown in Fig. 2 are usually employed during these discussions. 

3 

Ultrafast Dynamics of Cr(III) Ligand Field Excited States 

Ultrafast time-resolved absorption spectroscopy constitutes one of the most 

exciting and promising new frontiers in transition metal photochemistry and 

photophysics. The term ultrafast is applied to photoprocesses that occur on


the time scale of nuclear motion, and have until recently been most frequently 

associated with intramolecular processes such as vibrational equilibration 

and conformational dynamics and with medium effects such as solvation 

shell reorientation. The time frames involved are in the femtosecond to low 

picosecond range, and because of the challenging experimental demands, the 

field of ultrafast transition metal spectroscopy is still in its infancy [35–37]. 

As depicted earlier (Fig. 2), for the Franck–Condon excited level generated 

by initial light absorption, evolution down to lower-energy excited states 

involves processes such as IC, ISC, and vibrational relaxation (VR). Conventional 

wisdom based on organic experience suggests that the rate constants 

for these three processes would normally be in the order: kVR >kIC > 

kISC [38]. However, the observation that Cr(III) 2 Eg → 4 A2g phosphorescence 

yields and emission/reaction quenching ratios showed some dependence on 

the wavelength chosen for 4 A2g → 4 T2g excitation led to the suggestion that 

4 T2g → 2 Eg ISC may compete successfully with 4 T2g vibrational cooling [39– 

43]. This possibility received support from picosecond pulse laser studies 

that showed that the rise time for 2 Eg excited state transient absorption was 

shorter than the instrument time response [12–14]. 

3.1 

Ultrafast Dynamics of 2 Eg State Formation in Cr(acac)3 

The first Cr(III) femtosecond spectroscopic study addressing the questions 

raised above has been very recently reported in an elegant study by Juban 

and McCusker for the test case of [Cr(acac)3] (where acac = acetylaceto- 

Fig. 3 Simplified Jablonski state energy level diagram for [Cr(acac)3]


Fig. 4 A one-electron representation of the orbital electron populations for the 4 LMCT 

and 2 LMCT excited states of [Cr(acac)3] 

nate) [15, 37]. The state and orbital energy level diagrams for this molecule 

are shown in Figs. 3 and 4, respectively. 

Both ligand-field 4 A2 → 4 T2 and charge transfer 4 A2 → 4 LMCT pulse excitation 

experiments were performed, the former being the first such study 

for any transition metal system. Time evolution for 2 E excited state formation 

in CH3CN solution at ambient temperature was followed by monitoring 

the transient signal associated with the 2 E → 2 LMCT transition. 

Data analysis yielded the rate constants shown in Fig. 4, and provides convincing 

evidence that 4 T2 → 2 E ISC competes effectively with vibrational 

relaxation in the initially formed 4 T2 state. Coupling these results with 

those from related studies on [Fe(tren(py)3)] 2+ (where tren is tris(2-pyridylmethyliminoethyl)amine) 

[44], the authors argue that for transition metal 

systems the relative nuclear equilibrium displacements of potential energy 

surfaces and the high density of states may have a larger influence on the 

time-course of Franck–Condon excited state relaxation than spin selection 

rules [15, 37]. 

4 

Photosubstitution Studies 

A survey of the literature since 1999 reveals a marked decrease in activity in 

this foundation area of transition metal photochemistry, concomitant with 

the rapid development of the new areas of interest identified in Chap. 1 of this 

volume. For the case of octahedral or pseudo-octahedral Cr(III) species, only 

ten articles have been identified where ligand photosubstitution studies were 

the primary activity investigated [5, 6, 45–52]. Highlights from some of these 

papers are presented below. 

4.1 

[Cr(phen)3] 3+ Photoracemization/Hydrolysis 

The loss of optical activity of Λ-[Cr(phen)3] 3+ upon photolysis in aqueous 

solution exhibits a strong pH dependence [31, 32]. Under acidic conditions,


the rate of direct racemization is much larger than that of acid hydrolysis 

to a cis-[Cr(phen)2(H2O)2] 3+ product. In contrast, at pH ≥ 11, the rate 

of optical activity loss matches closely that for base hydrolysis to a cis- 

[Cr(phen)2(OH)2] + product,anditwassuggestedthatthelossofoptical 

activity under basic conditions occurs primarily via the hydrolysis path. However, 

these data did not preclude the possibility that a substantial fraction of 

rotation loss might occur via direct racemization, provided there is significant 

retention of optical configuration in the base hydrolysis reaction. 

A recent paper demonstrates that chiral capillary electrophoresis (CE) 

provides a very effective direct probe of the extent of racemization of 

parent Λ-[Cr(phen)3] 3+ , while simultaneously determining the optical purity 

of the hydrolysis product [49]. The electropherogram shown in Fig. 5 

(electropherogram A) is for a mixture of rac-[Cr(phen)3] 3+ and cis-rac- 

[Cr(phen)2(H2O)2] 3+ (where 50 mM dibenzoyl-l-tartrate was employed as 

the capillary chiral additive), while the electropherogram for parent Λ- 

[Cr(phen)3] 3+ prior to photolysis is provided in Fig. 5 (electropherogram B). 

Fig. 5 Electropherograms of A an aqueous mixture of 1 mM rac-[Cr(phen)3] 3+ and 1 mM 

cis-rac-[Cr(phen)2(H2O)2] 3+ ,andB a 1 mM aqueous solution of Λ-[Cr(phen)3] 3+ ,using 

50 mM sodium dibenzoyl-l-tartrate as chiral additive in 25 mM borate buffer, pH 9.2 

(20 ◦ C). Detection wavelength = 320 nm 

The corresponding electropherograms for Λ-[Cr(phen)3] 3+ solutions irradiated 

for 24 min at 350 nm at pH 2.2 and pH 11.6, respectively, are presented 

in Fig. 6. 

The pH 2.2 data (Fig. 6) show significant formation of ∆-[Cr(phen)3] 3+ , 

but no bands (in agreement with expectation) for cis-[Cr(phen)2(H2O)2] 3+ 

product. The corresponding pH 11.6 results (Fig. 6) are strikingly different. 

ThereisnoevidencefordirectΛ-[Cr(phen)3] 3+ racemization, while extensive 

hydrolysis is observed with no apparent retention of absolute configuration. 

In a control experiment, no loss of optical activity was observed on irradiating 

a ∆-cis-[Cr(phen)2(OH)2] + solution for 60 min at pH 11.6. These results 

provide direct verification that optical rotation loss for Λ-[Cr(phen)3] 3+


Fig. 6 Electropherograms of 5 mM aqueous solutions of Λ-[Cr(phen)3] 3+ after 350 nm 

photolysis for 24 min at pH 2.2 and pH 11.6, using 50 mM sodium dibenzoyl-l-tartrate as 

chiral additive in 25 mM borate buffer, pH 9.2 (20 ◦ C). Detection wavelength = 320 nm 

under strongly basic conditions is predominantly a consequence of hydrolysis. 

4.2 

Axial Ligand Photodissociation in Cr(III) Porphyrins 

In a series of nanosecond laser photolysis studies, Inamo and coworkers 

have recently explored the details of axial ligand photosubstitution 

(and recombination) for a range of Cr(III) porphyrin systems of the type 

[Cr(porphyrin)(Cl)(L)] in toluene and dichloromethane solution [46, 47, 50, 

51]. Interest in this area derives from the possible biological implications and 

the role of Cr(III) porphyrins in a variety of photocatalytic reactions. 

The presence of the highly conjugated porphyrin ring leads to orbital and 

state energy level diagrams considerably different from those normally en- 

countered for Cr(III) complexes. Iterative extended Huckel calculations [52] 

predict that a vacant d 2 

z and half-filled dxy, dxz,anddyz orbitals of the Cr(III) 

center are located between the HOMO π and LUMO π∗ orbitals of the por- 

phyrin ring, while the empty Cr d 2 

x – 2 

y orbital lies well above the porphyrin 

LUMO π∗ orbital. Weak coupling of the porphyrin (π,π∗ )stateswiththe 

paramagnetic Cr(III) center results in the singlet ground and excited 1 (π,π∗ ) 

states becoming 4S0 and 4S1 levels, respectively, whereas the excited triplet 

3 (π,π∗ )stateissplitintotripdoublet2T1, tripquartet4T1, andtripsextet6T1 levels. The resultant state energy level diagram is shown in Fig. 7 [52]. 

In the studies by Inamo and coworkers, several porphyrin ring systems 

were utilized (tetraphenylporphyrin, octaethylporphyrin, and tetramesitylporphyrin) 

as well as a range of leaving axial ligands (L = H2O, pyridine, 

piperidine, 1-methylimidazole, triphenylphosphine, and triphenylphosphine 

oxide). Analysis of the transient spectra observed, following initial pulse exci-


Fig. 7 State energy level diagram for [Cr(porphyrin)(Cl)(L)] complexes, showing the porphyrin 

(π,π ∗ ) levels following weak coupling with the d orbitals of the paramagnetic 

Cr(III) center 

tation of [Cr(porphyrin)(Cl)(L)] into the 4 S1 (π–π ∗ ) excited state, confirmed 

the formation of the five-coordinate complex, [Cr(porphyrin)(Cl)], produced 

by the photodissociation of the axial ligand L. Spectral evidence was also 

found for generation of the thermally equilibrated 4 T1 and 6 T1 excited states. 

The quantum yield, φ diss, for the photodissociation of L from [Cr(porphyrin)(Cl)(L)] 

0 was found to asymptotically decrease with increasing dissolved 

O2 concentrations towards a constant value. This suggested the presence 

of a quenchable dissociation pathway attributed to the longer-lived 4 T1 

and 6 T1 levels, and a non-quenchable reaction component associated with 

the short-lived (≪ 1 ns) 4 S1 level. The φ diss values were also found to vary 

markedly with the porphyrin ring and axial ligands present. Figure 8 summarizes 

the electronic energy dissipation processes proposed for these Cr(III) 

porphyrin systems. 

Fig. 8 State energy level diagram for [Cr(porphyrin)(Cl)(L)] complexes showing the principal 

relaxation processes following 4 S0 → 4 S1 excitation. Full arrows represent radiative 

processes, whereas wavy arrows refer to radiationless decay pathways


As shown in Fig. 8, the quenchable and non-quenchable dissociation pathways 

are both thought to proceed through lower-lying porphyrin π–Cr dπ 

charge transfer (CT) states [48]. For example, an increase in the electron 

density in a Cr dπ orbital with a z-axis component should lead to a weakening 

in the Cr-axial ligand bond. The rich photobehavior of these systems suggests 

that they would make good candidates for future ultrafast spectroscopic 

studies. 

5 

Thermally Activated 2 Eg Excited State Relaxation Studies 

of Sterically Constrained Systems 

Early studies on Cr(III) complexes of the type [Cr(N4)X2] n+ where N4 is 

the macrocyclic ligand cyclam or tet a (see Fig. 9) and X = Cl – [53, 54], 

CN – [55, 56], NH3 [56–58], and F – [59] revealed a marked difference in the 

photobehavior of the geometric isomers. All the trans isomers are photoinert 

while the cis species are photoreactive. The cyano, ammine, and fluoro systems 

drew particular attention because of their strong emission in rt solution, 

with accompanying lifetimes almost identical to those reported at 77 K. These 

observations were attributed to the steric rigidity of the macrocyclic ring restricting 

access to the thermally activated photochemical relaxation channels 

available to their non-macrocylic analogs. 

Fig. 9 Macrocyclic-N4 ligands, cyclam and tet a 

An extensive literature now exists on the effects of ligand steric constraint 

on 2 Eg excited state relaxation [4, 60–71], with studies by Endicott and coworkers 

being especially noteworthy. Hexaam(m)ine Cr(III) systems have been 

one key area of study of Endicott’s group, where a range of complexes were 

synthesized containing ligands that would be trigonally strained if coordinated 

octahedrally to Cr(III). Their studies provided convincing evidence that 

the more trigonally strained ligand systems underwent more rapid 2 Eg deactivation. 

In an illustrative example [63], the photobehavior of [Cr(en)3] 3+ was


compared with that of the quasi-cage complex [Cr(sen)] 3+ ,where[Cr(sen)] 3+ 

can be regarded as a [Cr(en)3] 3+ analog with a neopentyl cap bonded in a facial 

position. 

X-ray crystallographic data revealed that the CrN6 microsymmetry is virtually 

identical for these two complexes, with the NCrN bond angles in 

[Cr(sen)] 3+ being slightly closer to octahedral. These data and MM2 calculations 

also established considerable trigonal strain in the neopentyl cap for 

[Cr(sen)] 3+ . 

Both compounds were found to have similar 2 Eg → 4 A2g emission lifetimes 

at 77 K(120 µs and171 µs for the en and sen complexes, respectively), 

and fairly comparable quantum yields for photoaquation in rt solution (0.27 

and 0.10, respectively). However, the 2 Eg lifetime for [Cr(sen)] 3+ in ambient 

solution was a factor of 10 4 times shorter than that for [Cr(en)3] 3+ .The 

authors attributed this difference to a thermally activated 2 Eg deactivation 

channel promoted by steric factors associated with the sen complex. The general 

conclusion from this body of work was that large amplitude trigonal 

twists can facilitate thermally activated 2 Eg relaxationforarangeofsterically 

constrained hexaam(m)ine Cr(III) complexes [64]. The authors also 

suggest that this relaxation pathway may have mechanistic implications for 

the photoracemization of Cr(III) species with D3 symmetry [63]. Such a reaction 

channel could, for example, facilitate the trigonal twist pathway invoked 

for the observed photoinversion of Λ-fac-[Cr(S-trp)3] to ∆-fac-[Cr(S-trp)3] 

(where S-trp is the bidentate amino acid ligand S-tryptophan) [72]. 

Fig. 10 Quasi-cage N6 ligand, sen 

The earlier studies on macrocyclic cis- andtrans-[Cr(N4)X2] n+ complexes 

(where X is NH3 or CN – )werealsoexpandedbyEndicott’sgrouptoinclude 

systems where stereochemical perturbations were introduced by the 

presence of methyl substituents in the macrocylic ring in positions near the 

Cr–X coordination sites [65]. Their analysis of X-ray data and MM2 calculations 

supported the hypothesis that the more facile thermally activated 2 Eg 

relaxation of the cis-[Cr(N4)X2] n+ systems is predominantly a stereochemi-


cal effect. It was also argued that 2 Eg back-intersystem crossing (BISC) was 

not likely to be an important component of 2 Eg excited state deactivation 

for Cr(III) complexes with N6 or N4C2 chromophores [64]. This latter conclusion 

has been questioned by Kirk [4, 68], and in Sect. 5.1 the subject is 

discussed further. In Sect. 5.2 some very recent work by the Wagenknecht 

group employing a new series of sterically constrained N4-macrocycles is featured 

[69–71]. 

5.1 

[Cr(sen)] 3+ and [Cr[18]aneN6] 3+ 

5.1.1 

[Cr(sen)3] 3+ 

In a recent report, Kirk and coworkers have reinvestigated the photobehavior 

of [Cr(sen)] 3+ , and compared it with that for the macrocyclic complex, 

[Cr[18]aneN6] 3+ [68]. 

The data obtained for [Cr(sen)] 3+ supported the earlier report of a very 

short doublet lifetime in rt aqueous solution, and the photoaquation quantum 

yield of 0.10 determined upon 4 A2g → 4 T2g excitation was in excellent 

agreement with that recorded earlier. 

However, based on a more detailed investigation of Cr(sen)] 3+ photoaquation, 

it was proposed that this process occurs via the 4 T2g excited state after 

back-intersystem crossing (BISC). The more convincing argument presented 

was that direct irradiation into the 2 Eg state yielded a photoaquation quantum 

yield 22% lower than that for 4 T2g excitation excitation. However, as 

noted by the authors, direct spin-forbidden doublet excitation experiments 

are fraught with difficulties. Their second argument was based on a deter- 

Fig. 11 Macrocyclic-N6 ligand, [18]aneN6


mination of the stereochemistry of the [Cr(sen-NH)(H2O)] 4+ photoaquation 

product via chiral capillary electrophoresis analysis (CE), employing 

d-tartrate as the chiral capillary additive. The primary aquation product 

exhibited only a single peak, consistent with the presence of trans product – 

a result anticipated by AOM theory assuming quartet excited state reactivity. 

However, a control thermal aquation experiment (where isomerism is not expected) 

also yielded the same data. Although an explanation was offered for 

this latter result, the reviewer notes from experience [49, 73] that the CE separation 

of the ∆ and Λ isomers of cis hydrolysis products is often difficult, 

and a definitive assignment of a single peak to the trans isomer can be made 

confidently only after several chiral additives have been tested in the capillary 

buffer medium. 

5.1.2 

[Cr[18]aneN6] 3+ 

No emission was detectable from this compound in rt solution, despite 

the presence of strong, long-lived 2 E → 4 A2g phosphorescence (162 µs) at 

77 K [67]. A temperature dependence study of the lifetime for this transition 

showed the usual low and high temperature regimes, with a single-exponential 

fit to the high temperature region giving an apparent activation energy of 

34 kJ mol –1 . Interestingly, however, the compound was also photoinert in ambient 

solution. X-ray crystallographic data on [Cr[18]aneN6] 3+ indicated S6 

point group symmetry for the complex, with no evidence for trigonal twist 

strain in the [18]aneN6 ligand. The authors argue, therefore, that the Endicott 

thermally activated 2 Eg relaxation model is unlikely to be operative in 

this case. Instead, they propose that fast radiationless decay at rt is a consequence 

of the S6 distortion from octahedral geometry, which leads to a mixing 

of states with doublet and quartet character and a facilitation of 2 Eg ISC to 

the ground state. In the light of data to be presented in Sect. 5.2, one could 

also conjecture whether a contributing factor to the short rt lifetimes might be 

a non-productive reaction pathway involving transient Cr – Nbondcleavage. 

Finally, note is made of the recent communication by Sargeson and coworkers 

on the remarkable photobehavior of the caged hexamine complex, 

[Cr(fac-Me5-D3htricosaneN6] 3+ [74]. This photoinert compound exhibits 

unique photophysical behavior for an N6 chromophoric Cr(III) species 

in rt aqueous solution. In addition to displaying an exceptionally long 2 Eg 

state lifetime (τ = 235 µs), the emission shows a very strong isotope effect 

upon N – Hdeuteration(τ = 1.5 ms). These observations demonstrate that 

2 Eg excited state decay in solution at ambient temperature is dominated by 

2 Eg → 4 A2g radiationless deactivation, promoted by high frequency N – H 

stretching acceptor modes. Importantly, the results also argue against thermally 

activated back-intersystem crossing being a significant 2 Eg deactivation 

pathway for this CrN6 system.


5.2 

trans-[Cr(N4)(CN)2] + (where N4 = cyclam, 1,11-C3-cyclam, and 1,4-C2-cyclam) 

The Wagenknecht group has very recently studied a set of trans-dicyanochromium(III) 

complexes of topologically constrained tetraazamacrocycles, 

namely trans-[Cr(1,11-C3-cyclam)(CN)2] + and trans-[Cr(1,4-C2-cyclam) 

(CN)2] + (Fig. 12) to determine the effect that the additional strap has on 

the overall chemistry and photophysics relative to the cyclam parent complex 

[69–71]. 

Fig. 12 Parent cyclam ligand, and the strapped derivatives 1,4-C2-cyclam and 1,11-C3cyclam 

In their initial work [69, 70], differences in thermal reactivity, UV-visible absorption 

spectra, and low temperature photophysics were adequately explained 

on the basis of steric and symmetry arguments, and differences in numbers 

of N – H oscillators in the molecule. However, marked differences in their rt 

photobehavior eluded explanation. For example, the 1,11-C3-cyclam and 1,4- 

C2-cyclam complexes have rt 2 Eg excited state lifetimes one and three orders of 

magnitude lower, respectively, than the corresponding cyclam complex. 

Furthermore, the lifetimes for complexes with the topologically constrained 

ligands are strongly temperature dependent near rt in acidified aqueous solution 

and the Arrhenius plots are linear [70]. Potential radiationless deactivation 

pathways for the 2 Eg level in these systems are depicted in Fig. 13. 

Of these possibilities, back-intersystem crossing (BISC) was considered 

unlikely on energetic grounds, while net photoreaction was rejected as a significant 

relaxation pathway due to the very low quantum yields for photoaquation 

for all three complexes. Additionally, MM2 studies suggested that 

neither solvent association nor symmetry destroying molecular “twists” are 

likely causes for the data in the temperature-dependent regime [70]. In their 

most recent paper [71], the authors present evidence that a photodissociation 

pathway involving transient Cr-macrocyclic N-bond cleavage (followed 

by rapid ring closure) was the most plausible explanation for the thermally 

activated 2 Eg relaxation. This conclusion received strong support from the 

observation of photodeuteration of the NH protons upon photolysis of the 

cyclam complex in acidified D2O (where thermal deuteration was shown to


Fig. 13 Qualitative potential energy level diagram for Oh Cr(III) complexes showing possible 

radiationless 2 Eg relaxation processes, including a direct deactivation to the ground 

state, b back-intersystem crossing (BISC), c direct doublet reaction, or d surface crossing 

to a ground state intermediate surface (GSI) 

be minimal). The proposed mechanism for this photodeuteration is shown in 

Fig. 14. 

Fig. 14 Possible mechanism for photoinitiated macrocyclic N – H deuteration in acidic 

aqueous solution 

A related paper on the corresponding difluorosystems has recently been 

published [75]. 

6 

Energy Transfer Studies 

Cr(III) complexes were employed as acceptor species in room temperature 

energy transfer experiments between transition metal complexes as early as 

1972 [21, 76], and several years later the first cases appeared where the donor 

and acceptor were both Cr(III) compounds [77, 78]. In the survey since 1999, 

eight articles were identified where energy transfer studies involving Cr(III) 

species were the primary research activity [79–86]. The paper chosen for 

discussion in Sect. 6.1 describes the first report of electronic energy selfexchange 

between Cr(III) complexes [81].


6.1 

Self-Exchange Energy Transfer Between Identical Chromophores 

As noted earlier (Chap. 1 of this volume, Sect. 4.4.2), electronic energy transfer 

involving Cr(III) complexes is expected to proceed via an exchange 

mechanism, and thus effective donor–acceptor orbital overlap is a necessity. 

A large number of cross-exchange energy transfer studies utilizing Cr(III) 

donors and/or acceptors have been undertaken with the objective of determining 

the relative importance of thermodynamics, electronic factors (such 

as orbital overlap), and nuclear factors associated with Franck–Condon restrictions 

[87–92]. 

The study of self-exchange energy transfer is an attractive complementary 

approach, since the effect of thermodynamics on the rate is eliminated. 

However, monitoring self-exchange has proven a serious experimental challenge, 

since the absorption and emission characteristics of the donor and 

acceptor are identical. The only prior report of virtual self-exchange involving 

transition metal systems is that of Balzani and coworkers on several Ru(II) 

polypyridyl systems [93, 94]. In the paper to be discussed [81], advantage 

was taken of the marked enhancements in emission lifetimes and steadystate 

intensities in rt solution for the Cr(III) complexes listed in Table 1 upon 

deuteration of the amine N – Hprotons. 

For each complex, the solution absorption and emission maxima of the 

deuterated and undeuterated compounds were essentially identical, indicating 

the presence of effectively identical chromophores. Irradiation of acidified 

mixtures of the isotopically labeled and unlabeled chromophores leads to the 

Table 1 Emission lifetimes of Cr(III) complexes at 20 ◦ C 

Complex Solvent τH τD Refs. 

(µs) (µs) 

trans-[Cr(cyclam)(CN)2] + H2O 335 1500 [55] 

trans-[Cr(cyclam)(NH3)2] 3+ DMSO 135 1620 [57] 

trans-[Cr(tet a)F2] + H2O 30 234 [59] 

Scheme 1 Energy transfer between long-lived (CrL) andshort-lived(CrS) complexes


reaction sequence shown in Scheme 1. In this scheme, the long-lived and 

short-lived Cr(III) species are labeled CrL and CrS, respectively, while kET 

and k–ET are the corresponding rate constants for forward and reverse energy 

transfer. Likewise, the terms kL and kS are the reciprocals of the lifetimes of 

CrL and CrS, respectively, in the absence of energy transfer. Emission quenching 

of CrL and CrS couldthenbefollowedbyanalyzingthedecayprofile 

following pulsed excitation according to the mathematical formulation developed 

by Maharaj and Winnik [95]. 

For the trans-dicyano and trans-diammine systems, energy transfer rate 

constants at 20 ◦ C(µ = 1.0) were determined to be kET ≫ 7 × 10 6 M –1 s –1 

and 9.7 × 10 6 M –1 s –1 , respectively. However, for trans-[Cr(tet a)F2] + no 

energy transfer was observed, which implied that the rate constant was 

≪ 3 × 10 5 M –1 s –1 . Analysis of these results using Marcus theory lead to 

the important conclusion that electronic effects play a significant role in 

determining the rates of energy transfer self-exchange for these series of 

complexes. 

7 

Photoredox Behavior of [Cr(diimine)3] 3+ Systems 

Arguably, the seminal report by Gafney and Adamson in 1972 [96] that the 

3 MLCT excited state of [Ru(bpy)3] 2+ (where bpy is 2,2 ′ -bipyridine) could 

function as an electron transfer agent was the catalytic event that led to the 

extraordinary growth of transition metal photochemistry and photophysics 

over the last three decades [94, 97–99]. Today, the polypyridyl compounds 

of Ru(II) still hold a favored status, due to a coalescence of desirable properties 

including an intense, relatively long-lived luminescence signature, and 

a remarkable thermal robustness in a range of oxidation states. 

The analogous polypyridyl complexes of Cr(III) are the next most investigated 

[M(diimine)3] n+ systems. A few years after the Gafney and Adamson 

article appeared, Bolletta et al. presented the first evidence that the ligandfield 

2 Eg excited state of [Cr(bpy)3] 3+ was a strong one-electron photooxidant 

[100]. This involvement of polypyridyl Cr(III) species in direct bimolecular 

electron transfer reactions of the generic type represented in Eq. 1 has 

since been thoroughly documented for numerous substrates, Q [101–106]: 

( 2 Eg)Cr 3+ +Q→ Cr 2+ +Q + . (1) 

Importantly, [Cr(diimine)3] 3+ complexes are more powerful photooxidants 

than their [Ru(diimine)3] 2+ analogs. The oxidizing power of the Cr(III) 2 Eg 

excited state can be assessed from the value of the 2 Eg excited state reduction 

potential, E o ( ∗ Cr 3+ /Cr 2+ ). It has been shown [102, 103] that this latter quantity 

can be reliably estimated from the difference between the 2 Eg → 4 A2g


emission energies in eV units and the ground state standard reduction potentials, 

E o (Cr 3+ /Cr 2+ ), obtained from cyclic voltammetry (CV) measurements. 

A representative illustration of the relevant energetics is shown in Fig. 15 

for the case of [Cr(bpy)3] 3+ ,fromwhichE o ( ∗ Cr 3+ /Cr 2+ ) is determined to be 

1.44 V versus NHE in aqueous solution [103]. 

Fig. 15 Energetics associated with 2 Eg excited state oxidizing power 

In contrast, the corresponding E o ( ∗ Ru 2+ /Ru + ) value for [Ru(bpy)3] 2+ 

(where ∗ Ru 2+ is the 3 MLCT excited state) is reported as 0.84 V [107]. The 

primary reason for these differences is the relatively minor energetic cost 

of ground state M n+ → M (n–1)+ reduction in the Cr(III) case, which leaves 

approximately 85% of the free energy of the 2 Eg excited state available for 

photoredox (as opposed to 40% for the Ru(II) analog). 

Another important observation is that the 2 Eg → 4 A2g emission signal 

of [Cr(diimine)3] 3+ complexes in ambient solution is significantly quenched 

by the presence of dissolved oxygen, 3 O2, as the result of an energy transfer 

process generating excited state singlet oxygen ( 1 O2) [103, 105, 108]: 

( 2 Eg)Cr 3+ + 3 O2 → ( 4 A2g)Cr 3+ + 1 O2 . (2) 

SingletoxygenproductioninEq.2thenprovidesanalternativemethodfor 

substrate oxidation, where the Cr(III) 2 Eg excited state is functioning as 

a photocatalyst. During the present review period, Pagliero and Argüello 

examined the role of O2 in the photooxidation of phenols in aqueous solution, 

employing [Cr(phen)3] 3+ as the photocatalyst [109]. Although direct 

phenol oxidation according to Eq. 1 is thermodynamically feasible, under airsaturated 

conditions the net photochemistry is dominated by a singlet oxygen 

mediated pathway leading to benzoquinone as the sole organic product. The 

results confirm and amplify the observations from an earlier study [110], and 

have practical relevance to the emerging field of photoremediation of waste 

waters [111]. 

Despite the large number of molecules that have been shown to quench 

the 2 Eg excited state of [Cr(diimine)3] 3+ complexes via Eq. 1, biological substrates 

have very rarely been employed in this role. Several recent studies


utilizing DNA as the potential quenching species are highlighted in the following 

section. 

7.1 

DNA Interactions 

The last 20 years have witnessed the emergence of a rich chemistry associated 

with the non-covalent interaction of chiral [M(diimine)3] n+ complexes 

with duplex DNA, with the ultimate goal of developing new diagnostic and 

therapeutic agents [112, 113]. Of particular interest in the present context is 

the potential utility of [M(diimine)3] n+ systems as DNA photocleavage agents 

via excited state redox processes, which could lead to applications in the general 

field of photodynamic therapy [111]. The most widely explored systems 

have been [Ru(diimine)3] 2+ species. However, except for a few notable exceptions 

[114], values for E 0 ( ∗ Ru 2+ /Ru + )fallwellshortofthe1.2 Vvalue 

required for direct one-electron oxidation of guanine (the most readily oxidized 

nucleobase [115]) via a reaction pathway analogous to Eq. 1 above. 

Although [Ru(diimine)3] 2+ systems are known to photoinitiate DNA oxidation, 

this damage normally occurs via the intermediacy of a singlet oxygen 

pathway analogous to Eq. 2 [116, 117]. 

In view of the markedly higher oxidative power of the 2 Eg excited state 

of Cr(III) polypyridyl complexes (approximately 1.4 V versus NHE), such 

species would appear to be more attractive candidates for photoinitiated direct 

oxidation of DNA via Eq. 1 (where Q = DNA). Another potential advantage 

of these d 3 systems with regard to bimolecular redox activity is their longerlived 

2 Eg → 4 A2g emission (often two orders of magnitude greater than that 

for the analogous Ru(II) 3 MLCT emission signals [106]). These photoredox 

expectations have been experimentally confirmed in several reports in the 

present review period [118–120]. 

In the first of these studies, the interaction of the complexes [Cr(phen)3] 3+ 

and [Cr(bpy)3] 3+ with duplex DNA and a range of mononucleotides was explored 

[118]. A key observation was that the Cr(III) emission signals were 

strongly quenched in the presence of guanine-containing nucleotides, but not 

by the mononucleotides of adenine, cytosine, or thymidine, nor by the synthetic 

polynucleotide, poly(dA-dT) · poly(dA-dT). A representative example of 

Cr(III) emission quenching in shown in Fig. 16 for the case of [Cr(phen)3] 3+ in 

the presence of calf thymus B-DNA (which has 40%GCbasepairs). 

Such behavior provides strong evidence for direct oxidation of the guanine 

base of DNA via Eq. 1, since the corresponding oxidation of the other 

nucleobases is thermodynamically more difficult [115]. Since guanine oxidation 

has been shown in other systems to serve as a genesis point for DNA 

strand scission [115], these Cr(III) complexes show potential as a new class 

of DNA photocleavage agents (photonucleases). This expectation receives 

support from our recent observation of permanent DNA damage (strand


Fig. 16 Quenching of [Cr(phen)3] 3+ steady-state emission in air-saturated 50 mM Tris- 

HCl buffer (pH 7.4) by calf thymus B-DNA (22 ◦ C) 

scission) from agarose gel electrophoresis studies of [Cr(phen)3] 3+ bound 

to supercoiled ΦX174 RF plasmid DNA following photolysis at 350 nm (see 

Fig. 17) (Barnett et al., unpublished observations). In such studies, the detection 

of open-circular DNA following photolysis is clear evidence for a single 

strand break in the DNA [116]. 

Fig. 17 Photoactivated cleavage at pH 7.4 of 5 µM ΦX174 plasmid DNA by 200 µM 

[Cr(phen)3] 3+ following irradiation at 350 nm (Rayonet). Samples were subjected to electrophoresis 

on 1% agarose gels for 3 hat 70 V, followed by staining with ethidium 

bromide 

It is also noteworthy that while photodamage in the analogous Ru(II) cases 

is dramatically decreased in the absence of O2 (consistent with a singlet O2 

pathway), photodamage by [Cr(phen)3] 3+ is considerably greater under a N2 

atmosphere (Barnett et al., unpublished observations). Since many cancer


cells are hypoxic [111], the increase in photodamage at lower O2 levels may 

provide a selectivity advantage for these [Cr(diimine)3] 3+ reagents in terms 

of their future potential as phototherapeutic agents. 

In another aspect of this study [118], a mathematical analysis of the emission 

quenching data was undertaken. Representative steady-state intensity 

and lifetime Stern–Volmer (SV) plots for quenching of [Cr(phen)3] 3+ emissionbycalfthymusB-DNAareshowninFig.18.Fromthelifetimedata, 

a bimolecular quenching rate constant of 1.1 × 10 8 M –1 s –1 was extracted 

(a value close to that anticipated for a diffusion controlled process). In contrast, 

the steady-state SV plot showed strong upward curvature at higher DNA 

concentrations. This observation was attributed to the formation of a nonluminescent 

[Cr(phen)3] 3+ /DNA ion pair, and allowed an estimation to be 

made for the binding constant with DNA (KDNA ≈ 4000 M –1 ). 

Fig. 18 Stern–Volmer plots for [Cr(phen)3] 3+ emission quenching in air-saturated 50 mM 

Tris-HCl buffer (pH 7.4) by calf thymus B-DNA at 22 ◦ C: • steady-state data, � lifetime 

data 

A limitation of this initial work with [Cr(bpy)3] 3+ and [Cr(phen)3] 3+ 

is the relatively small binding constants of these compounds with DNA. 

In a subsequent study [119], the photoredox behavior of the complex 

[Cr(phen)2(DPPZ)] 3+ with DNA was investigated, where the third diimine 

ligand is dipyridophenazine, DPPZ. The value of KDNA increased by two 

orders of magnitude, consistent with the known ability of the DPPZ ligand 

to intercalate into DNA base stacks [113]. Perhaps more importantly, the 

complex was found to have an E o ( ∗ Cr 3+ /Cr 2+ )value80 mV more positive 

than that for [Cr(phen)3] 3+ , which placed it in the thermodynamic threshold 

range required for direct oxidation of the nucleobase adenine [115]. In accord 

with this thermodynamic argument, SV plots of the quenching of the emission 

lifetime of [Cr(phen)2(DPPZ)] 3+ in the presence of deoxyguanosine-5 ′ - 

monophosphate and deoxyadenosine-5 ′ -monophosphate yielded quenching 

rate constants of 2.4 × 10 9 M –1 s –1 and 1.8 × 10 7 M –1 s –1 , respectively [119]. 

More recently [120], a report by Vaidyanathan and Nair describes nucleobase 

photooxidation by the terpyridine Cr(III) derivatives [Cr(ttpy)2] 3+


and [Cr(Brphtpy)2] 3+ (where ttpy = p-tolylterpyridine and Brphtpy = pbromophenylterpyridine). 

The two complexes were reported to emit strongly 

in rt aqueous solution, although no emission lifetimes or spectra (except 

wavelength maxima) were provided. Such emission is quite remarkable in 

view of the exceedingly weak emission and very short lifetime (≈ 0.05 µs) 

found for the parent terpyridine complex, [Cr(tpy)2] 3+ [103]. Based on CV 

data and the reported emission spectral maxima, exceptionally high values 

for E o ( ∗ Cr 3+ /Cr 2+ ) were assessed for [Cr(ttpy)2] 3+ and [Cr(Brphtpy)2] 3+ 

(1.65 Vand1.85 V, respectively). Consistent with these thermodynamic observations, 

both complexes were demonstrated to be very powerful photooxidants. 

This was especially true for [Cr(Brphtpy)2] 3+ ,whereitsemission 

was quenched by all four mononucleotides (including deoxythymidine- 

5 ′ -monophosphate). This statement, however, requires that the labels for 

Figs. 4A and B in the paper were accidentally reversed. 

8 

Photoredox Involving Coordinated Ligands 

Whereas Sect. 7 was concerned with examples of intermolecular electron 

transfer between Cr(III) excited states and external substrates, attention 

is directed in the present section to cases of intramolecular redox chemistry 

involving the coordinated ligands. These studies have usually involved 

photoexcitation into high-energy LMCT excited states involving 

the ligand in question, which often results in the transient formation of 

a Cr(II)/ligand radical pair. The subject has been reviewed by Kirk [4], 

and some representative examples of molecules previously investigated are 

[Cr(NH3)5Br] 2+ [121] and trans-[Cr(tfa)3] (where tfa is the anion of 1,1,1trifluoro-2,4-pentanedione) 

[122]. Some of the more recent contributions in 

this area are discussed in the following two sections. 

8.1 

Photolabilization of NO from Cr(III)-Coordinated Nitrite 

It has been recently established that nitric oxide (NO) regulates a number 

of mammalian biological processes, including blood pressure, neurotransmission, 

and smooth muscle relaxation [123]. Additionally, tumor cells are 

particularly sensitive to NO, which induces programmed cell death [124] 

and limits metasis [125]. In response to these findings, Ford and coworkers 

have developed a range of air-stable, water-soluble nitrito-Cr(III) macrocyclic 

complexes, which display photochemically activated NO release [126–128]. 

The initial study involved the complex trans-[Cr(cyclam)(ONO)2] + [126], 

which for convenience is labeled structure I in Fig. 19.


Fig. 19 Representative trans-[Cr(macrocycle)(ONO)2] + complexes 

The only product of 436 nm continuous photolysis of I in deaerated pH 7 

aqueous solution was trans-[Cr(cyclam)(H2O)(ONO)] 2+ ,formedinasubstitution 

step in only low quantum yield (φaq = 0.009). However, when the 

same photolysis was performed in air-saturated solution, a very different 

product was formed in much higher yield (φO2 = 0.27). On the basis of 

mass spectral and EPR evidence, this Cr final product was formulated as the 

oxo-Cr(V) species, trans-[Cr(cyclam)(O)(ONO)] 2+ . In addition, NO gas release 

was confirmed employing an NO-specific electrode sensor. Transient 

absorption spectral studies indicated that NO release occurred in a rapidly 

reversible earlier step (see Scheme 2) involving homolytic cleavage of coordinated 

nitrite ion in I to yield the transitory oxo-Cr(IV) product, trans- 

[Cr(cyclam)(O)(ONO)] + . 

Scheme 2 Photoinitiated reaction scheme for trans-[Cr(cyclam)(ONO)2] + 

In deaerated solution, the transient rapidly reforms the parent complex, 

resulting in simple photoaquation being the only observed net reaction. 

Under air-saturated conditions, however, the transient species is very rapidly 

scavenged by dissolved O2 to generate the oxo-Cr(V) final product, leading to 

the net release of NO gas. The overall mechanism is summarized in Scheme 2. 

In an effort to utilize this reaction scheme in a more practical NO delivery 

system, the Ford group subsequently synthesized a series of complexes where


Fig. 20 Mechanism of NO release following light absorption by a pendant aromatic antennae 

strongly absorbing pendant aromatic chromophores were attached to the 

macrocyclic ring [128–130]. Two of these second-generation Cr(III)nitrito 

systems are shown in Fig. 19, where molecules II and III have anthracenyl 

and pyrenyl pendant arms, respectively. The normally strong fluorescence 

of these tethered aromatics was largely quenched upon Cr(III) coordination, 

consistent with fast intramolecular energy transfer to the lower lying 

Cr(III) ligand field excited states followed by NO gas release according to 

Scheme 2 [128]. The overall NO-generating process for these promising lightgathering 

antennae systems is depicted in Fig. 20 for the pyrenyl-pendant 

complex (molecule III). 

8.2 

Photogeneration of Nitrido Complexes from Cr(III) Coordinated Azide 

The complex [Cr(NH3)5N3] 2+ containing the azido ligand, N3 – ,wasthesubject 

of several photochemical studies during the 1970s [131–134]. The results 

from irradiations in the LMCT region (λ ≤ 330 nm)identifiedthepresenceof 

two competing processes, involving the formation of azide radical, N3 · and 

nitrene, N – , intermediates (Eqs. 3 and 4, respectively) [132–134]: 

[CrIII (NH3)5N3] 2+ + hν → [CrII (NH3)5(N3·)] 2+ → 1.5N2 

[CrIII (NH3)5N3] 2+ + hν → [CrIII (NH3)5N] 2+ +N2 

Product analysis was complicated by the thermal reactions of the radical 

species generated. However, based in part on the differences expected in the 

N2 gas yields for the two processes, Katz and Gafney [132, 134] concluded that 

initial nitrene formation was the dominant reaction pathway. Although the fi- 

(3) 

(4)


nal fate of the nitrene intermediate was not established, Sriram and Endicott 

noted that quasi-thermodynamic calculations suggested the lowest energy 

product ground state for this system would be a Cr(V) species [133]. 

More recent photochemical investigations on azido complexes of Cr(III) 

have focused on systems containing tetradentate ligands such as N2O4 Schiffbases 

[135] and N3 or N4 macrocyclic ligands [136, 137] as stable non-leaving 

groups. For many of these systems, air-stable, solid products have been isolated, 

and have been fully characterized by a variety of spectroscopic probes 

(including X-ray crystallography). These studies provide convincing evidence 

for the formation of stable Cr(V) complexes containing the nitrido ligand, 

N3– , formed via the generic photoreaction shown in Eq. 5: 

[CrIII – N3] 2+ + hν → [CrV ≡ N] 2+ +N2 

(5) 

Evidence for a Cr(V) product is based on the diagnostic EPR signature displayed 

by this d1 metal ion. The presence of a Cr ≡ Ntriplebondinthe 

product is also in accord with the short Cr – N bond distances observed in Xray 

crystallographic studies, and the presence of a strong infrared absorption 

in the 1020–1150 cm –1 region [135–138]. 

During the present review period, the charge transfer photochemistry of 

several azido-Cr(III) complexes containing new Schiff-base ligands as the 

non-leaving groups were examined [139, 140]. In the first of these papers, 

no crystallographic evidence was presented for Cr(V)-nitrido product formation, 

but this product assignment was strongly supported by EPR and infrared 

spectral results [139]. In the second contribution, a potentially valuable 

biochemical application of azido-Cr(III) photochemistry is reported by Shrivastava 

and Nair [140]. An azido-Cr(III) Schiff-base complex was irradiated 

in the presence of bovine serum albumin (BSA), and the photolyte examined 

by sodium dodecyl sulfate-polyacrylamide disc electrophoresis (SDS-PAGE). 

The SDS-PAGE results revealed that the BSA protein was cleaved at multiple 

sites, non-specifically into smaller peptide fragments. The protein cleavage 

was attributed to the azido-Cr(III) complex binding at multiple sites, and being 

subsequently converted to the reactive nitrido-Cr(V) species upon light 

activation. This light-promoted protease activity bears some analogies with 

the photonuclease activity discussed earlier for [Cr(diimine)3] 3+ interactions 

with DNA (Sect. 7.1). A non-selective photonuclease could be utilized in 

a variety of applications, including protein sequencing. 

9 

Final Comments 

In this chapter, the author has attempted to provide an overview of recent 

progress in the field of Cr(III) photochemistry and photophysics, with a more 

detailed focus on certain topics of interest. For cohesiveness, it was not pos-


sible to cover some aspects of the field that did not naturally fit within the 

scope of those focus areas. 

Included in the material not covered, is the recent contribution by Ronco 

and coworkers [141], where advantage is taken of the sometimes exquisite dependence 

on environmental factors of the emission intensity and lifetime of 

Cr(III) polypyridyls. This sensitivity was used to probe hydrophobic sites in 

anionic polyelectrolytes, where such information may provide useful guidance 

in attempts to enhance the rates of photoactivated electron transfer 

processes and/or retard recombination events. More recently [142], in an effort 

to more finely tune 2 Eg excited state properties, their group synthesized 

a range of mixed ligand polypyridyls of Cr(III), using a procedure we had 

developed earlier [143]. Another area not addressed is the increasing use of 

Cr(III) complexes in spectral hole-burning experiments to overcome spectral 

broadening in condensed phases [144, 145]. Finally, one of the more intriguing 

topics omitted is a report in Nature in 2000 describing an experimental 

confirmation [146] of a theoretical prediction, termed magnetochiral dichroism, 

that a chiral medium would absorb light traveling parallel to a magnetic 

field differently from light traveling antiparallel [147]. The compound investigated 

was [Cr(oxalate)3] 3– , and a very small, strongly excitation wavelengthdependent, 

induction of optical activity was observed on laser irradiation in 

a very powerful magnetic field (up to 15 Tesla). 

In conclusion, it is noted that although the subject of Cr(III) photochemistry 

and photophysics is unlikely to reassume the degree of prominence it 

held up until the early 1970s, the present condition of the field is good and the 

long-term prognosis is excellent. Who is to say that the sign on my laboratory 

door which boldly states: “Chromium – The Final Frontier”, will not one day 

be more than just a catchy phrase? 

Acknowledgements The author gratefully acknowledges stimulating discussions with Paul 

Wagenknecht and John Wheeler during the preparation of this chapter. In early 2006, the 

field of Cr(III) photochemistry and photophysics lost one of its young luminaries, Marc 

Perkovic. This chapter is dedicated to his memory. 

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Top Curr Chem (2007) 280: 69–115 

DOI 10.1007/128_2007_128 


Published online: 24 May 2007 

Photochemistry and Photophysics of Coordination 

Compounds: Copper 

Nicola Armaroli (✉) · Gianluca Accorsi · François Cardinali · Andrea Listorti 

Molecular Photoscience Group, Istituto per la Sintesi Organica e la Fotoreattività, 

Consiglio Nazionale delle Ricerche, Via Gobetti 101, 40129 Bologna, Italy 

nicola.armaroli@isof.cnr.it 

1 AnOverviewofCopper............................. 70 

1.1 HistoricalNotes,CurrentUse,ConsumptionTrends ............. 70 

1.2 Chemical Properties, Coordination Geometries, 

ExcitedStates—Cu(I)vs.Cu(II) ........................ 71 

1.3 CopperinBiology................................ 73 

1.4 Cu(I)inSupramolecularChemistry ...................... 78 

2 Cu(I)-Bisphenanthroline Complexes ...................... 81 

2.1 GroundStateGeometry............................. 81 

2.2 AbsorptionSpectra ............................... 83 

2.3 Excited State Distortion: Pulsed X-ray 

andTransientAbsorptionSpectroscopy.................... 86 

2.4 EmissiveExcitedState(s)andLuminescenceSpectra............. 88 

2.5 Photoinduced Processes in Multicomponent Systems Based on 

[Cu(NN)2] + Complexes............................. 92 

2.6 BimolecularQuenchingProcesses ....................... 94 

3 Heteroleptic Diimine/Diphosphine [Cu(NN)(PP)] + Complexes ....... 95 

3.1 PhotophysicalProperties ............................ 95 

3.2 OLEDandLECDevices............................. 99 

4 Cuprous Clusters ................................ 101 

4.1 CuprousHalideClusters ............................ 101 

4.2 CuprousIodideClusters ............................ 102 

4.3 OtherCopperClusters ............................. 105 

5 Miscellanea of Cu(I) Luminescent Complexes ................ 107 

6 Conclusions and Perspectives ......................... 109 

References ....................................... 110 

Abstract Cu(I) complexes and clusters are the largest class of compounds of relevant 

photochemical and photophysical interest based on a relatively abundant metal element. 

Interestingly, Nature has given an essential role to copper compounds in some biological 

systems, relying on their kinetic lability and versatile coordination environment. Some 

basic properties of Cu(I) and Cu(II) such as their coordination geometries and electronic 

levels are compared, pointing out the limited significance of Cu(II) compounds 

(d 9 configuration) in terms of photophysical properties. Well-established synthetic protocols 

are available to build up a variety of molecular and supramolecular architectures

70 N. Armaroli et al. 

(e.g. catenanes, rotaxanes, knots, helices, dendrimers, cages, grids, racks, etc.) containing 

Cu(I)-based centers and exhibiting photo- and electroluminescence as well as 

light-induced intercomponent processes. By far the largest class of copper complexes investigated 

to date is that of Cu(I)-bisphenanthrolines ([Cu(NN)2] + ) and recent progress 

in the rationalization of their metal-to-ligand charge-transfer (MLCT) absorption and luminescence 

properties are critically reviewed, pointing out the criteria by which it is now 

possible to successfully design highly emissive [Cu(NN)2] + compounds, a rather elusive 

goal for a long time. To this end the development of spectroscopic techniques such as 

light-initiated time-resolved X-ray absorption spectroscopy (LITR-XAS) and femtosecond 

transient absorption have been rather fruitful since they have allowed us to firmly ground 

the indirect proofs of the molecular rearrangements following light absorption that had 

accumulated in the past 20 years. A substantial breakthrough towards highly emissive 

Cu(I) coordination compounds is constituted by heteroleptic Cu(I) complexes containing 

both N- and P-coordinating ligands ([Cu(NN)(PP)] + ) which may exhibit luminescence 

quantum yields close to 30% in deaerated CH2Cl2 solution and have been successfully 

employed as active materials in OLED and LEC optoelectronic devices. Also copper clusters 

may exhibit luminescence bands of halide-to-metal charge transfer (XMCT) and/or 

cluster centered (CC) character and they are briefly reviewed along with miscellaneous 

Cu(I) compounds that recently appeared in the literature, which show luminescence 

bands ranging from the blue to the red spectral region. 

Keywords Clusters · Copper · Electron transfer · Energy transfer · Luminescence · 

OLED · Phenanthroline 

1 

An Overview of Copper 

1.1 

Historical Notes, Current Use, Consumption Trends 

Copper was known to some of the oldest civilizations on record, and has a historyofusethatisatleast10 

000 years old. A copper pendant was found in 

what is now northern Iraq that dates to 8700 B.C. and by 5000 B.C. there are 

signs of copper smelting from simple copper compounds such as malachite or 

azurite. This process appears to have been developed independently in several 

parts of the world since several centuries B.C., including Anatolia, China, 

Central America and West Africa. The Egyptians found that, upon addition 

of small amounts of tin, copper becomes easier to cast, and bronze alloys 

were extensively found in the Nile valley. The use of bronze was so pervasive 

in a certain era of civilization that the period spanning from 2500 to 600 

B.C. is named the Bronze Age. In Roman times, copper became known as aes 

Cyprium, aes being the generic Latin term for copper alloys such as bronze 

or other metals, and Cyprium because so much of it was mined in the island 

of Cyprus. From this, the phrase was simplified to cuprum (originating the 

current chemical symbol) and then eventually Anglicized into copper.

Photochemistry and Photophysics of Coordination Compounds: Copper 71 

Copper is usually found in Nature in association with sulfur. Pure copper 

metal is generally produced from a multistage process, beginning with the 

mining and concentrating of low-grade ores containing copper sulfide minerals, 

and followed by smelting and electrolytic refining to produce a pure 

copper cathode. An increasing share of this metal is produced from acid 

leaching of oxidized ores. Because of its properties which include high ductility, 

malleability, thermal and electrical conductivity, and resistance to corrosion, 

copper has become a major industrial metal, ranking third after iron 

and aluminum in terms of quantities consumed. Electrical uses of copper, 

including power transmission and generation, building wiring, telecommunication, 

and electronic products, account for about three quarters of total 

employment.Today,copperby-productsfrommanufacturingandobsolete 

copper products are readily recycled and contribute significantly to supply. 

This is becoming a necessity due to the increasing difficulty of production to 

meet current world demand which has led to a quintuplication of the copper 

price during the last seven years, rising from $0.60/pound in June 1999 

to $3.75/pound in May 2006. In 2005 14.9 million tons of copper were mined 

around the world; the global world reserves, economically recoverable with 

current technologies, amount to 470 million tons [1]. It has been recently 

estimated that ca. 25% of the copper stock initially available in the lithosphere 

has been already placed in use or in wastes from which it will probably 

never be recovered. This poses concern about the sustainability of current 

consumption trends of such a valuable commodity in the mid-long term [2]. 

1.2 

Chemical Properties, Coordination Geometries, Excited States—Cu(I) vs. Cu(II) 

Copper is a transition element belonging to the same group of the periodic 

table as gold and silver, these elements are sometimes referred to as the coinage 

metals in recognition of their historically widespread use in stamping coins. 

Copper has a single s electron outside the filled 3d shell but its properties have 

essentially nothing in common with alkali metals except for the possibility of 

assuming the +1 oxidation state. The filled d shell is not very effective in shielding 

the s electron from the nuclear charge, so the first ionization enthalpy of Cu 

is higher than that of the alkali metals. Since the electrons of the d shell are also 

involved in metallic bonding, the heat of sublimation and the melting point of 

Cu are also much higher than those of the alkalis. The above factors, taken together, 

are responsible for the noble character of copper. Indeed copper is the 

only industrial pure metal, used on a massive scale, exhibiting a positive electrochemical 

potential: for this reason it is not corroded by acids, unless they 

are strongly oxidizing like HNO3 and H2SO4. 

Copper in solution has two common oxidation states: + 1 and + 2. Because 

of their intrinsically superior photochemical and photophysical properties 

(vide infra), in this review our attention is focused on Cu(I) complexes, which


can be classified in three main categories, i.e. anionic complexes (e.g. alocomplexes), 

neutral clusters and cationic complexes. The photochemistry of 

Cu(I) complexes, also related to environmental aspects, has already been 

reviewed [3, 4], here we will essentially focus on photophysics. Anionic complexes 

do not exhibit attractive photophysical properties (e.g. luminescence), 

unlike cluster and cationic complexes which show a very rich photophysical 

behavior. Among the latter, the most extensively investigated are NN-type 

(where NN indicates a chelating imine ligand, typically 1,10-phenanthroline) 

or PP-type (where PP denotes a bisphosphine ligand). Both homoleptic 

[Cu(NN)2] + and heteroleptic [Cu(NN)(PP)] + motifs have been investigated. 

The coordination behavior of Cu(I) is strictly related to its electronic 

configuration. The complete filling of d orbitals (d 10 configuration) leads 

to a symmetric localization of the electronic charge. This situation favors 

a tetrahedral disposition of the ligands around the metal center in order to 

locate the coordinative sites far from one another and minimize electrostatic 

repulsions (Fig. 1). Clearly, the complete filling of d orbitals prevents d-d 

metal-centered electronic transitions in Cu(I) compounds. On the contrary, 

such transitions are exhibited by d 9 Cu(II) complexes and cause relatively intense 

absorption bands in the visible (VIS) spectral window. The lowest ones 

extend into the near infrared (NIR) region (above 800 nm for the Cu(II) aqua 

ion) [5] and deactivate via ultrafast non-radiative processes. The fact that 

the lowest electronic states of Cu(II) complexes are ultra-short lived make 

them far less interesting than Cu(I) complexes from the photophysical point 

of view. 

Fig. 1 Tetrahedral coordination environment typical of Cu(I) complexes


Cu(I) cluster compounds are characterized by a variety of emitting electronic 

levels whereas Cu(I) cationic complexes show only luminescence originating 

from metal-to-ligand charge-transfer (MLCT) states, as long as empty 

π orbitals are easily accessible in the ligands. Such MLCT transitions, which 

clearly take advantage of the low oxidation potential of Cu(I), arealsocommonly 

observed in other classes of coordination compounds, for example 

those of d 6 metals like Ru(II)-bipyridines [6] and Ir(III)-phenylpyridine complexes 

[7]. 

MLCT electronic transitions in coordination compounds are normally 

more intense when compared to MC (metal-centered) ones since they do not 

undergo the same prohibitions by orbital symmetry; accordingly MLCT absorption 

bands exhibit relatively high molar extinction coefficients. As far as 

emission is concerned, when MLCT excited states are the lowest-lying, they 

are generally characterized by long lifetimes, and potentially intense luminescence, 

even though exceptions are possible (vide infra). Complexes exhibiting 

long-lived MLCT excited states have been extensively investigated in the last 

decades both for a better comprehension of fundamental phenomena [8, 9] 

and for potential applications related to solar light harvesting and conversion 

[10–12]. Among them the highest attention was probably devoted to 

Ru(II) [13], Os(II) [14] and, more recently, Ir(III) [7] complexes, however, 

economical and environmental considerations make Cu(I) compounds interesting 

alternatives [15]. 

As extensively discussed in the literature, long-lived luminescent MLCT 

excited states of d 6 metal complexes, in particular those of Ru(II), can be 

strongly affected by the presence of upper lying MC levels. The latter can 

be partially populated through thermal activation from the MLCT states and 

prompt non-radiative deactivation pathways and photochemical degradation 

[6, 16]. Closed shell d 10 copper(I) complexes cannot suffer these kinds of 

problems, but undesired non-radiative deactivation channels of their MLCT 

levels can be favored by other factors, as will be discussed in detail further 

on in this review. An orbital diagram illustrating the electronic transitions of 

Ru(II) and Cu(I) complexes is reported in Fig. 2. 

1.3 

Copper in Biology 

Copper, even if present in traces, is an essential metal for the growth and development 

of biological systems. Copper plays a fundamental role in cerebral 

activity, nervous and cardiovascular systems, oxygen transport and cell protection 

against oxidation. Copper is important to strengthen the bones and to 

guarantee the performances of the immune system [17]. 

Metals are commonly found as natural constituents of proteins and, in 

thecourseofevolution,Naturehaslearnedhowtousethespecialproperties 

of metal ions to perform a wide variety of specific functions associated


Fig. 2 Qualitative comparison of orbitals and related electronic transitions in metal complexes 

having d 6 (e.g. Ru(II)) and d 10 (e.g. Cu(I)) configurations 

with life processes. It is puzzling that only a limited number of transition 

elements of the periodic table are utilized in biological systems, among them 

iron, copper, and zinc are of key importance. The criteria by which Nature 

chooses metals in biological systems are rather intriguing. One factor that


seems to be quite important is their relative abundance on Earth. A second 

factor is related to the fact that active centers of metalloproteins consist of 

kinetically labile and thermodynamically stable units. Kinetic lability facilitates 

rapid assembly/disassembly of the metal centers as well as fast association/dissociation 

of substrates. Both the above criteria are fulfilled by copper, 

which has been present in living organisms since the early stages of evolution, 

representing a fundamental constituent of many biological systems, particularly 

proteins, with the function of transporting oxygen and transferring 

electrons. 

A key diversity between Cu(I) and Cu(II) is the different preferential coordination 

geometry. Cu(I) prefers tetrahedral four-coordinate geometries 

whereas Cu(II) complexes are typically square-planar or, in some biosystems, 

trigonal planar; occasionally, square planar compounds bind two additional 

weakly bonded axial ligands. In metalloproteins undergoing electron 

transfer processes, copper experiences a wealth of slightly different coordination 

environments: a tetrahedral ligand arrangement usually stabilizes 

Cu(I) over Cu(II), decreasingtheCu(II)/Cu(I) reduction potential, whereas 

high reduction potentials are achieved through distortion towards trigonal 

planar. In general, the thermodynamic stability of a metal center in biological 

environments is determined not only by inherent preferences of the 

metal for a particular oxidation state, ligand donor set, and coordination 

geometry, but also by the ability of the biopolymer to control, through its 

three-dimensional structure, the stereochemistry and the actual nearby ligand 

available for coordination. Non-coordinating residues also contribute to 

shape the local environment via hydrophilic/hydrophobic effects or steric 

blocking of coordination sites [17]. The complex pattern of factors occurring 

in biological systems make it possible to reach coordination geometries, 

such as trigonal planar, which can hardly be reproduced via synthetic 

chemistry. 

Two examples of copper containing metalloproteins, namely the blue copper 

site and the mixed-valence binuclear CuA center, can be illustrated to 

better understand how Nature organizes metal complexed centers, with the 

aim of optimizing their properties for a specific function, in this case electron 

transfer [18]. 

In the blue copper site, which occurs in the plastocyanin that couples photosystem 

I with photosystem II through electron transfer (ET) [19], the X-ray 

geometrical structure of the Cu(II) center is distorted tetrahedral and not 

square planar, as normally observed for cupric complexes. The coordination 

environment is provided by two histidine nitrogen atoms giving 2.05 ˚A long 

N-Cubonds,onethiolatesulfurofcysteinewithashortCu– Sbondof 

≈ 2.1 ˚A length and one thioether methionine at a longer distance (S – Cu 

≈ 2.9 ˚A), Fig. 3. 

The unusual geometry and ligation are responsible for the unique spectroscopic 

features of the blue copper site. In contrast to the weak d-d tran-


Fig. 3 Thebluecoppersiteinsideplastocyanin 

sitions of normal tetragonal Cu(II) complexes with ε ≈ 40 M –1 cm –1 at ca. 

16 000 cm –1 (≈ 620 nm),thebluecoppersitehasanintenseabsorptionband 

at 16 000 cm –1 with ε ≈ 5000 M –1 cm –1 Fig. 4 [20]. This result is a consequence 

of an inversion of the ligand-to-metal charge transfer (LMCT) pattern for 

the blue copper site that rises from its particular ligands distribution. As can 

be seen in Fig. 5 variation of the typical overlapping between the orbitals of 

copper and those of the ligands leads to an inversion of the relative absorption 

intensity, the final result is an enhancement of the absorption on the low 

energy side [21]. 

Fig. 4 Absorption spectrum of the blue copper site in plastocyanin (Reprinted from [20] 

with permission, © (2006) American Chemical Society)


Fig. 5 Inverted intensity pattern of ligand-to-metal charge transfer absorption transitions 

for the blue copper site compared to a regular Cu(II) complex. L = generic organic ligand; 

S = sulfur coordinating site of a cysteine residue 

In photosynthesis, plastocyanin functions as an electron transfer relay 

between cytochrome f (inside cytochrome b6f complex) and P700 + .Cytochrome 

b6f complex (from photosystem II) and P700 + (from photosystem 

I) are both membrane-bound proteins with exposed residues on the lumenside 

of the thylakoid membrane of chloroplasts. Cytochrome f acts as an 

electron donor while P700+ accepts electrons from reduced plastocyanin 

(Fig. 6) [18]. 

Fig. 6 The so-called Z-scheme of photosynthesis 

Plastocyanin (Cu 2+ Pc) is reduced by cytochrome f to Cu + Pc which eventually 

diffuses through the lumen until recognition/binding occurs with P700 + , 

which oxidizes Cu + Pc back to Cu 2+ Pc. The electronic structure of the blue 

copper is crucial for an efficient electron transfer in which Cu(II) is reduced 

to Cu(I). The tetrahedral organization of the Cu(II) site minimizes the reorganizational 

energy λ increasing the rate of the process, according to Marcus 

theory [22].


In the CuA mixed-valence binuclear site (Fig. 7), present for example in 

the terminal aerobic respiration enzyme (cytochrome c oxidase), a similar 

situation occurs [23]. The efficient electron transfer is promoted by the threedimensional 

organization and by electronic factors. The presence of two 

coordinated anionic cysteine thiolates in a monomeric Cu complex, however, 

would severely decrease the rate of the electron transfer process by stabilizing 

the oxidized Cu 2+ state and making the reduction potential too negative 

(Fig. 7). 

Fig. 7 Schematic structure of the CuA mixed valence dinuclear site, present in some 

terminal aerobic respiration enzymes 

In CuA this is avoided by weakening axial bonding interactions and by 

delocalizing the charge over two Cu ions [24]. 

In conclusion, Nature makes extensive use of the coordination flexibility 

of copper complexes and of the related tuning of electronic properties, to 

optimize processes of crucial importance in living organisms. 

1.4 

Cu(I) in Supramolecular Chemistry 

In the frame of the spectacular development of synthetic supramolecular 

chemistry over the last two decades [25], coordination chemistry has played 

a primary role [26] and, in this context, bisphenanthroline Cu(I) complexes 

(hereafter indicated as [Cu(NN)2] + ) have been major players [15]. Cu(I)


has a strong tendency to bind phenanthroline-type ligands [27] originating 

a wealth of simple tetrahedral [Cu(NN)2] + complexeswithhighyields. 

The parent compound [Cu(phen)2] + (phen = 1,10-phenanthroline) has been 

scarcely studied, probably due to the lack of long-lived electronic excited 

states in solution, a problem that is partially avoided in the solid state, where 

some emission has been detected [28]. The most common [Cu(NN)2] + complexes 

are those 2,9 or 4,7 disubstituted phenanthrolines, due to an easier 

synthetic accessibility of the related ligands. 

The development of sophisticated synthetic strategies, which take advantage 

of this metal-ligand affinity, has afforded a number of complicated 

molecular architectures like catenanes [29–31], rotaxanes [7, 32, 33], 

Fig. 8 Selected examples of multicomponent arrays containing Cu(I)-bisphenanthroline 

centers: A catenane, B rotaxane (R=4-[tris-(4-tert-butyl-phenyl)-methyl]-phenolato), 

C grid, D dendrimer (R=C8H17)


pseudo-rotaxanes [34, 35], knots [36, 37], dendrimers [38, 39], helices [40– 

42], polynuclear hosts [43] etc. as originally developed by Sauvage, Dietrich- 

Buchecker and coworkers [44]. Some of these fascinating structures are depicted 

in Fig. 8. 

Most notably, some suitably engineered supramolecular architectures 

based on [Cu(NN)2] + cores are able to carry out motion at the molecular level 

upon chemical [45], or electrochemical/photochemical stimulation [32, 46] 

behaving as molecular machine prototypes [47, 48]. For instance a [2]-catenate 

made of two different rings, one with a phenanthroline fragment the 

other bearing both a phenanthroline and a terpyridine unit, undergoes spontaneous 

and reversible molecular rearrangements (Fig. 9 steps (B) and (D)) 

upon oxidation (step A) and subsequent reduction (step C). Rearrangements 

are driven by the different preferential coordination geometries of Cu(I) and 

Cu(II), i.e. tetra- vs. pentacoordination [46]. 

Fig. 9 Electrochemically induced molecular motions in a catenane containing a [Cu(NN)2] + 

center and a free tpy ligand. The spontaneous motion is driven by the different preferential 

coordination geometry of Cu(I) vs. Cu(II) 

More recently, Schmittel and coworkers have made new supramolecular 

systems based on [Cu(NN)2] + -type building blocks such as racks [49], 

grids [50], boxes [51], and macrocycles [52]. Control of the sophisticated 

heteroleptic architectures has been achieved by exploiting the HETPHEN 

(HETeroleptic bisPHENanthroline) concept [53]. This approach is based on 

the kinetic control of the metal complexation equilibrium and, in the target 

complex, the Cu(I) ionturnsouttobeboundtoasimpleandabulky 

phenanthroline ligand; this concept is schematically illustrated in Fig. 10.


Fig. 10 Kinetic control in the formation of heteroleptic [CuL 1 L 2 ] + complexes (HETPHEN 

approach) 

Simple and supramolecular Cu(I)-bisphenanthroline complexes exhibit 

very interesting and largely tunable photophysical properties that will be illustrated 

in the next sections. 

2 

Cu(I)-Bisphenanthroline Complexes 

2.1 

Ground State Geometry 

Cu(I)-bisphenanthroline complexes generally display distorted tetrahedral 

geometries. The distortion from D2d symmetry can be visualized with the aid 

of Fig. 11 [54]. 

θx, θy, andθz define the interligand angles based on the CuN4 core of the 

complex. When a molecule possesses a perfect tetrahedral geometry (D2d), θx 

= θy = θz = 90 ◦ ,whereasthesquareplanargeometryD2 implies θx = θy = 90 ◦ 

and θz = 0 ◦ .Practically,θz is the dihedral angle between the ligand planes and 

a decrease from 90 ◦ indicates a flattening distortion of the molecule that progressively 

lowers its symmetry to D2.Theθx and θy values indicate the degree 

of “rocking” and “wagging” distortions [55]. 

These distortions are due to intra- and intermolecular (in solid state crystals) 

π - stacking interactions which also cause considerable displacement 

from D2d symmetry. In practice, combinations of various types of distortions 

occur in [Cu(NN)2] + complexes and their extent is dictated by the size, 

chemical nature, and positions of the phenanthroline substituents. Recently, 

the parameter ξCD has been proposed to quantify the degree of distortion as


Fig. 11 Relative orientation of the two ligand planes (xz and yz) and of the reference θx 

(rocking), θy (wagging), and θz (flattening) angles. θz is on the plane of the sheet; the 

grey circles represent the N phenanthroline atoms; the Cu ion is centered on the origin 

of the reference axes; the vector ξ lies on the yz plane, the vector η is perpendicular to it. 

In this schematic representation the phenanthroline on the left-hand side is assumed to 

be placed on the plane of the sheet (yz) and that on the right-hand side on the xz plane 

perpendicular to it 

a combination of θx, θy and θz (Eq. 1) [56]: 

� �� 

90◦ + θx 90◦ + θy 90◦ + θz 

ξCD = 

1803 where CD stands for “combined distortion”. 

Detailed X-ray crystallographic studies have shown that the specific geometry 

in the solid state is dictated by packing forces and considerable variation 

is found for the same complex as a function of the counteranion. In the case 

of [Cu(1)2] (Fig. 12) θz varies from 88◦ in the tetrafluoroborate and tosylate 

salts to 73◦ in the picrate [57]. 

As an example, the θz dihedral angles between the two phenanthroline 

planes in [Cu(1)2]PF6, [Cu(2)2]PF6 and [Cu(3)2]PF6 are 79.4◦ [57], 87.5◦ [56, 

58] and 79.8◦ [59], respectively, according to X-ray crystal structures; ligands 

1, 2 and 3 are depicted in Fig. 12. 

(1)


Fig. 12 Ligands 1 (2,9-dimethyl-1,10-phenanthroline), 2 (2,9-ditrifluoromethyl-1,10-phenanthroline), 

3 (2,9-diphenyl-1,10-phenanthroline) 

Substantial geometric distortion is possible with small methyl residues 

in [Cu(1)2]PF6, whereas bulkier trifluoromethyl groups force a more rigid 

quasi-tetrahedral arrangement in [Cu(2)2]PF6. Inthecaseof[Cu(3)2]PF6, 

instead, the distortion is dictated by intramolecular π-π stacking interactions 

between phenyl rings and phenanthroline cores of the other ligand. The 

strong and sometimes hardly predictable effects of steric and/or electronic 

factors on the ground state geometry of [Cu(NN)2] + complexesisreflectedin 

the wide tuning of electronic absorption spectra. 

2.2 

Absorption Spectra 

In Fig. 13 are depicted the absorption spectra of three homoleptic complexes 

of 2,9-disubstituted phenanthrolines in CH2Cl2 solution, namely [Cu(1)2] + , 

[Cu(4)2] + ,and[Cu(5)2] + . Ligands 1, 4, and5 (Figs. 12 and 14) have alkyl- or 

aryl-type substituents and serve as paradigmatic cases. 

The ultraviolet (UV) portion of the spectra are characterized by the intense 

ligand-centered (LC) bands typical of the ππ transitions of the phenanthroline 

ligands [60]; the molar absorption coefficients (ε) areoftheorderof 

50 000–60 000 M –1 cm –1 . Some mixing with MLCT states cannot be excluded 

according to density functional theory (DFT) calculations [61]. The bands 

lying in the VIS spectral region are much weaker than those in the UV (ε 

typically below 10 000 M –1 cm –1 ) and have been assigned to metal-to-ligand 

charge-transfer (MLCT) electronic transitions [61–64]. These levels occur at 

low energy because the Cu + ion can be easily oxidized [65] and the phentype 

ligands possess low-energy empty π orbitals. Direct evidence of the 

localized nature of the lowest-lying MLCT state of Cu(I)-bisphenanthrolines 

was achieved via resonance Raman [66] and transient absorption spectroscopy 

[67]. 

In a number of papers McMillin et al. [68–71] and others [72–74] have presented 

and discussed the absorption spectra of several mononuclear Cu(I)-


Fig. 13 Absorption spectra of [Cu(1)2] + (full black line), [Cu(4)2] + (dashed line) and 

[Cu(5)2] + (full grey line) in CH2Cl2 solution at room temperature 

Fig. 14 Ligands 4 (2,9-di-p-tolyl-1,10-phenanthroline), 5 (2,9-bis(3,5-di-tert-butyl-4methoxyphenyl)-1,10-phenanthroline), 

6 (2,9-Diphenyl-3,4,7,8-tetramethyl-1,10-phenanthroline) 

bisphenanthrolines. In general, at least three MLCT bands can be identified 

in the VIS spectral region [68]. They are termed band I (above 500 nm), band 

II (maximum around 430–480 nm, the most prominent, attributed to S0→S3 

transitions [61]), and band III (390–420 nm, often hidden by the onset of 

band II). The envelope of such MLCT bands defines the shape of the VIS absorption 

spectrum (400–700 nm). Spectral intensities are strictly related to 

the symmetry of the complex (D2d vs. D2, Fig.15)that,inturn,isaffectedby 

the distortion from the tetrahedral geometry (see above).


Fig. 15 Schematic orbital splitting diagrams for [Cu(NN)2] + . Left, D2d symmetry; right, 

D2 symmetry. Grey arrows represent the transitions leading to bands I (— —), band II 

(···), and band III (– – –) 

A simple picture able to rationalize the MLCT absorption patterns of 

these complexes in solution is difficult, nevertheless some general trends have 

been found by analyzing the MLCT absorption maxima and molar extinction 

coefficients of series of [Cu(NN)2] + compounds and have been reported 

elsewhere [15]. 

The spectrum of [Cu(4)2] + in Fig. 13 exhibits a well-established fingerprint 

for complexes with 2,9-aryl substitution of the phenanthroline ring, i.e. 

a pronounced low-energy shoulder extending down to 650 nm (band I, see 

above), which is practically absent in the case of [Cu(1)2] + .Thispatternisrelated 

to the above mentioned intramolecular π-stacking interactions, which 

make the transition corresponding to band I more permitted. The absorption 

spectrum of [Cu(5)2] + is somewhat different if compared to both [Cu(4)2] + 

and [Cu(1)2] + . The low energy band is wider and more intense and the peak 

around 440, which is typically the most intense in [Cu(NN)2] + complexes, appears 

as just a weak shoulder [74]. This trend is due to the presence of the 

cumbersome tert-butyl groups on the phenyl residues, that limit π-π stacking 

interactions and make the structure of the complex particularly rigid. 

The MLCT absorption profile of [Cu(5)2] + is found to be similar to that of 

aCu(I)-catenate complex made of two interlocked 27-membered rings containing 

a phenanthroline moiety and exhibiting a very rigid coordination 

environment [75]. The structural peculiarity of [Cu(5)2] + , as revealed by the 

absorption spectrum, is in accord with its extraordinary kinetic inertness towards 

demetallation [74]. 

Interestingly, it has been found that the complex [Cu(6)2] + (Fig. 14) exhibits 

a very weak band above 500 nm despite the presence of phenyl rings in 

the 2 and 9 positions [71]. Indeed this confirms the above described model: 

the methyl groups in the 3 and 8 position contrast the flattening distortion,


leading to a coordination geometry (and a spectrum) very similar to that of 

the much simpler [Cu(1)2] + complex. 

Some Cu(I)-bisphenanthroline compounds have also been utilized as receptors 

for dicarboxylic acids [76] and spherical inorganic anions [77]. 

The recognition motif is established by suitably functionalizing one phenyl 

residue in the 2 or 9 position of the phenanthroline chelating units, which 

are made able to pinch the pertinent substrate. The formation of the supramolecular 

adducts is monitored through the substantial changes of the MLCT 

absorption bands that occur as a consequence of the modification of the coordination 

geometry and related symmetry [76]. 

2.3 

Excited State Distortion: Pulsed X-ray and Transient Absorption Spectroscopy 

Upon light excitation of [Cu(NN)2] + complexes the lowest MLCT excited state 

is populated, thus the metal center changes its formal oxidation state from 

Cu(I) to Cu(II) [15]. The Cu(I) MLCT excited complex undergoes further flattening 

compared to its ground state and assumes a geometry similar to that 

of ground state Cu(II)-bisphenanthroline complexes [59]. In this excited state 

flattened tetrahedral structure a fifth coordination site is made available for 

the Cu(II) d 9 ion, that can be filled by nucleophilic species such as solvent 

molecules and counterions. The intermediate species thus obtained is termed 

“pentacoordinated exciplex”. The process, which is schematically depicted in 

Fig. 16, had been proposed by McMillin and coworkers nearly 20 years ago 

on the basis of classical photochemical experiments on series of [Cu(NN)2] + 

complexes with increasingly nucleophilic counteranions [78]. 

Fig. 16 Flattening distortion and subsequent nucleophilic attack by solvent, counterion, 

or other molecules following light excitation in Cu(I)-phenanthrolines. The size (and position) 

of the R substituents is of paramount importance in determining both the extent of 

the distortion and the protection of the newly formed Cu(II) ion from nucleophiles 

In recent years this hypothesis has been nicely confirmed thanks to the 

development of light-initiated time-resolved X-ray absorption spectroscopy


(LITR-XAS) [79]. This pump-and-probe technique allows one to catch the 

transient oxidation state of a metal atom as well as its surrounding structures 

following photoexcitation via an ultrafast laser source; accordingly, it is particularly 

suited to investigating the process depicted in Fig. 16. Practically, the 

information obtained via LITR-XAS is a sort of snapshot of electronic excited 

states in disordered media (e.g. solution), which are generated via a UV-VIS 

femtosecond pump laser and subsequently probed with a 30–100 ps intense 

X-ray pulse produced by 3rd generation large synchrotron facilities [80]. 

This technique has been applied in solution studies to [Cu(1)2] + and unambiguously 

confirmed that, in the thermally equilibrated MLCT excited 

state, the copper ion is pentacoordinated both in poorly donor (toluene) [81] 

or highly donor (CH3CN) [82] solvents; in addition, the copper ion has 

thesameoxidationstateasthecorrespondinggroundstateCu(II) complex 

in both cases. Analogous investigations have been carried out also on 

Cu(I) complexes as solid crystals (“photocrystallography”) [55, 83]. LITR- 

XAS studies of [Cu(1)2] + in solution have been complemented by optical 

time-resolved spectroscopy, which evidenced spectroscopic features in the ps 

timescale, associated to excited state structural rearrangements, possibly flattening 

distortion [82]. 

We have also carried out femtosecond transient absorption studies on 

[Cu(7)2] + and [Cu(8)2] + in CH2Cl2 (Fig. 17) [84]. These complexes are characterized 

by alkyl- and more cumbersome phenyl-residues in the 2 and 9 

position of the phenanthroline ligand, which imparts rather different photophysical 

properties (i.e. shape of UV-VIS absorption, luminescence spectra, 

excited state lifetime) [85]. Despite this diversity, femtosecond transient absorption 

spectra have revealed a dynamic process lasting 15 ps in both cases 

Fig. 18. 

Fig. 17 Ligands 7 (2,9-bis(4-n-butylphenyl)-1,10-phenanthroline) and 8 (2,9-di-n-hexyl- 

1,10-phenanthroline) 

Specific assignment of the observed spectral variation to (i) flattening distortion 

or (ii) extra ligand pick-up, two processes that might also occur sim-


Fig. 18 Transient absorption spectral changes observed for [Cu(8)2] + in CH2Cl2 at λexs 

= 400 nm (150 fs Ti:Sapphire laser pulse). The spectra were recorded at delays of 2, 5, 10, 

25, 100, 1000 ps following the excitation pulse. In the inset are depicted spectral decays at 

two selected wavelengths, λobs = 520 (full circles) and 590 (half-empty circles) 

ultaneously, is not straightforward. Access to the fifth coordinating position 

is likely to be less favored for the more congested phenyl–phenanthroline 

complex [Cu(7)2] + . Hence the identical rate constant observed for the two 

compounds in CH2Cl2, aswellasinCH3CN for another [Cu(NN)2] + complex 

[82], is likely to be associated with the flattening distortion which is 

expected to be less solvent- and ligand-dependent than picking up an external 

unit for coordination expansion. Transient absorption studies leading 

to similar results have also been carried out with monophenanthroline complexes 

[86]. 

2.4 

Emissive Excited State(s) and Luminescence Spectra 

The first report on [Cu(NN)2] + luminescence in fluid solution dates back to 

1980, when it was shown that, upon excitation into the MLCT band region, 

[Cu(1)2] + exhibits a luminescence spectrum peaking around 700 nm and an 

excited state lifetime of 54 ns in air-equilibrated CH2Cl2 [87]. Luminescence 

from [Cu(NN)2] + complexes is observed in poorly electron donor solvents, 

typically CH2Cl2. At room temperature, the emission band is wide and exhibit 

λmax peaking between 680 and 740 nm, with rather low quantum yield 

(Φem 10 –3 –10 –4 ) [15]. Excited state lifetimes in CH2Cl2 solution are strongly 

dependent on the degree of excited state distortion and the protection to-


wards exciplex quenching. It has been hypothesized that emission stems from 

the pentacoordinated exciplex itself that deactivates to a pentacoordinated 

groundstatespecieswhicheventuallylosesthe“fifth”nucleophilicligandto 

regenerate the initial ground state pseudotetrahedral complex [81]. In recent 

years it has also been evidenced that some [Cu(NN)2] + complexes exhibit 

a prompt luminescence signal with a lifetime of the order of 13–16 ps, which 

is attributed to deactivation of 1 MLCT [88], whereas the long-lived component 

(above 50 ns) would be phosphorescence from 3 MLCT, which borrows 

intensity from upper lying singlet levels [89]. 

The shortest and longest values reported to date (oxygen-free CH2Cl2 

solution, longer-lived component) are 80 [68] and 930 ns [71] and refer to homoleptic 

[Cu(NN)2] + complexes of the two phenanthroline ligands depicted 

in Fig. 19. 

Fig. 19 Ligand 9 (2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline) and 10 (2,9-di-n-butyl- 

3,4,7,8-tetramethyl-1,10-phenanthroline). Lifetime values of their [Cu(NN)2] + complexes 

differ by more than one order of magnitude 

The value for [Cu(9)2] + is very similar to that of [Cu(1)2] + under the 

same conditions (80 ns), suggesting that electronic delocalization of the ligands 

[73], very strong for 9, is less important than steric factors in determining 

the lifetimes value. Notably, substitution on the 3 and 8 position 

with a simple methyl residue in 10 is particularly effective to limit exciplex 

quenching and yields a lifetime of almost 1 µs. In general, the large majority 

of [Cu(NN)2] + homoleptic complexes exhibit excited state lifetimes in the 

range 80–350 ns in oxygen-free solution [15]. A longer lifetime (730 ns, Φem 

= 0.01, oxygen-free CH2Cl2) has been found with the suitably designed heteroleptic 

complex [Cu(1)(11)] + [90], Fig. 20, in which excited state distortion 

is strongly limited by the cumbersome tert-butyl substituent. Interestingly, 

steric contraints make the formation of the homoleptic analogue [Cu(11)2] + 

very difficult. 

The picture describing the luminescent excited states of Cu(I)-bisphenanthrolinesisnotstraightforwardalthoughParkeretal.,inlightoftheob-


Fig. 20 Ligand 11 2,9-di-tert-butyl-1,10-phenanthroline 

served large Stokes shift (over 5000 cm –1 )hadattributedittothelowest 

3 MLCT excited state [91], likewise the popular family of octahedral Ru(II)polypyridines 

[6]. McMillin and coworkers, instead, suggested that emission 

of [Cu(NN)2] + compoundsarisesfromtwoMLCTexcitedstatesinthermal 

equilibrium, i.e. a singlet ( 1 MLCT)andatriplet( 3 MLCT) [92]. The energy 

gap between these states was found to be about 1500–2000 cm –1 and, at room 

temperature, the population of the lower lying 3 MLCT level exceeds that of 

1 MLCT. At 77 K where the excited state population is largely frozen in the 

triplet, the emission band is red-shifted and much weaker compared to room 

temperature, a rather unusual trend. Recent studies have confirmed and refined 

this rationale [81, 85, 89]. 

A few years ago our group discussed detailed temperature-dependent luminescence 

studies of a series of [Cu(NN)2] + complexes of 2,9-disubstituted 

phenanthroline ligands [85]. The above-described two-level model, which implies 

red-shift and intensity decrease of the emission band upon temperature 

lowering,isalwaysobeyedexceptwhenlongalkylchainsareutilizedassubstituents 

of the phenanthroline chelating agent. In this case the “regular” 

trend is obeyed only until the matrix remains fluid (T>150 K) but, when the 

matrix becomes rigid (T


Fig. 21 Temperature dependence of the luminescence spectra of [Cu(7)2] + (top) and 

[Cu(8)2] + (bottom) in CH2Cl2 MeOH 1:1 (v/v). In the fluid domain (up to 170 K) emission 

intensity decrease and spectral red-shifting is observed by lowering temperature in 

both cases. By contrast when the solvent matrix becomes rigid (around 120 K), the two 

compounds behave differently. For the 2,9-dialkylphenanthroline complex (bottom panel) 

a complete reversal of the previous trend is observed with intensity recovery and blue 

shift. At 96 K a very strong luminescence band is recorded


Fig. 22 Ligand 12, 2-(3,5-di-tert-butyl-4-methoxyphenyl)-9-(2,4,6-trimethylphenyl)-1,10phenanthroline 

(Fig. 22). This finding gives further support to the notion that steric factors 

are by far more important than electronic factors. 

The room temperature lifetime of [Cu(12)2] + in oxygen free solution (285 

ns) is scarcely affected under air-equilibrated conditions (266 ns) and is remarkably 

high also in CH3OH (182 ns), a solvent where most [Cu(NN)2] + 

do not show any luminescence. This rather unusual solvent insensitivity of 

the lifetime is observed also for [Cu(5)2] + (Fig. 14) suggesting that tert-butyl 

groups are very effective in preventing the formation of the pentacoordinated 

exciplex at room temperature [74]. On the other hand, in striking contrast 

with [Cu(12)2] + , [Cu(5)2] + is virtually non-luminescent in 77 K rigid matrix, 

highlighting once again the subtle factors affecting the molecular structure 

and, accordingly, the emission performance. Probably, in [Cu(12)2] + ,thekey 

structural features causing good luminescence performance are the methyl 

residues on the 2 and 6 position of a phenyl substituent (Fig. 22). 

Solid state measurements confirm the strong dependence of the excited 

state lifetimes of [Cu(NN)2] + complexes on geometric distortions. In particular 

it has been found that there is a good linear correlation between the 

ξCD parameter (see Sect. 2.1, Eq. 1) and the measured lifetime both at room 

temperature and at 17 K: the smaller the distortion from the ideal pseudotetrahedral 

geometry (high ξCD parameter), the longer the lifetime [56]. 

2.5 

Photoinduced Processes in Multicomponent Systems Based on 

[Cu(NN)2] + Complexes 

Templated synthesis of Cu(I)-bisphenanthrolines has prompted the design 

and construction of sophisticated multicomponent architectures comprising 

one or more [Cu(NN)2] + centers in tandem with other chromophores, typ-


ically porphyrins and fullerenes. Intercomponent light-induced energy- and 

electron-transfer processes have been investigated in [2]- [93, 94] and [3]catenanes 

[29, 95, 96], dinuclear knots [97], fullerene- [98] and porphyrinstoppered 

rotaxanes [99–108], dendrimers with [Cu(NN)2] + cores [38], and 

helicates with Cu(I)-complexed cores and peripheral methano- [109] or 

bismethano-fullerenes [41, 109, 110]. The latter, which constitute the most recently 

investigated systems, are depicted in Fig. 23. 

Fig. 23 Fullerohelicates based on a dinuclear [Cu(NN)2] + complex (Z=C8H17,R=C12H25). 

The peripheral moieties are different in the two cases, namely bismethano (left) vs. 

methanofullerenes (right) 

Upon excitation of the Cu(I)-complexed moiety and population of the related 

MLCT level, electron transfer to the fullerene subunit is observed for 

both compounds shown in Fig. 23. By contrast, although the same process 

is thermodynamically allowed also by populating the fullerene lowest singlet 

state, it is observed only in the case of the methanofullerene system. This 

is related to the inherently different electronic structure of the two fullerene 

derivatives. By means of an analysis of their fluorescence spectra, which 

are substantially different, it was possible to conclude that the singlet excited 

state of methanofullerenes is more prone to undergo electron transfer than 

that of bismethanofullerenes, thanks to the associated smaller internal reorganization 

energy [109]. In addition, methanofullerenes are slightly easier to 

reduce than bismethanofullerenes, giving also a thermodynamic advantage 

for electron transfer in multicomponent arrays containing the monofunc-


tionalized derivative. The combined effect of these two factors (kinetic and 

thermodynamic) can explain the different and unexpected trend in photoprocesses 

of multicomponent arrays containing Cu(I)-phenanthrolines linked to 

methanofullerenes vs. bismethanofullerenes, which has been found in a variety 

of molecular architectures such as dendrimers [38], rotaxanes [98] and 

sandwich-type dyads [110]. 

Exhaustive review articles presenting photophysical investigations on 

fullerene- and porphyrin-type arrays built-up around [Cu(NN)2] + centers 

have been published recently and we suggest the reader refers to these papers 

for a comprehensive and updated overview on this topic [15, 25, 111, 112]. 

2.6 

Bimolecular Quenching Processes 

Excited state electrochemical potentials can be obtained from the ground 

state monoelectronic electrochemical potentials and the spectroscopic energy 

(E ◦◦ in eV units, to be considered divided by a unitary charge) related to the 

involved transition, according to Eqs 2 and 3 [6]: 

E(A + / ∗ A)=E(A + /A)–E ◦◦ 

E( ∗ A/A – )=E(A/A – )+E ◦◦ 

Hence the variation of the electron-donating or accepting capability of a given 

molecule A, upon light excitation, can be easily assessed. In Eqs 2 and 3: ∗ A 

denotes the lowest-lying electronically excited state of A and its spectroscopic 

energy (E ◦◦ ) can be estimated from the onset of emission spectra [6]. 

Oxidation from Cu(I) to Cu(II) is easily accomplished and the MLCT 

excited states of Cu(I)-bisphenanthrolines are, therefore, potent reductants. 

For example [Cu(3)2] + is a more powerful reductant than the very popular 

photosensitizer [Ru(bpy)3] 2+ (A + /A = – 1.11 and – 0.85 V, respectively) 

owing to its more favorable ground state 2+/+ potential (+ 0.69 vs. + 1.27 V), 

that largely compensates the lower content of excited state energy (1.80 vs. 

2.12 eV) [15]. By contrast reduction of Cu(I)-bisphenanthrolines is strongly 

disfavored and they are mild excited state oxidants; accordingly, only a few examples 

of reductive quenching of [Cu(NN)2] + complexes are reported in the 

literature, with ferrocenes as donors [113, 114]. 

Oxidative quenching of [Cu(NN)2] + ’s by Co(III) and Cr(III) complexes as 

well as nitroaromatic compounds and viologens has been reported and comprehensively 

reviewed [115]. Some attempts to sensitize wide band-gap semiconductors 

with Cu(I) complexes were also carried out [115] but so far they 

do not seem to be competitive in terms of stability and efficiency with those 

based on Ru(II) complexes [12]. Energy transfer quenching to molecules 

possessing low-lying triplets such as anthracene has been demonstrated via 

transient absorption spectroscopy [116, 117], whereas oxygen quenching, 

(2) 

(3)


which in principle can occur both via energy- and electron transfer, was evidenced 

by monitoring sensitized singlet oxygen luminescence in the NIR 

region [29, 36]. Optically pure dicopper trefoil knots with [Cu(NN)2] + -type 

cores have been reported to quench the emission of the Λ or ∆ forms of 

Tb(III) and Eu(III) complexes, a very rare example of enantioselective luminescence 

quenching [118]. 

[Cu(NN)2] + complexes have also been used as substrates for DNA binding, 

trying to take advantage of the sensitivity of the luminescence of Cu(I)phenanthrolines 

to the local environment [63]. The structure of the associates 

has not been clarified: both electrostatic binding and intercalation of the aromatic 

ligands between adjacent bases are possible. Cu(I)-porphyrins seem to 

be more promising substrates for DNA [63]. 

3 

Heteroleptic Diimine/Diphosphine [Cu(NN)(PP)] + Complexes 

3.1 

Photophysical Properties 

Heteroleptic Cu(I) complexes containing both N- and P-coordinating ligands, 

[Cu(NN)(PP)] + , have been studied since the late 1970s [119]. The replacement 

of one N-N ligand with a P-P unit is often aimed at improving the 

emission properties. Accordingly, the relentless quest for highly performing 

luminescent metal complexes [7] has sparked revived interest in these compounds 

in recent years [120–122]. 

The absorption and luminescence spectrum of [Cu(dbp)(POP)] + (dbp = 

2,9-butyl-1,10-phenanthroline and POP = bis[2-(diphenylphosphino)phenyl] 

ether) is reported in Fig. 24, as a representative example for this class of compounds 

[123]. Substantial blue-shifts of the lower-energy bands are observed 

compared to typical spectra of [Cu(NN)2] + compounds (see Sect. 2). 

UV spectral features above 350 nm are due to ligand-centered transitions 

whereas those in the 350–450 nm window are attributed to MLCT 

levels. [Cu(NN)(PP)] + complexes are subject to dramatic oxygen quenching, 

as deduced from the strong difference in excited state lifetimes passing 

from air-equilibrated to oxygen-free CH2Cl2 solution, 250 ns and 17 600 ns 

in the case of [Cu(dbp)(POP)] + [123]. The character of the emitting state 

in [Cu(NN)(PP)] + complexes has been discussed since their first characterization 

[119] and now its MLCT nature is established experimentally and 

theoretically [120, 124, 125]. The electron-withdrawing effect of the P–P unit 

on the metal center tends to disfavor the Cu(I)→N–N electron donation, as 

also reflected by the higher oxidation potential of the Cu(I) center compared 

to [Cu(NN)2] + compounds [126], leading to a blue shift of MLCT transitions. 

This, according to the energy gap law [127], explains the emission enhance-


Fig. 24 Absorption and (inset) emission spectra of [Cu(dbp)(POP)] + in CH2Cl2 

ment of [Cu(NN)(PP)] + , that typically falls in the green spectral window, 

compared to weaker red-emitting [Cu(NN)2] + complexes. 

The luminescence efficiency of MLCT excited states in [Cu(NN)(PP)] + 

compounds is strongly solvent- and oxygen-dependent because it can be 

decreased by exciplex quenching [128, 129], in line with what is observed 

for the [Cu(NN)2] + analogues (see above). Therefore, the geometry of 

[Cu(NN)(PP)] + complexes plays a central role in addressing the extent of 

luminescence efficiency, even though this is hard to predict a priori. 

A variety of bidentate phosphine ligands has been prepared to coordinate 

Cu(I) in tandem with phenanthroline-type units: bis[2-(diphenylphosphino) 

phenyl]ether (POP), triphenylphosphine (PPh3), bis(diphenylphosphino) 

ethane (dppe), and bis(diphenyl-phosphino)methane (dppm), represent 

some recent examples [120, 122, 123, 130, 131], Fig. 25. 

Among them, the family of mononuclear [Cu(phen)(POP)] + complexes 

proposed by McMillin (see Fig. 25 for the PP-type ligands), where phen indicates 

a variably substituted 1,10 phenanthroline, shows an impressive emission 

efficiency compared to [Cu(NN)2] + compounds [120]. Especially on 

passing from pristine phenanthroline to dimethyl- or diphenyl-substituted 

analogues, and thanks to the efficient steric and electron-withdrawing effects 

of the POP ligand, remarkable emission quantum yields (Φem ∼ 0.15 

in CH2Cl2 oxygen-free solution) and long lifetimes (∼ 15 µs) have been 

measured. On the contrary, the replacement of the POP ligand with two 

PPh3 units, gives less remarkable results due to the lower geometric rigidity 

which leads to weak and red-shifted emissions comparable to those of


Fig. 25 Some ligands typically used as P–P units in [Cu(NN)(PP)] + complexes 

bis-phenanthroline-type complexes. Importantly, the P–Cu–P angle decreases 

from 122.7 ◦ in [Cu(dmp)(PPh3)2] + to 116.4 ◦ in [Cu(dmp)(POP)] + also allowing 

easier access for exciplex quenching over the fifth coordination position 

in the former. This example highlights the importance of having both conditions 

(i.e. steric protection and increased electron-withdrawing character 

of the P–P ligand) simultaneously satisfied for optimized photoluminescence 

performance of [Cu(NN)(PP)] + compounds. 

The importance of the choice of the P–P ligand for the coordination of 

the metal ion is evidenced also by the systems recently investigated by Wang 

et al. [121], in which ligands other than phenanthroline have been utilized 

(Fig. 26). 

By keeping the N–N ligand unchanged, the luminescence properties of the 

complexes (solid matrix, RT) increase on passing from dppe to POP to, sur- 

Fig. 26 General structure of [Cu(ppb)(P)2] complexes (pbb = 2-(2′-pyridyl)benzimidazolylbenzene)


prisingly, PPh3. Although POP provides the best emission performance when 

combined with phenanthroline ligands, a better result is found here for PPh3. 

This shows that subtle and combined steric and electronic effects of both P–P 

and N–N ligands are crucial for an enhanced light output, highlighting that 

general rules able to predict the photophysical behavior of [Cu(NN)(PP)] + 

complexes are not easy to draw. From an electronic point of view, both POP 

and PPh3 units promote the usual blue shift of the MLCT state compared to 

[Cu(NN)2] + compounds, as predictable by the substantially higher oxidation 

potential of the Cu(I) center of heteroleptic [Cu(NN)(PP)] + complexes. 

Dinuclear Cu(I) complexes have also been synthesized and investigated, 

two of them (A and B) are depicted in Fig. 27. Despite the presence of a P– 

P-type ligand, complex A shows a luminescence band peaked at 700 nm with 

a lifetime of 320 ns in the solid state [132]. The X-ray crystal structure indicates 

a distorted tetrahedral geometry which, combined to the scarcely 

protective 2,5-bppz N–N ligand (2,5-bis(2-pyridil)pyrazine) leads to a weakly 

red-emitting compound. By changing the N-N ligand (Fig. 27, B), a stronger 

Fig. 27 Chemical structures of heteroleptic Cu(I) complexes A and B


green emission (λmax = 550 nm) is detected with a quantum yield of 0.17 at 

77 KinCH2Cl2 matrix [133]. For complex B, althoughthegeometricstructure 

is similar to A, the oxidation potentials of the N–N ligand are substantially 

greater, pushing the MLCT levels at higher energy. 

3.2 

OLED and LEC Devices 

The outstanding photophysical performances of some [Cu(NN)(PP)] + complexes 

make them potentially attractive for optoelectronic devices requiring 

highly luminescent materials [134–137]. This interest is also related to the 

lower cost and higher relative abundance of copper compared to more classical 

emitting metals such as europium or iridium. Some research groups have 

recently fabricated OLED devices with [Cu(NN)(PP)] + complexes [138]. It has 

been shown that they can be profitably used as electrophosphorescent emitters 

and provide device efficiency comparable to that of Ir(III) complexes in 

similar device structures (11.0 cd/A at1.0 mA/cm 2 , 23% wtCu(I)-complex 

dispersed in PVK matrix). Also Li et al. have obtained a highly efficient electrophosphorescent 

OLED with the complex reported in Fig. 28 [139]. 

Fig. 28 The complex [Cu(Dicnq)(POP)] + BF4 used by Li et al. to make a highly efficient 

electroluminescent OLED device (Dicnq = 6,7-Dicyanodipyrido[2,2-d:2 ′ ,3 ′ -f ]quinoxaline) 

The performances of OLEDs fabricated by the vacuum vapor deposition 

technique with this complex are among the best reported for devices incorporating 

Cu(I) complexes as emitters. A low turn-on voltage of 4 V, a maximum 

current efficiency up to 11.3 cd/A, and a peak brightness of 2322 cd/m 2 have 

been achieved.


A different type of electroluminescent device is a light-emitting electrochemical 

cell (LEC). LECs are substantially different from OLEDs due to 

the fact that mobile ions in the electroluminescent layer drift towards the 

electrodes when a voltage is applied over the device, thereby facilitating 

charge-carrier injection from the electrodes. This results in two important 

advantages compared to traditional OLEDs: (i) thick electroactive layers can 

be used without severe voltage penalties and shorts can be eliminated even for 

large-area pixels; (ii) matching of the work function of the electrodes with the 

energy levels of the electroluminescent material is not required. 

We have recently described novel Cu(I) complexes with excellent PL performance 

(Q.Y. up to 0.28 in oxygen-free CH2Cl2) andthefirstLECdevice 

made with a Cu(I) complex, Fig. 29 [123]. 

Fig. 29 Chemical structure of the complex used to make the first LEC device based 

on a Cu(I)-complex, R = n-butyl. A schematic representation of the device structure is 

also depicted; in the electroluminescent layer the complex is dispersed in a polymethylmetacrylate 

matrix (PMMA) 

The device efficiency turned out to be moderate but comparable to LEC 

devices made with Ru(II)-type compounds [134]. Wang et al. used the same 

complex but, changing experimental conditions, could make a more efficient 

green light emitting device (CIE coordinates: 0.25, 0.60) with a maximum 

current efficiency of 56 cd/A at 4.0 V, corresponding to an external quantum 

yield of 16% [140]. This work notes the importance of the optimization 

of LEC device parameters such as the response time, which greatly depends 

on the counterion, driving voltage, and thickness of the emitting layer. Further 

efforts are needed to substantially improve the device stability and light 

output in order to take advantage of the low-cost and limited environmental 

damaging effects of copper materials. 

Finally, it is worth pointing out that also the family of cuprous cluster (described 

in the next paragraph) has been tested in devices. In the late 1990s 

Ma et al. described the electroluminescence properties of a LED containing 

a tetranuclear Cu(I) cluster as the active component contributing to broaden 

the pool of electroluminescent materials outside the traditional boundaries of 

organic dyes and polymers [141].


4 

Cuprous Clusters 

4.1 

Cuprous Halide Clusters 

Cluster compounds contain a group of two or more metal atoms where direct 

and substantial metal-metal bonding is present. Cuprous halide clusters have 

been known for about 100 years [142], their general formula is CunXnLm(X 

=Cl – ,Br – or I – ; L = N or P belonging to an organic molecule). For instance, 

in solution, mixtures of Cu(I) salts, iodine (I) and pyridine-type molecules 

(py) are primarily present as tetrahedral clusters Cu4I4py4 and give origin to 

mononuclear or dinuclear structures only if forced by mass action law under 

high pyridine concentration. Normally, copper(I) complexes in solution are 

quite labile towards ligand substitution and the formation of new species is 

driven by thermodynamic stability rather than kinetic control. 

Fig. 30 Illustrations of Cu4I4py4 (A), Cu2I2py4 (B), [Cu3(µ-dppm)3(µ3 – η 1 -CΞC-benzo- 

15-crown-5)2] + (C), and the repeating unit of a “stairstep” polymer [CuIpy]n (D)redrawn 

from the structural data. Black circles =copperatoms;grey circles =iodine;white rings = 

pyridine residues


Cuprous clusters display many structural formats that are characterized by 

largely different emission behavior (vide infra). The variety of structural motifs 

and stoichiometries is related to the remarkable flatness of their ground 

state potential energy surfaces [143]. The most extensive studies carried 

out on these Cu(I) complexes concern cubane-type clusters of general formula 

[CuXL]4 [144, 145]. The solid state structural variety observed among 

cuprous clusters can be illustrated by examining some crystallographic data 

reported by Ford and co-workers, who found that compounds containing 

Cu(I), iodine, and pyridine generate different structures depending on the 

reaction stoichiometry, Fig. 30. 

For a 1:1:1 Cu:I:L ratio (Fig. 30 A), the most common motif is the tetranuclear 

“cubane” structure (Cu4I4L4) in which a tetrahedron of copper atoms is 

included by a larger I4 tetrahedron where each iodine is placed on a triangular 

face of the Cu4 cluster and the fourth coordination site of each copper is occupied 

by the ligand (L). For stoichiometry 1:1:2 (Fig. 30 B), the most common 

structure is an isolated rhombohedron of Cu2I2 with alternating copper and 

halide atoms. Sometimes, clusters with stoichiometry (1:1:1) exist in more 

than one crystalline structure. For example (Fig. 30 C) Cu4I4py4 can also give 

rise to a polymeric “stair” made of an infinite chain of steps [146]. 

4.2 

Cuprous Iodide Clusters 

The interest in the luminescence properties of Cu(I) iodide clusters goes back 

to the pioneering work of Hardt and co-workers [144]. They found that the 

emission spectra of solid samples of [CuxIy(py)z] are markedly temperaturedependent 

and defined the term “luminescent thermochromism”. 

In some cases cuprous iodide clusters exhibit two emission bands termed 

HE (high energy) and LE (low energy), which sharply change their relative intensities 

upon temperature variation. As an example, in Fig. 31 are depicted 

the temperature-dependent emission spectra of Cu4I4(4 – phenylpyridine)4 

[147]. The LE band dominates at room temperature, while the HE band is by 

far the strongest at temperatures below 80 K. 

The HE band dominating at low temperature has been attributed, on the 

basis of ab initio calculations and experimental work, to ligand-to-ligand 

(I – → phenylpyridine) charge transfer states, also indicated as XLCT. The LE 

emission dominating at room temperature has been assigned to an excited 

state of mixed halide-to-metal charge transfer (XMCT) and d → s,p metalcentered 

character which is usually referred to as “cluster-centered” (CC). 

This term was introduced to highlight that these transitions are localized on 

the Cu4I4 cluster and are essentially independent on ligand L. The Cu–Cu distance 

is a fundamental parameter to allow the presence of CC bands and must 

be shorter than the orbital interaction radius, estimated to be 2.8 ˚A. Ifthe 

distance between the two metal centers exceeds this critical value the metal


Fig. 31 Temperature dependence of the emission spectrum of Cu4I4(4 – phenylpyridine)4 

in toluene solution with relative intensities normalized to 1 in each case (Reprinted 

from [147] with permission, © (1991) American Chemical Society) 

orbitals do not interact and the CC emission bands are not observed [148]. In 

Fig. 32 the relative position of the above-described excited states is schematically 

represented by means of potential energy curves. 

When the amine aromatic ligand py is replaced by the aliphatic one piperidine 

(pip), the corresponding cluster Cu4I4(pip)4 preserves the CC band but 

does not display the high energy XLCT emission owing to the absence of ligands 

possessing π orbitals. Thus, luminescence thermochromism is the norm 

for Cu4I4L4 clusters, but only when L is π-unsaturated. 

Another factor contributing to the complicated pattern of the luminescence 

properties of Cu4I4(4-phenylpyridine)4 is the dramatic red-shift of the 

CC band in going from solid or frozen solution samples to fluid solutions, indicating 

that also rigidochromism effects are operative. Ab initio calculations 

and the Stokes shifts for Cu4I4(4-phenylpyridine)4 (up to 16 300 cm –1 for the 

CC band in 296 Ktoluenesolution;7600 cm –1 for the XLCT band under the


Fig. 32 Schematic representation describing the relative positions of the potential energy 

curves related to the emitting states of Cu4I4py4 

same conditions) also indicate that CC excited states are quite distorted relative 

to both the GS and XLCT levels. Such distortion was proposed as the 

cause for the lack of communication between CC and XLCT states, for the 

rigidochromism of the low CC energy band, and for the large reorganization 

energies of electron transfer reactions involving CC excited states [143]. 

In an interesting recent work on clusters of general formula CuXL (L = N– 

heteroaromatic ligands), it was shown that the energy of the emitting level 

can be finely tuned [149]. The emission is attributed to a MLCT charge transfer 

state, because no clear correlation between Cu–Cu distances and emission 

maxima was observed and also because the effects of bridging halides were 

smaller than those of N-heteroaromatic ligands, therefore the position of the 

luminescence band can be varied by increasing the electron-accepting character 

of the ligand L, Fig. 33. In these compounds Cu–Cu distances are in 

the range 2.9–3.3 ˚A, accordingly the weak metal interaction prevent clustercentered 

luminescence. Very recently, density functional theory calculations 

have confirmed the involvement of the triplet cluster-centered (CC) and 

triplet XLCT excited states as the origin of the dual emission [151]. 

It must be pointed out, however, that short Cu–Cu separation does not automatically 

imply the establishment of metal-metal bonds and the effect of 

the bridging ligands has to be taken into account. For example Cotton et al.


Fig. 33 Emission spectra of clusters CuXL (X = Br – ) at room temperature. The emission 

maxima of the complexes cover a wide range, from 450 to 740 nm depending on the Nheteroaromatic 

ligands. bpy = 4,4-bipyridine; pyz = pyrazine; pym = pymidine; pip = 

piperazine; 1,5-nap = 1,5-naphthyridine; 1,6-nap = 1,6-naphthyridine; quina = quinazoline; 

dmap = N,N-dimethyl-amino-pyridine; 3-bzpy = 3-benzoyl-pyridine; 4-bzpy = 

4-benzoyl-pyridine. (Reprinted from [149] with permission, © (2006) American Chemical 

Society) 

have carried out DTF calculations on [Cu2(hpp)2], (where hpp – = 1,3,4,6,7,8hexahydro-2Hpyrimido[1,2-a]pyrimidinate) 

to investigate the possibility of 

metal-metal bonding in a complex where short metal–metal separations are 

present [dCu···Cu = 2.497(2) ˚A]. They concluded that there is no Cu–Cu 

bond and the short intermetal distance is related to the strong Cu–N bonds 

and the small bite angle of the bridging ligand [150]. 

4.3 

OtherCopperClusters 

Recently, the synthesis of several polynuclear copper(I) alkynyl clusters has 

been reported and their luminescence properties investigated in detail [152, 

153]; these compounds exhibit intense and long-lived luminescence upon 

photoexcitation. For instance, the tetranuclear copper(I) alkynyl complex 

[Cu4(PPh3)4(L)3]PF6, in Fig. 34 is characterized by an unusual open-cube 

structure, and exhibits a strong structured emission with two different max-


Fig. 34 A tetranuclear copper(I) alkynyl “open-cube” cluster 

ima at 445 and 630 nm in solid state at 298 K and a single band with λmax 

= 445 nm in rigid matrix at 77 K [154, 155]. 

For some of these open-cube compounds, an additional low-energy 

emission band at λ > 623–665 nm was observed in the solid-state spectra, 

similarly to what was observed for [Cu4I4L4] systems described above. 

In dichloromethane solution at ambient temperature they exhibit only 

an orange phosphorescence and the spectrum of [Cu4(PPh3)4(L)3]PF6 (L 

= p – nOctC6H4) is depicted in Fig. 35 as a representative example for this 

class of compounds. 

Fig. 35 Emission spectrum of [Cu4(PPh3)4(L)3]PF6 (L = p – nOctC6H4) in degassed 

dichloromethane at 298 K (Reprinted from [155] with permission, © (2006) Wiley)


There are other examples of luminescent clusters, for instance trinuclear 

copper(I) pyrazolates displaying emission bands over a wide spectral 

range [156], or others with a core made of four Cu(I) and four sulfur 

atoms [157, 158]. There are examples of Cu(I) luminescent clusters with 

a higher nuclearity, some of them are heterometallic (6–8 metal centers) [159] 

while others are homometallic but with a higher nuclearity (16–20 metal centers). 

In the latter case, which bear alkynyl ligands [160], an emission band is 

observed in the UV spectral region. Finally, the possible use of copper cluster 

units to assemble polymeric compounds with a wide range of possible 

structures, from one- to three-dimensional should be noted [161, 162]. 

5 

Miscellanea of Cu(I) Luminescent Complexes 

IntheprevioussectionswehavepresentedthethreemainclassesofCu(I) 

compounds exhibiting interesting photophysical properties, namely Cu(I)bisphenanthrolines, 

[Cu(NN)(PP)] + complexes and cuprous clusters. However, 

especially in recent years, a growing number of Cu(I) luminescent complexes 

with less conventional ligands have appeared in the literature and some 

of them will be now briefly presented. 

The homoleptic Cu(I) complexes of the benzo[h]quinoline ligands (BHQ) 

depicted in Fig. 36 exhibit excellent luminescence properties in CH2Cl2 

withquantumyieldsashighas0.10andτ = 5.3 µs (ligandC in Fig. 36), 

Fig. 36 Benzo[h]quinoline ligands which, upon complexation with Cu(I), provide highly 

luminescent complexes


which are values comparable to those of [Cu(NN)(POP)] + compounds (see 

Sect. 3.1) [163]. 

This relevant result has been rationalized assuming that the specific complex 

structure imposes minimal geometrical changes in the deactivation 

process of the luminescent excited state back to the ground state, while maintaining 

a significant energy gap. The BHQ ligands accomplish exactly this 

requirement by providing considerable steric congestion in the vicinity of 

chelation, which stabilizes Cu(I), while also providing interligand π-stacking 

that distorts the ground-state geometry and favors the (formal) oxidation of 

the metal with little structural change [163]. The same authors later proposed 

Cu(I) complexes made of bisphenanthroline ligands with structures identical 

to A, B and C (Fig. 36), but no emission data were presented [164]. 

2-Hydroxy-1,10-phenanthroline (Hophen) is a novel kind of substituted 

phenanthroline that was recently proposed (Fig. 37). With Cu(I) it gives origin 

to several compounds, as evidenced by X-ray crystallography, including 

an unusual neutral dinuclear complex [Cu2(ophen)2] which exists in three 

supramolecular isomeric forms and exhibits a broad and weak luminescencebandcenteredaround630 

nm, which has been tentatively attributed 

to deactivation of an MLCT state [165]. The same ligand, which is characterized 

by complicated coordination modes involving both the regular nitrogen 

sites and oxygen binding another metal ion (Fig. 37), was also used 

to make metal complexes of other d 10 metal ions, i.e. Zn(II), Cd(II) and 

Hg(II), which show ligand centered (LC) emission bands in the blue-green 

region [166]. 

Fig. 37 The proposed ketone and hydroxy tautomers of Hophen 

Vogler et al. have made several Cu(I) complexes exhibiting emission in different 

regions of the VIS spectral window, including blue and red, and having 

pure MLCT or mixed MLCT/LLCT character [167, 168]. For instance the complex 

depicted in Fig. 38 which is easily accessible from commercially available 

productsshowsaweakbutdistinctMLCTredluminescencepeakingat600 

nm [167]. 

Several other unconventional ligands have been utilized recently to make 

luminescent Cu(I) complexes such as thia-calix[3]pyridine (orange MLCT


Fig. 38 The red-emitting complex [Cu(BTA)(hfac)] with BTA = bis(trimethylsilyl)acetylene 

and hfac = 1,1,1,5,5,5-hexa-fluoroacetyl-acetonate 

phosphorescence) [169], mononuclear and dinuclear azaindole-containing 

systems (blue LC) [170, 171], heteroleptic diphosphine/diisocyanide ligands 

(blue MLCT) [172], pyrazolylborates (blue LC phosphorescence) [173]. 

6 

Conclusions and Perspectives 

In recent years, the rationalization of the electronic and photophysical properties 

of Cu(I) compounds has made considerable progress, in parallel with 

a significant implementation of synthetic protocols to prepare both simple 

and complex Cu(I)-based structures with satisfactory yields. Now we know 

key design principles that allow one to make highly luminescent Cu(I) compounds 

and supramolecular architectures with programmable cascades of 

photoinduced processes. Such trends have taken Cu(I) complexes among the 

key players in the realm of photoactive complexes, where other metals such 

as Ru(II) and, more recently Ir(III) have traditionally played a prominent 

role. The relentless quest for luminescent metal compounds to be utilized in 

a variety of applications can find interesting answers among some classes of 

Cu(I) complexes, as pointed out in this review article. Obviously, the need for 

abundant, cheap, and environmentally friendly smart materials makes copper 

compounds an attractive alternative to more traditional choices based on 

precious metals. The current trends in literature and patenting [174] indeed 

suggest that Cu(I) complexes are attracting increasing attention for technological 

applications (e.g. OLEDs) and, although we are still at the level of 

prototypes and proofs of principles, further important breakthroughs may be 

anticipated in the years to come.


Acknowledgements We thank the CNR (Progetto “Sistemi nanoorganizzati con proprietà 

elettroniche, fotoniche e magnetiche, commessa PM.P04.010 (MACOL)”) and the 

EC through the Integrated Project OLLA (contract no. IST-2002-004607) for financial 

support. Over the years we worked on several collaborative projects related to Cu(I) 

complexes and, in this regard, we wish to thank Jean-François Nierengarten (Toulouse, 

France), Jean-Pierre Sauvage (Strasbourg, France) and Michael Schmittel (Siegen, Germany) 

along with many other colleagues from their research groups, whose names are 

cited in the references. 

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Top Curr Chem (2007) 280: 117–214 

DOI 10.1007/128_2007_133 




of Coordination Compounds: Ruthenium 

Sebastiano Campagna 1 (✉)·FaustoPuntoriero 1 ·FrancescoNastasi 1 · 

Giacomo Bergamini 2 · Vincenzo Balzani 2 

1Dipartimento di Chimica Inorganica, Chimica Analitica e Chimica Fisica, 

Università di Messina, Via Sperone 31, 98166 Messina, Italy 

campagna@unime.it 

2Dipartimento di Chimica “G. Ciamician”, Università di Bologna, Via Selmi 2, 

40126 Bologna, Italy 

1 Introduction .................................. 118 

2 Structure, Bonding, and Excited States of Ru(II) Polypyridine Complexes 119 

3 [Ru(bpy)3] 2+ : The Prototype ......................... 123 

3.1 AbsorptionSpectrum ............................. 123 

3.2 DeactivationofUpperExcitedStates..................... 124 

3.3 EmissionProperties .............................. 124 

3.4 PhotosubstitutionandPhotoracemizationProcesses ............ 127 

3.5 Quenching of the 3 MLCT Excited State: 

EnergyandElectronTransferProcesses ................... 128 

3.6 ChemiluminescenceandElectrochemiluminescenceProcesses....... 131 

4 Some Important Features of Ru(II) Polypyridine Complexes ....... 133 

4.1 Nonradiative Decay Rate Constants and Emission Spectral Profiles 

ofRu(II)PolypyridineComplexes ...................... 133 

4.2 Ultrafast Time-Resolved Spectroscopy 

andLocalization/DelocalizationIssues .................... 135 

4.3 Ru(II)ComplexesBasedonTridentatePolypyridineLigands........ 136 

4.4 Interplay Between Multiple Low-Lying MLCT States 

InvolvingaSinglePolypyridineLigand.................... 138 

5 Ruthenium and Supramolecular Photochemistry .............. 141 

5.1 Photoinduced Electron/Energy Transfer Across Molecular Bridges 

inDinuclearMetalComplexes......................... 142 

5.2 Photoactive Multinuclear Ruthenium Species 

ExhibitingParticularTopologies ....................... 153 

5.2.1 RacksandGrids ................................ 153 

5.2.2 Dendrimers................................... 155 

5.3 Donor–Chromophore–AcceptorTriads.................... 164 

5.4 PolyadsBasedonOligoprolineAssemblies.................. 170 

5.5 Multi-ruthenium Assemblies Based on Derivatized Polystyrene . . . . . . 172 

5.6 Photoinduced Collection of Electrons 

intoaSingleSiteofaMetalComplex..................... 174 

5.7 PhotoinducedMultiholeStorage:MixedRu–MnComplexes ........ 177 

5.8 PhotocatalyticProcessesOperatedbySupramolecularSpecies....... 180

118 S. Campagna et al. 

5.8.1 PhotogenerationofHydrogen......................... 180 

5.8.2 OtherPhotocatalyticSystems ......................... 182 

5.9 Photoactive Molecular Machines Able to Perform Nuclear Motions . . . . 183 

6 Ruthenium Complexes and Biological Systems ............... 185 

7 Dye-Sensitized Photoelectrochemical Solar Cells .............. 188 

7.1 GeneralConcepts................................ 188 

7.2 Ruthenium-SensitizedPhotoelectrochemicalSolarCells .......... 191 

7.3 SupramolecularSensitizers .......................... 193 

8 Miscellanea ................................... 196 

References ....................................... 200 

Abstract Ruthenium compounds, particularly Ru(II) polypyridine complexes, are the 

class of transition metal complexes which has been most deeply investigated from 

a photochemical viewpoint. The reason for such great interest stems from a unique 

combination of chemical stability, redox properties, excited-state reactivity, luminescence 

emission, and excited-state lifetime. Ruthenium polypyridine complexes are indeed good 

visible light absorbers, feature relatively intense and long-lived luminescence, and can 

undergo reversible redox processes in both the ground and excited states. This chapter 

presents some general concepts on the photochemical properties of Ru(II) polypyridine 

complexes and gives an overview of various research topics involving ruthenium photochemistry 

which have emerged in the last 15 years. In particular, aspects connected to 

supramolecular photochemistry and photophysics are discussed, such as multicomponent 

systems for light harvesting and photoinduced charge separation, systems for photoinduced 

multielectron/hole storage, and photocatalytic processes based on supramolecular 

Ru(II) polypyridine species. Interaction with biological systems and dye-sensitized photoelectrochemical 

cells are also briefly discussed. 

Keywords Ruthenium · Luminescence · Electron transfer · Energy transfer · 

Solar energy conversion · Light-powered molecular machines · Dye-sensitized solar cells 

1 

Introduction 

The photochemistry of ruthenium complexes has undergone an impressive 

growth in the last few decades. The prototype compound [Ru(bpy)3] 2+ (bpy 

=2,2 ′ -bipyridine) has certainly been one of the molecules most extensively 

studied and widely used in research laboratories during the last 30 years. 

A unique combination of chemical stability, redox properties, excited-state reactivity, 

luminescence emission, and excited-state lifetime has attracted the 

attention of many researchers, first on this molecule and then on some hundreds 

of its derivatives. The study of this class of complexes has stimulated the 

growth of several branches of chemistry. In particular, Ru(II) polypyridine 

complexes have played and are still playing a key role in the development of

Photochemistry and Photophysics of Coordination Compounds: Ruthenium 119 

photochemistry, photophysics, photocatalysis, electrochemistry, photoelectrochemistry, 

chemi- and electrochemiluminescence, and electron and energy 

transfer. Mostly in the last 15 years, Ru(II) polypyridine complexes have 

also contributed highly to the development of supramolecular photochemistry, 

and in particular to its aspects related to photoinduced electron and energy 

transfer processes within multicomponent (supramolecular) assemblies, 

including luminescent polynuclear metal complexes, light-active dendrimers, 

artificial light-harvesting antennae, photoinduced charge-separation devices, 

luminescent sensors, and light-powered molecular machines. 

Because of the enormous number of Ru(II) complexes investigated from 

a photochemical viewpoint and the variety of multicomponent structures 

prepared and light-based functions explored, it is impossible to make an exhaustive 

review. In this chapter, we recall some basic concepts on ruthenium 

photochemistry and discuss in some detail a few selected topics, particularly 

those that have developed or emerged during the last 15 years. In this way we 

also hope to give an overview of some research directions which ruthenium 

photochemistry allows to be explored. An exhaustive review [1] published 

about 20 years ago collects photochemical, photophysical, and redox data of 

several hundreds of Ru(II) polypyridine complexes. Another extensive review 

was published about 10 years ago [2], dealing with the luminescence 

properties of polynuclear transition metal complexes, most of them containing 

Ru(II) polypyridine subunits (interestingly, in the former review [1] less 

than ten polynuclear Ru complexes were reported). A review focused on the 

photophysical properties of Ru(II) complexes with tridentate polypyridine 

ligands [3] has also been published. All these review articles contain more or 

less comprehensive tables of data. Enlightening articles on some basic properties 

of Ru(II) polypyridine complexes are also available [4–8]. 

The very large majority of photochemical investigations on ruthenium 

complexes deal with Ru(II) polypyridine species. For such a reason, as also 

implicitly suggested above, we will limit our discussion to these species. Other 

photoactive compounds containing ruthenium metals, including ruthenium 

porphyrins, are not included in this article. 

2 

Structure, Bonding, and Excited States of Ru(II) Polypyridine Complexes 

Ru2+ is a d6 system and the polypyridine ligands are usually colorless molecules 

possessing σ donor orbitals localized on the nitrogen atoms and 

π donor and π∗ acceptor orbitals more or less delocalized on aromatic rings. 

Following a single-configuration one-electron description of the excited state 

in octahedral symmetry (Fig. 1a), promotion of an electron from a πM metal 

ligand orbitals gives rise to metal-to-ligand charge transfer 

orbital to the π∗ L 

(MLCT) excited states, whereas promotion of an electron from πM to σ∗ M or-


Fig. 1 a Simplified molecular orbital diagram for Ru(II) polypyridine complexes in octahedral 

symmetry showing the three types of electronic transitions occurring at low 

energies. b Detailed representation of the MLCT transition in D3 symmetry 

bitals gives rise to metal-centered (MC) excited states. Ligand-centered (LC) 

excited states can be obtained by promoting an electron from πL to π ∗ L .All 

these excited states may have singlet or triplet multiplicity, although spin– 

orbit coupling causes large singlet–triplet mixing, particularly in MC and 

MLCT excited states [6, 9–11]. 

The prototype [Ru(bpy)3] 2+ (Fig. 2), as well as most of the Ru(LL)3 2+ complexes 

(LL = bidentate polypyridine ligand), exhibits a D3 symmetry [12]. 

Following Orgel’s notation [13], the π ∗ orbitals may be symmetrical (χ) or 

Fig. 2 Molecular structural formula of [Ru(bpy)3] 2+


antisymmetrical (Ψ ) with respect to rotation around the C2 axis retained by 

each Ru(bpy) unit. A more detailed picture of the highest occupied molecular 

orbitals (HOMOs) and lowest unoccupied molecular orbitals (LUMOs) is 

shown in Fig. 1b [14–16]. The HOMOs are πMa1(d) and πMe(d), which are 

mainly localized on the metal; the LUMOs are π ∗ L a2(Ψ )andπ ∗ L 

e(Ψ ), which 

are mainly localized on the ligands. The ground state of the complex is a singlet, 

derived from the πMe(d) 4 πMa1(d) 2 electronic configuration. 

According to Kasha’s rule, only the lowest excited state and the upper states 

that can be populated on the basis of the Boltzmann equilibrium distribution 

may play a role in determining the photochemical and photophysical properties. 

The MC excited states of d 6 octahedral complexes are strongly displaced 

with respect to the ground-state geometry along metal–ligand vibration coordinates 

[17, 18]. 

When the lowest excited state is MC, it undergoes fast radiationless deactivation 

to the ground state and/or ligand dissociation reactions (Fig. 3). As 

a consequence, at room temperature the excited-state lifetime is very short, 

no luminescence emission can be observed [19], and very rarely bimolecular 

(or supramolecular) reactions can take place. LC and MLCT excited states 

are usually not strongly displaced compared to the ground-state geometry. 

Thus, when the lowest excited state is LC or MLCT (Fig. 3) it does not undergo 

fast radiationless decay to the ground state and luminescence can usually be 

observed. The radiative deactivation rate constant is somewhat higher for 

3 MLCT than for 3 LC because of the larger spin–orbit coupling effect. For this 

reason, the 3 LC excited states are longer lived at low temperature in a rigid 

matrix and the 3 MLCT excited states are more likely to exhibit luminescence 

at room temperature in fluid solution. 

Fig. 3 Schematic representation of two limiting cases for the relative positions of 3 MC and 

3 LC (or 3 MLCT) excited states


From the above discussion, it is clear that the excited-state properties of 

a complex are related to the energy ordering of its low-energy excited states 

and, particularly, to the orbital nature of its lowest excited state. The energy 

positions of the MC, MLCT, and LC excited states depend on the ligand field 

strength, the redox properties of metal and ligands, and intrinsic properties 

of the ligands, respectively [1, 2, 6]. Thus, in a series of complexes of the same 

metal ion, the energy ordering of the various excited states, and particularly 

the orbital nature of the lowest excited state, can be controlled by the choice of 

suitable ligands [1, 2, 5, 6]. It is therefore possible to design complexes having, 

at least to a certain degree, desired properties. 

For most Ru(II) polypyridine complexes, the lowest excited state is 

a 3 MLCT level (or, better, a cluster [6] of closely spaced 3 MLCT levels, see 

later) which undergoes relatively slow radiationless transitions and thus exhibits 

relatively long lifetime and intense luminescence emission. Such a state 

is obtained by promoting an electron from a metal πM orbital to a ligand π∗ L 

orbital (Fig. 1). The same π∗ L orbital is usually involved in the one-electron 

reduction process. For a long time it has been discussed whether in homoleptic 

complexes the emitting 3 MLCT state is best described with a multichelate 

ring-delocalized orbital (Fig. 4a) or a single chelate ring-localized orbital with 

a small amount of interligand interaction (Fig. 4b) [20]. This problem has 

been tackled with a variety of techniques on both reduced and excited complexes. 

Compelling evidence for “spatially isolated” [21] redox orbitals has 

been obtained from low-temperature cyclic voltammetry [22, 23], electron 

spin resonance [24], electronic absorption spectra of reduced species [25, 26], 

nuclear magnetic resonance [27], resonance Raman spectra [28, 29], and 

time-resolved infrared spectroscopy [30]. In the last 10 years, with the coming 

into play of ultrafast spectroscopic techniques, it has also been possible to 

investigate the nature of the Franck–Condon state and the rate constants of 

the localization/delocalization processes, as well as the interligand hopping 

(sometimes called “randomization of the excitation”) in the MLCT excited 

state. These issues will be discussed in more detail later. 

Fig. 4 Pictorial description of the electron promoted to the π ∗ L 

delocalized orbital; b single chelate ring-localized orbital 

orbital: a multichelate ring


3 

[Ru(bpy)3] 2+ : The Prototype 

To discuss the general properties of Ru(II) polypyridine complexes, it is convenient 

to refer to the properties of the prototype of this class of compounds, 

that is, [Ru(bpy)3] 2+ . 

3.1 

Absorption Spectrum 

The absorption spectrum of [Ru(bpy)3] 2+ is shown in Fig. 5 along with the 

proposed assignments [1, 4, 6, 14–16, 31]. The bands at 185 nm (not shown in 

the figure) and 285 nm have been assigned to spin-allowed LC π → π ∗ transitions 

by comparison with the spectrum of protonated bipyridine [32]. The 

two remaining intense bands at 240 and 450 nm have been assigned to spinallowed 

MLCT d → π ∗ transitions. The shoulders at 322 and 344 nm might 

be MC transitions. In the long-wavelength tail of the absorption spectrum 

ashoulderispresentatabout550 nm (ε ∼ 600 M –1 cm –1 )inanethanol– 

methanol glass at 77 K [33]. This absorption feature is thought to be due to 

spin-forbidden MLCT transition(s). 

In spite of the presence of the heavy Ru atom, it has been established that 

it is reasonable to assign the electronic transitions of [Ru(bpy)3] 2+ as being 

due to “singlet” or “triplet” states. In particular, a singlet character ≤ 10% 

has been estimated [10, 34] for the lowest-lying excited states of [Ru(bpy)3] 2+ . 

The maximum of the 1 MLCT band at ∼ 450 nm is slightly sensitive to solvent, 

suggesting an instantaneous sensing of the formation of the dipolar excitedstate 

[Ru 3+ (bpy)2(bpy) – ] 2+ [35]. 

Fig. 5 Electronic absorption spectrum of [Ru(bpy)3] 2+ in alcoholic solution


3.2 

Deactivation of Upper Excited States 

As mentioned in the Introduction, the upper excited states of transition 

metal complexes usually undergo radiationless deactivation to the lowest 

excited state. For [Ru(bpy)3] 2+ the lowest excited state (or, better, the cluster 

of lowest excited states) is relatively long livedm and its formation and 

disappearance can be easily monitored by flash spectroscopy and luminescence 

decay. The absorption spectrum of the lowest excited state is shown in 

Fig. 6 [36–39]. 

Fig. 6 Electronic absorption spectrum of the lowest excited state of [Ru(bpy)3] 2+ in alcoholic 

solution 

The risetime of the lowest excited state upon excitation of spin-allowed 

excited states was initially estimated to be ≪ 1 ns for [Ru(bpy)3] 2+ [40, 41]; 

successively, available ultrafast spectroscopic techniques demonstrated that 

intersystem crossing occurs in the subpicosecond timescale (see later). The 

efficiency of formation of the lowest excited state, Φ( 3 MLCT) (and thus, the 

efficiency of intersystem crossing from the upper singlets obtained by excitation 

to the lowest triplet, ηisc), is essentially unity [36, 37, 42–44]. 

3.3 

Emission Properties 

Excitation of [Ru(bpy)3] 2+ in any of its absorption bands leads to a luminescence 

emission (Fig. 7) whose intensity, lifetime, and energy position 

are more or less temperature dependent. Detailed studies on the temperature 

dependence [1, 4, 6, 45–49] of the luminescence lifetime and quantum 

yield in the temperature range 2–70 K showed that luminescence originates 

from a set of three closely spaced levels (∆E, 10, and 61 cm –1 )inther-


Fig. 7 Emission spectrum of ∗ [Ru(bpy)3] 2+ in alcoholic solution at 77 K(solid line) and 

at room temperature (dashed line) 

mal equilibrium. This cluster of luminescent, closely spaced excited states 

will be indicated in the following by ∗ [Ru(bpy)3] 2+ or as the 3MLCT state. 

∗ [Ru(bpy)3] 2+ has a substantial triplet character and a single ligand localized 

excitation. 

In rigid glass at 77 K the emission lifetime of ∗ [Ru(bpy)3] 2+ is ∼ 5 µs and 

the emission quantum yield is ∼ 0.4 [1, 6, 8]. Taken together with the unitary 

intersystem crossing efficiency, these figures yield a value of ∼ 13 µs for 

the radiative lifetime. Values of this order of magnitude have been found for 

MLCT excited states of other transition metal complexes [50–53]. LC excited 

states of transition metal complexes usually exhibit radiative lifetimes in the 

millisecond range [1, 6, 49, 50, 53–60]. 

Fig. 8 Temperature dependence of the emission lifetime of ∗ [Ru(bpy)3] 2+ in nitrile solution


With increasing temperature, the emission lifetime (Fig. 8) and quantum 

yield decrease [1, 4, 6, 8, 32, 61–79]. This behavior may be accounted for by 

a stepwise term and two Arrhenius terms [1–3]: 

B 

1/τ =k0 + 

1+exp[C(1/T –1/TB)] + A1 exp(– ∆E1/RT) 

+ A2 exp(– ∆E2/RT). (1) 

The value of the various parameters is somewhat dependent on the nature 

of the solvent. In propionitrile–butyronitrile (4 : 5 v/v) the values are as 

follows [70, 71]: k0 = 2 × 10 5 s –1 ; B = 2.1 × 10 5 s –1 ; A1 = 5.6 × 10 5 s –1 ; ∆E1 = 

90 cm –1 ; A2 = 1.3 × 10 14 s –1 ; ∆E2 = 3960 cm –1 .Includedink0 are the radiative 

k0(r) and nonradiative k0(nr) rate constants at 84 K. The stepwise term B is 

due to the melting of the matrix (100–150 K) and corresponds to the coming 

into play of vibrations capable of facilitating radiationless deactivation [8, 71]. 

Inthesametemperaturerangearedshiftof∼ 1000 cm –1 is observed in the 

maximum of the emission band, and it is mainly attributed to reorganization 

of solvent molecules around the excited state in fluid solution before 

emission takes place [8, 71]. The Arrhenius term with A1 = 5.6 × 10 5 s –1 and 

∆E1 = 90 cm –1 is thought to correspond to the thermal equilibration with 

a level lying at slightly higher energy and having the same electronic nature 

(so it would be a fourth MLCT state [6], considering the lowest-lying MLCT 

state is made of three sublevels as described before). The second Arrhenius 

term corresponds to a thermally activated surface crossing to an upper-lying 

3 MC level which undergoes fast deactivation. Identification of this higher 

level as a 3 MC state is based upon the observed photosubstitution behavior 

at elevated temperatures [61], consistent with established photoreactivity 

patterns for d 6 metal complexes [17, 52]. 

Experiments carried out with [Ru(bpy)3] 2+ and [Ru(bpy-d8)3] 2+ in H2O 

and D2O [61, 80, 81] indicate that k0(nr) is sensitive to deuteration, as expected 

for a weak-coupled radiationless process [6, 82–84]. By contrast, A2 

is insensitive to deuteration, supporting a strong-coupled (surface crossing) 

deactivation pathway, which may be related to the observed photosensitivity. 

It should be noted that the decrease in lifetime on melting has also been explained 

on the basis of the energy gap law because of the corresponding red 

shift in the emission band [6]. 

Finally, it should be noted that at 77 K the emission spectrum of 

[Ru(bpy)3] 2+ , as well as that of most Ru(II) polypyridine complexes, exhibits 

a vibrational structure (see Fig. 7). This structure is assigned to the vibrational 

progression, and its energy spacing is about 1300 cm –1 ,equivalentto 

the C – NandC– C stretching energy of the aromatic rings, thus indicating 

that such stretchings are the dominant accepting modes for deactivation of 

the 3 MLCT state.


3.4 

Photosubstitution and Photoracemization Processes 

Although [Ru(bpy)3] 2+ is normally considered as photochemically inert toward 

ligand substitution, this is not strictly true [1, 6, 14, 15]. In aqueous 

solution the quantum yield of [Ru(bpy)3] 2+ disappearance is in the range 

10 –5 –10 –3 , depending on both the pH of the solution and temperature [1]. 

In chlorinated solvents such as CH2Cl2, the photochemistry of [Ru(bpy)3]X2 

(X = Cl – ,Br – ,NCS – ) is well behaved [64, 85], giving rise to [Ru(bpy)2X2] as 

the final product. The quantum yields are in the range 10 –1 –10 –2 .ThePF – 6 salt 

is photoinert. A substantial difference between aqueous and CH2Cl2 solutions 

is that salts of [Ru(bpy)3] 2+ are completely ion-paired in the latter medium. 

A detailed mechanism for the ligand photosubstitution reaction of 

[Ru(bpy)2X2] has been proposed [6, 64] (Fig. 9). According to this mechanism, 

thermally activated formation of a 3 MC excited state (vide supra) 

leads to the cleavage of a Ru – N bond, with formation of a five-coordinate 

square pyramidal species. In the absence of coordinating ions, as with the 

PF – 6 salt, this square pyramidal species returns to [Ru(bpy)3] 2+ .Whencoordinating 

anions are present, as in the Cl – salt, a hexacoordinated monodentate 

bpy intermediate is formed. Once formed, this monodentate bpy species can 

undergo loss of bpy and formation of [Ru(bpy)2X2], or a “self-annealing” 

process (chelate ring closure), with re-formation of [Ru(bpy)3] 2+ .The“selfannealing” 

protective step is favored in aqueous solution, presumably because 

Fig. 9 Scheme of the proposed mechanism for ligand photosubstitution reactions of 

[Ru(bpy)3]X2


of stabilization of the cationic [Ru(bpy)3] 2+ species, whereas formation of 

neutral [Ru(bpy)2X2] complexes is favored in low-polarity solvents. Photoracemization 

of [Ru(bpy)3] 2+ [86] also occurs with low quantum yield 

(2.9 × 10 –4 in water at 25 ◦ C). This process can be accounted for by a rearrangement 

of the square pyramidal primary photoproduct into a trigonal 

bipyramidal intermediate which can lead back to either the ∆ or the Λ isomer 

[64]. 

Ligand photodissociation is, of course, a drawback for the use of [Ru 

(bpy)3] 2+ in practical applications. To avoid ligand photodissociation one 

should prevent population of 3 MC and/or ligand dissociation from 3 MC. 

Population of 3 MC can be prevented or at least reduced by: (a) addition of 

sufficient quencher to capture 3 MLCT before surface crossing to 3 MC can 

occur; (b) working at low temperature; (c) increasing the energy gap between 

3 MLCT and 3 MC; and (d) increasing pressure [87, 88]. Ligand dissociation 

from 3 MC can also be reduced by (e) avoiding coordinating anions in solvent 

of low dielectric constant and (f) linking together the three bpy ligands so 

as to form a single caging ligand which encapsulates the metal ion. Point (a) 

is experimentally difficult, since thermal equilibration is quite a fast process. 

Points (c) and (f) are particularly interesting and much effort has been 

made along such directions [1, 89, 90]. It should be considered that in most 

of the [Ru(bpy)3] 2+ derivatives, the 3 MLCT state is shifted to lower energies 

[1], whereas the energy of the 3 MC state usually does not change. This 

leads to an increased energy gap between MLCT and MC states and decreased 

photolability. As a consequence, photosubstitution is a minor problem in 

most ruthenium polypyridine complexes. It should be considered, however, 

that decreasing the energy of the 3 MLCT level increases the Franck–Condon 

factors for radiationless decay to the ground state, leading to decreased luminescence 

lifetimes and quantum yields. The rate of radiationless decay can 

be decreased by extending the delocalization of the promoted electron on 

suitable aromatic ligands [78, 91, 92]. 

Finally, it should also be noted that the photolabilization of ligands can 

be a profitable photochemical process: for example, a synthetic route to trisheteroleptic 

Ru complexes involves photosubstitution of ligands [93] and 

photochemical, reversible ligand exchange has been proposed to be used to 

photoswitch the complexation activity in a ruthenium complex containing 

a scorpionate terpyridine ligand [94]. 

3.5 

Quenching of the 3 MLCT Excited State: 

Energy and Electron Transfer Processes 

The lowest 3 MLCT excited state of [Ru(bpy)3] 2+ lives long enough to encounter 

other solute molecules (even when these are present at relatively low 

concentration) and possesses suitable properties to play the role of energy


Fig. 10 Molecular quantities of [Ru(bpy)3] 2+ relevant for energy and electron transfer 

processes. ∗∗ [Ru(bpy)3] 2+ indicates higher-energy spin-allowed excited states and 

∗ [Ru(bpy)3] 2+ indicates the lowest spin-forbidden excited state ( 3 MLCT). Reported potentials 

are in aqueous solution vs SCE 

donor, electron donor, or electron acceptor. As is also shown in Fig. 10, the energy 

available to ∗ [Ru(bpy)3] 2+ for energy transfer processes is 2.12 eV and its 

reduction and oxidation potentials are + 0.84 and – 0.86 V(aqueoussolution, 

vs SCE). It follows that ∗ [Ru(bpy)3] 2+ is at the same time a good energy donor 

(Eq. 2), a good electron donor (Eq. 3), and a good electron acceptor (Eq. 4): 

∗ [Ru(bpy)3] 2+ +Q→ [Ru(bpy)3] 2+ + ∗ Q energy transfer (2) 

∗ [Ru(bpy)3] 2+ +Q→ [Ru(bpy)3] 3+ +Q – 

∗ [Ru(bpy)3] 2+ +Q→ [Ru(bpy)3] + +Q + 

oxidative quenching (3) 

reductive quenching . (4) 

The direct observation of redox products represents the strongest evidence to 

support the occurrence of oxidative and reductive quenching mechanisms. 

These observations can be performed in a few cases with continuous irradiation 

[95, 96] and more often in flash photolysis experiments, because 

usually the redox products rapidly decay either by back electron transfer reactions 

to re-form the starting materials or by secondary reactions to form 

other products. In practice, the possibility to observe transient absorptions 

is related to the changes in the optical density of the solution caused by the 

photoreaction. Bleaching and recovering of the [Ru(bpy)3] 2+ spectrum can 

be used for kinetic measurements. The absorption band at 680 nm typical of 

[Ru(bpy)3] 3+ is too weak to detect small [Ru(bpy)3] 3+ concentrations, so that 

in oxidative quenching processes one is forced to use the absorption spec-


trum of Q – to monitor product formation. By contrast, [Ru(bpy)3] + exhibits 

a strong absorption band at 510 nm, which is particularly useful to investigate 

reductive quenching reactions. 

Following some pioneering works [96–100], literally hundreds of bimolecular 

excited-state reactions of [Ru(bpy)3] 2+ and of its derivatives have been 

studied [101]. Here we only illustrate a few examples to show that these 

excited-state reactions can be used for mechanistic studies as well as for potential 

applications of the greatest interest. 

The early interest in [Ru(bpy)3] 2+ photochemistry arose from the possibility 

of using its long-lived excited state as energy donor in energy transfer 

processes. Although several sensitized reactions attributed to energy transfer 

processes (see, e.g., [102, 103]) were later shown to proceed via electron transfer 

[100], there are some very interesting cases in which energy transfer has 

been firmly demonstrated. A clear example is the quenching of ∗ [Ru(bpy)3] 2+ 

by [Cr(CN)6] 3– , where sensitized phosphorescence of the chromium complex 

has been observed both in fluid solution [104–108] and in the solid 

state [109–111]: 

∗ 

[Ru(bpy)3] 2+ + [Cr(CN)6] 3– → [Ru(bpy)3] 2+ +( 2 Eg)[Cr(CN)6] 3– (5) 

( 2 Eg)[Cr(CN)6] 3– → [Cr(CN)6] 3– + hν . (6) 

Energy transfer from ∗ [Ru(bpy)3] 2+ to [Cr(CN)6] 3– was also used to demonstrate 

that the photosolvation reaction observed upon direct excitation of 

[Cr(CN)6] 3– does not originate from the luminescent 2 Eg state of the chromium 

complex [104, 112]. 

It should be pointed out that both reductive and oxidative ∗ [Ru(bpy)3] 2+ 

electron transfer quenchings by [Cr(CN)6] 3– are thermodynamically forbidden 

because it is very difficult to reduce or oxidize [Cr(CN)6] 3– [108]. 

[Cr(bpy)3] 3+ , by contrast, can be very easily reduced and with this quencher 

oxidative electron transfer prevails over energy transfer 

∗ [Ru(bpy)3] 2+ + [Cr(bpy)3] 3+ 

k=3.3×10 9 M –1 s –1 

–––––––––––––––––––→[Ru(bpy)3] 3+ + [Cr(bpy)3] 2+ , (7) 

as is shown by the appearance of the [Cr(bpy)3] 2+ absorption spectrum in 

flash photolysis experiments [113]. Equation 7 converts 71% ofthespectroscopic 

energy (2.12 eV) of the excited-state reactant into chemical energy of 

the products. As usually happens in these simple homogeneous systems, the 

converted energy cannot be stored but is immediately dissipated into heat by 

the back electron transfer reaction: 

[Ru(bpy)3] 3+ + [Cr(bpy)3] 2+ 

k=2×10 9 M –1 s –1 

–––––––––––––––––→[Ru(bpy)3] 2+ + [Cr(bpy)3] 3+ . (8)


A carefully studied example of reductive electron transfer quenching (Eq. 9) 

is that involving Eu2+ aq as a quencher [114, 115]: 

∗ [Ru(bpy)3] 2+ +Euaq 2+ k=2.8×107 M –1 s –1 

–––––––––––––––––––→[Ru(bpy)3] + +Euaq 3+ . (9) 

The difference spectrum obtained by flash photolysis after a 30-ns light 

pulse shows a bleaching in the region around 430 nm due to depletion of 

[Ru(bpy)3] 2+ and an increased absorption around 500 nm due to the forma- 

tion of [Ru(bpy)3] + (note that both Eu 2+ 

aq 

and Eu3+ aq 

are transparent in this 

spectral region). Clear kinetic evidence for reductive quenching comes from 

the observation that the growth of the absorption at 500 nm occurs at a rate 

equal to the rate of decay of the luminescence emission of ∗ [Ru(bpy)3] 2+ .As 

it may happen in excited-state reactions, the products of Eq. 9 have a high 

energy content and thus they give rise to a back electron transfer reaction 

[Ru(bpy)3] + +Euaq 3+ k=2.7×107 M –1 s –1 

–––––––––––––––––––→[Ru(bpy)3] 2+ +Euaq 2+ , (10) 

which can be monitored (on a longer timescale) through the recovery of the 

430-nm absorption or the disappearance of the 500-nm absorption. 

In several cases direct evidence for energy transfer quenching (i.e., sensitized 

luminescence or absorption spectrum of the excited acceptor) or 

electron transfer quenching (i.e., absorption spectrum of redox products) is 

difficult or even impossible to obtain for bimolecular processes. In such cases, 

free energy correlations of rate constants are quite useful to elucidate the reaction 

mechanism [108, 116–118]. As we will see later, photoinduced energy 

and electron transfer processes can take place very easily in suitably organized 

supramolecular systems. 

3.6 

Chemiluminescence and Electrochemiluminescence Processes 

As mentioned in the introductory chapter (Balzani et al. 2007, in this volume) 

[119], excited states can be generated in very exergonic electron transfer 

reactions. Formation of excited states can be easily demonstrated when 

the excited states are luminescent species. Because of its stability in the reduced 

and oxidized forms and the strong luminescence of its excited state, 

[Ru(bpy)3] 2+ is an extremely versatile reactant for a variety of chemiluminescent 

processes [32, 120–124]. 

In principle, there are two ways to generate the luminescent ∗ [Ru(bpy)3] 2+ 

excited state in chemical reactions. One way (Eq. 11) is to oxidize [Ru(bpy)3] + 

with a species X having reduction potential E 0 (X/X – )morepositivethan 

0.84 V, and another way (Eq. 12) is to reduce [Ru(bpy)3] 3+ with a species Y –


whose potential E 0 (Y/Y – )ismorenegativethan–0.86 V (see also Fig. 10). 

[Ru(bpy)3] + +X→ ∗ [Ru(bpy)3] 2+ +X – (11) 

[Ru(bpy)3] 3+ +Y – → ∗ [Ru(bpy)3] 2+ +Y (12) 

∗ 

[Ru(bpy)3] 2+ → [Ru(bpy)3] 2+ + hν . (13) 

A variety of oxidants (e.g., S2O8 2– [125, 126]) and reductants (e.g., e – aq [127], 

hydrazine and hydroxyl anion [128], oxalate ion [129, 130]) have been used 

in these chemiluminescent processes. In some cases (e.g., with OH – ), the 

reaction mechanism cannot be a simple outer sphere electron transfer reaction 

and the emitting species could be a slightly modified (on the ligands) 

complex. It should also be pointed out that minor amounts of oxidizing and 

reducing impurities are sufficient to produce luminescence in chemiluminescence 

and electrochemiluminescence experiments [131]. 

The most interesting way [132] to obtain chemiluminescence from 

[Ru(bpy)3] 2+ solutions is probably to produce the oxidized and/or reduced 

form of the complex “in situ” by electrochemical methods. Three classical 

experiments of this type can be performed: 

(a) To pulse the potential applied to a working electrode between the oxidation 

and reduction potentials of [Ru(bpy)3] 2+ in a suitable solvent [132, 

133]. In such a way the reduced and oxidized forms produced in the same 

region of space can undergo a comproportionation reaction where enough 

energy is available to produce an excited state and a ground state (see also 

Fig. 10): 

[Ru(bpy)3] 2+ +e – → [Ru(bpy)3] + (14) 

[Ru(bpy)3] 2+ –e – → [Ru(bpy)3] 3+ (15) 

[Ru(bpy)3] 3+ + [Ru(bpy)3] + → ∗ [Ru(bpy)3] 2+ + [Ru(bpy)3] 2+ . (16) 

(b) To reduce [Ru(bpy)3] 2+ in the presence of a strong oxidant (reductive 

oxidation). For example, luminescence is obtained upon continuous reduction 

of [Ru(bpy)3] 2+ at a working electrode in the presence of S2O8 2– [125, 

126]. This oxidant in a first one-electron oxidation reaction generates the very 

powerful oxidant SO4 – that can either oxidize [Ru(bpy)3] + to ∗ [Ru(bpy)3] 2+ 

(Eq. 18) or [Ru(bpy)3] 2+ to [Ru(bpy)3] 3+ (Eq. 19), which then reacts with 

[Ru(bpy)3] + (Eq. 16) to yield the luminescent excited state: 

[Ru(bpy)3] 2+ +e – → [Ru(bpy)3] + (14) 

[Ru(bpy)3] + +S2O8 2– → [Ru(bpy)3] 2+ +SO4 – +SO4 2– (17) 

[Ru(bpy)3] + +SO4 – → ∗ [Ru(bpy)3] 2+ +SO4 2– (18) 

[Ru(bpy)3] 2+ +SO4 – → [Ru(bpy)3] 3+ +SO4 2– (19) 

[Ru(bpy)3] + + [Ru(bpy)3] 3+ → ∗ [Ru(bpy)3] 2+ + [Ru(bpy)3] 2+ . (16)


(c) To oxidize [Ru(bpy)3] 2+ in the presence of a strong reductant (oxidative 

reduction). For example, light is generated upon continuous oxidation 

of [Ru(bpy)3] 2+ at a working electrode in the presence of C2O4 2– [129, 130]. 

This reductant in a first one-electron reaction generates the strongly reducing 

CO2 – radical that can reduce [Ru(bpy)3] 3+ to the excited ∗ [Ru(bpy)3] 2+ 

[Ru(bpy)3] 2+ –e – → [Ru(bpy)3] 3+ (15) 

[Ru(bpy)3] 3+ +C2O4 2– → [Ru(bpy)3] 2+ +CO2 +CO2 – (20) 

[Ru(bpy)3] 3+ +CO2 – → ∗ [Ru(bpy)3] 2+ +CO2 . (21) 

These chemiluminescent electron transfer reactions are quite interesting 

from an applicative [134–136] as well as from a theoretical viewpoint. Actually, 

method a is at the basis of electroluminescent materials, such as organic 

light-emitting diodes (OLEDs) and similar devices, which are receiving increasing 

interest for practical applications [137–141]. 

4 

Some Important Features of Ru(II) Polypyridine Complexes 

4.1 

Nonradiative Decay Rate Constants and Emission Spectral Profiles 

of Ru(II) Polypyridine Complexes 

Radiationless decay from MLCT states of metal polypyridine complexes occurs 

with energy release into medium-frequency (polypyridyl-based) modes 

and, to a lower degree, low-frequency modes and solvent [4, 142–149]. Averaging 

the medium-frequency modes which mainly promote the transition 

and combining low-frequency modes, including solvent, into a single mode, 

treated classically, the rate constant for radiationless decay knr is predicted to 

follow the so-called energy gap law [150–154]. Most of the work to define this 

topic has been made by using Ru(II) polypyridine complexes as models; however, 

the approach also applies to any MLCT emitter, as largely demonstrated 

for Os(II) [146, 147, 155] and Re(I) polypyridine [147, 149, 156] complexes. 

Actually, the energy gap law can be expressed by Eq. 22, where β0 includes the 

vibrationally induced electronic matrix element and F(calc) is the vibrational 

overlap factor (the quantity 1s in Eq. 22 is used to give unitless expression): 

ln(knr · 1s)=lnβ0 +ln[F(calc)] . (22) 

In a simplified version, F(calc) can be expressed as in Eq. 23 [157]: 

� � 

– γ E0 

F(calc) ∝ 

�ω 

� � 

E0 

γ =ln –1. 

SM�ω 

(23)


In Eq. 23, the energy gap E0 is related to the energy separation of the two 

coupled surfaces the �ω term describes, assuming a single configurational 

coordinate model, the average energy of the medium-frequency vibrational 

(accepting) modes that couple the MLCT and ground states, that is, the vibrational 

spacing of the ground state, and SM is the electron-vibrational coupling 

constant (Huang–Rhys factor). E0, �ω, andSM are expressed in cm –1 ,aswell 

as ∆ν1/2, the “classical” bandwidth that takes into account the low-frequency 

modes and is present in the detailed expression for F(calc) [146, 147], not 

shown here. If β0, the electronic term, remains roughly constant for a series 

of related complexes, Eq. 22 yields a straight line with intercept ln β0 [146]. 

It is interesting to note that the parameters E0, �ω, SM, and∆ν1/2 also define 

the emission spectral profile, so they can be obtained by a single-mode 

Franck–Condon analysis of the emission spectra, using Eq. 24: 

5� 

��E0 �3 � 

– x�ω Sx � 

M 

I(ν)= 

E0 x! 

x=0 

� � � � �� 

2 

ν – E0 + x�ω 

exp –4ln2 

. 

∆ν1/2 

(24) 

In Eq. 24, I(ν) is the relative emission intensity at energy ν (in cm –1 ), E0 is 

the energy of the zero–zero transition (i.e., the energy of the emitting 3 MLCT 

state), �ω is the average of medium-frequency acceptor modes coupled to the 

MLCT transition, x is the quantum number of such an averaged mediumfrequency 

mode which serves as the final vibronic state (note that x is usually 

limited to 5), ∆ν1/2 is the half-width of the individual vibronic bands, and SM 

is the Huang–Rhys factor. 

Application of the above equations to Ru(II) polypyridine complexes allows 

important information to be obtained on the excited-state properties. It 

should be considered, however, that Eqs. 22–24 are based on several assumptions, 

the most important being the following. (1) For radiationless decay, the 

thermal population of higher-energy excited states is neglected; when such 

an activated route cannot be disregarded, Eq. 22 only gives a contribution to 

the observed radiationless rate constant, the one related to k0 of Eq. 1. (2) The 

Franck–Condon analysis of the emission spectral profile here shown is based 

on a single coordinate; when more coordinates need to be considered, fitting 

of the spectral profile following Eq. 24 can give uncorrected parameter values. 

A simple refinement of Eq. 24 requires inclusion of a second (low energy) 

frequency acceptor mode due to solvent contributions [158–161]. 

More sophisticated theoretical methods to analyze the emission spectra 

of Ru(II) complexes have been introduced [162–166]. In particular, these 

methods allow for the detailed characterization of the high-frequency vibronic 

contributions to the emission spectra and the dependence of such 

contributions on various factors, such as the energy gap between ground and 

excited states. Extensive discussion can be found in the cited references.


Emission spectral profiles calculated by equations such as Eq. 24 have also 

been used, along with theoretical quantum mechanical expressions and experimentally 

determined rate constant values, to estimate the electronic coupling 

matrix element for such intercomponent processes for photoinduced 

energy transfer in dinuclear Ru(II) polypyridine complexes [160]. 

4.2 

Ultrafast Time-Resolved Spectroscopy 

and Localization/Delocalization Issues 

In the last 20 years, ultrafast pump–probe spectroscopy has become accessible 

to several research laboratories, including research groups interested 

in Ru photochemistry. The possibility of investigating the excited-state dynamics 

at very short time delays after the excitation pulse allowed clarification 

of several problems and shed light on many aspects of ruthenium 

photochemistry. New features, sometimes unexpected, have also been revealed 

and new questions and research topics have emerged. For example, 

as discussed in other parts of this chapter, it was found that singlet MLCT 

states can be involved in electron transfer and energy transfer processes, 

even before intersystem crossing and/or thermal relaxation. This is the case 

of photoinduced electron injection in semiconductors [167] and energy 

transfer/migration between Ru subunits of large, strongly coupled dendriticshaped 

systems [168–170]. Fluorescence from ruthenium complexes has also 

been detected [171]. 

Powered by the availability of ultrafast techniques, the long-term issue of 

localization/delocalization of MLCT states has also been revitalized. As previously 

stated, the general view is that the emissive state is localized on a single 

ligand, even for homoleptic species. However, open questions remain concerning 

the nature of the Franck–Condon state and the early-time dynamics 

which leads to the emissive state. 

As for the early-time dynamics, it is largely accepted that in [Ru(bpy)3] 2+ 

and analogous homoleptic species light excitation in the MLCT singlet manifold 

initially produces a Franck–Condon state that is delocalized, which is 

where the promoted electron is shared by all the polypyridine ligands. Then 

on the timescale of tens of femtoseconds, the promoted electron becomes 

localized on a single ligand, due to coupling with local solvent dipoles. Intersystem 

crossing then takes place in about 100 fs, producing a localized 

triplet state. The triplet MLCT state becomes “randomized” by interligand 

hopping on the timescale of 10 ps, the same scale of thermal (including vibrational 

and solvent reorganization) relaxation of the 3 MLCT state. This 

general figure is schematized in Fig. 11, and is based on results dealing with 

many Ru(II) polypyridine complexes, taking advantage of various experimental 

techniques (transient absorption anisotropy, time-resolved resonance 

Raman, pump–probe femtosecond transient absorption spectroscopy).


However, some experimental data contrast with the scheme shown in 

Fig. 11. For example, a time-resolved resonance Raman study indicates that 

in [Ru(bpy)3] 2+ even the initial excitation is localized [172]. Moreover, the recently 

reported femtosecond transient absorption spectrum of [Ru(bpy)3] 2+ 

in the UV region suggests that complete relaxation within the emitting triplet 

MLCT state takes several tens of picoseconds, and that randomization of 

triplet MLCT is complete in less than 500 fs [173]. If this latter point is correct, 

interligand hopping should largely occur from nonrelaxed states, probably 

even partly in the singlet state. In the relaxed triplet MLCT state, interligand 

hopping could be slower, but it would be difficult to measure since randomization 

would already have happened. 

Fig. 11 Picture of the early-time dynamics of light excitation in the MLCT singlet of 

[Ru(bpy)3] 2+ . A delocalizated Franck–Condon state is formed (a), which becomes localized 

on a single ligand (b) and then becomes “randomized” by interligand hopping (c) 

Related to the excited-state dynamics at short times after excitation, broadband 

femtosecond fluorescence spectroscopy of [Ru(bpy)3] 2+ has been recently 

reported, as already mentioned [171]. The authors get 15 ± 10 fs as the 

lifetime for the singlet emission, which is centered at about 520 nm. 

4.3 

Ru(II) Complexes Based on Tridentate Polypyridine Ligands 

An important family of Ru(II) polypyridine complexes is that based on 

tridentate ligands, with [Ru(terpy)2] 2+ as a prototype (terpy = 2,2 ′ :6 ′ ,2 ′′ - 

terpyridine). The absorption, emission, and redox properties of [Ru(terpy)2] 2+ 

are similar to those of [Ru(bpy)3] 2+ ,exceptthat[Ru(terpy)2] 2+ is essentially 

nonluminescent at room temperature, with a lifetime of the 3 MLCT 

state in degassed acetonitrile at room temperature of about 250 ps (meas-


ured by transient absorption spectroscopy [3]), compared with a value of 

about 1 µs exhibitedby[Ru(bpy)3] 2+ under the same conditions [1]. Such 

a short excited-state lifetime is very disappointing, as [Ru(terpy)2] 2+ has 

some advantage over [Ru(bpy)3] 2+ from a structural point of view. Whereas 

[Ru(bpy)3] 2+ can exist as a mixture of Λ and ∆ isomers, and the isomer 

problem can become even more complicated for polynuclear species based on 

“asymmetric” bidentate ligands such as 2,3-bis(2 ′ -pyridyl)pyrazine (2,3-dpp), 

[Ru(bpy)3] 2+ is achiral. Moreover, by taking advantage of para substituents 

on the central pyridine of the terpy ligand, [Ru(terpy)2] 2+ can give rise to 

supramolecular architectures perfectly characterized from a structural viewpoint, 

in particular to multinuclear one-dimensional (“wire”-like) species. 

The reason for the poor photophysical properties of Ru(II) complexes with 

tridentate polypyridine ligands at room temperature, compared to Ru(II) 

species with bidentate chelating polypyridine, stems from the bite angle of 

the tridentate ligand that leads to a weaker ligand field strength and thus to 

lower-energy MC states as compared to Ru(II) complexes of bpy. The thermally 

activated process from the potentially emitting 3 MLCT state to the 

higher-lying 3 MC state is therefore more efficient in [Ru(terpy)2] 2+ and its 

derivatives and leads to fast deactivation of the excited state by nonradiative 

processes [1, 3, 4], although terpy-type Ru complexes are inherently more 

photostable than bpy-type ones because of a stronger chelating effect. 

Much effort has been devoted to the design and synthesis of tridentate 

polypyridine ligands, leading to Ru(II) complexes with improved photophysical 

properties [3, 78, 92, 174–181]. For example, the use of ligands containing 

electron-withdrawing and -donor substituents on tpy increases the 

gap between the 3 MLCT and the 3 MC states [174]. An increase in such 

an energy gap has also been obtained by the use of cyclometallating ligands 

[177]. Unavoidably, the stabilization of 3 MLCT states causes an increase 

of the rate constant for radiationless decay to the ground state. This latter 

effect can be balanced by extension of the π ∗ orbital by appropriate substituents, 

which increases the delocalization of the acceptor ligand of the 

MLCT excited state leading to a smaller Franck–Condon factor for nonradiative 

decay [78, 175, 176, 178, 179, 182–188]. In this regard, species based on 

ethynyl-substituted terpy ligands feature particularly interesting photophysical 

properties [175, 176, 182]. Various approaches to improve the photophysical 

properties of Ru(II) complexes with tridentate polypyridine ligands have 

been reviewed [175, 182]. 

The bis-tridentate Ru(II) polypyridine complex with the best photophysical 

properties reported up to now is probably the species 1, based on 

the 2,6-bis(8 ′ -quinolinyl)pyridine ligand [189]. This species exhibits 3 MLCT 

emission with a maximum at 700 nm, with a lifetime of 3.0 µs and a quantum 

yield of 0.02 in deoxygenated methanol–ethanol solution at room temperature. 

The emission maximum blue-shifts to 673 nm at 77 Kinthesame 

solvent mixture, exhibiting a luminescence lifetime of 8.5 µs and a quantum


yield of 0.06. The authors attribute these excellent (particularly at room temperature) 

photophysical properties to the relief of structural distortion from 

the ideal octahedral geometry, due to ligand design. Actually, X-ray characterization 

of the compound reveals a quasi-ideal octahedral geometry around 

the metal center. 

4.4 

Interplay Between Multiple Low-Lying MLCT States 

Involving a Single Polypyridine Ligand 

Usually, there is a linear relationship between redox data, namely first oxidation 

and reduction potentials of Ru complexes, and spectroscopic parameters 

such as MLCT absorption and emission bands, provided that the considered 

compounds constitute a homogeneous series [1, 4, 190–192]. This relationship 

is based on the fact that the orbitals involved in metal-based oxidation 

and ligand-based reduction processes are the same (to a first approximation) 

as those involved in the MLCT absorption and emission transitions. 

Differences in solvent effects for redox and spectroscopic processes should 

be constant, so that the relationship is still linear, although the slops are not 

unitary [1, 191]. However, whereas until 20 years ago this relationship was followed 

by almost all the Ru complexes reported at that time, and exceptions 

were rare [193] and partly unexplained, Ru complexes which do not follow 

the rule, in particular as far as the absorption spectra are concerned, have 

become quite common in recent years. The availability of several examples allowed 

the development of a general interpretation of this behavior. In all the 

cases that do not obey the linear relationship, ligands characterized by a large 

aromatic framework are present. 

It is now clear that the apparent mismatched relationship is linked to the 

presence of multiple low-energy MLCT transitions to a single polypyridine 

ligand, with one such transition being essentially almost invisible spectroscopically. 

The key feature here is that the “single” polypyridine ligand can 

actually be viewed as being made of two “separated” subunits (Fig. 12) with 

the LUMO centered on a part of the ligand framework which is not significantly 

coupled with the metal-based HOMOs (in other words, the LUMO does


Fig. 12 Structural formula of two possible MLCT transitions in ruthenium complexes with 

large polypyridine ligands 

not receive a significant contribution from the chelating nitrogens). In these 

systems, the lowest-energy MLCT transition (MLCT0)canhavevanishingoscillator 

strength and thus it does not significantly contribute to the absorption 

spectrum. On the contrary, a closely lying LUMO+1 (centered on a different 

moiety of the large ligand) receives a significant contribution from the 

chelating nitrogens, so it is largely coupled with the metal-based HOMO(s); 

as a consequence, its corresponding MLCT transition (the MLCT1 transition) 

dominates the absorption spectrum. Since reduction takes place in the 

LUMO, the linear relationship between absorption spectra and redox potential 

cannot be followed. This case will also be discussed in Sects. 5 and 6, for 

specific systems. 

As far as the relationship between emission spectra and redox potentials is 

concerned, whether it is followed or not depends on how fast the interconversion 

between the MLCT1 and MLCT0 states is, compared to the intrinsic decay 

of the MLCT1 excited state (here it is assumed that MLCT0 is lower in energy 

than MLCT1; otherwise, the relationship is always followed, except for very 

particular cases). Solvent, temperature, driving force, and medium effects are 

very important in this regard. For example, at room temperature in fluid


solution the mononuclear complex [(phen)2Ru(tpphz)] 2+ (2, phen = 1,10phenanthroline; 

tpphz = tetrapyrido[3,2-a:2 ′ ,3 ′ -c:3 ′′ ,2 ′′ -h:2 ′′ ,3 ′′ -j]phenazine) 

exhibits emission from its MLCT1 level (λmax = 625 nm, τ = 1.25 ms, Φ = 

0.07), while the dinuclear species [(phen)2Ru(tpphz)Ru(phen)2] 4+ (3) emits 

from its MLCT0 level (λmax = 710 nm, τ = 0.100 ms, Φ = 0.005) [194]. The 

absorption spectra of both compounds in the visible region are very similar 

to one another (apart from the intensity), with the lowest-energy MLCT 

band maximizing at about 440 nm in both cases. In a rigid matrix at 77 K, 

both the mononuclear and dinuclear metal complexes exhibit emission at 

about 585 nm (lifetime in the microsecond timescale), typical of the MLCT1 

level. Such results are interpreted on considering that the ligand tpphz has 

two empty orbitals close in energy: the LUMO is centered on the central 

pyrazine, with negligible contribution from the chelating nitrogen atoms, and 

the LUMO+1 is essentially a bpy-type orbital. Reduction potential data of 

the complexes indicate that LUMO+1 of tpphz is hardly affected on passing 

from mononuclear to dinuclear species, whereas the LUMO of tpphz is stabilized. 

For both [(phen)2Ru(tpphz)] 2+ and [(phen)2Ru(tpphz)Ru(phen)2] 4+ , 

the absorption spectrum is dominated by Ru-to-tpphzLUMO+1 charge transfer 

(i.e., MLCT1) transition—almost coincident to the Ru-to-phen charge transfer 

transition—which occurs at roughly the same energy in mononuclear and 

dinuclear species, with the Ru-to-tpphzLUMO charge transfer (i.e., MLCT0) 

transition not contributing to the absorption feature. Because of the different 

stabilization of MLCT1 and MLCT0 on passing from mononuclear to 

dinuclear species (see above), the driving force for the MLCT1-to-MLCT0 interconversion 

is more favorable, and therefore faster, in the dinuclear species. 

As a consequence, MLCT1-to-MLCT0 decay does not compete with the direct 

decay of MLCT1 to the ground state in the mononuclear species, whereas it is


faster and efficient in the dinuclear species. However, at 77 KtheMLCT1-to- 

MLCT0 interconversion, which cannot occur without solvent reorganization, 

becomes inefficient in both systems. Essentially the same experimental behavior 

is featured by many other Ru(II) polypyridine complexes, and can be 

interpreted in a similar way [195–205]. 

Time-resolved transient absorption spectroscopy [206, 207] confirmed 

that the MLCT1-to-MLCT0 excited-state conversion in [(phen)2Ru(tpphz)- 

Ru(phen)2] 4+ at room temperature is solvent dependent. Indeed, it was 120 ps 

in dichloromethane and faster than 40 ps in acetonitrile [206, 207]. The solvent 

dependence can be attributed to the difference in the MLCT1/MLCT0 

energy gap in the two solvents. It has to be considered, in fact, that the 

“charge separation” between donor and acceptor orbitals, and, as a consequence, 

the coulombic stabilization, is quite different in the two types of 

MLCT states. A nonnegligible reorganization energy is therefore expected for 

the MLCT1-to-MLCT0 transition. A detailed study of the temperature dependence 

of the luminescence properties of species exhibiting this interesting 

behavior would be quite useful, but to our knowledge it has not yet been 

reported. 

5 

Ruthenium and Supramolecular Photochemistry 

Supramolecular photochemistry has played a prominent role in chemical 

research since its definition in the late 1980s [208, 209]. The operational 

definition of supramolecular species is discussed (Balzani et al. 2007, in this 

volume) [119], so it will not be further commented on here. Since Ru(II) 

polypyridine complexes exhibit very interesting photochemical properties 

and can be prepared by relatively easy synthetic methods, even with madeto-order 

properties, the number of photoactive supramolecular species based 

on Ru(II) complexes has rapidly become extraordinarily large. Supramolecular 

systems in which donor and acceptor units are placed at designed 

distances can undergo photoinduced energy and electron transfer process 

(first-order kinetics) even in the case of short-lived excited states [208, 

209]. 

Indeed, Ru(II) polypyridine compounds have been extensively used as 

photoactive units in supramolecular systems either exclusively made of 

metal-based components, such as molecular racks, grids, and dendrimers, or 

in systems whose other active components of the assemblies are of an organic 

nature. In both cases, the final goals of the supramolecular systems are 

essentially two, reflecting the nature of the whole of photochemical science: 

(1) systems designed for the conversion of light energy into other forms of 

energy, essentially chemical energy or electricity; and (2) systems focused on 

the elaboration of the information, including sensors. Quite often these two


aspects are intertwined; for example, long-range photoinduced electron and 

energy transfer processes are important both for the elaboration of optical 

information signals and for light-harvesting systems. 

5.1 

Photoinduced Electron/Energy Transfer Across Molecular Bridges 

in Dinuclear Metal Complexes 

Dinuclear metal complexes containing Ru(II) polypyridine subunits, where 

the metal centers are separated by molecular components (bridges), are particularly 

suited to investigating photoinduced electron and energy transfer 

processes, whose rate constants can give information on the electronic coupling 

mediated by the bridge. The latter topic has been recently reviewed and 

deeply discussed [210]. The number of photoactive (usually, luminescent) 

dinuclear metal complexes based on Ru(II) subunits is very large. The last 

exhaustive review dealing with such species was published about 10 years 

ago [2]. Today it is impossible to be exhaustive even in this relatively narrow 

field. Therefore, we will only present a few examples. In most cases, Ru(II) 

subunits, which play the role of donors, are coupled to Os(II) units, which 

play the role of acceptors in photoinduced energy transfer processes. In all 

cases, the dinuclear homometallic Ru(II) species have also been investigated 

for comparison purposes. Their photophysical properties can be found in the 

original references. 

It is important to note that to have control of the distance between the 

metal centers, the bridges have to be rigid as much as possible. Therefore, 

it is not surprising that oligophenylenes have often been employed. In the 

series of dinuclear dyads 4 [211], having the general formula [Ru(bpy)3] 2+ - 

(ph)n-(R2ph)-(ph)n-[Os(bpy)3] 2+ (ph = 1,4-phenylene; n = 1, 2, 3), excitation 

of the [Ru(bpy)3] 2+ unit is followed by energy transfer to the [Os(bpy)3] 2+ 

unit, as shown by the sensitized emission of the latter. For the compound with 

n = 3, with a total of seven phenylene spacers, the rate constant ken for energy 

transfer over the 4.2-nm metal-to-metal distance is 1.3 × 10 6 s –1 in acetonitrile 

solution at room temperature. This was probably the first example of


a systematic study on the distance dependence of energy transfer rates for 

Ru(II)–Os(II) dyads. A Dexter-type mechanism for the Ru(II)–Os(II) energy 

transfer was proposed, and an attenuation factor β of 0.32 ˚A –1 for photoinduced 

energy transfer was obtained from the ln ken vs metal–metal distance 

plot. 

In the [Ru(bpy)3] 2+ -(ph)n-[Os(bpy)3] 3+ compounds, obtained by chemical 

oxidation of the Os-based moiety, photoexcitation of the [Ru(bpy)3] 2+ 

unit causes the transfer of an electron to the Os-based one with a rate 

constant (kel) of3.4 × 10 7 s –1 for n = 3. Unless the electron added to the 

[Os(bpy)3] 3+ unit is rapidly removed, a back electron transfer reaction (rate 

constant 2.7 × 10 5 s –1 for n = 3) takes place from the [Os(bpy)3] 2+ unit to the 

[Ru(bpy)3] 3+ one [211]. The rate constants of all the transfer processes in 

the series of complexes decrease, as expected, with decreasing length of the 

oligophenylene spacer, whereas they were practically unaffected by temperature. 

Interestingly, a series of analogous dyads missing the central substituted 

phenylene (5) was successively prepared [212, 213] and the results of the 

two series have been compared: for the dyads containing bridges made of 

a total of three and five phenylene spacers, the rate constants of photoinduced 

energy transfer are higher in the nonsubstituted phenyl series. This was attributed 

to effects of inter-phenylene twist angle on the electronic coupling 

between donor and acceptor subunits [213]. 

Thepresenceofmeta substitution in oligophenylene bridges versus the 

all-para systems have been evidenced by the dyads 6 [210, 213]. In these 

species, the photoinduced energy transfer rate constants are lower than 

the rates for the respective compounds containing all-para phenylene units: 

for example, for spacers made of three and five phenylenes, respectively, 

ken is 1.32 × 10 9 and 6.67 × 10 7 s –1 for the meta series and 2.77 × 10 10 and 

4.90 × 10 8 s –1 for the para series. A related result has been reported for the 

tetranuclear Ir(III)/Ru(II) mixed-metal species 7 (although not linear, this 

species is briefly discussed here for convenience reasons) [214]. In 7,bothIrbased 

emission (λ = 572 nm, τ = 2.9 µs) and Ru-based emission (λ = 682 nm, 

τ = 82 ns) are present, showing that photoinduced energy transfer from the


Ir(III) chromophores to the Ru(II) units is inefficient at room temperature in 

fluid solution. This suggests that the Ir-to-Ru photoinduced energy transfer 

rate constant in this tetranuclear species is lower than the intrinsic rate constant 

for Ir decay (about 3.7 × 10 5 s –1 ), in spite of the nonnegligible driving 

force (about 0.3 eV, from emission data). At 77 K, energy transfer from Irbased 

to Ru-based chromophores is quantitative because of the much longer 

lifetime (205 µs) of the excited state of the Ir-based units. Indirectly, the 

room- and low-temperature results tend to suggest that Ir-to-Ru energy transfer 

in the tetranuclear mixed-metal species would occur with a rate constant 

of the order of 10 4 s –1 . The apparent discrepancy with the relatively fast energy 

transfer rate constant for the Ru–Os species with three phenylene unit 

bridges of the meta series discussed above (having a similar bridge to the 

Ir/Ru tetranuclear system here discussed) shows that the energy transfer


rates are significantly affected by the partner properties, as expected for both 

coulombic and (superexchange-assisted) Dexter-type mechanisms. 

In the series of Ru–Os dyads with tridentate ligands 8 [215–217], both 

the donor Ru-based and acceptor Os-based MLCT states taking part in the 

energy transfer process involve the bridging ligand, while in the analogous, 

cyclometallated species 9 [215, 218] the donor and acceptor MLCT states are 

based on the peripheral ligands. In the larger systems, with two phenylene 

spacers, ken for energy transfer from the Ru-based chromophore to the Osbased 

one is 5 × 10 10 s –1 for the non-cyclometallated species [215–217] and 

< 2 × 10 7 s –1 for the cyclometallated species [215, 218]. These different results 

are mainly attributed to the fact that the energy transfer pathway is longer for 

the cyclometallated system, although it is suggested that the different nature 

of the bridge could also play a role. 

Possible effects of excited-state localization on intramolecular energy 

transfer kinetics are also shown by the results obtained for the two isomeric 

dinuclear Ru(II) species 10 and 11 [219]. In both complexes, energy transfer 

from the non-cyclometallated Ru subunit to the cyclometallated Ru subunit 

takes place by a Dexter mechanism. Ultrafast spectroscopic measurements 

yield different energy transfer time constants for the two isomers, with that 

related to the bridge-cyclometallated complex (2.7 ps) being faster than that 

related to the terminal-cyclometallated one (8.0 ps). This difference is explained 

in terms of different electronic factors for Dexter energy transfer. The 

lowest MLCT excited state in the Ru cyclometallated unit of the dinuclear


complexes (that is, the acceptor state of the energy transfer process) has the 

promoted electron on the non-cyclometallating ligand, i.e., on the bridging 

ligand for 10 and on the terminal ligand for 11. The lowest MLCT excited state 

of the non-cyclometallated Ru(II) subunit (that is, the donor state of the energy 

transfer process) has the promoted electron on the bridging ligand in 

both cases. The energy transfer process, in a Dexter mechanism, is equivalent 

to simultaneous electron–hole transfer between the molecular components. 

The hole transfer process is the same (metal-to-metal) for both isomers. The 

electron transfer, on the other hand, is different for the two isomers, taking 

place between the two halves of the bridging ligand for 10 and from the 

bridging ligand of one unit to the terminal ligand of the other unit in the 

case of 11. The exchange electronic coupling is clearly expected to be higher 

in the former than in the latter case. Interestingly, in both cases the energy 

transfer processes also have a slower component of about 40 ps, which was 

tentatively assigned to roughly isoenergetic electron hopping between terminal 

and bridging ligands in the non-cyclometallated Ru chromophore, in 

agreement with reported rate constants for isoenergetic electron hopping in 

Ru(II) polypyridine complexes, as discussed in Sect. 4.2. This study clearly 

highlights the peculiar intricacies of intramolecular energy transfer in inorganic 

dyads involving MLCT excited states. 

Oligophenylene bridges have also been employed for studying photoinduced 

electron transfer from Ru(II) chromophores to Rh(III) subunits. In 

this type of multicomponent species, electron transfer from the Ru-based 

MLCT state to the Rh(III) component takes place, followed by charge recombination. 

The compounds 12–17 are an interesting series of homologous 

systems [220, 221]. The rate constants of the photoinduced electron transfer 

processes reported confirm the distance dependence of the process, as well as 

the effect of the twist angle between adjacent spacers [106, 213, 222, 224, 225] 

on the electronic coupling (and, therefore, on the electron transfer kinetics) 

across the spacer.

Photochemistry and Photophysics of Coordination Compounds: Ruthenium 147


Another spacer subunit which allows for rigidity and controlled directionality 

is the alkynyl group. Dinuclear species incorporating an unsaturated 

polyacetylenic backbone in the spacer (18) have been extensively investigated 

[175, 226]. Energy transfer (electron exchange mechanism) from the 

Ru(II) chromophore to the Os(II) one takes place with a rate constant of 

7.1 × 10 10 and 5.0 × 10 10 s –1 for n =1 and n = 2, respectively [175]. The β 

attenuation factor [227] for the polyacetylenic systems was calculated to 

be 0.17 ˚A –1 , indicating that the “electron conduction” for energy transfer 

through alkyne bridges is more efficient than that through oligophenylenic 

spacers [228]. 

Several other differently connected multicomponent species based on 

Ru(II) chromophores as donors and incorporating polyacetylenic bridges 

have been studied. Interested readers can find information in [175, 176, 226, 

229]. A special comment is warranted by the species 19–21 [230]. These compounds 

indicate the effect of the incorporation of additional units into the


polyacetylenic backbone. For the phenyl-containing bridge system, the triplet 

state of the bridge is higher in energy than both the Ru(II) and Os(II) MLCT 

levels. Energy transfer takes place directly from the Ru(II) chromophore 

to the Os(II) one via a superexchange-assisted Dexter mechanism. In the 

naphthyl-bridged Ru–Os species, the triplet state of the bridge is intermediate 

in energy between donor and acceptor levels: the energy transfer from the 

Ru(II) subunit to the Os(II) one occurs in a stepwise manner, first to the central 

bis(alkyl)naphthalene unit of the bridge and then to the Os(II) site. In 

the anthryl-bridged species, the bis(alkyl)anthracene triplet is lower in energy 

than both Ru- and Os-based MLCT states, and the bridge plays the role 

of an energy trap [230]. 

The last Ru–Os compound discussed above has some similarity with the 

Ru–anthracene–Os species 22 [231, 232]. In this species, missing the ethynyl 

groups, the anthracene triplet lies in between the Ru donor and Os acceptor 

energy transfer subunits, so the behavior of the bridge is similar to that of 

the naphthyl-bridged species mentioned above. However, in air-equilibrated 

solution the energy transfer rate constant significantly decreases with increasing 

irradiation time. This effect is due to the formation of singlet oxygen by 

bimolecular energy transfer from the Os(II) excited state. The singlet oxygen 

reacts with the anthracene unit to give a peroxide species which cannot behave 

as the intermediate “station” for energy transfer, so that the overall process 

is significantly slowed down. This complex was called a “self-poisoning” 

species [231, 232].


Another example showing an “active” role of the bridge in mediating intercomponent 

transfer processes involving Ru(II) species is evidenced by 

23 [233]. In this species, there are two close-lying MLCT states per metal 

center involving the bridging ligand (leaving aside the MLCT state involving 

the peripheral ligands), because of the particular nature of the bridge 

(see Sect. 5.9). The higher energy of such MLCT states (MLCT1) involves 

a bridging ligand orbital mainly centered in the bpy-like coordinating site 

(LUMO+1), and the lower energy one (MLCT0) islocalizedonthecentral 

phenazine-like site (LUMO). Light excitation of the Ru-based chromophore 

populates the singlet MLCT1 state, which rapidly decays to its triplet counterpart. 

Direct light excitation into the singlet MLCT0 level (and successive 

population of its triplet) is inefficient because of the negligible oscillator 

strength of the transition. For Ru-to-Os energy transfer, two possible pathways 

are possible: (1) Ru-to-Os energy transfer at the 3 MLCT1 level (EnT), 

followed by 3 MLCT1-to- 3 MLCT0 relaxation within the Os(II) chromophore 

(a sort of intraligand electron transfer, ILET, within the Os(II) subunit); and 

(2) 3 MLCT1-to- 3 MLCT0 relaxation within the Ru(II) chromophore (ILET in 

Ru(II) subunit), followed by Ru-to-Os energy transfer at the 3 MLCT0 level. 

The situation is schematized in Fig. 13 [210, 233]. 

Interestingly, ultrafast spectroscopy shows that pathway 1 is followed in 

dichloromethane and pathway 2 prevails in the more polar acetonitrile solvent. 

Oligophenyl bridges are reported to play “active” roles in the dinuclear 

Ir(III)–Ru(II) species 24–27 [234]. In this series of complexes, the Ru-based 

component is the energy transfer acceptor subunit. Indeed, Ru-based emission 

takes place in all the species at about 625 nm (lifetime about 200 ns) in 

aerated acetonitrile at room temperature and at about 590 nm (lifetime about 

6 µs) in butyronitrile at 77 K, whereas the high-energy Ir-based chromophore 

has a very short excited-state lifetime, determined by time-resolved emission 

and subpicosecond transient absorption spectroscopy, slightly dependent on 

the bridge. The energy transfer rate constant is very weakly slowed down by 

increasing the bridge length, passing from 8.3 × 10 11 s –1 for the species with 

two phenyls as spacer to 3.3 × 10 11 s –1 for the species with five interposed 

phenyls. The apparent attenuation parameter β for energy transfer rate con-


Fig. 13 Energy transfer pathways in a dinuclear Ru(II)–Os(II) species containing a πextended 

bridge 

stant would be 0.07 ˚A –1 . However, increasing the bridge length changes the 

excited-state energy level of the bridge itself, and it is proposed that the MLCT 

excited state of the Ir center assumes an increasing LC character involving 

the bridge orbitals as the number of phenyls increases. As a consequence, 

metal–metal separation does not reflect the effective donor–acceptor separation 

for the energy transfer process; with the donor excited state largely 

involving the oligophenyl spacer, the energy transfer takes place by an in-


coherent hopping mechanism, and cannot be considered a through-bond, 

superexchange-assisted energy transfer. Interestingly, the use of the same 

spacers has already been mentioned for the couple Ru/Os (see above), where 

a normal through-bond superexchange mechanism was proposed to be operative: 

the reason for the difference between Ru/Os and Ir/Ru series is the 

energy level of the donor excited state. In Ir(III) complexes, such a state is 

much higher in energy and can interact significantly with oligophenyl-based 

excited states. 

An interesting series of papers dealing with photoinduced electron transfer 

in a series of dinuclear Ru(II)–Co(III) species allowed the accumulation 

of further information on the role of the bridging ligand [207, 235, 236]. Typical 

studied complexes were [(terpy)Ru(terpy-terpy)Co(terpy)] 5+ , [(terpy)Ru 

(terpy-ph-terpy)Co(terpy)] 5+ , and [(bpy)2Ru(tpphz)Co(bpy)2] 5+ (terpy-terpy 

=6,6 ′ -bis(2-pyridyl)-2,2 ′ :4 ′ ,4 ′′ :2 ′′ ,2 ′′′ -quarterpyridine, that is, the bridge with 

n =0in5; terpy-ph-terpy is the bridge with n =1in5; tpphzisthebridgein 

3). In these studies, the quantum yields of the thermally equilibrated product 

of the photoinduced electron transfer from the Ru-based MLCT state to 

the Co(III) subunit were carefully measured [207]. Interestingly, ∗ Ru(II)-to- 

Co(III) electron transfer takes place in less than 10 ps in all three abovementioned 

species; however, the quantum yields of the thermally equilibrated 

electron transfer product were quite different in the series, with the tpphzbridged 

dinuclear species exhibiting a quantum yield of about 0.8 and the 

other two compounds featuring significantly smaller values (0.53 and 0.41 for 

the terpy-terpy and the terpy-ph-terpy species, respectively) in butyronitrile 

at 298 K [207]. The authors proposed that the relatively low yield of photoinduced 

electron transfer in the two terpy-based complexes is due to formation 

of the d–d (MC) excited state of the [Co(terpy)2] 3+ moiety during solvent


relaxation within the electron transfer excited state. In fact, fast tunneling 

transition of the nonrelaxed electron transfer product to the lowest d–d excited 

states of the [Co(terpy)2] 3+ moiety can take place via hole transfer from 

[Ru(terpy)2] 3+ to the [Co(terpy)2] 2+ , generating a dπ 6 dσ ∗ configuration. 

Strong through-ligand electronic coupling of dπ(Ru)–dπ(Co), as estimated 

from the strong intensity of the intervalence band of [(terpy)Ru(terpyterpy)Ru(terpy)] 

5+ , can effectively mediate the fast hole transfer process. 

For the tpphz-bridged system, through-ligand electronic coupling between 

dπ(Ru III ) and dπ(Co II ) orbitals is much smaller, as suggested by the absence 

of any sizeable intervalence band in [(bpy)2Ru(tpphz)Ru(bpy)2] 5+ [207]. It 

turns out that the weak tpphz superexchange interaction between dπ(Ru III ) 

and dπ(Co II ) orbitals may be unable to open the channel of hole transfer 

during the relaxation of the electron transfer product, leading to a higher 

quantum yield of the charge-separated, thermally equilibrated product. However, 

the charge recombination rate constant was fast in all cases: in butyronitrile 

at room temperature it was 2.1 × 10 7 s –1 for the tpphz species and 

biphasic and faster for the other two compounds (81 × 10 9 and 5 × 10 9 s –1 

for the terpy-ph-terpy containing species and 52 × 10 10 and 3 × 10 10 s –1 for 

the terpy-terpy species). Even in the charge recombination (back electron 

transfer) process, the different coupling offered by the bridging ligands could 

explain the results. 

5.2 

Photoactive Multinuclear Ruthenium Species 

Exhibiting Particular Topologies 

5.2.1 

Racks and Grids 

Rack-type metal complexes are linearly arranged species [237], but differ 

from the species discussed in the former section since they are made of 

several repeating, roughly identical, metal-based subunits orthogonally appended 

to a roughly linear and rigid polytopic molecular strand. The metal 

centers are never aligned along the main axis of the bridging ligand. 

The first rack-type Ru(II) polypyridine complex investigated from a photochemical 

viewpoint is 28 [238]. In this species, the anthryl group has only 

the function of absorbing additional light energy; in fact, its triplet state is 

higher in energy than the MLCT triplet state(s) of the Ru(II) subunits (here, 

the lowest-lying MLCT states involve the bridging ligand, which is very easily 

reduced). For 28, near-IR emission occurs (λem = 845 nm) with a relatively 

long lifetime (60 ns). Such an emission is totally quenched in the somewhat 

related tetranuclear Ru – Fe grid 29 [239], where energy transfer from the Rubased 

MLCT state to the Fe-based MC levels is likely to occur. Kinetic data for 

the quenching processes were not reported.


Ru(II) racks based on different molecular strands (30, 31) havealsobeen 

recently studied [240] (S Campagna, unpublished results). The pyrimidinecontaining 

Ru complex exhibits a 3 MLCT emission with maximum at 758 nm


in acetonitrile at room temperature (τ = 30 ns), which moves to 740 nm 

in nitrile glass at 77 K(τ = 335 ns) [240]. The pyrazine-containing species 

exhibits very similar emission properties (room temperature, acetonitrile: 

λmax = 765 nm; τ = 60 ns; 77 K, nitrile glass: 750 nm; τ = 400 ns) (S Campagna, 

unpublished results). For these species, the lowest (emitting) MLCT 

state(s) involve(s) the bridging ligands, as for the former rack-type complex 

discussed above. 

5.2.2 

Dendrimers 

Luminescent Ru(II) dendrimers have been deeply investigated and the field 

has been reviewed recently [2, 241–245]. We will only mention some examples. 

5.2.2.1 

Dendrimers Containing Only One Metal Center Unit 

The ruthenium compound 32 is a classical example of a dendrimer containing 

a luminescent ruthenium complex core surrounded by organic wedges. 

In this dendrimer, the 2,2 ′ -bipyridine (bpy) ligands of the {Ru(bpy)3} 2+ -type 

core carry branches containing 1,3-dimethoxybenzene- and 2-naphthyl-type 

chromophoric units [246]. All three types of chromophoric groups present in 

the dendrimer, namely, {Ru(bpy)3} 2+ , dimethoxybenzene, and naphthalene, 

are potentially luminescent species. In 32, however, the fluorescence of the 

dimethoxybenzene- and naphthyl-type units is almost completely quenched 

in acetonitrile solution, with concomitant sensitization of the {Ru(bpy)3} 2+ 

core luminescence. These results show that very efficient energy transfer processes 

take place, converting the very short-lived (nanosecond timescale) UV 

fluorescence of the aromatic units of the wedges to the relatively long-lived 

(microsecond timescale) orange luminescence of the metal-based dendritic 

core. This dendrimer is therefore an excellent example of a light-harvesting 

antenna system as well as of a species capable of acting as a frequency converter. 

It should also be noted that in aerated solution the phosphorescence 

intensity of the dendritic core is more than twice as intense as that of the


[Ru(bpy)3] 2+ parent compound, because the dendrimer branches protect the 

Ru–bpy-based core from dioxygen quenching [247]. 

More recently, dendrimers based on {Ru(phen)3} 2+ or {Ru(bpy)2(phen)} 2+ 

(phen = 1,10-phenanthroline) cores with appended carbazole chromophoric 

units have been investigated (see, e.g., 33) [248]. In these compounds, energy 

transfer from the peripheral carbazole units to the metal-based core occurs 

with ca. 100% efficiency, with sensitization of the 3 MLCT luminescence at ca. 

630 nm. 

The {Ru(bpy)3} 2+ core was used to build a first-generation dendrimer 

containing 12 coumarin 450 units in the periphery (34). Inacetonitrilesolution, 

excitation of the coumarin 450 chromophores resulted in luminescence 

emission at 625 nm, typical of the 3 MLCT excited state of the {Ru(bpy)3} 2+ 

core, with only a minor residual fluorescence of the initially excited chromophores. 

An antenna effect is thus operative, with an estimated intramolecular 

energy transfer efficiency close to unity [249]. Using the same dendrimer 

and the [Ru(dmb)3] 2+ (dmb = 4,4 ′ -methyl-2,2 ′ -bipyridine) complex, which 

served as a model for the “naked” dendrimer core, it was possible to investigate 

the shielding effect of the dendritic structure on the intermolecular energy 

and electron transfer processes [250]. Bimolecular quenching constants 

were measured in acetonitrile solution for dioxygen, 9-methylanthracene 

(MA), phenothiazine (PTZ), and methyl viologen (MV 2+ ), using three dif-


ferent techniques: intensity and lifetime quenching, and transient absorption 

spectroscopy. These quenchers were chosen to explore different quenching 

mechanisms. Whereas in the case of dioxygen the quenching mechanism is 

still an object of controversy, and should involve both energy and electron 

transfer, it is known that the [Ru(bpy)3] 2+ excited state is quenched by MA, 

PTZ, and MV 2+ via energy transfer, reductive and oxidative electron transfer 

mechanisms, respectively. The results obtained with the quenchers MA, PTZ,


and MV 2+ indicated that the first-generation dendritic structure of this class 

of dendrimers is unable to shield the Ru-based core from bimolecular energy 

and electron transfer reactions. On the contrary, quenching by dioxygen was 

attenuated in going from [Ru(dmb)3] 2+ to larger dendrimers, suggesting an 

effect of the dendritic structure. As this effect is not observed with MA, PTZ, 

and MV 2+ , which are certainly much bulkier species, the shielding effect observedinthecaseofdioxygenwasattributedtolowerO2 

solubility within the 

dendritic structure [250]. 

5.2.2.2 

Multimetallic Dendrimers 

For the class of dendrimers where metal complexes are the branching centers, 

a key role is played by polytopic chelating ligands (bridging ligands), which 

can control the shape of the polynuclear array and the electronic interaction 

between metal chromophores. 

The largest family of these dendrimers is based on the 2,3-bis(2 ′ - 

pyridyl)pyrazine (dpp) bridging ligand. Within such a series, the largest 

species contain 22 metal centers [251–253]. The decanuclear compound 35 is


a second-generation dendrimer of this family (in the sketch of the compound, 

the peripheral ligands, schematized as NˆN, stand for 2,2 ′ -bipyridine) [254]. 

For the dendrimers of this series, the modular synthetic strategy [252] allows 

a high degree of synthetic control in terms of the nature and position of metal 

centers, bridging ligands, and terminal ligands. Since the excited-state level 

of each metal center in the dendrimer depends on the nature of the metal, of 

its coordination sphere (which in its turn depends on the metal position, inner 

or outer, within the dendritic array) and on the ligands, each metal-based 

subunit is characterized by specific excited-state properties, which because of 

the symmetry of the dendritic structure are usually identical for each metalbased 

subunit belonging to the same dendritic layer. Therefore, the synthetic 

control translates into control of specific properties, such as the direction 

of electronic energy flow within the dendritic array (antenna effect) [245]. 

For example, in 35 the lowest excited-state level involves the peripheral subunit(s), 

and the emission of the species (acetonitrile, room temperature: 

λmax = 780 nm; τ = 60 ns; Φ = 3 × 10 –3 ) is assigned to a (bpy)2Ru→ µ-dpp 

MLCT triplet state [254]. Excitation spectroscopy indicates that quantitative 

energy transfer takes place from inner subunits to the peripheral ones [254]. 

Because of the energy gradient between the dendritic layers, the energy transfer 

is ultrafast (see later), occurring in the femtosecond timescale. On the 

basis of the above discussion, it is not surprising that all the homometallic 

dendrimers of the same family, independent of the number of Ru subunits 

(i.e, tetranuclear [255, 256], decanuclear [253, 254], and docosanuclear [251– 

253], as well as hexanuclear [257, 258], heptanuclear [259], and tridecanuclear 

species [260], which have particular connections/geometries), exhibit 

practically identical photophysical properties, since the lowest-energy subunit(s) 

is in all cases the identical peripheral (bpy)2Ru(µ-dpp) MLCT state(s). 

A nonanuclear species has also been prepared [261], but its photophysical 

properties have not yet been reported. 

On increasing nuclearity, a unidirectional gradient (center-to-periphery 

or vice versa) for energy transfer is hardly obtained with only two types of 

metals (commonly, Ru(II) and Os(II)) and ligands (bpy and 2,3-dpp). In fact, 

by using a divergent synthetic approach starting from a metal-based core 

it becomes unavoidable that metal-based building blocks with high-energy 

excited states (high-energy subunits) are interposed between donor and acceptor 

subunits of the energy transfer processes [245]. For example, while in 

the tetranuclear [Os{(µ-dpp)Ru(bpy)2}3] 8+ (OsRu3) species, in which a central 

{Os(µ-dpp)3} 2+ subunit is surrounded by three {Ru(bpy)2} 2+ subunits, 

only the osmium-based core emission is obtained (acetonitrile, room temperature: 

λmax = 860 nm; τ = 18 ns; Φ = 1 × 10 –3 ) [262], indicating quantitative 

energy transfer from the peripheral Ru-based chromophore to the central 

Os-based site; for the larger systems the peripheral Ru-based emission is not 

quenched [245, 253]. This result highlights that although downhill or even 

isoergonic energy transfer between nearby building blocks in the dendrimers


based on the 2,3-dpp bridging ligand is fast and efficient, direct downhill energy 

transfer between partners separated by high-energy subunits is much 

slower and can be highly inefficient. This problem has been overcome (1) by 

using a third type of metal center, namely a Pt(II) one, to prepare decanuclear 

species (second-generation dendrimers) having different metal centers 

in each “generation” layer (schematically, OsRu3Pt6 species) [263] or, more 

recently, (2) in a heptanuclear dendron where the barrier made of highenergy 

subunits is bypassed via the occurrence of consecutive electron transfer 

steps [264]. Quite interestingly, this latter study suggests that long-range 

photoinduced electron transfer processes do not appear to be dramatically 

slowed down by interposed high-energy subunits in this class of dendrimers. 

The efficiency of energy migration in 2,3-dpp-based dendrimers has attracted 

a large interest for the potential use of these species as synthetic 

antennae in artificial photosynthesis processes, and this has stimulated detailed 

kinetic investigations by means of ultrafast techniques. Studies on dinuclear 

model compounds have shown that esoergonic and isoergonic energy 

transfer between nearby units occurs within 200 fs, probably from nonthermalized 

excited states [168]. A direct consequence of such results is that 

energy transfer involving singlet states can compete with intersystem crossing. 

This conclusion is supported by the fact that the energy transfer from 

the peripheral Ru(II) subunits to the central Os(II) core in a tetranuclear 

OsRu3 dendrimer takes place both by a singlet–singlet pathway, with a lifetime 

of less than 60 fs, and by triplet–triplet energy transfer, with a lifetime 

of 600 fs [169, 170]. The finding of singlet–singlet energy transfer is a particularly 

important result, since it indicates that the idea that any excited-state 

process involving metal polypyridine complexes had to be ascribed only to 

triplet states should be taken with caution when a significant electronic coupling 

between donor and acceptor is present. In some way, this finding also 

parallels the results obtained for photoinduced injection of electrons into 

semiconductors [167, 265–271]. 

An extension of this kind of antenna is a first-generation heterometallic 

dendrimer with appended organic chromophores like pyrenyl units 

(36) [272]. In this species, consisting of an Os(II)-based core surrounded by 

three Ru(II)-based moieties and six pyrenyl units in the periphery, 100% efficient 

energy transfer to the Os(II) core is observed, regardless of the light 

absorbing unit. A detailed investigation of the excited-state dynamics occurring 

in this multicomponent species on exciting in the UV region (267 nm) 

has also been performed [273]. Transient absorption spectra (in the range 

420–700 nm) for the various intermediates have been reported by the acquisition 

of evolution-associated difference spectra. 

Energy transfer processes from the nonrelaxed and relaxed S1 state of the 

peripheral pyrenyl chromophores to the lowest-lying Os-based MLCT triplet 

excited state occur with lifetimes of about 6 and 45 ps, respectively [273]. Subpicosecond 

energy transfer from the excited Ru manifold to the Os-based


chromophore and interconversion between the initially prepared S3 state 

and the low-lying S1 level within the pyrenyl subunits have also been evidenced. 

The rate constant of the energy transfer from the pyrenyl groups 

to the Ru/Os excited state manifold is in good agreement with the Förster 

mechanism when the relaxed S1 pyrene state is taken into account. Energy 

transfer from the nonrelaxed state most likely involves folded conformations 

in which the pyrenyl subunits are strongly interacting with inner subunits of 

the tetranuclear core. Such interactions were also suggested by the groundstate 

absorption spectrum of the compound [272]. 

Because octahedral metal complexes can exist in two chiral forms, Λ and 

∆, it could be expected that the photophysical properties of dendrimers 

containing metal complexes as branching centers could be different for the 

various isomers (the situation can be even more complicated in the case of 

geometrical isomers). However, the investigation of optically pure isomers of 

dinuclear and dendritic-shaped tetranuclear species (37 is the general structural 

formula of the tetranuclear systems: optical geometry is not evidenced) 

has shown that stereochemical isomerism does not cause any sizeable difference, 

at least for the studied compounds [194]. 

In 37 the emissive state, which involves the peripheral Ru(II) centers, does 

not have a sizeable absorption counterpart, since it is a special type of chargeseparated 

state, with the formal “hole” localized on a peripheral Ru(II) center 

and the “electron” localized on an orbital mainly centered on the pyrazine 

moiety of the bridging ligand: the absorption related to such a state has negligible 

oscillator strength, due to the poor overlap of the orbitals involved


(for similar systems, see Sect. 4.4) [194]. A recently investigated, closely related 

tetranuclear dendritic-shaped compound, where the bridging ligand 

is the asymmetric PHEHAT ligand (PHEHAT = 1,10-phenanthrolino[5,6b]-1,4,5,8,9,12-hexaazatriphenylene), 

displays similar photophysical properties 

[274]. 

Mixed-metal Os(II)–Ru(II) dendrimers whose bridging ligands contain 

ether linkages have also been reported: compounds 38 and 39 are two examples 

[275, 276]. The tetranuclear species in 38 exhibits quantitative energy 

transfer from the Ru(II) chromophores to the Os(II) core [275], whereas 

for 39 the efficiency of the energy transfer process is highly temperature 

dependent; in fact the Ru-to-Os energy transfer is highly efficient at low temperature, 

whereas at room temperature it does not compete with the intrinsic 

decay of the Ru(II)-based subunits [276]. 

In most of the mixed-metal dendrimers featuring energy transfer and 

containing Ru(II) subunits, the Ru(II) centers play the role of the energy 

donor components (and usually Os(II) centers are the acceptors). Examples 

of photoactive dendrimers in which Ru(II) subunits behave as acceptor components 

are the tetranuclear compound 40 [277] and its higher-generation, 

Y-shaped octanuclear species, in which four additional Ir(III) chromophores 

have been connected at the two peripheral Ir(III) subunits of the compound 

40 (JAG Williams, personal communication). Here, efficient energy transfer 

takes place for the Ir(III) cyclometallated subunits toward the single Ru(II) 

polypyridine chromophore, which acts as the energy trap of the assemblies. 

The Ru(II) subunit then emits with relatively high quantum yield (0.12 in 

degassed acetonitrile at room temperature) and long lifetime (1.6 µs) [277].


The fluoro substituents which are present on the peripheral Ir chromophores 

have the role of increasing the excited-state energy levels of the outer Ir chromophores 

with regard to the inner ones, so generating the correct energy 

gradient. 

Multi-ruthenium dendrimers of the 2,3-dpp family discussed above have 

also been functionalized with organic electron donor subunits, to yield integrated 

light-harvesting antennae/electron donor systems for charge separation. 

In particular, a triruthenium dendron has been coupled with a tetrathiafulvalene 

(TTF) derivative [278] and the tetranuclear OsRu3 has been functionalized 

at the peripheral bpy ligands with up to six phenothiazine subunits 

[279]. In both cases, the light-harvesting antenna emission was totally 

quenched by reductive electron transfer from the electron donor subunit. 

Interestingly, the electron donor quenchers were not directly linked to the energy 

trap of the antenna; however, the electron transfer process was fast and


efficient, which confirms that in this type of artificial antenna metallodendrimer, 

long-range electron transfer can be quite effective, as in the case of 

the heptanuclear complex mentioned above [264], and could suggest interesting 

options to build up integrated donor–antenna–acceptor systems. This 

discussion leads us directly to the next section. 

5.3 

Donor–Chromophore–Acceptor Triads 

Triad systems (Fig. 14) are key components of the early events in artificial 

photosynthesis: the light energy collected by the chromophore (P) is transformed 

into chemical (redox) energy by a sequence of electron transfer steps 

involving electron donor (D) and electron acceptor (A) units, ultimately leading 

to charge separation [208, 209, 228]. Charge separation is probably the 

most important photoinduced process on Earth, so it is not surprising that 

many triads based on Ru(II) complexes have been prepared and studied in 

the last 20 years [228, 280]. It should be noted that there are literally dozens 

of dyads based on Ru polypyridine complexes [228, 281]. Only some examples 

of triads (that is, species where Ru(II) chromophores are simultaneously 

coupled to electron donor and acceptor units) are discussed here.


Fig. 14 Schematization of a triad for photoinduced charge separation. P, chromophore; D, 

donor; A, acceptor; cs, primary charge separation; cr, primary charge recombination; cs ′ , 

secondary charge separation; cr ′ , final charge recombination 

Structurally speaking, Ru complexes with tridentate ligands like terpy are 

ideal systems for molecular triads: indeed, substitution at the 4 position of 

the central pyridine ring of terpy-like ligands allows a linear arrangement 

of subunits, with control of geometry and distance. Some of the first ruthenium 

triads based on such an arrangement are shown in Fig. 15 [282]. 

Whereas fully developed charge separation, with formation of the D + -P-A – 

Fig. 15 Structural formulae of Ru(II) terpyridine triads containing acceptor (A) and 

donor (D) components


charge-separated state, did not take place in the triad with D = PTZ (PTZ = 

phenothiazine subunit), the formation of such a charge-separated species was 

inferred for the species with D = DPPA (DPPA = diphenylamino moiety) at 

150 K, although it could not be evidenced spectroscopically because it did not 

accumulate as a consequence of a fast recombination rate [282]. 

In spite of less control of distance and orientation between donor and acceptor, 

better results have been reported for a series of systems exemplified 

by 42 [283–285]. The components of this series of triads are a tris-bipyridine 

Ru(II) chromophore covalently linked to one or two phenothiazine electron 

donors and to quaternized bipyridinium electron acceptors. The saturated 

alkyl chains bridging the molecular components are electrically insulating 

and flexible. This latter point is apparently a drawback since it does not allow 

for control of geometry. Moreover, even the octahedral arrangement of 

the bpy subunits adds some difficulties in defining the real structure: for 

example, geometrical isomers can also exist, since each bpy of 42 is nonsymmetric. 

The compound 42 exhibited formation of the D + –P–A – chargeseparated 

state in dichloromethane at room temperature with an initially 

reported efficiency of about 26%. Such a value was later corrected to about 

86% by using a slightly different solvent (1,2-dichloroethane) [283, 286, 287]. 

Once formed, the charge-separated state decayed with a relatively fast rate 

(kCR = 6.3 × 10 6 s –1 ), corresponding to a lifetime of about 160 ns. On the basis 

of redox data, the charge-separated state stored about 1.3 eV. 

Similar complexes were prepared that differed from one another by the 

length of the alkyl chains connecting the subunits and/or by changing


the methylene chains connecting the quaternary nitrogens (and, as a consequence, 

the reduction potential of the electron acceptor) [283–287]. In 

a homogeneous series of experiments performed on such species, however, 

the efficiency of charge separation does not change appreciably, remaining 

larger than 0.80, although the driving forces and the rate constants of the various 

electron transfer steps, as obtained by independent studies performed on 

isolated dyads of the type D–P or P–A, were different. 

In D–P ∗ –A systems, the fully developed charge-separated state can be obtained, 

in principle, by two different routes (excluding direct electron transfer 

from D to A): (1) a route initiated by oxidative quenching, that is, the series 

of events described by the sequence D–P ∗ –A, D–P + –A – ,D + –P–A – ;and 

(2) a route initiated by reductive quenching, described by the sequence D– 

P ∗ –A, D + –P – –A, D + –P–A – . Both routes can also take place simultaneously. 

The comparison between the photophysical properties of the various triads 

and the corresponding isolated dyads of this family of compounds indicated 

that the emission decay rates of any D–P–A triad never differed by more 

than a factor of two from those of the P–A dyads, although the absolute decay 

rate values changed by over a factor of 10 3 (over the whole collection 

of compounds). This prompted the authors to attribute the initial quenching 

event in all the D–P–A triads of this family to oxidative electron transfer, with 

formation of the D–P + –A – intermediate, with the route initiated by reductive 

quenching playing a negligible role [288]. However, in all the P–A dyad 

systems, it was always impossible to detect the A – radical anion [284], indicating 

that back electron transfer in the P–A dyads was faster than the 

forward, oxidative electron transfer. This posed some problems in justifying 

the efficiency of formation of the fully developed charge-separated state, 

where apparently reduction of P + from D in D–P + –A – species efficiently 

competes with back electron transfer in the intermediate. In fact, this looks 

somewhat puzzling because the reductive electron transfer in D–P ∗ dyads is 

reported to be of the order of 10 6 s –1 [289], while oxidative electron transfer 

in P ∗ –A dyads ranges from 10 10 to 10 7 s –1 [284, 285, 290–293] and, based 

on the circumstances mentioned above, back electron transfer in P + –A – (and 

for extension in D–P + –A – ), opposing the formation of the fully developed 

charge-separated state, could be even faster. To justify the experimental data, 

electron transfer from D to P + in D–P + –A – should be about 1 × 10 10 s –1 or 

faster. Therefore, the exceptional properties of these compounds as far as the 

efficiency of charge separation is concerned remained largely unexplained. 

A recent paper has shed light on the photophysical behavior of these 

triads [288]. A series of new experiments, including transient absorption 

measurements, emission decay, and a careful examination of the ground-state 

absorption spectra of the triads and of various separated dyad components, 

suggested that in the D–P–A triads of this family an association between the 

tethered phenothiazine electron donor subunit and the Ru(II) chromophore 

takes place, in a folded conformation. The association is already present in the


ground state, before excitation and any electron transfer process. The triad 

would then be better defined as a D/P–A system, and electron transfer between 

D and P + subunits in the intermediate state D/P + –A – can be largely 

faster than that reported for the D–P ∗ dyad, and even faster than charge 

recombination between C + and A – in the assembly, justifying the high efficiency 

of formation of the fully developed charge separation. Ground-state 

association of tethered aromatic ligands (with flexible linkages) with Ru(II) 

chromophores was also known in a tetranuclear dendrimer [272, 273], further 

supporting these conclusions. 

The triad 43 involves components similar to those used in the former 

discussed systems, but the connecting scheme is different [294]. Here, the 

chromophore and the electron donor and acceptor subunits are all linked to 

a lysine moiety. The charge-separated state of this triad stores 1.17 eV and 

lives for 108 ns (rate constant of charge recombination kCR = 9.26 × 10 6 s –1 ) 

in acetonitrile, as observed by transient spectroscopy. The efficiency of formation 

of the charge-separated state is about 34%. On the basis of detailed 

photophysical studies of isolated dyads [294], the authors believe that the only 

route leading to fully developed charge separation is reductive quenching of 

D–P ∗ –A leading to D + –P – –A, followed by a second electron transfer leading 

to D + –P–A – , whereas the route passing through the population of the D–P + – 

A – intermediate does not lead to the full charge-separated species, but leads 

directly to the ground state. Therefore, the occurrence of oxidative electron 

transfer of P* would be the main reason for the efficiency loss in the whole 

process. Better results were obtained by modifying the acceptor site of the 

triad, as shown in 44 [295]. In this species, the energy stored by the chargeseparated 

species is higher (1.54 eV), since the anthraquinone has a less negative 

reduction potential than the methyl viologen subunit, and the lifetime 

of the charge-separated state is longer (174 ns, kCR = 5.7 × 10 6 s –1 ), but the


quantum yield is slightly smaller (26%). The longer lifetime of the chargeseparated 

species of the anthraquinone-containing triad was attributed to the 

charge-recombination process being further in the inverted region (a similar 

reorganization energy λ,about0.80 eV, is assumed in both cases). Even in 

this case, the main losses in the efficiency of formation of the fully developed 

charge separation were attributed to the inefficiency of the route in which the 

initial electron transfer is oxidative to produce the final D + –C–A – state. 

Within the field of molecular triads, a peculiar example is 45 [296]. In this 

species, the Ru(II) subunit playing the role of the chromophore is mechanically 

linked with a cyclobis(paraquat-p-phenylene) unit (BV 4+ ,acceptor)and 

covalently linked with a protoheme unit (the donor), located in a myoglobin 

pocket (not shown in figure). The overall system is therefore a reconstituted 

protein bearing a molecular triad. Excitation of the ruthenium chromophore 

is followed by a series of photoinduced electron transfer steps (the first 

one being electron transfer to the electron acceptor), leading to the chargeseparated 

species containing the porphyrin radical cation and the paraquatbased 

radical anion, Mb(Fe III OH2) + -Ru 2+ -BV 3+ . This species successively undergoes 

a series of deprotonation processes ultimately leading to another 

charge-separated species, identified as Mb(Fe IV =O)-Ru 2+ -BV 3+ ,withanap-


parent first-order rate constant of 6.6 × 10 3 s –1 . This species is produced with 

alowquantumyield(0.5%), stores about 1.3 eV, and recombines to the 

ground state with a lifetime exceeding 2 ms. In spite of the low efficiency 

of the charge-separation process, there are several interesting points: (1) in 

the absence of myoglobin, charge separation is not obtained at all; (2) the 

complete process is pH dependent; (3) the final charge-separated state lifetime 

is comparable with that of natural photosynthetic reaction centers; and 

(4) back electron transfer is regulated by protonation/deprotonation of distal 

histidine moieties, which appears to be needed to reduce the Mb(Fe IV =O) 

subunit. The low efficiency of the overall process is mainly attributed to 

charge recombination within the Mb(Fe III OH2) + -Ru 2+ -BV 3+ state, which efficiently 

competes with the deprotonation processes. This study highlights the 

potential of mixed synthetic–natural systems for obtaining long-lived charge 

separation. 

5.4 

Polyads Based on Oligoproline Assemblies 

The interesting results obtained by organizing D–P–A triads on the structure 

of the amino acid lysine (Sect. 5.3) prompted the preparation of D–P–A systems 

assembled on oligoproline scaffolds, by means of solid-state peptide 

synthesis [294, 297–299]. One such system is 46. In this species, a phenothiazine 

(PTZ) group acts as the electron donor and an anthraquinone (Anq) 

subunit plays the role of the electron acceptor. Oligoprolines were selected, 

since it is known that oligoproline chains of nine or more proline units 

fold into stable helices even with large functional groups on the proline 

units. The terminal segments allow the helix to begin and end with capped 

Pro3 turns which prevent unwinding of the helix. For 46, a fully developed 

charge-separated state is gained in acetonitrile solution with good efficiency 

(53%). The charge-separated state stores 1.65 eV relative to the ground 

state and returns to the ground state with a rate constant of 5.7 × 10 6 s –1 

(τ = 175 ns) [299, 300]. Quenching of the Ru-based excited state is domi-


nated by reductive electron transfer involving the PTZ electron donor, in 

a process which is largely solvent dependent, as is the driving force of the 

process. Then, there is a fast electron transfer from the reduced metal chromophore 

to the Anq subunit, which yields the charge-separated state. There 

is a strong solvent-dependent competition between such a second electron 

transfer, which allows for fully developed charge separation and back electron 

transfer in the D + –P – –A intermediate. The consequence of this solvent 

dependence is that going from 1,2-dichloroethane to dimethylacetamide, the 

efficiency of charge separation changes from 33 to 96%. Also, the charge recombination 

is solvent dependent, and the electronic coupling between PTZ + 

and Anq – was calulated to be about 0.13 cm –1 [300]. 

A more elaborated polyad based on the formerly described systems is 

the D–P–P–A tetrad 47 [301]. This is the evolution of a system quite related 

to 46, where substituents on the terminal bpy ligands of the metal 

chromophore are used to favor the thermodynamics of the (reductive) first 

electron transfer step. This modification led to an efficiency of charge separation 

of 90% in acetonitrile for the corresponding triad. In the tetrad, 13 

proline spacers are present between PTZ and Anq. The efficiency of formation 

of the charge-separated state in the tetrad is 60% and its lifetime is 2 µs 

(kCR = 5.0 × 10 5 s –1 ). Excitation can occur in both the Ru chromophores, but 

apparently the result is not identical. Excitation of the Ru(II) complex adjacent 

to the PTZ electron donor gives the D + –P – –P–A system. To produce the 

fully developed D + –P–P–A + species, it is proposed that a stepwise mechanism 

occurs, with the species D + –P–P – –A as an intermediate. Efficient, isoergonic 

electron transfer between the two chromophores is therefore foreseen. Excitation 

of the Ru(II) complex adjacent to the electron acceptor Anq subunit 

would be unproductive in a direct sense, since oxidative electron transfer by 

Anq is unfavorable thermodynamically and direct quenching from the PTZ 

unit is unlikely because of the large distance. However, even excitation of this 

Ru(II) chromophore can become productive, provided that isoergonic energy 

transfer to the Ru(II) chromophore adjacent to the PTZ unit takes place. Since 

the quantum efficiency of formation of the charge-separated state is 60%, 

and considering that excitation of the two identical Ru(II) chromophores


is equivalent (that is, 50% each), clearly interchromophoric energy transfer 

takes place, although not with a high efficiency. 

As far as the mechanism of the intrastrand energy and electron transfer 

in such oligoprolines is concerned, through-bond and through-space pathways 

can be considered. An important step to elucidate the mechanism was 

a systematic investigation of the reductive electron transfer in oligoprolines 

containing only PTZ and one Ru(II) chromophore [302]. From such a study, 

when the number of interposed prolines varies from two to five, it was 

demonstrated that a through-space mechanism is operative, with an apparent 

β value of 0.41 ˚A –1 . Superexchange coupling with the solvent and oligoproline 

scaffold are proposed to play important roles in promoting electronic 

coupling [302]. 

To complete the overview of these oligoproline-based systems, it has to 

be mentioned that also pentads of type D–P–P–P–A have been prepared and 

studied, where each functional subunit is separated from the adjacent ones by 

two prolines [297, 303]. The results obtained indicate that interchromophoric 

energy transfer is relatively slow across five proline units, but rapid across two 

prolines [297]. 

5.5 

Multi-ruthenium Assemblies Based on Derivatized Polystyrene 

A suitable choice to assemble a large number of chromophores is polymer 

derivatization: within this class of compounds, probably the largest 

series deals with polystyrene polymers. Indeed, up to 30 Ru–bpy-type chromophores 

have been linked to soluble styrene polymers [297, 304–308, 310, 

318]. An early synthetic strategy [309] involved attachment of the chromophores 

to polystyrene by an ether linkage (48). Successively [310], the


same authors developed an alternative route based on amide linkage (49), 

finding a dramatic enhancement in the ability of the polymeric arrays to 

promote intrastrand energy transfer. In the (amide-functionalized) mixed 

polymers containing a 3 : 13 ratio between the lower-energy Os-based chromophores 

and the higher-energy Ru-based ones, triplet–triplet energy transfer 

was found to occur with an efficiency higher than 0.90 in acetonitrile 

solution. 

It was pointed out [310] that energy transfer from the excited Ru-based 

moieties to the ground-state Os-based moieties requires two processes: energy 

migration among the Ru-based units (site-to-site energy hopping) and 

a final energy transfer from a Ru-based to a nearby Os-based unit. For both 

processes the rate constants exceeded 2 × 10 8 s –1 for the amide-linked polymer, 

whereas in the ether-linked polymer the rate constant for intrastrand 

energy migration from ∗ Ru to Ru was orders of magnitude slower. The 

rate constants for the amide-linked species, coupled with the relatively long 

excited-state lifetime of the Ru-based chromophores (910 ns), account for the 

ability of the polymer arrays containing the amide-linked chromophores to 

act as efficient antennae. The reason for the different behavior of the etherlinked 

and amide-linked arrays lies in the direction of the excited-state MLCT 

dipole of the chromophores involved in the energy transfer processes. This 

dipole is directed toward the polymer backbone in the most effective amidelinked 

antenna systems, whereas it is out from the polymer backbone in the 

less efficient ether-linked arrays. This difference affects the electronic coupling 

between donor and acceptor sites of the energy migration processes. 

Ru(II) chromophores with appended electron acceptor (a methyl viologen 

species, A) and donor (a phenothiazine group, D) subunits have been incorporated 

within the antenna polystyrene system (50) [311], to play the role 

of “reaction center” (RC) units. In such an integrated antenna–RC polymer,


containing 17 normal Ru chromophores and three RCs, the D + –A + chargeseparated 

state is formed, as shown by transient absorption spectroscopy. 

Emission at the Ru(II) chromophores was quenched by about 34% compared 

to the homopolymer containing 20 “normal” Ru chromophores. Energy 

transfer from the normal Ru chromophores to the RC sites was favored by 

– 0.1 eV. It was shown that about 50% of the charge-separated state was 

formed during the 7-ns laser pulse, indicating intrastrand sensitization, with 

the charge-separated state formed by direct excitation at the RC complex and 

by excitation to nonadjacent Ru chromophores followed by energy migration 

to the RC sites. In this system, 1.15 eV is stored in the charge-separated state 

and the efficiency of the process varies from 12 to 18% dependingonlaser 

irradiance, indicating excited-state annihilation at high irradiance. Charge 

recombination is similar to that of the “isolated” RC, but an additional longlivedtransient(formedinlowefficiency,about0.5%) 

was observed, which 

decayed by second-order kinetics with k = 48 M –1 s –1 .Thislong-livedtransient 

was attributed to polymers in which D + and A – were formed on different 

RC units, by invoking mechanisms in which electron transfer quenching by 

oxidative or reductive electron transfer in a RC site is followed by intrastrand 

hole or electron transfer to a second RC site [311]. 

5.6 

Photoinduced Collection of Electrons 

into a Single Site of a Metal Complex 

An essential property of natural photosynthesis is the collection of multiredox 

equivalents at specific sites. Indeed, all the important light energy storage


processes require more than one electron to operate: for example, reduction 

of H + to H2 is bielectronic, and oxidation of oxygen in water to produce O2 is 

a four-electron process. Reduction of CO2 to the high-energy content glucose 

species is also a multielectron process. Whereas artificial systems capable of 

performing photoinduced charge separation have been reported, species able 

to collect, by successive photoinduced processes, more than one single electron 

(or hole) in one specific site of their structure are very rare. These species 

differ from polymers or dendritic species, which are also able to reversibly 

store more than one single electron (or hole) in their structure (in several, 

roughly identical, but spatially separated sites), since the accumulated charges 

should be located in a single subunit and, at least in principle, could be more 

easily delivered simultaneously to a unique substrate. 

A breakthrough in this field was the study of the two dinuclear Ru complexes 

51 and 52 [312]. These complexes are indeed able to collect two electrons 

(and two protons) and four electrons (and four protons), respectively, 

within their bridging ligand moieties upon successive light excitation and 

in the presence of sacrificial donor species. In a typical (schematized) sequence 

of events involving 51: (1) light excitation produces a MLCT state 

involving the bpy-like subunit of the bridge; (2) a charge shift takes place 

from the bpy-like bridge moiety to the inner, phenazine-like portion of the 

bridge, so producing a sort of charge-separated state; (3) the sacrificial donor, 

a triethylamino (TEA) species, reduces the Ru(III) center, so restoring the 

chromophore; (4) the reduced central moiety of the bridge adds a proton 

(originated from irreversible TEA oxidation), so reaching charge neutrality; 

and (5) the sequence of events 1–4 is repeated and two electrons and 

two protons are collected [312]. However, a recent refinement of the ultrafast 

spectroscopic results has evidenced that the product of step 2, initially 

identified as a sort of charge-separated state [313], receives a significant contribution 

also from a bridge-centered triplet state [314]. The overall process 

is perfectly reversible, and 51 is fully restored on leaving molecular oxygen 

reaching the complex [312]. For 52, the formerly described sequence of events 

is repeated four times, thanks to the presence of the quinone subunits responsible 

for the addition of two extra electron/proton couples [312]. All the 

various steps of the multielectron processes occurring in 51 have also been 

characterized by UV/Vis spectroscopy and each intermediate has a unique


signature. This characterization was made by extensive work (including spectroelectrochemistry 

in various solvents, as well as at different pH values in 

aqueous solution) to identify the intermediates and to clarify the effect of 

pH on the processes [315–317]. Indeed, it has been demonstrated that in 

some solvents, processes 2 and 4 take place simultaneously, so it would be 

more correct to talk of proton-coupled electron transfer instead of successive 

electron/proton transfer events. At pH 4, only a fully reversible, coupled twoelectron/two-proton 

transfer process is observed for 51. This is a rare example 

of a proton-coupled multielectron transfer reaction [317] (FM MacDonnell, 

personal communication). 

Another interesting example of photoinduced multielectron collection, 

this time at a metal center rather than at a ligand site, has been reported for 

a series of trimetallic mixed-metal species [318–320]. The most recent example 

of the series is 53 [320]. In this species, two Ru(II) chromophores (the 

light absorber units, LA) are linked to one Rh(III) center, which represents the 

electron collection (EC) core, through polyazine bridging ligands (BLs). The 

absorption spectrum of this species is dominated by the absorption of the Ru 

LA subunits, while the reduction properties identify the Rh(dσ ∗ ) orbital as 

the LUMO [319]. Upon excitation of the peripheral Ru(II) chromophore, an 

oxidative electron transfer to the Rh(III) center takes place (k = 1.2 × 10 8 s –1 ), 

populating a triplet Ru(II)-to-Rh(III) charge transfer state. In the presence of 

a sacrificial donor, dimethylaniline, which restores the Ru(II) center(s), the 

starting compound undergoes a net photoreduction process with formation 

of [{(bpy)2Ru(dpp)}2Rh I ] 5+ , as also demonstrated by spectroelectrochemistry, 

in which two chlorides have been lost (probably by irreversible chlo-


ride oxidation), and a two-electron reduced unit Rh(I) is formed [320]. The 

Rh center should simultaneously undergo a structural reorganization (most 

likely, octahedral/square planar). Interestingly, the photoreduced species is 

coordinatively unsaturated and therefore could be available to interact with 

substrates. It warrants mentioning that photoinitiated electron collection is 

obtained in a trimetallic species having an Ir(III) redox-active center instead 

of a Rh(III) one and similar Ru-based LA units [318]. 

5.7 

Photoinduced Multihole Storage: Mixed Ru–Mn Complexes 

Complementary to the topic discussed in the previous section (that is, to accumulate 

multiple electrons in a single site of a (supra)molecular species) 

is the development of systems capable of accumulating holes, as happens in 

the oxygen evolving systems of natural photosynthesis. The source of inspiration 

is the photosystem II [321], where the excited primary chlorophyll donor, 

∗ P680, one of the most effective photooxidants of natural systems, is able to extract 

up to four electrons in consecutive steps from the so-called manganese 

cluster, whose structure—at least for a specific natural system—has recently 

been revealed [322–324]. The four-times oxidized manganese cluster successively 

produces molecular oxygen, thus returning to its initial state, ready for 

another photoinduced catalytic cycle. 

Since the inspiration is photosystem II, it is not surprising that the largest 

family of complexes made to photochemically accumulate “holes” are Ru(II) 

polypyridine complexes coupled to manganese species. The field has been 

recently reviewed [325]. 

Several Ru–Mn dyads were initially studied to investigate some specific 

parameters for electron transfer (see for example 54–57 [326]). In a typical 

experiment, the excited Ru(II) chromophore is quenched via a bimolecular 

oxidative electron transfer by a sacrificial acceptor (usually methyl viologen), 

and the oxidized Ru(III) species oxidizes the attached Mn(II) subunit to 

Mn(III). Time constants of these latter processes spanned a large range, from


< 50 ns to 10 µs, depending on the complex [326]. This study demonstrated 

that the reorganization energy for the Mn(II)-to-Ru(III) electron transfer was 

quite large (1.4–2.0 eV), suggesting significant inner reorganization of the 

manganese moiety during the process [327]. As an obvious consequence, 

very fast Mn(II)-to-Ru(III) electron transfer could not be expected. A further 

complication was that the manganese subunit could directly quench the 

excited ruthenium chromophore by Dexter energy transfer, competing with 

electron transfer quenching by the sacrificial acceptor for short Ru–Mn distances. 

These arguments led to the preparation of more elaborated systems 

in which intermediate donor species were interposed between ruthenium and


manganese subunits [328–330], with phenolate and tyrosine moieties playing 

the role of an intermediate donor. In most cases, proton-coupled electron 

transfer processes took place. 

To achieve multielectron catalysts, more than one manganese ion was included 

in the systems. Figure 16 shows some examples [329, 331, 332] of 

Ru(II) species covalently linked to Mn dimers or trimers via phenolate lig- 

ands. In particular, for the Ru–Mn II,II 

2 

complex 58 reported in the figure, 

repeated flashes in the presence of a Co(III) sacrificial electron acceptor allowed 

three successive one-electron oxidations of the manganese moiety by 

the photooxidized Ru(III) subunit [333], as evidenced by the disappearance 

of the characteristic Mn2 II,II signals and the appearance of the characteristic 

Mn2 III,IV signals in EPR experiments. Manganese oxidation was suggested to 

involve a ligand exchange, in which acetate is released and water molecules 

are bound to form a di-µ-oxo bridge (see the reaction scheme in Fig. 16). According 

to the authors, this was the first example of a light-driven, multiple 

oxidation of a manganese complex attached to a photosensitizer. The ligand 

exchange at the manganese sites (presumably occurring in the Mn2 III,III 

state, that is, after second electron release from the initial Mn2 II,II center) 

is functional to the overall process, as it allows introduction of negative 

Fig. 16 Electron transfer from the manganese moiety to the photooxidized Ru(III) in a 

aRu– Mn2 II,II complex and b aRu– Mn3 II,II,II complex


charges to the complex thus making feasible oxidation to the Mn2 III,IV state, 

which would have otherwise been impossible on thermodynamic grounds. 

The ligand exchange and reorganization of the manganese ion coordination 

sphere, which has a nonneutral effect from the viewpoint of the charge, would 

be a charge compensating process, analogous to proton release occurring 

in most of the oxidation states of the “manganese cluster” in natural systems 

[321, 325]. It can be inferred that charge compensating processes are 

needed requirements to maintain the oxidation potential of the redox-active 

catalytic site roughly constant when moving along the various steps of the 

overall hole accumulating process, an aspect that should be well taken into 

account in designing new systems. 

Recently a mixed Ru–Mn2 species featuring a photoinduced chargeseparation 

state with an impressive lifetime (0.6 ms at room temperature 

and 0.1–1 sat140 K, comparable to many of the naturally occurring chargeseparated 

states in photosynthetic systems) has been reported [334]. The 

slow charge-recombination rate obtained for such a species has been mainly 

attributed to the large reorganization energy connected with the inner reorganization 

of the manganese subunit already mentioned (about 2 eV for the 

compound in [334]). This would suggest that there is no needed to look for 

charge-recombination processes occurring in the Marcus inverted region to 

obtain long-lived separated states, since the large inner reorganization energy 

typical of the manganese systems could lead to the same (or better) result. 

5.8 

Photocatalytic Processes Operated by Supramolecular Species 

5.8.1 

Photogeneration of Hydrogen 

Since the early papers appeared in the 1970s [335–339], Ru(II) polypyridine 

complexes have been extensively used to produce hydrogen in heterogeneous 

cycles under light irradiation, by using sacrificial donor species (most commonly 

amines), electron acceptor relays (usually methyl viologens), and colloidal 

metal catalysts (Pt, Rh, etc.). This aspect of Ru(II) photochemistry has 

been extensively reviewed [1, 340] and will not be discussed in detail here. We 

will discuss some recent papers in which a (supramolecular) multicomponent 

approach is used. 

One of the important steps in designing a multicomponent hydrogen 

evolving system would be to assemble in the same (supramolecular) system 

as many key components as possible. Key components would be (based on the 

systems operating in heterogeneous schemes): (a) light-harvesting units (antennae); 

(b) a charge-separation unit made of a photosensitizer (the energy 

trap of the antenna, if an antenna is present), an electron acceptor, and an 

electron donor; and (c) a catalyst [280, 297, 341–345]. The compound 60 [346]


is a quite interesting example toward the preparation of functional supramolecular 

species of this type, where all the key components would be integrated 

into a single multicomponent system. Under visible irradiation, 60 evolves 

molecular hydrogen from water (in the presence of EDTA as sacrificial donor) 

with an efficiency of about 1%. A relatively low turnover number (4.8) is estimated 

on the basis of the total amount of H2 evolved after 10 h(2.4 µmol) and 

the amount of the complex used (1 µmol). The hydrogen formation is immediate 

and is rather higher in its rate when UV light is eliminated by suitable 

filters. This latter observation suggests some photoinstability of the system 

upon UV irradiation. 

The wavelength dependence in the visible region of the rate of H2 formation 

agrees with the absorption spectrum of the complex. Moreover, the 

rate of H2 formation increases linearly with the photon flux, indicating that 

one-photon excitation of the molecule operates. This multicomponent system 

integrates in its structure the light harvester/photosensitizer (the Ru(II) chromophore) 

and the electron acceptor, which also plays the role of the catalyst 

(the Pt(II) subunit). Compared to the formerly investigated systems [1, 335– 

337, 339, 456], neither the electron relay (usually a methyl viologen species) or 

the solid-state catalyst (usually colloidal platinum or rhodium) are required. 

Mechanistic aspects have not been discussed yet. 

Another multicomponent system (61) has been recently reported [347]. 

When excited at 470 nm in acetonitrile and in the presence of triethylamine 

(TEA, 2.08 mol L –1 ), such a species evolves H2 from the solution with a turnover 

number of about 50 (mol of H2 per mol of compound). No H2 evolution is ob-


tained in the dark or when one of the key components (the Ru(II) chromophore, 

the Pd catalyst, or the particular bridging ligand) is missing. Interestingly, 

the same compound where the bridging ligand between the two metal sites is 

a bipyrimidine unit does not evolve molecular hydrogen. It is also reported 

that the amount of photocatalytically formed hydrogen depends strongly on 

the TEA concentration and the exposure time, and chloride ions inhibit the 

reaction. The amount of hydrogen produced increases steadily and levels off 

after 1200 min. After about 1800 min no more hydrogen is produced. The 

rate of hydrogen formation increases with increasing TEA concentration for 

low TEA concentration, but becomes independent at a TEA concentration 

> 0.86 mol L –1 , where it is about 1600 nmol min –1 . Although detailed mechanistic 

data are not available, the authors suggest as a first step of the process 

a twofold photoinduced reduction of the compound by TEA, analogously to 

what was reported for the related photoinduced electron collection system 51. 

Probably reduction is concomitant with proton extraction from TEA oxidation 

products: TEA should therefore be the proton source. The successive step 

should be reduction of the protons at the nearby Pd center. This latter step probably 

passes through a temporary chloride loss. The same paper also reports the 

photocatalyzed selective hydrogenation of tolane to cis-stilbene, accomplished 

by the same compound. Analogously to 60, the Ru(II) chromophore of 61 acts 

as the light harvester/photosensitizer (which also contains in its structure the 

electron acceptor subunit, that is, the phenazine moiety of the bridging ligand), 

while the role of the catalytic unit is here played by the Pd(II) center. 

Molecular hydrogen evolution under visible light irradiation has also been 

reported for trimetallic species like 53 [348]. Mechanistic details are not available. 

5.8.2 

Other Photocatalytic Systems 

The catalytic potential of heterometallic species containing Ru(II) and Re(I) 

chromophores for the conversion of CO2 to CO has been recently shown [349]. 

Compound 62 is one of the species in this regard. This investigation highlighted 

the fact that the photocatalytic activity is deeply influenced by the nature of 

both the bridging ligand and the peripheral ligands at the light-harvesting 

Ru(II) chromophore. The proposed mechanism is that upon light irradiation 

(λ > 480 nm) in DMF/triethanolamine (TEOA, acting as base) with 1-benzyl- 

1,4-dihydronicotinamide (BNAH) as sacrificial donor, the initially produced 

Ru-based MLCT state is reduced by BNAH. Then intrabridging ligand electron 

transfer occurs, with formation of the reduced rhenium subunit. This latter 

speciesisknowntoreactwithCO2 upon Cl ligand loss [350, 351]. The reduction 

of CO2 is bielectronic, so it is assumed that the second electron transfer follows 

a similar route. The most efficient species of this series of compounds, which is 

exactly 62, exhibits a turnover number of 170.


Two other dinuclear species (63 and 64) have been reported to show photocatalytic 

activity for specific reactions. The Ru–Pd dimer 63 is active for the 

photocatalytic dimerization of 1-methylstyrene [352], and the turnover numbers 

of the photocatalyzed reaction (> 90 within 4 h) and the high selectivity 

compete well with thermal catalytic systems. Compound 64 is active for the 

conversion of trans-4-cyanostilbene to its cis form [353]. Other aspects of the 

last mentioned works and of similar systems are also commented on in a very 

recent paper [354]. Photocatalytic processes based on photoelectrochemical 

cells in which the Ru chromophores are physically interfaced to electrodes or 

other solid systems are reported later. 

5.9 

Photoactive Molecular Machines Able to Perform Nuclear Motions 

In the last 10 years there has been great interest in designing molecular 

machines [280]. As machines of the macroscopic world, even molecular machines 

need energy to operate, and a suitable form of energy to power


molecular machines is light. It is therefore almost obvious that several photoactive 

molecular machines containing Ru(II) polypyridine complexes as 

the photoactive subunits have been prepared and studied. For an exhaustive 

discussion, the reader should consult [355–358]. A recent outstanding 

example [359] is discussed Balzani et al. 2007, in this volume [119]. 

We mention here the case of photoinduced ring motion in the catenane 

65 [360], illustrated in Fig. 17. Visible light excitation of this compound in 

acetonitrile leads to population of the MLCT triplet state and subsequent 

formation (via thermal activation) of the MC state which causes the decoordination 

of the sterically hindered bpy-type ligand. As a result, swinging of 

the bpy-containing ring occurs, and the catenane structure made of discon- 

Fig. 17 Structural formula and photochemically and thermally induced motions of 

a Ru(II) catenane complex


nected rings is obtained (66). Heating regenerates the starting catenane. The 

process can be repeated at will, since both the reactions (coordination and 

decoordination) are quantitative. It can be noted that in this system the photosubstitution 

process, usually considered as an undesired property for Ru(II) 

complexes, is instead used to perform the desired function. More recently, 

light-induced motion on a rotaxane system has also been obtained by using 

the same strategy [361]. 

6 

Ruthenium Complexes and Biological Systems 

The effects of the interaction of photoactive ruthenium complexes with biological 

structures have been extensively studied. Because of the outstanding 

excited-state properties of Ru(II) polypyridine complexes, these systems 

have been employed as probes of biological sites, as well as photocleavage 

agents and, in recent times, as inhibitors of biological functions [362–369]. 

Among the species used as luminescent probes, one of the most studied compounds 

in the last 15 years is 67 [200–204, 362–379]. This complex is very 

weakly emissive in aqueous solution, but becomes strongly emitting in the 

presence of DNA, giving rise to the so-called light-switch effect [200, 201]. 

The reason for such a behavior lies in the electronic properties of this specific 

chromophore and in the intercalation ability of the dipyrido[3,2-a:2 ′ ,3 ′ - 

c]phenazine (dppz) ligand. In this species, there are several triplet excited 

states quite close in energy: (1) a MLCT state directly populated by light excitation, 

in which the excited electron resides in the LUMO+1 centered on 

the “bpy-like” portion of dppz ligand; (2) a MLCT state in which the excited 

electron is located in the LUMO centered on the “phenazine-like” portion of 

the dppz ligand; and (3) a ligand-centered (dppz-based) excited state. This 

compound represents another example of interplay between multiple MLCT 

states, discussed in more detail in Sect. 4.4. The energy gap and order of the 

three low-lying excited states mentioned above, as well as their dynamics, 

can be modulated by various parameters, including solvent dielectrics, protic 

ability of the solvent, and hydrophobic interactions. In a simplified schema-


tization, in aqueous solution the dominant (lowest energy) excited state is 

the MLCT involving the phenazine-like dppz subunit (populated by a sort 

of “charge shift” decay from the directly excited MLCT level), which deactivates 

largely by nonradiative processes. When the complex interacts with 

DNA, the MLCT state involving the bpy-like dppz subunit becomes dominant, 

and since such a state has better luminescent properties, the luminescence 

of the complex is switched on. Looking in more detail, the interplay 

among the various states, in this and related species and in the absence 

and presence of DNA, is more complicated, as demonstrated by various theoretical 

[379] and experimental techniques, including transient absorption 

femtosecond spectroscopy [370, 371, 378, 380, 381] and time-resolved resonance 

Raman spectroscopy [372, 373, 382]. Details can be found in the original 

references. 

Besides 67, many other Ru(II) polypyridine complexes have been reported 

to exhibit luminescence enhancement in the presence of DNA. In most cases, 

the luminescence enhancement is moderate and can be assigned to the protection 

offered toward oxygen quenching by DNA structures to the surfaceattached 

Ru(II) complex (which can bind, essentially for electrostatic reasons, 

to the major or minor grooves). If the interaction is limited to surface binding, 

the luminescence enhancement is usually within 20–40% in the presence 

of oxygen, whereas it is negligible in deoxygenated samples. Several compounds, 

however, exhibit noticeable luminescence enhancement (one order of 

magnitude or higher): in most of these cases, the compounds quite often have 

a ligand with a large, flat framework and intercalation takes place. The compound 

68 [383], which is nonemissive in water solution and strongly emissive 

in organic solvents (λ = 610 nm; τ = 1.1 µs; Φ = 0.12) is an exception. In water, 

the presence of DNA switches the luminescence on. The authors suggest 

that in water solvent-specific interactions with the amido moiety promote 

radiationless decay of the (potentially) emitting MLCT state, and that the protection 

offered by DNA versus solvent interaction restores MLCT emission. 

Ru(II) complexes whose luminescence is significantly quenched in the 

presence of DNA have also been reported. Usually, the excited state of these 

species is a very good oxidant, and photoinduced reductive electron transfer 

involving guanine residues is responsible for the luminescence quenching 

[362, 369, 384]. In some cases, it has been proposed that the quenching


process can occur via photoinduced proton-coupled electron transfer with 

guanosine-5 ′ -monophosphate. 

Besides being used as luminescent probes, ruthenium complexes have 

been reported to form photoadducts with DNA and other species of biological 

relevance. The most studied photoadducts are probably the ones formed 

by Ru(II) complexes containing 1,4,5,8-tetraazaphenanthrene (TAP) as ligand 

and guanine residues on DNA strands (see for example 69) [386]. The 

mechanism of photoadduct formation has been extensively investigated. The 

initially formed MLCT state undergoes reductive electron transfer from guanine. 

This process is followed by fast formation of a covalent bond between 

the electron donor and acceptor, which leads to an adduct between the metallic 

complex and the nucleobase [386]. Such photoadduct formation has also 

been used to induce photocrosslinks between two nucleotide strands when 

one of the strands was chemically derivatized by the photoreactive metal 

complex and the complementary strand contained a guanine base in the 

proximity of the tethered complex [387]. The necessary requirement for this 

photoreaction to occur is a MLCT excited state which is a very strong oxidant, 

as guaranteed by the TAP ligand. 

More recently, a photoadduct between similar Ru complexes and the 

amino acid tryptophan have also been reported [388]. The authors mention 

that this photoreaction appears very promising for a wide range of applications 

to peptides and proteins. 

Ru(II) complexes have also been inserted into synthetic oligonucleotides to 

obtain specific information on the properties of DNA strands and/or to prepare 

particular (super)structures [389–393]. For example, Ru(II)-derivatized 

oligonucleotides have been used to investigate the distance dependence of the 

quenching of suitable Ru luminescence by guanine residues [393]. Oligonucleotide 

conjugates containing Ru(II) polypyridine units as photosensitizers 

have also been reported to induce photodamage on single-stranded DNA 

sites [394]. 

The potential of Ru(II)-derivatized oligonucleotides has been explored 

to synthesize novel, interesting, and beautiful nanometer-sized luminescent 

structures in which the DNA strands act as templates and the Ru complexes 

act as both template and photoactive units [395–397], giving rise to


3D-networked structures. In these systems, the Ru(II) polypyridine subunits 

carry their own photoluminescence properties in the networked assemblies. 

Finally, it has been demonstrated that the photoexcitation of suitable 

Ru(II) complexes can inhibit biological functions; for example, in Ru(II)labeled 

oligonucleotides DNA polymerase is inhibited by a photocrosslinking 

process [387]. On the basis of these and similar results, which indicate strong 

and even complete inhibition of gene transcription by photoexcited Ru(II) 

complexes [398], it has been proposed that properly designed compounds 

can be ideal candidates for a phototherapy with implemented fiber-optic light 

source [398–400]. It should be noted that the photoactivity of Ru(II) complexes 

for phototherapy does not depend on the presence of oxygen: this 

could represent a real advantage as compared to other dyes used in photodynamic 

therapy [398]. 

7 

Dye-Sensitized Photoelectrochemical Solar Cells 

One of the most important developments involving Ru(II) polypyridine complexes 

in the last two decades is related to the design of dye-sensitized photoelectrochemical 

solar cells, which have outstanding properties for application 

in the field of solar energy conversion, in particular photovoltaics. Since their 

appearance in the early 1990s [401, 402], dye-sensitized photoelectrochemical 

solar cells based on the principle of sensitization of wide-bandgap mesoporous 

semiconductors have indeed attracted the interest of the scientific 

community, due to their performances which started the vision of a promising 

alternative to conventional junction-based photovoltaic devices. For the 

first time a solar energy device operating on a molecular level showed the stability 

and the efficiency required for potential practical applications. In the 

last few years, several excellent review articles have been published in this 

field [403–405]. These articles indicated that research on dye-sensitized solar 

cells is strongly multidisciplinary, involving areas such as nanotechnology, 

materials science, interfacial electron transfer, and supramolecular photochemistry 

and electrochemistry [405]. Here we mention the main basic aspects 

and describe a few of recent Ru(II) photosensitizers which exhibit quite 

interesting performances. 

7.1 

General Concepts 

The principle of dye sensitization of semiconductors can be traced back to 

the end of the 1960s [406]. However, the practical use of these systems was 

limited for a long time because the efficiencies obtained with single-crystal 

substrates were too low due to the poor light absorption of the adsorbed


monolayer of dye molecules. The breakthrough in the field was brought about 

by the introduction of mesoscopic films made of sintered nanoparticles of 

a semiconductor metal oxide with a large surface area, which allowed the 

adsorption, at monolayer coverage, of a much larger number of sensitizer 

molecules leading to absorbance values, of thin films of a few microns, well 

above unity [407]. 

Wide-bandgap semiconductor materials such as TiO2 show a separation 

between the energy levels of the valence and conduction bands of the order 

of 3 eV, which means that the electron–hole pair needs to be produced by irradiation 

with light having a wavelength shorter than 400 nm, i.e., UV light. 

In order to use sunlight, mainly visible and near-IR, two general approaches 

have been developed: doping and molecular sensitization. The doping approach 

is the preferred choice for conventional photovoltaic devices [405]. 

Dye-sensitized photelectrochemical cells rely on photosensitization. In this 

process, a photoexcited species, the sensitizer (S), is capable of injecting an 

electron into the conduction band (CB) or a hole into the valence band (VB) 

of the semiconductor (Fig. 18). 

Fig. 18 Schemes for sensitized charge injection in the photoelectrochemical solar cells: 

a electron injection, b hole injection 

In fact, when the excited-state energy level of the sensitizer is higher with 

respect to the bottom of the conduction band, an electron can be injected 

with no thermal activation barrier in the semiconductor, leaving the sensitizer 

in its one-electron oxidized form (Fig. 18a). When the excited state 

is lower in energy with respect to the top of the valence band, an electron 

transfer (formally a hole transfer) between the semiconductor and the sensitizer 

can take place, leaving the molecule in its one-electron reduced form 

(Fig. 18b) [408]. 

The operation of a dye-sensitized solar cell is schematized in Fig. 19 [403, 

405]. The system is comprised of two facing electrodes, a photoanode and 

a counter electrode, with an electrolyte in between. The transparent conductive 

photoanode is covered with a thin film (7–10 µm) of a mesoporous


Fig. 19 Schematic operation principle of a dye-sensitized solar cell 

semiconductor oxide obtained via a sol–gel procedure. Dye coverage of semiconductor 

nanoparticles is generally obtained from alcoholic solutions of the 

sensitizer, in which the sintered film is left immersed for a few hours. Sensitizers 

are usually designed to have functional groups such as – COOH, – PO3H2, 

or – B(OH)2 for stable adsorption onto the semiconductor substrate. The dyecovered 

film is in intimate contact with an electrolytic solution containing 

a redox couple dissolved in a suitable solvent. The electron donor member of 

the redox couple must reduce quickly and quantitatively the oxidized sensitizer, 

so closing the circuit. A variety of solvents with different viscosity and of 

redox mediators have been the object of intense studies, the most commonly 

used being the couple I – 3 /I– in acetonitrile or methoxypropionitrile solution. 

The counter electrode is a conductive glass covered with a few clusters of 

metallic platinum, which has a catalytic effect in the reduction process of the 

electron mediator. Further details on the cells and on their preparation can be 

found in the literature [403–405]. 

The complete photoelectrochemical cycle of the device can be outlined as 

follows. The adsorbed sensitizer molecules (S) are brought into their excited 

state (S∗ ) by photon absorption and inject one electron into the empty conduction 

band of the semiconductor in a timescale of femtoseconds. Injected 

electrons percolate through the nanoparticle network and are collected by the 

conductive layer of the photoanode electrode, while the oxidized sensitizer 

(S + ) in its ground state is rapidly reduced by I – ions in solution. Photoinjected 

electrons flow in the external circuit where useful electric work is produced 

and are available at the counter electrode for the reduction of the electron 

mediator acceptor I – 3 . The entire cycle consists in the quantum conversion of 

photons to electrons. 

S+hν → S∗ photoexcitation (25) 

S∗ +TiO2→S + +(e – ,TiO2) electron injection (26) 

2S + +3I – → 2S+I3 – sensitizer regeneration (27) 

I3 – +2e – → 3I – electron donor regeneration . (28)


Photoinjected electrons should escape from any recombination process in 

order to have a unit charge collection efficiency at the photoelectrode back 

contact. The two major waste processes in a dye-sensitized solar cell are 

due to (1) back electron transfer, at the semiconductor/electrolyte interface, 

between electrons in the conduction band and the oxidized dye molecules 

(Eq. 29), and (2) reduction of the electron relay (I – 3 , in this case) at the semiconductor 

nanoparticle surface (Eq. 30). 

S + +(e – ,TiO2) → S back electron transfer (29) 

I3 – +2(e – ,TiO2) → 3I – electron capture from mediator . (30) 

A detailed knowledge of all the kinetic mechanisms occurring in a photoelectrochemical 

cell under irradiation is an essential feature toward optimization 

of the process. 

7.2 

Ruthenium-Sensitized Photoelectrochemical Solar Cells 

A major breakthrough in the field relied on the performance of dye-sensitized 

solar cells employing Ru(II) complexes as sensitizers [401–405, 409–411]. 

Several reasons are at the basis of the success of Ru(II) polypyridine complexes 

in playing this leading role: 

1. Strong absorption throughout all the visible region, which can also extend 

to the near-IR. This result is obtained by means of intense MLCT bands 

due to a judicious choice and combination of ligands [1]. 

2. Strong electronic coupling between the MLCT excited state of the chromophore 

and the semiconductor conduction band. To fulfill this requirement, 

it has to be noted that the polypyridine ligand connected to the 

semiconductor via suitable functionalization of the ligand (usually carboxylated 

ligands) must be that involved in the lowest-lying MLCT state. 

3. Tunability of the excited-state redox properties. This allows the preparation 

of compounds whose excited-state oxidation potential can ensure an 

efficient electron injection in the semiconductor conduction band. In this 

regard, it should be considered that to estimate a “reduction potential” 

(Ecb) for the semiconductor conduction band is not an easy task [404, 412– 

414], and in nonaqueous solvents adsorption of cations, which are present 

as electrolytes, also has a significant effect on Ecb values. For example, 

Ecb for nanostructured TiO2 has been reported to be – 1.0 VvsSCEin 

0.1M LiClO4/acetonitrile and about – 2.0 VwhenLi + cations are replaced 

by tetrabutylammonium [413, 414]. 

4. Stability of the Ru(II) polypyridine complexes, in the ground state as well 

as in the excited and redox states. However, it is useful to note that photostability 

is not a strict requisite here, since the excited state is rapidly 

deactivated by electron injection. The same applies to chromophores hav-


ing intrinsic short excited-state lifetimes, provided that their intrinsic 

lifetime (that is, the reverse of the summation of the radiative and radiationless 

decay of the “isolated” chromophores) does not compete with the 

timescale for photoinjection. 

The electronic coupling between MLCT states and TiO2 conduction band 

is so efficient that electron injection takes place in the ultrafast regime. 

In particular, for the complex [Ru(dcbpy)2(NCS)2] (dcbpy=4,4 ′ -carboxylbipyridine), 

also called N3 or “red dye” (70), biphasic kinetics was reported 

for photoinduced electron injection [266, 267, 270]: the first ultrafast component 

was estimated to have a risetime of 28 fs and the slower component was 

reported to occur within the 1–50 ps time range [270, 271]. This behavior was 

initially interpreted on the basis of a two-state mechanism, the fast and slow 

components being attributed to injection from the MLCT singlet and triplet 

states, respectively. Also, the singlet-state injection was from nonthermalized 

vibronic states. Whereas injection from the singlet state was later confirmed 

(although with risetime slightly different, shorter than 20 fs), the origin of the 

picosecond component has been questioned: it has been recently proposed 

that such a “slow” component of electron injection arises from sensitizer 

molecules which are loosely attached to the semiconductor or are present in 

aggregated forms [271]. 

Dozens of Ru(II) complexes have been explored as sensitizers in dyesensitized 

solar cells of this type. We mention here some of the species 

exhibiting the best performances. The above mentioned N3 complex has 

very interesting properties: it shows a photoaction spectrum dominating almost 

the entire visible region, with incident photon-to-current conversion 

efficiency (IPCE) of the order of 90% between 500–600 nm. Short-circuit photocurrents 

exceeding 17 mA/cm 2 in simulated A.M. 1.5 sunlight and opencircuit 

photovoltages of the order of 0.7 V were obtained by using the couple 

I – 3 /I– as redox electrolyte [415]. For the first time a photoelectrochemical device 

was found to give an overall conversion efficiency of 10%. These performances, 

in part expected for the high reducing ability of the 3 MLCT state (ca. 

– 1 eV vs SCE) and the positive ground-state oxidation potential (+ 0.85 eV


vs SCE), contrast with the lower IPCE observed for other sensitizers having 

comparable ground- and excited-state properties [416]. It has been suggested 

that a peculiar molecular level property of the cis-[Ru(dcbpy)2(NCS)2] complex 

could affect one of the key processes of the cell mechanism. This view is 

consistent with the results of photoelectron spectroscopy and INDO/S calculations 

indicating that the Ru d orbitals interact strongly with the π orbitals 

of NCS, resulting in MOs of mixed nature [417]. In particular, the calculations 

show that the sulfur 3p orbitals give a considerable contribution to the 

HOMO of the complex. Hole delocalization across the NCS ligands can thus 

be responsible for an increased electronic matrix element for the electron 

transfer reaction involving TiO2/Ru III NCS and I – . This would lead to an increase 

of the rate constant of the reductive process, and as a consequence 

of IPCE. 

Even better photoelectrochemical performances, compared to those given 

by the complex [Ru(dcbpy)2(NCS)2], are featured by a species based on 

the terpyridine ligand [418]: TiO2 electrodes covered with the complex 

[Ru(L)(NCS)3] (L=4,4 ′ ,4 ′′ -tricarboxy-2,2 ′ :6 ′ ,2 ′′ -terpyridine) display very efficient 

panchromatic sensitization covering the whole visible spectrum and 

extend the spectral response at the near-IR region up to 920 nm, with maximum 

IPCE values comparable to that obtained with the dithiocyanate complex. 

Another species based on a substituted terpyridine is the mixed ligand 

complex [Ru(HP-terpy)(dmb)(NCS)], where P-terpy = 4-phosphonato- 

2,2 ′ :6,2 ′′ -terpyridine and dmb = 4,4 ′ -dimethyl-2,2 ′ -bipyridine [419]. A quantitative 

study of dye adsorption on TiO2 has shown that complexes containing 

the phosphonated terpyridine ligand adsorb more efficiently and strongly, 

giving an adsorption constant about 80 times larger than that for the dicarboxy 

bipyridine compounds. Since one of the problems encountered with the 

carboxy polypyridine class of sensitizers is the desorption from the semiconductor 

surface in the presence of water, the search for new anchoring 

groups is advisable. Along this line of research, complexes based on the 

derivatization of 2,2 ′ -bipyridine with a phenylboronic functionality were prepared. 

The photoaction spectra of TiO2 electrodes sensitized with the [Ru(4phenylboronic-2,2 

′ -bipyridine)2(CN)2] complex showed IPCE values comparable 

to those observed for [Ru(dcbH2)2(CN)2], indicating that the new type 

of linkage does not reduce the electronic coupling between sensitizer and 

semiconductor [405]. 

7.3 

Supramolecular Sensitizers 

Besides mononuclear Ru(II) complexes, multinuclear (supramolecular) compounds, 

as well as chromophore–acceptor or chromophore–donor dyads 

made of Ru(II) species and organic quenchers, have been used as sensitizers. 

There are several aims that inspired the design of such systems:


1. To increase the absorption properties of the (multicomponent) sensitizer, 

by using systems featuring the antenna effect, with the energy trap of the 

antenna being the Ru(II) unit directly connected to the semiconductor. 

One example is the trinuclear Ru(II) species 71 showninFig.20;in71,the 

light absorbed by the peripheral Ru(II) chromophores is transferred quantitatively 

to the central Ru(II) chromophore, from which electron injection 

takes place [420]. Experiments on this complex adsorbed on polycrystalline 

TiO2 gave an overall conversion efficiency of ca. 7% withturnover 

numbers of at least 1 × 10 6 . The antenna effect is expected to be of relevance 

for applications requiring very thin TiO2 layers. 

2. To spatially separate the injected electron and the hole on the sensitizer, 

so decreasing losses due to charge recombination. A system designed for 

this aim (72) is shown in Fig. 21, where the possible electron transfer steps 

are indicated [421]. In 72, the MLCT state of the Ru(II) unit is rapidly 

quenched by the Rh(III) species (step k1 in Fig. 21), followed by injection 

of the electron onto the semiconductor conductance band from the reduced 

Rh unit (step k2, in competition with step k4). The recombination 

process (k5) is slow because of a very weak electronic coupling. To reach 

thesameaim,thespecies73 shown in Fig. 22 has been designed [422]. 

Here, the first step is the electron injection from the Ru unit. Then, electron 

transfer from the phenothiazine unit to the oxidized Ru unit takes 

place, resulting in a charge-separated species which decays to the ground 

state with a rate of 3.6 × 10 3 s –1 (lifetime, 0.3 ms). The dyad and model 

molecules were also tested in solar cells, with iodide as an electron donor. 

While the observed IPCE was of the order of 45% for both systems, the 

open circuit photovoltage was higher for the dyad by 100 mV. The effect 

was more pronounced in the absence of iodide with Voc = 180 mV. Applying 

the measured interfacial electron transfer rates to the diode equation 

Fig. 20 Structural formula of a branched antenna system and schematization of energy 

transfer and charge injection in TiO2. Complex charge is omitted for clarity


Fig. 21 Structural formula of a linear antenna system and schematization of energy transfer 

and charge injection in TiO2. Complex charge is omitted for clarity 

Fig. 22 Structural formula of TiO2/Ru-PTZ heterotriad system and schematization of the 

electron transfer steps. Complex charge is omitted for clarity


gave the predicted increase of Voc of 200 mV, which was in agreement 

with the obtained value (180 mV) [422]. It is interesting that an increase 

of the lifetime of the interfacial charge-separated state TiO2(e – )–Ru(II)– 

PTZ + has a direct influence on the overall efficiency of the cell. A similar 

approach inspired the design of the supramolecular species 74, basedon 

the “red dye” N3 sensitizer. Optical excitation of a nanocrystalline TiO2 

film dye coated with such a species showed a long-lived charge-separated 

state [423]. 

8 

Miscellanea 

The fields that have been recently powered by Ru photochemistry are much 

more than those reported in some detail in this article. A few of those that are 

not discussed above will be briefly mentioned. 

Ru(II)-based chromophores have been linked to a plethora of receptor 

species, like calixarenes, crowns, and azacrowns, essentially for sensing 

purposes [280, 424, 425]. Ru(II) chromophores have also been embedded in 

oxygen-permeating polymers to yield luminescent sensors for molecular oxygen 

determination in atmosphere [426–429]. New systems have been designed 

and studied for obtaining OLED materials. In this regard, a dinuclear 

Ru complex has been used in conjunction with an organic luminophore to 

generate two-color electroluminescence [430]. 

Multichromophoric species made of Ru(II) chromophores interfaced with 

organic aromatics having suitable triplet-state levels have been studied to extend 

the lifetime of the MLCT excited state by a sort of delayed luminescence 

involving intercomponent energy transfer, with the organic triplet states used 

as excited-state energy storage systems [431–440, 442]. A few such species are 

compounds 75 (which is the first reported example of such a behavior [431]), 

76 [437], and 77 [435]. Compound 77 is one of the species featuring the most 

outstanding behavior: its emission in fluid solution at room temperature 

(with maximum at about 600 nm) has lifetimes ranging from 43 µs (acetone 

solution) to 61 µs (acetonitrile)to115 µs (DMSO solution) [435]. With 

the aim of increasing the excited-state lifetime as well as the luminescence


quantum yield, a goal which is missed by the formerly mentioned multichromophoric 

species, several Ru(II) chromophores having polypyridine ligands 

able to feature extended electron delocalization in their structure have been 

prepared [441]. In this case, the improved photophysical properties are due to 

reduced Franck–Condon factors for radiationless decay, a consequence of extended 

delocalization of the emitting MLCT state [78, 157, 443–446]. A typical 

example is the compound 78 shown in Fig. 23 [445]. In 78, in spite of the quite 

low emission energy at room temperature (820 nm), a relatively long luminescence 

lifetime and high quantum yield are found (420 ns and about 0.01, 

respectively). 

Hydrogen-bonded or, generally, noncovalently linked supramolecular 

species exhibiting photoinduced electron and/or energy transfer have also 

been prepared to mimic natural systems (see, for example, 79–81) [447–452]. 

Efficient intercomponent energy transfer through the noncovalently linked 

frameworks is usually obtained. However, in some cases the interest in the 

potential application of these systems is reduced by the small value of the 

association constants.


Fig. 23 Thermal ellipsoid views and structural formula of a dinuclear Ru(II) complex 

Multiple emissions from Ru(II) polypyridine complexes have been reported. 

Besides multiple emission connected to supramolecular species featuring 

nonquantitative interchromophoric energy transfer (a relatively common 

and in some way an expected behavior), in some cases multiple emission 

from the same Ru(II) subunit has also been proposed at room temperature 

[453]. This behavior has not been fully explained yet. 

Many efforts have aimed to take advantage of the photophysical properties 

of Ru(II) chromophores in electropolymerized thin film structures and


in SiO2-based sols–gels [342]. In some cases, these solid-interfaced systems 

allowed interesting photocatalytic results to be obtained, such as (1) the dehydrogenation 

of 2-propanol to acetone and molecular hydrogen, obtained 

in a photoelectrochemical cell in which a dinuclear Ru complex is adsorbed


to TiO2 [454], and (2) the dehydrogenation of hydroquinone to quinone, by 

means of two Ru chromophores coadsorbed on TiO2 at the anode of a photoelectrochemical 

cell, with molecular hydrogen produced at a platinizedplatinum 

cathode [455]. These fields have been recently reviewed and several 

details can be found in [342]. Quite recently photoinduced charge transfer between 

CdSe nanocrystal quantum dots and Ru(II) complexes has also been 

reported [456]. 

Interesting results, from the photocatalysis point of view, have been reported 

for other heterogeneous systems. For example, the sensitization of platinized 

layered metal oxide semiconductors with Ru(II) polypyridine dyes enabled 

photolysis of aqueous hydrogen iodide to molecular hydrogen and triiodide 

using visible light [457–459]. Photocatalytic water oxidation has been 

accomplished with good efficiency in a system based on [Ru(bpy)3] 2+ as the 

photosensitizer and using a colloidal solution of IrO2 as a catalyst [460, 461]. 

Ru(II) polypyridine complexes containing ligands which can be protonated/deprotonated 

have been extensively studied. Interesting effects of electronic 

coupling between metal centers in dinuclear systems with protonable/deprotonable 

bridging ligands have been reported [462–464], as well 

improved emissive properties in Ru(terpy)2-like complexes [188]. Quite interesting 

light-controlled electronic coupling between the Ru(II) centers of 

dinuclear systems has also been obtained by inserting photoisomerizable 

subunits with the bridging ligands [465, 466]. 

Finally, the recently published X-ray time-resolved spectra of [Ru(bpy)3] 2+ , 

obtained by different techniques [467–469], warrants recalling. These studies 

give information on the structural changes induced by excitation. Interestingly, 

they confirm the very small distortion of the 3 MLCT state in comparison 

with the ground state, as formerly predicted on the basis of other 

experimental results and theoretical arguments. 

Acknowledgements We acknowledge MIUR (PRIN projects nos. 2006034123 and 2006- 

030320), the University of Bologna, and the University of Messina for financial support. 

We also wish to thank our colleagues F. Barigelletti, L. Hammarström, and C.A. Bignozzi 

for useful discussions. 

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DOI 10.1007/128_2007_137 




of Coordination Compounds: Rhodium 

Maria Teresa Indelli · Claudio Chiorboli · Franco Scandola (✉) 

Dipartimento di Chimica dell’Università, ISOF-CNR sezione di Ferrara, 44100 Ferrara, 

Italy 

snf@unife.it 

1 Introduction ................................... 216 

2 Mononuclear Species .............................. 218 

2.1 PolypyridineComplexes ............................ 218 

2.2 CyclometalatedComplexes ........................... 223 

3 Polynuclear and Supramolecular Species ................... 226 

3.1 HomobinuclearComplexes........................... 226 

3.2 Dyads....................................... 227 

3.2.1 Photoinduced Electron Transfer in Ru(II)-Rh(III) Polypyridine Dyads . . . 228 

3.2.2 Photoinduced Electron Transfer in Porphyrin-Rh(III) Conjugates . . . . . 234 

3.3 TriadsandOtherComplexSystems ...................... 235 

3.4 PhotoinducedElectronCollection ....................... 239 

4 RhodiumComplexesasDNAIntercalators .................. 241 

4.1 SpecificBindingtoDNAandPhotocleavage.................. 241 

4.2 Rh(III) Complexes in DNA-Mediated Long-Range Electron Transfer . . . . 245 

4.2.1 Rh(III) Complexes as Acceptors in Electron Transfer Reactions ....... 245 

4.2.2 Long Range Oxidative DNA Damage by Excited Rh(III) Complexes . . . . 248 

5 Conclusion .................................... 250 

References ....................................... 251 

Abstract Rhodium(III) polypyridine complexes and their cyclometalated analogues display 

photophysical properties of considerable interest, both from a fundamental viewpoint 

and in terms of the possible applications. In mononuclear polypyridine complexes, 

the photophysics and photochemistry are determined by the interplay between LC and 

MC excited states, with relative energies depending critically on the metal coordination 

environment. In cyclometalated complexes, the covalent character of the C – Rh bonds 

makes the lowest excited state classification less clear cut, with strong mixing of LC, 

MLCT, and LLCT character being usually observed. In redox reactions, Rh(III) polypyridine 

units can behave as good electron acceptors and strong photo-oxidants. These 

properties are exploited in polynuclear complexes and supramolecular systems containing 

these units. In particular, Ru(II)-Rh(III) dyads have been actively investigated for the 

study of photoinduced electron transfer, with specific interest in driving force, distance, 

and bridging ligand effects. Among systems of higher nuclearity undergoing photoinduced 

electron transfer, of particular interest are polynuclear complexes where rhodium 

dihalo polypyridine units, thanks to their Rh(III)/Rh(I) redox behavior, can act as twoelectron 

storage components. A large amount of work has been devoted to the use of

216 M.T. Indelli et al. 

Rh(III) polypyridine complexes as intercalators for DNA. In this role, they have proven 

to be very versatile, being used for direct strand photocleavage marking the site of intercalation, 

to induce long-distance photochemical damage or dimer repair, or to act as 

electron acceptors in long-range electron transfer processes. 

Keywords DNA intercalators · Electron transfer · Photophysics · 

Polynuclear complexes · Rhodium 

Abbreviations 

bpy 2,2 ′ -bipyridine 

bzq Benzo(h)-quinoline 

chrysi 5,6-chrysenequinone diimine 

dpb 2,3-bis(2-pyridyl)benzoquinoxaline 

DPB 4,4 ′ -diphenylbipyridine 

dpp 2,3-bis(2-pyridyl)pyrazine 

dppz dipyridophenazine 

dpq 2,3-bis(2-pyridyl)quinoxaline 

HAT 1,4,5,8,9,12-hexaazatriphenylene 

Me2bpy 4,4 ′ -dimethyl-2,2 ′ -bipyridine 

Me2phen 4,7-dimethyl-1,10-phenanthroline 

Me2trien diamino-4,7-diazadecane 

ox Oxalato 

phen 1,10-phenanthroline 

phi 9,10-phenanthrenequinonediimine 

PPh3 triphenylphosphine 

ppy 2-phenylpyridine, 

TAP 1,4,5,8-tetraazaphenanthrene 

thpy 2-(2-thienyl)-pyridine 

tpy 2,2 ′ :6 ′ ,2 ′′ -terpyridine 

1 

Introduction 

Although not as popular as other transition metals, e.g., ruthenium, rhodium 

has received considerable attention in the field of inorganic photochemistry. 

Few specific reviews on rhodium photochemistry are available, however. The 

literature preceding 1970 was reviewed in the classical book of Carassiti and 

Balzani [1]. The photochemistry of polypyridine metal complexes, including 

those of rhodium, has been reviewed by Kalyanasundaram in 1992 [2]. Several 

rhodium-containing species are considered in the extensive review written in 

1996 by Balzani and coworkers on luminescent and redox active polynuclear 

complexes [3]. A number of photochemical investigations are included in the 

1997 review article of Hannon on rhodium complexes [4]. Rhodium complexes 

are included in more recent reviews dealing with photoinduced processes in 

covalently linked systems containing metal complexes [5, 6].

Photochemistry and Photophysics of Coordination Compounds: Rhodium 217 

In general terms, three main classes of rhodium complexes have attracted 

the attention of photochemists: (i) amino complexes and substituted derivatives; 

(ii) multiply bridged dirhodium complexes; (iii) rhodium polypyridine 

and related complexes. 

Rhodium(III) halo/amino complexes have been actively investigated in the 

late 1970s and in the 1980s. They have low-lying metal centered (MC) excited 

stats of d – d type, and can be considered paradigmatic representatives 

of the ligand-field photochemistry of d 6 metal complexes. The subject has 

been summarized and clearly discussed by Ford and coworkers in their 1983 

review articles [7, 8]. In more recent times, however, despite a number of interesting 

investigations [9–18], the activity in the field seems to have slowed 

down considerably. 

Multiply bridged dirhodium complexes constitute a large family of compounds, 

the structure and properties of which depend strongly upon the oxidation 

state of the metals. The Rh(I)-Rh(I) species of formula Rh2(bridge)4 2+ , 

where the two d 8 metal centers are bridged by four bidentate ligands (e.g., 

diisocyanoalkanes) in square planar coordination, have dσ ∗ → pσ excited 

states with a greatly shortened metal–metal bond [19] that emit efficiently in 

fluid solution [20]. In the Rh(II)-Rh(II) species of formula Rh2(bridge)4X2 n+ , 

the two d 7 metal centers are bridged by four bidentate ligands (e.g., diisocyanoalkanes, 

acetate) and complete their pseudo-octahedral coordination 

with a metal–metal bond and two-axial monodentate ligands (e.g., X 

=Cl,Brn = 2). These dirhodium complexes have long-lived excited states 

of dπ ∗ → dσ ∗ type, which do not emit in fluid solution but can undergo 

a variety of bimolecular energy and electron transfer reactions [21]. 

Dirhodium tetracarboxylato units of this type have also been used as building 

blocks for a variety of supramolecular systems of photophysical interest 

[22–24]. Particularly interesting triply bridged dirhodium complexes of 

type X2Rh(bridge)3RhX2, LRh(bridge)3RhX2, andLRh(bridge)3RhL (bridge 

= bis(difluorophosphino)methylamine, X = Br, L = PPh3) have been developed 

recently by Nocera [25]. These Rh(II)-Rh(II), Rh(0)-Rh(II) and 

Rh(0)-Rh(0) species, all possessing excited states of dπ ∗ → dσ ∗ type, can be 

interconverted photochemically by means of two-electron redox processes. 

Such two-electron photoprocesses provide the basis for a recently developed 

light-driven hydrogen production system [26], with a Rh(0)-Rh(II) mixed 

valence species playing the role of key photocatalysts [27]. The interest in 

multiply bridged dirhodium systems is now largely driven by their potential 

and implications for photocatalytic purposes. 

As for other transition metals, polyimine ligands (in particular, polypyridines 

and their cyclometalated analogues) have played a major role in the design 

of rhodium complexes of photophysical interest. This is due to an ensemble 

of factors, including chemical robustness, synthetic flexibility, electronic 

structure, excited-state and redox tunability. Thus, rhodium polypyridine 

and related complexes have been extensively studied from a photophysical


viewpoint, both as simple molecular species or as components of supramolecular 

systems featuring energy/electron transfer processes. Also, rhodium 

polypyridine complexes have played a major role in the active research field 

of DNA metal complex interactions. In recognition of the relevance of these 

systems, this review will be essentially focused on the photochemistry and 

photophysics of complexes of rhodium with polypyridine-type ligands and 

of supramolecular systems that that contain such units as molecular components. 

2 

Mononuclear Species 

2.1 

Polypyridine Complexes 

The fundamental features of the photophysics of Rh(III) polypyridine 

complexes have been extensively discussed in the book of Kalyanasundaram 

[2] and only a few general aspects are recalled here. The tris(1,10phenanthroline)rhodium(III) 

ion, Rh(phen)3 3+ (1), can be used to exemplify 

the typical photophysical behavior of this class of complexes. Rh(phen)3 3+ 

exhibits in 77 Kmatricesanintense(Φ,ca.1),long-lived(τ,ca.50 ms), structured 

emission (λ = 465, 485, 524, 571 nm) assigned as ligand-centered (LC) 

phosphorescence, i.e., emission from a π–π ∗ triplet state essentially localized 

on the phenanthroline ligands [28–31]. As is shown by high-resolution 

spectroscopy, the LC excitation is not delocalized, but rather confined to 

a single ligand [32, 33]. In room-temperature fluid solutions, Rh(phen)3 3+ is 

practically non-emitting (see below). The LC triplet state can be nevertheless 

easily monitored by transient absorption spectroscopy (λmax = 490 nm, 

εmax = 4000 M –1 cm –1 , τ = 250 ns) [34]. The temperature-dependent behavior 

is explained on the basis of decay of the LC triplet via a thermally activated 

process involving an upper metal-centered (MC) state [34–36]. Indeed, the


properties of the very weak emission measured from room-temperature solutions 

of Rh(phen)3 3+ (bandshape, lifetimes) are consistent with a small 

amount of MC excited state being in thermal equilibrium with the lowest LC 

state [34]. In related 2,2 ′ -bipyridine complexes, changes in LC-MC energy gap 

and emission spectral profile can be induced by methyl substitution in the 

3,3 ′ positions, as a consequence of changes in the degree of planarity of the 

ligand [37]. 

The interplay of LC and MC triplets in the photophysics of this type of 

complexes has been investigated in detail by studying the series of mixedligand 

complexes cis-Rh(phen)2XY n+ (X=Y=CN,n =1;X=Y=NH3, n =3; 

X=NH3, Y=Cl,n = 2), where the energy of the MC states is controlled by 

the ligand field strength of the X, Y ancillary ligands [38]. While at room temperature 

all the complexes are very weakly emissive, at 77 K, the di-cyano and 

di-amino complexes give the typical, structured LC emission, whereas the 

amino-chloro complex exhibits a broad emission of MC type (Fig. 1a). This 

can be readily explained on the basis of the energy diagram of Fig. 1b, where 

the relative energies of the LC triplet (appreciably constant for the three complexes) 

and of the lowest MC state (dependent on the ligand field strength of 

the ancillary ligands) are schematically depicted. For the amino-chloro complex 

the MC state is the lowest excited state of the system. For the di-amino 

case the situation is similar to that of the Rh(phen)3 3+ complex, with LC as 

the lowest excited state but with MC sufficiently close in energy to provide 

an efficient thermally activated decay path for LC (actually, in an appropriate 

temperature regime the two states are in equilibrium). In the di-cyano 

complex, the MC state is sufficiently high in energy that the LC state has 

a substantial lifetime (1.2 µs) even in room-temperature solution [38]. The 

actual energy gap between the LC and MC states (2000 cm –1 for the di-cyano 

Fig. 1 a 77 K emission spectra of cis-Rh(phen)2XY n+ complexes with different X, Y ancillary 

ligands. b Rationalization of the emission properties in terms of relative energies of 

ligand-centered (LC) and metal-centered (MC) excited states (adapted from [38])


complex) can be evaluated by measuring the activation energy of the photosolvation 

reaction originating from the MC state in low-temperature glycerol 

matrices [39]. 

As it is obvious from the trend sketched in Fig. 1b, the cis-Rh(phen)2Cl2 + 

complex has again a lowest MC excited state. It emits at ca. 710 nm [31] and, in 

keeping with the typical ligand-field photoreactivity of d 6 metal complexes [7], 

undergoes in fluid solution photosolvation of the chloride ligands [40–42]. 

This type of reactivity that has been exploited by Morrison and coworkers [43] 

in an extensive series of studies on photoinduced binding to DNA and potential 

applications of this class of complexes as photo-toxic agents. 

Anumberofanaloguesofthecis-Rh(NN)2Cl2 + complexes, where the NN 

represents various bidentate nitrogen donors, such as e.g., 2, 3 behave similarly 

to cis-Rh(phen)2Cl2 + , i.e., have lowest excited states of MC character and 

emit accordingly [44, 45]. 

Thesamelineofreasoningcanbeappliedtorationalizethephotophysical 

properties of the bis(2,2 ′ :6 ′ ,2 ′′ -terpyridine)rhodium(III) ion, Rh(tpy) 3+ 

2 (4), 

and analogous complexes. It is well known that, owing to a more unfavorable


bite angle, two tpy ligands provide a lower ligand field strength compared 

to three phen (or bpy) ligands [46]. Accordingly, whereas Rh(bpy) 3+ 

3 is a LC 

emitter, Rh(tpy) 3+ and related compounds only show at 77 Kbroadstructure- 

2 

less emissions of MC type [47]. 

In heteroleptic bis-imine Rh(III) complexes, multiple emissions originating 

from LC states localized on different ligands can be present at 

77 K. For example, in complexes of type [Rh(bpy)n(phen)3–n] 3+ , the LC 

emissions localized on bpy and phen are spectrally very similar but can 

be distinguished on the basis of their different lifetimes in time-resolved 

experiments [30, 48]. Clear indication that the excitation is trapped at either 

a bipyridyl or a phenanthroline ligand in the phosphorescent triplet state 

can also be obtained from phosphorescence microwave double resonance 

(PMDR) experiments [49]. The heteroleptic complex Rh(phi)2(phen) 3+ (5) 

has been particularly developed as a DNA photocleavage agent (see Sect. 4). 

The complexes absorb strongly in UV region with a significant broad absorption 

in the visible range, which tails to ca. 500 nm [50]. The spectrum 

is dominated by overlapping of the ligand centered (LC) transitions of the 

component ligands with the phi centered bands at lower energy. These bands 

are strongly pH dependent with shifts to the blue upon increasing the pH. 

At 77 K the complex exhibits ligand centered (LC) dual emission from both 

phi and phen ligands. At room temperature no emission can be detected 

whereas a long-lived excited state (τ ≈ 200 ns in polar solvent) has been 

observed by transient absorption. On the basis of several experimental evidences 

this excited state, lying in energy at ca. 2 eV above the ground state, 

is assigned by the authors to be intraligand charge transfer in nature (ILCT). 

Quenching experiments with organic electron donors clearly indicate that the 

ILCT triplet state is a strong oxidizing agent with E1/2( ∗Rh3+ /Rh2+ )=2.0 V 

vs. NHE [50]. Multiple LC emissions have also been suggested to occur in 

heteroleptic complexes containing bipyridine or phenanthroline and pyridyl 

triazole ligands [51]. 

While for the vast majority of Rh(III) polypyridine complexes the photophysics 

and photochemistry are dominated by LC and MC states, in a few


cases ligand-to-metal charge transfer (LMCT) photochemistry is observed. 

A clear example is provided by the Rh(bpy)2(ox) + complex (6) [52]. 

The spectrum of the colorless complex 6 is characterized by an intense 

band at ca. 300 nm assigned to oxalato-to-rhodium LMCT transitions. Upon 

UV irradiation, the following photoreaction is observed: 

Rh III (ox)(bpy)2 + + hν → Rh I (bpy)2 + + 2CO2 . (1) 

The Rh(bpy)2 + product is formed rapidly (risetime in pulsed experiments, 

ca. 10 ns), probably via a sequence of processes comprising photochemical intramolecular 

electron transfer from the oxalate ligand to Rh(III) followed by 

the decomposition of the oxidized ligand into CO2 and CO2 – radical (Eq. 2) 

and thermal reduction of the Rh(II) center to Rh(I) by the reactive CO2 – radical 

(Eq. 3) [52]. 

RhIII (ox)(bpy)2 + + hν → RhII (CO2 – )(bpy)2 + +CO2 

(2) 

RhII (CO2 – )(bpy)2 + → RhI (bpy)2 + +CO2 . (3) 

The violet Rh(bpy)2 + product, with intense MLCT visible absorption, is 

a tetrahedrally distorted d 8 square planar complex [53]. This Rh(I) species, 

which can also be obtained by chemical [54], electrochemical [55], or radiation 

chemical [56, 57] reduction of Rh(III) complexes, is of great interest from 

the catalytic viewpoint. It undergoes facile oxidative addition by molecular 

hydrogen [58], to give the corresponding Rh(III) dihydride (Eq. 4). The reaction 

is fully reversible upon 

Rh(bpy)2 + +H2 ⇆ cis-Rh III (bpy)2(H)2 + (4) 

removal of molecular hydrogen from the system. Interestingly, the release of 

molecular hydrogen from the dihydride complex can be obtained photochemically 

(Eq. 5). This photoreaction provides a 

cis-RhIII (bpy)2(H)2 –→Rh(bpy)2 + +H2 

hν 

(5)


convenient means to perturb the equilibrium of Eq. 4 and to study the kinetics 

of hydrogen uptake in the thermal relaxation of the perturbed system. 

A detailed picture of the transition state of this interesting reaction has been 

obtained by such type of experiments [53]. 

2.2 

Cyclometalated Complexes 

Ligands such as 2-phenylpyridine, 2-(2-thienyl)-pyridine, or benzo(h)quinoline, 

in their ortho-deprotonated forms (ppy, 7;thpy,8;bzq,9), can bind 

in a bidentate N^C fashion to a variety of transition metals, including Rh(III). 

These complexes are generally indicated as cyclometalated (or orthometalated) 

complexes. The spectroscopy and photophysics of cyclometalated 

complexes [59] are usually very different form those of the corresponding 

polypyridine complexes, the main reason being the much stronger σ donor 

character of C – relative to N. The consequences are (i) a high degree of covalency 

in the carbon–metal bond, (ii) a strongly enhanced ease of oxidation 

of the metal, (iii) high-energy MC excited states, (iv) relatively low-energy 

MLCT excited states. 

The photophysics of Rh(III) cyclometalated complexes, though not as 

developed as that of analogous Ir(III) species (see Chap. 9), has been actively 

investigated in the last two decades. While some tris- [60] and monocyclometalated 

[61] compounds have been synthesized and studied, for synthetic 

reasons bis-cyclometalated complexes of Rh(III) are by far more common 

in the literature. All the bis-cyclometalated complexes have a C,C cis 

geometry [62]. The Rh(ppy)2(bpy) + (10) complex can be used here to exemplify 

the main photophysical features of this class of compounds.


The absorption spectrum of 10 (Fig. 2) shows LC transitions of the bpy 

and ppy ligands in the 240–310 nm range. In addition, a prominent band 

is present at 366 nm, which is attributed to MLCT transitions [63, 64]. The 

presence of MLCT transitions at relatively low energy, a feature completely 

absent in analogous polypyridine complexes, is the result of the strongly 

electron donating character of the cyclometalating ligand, which makes the 

formally Rh(III) center relatively easy to oxidize (irreversible wave observed 

at ca. + 1.1 V vs. NHE) [63]. The complex, which is only very weakly emissive 

at room temperature, exhibits an intense, long-lived (τ, 170 µs), structured 

emission at 77 K (Fig. 2). The emission has been attributed to ppy-based LC 

phosphorescence, although various degrees of mixing between LC and with 

MLCT triplet states have been invoked on the basis of absence of dual emissions 

[64], lifetime considerations [63], and high-resolution spectroscopy [65, 

66]. In fact, because of the strong covalency of the carbon–metal bonds, the 

HOMO in this class of complexes has a mixed character, being delocalized 

on the central metal and on the cyclometalating ligand. This has been clearly 

shown by a number of recent TD/DFT calculations performed on Rh(III) [67] 

and related Ir(III) [68, 69] cyclometalated complexes. Therefore, a classifica- 

Fig. 2 Absorption spectrum (dashed line) and emission spectrum (room-temperature, 

continuous line; 77 K, dotted line) of Rh(ppy)2(bpy) + in 4/1 EtOH/MeOH (adapted 

from [64])


tion of the excited states in classical, localized terms (MC, LC, MLCT) is not 

generally applicable for cyclometalated complexes. 

The general behavior of other Rh(N^C)2(N^N) + complexesissimilar 

to that described above for Rh(ppy)2(bpy) + , with some easily understandable 

specific differences. For instance, the main effect of substituting thpy 

(8) for ppy is an enhancement of emission intensity and lifetime in roomtemperature 

solutions [63]. This is likely a result of the lower energy of 

the emission and the consequent lower efficiency of the thermally activated 

decay [70] via higher MC states. The remarkably long-lived (4.4–6.6 µs) 

emission observed in fluid solution for an analog of 10 with aldehyde substituents 

on the phenyl ring of the cyclometalating ligand [71] is probably 

justified by the same type of argument. On the other hand, the photophysical 

behavior of 10 is only slightly affected by substitution of bpy with similar 

N^N ligands, such as, e.g., 1,10-phenanthroline, 2,2 ′ -biquinoline [72], 

or 4-amino-3,5-bis(2-pyridyl)-4H-1,2,4-triazole) (11) [73]. However, when 

strong π-deficient ligands such as 1,4,5,8-tetraazaphenanthrene (TAP, 12) 

or 1,4,5,8,9,12-hexaazatriphenylene (HAT, 13) are used as N^N ligand [74], 

a definite switch in behavior is observed, with the presence of broad structureless 

77 K emissions that have a clear charge transfer character. Indeed, for 

this type of complexes, TD/DFT calculations show that the HOMO involves 

the metal and the cyclometalating ligand-carbon bonds, but the LUMO is now 

exclusively localized on the non-cyclometalating ligand (Fig. 3) [67]. Thus, 

in this case the emission is best considered as having a mixed LLCT/MLCT 

character.


Fig. 3 Frontier orbitals of Rh(ppy)2TAP + . From [67] 

3 

Polynuclear and Supramolecular Species 

3.1 

Homobinuclear Complexes 

A few homobinuclear ligand-bridged Rh(III) polypyridine complexes have 

been studied [3, 75, 76]. Their photochemical interest is rather limited, however, 

as they behave generally like their mononuclear analogues, with minor 

differences in spectral shifts and lifetimes. An interesting type of systems, 

which bring together the complexities of ligand-bridged species and multiply 

bridged metal–metal bonded rhodium dimers, has recently been reported by 

Campagna et al. [77]. In (14) two quadruply bridged Rh(II)-Rh(II) dimers are 

the “molecular components” of a higher-order two-component system held 

together by the complex bis-naphthyridine-type ligand. As already observed 

for some related simple rhodium dimers (e.g., Rh2(CH3COO)4(PPh3)2) [21], 

the “binuclear” compound has a non-emissive but long-lived excited state in 

room-temperature solution. In the case of 14, the long-lived state has been assigned 

as an MLCT (metal–metal π ∗ to naphthyridine π ∗ )excitedstate[77].


3.2 

Dyads 

Heteronuclear bimetallic species containing rhodium polypyridine complexes 

are more interesting, as the Rh(III) unit can be involved in intercomponent 

processes, particularly of the electron transfer type. The thermodynamic 

requirements for the participation of Rh(III) polypyridine complexes in electron 

transfer processes are summarized in Fig. 4, where Rh(III), ∗ Rh(III), and 

Rh(II) represent the ground state, the triplet LC excited state (see Sect. 2.1), 

and the one-electron reduced form, respectively, and the values of excitedstate 

energy [31] and reduction potential [78] refer to Rh(phen)3 3+ (1). From 

these figures, it is apparent that Rh(III) polypyridine complexes can behave 

as extremely powerful photochemical oxidants and relatively good electron 

transfer quenchers. On the other hand, because of the high excited-state energy, 

these complexes are also good potential energy donors. 

Fig. 4 Typical redox energy level diagram for Rh(III) polypyridine complexes. Values 

(reduction potential vs. SCE) appropriate for Rh(phen)3 3+ (1) 

As a matter of fact, Rh(III) polypyridine complexes have been extensively 

used in bimolecular electron transfer processes, either as photoexcited 

molecule [79, 80] or as quencher [81–83], with motivations of both fundamental 

(testing electron transfer-free energy relationships) [80] and applied 

nature (photoinduced hydrogen evolution from water) [81, 82]. Here, on the 

other hand, we focus our attention on photoinduced processes where the reactants 

are pre-assembled in some kind of supramolecular system. The most 

common photoinduced processes taking place in simple two-component systems 

(often called “dyads”) involving a Rh(III) polypyridine unit are shown 

in Eqs. 6–8: 

∗ Rh(III) –Q → Rh(III) – ∗ Q (6) 

∗ Rh(III) –Q → Rh(II) –Q + (7) 

∗ P – Rh(III) → P + – Rh(II) . (8)


The dyads generically indicated in Eqs. 6–8 are actually heterobinuclear complexes 

when, as it is often the case, the photosensitizer P or the quencher Q 

are transition metal complex moieties, and the chemical linkage is provided 

by a bridging ligand. A number of examples of such processes are discussed 

in the following sections. 

3.2.1 

Photoinduced Electron Transfer in Ru(II)-Rh(III) Polypyridine Dyads 

Though not exclusively, dyads containing Rh(III) polypyridine units have 

often involved as P or Q (Eqs. 7, 8) the chromophores par excellence of inorganic 

photochemistry, namely Ru(II) polypyridine complexes. The general 

behavior of Ru(II)-Rh(III) polypyridine dyads can be discussed taking dyad 

15 as an example [84]. 

The absorption spectrum of dyad 15, as compared with those of the 

Ru(Me2phen)2(Me2bpy) 2+ and Rh(Me2bpy)3 3+ molecular components, is 

shown in Fig. 5. It shows that the spectra of the molecular components are 

strictly additive, as expected for weak intercomponent interaction, and that 

selective (100%) excitation of the Ru(II) chromophore can be easily performed 

in the visible region, whereas partial excitation of the Rh(III) component 

(ca. 70% at300 nm) can be achieved in the ultraviolet. The energy 

level diagram for this dyad (Fig. 6) shows that, besides the photophysical 

processes taking place within each molecular component, a number of intercomponent 

processes are thermodynamically allowed. They include: 

∗Ru(II)-Rh(III) → Ru(III)-Rh(II) electron transfer from excited Ru(II) 

(a) 

Ru(II)-∗Rh(III) → Ru(III)-Rh(II) electron transfer to excited Rh(III) 

(b) 

Ru(II)-∗Rh(III) → ∗Ru(II)-Rh(III) energy transfer from Rh(III) to Ru(II) 

(c) 

Ru(III)-Rh(II) → Ru(II)-Rh(III) back electron transfer . (d)


Fig. 5 Absorption spectra of the Ru(II)-Rh(III) dyad 15 (dotted line, c), and of the 

Ru(Me2phen)2(Me2bpy) 2+ (dashed line, b) andRh(Me2bpy)3 3+ (continuous line, a) molecular 

components 

Fig. 6 Energy level diagram and photophysical processes for the Ru(II)-Rh(III) dyad 15 

For dyad 15, all these processes could be time-resolved by nanosecond and picosecond 

techniques under the appropriate experimental conditions (visible 

excitation for a, UV excitation for b and d, rigid matrix for c), leading to the


detailed kinetic picture of Fig. 6 [84]. The widely different rates of the three 

ET processes (a, b, d) can be rationalized in terms of predominant driving 

force effects [84], as shown schematically in Fig. 7. 

Fig. 7 Free-energy correlation of rate constants for the three electron transfer processes 

of dyad 15 

Since most Ru(II)-Rh(III) polypyridine dyads have very similar energetics, 

the qualitative features illustrated above for dyad 15 can be safely generalized 

to this whole class of compounds. For example, in dyad 16 [85]therateconstants 

of processes a, b,andd are slower by a factor of ca. 3 but have the same 

relative magnitudes as for dyad 15. The slower rates are likely related to the 

longer aliphatic bridge, although for this and related [86] dyads, the flexibility 

of the bridges limits the validity of such comparisons. 

Within this general type of behavior of Ru(II)-Rh(III) dyads, a number of 

experimental studies have been specifically aimed at investigating the role of 

the bridge in determining electron transfer rates. As has been the case for 

other types of bimetallic dyads [87–90], particular attention has been devoted 

to Ru(II)-Rh(III) dyads with modular bridges involving p-phenylene 

spacer units [6, 91, 92]. The dyads in Chart 1 provide a homogeneous series


with appreciably constant energetics but differing in the number (1–3) of 

p-phenylene spacers in the bridge and, in one of the two dyads with three 

spacers, for the presence of alkyl substituents on the central spacer. Upon 

excitation of the Ru(II)-based chromophore, the rate constants of photoinduced 

electron transfer (process a in the above general scheme) have been 

measured by time-resolved emission and transient absorption techniques in 

the nanosecond and picosecond time domains [6, 92]. The values for Ru-ph- 

Rh, Ru-ph2-Rh, andRu-ph3-Rh (Chart 1), when plotted as a function of the 

metal–metal distance r (Fig. 8), display an exponential decrease (Eq. 9): 

k = k(0) exp(– βr). (9) 

Chart 1


Fig. 8 Distance dependence of photoinduced electron transfer rates in the dyads of 

Chart 1: Ru-ph-Rh, Ru-ph2-Rh, Ru-ph3-Rh (dots), Ru-ph ′ 3-Rh (triangle) 

This is the behavior predicted for electron transfer in the superexchange 

regime [5,93,95] if the distance dependence of the reorganizational energy 

term can be neglected. The β valueobtainedfromtheslopeofthelinein 

Fig. 8, 0.65 ˚A –1 , should be regarded as an upper limiting value for the attenuation 

factor of the intercomponent electronic coupling (Eq. 9). This β value 

is in the range found for other oligophenylene-containing systems (organic 

dyads [96, 97], metal–molecules–metal junctions [98]). This underlines the 

goodabilityofthistypeofbridgestomediatedonor–acceptorelectroniccoupling 

(for comparison, β is typically 0.8–1.2 ˚A –1 for rigid aliphatic bridges). In 

this regard, it is instructive to compare the electron transfer rate constant observed 

for Ru-ph-Rh (k = 3.0 × 109 s –1 ) with that mentioned above for dyad 

15 containing an aliphatic bis-methylene bridge (k = 1.7 × 108 s –1 ). Despite the 

longer metal–metal distance (15.5 ˚A for Ru-ph-Rh relative to 13.5 ˚A for 15), 

the reaction is faster across the phenylene spacer by more than one order of 

magnitude. 

An interesting result [6, 92] is the fact that dyad Ru-ph ′ 3-Rh, which is iden- 

tical to Ru-ph3-Rh except for the presence of two solubilizing hexyl groups on 

the central phenylene ring, is one order of magnitude slower than its unsubstituted 

analog (Fig. 8). This is related to the notion that in a superexchange 

mechanism the rate is sensitive to the electronic coupling between adjacent 

modules of the spacer [5, 93, 95], and that in polyphenylene bridges this coupling 

is a sensitive function of the twist angle between adjacent spacers [99]. 

While the planes of unsubstituted adjacent phenylene units form angles of. 

20 ◦ –40 ◦ [100, 101], ring substitution leads to a substantial increase in the 

twist angle (to ca. 70 ◦ ) [100] and, as a consequence, to a slowing down of the 

electron transfer process. 

A number of Ru(II)-Rh(III) dyads have been reported where little or any 

photoinduced electron transfer quenching of the Ru(II)-based MLCT emis-


sion takes place [75, 76, 102]. In the case of dyad 17, the plausible reason is 

that, owing to the comparable reduction potentials of the diphenylpyrazine 

bridging ligand and Ru(III) center, the driving force for intramolecular electron 

transfer is too small [102]. In the cases of dyads 18 and 19, the presence 

of cyclometalated ancillary ligands makes the formally Rh(III) center very 

difficult to reduce and relatively easy to oxidize, thus yielding MLCT states at 

comparable energies on the two units [75, 76]. 

Low driving force arguments could also apply to the dyad 20, where 

relatively slow quenching of the Ru(II) MLCT emission (estimated k, 

ca. 3.5 × 10 7 s –1 ) was observed and attributed to intramolecular electron 

transfer [103]. Here, however, a relevant aspect is also the presence a Rh(III)


moiety with of coordinated chloride ligands. In fact, contrary to what happens 

for common Rh(III) polypyridine units (e.g., Rh(bpy)3 3+ ,Rh(phen)3 3+ ) 

where one-electron reduction is a quasi-reversible process, mixed-ligand 

units containing halide ions (e.g., Rh(bpy)2Cl2 + ,Rh(phen)2Cl2 + )undergo 

strongly irreversible two-electron reductions accompanied by prompt halide 

ligand loss [78, 104]. While the use of these units as electron acceptors can 

be of interest towards photoinduced electron collection and multi-electron 

catalysis (see Sect. 3.4), from a kinetic viewpoint the large reorganizational 

energies involved are likely to lead to slow electron transfer rates. 

3.2.2 

Photoinduced Electron Transfer in Porphyrin-Rh(III) Conjugates 

Though structurally quite different, the porphyrin-Rh(III) conjugates thoroughly 

studied by Harriman et al. [105] behave with regard to photoinduced 

electron transfer rather similarly to the above-discussed Ru(II)-Rh(III) dyads. 

The systems contain a zinc porphyrin unit connected directly with one (21)or 

via a phenylene spacer with two (22) rhodium terpyridine units. 

The electron transfer processes thermodynamically allowed in these systems 

are indicated in Eqs. 10–14, where both 21 and 22 are schematized as


dyads (indeed, 22 is a triad only in a formal sense), ZnP, Rh, and ph represent 

the zinc porphyrin and rhodium terpyridine molecular components and the 

phenylene spacer, respectively. The singlet excited state is considered for the 

zinc porphyrin chromophore and the triplet state for the rhodium terpyridine 

unit. 

∗ + ◦ 

ZnP-ph-Rh(III) → ZnP -ph-Rh(II) ∆G =–0.71 eV (10) 

ZnP-ph-Rh(III) ∗ → ZnP + -ph-Rh(II) ∆G ◦ =–0.81 eV (11) 

ZnP + -ph-Rh(II) → ZnP-ph-Rh(III) ∆G ◦ =–1.47 eV (12) 

∗ + ◦ 

ZnP-Rh(III) → ZnP -Rh(II) ∆G =–0.58 eV (13) 

ZnP-Rh(III) ∗ → ZnP + -Rh(II) ∆G ◦ =–0.80 eV (14) 

In 22, where a phenylene spacer is interposed between the two molecular 

components, both photoinduced electron transfer following excitation of the 

zinc porphyrin (Eq. 10) and charge recombination (Eq. 12) have been time 

resolved, with values in acetonitrile of 3.2 × 1011 s –1 and 8.3 × 109 s –1 ,respectively. 

The wide difference in rates is attributed to the different kinetic 

regimes of the two processes, photoinduced electron transfer (Eq. 10) being 

almost activationless, and charge recombination (Eq. 12) lying deep into the 

Marcus inverted region [105]. For the directly linked system 21 (as well as for 

its free-base analogue), the disappearance of the porphyrin excited state, presumablybyphotoinducedelectrontransfer(Eq.13),isextremelyfast.Infact, 

the excited state lifetime of 21, ca.0.7 ps in acetonitrile, is comparable to the 

longitudinal relaxation time of the solvent, implying that the electron transfer 

process in this system is controlled by solvent reorientation [105]. 

3.3 

Triads and Other Complex Systems 

In dyads, including those discussed in the previous sections, photoinduced 

electron transfer is always followed by fast charge recombination. This greatly 

limits the use of dyads for practical purposes such as, e.g., conversion of light


into chemical energy. A strategy to overcome this problem, largely inspired 

by the architecture of natural photosynthetic reaction centers, is that of going 

to more complex supramolecular systems, triads, etc., in which a sequence of 

electron transfer steps is used to achieve long-range charge separation. The 

simplest of such system is a triad, as schematically illustrated in Fig. 9 for two 

possible reaction schemes. In Fig. 9a, the consecutive ET steps are from the 

excited chromophore, P, to an acceptor molecular component, A, and from 

a donor unit, D, to the oxidized chromophore. In Fig. 9b, the two consecutive 

electron transfer steps are from the excited chromophore, P, to a primary 

acceptor, A, and from the primary to a secondary acceptor unit, A ′ .These 

strategies have been extensively implemented using organic [106, 107] and, to 

a lesser extent, inorganic [108–110] molecular components. 

Fig. 9 Two types of triads for photoinduced charge separation. Molecular components: 

P (chromophore), D (donor), A (acceptor), A ′ (secondary acceptor). Electron transfer 

processes: cs (primary PET), cr (primary charge recombination), cs ′ (secondary charge 

separation), cr ′ (final charge recombination) 

A number of systems involving Rh(III) molecular components that behave 

in some respects as triads (or pseudo-triads) are discussed in this section. 

The trimetallic species 23 has been synthesized and studied by Petersen 

and coworkers [111] as a possible supramolecular system for photoinduced 

multi-step charge separation. This system comprises a Fe(II) electron donor, 

a Ru(II) photoexcitable chromophore, and a Rh(III) unit carrying a “monoquat” 

acceptor as ligand. Two different types of bridging ligand are present 

in 23, a bipyrimidine between Fe(II) and Ru(II) and a dipyridylpyrazine between 

Ru(II) and Rh(III). All the other combinations of bridging ligands be-


tween the three metal centers have been produced as well [111]. The ideal aim 

of the molecular design was to achieve complete photoinduced charge separation, 

with the positive hole on the iron donor and the electron on monoquat 

acceptor. Apparently, however, the redox properties of the molecular components 

were not quite ideal. Thus, the Ru(II)-based MLCT excited state is 

efficiently quenched. In the transient product obtained, however, the oxidized 

site is indeed the iron center but the reduced site seems to be a polypyridine 

ligand (in 23, either the bridging dipyridylpyrazine or the terpyridine ligand). 

In 23, this charge transfer state reverts to the ground state in 37 ns [111]. 

The simple chromophore-quencher system 24 also contains a quaternarized 

electron acceptor attached to a Rh(III) polypyridine unit. This dyad 

was designed [112] to study intramolecular charge shift processes, using 

a photochemical inter/intramolecular reaction scheme of the type shown in 

Eqs. 15–19. 

The dyad, schematized as Rh(III)-DQ, undergoes Rh(III)-localized photoexcitation 

(Eq. 15). The excited state is then involved in reductive quenching 

(Eq. 16) by a suitable electron donor, 1,3,5-trimethoxybenzene, indicated 

as TMB. The reduced dyad originates from the quenching process with the extra 

electron on the metal complex moiety, i.e., on the thermodynamically less


favored (by ca. 0.2 eV) site. Therefore, in competition with primary bimolecular 

charge recombination (Eq. 17), it is expected to undergo intramolecular 

electron transfer (charge shift process, Eq. 18) from the metal complex to DQ. 

The system will be finally converted back to ground-state reactants by secondary 

charge recombination (Eq. 19): 

Rh(III)-DQ + hν → ∗ Rh(III)-DQ (15) 

∗ + 

Rh(III)-DQ + TMB → Rh(II)-DQ + TMB (16) 

Rh(II)-DQ + TMB + → Rh(III)-DQ + TMB (17) 

Rh(II)-DQ → Rh(III)-DQ – 

(18) 

Rh(III)-DQ – +TMB + → Rh(III)-DQ + TMB . (19) 

The system performs indeed as predicted. The rate constants of all the processes 

in the above scheme have been experimentally determined, except for 

that of Eq. 17, inferred from experiments on appropriate model rhodium 

systems (without DQ pendant unit) [112]. In particular, the intramolecular 

charge shift process (Eq. 18) has been observed and time-resolved (k = 

3 × 107 s –1 ) by laser flash photolysis. It can be noticed that, from a formal 

viewpoint, the stepwise photoinduced electron transfer taking place in this 

dyad/quencher system is reminiscent of that of a triad for charge separation. 

A system that, despite the chemical and physical differences, bears a close 

similarity to charge separating triads, is the heterogeneous assembly depicted 

in Fig. 10 [113]. It is based on a Ru(II)-Rh(III) polypyridine dyad of the 

same type as 15, that has been functionalized with carboxyl groups at the 

Rh(III) units. This gives the dyad the capability to adsorb on nanocrystalline 

titanium dioxide and to act as a photosensitizer in Graetzel-type photoelec- 

Fig. 10 Schematic picture of a Rh(III)-Ru(II) dyad anchored on nanocrystalline TiO2 and 

of its behavior as a heterotriad system (from [113])


Fig. 11 Energy-level diagram and photophysical processes for the “heterotriad” of Fig. 10 

trochemical cells. When photocurrent action spectra are measured with this 

dyad sensitizer, it is seen that light absorption by the Ru(II) chromophore 

leads to electron injection into the semiconductor. Furthermore, a detailed 

analysis of the transient behavior of the system indicates that the dyad performs 

a stepwise charge injection process, i.e., intramolecular Ru → Rh 

electron transfer followed by electron injection from the Rh unit into the 

semiconductor (Fig. 10). The first process has comparable rates and efficiencies 

as for the free dyads in solution. The second step is 40% efficient, because 

of competing primary recombination (Fig. 11). When the final recombination 

between injected electrons and oxidized Ru(III) centers is studied, a remarkable 

slowing down is obtained relative to standard systems containing simple 

mononuclear sensitizers. Stepwise charge separation and slow recombination 

between remote sites are distinctive features of charge separating triads 

(Fig. 9b). Therefore, the system can be considered as a “heterotriad” with the 

TiO2 nanocrystal playing the role of the terminal electron acceptor [113]. 

3.4 

Photoinduced Electron Collection 

Central to the problem of light energy conversion into fuels (e.g., solar water 

splitting or light-driven carbon dioxide reduction) is the concept that the 

fuel-generating reactions are multielectron processes [3]. Progress in this 

field is therefore related to the design of systems capable of performing 

photoinduced electron collection [114]. A supramolecular system for photoinduced 

electron collection (PEC) can be constructed by coupling components 

capable of causing photoinduced electron transfer processes with compo-


Fig. 12 Block diagram representation of a photochemical molecular device for photoinduced 

electron collection and two-electron redox catalysis 

nents capable of storing electrons and using them in multielectron redox 

processes. A possible PEC scheme is shown in Fig. 12. 

In this scheme, P are electron transfer photosensitizers, C an electron store 

component, and BL rigid bridging ligands. D is a sacrificial electron donor, 

and A2 is a two-electron reduced product (e.g., H2 starting from 2H + ). The 

key molecular component C must have the ability to store two electrons following 

photoinduced electron transfer from P, and to deliver them to the 

substrate A + in a low-activation two-electron process that leads to the desired 

product A2. 

Very few homogeneous systems for photoinduced electron collection 

(PEC) have been reported by now [115–122]. Trimetallic complexes incorporating 

polyazine bridging ligands have been designed and studied by 

Brewer’s group for potential applications as PEC devices [123–126]. Most 

of these complexes have general formula [(bpy)2Ru(BL)]2MCl2 n+ with BL = 

2,3-bis(2-pyridyl)pyrazine (dpp), 2,3-bis(2-pyridyl)quinoxaline (dpq), or 2,3bis(2-pyridyl)benzoquinoxaline 

(dpb) and M = Ir(III) or Rh(III) [123, 125]. 

The first functioning PEC system, [(bpy)2Ru(dpb)]2IrCl2 5+ ,wasreported 

in 1994, employing π systems of polyazine bridging ligands to collect electrons 

[123]. Very recently, an analogous supramolecular trimetallic species 

has been reported where the central Ir-based moiety is replaced by a Rh(III) 

complex [127]. This new system, [(bpy)2Ru(dpp)]2RhCl2 5+ (25), was obtained 

coupling two Ru chromophoric units which play the role of P, through


polyazine bridges (BL), to a central Rh core (C). In the presence of dimethylaniline 

(DMA) as sacrificial electron donor (D), 25 undergoes two-electron 

photoreduction at the rhodium center, producing the stable Rh(I) form 

[(bpy)2Ru(dpp)]2Rh 5+ via loss of two chlorides. This result is demonstrated 

by the observation that the spectroscopic changes associated with the photochemical 

reduction are identical with those seen in the electrochemical reduction 

experiments. On the basis of quenching experiments in the presence 

of different concentrations of DMA, the authors suggest that the monoreduced 

Rh(II) species can be formed by two alternative pathways following 

excitation of the peripheral Ru(II)-base chromophores: i) photoinduced electron 

transfer from the excited Ru(II)-based units followed by regeneration of 

the Ru(II)-based chromophores by oxidation of the sacrificial donor or ii) bimolecular 

quenching of the excited Ru(II)-based units by the sacrificial donor 

followed by reduction of the central Rh(III). No clear indication is given as 

to the mechanism for the formation of the stable two-electron-reduced Rh(I) 

product from the Rh(II) intermediate. According to the authors, the ability 

of [(bpy)2Ru(dpp)]2RhCl2 5+ to undergo photoreduction at the rhodium 

center by multiple electrons and the fact that the photoreduced product 

[(bpy)2Ru(dpp)]2Rh 5+ is coordinatively unsaturated and thus available to interact 

with substrates are promising features in view of potential applications 

in light energy harvesting to produce fuels [127]. 

4 

Rhodium Complexes as DNA Intercalators 

4.1 

Specific Binding to DNA and Photocleavage 

The development and the study of transition metal complexes able to bind 

selectively to DNA sites, emulating the behavior of the DNA-binding proteins, 

ranks among the most fascinating and challenging issues in the field of 

current chemical and biological research [128–131]. This topic has been extensively 

investigated by Barton and coworkers, who devoted an impressive 

number of studies to the use of Rh(III) complexes with ligands containing 

nitrogen donors as DNA binding agents [129, 130]. Most of these studies center 

around complexes of the ligand 9,10-phenanthrenequinonediimine (phi) 

(26) [130, 132–138]. 

The phi Rh(III) complexes are indeed excellent DNA intercalators given 

the flat aromatic heterocyclic moiety of the phi ligand that deeply inserts and 

stacks in between the DNA base pairs (binding affinity constants range from 

10 6 –10 9 M –1 ) [132, 139, 140]. The photophysical properties of phi Rh(III) 

complexes [50] have been discussed in Sect. 2.1. When bound to DNA, upon 

photoactivation with UV light, they are able to promote DNA strand cleav-


age, thanks to the outstanding oxidant properties of their excited states. The 

photocleavage ability offers a strategy for the use of these rhodium complexes 

as DNA targets. The approach used by the Barton’s laboratory is the following: 

i) upon ultraviolet excitation, the excited state of DNA-bound rhodium 

complex promotes the scission of DNA sugar-phosphate backbone through 

oxidative degradation of the sugar moiety; ii) biochemical methods (e.g., gel 

electrophoresis) are used to determine where the strand scission occurred 

and therefore where, along the strand, the complex was bound. This method 

provides a powerful tool to mark specifically the sites of binding [129]. 

A variety of articles focused on DNA photocleavage by phi complexes containing 

different ligands in ancillary positions [130, 139–142]. The structural 

formula of the most extensively characterized complexes are reported below 

(27, 28, 29, 30). 

Irradiation with UV light of Rh(phen)2(phi) 3+ (28) andRh(bpy)(phi)2 3+ 

(30) intercalated in DNA leads to direct DNA strand scission with products


consistent with 3 ′ -hydrogen abstraction from the deoxyribose sugar adjacent 

to the binding site [140, 141]. The photochemistry of these phi complexes 

intercalated in DNA has also been studied as a function of irradiation 

wavelength [143, 144]. Interestingly, the results showed that light of different 

wavelengths induces selectively different chemical reactions. In particular, 

the irradiation of the DNA-bound Rh(phi)2(L) 3+ complexes with UV light 

(λ = 313 nm) leads, as discussed above, to direct scission of the DNA-sugar 

backbone [141, 144]. If instead the complexes are excited with low-energy 

light (λ ≥ 365 nm) oxidative damage to the DNA bases is observed. The mechanism 

of these two photoprocesses is not discussed in great detail [130, 143]. 

It is proposed, however, which direct DNA scission takes place by hydrogen 

abstraction from the sugar by the phi ligand radical of a ligand-to-metal 

charge transfer (LMCT) state [130]. On the other hand, the oxidative damage 

is attributed to the population of a powerful oxidizing excited state (ILCT [50] 

or LC [143]) with longer wavelength light. 

Photocleavage experiments have been used profitably for establishing how 

the phi complexes are associated to DNA [129, 130, 141]. Confirmation of 

site selectivity and greater structural definition were obtained later from 

high-resolution NMR studies [134–138, 145]. The important result is that 

all the phi complexes bind DNA noncovalently through intercalation in the 

major groove where the phi ligand is inserted between the base pairs so 

as to maximize stacking interactions. More recently a full crystal structure 

of ∆– Rh ((R,R)-Me2trien)2(phi) 3+ ((R,R)-Me2trien = 2R,9R-diamino-4,7diazadecane) 

bound to a DNA octamer provided a direct evidence of the


intercalation through the major groove [146]. A series of systematic NMR and 

photocleavage studies clearly showed that the binding of complexes containing 

different ancillary ligands occurs at a different specific DNA sequence. 

This site specificity results from both shape-selective steric interactions as 

well as stabilizing van der Waals and hydrogen bonding contacts. In particular, 

Rh(NH3)4phi 3+ and related amine complexes bind to d(TGGCCA)2 

duplex through hydrogen bonding between the ancillary amine ligands and 

DNA bases [130, 136]. Evidence for specific intercalation was found also for 

Rh(phen)2phi 3+ in the hexanucleotide d(GTCGAC)2 [134]. In this case Barton 

proposed that the site specificity was based upon shape-selection. Since 

thephenanthrolineligandsprovidestericbulkaboveandbelowtheplaneof 

the phi ligand, the stacking occurs at sites which are more open in the major 

grove. The most striking example of site-specific recognition by shape 

selection with bulky ancillary ligands was found for Rh(DPB)2phi 3+ (DPB 

=4,4 ′ -diphenylbpy) [140]. For all the complexes studied enantioselectivity 

favoring the intercalation of the ∆-isomer was observed [130, 147]. Further 

control of sequence specificity has been achieved by using derivatives of 

Rh(phen)(phi)2 3+ complexes where the nonintercalating phenanthroline ligand 

has been functionalized with pendant guanidinium group or with short 

oligopeptides [148, 149]. For metal-peptide complexes photocleavage experiments 

showed that the polypeptide chain is essential in directing the complex 

to a specific DNA sequence [149]. 

Among the rhodium intercalators explored as probes of DNA structure, 

Barton selected the Rh(bpy)2chrysi 3+ (chrysi = 5,6-chrysenequinone diimine, 

31) complex as an ideal candidate for mismatches recognition [150–152]. 

The specific recognition is based on the size of the intercalating ligand: the 

chrysene ring system is too large to intercalate in normal B-form DNA but it 

can do so at destabilized mismatch sites. The authors point out that sterically 

demanding intercalators such as Rh(bpy)2chrysi 3+ may have application both 

in mutation detection systems and as mismatch-specific chemotherapeutic 

agents. 

Recently mixed-metal trimetallic complexes have been designed and 

studied by Brewer to obtain supramolecular system capable of DNA photocleavage 

[153, 154]. These complexes of general formula [{(bpy)2M(dpp)}2 

RhCl2](PF6)5 with M = Ru(II) or Os(II) couple ruthenium or osmium chromophoric 

units to a central rhodium(III) core. When excited with visible light 

into their intense MLCT bands, these complexes exhibit DNA photocleavage


property. The authors discussed the role of the supramolecular architecture 

and in particular of the rhodium(III) unit on the photoreactivity 

4.2 

Rh(III) Complexes in DNA-Mediated Long-Range Electron Transfer 

DNA-mediated electron transfer has been an active and much debated topic 

of research [155]. Several articles have dealt with DNA-mediated photoinduced 

electron transfer reactions involving metal complexes as photoactive 

units [156]. In this context, of particular interest are the studies of Barton 

and coworkers that used rhodium(III) intercalator complexes not only as 

ground-state but also as excited-state electron acceptor in electron transfer 

(ET) reactions through the DNA [144]. 

4.2.1 

Rh(III) Complexes as Acceptors in Electron Transfer Reactions 

In 1993, Murphy et al. [157]. reported the surprising result that an efficient 

and rapid photoinduced electron transfer occurs over a large separation distance 

(> 40 ˚A) between DNA metallointercalators that are covalently tethered 

to opposite 5 ′ -ends of a 15-base pair DNA duplex (Fig. 13). 

In this oligomeric assembly Ru(phen)2dppz 2+ (dppz = dipyridophenazine) 

plays the role of excited electron donor and the Rh(phi)2(phen) 3+ is the electron 

acceptor. Both donor and acceptor bind to DNA with high affinities 

(> 10 6 M –1 ) by intercalation through the dppz [158, 159] and phi ligands, re- 

Fig. 13 A 15-base pair DNA duplex carrying covalently tethered Ru(II) and Rh(III) intercalators


spectively. A clear advantage of the tethered Ru/DNA/Rh system is that both 

the donor and acceptor are covalently held in a well-defined fixed distance 

range. On the other hand, the report of Murphy et al. was limited by the 

exclusive use of steady-state emission spectroscopy. A lower limit for the photoinduced 

electron transfer rate (> 3 × 10 9 s –1 ) has been obtained measuring 

the quenching of the Ru(II) metal-to-ligand charge transfer (MLCT) emission 

by the tethered Rh(III) acceptor. 

In a subsequent investigation, untethered Ru/DNA/Rh and related systems 

were studied by Barton et al. [160] using ultrafast laser spectroscopy. The 

study was focused mainly on the system shown in Fig. 14 constituted by ∆- 

Ru(phen)2dppz 2+ as excited donor, ∆-Rh(phi)2bpy 3+ as acceptor intercalated 

in the calf thymus DNA with the aim to determine the rate of excited-state 

electron transfer (ket) that occurs from the lowest-lying MLCT state of the Ru 

donor, and the recombination electron transfer reaction (krec). 

Fig. 14 Photoinduced electron transfer processes taking place between Ru(phen)2dppz 2+ 

and Rh(phi)2bpy 3+ DNA intercalators 

Efficient and rapid quenching of luminescence of the Ru complex in the 

presence of Rh complex, even at surprisingly low acceptor loading on the 

DNA duplex was observed. All the experimental observations were consistent 

with complete intercalation of the donor and acceptor in DNA. A comparative 

experiment employing Ru(NH3) 3+ 

6 complex as electron acceptor, clearly 

indicates that much less efficient quenching is observed when the quencher 

is groove bound rather intercalated. To deepen the understanding of the 

mechanism of the electron transfer processes, the authors examined the photoinduced 

charge separation (ket) and recombination electron transfer (krec) 

reactions on the picosecond time scale by monitoring both the kinetics of 

the emission decay and the kinetics of the recovery of ground state absorption 

of Ru(II) donor (Fig. 14). Time-correlated single photon counting failed 

to detect the lifetime of the excited state, clearly indicating that luminescent 

quenching by electron transfer proceeds faster (ket > 3 × 10 10 s –1 ) than the 

time resolution of the instrument (ca. 50 ps). Ultrafast transient absorption 

measurements, on the other hand, revealed bleaching of the MLCT band of 

the Ru(II) complex in a picosecond time scale, assigned by the authors to the


Table 1 Rate constants for charge recombination following electron transfer from DNAbound, 

photoexcited donors to ∆-Rh(phi)2(bpy) 3+ a 

Donor DNA krec [10 9 s –1 ] –∆G ◦ [V] 

∆-Ru(phen)2(dppz) 2+ Calf thymus 9.2 1.66 

rac-Ru(bpy)2(dppz) 2+ Calf thymus 7.1 1.69 

∆-Ru(dmp)2(dppz) 2+ Calf thymus 11 1.59 

∆-Ru(phen)2(F2dppz) 2+ Calf thymus 7.7 1.68 

rac-Ru(phen)2(Me2dppz) 2+ Calf thymus 9.2 1.67 

∆-Os(phen)2(dppz) 2+ Calf thymus 11 1.21 

Λ-Ru(phen)2(dppz) 2+ Calf thymus 4.5 

∆-Ru(phen)2(dppz) 2+ Poly-d(AT) 7.4 

∆-Ru(phen)2(dppz) 2+ Poly-d(GC) 0.21 

a Based on [144] 

presence of Ru(III) oxidized donor. The rate constants for charge recombination 

process (krec) were obtained from the decay of this signal. The data 

for seven donor–acceptor pairs are given in Table 1. Within this series, the 

driving force (∆G ◦ ) is comparable but the donors vary with respect to intercalating 

ligand, ancillary ligands, chirality, and metal center. Despite such 

a range of chemical properties, the rate observed is centered around 10 10 s –1 . 

A significant difference in rate is observed, however, when the absolute 

configuration of the donor is varied. For the right-handed ∆-Ru(phen)2dppz 2+ 

the value is 2.5 times higher with respect the left-handed enantiomer indicating 

a deeper stacking of this complex into the double helix. This result, 

according to the authors, clearly suggests that the electron transfer process 

required the intervening aromatic base pairs. The notion of highly efficient 

ET through the stack of DNA base is also strongly supported by the finding 

that the largest change in electron transfer rate is observed when the sequence 

of the DNA bridge is changed: for the same donor and acceptor reactants the 

rate with poly d(AT) is 30 times higher than with poly d(GC). This is an importantresultthatindicatesthattheπ-stacked 

bases of the DNA provide an 

effective pathway for electron transfer reactions. However, the crucial point of 

this study that involves an untethered Ru/DNA/Rh system concerns the distances 

over which fast ET occurs. The question is: do the donor and acceptor 

complexes contact each other, or does electron transfer occur at long range? 

Two models were considered by the authors to interpret the experimental results: 

i) a cooperative binding model with ET over short D–A distance and 

ii) a random binding model with ET over long distances. On the basis of 

DNA photocleavage experiments, the first hypothesis was reject in favor of 

a rapid long range ET with a shallow distance dependence [160]. On the other 

hand, soon thereafter, Barbara [161] reinterpreted the experimental results on 

a quantitative basis using computational simulation procedures and demon-


strated the failure of long-distance electron transfer model to account for the 

data. Concurrently, Tuite and coworkers [162] arrived at a similar conclusion 

for the ET quenching of Ru(phen)2dppz 2+ emission in a very similar untethered 

DNA/metallointercalator system. In summary, considerable controversy 

persists in the estimates of the distances over which fast ET may occur in such 

type of untethered systems [144]. 

4.2.2 

Long Range Oxidative DNA Damage by Excited Rh(III) Complexes 

It is well known from a large variety of experimental studies and calculations 

that guanine (G) is the most easily oxidized of the nucleic acid 

bases [144, 163, 164]. Barton and coworkers have extensively exploited the 

ability of Rh(III)-phi complexes to induce oxidative damage specifically at 

the 5 ′ -G of the 5 ′ -GG-3 ′ doublets, when irradiated with low-energy light. 

A first investigation was carried out using a 15-base duplex (Rh-DNA) which 

possesses an end-tethered Rh(phi)2bpy 3+ complex in one strand and two 

5 ′ -GG-3 ′ sites in the complementary strand. (Fig. 15). The peculiarity of 

this Rh-DNA assembly is that the rhodium complex is spatially separated 

in a well-defined manner from the potential sites of oxidation. Damage to 

DNA was demonstrated to occurred as a result of excitation of the intercalated 

rhodium complex, followed by long-range (30–40 ˚A) electron transfer 

through the DNA base pair stack [165]. The strategy used to analyze the 

mechanism is illustrated in Fig. 16. 

Fig. 15 A15-base duplex with an end-tethered Rh(phi)2bpy 3+ complex in one strand and 

two 5 ′ -GG-3 ′ sites in the complementary strand [165] 

The Rh-DNA assembly was first irradiated at 313 nm to induce direct 

strand cleavage. This photocleavage step marks the site of intercalation, and 

permits determination of the distance separating the rhodium complex from 

potential sites of damage. Rh-DNA samples were then irradiated with low 

energy light at 365 nm, treated with hot piperidine, which promotes strand 

cleavage at the damaged sites, and examined by gel electrophoresis. This 

treatment reveals the position and yield of damage. The results clearly in-


Fig. 16 Use of the tethered Rh(III) complex to (i) mark the site of intercalation by direct 

strand cleavage (313-nm irradiation) and (ii) promote damage via long-range electron 

transfer (365-nm irradiation) [165] 

dicated that both the proximal and distal 5 ′ -GG-3 ′ doublets were equally 

damaged and the reaction was intraduplex. Two possible mechanisms for this 

process were discussed: i) concerted long-range electron transfer; and ii) oxidation 

of a base near the intercalated Rh acceptor followed by hole migration 

to the two GG sites. Sensitivity of the reaction to the intervening base pair 

stack was also observed. In subsequent studies, oxidation has been reported 

at sites that are up to 200 ˚A away from the site of intercalation of the photoactive 

rhodium complex [166]. 

The photooxidant properties of the phi rhodium (III) complexes have also 

been used to repair thymine dimers [167, 168], the most common photo- 

Fig. 17 DNA duplex containing a thymine dimer with tethered Rh(III) complex for photoinduced 

repair studies [167, 168]


chemical lesion in DNA. Investigations of photoinitiated repair of duplexes 

containing a single thymine dimer lesion were carried out with visible light 

(400 nm) using both nontethered and tethered complexes (Fig. 17). 

The quantum yield for photorepair with a Rh(III)-tethered complex is substantially 

(about ca. 30 fold) reduced compared to the noncovalently bound 

complex. Since the repair efficiency does not appear to be very sensitive to the 

distance between intercalated rhodium complex and the thymine dimer, the 

authors suggest that the observed disparity likely results from differences in 

π-stacking. In addition, evidences that the repair efficiency diminished with 

disruption of the intervening π-stack confirm that the DNA helix mediates 

this long-range oxidative repair reaction. 

5 

Conclusion 

A large number of rhodium(III) polypyridine complexes and their cyclometalated 

analogues have been investigated from the viewpoint of photochemistry, 

photophysics and of their possible applications. 

As mononuclear species, Rh(III) polypyridine complexes display interesting 

photophysical properties, with lowest excited states of LC type for tris 

bis-chelated species, and increasing role of MC states for mixed-ligand halopolypyridine 

species. In Rh(III) cyclometalated complexes, the covalent character 

of the C – Rh bonds makes the excited state classification less clearcut, 

with strong mixing of LC, MLCT, and LLCT character. 

Many polynuclear and supramolecular systems containing Rh(III) polypyridine 

and related units have been synthesized and studied, taking advantage 

of the favorable properties of these units as good electron acceptors and 

strong photo-oxidants. In particular, Ru(II)-Rh(IIII) dyads have been actively 

investigated for the study of photoinduced electron transfer, with specific 

interest in driving force, distance, and bridging ligand effects. A limited number 

of supramolecular systems of higher nuclearity have also been produced. 

Among these, of particular interest are trinuclear species containing rhodium 

dihalo polypyridine units, which can act as two-electron storage components 

thanks to their Rh(III)/Rh(I) redox behavior. 

Finally, a large amount of work has been devoted to the use of Rh(III) 

polypyridine complexes as intercalators for DNA. In this role, they have 

shown a very versatile behavior, being used for direct strand photocleavage 

marking the site of intercalation, to induce long-distance photochemical 

damage or dimer repair, or to act as electron acceptors in long-range electron 

transfer processes. 

Acknowledgements Financial support from MUR (PRIN 2006) is gratefully acknowledged.


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Author Index Volumes 251–280 

Author Index Vols. 26–50 see Vol. 50 





Thevolumenumbersareprintedinitalics 

Accorsi G, see Armaroli N (2007) 280: 69–115 

Ajayaghosh A, George SJ, Schenning APHJ (2005) Hydrogen-Bonded Assemblies of Dyes 

and Extended π-Conjugated Systems. 258: 83–118 

Akai S, Kita Y (2007) Recent Advances in Pummerer Reactions. 274: 35–76 

Albert M, Fensterbank L, Lacôte E, Malacria M (2006) Tandem Radical Reactions. 264:1–62 

Alberto R (2005) New Organometallic Technetium Complexes for Radiopharmaceutical 

Imaging. 252:1–44 

Alegret S, see Pividori MI (2005) 260:1–36 

Alfaro JA, see Schuman B (2007) 272: 217–258 

Amabilino DB, Veciana J (2006) Supramolecular Chiral Functional Materials. 265: 253–302 

Anderson CJ, see Li WP (2005) 252: 179–192 

Anslyn EV, see Collins BE (2007) 277: 181–218 

Anslyn EV, see Houk RJT (2005) 255: 199–229 

Appukkuttan P, Van der Eycken E (2006) Microwave-Assisted Natural Product Chemistry. 

266: 1–47 

Araki K, Yoshikawa I (2005) Nucleobase-Containing Gelators. 256: 133–165 

Armaroli N, Accorsi G, Cardinali Fç, Listorti A (2007) Photochemistry and Photophysics of 

Coordination Compounds: Copper. 280: 69–115 

Armitage BA (2005) Cyanine Dye–DNA Interactions: Intercalation, Groove Binding and 

Aggregation. 253: 55–76 

Arya DP (2005) Aminoglycoside–Nucleic Acid Interactions: The Case for Neomycin. 253: 

149–178 

Bailly C, see Dias N (2005) 253: 89–108 

Balaban TS, Tamiaki H, Holzwarth AR (2005) Chlorins Programmed for Self-Assembly. 258: 

1–38 

Baltzer L (2007) Polypeptide Conjugate Binders for Protein Recognition. 277: 89–106 

Balzani V, Bergamini G, Campagna S, Puntoriero F (2007) Photochemistry and Photophysics 

of Coordination Compounds: Overview and General Concepts. 280:1–36 

Balzani V, Credi A, Ferrer B, Silvi S, Venturi M (2005) Artificial Molecular Motors and 

Machines: Design Principles and Prototype Systems. 262:1–27 

Balzani V, see Campagna S (2007) 280: 117–214 

Barbieri CM, see Pilch DS (2005) 253: 179–204 

Barchuk A, see Daasbjerg K (2006) 263: 39–70 

Bargon J, see Kuhn LT (2007) 276: 25–68 

Bargon J, see Kuhn LT (2007) 276: 125–154 

Bayly SR, see Beer PD (2005) 255: 125–162

258 Author Index Volumes 251–280 

Beck-Sickinger AG, see Haack M (2007) 278: 243–288 

Beer PD, Bayly SR (2005) Anion Sensing by Metal-Based Receptors. 255: 125–162 

Bergamini G, see Balzani V (2007) 280:1–36 

Bergamini G, see Campagna S (2007) 280: 117–214 

Bertini L, Bruschi M, de Gioia L, Fantucci P, Greco C, Zampella G (2007) Quantum Chemical 

Investigations of Reaction Paths of Metalloenzymes and Biomimetic Models – The 

Hydrogenase Example. 268:1–46 

Bier FF, see Heise C (2005) 261:1–25 

Blum LJ, see Marquette CA (2005) 261: 113–129 

Boiteau L, see Pascal R (2005) 259: 69–122 

Bolhuis PG, see Dellago C (2007) 268: 291–317 

Borovkov VV, Inoue Y (2006) Supramolecular Chirogenesis in Host–Guest Systems Containing 

Porphyrinoids. 265: 89–146 

Boschi A, Duatti A, Uccelli L (2005) Development of Technetium-99m and Rhenium-188 Radiopharmaceuticals 

Containing a Terminal Metal–Nitrido Multiple Bond for Diagnosis 

and Therapy. 252: 85–115 

Braga D, D’Addario D, Giaffreda SL, Maini L, Polito M, Grepioni F (2005) Intra-Solid and 

Inter-Solid Reactions of Molecular Crystals: a Green Route to Crystal Engineering. 254: 

71–94 

Bräse S, see Jung N (2007) 278:1–88 

Braverman S, Cherkinsky M (2007) [2,3]Sigmatropic Rearrangements of Propargylic and 

Allenic Systems. 275: 67–101 

Brebion F, see Crich D (2006) 263:1–38 

Breinbauer R, see Mentel M (2007) 278: 209–241 

Breit B (2007) Recent Advances in Alkene Hydroformylation. 279: 139–172 

Brizard A, Oda R, Huc I (2005) Chirality Effects in Self-assembled Fibrillar Networks. 256: 

167–218 

Broene RD (2007) Reductive Coupling of Unactivated Alkenes and Alkynes. 279: 209–248 

Bromfield K, see Ljungdahl N (2007) 278: 89–134 

Bruce IJ, see del Campo A (2005) 260: 77–111 

Bruschi M, see Bertini L (2007) 268:1–46 

Bur SK (2007) 1,3-Sulfur Shifts: Mechanism and Synthetic Utility. 274: 125–171 

Campagna S, Puntoriero F, Nastasi F, Bergamini G, Balzani V (2007) Photochemistry and 

Photophysics of Coordination Compounds: Ruthenium. 280: 117–214 

Campagna S, see Balzani V (2007) 280:1–36 

del Campo A, Bruce IJ (2005) Substrate Patterning and Activation Strategies for DNA Chip 

Fabrication. 260: 77–111 

Cardinali F, see Armaroli N (2007) 280: 69–115 

Carney CK, Harry SR, Sewell SL, Wright DW (2007) Detoxification Biominerals. 270: 155–185 

Castagner B, Seeberger PH (2007) Automated Solid Phase Oligosaccharide Synthesis. 278: 

289–309 

Chaires JB (2005) Structural Selectivity of Drug-Nucleic Acid Interactions Probed by Competition 

Dialysis. 253: 33–53 

Cherkinsky M, see Braverman S (2007) 275: 67–101 

Chiorboli C, Indelli MT, Scandola F (2005) Photoinduced Electron/Energy Transfer Across 

Molecular Bridges in Binuclear Metal Complexes. 257: 63–102 

Chiorboli C, see Indelli MT (2007) 280: 215–255 

Coleman AW, Perret F, Moussa A, Dupin M, Guo Y, Perron H (2007) Calix[n]arenes as 

Protein Sensors. 277: 31–88

Author Index Volumes 251–280 259 

Cölfen H (2007) Bio-inspired Mineralization Using Hydrophilic Polymers. 271:1–77 

Collin J-P, Heitz V, Sauvage J-P (2005) Transition-Metal-Complexed Catenanes and Rotaxanes 

in Motion: Towards Molecular Machines. 262: 29–62 

Collins BE, Wright AT, Anslyn EV (2007) Combining Molecular Recognition, Optical Detection, 

and Chemometric Analysis. 277: 181–218 

Collyer SD, see Davis F (2005) 255: 97–124 

Commeyras A, see Pascal R (2005) 259: 69–122 

Coquerel G (2007) Preferential Crystallization. 269:1–51 

Correia JDG, see Santos I (2005) 252: 45–84 

Costanzo G, see Saladino R (2005) 259: 29–68 

Cotarca L, see Zonta C (2007) 275: 131–161 

Credi A, see Balzani V (2005) 262:1–27 

Crestini C, see Saladino R (2005) 259: 29–68 

Crich D, Brebion F, Suk D-H (2006) Generation of Alkene Radical Cations by Heterolysis 

of β-Substituted Radicals: Mechanism, Stereochemistry, and Applications in Synthesis. 

263:1–38 

Cuerva JM, Justicia J, Oller-López JL, Oltra JE (2006) Cp2TiCl in Natural Product Synthesis. 

264: 63–92 

DaasbjergK,SvithH,GrimmeS,GerenkampM,Mück-LichtenfeldC,GansäuerA,Barchuk 

A (2006) The Mechanism of Epoxide Opening through Electron Transfer: Experiment 

and Theory in Concert. 263: 39–70 

D’Addario D, see Braga D (2005) 254: 71–94 

Danishefsky SJ, see Warren JD (2007) 267: 109–141 

Darmency V, Renaud P (2006) Tin-Free Radical Reactions Mediated by Organoboron Compounds. 

263: 71–106 

Davis F, Collyer SD, Higson SPJ (2005) The Construction and Operation of Anion Sensors: 

Current Status and Future Perspectives. 255: 97–124 

Deamer DW, Dworkin JP (2005) Chemistry and Physics of Primitive Membranes. 259:1–27 

Debaene F, see Winssinger N (2007) 278: 311–342 

Dellago C, Bolhuis PG (2007) Transition Path Sampling Simulations of Biological Systems. 

268: 291–317 

Deng J-Y, see Zhang X-E (2005) 261: 169–190 

Dervan PB, Poulin-Kerstien AT, Fechter EJ, Edelson BS (2005) Regulation of Gene Expression 

by Synthetic DNA-Binding Ligands. 253:1–31 

Dias N, Vezin H, Lansiaux A, Bailly C (2005) Topoisomerase Inhibitors of Marine Origin 

and Their Potential Use as Anticancer Agents. 253: 89–108 

DiMauro E, see Saladino R (2005) 259: 29–68 

Dittrich M, Yu J, Schulten K (2007) PcrA Helicase, a Molecular Motor Studied from the 

Electronic to the Functional Level. 268: 319–347 

Dobrawa R, see You C-C (2005) 258: 39–82 

Du Q, Larsson O, Swerdlow H, Liang Z (2005) DNA Immobilization: Silanized Nucleic Acids 

and Nanoprinting. 261: 45–61 

Duatti A, see Boschi A (2005) 252: 85–115 

Dupin M, see Coleman AW (2007) 277: 31–88 

Dworkin JP, see Deamer DW (2005) 259:1–27 

Edelson BS, see Dervan PB (2005) 253:1–31 

Edwards DS, see Liu S (2005) 252: 193–216 

Ernst K-H (2006) Supramolecular Surface Chirality. 265: 209–252


Ersmark K, see Wannberg J (2006) 266: 167–197 

Escudé C, Sun J-S (2005) DNA Major Groove Binders: Triple Helix-Forming Oligonucleotides, 

Triple Helix-Specific DNA Ligands and Cleaving Agents. 253: 109–148 

Evans SV, see Schuman B (2007) 272: 217–258 

Van der Eycken E, see Appukkuttan P (2006) 266:1–47 

Fages F, Vögtle F, ˇZinić M (2005) Systematic Design of Amide- and Urea-Type Gelators with 

Tailored Properties. 256: 77–131 

Fages F, see Žinić M (2005) 256: 39–76 

Faigl F, Schindler J, Fogassy E (2007) Advantages of Structural Similarities of the Reactants 

in Optical Resolution Processes. 269: 133–157 

Fan C-A, see Gansäuer A (2007) 279: 25–52 

Fantucci P, see Bertini L (2007) 268:1–46 

Fechter EJ, see Dervan PB (2005) 253:1–31 

Fensterbank L, see Albert M (2006) 264:1–62 

Fernández JM, see Moonen NNP (2005) 262: 99–132 

Fernando C, see Szathmáry E (2005) 259: 167–211 

Ferrer B, see Balzani V (2005) 262:1–27 

De Feyter S, De Schryver F (2005) Two-Dimensional Dye Assemblies on Surfaces Studied 

by Scanning Tunneling Microscopy. 258: 205–255 

Fischer D, Geyer A (2007) NMR Analysis of Bioprotective Sugars: Sucrose and Oligomeric 

(1→2)-α-d-glucopyranosyl-(1→2)-β-d-fructofuranosides. 272: 169–186 

Flood AH, see Moonen NNP (2005) 262: 99–132 

Fogassy E, see Faigl F (2007) 269: 133–157 

Fricke M, Volkmer D (2007) Crystallization of Calcium Carbonate Beneath Insoluble Monolayers: 

Suitable Models of Mineral–Matrix Interactions in Biomineralization? 270:1–41 

Fujimoto D, see Tamura R (2007) 269: 53–82 

Fujiwara S-i, Kambe N (2005) Thio-, Seleno-, and Telluro-Carboxylic Acid Esters. 251: 87– 

140 

Gansäuer A, see Daasbjerg K (2006) 263: 39–70 

Garcia-Garibay MA, see Karlen SD (2005) 262: 179–227 

Gelinck GH, see Grozema FC (2005) 257: 135–164 

Geng X, see Warren JD (2007) 267: 109–141 

Gansäuer A, Justicia J, Fan C-A, Worgull D, Piestert F (2007) Reductive C–C Bond Formation 

after Epoxide Opening via Electron Transfer. 279: 25–52 

George SJ, see Ajayaghosh A (2005) 258: 83–118 

Gerenkamp M, see Daasbjerg K (2006) 263: 39–70 

Gevorgyan V, see Sromek AW (2007) 274: 77–124 

Geyer A, see Fischer D (2007) 272: 169–186 

Giaffreda SL, see Braga D (2005) 254: 71–94 

Giernoth R (2007) Homogeneous Catalysis in Ionic Liquids. 276:1–23 

de Gioia L, see Bertini L (2007) 268:1–46 

Di Giusto DA, King GC (2005) Special-Purpose Modifications and Immobilized Functional 

Nucleic Acids for Biomolecular Interactions. 261: 131–168 

Greco C, see Bertini L (2007) 268:1–46 

Greiner L, Laue S, Wöltinger J, Liese A (2007) Continuous Asymmetric Hydrogenation. 276: 

111–124 

Grepioni F, see Braga D (2005) 254: 71–94 

Grimme S, see Daasbjerg K (2006) 263: 39–70


Grozema FC, Siebbeles LDA, Gelinck GH, Warman JM (2005) The Opto-Electronic Properties 

of Isolated Phenylenevinylene Molecular Wires. 257: 135–164 

Guiseppi-Elie A, Lingerfelt L (2005) Impedimetric Detection of DNA Hybridization: Towards 

Near-Patient DNA Diagnostics. 260: 161–186 

Guo Y, see Coleman AW (2007) 277: 31–88 

Haack M, Beck-Sickinger AG (2007) Multiple Peptide Synthesis to Identify Bioactive Hormone 

Structures. 278: 243–288 

Haase C, Seitz O (2007) Chemical Synthesis of Glycopeptides. 267:1–36 

Hahn F, Schepers U (2007) Solid Phase Chemistry for the Directed Synthesis of Biologically 

Active Polyamine Analogs, Derivatives, and Conjugates. 278: 135–208 

Hansen SG, Skrydstrup T (2006) Modification of Amino Acids, Peptides, and Carbohydrates 

through Radical Chemistry. 264: 135–162 

Harmer NJ (2007) The Fibroblast Growth Factor (FGF) – FGF Receptor Complex: Progress 

Towards the Physiological State. 272: 83–116 

Harry SR, see Carney CK (2007) 270: 155–185 

Heise C, Bier FF (2005) Immobilization of DNA on Microarrays. 261:1–25 

Heitz V, see Collin J-P (2005) 262: 29–62 

Herrmann C, Reiher M (2007) First-Principles Approach to Vibrational Spectroscopy of 

Biomolecules. 268: 85–132 

Higson SPJ, see Davis F (2005) 255: 97–124 

Hirao T (2007) Catalytic Reductive Coupling of Carbonyl Compounds – The Pinacol Coupling 

Reaction and Beyond. 279: 53–75 

Hirayama N, see Sakai K (2007) 269: 233–271 

Hirst AR, Smith DK (2005) Dendritic Gelators. 256: 237–273 

Holzwarth AR, see Balaban TS (2005) 258:1–38 

Homans SW (2007) Dynamics and Thermodynamics of Ligand–Protein Interactions. 272: 

51–82 

Houk RJT, Tobey SL, Anslyn EV (2005) Abiotic Guanidinium Receptors for Anion Molecular 

Recognition and Sensing. 255: 199–229 

Huc I, see Brizard A (2005) 256: 167–218 

Ihmels H, Otto D (2005) Intercalation of Organic Dye Molecules into Double-Stranded DNA 

– General Principles and Recent Developments. 258: 161–204 

Iida H, Krische MJ (2007) Catalytic Reductive Coupling of Alkenes and Alkynes to Carbonyl 

Compounds and Imines Mediated by Hydrogen. 279: 77–104 

Imai H (2007) Self-Organized Formation of Hierarchical Structures. 270: 43–72 

Indelli MT, Chiorboli C, Scandola F (2007) Photochemistry and Photophysics of Coordination 

Compounds: Rhodium. 280: 215–255 

Indelli MT, see Chiorboli C (2005) 257: 63–102 

Inoue Y, see Borovkov VV (2006) 265: 89–146 

Ishii A, Nakayama J (2005) Carbodithioic Acid Esters. 251: 181–225 

Ishii A, Nakayama J (2005) Carboselenothioic and Carbodiselenoic Acid Derivatives and 

Related Compounds. 251: 227–246 

Ishi-i T, Shinkai S (2005) Dye-Based Organogels: Stimuli-Responsive Soft Materials Based 

on One-Dimensional Self-Assembling Aromatic Dyes. 258: 119–160 

James DK, Tour JM (2005) Molecular Wires. 257: 33–62 

James TD (2007) Saccharide-Selective Boronic Acid Based Photoinduced Electron Transfer 

(PET) Fluorescent Sensors. 277: 107–152


Jelinek R, Kolusheva S (2007) Biomolecular Sensing with Colorimetric Vesicles. 277: 155–180 

Jones W, see Trask AV (2005) 254: 41–70 

Jung N, Wiehn M, Bräse S (2007) Multifunctional Linkers for Combinatorial Solid Phase 

Synthesis. 278:1–88 

Justicia J, see Cuerva JM (2006) 264: 63–92 

Justicia J, see Gansäuer A (2007) 279: 25–52 

Kambe N, see Fujiwara S-i (2005) 251: 87–140 

Kane-Maguire NAP (2007) Photochemistry and Photophysics of Coordination Compounds: 

Chromium. 280: 37–67 

Kann N, see Ljungdahl N (2007) 278: 89–134 

Kano N, Kawashima T (2005) Dithiocarboxylic Acid Salts of Group 1–17 Elements (Except 

for Carbon). 251: 141–180 

Kappe CO, see Kremsner JM (2006) 266: 233–278 

Kaptein B, see Kellogg RM (2007) 269: 159–197 

Karlen SD, Garcia-Garibay MA (2005) Amphidynamic Crystals: Structural Blueprints for 

Molecular Machines. 262: 179–227 

Kato S, Niyomura O (2005) Group 1–17 Element (Except Carbon) Derivatives of Thio-, 

Seleno- and Telluro-Carboxylic Acids. 251: 19–85 

Kato S, see Niyomura O (2005) 251:1–12 

Kato T, Mizoshita N, Moriyama M, Kitamura T (2005) Gelation of Liquid Crystals with 

Self-Assembled Fibers. 256: 219–236 

Kaul M, see Pilch DS (2005) 253: 179–204 

Kaupp G (2005) Organic Solid-State Reactions with 100% Yield. 254: 95–183 

Kawasaki T, see Okahata Y (2005) 260: 57–75 

Kawashima T, see Kano N (2005) 251: 141–180 

Kay ER, Leigh DA (2005) Hydrogen Bond-Assembled Synthetic Molecular Motors and 

Machines. 262: 133–177 

Kellogg RM, Kaptein B, Vries TR (2007) Dutch Resolution of Racemates and the Roles of 

Solid Solution Formation and Nucleation Inhibition. 269: 159–197 

Kessler H, see Weide T (2007) 272:1–50 

Kimura M, Tamaru Y (2007) Nickel-Catalyzed Reductive Coupling of Dienes and Carbonyl 

Compounds. 279: 173–207 

King GC, see Di Giusto DA (2005) 261: 131–168 

Kirchner B, see Thar J (2007) 268: 133–171 

Kita Y, see Akai S (2007) 274: 35–76 

Kitamura T, see Kato T (2005) 256: 219–236 

Kniep R, Simon P (2007) Fluorapatite-Gelatine-Nanocomposites: Self-Organized Morphogenesis, 

Real Structure and Relations to Natural Hard Materials. 270: 73–125 

Koenig BW (2007) Residual Dipolar Couplings Report on the Active Conformation of 

Rhodopsin-Bound Protein Fragments. 272: 187–216 

Kolusheva S, see Jelinek R (2007) 277: 155–180 

Komatsu K (2005) The Mechanochemical Solid-State Reaction of Fullerenes. 254: 185–206 

Kremsner JM, Stadler A, Kappe CO (2006) The Scale-Up of Microwave-Assisted Organic 

Synthesis. 266: 233–278 

Kriegisch V, Lambert C (2005) Self-Assembled Monolayers of Chromophores on Gold Surfaces. 

258: 257–313 

Krische MJ, see Iida H (2007) 279: 77–104 

Kuhn LT, Bargon J (2007) Transfer of Parahydrogen-Induced Hyperpolarization to Heteronuclei. 

276: 25–68


Kuhn LT, Bargon J (2007) Exploiting Nuclear Spin Polarization to Investigate Free Radical 

Reactions via in situ NMR. 276: 125–154 

Lacôte E, see Albert M (2006) 264:1–62 

Lahav M, see Weissbuch I (2005) 259: 123–165 

Lambert C, see Kriegisch V (2005) 258: 257–313 

Lansiaux A, see Dias N (2005) 253: 89–108 

LaPlante SR (2007) Exploiting Ligand and Receptor Adaptability in Rational Drug Design 

Using Dynamics and Structure-Based Strategies. 272: 259–296 

Larhed M, see Nilsson P (2006) 266: 103–144 

Larhed M, see Wannberg J (2006) 266: 167–197 

Larsson O, see Du Q (2005) 261: 45–61 

Laue S, see Greiner L (2007) 276: 111–124 

Leigh DA, Pérez EM (2006) Dynamic Chirality: Molecular Shuttles and Motors. 265: 185–208 

Leigh DA, see Kay ER (2005) 262: 133–177 

Leiserowitz L, see Weissbuch I (2005) 259: 123–165 

Lhoták P (2005) Anion Receptors Based on Calixarenes. 255: 65–95 

Li WP, Meyer LA, Anderson CJ (2005) Radiopharmaceuticals for Positron Emission Tomography 

Imaging of Somatostatin Receptor Positive Tumors. 252: 179–192 

Liang Z, see Du Q (2005) 261: 45–61 

Liese A, see Greiner L (2007) 276: 111–124 

Lingerfelt L, see Guiseppi-Elie A (2005) 260: 161–186 

Listorti A, see Armaroli N (2007) 280: 69–115 

Litvinchuk S, see Matile S (2007) 277: 219–250 

Liu S (2005) 6-Hydrazinonicotinamide Derivatives as Bifunctional Coupling Agents for 

99m Tc-Labeling of Small Biomolecules. 252: 117–153 

Liu S, Robinson SP, Edwards DS (2005) Radiolabeled Integrin αvβ3 Antagonists as Radiopharmaceuticals 

for Tumor Radiotherapy. 252: 193–216 

Liu XY (2005) Gelation with Small Molecules: from Formation Mechanism to Nanostructure 

Architecture. 256:1–37 

Ljungdahl N, Bromfield K, Kann N (2007) Solid Phase Organometallic Chemistry. 278: 

89–134 

De Lucchi O, see Zonta C (2007) 275: 131–161 

Luderer F, Walschus U (2005) Immobilization of Oligonucleotides for Biochemical Sensing 

by Self-Assembled Monolayers: Thiol-Organic Bonding on Gold and Silanization on 

Silica Surfaces. 260: 37–56 

Maeda K, Yashima E (2006) Dynamic Helical Structures: Detection and Amplification of 

Chirality. 265: 47–88 

Magnera TF, Michl J (2005) Altitudinal Surface-Mounted Molecular Rotors. 262: 63–97 

Maini L, see Braga D (2005) 254: 71–94 

Malacria M, see Albert M (2006) 264:1–62 

Marquette CA, Blum LJ (2005) Beads Arraying and Beads Used in DNA Chips. 261: 113–129 

Mascini M, see Palchetti I (2005) 261: 27–43 

Matile S, Tanaka H, Litvinchuk S (2007) Analyte Sensing Across Membranes with Artificial 

Pores. 277: 219–250 

Matsumoto A (2005) Reactions of 1,3-Diene Compounds in the Crystalline State. 254: 263– 

305 

McGhee AM, Procter DJ (2006) Radical Chemistry on Solid Support. 264: 93–134


Mentel M, Breinbauer R (2007) Combinatorial Solid-Phase Natural Product Chemistry. 278: 

209–241 

Meyer B, Möller H (2007) Conformation of Glycopeptides and Glycoproteins. 267: 187–251 

Meyer LA, see Li WP (2005) 252: 179–192 

Michl J, see Magnera TF (2005) 262: 63–97 

Milea JS, see Smith CL (2005) 261: 63–90 

Mizoshita N, see Kato T (2005) 256: 219–236 

Modlinger A, see Weide T (2007) 272:1–50 

Möller H, see Meyer B (2007) 267: 187–251 

Montgomery J, Sormunen GJ (2007) Nickel-Catalyzed Reductive Couplings of Aldehydes 

and Alkynes. 279:1–23 

Moonen NNP, Flood AH, Fernández JM, Stoddart JF (2005) Towards a Rational Design of 

Molecular Switches and Sensors from their Basic Building Blocks. 262: 99–132 

Moriyama M, see Kato T (2005) 256: 219–236 

Moussa A, see Coleman AW (2007) 277: 31–88 

Murai T (2005) Thio-, Seleno-, Telluro-Amides. 251: 247–272 

Murakami H (2007) From Racemates to Single Enantiomers – Chiral Synthetic Drugs over 

the last 20 Years. 269: 273–299 

Mutule I, see Suna E (2006) 266: 49–101 

Naka K (2007) Delayed Action of Synthetic Polymers for Controlled Mineralization of 

Calcium Carbonate. 271: 119–154 

Nakayama J, see Ishii A (2005) 251: 181–225 

Nakayama J, see Ishii A (2005) 251: 227–246 

Narayanan S, see Reif B (2007) 272: 117–168 

Nastasi F, see Campagna S (2007) 280: 117–214 

Neese F, see Sinnecker S (2007) 268: 47–83 

Nguyen GH, see Smith CL (2005) 261: 63–90 

Nicolau DV, Sawant PD (2005) Scanning Probe Microscopy Studies of Surface-Immobilised 

DNA/Oligonucleotide Molecules. 260: 113–160 

Niessen HG, Woelk K (2007) Investigations in Supercritical Fluids. 276: 69–110 

Nilsson P, Olofsson K, Larhed M (2006) Microwave-Assisted and Metal-Catalyzed Coupling 

Reactions. 266: 103–144 

Nishiyama H, Shiomi T (2007) Reductive Aldol, Michael, and Mannich Reactions. 279: 

105–137 

Niyomura O, Kato S (2005) Chalcogenocarboxylic Acids. 251:1–12 

Niyomura O, see Kato S (2005) 251: 19–85 

Nohira H, see Sakai K (2007) 269: 199–231 

Oda R, see Brizard A (2005) 256: 167–218 

Okahata Y, Kawasaki T (2005) Preparation and Electron Conductivity of DNA-Aligned Cast 

and LB Films from DNA-Lipid Complexes. 260: 57–75 

Okamura T, see Ueyama N (2007) 271: 155–193 

Oller-López JL, see Cuerva JM (2006) 264: 63–92 

Olofsson K, see Nilsson P (2006) 266: 103–144 

Oltra JE, see Cuerva JM (2006) 264: 63–92 

Onoda A, see Ueyama N (2007) 271: 155–193 

Otto D, see Ihmels H (2005) 258: 161–204 

Otto S, Severin K (2007) Dynamic Combinatorial Libraries for the Development of Synthetic 

Receptors and Sensors. 277: 267–288


Palchetti I, Mascini M (2005) Electrochemical Adsorption Technique for Immobilization of 

Single-Stranded Oligonucleotides onto Carbon Screen-Printed Electrodes. 261: 27–43 

Pascal R, Boiteau L, Commeyras A (2005) From the Prebiotic Synthesis of α-Amino Acids 

Towards a Primitive Translation Apparatus for the Synthesis of Peptides. 259: 69–122 

Paulo A, see Santos I (2005) 252: 45–84 

Pérez EM, see Leigh DA (2006) 265: 185–208 

Perret F, see Coleman AW (2007) 277: 31–88 

Perron H, see Coleman AW (2007) 277: 31–88 

Pianowski Z, see Winssinger N (2007) 278: 311–342 

Piestert F, see Gansäuer A (2007) 279: 25–52 

Pilch DS, Kaul M, Barbieri CM (2005) Ribosomal RNA Recognition by Aminoglycoside 

Antibiotics. 253: 179–204 

Pividori MI, Alegret S (2005) DNA Adsorption on Carbonaceous Materials. 260:1–36 

Piwnica-Worms D, see Sharma V (2005) 252: 155–178 

Plesniak K, Zarecki A, Wicha J (2007) The Smiles Rearrangement and the Julia–Kocienski 

Olefination Reaction. 275: 163–250 

Polito M, see Braga D (2005) 254: 71–94 

Poulin-Kerstien AT, see Dervan PB (2005) 253:1–31 

de la Pradilla RF, Tortosa M, Viso A (2007) Sulfur Participation in [3,3]-Sigmatropic Rearrangements. 

275: 103–129 

Procter DJ, see McGhee AM (2006) 264: 93–134 

Puntoriero F, see Balzani V (2007) 280:1–36 

Puntoriero F, see Campagna S (2007) 280: 117–214 

Quiclet-Sire B, Zard SZ (2006) The Degenerative Radical Transfer of Xanthates and Related 

Derivatives: An Unusually Powerful Tool for the Creation of Carbon–Carbon Bonds. 264: 

201–236 

Ratner MA, see Weiss EA (2005) 257: 103–133 

Raymond KN, see Seeber G (2006) 265: 147–184 

Rebek Jr J, see Scarso A (2006) 265:1–46 

Reckien W, see Thar J (2007) 268: 133–171 

Reggelin M (2007) [2,3]-Sigmatropic Rearrangements of Allylic Sulfur Compounds. 275: 

1–65 

Reif B, Narayanan S (2007) Characterization of Interactions Between Misfolding Proteins 

and Molecular Chaperones by NMR Spectroscopy. 272: 117–168 

Reiher M, see Herrmann C (2007) 268: 85–132 

Renaud P, see Darmency V (2006) 263: 71–106 

Revell JD, Wennemers H (2007) Identification of Catalysts in Combinatorial Libraries. 277: 

251–266 

Robinson SP, see Liu S (2005) 252: 193–216 

Saha-Möller CR, see You C-C (2005) 258: 39–82 

Sakai K, Sakurai R, Hirayama N (2007) Molecular Mechanisms of Dielectrically Controlled 

Resolution (DCR). 269: 233–271 

Sakai K, Sakurai R, Nohira H (2007) New Resolution Technologies Controlled by Chiral 

Discrimination Mechanisms. 269: 199–231 

Sakamoto M (2005) Photochemical Aspects of Thiocarbonyl Compounds in the Solid-State. 

254: 207–232 

Sakurai R, see Sakai K (2007) 269: 199–231


Sakurai R, see Sakai K (2007) 269: 233–271 

Saladino R, Crestini C, Costanzo G, DiMauro E (2005) On the Prebiotic Synthesis of Nucleobases, 

Nucleotides, Oligonucleotides, Pre-RNA and Pre-DNA Molecules. 259: 29–68 

Santos I, Paulo A, Correia JDG (2005) Rhenium and Technetium Complexes Anchored by 

Phosphines and Scorpionates for Radiopharmaceutical Applications. 252: 45–84 

Santos M, see Szathmáry E (2005) 259: 167–211 

Sato K (2007) Inorganic-Organic Interfacial Interactions in Hydroxyapatite Mineralization 

Processes. 270: 127–153 

Sauvage J-P, see Collin J-P (2005) 262: 29–62 

Sawant PD, see Nicolau DV (2005) 260: 113–160 

Scandola F, see Chiorboli C (2005) 257: 63–102 

Scarso A, Rebek Jr J (2006) Chiral Spaces in Supramolecular Assemblies. 265:1–46 

Schaumann E (2007) Sulfur is More Than the Fat Brother of Oxygen. An Overview of 

Organosulfur Chemistry. 274:1–34 

Scheffer JR, Xia W (2005) Asymmetric Induction in Organic Photochemistry via the Solid- 

State Ionic Chiral Auxiliary Approach. 254: 233–262 

Schenning APHJ, see Ajayaghosh A (2005) 258: 83–118 

Schepers U, see Hahn F (2007) 278: 135–208 

Schindler J, see Faigl F (2007) 269: 133–157 

Schmidtchen FP (2005) Artificial Host Molecules for the Sensing of Anions. 255:1–29Author 

Index Volumes 251–255 

Scandola F, see Indelli MT (2007) 280: 215–255 

Schmuck C, Wich P (2007) The Development of Artificial Receptors for Small Peptides Using 

Combinatorial Approaches. 277:3–30 

Schoof S, see Wolter F (2007) 267: 143–185 

De Schryver F, see De Feyter S (2005) 258: 205–255 

Schulten K, see Dittrich M (2007) 268: 319–347 

Schuman B, Alfaro JA, Evans SV (2007) Glycosyltransferase Structure and Function. 272: 

217–258 

Seeber G, Tiedemann BEF, Raymond KN (2006) Supramolecular Chirality in Coordination 

Chemistry. 265: 147–184 

Seeberger PH, see Castagner B (2007) 278: 289–309 

Seitz O, see Haase C (2007) 267:1–36 

Senn HM, Thiel W (2007) QM/MM Methods for Biological Systems. 268: 173–289 

Severin K, see Otto S (2007) 277: 267–288 

Sewell SL, see Carney CK (2007) 270: 155–185 

Sharma V, Piwnica-Worms D (2005) Monitoring Multidrug Resistance P-Glycoprotein Drug 

Transport Activity with Single-Photon-Emission Computed Tomography and Positron 

Emission Tomography Radiopharmaceuticals. 252: 155–178 

Shinkai S, see Ishi-i T (2005) 258: 119–160 

Shiomi T, see Nishiyama H (2007) 279: 105–137 

Sibi MP, see Zimmerman J (2006) 263: 107–162 

Siebbeles LDA, see Grozema FC (2005) 257: 135–164 

Silvi S, see Balzani V (2005) 262:1–27 

Simon P, see Kniep R (2007) 270: 73–125 

Sinnecker S, Neese F (2007) Theoretical Bioinorganic Spectroscopy. 268: 47–83 

Skrydstrup T, see Hansen SG (2006) 264: 135–162 

Smith CL, Milea JS, Nguyen GH (2005) Immobilization of Nucleic Acids Using Biotin- 

Strept(avidin) Systems. 261: 63–90 

Smith DK, see Hirst AR (2005) 256: 237–273


Sormunen GJ, see Montgomery J (2007) 279:1–23 

Specker D, Wittmann V (2007) Synthesis and Application of Glycopeptide and Glycoprotein 

Mimetics. 267: 65–107 

Sromek AW, Gevorgyan V (2007) 1,2-Sulfur Migrations. 274: 77–124 

Stadler A, see Kremsner JM (2006) 266: 233–278 

Stibor I, Zlatuˇsková P (2005) Chiral Recognition of Anions. 255: 31–63 

Stoddart JF, see Moonen NNP (2005) 262: 99–132 

Strauss CR, Varma RS (2006) Microwaves in Green and Sustainable Chemistry. 266: 199–231 

Suk D-H, see Crich D (2006) 263:1–38 

Suksai C, Tuntulani T (2005) Chromogenetic Anion Sensors. 255: 163–198 

Sun J-S, see Escudé C (2005) 253: 109–148 

Suna E, Mutule I (2006) Microwave-assisted Heterocyclic Chemistry. 266: 49–101 

Süssmuth RD, see Wolter F (2007) 267: 143–185 

Svith H, see Daasbjerg K (2006) 263: 39–70 

Swerdlow H, see Du Q (2005) 261: 45–61 

Szathmáry E, Santos M, Fernando C (2005) Evolutionary Potential and Requirements for 

Minimal Protocells. 259: 167–211 

Taira S, see Yokoyama K (2005) 261: 91–112 

Takahashi H, see Tamura R (2007) 269: 53–82 

Takahashi K, see Ueyama N (2007) 271: 155–193 

Tamiaki H, see Balaban TS (2005) 258:1–38 

Tamaru Y, see Kimura M (2007) 279: 173–207 

Tamura R, Takahashi H, Fujimoto D, Ushio T (2007) Mechanism and Scope of Preferential 

Enrichment, a Symmetry-Breaking Enantiomeric Resolution Phenomenon. 269: 53–82 

Tanaka H, see Matile S (2007) 277: 219–250 

Thar J, Reckien W, Kirchner B (2007) Car–Parrinello Molecular Dynamics Simulations and 

Biological Systems. 268: 133–171 

Thayer DA, Wong C-H (2007) Enzymatic Synthesis of Glycopeptides and Glycoproteins. 

267: 37–63 

Thiel W, see Senn HM (2007) 268: 173–289 

Tiedemann BEF, see Seeber G (2006) 265: 147–184 

Tobey SL, see Houk RJT (2005) 255: 199–229 

Toda F (2005) Thermal and Photochemical Reactions in the Solid-State. 254:1–40 

Tortosa M, see de la Pradilla RF (2007) 275: 103–129 

Tour JM, see James DK (2005) 257: 33–62 

Trask AV, Jones W (2005) Crystal Engineering of Organic Cocrystals by the Solid-State 

Grinding Approach. 254: 41–70 

Tuntulani T, see Suksai C (2005) 255: 163–198 

Uccelli L, see Boschi A (2005) 252: 85–115 

Ueyama N, Takahashi K, Onoda A, Okamura T, Yamamoto H (2007) Inorganic–Organic 

Calcium Carbonate Composite of Synthetic Polymer Ligands with an Intramolecular 

NH···OHydrogenBond.271: 155–193 

Ushio T, see Tamura R (2007) 269: 53–82 

Varma RS, see Strauss CR (2006) 266: 199–231 

Veciana J, see Amabilino DB (2006) 265: 253–302 

Venturi M, see Balzani V (2005) 262:1–27 

Vezin H, see Dias N (2005) 253: 89–108


Viso A, see de la Pradilla RF (2007) 275: 103–129 

Vögtle F, see Fages F (2005) 256: 77–131 

Vögtle M, see Žinić M (2005) 256: 39–76 

Volkmer D, see Fricke M (2007) 270:1–41 

Volpicelli R, see Zonta C (2007) 275: 131–161 

Vries TR, see Kellogg RM (2007) 269: 159–197 

Walschus U, see Luderer F (2005) 260: 37–56 

Walton JC (2006) Unusual Radical Cyclisations. 264: 163–200 

Wannberg J, Ersmark K, Larhed M (2006) Microwave-Accelerated Synthesis of Protease 

Inhibitors. 266: 167–197 

Warman JM, see Grozema FC (2005) 257: 135–164 

Warren JD, Geng X, Danishefsky SJ (2007) Synthetic Glycopeptide-Based Vaccines. 267: 

109–141 

Wasielewski MR, see Weiss EA (2005) 257: 103–133 

Weide T, Modlinger A, Kessler H (2007) Spatial Screening for the Identification of the 

Bioactive Conformation of Integrin Ligands. 272:1–50 

Weiss EA, Wasielewski MR, Ratner MA (2005) Molecules as Wires: Molecule-Assisted Movement 

of Charge and Energy. 257: 103–133 

Weissbuch I, Leiserowitz L, Lahav M (2005) Stochastic “Mirror Symmetry Breaking” via Self- 

Assembly, Reactivity and Amplification of Chirality: Relevance to Abiotic Conditions. 

259: 123–165 

Wennemers H, see Revell JD (2007) 277: 251–266 

Wich P, see Schmuck C (2007) 277:3–30 

Wicha J, see Plesniak K (2007) 275: 163–250 

Wiehn M, see Jung N (2007) 278:1–88 

Williams LD (2005) Between Objectivity and Whim: Nucleic Acid Structural Biology. 253: 

77–88 

Winssinger N, Pianowski Z, Debaene F (2007) Probing Biology with Small Molecule Microarrays 

(SMM). 278: 311–342 

Wittmann V, see Specker D (2007) 267: 65–107 

Wright DW, see Carney CK (2007) 270: 155–185 

Woelk K, see Niessen HG (2007) 276: 69–110 

Wolter F, Schoof S, Süssmuth RD (2007) Synopsis of Structural, Biosynthetic, and Chemical 

Aspects of Glycopeptide Antibiotics. 267: 143–185 

Wöltinger J, see Greiner L (2007) 276: 111–124 

Wong C-H, see Thayer DA (2007) 267: 37–63 

Wong KM-C, see Yam VW-W (2005) 257:1–32 

Worgull D, see Gansäuer A (2007) 279: 25–52 

Wright AT, see Collins BE (2007) 277: 181–218 

Würthner F, see You C-C (2005) 258: 39–82 

Xia W, see Scheffer JR (2005) 254: 233–262 

Yam VW-W, Wong KM-C (2005) Luminescent Molecular Rods – Transition-Metal Alkynyl 

Complexes. 257:1–32 

Yamamoto H, see Ueyama N (2007) 271: 155–193 

Yashima E, see Maeda K (2006) 265: 47–88 

Yokoyama K, Taira S (2005) Self-Assembly DNA-Conjugated Polymer for DNA Immobilization 

on Chip. 261: 91–112


Yoshikawa I, see Araki K (2005) 256: 133–165 

Yoshioka R (2007) Racemization, Optical Resolution and Crystallization-Induced Asymmetric 

Transformation of Amino Acids and Pharmaceutical Intermediates. 269: 83–132 

You C-C, Dobrawa R, Saha-Möller CR, Würthner F (2005) Metallosupramolecular Dye 

Assemblies. 258: 39–82 

Yu J, see Dittrich M (2007) 268: 319–347 

Yu S-H (2007) Bio-inspired Crystal Growth by Synthetic Templates. 271: 79–118 

Zampella G, see Bertini L (2007) 268:1–46 

Zard SZ, see Quiclet-Sire B (2006) 264: 201–236 

Zarecki A, see Plesniak K (2007) 275: 163–250 

Zhang W (2006) Microwave-Enhanced High-Speed Fluorous Synthesis. 266: 145–166 

Zhang X-E, Deng J-Y (2005) Detection of Mutations in Rifampin-Resistant Mycobacterium 

Tuberculosis by Short Oligonucleotide Ligation Assay on DNA Chips (SOLAC). 261: 

169–190 

Zimmerman J, Sibi MP (2006) Enantioselective Radical Reactions. 263: 107–162 

ˇZinić M, see Fages F (2005) 256: 77–131 

Žinić M, Vögtle F, Fages F (2005) Cholesterol-Based Gelators. 256: 39–76 

Zipse H (2006) Radical Stability—A Theoretical Perspective. 263: 163–190 

Zlatuˇsková P, see Stibor I (2005) 255: 31–63 

Zonta C,DeLucchi O,Volpicelli R,Cotarca L(2007)Thione–Thiol Rearrangement:Miyazaki– 

Newman–Kwart Rearrangement and Others. 275: 131–161

Subject Index 

Alkyne bridges 148 

Angular overlap model (AOM) 41 

Antenna system 26 

Antennae, artificial light-harvesting 

119 

Azido-Cr(III) 62 

Back-intersystem crossing (BISC) 51 

Bimolecular processes 10 

–, metal complexes 11 

–, quenching 94 

Bis(alkyl)naphthalene 149 

Bis[2-(diphenylphosphino)phenyl] ether) 

95 

Bisphenanthroline Cu(I) complexes 78 

trans-Chalcone 29 

Chemiluminescence 131 

Chromium 37 

–, coordination compounds, 

photochemistry/photophysics 37 

Cluster centered (CC) character 70 

Clusters 69 

Coordination compounds, chromium, 

photochemistry/photophysics 37 

–, photochemical molecular 

devices/machines 24 

Copper 70 

–, biology 73 

Coulombic mechanism 22 

Cr(acac)3 42 

[Cr[18]aneN6] 3+ 49 

[Cr(diimine)3] 3+ systems, photoredox 

behavior 54 

[Cr(N4)(CN)2] + 51 

[Cu(NN)2] + complexes 92 

[Cr(phen)3] 3+ 

photoracemization/hydrolysis 43 

[Cr(sen)3] 3+ 49 

Cr(III) ligand field excited states, ultrafast 

dynamics 41 

Cr(III) porphyrins, axial ligand 

photodissociation 45 

Cu(I) 71 

–, luminescent complexes 107 

–, supramolecular chemistry 78 

Cu(I)-bisphenanthroline 79, 81 

Cu(II) 71 

Cuprous halide clusters 101 

Cyclam 47 

Cytochrome c oxidase 78 

Dendrimers 26, 155 

Diimine/diphosphine [Cu(NN)(PP)] + 

complexes 95 

Diphosphine 95 

DNA binding, rhodium complexes, 

photocleavage 241 

DNA damage, long-range oxidative, excited 

Rh(III) complexes 248 

DNA interactions 56 

DNA intercalators 215, 241 

DNA photocleavage 242 

Donor–chromophore–acceptor triads 164 

Dyads 227 

Dye-sensitized solar cells 117 

Electrochemiluminescence 131 

Electron collection, photoinduced 239 

Electron transfer 16, 69 

Emissive excited state(s), luminescence 

spectra 88 

Energy transfer 21, 37, 53, 69, 117, 215 

–, self-exchange between identical 

chromophores 53 

Eu(III) complexes 95 

Exchange mechanism 23 

Excimers 15

272 Subject Index 

Exciplexes 15 

Excited-state decay, intramolecular 8 

Excited-state distortion 86 

Extension cable 28 

Fe(III) porphyrin 13 

Formaldehyde 3,4 

Grids 153 

Halide-to-metal charge transfer (XMCT) 

70, 102 

Intraligand charge transfer (ILCT) 221 

Jablonski diagram, [Cr(acac)3] 42 

–, light absorption 5 

Lanthanide ions, long-lived luminescence 

26 

LEC devices 99, 100 

Ligand-centered (LC) transitions 6, 120, 

218 

Ligand-to-metal charge-transfer (LMCT) 

transitions 6, 76 

Light absorption 8 

Light-initiated time-resolved X-ray 

absorption spectroscopy (LITR-XAS) 

70 

Light-powered molecular machines 117 

Luminescence 69, 117 

Machines, light-powered molecular 

117 

Marcus inverted region 12 

Marcus theory 16 

Metal complexes 5 

Metal-centered (MC) transitions 6, 120, 

217 

Metalloproteins, copper 75 

Metal-to-ligand charge-transfer (MLCT) 

transitions 6, 70, 73, 119 

4 ′ -Methoxyflavylium ion 29 

MLCT excited states 128 

–, multiple low-lying, polypyridine ligand 

138 

Molecular machines, light-powered 117 

Molecular wires 24 

Multihole storage, photoinduced, mixed 

Ru–Mn 177 

Nanomotor, sunlight-powered 30 

Naphthalene 10 

Nitric oxide (NO) 60 

Nitrido complexes, Cr(III) coordinated 

azide, photogeneration 61 

Nitrido-Cr(V) 62 

NO, Cr(III)-coordinated nitrite, 

photolabilization 60 

Nuclear motions, photoactive molecular 

machines 183 

OLED devices 69, 99, 100, 133 

Oligophenylene bridges 145 

Optical electron transfer 20 

Os(II) bipyridine-type complexes 11 

9,10-Phenanthrenequinonediimine 241 

Phenanthroline 69 

Phosphorescence microwave double 

resonance (PMDR) 221 

Photocatalytic processes, supramolecular 

species 180 

Photochemistry, molecular 3 

–, supramolecular 12 

Photogeneration, hydrogen 180 

Photoinduced processes, supramolecular 

systems 15 

Photonuclease 63 

Photoracemization, [Ru(bpy)3] 2+ 127 

Photoredox 37 

Photosubstitution 37, 43 

Photosynthesis, Z-scheme 77 

Plastocyanin, blue copper site 75 

Polyacetylenic bridges 148 

Polyads, oligoproline assemblies 170 

Polynuclear complexes 215 

Polystyrene, multi-ruthenium assemblies 

172 

POP 95 

Porphyrin-Rh(III) conjugates, 

photoinduced electron transfer 234 

Pseudorotaxanes 28 

Quantum mechanical theory 19 

Racks, Ru(II) 153 

Rh(III) complexes, as acceptors in electron 

transfer reactions 245 

–, DNA-mediated long-range electron 

transfer 245

Subject Index 273 

Rh(III) cyclometalated complexes 223 

Rh-DNA 248 

Rhodium 215 

–, complexes, DNA intercalators 241 

–, cyclometalated complexes 223 

–, homobinuclear complexes 226 

–, mononuclear species 218 

–, polynuclear/supramolecular species 

226 

Rhodium polypyridine complexes 215, 

218 

[Ru(bpy)3] 2+ 120, 123 

Ru(II) complexes, tridentate polypyridine 

ligands 136 

Ru(II) dendrimers, luminescent 155 

Ru(II) polypyridine complexes 117, 119 

–, nonradiative decay 133 

Ru(II) racks 154 

Ru(II)-Rh(III) polypyridine dyads, 

photoinduced electron transfer 228 

Ru–Os dyads, tridentate ligands 145 

Ruthenium 117 

–, complexes, biological systems 185 

–, species, photoactive multinuclear 153 

–, supramolecular photochemistry 141 

Solar cells, photoelectrochemical, 

dye-sensitized 188 

–, photoelectrochemical, 

ruthenium-sensitized 191 

State energy levels/conversion 38, 117 

Sunlight-powered nanomotor 30 

Supramolecular species/sensitizers 12, 

180, 193 

Tb(III) complexes 95 

Thermal excited state relaxation 37 

Ultrafast dynamics 37 

Wires, molecular 24 

XMCT 70, 102 

XOR logic gate 29 

Zn(II) porphyrin 13

Photochemistry and Photophysics of Coordination Compounds

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