Photochemistry and Photophysics of Coordination Compounds

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238 M.T. Indelli et al. 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])
Photochemistry and Photophysics of Coordination Compounds: Rhodium 239 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-
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280 Topics in Current Chemistry Edi
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Photochemistry and Photophysics of
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Volume Editors Prof. Vincenzo Balza
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Topics in Current Chemistry Also Av
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X Preface nation Compounds. The ven
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Contents Photochemistry and Photoph
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Page 279 and 280: Subject Index Alkyne bridges 148 An
Page 281: Subject Index 273 Rh(III) cyclometa
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