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168 S. Campagna et al. 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
Photochemistry and Photophysics of Coordination Compounds: Ruthenium 169 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-
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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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258 Author Index Volumes 251-280 Be
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Subject Index Alkyne bridges 148 An
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Subject Index 273 Rh(III) cyclometa
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