Photochemistry and Photophysics of Coordination Compounds

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152 S. Campagna et al. 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
Photochemistry and Photophysics of Coordination Compounds: Ruthenium 153 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.
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280 Topics in Current Chemistry Edi
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Subject Index Alkyne bridges 148 An
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Subject Index 273 Rh(III) cyclometa
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