Photochemistry and Photophysics of Coordination Compounds

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172 S. Campagna et al. 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
Photochemistry and Photophysics of Coordination Compounds: Ruthenium 173 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,
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280 Topics in Current Chemistry Edi
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Photochemistry and Photophysics of
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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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