Photochemistry and Photophysics of Coordination Compounds

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148 S. Campagna et al. 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
Photochemistry and Photophysics of Coordination Compounds: Ruthenium 149 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].
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280 Topics in Current Chemistry Edi
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Photochemistry and Photophysics of
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X Preface nation Compounds. The ven
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Subject Index Alkyne bridges 148 An
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Subject Index 273 Rh(III) cyclometa
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