Photochemistry and Photophysics of Coordination Compounds

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178 S. Campagna et al. < 50 ns to 10 µs, depending on the complex [326]. This study demonstrated that the reorganization energy for the Mn(II)-to-Ru(III) electron transfer was quite large (1.4–2.0 eV), suggesting significant inner reorganization of the manganese moiety during the process [327]. As an obvious consequence, very fast Mn(II)-to-Ru(III) electron transfer could not be expected. A further complication was that the manganese subunit could directly quench the excited ruthenium chromophore by Dexter energy transfer, competing with electron transfer quenching by the sacrificial acceptor for short Ru–Mn distances. These arguments led to the preparation of more elaborated systems in which intermediate donor species were interposed between ruthenium and
Photochemistry and Photophysics of Coordination Compounds: Ruthenium 179 manganese subunits [328–330], with phenolate and tyrosine moieties playing the role of an intermediate donor. In most cases, proton-coupled electron transfer processes took place. To achieve multielectron catalysts, more than one manganese ion was included in the systems. Figure 16 shows some examples [329, 331, 332] of Ru(II) species covalently linked to Mn dimers or trimers via phenolate lig- ands. In particular, for the Ru–Mn II,II 2 complex 58 reported in the figure, repeated flashes in the presence of a Co(III) sacrificial electron acceptor allowed three successive one-electron oxidations of the manganese moiety by the photooxidized Ru(III) subunit [333], as evidenced by the disappearance of the characteristic Mn2 II,II signals and the appearance of the characteristic Mn2 III,IV signals in EPR experiments. Manganese oxidation was suggested to involve a ligand exchange, in which acetate is released and water molecules are bound to form a di-µ-oxo bridge (see the reaction scheme in Fig. 16). According to the authors, this was the first example of a light-driven, multiple oxidation of a manganese complex attached to a photosensitizer. The ligand exchange at the manganese sites (presumably occurring in the Mn2 III,III state, that is, after second electron release from the initial Mn2 II,II center) is functional to the overall process, as it allows introduction of negative Fig. 16 Electron transfer from the manganese moiety to the photooxidized Ru(III) in a aRu– Mn2 II,II complex and b aRu– Mn3 II,II,II complex
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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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