Photochemistry and Photophysics of Coordination Compounds

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180 S. Campagna et al. charges to the complex thus making feasible oxidation to the Mn2 III,IV state, which would have otherwise been impossible on thermodynamic grounds. The ligand exchange and reorganization of the manganese ion coordination sphere, which has a nonneutral effect from the viewpoint of the charge, would be a charge compensating process, analogous to proton release occurring in most of the oxidation states of the “manganese cluster” in natural systems [321, 325]. It can be inferred that charge compensating processes are needed requirements to maintain the oxidation potential of the redox-active catalytic site roughly constant when moving along the various steps of the overall hole accumulating process, an aspect that should be well taken into account in designing new systems. Recently a mixed Ru–Mn2 species featuring a photoinduced chargeseparation state with an impressive lifetime (0.6 ms at room temperature and 0.1–1 sat140 K, comparable to many of the naturally occurring chargeseparated states in photosynthetic systems) has been reported [334]. The slow charge-recombination rate obtained for such a species has been mainly attributed to the large reorganization energy connected with the inner reorganization of the manganese subunit already mentioned (about 2 eV for the compound in [334]). This would suggest that there is no needed to look for charge-recombination processes occurring in the Marcus inverted region to obtain long-lived separated states, since the large inner reorganization energy typical of the manganese systems could lead to the same (or better) result. 5.8 Photocatalytic Processes Operated by Supramolecular Species 5.8.1 Photogeneration of Hydrogen Since the early papers appeared in the 1970s [335–339], Ru(II) polypyridine complexes have been extensively used to produce hydrogen in heterogeneous cycles under light irradiation, by using sacrificial donor species (most commonly amines), electron acceptor relays (usually methyl viologens), and colloidal metal catalysts (Pt, Rh, etc.). This aspect of Ru(II) photochemistry has been extensively reviewed [1, 340] and will not be discussed in detail here. We will discuss some recent papers in which a (supramolecular) multicomponent approach is used. One of the important steps in designing a multicomponent hydrogen evolving system would be to assemble in the same (supramolecular) system as many key components as possible. Key components would be (based on the systems operating in heterogeneous schemes): (a) light-harvesting units (antennae); (b) a charge-separation unit made of a photosensitizer (the energy trap of the antenna, if an antenna is present), an electron acceptor, and an electron donor; and (c) a catalyst [280, 297, 341–345]. The compound 60 [346]
Photochemistry and Photophysics of Coordination Compounds: Ruthenium 181 is a quite interesting example toward the preparation of functional supramolecular species of this type, where all the key components would be integrated into a single multicomponent system. Under visible irradiation, 60 evolves molecular hydrogen from water (in the presence of EDTA as sacrificial donor) with an efficiency of about 1%. A relatively low turnover number (4.8) is estimated on the basis of the total amount of H2 evolved after 10 h(2.4 µmol) and the amount of the complex used (1 µmol). The hydrogen formation is immediate and is rather higher in its rate when UV light is eliminated by suitable filters. This latter observation suggests some photoinstability of the system upon UV irradiation. The wavelength dependence in the visible region of the rate of H2 formation agrees with the absorption spectrum of the complex. Moreover, the rate of H2 formation increases linearly with the photon flux, indicating that one-photon excitation of the molecule operates. This multicomponent system integrates in its structure the light harvester/photosensitizer (the Ru(II) chromophore) and the electron acceptor, which also plays the role of the catalyst (the Pt(II) subunit). Compared to the formerly investigated systems [1, 335– 337, 339, 456], neither the electron relay (usually a methyl viologen species) or the solid-state catalyst (usually colloidal platinum or rhodium) are required. Mechanistic aspects have not been discussed yet. Another multicomponent system (61) has been recently reported [347]. When excited at 470 nm in acetonitrile and in the presence of triethylamine (TEA, 2.08 mol L –1 ), such a species evolves H2 from the solution with a turnover number of about 50 (mol of H2 per mol of compound). No H2 evolution is ob-
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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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