Photochemistry and Photophysics of Coordination Compounds

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226 M.T. Indelli et al. Fig. 3 Frontier orbitals of Rh(ppy)2TAP + . From [67] 3 Polynuclear and Supramolecular Species 3.1 Homobinuclear Complexes A few homobinuclear ligand-bridged Rh(III) polypyridine complexes have been studied [3, 75, 76]. Their photochemical interest is rather limited, however, as they behave generally like their mononuclear analogues, with minor differences in spectral shifts and lifetimes. An interesting type of systems, which bring together the complexities of ligand-bridged species and multiply bridged metal–metal bonded rhodium dimers, has recently been reported by Campagna et al. [77]. In (14) two quadruply bridged Rh(II)-Rh(II) dimers are the “molecular components” of a higher-order two-component system held together by the complex bis-naphthyridine-type ligand. As already observed for some related simple rhodium dimers (e.g., Rh2(CH3COO)4(PPh3)2) [21], the “binuclear” compound has a non-emissive but long-lived excited state in room-temperature solution. In the case of 14, the long-lived state has been assigned as an MLCT (metal–metal π ∗ to naphthyridine π ∗ )excitedstate[77].
Photochemistry and Photophysics of Coordination Compounds: Rhodium 227 3.2 Dyads Heteronuclear bimetallic species containing rhodium polypyridine complexes are more interesting, as the Rh(III) unit can be involved in intercomponent processes, particularly of the electron transfer type. The thermodynamic requirements for the participation of Rh(III) polypyridine complexes in electron transfer processes are summarized in Fig. 4, where Rh(III), ∗ Rh(III), and Rh(II) represent the ground state, the triplet LC excited state (see Sect. 2.1), and the one-electron reduced form, respectively, and the values of excitedstate energy [31] and reduction potential [78] refer to Rh(phen)3 3+ (1). From these figures, it is apparent that Rh(III) polypyridine complexes can behave as extremely powerful photochemical oxidants and relatively good electron transfer quenchers. On the other hand, because of the high excited-state energy, these complexes are also good potential energy donors. Fig. 4 Typical redox energy level diagram for Rh(III) polypyridine complexes. Values (reduction potential vs. SCE) appropriate for Rh(phen)3 3+ (1) As a matter of fact, Rh(III) polypyridine complexes have been extensively used in bimolecular electron transfer processes, either as photoexcited molecule [79, 80] or as quencher [81–83], with motivations of both fundamental (testing electron transfer-free energy relationships) [80] and applied nature (photoinduced hydrogen evolution from water) [81, 82]. Here, on the other hand, we focus our attention on photoinduced processes where the reactants are pre-assembled in some kind of supramolecular system. The most common photoinduced processes taking place in simple two-component systems (often called “dyads”) involving a Rh(III) polypyridine unit are shown in Eqs. 6–8: ∗ Rh(III) –Q → Rh(III) – ∗ Q (6) ∗ Rh(III) –Q → Rh(II) –Q + (7) ∗ P – Rh(III) → P + – Rh(II) . (8)
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280 Topics in Current Chemistry Edi
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X Preface nation Compounds. The ven
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Photochemistry and Photophysics of Coordination Compounds

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