Photochemistry and Photophysics of Coordination Compounds

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234 M.T. Indelli et al. moiety with of coordinated chloride ligands. In fact, contrary to what happens for common Rh(III) polypyridine units (e.g., Rh(bpy)3 3+ ,Rh(phen)3 3+ ) where one-electron reduction is a quasi-reversible process, mixed-ligand units containing halide ions (e.g., Rh(bpy)2Cl2 + ,Rh(phen)2Cl2 + )undergo strongly irreversible two-electron reductions accompanied by prompt halide ligand loss [78, 104]. While the use of these units as electron acceptors can be of interest towards photoinduced electron collection and multi-electron catalysis (see Sect. 3.4), from a kinetic viewpoint the large reorganizational energies involved are likely to lead to slow electron transfer rates. 3.2.2 Photoinduced Electron Transfer in Porphyrin-Rh(III) Conjugates Though structurally quite different, the porphyrin-Rh(III) conjugates thoroughly studied by Harriman et al. [105] behave with regard to photoinduced electron transfer rather similarly to the above-discussed Ru(II)-Rh(III) dyads. The systems contain a zinc porphyrin unit connected directly with one (21)or via a phenylene spacer with two (22) rhodium terpyridine units. The electron transfer processes thermodynamically allowed in these systems are indicated in Eqs. 10–14, where both 21 and 22 are schematized as
Photochemistry and Photophysics of Coordination Compounds: Rhodium 235 dyads (indeed, 22 is a triad only in a formal sense), ZnP, Rh, and ph represent the zinc porphyrin and rhodium terpyridine molecular components and the phenylene spacer, respectively. The singlet excited state is considered for the zinc porphyrin chromophore and the triplet state for the rhodium terpyridine unit. ∗ + ◦ ZnP-ph-Rh(III) → ZnP -ph-Rh(II) ∆G =–0.71 eV (10) ZnP-ph-Rh(III) ∗ → ZnP + -ph-Rh(II) ∆G ◦ =–0.81 eV (11) ZnP + -ph-Rh(II) → ZnP-ph-Rh(III) ∆G ◦ =–1.47 eV (12) ∗ + ◦ ZnP-Rh(III) → ZnP -Rh(II) ∆G =–0.58 eV (13) ZnP-Rh(III) ∗ → ZnP + -Rh(II) ∆G ◦ =–0.80 eV (14) In 22, where a phenylene spacer is interposed between the two molecular components, both photoinduced electron transfer following excitation of the zinc porphyrin (Eq. 10) and charge recombination (Eq. 12) have been time resolved, with values in acetonitrile of 3.2 × 1011 s –1 and 8.3 × 109 s –1 ,respectively. The wide difference in rates is attributed to the different kinetic regimes of the two processes, photoinduced electron transfer (Eq. 10) being almost activationless, and charge recombination (Eq. 12) lying deep into the Marcus inverted region [105]. For the directly linked system 21 (as well as for its free-base analogue), the disappearance of the porphyrin excited state, presumablybyphotoinducedelectrontransfer(Eq.13),isextremelyfast.Infact, the excited state lifetime of 21, ca.0.7 ps in acetonitrile, is comparable to the longitudinal relaxation time of the solvent, implying that the electron transfer process in this system is controlled by solvent reorientation [105]. 3.3 Triads and Other Complex Systems In dyads, including those discussed in the previous sections, photoinduced electron transfer is always followed by fast charge recombination. This greatly limits the use of dyads for practical purposes such as, e.g., conversion of light
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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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Page 281: Subject Index 273 Rh(III) cyclometa
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Photochemistry and Photophysics of Coordination Compounds

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