Problems of back electron transfer in electron transfer sensitization

Abstract

The problems of back electron transfer in electron transfer sensitization were discussed and methods of preventing back electron transfer to improve the quantum efficiency of charge separation in homogeneous systems were proposed. Although complete quenching of an excited sensitizer by an electron donor or acceptor is not difficult, chemical yields of oxidized or reduced species are in general low. The importance of the coulombic effect was demonstrated for benzophenone/leuco crystal violet, pyrene/methylviologen (MV 2+)/ethylenediaminetetraacetic acid and phenothiazine/viologen analogue systems. Then the discussion was extended to the quenching of Ru(bpy) 3 2+* (bpy ≡ 2,2′-bipyridine) by organic acceptors and donors in acetonitrile. The magnitude of the quenching constant k q as a function of exoergicity could be explainable by the Rehm—Weller equation in both cases: Δ H ‡ is apparently negative for oxidative quenching (RuL 3 2+* + A → RuL 3 3+ + −; in contrast, for reductive quenching (RuL 3 2+* + D → RuL 3 + + D +) Δ H ‡ is normal and control k q. A detailed kinetic mechanistic study leads to the conclusion that the attraction between RuL 3 3+ and A − in oxidative quenching brings about back electron transfer to the excited state, and the charge separation yield is found is found to be lower than that for reductive quenching as would be expected. By modulation of the ligand structure from 2,2′-bipyridine to 2,2′-bipyrazine and other ligands, the redox properties of RuL 3 2+* can be modified. Thus, the photo-reaction of the RuL 3 2+/MV 2+/triethanolamine (TEOA) system proceeds via reductive quenching when L is 2,2-bipyrazine or some other ligands (RuL 3 2+* + TEOA → RuL 3 +; RuL 3 + + MV 2+ → RuL 3 2+ + MV +. The quantum yield of MV + formation approaches 100% by tris(2,2′-bipyrazine)-ruthenium(II) whereas the conventional Ru(bpy) 2 2+ sensitizer reacting via the oxidative quenching mechanism (RuL 3 2+* + MV 2+ → RuL 3 3+ + MV +; RuL 3 3+ + TEOA → RuL 3 2+) gives a quantum yield of MV + formation of only about 20%.