Charge-transfer reactions of methoxybenzenes with aromatic carbonyl triplets. A laser flash photolytic study
Using 337.1 nm laser flash photolysis and kinetic spectrophotometry, the quenching of aromatic carbonyl triplets by a number of methoxybenzenes, including several trimethoxybenzoic acids and methoxy-NN-dimethylanilines, has been examined in detail. The observed quenching rate constants (kq) are in the range 1×105-1×1010 dm3 mol–1 s–1. There is an enhancement in kq, usually by a factor of two to four, on going from benzene to acetonitrile. In benzene, with methoxybenzenes and trimethoxybenzoic acids as quenchers, no transient photoproducts identifiable as ketyl radicals or radical anions/cations are formed in significant amounts as a result of the quenching of the benzophenone triplet. In acetonitrile, with quenchers that are relatively efficient, a small fraction (0.1–0.4) of the quenching events lead to photoreduction of the benzophenone triplet (photoreduction through quenching by methoxy-NN-dimethylanilines is nearly quantitative). Below the diffusion-controlled limit, the quenching rate constants correlate very well with the oxidation potentials of the methoxybenzenes giving slopes of –6.8 and –6.6 eV–1 for the benzophenone triplet in benzene and acetonitrile, respectively, and –12.1 eV–1 for the benzil triplet in acetonitrile. The results suggest modest involvement of charge transfer in the quenching processes. No correlation is observed in the dependence of quenching rate constants on the reduction potentials of the carbonyl triplets under study.