Photosensitized Reduction of Sulfonium Salts: Evidence for Nondissociative Electron Transfer

Abstract

The photosensitized reductions of 4-cyanobenzylmethylphenyl sulfonium tetrafluoroborate (1-BF4) by 9-phenylanthracene, 2-ethyl-9,10-dimethoxyanthracene, or perylene excited singlet states gives quantitative yields of phenyl methyl sulfide. Fluorescence quenching and the quantum yields for product formation, as functions of [1-BF4], give bimolecular rate constants (0.58−1.6 × 1010 M-1 s-1) for reaction of the excited sensitizers with the sulfonium salt. The limiting quantum yields, corresponding to infinite [1-BF4], are 0.65−0.77 for the three sensitizers, revealing significant inefficiencies in the photoreduction. These inefficiencies are assigned to the partitioning of a sulfuranyl radical intermediate in a two-step associative electron-transfer mechanism.