Photoelectrochemical electron spin resonance. Part 4.—The ‘photo-ECE’ reaction and the reduction of 1-halogenoanthraquinones

Abstract

The reduction of 1-bromoanthraquinone in acetonitrile solvent in the presence of tetrabutylammonium perchlorate, at irradiated (565 nm) gold electrodes is shown to produce the radical anion of anthraquinone. A combination of photoelectrochemical hydrodynamic voltammetry, in situ electrochemical ESR and preparative experiments are used to deduce that the mechanism is a ‘photo-ECE’ reaction, in which the first electron transfer produces the radical anion of 1-bromoanthraquinone. On irradiation, this anion dissociates. Bromide ions and anthraquinone are formed and the latter undergoes a further one-electron reduction at the electrode resulting in the detection of significant photocurrents. The H atom abstracted in the formation of the anthraquinone is shown to arise from the tetrabutylammonium cations and not from the solvent as previously suggested in analogous processes. The related species, the 1-chloroanthraquinone radical anion, is shown to be stable towards irradiation on the electrochemical timescale, whereas 1-iodoanthraquinone is found to undergo reduction at gold in the absence of light in an analogous mechanistic manner to 1-bromoanthraquinone in the light.