Photoelectrochemical electron spin resonance. Part 3.—The reduction of fluorescein: a ‘photo-DISP2’ reaction

Abstract

The reduction of the dye fluorescein in an aqueous solution of high pH (13), at mercury-plated electrodes, has been investigated using in situ electrochemical e.s.r. and a channel electrode flow cell. In the dark, a simple reversible one-electron reduction is observed leading to the formation of the stable semi-fluorescein radical anion. On irradiation with 390 nm light, appreciable photocurrents are observed and the process takes on the characteristics of a DISP reaction with the formation of leucofluorescein: the two-electron reduction product. Analysis of the e.s.r. signal–photocurrent–solution flow-rate data allows the precise mechanism of the reaction to be deduced, and this is shown to involve a rate-determining electron transfer between excited and ground-state semi-fluorescein molecules.