Self-Repairing Photosensitized Electron Transfer from Thiones to Methyl Viologen in Aqueous Media

Abstract

Reversible formation of the persistent methyl viologen radical cation (MV•+) was observed by the steady-state photoillumination of thiones in the presence of MV2+ in deaerated aqueous solution. Formation yield and lifetime of MV•+ are favorable at a pH higher than 3.00. By laser flash photolysis, it was observed that fast electron transfer occurs from triplet states of thiones (3PT*) to MV2+, yielding the radical cation of thiones (PT•+) and MV•+. The initial rapid decay of MV•+ is attributed to back electron transfer from MV•+ to PT•+. On the other hand, the slow decay part of MV•+ can be attributed to endothermic electron transfer from MV•+ to disulfide, which is formed by the coupling of the thio radical produced after deprotonation of PT•+. The ratio of slow decay component and fast decay component of MV•+ in the short time scale is a good measure of the yields of persistent MV•+ with changing thiones and pH. A cyclic mechanism interpreting reversible electron transfer was proposed in which thiones act as self-repairing photosensitizers.