Self-repairing photosensitized electron transfer from thiones to methyl viologen in aqueous media

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 ((PT)-P-3*) 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.