Photoredox Reactions of Thionine

Abstract

The photoredox reaction between thionine and iron (II) has been studied by flash photolysis techniques. The principal reaction intermediate is the semithionine radical anion which decays via a dismutation process forming ground state thionine and the fully reduced leucothionine. Subsequently, leucothionine is reoxidised slowly by iron (III) which is formed in the primary electron transfer reaction. Rate constants for all the reaction steps have been determined. The reaction has been extended to include reduction with cobalt (II) and manganese (II). Here, the ground state quenchers do not possess the necessary reducing power to form the semithionine radical anion, and reaction leads to enhanced deactivation of triplet thionine. However, when the transition metal ion is excited directly, full electron transfer takes place with concomitant formation of the radical ions.