Mechanistic Studies on the Photochemical Oxidation of Phenothiazine with Oxygen in Ethanol

Abstract

It has been established that phenothiazine (PTH) undergoes photooxidation, PTH+O2\oversethν→PT·+HO2 in the aerated ethanol (dehydrated!) yielding a neutral radical as a stable product. The reaction is a one photon process and the reactive state is triplet PTH. It has further been established that the reaction occurs via a molecular complex (PTH-O2) most likely of C-T type, which has the absorption at ∼385 nm and decays with the first order rate constant of 1.6×102 sec−1. A similar transient C-T complex has been found to be produced from triplet PTH and dimethylisophthalate (weak electron acceptor) (absorption ∼385 nm, decay constant 2×103sec−1) with almost the diffusion controlled rate constant, although the eventual reaction does not occur. Thus in case of (PTH-O2), reaction (PTH-O2)→PT·+HO2 occurs while in case of (PTH-DMIP), the analogous reaction does not. Behavior of radical species (containing radical cation and (PTH-O2)) produced in the rigid aerated ethanol at 77°K has also been studied. The present C-T complex is interesting in the point that it is produced via triplet state of donor and has a rather long lifetime.