Proton transfer in phenothiazine photochemical oxidation: Laser flash photolysis and fluorescence studies

Abstract

Phenothiazine (PTH) cation and neutral radical have been characterized by nanosecond laser flash photolysis in basic and acid acetonitrile. In the presence of water or small amounts of base, the radical cation (PTH + ) was not detected and the neutral radical (PT ) was the only observed radical in photoionization process. Triplet PTH ( 3PTH ∗) was quenched by diethyl fumarate in acetonitrile at a rapid rate ( k q T = 6.1 × 10 9 M - 1 s - 1 ) , and then, it was observed a specific absorption around 410 nm that is assigned to PT formed via photoinduced electron/proton transfer. However, no PT was detected on 3PTH ∗quenching by cinnamonitrile (CN). On the basis of the results, the fast proton transfer in the oxidation process is mainly dependent on the basic groups, such as the carbonyl group in electron acceptors. The quenching rate constants ( k q T ) of 3PTH ∗ by various electron acceptors have been determined at 470 nm as well as fluorescence quenching rate constants k q S obtained by Stern–Volmer equation. All k q T and k q S values approach the diffusion limit. The driving force dependence of bimolecular quenching rate constants was evaluated in light of Rehm–Weller analysis. In addition, disharmonic k q values by CN in endergonic region might be the contribution of charge transfer.