Structures and dynamics of the lowest excited triplet states and cation radicals of phenothiazine and 2-chlorophenothiazine: transient resonance Raman and absorption study

Abstract

Transient resonance Raman and absorption spectra of the lowest excited triplet states T 1 and the cation radicals of phenothiazine and 2-chlorophenothiazine were measured. It was found that in the photoreaction of 2-chlorophenothiazine a transient exhibiting an absorption band at 556 nm was generated from the cation radical. The corresponding transient was not observed in the absorption spectrum of phenothiazine, a fact which suggests a possibility of phenothiazinyl radical generation for 2-chlorophenothiazine by photoinduced dechlorination. Vibrational assignments of the T 1 states and the cation radicals of the both compounds were made based on the frequency shifts on isotopic substitutions. Unusually large low-frequency shifts of the phenyl 8a and 8b modes were observed in the T 1 state but no appreciable shifts were detected in the cation radical, indicating that the phenyl rings are drastically weakened, and therefore, the phenyl C-C bonds are very much lengthened in the T 1 state. This implies that the excitation is strongly localized on the phenyl rings and the T 1 state has an n − π character.