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

Article abstract of DOI:10.1016/S0022-2860(97)00060-4

Transient resonance Raman and absorption spectra of the lowest excited triplet states T₁ 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 phenothlazinyl radical generation for 2-chlorophenothiazine by photoinduced dechlorination. Vibrational assignments of the T₁ 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 T₁ 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 T₁ state. This implies that the excitation is strongly localized on the phenyl rings and the T₁ state has an n-π* character.