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