Abstract
The photoisomerization of the merocyanine dyes, 5-[2-(3-ethyl-2-benzothiazolylidene)-1-methyl-ethylidene]-3-ethyl-2-thioxo-4-thiazolidinone (Et(Me)MD) and 5-[2-(3-ethyl-2-benzothiazolylidene)ethylidene]-3-ethyl-2-thioxo-4-thiazolidinone (EtMD), in solution was studied by visible absorption, resonance Raman and IR spectroscopies. It was found that the most stable conformers for Et(Me)MD and EtMD of the four possible isomers A–D were different. For Et(Me)MD, isomer A, which was stable in cyclohexane and carbon tetrachloride, was photoisomerized to isomer C by 514.5 nm laser irradiation. Isomer C returned to isomer A slowly in the dark or rapidly on 488.0 nm laser irradiation. In contrast, Isomer C for EtMD, which was stable in the solvents, was photoisomerized to isomer A by 488.0 or 457.9 nm laser irradiation. Isomer A returned to isomer C slowly in the dark or rapidly on 514.5 nm laser irradiation in cyclohexane. Possible photoisomerization mechanisms are discussed based on the rotational motions around the two CC double bonds of the central conjugated system.
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