CONFORMATIONAL-ANALYSIS AND PHOTOISOMERIZATION OF MEROCYANINE DYES STUDIED BY VISIBLE ABSORPTION, RESONANCE RAMAN AND IR SPECTROSCOPIES

The photoisomerization of the merocyanine dyes, thyl-ethylidene]-3-eth yl-2-thioxo-4-thiazolidinone (Et(Me)MD) and dene)ethylidene]-3-ethyl-2 -thioxo-4-thiazolidinone (EtMD), in solution was studied by visible ab sorption, resonance Raman and IR spectroscopies. It was found that the most stable conformers for Et(Me)MD and EtMD of the four possible iso mers A-D were different. For Et(Me)MD, isomer A, which was stable in c yclohexane and carbon tetrachloride, was photoisomerized to isomer C b y 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 i n cyclohexane. Possible photoisomerization mechanisms are discussed ba sed on the rotational motions around the two C=C double bonds of the c entral conjugated system. (C) 1997 Elsevier Science B.V.