THE STABLE AND PHOTOCHEMICAL ISOMERS OF SOME MEROCYANINES - A H-1-NMRAND THEORETICAL CS INDO STUDY OF THE STRUCTURES AND ELECTRONIC-SPECTRA

The conformations and electron distributions of four merocyanines were investigated by H-1 NMR spectroscopy in low-polarity solvents and CS INDO calculations on the isolated molecules. The two approaches gave c onsistent results. Some information about the torsional dynamics of th ese dyes in the ground state was obtained from a rough H-1 NMR line sh ape analysis, and H-1 NMR experiments performed on photolysed samples made it possible to identify the conformations of the longest-lived ph otoisomers of three compounds and to provide an estimate of their life times. The effects of an increase in the solvent polarity on the struc tures and absorption spectra of the four dyes were investigated by, re spectively, measuring their H-1 NMR spectra in three more solvents wit h increasing dielectric constants, and CS INDO CI calculations of the electronic spectra, including solvent shift effects, evaluated within the classical solvaton model. All the results related to the medium po larity effects could be interpreted according to the generally accepte d description of the merocyanine chromophore as the resonance hybrid o f a neutral and a charge-separated form. (C) 1997 Elsevier Science S.A .