The stable and photochemical isomers of some merocyanines: A H-1 NMR and 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 consistent results. Some information about the torsional dynamics of these dyes in the ground state was obtained from a rough H-1 NMR line shape analysis, and H-1 NMR experiments performed on photolysed samples made it possible to identify the conformations of the longest-lived photoisomers of three compounds and to provide an estimate of their lifetimes. The effects of an increase in the solvent polarity on the structures and absorption spectra of the four dyes were investigated by, respectively, measuring their H-1 NMR spectra in three more solvents with 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 polarity effects could be interpreted according to the generally accepted description of the merocyanine chromophore as the resonance hybrid of a neutral and a charge-separated form.