A theoretical investigation of solvatochromism. Application to merocyanines similar to colored forms obtained by flash-photolysis of spiropyrans Original Research Article

Solvatochromism of colored photomerocyanines was discussed within the framework of the CNDO/S-CI method including solvent effects through an extension of a virtual-charge model to an indifferent state (ground or excited). These calculations were applied to the study of the solvent effects on the molecular structure and shifts of n → π* and π → π* absorption bands of α-oxo and γ-oxo benzothiazolinic, benzimidazolinic and benzodithiolic merocyanines. A detailed analysis of the solvent-induced change in the intramolecular charge-transfer bands and the differences Δμ between dipole moments of excited and ground states allowed to explain the inverted solvatochromism observed for the γ-oxo benzodithiolic merocyanine in comparison with the other ones. Our results confirmed and completed the rough experimental data available for these molecules.