A THEORETICAL INVESTIGATION OF SOLVATOCHROMISM - APPLICATION TO MEROCYANINES SIMILAR TO COLORED FORMS OBTAINED BY FLASH-PHOTOLYSIS OF SPIROPYRANS

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 o r excited). These calculations were applied to the study of the solven t effects on the molecular structure and shifts of n --> pi and pi - -> pi absorption bands of alpha-oxo and gamma-oxo benzothiazolinic, benzimidazolinic and benzodithiolic merocyanines. A detailed analysis of the solvent-induced change in the intramolecular charge-transfer ba nds and the differences Delta mu between dipole moments of excited and ground states allowed to explain the inverted solvatochromism observe d for the gamma-oxo benzodithiolic merocyanine in comparison with the other ones. Our results confirmed and completed the rough experimental data available for these molecules.