Solvent effects within the CS INDO method. Geometrical distortion and solvatochromism of merocyanine dyes

This work represents the first step of a theoretical study aiming at explicitly including solvation in the CS INDO CI calculations of ground and excited state properties of organic dyes. The reported theoretical treatment falls within the electrostatic continuum theories and introduces solute-solvent interaction in the SCF calculation according to Klopman's solvaton model. The procedure was tested on two merocyanines characterized by opposite solvatochromic behaviours. It was found that solvaton systems most suitable for pi-conjugated donor-acceptor dyes, like merocyanines, can be built from the net pi-electron charges. The results show that the combined solvaton/CS INDO scheme is able to describe rather well the modifications of both geometries and absorption spectra of merocyanines as a function of the medium polarity. The advantages of the proposed approach with respect to other semi-empirical procedures are discussed. (C) 1998 Elsevier Science B.V. All rights reserved.