Solvent Effects on Linear and Nonlinear Optical Properties of Donor−Acceptor Polyenes:  Investigation of Electronic and Vibrational Components in Terms of Structure and Charge Distribution Changes

We investigate the influence of solvation media upon the relationship among structure, spatial distribution of electron density, and linear and nonlinear electric properties for two series of push−pull π-conjugated molecules. The analysis is performed on both electronic and vibrational components of static polarizability and first hyperpolarizability, and the effects solvent induce on them are analyzed singularly within the framework of the polarization continuum model. Solvent is found to affect the extent of charge separation induced in the ground state of these molecules. This charge separation leads to a geometric distortion, measured by the bond-length-alternation (BLA) parameter, which shows a solvent-induced evolution of the molecular geometry from a neutral, bond-alternated polyene-like structure, to a partially ionic cyanine-like structure, and ultimately to an ionic bond-alternated structure. As a consequence large changes in the linear and nonlinear response properties are found, in both their electronic and vibrational contributions. Regarding the latter, we recall that studies on vibrational (hyper)polarizabilities for molecular systems in solution are presented here for the first time.