Solvent effects on solute electronic structure and properties: Theoretical study of a betaine dye molecule in polar solvents

The electronic structure of the betaine dye molecule, pyridinium- N-phenoxide [4-(1-pyridinio)phenolate] including the effects of geometry and polar solvents, has been studied at an ab initio level using the reference interaction site model self-consistent-field (RISM-SCF) method. Acetonitrile (CH3CN) and water (H2O) were selected as polar solvents. We obtain both the optimized solute geometry in solution and the total free energy profile with respect to variation in the torsion angle between the pyridinium and phenoxide rings and analyze the various electronic and solvation contributions. The betaine molecule in the gas phase has a twisted geometry, which is slightly more twisted in solution. In acetonitrile, the calculated structure shows good agreement with earlier semiempirical results for the minimum free energy structure. It is shown that the solute dipole moment is strongly enhanced in polar solution, also in accord with earlier semiempirical calculations. However, in solution, there is relatively little change in dipole moment with changes in the torsion angle, in contrast to the marked variation in the gas phase. Correspondingly, the solvation free energy is only weakly more negative with increasing twist. Electron correlation in the solute molecule is shown to play an important role in the torsional free energy, destabilizing the twisted form. This destabilization decreases by a factor of 4 from the gas phase to water, with increasing charge localization induced by the solvent. The implications of these results for interaction site models of charge-separated conjugated molecules in solution are discussed.