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

The electronic structure of the betaine dye molecule, pyridinium- N-phenoxi de [4-(1-pyridinio)phenolate] including the effects of geometry and polar s olvents, has been studied at an ab initio level using the reference interac tion site model self-consistent-field (RISM-SCF) method. Acetonitrile (CH3C N) and water (H2O) were selected as polar solvents. We obtain both the opti mized solute geometry in solution and the total free energy profile with re spect to variation in the torsion angle between the pyridinium and phenoxid e rings and analyze the various electronic and solvation contributions. The betaine molecule in the gas phase has a twisted geometry, which is slightl y more twisted in solution. In acetonitrile, the calculated structure shows good agreement with earlier semiempirical results for the minimum free ene rgy structure. It is shown that the solute dipole moment is strongly enhanc ed in polar solution, also in accord with earlier semiempirical calculation s. 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 mo re negative with increasing twist. Electron correlation in the solute molec ule is shown to play an important role in the torsional free energy, destab ilizing the twisted form. This destabilization decreases by a factor of 4 f rom 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.