Consequences of strong coupling between solvation and electronic structure in the excited state of a betaine dye

The electronic ground and excited-state structures of the betaine dye molecule pyridinium- N-phenoxide [4-(1-pyridinio)phenolate] are investigated both in the gas phase and in aqueous solution, using the reference interaction site model self-consistent-field (RISM-SCF) procedure within a CASSCF framework. We obtain the total free energy profiles in both the ground and excited states with respect to variation in the torsion angle between the phenoxide and pyridinium rings. We analyze the effect of solvent on the variation of the solute dipole moment and on the charge transfer character in the excited state. In the gas phase, it is shown that the potential energy profile in the excited-state decreases monotonically toward a perpendicular ring orientation and the dipole moment decreases along with decreasing charge localization. In water, the free energy surface for twisting is better characterized as nearly flat along the same coordinate for sterically accessible angles. These results are analyzed in terms of contributions of the solvation free energy, the solute electronic energy, and their coupling. Correspondingly, the dependence of the charge transfer character on solute geometry and solvation are analyzed, and the important roles in the excitation and subsequent relaxation processes for the betaine dye are discussed. It is found that there is considerable solute electronic reorganization associated with the evolution of solvation in the excited state, and it is suggested that this reorganization may contribute significantly to the early time evolution of transient spectra following photoexcitation.