Solvatochromism and preferential solvation of Brooker's merocyanine in water–methanol mixtures

The excitation energy of Brooker's merocyanine in water–methanol mixtures shows nonlinear behavior with respect to the mole fraction of methanol, and it was suggested that this behavior is related to preferential solvation by methanol. We investigated the origin of this behavior and its relation to preferential solvation using the three‐dimensional reference interaction site model self‐consistent field method and time‐dependent density functional theory. The calculated excitation energies were in good agreement with the experimental behavior. Analysis of the coordination numbers revealed preferential solvation by methanol. The free energy component analysis implied that solvent reorganization and solvation entropy drive the preferential solvation by methanol, while the direct solute–solvent interaction promotes solvation by water. The difference in the preferential solvation effect on the ground and excited states causes the nonlinear excitation energy shift. © 2017 Wiley Periodicals, Inc. The solvatochromism of Brooker's merocyanine (BM) in water–methanol mixtures was investigated using the three‐dimensional reference interaction site model self‐consistent field (3D‐RISM‐SCF) method. Three‐dimensional‐RISM‐SCF reproduced the experimental nonlinear behavior of the excitation energy with respect to the mole fraction of methanol. Analyses of the solute–solvent binding energies and coordination numbers revealed that the origin of the nonlinear behavior is preferential solvation by methanol.