Solute-solvent and solvent-solvent interactions in the preferential solvation of Brooker's merocyanine in binary solvent mixtures

The ET polarity values of 4-[(1-methyl-4(1H)-pyridinylidene)ethylidene]-2,5-cyclohexadien-1-one, better known as Brooker's merocyanine, were collected in binary mixtures comprising a non-protic [acetonitrile, dimethyl sulfoxide (DMSO) and acetone] and a protic (water, methanol, ethanol, propan-2-ol and butan-1-ol) solvent. Data referring to mixed systems involving water and alcohols (methanol, ethanol, propan-2-ol and 2-methylpropan-2-ol) were also collected. These data were used in the investigation of the preferential solvation of the dye. Each solvent system was analyzed in terms of both solute–solvent and solvent–solvent interactions. These latter interactions are responsible for the synergistic effects observed for the ET parameter in many binary mixtures. All data were successfully fitted to a model based on solvent exchange equilibria, which allowed the separation of the different contributions of the solvent species in the solvation shell of the dye. This model was then successfully applied to explain the kinetic behavior of the methylation of sodium 4-nitrophenoxide in acetone–water mixtures. This result suggests that this solvatochromic strategy can be successful in the analysis of many other solvent-dependent kinetic and thermodynamic processes occurring in mixed solvents. Copyright © 2002 John Wiley & Sons, Ltd.