Preferential solvation of styrylpyridinium dyes in binary mixtures of alcohols with hexane, dioxane, and dichloromethane.

The absorption maxima of N-alkyl(methyl, hexyl, and hexadecyl)-4-[(4-N,N-dimethylamino)styryl]pyridinium halides in binary solvents were analyzed for probing preferential solvation by any one of the solvents. The probes have a donor-acceptor system and the corresponding absorption bands are found to be solvent-sensitive. In neat solvents, excluding a few, reversal in solvatochromism was observed, identifying a polarity scale for solvatochromic switch, which appears around 45-50 E(T)(30) scale. The change in apolarity due to change in alkyl chain in the probe could not sway the scale, as the solvatochromism in neat solution is due to the chromophoric group only. When one solvent is considered as the parent solvent (hexane, dichloromethane, and dioxane) and the other as cosolvent (10 different alcohols), the solvation phenomena of these probes have been investigated. Due to solvent-solvent interactions, in some cases, hypo- and hyperpolarity in the solvent cage were observed. The preferential solvations of the dyes by these solvents were analyzed by consideration of the interactions of solvents with hydrophilic and hydrophobic groups of the probes. Tentative orientations of the solvent molecules around the probes were proposed. Disorder of the solvents around the probes was also considered for preferential solvation phenomena.