Solvatochromism in pure and binary solvent mixtures: effects of the molecular structure of the zwitterionic probe

Abstract

The solvatochromic behavior of 2,6-dichloro-4-(2,4,6-triphenyl-1-pyridinio)phenolate (WB) was studied by UV–visible spectrophotometry in 32 pure solvents, in binary mixtures of 1-butanol–cyclohexane (BuOH–Cyhx), and of water with methanol, ethanol, 1-propanol, 2-butoxyethanol (2-BE), acetonitrile, 1,4-dioxane and THF. The solvent polarity, ET(33) in kcal mol−1, was calculated from the position of the longest-wavelength intramolecular charge-transfer absorption band of WB and the results were compared with those for 2,6-diphenyl-4-(2,4,6-triphenyl-1-pyridinio)phenolate [RB, ET(30)] and of 1-methyl-8-oxyquinolinium betaine [QB, ET(QB)]. For pure solvents, ET(33) is a linear function of ET(30), with a slope of practically unity. Steric crowding from the two ortho phenyl rings of RB hinders the formation of H-bonds with solvents, which results in similar susceptibilities of WB and RB to solvent acidity. For binary solvent mixtures, all plots of ET versus the mole fraction of 1-butanol or water are non-linear owing to preferential solvation of the probe by one component of the mixed solvent and, when applicable, to solvent micro-heterogeneity. Preferential solvation due to non-specific and specific probe–solvent interactions was calculated for BuOH–Cyhx and water–acetonitrile. Both solvation mechanisms contribute to the non-ideal behavior in the former binary mixture, whereas probe–solvent specific interactions dominate the solvatochromic behavior in the latter. The composition of the probe solvation shell was calculated. In aqueous alcohols, preferential solvation is by the alcohol. In water–aprotic solvent mixtures, preferential solvation of RB and WB is by the solvent which is present in lower concentration, whereas QB seems to form its own, water-rich solvation shell over a wide range of water concentration. Copyright © 2000 John Wiley & Sons, Ltd.