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

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 solve nts, in binary mixtures of l-butanol-cyclohexane (BuOH-Cyhx), and of water with methanol, ethanol, I-propanol, 2-butoxyethanol (2-BE), acetonitrile, 1 ,4-dioxane and THF. The solvent polarity, E-T(33) in kcal mol(-1), was calc ulated from the position of the longest-wavelength intramolecular charge-tr ansfer absorption band of WE and the results were compared with those for 2 ,6-diphenyl-4-(2,4,6-triphenyl-1-pyridinio)phenolate [RB, E-T(30)] add of 1 -methyl-8-oxyquinolinium betaine [QB, E-T(QB)]. For pure solvents, ET(33) i s a linear function of ET(30), with a slope of practically unity. Steric cr owding from the two ortho phenyl rings of RE hinders the formation of H-bon ds with solvents, which results in similar susceptibilities of WE and RE to solvent acidity. For binary solvent mixtures, all plots of ET versus the m ole fraction of l-butanol or water are nonlinear owing to preferential solv ation of the probe by one component of the mixed solvent and, when applicab le, to solvent micro-heterogeneity. Preferential solvation due to non-speci fic and specific probe-solvent interactions was calculated for BuOH-Cyhx an d water-acetonitrile. Both solvation mechanisms contribute to the non-ideal behavior in the former binary mixture, whereas probe-solvent specific inte ractions dominate the solvatochromic behavior in the latter. The compositio n of the probe solvation shell was calculated. In aqueous alcohols, prefere ntial solvation is by the alcohol. In water-aprotic solvent mixtures, prefe rential solvation of RE and WE 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 (C) 2000 John Wiley & Sons, Ltd.