Thermo-solvatochromism of zwitterionic probes in aqueous alcohols: effects of the properties of the probe and the alcohol

Abstract

Thermo-solvatochromism of 2,6-dichloro-4-(2,4,6-triphenylpyridinium-1-yl)phenolate, 1-methylquinolinium-8-olate and 4-[2-(1-methylpyridinium-4-yl)ethenyl]phenolate, in the temperature range 10 to 60 °C (ethanol and 2-methoxyethanol) and 25 to 60 °C (2-methyl-2-propanol), has been investigated in binary water–alcohol mixtures. Thermo-solvatochromic data have been treated according to a recently introduced model that explicitly considers the presence of 1∶1 alcohol–water H-bonded species, ROH–W, in bulk solution, and its exchange equilibria with water and alcohol in the probe solvation micro-sphere. The composition of the latter is given in terms of the appropriate set of solvent fractionation factors. These indicate that all probes are more solvated by alcohol than by water. Additionally, solvation by ROH–W is favoured over solvation by either of the two precursor solvents. Comparison of the data of six alcohols (three from a recent study, methanol, 1-propanol and 2-propanol) showed that solvation by alcohols is affected more by probe–ROH hydrophobic interactions than by H-bonding to the probe phenoxy oxygen. Temperature increase results in a gradual desolvation of all probes, due to a decrease in the H-bonding of all components of the binary solvent mixture. The energy of the temperature-induced desolvation (10 to 60 °C) was found to range from 0.8 to 2.5 kcal mol⁻¹.