Thermosolvatochromism of merocyanine polarity indicators in pure and aqueous solvents: Relevance of solvent lipophilicity

摘要

The following novel solvatochromic probes were synthesized: 2,6-dibromo-4-[( E)-2-( 1-alkylpyridinium-4-yl) ethenyl] phenolate, where the alkyl groups are methyl, n-butyl, n-hexyl, and n-octyl, respectively. Solvatochromism of three of these probes ( C-1, C-4, and C-8) was studied in 36 protic and aprotic solvents. A modified linear solvation energy relationship has been applied to the data obtained at 25 degrees C. Correlation of ( empirical) polarities with other solvent properties showed more dependence on lipophilicity than on basicity. A similar conclusion has been reached for a series of other solvatochromic indicators. Exceptions are those that carry acidic hydrogens, being biased toward solvent basicity. Thermosolvatochromism has been studied in mixtures of water with methanol, 1-propanol, acetonitrile, and DMSO. Thermosolvatochromic data have been treated according to a model that explicitly considers the presence in bulk solution of three "species": water, organic component, and solvent-water hydrogen-bonded aggregate. Solvation by the latter is favored over solvation by either of the two precursor solvents ( aqueous DMSO is an exception). Temperature increase resulted in desolvation of the probes, due to concomitant decrease of the structures of the component solvents. The above-mentioned modified solvation equation has been successfully applied to solvatochromism in aqueous methanol and aqueous 1-propanol.