Water-induced quenching of salicylic anion fluorescence

Salicylic anion absorption and emission are studied in a variety of solvents and solvent mixtures. The large Stokes shift observed for this anion is taken to be indicative of a rapid excited state proton transfer reaction to its keto form. The changes in the Stokes shift in the various solvents can be well-correlated with changes in polarity/polarizability and hydrogen-bonding acidity. The time-resolved data can for the most part also be well-correlated with these properties. A notable exception is the behavior in water and water-rich mixtures. A significant decrease in fluorescence lifetime is observed, and the influence of temperature in pure water is much larger than in other neat liquids. As an explanation for these effects, an excited state intermolecular proton transfer reaction is suggested, from larger-sized water clusters, to the anion.