Ground- and excited-state solvation of a squaraine dye by water in dioxane

Dioxane/water mixtures were used in order to assess the water effect on the photophysics of two squaraine dyes. It was found that a long-range hydrogen-bonding interaction exists in the ground state, without preferential solvation by water. However, in the excited state a more specific interaction is responsible for a pronounced quenching effect. Indeed, in the unsubstituted squaraine bis[4-(dimethylamine)phenyl] squaraine an intermolecular hydrogen bonding leads to an increase of internal conversion due to accepting vibrational modes created by this interaction, while in the symmetrically dihydroxy-substituted squaraine, an intramolecular hydrogen bond prevents the occurrence of the intermolecular interaction with water.