Excited-State Reactions of Coumarins. VII. The Solvent-Dependent Fluorescence of 7-Hydroxycoumarins

Abstract The excited-state reactions of 7-hydroxycoumarin (7HC) and 7-hydroxy-4-methylcoumarin (7H4MC) have been studied by measuring the absorption and fluorescence spectra in aqueous and organic solvents under acidic and basic conditions. Based on the solvent-dependent fluorescence of the fluorophores, the solvents could be classified into several groups: hydroxylic solvents, nonhydroxylic solvents with low dielectric constants, and those with high dielectric constants. In the hydroxylic solvent, protonation, deprotonation, and tautomerization were the main reactions, while in the nonhydroxylic solvent the formation of hydrogen bonding and ion pairs was essential. The fluorescence in water–ethanol mixtures and in aqueous sodium dodecyl sulfate solutions has also been examined in relation to the solvent-dependent spectra in pure hydroxylic and nonhydroxylic solvents.