Solvatochromic properties of two related N-salicylidenanilines with dual fluorescence

The photophysical properties of two N-salicylidene p-X-anilines (XCN, COOH) were studied in a series of solvents with varying polarity and proticity by steady state absorption and emission spectroscopy. The two compounds are dually fluorescent in solution due to an excited state proton transfer. The solvatochromic shifts were used to connect the solvent polarity parameter to the dipole moment in excited state using several approaches. The multiparameter Kamlet-Taft and Catalán solvent scales with four parameters have been employed to analyze the specific and non-specific solvent-solute interactions. The absorption spectra are less sensitive to the substitution at the aniline ring and to the solvent properties than the fluorescence spectra. Especially the ratio between the normal and abnormal Stokes shifted fluorescence intensities is dependent on the polarity and hydrogen bonding abilities of the solvent. The increasing solvent viscosity led to a decrease in the ratio of the intensities of the two emissions.