Excitation energy effects on the fluorescence quantum yield in solution. Deactivation pathways for aromatic amines

Abstract

The dependence of the fluorescence quantum yield, Q_F, on the energy of the exciting light was examined for a number of aromatic amines (aniline, N,N-dimethylaniline, 2-phenylethylamine, etc.) in various polar and non-polar solvents down to 200 nm. Q_F of aniline is constant throughout the first absorption system but decreases when the excitation reaches the second band. This effect is most pronounced in hydrocarbon solutions but small in polar solvents capable of forming hydrogen bonds, with the only exception being water. For N,N-dimethylaniline a wavelength dependence of Q_F is observed only in aqueous solutions. Concomitant to the Q_F decrease the H₂ evolution in hydrocarbons and electron ejection in water increase. These effects are also observed for solutions of 2-phenylethylamine and are explained by the formation of weak intramolecular complexes. A further pathway, characteristic of the benzoic ring, is assumed to be responsible for the wavelength dependence of protonated anilines. The derived deactivation pattern is of general validity for simple polar aromatics.