Solvent effects on the fluorescence properties of anilines

Fluorescence spectra, quantum yields and lifetimes are comparatively studied for aniline, N,N-dimethylaniline (DMA) and some further alkylsubstituted and rigid aniline derivatives in various solvents. A good correlation of Stokes shift with macroscopic solvent parameters is found for DMA but not for aniline. General and specific solvent effects are distinguished by studying the functional dependence of various spectroscopic and photophysical parameters on the polar co-solvent concentration in binary solvent mixtures. Excited state complexation of primary and secondary amines with alcohols is demonstrated by the double-exponential decay of the monomer fluorescence and the grow-in of exciplex emission found in aniline—ethanol—n-hexane ternary systems. Exciplex formation is attributed to solvent—donor hydrogen bonding and primarily causes enhanced intersystem crossing compared with that in the free molecule, an effect also evoked by N-methylation. Non-specific long-range interactions give rise to a general reduction in the non-radiative deactivation rate, with the exception of o,o′-dimethyl-substituted anilines, for which an increase in this rate was found as the bulk becomes polar.