Interaction of Excited Aromatic Hydrocarbons with Aromatic Amines

Abstract

Abstract It has been established that many aromatic hydrocarbons in N,N-dimethyl aromatic amine solvents show their fluorescence spectra at much longer wavelengths than in such inactive solvents as n-hexane and toluene. Because the absorption spectra of the hydrocarbons in the amines do not show any essential change, it is confirmed that specific interaction between a hydrocarbon and an amine occurs only in the excited electronic state (the fluorescent state). It has been clearly demonstrated, from the relation between the frequencies of the long-wavelength fluorescence and the electron affinities of the excited aromatic hydrocarbons, the ionization potentials of the amines, etc., that this interaction can be ascribed to the charge transfer from the amine to the excited hydrocarbon, and that the long-wavelength fluorescence is a kind of charge-transfer fluorescence. At the temperature of liquid nitrogen, the interaction becomes much weaker because the rearrangement motions of molecules during the lifetime of the excited state, which are necessary for the charge-transfer complex formation, are more or less hindered in a rigid solvent at low temperatures. It has been confirmed that the interaction is still weaker in the phosphorescent state. This phenomenon has been explained by reference to the electronic processes in the complex.