Reinterpretation of the anomalous fluorescense of p-n,n-dimethylamino-benzonitrile

Two fluorescense bands of p-N,N-dimethylamino-benzonitrile (DAB, I) have previously been ascribed to two emitting states, the strongly polar 1Lb and the less polar 1Lb, or to the presence of an excimer. However, the parallel polarization of both emission bands in derivatives II and III, the effect of substituents, especially the steric effect on the dimethylamino group, and the lack of concentration effects in a broad concentration range, invalidate these hypotheses. We tentatively assign the emissions to the existence of two excited species differing in polarity and in the orientation of the —N(CH3)2 group: a lone pair perpendicular or parallel to the plane of benzene ring; in the latter case π*y orbitals of the —CN group seem to be involved in the excitation.