The Theory of Vibronic Interactions in Four Benzene Derivatives

The vibronic theory, developed to explain the presence of forbidden character in allowed electronic transitions, is reviewed and slightly expanded in light of new experimental data. On the basis of this theory, the changing polarizations of absorption and emission in some aryl amines are interpreted as evidence for vibronic mixing in the first excited singlet state. The experimental data are treated by the theory to extract the perturbation energy matrix elements. These are compared with the theoretically calculated perturbation energies. This assumes the apparently dominant vibronic interaction of the lowest singlet with the nearest allowed singlet, here the second excited state, from which the forbidden intensity is obtained. It is found that the vibronic perturbation energies for the amines in absorption are twice those of benzene in the first excited state. The calculations suggest that, unlike the lowest forbidden transition in benzene, the 1600 cm−1 b3g vibration is vibronically active. This reflects on the nature of the wave functions in the benzene derivatives. The pertubation energies for emission are smaller than for absorption, a fact that can be explained in terms of an expanded nuclear framework of the equilibrium excited state. A qualitative treatment of the triplet state shows that vibronically mixed triplets are spin‐orbit coupled to the singlet states, as in benzene. The great utility of vibronic theory and complete polarization measurements is therefore demonstrated.