Theoretical Study of the Internal Charge Transfer in Aminobenzonitriles

The lower excited states for the molecules aminobenzonitrile (ABN) and (dimethylamino)benzonitrile (DMABN) have been studied as a function of the twisting and wagging motion of the amino group. Theoretical calculations have been performed using the complete active space (GAS) SCF method in combination with multiconfigurational second Order perturbation theory (CASPT2): Basis sets of the ANO-type (C,N/3s2p1d and H/2s) were employed. Ground state geometries were optimized at the CASSCF level. The excitation energies were computed as function of a twist angle, where the amino group is rotated with respect to the benzonitrile plane, and for two values of the wagging angle (0 and 21 degrees). The influence of the wagging angle in the nontwisted molecules was also analyzed. The results fully confirm the twist intramolecular charge transfer (TICT) model proposed to explain the dual fluorescence phenomena occurring in DMABN. The absence of the low frequency part of the fluorescence spectrum in ABN in explained by the shape of the potential energy surface along the isomerization path, due to the large energy gaps among the interacting states, which prevents the amino group from rotating into the TICT state. Calculated transition energies (absorption and emission), structural, and electrical properties of the ground and excited states are in agreement with available experimental information.