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 o f the twisting and wagging motion of the amino group. Theoretical calc ulations have been performed using the complete active space (GAS) SCF method in combination with multiconfigurational second Order perturba tion theory (CASPT2): Basis sets of the ANO-type (C,N/3s2p1d and H/2s) were employed. Ground state geometries were optimized at the CASSCF l evel. The excitation energies were computed as function of a twist ang le, 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 a nalyzed. The results fully confirm the twist intramolecular charge tra nsfer (TICT) model proposed to explain the dual fluorescence phenomena occurring in DMABN. The absence of the low frequency part of the fluo rescence spectrum in ABN in explained by the shape of the potential en ergy surface along the isomerization path, due to the large energy gap s among the interacting states, which prevents the amino group from ro tating into the TICT state. Calculated transition energies (absorption and emission), structural, and electrical properties of the ground an d excited states are in agreement with available experimental informat ion.