Ab initio investigation of reaction pathways for intramolecular charge transfer in dimethylanilino derivatives

Minimum-energy paths relevant for intramolecular charge transfer (ICT) in 4-(dimethylamino)benzonitrile (DMABN) and in 4-(dimethylamino)benzethyne (DMABE) have been theoretically investigated. They are expected to characterize the "twisted"-ICT (TICT) and the "rehybridized"-ICT (RICT) photoprocesses. Reaction paths for the L_a and L_b states and the RICT state have been determined at the CIS level. The energy profiles along the reaction paths have been calculated with the CIS, CASSCF, and CASPT2 electronic structure theory methods. It is concluded that stabilization of the L_a state along the TICT reaction path is responsible for appearance of the CT emission in DMABN in polar solvents. The absence of CT fluorescence in DMABE in polar solvents is tentatively attributed to fluorescence quenching by the RICT state.