Intramolecular Charge-Transfer Excited-State Processes in 4-(N,N-Dimethylamino)benzonitrile: The Role of Twisting and the πσ* State

The structural processes leading to dual fluorescence of 4-(dimethylamino)benzonitrile in the gas phase and acetonitrile solvent were investigated using a combination of multireference configuration interaction (MRCI) and the second-order algebraic diagrammatic construction (ADC(2)) methods. Solvent effects were included on the bash of the conductor-like screening model. The MRCI method was used far computing the nonadiabatic interaction between the two lowest excited pi pi* states (S-2(L-a, CT) and S-1(L-b, LE)) and the corresponding minimum on the crossing seam (MXS) whereas the ADC(2) calculations were dedicated to assessing the role of the pi sigma* State. The MXS structure was found to have a twisting angle of similar to 50 degrees. The branching space does not contain the twisting motion of the dimethylamino group and thus is not directly involved in the deactivation process from S-2 to S-1. Polar solvent effects are not found to have a significant influence on this situation. Applying C-s symmetry restrictions, the ADC(2) calculations show that CCN bending leads to a strong stabilization and to significant charge transfer (CT). Nevertheless, this structure is not a minimum but converts to the local excitation (LE) structure on releasing the symmetry constraint. These findings suggest that the main role in the dynamics is played by the nonadiabatic interaction of the LE and CT states and that the main source for the dual fluorescence is the twisted internal charge-transfer state in addition to the LE state.