title: Intramolecular Charge-Transfer Excited-State Processes in 4-(N,N-Dimethylamino)benzonitrile: The Role of Twisting and the πσ* State.
abstract: The structural processes leading to dual fluorescence of 4-(dimethylamino)benzonitrile in the gas phase and in 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 basis of the conductor-like screening model. The MRCI method was used for computing the nonadiabatic interaction between the two lowest excited ππ* states (S2(La, CT) and S1(Lb, LE)) and the corresponding minimum on the crossing seam (MXS) whereas the ADC(2) calculations were dedicated to assessing the role of the πσ* state. The MXS structure was found to have a twisting angle of ∼50°. The branching space does not contain the twisting motion of the dimethylamino group and thus is not directly involved in the deactivation process from S2 to S1. Polar solvent effects are not found to have a significant influence on this situation. Applying Cs 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.
authors.affiliation: †Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, Sofia, Bulgaria.
authors.forename: Ivelina
authors.initials: I
authors.lastname: Georgieva
authors.affiliation: ‡Department of Chemistry and Biochemistry, Texas Tech University, Lubbock, Texas 79409-1061, United States.|§Institute for Theoretical Chemistry, University of Vienna, A-1090 Vienna, Austria.
authors.forename: Adélia J A
authors.initials: AJ
authors.lastname: Aquino
authors.affiliation: §Institute for Theoretical Chemistry, University of Vienna, A-1090 Vienna, Austria.
authors.forename: Felix
authors.initials: F
authors.lastname: Plasser
authors.affiliation: †Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, Sofia, Bulgaria.
authors.forename: Natasha
authors.initials: N
authors.lastname: Trendafilova
authors.affiliation: ∥Institute for Theoretical Chemistry, University of Stuttgart, 70569 Stuttgart, Germany.
authors.forename: Andreas
authors.initials: A
authors.lastname: Köhn
authors.affiliation: ‡Department of Chemistry and Biochemistry, Texas Tech University, Lubbock, Texas 79409-1061, United States.|§Institute for Theoretical Chemistry, University of Vienna, A-1090 Vienna, Austria.
authors.forename: Hans
authors.initials: H
authors.lastname: Lischka