Theoretical Study of the Dual Fluorescence of 4-(N,N-Dimethylamino)benzonitrile in Solution

We have performed high-level quantum calculations of absorption and emission properties of 4-(N,N-dimethylamino)benzonitrile (DMABN) in gas phase and in polar solvents, including the solvent effects with an explicit mean field model. Two excited states of DMABN have been found and optimized, corresponding to the two excited states assumed by the generally accepted hypotheses for the dual fluorescence of this molecule: a locally excited (LE) state and a charge transfer (CT) state. The results show that, in the gas phase, the charge transfer state is severely distorted and higher in energy than the locally excited state, while in polar solvents, it becomes almost an ideal twisted intramolecular charge transfer state (TICT) and is stabilized with respect to the locally excited state and the Franck-Condon point. The relative free energies calculated for the two excited states in solution suggest that both states are accessible from initial Franck-Condon excitation, the charge transfer state being highly favored in aqueous solution and less so in tetrahydrofuran. The results support the validity of the TICT model in this system.