Molecular-dynamics simulations of solvent effects in the intramolecular charge transfer of 4-(N,N-dimethylamino)benzonitrile

Abstract

The microscopic mechanism of solvation of the ground, locally-excited and charge-transfer singlet states of 4-(N,N-dimethylamino)benzonitrile in cyclopentane and acetonitrile is examined. These states play the decisive role in the dual fluorescence observed for this compound in polar solvents. Minimum-energy paths, potential-energy profiles and atomic charges along the dimethylamino twist obtained by high-level ab initio calculations in vacuum were combined with molecular-dynamics simulations in the solvent. Electronic energies, energy gaps and pair-distribution functions were evaluated using equilibrium molecular-dynamics simulations. Solvation effects in cyclopentane turn out to be negligible. In acetonitrile, the charge-transfer state is much more stabilized than the locally-excited state and develops an electronic energy minimum at the fully-twisted geometry. Fluorescence at this geometry is strongly red-shifted, mainly due to the increase of the ground-state energy upon twisting. The calculated vertical excitation energies are in good agreement with experiment, whereas the Stokes' shift in acetonitrile is underestimated by at least 0.3 eV. The state-dependent solvation in acetonitrile is found to be mainly caused by a more pronounced orientation of the solvent towards the donor and acceptor groups in the charge-transfer state. The solvation dynamics in acetonitrile directly after absorption was investigated by nonequilibrium simulations. They reveal the solvent-driven change of the excited-state energy-level pattern. The solvent response is almost linear, and the initial response is very rapid with an average solvation time of 0.2 ps. The present calculations support the twisted intramolecular charge-transfer mechanism for the occurrence of dual fluorescence, although a final confirmation has to await further studies.