Solvation dynamics of benzonitrile excited state in polar solvents: A time-dependent reference interaction site model self-consistent field approach

摘要

The solvation dynamics of benzonitrile (C_6H_5CN) after the 2 ~1A_1 <- 1 ~1A_1 vertical transition in water (H_2O), methanol (CH_3OH), and acetonitrile (CH_3CN) solvents is studied with the reference interaction site model self-consistent field (RISM-CSF) method. The evolution of solute electronic states associated with the solvent relaxation is described by a time-dependent RISM-SCF method, incorporating the time-dependent solute-solvent site-site radial distribution functions, which are derived from the surrogate linear response theory. Ab initio electronic structure calculations reveal that the 2 ~1A_1 state is of ionic nature whose dipole moment is larger by 2.41 D than that of the ground state. It is found that the excited state dipole moment is enhanced in the solutions, which provides the red shift of approx 6000 cm~(-1) in the vertical excitation energy. The solvent relaxation further increases the charge polarization in solute, indicating the electronic state of excited C_6H_5CN is sensitive to the electrostatic field coming from the solvent. The dynamic Stokes shift is characterized by the solvation time correlation function (STCF). The calculated STCFs show that the solvent relaxation exhibits a nonexponential behavior and almost completes within 5 ps in H_2O and CH_3CN while a long-time tail is observed up to 20 ps in CH_3OH. The slow component of the decay rate is consistent with other simulation calculations though the fast one is smaller. In order to analyze the solute charge polarization during the solvent relaxation, the solute charge time correlation function (CTCF) is calculated and the resultant CTCFs are discussed in terms of the solvent charge polarization in the vicinity of solute molecule.