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

The solvation dynamics of benzonitrile (C6H5CN) after the 2 (1)A(1) <-- 1 ( 1)A(1) vertical transition in water (H2O), methanol (CH3OH), and acetonitri le (CH3CN) solvents is studied with the reference interaction site model se lf-consistent field (RISM-SCF) method. The evolution of solute electronic s tates associated with the solvent relaxation is described by a time-depende nt RISM-SCF method, incorporating the time-dependent solute-solvent site-si te radial distribution functions, which are derived from the surrogate line ar response theory. Ab initio electronic structure calculations reveal that the 2 (1)A(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 dipo le moment is enhanced in the solutions, which provides the red shift of sim ilar to 6000 cm(-1) in the vertical excitation energy. The solvent relaxati on further increases the charge polarization in solute, indicating the elec tronic state of excited C6H5CN is sensitive to the electrostatic field comi ng from the solvent. The dynamic Stokes shift is characterized by the solva tion time correlation function (STCF). The calculated STCFs show that the s olvent relaxation exhibits a nonexponential behavior and almost completes w ithin 5 ps in H2O and CH3CN while a long-time tail is observed up to 20 ps in CH3OH. The slow component of the decay rate is consistent with other sim ulation calculations though the fast one is smaller. In order to analyze th e solute charge polarization during the solvent relaxation, the solute char ge time correlation function (CTCF) is calculated and the resultant CTCFs a re discussed in terms of the solvent charge polarization in the vicinity of solute molecule. (C) 1999 American Institute of Physics. [S0021-9606(99)50 923-9].