Solvation Dynamics in Dipolar−Quadrupolar Mixtures:  A Computer Simulation Study of Dipole Creation in Mixtures of Acetonitrile and Benzene

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Molecules, spectroscopy, kinetics, environment, & general theory, 2002-08, Vol.106 (30), p.6922-6934
• Main Authors: Ladanyi, Branka M, Perng, Baw-Ching
• Format: Article
• Language: English
• ISSN: 1089-5639; 1520-5215
• DOI: 10.1021/jp013580a

We present here the results of molecular dynamics simulation of solvation dynamics (SD) in benzene, acetonitrile, and their mixtures corresponding to three sets of acetonitrile mole fractions, x ac = 0.20, 0.50, and 0.75, at temperature and densities appropriate for ambient conditions. The change in solute−solvent interactions triggered by solute electronic S0 → S1 excitation is represented as dipole creation in a benzene-like solute. We find that both solvent components are active participants in the SD event, with electrostatic interactions of the dipolar solute with quadrupolar benzene molecules making an important contribution to the solvation mechanism and the steady-state Stokes shift in the fluorescence spectrum. Our model solute is preferentially solvated by acetonitrile in its S1 state and the enhancement in the local acetonitrile concentration contributes significantly to the solvation time scale, especially in the benzene-rich mixture, where this process becomes considerably slower than the solvation coordinate relaxation in either solvent, in agreement with experimental findings. We investigate the contributions to SD from concentration fluctuations by monitoring the time evolution in the solvation structure. We find that in many respects the way that these fluctuations contribute to SD in benzene−acetonitrile mixtures resembles their contributions to the SD mechanism in mixtures of dipolar molecules.