Electronic absorption spectra in a polar fluid: Theory and simulation

Abstract

We present calculations of the inhomogeneously broadened absorption line shape associated with a single vibronic transition in a polar chromophore at infinite dilution in a polar solvent. The permanent electric dipole moment of the solute is assumed to change upon electronic excitation. The line shape is related to the dipolar solvation free energy of a fictitious solute with a complex-valued dipole moment. This relation allows methods of equilibrium fluid theory to be applied to the line shape calculation. In order to assess the accuracy of theoretical predictions, we determine line shapes from Monte Carlo simulations for a dipolar hard sphere solute in a dipolar hard sphere solvent. Simulated line shapes are compared to predictions of the mean spherical approximation (MSA) and to a simplified nonlinear solvation theory. The MSA is found to show semiquantitative agreement with simulation results, despite the large value of the solute’s ground-state dipole moment and of its dipole moment change upon excitation. Simulation results are also compared to the relation between the first and second moments of the spectrum that is predicted by any linearized solvation theory. The comparison suggests that an ‘‘exact’’ linearized theory would provide accurate predictions for absorption spectra in polar solutions.