Coupled molecular dynamics/semiempirical simulation of organic solutes in polar liquids. II. Coumarin 153 in methanol and acetonitrile

In this paper we present a coupled molecular dynamics/semiempirical simulation of the solvation of the dye Coumarin 153 (C153) in two solvents, methanol and acetonitrile. In order to account for the solute electronic polarizability we use a semiempirical description to determine the charge distribution of the dye during the simulation. Solute-solvent and solvent-solvent interactions are described by empirical potentials. We examine the structure of the solvation shell, the purely electrostatic part of the solute-solvent interactions, shifts of the absorption and emissions spectra and the solvation dynamics of C153 in both solvents. In contrast with our first study of naphthalene in acetonitrile, the equilibrium simulations show structural changes in the solvation shell when electronic polarizability is included. The inclusion of electronic polarizability also enhances solute-solvent electrostatic interactions. Therefore, an increase of absorption and emission redshifts is observed compared to simulations with a nonpolarizable solute. While the computed absorption shift is in excellent agreement with experimental data, the emission shift calculations fail. Possible reasons for this failure are discussed. The solvation dynamics shows a considerable slowing down compared to the nonpolarizable solute. Nonetheless, linear response is still valid in these systems. Some differences with the continuum model of solvation are pointed out. (C) 2001 American Institute of Physics.