HYBRID SIMULATIONS OF SOLVATION EFFECTS ON ELECTRONIC-SPECTRA - INDOLES IN WATER

Solute-solvent interactions and dynamics are simulated with a fully mo lecular hybrid method consisting of a semiempirical quantum mechanical method with singly excited configurations for the solute and classica l molecular dynamics (MD) for the solvent(H2O). The interactions are p urely electrostatic, with the solute being polarizable and sharing its charge information with the MD at 5 fs intervals. The solvent charges are fixed and the results are not sensitive to the point charges used . For the solute, the results depend on the;dipole moment much more th an on the point charge magnitudes leading to a given dipole. This meth od is applied to the spectral shifts, dynamics, linewidths, and free e nergies of indole and 3-methylindole (3MI) in water at 300 K, includin g the effect of geometry changes and clarifications concerning vertica l vs 0-0 transition predictions. Large fluorescence Stokes shifts are predicted, in fair agreement with observed values. The (1)L(a) excited state dipole is calculated to be about 12 D after solvent relaxation following excitation. This increase of about 5 D above that calculated in vacuum is caused by the solvent reaction field, and approximately doubles the calculated shift compared to that using the vacuum dipoles . There does not seem to be a need to invoke a solute-solvent excited state charge transfer complex (exciplex) to account for the large shif ts. About 50% of the Stokes shift occurs in similar to 15 fs with a Ga ussian response function, and the remainder is approximately an expone ntial with tau=400 fs. The fast component is created by small rotation al deviations in the trajectories of a few nearby waters. The change i n free energy of solvation upon excitation is found to be half the sum of the absorption and fluorescence shifts.