Excited state hydrogen bond dynamics: Coumarin 102 in acetonitrile-water binary mixtures

摘要

The time dependent change in the intermolecular response of solvent molecules following photoexcitation of Coumarin 102 (C102) has been measured in acetonitrile-water binary mixtures. Experiments were per-formed on mixtures of composition X-CH3CN 0.25, 0.50, 0.75, and 1.00. At low water concentrations (x(H2O) <= 0.25) the solvent response is consistent with previous measurements probing dipolar solvation. With increasing water concentration (X-H2O >= 0.50) an additional response is found subsequent to dipolar solvation, exhibited as a rapid gain. in the solvent's polarizability on a similar to 250 fs time scale. Monte Carlo simulations of the C102: binary mixture system were performed to quantify the number of hydrogen-bonding interactions between C102 and water. These simulations indicate that the probability of the C102 solute being hydrogen bound with two water molecules, both as donors at the carbonyl site, increases in a correlated fashion with the amplitude of the additional response in the measurements. We conclude that excitation of C 102 simultaneously weakens and strengthens hydrogen bonding in complexes with two inequivalently bound waters.