Excited State Hydrogen Bond Dynamics:  Coumarin 102 in Acetonitrile−Water Binary Mixtures

Nathan P. Wells, Matthew J. McGrath, J. Ilja Siepmann, David F. Underwood, and David A. Blank*
Department of Chemistry, University of Minnesota, 207 Pleasant Street SE, Minneapolis, Minnesota 55455
J. Phys. Chem. A, 2008, 112 (12), pp 2511–2514
DOI: 10.1021/jp710889t
Publication Date (Web): February 15, 2008
Copyright © 2008 American Chemical Society
*

 Corresponding author. E-mail:  blank@umn.edu.

Abstract

Abstract Image

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 performed on mixtures of composition xCH3CN = 0.25, 0.50, 0.75, and 1.00. At low water concentrations (xH2O ≤ 0.25) the solvent response is consistent with previous measurements probing dipolar solvation. With increasing water concentration (xH2O ≥ 0.50) an additional response is found subsequent to dipolar solvation, exhibited as a rapid gain in the solvent's polarizability on a 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 C102 simultaneously weakens and strengthens hydrogen bonding in complexes with two inequivalently bound waters.

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History

  • Published In Issue March 27, 2008
  • Received November 14, 2007
    Revised December 20, 2007

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