We present results of molecular dynamics simulations of solvation dynamics of coumarin 153 in dimethylsulfoxide ͑DMSO͒-water mixtures of different compositions (x D ϭ0.00, 0.25, 0.32, 0.50, 0.75, and 1.00͒ using an all-atom model for the solute probe. Results are reported for the global solvation responses of the simulated systems, as well as for the separate contributions from each cosolvent and the individual solute-site couplings to water and DMSO. The solvation dynamics is predominantly given by DMSO's contribution, even at low ͑25%͒ DMSO content, because of the preferential solvation of the probe. We find that the water molecules are only mildly coupled to the charge transfer in the coumarin, resulting in a small, largely diffusive, water relaxation component. Simulation results, including solvation responses, characteristic times, and Stokes shifts are compared with recent fluorescence upconversion experimental measurements showing good agreement for the relaxation but significant differences for the shifts.
We report the results of a molecular dynamics (MD) simulation study of solvation dynamics associated with electronic excitation of the coumarin 343 (C343) dye at the interface of water and zirconia (ZrO 2 ). We use an all-atom representation of the species present in this system. The ZrO 2 partial charges and C343 geometries and charge distributions in the ground and excited electronic states have been obtained from electronic structure calculations. Our work is inspired by recent time-resolved fluorescence experiments involving C343 adsorbed at the surface of zirconia nanoparticles dissolved in H 2 O and D 2 O (Pant, D.; Levinger, N. E. J. Phys. Chem. B 1999, 103, 7846). In addition to simulation of solvation dynamics, we investigate the structure and dynamics of the dye and water in the presence of a planar ZrO 2 interface. We also address several issues relevant to the interpretation of experiments, including the solvent isotope effects and the ionization state of the carboxylate group of C343 on solvation dynamics. We find that the neutral form (C343) of the dye is more strongly adsorbed at the ZrO 2 interface and that the water portion of the solvation response for this form of the dye is significantly faster than for the deprotonated form, C343 -. We also find that D 2 O-H 2 O solvent isotope effects on the solvation response of either form of the dye are quite modest and independent of the presence of zirconia. Rotational motion of the solute relative to the ZrO 2 surface contributes a significant, very slowly relaxing component to the interfacial solvation dynamics. We discuss the implications of our findings for the interpretation of the experimental data.
A molecular dynamics simulation study of the structure and dynamics of aqueous solutions of the squarate oxocarbon dianion (C 4 O 4 2-) is presented. Analyses of the solute-solvent radial distribution functions and hydrogen (H)-bonding distributions indicate a well-defined hydration shell consisting of approximately 18 water molecules. About 12 of these molecules are tightly H-bonded to the oxocarbon (an average of three molecules per oxygen atom) forming a highly symmetric solute-solvent aggregate, whereas the remaining six water molecules (not bonded to the ion) are more loosely distributed above and below the oxocarbon plane. The mean residence time for molecules that are H-bonded to the solute is estimated to be larger than 20 ps, whereas molecules that are not directly bonded to the ion are frequently exchanged with the bulk and remain within the first solvation shell for times of the order of a few picoseconds. For the dynamics, we find that the translational motions projected along the squarate plane and perpendicular to it are comparable to each other. The rotational diffusion coefficients for the main symmetry axes also indicate that the spinning and tumbling motions of the oxocarbon are roughly isotropic despite the shape of the solute. The squarate is found to perform fast librational motions inside the solvente cage, with a characteristic frequency of approximately 70 cm -1 , in close agreement with recent experimental Raman band shape analysis. These results are discussed in the light of the hydration structure.
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