Equilibrium and dynamical aspects of solvation of Coumarin-151 in polar nanoclusters

Molecular dynamics experiments have been performed to study equilibrium aspects of the solvation of Coumarin-151 in polar nanoclusters containing N-s = 5, 10, and 50 water and methanol molecules. In small aggregates, both solvents show preferential solvation of the amino group of the solute. Whereas in aqueous aggregates the dye molecule shows a propensity to reside on the cluster surface for all sizes investigated, the 50 methanol cluster exhibits clear signs of more uniform, bulklike solvation. Using nonequilibrium simulations, we also studied the solvation dynamics upon an electronic excitation of the probe. At temperatures close to T = 200 K, small clusters exhibit two well differentiated equilibrium solvation structures for the excited states of the solute. Interconversions between these structures take place in a time scale much longer than the one characterizing, the solvation relaxation. In N-s = 5 clusters, the nonequilibrium responses are exclusively dominated by a fast inertial relaxation lasting less that 0.1 ps; for N-s = 50, the relaxations exhibit a slow diffusional regime that dominates the long time behavior as well. The overall response of the largest clusters is also analyzed in terms of linear response theories.