Short- and Long-Range Solvation Effects on the Transient UV-Vis Absorption Spectra of a Ru(II)-Polypyridine Complex Disentangled by Nonequilibrium Molecular Dynamics

Evidence of subtle effects in the dynamic reorganization of a protic solvent in its first- and farther-neighbor shells, in response to the sudden change in the solute's electronic distribution upon excitation, is unveiled by a multilevel computational approach. Through the combination of nonequilibrium molecular dynamics and quantum mechanical calculations, the experimental time evolution of the transient T-1 absorption spectra of a heteroleptic Ru(II)-polypyridine complex in ethanol or dimethyl sulfoxide solution is reproduced and rationalized in terms of both fast and slow solvent reequilibration processes, which are found responsible for the red shift and broadening experimentally observed only in the protic medium. Solvent orientational correlation functions and a time-dependent analysis of the solvation structure confirm that the initial, fast observed red shift can be traced back to the destruction-formation of hydrogen bond networks in the first-neighbor shell, whereas the subsequent shift, evident in the [20-500] ps range and accompanied by a large broadening of the signal, is connected to a collective reorientation of the second and farther solvation shells, which significantly changes the electrostatic embedding felt by the excited solute.