Picosecond reorientational dynamics of resorufin: Correlations of dynamics and liquid structure

We describe the reorientational dynamics in polar fluids of resorufin, a rigid aromatic probe molecule, whose anionic site permits several specific intermolecular interactions to be investigated. Using the novel technique of difference-frequency multiple modulation picosecond absorption spectroscopy, we report the first measurements on the ground-state dynamics of resorufin in a series of protic alcohols, aprotic fluids, and the binary system of propanol:water. Whereas the reorientation times τ_rot in the alcohols can be well described by Debye-Stokes-Einstein equation with stick boundary conditions, the τ_rot data in acetonitrile, dimethylformamide, and dimethyl sulfoxide show faster reorientation times and quite different behavior from hydrodynamic expectations, even if slip boundary conditions were invoked. Finally, resorufin in formamide, water, and water:propanol systems shows considerable deviation from the τ_rot pattern established in the alcohols. A model is proposed in which the lifetime of the local and site-specific intermolecular interaction between resorufin and its nearest-neighbor solvent molecules is compared to the cooperative relaxation time of the overall three-dimensional or two-dimensional network of solvent interactions typical of hydrogen-bonded systems. We conclude that both long-wavelength hydrodynamic effects and the nature and behavior of the first solvation shell influence rotational dynamics.