Effects of Solubilized Water on the Relaxation Dynamics Surrounding 6-Propionyl-2-(N,N-dimethylamino)naphthalene Dissolved in 1-Butyl-3-methylimidazolium Hexafluorophosphate at 298 K

We report on the picosecond time-resolved fluorescence of 6-propionyl-2-(N,N-dimethylamino)naphthalene (PRODAN) dissolved in 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF₆]) at 298 K as a function of solubilized water in the [bmim][PF₆] phase. The observed solvent relaxation dynamics can be described by three components with apparent relaxation times that occur over a large time regime (< 15 ps to > 10 ns). The average relaxation dynamics become faster as the water concentration in the [bmim][PF₆] phase increases. Libration and vibration, ion ballistic motion, ion local basin exploration, and ion basin hopping, ion diffusion, and/or the ultrafast relaxation from water (or other small molecules/impurities) are suggested as possible reasons for the yet unquantified sub-15-ps dynamics. The sub-nanosecond dynamics are consistent with [PF₆] anion relaxation. This process was found to be water-dependent, slowing as the amount of solubilized water in the [bmim][PF₆] phase increased. We speculate that this slowing arises from the formation of 1:2 H-bonded (PF₆]⋯HOH⋯[PF ₆] complexes. The nanosecond dynamics are consistent with the cation, decreasing slightly with an increase in the amount of solubilized water. We suggest that the decrease in this relaxation time arises from a decrease in the bulk viscosity on adding water.