文摘
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 solventrelaxation dynamics can be described by three components with apparent relaxation times thatoccur over a large time regime (<15 ps to >10 ns). The average relaxation dynamics becomefaster 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 theultrafast relaxation from water (or other small molecules/impurities) are suggested as possiblereasons for the yet unquantified sub-15-ps dynamics. The sub-nanosecond dynamics areconsistent with [PF6] anion relaxation. This process was found to be water-dependent, slowingas the amount of solubilized water in the [bmim][PF6] phase increased. We speculate that thisslowing arises from the formation of 1:2 H-bonded [PF6]···HOH···[PF6] complexes. Thenanosecond dynamics are consistent with the cation, decreasing slightly with an increase inthe amount of solubilized water. We suggest that the decrease in this relaxation time arisesfrom a decrease in the bulk viscosity on adding water.