Solvation dynamics of room-temperature ionic liquids: evidence for collective solvent motion on sub-picosecond timescales

Little is known about the microscopic mechanism of solvation dynamics in room-temperature ionic liquids, but experimental studies have found that the solvent response has both sub-picosecond and nanosecond timescale components. We present the results of molecular dynamics calculations of the time-resolved fluorescence response of a chromophore in an ionic liquid, and analyze the solute–solvent interactions responsible for the observed signal. We find evidence for collective cation–anion motion on sub-picosecond timescales, contradicting earlier work suggesting the sub-picosecond response is purely anionic. We present an alternative hypothesis to explain our results and the widely disparate timescales for solvation response.