Solute rotation and solvation dynamics in a room-temperature ionic liquid

Steady-state spectra, rotation times, and time-resolved emission spectra of the probe 4-aminophthalimide (4-AP) in the ionic liquid 1-n-butyl-3-methylimidazolium hexafluorophosphate ([bmim⁺][PF₆^-]) were measured over the temperature range 298-355 K. The steady-state spectroscopy indicates that the solvation energetics of 4-AP in [bmim⁺][PF₆^-] are comparable to those of 4-AP in highly polar but aprotic solvents such as dimethylformamide and acetonitrile (π* ∼ 0.8, E_T^N ∼ 0.4). The rotation of 4-AP in [bmim⁺][PF₆^-] and in more conventional aprotic solvents generally conforms to the expectations of simple hydrodynamic models. Other than the fact that [bmim⁺][PF₆^-] is highly viscous, nothing distinguishes the rotation of 4-AP in this ionic liquid from its rotation in more conventional polar aprotic solvents. Time-dependent emission spectra, recorded with an instrumental response of 25 ps, indicate that solvation dynamics in [bmim⁺][PF₆^-] occur in two well-separated time regimes. Near to room temperature, the observable response takes place in the 0.1-2 ns time range. This component can be described by a stretched exponential time dependence with an exponent of 0.6-0.7, indicative of strongly nonexponential relaxation. The integral time of the observed component of solvation is proportional to the rotation time of 4-AP and to solvent viscosity, suggesting the involvement of substantial solvent rearrangement. In addition to this relatively slow component, more than half of the solvation response in [bmim⁺][PF₆^-] is faster than can be detected in these experiments, that is, takes place in <5 ps.