Dynamic Heterogeneity in Ionic Liquids near and below the Glass Transition: Rotational Diffusion of Probes in 1-Butyl-3-methylimidazolium Hexafluorophosphate

Dynamic heterogeneity in the ionic liquid, 1-butyl-3-methylimidazolium hexafluorophosphate ([Bmim][PF$_{6}$]) ($T_{\mathrm{g}}=$ 196 K) near glass transition is investigated by conducting probe rotational diffusion experiments on rubrene and tetracene in [Bmim][PF$_{6}$] using the fluorescence recovery after photobleaching (FRAP) technique. Rotational anisotropy decays for the probes in the temperature range $T_{\mathrm{g}}$--6 to $T_{\mathrm{g}}+$4 are well described by the stretched exponential function. The stretching parameter is found to be constant for both probes with $\beta _{\mathrm{tetracene}}=$0.71 and $\beta_{\mathrm{rubrene}}=$ 0.88. The viscosity of [Bmim][PF$_{6}$] at high temperatures from 353 K to 283 K and near $T_{\mathrm{g}}$ from 203 K to 196 K can be fit by a single VFT equation. In the temperature range of our measurements, the rotational diffusion of these probes in [Bmim][PF$_{6}$] is decoupled from structural relaxation with a rotational correlation time following a fractional Debye-Stokes-Einstein (DSE) relation.