Heterogeneous dynamics in ionic liquids at the glass transition: Fluorescence recovery after photobleaching measurements of probe rotational motion from Tg− 6 K to Tg+ 4 K

Abstract The rotational dynamics of tetracene and rubrene in the ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate ([C 4 C 1 im][PF 6 ]) at the glass transition ( T g = 196 K), from T g − 6 K to T g + 4 K, were measured using the technique of fluorescence recovery after photobleaching. The rotational anisotropy decays of these probes in [C 4 C 1 im][PF 6 ] were found to be non-exponential and well fit by the Kohlrausch–Williams–Watts (KWW) function with the stretching parameter β KWW equal to 0.70 ± 0.03 for tetracene and 0.88 ± 0.04 for rubrene in the temperature range of the measurements. The rotational correlation time τ c at T g is equal to 19 ± 1 s for the smaller probe tetracene and 180 ± 40 s for the larger probe rubrene. Below T g , τ c shows a slight decoupling from the extrapolation of fits of the Vogel–Fulcher–Tammann equation to the viscosity η . This decoupling is characterized by a fractional Debye–Stokes–Einstein relation, τ c ∝ η ξ / T , with ξ equal to 0.78 ± 0.02 for rubrene and 0.85 ± 0.01 for tetracene. The dependence of β KWW on probe size is consistent with the dynamics in [C 4 C 1 im][PF 6 ] being heterogeneous and is rationalized in terms of the time scale of the probe rotational motion compared to the domain exchange time.