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

The rotational dynamics of tetracene and rubrene in the ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate ([C₄C₁im][PF₆]) 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 [C₄C₁im][PF₆] 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 [C₄C₁im][PF₆] being heterogeneous and is rationalized in terms of the time scale of the probe rotational motion compared to the domain exchange time.