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

The rotational dynamics of tetracene and rubrene in the ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate ([C4C1im][PF6]) at the glass transition (Tg = 196 K), from Tg − 6 K to Tg + 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 βKWW 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 τc at Tg is equal to 19 ± 1 s for the smaller probe tetracene and 180 ± 40 s for the larger probe rubrene. Below Tg, τc 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, τc ∝ ηξ/T, with ξ equal to 0.78 ± 0.02 for rubrene and 0.85 ± 0.01 for tetracene. The dependence of βKWW 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.