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 ([C(4)C(1)im][PF6]) at the glass transition (T-g = 196 K), from T-g - 6 K to T-g + 4K, were measured using the technique of fluorescence recovery after photobleaching. The rotational anisotropy decays of these probes in [C(4)C(1)im][PF6] were found to be non-exponential and well fit by the Kohlrausch-Williams-Watts (KWW) function with the stretching parameter beta(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 tau(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, tau(c) shows a slight decoupling from the extrapolation of fits of the Vogel-Fulcher-Tammann equation to the viscosity eta. This decoupling is characterized by a fractional Debye-Stokes-Einstein relation, tau(c) proportional to eta(xi)/T, with xi equal to 0.78 +/- 0.02 for rubrene and 0.85 +/- 0.01 for tetracene. The dependence of beta(KWW) on probe size is consistent with the dynamics in [C(4)C(1)im][PF6] being heterogeneous and is rationalized in terms of the time scale of the probe rotational motion compared to the domain exchange time. (c) 2014 Elsevier B.V. All rights reserved.