The rotational diffusion of tetracene and rubrene in the ionic liquid (IL) 1-butyl-3-methylimidazolium hexafluorophosphate ([C4C1im][PF6]) near the glass transition (Tg = 196 K) from Tg - 5K to Tg + 2K has been measured using the fluorescence recovery after photobleaching (FRAP) technique.  Rotational correlation functions for these probes in this IL were fit by a KWW function.  From these fits, the KWW � parameter and the average rotational time <trot> were obtained.  Whereas � is temperature independent and ~ 1 for rubrene in this temperature range, � is temperature dependent for tetracene, decreasing from � ~ 1 at Tg + 2K to � ~ 0.6 at Tg - 5K.  For both tetracene and rubrene, <trot> is found to exhibit a weaker temperature dependence than predicted by the Debye-Stokes-Einstein equation.  The difference in temperature dependences of � and the decoupling of rotational diffusion from viscosity for these probes are discussed within the context of spatial heterogeneity in this IL.

Rotational diffusion of probe molecules in ionic liquids near the glass transition: Fluorescence recovery after photobleaching measurements

244th National Meeting (2012)

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