Browsing by Author "Quitevis, EL"

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    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
    Bardak, F; Rajian, JR; Son, P; Quitevis, EL
    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.
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    Comparative study of the intermolecular dynamics of imidazolium-based ionic liquids with linear and branched alkyl chains: OHD-RIKES measurements
    Xue, LJ; Bardak, F; Tamas, G; Quitevis, EL
    This article describes a comparative study of the low-frequency (0-450 cm(-1)) Kerr spectra of the branched 1-(iso-alkyl)-3-methylimidazolium bis(trifluoromethylsulfonyl) amide ([(N - 2) mC(N) (1)C(1)im][NTf2] with N = 3-7) ionic liquids (ILs) and that of the linear 1-(n-alkyl)-3-methylimidazolium bis(trifluoromethylsulfonyl) amide ([C(N)C(1)im][NTf2] with N = 2-7) ILs. The spectra were obtained by use of femtosecond optical heterodyne-detected Raman-induced Kerr effect spectroscopy (OHD-RIKES). The intermolecular spectrum of a branched IL is similar to that of a linear IL that is of the same alkyl chain length rather than of the same number of carbon atoms in the alkyl chain. This similarity and the lack of a correlation of the first spectral moments and widths of the intermolecular spectra with chain length is mainly attributed to the increase in the dispersion contribution to the total molar cohesive energy being compensated by stretching of the ionic network due to the increasing size of the nonpolar domains, which is dependent only on the length of the alkyl chain.