Fluorescence probing of temperature-dependent dynamics and friction in ionic liquid local environments

Alison M. Funston, Tatiana A. Fadeeva, James F. Wishart, Edward W. Castner

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151 Citations (Scopus)

Abstract

The solvation dynamics and local orientational friction for a series of four ionic liquids have been probed using coumarin 153 (C153) as a function of temperature. These ionic liquids are comprised of nonaromatic organic cations paired with a common anion, bis(trifluoromethylsulfonyl)imide (NTf 2-). The specific liquids are as follows: N-methyl-tri-N-butylammonium NTf2- (N1444 +/NTf2-), N-hexyl-tri-N-butylammonium NTf 2- (N6444+/NTf2 -), N-methyl-N-butylpyrrolidinium NTf2- (Pyrr14+/NTf2-), and N-methyl-N-ethoxyethylpyrrolidinium NTf2- (Pyrr 1(2O2)+/NTf2-). The observed solvation dynamics and fluorescence depolarization dynamics occur over a broad range of time scales that can only be adequately fit by functions including three or more exponential components. Stretched exponential distributions cannot adequately fit our data. The solvation and reorientational dynamics of the C153 probe are studied over a range of temperatures from 278.2 to 353.2 K. For both the solvation dynamics and the probe reorientational dynamics, the observed temperature dependence is well fit by a Vogel-Tammann-Fulcher law. To correlate the observed microscopic dynamics with macroscopic physical properties, temperature-dependent viscosities are also measured. Differential scanning calorimetry is used to study the thermodynamics of the phase transitions from the liquid to supercooled liquid to glassy states. For the two tetraalkylammonium liquids, the observed melting transitions occur near 300 K, so we are able to study the dynamics in a clearly supercooled regime. Very long time scale orientational relaxation time constants dynamics on the order of 100 ns are observed in the C153 fluorescence anisotropy. These are interpreted to arise from long-lived local structures in the environment surrounding the C153 probe.

Original languageEnglish
Pages (from-to)4963-4977
Number of pages15
JournalJournal of Physical Chemistry B
Volume111
Issue number18
DOIs
Publication statusPublished - May 10 2007

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ASJC Scopus subject areas

  • Physical and Theoretical Chemistry
  • Surfaces, Coatings and Films
  • Materials Chemistry

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