Web Release Date: October 4,
Solvation Dynamics and Electric Field Relaxation in an Imidazolium-PF6 Ionic Liquid:
from Room Temperature to the Glass Transition
Department of Chemistry and Biochemistry, Arizona State University, Tempe, Arizona 85287-1604
Received: June 26, 2006
In Final Form: July 26, 2006
Abstract:
Time-resolved phosphorescence spectra and anisotropy of quinoxaline were measured in an ionic liquid, 1-butyl-3-methylimidazolium hexafluorophosphate (BMIM-HFP), in its supercooled state near the glass-transition
temperature. The solvation dynamics results are compared with the rotational motion of the probe and with
the dielectric behavior of the neat ionic liquid. The dynamics in the viscous state are highly dispersive and
show a super-Arrhenius temperature dependence, as typical for glass-forming materials. Combined with room-temperature results, solvation dynamics is observed to follow the structural relaxation times in terms of 
/T
for more than 10 decades, from subnanoseconds at room temperature to seconds near the glass-transition
temperature Tg. The dielectric modulus relaxation follows this trend only for temperatures T > 1.2Tg and
departs significantly from /T in the 1.1Tg > T > Tg range. This deviation is reminiscent of the enhanced
translational diffusion or fractional Stokes-Einstein behavior observed in many fragile supercooled liquids.
Because the electric field relaxation in BMIM-HFP includes dc conductivity, this correlation function involves
translational motion and thus displays the effect of enhanced diffusivity. A microscopic model is required for
rationalizing the decoupling of solvation dynamics from the longitudinal time scales and the limitation of this
effect to the viscous regime with T < 1.2Tg.
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