Ultrafast Torsional Relaxation of Thioflavin‑T
in Tris(pentafluoroethyl)trifluorophosphate (FAP) Anion-Based Ionic
Liquids
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Abstract
Ultrafast
spectroscopy on solutes, whose dynamics is very sensitive
to the friction in its local environment, has strong potential to
report on the microenvironment existing in complex fluids such as
ionic liquids. In this work, the torsional relaxation dynamics of
Thioflavin-T (ThT), an ultrafast molecular rotor, is investigated
in two fluoroalkylphosphate ([FAP])-based ionic liquids, namely, 1-ethyl-3-methylimidazolium
tris(pentafluoroethyl)trifluorophosphate ([EMIM][FAP]) and 1-(2-hydroxyethyl)-3-methylimidazolium
tris(pentafluoroethyl)trifluorophosphate ([OHEMIM][FAP]), using ultrafast
fluorescence up-conversion spectroscopy. The emission quantum yield
and the excited-state fluorescence lifetime measurement suggest that
the torsional relaxation of Thioflavin-T, in this class of ionic liquids,
is guided by the viscosity of the medium. The temporal profile of
the dynamic Stokes’ shift of ThT, measured from time-resolved
emission spectrum (TRES), displays a multiexponential behavior in
both ionic liquids. The long time dynamics of the Stokes’ shift
is reasonably slower for the hydroxyethyl derivative as compared to
the ethyl derivative, which is in accordance with their measured shear
viscosity. However, the short time dynamics of Stokes’ shift
is very similar in both the ionic liquids, and seems to be independent
of the measured shear viscosity of the ionic liquid. We rationalize
these observations in terms of different locations of ThT in these
ionic liquids. These results suggest that despite having a higher
bulk viscosity in the ionic liquid, they can provide unique microenvironment
in their complex structure, where the reaction can be faster than
expected from their measured shear viscosity