Ultrafast Torsional Relaxation of Thioflavin-T in Tris(pentafluoroethyl)trifluorophosphate (FAP) Anion-Based Ionic Liquids.

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.