Ultrafast Torsional Relaxation of Thioflavin-Tin Tris(pentafluoroethyl)trifluorophosphate (FAP) Anion-Based IonicLiquids.

Ultrafastspectroscopy on solutes, whose dynamics is very sensitiveto the friction in its local environment, has strong potential toreport on the microenvironment existing in complex fluids such asionic liquids. In this work, the torsional relaxation dynamics ofThioflavin-T (ThT), an ultrafast molecular rotor, is investigatedin two fluoroalkylphosphate ([FAP])-based ionic liquids, namely, 1-ethyl-3-methylimidazoliumtris(pentafluoroethyl)trifluorophosphate ([EMIM][FAP]) and 1-(2-hydroxyethyl)-3-methylimidazoliumtris(pentafluoroethyl)trifluorophosphate ([OHEMIM][FAP]), using ultrafastfluorescence up-conversion spectroscopy. The emission quantum yieldand the excited-state fluorescence lifetime measurement suggest thatthe torsional relaxation of Thioflavin-T, in this class of ionic liquids,is guided by the viscosity of the medium. The temporal profile ofthe dynamic Stokes’ shift of ThT, measured from time-resolvedemission spectrum (TRES), displays a multiexponential behavior inboth ionic liquids. The long time dynamics of the Stokes’ shiftis reasonably slower for the hydroxyethyl derivative as compared tothe ethyl derivative, which is in accordance with their measured shearviscosity. However, the short time dynamics of Stokes’ shiftis very similar in both the ionic liquids, and seems to be independentof the measured shear viscosity of the ionic liquid. We rationalizethese observations in terms of different locations of ThT in theseionic liquids. These results suggest that despite having a higherbulk viscosity in the ionic liquid, they can provide unique microenvironmentin their complex structure, where the reaction can be faster thanexpected from their measured shear viscosity. [ABSTRACT FROM AUTHOR]