Rotational dynamics of positively and negatively charged solutes in ionic liquid and viscous molecular solvent studied by time-resolved fluorescence anisotropy measurements

Rotational dynamics of two negatively and positively charged solutes, 1-anilinonaphthalene-8-sulfonate (ANS) and ethidium bromide (EB), respectively, have been studied in an ionic liquid, 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF₄]) and a conventional viscous solvent, glycerol, at different temperatures to obtain insight into the nature of various forces that operate in the ionic liquid. The fluorescence anisotropy of the systems decays exponentially with time in both the solvents. Under isoviscous conditions, the reorientation time of each probe molecule is found to be very similar in ionic liquid and glycerol indicating that the electrostatic forces exert negligible influence on the charged solute molecules. Analysis of the experimentally measured reorientation times of the two solutes using the Stokes-Einstein-Debye hydrodynamic theory shows that the rotational diffusion of ANS is best represented by a behavior that falls in-between the stick and slip boundary conditions, whereas EB exhibits a superstick behavior indicating its strong association with the solvent molecules. The association between the positively charged solute EB with ionic liquid and glycerol, which is also evident from a much higher value of the rotational coupling constant, appears to be mediated by hydrogen bonding interactions.