Diffusion–Viscosity Decoupling in Solute Rotation and Solvent Relaxation of Coumarin153 in Ionic Liquids Containing Fluoroalkylphosphate (FAP) Anion: A Thermophysical and Photophysical Study

Abstract

Steady state and time-resolved fluorescence behavior of coumarin153 (C153) has been investigated in two ionic liquids (ILs), namely 1-(2-methoxyethyl)-1-methylpyrrolidinium tris(pentafluoroethyl)trifluorophosphate ([MOEMPL][FAP]) and 1-(2-methoxyethyl)-1-methylmorpholinium tris(pentafluoroethyl)trifluorophosphate ([MOEMMO][FAP]) in order to find out the viscosity–diffusion decoupling during solvation and rotational relaxation of C153. Thermophysical studies have also been carried out to understand the physicochemical properties of the media. At 293 K, the measured viscosity of [MOEMMO][FAP] is 8 times higher than that of [MOEMPL][FAP]. The data obtained from steady state and time-resolved fluorescence measurements show the deviation of average solvation and rotation times from conventional hydrodynamics. The decoupling of solute and solvent dynamics from medium viscosity is manifested through fractional viscosity dependence (η) of the measured average solvation (⟨τs⟩) and rotation (⟨τr⟩) times: ⟨τx⟩ ∝ (η/T)p (x denotes solvation or rotation and T is the temperature) covering the p value 0.69 < p < 0.85 for solvent relaxation and 0.48 < p < 1.10 for solute rotation. The excitation wavelength dependent fluorescence studies have been performed to correlate the experimental findings with the heterogeneity of the medium. While the excitation wavelength dependent time-resolved fluorescence studies of coumarin153 reveal a very similar variation of average solvation time with a change in excitation wavelengths for both the ionic liquids, the steady state excitation wavelength dependent measurements of 2-amino-7-nitrofluorene (ANF) show a higher (630 cm–1) shift of the fluorescence maximum for highly viscous ionic liquid as compared to that (430 cm–1) of another much less viscous ionic liquid. The recent theoretical (Chem. Phys. Lett.2011, 517, 180) and experimental (J. Chem. Phys.2012, 136, 174503) findings and the outcome of the excitation wavelength dependent fluorescence measurements in the present case seem to suggest that both static and dynamic heterogeneity may play an important role in the observed viscosity–diffusion (d–η) decoupling for highly viscous ionic liquid.