Solute Rotation and Solvation Dynamics in a Room-Temperature Ionic Liquid

Abstract

Steady-state spectra, rotation times, and time-resolved emission spectra of the probe 4-aminophthalimide (4-AP) in the ionic liquid 1-n-butyl-3-methylimidazolium hexafluorophosphate ([bmim+][PF6 -]) were measured over the temperature range 298−355 K. The steady-state spectroscopy indicates that the solvation energetics of 4-AP in [bmim+][PF6 -] are comparable to those of 4-AP in highly polar but aprotic solvents such as dimethylformamide and acetonitrile (π* ∼ 0.8, ∼ 0.4). The rotation of 4-AP in [bmim+][PF6 -] and in more conventional aprotic solvents generally conforms to the expectations of simple hydrodynamic models. Other than the fact that [bmim+][PF6 -] is highly viscous, nothing distinguishes the rotation of 4-AP in this ionic liquid from its rotation in more conventional polar aprotic solvents. Time-dependent emission spectra, recorded with an instrumental response of 25 ps, indicate that solvation dynamics in [bmim+][PF6 -] occur in two well-separated time regimes. Near to room temperature, the observable response takes place in the 0.1−2 ns time range. This component can be described by a stretched exponential time dependence with an exponent of 0.6−0.7, indicative of strongly nonexponential relaxation. The integral time of the observed component of solvation is proportional to the rotation time of 4-AP and to solvent viscosity, suggesting the involvement of substantial solvent rearrangement. In addition to this relatively slow component, more than half of the solvation response in [bmim+][PF6 -] is faster than can be detected in these experiments, that is, takes place in <5 ps.

Translational and Rotational Motions for TFSA-Based Ionic Liquids Studied by NMR Spectroscopy

“…33,34 Experimental studies on the rotational motion of small molecules dissolved in ILs have also been reported. [35][36][37][38][39] We calculated  2 in Eq. 7 by using the measured viscosity data and assuming V of the van der Waals volumes of the cations in Table 3.…”

Dynamics in Organic Ionic Liquids in Distinct Regions Using Charged and Uncharged Orientational Relaxation Probes

“…Previous fluorescence studies of orientational relaxation of RTILs have seen Debye-Stokes-Einstein (DSE) behavior 14 and possibly deviations from DSE behavior. [15][16][17] Most of the studies examined very polar and charged dye molecules.…”

Influence of Hydrogen Bonds and Nonspecific Interactions on the Spectral and Photophysical Properties of the Excited Singlet States of 4‐Aminophthalimide in Amine Solution

“…The 4-aminophthalimide molecule (4-AP, Figures 1 and 2) and its N-methyl derivative (NM-4-AP) have been widely used as fluorescence probes in investigations of the properties of many complex systems such as mixtures of solvents, [1] micelles, [2][3][4] cyclodextrines, [5] ionic liquids [6,7] and others. [8][9][10][11] The usefulness of 4-AP and NM-4-AP as probes is related to the strong dependence of their fluorescence maxima (n F max ), their quantum yields of fluorescence (f F ) and their fluorescence decay times (t F ) on the properties of their environment.…”