Solvation and Rotational Dynamics of Coumarin 153 in Ionic Liquids:  Comparisons to Conventional Solvents

Abstract

Steady-state and time-resolved emission spectroscopy with 25 ps resolution are used to measure equilibrium and dynamic aspects of the solvation of coumarin 153 (C153) in a diverse collection of 21 room-temperature ionic liquids. The ionic liquids studied here include several phosphonium and imidazolium liquids previously reported as well as 12 new ionic liquids that incorporate two homologous series of ammonium and pyrrolidinium cations. Steady-state absorption and emission spectra are used to extract solvation free energies and reorganization energies associated with the S0 <--> S1 transition of C153. These quantities, especially the solvation free energy, vary relatively little in ionic liquids compared to conventional solvents. Some correlation is found between these quantities and the mean separation between ions (or molar volume). Time-resolved anisotropies are used to observe solute rotation. Rotation times measured in ionic liquids correlate with solvent viscosity in much the same way that they do in conventional polar solvents. No special frictional coupling between the C153 and the ionic liquid solvents is indicated by these times. But, in contrast to what is observed in most low-viscosity conventional solvents, rotational correlation functions in ionic liquids are nonexponential. Time-resolved Stokes shift measurements are used to characterize solvation dynamics. The solvation response functions in ionic liquids are also nonexponential and can be reasonably represented by stretched-exponential functions of time. The solvation times observed are correlated with the solvent viscosity, and the much slower solvation in ionic liquids compared to dipolar solvents can be attributed to their much larger viscosities. Solvation times of the majority of ionic liquids studied appear to follow a single correlation with solvent viscosity. Only liquids incorporating the largest phosphonium cation appear to follow a distinctly different correlation.

Bimolecular Photoinduced Electron Transfer in Imidazolium-Based Room-Temperature Ionic Liquids Is Not Faster than in Conventional Solvents

“…The so-obtained τ L values scale with the viscosity of the RTILs and are comparable to the shortest and medium solvent relaxation times determined experimentally for RTILs but not to the average solvation times. 35,51 This can be due to the fact that in such complex liquids, the relaxation mode associated with the ET reaction may not be unique, as assumed in our model. The variation of the relaxation time with the composition of the DMSO/GLY mixture is still a matter of research, but it has so far been successfully simulated with a single time.…”

Spectroscopic Evidence for Unusual Microviscosity in Imidazolium Ionic Liquid and Tetraethylene Glycol Dimethyl Ether Cosolvent Mixtures

“…C153 is considered to be an ideal probe because the dynamics probed by C153 have been found to be primarily related to nonspecific solute−solvent interactions in many RTIL systems. 17,52,53,58 BPP is a particularly sensitive probe of microviscosity especially in the heterogeneous environment such as ionic liquid systems. 55−57,59−62 As a result, the microviscosity enhancement effects in the [bmim] [PF 6 ] and TEGDME hybrid system were observed by probing the microviscosity through the two complementary experimental methods.…”

Ultrafast dynamics of solvation: the story so far

“…101 Utilizing OKE and TRFS ultrafast spectroscopy techniques, the fundamental solvation timescales have been characterized for many ionic liquids (e.g., imidazolium-and phosphonium-based ionic liquids), and a few trends are beginning to be observed. [102][103][104][105][106][107][108][109] First, solvation dynamics in room-temperature ionic liquids are much slower than observed for conventional liquids due to the high viscosity of ionic liquids (Figure 4). Second, temporal profiles are highly non-exponential.…”