Microviscosity and Micropolarity Effects of Imidazolium Based Ionic Liquids Investigated by Spin Probes Their Diffusion and Spin Exchange

Abstract

Different polar common spin probes (TEMPO, TEMPOL, and CAT-1) as well as 15N spin probes (15N-TEMPO and 15N-TEMPOL-D17) were investigated to get information about microviscosity and micropolarity of ionic liquids. Rotational correlation times and hyperfine coupling constants of the spin probes were obtained by complete simulation of the ESR spectra. Microviscosity effects as shown by the Gierer–Wirtz theory may explain the spin probe behavior. Investigation of spin exchange of TEMPO, TEMPOL, and CAT-1 dissolved in ionic liquids shows an increased tendency of aggregation in the case of the nonpolar spin probe TEMPO. Two different kinds of species (isolated and aggregated species) were observed in the case of the more polar spin probes TEMPOL and CAT-1. ESR tomographic investigation of lateral diffusion of selected spin probes in an ionic liquid corresponds to the results obtained in rotational diffusion experiments. Furthermore, the strongly decreased mobility of radicals in ionic liquids makes detection of a polymer radical possible that was observed during thermal induced free radical polymerization of a methacrylate substituted by a sulfobetaine structure.

Probing Microenvironment in Ionic Liquids by Time-Resolved EPR of Photoexcited Triplets

“…The mobility of stable nitroxide radical provides information on local viscosity and environment, and this approach is widely used in various fields of science, including 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 studies of ILs. [16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34] However, in most cases previous CW EPR data were obtained at temperatures close to ambient and well above melting temperatures of corresponding ILs, and only a few works addressed the low temperature regions between glass transition (T g ) and melting (T m ) temperatures.…”

“…A number of EPR studies in ILs using spin probes have been accomplished, both in neat ILs and in IL/water mixtures. [16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34] In particular, it was shown that nanostructuring of ILs in water can be monitored via polaritydependent hyperfine splitting. However, in general, the shape of the nitroxide spectrum was found to be not very sensitive to the effects of nanostructuring, therefore developing other types of probes for heterogeneities in ILs would be a topical task.…”

Study of Nanostructural Organization of Ionic Liquids by Electron Paramagnetic Resonance Spectroscopy

“…The hyperfine coupling spacing of a nitroxide spin probe is a good measure of the polarity of a solvent, 14 and as such, it has been applied to study the polarity of a variety of ILs. 15–18 Using the EPR spin probe method, Pergushov et al have detected structural and dynamic heterogeneity in a 1-octyl-3-methylimidazolium tetrafluoroborate solution. 19 In 1992, Noel et al 20 studied the solvation of two nitoxide spin probes, 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) and 4-amino-2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPAMINE), in ionic liquids.…”

Isotope Effects in ESR Spectroscopy

“…The isotopic marking of the spin probes 15 N-2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO) and 4-hydroxy-2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPOL) allows the observation of their lateral diffusion in opposite directions in a one-phase system by means of ESR tomography [163]. Figure 13 displays the profiles of the signal amplitudes after 10 and 135 min, respectively, for the diffusion of the spin probes in the ionic liquid butylmethylimidazolium tetrafluoroborate (BuMeImBF 4 ) [163]. By means of the marking the concentration profiles can be assigned unambiguously and specifically for the respective probe including their spatial distribution.…”