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.