Radical ionic liquid: an efficient self-probe to study heterogeneous structure in glassy state using EPR spectroscopy

Ionic liquids (ILs) exhibit a number of unique properties, including formation of various heterogeneities in their liquid and glassy states. Electron Paramagnetic Resonance (EPR) spectroscopy has proven informative in studies of such heterogeneous nanostructures; however it employs additional small paramagnetic molecules – spin probes – dissolved in ILs as reporters. In this work we synthesized two radical-ILs, which include integrated paramagnetic fragments in their structures, and employed them in continuous wave (CW) and pulse EPR studies of structural peculiarities in IL glasses. In this approach no external-to-IL spin probe is required. We have shown that the trends observed with radical-ILs are similar to those obtained previously using external-to-IL spin probes. In particular, unusual structural anomalies near glass transition temperatures, which were previously found using external-to-IL spin probes, have been now observed using radical-ILs, thus confirming their genuine origin. In general, radical-ILs can be considered as efficient tools to study heterogeneous structure of ILs using EPR.