The solvation of nitroxide radicals in ionic liquids studied by high-field EPR spectroscopy

Ionic liquids (ILs) feature a variety of properties that make them a unique class of solvents. To gain a better understanding of how ILs solvate compounds of different chemical structure, we used pulsed high-field electron paramagnetic resonance (EPR) spectroscopy at W-band (approximately 94 GHz) and continuous wave EPR at X-band (approximately 9.4 GHz) on three TEMPO-based spin probes with different substitutions at the 4-position: 4-R-2,2,6,6-tetramethylpiperidine-1-oxyl, with R = N(CH(3))(3)(+), Cat-1, R = COO(-), TEMPO-4-carboxylate, and R = OH, TEMPOL. The spin probes are dissolved in imidazolium based ILs with different alkyl chain lengths (-C(2)H(5), -C(4)H(9), -C(6)H(13)) and anions (BF(4)(-), PF(6)(-)) and also in molecular solvents (methanol, water-glycerol). X-Band EPR at RT shows that the reorientational motion of the charged spin probes in ILs is about fivefold slower than that of the TEMPOL. Moreover, anion variation from BF(4)(-) to PF(6)(-) in ILs most strongly slows down the rotational motion (as measured by the rotational correlation time tau(r)) of Cat-1, followed by TEMPOL, while tau(r) of TEMPO-4-carboxylate is least affected. The EPR parameters g(xx) and A(zz) (tensor elements of the g- and hyperfine tensor) are sensitive to environmental effects and are only fully resolved at the high field used in this study. Changes of g(xx) and A(zz) values of the Cat-1 in ILs and methanol are very small especially compared to that of TEMPO-4-carboxylate, indicating that Cat-1 is located in a polar region of the ILs resembling the situation in methanol. On the other hand, the g(xx) value of TEMPO-4-carboxylate is sensitive to the length of alkyl group which shows that TEMPO-4-carboxylate is close to the nonpolar region of ILs. The small differences in the chemical substitution of the spin probes used here are sufficient for the molecules to reside in different domains of different dielectric properties in ILs. Our combined results are in good agreement with a picture of a nanophase separation, in which the charged cations and anions form polar regions and the hydrophobic alkyl chains of the IL cations form non-polar regions.