Comparison of electron paramagnetic resonance line shapes and electron spin relaxation rates for C-60(-) and C-60(3-) in 4:1 toluene:acetonitrile and dimethyl sulfoxide

To assess the effect of solvent on the properties of C-60 anions, solutions of C-60(-) and C-60(3-) were generated electrochemically in 4:1 toluene:acetonitrile or DMSO containing 0.1 M tetrabutylammonium hexafluorophosphate. The anions were characterized by NIR spectroscopy and square-wave voltammetry. In both solvents continuous wave (CW) electron paramagnetic resonance (EPR) spectra at 8 K are axial for C-60(-) and rhombic for C-60(3-), consistent with increasing distortion of the anion with increasing addition of electrons to the LUMO of C-60. The line shapes in the wings of the spectra, decreasing anisotropy of the spectra with increasing temperature, faster phase memory relaxation rates in the wings of the spectra, and distribution of values of the spin-lattice relaxation rate (1/T-1) provide evidence for a distribution of Jahn-Teller distorted forms and increasing rates of interconversion between these forms with increasing temperature. These properties are similar in the two solvents, despite substantial differences in solvent polarity. Extrapolation of the temperature dependence of 1/T-1 measured below 30 K suggests that the line widths above 70 K are relaxation determined. Above about 70 K the line widths of the CW spectra broaden as increasing mobility of the solute/solvent increases the rate of spin-lattice relaxation.