Studies on the solution-phase ESR spectra of the C₆₀ monoanion under varying experimental conditions

ESR spectroscopic studies on C₆₀^-· electrochemically generated from C₆₀ in toluene-acetonitrile solutions (with Bu₄NPF₆ as the supporting electrolyte) were performed under varying time (0-35 days), temperature (293-133 K), concentration (0.2-1.0 mM), oxygen content, purity and photolytic conditions. It was found that the ESR signals frequently referred to as sharp (ΔH_pp ≈ 0.1 mT), that are present in low abundance (∼1-2%) in most samples of C₆₀^-· (ΔH_pp ≈ 3-6 mT), can exist in several different forms with distinct linewidths and g-values depending on the temperature, the oxygen content of solution and the time following the generation of C₆₀^-·. Only one sharp signal (in addition to the broad signal) was evident immediately after the electrochemical generation of C₆₀^-·, but with increasing time (hours-days), in the presence of light and/or in the presence of atmospheric air, other sharp signals appeared, some of which were attributable to the formation of precipitate radicals. Data obtained from a number of experiments suggest that the sharp linewidth signals behave independently of the broad linewidth signal, which reduces the likelihood that the species responsible for the two types of signals (broad and sharp) are in equilibrium, or that the paramagnetic species responsible for the broad signal converts into the species responsible for the sharp signals. Instead, the data are consistent with the sharp linewidth signals originating from an impurity in the original C₆₀ that is converted to a radical species upon reduction of C₆₀, which subsequently undergoes further chemical reactions to form other paramagnetic species.