Voltammetric, EPR and UV–VIS–NIR spectroscopic studies associated with the one-electron oxidation of C60 and C70 in 1,1′,2,2′-tetrachloroethane containing trifluoromethanesulfonic acid
Cyclic voltammetry experiments performed at a scan rate of 100 mV s−1 on C60 and C70 in 1,1′,2,2′-tetrachloroethane (TCE) containing 1–10% (v/v) CF3SO3H and 0.5 M Bu4NPF6 indicated one-electron chemically reversible processes at 1.26 V (C60) and 1.21 V (C70) s. ferrocene/ferrocinium, suggesting the formation of persistent C60•+ and C70•+ cations. The addition of CF3SO3H to the TCE solutions had an immediate positive influence on the ability to observe a one-electron oxidation process for the fullerenes, thereby indicating that the presence of acid aided the longevity of the cation radicals in a non-ultra-dry solvent. Solution phase UV–VIS–NIR spectra were obtained by the one-electron oxidation of C60 in an optically transparent thin layer electrochemical cell at 233 K in TCE containing 5–10% CF3SO3H (0.5 M Bu4NPF6). The series of UV–VIS–NIR spectra showed several isosbestic points indicating stability of the oxidised compound over the time-frame of the experiment (3–4 h) and characteristic NIR bands associated with C60•+ were detected at 10170 cm−1 (ε = 11000 L cm−1 mol−1) and 11820 cm−1 (ε = 3500 L cm−1 mol−1) in addition to a less intense band at 8950 cm−1 (ε = 750 L cm−1 mol−1) (with a shoulder at ∽7500 cm−1). Solutions of C60 were bulk electrochemically oxidised by one-electron in a controlled potential electrolysis cell at 233 K in TCE/CF3SO3H and then transferred under vacuum into an EPR tube and immediately frozen in liquid nitrogen. Cyclic voltammograms performed at the completion of the bulk one-electron oxidation indicated stability of the formed C60•+ under long-term electrolysis conditions (3–4 h at 233 K), although at higher temperatures and/or longer times decomposition occurred. Continuous wave X-band EPR spectra of frozen solutions of C60•+ at temperatures between 5 and ∽200 K showed one well-resolved rhombic shaped signal (gx = 2.0054, gy = 2.0030 and gz = 1.9995 at 133 K) that transformed into an isotropically shaped line at higher temperatures. The electrochemical and spectroscopic data obtained during the long term (hours) oxidation of C70 were more complicated than the C60 case and indicated instability of the C70•+ cation in the TCE/CF3SO3H media, even at low temperatures (T = 233 K).