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

Abstract

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 (g_x = 2.0054, g_y = 2.0030 and g_z = 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).