Electrochemical, Spectroscopic, and DFT Study of C₆₀(CF₃)<i>_n</i> Frontier Orbitals (<i>n</i> = 2−18): The Link between Double Bonds in Pentagons and Reduction Potentials

The frontier orbitals of 22 isolated and characterized C60(CF3)n derivatives, including seven reported here for the first time, have been investigated by electronic spectroscopy (n = 2 [1], 4 [1], 6 [2], 8 [5], 10 [6], 12 [3]; the number of isomers for each composition is shown in square brackets) fluorescence spectroscopy (n = 10 [4]), cyclic voltammetry under air-free conditions (all compounds with n ≤ 12), ESR spectroscopy of C60(CF3)n- radical anions at 25 °C (n = 4 [1] and 10 [1]), and quantum chemical calculations at the DFT level of theory (all compounds including n = 16 [3] and 18 [2]). DFT calculations are also reported for several hypothetical C60(CF3)n derivatives. The X-ray structure of one of the compounds, 1,6,11,16,18,26,36,41,44,57-C60(CF3)10, is reported here for the first time. Most of the compounds with n ≤ 12 exhibit two or three quasi-reversible reductions at scan rates from 20 mV s-1 up to 5.0 V s-1, respectively. The 18 experimental 0/− E1/2 values (vs C600/-) are a linear function of the DFT-predicted LUMO energies (average E1/2 deviation from the least-squares line is 0.02 V). This linear relationship was used to predict the 0/− E1/2 values for the n = 16 and 18 derivatives, and none of the predicted values is more positive than the 0/− E1/2 value for one of the isomers of C60(CF3)10. In general, reduction potentials for the 0/− couple are shifted anodically relative to the C600/- couple. However, the 0/− E1/2 values for a given composition are strongly dependent on the addition pattern of the CF3 groups. In addition, LUMO energies for isomers of C60(X)n (n = 2, 4, 6, 8, 10, and 12) that are structurally related to many of the CF3 derivatives were calculated and compared for X = CH3, H, Ph, NH2, CH2F, CHF2, F, NO2, and CN. The experimental and computational results for the C60(CF3)n compounds and the computational results for more than 50 additional C60(X)n compounds provide new insights about the frontier orbitals of C60(X)n derivatives. For a given substituent, X, the addition pattern is as important, if not more important in many cases, than the number of substituents, n, in determining E1/2 values. Those addition patterns with double bonds in pentagons having two C(sp2) nearest neighbors result in the strongest electron acceptors.