Electrochemical, Spectroscopic, and DFT Study of C60(CF3) n Frontier Orbitals (n = 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/− E 1/2 values (vs C60 0/-) are a linear function of the DFT-predicted LUMO energies (average E 1/2 deviation from the least-squares line is 0.02 V). This linear relationship was used to predict the 0/− E 1/2 values for the n = 16 and 18 derivatives, and none of the predicted values is more positive than the 0/− E 1/2 value for one of the isomers of C60(CF3)10. In general, reduction potentials for the 0/− couple are shifted anodically relative to the C60 0/- couple. However, the 0/− E 1/2 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 E 1/2 values. Those addition patterns with double bonds in pentagons having two C(sp2) nearest neighbors result in the strongest electron acceptors.