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 C-60(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 (51, 10 (6], 12 [31; 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 C-60(CF3)(n)(-) radical anions at 25 degrees 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 C-60(CF3)(n) derivatives. The X-ray structure of one of the compounds, 1,6,11,11 6,18,26,36,41,44,57-C-60(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-12 values (vs C-60(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 C-60(CF3)(10). In general, reduction potentials for the 0/- couple are shifted anodically relative to the C-60(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 C-60(CF3)(n) compounds and the computational results for more than 50 additional C-60(X)(n) compounds provide new insights about the frontier orbitals of C-60(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(Sp(2)) nearest neighbors result in the strongest electron acceptors.