Substituent effects in a series of 1,7-C60(RF)2 compounds (RF = CF3, C2F5, n-C3F7, i-C3F7, n-C4F9, s-C4F9, n-C8F17): electron affinities, reduction potentials and E(LUMO) values are not always correlated

Abstract

A series of seven structurally-similar compounds with different pairs of R_F groups were prepared, characterized spectroscopically, and studied by electrochemical methods (cyclic and square-wave voltammetry), low-temperature anion photoelectron spectroscopy, and DFT calculations (five of the compounds are reported here for the first time). This is the first time that a set of seven R_F groups have been compared with respect to their relative effects on E_1/2(0/−), electron affinity (EA), and the DFT-calculated LUMO energy. The compounds, 1,7-C₆₀(R_F)₂ (R_F = CF₃, C₂F₅, i-C₃F₇, n-C₃F₇, s-C₄F₉, n-C₄F₉ and n-C₈F₂₁), were found to have statistically different electron affinities (EA), at the ±10 meV level of uncertainty, but virtually identical first reduction potentials, at the ±10 mV level of uncertainty. The lack of a correlation between EA and E_1/2(0/−), and between E(LUMO) and E_1/2(0/−), for such similar compounds is unprecedented and suggests that explanations for differences in figures of merit for materials and/or devices that are based on equating easily measurable E_1/2(0/−) values with EAs or E(LUMO) values should be viewed with caution. The solubilities of the seven compounds in toluene varied by nearly a factor of six, but in an unpredictable way, with the C₂F₅ and s-C₄F₉ compounds being the most soluble and the i-C₃F₇ compound being the least soluble. The effects of the different R_F groups on EAs, E(LUMO) values, and solubilities should help fluorine chemists choose the right R_F group to design new materials with improved morphological, electronic, optical, and/or magnetic properties.