Effects of Lowering Symmetry on the ESR Spectra of Radical Anions of Fullerene Derivatives and the Reduction Potentials

Abstract

Radical anions of dibenzyl adducts of C60, 1,4-(C6H5CH2)2C60•- and 1,2-(C6H5CH2)2C60•-, give ESR spectra which have larger g values (2.0004 and 2.0001, respectively) and much smaller line widths (ΔHmsl = 2.5 and 3.3 G, respectively, at 213 K) than the g value (1.9984) and the ΔHmsl value (30.9 G at 213 K) of C60•-. An even smaller ΔHmsl value (0.17 G) and a larger g value (2.0011) are observed for the tetrabenzyl C60 adduct radical anion, 1,4,10,24-(C6H5CH2)4C60•- and this is ascribed to a large splitting of the degenerate t1u orbitals caused by introduction of four benzyl groups to C60. In this case, a hyperfine structure, due to two nonequivalent protons of only one benzyl group (aH1 = 0.31 G, aH2 = 0.11 G), is observed and this is consistent with the predicted localized spin density at the C2 position next to the C1 carbon to which a benzyl group is attached. The radical anions of the mono- and bisadducts formed in the Diels−Alder cycloaddition reaction of C60 with 9,10-dimethylanthracene show ESR signals at different g values (2.0003 for the monoadduct and 2.0009 for the bisadduct). The relationship between lower symmetry and the ESR spectra of radical anions of various C60 derivatives is discussed in terms of the g values and the line widths. The energy gap (δ) between the singly occupied orbital and the two other orbitals which had a t1u symmetry prior to introduction of addends to C60 is derived from differences in the g values of various C60•- derivatives from the free spin value (2.0023). A linear correlation is shown to exist between log ΔHmsl and −δ. The δ values are also obtained from Arrhenius plots of ln ΔHmsl vs T-1 and they agree well with the values derived from the g values and the reduction potentials.