Redox states of C60 and C70 measured by EPR and optical absorption spectroscopy, Part II.

Characteristic VIS-NIR absorption bands of the anions of C60 and C70 can be used for the clear assignment of individual redox states and their mixtures in solution. These optical transitions reflect the energy separation of the HOMO-LUMO orbitals which partially lose their degeneracy upon charging. The assignment of one sharp EPR signal in solution to C60− (g = 2.0000, ΔH = 0.025−0.05 mT) is supported by the same findings for a monosubstituted derivative, where a small broadening of this signal is found from additional 1H hyperfine interaction (g = 1.9999, ΔH = 0.1 mT). Furthermore, the g-values of the radical anions of C60 increase with the amount of charges, indicating largest contributions from spin-orbit-coupling for the monoanion. Both molecules show a biradical dianion with comparable zfs-components (C602− D = 2.7 mT; C702−: D = 3.1 mT)