Metallofullerenes: their formation and characterization

Endohedral metallofullerenes are novel forms of fullerene-related materials which encage metal atom(s) in a various size of fullerene cages. In the present article, recent advances on the production, separation (isolation) and various spectroscopic characterization of endohedral metallofullerenes are presented in an effort to clarify their structural and electronic properties. Endohedral metallofullerenes are produced by the DC arc-discharge method with metal/graphite composite rods as positive electrodes. The endohedral metallofullerenes, such as Y rate at C82, Sc rate at C82, Sc2 rate at C84 and Sc3 rate at C82, have been purified and isolated by using the two-stage high performance liquid chromatography (HPLC). By using the purified metallofullerene samples, spectroscopic and structural features of the endohedral metallofullerenes are investigated. The ESR hyperfine splittings observed in the paramagnetic metallofullerenes such as Y rate at C82, Sc rate at C82 and Sc3 rate at C82 can provide us information on electron transfer nature from the encaged metal atoms to the C82 fullerene cage. Direct STM imaging of the metallofullerenes on clean surfaces in an ultra-high vacuum environment shows that the endohedral metallofullerenes are stacked as close-packed arrays on the clean surfaces that uniform fcc (111) faces can be grown. Synchrotron X-ray diffraction study of Y rate at C82 can give us a direct structural evidence that the yttrium atom is encapsulated within the C82 cage. It shows that the encaged yttrium atom is not in the center of the cage but very close to the carbon network. The observed endohedral nature is consistent with the presence a strong electron transfer interaction between encaged atom(s) and fullerene cages. (orig.)