Endohedral metallofullerenes

Endohedral metallofullerenes (fullerenes with metal atom(s) encapsulated) are novel forms of fullerene-based materials which have attracted wide interest during the past eight years, not only in physics and chemistry but also in such interdisciplinary areas as materials and biological sciences. In this paper, advances in the production, separation (isolation) and various spectroscopic characterizations of endohedral metallofullerenes are presented in an attempt to clarify their structural, electronic and solid state properties. Endohedral metallofullerenes are normally produced by DC electric arc discharge of metal/graphite composite rods used as positive electrodes. The metallofullerenes can also be produced by the so-called laser furnace method which incorporates laser vaporization of the composite rods under high temperature (ca 1000 °C). The endohedral metallofullerenes so far produced are centred on group 2 and 3 metallofullerenes such as Sc, Y, La, Ca, Sr and Ba as well as lanthanide metallofullerenes (Ce-Lu). These metal atoms have been encapsulated in higher fullerenes, especially in C₈₂. These metallofullerenes have easily been extracted by solvents from primary soot. By using an elaborate high-performance liquid chromatography technique, the metallofullerenes are completely purified and isolated like C₆₀ and C₇₀. Synchrotron x-ray diffraction, ¹³C NMR and ultra-high vacuum scanning tunnelling microscopy (UHV-STM) studies have revealed that metal atoms are indeed encapsulated by the carbon cage and that the metal atoms are not in the centre of the fullerene cage but very close to the carbon cage, indicating the presence of a strong metal-cage interaction. It has been revealed by electron spin resonance and also by theoretical calculations that substantial electron transfers take place from the encaged metal atom to the carbon cage: intrafullerene electron transfers. Good examples are lanthanum and yttrium metallofullerenes which have the charge states of La³⁺@C₈₂^3- and Y³⁺@C₈₂^3-. One of the most distinct features of such a metallofullerene is superatom character, by which the metallofullerene can be viewed as a positively charged core metal surrounded by a negatively charged carbon cage. Such a molecule has a great similarity to the superatom concept first proposed by Watanabe and Inoshita in a semiconductor heterostructure comprising a spherically symmetric positively charged core. Structural and electronic analyses based on x-ray diffraction and UHV-STM measurements indeed provide evidence of such character. Other electronic and magnetic properties of the metallofullerenes are also intriguing. Endohedral metallofullerenes will be metal, small-gap semiconductors or insulators depending upon the fullerene size and the kind and the number of metal atom encapsulated. Finally, some prospective applications of metallofullerenes are presented.