ABSTRACT
Almost simultaneously with the discovery of fullerenes (Kroto et al. 1985; Krätschmer et al. 1990; Hirsch and Brettreich 2005), it was found that these hollow structures can act as host molecules encapsulating guests such as small atoms, molecules, or ions within their cavity, thereby forming endohedral fullerenes (represented as M@C2n) (Kikuchi et al. 1993, 1994; Saunders et al. 1993, 1994, 1996; Shinohara et al. 1993, 1994; Jimenez-Vasquez et al. 1994; Akasaka et al. 1995; Suzuki et al. 1995; Ding and Yang 1996; Kirbach and Dunsch 1996; Kubozono et al. 1996a,b; Murphy et al. 1996; Nagase et al. 1996; Tellgmann et al. 1996; Shimshi et al. 1997; Rubin 1999; Stevenson et al. 1999; Khong et al. 2000; Liu and Sun 2000; Shinohara 2000; Hirsch 2001; Akasaka and Nagase 2002; Syamala et al. 2002; Kato et al. 2003; Komatsu et al. 2005a,b; Murata et al. 2006). Interestingly, the inner surface of the fullerene cage, in contrast to its exterior, is chemically inert (Murphy et al. 1996). Fullerenes enclosing metal atoms (endohedral metallofullerenes) bear significantly altered physical properties compared with those of empty fullerenes. This is because of electron transfer from the encaged metal atom to the fullerene core. A transition metal atom inside the fullerene cage (diameter ~3.5 Å) may, for example, result in higher temperature superconductors or lead to less cytotoxic contrast agents for magnetic resonance imaging (Kato et al. 2003).
