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Fullerenes

 Fullerene is a spherical carbon compound and is an allotrope of carbon such as diamond, graphite and carbon nanotubes. Fullerenes of C60, C70 and C84 are well known. They are isolable carbon compounds in a sole molecular species. Among them, the C60 is a representative species. Kroto, Smalley and Curl et al. first observed the C60 in which the 60 carbon atoms consist of 12 five-membered rings and 20 six-membered rings.1) Kroto, Smalley and Curl won their joint Nobel prizes in chemistry in 1996 for their contributions. Osawa predicted existence of fullerene in 1970, earlier than the first observation of fullerene.2)
 The most specific feature of fullerene is that it is an excellent electron acceptor. Any fullerenes are n-type semiconductors, which are suitable for organic electronic materials with electron carriers. Rubidium- and cesium-doped fullerenes can be superconductors with electron carriers. These superconducting transitions occur at more than 30 K.3,4)
 Addition reactions and other chemical modifications of fullerenes easily produce fullerene derivatives. Precise structure analyses of these derivatives are possible because they are molecular species. Non-derivatized fullerenes are poorly soluble in similarity to the other nanocarbon materials. However, we can introduce soluble functional groups to form solution-processible electronic materials. Phenyl-C61-butyric acid methyl ester ([60]PCBM) and indene-C60 bisadduct (ICBA) are useful organic semiconductors for fabricating a solution-processible electronic device.5,6) These fullerene derivatives are n-type organic semiconductors for organic photovoltaics (OPV) by mixing with a p-type conjugated polymer.7) An application of a fullerene derivative for organic transistors was also reported.8) A complexation of C60 with tetrakis(dimethylamino)ethylene (TDAE) gives a charge transfer complex (TDAE-C60), which is an organic magnet at low temperature.9) 
 Although a chemical modification of the outer surface of fullerene provides PCBM or ICBA, we can introduce a small component to the inner side of fullerene. For instance, fullerenes can encapsulate a metal atom on the inner side, when the fullerenes are produced in the presence of the metal. This is the so-called metal-encapsulated fullerene described as M@C60. The encapsulation modifies the electronic state10) and chemical reactivity of fullerene.11) On the other hand, water-encapsulated fullerene (H2O@C60) was also reported. A publication described that an organic synthetic procedure gave open-caged C60 which could then encapsulate one water molecule. A following chemical modification could close the cage to from water-encapsulated fullerene H2O@C60.12)

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