Fullerenes

Abstract

Fullerenes are closed cage carbon molecules C 2 n ( n ≥ 10) comprising a combination of n carbon atoms of sp 2 (trigonal) hybridisation arranged into 12 pentagons and ( n ‐ 20)/2 hexagons. They are sometimes referred to as the “third form of carbon” (after diamond and graphite). Unlike the other forms of carbon, fullerenes are soluble in many organic solvents and undergo a wide range of standard chemical reactions. They can be formed by various procedures, but most commonly by vaporization of graphite either by use of lasers or electric arc discharge in an inert atmosphere. They do not occur naturally because of their instability arising from fairly rapid air oxidation. For a given value of n , very many isomers (having a wide range of stabilities) are possible, the number increasing geometrically with increasing size of n . Only a few are isolable. The most abundant fullerene is the spherical I h isomer of C 60 (there are 1819 other isomers, all unstable), which has a diameter of ∼10.0 Å. The next most abundant is C 70 , which is shaped like a kiwi fruit, and fullerenes containing >70 atoms are known as higher fullerenes. The geodesic structure of the molecules led to C 60 being named as “Buckminsterfullerene” after R. Buckminsterfullerene who popularized the use of geodesic domes in building construction. This term, now rarely used, gave rise to the abbreviated “fullerenes” for describing the whole class of materials. Because of the curved surfaces of the fullerenes, the π‐electrons are poorly delocalized so that C 60 is not very aromatic and contains essentially 30 double bonds and 60 single bonds. The main chemical reactions of fullerenes are therefore additions, and because of the cage curvature and hybridization state of the carbons, they are strongly electron deficient, reactions with electron‐rich nucleophiles being especially favored. Many derivatives having electron‐donor groups attached to the cage are being investigated with regard to their photoinduced electron‐transfer properties with potential for conversion of light into electricity. The cages are able to enclose a wide variety of atoms and molecules and these incar‐ or endohedral fullerenes have properties that differ from those of the empty cages. Larger fullerenes can also incorporate smaller ones and these multiwall or “nested” fullerenes are known as “onions”. Very elongated cages are known as nanotubes, and are potentially the strongest known constructional materials. These may consist of either a single tube (single‐wall nanotube, SWNT) or numerous concentric tubes within each other (multiwall nanotubes, MWNT), and various elements and molecules have been incorporated inside these also. Aza phospha‐ and borafullerenes have one or more nitrogen, phosphorus, or boron atom, respectively, replacing the corresponding number of carbon atoms in the cage structure. Nanotubes containing both nitrogen and boron atoms are also known. Fullerenes may be joined together in various ways (eg, by the use of high pressure) to give polymeric structures, but these are difficult to process and analyse because of very low solubility. Some fullerenes with alkaline metals incorporated into the lattices show superconducting properties in the solid state, but they are pyrophoric. Although fullerenes are insoluble in water, some derivatives are very soluble and this has led to investigations of biological properties. In particular, their use in connection with removal of free radicals, anti‐ human immunodeficiency virus (HIV) properties, antiviral behavior, and photodynamic therapy are all under investigation.