What Are Fullerenes?

Fullerenes are a family of carbon allotropes in which carbon atoms form closed, hollow cage structures composed entirely of pentagons and hexagons. They were discovered in 1985 by Harold Kroto, Robert Curl, and Richard Smalley through laser vaporization of graphite, work that earned the 1996 Nobel Prize in Chemistry. The simplest and most studied member of the family is C60, buckminsterfullerene, but the family extends to C70, C76, C84, and higher-order cages, as well as to carbon nanotubes when the concept is generalized to cylindrical closed structures. Fullerenes share with graphite an sp2-hybridized bonding arrangement, but the curvature introduced by pentagonal rings gives them a three-dimensional cage geometry not found in planar graphite.

The Ossila introduction to fullerene materials identifies the family broadly as including spherical, ellipsoidal, and elongated forms, with properties that depend strongly on cage size, symmetry, and degree of chemical functionalization.

Structural Variety

The isolated pentagon rule governs which fullerene cages are chemically stable: no two pentagonal faces may be adjacent, because fused pentagons concentrate strain and destabilize the structure. This rule limits stable fullerenes to specific cage sizes; C60 satisfies it with 12 non-adjacent pentagons interspersed among 20 hexagons, while C70 adds a band of 10 additional hexagons to produce a slightly elongated ellipsoidal cage. Above C70, many isomers satisfying the isolated pentagon rule become possible, so higher fullerenes are mixtures of isomers that must be separated chromatographically. Endohedral fullerenes enclose metal atoms or clusters inside the cage, and heterofullerenes substitute nitrogen or boron atoms for one or more carbon positions.

Carbon nanotubes, which can be conceptualized as rolled graphene sheets capped at each end by hemispherical fullerene domes, are sometimes grouped within the extended fullerene family. Single-walled nanotubes and multiwalled nanotubes derive their electronic and mechanical properties from the same sp2-bonding network, making the fullerene concept a unifying framework for a broad range of curved carbon nanostructures.

Synthesis and Separation

Fullerenes are produced in bulk by arc discharge, combustion synthesis, or laser ablation of carbon-rich sources. Arc discharge between graphite electrodes in a helium atmosphere generates a fullerene-containing soot from which C60 and C70 are extracted with organic solvents and separated by high-performance liquid chromatography. The synthesis of endohedral metallofullerenes, such as Sc3N@C80, requires the presence of a nitrogen source or a co-vaporized metal during arc discharge. The relative proportions of different cage sizes in the crude product depend on temperature, pressure, and gas flow, parameters that researchers have worked to optimize for selective production.

Electronic and Chemical Properties

Fullerenes are electron acceptors with relatively high electron affinities among organic molecules. C60 can be reversibly reduced to C606- electrochemically, and this electron-accepting character is the basis for its use in organic semiconductor devices. Computational studies of fullerene structure-property relationships show that the HOMO-LUMO gap decreases with increasing cage size, enabling tuning of optical absorption and reduction potential by cage selection or functionalization. The chemical modification most consequential for applications is cyclopropanation, which converts the fullerene into derivatives such as PCBM with improved solubility and tailored energy levels for organic photovoltaic devices. Research on buckminsterfullerene chemistry covers the wide range of addition reactions used to attach functional groups to the cage, from Diels-Alder cycloadditions to azide additions.

Applications

Fullerenes have applications in a range of fields, including:

• Organic photovoltaics, where functionalized fullerene derivatives serve as electron-acceptor layers
• Organic transistors and molecular electronics exploiting high electron mobility
• Biomedicine, including drug delivery and photodynamic therapy agents
• Tribological coatings and lubricant additives that reduce friction
• Quantum information science using endohedral metallofullerenes as qubit candidates
• Superconducting materials, where alkali-metal-doped fullerides such as K3C60 exhibit superconductivity below 18 K