Fullerene

Rotating view of Buckminsterfullerene C₆₀.

The Icosahedral fullerene C₅₄₀

A fullerene is any molecule composed entirely of carbon, in the form of a hollow sphere, ellipsoid, or tube. Spherical fullerenes are also called buckyballs, and cylindrical ones are called carbon nanotubes or buckytubes. Fullerenes are similar in structure to graphite, which is composed of stacked graphene sheets of linked hexagonal rings; but they may also contain pentagonal (or sometimes heptagonal) rings.

The first fullerene to be discovered, and the family's namesake, was buckminsterfullerene C₆₀, made in 1985 by Robert Curl, Harold Kroto and Richard Smalley. The name was an homage to Richard Buckminster Fuller, whose geodesic domes it resembles. Fullerenes have since been found to occur (if rarely) in nature.

The discovery of fullerenes greatly expanded the number of known carbon allotropes, which until recently were limited to graphite, diamond, and amorphous carbon such as soot and charcoal. Buckyballs and buckytubes have been the subject of intense research, both for their unique chemistry and for their technological applications, especially in materials science, electronics, and nanotechnology .

Prediction and discovery

The existence of C₆₀ was predicted by Eiji Osawa of Toyohashi University of Technology in a Japanese magazine in 1970. He noticed that the structure of a corannulene molecule was a subset of a soccer-ball shape, and he made the hypothesis that a full ball shape could also exist. His idea was reported in Japanese magazines, but did not reach Europe or America.

Also in 1970, R.W.Henson (then of the Atomic Energy Research Establishment) proposed the structure and made a model of C₆₀. The evidence for this new form of carbon was very weak and was not accepted, even by his colleagues. The results were never published but were acknowledged in the Carbon journal in 1999.

With mass spectrometry, discrete peaks were observed corresponding to molecules with the exact mass of sixty or seventy or more carbon atoms. In 1985, Harold Kroto (then of the University of Sussex), James R. Heath, Sean O'Brien, Robert Curl and Richard Smalley, from Rice University, discovered C₆₀, and shortly thereafter came to discover the fullerenes. Kroto, Curl, and Smalley were awarded the 1996 Nobel Prize in Chemistry for their roles in the discovery of this class of compounds. C₆₀ and other fullerenes were later noticed occurring outside the laboratory (e.g., in normal candle soot). By 1991, it was relatively easy to produce gram-sized samples of fullerene powder using the techniques of Donald Huffman and Wolfgang Krätschmer. Fullerene purification remains a challenge to chemists and to a large extent determines fullerene prices. So-called endohedral fullerenes have ions or small molecules incorporated inside the cage atoms. Fullerene is an unusual reactant in many organic reactions such as the Bingel reaction discovered in 1993. The first nanotubes were obtained in 1991.

Minute quantities of the fullerenes, in the form of C₆₀, C₇₀, C₇₆, and C₈₄ molecules, are produced in nature, hidden in soot and formed by lightning discharges in the atmosphere. Recently, fullerenes were found in a family of minerals known as Shungites in Karelia, Russia.

Naming

Buckminsterfullerene (C₆₀) was named after Richard Buckminster Fuller, a noted architectural modeler who popularized the geodesic dome. Since buckminsterfullerenes have a similar shape to that sort of dome, the name was thought to be appropriate. As the discovery of the fullerene family came after buckminsterfullerene, the shortened name 'fullerene' was used to refer to the family of fullerenes. The suffix “ene” indicates that each C atom is covalently bonded to three others (instead of the maximum of four), a situation that classically would correspond to the existence of bonds involving two pairs of electrons (“double bonds”).

Variations

Since the discovery of fullerenes in 1985, structural variations on fullerenes have evolved well beyond the individual clusters themselves. Examples include:

• buckyball clusters: smallest member is C
  ₂₀ (unsaturated version of dodecahedrane) and the most common is C
  ₆₀;
• nanotubes: hollow tubes of very small dimensions, having single or multiple walls; potential applications in electronics industry;
• megatubes: larger in diameter than nanotubes and prepared with walls of different thickness; potentially used for the transport of a variety of molecules of different sizes;
• polymers: chain, two-dimensional and three-dimensional polymers are formed under high pressure high temperature conditions
• nano"onions": spherical particles based on multiple carbon layers surrounding a buckyball core; proposed for lubricants;
• linked "ball-and-chain" dimers: two buckyballs linked by a carbon chain;
• fullerene rings

"Buckyball"

C₆₀ with isosurface of ground state electron density as calculated with DFT

An association football is a model of the Buckminsterfullerene C₆₀

Buckminsterfullerene

Buckminsterfullerene (IUPAC name (C₆₀-I_h)[5,6]fullerene) is the smallest fullerene molecule in which no two pentagons share an edge (which can be destabilizing, as in pentalene). It is also the most common in terms of natural occurrence, as it can often be found in soot.

The structure of C₆₀ is a truncated (T = 3) icosahedron, which resembles a soccer ball of the type made of twenty hexagons and twelve pentagons, with a carbon atom at the vertices of each polygon and a bond along each polygon edge.

The van der Waals diameter of a C₆₀ molecule is about 1 nanometer (nm). The nucleus to nucleus diameter of a C₆₀ molecule is about 0.71 nm.

The C₆₀ molecule has two bond lengths. The 6:6 ring bonds (between two hexagons) can be considered "double bonds" and are shorter than the 6:5 bonds (between a hexagon and a pentagon). Its average bond length is 1.4 angstroms.

Silicon buckyballs have been created around metal ions.

Boron buckyball

A new type of buckyball utilizing boron atoms instead of the usual carbon has been predicted and described in 2007. The B₈₀ structure, with each atom forming 5 or 6 bonds, is predicted to be more stable than the C₆₀ buckyball. One reason for this given by the researchers is that the B-80 is actually more like the original geodesic dome structure popularized by Buckminster Fuller which utilizes triangles rather than hexagons. However, this work has been subject to much criticism by quantum chemists as it was concluded that the predicted Ih symmetric structure was vibrationally unstable and the resulting cage undergoes a spontaneous symmetry break yielding a puckered cage with rare Th symmetry (symmetry of a volleyball). The number of six atom rings in this molecule is 20 and number of five member rings is 12. There is an additional atom in the center of each six member ring, bonded to each atom surrounding it.

Variations of buckyballs

Another fairly common buckminsterfullerene is C₇₀, but fullerenes with 72, 76, 84 and even up to 100 carbon atoms are commonly obtained.

In mathematical terms, the structure of a fullerene is a trivalent convex polyhedron with pentagonal and hexagonal faces. In graph theory, the term fullerene refers to any 3-regular, planar graph with all faces of size 5 or 6 (including the external face). It follows from Euler's polyhedron formula, |V|-|E|+|F| = 2, (where |V|, |E|, |F| indicate the number of vertices, edges, and faces), that there are exactly 12 pentagons in a fullerene and |V|/2-10 hexagons.

20-fullerene (dodecahedral graph)	26-fullerene graph	60-fullerene (truncated icosahedral graph)	70-fullerene graph

The smallest fullerene is the dodecahedron – the unique C₂₀. There are no fullerenes with 22 vertices. The number of fullerenes C_2n grows with increasing n = 12,13,14..., roughly in proportion to n⁹ (sequence A007894 in the OEIS). For instance, there are 1812 non-isomorphic fullerenes C₆₀. Note that only one form of C₆₀, the buckminsterfullerene alias truncated icosahedron, has no pair of adjacent pentagons (the smallest such fullerene). To further illustrate the growth, there are 214,127,713 non-isomorphic fullerenes C₂₀₀, 15,655,672 of which have no adjacent pentagons.

Trimetasphere carbon nanomaterials were discovered by researchers at Virginia Tech and licensed exclusively to Luna Innovations. This class of novel molecules comprises 80 carbon atoms (C
₈₀) forming a sphere which encloses a complex of three metal atoms and one nitrogen atom. These fullerenes encapsulate metals which puts them in the subset referred to as metallofullerenes. Trimetaspheres have the potential for use in diagnostics (as safe imaging agents), therapeutics and in organic solar cells.

Carbon nanotubes

This model of a rotating Carbon nanotube shows its 3D structure.

Main article: Carbon nanotube

Nanotubes are cylindrical fullerenes. These tubes of carbon are usually only a few nanometres wide, but they can range from less than a micrometer to several millimeters in length. They often have closed ends, but can be open-ended as well. There are also cases in which the tube reduces in diameter before closing off. Their unique molecular structure results in extraordinary macroscopic properties, including high tensile strength, high electrical conductivity, high ductility, high resistance to heat, and relative chemical inactivity (as it is cylindrical and "planar" — that is, it has no "exposed" atoms that can be easily displaced). One proposed use of carbon nanotubes is in paper batteries, developed in 2007 by researchers at Rensselaer Polytechnic Institute. Another proposed use in the field of space technologies and science fiction is to produce high-tensile carbon cables required by a space elevator.

Carbon nanobuds

Main article: Carbon nanobud

Nanobuds have been obtained by adding fullerenes to carbon nanotubes.

Properties

For the past decade, the chemical and physical properties of fullerenes have been a hot topic in the field of research and development, and are likely to continue to be for a long time. Popular Science has published articles about the possible uses of fullerenes in armor. In April 2003, fullerenes were under study for potential medicinal use: binding specific antibiotics to the structure to target resistant bacteria and even target certain cancer cells such as melanoma. The October 2005 issue of Chemistry and Biology contains an article describing the use of fullerenes as light-activated antimicrobial agents.

In the field of nanotechnology, heat resistance and superconductivity are some of the more heavily studied properties.

A common method used to produce fullerenes is to send a large current between two nearby graphite electrodes in an inert atmosphere. The resulting carbon plasma arc between the electrodes cools into sooty residue from which many fullerenes can be isolated.

There are many calculations that have been done using ab-initio quantum methods applied to fullerenes. By DFT and TD-DFT methods one can obtain IR, Raman and UV spectra. Results of such calculations can be compared with experimental results.

Aromaticity

Researchers have been able to increase the reactivity of fullerenes by attaching active groups to their surfaces. Buckminsterfullerene does not exhibit "superaromaticity": that is, the electrons in the hexagonal rings do not delocalize over the whole molecule.

A spherical fullerene of n carbon atoms has n pi-bonding electrons. These should try to delocalize over the whole molecule. The quantum mechanics of such an arrangement should be like one shell only of the well-known quantum mechanical structure of a single atom, with a stable filled shell for n = 2, 8, 18, 32, 50, 72, 98, 128, etc.; i.e. twice a perfect square number; but this series does not include 60. As a result, C₆₀ in water tends to pick up two more electrons and become an anion. The nC₆₀ described below may be the result of C₆₀ trying to form a loose metallic bonding.

Chemistry

Main article: Fullerene chemistry

Fullerenes are stable, but not totally unreactive. The sp²-hybridized carbon atoms, which are at their energy minimum in planar graphite, must be bent to form the closed sphere or tube, which produces angle strain. The characteristic reaction of fullerenes is electrophilic addition at 6,6-double bonds, which reduces angle strain by changing sp²-hybridized carbons into sp³-hybridized ones. The change in hybridized orbitals causes the bond angles to decrease from about 120° in the sp² orbitals to about 109.5° in the sp³ orbitals. This decrease in bond angles allows for the bonds to bend less when closing the sphere or tube, and thus, the molecule becomes more stable.

Other atoms can be trapped inside fullerenes to form inclusion compounds known as endohedral fullerenes. An unusual example is the egg shaped fullerene Tb₃N@C₈₄, which violates the isolated pentagon rule. Recent evidence for a meteor impact at the end of the Permian period was found by analyzing noble gases so preserved. Metallofullerene-based inoculates using the rhonditic steel process are beginning production as one of the first commercially-viable uses of buckyballs.

Solubility

Fullerenes are sparingly soluble in many solvents. Common solvents for the fullerenes include aromatics, such as toluene, and others like carbon disulfide. Solutions of pure buckminsterfullerene have a deep purple color. Solutions of C₇₀ are a reddish brown. The higher fullerenes C₇₆ to C₈₄ have a variety of colors. C₇₆ has two optical forms, while other higher fullerenes have several structural isomers. Fullerenes are the only known allotrope of carbon that can be dissolved in common solvents at room temperature.

Some fullerene structures are not soluble because they have a small band gap between the ground and excited states. These include the small fullerenes C₂₈, C₃₆ and C₅₀. The C₇₂ structure is also in this class, but the endohedral version with a trapped lanthanide-group atom is soluble due to the interaction of the metal atom and the electronic states of the fullerene. Researchers had originally been puzzled by C₇₂ being absent in fullerene plasma-generated soot extract, but found in endohedral samples. Small band gap fullerenes are highly reactive and bind to other fullerenes or to soot particles.

Solvents that are able to dissolve buckminsterfullerene (C₆₀) are listed below in order from highest solubility. The value in parentheses is the approximate saturated concentration.

1. 1-chloronaphthalene (51 mg/mL)
2. 1-methylnaphthalene (33 mg/mL)
3. 1,2-dichlorobenzene (24 mg/mL)
4. 1,2,4-trimethylbenzene (18 mg/mL)
5. tetrahydronaphthalene (16 mg/mL)
6. carbon disulfide (8 mg/mL)
7. 1,2,3-tribromopropane (8 mg/mL)
8. bromoform (5 mg/mL)
9. cumene (4 mg/mL)
10. toluene (3 mg/mL)
11. benzene (1.5 mg/mL)
12. cyclohexane (1.2 mg/mL)
13. carbon tetrachloride (0.4 mg/mL)
14. chloroform (0.25 mg/mL)
15. n-hexane (0.046 mg/mL)
16. tetrahydrofuran (0.006 mg/mL)
17. acetonitrile (0.004 mg/mL)
18. methanol (0.000 04 mg/mL)
19. water (1.3×10⁻¹¹ mg/mL)

Solubility of C₆₀ in some solvents shows unusual behaviour due to existence of solvate phases (analogues of crystallohydrates). For example, solubility of C₆₀ in benzene solution shows maximum at about 313 K. Crystallization from benzene solution at temperatures below maximum results in formation of triclinic solid solvate with four benzene molecules C₆₀•4C₆H₆ which is rather unstable in air. Out of solution, this structure decomposes into usual fcc C₆₀ in few minutes' time. At temperatures above solubility maximum the solvate is not stable even when immersed in saturated solution and melts with formation of fcc C₆₀. Crystallization at temperatures above the solubility maximum results in formation of pure fcc C₆₀. Millimeter-sized crystals of C₆₀ and C₇₀ can be grown from solution both for solvates and for pure fullerenes.

Quantum mechanics

In 1999, researchers from the University of Vienna demonstrated that wave-particle duality applied to molecules such as fullerene. One of the co-authors of this research, Julian Voss-Andreae, has since created several sculptures symbolizing wave-particle duality in fullerenes (see Fullerenes in popular culture for more detail).

Science writer Marcus Chown stated on the CBC radio show Quirks and Quarks in May 2006 that scientists are trying to make buckyballs exhibit the quantum behavior of existing in two places at once (quantum superposition).

Safety and toxicity

When considering toxicological data, care must be taken to distinguish as necessary between what are normally referred to as fullerenes: (C₆₀, C₇₀,...); fullerene derivatives: C₆₀ or other fullerenes with covalently bonded chemical groups; fullerene complexes (e.g., water-solubilized with surfactants, such as C₆₀-PVP; host-guest complexes, such as with cyclodextrin; ), where the fullerene is physically bound to another molecule; C₆₀ nanoparticles, which are extended solid-phase aggregates of C₆₀ crystallites; and nanotubes, which are generally much larger (in terms of molecular weight and size) compounds, and are different in shape to the spheroidal fullerenes C₆₀ and C₇₀, as well as having different chemical and physical properties.

The above different compounds span the range from insoluble materials in either hydrophilic or lipophilic media, to hydrophilic, lipophilic, or even amphiphilic compounds, and with other varying physical and chemical properties. Therefore any broad generalization extrapolating for example results from C₆₀ to nanotubes or vice versa is not possible, though technically all are fullerenes, as the term is defined as a close-caged all-carbon molecule. Any extrapolation of results from one compound to other compounds must take into account considerations based on a Quantitative Structural Analysis Relationship Study (QSARS), which mostly depends on how close the compounds under consideration are in physical and chemical properties.

In 1996 and 1997, Moussa et al. studied the in vivo toxicity of C₆₀ after intra-peritoneal administration of large doses. No evidence of toxicity was found and the mice tolerated a dose of 5 000 mg/kg of body weight (BW). Mori et al. (2006) could not find toxicity in rodents for C₆₀ and C₇₀ mixtures after oral administration of a dose of 2 000 mg/kg BW and did not observe evidence of genotoxic or mutagenic potential in vitro. Other studies could not establish the toxicity of fullerenes: on the contrary, the work of Gharbi et al. (2005) suggested that aqueous C₆₀ suspensions failing to produce acute or subacute toxicity in rodents could also protect their livers in a dose-dependent manner against free-radical damage.

A comprehensive and recent review on fullerene toxicity is given by Kolosnjaj et al. (2007a,b, c). These authors review the works on fullerene toxicity beginning in the early 1990s to present, and conclude that very little evidence gathered since the discovery of fullerenes indicate that C₆₀ is toxic.

With reference to nanotubes, a recent study by Poland et al. (2008) on carbon nanotubes introduced into the abdominal cavity of mice led the authors to suggest comparisons to "asbestos-like pathogenicity". It should be noted that this was not an inhalation study, though there have been several performed in the past, therefore it is premature to conclude that nanotubes should be considered to have a toxicological profile similar to asbestos. Conversely, and perhaps illustrative of how the various classes of compounds which fall under the general term fullerene cover a wide range of properties, Sayes et al. found that in vivo inhalation of C₆₀(OH)₂₄ and nano-C₆₀ in rats gave no effect, whereas in comparison quartz particles produced an inflammatory response under the same conditions. As stated above, nanotubes are quite different in chemical and physical properties to C₆₀, i.e., molecular weight, shape, size, physical properties (such as solubility) all are very different, so from a toxicological standpoint, different results for C₆₀ and nanotubes are not suggestive of any discrepancy in the findings.

Superconductivity

After the synthesis of macroscopic amounts of fullerenes, their physical properties could be investigated. Very soon Haddon et al. found that intercalation of alkali-metal atoms in solid C₆₀ leads to metallic behavior. In 1991, it was revealed that potassium-doped C₆₀ becomes superconducting at 18 K. This was the highest transition temperature for a molecular superconductor. Since then, superconductivity has been reported in fullerene doped with various other alkali metals. It has been shown that the superconducting transition temperature in alkaline-metal-doped fullerene increases with the unit-cell volume V. As caesium forms the largest alkali ion, caesium-doped fullerene is an important material in this family. Recently, superconductivity at 38 K has been reported in bulk Cs₃C₆₀, but only under applied pressure. The highest superconducting transition temperature of 33 K at ambient pressure is reported for Cs₂RbC₆₀.

The increase of transition temperature with the unit-cell volume had been believed to be evidence for the BCS mechanism of C₆₀ solid superconductivity, because inter C₆₀ separation can be related to an increase in the density of states on the Fermi level, N(ε_F). Therefore, there have been many efforts to increase the interfullerene separation, in particular, intercalating neutral molecules into the A₃C₆₀ lattice to increase the interfullerene spacing while the valence of C₆₀ is kept unchanged. However, this ammoniation technique has revealed a new aspect of fullerene intercalation compounds: the Mott-Hubbard transition and the correlation between the orientation/orbital order of C₆₀ molecules and the magnetic structure.

The C₆₀ molecules compose a solid of weakly bound molecules. The fullerites are therefore molecular solids, in which the molecular properties still survive. The discrete levels of a free C₆₀ molecule are only weakly broadened in the solid, which leads to a set of essentially nonoverlapping bands with a narrow width of about 0.5 eV. For an undoped C₆₀ solid, the 5-fold h_u band is the HOMO level, and the 3-fold t_1u band is the empty LUMO level, and this system is a band insulator. But when the C₆₀ solid is doped with metal atoms, the metal atoms give electrons to the t_1u band or the upper 3-fold t_1g band. This partial electron occupation of the band leads to sometimes metallic behavior. However, A₄C₆₀ is an insulator, although the t_1u band is only partially filled and it should be a metal according to band theory. This unpredicted behavior may be explained by the Jahn-Teller effect, where spontaneous deformations of high-symmetry molecules induce the splitting of degenerate levels to gain the electronic energy. The Jahn-Teller type electron-phonon interaction is strong enough in C₆₀ solids to destroy the band picture for particular valence states.

A narrow band or strongly correlated electronic system and degenerated ground states are important points to understand in explaining superconductivity in fullerene solids. When the inter-electron repulsion U is greater than the bandwidth, an insulating localized electron ground state is produced in the simple Mott-Hubbard model. This explains the absence of superconductivity at ambient pressure in caesium-doped C₆₀ solids. Electron-correlation-driven localization of the t_1u electrons exceeds the critical value, leading to the Mott insulator. The application of high pressure decreases the interfullerene spacing, therefore caesium-doped C₆₀ solids turn to metallic and superconducting.

A fully developed theory of C₆₀ solids superconductivity is still lacking, but it has been widely accepted that strong electronic correlations and the Jahn-Teller electron-phonon coupling produce local electron-pairings that show a high transition temperature close to the insulator-metal transition.

Chirality

Few fullerenes (e.g. C₇₆, C₇₈, C₈₀, and C₈₄) are inherently chiral because they are D₂-symmetric and have been successfully resolved. Research efforts are ongoing to develop specific sensors for their enantiomers.

Examples of fullerenes in popular culture

Main article: Fullerenes in popular culture

Examples of fullerenes in popular culture are numerous. Fullerenes appeared in fiction well before scientists took serious interest in them. In New Scientist there used to be a weekly column called "Daedalus" written by David Jones, which contained humorous descriptions of unlikely technologies. In 1966 Jones suggested that it may be possible to create giant hollow carbon molecules by distorting a plane hexagonal net by the addition of impurity atoms.

Fullerite (solid state)

The C₆₀ fullerene in crystalline form

Fullerites are the solid-state manifestation of fullerenes and related compounds and materials.

Ultrahard fullerite, buckyball

Main article: Aggregated diamond nanorod

"Ultrahard fullerite" is a coined term frequently used to describe material produced by high-pressure high-temperature (HPHT) processing of fullerite. Such treatment converts fullerite into a nanocrystalline form of diamond which exhibits remarkable mechanical properties.

See also

• Buckypaper
• Truncated rhombic triacontahedron
• Dodecahedrane
• Graphite
• Diamond
• Graphene
• Carbon

Further reading

• Aldersey-Williams, Hugh (1995). The Most Beautiful Molecule: The Discovery of the Buckyball. John Wiley & Sons. ISBN 0-471-19333-X.

External links

Pharmacological Studies on Fullerene (C60), a Novel Carbon Allotrope, and Its Derivatives

Fullerenes in solution: about anomalous temperature dependence of solubility and solid solvates of fullerenes

Fullerene and nanotube Gallery

Properties of C60 fullerene

CBC Radio|Quirks & Quarks|June 17, 2006 at www.cbc.ca

Buckyball Workshops by Sir Harry Kroto and the Vega Science Trust

Center for Nanoscale Science and Technology

Center for Biological and Environmental Nanotechnology

Dr. Smalley's brief autobiography

Dr. Smalley's webpage

Sir Harry Kroto's webpage

Interview with James R. Heath, discussing the discovery of C₆₀

Diffraction and Interference with Fullerenes: Wave-particle duality of C60, University of Vienna

Fullerene-based architectures for quantum computing in Germany and in Great Britain at the QIP IRC

Computational Chemistry Wiki

A Spherical Revelation

C₆₀ 3D-view and pdb-file

Simple model of Fullerene.

Story on "Buckyeggs" (UC Davis website)

Stainless Steel Buckminster Fullerenes

Rhonditic Steel

Introduction to fullerites

Introduction to P-SWNT

Bucky Balls, a short video explaining the structure of C60 by the Vega Science Trust

Giant Fullerenes, a short video looking at Giant Fullerenes