Afullerene is anallotrope of carbon whose molecules consist of carbon atoms connected by single and double bonds so as to form a closed or partially closedmesh, withfused rings of five to six atoms. The molecules may have hollowsphere- andellipsoid-like forms,tubes, or other shapes.
Fullerenes with a closed mesh topology are informally denoted by theirempirical formula Cn, often written Cn, wheren is the number of carbon atoms. However, for some values ofn there may be more than oneisomer.
The family is named afterbuckminsterfullerene (C60), the most famous member, which in turn is named afterBuckminster Fuller. The closed fullerenes, especially C60, are also informally calledbuckyballs for their resemblance to the standardball ofassociation football. Nested closed fullerenes have been namedbucky onions. Cylindrical fullerenes are also calledcarbon nanotubes orbuckytubes.[1] The bulk solid form of pure or mixed fullerenes is calledfullerite.[2]
Fullerenes had been predicted for some time, but only after their accidental synthesis in 1985 were they detected in nature[3][4] and outer space.[5][6] The discovery of fullerenes greatly expanded the number of knownallotropes of carbon, which had previously been limited tographite,diamond, andamorphous carbon such assoot andcharcoal. They have been the subject of intense research, both for their chemistry and for their technological applications, especially inmaterials science,electronics, andnanotechnology.[7]
IUPAC defines fullerenes as "polyhedral closed cages made up entirely of n three-coordinate carbon atoms and having 12 pentagonal and (n/2-10) hexagonal faces, where n ≥ 20."[8]
The icosahedralC 60H 60 cage was mentioned in 1965 as a possible topological structure.[9]Eiji Osawa predicted the existence ofC 60 in 1970.[10][11] He noticed that the structure of acorannulene molecule was a subset of the shape of a football, and hypothesised that a full ball shape could also exist. Japanese scientific journals reported his idea, but neither it nor any translations of it reached Europe or the Americas.
Also in 1970,R. W. Henson (former member of theUKAtomic Energy Research Establishment[12]) proposed theC 60 structure and made a model of it. Unfortunately, the evidence for that new form of carbon was very weak at the time, so the proposal was met with skepticism, and was never published. It was acknowledged only in 1999.[13][14]
In 1973, independently from Henson, D. A. Bochvar and E. G. Galpern made a quantum-chemical analysis of the stability ofC 60 and calculated its electronic structure. The paper was published in 1973,[15] but the scientific community did not give much importance to this theoretical prediction.
Around 1980,Sumio Iijima identified the molecule ofC 60 from an electron microscope image ofcarbon black, where it formed the core of a particle with the structure of a "bucky onion".[16]
Also in the 1980s at MIT,Mildred Dresselhaus andMorinobu Endo, collaborating with T. Venkatesan, directed studies blasting graphite with lasers, producing carbon clusters of atoms, which would be later identified as "fullerenes."[17]
The name "buckminsterfullerene" was eventually chosen forC 60 by the discoverers as an homage toAmericanarchitectBuckminster Fuller for the vague similarity of the structure to thegeodesic domes which he popularized; which, if they were extended to a full sphere, would also have the icosahedral symmetry group.[19] The "ene" ending was chosen to indicate that the carbons areunsaturated, being connected to only three other atoms instead of the normal four. The shortened name "fullerene" eventually came to be applied to the whole family.
Kroto, Curl, and Smalley were awarded the 1996Nobel Prize in Chemistry[20] for their roles in the discovery of this class of molecules.
Kroto and the Rice team already discovered other fullerenes besides C60,[18] and the list was much expanded in the following years. Carbon nanotubeswere first discovered and synthesized in 1991.[21][22]
After their discovery, minute quantities of fullerenes were found to be produced insooty flames,[23] and bylightning discharges in the atmosphere.[4] In 1992, fullerenes were found in a family of mineraloids known asshungites inKarelia, Russia.[3]
In 2010, thespectral signatures of C60 and C70 were observed by NASA'sSpitzer infrared telescope in a cloud of cosmic dust surrounding a star 6500 light years away.[5] Kroto commented: "This most exciting breakthrough provides convincing evidence that the buckyball has, as I long suspected, existed since time immemorial in the dark recesses of our galaxy."[6] According to astronomer Letizia Stanghellini, "It's possible that buckyballs from outer space provided seeds for life on Earth."[25] In 2019, ionized C60 molecules were detected with theHubble Space Telescope in the space between those stars.[26][27]
There are two major families of fullerenes, with fairly distinct properties and applications: the closed buckyballs and the open-ended cylindrical carbon nanotubes.[28] However, hybrid structures exist between those two classes, such ascarbon nanobuds — nanotubes capped byhemispherical meshes or larger "buckybuds".
Buckminsterfullerene is the smallest fullerene molecule containing pentagonal and hexagonal rings in which no two pentagons share an edge (which can be destabilizing, as inpentalene). It is also most common in terms of natural occurrence, as it can often be found insoot.
The empirical formula of buckminsterfullerene isC 60 and its structure is atruncated icosahedron, which resembles anassociation football ball of the type made of twenty(20) hexagons and twelve (12) pentagons, with a carbon atom at the vertices of each polygon and a bond along each polygon edge.
Thevan der Waals diameter of a buckminsterfullerene molecule is about 1.1nanometers (nm).[29] The nucleus to nucleus diameter of a buckminsterfullerene molecule is about 0.71 nm.
The buckminsterfullerene molecule has two bond lengths. The 6:6 ring bonds (between two hexagons) can be considered "double bonds" and are shorter (1.401 Å) than the 6:5 bonds (1.458 Å, between a hexagon and a pentagon). The weighted average bond length is 1.44 Å.[30]
C 70 has 10 additional atoms (shown in red) added toC 60 and a hemisphere rotated to fit
Another fairly common fullerene has empirical formulaC 70,[31] but fullerenes with 72, 76, 84 and even up to 100 carbon atoms are commonly obtained.
The smallest possible fullerene is thedodecahedralC 20. There are no fullerenes with 22 vertices.[32] The number of different fullerenes C2n grows with increasingn = 12, 13, 14, ..., roughly in proportion ton9 (sequenceA007894 in theOEIS). For instance, there are 1812 non-isomorphic fullerenesC 60. Note that only one form ofC 60, buckminsterfullerene, has no pair of adjacent pentagons (the smallest such fullerene). To further illustrate the growth, there are 214,127,713 non-isomorphic fullerenesC 200, 15,655,672 of which have no adjacent pentagons. Optimized structures of many fullerene isomers are published and listed on the web.[33]
Heterofullerenes have heteroatoms substituting carbons in cage or tube-shaped structures. They were discovered in 1993[34] and greatly expand the overall fullerene class of compounds and can have dangling bonds on their surfaces. Notable examples include boron, nitrogen (azafullerene), oxygen, and phosphorus derivatives.
This rotating model of acarbon nanotube shows its 3D structure.
Carbon 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 hightensile strength, highelectrical conductivity, highductility, highheat conductivity, and relativechemical 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 inpaper batteries, developed in 2007 by researchers atRensselaer Polytechnic Institute.[35] Another highly speculative proposed use in the field of space technologies is to produce high-tensile carbon cables required by aspace elevator.
After the discovery of C60, many fullerenes have been synthesized (or studied theoretically bymolecular modeling methods) in which some or all the carbon atoms are replaced by other elements.Non-carbon nanotubes, in particular, have attracted much attention.
A type of buckyball which usesboron atoms, instead of the usual carbon, was predicted and described in 2007. TheB 80 structure, with each atom forming 5 or 6 bonds, was predicted to be more stable than theC 60 buckyball.[41] However, subsequent analysis found that the predicted Ih symmetric structure was vibrationally unstable and the resulting cage would undergo a spontaneous symmetry break, yielding a puckered cage with rare Th symmetry (symmetry of avolleyball).[42] The number of six-member 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. By employing a systematic global search algorithm, it was later found that the previously proposedB 80 fullerene is not a global maximum for 80-atom boron clusters and hence cannot be found in nature; the most stable configurations have complex geometries.[43] The same paper concluded that boron's energy landscape, unlike others, has many disordered low-energy structures, hence pure boron fullerenes are unlikely to exist in nature.[43]
However, an irregularB 40 complex dubbedborospherene was prepared in 2014. This complex has two hexagonal faces and four heptagonal faces with in D2d symmetry interleaved with a network of 48 triangles.[44]
B 80 was experimentally obtained in 2024, i.e. 17 years after theoretical prediction by Gonzalez Szwackiet al..[45]
Icosahedral or distorted-icosahedral fullerene-like complexes have also been prepared forgermanium,tin, andlead; some of these complexes are spacious enough to hold most transition metal atoms.[47][48]
Below is a table of main closed carbon fullerenes synthesized and characterized so far, with theirCAS number when known.[49] Fullerenes with fewer than 60 carbon atoms have been called "lower fullerenes",[50] and those with more than 70 atoms "higher fullerenes".[51]
In the table, "Num.Isom." is the number of possibleisomers within the "isolated pentagon rule", which states that two pentagons in a fullerene should not share edges.[53][54] "Mol.Symm." is the symmetry of the molecule,[54][55] whereas "Cryst.Symm." is that of the crystalline framework in the solid state. Both are specified for the most experimentally abundant form(s). The asterisk * marks symmetries with more than one chiral form.
WhenC 76 orC 82 crystals are grown from toluene solution they have a monoclinic symmetry. The crystal structure contains toluene molecules packed between the spheres of the fullerene. However, evaporation of the solvent fromC 76 transforms it into a face-centered cubic form.[56] Both monoclinic andface-centered cubic (fcc) phases are known for better-characterizedC 60 andC 70 fullerenes.
Schlegel diagrams are often used to clarify the 3D structure of closed-shell fullerenes, as 2D projections are often not ideal in this sense.[57]
In mathematical terms, thecombinatorial topology (that is, the carbon atoms and the bonds between them, ignoring their positions and distances) of a closed-shell fullerene with a simple sphere-like mean surface (orientable,genus zero) can be represented as a convexpolyhedron; more precisely, itsone-dimensional skeleton, consisting of its vertices and edges. The Schlegel diagram is a projection of that skeleton onto one of the faces of the polyhedron, through a point just outside that face; so that all other vertices project inside that face.[58]
The Schlegel diagram of a closed fullerene is agraph that isplanar and3-regular (or "cubic"; meaning that all vertices havedegree 3).
A closed fullerene with sphere-like shell must have at least some cycles that are pentagons or heptagons. More precisely, if all the faces have 5 or 6 sides, it follows fromEuler's polyhedron formula,V−E+F=2 (whereV,E,F are the numbers of vertices, edges, and faces), thatV must be even, and that there must be exactly 12 pentagons andV/2−10 hexagons. Similar constraints exist if the fullerene has heptagonal (seven-atom) cycles.[59]
Additional atoms, ions, clusters, or small molecules can be trapped inside fullerenes to forminclusion compounds known asendohedral fullerenes. An unusual example is the egg-shaped fullerene Tb3N@C 84, which violates the isolated pentagon rule.[62] Evidence for a meteor impact at the end of thePermian period was found by analyzingnoble gases preserved by being trapped in fullerenes.[63]
There are many calculations that have been done usingab-initio quantum methods applied to fullerenes. ByDFT andTD-DFT methods one can obtainIR,Raman andUV spectra. Results of such calculations can be compared with experimental results.
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 notdelocalize over the whole molecule.
A spherical fullerene ofn carbon atoms hasnpi-bonding electrons, free to delocalize. These should try to delocalize over the whole molecule. The quantum mechanics of such an arrangement should be like only one shell of the well-known quantum mechanical structure of a single atom, with a stable filled shell forn = 2, 8, 18, 32, 50, 72, 98, 128, etc. (i.e., twice a perfectsquare number), but this series does not include 60. This 2(N + 1)2 rule (withN integer) forspherical aromaticity is the three-dimensional analogue ofHückel's rule. The 10+cation would satisfy this rule, and should be aromatic. This has been shown to be the case usingquantum chemical modelling, which showed the existence of strong diamagnetic sphere currents in the cation.[64]
As a result,C 60 in water tends to pick up two more electrons and become ananion. ThenC 60 described below may be the result ofC 60 trying to form a loosemetallic bond.
Under high pressure and temperature, buckyballs collapse to form various one-, two-, or three-dimensional carbon frameworks. Single-strand polymers are formed using theAtom Transfer Radical Addition Polymerization (ATRAP) route.[65]
"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 ofdiamond which has been reported to exhibit remarkable mechanical properties.[66]
Fullerenes are stable, but not totally unreactive. The sp2-hybridized carbon atoms, which are at their energy minimum in planargraphite, must be bent to form the closed sphere or tube, which producesangle strain. The characteristic reaction of fullerenes iselectrophilic addition at 6,6-double bonds, which reduces angle strain by changing sp2-hybridized carbons into sp3-hybridized ones. The change in hybridizedorbitals 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.
Solutions of pure buckminsterfullerene have a deep purple color. Solutions ofC 70 are a reddish brown. Thehigher fullerenesC 76 toC 84 have a variety of colors.
Millimeter-sized crystals ofC 60 andC 70, both pure and solvated, can be grown from benzene solution. Crystallization ofC 60 from benzene solution below 30 °C (when solubility is maximum) yields atriclinic solidsolvateC 60·4C 6H 6. Above 30 °C one obtains solvate-freefccC 60.[72][73]
Fullerenes are normally electrical insulators, but when crystallized with alkali metals, the resultant compound can be conducting or even superconducting.[75]
Some fullerenes (e.g.C 76,C 78,C 80, andC 84) areinherently chiral because they are D2-symmetric, and have been successfully resolved. Research efforts are ongoing to develop specific sensors for their enantiomers.
Two theories have been proposed to describe the molecular mechanisms that make fullerenes. The older, "bottom-up" theory proposes that they are built atom-by-atom. The alternative "top-down" approach claims that fullerenes form when much larger structures break into constituent parts.[76]
In 2013 researchers discovered that asymmetrical fullerenes formed from larger structures settle into stable fullerenes. The synthesized substance was a particularmetallofullerene consisting of 84 carbon atoms with two additional carbon atoms and twoyttrium atoms inside the cage. The process produced approximately 100 micrograms.[76]
However, they found that the asymmetrical molecule could theoretically collapse to form nearly every known fullerene and metallofullerene. Minor perturbations involving the breaking of a few molecular bonds cause the cage to become highly symmetrical and stable. This insight supports the theory that fullerenes can be formed from graphene when the appropriate molecular bonds are severed.[76][77]
According to theIUPAC, to name a fullerene, one must cite the number of member atoms for the rings which comprise the fullerene, itssymmetry point group in theSchoenflies notation, and the total number of atoms. For example, buckminsterfullerene C60 is systematically named (C 60-Ih)[5,6]fullerene. The name of the point group should be retained in any derivative of said fullerene, even if that symmetry is lost by the derivation.
To indicate the position of substituted or attached elements, the fullerene atoms are usually numbered in a spiral path, usually starting with the ring on one of the main axes. If the structure of the fullerene does not allow such numbering, another starting atom was chosen to still achieve a spiral path sequence.
The latter is the case for C70, which is (C 70-D5h(6))[5,6]fullerene in IUPAC notation. The symmetryD5h(6) means that this is the isomer where theC5 axis goes through a pentagon surrounded by hexagons rather than pentagons.[57]
(C 60-Ih)[5,6]fullerene Carbon numbering.
(C 70-D5h(6))[5,6]fullerene Carbon numbering.
(C 70-D5h(6))[5,6]fullerene Non-equivalent bonds shown by different colours.
3'H-Cyclopropa[1,2](C 70-D5h(6))[5,6]fullerene.
3'H-Cyclopropa[2,12](C 70-D5h(6))[5,6]fullerene.
C 71-PCBM, [1,2]-isomer. IUPAC name is methyl 4-(3'-phenyl-3'H-cyclopropa[1,2](C 70-D5h(6))[5,6]fullerene-3'-yl)butyrate.
In IUPAC's nomenclature, fully saturated analogues of fullerenes are calledfulleranes. If the mesh hasother element(s) substituted for one or more carbons, the compound is named aheterofullerene. If a double bond is replaced by amethylene bridge−CH2−, the resulting structure is ahomofullerene. If an atom is fully deleted and missing valences saturated with hydrogen atoms, it is anorfullerene. When bonds are removed (both sigma and pi), the compound becomessecofullerene; if some new bonds are added in an unconventional order, it is acyclofullerene.[57]
Fullerene production generally starts by producing fullerene-rich soot. The original (and still current) method was to send a large electric current between two nearbygraphite electrodes in aninert atmosphere. The resultingelectric arc vaporizes the carbon into aplasma that then cools into sooty residue.[18] Alternatively, soot is produced bylaser ablation of graphite orpyrolysis ofaromatic hydrocarbons.[78][citation needed] Combustion of benzene is the most efficient process, developed atMIT.[79][80]
These processes yield a mixture of various fullerenes and other forms of carbon. The fullerenes are then extracted from the soot usingappropriate organic solvents and separated bychromatography.[81]: p.369 One can obtain milligram quantities of fullerenes with 80 atoms or more. C76, C78 and C84 are available commercially.
Functionalized fullerenes have been researched extensively for several potential biomedical applications including high-performance MRIcontrast agents, X-ray imaging contrast agents,photodynamic therapy for tumor treatment,[82][83] and drug and gene delivery.[84][85]
In 2013, a comprehensive review on the toxicity of fullerene was published reviewing work beginning in the early 1990s to present and concluded that very little evidence gathered since the discovery of fullerenes indicate thatC 60 is toxic.[84] The toxicity of these carbonnanoparticles is not only dose- and time-dependent, but also depends on a number of other factors such as:
type (e.g.:C 60,C 70, M@C 60, M@C 82)
functional groups used to water-solubilize these nanoparticles (e.g.: OH, COOH)
method of administration (e.g.: intravenous, intraperitoneal)
It was recommended to assess the pharmacology of every new fullerene- or metallofullerene-based complex individually as a different compound.
Examples of fullerenes appear frequently inpopular culture. Fullerenes appeared in fiction well before scientists took serious interest in them. In a humorously speculative 1966 column forNew Scientist,David Jones suggested the possibility of making giant hollow carbon molecules by distorting a plane hexagonal net with the addition of impurity atoms.[87]
^ab"The allotropes of carbon".Interactive Nano-Visualization in Science & Engineering Education. Archived fromthe original on 18 June 2010. Retrieved29 August 2010.
^Bochvar, D.A.; Galpern, E.G. (1973). "О гипотетических системах: карбододекаэдре, s-икосаэдре и карбо-s-икосаэдре" [On hypothetical systems: carbon dodecahedron, S-icosahedron and carbon-S-icosahedron].Dokl. Akad. Nauk SSSR.209: 610.
^Iijima, S (1980). "Direct observation of the tetrahedral bonding in graphitized carbon black by high resolution electron microscopy".Journal of Crystal Growth.50 (3):675–683.Bibcode:1980JCrGr..50..675I.doi:10.1016/0022-0248(80)90013-5.
^Reilly, P. T. A.; Gieray, R. A.; Whitten, W. B.; Ramsey, J. M. (2000). "Fullerene Evolution in Flame-Generated Soot".Journal of the American Chemical Society.122 (47):11596–11601.Bibcode:2000JAChS.12211596R.doi:10.1021/ja003521v.ISSN0002-7863.
^Thilgen, Carlo; Herrmann, Andreas; Diederich, François (14 November 1997). "The Covalent Chemistry of Higher Fullerenes: C70 and Beyond".Angewandte Chemie International Edition in English.36 (21):2268–2280.doi:10.1002/anie.199722681.
^Margadonna, Serena; Brown, Craig M.; Dennis, T. John S.; et al. (July 1998). "Crystal Structure of the Higher Fullerene C".Chemistry of Materials.10 (7):1742–1744.doi:10.1021/cm980183c.
^abDiederich, Francois; Whetten, Robert L. (1992). "Beyond C60: The higher fullerenes".Accounts of Chemical Research.25 (3): 119.doi:10.1021/ar00015a004.
^Sivaraman, N.; Dhamodaran, R.; Kaliappan, I.; et al. (1994). "Solubility ofC 70 in Organic Solvents".Fullerene Science and Technology.2 (3):233–246.doi:10.1080/15363839408009549.
^Semenov, K. N.; Charykov, N. A.; Keskinov, V. A.; et al. (2010). "Solubility of Light Fullerenes in Organic Solvents".Journal of Chemical & Engineering Data.55:13–36.doi:10.1021/je900296s.
^Zhang, J.; Bowles, F. L.; Bearden, D. W.; et al. (2013). "A missing link in the transformation from asymmetric to symmetric metallofullerene cages implies a top-down fullerene formation mechanism".Nature Chemistry.5 (10):880–885.Bibcode:2013NatCh...5..880Z.doi:10.1038/nchem.1748.PMID24056346.
^Arikawa, Mineyuki (2006). "Fullerenes—an attractive nano carbon material and its production technology".Nanotechnology Perceptions.2 (3):121–128.ISSN1660-6795.
^Hu, Zhen; Zhang, Chunhua; Huang, Yudong; et al. (2012). "Photodynamic anticancer activities of water-solubleC 60 derivatives and their biological consequences in a HeLa cell line".Chemico-Biological Interactions.195 (1):86–94.doi:10.1016/j.cbi.2011.11.003.PMID22108244.
Nanocarbon: From Graphene to Buckyballs Interactive 3D models of cyclohexane, benzene, graphene, graphite, chiral & non-chiral nanotubes, and C60 Buckyballs – WeCanFigureThisOut.org.
![(C 60-Ih)[5,6]fullerene Carbon numbering.](/mt/?noimg=&dark=on&url=http%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aBuckminsterfullerene-2D-skeletal_numbered.svg)
![(C 70-D5h(6))[5,6]fullerene Carbon numbering.](/mt/?noimg=&dark=on&url=http%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aC70fullerene-2D-skeletal_numbered.svg)
![(C 70-D5h(6))[5,6]fullerene Non-equivalent bonds shown by different colours.](/mt/?noimg=&dark=on&url=http%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aC70fullerene-2D-skeletal_numbered_isobonds.svg)
)[5,6]fullerene.](/mt/?noimg=&dark=on&url=http%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aCyclopropa12_C70fullerene-2D-skeletal_renumbered.svg)
)[5,6]fullerene.](/mt/?noimg=&dark=on&url=http%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aCyclopropa212_C70fullerene-2D-skeletal_renumbered.svg)
![C 71-PCBM, [1,2]-isomer. IUPAC name is methyl 4-(3'-phenyl-3'H-cyclopropa[1,2](C 70-D5h(6))[5,6]fullerene-3'-yl)butyrate.](/mt/?noimg=&dark=on&url=http%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aPC71BM.svg)
