Fullerene

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Model of the C60 fullerene (buckminsterfullerene). C60a.png
Model of the C60 fullerene (buckminsterfullerene).
Model of the C20 fullerene. C20 Fullerene.png
Model of the C20 fullerene.
Model of a carbon nanotube. Carbon nanotube zigzag povray cropped.PNG
Model of a carbon nanotube.
C60 fullerite (bulk solid C60). C60-Fulleren-kristallin.JPG
C60 fullerite (bulk solid C60).

A fullerene is an allotrope of carbon whose molecule consists of carbon atoms connected by single and double bonds so as to form a closed or partially closed mesh, with fused rings of five to seven atoms. The molecule may be a hollow sphere, ellipsoid, tube, or many other shapes and sizes. Graphene (isolated atomic layers of graphite), which is a flat mesh of regular hexagonal rings, can be seen as an extreme member of the family.

Allotropes of carbon Materials made only out of carbon

Carbon is capable of forming many allotropes due to its valency. Well-known forms of carbon include diamond and graphite. In recent decades, many more allotropes have been discovered and researched including ball shapes such as buckminsterfullerene and sheets such as graphene. Larger scale structures of carbon include nanotubes, nanobuds and nanoribbons. Other unusual forms of carbon exist at very high temperatures or extreme pressures. Around 500 hypothetical 3-periodic allotropes of carbon are known at the present time, according to the Samara Carbon Allotrope Database (SACADA).

Carbon Chemical element with atomic number 6

Carbon is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds. It belongs to group 14 of the periodic table. Three isotopes occur naturally, 12C and 13C being stable, while 14C is a radionuclide, decaying with a half-life of about 5,730 years. Carbon is one of the few elements known since antiquity.

Sphere round geometrical and circular object in three-dimensional space; special case of spheroid

A sphere is a perfectly round geometrical object in three-dimensional space that is the surface of a completely round ball.

Contents

Fullerenes with a closed mesh topology are informally denoted by their empirical formula Cn, often written Cn, where n is the number of carbon atoms. However, for some values of n there maybe more than one isomer.

In chemistry, the empirical formula of a chemical compound is the simplest positive integer ratio of atoms present in a compound. A simple example of this concept is that the empirical formula of sulfur monoxide, or SO, would simply be SO, as is the empirical formula of disulfur dioxide, S2O2. Thus, sulfur monoxide and disulfur dioxide, both compounds of sulfur and oxygen, have the same empirical formula. However, their molecular formulas, which express the number of atoms in each molecule of a chemical compound, are not the same.

In chemistry, isomers are ions or molecules with identical formulas but distinct structures. Isomers do not necessarily share similar properties. Two main forms of isomerism are structural isomerism and stereoisomerism.

The family is named after buckminsterfullerene (C60), the most famous member, which in turn is named after Buckminster Fuller. The closed fullerenes, especially C60, are also informally called buckyballs for their resemblance to the standard ball of association football ("soccer"). Nested closed fullerenes have been named bucky onions. Cylindrical fullerenes are also called carbon nanotubes or buckytubes. The bulk solid form of pure or mixed fullerenes is called fullerite.[ not verified in body ]

Buckminsterfullerene chemical compound

Buckminsterfullerene is a type of fullerene with the formula C60. It has a cage-like fused-ring structure (truncated icosahedron) that resembles a soccer ball, made of twenty hexagons and twelve pentagons, with a carbon atom which has one π bond and two single bonds at each corner of the shape to create a universal vertex.

Buckminster Fuller American architect, systems theorist, author, designer, inventor and futurist

Richard Buckminster Fuller was an American architect, systems theorist, author, designer, inventor, and futurist. Fuller published more than 30 books, coining or popularizing terms such as "Spaceship Earth", "Dymaxion" house/car, ephemeralization, synergetic, and "tensegrity". He also developed numerous inventions, mainly architectural designs, and popularized the widely known geodesic dome. Carbon molecules known as fullerenes were later named by scientists for their structural and mathematical resemblance to geodesic spheres.

Ball (association football) ball used in the sport of association football

A football, soccer-ball, football ball, or association footballball is the ball used in the sport of association football. The name of the ball varies according to whether the sport is called "football", "soccer", or "association football". The ball's spherical shape, as well as its size, weight, and material composition, are specified by Law 2 of the Laws of the Game maintained by the International Football Association Board. Additional, more stringent, standards are specified by FIFA and subordinate governing bodies for the balls used in the competitions they sanction.

Fullerenes had been predicted for some time, but only after their accidental synthesis in 1985 were they detected in nature [1] [2] and outer space. [3] [4] The discovery of fullerenes greatly expanded the number of known allotropes of carbon, which had previously been limited to graphite, diamond, and amorphous carbon such as soot and charcoal. They have been the subject of intense research, both for their chemistry and for their technological applications, especially in materials science, electronics, and nanotechnology. [5]

Diamond Allotrope of carbon often used as a gemstone and an abrasive

Diamond is a solid form of the element carbon with its atoms arranged in a crystal structure called diamond cubic. At room temperature and pressure, another solid form of carbon known as graphite is the chemically stable form, but diamond almost never converts to it. Diamond has the highest hardness and thermal conductivity of any natural material, properties that are utilized in major industrial applications such as cutting and polishing tools. They are also the reason that diamond anvil cells can subject materials to pressures found deep in the Earth.

Amorphous solid crystal system

In condensed matter physics and materials science, an amorphous or non-crystalline solid is a solid that lacks the long-range order that is characteristic of a crystal. In some older books, the term has been used synonymously with glass. Nowadays, "glassy solid" or "amorphous solid" is considered to be the overarching concept, and glass the more special case: Glass is an amorphous solid that exhibits a glass transition. Polymers are often amorphous. Other types of amorphous solids include gels, thin films, and nanostructured materials such as glass.

Soot Impure carbon particles resulting from the incomplete combustion of hydrocarbons

Soot is a mass of impure carbon particles resulting from the incomplete combustion of hydrocarbons. It is more properly restricted to the product of the gas-phase combustion process but is commonly extended to include the residual pyrolysed fuel particles such as coal, cenospheres, charred wood, and petroleum coke that may become airborne during pyrolysis and that are more properly identified as cokes or char.

History

The icosahedral fullerene C
540, another member of the family of fullerenes Fullerene c540.png
The icosahedral fullerene C
540
, another member of the family of fullerenes

Predictions and limited observations

The icosahedral C
60
H
60
cage was mentioned in 1965 as a possible topological structure. [6] Eiji Osawa currently of Toyohashi University of Technology predicted the existence of C
60
in 1970. [7] [8] He noticed that the structure of a corannulene molecule was a subset of the shape of a soccer ball, 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.

Eiji Osawa Japanese chemist

Eiji Osawa (Japanese: 大澤 映二 Hepburn: Ōsawa Eiji, born June 6, 1935 in Toyama, Japan) is a former professor of computational chemistry, noted for his prediction of the C60 molecule in 1970.

Toyohashi University of Technology Higher education institution in Aichi Prefecture, Japan

Toyohashi University of Technology, often abbreviated to Toyohashi Tech, or TUT, is a national engineering university located in Toyohashi, Aichi, Japan. Distinguished for the upper-division student body where over 80% of them are transfer students from 5-year Technical Colleges called Kōsens, the Toyohashi Tech is one of the only two Universities of Technology, a form of universities in Japan, the other being Nagaoka University of Technology. Toyohashi Tech is also noted for the fact that majority of the students proceed to graduate schools. The university is locally nicknamed Gikadai (技科大).

Corannulene chemical compound

Corannulene is a polycyclic aromatic hydrocarbon with chemical formula C20H10. The molecule consists of a cyclopentane ring fused with 5 benzene rings, so another name for it is [5]circulene. It is of scientific interest because it is a geodesic polyarene and can be considered a fragment of buckminsterfullerene. Due to this connection and also its bowl shape, corannulene is also known as a buckybowl. Corannulene exhibits a bowl-to-bowl inversion with an inversion barrier of 10.2 kcal/mol (42.7 kJ/mol) at −64 °C.

Also in 1970, R. W. Henson (then of the UK Atomic Energy Research Establishment) proposed the C
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. [9] [10]

United Kingdom Country in Europe

The United Kingdom of Great Britain and Northern Ireland, commonly known as the United Kingdom or Britain, is a sovereign country located off the north­western coast of the European mainland. The United Kingdom includes the island of Great Britain, the north­eastern part of the island of Ireland, and many smaller islands. Northern Ireland is the only part of the United Kingdom that shares a land border with another sovereign state, the Republic of Ireland. Apart from this land border, the United Kingdom is surrounded by the Atlantic Ocean, with the North Sea to the east, the English Channel to the south and the Celtic Sea to the south-west, giving it the 12th-longest coastline in the world. The Irish Sea separates Great Britain and Ireland. The United Kingdom's 242,500 square kilometres (93,600 sq mi) were home to an estimated 66.0 million inhabitants in 2017.

The Atomic Energy Research Establishment, known as AERE or colloquially Harwell Laboratory, near Harwell, Oxfordshire, was the main centre for atomic energy research and development in the United Kingdom from the 1940s to the 1990s.

In 1973, independently from Henson, a group of scientists from the USSR made a quantum-chemical analysis of the stability of C
60
and calculated its electronic structure. The paper was published in 1973, [11] but the scientific community did not give much importance to this theoretical prediction.

Around 1980, Sumio Iijima identified the molecule of C
60
from an electron microscope image of carbon black, where it formed the core of a particle with the structure of a "bucky onion". [12]

Discovery of C
60

In 1985 Harold Kroto of the University of Sussex, working with James R. Heath, Sean O'Brien, Robert Curl and Richard Smalley from Rice University, discovered fullerenes in the sooty residue created by vaporising carbon in a helium atmosphere. In the mass spectrum of the product, discrete peaks appeared corresponding to molecules with the exact mass of sixty or seventy or more carbon atoms, namely C
60
and C
70
. The team identified their structure as the now familiar "buckyballs". [13]

The name "buckminsterfullerene" was eventually chosen for C
60
by the discoverers as an homage to American architect Buckminster Fuller for the vague similarity of the structure to the geodesic domes which he popularized; which, if they were extended to a full sphere, would also have the icosahedral symmetry group. [14] The "ene" ending was chosen to indicate that the carbons are unsaturated, being connected to only three other atoms instead of the normal four. The shortened named "fullerene" eventually came to be applied to the whole family.

Kroto, Curl, and Smalley were awarded the 1996 Nobel Prize in Chemistry [15] for their roles in the discovery of this class of molecules.

Further developments

Kroto and the Rice team already discovered other fullerenes besides C60, [13] and the list was much expanded in the following years. Carbon nanotubes were first discovered and synthesized in 1991. [16] [17]

After their discovery, minute quantities of fullerenes were found to be produced in sooty flames,[ citation needed ] and by lightning discharges in the atmosphere. [2] In 1992, fullerenes were found in a family of minerals known as shungites in Karelia, Russia. [1] .

The production techniques were improved by many scientists, including Donald Huffman, Wolfgang Krätschmer, Lowell D. Lamb, and Konstantinos Fostiropoulos.[ citation needed ] Thanks to their efforts, by 1990 it was relatively easy to produce gram-sized samples of fullerene powder. Fullerene purification remains a challenge to chemists and to a large extent determines fullerene prices.

In 2010, the spectral signatures of C60 and C70 were observed by NASA's Spitzer infrared telescope in a cloud of cosmic dust surrounding a star 6500 light years away. [3] 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." [4] According to astronomer Letizia Stanghellini, "It’s possible that buckyballs from outer space provided seeds for life on Earth." [18] In 2019, ionized C60 molecules were detected with the Hubble Space Telescope in the space between those stars. [19] [20]

Types

There are two major families of fullerenes, with fairly distinct properties and applications: the closed buckyballs and the open-ended cylindrical carbon nanotubes. [21] However, hybrid structures exist between those two classes, such as carbon nanobuds — nanotubes capped by hemispherical meshes or larger "buckybuds".

Buckyballs

C
60 with isosurface of ground state electron density as calculated with DFT C60 isosurface.png
C
60
with isosurface of ground state electron density as calculated with DFT
Rotating view of C
60, one kind of fullerene C60 Buckyball.gif
Rotating view of C
60
, one kind of fullerene

Buckminsterfullerene

Buckminsterfullerene is the smallest fullerene molecule containing pentagonal and hexagonal rings in which no two pentagons share an edge (which can be destabilizing, as in pentalene). It is also most common in terms of natural occurrence, as it can often be found in soot.

The empirical formula of buckminsterfullerene is C
60
and its structure is a truncated icosahedron, which resembles an association football 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 buckminsterfullerene molecule is about 1.1 nanometers (nm). [22] 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 than the 6:5 bonds (between a hexagon and a pentagon). Its average bond length is 1.4 angstroms.

Other fullerenes

Another fairly common fullerene has empirical formula C
70
, [23] but fullerenes with 72, 76, 84 and even up to 100 carbon atoms are commonly obtained.

The smallest possible fullerene is the dodecahedral C
20
. There are no fullerenes with 22 vertices. [24] The number of different fullerenes C2n 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 C
60
. Note that only one form of C
60
, buckminsterfullerene, has no pair of adjacent pentagons (the smallest such fullerene). To further illustrate the growth, there are 214,127,713 non-isomorphic fullerenes C
200
, 15,655,672 of which have no adjacent pentagons. Optimized structures of many fullerene isomers are published and listed on the web. [25]

Heterofullerenes have heteroatoms substituting carbons in cage or tube-shaped structures. They were discovered in 1993 [26] and greatly expand the overall fullerene class of compounds. Notable examples include boron, nitrogen (azafullerene), oxygen, and phosphorus derivatives.

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 [27] and in organic solar cells. [28]

Carbon nanotubes

This rotating model of a carbon nanotube shows its 3D structure. Kohlenstoffnanoroehre Animation.gif
This rotating model of a carbon 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 high tensile strength, high electrical conductivity, high ductility, high heat conductivity, 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. [29] Another highly speculative proposed use in the field of space technologies is to produce high-tensile carbon cables required by a space elevator.

Derivatives

Buckyballs and carbon nanotubes have been used as building blocks for a great variety of derivatives and larger structures, such as [21]

Heterofullerenes and non-carbon fullerenes

After the discovery of C60, many fullerenes have been synthesized (or studied theoretically by molecular modeling methods) in which some or all the carbon atoms are replaced by other elements. Inorganic nanotubes, in particular, have attracted much attention.

Silicon

Silicon buckyballs have been created around metal ions.

Boron

A type of buckyball which uses boron atoms, instead of the usual carbon, was predicted and described in 2007. The B
80
structure, with each atom forming 5 or 6 bonds, is predicted to be more stable than the C
60
buckyball. [35] One reason for this given by the researchers is that B
80
is actually more like the original geodesic dome structure popularized by Buckminster Fuller, which uses triangles rather than hexagons. However, this work has been subject to much criticism by quantum chemists [36] [37] 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). [36] 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, later it was found that the previously proposed B80 fullerene is not global minimum for 80 atom boron clusters and hence can not be found in nature. [38] In the same paper by Sandip De et al., it was concluded that boron's energy landscape is significantly different from other fullerenes already found in nature hence pure boron fullerenes are unlikely to exist in nature.

Other elements

Inorganic (carbon-free) fullerene-type structures have been built with the disulfides of molybdenum (MoS2), long used as a graphite-like lubricant, tungsten (WS2), titanium (TiS2) and niobium (NbS2). These materials were found to be stable up to at least 350 tons/cm2 (34.3 GPa). [39]

Main fullerenes

Below is a table of main closed carbon fullerenes synthesized and characterized so far, with their CAS number when known. [40] Fullerenes with fewer than 60 carbon atoms have been called "lower fullerenes", and those with more than 70 atoms "higher fullerenes".[ citation needed ]

FormulaNum.
Isom.
Mol.
Symm.
Cryst.
Symm.
Space group NoPearson
symbol
a (nm)b (nm)c (nm)β°Zρ
(g/cm3)
C
20
1Ih
C
60
1Ih
C
70
1D5h
C
72
1D6h
C
74
1D3h
C
76
2D2* Monoclinic P214mP21.1021.1081.768108.1021.48
Cubic Fm3m225cF41.54751.54751.54759041.64
C
78
5D2v
C
80
7
C
82
9C
2
, C2v, C3v
Monoclinic P214mP21.1411.13551.8355108.072
C
84
24D2*, D2d Cubic Fm3m1.5817 [41] 1.58171.581790
C
86
19
C
88
35
C
90
46
C
3996

In the table, "Num.Isom." is the number of possible isomers within the "isolated pentagon rule", which states that two pentagons in a fullerene should not share edges. [42] "Mol.Symm." is the symmetry of the molecule, [43] [44] , 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.

When C
76
or C
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 from C
76
transforms it into a face-centered cubic form. [45] Both monoclinic and face-centered cubic (fcc) phases are known for better-characterized C
60
and C
70
fullerenes
.

Properties

Topology

Schlegel diagrams are often used to clarify the 3D structure of closed-shell fullerenes, as 2D projections are often not ideal in this sense. [46]

In mathematical terms, the combinatorial 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 convex polyhedron; more precisely, its one-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.

The Schlegel diagram of a closed fullerene is a graph that is planar and 3-regular (or "cubic"; meaning that all vertices have degree 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 from Euler's polyhedron formula, VE+F=2 (where V, E, F are the numbers of vertices, edges, and faces), that V must be even, and that there must be exactly 12 pentagons and V/2−10 hexagons. Similar constraints exist if the fullerene has heptagonal (seven-atom) cycles. [47]

Open fullerenes, like carbon nanotubes and graphene, can consist entirely of hexagonal rings. In theory, a long nanotube with ends joined to form a closed torus-like sheet could also consist entirely of hexagons.

Bonding

Since each carbon atom is connected to only three neighbors, instead of the usual four, it is customary to describe those bonds as being a mixture of single and double covalent bonds.

Encapsulation

So-called endohedral fullerenes have ions or small molecules incorporated inside the cage atoms.

Research

In the early 2000s, the chemical and physical properties of fullerenes were a hot topic in the field of research and development. Popular Science discussed possible uses of fullerenes (graphene) in armor. [48]

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

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.

Fullerene is an unusual reactant in many organic reactions such as the Bingel reaction discovered in 1993.

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, free to delocalize. 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. This 2(N + 1)2 rule (with N integer) for spherical aromaticity is the three-dimensional analogue of Hückel's rule. The 10+ cation would satisfy this rule, and should be aromatic. This has been shown to be the case using quantum chemical modelling, which showed the existence of strong diamagnetic sphere currents in the cation. [49]

As a result, C
60
in water tends to pick up two more electrons and become an anion. The nC
60
described below may be the result of C
60
trying to form a loose metallic bond.

Reactions

Polymerization

Under high pressure and temperature, buckyballs collapse to form various one-, two-, or three-dimensional carbon frameworks. Single-strand polymers are formed using the Atom Transfer Radical Addition Polymerization (ATRAP) route [50]

"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 has been reported to exhibit remarkable mechanical properties. [51]

Fullerite (scanning electron microscope image) C60 SEM.jpg
Fullerite (scanning electron microscope image)

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@C
84
, which violates the isolated pentagon rule. [52] Recent evidence for a meteor impact at the end of the Permian period was found by analyzing noble gases so preserved. [53] Metallofullerene-based inoculates using the rhonditic steel process are beginning production as one of the first commercially viable uses of buckyballs.

Solubility

C
60 in solution C60 Fullerene solution.jpg
C
60
in solution
C
60 in extra virgin olive oil showing the characteristic purple color of pristine C
60 solutions Carbon 60 Olive Oil Solution.JPG
C
60
in extra virgin olive oil showing the characteristic purple color of pristine C
60
solutions

Fullerenes are soluble in many organic solvents, such as toluene, chlorobenzene, and 1,2,3-trichloropropane. Solubilities are generally rather low, like 8 g/L for C60 in carbon disulfide. Still, fullerenes are the only known allotrope of carbon that can be dissolved in common solvents at room temperature. [54] [55] [56] [57] [58] Among the best solvents is 1-chloronaphthalene, which will dissolve 51 g/L of C60.

Solutions of pure buckminsterfullerene have a deep purple color. Solutions of C
70
are a reddish brown. The higher fullerenes C
76
to C
84
have a variety of colors.

Millimeter-sized crystals of C
60
and C
70
, both pure and solvated, can be grown from benzene solution. Crystallization of C
60
from benzene solution below 30 °C (when solubility is maximum) yields a triclinic solid solvate C
60
·4C
6
H
6
. Above 30 °C one obtains solvate-free fcc C
60
. [59] [60]

Quantum mechanics

In 1999, researchers from the University of Vienna demonstrated that wave-particle duality applied to molecules such as fullerene. [61]

Superconductivity

Fullerenes are normally electrical insulators, but when crystallized with alkali metals, the resultant compound can be conducting or even superconducting. [62]

Chirality

Some fullerenes (e.g. C
76
, C
78
, C
80
, and C
84
) are inherently chiral because they are D2-symmetric, and have been successfully resolved. Research efforts are ongoing to develop specific sensors for their enantiomers.

Stability

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. [63]

In 2013 researchers discovered that asymmetrical fullerenes formed from larger structures settle into stable fullerenes. The synthesized substance was a particular metallofullerene consisting of 84 carbon atoms with two additional carbon atoms and two yttrium atoms inside the cage. The process produced approximately 100 micrograms. [63]

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. [63] [64]

Systematic naming

According to the IUPAC, to name a fullerene, one must cite the number of member atoms for the rings which comprise the fullerene, its symmetry point group in the Schoenflies 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 spiral pathway, 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 symmetry D5h(6) means that this is the isomer where the C5 axis goes through a pentagon surrounded by hexagons rather than pentagons. [46]

In IUPAC's nomenclature, fully saturated analogues of fullerenes are called fulleranes. If the mesh has other element(s) substituted for one or more carbons, the compound is named a heterofullerene. If a double bond is replaced by a methylene bridge –CH
2
, the resulting structure is a homofullerene. If an atom is fully deleted and missing valences saturated with hydrogen atoms, it is a norfullerene. When bonds are removed (both sigma and pi), the compound becomes secofullerene; if some new bonds are added in an unconventional order, it is a cyclofullerene. [46]

Production

Fullerene production generally starts by producing fullerene-rich soot. The original (and still current) method was to send a large electric current between two nearby graphite electrodes in an inert atmosphere. The resulting electric arc vaporizes the carbon into a plasma that then cools into sooty residue. [13] Alternatively, soot is produced by laser ablation of graphite or pyrolysis of aromatic hydrocarbons.[ citation needed ] Combustion is the most efficient process, developed at MIT. [65] [66]

These processes yield a mixture of various fullerenes and other forms of carbon. The fullerenes are then extracted from the soot using appropriate organic solvents and separated by chromatography. [67] :p.369 One can obtain milligram quantities of fullerenes with 80 atoms or more. C76, C78 and C84 are available commercially.

Applications

Fullerenes have been extensively used for several biomedical applications including the design of high-performance MRI contrast agents, X-ray imaging contrast agents, photodynamic therapy and drug and gene delivery, summarized in several comprehensive reviews. [68]

Medical research

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 & Biology contained an article describing the use of fullerenes as light-activated antimicrobial agents. [69]

Tumor research

While past cancer research has involved radiation therapy, photodynamic therapy is important to study because breakthroughs in treatments for tumor cells will give more options to patients with different conditions. Recent experiments using HeLa cells in cancer research involves the development of new photosensitizers with increased ability to be absorbed by cancer cells and still trigger cell death. It is also important that a new photosensitizer does not stay in the body for a long time to prevent unwanted cell damage. [70]

Fullerenes can be made to be absorbed by HeLa cells. The C
60
derivatives can be delivered to the cells by using the functional groups L-phenylalanine, folic acid, and L-arginine among others. [71]

Functionalizing the fullerenes aims to increase the solubility of the molecule by the cancer cells. Cancer cells take up these molecules at an increased rate because of an upregulation of transporters in the cancer cell, in this case amino acid transporters will bring in the L-arginine and L-phenylalanine functional groups of the fullerenes. [72]

Once absorbed by the cells, the C
60
derivatives would react to light radiation by turning molecular oxygen into reactive oxygen which triggers apoptosis in the HeLa cells and other cancer cells that can absorb the fullerene molecule. This research shows that a reactive substance can target cancer cells and then be triggered by light radiation, minimizing damage to surrounding tissues while undergoing treatment. [73]

When absorbed by cancer cells and exposed to light radiation, the reaction that creates reactive oxygen damages the DNA, proteins, and lipids that make up the cancer cell. This cellular damage forces the cancerous cell to go through apoptosis, which can lead to the reduction in size of a tumor. Once the light radiation treatment is finished the fullerene will reabsorb the free radicals to prevent damage of other tissues. [74] Since this treatment focuses on cancer cells, it is a good option for patients whose cancer cells are within reach of light radiation. As this research continues, the treatment may penetrate deeper into the body and be absorbed by cancer cells more effectively. [70]

Safety and toxicity

Lalwani et al. published a comprehensive review on fullerene toxicity in 2013. [68] 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 C
60
is toxic. The toxicity of these carbon nanoparticles is not only dose- and time-dependent, but also depends on a number of other factors such as:

The authors therefore recommend assessing the pharmacology of every new fullerene- or metallofullerene-based complex individually as a different compound.

Examples of fullerenes in popular culture are numerous. Fullerenes appeared in fiction well before scientists took serious interest in them. In a humorously speculative 1966 column for New Scientist , David 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. [75]

See also

Related Research Articles

Carbon nanotube allotropes of carbon with a cylindrical nanostructure

Carbon nanotubes (CNTs) are tubes made of carbon with diameters typically measured in nanometers.

Truncated icosahedron Archimedean solid

In geometry, the truncated icosahedron is an Archimedean solid, one of 13 convex isogonal nonprismatic solids whose faces are two or more types of regular polygons.

Sigma bond strongest type of covalent chemical bond; formed by head-on overlapping between atomic orbitals. Sigma bonding for diatomic molecules (using the language and tools of symmetry groups):σ-bond is symmetrical with respect to rotation about the bond axis

In chemistry, sigma bonds are the strongest type of covalent chemical bond. They are formed by head-on overlapping between atomic orbitals. Sigma bonding is most simply defined for diatomic molecules using the language and tools of symmetry groups. In this formal approach, a σ-bond is symmetrical with respect to rotation about the bond axis. By this definition, common forms of sigma bonds are s+s, pz+pz, s+pz and dz2+dz2 . Quantum theory also indicates that molecular orbitals (MO) of identical symmetry actually mix or hybridize. As a practical consequence of this mixing of diatomic molecules, the wavefunctions s+s and pz+pz molecular orbitals become blended. The extent of this mixing depends on the relative energies of the MOs of like symmetry.

Platonic hydrocarbon

A Platonic hydrocarbon is a hydrocarbon (molecule) whose structure matches one of the five Platonic solids, with carbon atoms replacing its vertices, carbon–carbon bonds replacing its edges, and hydrogen atoms as needed.

Dodecahedrane is a chemical compound, a hydrocarbon with formula C
20
H
20
, whose carbon atoms are arranged as the vertices (corners) of a regular dodecahedron. Each carbon is bound to three neighbouring carbon atoms and to a hydrogen atom. This compound is one of the three possible Platonic hydrocarbons, the other two being cubane and tetrahedrane.

Endohedral fullerene class of chemical compounds

Endohedral fullerenes, also called endofullerenes, are fullerenes that have additional atoms, ions, or clusters enclosed within their inner spheres. The first lanthanum C60 complex was synthesized in 1985 and called La@C60. The @ (at sign) in the name reflects the notion of a small molecule trapped inside a shell. Two types of endohedral complexes exist: endohedral metallofullerenes and non-metal doped fullerenes.

Stone–Wales defect

A Stone–Wales defect is a crystallographic defect that involves the change of connectivity of two π-bonded carbon atoms, leading to their rotation by 90° with respect to the midpoint of their bond. The reaction commonly involves conversion between a naphthalene-like structure into a fulvalene-like structure, that is, two rings that share an edge vs two separate rings that have vertices bonded to each other.

Fullerene chemistry

Fullerene chemistry is a field of organic chemistry devoted to the chemical properties of fullerenes. Research in this field is driven by the need to functionalize fullerenes and tune their properties. For example, fullerene is notoriously insoluble and adding a suitable group can enhance solubility. By adding a polymerizable group, a fullerene polymer can be obtained. Functionalized fullerenes are divided into two classes: exohedral fullerenes with substituents outside the cage and endohedral fullerenes with trapped molecules inside the cage.

Geodesic polyarene

A geodesic polyarene in organic chemistry is a polycyclic aromatic hydrocarbon with curved convex or concave surfaces. Examples include fullerenes, nanotubes, corannulenes, helicenes and sumanene. The molecular orbitals of the carbon atoms in these systems are to some extent pyramidalized resulting a different pi electron density on either side of the molecule with consequences for reactivity.

C<sub>70</sub> fullerene

C70 fullerene is the fullerene molecule consisting of 70 carbon atoms. It is a cage-like fused-ring structure which resembles a rugby ball, made of 25 hexagons and 12 pentagons, with a carbon atom at the vertices of each polygon and a bond along each polygon edge. A related fullerene molecule, named buckminsterfullerene (C60 fullerene), consists of 60 carbon atoms.

In chemistry, a metallofullerene is a molecule composed of a metal atom trapped inside a fullerene cage.

Azafullerenes are a class of heterofullerenes in which the element substituting for carbon is nitrogen. They can be in the form of a hollow sphere, ellipsoid, tube, and many other shapes. Spherical azafullerenes resemble the balls used in football (soccer). They are also a member of the carbon nitride class of materials that include beta carbon nitride (β-C3N4), predicted to be harder than diamond. Besides the pioneering work of a couple of academic groups, this class of compounds has so far garnered little attention from the broader fullerene research community. Many properties and structures are yet to be discovered for the highly-nitrogen substituted subset of molecules.

Borospherene chemical compound

Borospherene (B40) is a cluster molecule containing 40 boron atoms. It is similar to buckminsterfullerene, the "spherical" carbon structure, but with a different symmetry. The discovery of borospherene was announced in July 2014, and is described in the journal Nature Chemistry. Borospherene is the latest in a series of cluster molecules, including buckminsterfullerene (C60), stannaspherene, and plumbaspherene. The newly discovered molecule includes unusual heptagonal faces.

Carbon nanothread

A carbon nanothread is a sp3-bonded, one-dimensional carbon crystalline nanomaterial. The tetrahedral sp3-bonding of its carbon is similar to that of diamond. Nanothreads are only a few atoms across, more than 20,000 times thinner than a human hair. They consist of a stiff, strong carbon core surrounded by hydrogen atoms. Carbon nanotubes, although also one-dimensional nanomaterials, in contrast have sp2-carbon bonding as is found in graphite.

Carbon peapod

Carbon peapod is a hybrid nanomaterial consisting of spheroidal fullerenes encapsulated within a carbon nanotube. It is named due to their resemblance to the seedpod of the pea plant. Since the properties of carbon peapods differ from those of nanotubes and fullerenes, the carbon peapod can be recognized as a new type of a self-assembled graphitic structure. Possible applications of nano-peapods include nanoscale lasers, single electron transistors, spin-qubit arrays for quantum computing, nanopipettes, and data storage devices thanks to the memory effects and superconductivity of nano-peapods.

Volleyballene chemical compound

Volleyballene term refers to a chemical compound that is a new type of 3D hollow molecule composed of carbon and transition metals, the name is a reference to fullerenes. It is the first buckyball compound to be spiked with scandium atoms. The main feature of these substances is that metal atoms are part of the framework and they are not deposited on the surface of the molecule. The incorporation of the metal atoms avoids their clustering and confers to volleyballene with sites to attach hydrogen mainly. The history of volleyballenes dates from its first prediction in 2016 by Jing Wang et al. A further study based on Density functional Theory (DFT) carried out by Tlahuice-Flores in the same year supports the prediction and provides with Infrared, Raman and UV spectra of the structure for its experimental detection. The structure is described as one Sc8 cluster holding 12 scandium atoms linked to six C10 units on each face. The chemical formula C60Sc20 is close related to C80 fullerene and it has a large HOMO-LUMO gap of 1.47 eV. Further hydrogenation of volleyballene reported a 70-H structure with an adsorption energy of circa -0.11 eV/H2. Moreover, it is expected that the adsorption-desorption reaction can be reached at ambient temperature. Potential use of volleyballenes is hydrogen storage even at ambient conditions.  

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