Buckminsterfullerene

Template:Short description Template:Chembox Template:Nanomaterials Buckminsterfullerene is a type of fullerene with the formula Template:Chem. It has a cage-like fused-ring structure (truncated icosahedron) made of twenty hexagons and twelve pentagons, and resembles a football. Each of its 60 carbon atoms is bonded to its three neighbors.

Buckminsterfullerene is a black solid that dissolves in hydrocarbon solvents to produce a purple solution. The substance was discovered in 1985 and has received intense study, although few real world applications have been found.

Molecules of buckminsterfullerene (or of fullerenes in general) are commonly nicknamed buckyballs.

Occurrence

Buckminsterfullerene is the most common naturally occurring fullerene. Small quantities of it can be found in soot.

It also exists in space. Neutral Template:Chem has been observed in planetary nebulae and several types of star. The ionised form, Template:Chem, has been identified in the interstellar medium, where it is the cause of several absorption features known as diffuse interstellar bands in the near-infrared.

History

File:Football Pallo valmiina-cropped.jpg

Many footballs have the same arrangement of polygons as buckminsterfullerene, Template:Chem.

Theoretical predictions of buckminsterfullerene molecules appeared in the late 1960s and early 1970s. It was first generated in 1984 by Eric Rohlfing, Donald Cox, and Andrew Kaldor, using a laser to vaporize carbon in a supersonic helium beam, although the group did not realize that buckminsterfullerene had been produced. In 1985 their work was repeated by Harold Kroto, James R. Heath, Sean C. O'Brien, Robert Curl, and Richard Smalley at Rice University, who recognized the structure of Template:Chem as buckminsterfullerene.

Concurrent but unconnected to the Kroto-Smalley work, astrophysicists were working with spectroscopists to study infrared emissions from giant red carbon stars. Smalley and team were able to use a laser vaporization technique to create carbon clusters which could potentially emit infrared at the same wavelength as had been emitted by the red carbon star. Hence, the inspiration came to Smalley and team to use the laser technique on graphite to generate fullerenes.

Using laser evaporation of graphite the Smalley team found C_n clusters (where n > 20 and even) of which the most common were Template:Chem and Template:Chem. A solid rotating graphite disk was used as the surface from which carbon was vaporized using a laser beam creating hot plasma that was then passed through a stream of high-density helium gas. The carbon species were subsequently cooled and ionized resulting in the formation of clusters. Clusters ranged in molecular masses, but Kroto and Smalley found predominance in a Template:Chem cluster that could be enhanced further by allowing the plasma to react longer. They also discovered that Template:Chem is a cage-like molecule, a regular truncated icosahedron.

The experimental evidence, a strong peak at 720 daltons, indicated that a carbon molecule with 60 carbon atoms was forming, but provided no structural information. The research group concluded after reactivity experiments, that the most likely structure was a spheroidal molecule. The idea was quickly rationalized as the basis of an icosahedral symmetry closed cage structure.

Kroto, Curl, and Smalley were awarded the 1996 Nobel Prize in Chemistry for their roles in the discovery of buckminsterfullerene and the related class of molecules, the fullerenes.

In 1989 physicists Wolfgang Krätschmer, Konstantinos Fostiropoulos, and Donald R. Huffman observed unusual optical absorptions in thin films of carbon dust (soot). The soot had been generated by an arc-process between two graphite electrodes in a helium atmosphere where the electrode material evaporates and condenses forming soot in the quenching atmosphere. Among other features, the IR spectra of the soot showed four discrete bands in close agreement to those proposed for Template:Chem.

Another paper on the characterization and verification of the molecular structure followed on in the same year (1990) from their thin film experiments, and detailed also the extraction of an evaporable as well as benzene-soluble material from the arc-generated soot. This extract had TEM and X-ray crystal analysis consistent with arrays of spherical Template:Chem molecules, approximately 1.0 nm in van der Waals diameter as well as the expected molecular mass of 720 Da for Template:Chem (and 840 Da for Template:Chem) in their mass spectra. The method was simple and efficient to prepare the material in gram amounts per day (1990) which has boosted the fullerene research and is even today applied for the commercial production of fullerenes.

The discovery of practical routes to Template:Chem led to the exploration of a new field of chemistry involving the study of fullerenes.

Etymology

The discoverers of the allotrope named the newfound molecule after American architect R. Buckminster Fuller, who designed many geodesic dome structures that look similar to Template:Chem and who had died in 1983, the year before discovery. Another common name for buckminsterfullerene is "buckyballs".

Synthesis

Soot is produced by laser ablation of graphite or pyrolysis of aromatic hydrocarbons. Fullerenes are extracted from the soot with organic solvents using a Soxhlet extractor. This step yields a solution containing up to 75% of Template:Chem, as well as other fullerenes. These fractions are separated using chromatography. Generally, the fullerenes are dissolved in a hydrocarbon or halogenated hydrocarbon and separated using alumina columns.

Synthesis using the techniques of "classical organic chemistry" is possible, but not economic.

Structure

Buckminsterfullerene is a truncated icosahedron with 60 vertices, 32 faces (20 hexagons and 12 pentagons where no pentagons share a vertex), and 90 edges (60 edges between 5-membered & 6-membered rings and 30 edges are shared between 6-membered & 6-membered rings), with a carbon atom at the vertices of each polygon and a bond along each polygon edge. The van der Waals diameter of a Template:Chem molecule is about 1.01 nanometers (nm). The nucleus to nucleus diameter of a Template:Chem molecule is about 0.71 nm. The Template:Chem 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 0.14 nm. Each carbon atom in the structure is bonded covalently with 3 others. A carbon atom in the Template:Chem can be substituted by a nitrogen or boron atom yielding a Template:Chem or Template:Chem respectively.

File:Buckball-electronic-str en.svg

Energy level diagram for Template:Chem under "ideal" spherical (left) and "real" icosahedral symmetry (right).

Properties

Orthogonal projections

Centered by	Vertex	Edge 5–6	Edge 6–6	Face Hexagon	Face Pentagon
Image	File:Dodecahedron t12 v.png	File:Dodecahedron t12 e56.png	File:Dodecahedron t12 e66.png	File:Icosahedron t01 A2.png	File:Icosahedron t01 H3.png
Projective symmetry	[2]	[2]	[2]	[6]	[10]

For a time buckminsterfullerene was the largestTemplate:Quantify known molecule observed to exhibit wave–particle duality. In 2020 the dye molecule phthalocyanine exhibited the duality that is more famously attributed to light, electrons and other small particles and molecules.

Solution

File:C60 Fullerene solution.jpg

Dilute solution of Template:Chem in an aromatic solvent

Solubility of Template:Chem

Solvent	Solubility (g/L)
1-chloronaphthalene	51
1-methylnaphthalene	33
1,2-dichlorobenzene	24
1,2,4-trimethylbenzene	18
tetrahydronaphthalene	16
carbon disulfide	8
1,2,3-tribromopropane	8
xylene	5
bromoform	5
cumene	4
toluene	3
benzene	1.5
carbon tetrachloride	0.447
chloroform	0.25
hexane	0.046
cyclohexane	0.035
tetrahydrofuran	0.006
acetonitrile	0.004
methanol	0.00004
water	1.3 × 10−11
pentane	0.004
octane	0.025
isooctane	0.026
decane	0.070
dodecane	0.091
tetradecane	0.126
dioxane	0.0041
mesitylene	0.997
dichloromethane	0.254

File:UV-Vis C60.jpg

Optical absorption spectrum of Template:Chem solution, showing diminished absorption for the blue (~450 nm) and red (~700 nm) light that results in the purple color.

Fullerenes are sparingly soluble in aromatic solvents and carbon disulfide, but insoluble in water. Solutions of pure Template:Chem have a deep purple color which leaves a brown residue upon evaporation. The reason for this color change is the relatively narrow energy width of the band of molecular levels responsible for green light absorption by individual Template:Chem molecules. Thus individual molecules transmit some blue and red light resulting in a purple color. Upon drying, intermolecular interaction results in the overlap and broadening of the energy bands, thereby eliminating the blue light transmittance and causing the purple to brown color change.

Template:Chem crystallises with some solvents in the lattice ("solvates"). For example, crystallization of Template:Chem from benzene solution yields triclinic crystals with the formula Template:Chem2. Like other solvates, this one readily releases benzene to give the usual face-centred cubic Template:Chem. Millimeter-sized crystals of Template:Chem and Template:Chem can be grown from solution both for solvates and for pure fullerenes.

Solid

File:C60-Fulleren-kristallin.JPG

Micrograph of Template:Chem.

File:Fullerite structure.jpg

Packing of Template:Chem in crystal.

In solid buckminsterfullerene, the Template:Chem molecules adopt the fcc (face-centered cubic) motif. They start rotating at about −20 °C. This change is associated with a first-order phase transition to an fcc structure and a small, yet abrupt increase in the lattice constant from 1.411 to 1.4154 nm.

Template:Chem solid is as soft as graphite, but when compressed to less than 70% of its volume it transforms into a superhard form of diamond (see aggregated diamond nanorod). Template:Chem films and solution have strong non-linear optical properties; in particular, their optical absorption increases with light intensity (saturable absorption).

Template:Chem forms a brownish solid with an optical absorption threshold at ≈1.6 eV. It is an n-type semiconductor with a low activation energy of 0.1–0.3 eV; this conductivity is attributed to intrinsic or oxygen-related defects. Fcc Template:Chem contains voids at its octahedral and tetrahedral sites which are sufficiently large (0.6 and 0.2 nm respectively) to accommodate impurity atoms. When alkali metals are doped into these voids, Template:Chem converts from a semiconductor into a conductor or even superconductor.

Chemical reactions and properties

Template:Chem undergoes six reversible, one-electron reductions, ultimately generating Template:Chem. Its oxidation is irreversible. The first reduction occurs at ≈−1.0 V (Fc/Template:Chem), showing that Template:Chem is a reluctant electron acceptor. Template:Chem tends to avoid having double bonds in the pentagonal rings, which makes electron delocalization poor, and results in Template:Chem not being "superaromatic". Template:Chem behaves like an electron deficient alkene. For example, it reacts with some nucleophiles.

Hydrogenation

Template:Chem exhibits a small degree of aromatic character, but it still reflects localized double and single C–C bond characters. Therefore, Template:Chem can undergo addition with hydrogen to give polyhydrofullerenes. Template:Chem also undergoes Birch reduction. For example, Template:Chem reacts with lithium in liquid ammonia, followed by tert-butanol to give a mixture of polyhydrofullerenes such as Template:Chem2, Template:Chem2, Template:Chem2, with Template:Chem2 being the dominating product. This mixture of polyhydrofullerenes can be re-oxidized by 2,3-dichloro-5,6-dicyano-1,4-benzoquinone to give Template:Chem again.

A selective hydrogenation method exists. Reaction of Template:Chem with 9,9′,10,10′-dihydroanthracene under the same conditions, depending on the time of reaction, gives Template:Chem2 and Template:Chem2 respectively and selectively.

Halogenation

Addition of fluorine, chlorine, and bromine occurs for Template:Chem. Fluorine atoms are small enough for a 1,2-addition, while Template:Chem and Template:Chem add to remote C atoms due to steric factors. For example, in Template:Chem2 and Template:Chem2, the Br atoms are in 1,3- or 1,4-positions with respect to each other. Under various conditions a vast number of halogenated derivatives of Template:Chem can be produced, some with an extraordinary selectivity on one or two isomers over the other possible ones. Addition of fluorine and chlorine usually results in a flattening of the Template:Chem framework into a drum-shaped molecule.

Addition of oxygen atoms

Solutions of Template:Chem can be oxygenated to the epoxide Template:Chem2. Ozonation of Template:Chem in 1,2-xylene at 257K gives an intermediate ozonide Template:Chem2, which can be decomposed into 2 forms of Template:Chem2. Decomposition of Template:Chem2 at 296 K gives the epoxide, but photolysis gives a product in which the O atom bridges a 5,6-edge.

File:Addition of O atom into C60 Scheme.png

Cycloadditions

The Diels–Alder reaction is commonly employed to functionalize Template:Chem. Reaction of Template:Chem with appropriate substituted diene gives the corresponding adduct.

The Diels–Alder reaction between Template:Chem and 3,6-diaryl-1,2,4,5-tetrazines affordsTemplate:Chem. The Template:Chem has the structure in which a four-membered ring is surrounded by four six-membered rings.

File:3D structure of C62 derivative from C60 update.jpg

A Template:Chem derivative Template:Chem2 synthesized from Template:Chem and 3,6-bis(4-methylphenyl)-3,6-dihydro-1,2,4,5-tetrazine

The Template:Chem molecules can also be coupled through a [2+2] cycloaddition, giving the dumbbell-shaped compound Template:Chem. The coupling is achieved by high-speed vibrating milling of Template:Chem with a catalytic amount of KCN. The reaction is reversible as Template:Chem dissociates back to two Template:Chem molecules when heated at Template:Convert. Under high pressure and temperature, repeated [2+2] cycloaddition between Template:Chem results in polymerized fullerene chains and networks. These polymers remain stable at ambient pressure and temperature once formed, and have remarkably interesting electronic and magnetic properties, such as being ferromagnetic above room temperature.

Free radical reactions

Reactions of Template:Chem with free radicals readily occur. When Template:Chem is mixed with a disulfide RSSR, the radical Template:Chem2 forms spontaneously upon irradiation of the mixture.

Stability of the radical species Template:Chem2 depends largely on steric factors of Y. When tert-butyl halide is photolyzed and allowed to react with Template:Chem, a reversible inter-cage C–C bond is formed:

File:Free radical reaction of fullerene with tert-butyl radical.png

Cyclopropanation (Bingel reaction)

Cyclopropanation (the Bingel reaction) is another common method for functionalizing Template:Chem. Cyclopropanation of Template:Chem mostly occurs at the junction of 2 hexagons due to steric factors.

The first cyclopropanation was carried out by treating the β-bromomalonate with Template:Chem in the presence of a base. Cyclopropanation also occur readily with diazomethanes. For example, diphenyldiazomethane reacts readily with Template:Chem to give the compound Template:Chem2. [[Phenyl-C61-butyric acid methyl ester|Phenyl-Template:Chem-butyric acid methyl ester]] derivative prepared through cyclopropanation has been studied for use in organic solar cells.

Redox reactions

Template:Chem anions

The LUMO in Template:Chem is triply degenerate, with the HOMO–LUMO separation relatively small. This small gap suggests that reduction of Template:Chem should occur at mild potentials leading to fulleride anions, Template:Chem2 (n = 1–6). The midpoint potentials of 1-electron reduction of buckminsterfullerene and its anions is given in the table below:

Reduction potential of Template:Chem at 213 K
Half-reaction	E° (V)
Template:Chem2	−0.169
Template:Chem2	−0.599
Template:Chem2	−1.129
Template:Chem2	−1.579
Template:Chem2	−2.069
Template:Chem2	−2.479

Template:Chem forms a variety of charge-transfer complexes, for example with tetrakis(dimethylamino)ethylene:

Template:Chem2

This salt exhibits ferromagnetism at 16 K.

Template:Chem cations

Template:Chem oxidizes with difficulty. Three reversible oxidation processes have been observed by using cyclic voltammetry with ultra-dry methylene chloride and a supporting electrolyte with extremely high oxidation resistance and low nucleophilicity, such as Template:Chem2.

Reduction potentials of Template:Chem oxidation at low temperatures
Half-reaction	E° (V)
Template:Chem2	+1.27
Template:Chem2	+1.71
Template:Chem2	+2.14

Metal complexes

Template:Chem forms complexes akin to the more common alkenes. Complexes have been reported molybdenum, tungsten, platinum, palladium, iridium, and titanium. The pentacarbonyl species are produced by photochemical reactions.

Template:Chem2 (M = Mo, W)

In the case of platinum complex, the labile ethylene ligand is the leaving group in a thermal reaction:

Template:Chem2

Titanocene complexes have also been reported:

Template:Chem2

Coordinatively unsaturated precursors, such as Vaska's complex, for adducts with Template:Chem:

Template:Chem2

One such iridium complex, Template:Chem2 has been prepared where the metal center projects two electron-rich 'arms' that embrace the Template:Chem guest.

Endohedral fullerenes

Metal atoms or certain small molecules such as Template:Chem and noble gas can be encapsulated inside the Template:Chem cage. These endohedral fullerenes are usually synthesized by doping in the metal atoms in an arc reactor or by laser evaporation. These methods gives low yields of endohedral fullerenes, and a better method involves the opening of the cage, packing in the atoms or molecules, and closing the opening using certain organic reactions. This method, however, is still immature and only a few species have been synthesized this way.

Endohedral fullerenes show distinct and intriguing chemical properties that can be completely different from the encapsulated atom or molecule, as well as the fullerene itself. The encapsulated atoms have been shown to perform circular motions inside the Template:Chem cage, and their motion has been followed using NMR spectroscopy.

Potential applications in technology

The optical absorption properties of Template:Chem match the solar spectrum in a way that suggests that Template:Chem-based films could be useful for photovoltaic applications. Because of its high electronic affinity it is one of the most common electron acceptors used in donor/acceptor based solar cells. Conversion efficiencies up to 5.7% have been reported in Template:Chem–polymer cells.

Potential applications in health

Ingestion and risks

Template:Chem is sensitive to light, so leaving Template:Chem under light exposure causes it to degrade, becoming dangerous. The ingestion of Template:Chem solutions that have been exposed to light could lead to developing cancer (tumors). So the management of Template:Chem products for human ingestion requires cautionary measures such as: elaboration in very dark environments, encasing into bottles of great opacity, and storing in dark places, and others like consumption under low light conditions and using labels to warn about the problems with light.

Solutions of Template:Chem dissolved in olive oil or water, as long as they are preserved from light, have been found nontoxic to rodents.

Otherwise, a study found that Template:Chem remains in the body for a longer time than usual, especially in the liver, where it tends to be accumulated, and therefore has the potential to induce detrimental health effects.

Oils with C60 and risks

An experiment in 2011–2012 administered a solution of Template:Chem in olive oil to rats, achieving a 90% prolongation of their lifespan. Template:Chem in olive oil administered to mice resulted in no extension in lifespan. Template:Chem in olive oil administered to beagle dogs resulted in a large reduction of C-reactive protein, which is commonly elevated in cardiovascular disease and cerebrovascular disease.

Many oils with Template:Chem have been sold as antioxidant products, but it does not avoid the problem of their sensitivity to light, that can turn them toxic. A later research confirmed that exposure to light degrades solutions of Template:Chem in oil, making it toxic and leading to a "massive" increase of the risk of developing cancer (tumors) after its consumption.

To avoid the degradation by effect of light, Template:Chem oils must be made in very dark environments, encased into bottles of great opacity, and kept in darkness, consumed under low light conditions and accompanied by labels to warn about the dangers of light for Template:Chem.

Some producers have been able to dissolve Template:Chem in water to avoid possible problems with oils, but that would not protect Template:Chem from light, so the same cautions are needed.

Further reading

• – describing the original discovery of C₆₀
• – report describing the synthesis of Template:Chem with combustion research published in 2000 at the 28th International Symposium on Combustion

External links

• History of C₆₀'s discovery carried out by the Chemistry Department at Bristol University
• A brief overview of buckminsterfullerene described by the University of Wisconsin-Madison
• A report by Ming Kai College detailing the properties of buckminsterfullerene
• Donald R. Huffman and Wolfgang Krätschmer's paper pertaining to the synthesis of Template:Chem in Nature published in 1990
• A thorough description of Template:Chem by the Oak Ridge National Laboratory
• An article about buckminsterfullerene on Connexions Science Encyclopaedia
• Extensive statistical data compiled by the University of Sussex on the numerical quantitative properties of buckminsterfullerene
• A web portal dedicated to buckminsterfullerene, authored and supported by the University of Bristol
• Another web portal dedicated to buckminsterfullerene, authored and supported by the Chemistry Department at the University of Bristol
• A brief article entirely devoted to Template:Chem and its discovery, structure, production, properties, and applications
• American Chemical Society's complete article on buckminsterfullerene
• Buckminsterfullerene at The Periodic Table of Videos (University of Nottingham)