The molecular form of carbon or its allotropic modification, fullerene, is a long series of atomic clusters C n (n> 20), which are convex closed polyhedra built of carbon atoms and having pentagonal or hexagonal faces (there are very rare exceptions). The carbon atoms in unsubstituted fullerenes tend to be in the sp 2 -hybrid state with a coordination number of 3. Thus, a spherical conjugated unsaturated system is formed according to the theory of valence bonds.
general description
The most thermodynamically stable form of carbon under normal conditions is graphite, which looks like a stack of graphene sheets barely connected to each other: flat lattices consisting of hexagonal cells where carbon atoms are at the vertices. Each of them is associated with three neighboring atoms, and the fourth valence electron forms a pi system. This means that fullerene is just such a molecular form, that is, the picture of the sp 2 hybrid state is obvious. If geometric defects are introduced into the graphene sheet, a closed structure is inevitably formed. For example, five-membered cycles (pentagonal faces), similarly common along with hexagonal ones in carbon chemistry, serve as such defects.
Euler’s theorem states that obtaining a closed polyhedron with three-coordinated vertices is possible if twelve pentagons are introduced regardless of the number of hexagonal faces. Hence, the minimum fullerene size is formally the C 20 dodecahedron. However, the curvature of such structures with a small number of carbon atoms is high, and therefore it is not very beneficial for the sp 2 hybrid state, since carbon prefers planar coordination. That is why the smallest pure fullerene obtained in its pure form is C 60 , which has the structure of a truncated regular icosahedron. In it, all pentagonal are separated from each other by hexagonal faces. The chemistry of fullerenes calls this fact the rule of isolated pentagons, and every really accessible fullerene obeys it.
Formula
Hydrated fullerene is designated C 60 (C 60 HyFn). This is a strong supramolecular hydrophilic complex, which consists of one C 60 fullerene molecule in the first hydrant shell and twenty-four water molecules. This is fullerene, the formula of which is C 60 (H 2 O) 24 . It consists of twenty hexagons and twelve pentagons, outwardly resembling a soccer ball. Further, a series of fullerenes supplements C 70 , since there are no intermediate structures where all pentagons would be isolated. Starting with C 78, the fullerene structure allows one to observe several stable isomers in each carbon skeleton. It is theoretically possible that there are fullerenes with carbon atoms of any even number, "higher fullerenes."
Fullerenes are most often produced by the electric arc or electron beam method, as well as by laser spraying of graphite in a helium atmosphere. Soot that condenses on the surface of the reactor is treated in boiling benzene, toluene, xylene or other organic solvents. The solution is evaporated, forming a black condensate, consisting of ten to fifteen percent fullerenes C 70 and C 60 , as well as a very small amount of higher fullerenes.
The ratio between different furellenes can vary, it depends on the synthesis parameters, however, usually C 60 prevails, it is several times more. Among the highest, most of all are C 78 , C 76 and C 84 . In the overall picture, a drop in the share of fullerene in all synthesis products is observed. This is most likely due to the low probability of assembling large structures from initially evaporated small clusters.
Water with fullerenes
The most common and widely studied is C 60 , where twenty-four water molecules fall on one molecule of the substance itself. This fullerene molecule has a high symmetry, where all atoms are equivalent. Its spherical shape is formed by a distance from the nuclei of atoms to the center of about 0.36 nm and a radius of about 0.5 nm. A molecular crystal is formed, where the molecules make up a cubic face-centered lattice, a three-layer dense ball packing. At high temperature, C 60 sublimates without forming a liquid phase. It dissolves best in aromatic substances with solvents such as carbon disulfide, and in polar ones it is much worse. The elongated ellipsoidal shape of C 70 , like the higher fullerenes, has very close physical properties to C 60 .
And in the field of chemistry, they are all much more generous and give all sorts of opportunities to get different classes of their derivatives: metallofullerenes, heterofullerenes. The richest of all are the family of products of exoendral (external sphere) addition, because each carbon atom is an accessible reaction center. These are the properties of fullerenes. They are added to them, creating new products, atoms of halogens and hydrogen, organic radicals, even attachment of cycles can occur. So polymeric materials containing fullerenes, multisphere compounds are obtained. Up to forty-eight substituents are attached to C 60 without destroying the carbon skeleton. So, for example, C 60 F 48 is obtained, and many compounds have been mastered where fullerenes are the basis.
Application
Almost fullerenes are interesting in completely different areas. Given their electronic properties, we can consider them and their derivatives as semiconductors. Fullerenes well absorb ultraviolet radiation, they have a high electron-withdrawing ability. All these properties make it possible to use them in photovoltaics, photosensors, solar batteries, and devices of various molecular electronics. Fullerenes are involved in medicine as antiviral and antimicrobial agents, as agents in photodynamic therapy, etc.
Modern technologies have allowed a relatively rapid increase in the total number of plants in order to obtain more fullerenes, and their cleaning methods are working more and more efficiently. That is why the cost, for example, C 60 has decreased significantly over the past one and a half ten years (from ten thousand to ten to fifteen dollars per gram). Now we are faced with a situation where fullerenes also come to humans for real industrial use. Their application is growing and expanding day by day.
Discovery Information
Fullerenes are named after the famous architect and engineer Richard Buckminster Fuller, who invented geodesic structures based on the principle of their structure. At first, this class of compounds was limited to structures that included only hexagonal and pentagonal faces. If, in addition to carbon, atoms of other chemical elements are present in the fullerene molecule and they are located inside the carbon framework, then these fullerenes are called endohedral. If the atoms of other elements are located outside, then the fullerenes are obtained exohedral. This molecular form got its name not very long ago - in 1985, when a group of researchers unexpectedly made the discovery of fullerene. Small, Kroto, Curl and other chemists studied graphite vapors, which were obtained by laser ablation (irradiation) of a solid. Peaks were found that had a maximum amplitude corresponding to clusters of sixty and seventy carbon atoms.
Thus, C 60 and C 70 molecules were calculated, and the hypothesis was put forward that the first molecule was built in the shape of a truncated icosahedron. It is the most common C 60 that is called buckminsterfullerene, and the rest of the molecules are simply fullerenes.
Chemists were well versed in architecture and remembered Fuller, who built domes on buildings in this way - pentagons separated by hexagons, which is the basis of the structure of the molecular skeletons of absolutely all fullerenes. However, this is a fascinating story, and could not do without an interesting background. The possibility of the existence of such molecules was written earlier in Japan (1971), and a theoretical justification was obtained and published in the USSR (1973). Nevertheless, it was Kroto, Curl, and Small who received the Nobel Prize in Chemistry.
Nature and technology
The production of fullerenes in pure form is possible by artificial synthesis. These compounds continue to be intensively studied in different countries, establishing the conditions under which their formation occurs, and the structure of fullerenes and their properties are considered. The scope of their application is expanding. It turned out that a significant amount of fullerenes is contained in soot, which is formed on graphite electrodes in an arc discharge. No one has ever seen this fact before.
When fullerenes were obtained in the laboratory, carbon molecules began to be detected in nature. They were found in Karelia in samples of shungites, in India and the USA in furulgites. Also, there are many and often found carbon molecules in meteorites and sediments at the bottom, which are at least sixty-five million years old. On Earth, pure fullerenes can form during lightning discharges and during the combustion of natural gas. Air samples taken over the Mediterranean Sea were studied in 2011, and it turned out that fullerene is present in all samples taken - from Istanbul to Barcelona. The physical properties of this substance cause spontaneous formation. Also, its huge quantities are found in space - both in a gaseous state and in solid form.
Synthesis
The first experiments on the extraction of fullerenes occurred through condensed graphite vapors, which were obtained by laser irradiation with solid graphite samples. It was possible to get only traces of fullerenes. Only in 1990, chemists Huffman, Lamb and Kretchmer developed a new method for the extraction of fullerenes in gram quantities. It consisted of burning graphite electrodes with an electric arc in a helium atmosphere and at low pressure. Anode erosion occurred, and soot containing fullerenes appeared on the walls of the chamber.
Next, the soot was dissolved in toluene or benzene, and pure grams of C 70 and C 60 molecules were isolated in the resulting solution. The ratio is 1: 3. In addition, the solution also contained two percent of higher-order heavy fullerenes. Now the matter was small: to select the optimal parameters for evaporation - the composition of the atmosphere, pressure, diameter of the electrodes, current, and so on, in order to achieve the highest yield of fullerenes. They amounted to about twelve percent of the actual material of the anode. That is why fullerenes are so expensive.
Production
All attempts by experimental scientists at first were in vain: productive and cheap methods for producing fullerenes were not found. Neither burning in a flame of hydrocarbons, nor chemical synthesis led to success. The electric arc method remained the most productive, allowing one to get about one gram of fullerenes per hour. Mitsubishi has established industrial production by burning hydrocarbons, but their fullerenes are not clean - they contain oxygen molecules. And the mechanism of the formation of this substance is still unclear, because the processes of arc burning are extremely unstable from a thermodynamic point of view, and this greatly inhibits the consideration of the theory. The only incontrovertible facts are that fullerene collects individual carbon atoms, i.e. fragments of C 2 . However, a clear picture of the formation of this substance has not been formed.
The high cost of fullerenes is determined not only by the low yield during combustion. Isolation, purification, separation of fullerenes of different masses from soot - all these processes are quite complicated. This is especially true for the separation of the mixture into individual molecular fractions, which are carried out by liquid chromatography on columns and with high pressure. In the last step, solvent residues are removed from the already solid fullerene. For this, the sample is maintained under dynamic vacuum at a temperature of up to two hundred and fifty degrees. But the plus is that during the development of C 60 fullerene and its production in already small amounts, organic chemistry grew by an independent branch - the chemistry of fullerenes, which became incredibly popular.
Benefit
Derivatives of fullerenes are used in various fields of technology. Fullerene films and crystals are semiconductors with photoconductivity during optical irradiation. C 60 crystals, if doped with alkali metal atoms, go into a state of superconductivity. Fullerene solutions have nonlinear optical properties, therefore, they can be used as the basis of optical shutters, which are necessary for protection against intense radiation. Fullerene is also used as a catalyst for the synthesis of diamonds. Fullerenes are widely used in biology and medicine. Three properties of these molecules work here: lipophilicity determining membraneotropy, electron deficiency, which gives the ability to interact with free radicals, and the ability to transfer an ordinary excited state to an ordinary oxygen molecule and turn this oxygen into a singlet one.
Such active forms of the substance attack biomolecules: nucleic acids, proteins, lipids. Active oxygen species are used in photodynamic therapy to treat cancer. Photosensitizers are introduced into the patient’s blood, generating active oxygen species - fullerenes themselves or their derivatives. The blood flow in the tumor is weaker than in healthy tissues, and therefore photosensitizers accumulate in it, and after directed irradiation, the molecules are excited, generating active forms of oxygen. cancer cells experience apoptosis and the tumor collapses. Plus, fullerenes have antioxidant properties and capture reactive oxygen species.
Fullerene reduces the activity of HIV integrase, the protein that is responsible for the incorporation of the virus into DNA, interacting with it, changing the conformation and depriving it of its main harmful function. Some of the fullerene derivatives interact directly with DNA and inhibit the action of restriction enzymes.
More about medicine
In 2007, water-soluble fullerenes began to be used for their use as anti-allergic agents. Studies were conducted on human cells and blood, which were exposed to fullerene derivatives - C60 (NEt) x and C60 (OH) x. In experiments on living organisms - mice - the results were positive.
Already, this substance is used as a drug delivery vector, since water with fullerenes (recall the hydrophobicity of C 60 ) penetrates the cell membrane very easily. For example, erythropoietin, a hormone of the kidneys introduced directly into the blood, is degraded in a significant amount, and if used together with fullerenes, the concentration more than doubles, and therefore it enters the cell.