Fullerene - what is this? Properties and application of fullerenes

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 constructed of carbon atoms and having pentagonal or hexagonal faces (there are very few exceptions here). Atoms of carbon in unsubstituted fullerenes tend to be in the sp ² -hybrid state with coordination number 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 hardly connected to each other: flat lattices consisting of hexagonal cells, where carbon atoms are on the tops. Each of them is connected with three neighboring atoms, and the fourth valence electron forms a pi-system. Hence, fullerene is just such a molecular form, that is, the picture of the sp ² -hybrid state is obvious. If we introduce geometric defects into the graphene sheet, a closed structure is inevitably formed. For example, such defects are five-membered cycles (pentagonal faces), similarly distributed along with hexagonal in carbon chemistry.

Euler's theorem says that it is possible to obtain a closed polyhedron with three-coordinated vertices by introducing twelve pentagons, regardless of the number of hexagonal faces. Hence, the minimum size of fullerene is formally dodecahedron C ₂₀ . However, the curvature of such structures having a small number of carbon atoms is high, and therefore for the sp ² -hybrid state it is not very beneficial, since carbon prefers planar coordination. That's why the smallest in the pure form of the resulting fullerene is C ₆₀ , which has the structure of a truncated regular icosahedron. In it the hexagonal faces are separated from each other by all the pentagonal. The chemistry of fullerenes calls this fact the rule of isolated pentagons, and every real fullerene obeys it.

Formula

Hydrated fullerene is designated C ₆₀ (C ₆₀ HyFn). This is a strong supramolecular hydrophilic complex, which consists of one fullerene C ₆₀ molecule in the first hydrant shell and twenty-four water molecules. This is the 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 number of fullerenes complement C ₇₀ , because there are no intermediate structures where all pentagons would be isolated. Beginning with C _78, the fullerene structure makes it possible to observe several stable isomers in each carbon skeleton. Theoretically, it is quite possible that there are fullerenes with carbon atoms of any even number, "higher fullerenes".

The production of fullerenes occurs most often by electric arc or electron beam method, as well as by laser spraying of graphite in a helium atmosphere. The soot that condenses on the surface of the reactor is treated in boiling benzene, toluene, xylene or other organic solvents. The solution evaporates, forming a black condensate, ten to fifteen percent consisting of fullerenes C ₇₀ and C ₆₀ , as well as a very small number of higher fullerenes.

The relationship between different furellans can vary, it depends on the synthesis parameters, but usually C ₆₀ prevails, it is more than several times. Among the highest, the most are C ₇₈ , C ₇₆ and C ₈₄ . In the same picture, the share of fullerene in all synthesis products is falling. 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 there are twenty-four water molecules per molecule of the substance itself. This fullerene molecule has a high symmetry, there all the atoms are equivalent. Its spherical shape is formed by the distance from the nucleus of atoms to the center at about 0.36 nm and a radius of about 0.5 nm. A molecular crystal is formed, where the molecules form a cubic face-centered lattice, a three-layer densest spherical packing. At high temperatures, C ₆₀ sublimes without forming a liquid phase. Best of all, it dissolves in aromatic substances with solvents such as carbon disulfide, and in the polar - much worse. The elongated ellipsoidal C ₇₀ shape, 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 every opportunity to get different classes of their derivatives: metallofullerenes, heterofullerenes. The product family of exoendral (outer-outer) accession is richer than all, because each carbon atom is an accessible reaction center. These are the properties of fullerenes. They are joined, creating new products, atoms of halogens and hydrogen, organic radicals, even the addition of cycles can occur. Thus, polymeric materials containing fullerenes and multi-sphere compounds are obtained. K C ₆₀ joins up to forty-eight substituents without destroying the carbon skeleton. Thus, for example, C ₆₀ F _{48 is} obtained, and many such compounds have been developed, where the basis is fullerenes.

Application

Almost fullerenes are interesting in completely different areas. Given their electronic properties, we can consider them themselves and their derivatives as semiconductors. Fullerenes absorb ultraviolet radiation well, they have high electron-withdrawing ability. All these properties allow using them in photovoltaics, photosensors, solar batteries, devices of various molecular electronics. The fullerenes in medicine are used as antiviral and antimicrobial agents, as agents in photodynamic therapy, etc.

Technologies of the present have allowed to increase the total number of installations relatively quickly, in order to obtain more fullerenes, and methods for their purification are also working better and more efficiently. That is why the cost, for example, of C ₆₀ has significantly decreased over the last fifteen years (from ten thousand to ten or fifteen dollars per gram). Now we are faced with the situation, when for real industrial use, fullerenes fall into the person. Their application grows and spreads day by day.

Information about the opening

The fullerenes are named after the famous architect and engineer Richard Buckminster Fuller, who, according to the principle of their structure, invented geodesic constructions. At first this class of compounds was limited to structures that included only six- and pentagonal facets. If in the fullerene molecule besides carbon are present atoms of other chemical elements, and they are located inside the carcass of carbon, these fullerenes are called endohedral. If the atoms of other elements are located outside, then the fullerenes are exohedral. Their names this molecular form was received not so long ago - in 1985, when a group of researchers unexpectedly made the discovery of fullerene. Small, Kroto, Curle and other chemists studied graphite pairs, which were obtained by laser ablation (irradiation) of a solid. Peaks were detected that had a maximum amplitude corresponding to clusters of sixty and seventy carbon atoms.

So the molecules C ₆₀ and C ₇₀ have been calculated, and a hypothesis has been put forward that the first molecule is lined up in the form of a truncated icosahedron. It is the most common C ₆₀ called baxminsterfulleren, and the remaining molecules are simply fullerenes.

Scientists-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 molecular structure of absolutely all fullerenes. However, this is a fascinating story, and without an interesting prehistory it did not. Somewhat earlier it was written about the possibility of the existence of such molecules in Japan (1971), and a theoretical justification was obtained and published in the USSR (1973). Nevertheless, it was Kroto, Curl and Small that received the Nobel Prize in Chemistry.

Nature and technology

The preparation 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 takes place, and also the structure of fullerenes and their properties are considered. The sphere of their application is spreading ever wider. It turned out that a significant amount of fullerenes is contained in the soot, which is formed on graphite electrodes in an arc discharge. Earlier this fact was simply not seen.

When fullerenes were obtained under laboratory conditions, carbon molecules began to be found in nature. In Karelia they were found in samples of schungites, in India and the USA - in furulgites. Much and often there are carbon molecules in meteorites and sediments on the bottom, which are at least sixty five million years old. On the Earth, pure fullerenes can form during the discharge of lightning 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 amounts are found in space - both in the gaseous state and in a solid state.

Synthesis

The first experiments on the separation of fullerenes occurred through condensed graphite vapor, which were obtained by laser irradiation of solid graphite samples. It was possible to obtain only traces of fullerenes. Only in 1990 the chemists Huffman, Lamb and Kretchmer developed a new method for the production of fullerenes in gram quantities. It consisted in burning graphite electrodes with an electric arc in an atmosphere of helium and at low pressure. There was an erosion of the anode, and soot containing fullerenes appeared on the walls of the chamber.

Then the soot was dissolved in toluene or benzene, and in the resulting solution grams were isolated in pure form of the molecules C ₇₀ and C ₆₀ . The ratio is 1: 3. In addition, the solution contained two percent of heavy fullerenes of higher order. Now it was a small matter: to select the optimal parameters for evaporation-the composition of the atmosphere, the pressure, the diameter of the electrodes, the current, and so on, in order to achieve the greatest yield of fullerenes. They amounted to about twelve percent of the anode material itself. That's why the cost of fullerenes is so high.

Production

All attempts of experimental scientists at first were in vain: productive and cheap ways of obtaining fullerenes were not. Neither burning in the flame of hydrocarbons nor chemical synthesis led to success. The electric arc method remained the most productive, allowing one to obtain about one gram of fullerenes per hour. Mitsubishi has set up industrial production by burning hydrocarbons, but their fullerenes are not clean - they contain oxygen molecules. And the mechanism of the formation of this substance itself remains unclear, because the arc combustion processes are extremely unstable from the thermodynamic point of view, and this very much hinders the treatment of the theory. Confutable only the facts that fullerene collects individual carbon atoms, that is, fragments of C ₂ . However, a visual picture of the formation of this substance was not 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 rather complicated. Especially it concerns the separation of the mixture into separate molecular fractions, which are carried out by liquid chromatography on columns and with high pressure. At the last stage, the solvent residues from the already solid fullerene are removed. For this purpose, the sample is maintained under dynamic vacuum conditions at a temperature of up to two hundred and fifty degrees. But the plus is that during the development of fullerene C ₆₀ and its production in already macroquantities, organic chemistry has grown by an independent branch - the chemistry of fullerenes, which has become incredibly popular.

Benefit

Derivatives of fullerenes are used in various fields of engineering. Films and fullerene crystals are semiconductors that possess photoconductivity under optical irradiation. Crystals C ₆₀ , if they are doped with alkali metal atoms, become superconducting. Fullerene solutions have nonlinear optical properties, therefore they can be used as the basis of optical closures, which are necessary for protection against intense radiation. Also fullerene is used as a catalyst for the synthesis of diamonds. Widely used fullerenes in biology and medicine. Three properties of these molecules work here: the lipophilicity determining the membranotropy, the electron deficiency that gives the ability to interact with free radicals, and the ability to transfer the molecule of ordinary oxygen to their own excited state and convert this oxygen into a singlet one.

Such active forms of substance attack biomolecules: nucleic acids, proteins, lipids. Active forms of oxygen are used in photodynamic therapy for the treatment of cancer. In the patient's blood, photosensitizers are injected, generating active forms of oxygen-actually the fullerenes or their derivatives. 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 undergo apoptosis, and the tumor is destroyed. Plus to this - fullerenes have antioxidant properties and capture active forms of oxygen.

Fullerene lowers the activity of HIV integrase, a protein that is responsible for inserting the virus into DNA, interacting with it, changing the conformation and depriving it of its primary wrecking function. Some of the fullerene derivatives interact directly with DNA and interfere with the action of restitaz.

More about medicine

In 2007, water-soluble fullerenes were started to be used as antiallergic agents. The studies were carried out 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 (we recall the hydrophobicity of C ₆₀ ) penetrates the cell membrane very easily. For example, erythropoietin, a kidney hormone injected directly into the blood, is degraded in significant amounts, and if used together with fullerenes, the concentration increases more than twice, and therefore it enters the cell.