International Journal of ChemTech Research CODEN( USA): IJCRGG ISSN : 0974-4290 Vol.5, No.1, pp 167-171, Jan-Mar 2013 The Third Allotrope of Carbon: an Update

Shiv Bhadra Singh*, Aaditya Singh

Faculty of Pharmacy, Aryakul college of Pharmacy and Research Lucknow, India.

*Corres.author: [email protected]

Abstract: There were only three known forms or allotropes of carbon molecules - diamonds (tetrahedral arrangement which is the strongest making it a very hard substance), graphite or pencil lead (lattice sheets) and soot or charcoal which has no crystalline structure. The newly discovered allotrope of carbon i.e, fullerene family, are extremely useful in modern as well as biological applications such as nanotechnology, electronics, architecture, construction, optics, enzyme inhibition, antiviral activity, DNA cleavage, photodynamic therapy, electron transfer, and other in vitro and in vivo biological effects. are graphitic cage structures incorporating exactly twelve pentagons and undergo various chemical reactions. This review deals with various progresses in studies of fullerenes and their applications. Keywords: Third Allotrope, Carbon: Fullerene.

Introduction The discoveries of single-walled carbon nanotubes 1 have provided the opportunity to study their Ever since their experimental discovery in 1985, mechanical, optical and electronic properties. fullerenes have attracted considerable attention in Specifically, their electronic characteristics are different fields of sciences. The keen interest in predicted to vary depending upon the nanotube fullerenes is due to much of unusal physical and symmetry and diameter, thus giving either metallic chemical properties as well as diversity of their or semiconducting behaviour. It is very important behavior at various conditions. The discovery and for the creation of novel materials for availability in large quantity of fullerenes have nanoengineering. stimulated their extensive research in a wide variety Various extraction and chromatographic techniques of fields such as physics, chemistry, material are currently employed for the isolation of fullerenes science, biology etc. Their unique carbon cage because of their immense applications in various structure coupled with immense scope for fields.5,6 derivatization makes fullerenes a potential 2 therapeutic agent. Initially fullerenes are viewed as Fullerenes include - aromatic and quite chemically inert molecules, Fullerenes: The classical definition of a fullerene is a fullerenes can actually undergo various chemical compound composed solely of an even number of reactions3 such as nucleophilic addition, Diel’s carbon atoms which form a cage-like fused-ring Alder reaction, 1,3-dipolar cycloaddition, radical polycyclic system with 12 five-membered rings and addition, oxidation, reduction etc. the rest six-membered rings; the archetypal example Various solvent free mechanochemical reactions of is the 60-atom structure, where the atoms and bonds fullerenes have also been investigated. Most of the delineate a truncated icosahedron. However, the studied mechanochemical reactions of fullerenes term has been broadened to include any closed-cage were done by high energy high speed vibration structure having 20 or more carbon atoms consisting milling technique (HSVM), and many fewer entirely of 3 coordinate carbon atoms. reactions were done by low energy ball milling and hand grinding.4 Shiv Bhadra Singh et al /Int.J.ChemTech Res.2013,5(1) 168

Fulleranes: Fulleranes are fully saturated fullerenes, Kekulé structure count, the graph independence for example, the hydrocarbon C60H60. number, the number of spanning trees, the Fulleroids: Heterofullerenes, norfullerenes, combinatorial curvature, the bipartivity measure of homofullerenes, and secofullerenes have been called fullerene graphs, the occurrence number of different fulleroids (fullerene-like) because they resemble structural motifs in fullerenes. fullerenes in structure but do not conform to the In the majority of cases, for the stability prediction definition of a fullerene as given above. It is of lower fullerene isomers Cn with n ≤ 70 the convenient to refer to them as fulleroids and name pentagon adjacency index NP (the so-called them as modified fullerenes. minimal-NP criterion) is used and according to the minimal-NP rule it is supposed that fullerenes which Nomenclature minimize NP are more likely to be stable than those The general problem of fullerene nomenclature has that do not. Consequently, it is believed that the been first considered by R. Taylor. He proposed the buckminsterfullerene is the most stable C60 numbering of atoms in those fullerene isomer that fullerene, because this is the only one for which NP 13,14 were isolated so far. However the proposal was has a minimum value (NP = 0). intended mainly to bring some order into naming fullerene derivatives which were being produced at Types of fullerene an increasing rate. An approved IUPAC Structural variations on fullerenes have evolved well recommendations for naming and numbering the beyond the individual clusters themselves. Examples C60-Ih and C70-D5h(6) fullerenes and their derivatives include- was prepared in year 2002. There are two system for naming- a) Buckyball clusters: smallest member of these a) Systematic names The recommended groups include C20 and the most common is C60. systematic names for the icosahedral C60 and the b) Nanotubes: hollow tubes of very small D5h(6)-C70 fullerenes are (C60-Ih)[5,6]fullerene dimensions, having single or multiple walls. and (C70-D5h(6))[5,6]fullerene. The parenthetical c) Megatubes: larger in diameter than nanotubes prefix gives the carbon content and the point and prepared with walls of different thickness. group symbol; and the bracketed numbers d) Polymers: two-dimensional and three- indicate the ring sizes in the fullerene. The latter dimensional polymers are formed under high- is important in fullerenes with rings other than pressure high-temperature conditions. five- and six-membered. The subscript e) Nano"onions": spherical particles based on parenthetical (6) following the point group multiple carbon layers surrounding a buckyball symbol D5h in the latter name indicates that the core; proposed for lubricants. five-membered ring on the five-fold symmetry f) Linked "ball-and-chain" dimers: two buckyballs axis is surrounded by six-membered rings. This linked by a carbon chain. differentiates this fullerene from an isomeric g) Fullerene rings.15 (C70-D5h)[5,6]fullerene, which has five- membered rings surrounding the five-membered Structure ring on the five-fold symmetry axis, which Carbon bonds undergo catenation and give rise to would have the name (C70-D5h(5))[5,6]fullerene. different shapes, demonstrating different properties. This technique is being further evaluated to One of the famous substances containing carbon is determine its value for differentiating among diamond, a carbon allotrope, the bounds form an other fullerenes with degenerate point group inflexible three-dimensional lattice, and this unique symbols. shape accounts for the strength displayed by b) Trivial names The names [60-Ih]fullerene and diamond. Another carbon allotrope is graphite, in [70-D5h]fullerene given in the IUPAC which carbon atoms are planimetrically arranged but Preliminary Survey are names first introduced are stacked in layers. The third carbon allotrope is into the literature for the C60-Ih and C70-D5h(6) fullerene, it is a family of caged compounds fullerenes. Since important structural composed of only carbon elements. A fullerene Cn information is omitted from these names, they consists of 12 pentagonal rings16 and any number of are to be considered as trivial names to be used 7-12 hexagonal ones, m, such that m = (Cn – 20)/2 only for these specific compounds. (Euler’s Theorem)17 linked together to form a Classification geodesic dome. Structure of these fullerenes Several topological descriptors have been proposed resembles as a soccer ball and the typical diameter to evaluate and classify the topological structure of of a fullerene molecule is 0.7 nm.18 fullerene isomers: the pentagon adjacency index NP, the Wiener index WI, the resistance distance RT, the Shiv Bhadra Singh et al /Int.J.ChemTech Res.2013,5(1) 169

Fig- Allotropes of carbon a) Diamond b) Graphite c) Lonsdaleite d) C60 (e) C540 f) C70 g) Amorphous carbon h) single-walled carbon nanotube

Solubility A deeper understanding of the interaction of Chemical properties fullerenes with various solvents will allow rational At an early stage of investigations it was supposed choice of solvents capable of purifying fullerenes by that fullerenes ought to be chemically inactive and cheaper and scalable methods. relativity stable. However it has been later proved that fullerenes participate in various chemical Solubility of fullerenes in various solvents-19 reactions. Fullerenes are also proved to be strong Solvent Solubility acceptor of electrons in presence of many organic (mg/ml) and inorganic donors. There chemical activity results n- 0.005 from an easy breakage of one of the double bond. In n- 0.043 later experiments a way to break single bond has Cyclohexane 0.036 been found thus allowing preparation of various Choloroform 0.16 types of C60 adducts compound. Regioselective 0.26 formation of multiple adducts has allowed study of 1,2-dibromomethane 0.50 the changes in chemical and physical properties that 0.000 occur when the conjugated fullerene chromophore is reduced during an increase in functionalization. 0.001 Functionalized fullerenes are divided into two 1.7 classes: exohedral with substituents outside the cage Xylenes 5.2 and endohedral fullerenes with trapped molecules 1-methylnaphthalene 33 inside the cage. 1-pheylnaphthalene 50 Nucleophilic addition 20 Physical properties Fullerenes react as electrophiles with a host of Appearance Black/brown powder nucleophiles in nucleophilic additions. The Odor odorless intermediary formed carbanion is captured an Density 1.65 g/cm3 electrophile. Examples of nucleophiles are Grignard Standard heat of 9.08 k cal/mol reagents and organolithium reagents. For example formation the reaction of C60 with methylmagnesium Refractive index 2.2 (600nm) chloride.21,22 Boiling point Sublimes at 800K Resistivity 1014 ohms/m Hydroarylation reaction Crystal form Hexagonal cubic Fullerene reacts with chlorobenzene and aluminium Vapour pressure 5 × 10-6 torr at room chloride in a Friedel-Crafts alkylation type reaction. temperature In this hydroarylation the reaction product is the 1,2- Compressibility 6.9 × 10-12 cm2dyn addition adduct (Ar-CC-H).23 Young’s 15.9 GPa modulous Cycloaddition Debye 100 K The [6,6] bonds of fullerenes react as dienes or temperature dienophiles in cycloadditions for instance Diels- Bulk modulous 8.8 GPa Alder reactions. 4-membered rings can be obtained by [2+2]cycloadditions for instance with benzyne. Shiv Bhadra Singh et al /Int.J.ChemTech Res.2013,5(1) 170

An example of a 1,3-dipolar cycloaddition to a 5- and visible is about two. To purify the material, membered ring is the Prato reaction.24 remove the ubiquitous hydrocarbons before the concentration procedure is applied (for example, by Hydrogenation washing the initial soot with ether). Thin films and Fullerenes are easily hydrogenated by several powder samples of the new material can be handled methods with C60H18 and C60H36 being most studied without special precautions and seem to be stable in hydrofullerenes. However, completely hydrogenated air for at least several weeks, although there does C60H60 is only hypothetical because of large strain. seem to be some deterioration with time for reasons Highly hydrogenated fullerenes are not stable, that are as yet unclear. The material can be sublimed prolonged hydrogenation of fullerenes by direct repeatedly without decomposition. Using the reaction with hydrogen gas at high temperature apparatus described, one person can produce of the conditions results in collapse of cage structure with order of 100 mg of the purified material in a day.31 formation of polycyclic aromatic hydrocarbons. 25-27 Application Hydroxylation Starting soon after the production of fullerenes in Fullerenes can be hydroxylated to fullerenols or 1990, many efforts have been devoted to the fullerols by various methods. One method is application of C60 and its derivatives. In fact, fullerene reaction in diluted sulfuric acid and fullerene possesses a variety of interesting biological potassium nitrate to C60(OH)15. Another method is properties, such as HIV-P inhibition, DNA reaction in diluted sodium hydroxide catalysed by photocleavage, neuroprotection, apoptosis, etc.32 TBAH adding 24 to 26 hydroxyl groups.28 Unfortunately, the low solubility in biological fluids limits the use of these compounds as new Electrophilic additions pharmacophores for structure-activity relationship 33 Fullerenes react in electrophilic additions as well. studies in medicinal chemistry. Fullerenes are also The reaction with bromine can add up to 24 bromine used in electronic, optic, medical, and cosmetic atoms to the sphere. The record holder for fluorine applications. addition is C60F48. According to in silico predictions . Earlier experiments performed by Wudl and the as yet elusive C60F60 may have some of the coworkers demonstrated that there is inhibition fluorine atoms in endo positions (pointing inwards) of the HIV-P in the presence of C60 and are and may resemble a tube more than it does a therefore useful in fighting the HIV virus that sphere.29 leads to AIDS. . Easy photoexcitation of fullerenes results in Oxidation reaction formation of various species which helps in Although more difficult than reduction, oxidation of DNA photocleavage. fullerene is possible for instance with oxygen and . C60 could produce membrane disruptions by osmium tetraoxide.30 insertion into phospholipidic bilayers and hence have antibacterial activity.34 Method of production . Fullerenes are powerful antioxidants, reacting Pure graphitic carbon soot with a few per cent by readily and at a high rate with free radicals, which are often the cause of cell damage or weight of C60 molecules is produced by evaporating 35 graphite electrodes in an atmosphere of about 100 death. torr of helium. The resulting black soot is gently . Fullerenes have a property of neuroprotection, scraped from the collecting surfaces inside the they control the neurological damage of diseases evaporation chamber and dispersed in benzene. The such as Alzheimer's disease and Lou Gehrig's material giving rise to the spectral features attributed disease (ALS). . Fullerenes and fullerenic black are chemically to C60 dissolves to produce a wine-red to brown liquid, depending on the concentration. The liquid is reactive and can be added to polymer structures then separated from the soot and dried using gentle to create new copolymers with specific physical heat, leaving a residue of dark brown to black and mechanical properties. crystalline material. Other non-polar solvents, such . Fullerenes are used in formation of organic as carbon disulphide and carbon tetrachloride, can photovoltaics (OPVs). The most commonly used fullerene derivative in OPV applications is also dissolve the material. 36,37 An alternative concentration procedure is to heat the PCBM. soot to 400°C in a vacuum or in an inert atmosphere, . Fullerenes have marked ability to accept and to transfer hydrogen atoms, so they can be thus subliming the C60 out of the soot. The sublimed coatings are brown to grey, depending on the used as catalyst in hydrogenation and thickness. The refractive index in the near-infrared hydrodealkylations. Shiv Bhadra Singh et al /Int.J.ChemTech Res.2013,5(1) 171

. Fullerenes are used in purification & bio- . Fullerene derivatives are critically affects Mast hazard protection due to singlet oxygen catalysis cell responses and used in allergy treatment.38 of organics properties of C60. . Fullerene black/rubber compounds enhanced . Fullerenes are more sensitive MRI contrast durability of vehicles, lower the heat build-up agents. and give better fuel economy. . Fullerenes are exceptional free radical scavengers.

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