Introduction to carbon materials

Introduction to Carbon Materials Hybrid orbitals of carbon How did we get to know about fullerenes?  Nuclear physics researchers Hahn & Strassman in Germany noticed that carbon cluster ions up to C15+ were produced in a high frequency arc with a graphite electrode in the 1943  A Japanese physical organic chemist E G Osawa had perceived that carbon in the single layer closed cages structure would be aromatic and therefore stable, in early 1970  Gal’pern (Russian scientist) had completed the first of many Hückel calculations showing that it would be a closed shell molecule with a large HOMO-LUMO gap in 1973  Fullerenes were discovered experimentally for the first time by a group of scientists at Rice University, Houston, Texas, in September of 1985 Photograph of the research group that discovered the fullerenes at Rice University in September of 1985 standing: Curl, kneeling (left to right): O’Brian, Smalley, Kroto and Heath Nobel Prize in chemistry in 1996 Graphite and Diamond Graphite • • • • • • • • • • Most stable form of carbon under standard conditions ( Δ H = kJ mol − ) • Two modifi cations: hexagonal α - graphite and rhombohedral β - graphite • The carbon atoms are sp - hybridized, σ - bonds with three adjacent atoms within one layer (bond angle 120 ° ) Additional delocalized π - bonds within these layers Only weak van der Waals interaction between the graphene sheets • Considerable anisotropy of properties like electrical conductivity, modulus of elasticity, etc due to the layered structure • Despite chemical inertness several compounds are known, above all intercalation compounds with alkali metals or halogens Diamonds • • • • Metastable modifi cation of carbon Appears as cubic or as hexagonal diamond ( Lonsdaleite ) • Each C - atom is tetrahedrally connected to four adjacent atoms by σ bonds The C - atoms are sp - hybridized • Greatest hardness and highest thermal conductivity among all natural materials Electrical insulator, yet semiconductance achievable by doping • Chemically extremely inert, is only attacked by aggressive reagents like • chromosulfuric acid • • • Fullerenes  Kroto and co-workers discovered an entirely new form of carbon known as C60 or the fullerene molecule (only diamond and graphite were known before this)  The original discovery of C60 was in the soot produced from the laser ablation of graphite  Only in the early 1990s fullerenes could be synthesized in large enough quantities for significant research in this field to be undertaken  Fullerene cages are about 7-15 angstroms in diameter ( 1A° = 1010m)  In atomic terms, their sizes are enormous  But fullerenes are still small compared to many organic molecules  Chemically, they are quite stable; temperatures of over 10000 C breaking the balls requires  At much lower temperatures (a few hundred degrees C) fullerenes will "sublime“ Fullerene- C60 Buckminsterfullerene (C60) has 60 carbon atoms arranged in a spherical structure (resemblance of this shape to the geodesic domes designed and built by the architect R Buckminster Fuller)     Pentagon (C5) Hexagon (C6)    Pure C60 consists of 60 carbon atoms arranged as 12 pentagons and 20 hexagons Visually C60 it is quite different from both graphite and diamond It is a yellow powder, which turns pink when dissolved in certain solvents such as toluene When exposed to strong ultraviolet light, the buckyballs polymerize,forming bonds between adjacent balls In crystalline form C60 is cubic (at each lattice point of a cube, there is a buckyball) It is Electrically insulating It shows electro-negativity and forms compounds easily with alkali atoms Fullerene – C70  The rugby-ball shaped C70has 12 pentagons and 25 hexagons  Gives red colour with dichlorobenzene Applications • Electrooptical devices – fullerenes with conducting polymers • Composite materials for interesting electronic and photophysical properties • Polymer – fullerene composites( photoactive, electroactive and NLO materials) • Photoconducting, electroluminescent materials • Silicon alkoxide functionalized with fullerenes are used in stationary phase in HPLC This works good with organic and water media • (1gm of pure C60 ~ 100 euros) Preparation in higher quantity and pure forms limits their commercial application HIV Protease Inhibition by C60 o Derivatives of C60 are currently being investigated as potential inhibitors of the protease enzyme, which is specific to the HIV (virus) generation o Active site of the enzyme roughly described as an openended cylinder structure, which is with large hydrophobic amino acids o The C60 has app the same radius as cylinder & C60 and derivatives are primarily hydrophobic→can able to block the active side →reduce the HIV (virus) generation Hydrophobic sites C60 (as inhibitor) Cy l ica r e lind HIV protease site Cartoon for the C60 HIV protease inhibition  Other applications: Cancer treatments, antimicrobial agents, Hydrogen storage (H2@C60), Trapping Reactive species, Geochemistry & Astro-chemistry etc Additional information on HIV Protease inhibition The HIV Life Cycle Step 1: Binding Step 2: Reverse Transcription Step 3: Integration Step 4: Transcription Step 5: Translation Step 6: Viral Assembly and Maturation BINDING HIV has proteins on its envelope that are strongly attracted to the CD4+ surface receptor on the outside of the T4-cell When HIV binds to a CD4+ surface receptor, it activates other proteins on the cell's surface, allowing the HIV envelope to fuse to the outside of the cell Reverse Transcription After the binding process, the viral capsid (the inside of the virus which contains the RNA and important enzymes) is released into the host cell A viral enzyme called reverse transcriptase makes a DNA copy of the RNA This new DNA is called "proviral DNA." Integration The HIV DNA is then carried to the cell's nucleus (center), where the cell's DNA is kept Then, another viral enzyme called integrase hides the proviral DNA into the cell's DNA Then, when the cell tries to make new proteins, it can accidentally make new HIVs nucleus, it directs the cell to produce new HIV o The strands of viral DNA in the nucleus separate, and special enzymes create a complementary strand of genetic material called messenger RNA or mRNA o As each mRNA strand is processed, a corresponding string of proteins is made o This process continues until the mRNA strand has been transformed or "translated" into new viral proteins needed to make a new virus o Long strings of proteins are cut up by a viral enzyme called protease into smaller proteins o Once the new viral particles are assembled, they bud off the host cell, and create a new virus The virus then enters the maturation stage, which involves the processing of viral proteins o With viral assembly and maturation completed, the virus is able to infect new cells Each infected cell can produce a lot of new viruses T4 Cell infected with HIV Viral assembly can be blocked by Protease Inhibitors o The catalytic site of HIV-P contains two aspartic residues o Fullerene residues can bind to aspartic residues through electrostatic interactions Structure of fullerenes • • Fullerenes are a modifi cation of carbon with a cage - like structure • The surface curvature arises from incorporating fi ve - membered rings into the hexagonal network of graphene layers • In the most stable fullerenes, the fi ve - membered rings are evenly distributed across the surface and isolated from each other (isolated pentagon rule – IPR) • The double bonds are preferably located in the six - membered rings; double bonds in fi ve - membered rings are energetically unfavorable • • C 60 and C 70 are the most important species among the fullerenes • • • • • • • Physical properties of fullerenes • • • • • • • • • Fullerenes exhibit strong electron affi nity and act as “ radical sponges ” • The solubility in organic media depends on the type of solvent; aromatic and halogenated aromatic substances are the most suitable • The spectroscopic properties of fullerenes are closely related to their respective symmetry Structural information can be obtained from the number of bands, for example, in the IR - spectrum • In comparison to graphite, all fullerenes are more energetic, with increasing size, however, their standard enthalpy of formation approximates the value • of graphite Chemistry of fullerenes • • • • • • • • • • • • • • • • • Fullerenes behave like electron - defi cient polyolefi ns and not like aromatic compounds • They easily enter into addition reactions with nucleophiles • Functionalization can easily be achieved by cycloaddition reactions • Fullerenes add hydrogen or halogens and may be transformed into highly functionalized compounds • 1,2 - or 1,4 - addition occurs depending on the size of the addends • The regiochemistry of multiple additions is governed not only by steric factors but also by a tendency to avoid double bonds in fi ve - membered rings Thus a cis - - arrangement is observed with small addends, while larger addends prefer e - or trans - - positioning • The Bingel – Hirsch reaction is an important tool in fullerene functionalization It yields cyclopropanated fullerenes by means of deprotonated bromomalonates • Fullerenes may incorporate guests in their inner cavity, these compounds are called endofullerenes • • • Heterofullerenes are compounds with one or more heteroatom substituting for carbon atoms of the cage Possible applications of fullerenes and their derivatives • • • • • • Solar cells • Composite materials with interesting electronic properties • Derivatives of fullerenes for photodynamic tumor therapy • Chemical sensors • Endofullerenes as contrasting agent in MRI with reduced side effects [...]... with Toluene (C60+C70+impurities) → Deep Red colour o The column is first filled with carbon granules o Toluene is filled into the column until the level of Crude soot solution (in organic solvent) toluene equals to the height of carbon granules o Then the solution containing C60& C70 is added into the column through a dropper flask o ~20 mL fullerene solution→shaken vigorously →filtered →chromatography... o into waste C60 Band Colour starts to change to magenta after 20 to 25 C70 Band minutes – the fraction is collected as C60 impure band-1 After 20 to 30 minutes the solution with magenta colour stops coming out o Carbon granules The initial solution coming out of the column is collected separately in a conical flask and thrown o Silica (SiO2) At this stage, impure band-2 dichlorobenzene is added to. .. 100 to 200 mmHg - The soot from Arc method  The soot collected in the arc method contains a) Carbon clusters mixture = 25-30% b) C60 and C70 fullerenes = 1.2-2.5%  To extract the fullerenes from the soot Soxhlet extraction with toluene followed by extraction in a sonic bath is carried out  The C60+C70 mixture obtained is separated by column chromatography Purification of Fullerenes by Chromatography... (Gd@C82(OH)x), Ho@C82(OH)x Applications • Electrooptical devices – fullerenes with conducting polymers • Composite materials for interesting electronic and photophysical properties • Polymer – fullerene composites( photoactive, electroactive and NLO materials) • Photoconducting, electroluminescent materials • Silicon alkoxide functionalized with fullerenes are used in stationary phase in HPLC This works good... – substituted with one or more other Atoms (N, S, B) in the place of carbon atom Exohedral fullerenes • • • Hydrogenated, halogenated, other functionalities by addition reactions Exohedral fullerenes shows higher solubility Functionalization are useful to form suprmolecular compounds C60H 36 Exohedral fullerenes Endohedral fullerenes • Fullerenes filled with atoms, molecules inside the cavity are called... biopolymers • Hydrogen storage material Medicinal applications • Solubility of fullerenes increased by functionalisation with hydrophilic groups • It can act as radical scavengers ( It easily reacts with hydroxyl or peroxides which cause premature cell death) • in vitro inhibition of HIV protease • Photodynamic therapy – destruction of tumour tissue • Act as antioxidants to prevent apoptosis [ C60(OH)x]... cooled, its resistivity begins to drop sharply at about 18°K, indicating the onset of superconductivity (Hebard, 1991) o As larger alkali-metal cations are incorporated into the lattice and the fcc lattice parameter (a) increases, the superconducting transition temperature, T, also increases (see Figure ) o The T for Rb3 C60 rises to 28°K This rise in T may be related to an increase in the density of... inhibitors of the protease enzyme, which is specific to the HIV (virus) generation o Active site of the enzyme roughly described as an openended cylinder structure, which is with large hydrophobic amino acids o The C60 has app the same radius as cylinder & C60 and derivatives are primarily hydrophobic→can able to block the active side →reduce the HIV (virus) generation Hydrophobic sites C60 (as inhibitor)... Fermi level with increasing lattice constant (a) Different kinds of polymer materials that may be obtained from different modes of fullerene incorporation; (b) examples of fullerene – polymer composites Applications • Biological and medicine applications • Sensitizer in photochemical generation of singlet oxygen • Superconductors, chemical sensors • Self organized thin films for various application... of hydrocarbons (HCs) • Combustion – partial combustion of HCs • Arc discharge method – graphite • Resistance heating method • Rational synthesis • Pulse laser beam method - Graphite electrodes Thermal method - pyrolysis • Poly aromatic hydrocarbons(PAH) are suitable for synthesis of fullerenes by pyrolysis • Here the structural elements have the structural framework of fullerene cage • Hydrocarbons ... →filtered →chromatography o o into waste C60 Band Colour starts to change to magenta after 20 to 25 C70 Band minutes – the fraction is collected as C60 impure band-1 After 20 to 30 minutes the... Composite materials for interesting electronic and photophysical properties • Polymer – fullerene composites( photoactive, electroactive and NLO materials) • Photoconducting, electroluminescent materials. .. attracted to the CD4+ surface receptor on the outside of the T4-cell When HIV binds to a CD4+ surface receptor, it activates other proteins on the cell's surface, allowing the HIV envelope to fuse to