Introduction to carbon materials

Isolated pentagon rule- Energetically unfavourable pentalene, 8 π electrons can lead to resonance destabilisation b - meta arrangement are more favourable to avoid neighbouring pentagons

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

 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,

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

• Most stable form of carbon under standard conditions ( Δ H 0 = 0 kJ mol − 1 ).

• • Two modifi cations: hexagonal α - graphite and rhombohedral β - graphite.

• • The carbon atoms are sp 2 - 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.

• The C - atoms are sp 3 - 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.

 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 ° = 10m)

10- In atomic terms, their sizes are enormous

 But fullerenes are still small compared to many organic molecules

 Chemically, they are quite stable; breaking the balls requires temperatures of over 10000 C.

 At much lower temperatures (a few hundred degrees C) fullerenes will

"sublime“.

Fullerene- C60

 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

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)

With his geodetic domes, R Buckminster Fuller provided the architectonic

inspiration for the naming of the fullerenes The US pavilion for the Montr é al EXPO76 is just one example ( © Montr é alais)

A previously planar aromatic structure is getting curved by the closure of a fi ve - membered ring; (b) C 60 contains

12 fi ve - membered and 20 six – membered rings showing the characters of radialenes or cyclohexatrienes, respectively The bonds situated between two adjacent six - membered rings are called (6,6) - bonds, those between a fi ve - and a six - membered ring (5,6) - bonds

The formation of a fullerene cage The tendency toward minimizing the number of unsaturated bonding sites causes the generation of fi ve -

membered rings and thus an increasing curvature and fi nal closure of the structure to be a cage - like molecule

Units of C60

Corannulene

Radialene

Cyclohexatriene

Isolated pentagon rule

- Energetically unfavourable pentalene, 8 π electrons can lead to resonance destabilisation

b - meta arrangement are more favourable to avoid neighbouring pentagons and double bonds within five membered rings

Structure and Bonding

• C60 has 12 five and 20 six membered rings

• Not all the C-C bonds of C60 is of same length

• The double bonds are localized in 6 membered rings

• Five membered rings are evenly distributed on the surface and isolated (Isolated pentagon rule)

• Diameter of C60 molecule is 0.702 nm

• High symmetry molecule – easily identified by spectroscopy

Double bonds are preferably located in

6 membered rings and it is

energetically unfavourable in 5

membered ring

Derivatizations of C60

Exohedral fullerenes

• Hydrogenated, halogenated, other functionalities by addition reactions

• Exohedral fullerenes shows higher solubility

• Functionalization are useful to form suprmolecular compounds

C60H36

Exohedral fullerenes

Endohedral fullerenes

• Fullerenes filled with atoms, molecules

inside the cavity are called endohedral

fullerenes

• Metallofullerene - M@Cn ( M -Li, Ca, Sc,

Y, La, Ce, Eu)

• Nonmetallic endohedral fullerenes ( N, P)

• Noble gas containing fullerenes – X@C60 ( X = He, Ne, Ar, Kr, Xe)

Method of Preparation

• Thermal method – pyrolysis 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 already consisting of 5 and

6 membered rings

• Napthalene, corannulenes, PAH pyrolysed

at 1000oC in an inert atmosphere.

Resistance heating method

• Here the carbon electrodes touch each other

• Small area of contact and higher current density results in strong local heating

• Temperature reachs to 2500 – 3000 oC

• Smoke formation and cooling

• Requirement of inert (He, Ar) atmosphere (even nitrogen result in undesirable reaction)

• Pressure should be maintained (140 – 160 mbar)

• Higher yield of fullerenes ( 15%)

• On purification, C70:C60 – 0.02:0.18

Preparation of fullerene by Arc Method

It is basically a DC arc chamber reactor chamber with

- ensembles of electrodes (anode & cathode),

- cooling system reactor chamber,

- cooling system for electrode assembly

- vacuum system,

- electrical system and

- electronic system

The operating conditions are:

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

Soot is extracted 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

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 The initial solution coming out of the column is

collected separately in a conical flask and thrown

into waste.

o Colour starts to change to magenta after 20 to 25

minutes – the fraction is collected as C60

o After 20 to 30 minutes the solution with magenta

colour stops coming out

o At this stage, dichlorobenzene is added to the

column.

o A red colour separation will be seen for C70 fraction

will appear

o This fraction C70 is collected separately in a flask.

o From both the flasks, the solvent is evaporated to

obtain pure C60 and C70 fractions separately.

Silica (SiO 2 ) Carbon granules

Crude soot solution (in organic solvent)

C 60 Band

C 70 Band impure band-1 impure band-2

1 pure C 60 (magenta) in Toluene

2 pure C 70 in CH 2 Cl 2 (Red)

at Tc ~18 K

o K3C60→Partial filling of Conduction band

o It retains the basic FCC structure of C60

and lattice constants to accommodate the alkali ions

potassium-doped C60 has only a single stable superconducting phase, K3C60, with a transition temperature of 19.3 K.

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 states at the 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

• Biological and medicine applications

• Sensitizer in photochemical generation of singlet oxygen

• Self organized thin films for various application

• Electrooptical and light harvesting applications

• Attached with biopolymers

• Hydrogen storage material

Medicinal applications

hydrophilic groups

hydroxyl or peroxides which cause premature cell death)

contrasting agents in MRI, with reduced side effects

(Gd@C82(OH)x), Ho@C82(OH)x.

electroactive and NLO materials)

stationary phase in HPLC This works good with organic and water media

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

open-ended 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

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

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."

Transcription

o Once HIV's genetic material is inside the cell's

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

Viral Assembly and Maturation

o Long strings of proteins are cut up by a viral enzyme called protease

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

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.

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 - 1 - arrangement is observed with small addends, while larger

• addends prefer e - or trans - 3 - 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