Instructor: Instructor:

Dr. Marinella SandrosDr. Marinella Sandros

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NanochemistrNanochemistryy

NAN 601NAN 601

Lecture 15: Fullerenes

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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.

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Nobel Prize in chemistry in 1996

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Fullerenes are closed hollow cages consisting of carbon atoms interconnected in pentagonal and hexagonal rings.

Each carbon atom on the cage surface is bonded to three carbon neighbors therefore is sp2 hybridized.

The most famous fullerene is C60, known also by “ buckyball ".

Other relatively common clusters are C70, C72, C74, C76, C80, C82 and C84 (plenty of others, higher or lower than C60, exist too but less abundant in the experimentally produced mixture fullerene soot).

Dissolves in common solvents like benzene, toluene, hexane

Readily vaporizes in vacuum around 400°C Low thermal conductivity Pure C60 is an electrical insulator C60 doped with alkali metals shows a range of

electrical conductivity:◦ Insulator (K6 C60) to superconductor (K3 C60) < 30 K!

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How many bonds does carbon always form?

Four

These can be:

What is carbon’s electron orbital diagram?

• two single bonds and one double bond

• one single bond and one triple bond

• four single bonds

• two double bonds

Review: Carbon Chemistry

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Four single bonds

• Tetrahedral with bond angles of approximately 109º.

C Two single bonds and one double bond

• Planar with 120º bond angles.

C One single bond and one triple bond

• Linear with 180º bond angles.

C

=C= Two double bonds

• Linear with 180º bond angles.

Review: VSEPR Theory

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How are the carbon atoms arranged in diamond?

Each interior carbon is covalently bonded to four others in a tetrahedron.

Diamond

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How are carbon atoms arranged in graphite?

• arranged in planar layers (sheets)

• each interior carbon atom is covalently bonded to three others in a hexagonal pattern

• very weak forces exist between the layers (gray lines in the figure above)

• the individual layers extend indefinitely in two dimensions

Graphite

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Knowing that:1. carbon always forms four bonds; 2. each carbon atom in graphite is covalently bonded to three

other carbon atoms; and3. the graphite layers are flat.

Two single bonds and one double bondC C

C

C

What is the bonding pattern around a given carbon atom in graphite?

Graphite

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Scientists knew the substance was carbon, but it wasn’t graphite, diamond, or individual carbon atoms.

They proposed the formula of the material was C16.

How would chemists represent the structure of C16?

So, what was it?

Nanoparticles

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C16 fragment – a flat structure that does not contain hydrogen

What is wrong with this picture?

Hint: Remember, carbon always forms four bonds.

Nanoparticles

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The product obtained in the lab was identified by mass spectrometry. The mass spectrum of the product is shown below.

The evidence points to the formula C60 (mass 720 amu).

How many carbon atoms did the sample contain?

C ??

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•Given the predominance of the C60 peak in their mass spectra, Smalley, Kroto, and coworkers began to think about possible 60-atom structures that would exhibit unusually high stability.

•They believed that in the laser vaporization, fragments of graphite were torn from the surface.

•Graphite has a planar structure composed of fused six-membered rings. Each carbon is bonded to three other carbons in an infinite two-dimensional array.

•Small graphite fragments would contain many "unsatisfied valences" at the edges; the carbon atoms around the circumference of the fragment would be bonded to only two other carbons, rather than the more-desirable three carbon atoms.

Fullerene Structure

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Could the structure of C60 be flat?

• No – just like the C16 fragment, a planar C60 structure would also have “dangling bonds” on the outer edges.

C 60

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Clearly by adding a 5-membered ring, the structure takes on a bowl-like shape with curvature. Aha!

Notice that this molecule, corannulene (C20H10), possesses a single 5-membered ring in addition to five 6-membered rings.

Nanoparticles

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The mystery of C60 was finally solved.

The Nobel Prize in chemistry was awarded in 1996 for this work.

It soon became known as a “buckyball” because it resembles the famous architecture of Buckminster Fuller.

This material incorporates both 5-membered and 6-membered rings.

Buckyball

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When the firing of the vaporization laser was delayed until most of the helium pulse had passed, a roughly Gaussian distribution of large, even-numbered clusters with 38-120 carbon atoms resulted. The C60 peak was largest but not dominant.

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When the vaporization laser was fired at a time of maximum helium density, the C60 peak grew into a feature 5 times stronger than its neighbors (with the exception of C70)).

When these conditions were duplicated but the "integration cup" was added to increase the time between vaporization and cluster analysis.

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The explanation for these results is that when hot clusters are left in contact with high-density helium, they equilibrate toward the most stable species, which appears to be a unique cluster containing 60 atoms.

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Fullerenes can be made by vaporizing carbon within a gas medium.

(they could form spontaneously in a condensing carbon vapor)

R. E. Smalley, Nobel Prize lecture, December 7, 1996

• An electric arc is maintained between two nearly contacting graphite electrodes.

•Most of the power is dissipated in the arc and not in resistive heating of the rod.

•The entire electrode assembly is enclosed in a reaction kettle that is filled with ~ 100 torr pressure of helium.

•Black soot is produced, and extraction with organic solvents yields fullerenes.

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No other element has such wonderful properties as carbon.

Buckyballs are relatively cheap; carbon is everywhere!

Even though each carbon atom is only bonded with three other carbons (they are most comfortable with four bonds) in a fullerene molecule, dangling a single carbon atom next to the structure is not strong enough to break the structure of the fullerene molecule.

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In fullerenes, 12 pentagonal rings are necessary and sufficient to effect the cage closure.

Fullerenes contain carbon atoms arranged as a combination of 12 pentagonal rings and n hexagonal rings. The chemical formula is C20+2n.

Fullerene cages are about 7-15 Å in diameter, and are one carbon atom thick.

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Quite stable from chemical and physical points of view (breaking the balls requires temperatures of about 1000 °C).

Highest tensile strength of any known 2D structure or element.

Highest packing density of all known structures.

Impenetrable to all elements under normal circumstances, even to a helium atom with an energy of 5 eV.

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Fullerenes with material inside are called cage compounds, or endohedral compounds.

The formulas of endohedral compounds are shown as M@C60—where M represents the item inside of the cage.

Examples of known compounds include:N@C60 and La@C82

What possible applications might there be for endohedral buckyballs?

Fullerenes

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Exohedral compounds are those in which a wide variety of both inorganic and organic groups added to the exterior of the cage.

These materials offer the most exciting potential for useful applications of fullerene materials.

Fullerenes

Combination endo- and exohedral compounds have also been synthesized. An interesting example is:

The gadolinium (Gd) is inside the cage and the outside is covered with hydroxyl groups.

Gd@C82(OH)n is a possible enhancement material for magnetic resonance imaging, MRI.

Gd@C82(OH)n

Fullerenes

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Commercial and biological possibilities exist:

Sunscreens

Superconducting materials

Antibacterials

due to photophysical properties

due to redox and general chemical reactivity

due to physical properties

Fullerenes

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A plasma of carbon vapor is generated over the irradiated spot, where temperatures over 10,000°C are attained. In order to cool this superhot plasma and generate carbon clusters, a burst of helium gas is introduced from a pulsed gas nozzle. As the carbon atoms cool, clustering reactions occur. By adjusting the relative timing between the vaporization laser and the helium gas pulses, the residence time in the source can be extended to allow these growing clusters to aggregate in the soup of carbon atoms.

The growth process that generates the fullerenes probably begins with small linear chains that add other linear chains and carbon atoms until they become large enough to take the form of monocyclic rings. These rings, in turn, could grow through further additions of atoms or small chains until they were in the 25-35 atom size range. Then polycyclic networks like the open graphite sheets discussed earlier would begin to be favored.

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In order to explain the formation of fullerenes, the open graphitic sheets must rearrange to incorporate pentagons as well as hexagons in the bonding pattern. The pentagons would cause the sheet to curl and enable some of the edge carbon atoms with unsatisfied valences to bond together. The loss of p-p (-bond) overlap resulting from curling would be more than offset by the formation of good -bonds from coupling edge carbons.

Key to the success of this process is adequate "annealing time" so that the growing cluster can incorporate enough pentagons (12) to close. If the rate of cooling the carbon plasma is too rapid, most of the clusters will grow out well beyond C, becoming giant fullerenes or soot particles

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Early on, it was discovered that metal atoms could be placed inside C by simply carrying out the fullerene synthesis in the presence of metal atoms. For example, in the original Smalley/Kroto apparatus, if the graphite disk is impregnated with lanthanum by exposure to a boiling saturated solution of LaCl in water, carbon clusters of the form CLa (where n is an even number ranging from 44 to more than 76) are observed (Heath, 1985).

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X-ray powder diffraction has shown that fullerite adopts the face-centered cubic (fcc) close-packed structure with lattice constant a = 14.17 Å (Heiney, 1991).

This arrangement results in a ABCABC pattern and corresponds to a lattice with a face-centered cubic unit cell.

Spheres sit at the eight corners and at the

centers of the six sides of the cubic unit cell, which has an edge length ("lattice constant") of a.

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The energies of the Huckel -molecular orbitals for C have been calculated (Haddon, 1986, Vol.125).

As shown, the triply-degenerate LUMO (t symmetry) is rather low-lying, suggesting that C60 should be relatively easy to reduce.

In fact, treatment of C60 with three equivalents of alkali metal leads to the production of A3C60, which possesses a half-filled t electronic level, while treatment of C60 with six equivalents of alkali metal generates an A6C60 phase with a filled t level (Haddon, 1991).

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The A3C60 phase is a conductor at room temperature due to the partial filling of the t1u "conduction band". Electrons can move between C molecules through the radiating -orbitals.

At low temperature, A3C60 becomes super-conducting . In contrast, the A6C60 phase is an insulator due to the complete filling of the t1u orbitals.

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The A3C60 phase retains the basic face-

centered cubic (fcc) structure that was discussed earlier for C itself (Stephens, 1991).

The lattice constant increases slightly to accommodate the alkali metal cations. The reason that the fcc packing can be retained is that there are three "holes" per sphere in the fcc lattice, one octahedral hole and two tetrahedral holes.

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Location of octahedral holes (left) and tetrahedral holes (right) relative to lattice spheres in face-centered cubic unit cell.Note: The holes are gray and the lattice spheres are light blue in the unit cells above.

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The octahedral holes have local octahedral symmetry, i.e., they are surrounded by six nearest-neighbor spheres arranged octahedrally, while the tetrahedral holes have four nearest-neighbor spheres arranged tetrahedrally.

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+

antioxidants

hair growthinhibit allergic response

Imaging plaques

Treat plaques

Fullerene Derivatives

Photovoltaics are best known as a method for generating electric power by using solar cells to convert energy from the sun into a flow of electrons.

The photovoltaic effect refers to photons of light exciting electrons into a higher state of energy, allowing them to act as charge carriers for an electric current.

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Fullerenes show a high electron affinity resulting from an energetically deep LUMO level of −4.3 eV versus vacuum and capable of accepting as many as six electrons

Contact with molecules having a higher lying LUMO level, this high electron affinity can lead to a favored electron transfer process to the fullerene.

High symmetry that leads to a good contact with the neighboring molecules almost independent on the fullerene orientation.

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UV/Vis spectrum of FMH (black) and UV/Vis (solid lines) and PL spectra (dashed lines) of QD525 (green), QD565 (orange), and QD605 (red). A is absorbance, IPL is photoluminescence intensity.

Fullerene–malonic acid hexaadduct

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1) Coverglass cleaning

2) surfaceSilanization3) FMH coupling 4) linker coupling, 5) QD conjugation

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Electron-transfer rate kET versus linkerlength R for QD605–FMH dimers.

Electron-transfer rate versus QD size for QD–16AHT–FMH dimers.

Why are fullerenes ideal for solar cell applications?

What type of structure does Fullerene adopt?

Why after the addition of 3 alkali metals in fullerenes, the solid state structure is retained?

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