Vacuum state / undisturbed conjugation neutral solution / free radical positive solition /...
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vacuum state / undisturbed conjugation
neutral solution / free radical
positive solition / carbocation ( carbenium ion ; carbonium ion)
negative soliton / carbanion
positive polaron / radical cation
negative polaron / radical anion
positive bisolition (bipolaron) / carbodication
negative bisolition (bipolaron) / carbodianion
Frenkel exciton (bound electron-hole pair) / exited stateMolecular Chemistry versus Solid State Physics
Between two molecules with different electron affinity, charge redistribution can take place, anywhere from zero charge to a full electron transfer from one to the other.
Charge transfer of anywear from zero charge to one electron charge.Electron transfer means what it literally says: a hole electron is transferred. Pi-conjugated systems. Electron transfer and donor-acceptor compounds
Smalley, Kroto and Curl 1985
1996 Nobel Prize in ChemistryCurl Jr. Kroto SmalleyAutumn 1985
Buckminster Fullerene1967 World Exhibition in MontrealSpherical building by R. Buckminster Fullericosahedron truncated European football graphite diamond icosahedron
We know a whole family of fullerenes (ranging from C60 to ~ C100), unions (concentric carbon spheres of various shapes and sizes)carbon nanotubes (CNTs, bucky tubes), long, cigar-shaped all-carbon macromolecules that come in a variety of forms and shapessingle wall SWCNTsmulti-wall MWCNTs chiral and achiral
All of these new forms have in common that they are constructed from five and six-membered rings of sp2-hybridized carbon atomsThe geometrical structure of a fullerene must have exactly 12 pentagonal faces, but may have any number (except 1) of hexagonal faces.Fullerenes are described by the general chemical formula C20+2H where H is the number of hexagonal faces. Fullerene: Introduction
Total Synthesis ApproachesFullerene generation by vaporization of graphite or by combustion of hydrocarbons is very effective and certainly unbeatable what facile production in large quantities is concerned. However, total synthesis approaches are attractive because (a) specific fullerenes could be made selectively and exclusively(b) new endohedral fullerenes could be formed (c) heterofullerenes(d) other cluster modified fullerenes could be generated using relate synthesis protocolsFullerene: ProductionConversion og cyclophane into C60 in the gas phase in laser desorption mass spectrometrySynthesis of circumtrindene, representig 60% of C60Generation of C60 by cyclodehydrogenation of polyarene
Larger quantitiesHoffmann in Arizona and Kratchmer in Heidelberg in 1990Resistive heating of graphiteArc Heating of GraphiteInductive heating of graphite and other carbon sources (acetylene)Pyrolyses of hydrocarbons ( naphtalene)
One can draw 12500 resonance structures for fullerene C60 . Nevertheless only one structure really represents it most appropriately: the one which all c=c double bonds are in the hexagons.
Fullerene: ChemistryThe hexagons are much better be considered as cyclohexatrienes than as benzene rings.C60 is a strong electron acceptor.
Structure2 types of bonds1.38 1.45 C60M. Prato J. Mater. Chem. 1997, 7, 1097.20 hexagons + =12 pentagonsface centered cubic arrangement Crystal lattice7 180 3 Deviation from planarity
Properities of Bucky BallsBandgap of 1.68ev C60(purple)Bandgap of 1.76ev C70(red)Strong absorptions between 190 and 410Unique optical properties C60 decay very easily to low lying triplet stateNo reorganization energy.accelarates charge separation and hence long lived states(optical limiter and lubricant, MRI contrast reagent)
Fullerene: Chemistry 1,2 additionsDBU
Fullerene: Chemistry 1,3 additions
There are almost no limitations for R2For the -amino acid derivative, R1=H is not possibleFullerene: Chemistry 1,3 additions (Prato addition)
In-situ formation of diazo compounds from tosylhydrazonesFullerene: Chemistry 1,3 additions (diazomethane addition)
Fullerene: Chemistry 1,3 additions (diazomethane addition)
Fullerene: Chemistry 1,3 additions (addition of azides)Azides react in the same way as the diazo compounds, however, the reaction stops at the [5,6]-adduct. Theoretical investigations indicate stepwise mechanism in which the cleavage of the N-N single bond precedes the breaking of the C-N bond During extrusion of N2 the steric effect of the leaving N2 molecule prevents the addition of the nitrene substituent to the [6,6] bond and forces the addition to an adjacent [5,6] ring junction.
Fullerene: Chemistry Grignard addition
Fullerene: Chemistry Diels Alder additionsC60 is the dienophile, a reactive one since it is electrophilic.At elevated temperatures, the reaction is reversible. This seriously limits its applicationsThe monoadduct cannot be isolatedThe monoadducts can be isolated at low temperature
Fullerene: Formation of bis-adductsAfter the first addition, there are still 29 double bonds left in the C60 fragment, these bonds are still reactive.The reaction is usually stopped before it goes to completion, to get a maximum yield of the mono-adduct.The second addition to one of the other C=C bonds give rise to various isomeric adducts.
PhotovoltaicsBandgap of 1.68ev C60
Bandgap of 1.76ev C70
In -conjugated molecules, the frontier orbitals (HOMO and LUMO) are related to the -electron system. Therefore the color of the molecules is primarily determined by the HOMO-LUMO gap.
Especially when it comes to the design of -conjugated polymers, a set of parameters influencing the band-gap can be defined. The intrinsic band-gap of an isolated conjugated polymer chain can be described as a combination of contributions of energies, related to
1 bond length variation2 resonance stabilization energy3 inter-ring torsion angle4 inductive or resonance effect from substituents
Eg=Er+RE+E+ERPi-conjugated systems. Color and Charge transport
Hopping of the charge from one molecule to another. ( the electronic overlap between a charged molecule and a neutral neighbor)The way the charge is delocalized inside a molecule is of importance.
Speaking physics, charge transport means transport of either an electron or a hole.
For chemists it is essential to realize this is very different from moving the charges or electrons involved in cations, anions, or free radicals.Example: if a charge is to hop from a (carbo) cationic molecule to a neighboring neutral analogue, this would have to happen through proton transfer instead of an electron hopping from one to the other.Pi-conjugated systems. Color and Charge transport
During an intense working week in the autumn of 1985, Robert Curl, Harold Kroto and Richard Smalley made the completely unexpected discovery that the element carbon can also exist in the form of very stable spheres. They termed these new carbon balls fullerenes. The carbon balls are formed when graphite is evaporated in an inert atmosphere. Usually they contain 60 or 70 carbon atoms. A new carbon chemistry has developed around these spheres. It is possible to enclose metals and noble gases in them, to form new superconducting materials with them and to create new organic compounds and polymeric matter for them. The discovery of the fullerenes shows how unexpected and fascinating results can be created when scientists with different experience and research objectives collaborate.
An idea from outer spaceKroto's special interest in red giant stars rich in carbon led to the discovery of the fullerenes. For years, he had had the idea that long-chained molecules of carbon could form near such giant stars. To mimic this special environment in a laboratory, Curl suggested contact with Smalley who had built an apparatus which could evaporate and analyze almost any material with a laser beam. During the crucial week in Houston in 1985 the Nobel laureates, together with their younger co-workers J. R. Heath and J. C. O'Brien, starting from graphite, managed to produce clusters of carbon consisting mainly of 60 or 70 carbon atoms. These clusters proved to be stable and more interesting than long-chained molecules of carbon. Two questions immediately arose. How are these clusters built? Does a new form of carbon exist besides the two well-known forms graphite and diamond?
Fig 1. Schematic drawing of Smalley's cluster apparatus for the laser evaporation of graphite. The carbon clusters are formed in the helium gas flow and then analysed with mass spectrometry.
Fig 2. The read-out from the mass spectrometer shows how the peaks corresponding to C60 and C70 become more distinct when the experimental conditions are optimised.For the 1967 World Exhibition in Montreal the architect R. Buckminster Fuller designed a spherical building which 18 years later gave the clue to the structure of the carbon clusters. He used hexagons and a small number of pentagons to create "curved" surfaces. This year's Nobel Prize laureates assumed that the cluster of 60 carbon atoms - C60 - consists of 12 pentagons and 20 hexagons with carbon atoms at each corner, the same form as a European football. They called the new carbon ball, C60, buckminsterfullerene. In colloquial English the carbon balls became "buckyballs".
In Ancient Greece the regular polyhedra symbolised the different elements, earth, air, fire, water, and the universe. The cube was the sy