Molecular Chemistry versus Solid State Physics 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 state
Pi-conjugated systems. Electron transfer and donor-acceptor compounds 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.
1996 Nobel Prize in Chemistry Curl Jr. Kroto Smalley Autumn 1985
Buckminster Fullerene 1967 World Exhibition in Montreal Spherical building by R. Buckminster Fuller icosahedron truncated European football graphite diamond icosahedron
Fullerene: Introduction We know a whole family of fullerenes (ranging from C60 to ~ C100), ‘unions’ (concentric carbon spheres of various shapes and sizes) carbon nanotubes (CNT’s, bucky tubes), long, cigar-shaped all-carbon macromolecules that come in a variety of forms and shapes single wall SWCNT’s multi-wall MWCNT’s 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 atoms The 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: Production Total Synthesis Approaches Fullerene 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 protocols Conversion og cyclophane into C60 in the gas phase in laser desorption mass spectrometry Synthesis of circumtrindene, representig 60% of C60 Generation of C60 by cyclodehydrogenation of polyarene
Larger quantities Hoffmann in Arizona and Kratchmer in Heidelberg in 1990 Resistive heating of graphite Arc Heating of Graphite Inductive heating of graphite and other carbon sources (acetylene) Pyrolyses of hydrocarbons ( naphtalene)
Fullerene: Chemistry 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. The hexagons are much better be considered as cyclohexatrienes than as benzene rings. C60 is a strong electron acceptor.
Structure C60 2 types of bonds 1.38 Å 180 Å3 20 hexagons + = 12 pentagons 7Å 1.45 Å Deviation from planarity Crystal lattice face centered cubic arrangement M. Prato J. Mater. Chem.1997, 7, 1097.
Properities of Bucky Balls Bandgap of 1.68ev C60 (purple) Bandgap of 1.76ev C70 (red) Strong absorptions between 190 and 410 Unique optical properties C60 decay very easily to low lying triplet state No reorganization energy….accelarates charge separation and hence long lived states (optical limiter and lubricant, MRI contrast reagent)
Functionalisation exohedral endohedral ‘outside’ ‘inside’ Ce, Gd,Eu,Nd,Sm,Tb,Ho
Fullerene: Chemistry 1,3 additions (Prato addition) There are almost no limitations for R2 For the α-amino acid derivative, R1=H is not possible
Fullerene: Chemistry 1,3 additions (diazomethane addition) In-situ formation of diazo compounds from tosylhydrazones
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 bondDuring 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 Diels Alder additions C60 is the dienophile, a reactive one since it is electrophilic. At elevated temperatures, the reaction is reversible. This seriously limits its applications The monoadducts can be isolated at low temperature The monoadduct cannot be isolated
Fullerene: Formation of bis-adducts After 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.
Photovoltaics Bandgap of 1.68ev C60 Bandgap of 1.76ev C70