vacuum state / undisturbed conjugation neutral solution / free radical positive solition /...

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

Molecular Chemistry versus Solid State Physics

RR

RC R

R C+

R

R C-

R

RC

C+

R

RC

C-

R

RC

+

C+ R

RC

-

C- R

RC- C

+

R

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 Chemistry

Curl Jr. Kroto Smalley

Autumn 1985

Buckminster Fullerene

1967 World Exhibition in MontrealSpherical building by R. Buckminster Fuller

icosahedron 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 (CNT’s, bucky tubes), long, cigar-shaped all-

carbon macromolecules that come in a variety of forms and shapessingle wall SWCNT’smulti-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 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 protocols

Fullerene: Production

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 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: Chemistry

The hexagons are much better be considered as cyclohexatrienes than as benzene rings.

C60 is a strong electron acceptor.

Structure

2 types of bonds

1.38 Å

1.45 Å

C60

M. Prato J. Mater. Chem. 1997, 7, 1097.

20 hexagons + =12 pentagons

face centered

cubic arrangement

Crystal lattice

7Å

180 Å3

Deviation from planarity

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 stateNo reorganization energy….accelarates charge separation and hence long lived states

(optical limiter and lubricant, MRI contrast reagent)

Functionalisation

‘outside’ ‘inside’

exohedralendohedral

Ce, Gd,Eu,Nd,Sm,Tb,Ho

Fullerene: Chemistry 1,2 additions

R1

R2

Br+

R1 R2

base

DBU O

N

H

H

H

OH

HH

HO

NH

Br

O

O

DBU

R1R2

Br

+ B-

R2

C-

Br

R1

C-

Br

CH3 CH3 R1 R2

Fullerene: Chemistry 1,3 additions

X

Y

Z

+

- X

Y

Z+ -

X

Y

Z

NH

R1 O

OH

R2

O

H

+R2

H

N+

R1

O

O-

R2

H

N+

CH2-

R1

R2

C+

H

NCH2

-

R1

-CO2

R2

C+

H

NCH2

-

R1

R2

H

N

R1

There are almost no limitations for R2

For the α-amino acid derivative, R1=H is not possible

Fullerene: Chemistry 1,3 additions (Prato addition)

+

R1

R2

N2

R2R1

CH3

SO O

NH

N

CH3

SO O

N-

N

CH3

SO O

N-

N+

+

In-situ formation of diazo compounds from tosylhydrazones

Fullerene: Chemistry 1,3 additions (diazomethane addition)

N+

N-

R1

R2

C+

N

N-

R1

R2

N

N R1

R2

[6,6]-diazoline

N

N R1

R2R2

R1

60-70 oC

-N 2

R2

R1R2

R1

lightheatH+electrochemically

quantita tive,un imolucular reaction!

[6 ,6]-methanofu llerene

Fullerene: Chemistry 1,3 additions (diazomethane addition)

Fullerene: Chemistry 1,3 additions (addition of azides)

N

CH2

R

R CH2 N3

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 Grignard addition

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

O

1,3-d iphenylisobenzofuran

CH3

9-m ethylanthracene

CH3

CH3

9,10-d im ethylanthracene

The monoadduct cannot be isolated The monoadducts can be isolated at low temperature

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

Nanotube Biosensor

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+ER

Pi-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