lecture 5 oms

Lecture V.

. Organic Conductors Charge-transfer Complexes and Radical-ion Salts Other Low-dimensional materials Polymers

•Conductive organic molecules

•“Plastic can indeed, under certain circumstances, be made to behave

very like a metal - a discovery for which Alan J. Heeger, Alan G.

MacDiarmid and Hideki Shirakawa are to receive the Nobel Prize in

Chemistry 2000”.

•Molecular building blocks Single bond:

sp3

hybridization

Double bond:

sp2 hybridization

•p-AO o f neighborng i caronb

atoms form π-bonding

•Rigid bond, length of 134 pm

Carbon atom can form

four σ-bonds.

Free rotation is possible

with activaton i energy of 0.1

eV.

Bond length 154 pm

•Molecular building blocks

Cyclic polyenes with conjugation

•that spreads the entire ring are

•called aromatic or arenes

Stability and delocalization of π- •electrons maintained in fused

•rings (polycyclic aromatic molecules)

•Molecular building blocks

Molecules with more than one

double bond called polyenes.

Shape and properties of the

molecule depend on the position of

the double bond

Conjugated double bonds play a

particular role as π-electrones are

delocalized over the extent of the

conjugation

Isolated double bonds

Conjugated double bonds

Cumulated double bonds

•Molecular building blocks Molecules with smaller or larger rings or other atoms in the

•ring (heterocycles) possess the same delocalization

•properties if the number of π-electrons is six.

•Cyclopentadiene anion

•Cycloheptatriene cation

•Heterocycles

•Molecular building blocks

Molecules with a triple bond

are called alkynes

Here, the π-electrons form a

cylindrical cloud around σ-

bond

Very rigid, linear bond with

the length of 120 nm

Conjugated triple bonds

show the same

delocalization as double

bonds

Acetlene y

•Molecular wires Molecular wires are, generally, rod-like structures with delocalized p-system,

•the longer the structure the lesser the difference between the frontier orbitals

•and the Fermi level of the electrode

polyene – alternating system of single

and double bonds;

polythiophene

polyphenylenevinylene

polyphenyleneethynylene

thyophenylsubstituted benzene

Conductivity Of Organic Materials

There are Thermally-stable Good Insulators

We will concentrate on the good conductors

Charge Transfer Complexes

Radical-Ion Salts

Discovery of Conducting Organic Crystals

TTF-TCNQ

Uniform segregated stacks (1D system) Metallic conductivity Metal-insulator transition at TMI = 54 K

TTF-TCNQ ANALOGS

S

S

S

S

Se

Se

Se

Se

Se

Se

Se

Se

TSFHMTTF HMTSF

HMTTF-TCNQ TMI = 48, 43 K 2.38a x 2.78b x c r = 0.72

HMTSF-TCNQ T = 24 K Toward semi-metal a x 2.7b x c with r = 0.74

TSF-TCNQ : TMI = 29 K 2a x 3.15 b x c r = 0.63

TCNQ is not necessary ! Cation radical salts with spectator anions (Brˉ, BF4ˉ, ClO4ˉ, PF6ˉ, …) obtained by chemical (Br2, I2, …) or electrochemical oxidation (electrocristallization)

Other Low Dimensional Materials

Conductivity is controlled by the phthalocyanine ring. The metal core does not interfere the conductivity.

Phthalocyanine channel

I-

Chain length: Si:12030 Ge: 7440 Sn:10040

Polymers

Insulating Polymeric Donor Molecules

Some Chemical Types of Polymers

Conjugated Polymers

Polyacetylene

Electrical conductivity of polyacetylene

Cis-PA s = 1.7 x 10-9 S/cm Trans-PA s = 4.4 x 10-5 S/cm I2 doped s = 5.5 x 102 S/cm AsF5 doped s = 1.2 x 103 S/cm Electrochemical Oxidation s = 1 x 103 S/cm Li doped s = 2 x 102 S/cm Na doped s = 101-10-2 S/cm

Polyparaphenylene (PPP)

Polyaniline (PANI)

Conductor

Insulator

Conductivity Of Organic Materials