Lecture 5 oms
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Transcript of 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