Published on April 15, 2014
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
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 TSF HMTTF 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
Insulating Polymeric Donor Molecules
Some Chemical Types of Polymers
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
Polyaniline (PANI) Conductor Insulator
Conductivity Of Organic Materials
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