Lecture 5 oms

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Information about Lecture 5 oms

Published on April 15, 2014

Author: AllenHermann

Source: slideshare.net

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 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:12030 Ge: 7440 Sn:10040

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

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