Electrical- Organics Flashcards
When do organic materials not conduct?
When all electrons are localised in sp3 hybrid type orbitals, e.g polythene
Describe the bonding in benzene and graphite
There are overlapping pz (subscript z) orbitals (sp2 hybridisation) giving rise to delocalised electrons and aromaticity. They are π-electrons and π-bonds where the electrons overlap sideways. Aromatic compounds have additional chemical stability. For conductivity we require a conjugated system that contains a skeleton of alternate double and single C-C bonds.
Describe sp2 hybridisation in ethene
Starts [He]2s2 2p2 Then promotion: [He]2s1 2px1 2py1 2pz1 Then hybridisation: [He](sp2)^3 2pz1 Gives rise to trigonal geometry The 2pz1 electrons from carbons form π bond above and below sp2 hybrid C-C bond
Describe σ-bonding and π-bonding in benzene
The σ bond framework is built from 6 sp2 hybridised C atoms with two of the sp2 hybrids from each C used to bond to neighbouring C atoms. The third is used to bond to bond to a H atom using the H 1s orbital. There are 6 unhybridised 2pz orbitals orientated such that they can form π bonds. Each C contributes 1 p electron into a compound with 6 MO so there are empty orbitals into which the electrons can delocalise over the whole structure.
How is graphite a conductor?
Delocalised electrons from fused benzene rings give rise to conductivity along the layers. There is only low conductivity perpendicular to the layers.
Describe the structure of polyacetylene
There is a C backbone of alternate double and single bonds. The framework is based on a skeleton on sp2 hybrid σ bonds. The double bonds are formed by the overlap of π orbitals above and below the plane of the C chains. The cis isomer has the two carbon chains on the same side of a double bond ad they are on opposite sides for the trans isomer.
Describe Peierls distortion in polyacetylene
A linear chain of equispaced atoms found to be energetically unstable (this would have been a metal). The atoms undergo a slight distortion (Peierls distortion) with alternate double (short) and single (long) bonds and this produces an energy gap at the Fermi level. What would have been a half filled band from the undistorted structure is transformed into a filled lower band and an empty upper band.
What does Peierls distortion mean for conductivity of polyacetylene?
It disrupts the full delocalisation of the π electrons and it becomes a semiconductor. The electrons are semi localised but only a modest amount of thermal excitation gives rise to semiconductivity.
Equivalent conductivity of undoped cis and trans polyacetylene
Cis- Si
Trans- water
How does acceptor doping with halogens work with polyacetylene?
Use Cl2, Br2 or I2. They oxidise the polymer. Electrons removed from the filled band and used to form acceptor (halide) ions. This leaves holes at the top of the band and results in a p-type material. The halide resides between the molecular chains of the polymer.
How does donor doping with alkali metals work with polyacetylene?
They reduce the polymer. The metal donates its electron to the empty band, therefore producing an n-type material. The dopant resides between the polymer chains. Using Li increases the conductivity by several orders of magnitude than acceptor doping with I2 for trans. There is an abrupt transition from semiconductor to metallic conduction at a particular dopant level (1-5%) which is much higher than doping for Si-based semiconductors.
What is Li-doped polyacetylene used for?
An electrode in Li-batteries. It has mixed ionic-electronic conductivity. Electronic conduction from the polymer and Li+ ions can migrate through the spaces between the polymer chains.
