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Polymers with useful electronic properties (Read 1778 times)
Gerrit-Jan Linker
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Polymers with useful electronic properties
29.09.09 at 07:30:43
 
Polymers with useful electronic properties
 
Two classes of polymers with useful electronic properties:

  • Redox polymers:
    • These contain redox active subunits linked by unsaturated spacers.
    • They thus exist as independent building blocks
    • These independent blocks can be charged independently.
    • If electrostatic interactions are absent many electron transfers occur at the same potential.

  • Conducting polymers:
    • Conducting polymers are characterised by an extended pi conjugation.
       
      Conjugated polymers can be described in terms of energy band theory:
      The pz-orbitals of neighboring carbons overlap to form pi-bands
      This leads to semiconducting or metallic properties depending  upon whether the bands are filled or partially filled.
      Example:
      Consider n pz electrons in the unit cell. These n electrons will form a pi band that is split into n pi sub-bands.  
      The lowest n/2 sub-bands are called the binding pi-sub-bands.  
      The highest n/2 sub-bands are called the anti-binding pi*-sub-bands.
      A material is metallic if a sub-band is partially filled.  
      So, if the number of pz electrons is even the material is not metallic, if the number is uneven the material is metallic.
       
      When the chain of monomers is large, end points are not important and the pi electron transfer integral (beta in MO theory or t in tight binding theory) tends to delocalise the wavefunction over the entire chain.
      This tendency towards delocalisation is counterbalanced by disorder (disorder tends to localise the wavefunction) and by Coulomb interaction which binds the electron when transferred to a nearby monomer to the positive charge left behind; the hole.
       
      Energy band theory for conjugated polymers is incomplete for 3 reasons:
       

      • consequences of disorder
        Polymers are disordered in the solid state.
        The disorder leads to localisation of electronic states, band tailing and broadening of the optical transitions.
      • interconnection between electronic and chemical structures
        There is a deep interconnection betwee the electronic and chemical structure.
        Elementary excitations in semiconducting polymers involve lattice relaxation around the electrons and holes.
        Lattice relaxation in the excited state leads to self-localisation and the formation of solitons, polarons and bipolarons.
      • electron correlations
        The band description ignors the attractive Coulomb interaction between electrons in the pi*-band and holes in the pi-band.  
        This attraction causes the formation of excitons.

       
      The central issue is relates to the strength of the electron-electron interactions relative to the bandwidth, relative to the electron-phonon interaction and relative to the strength of the main disorder potential.
       
      Local description:
      Strong electron-electron interaction (electron-hole attraction) lead to the creation of  localised and strongly correlated negative and positve polaron pair; neutral polaron excitations.
       
      Band description:
      On the other side, well screened electrons and holes with associated lattice distortions (charged polarons) are more appropriately described by a band description supplemented by the electron-phonon interaction.
       
    • Successive electron transfers will lead to separate waves as a result of strong electrostatic effects.
    • Metallic polymers exhibit high reflectance (and thus look shiny) in the infrared, but they are semitransparent in the visible part of the spectrum. The residual absorption above the plasma frequency  arises primarily from interband (pi - pi*) transitions between the partially filled conduction band and the lowest energy unoccupied band.
       

 
 
Source:
Advances in Synthetic materials: 20 Years of Synthetic Metals - the role of synthesis, D. Marsitzky, K. Mullen
 
See also:
Exciton
http://www.oraxcel.com/cgi-bin/yabb2/YaBB.pl?num=1256375335/0#0
Polaron  
http://www.oraxcel.com/cgi-bin/yabb2/YaBB.pl?num=1256934928/0#0
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« Last Edit: 30.10.09 at 22:45:37 by Gerrit-Jan Linker »  

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