Charge bipolarons

Figure 6 Comparison of (a) a pair of solitons (b) a positively charged bipolaron and (c) an electron-hole pair bound by the lattice relaxation of the chain.

Figure 5-9. Band diagram of negatively charged bipolarons (BP ) in PPV.

That this procedure does indeed yield the fully oxidized charged bipolaron lattice was demonstrated by nuclear magnetic resonance (NMR). NMR spectra of the... 

Note that equation (8.8) is expected to be only approximate. Including the electron-electron repulsion associated with the double charge on the bipolaron, and the binding energy of the charged bipolaron to the counterions (2 b) [233,234], the following relationship holds ... 

Leucoemeraldine, which is an insulator with a large bandgap, showed an onset of n—n transition around 3 eV with a peak at 3.7—3.8 eV, whereas emeraldine base has a two principal absorptions (2 eV and 3.9 eV). However, overoxidized polyaniline exhibits intense absorption at 2.2 eV and a well-defined peak near 4 eV shown to be due to doubly charged bipolarons associated with a quinoid unit [164,227]. 

Figure 8. Sketch of the one-electron band-structure model for polythiophene (a) in the neutral state (b) in the presence of a positively charged polaron and (c) in the presence of a positively charged bipolaron. The possible subgap optical transitions induced upon doping are represented by arrows.

Bipolarons are formed by the combination of two polarons with the same charge. The bipolaron also has two levels in the energy gap. In the case of a negative bipolaron both levels are fully occupied and for a positive bipolaron both levels are empty. In either case the spin is zero. Because of their charge, bipolarons are assumed to be in close proximity to their coimterions. 

Overoxidation of polyaniline in sulfuric acid leads to conversion to the formation of an array of charged bipolarons on the polyaniline chains. The implied electronic structure was confirmed in detail by UV visible spectroscopy and by NMR spectra. 

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