Bipolaron lattice

The bipolaron-lattice-parameter model is based on the following assumptions ... 

The strength of the vibronic interaction is dependent on the amplitudes of the bipolaron and hole wavefunctions in bipolaron-lattice wavefunction The ground-... 

Figure 5. Ener level shematics at the metal/polymer interface. The formation of bipolaron lattice at the interface depends on the relative positions of the metal Fermi level and the bipolarons. At the interface, Ca will tend to form negative bipolarons (a), A1 will not form bipolarons (b) and Au will tend to form positive bipolarons (c). Although bipolarons extend over several monomeric units, they span only 4 units in this diagram (33). We iUustrated the bipolarons with PPV chains.

The spectra of leuoemeraldine, (lA)n, and the emeraldine base, [(1 A)(2A)]n, have been extensively studied elsewhere.28 29 We include the spectra of these two important forms of polyaniline obtained from films spin-cast from sulfuric acid (see Figure 5) in order that they can be directly and quantitatively compared with those of the emeraldne salt and the bipolaron lattice. Leucoemeraldine is an insulator with a large band gap. The ti-tc transition shows an onset at approximately 3 eV, with a peak at 3.7-3.8 eV. The emeraldine base shows two principal absorption bands (see Figure 5), with maxima at 2 eV and at 3.9 eV, respectively. 

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

Fig.6. Solution optical absorption spectra showing the formation of the fully oxidized bipolaron lattice form of polyanillne.

The stabilization of the bipolaron lattice structure provides an opportunity for studies of this new degenerate ground-state polymer, at least in solution. However, dthough the bipolaron lattice form appears to be stable in concentrated sulfuric acid, hydrolysis occurs when the solution is exposed to moisture from the atmosphere. As a result, attempts toward recovering (4) in the pure solid state have thus far been unsuccessful. Efforts in this direction are continuing in our laboratory. 

The available data provide the basis for an understanding of the electronic structure of the four principal forms of polyaniline the fully reduced leucoemeraldine, (lA)n the emeraldine base, [(lA)(2A)]n the oxidized and fully protonated emeraldine salt, [lS] (A )n and the fully oxidized bipolaron lattice, (-B-NH+=Q=NH+-)n. 

Figure 4-16. Schematic structures of polythiophene chains doped with electron acceptors (dopant content, 25 mole% per thiophene ring) and bonding electronic levels of positive polarons and bipolarons. la) Polaron lattice (b) bipolaron lattice. A. acceptor +. positive charge negative charge , electron —, electronic energy level.

Let us suppose an infinite nondegenerate polymer chain (e.g., polythiophene) doped heavily with electron acceptors. At a high dopant content, the polymer-chain structure and electronic structure of the doped polymer are radically different from those of the intact polymer. As typical cases, we will describe two kinds of lattice structures of doped polythiophene (dopant content, 25 mole% per thiophene ring) a polaron lattice and a bipolaron lattice. They are the regular infinite arrays of polarons and bipolarons. The schematic polymer-chain structures are shown in Figure 4-16. Band-structure calculations have been performed for polaron and/or bipolaron lattices of poly(p-phenylene) [124], polypyrrole [124], polyaniline [125], polythiophene [124, 126], and poly( p-phenylenevinylene) [127], with the valence-effective Hamiltonian pseudopotential method on the basis of geometries obtained by MO methods. The schematic electronic band structures shown in Figure 4-17... 

Figure 12.20. Polaron and bipolaron lattice, (a) Emeraldine salt in bipolar form, (b) Dissociation of the bipolaron into two polarons. (c) Rearrangement of the charges into a polaron lattice . (Adapted from references [222,225]).

Fig. 174. Schematic representations of the band structures of (a) soliton lattice, (b) bipolaron lattice, and (c) polaron lattice forms.

The sudden change of the susceptibility near 3.4 V vs Li (corresponding to a doping level of a few mol% per thiophene ring) is apparently related to a bend noted in the relationship between conductivity and dopant concentration for the same material [37] this bend also occurs at a doping level of a few mol% per ring [89]. This is suggestive of the transition associated with the formation of bipolaron lattices in PMT [155]. 

Figure 18. Schematic (a) bipolaron lattice and (b) polaron lattice in the emeraldine salt polymer.

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