Poly bipolaron state

Eig. 2. Lattice distortions associated with the neutral, polaron, and bipolaron states in poly(p-phenylene). 

During the past three years we have been studying the chemical (SbCl5) oxidation of well-characterized oligomers of polyacetylene, poly[p-phenylene vinylene] (PPV) and poly[2,5-thienylene vinylene] (PTV) in order to determine how polaron and bipolaron states can be preferentially formed and stabilized. 

Fig. 4.3 The bipolaron state in poly(p -phenylenevinylene) is shown schematically.

Although theoretical calculations have predicted that the bipolaron state is energetically more favored than the polaron, it is widely agreed that polarons are the charge carriers responsible for the high conductivity in poly-aniline. It has been proposed that the presence of coulombic interactions. 

But obviously the single oxygen atom in 3-alkoxy-4-methylthiophenes is not sufficient for an adequate mesomeric stabilization of the bipolaronic state in the doped polymer. For example, the poly-3-alkoxy-4-methylthiophenes described by Leclerc and Daoust (alkoxy=methoxy, n-butoxy) tend to be partially dedoped even by treatment with methanol, whereas poly-3,4-di-n-butoxythiophene... 

Fig. 10. Formation of the bipolaron (= diion) state in poly-p-phenylene upon reduction In the model it is assumed that the ionized states are stabilized by a local geometric distortion from a benzoid-like to a chinoid-Iike structure. Hereby one bipolaron should thermodynamically become more stable than two polarons despite the coulomb repulsion between two similar charges...

The electronic band structure of a neutral polyacetylene is characterized by an empty band gap, like in other intrinsic semiconductors. Defect sites (solitons, polarons, bipolarons) can be regarded as electronic states within the band gap. The conduction in low-doped poly acetylene is attributed mainly to the transport of solitons within and between chains, as described by the intersoliton-hopping model (IHM) . Polarons and bipolarons are important charge carriers at higher doping levels and with polymers other than polyacetylene. 

On the basis of their results the authors concluded that the crossover from the neutral to oxidised form of poly pyrrole, and vice versa, requires the number of spins to pass through a maximum, i.e. the creation and anihilation of bipolarons involves the passage through the polaron state. They expressed this as a two-step redox reaction ... 

The case U = 0 is of special interest. In such a case, making one bipolaron from two polarons does not cost energy. The maximum spin concentration is i, which means that 50% of the boxes possess unpaired spins. This result can be explained as follows. Since (1) at the maximum spin concentration one has q = 1, and (2) placing one or two balls in the box is equivalent, there are four equiprobable cases (1) neutral site (no ball), (2) polaron up, (3) polaron down, and (4) bipolaron (two balls). Two among four of these states are magnetic, which gives for the spin concentration. The room-temperature data for polypyrrole and poly aniline can almost be fitted with U 0, which means that in these compounds polarons and bipolarons would be degenerate. 

Poly(phenylene vinylene), PPV, and its soluble derivatives have emerged as the prototypical luminescent semiconducting polymers. Since PPV has a nondegenerate ground state, structural relaxation in the excited state leads to the formation of polarons, bipolarons, and neutral excitons. However, prior to treating the structural relaxation in the excited state, one needs to develop a satisfactory description of the electronic excited states. 

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