Bipolarons in poly

Figure 1. Examples of conducting polymers and species responsible for charge storage. Top, poly(acetylene) and soliton 2nd, poly-p-phenylene and bipolaron 3rd, poly- -phenylene sulfide 4th, poly(heterocycles) and bipolaron. Bipolarons in poly(furan) have not yet been established.

Fig. 2-12 Actual structures of polarons/bipolarons in Poly(p-phenylene).

Photoinduced absorption studies of the oligomer have shown that the dominant long-lived photocarriers are bipolarons formed by interchain charge separation. The lifetimes of the bipolarons in poly(l-(A-carbazo-lyl)penta-5-acetoxy-l,3-diyn) are of the order of seconds at 80 K. The polymer of 1-(A-carbazolyl)-penta-l,3-diyne-5-ol, which is an intermediate compound in the synthesis of (61), has a very low band gap energy (1.6 eV) exhibits a strong non-linear optical... 

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

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...

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

Scheme 5.J. The oxidation (doping) of poly p-phenylene). The bipolaron is the final oxidation product in poly(p-phenylene) and other polymers with nondegenerate ground states.

Fig. 18. Schematic representation of bipolaron hopping mechanism in poly(p-phen-ylene).

FIGURE 16,11 Illustration of polaron and bipolaron structures in poly(p-phenylene) and the proposed band structure for the oxidized (p-type) polymer. (Partially adapted from Cowan, D.O. and Wiygul, KM., The organic solid state, Chem. Eng. News, 28,1986. With permission from the American Chemical Society.)... 

We shall focus this discussion on polythiophene spectroelectrochemistry [260]. While simply monitoring the UV-vis spectrum of a coated ITO electrode can show evolution of polaron and bipolaron bands [255], for example, in poly(thiophene-3-methanol) [261], a DCVA experiment is even more revealing, showing that more regular and conjugated polymers generate bipolarons in much greater concentration early on in the CV [262, 263]. 

Soliton (Fig. 2c) is a term used by physicists in various connections. From the chemist s point of view, a soliton is a positive, negative, or neutral radical-like site and corresponding conformational kink in the simple conjugated system of polyacetylene separating two semiinfinite conjugated chains. As with the polaron and bipolaron in other CPs, the soliton is mobile in the chain and is thought to be the charge carrier in poly acetylene. 

Figure 4-3. Schematic structures of self-localized excitations in poly( p-phenylene) and trans-po y-acetylene. (a) Positive polaron (b) negative polaron (c) positive bipolaron (d) negative bipolaron (e) positive polaron (f) negative polaron (g) neutral soliton (h) positive soliton (i) negative soli-ton . D, donor A, acceptor -I-, positive charge negative charge , unpaired electron.

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