Polypyrrole bipolarons

Figure 3.72 Electronic structure diagrams for a polypyrrole chain containing (a) a polaron and (b) a bipolaron (c) The band structure obtained for highly oxidised (33 mol.0,, ) polypyrrole showing the presence of two broad bipolaron bands in the gap. After Bredas et al. (1984).

Hence the authors concluded that bipolarons are the major current carriers on the basis of the calculation showing them to be 0.49 eV more stable than two polarons, with polarons combining where possible to give bipolarons. This paper dominated the interpretation of experiments on polypyrrole for several years until in situ techniques became routinely applied,... 

This means that for a given polypyrrole chain if a unit cell (chosen as 6 monomer units on the basis of the observed maximum doping level of 2 charges per 6 monomer units) already has a polaron on it and a second charge is injected onto the same chain, it is more favourable to form a second polaron on another vacant unit cell than to force spin pairing to give a bipolaron. If, however, all the other unit cells are already occupied, then a bipolaron will be formed in the unit cell, rather than two polarons. 

Table 3.7 Polaron and bipolaron absorptions in polypyrrole (after Christensen and Hamnett, 1991) ...

Thus, it appears that the transition represented by the anodic peak in the cyclic voltammogram of polypyrrole is due to a changeover in the dominant carrier type and is accompanied by a dramatic contraction of the film. The authors strongly suspected that this contraction was due to electro-striction associated with bipolaron formation. As a further test they also carried out experiments intended to test if proton expulsion from the film occurred on oxidation. They found that it did indeed occur but monotonically at alt potentials > -0.6 F, in agreement with the extremely elegant work of Tsai et at. (1987), and so could not be responsible for the relatively sudden contraction at potentials > —0.2 V. 

The work of Christensen and Hamnett (1991) provided the first positive evidence for bipolarons and showed, for the first time, that the oxidation of polypyrrole was accompanied by a dramatic decrease in the film thickness, linking this with the generation of these carriers. Taken in addition to all the work discussed above their work provided some of the final pieces in a workable theory of the conduction mechanism in polypyrrole. 

The bipolarons are energetically described as spinless bipolaron levels (scheme (9.30a)) which are empty and which, at high doping levels, may overlap with the formation of bipolaronic bands (9.30b). Finally, for polymers with band gap, values smaller than that of polypyrrole - such as polythiophene - the bipolaronic bands may also overlap with the valence and conduction bands, thus approaching the metallic regime. 

On the basis of their results the authors concluded lhat the crossover from the neutral to oxidised form of polypyrrole, 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 ... 

However, while the evidence for the existence of polarons was extremely convincing, that for bipolarons was rather more problematical in that it was largely effectively negative in nature the absence of an absorption peak in the optical spectrum, the absence of a signal in epr studies on the decline of the observed signal. In essence, bipolarons had not been actually observed. This fact was remedied by the work of Christensen and Hamnett (1991) who employed ellipsometry and FTIR to study the growth and electrochemical cycling of polypyrrole in situ in aqueous solution. 

Aida and coworkers developed poly(pyrrole)-containing mesoporous silica films in both hexagonal and lamellar phases.34 The polypyrrole chains are highly constrained and insulated when incorporated within hexagonal mesoscopic channels and the possibility of the polarons recombining into bipolarons is significantly suppressed. In contrast, the two-dimensional lamellar phase affords spatial freedom for electron recombination. 

Many other air-stable conducting polymers followed (Fig. 12.10) polypyrrole, polythiophene, polyaniline (which had been known since the nineteenth century as "aniline black"), and so on (Table 12.4). These polymers are semiconducting, not metallic, when "doped" with electron donors or acceptors the individual conjugated chains have finite length, so the conductivity is limited by chain-to-chain hopping. Also, if the individual strands exceed four or so oligomers, the conjugation tends to decrease, as the strand tends to adopt a screw-type distortion. The transport within each strand is attributed to polarons and bipolarons. 

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. 

Figure 6. Band structure of polypyrrole as a function of doping level (schematic). (A), neutral polymer (B) polaron (+), spin = 1/2, 3 new transitions (C) bipolaron (++), spin = 0, 2 transitions (D) Heavily doped, bipolaron bands.

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