Leuco-emeraldine salt

The most electrically conductive state is the emeraldine salt (ES) that is between pernigraniline salt (Pas) at the high potential side and the leuco-emeraldine salt (LS) at the low potential side. The LS and the Pas are the most reduced and oxidised states, respectively, and have been found to be insulating. In an oxidation process from the LS to the ES, two electrons are withdrawn and two chloride ions are doped for every four benzene units. For the oxidation from the ES to the Pas, two electrons are withdrawn and two protons are released. The LS/ ES reaction is reversible, however, the hydrolysis occurs at the higher oxidised Pas states. 

FIG. 11.19 Oxidative doping (p-doping) of leuco-emeraldine base and protonic acid doping of emeraldine base, leading to the same final product, emeraldine salt. Reproduced from Fig. 34.3 in Mark (1996). Courtesy Springer Verlag. 

Figures 11.63 and 11.64 show that the contact of polyaniline with iron in the presence of water changes the polyaniline from the green to the yellow form, which can be understood as the reduction of polyaniline from the emeraldine salt to the leuco-emeraldine. This requires an equivalent oxidation of iron (to Fe " ). The back reaction shown in Figures 11.65 and 11.66 leads back to the normal polyaniline, although with some bluish component (emeraldine base or nigraniline) in the neutral environment. This back reaction is performed with the help of oxygen, as can be seen from the comparison of the spectra in Figures 11.69 and 11.70 with 11.65/11.66 and 11.71/11.72. In the de-...

Quite evidently, all states except leuco-emeraldine can be protonated. The states with no protonation are denoted as the "base" form, e.g. emeraldine base (which would be non-conductive). The reader may note tlie presence and alternation of "benzenoid" and "quinonoid" segments, as defined in an earlier Chapter, in the structures of Fig. 13-9. Upon protonation, the polymer is denoted as the salt, for example protonation of the emeraldine base form (Fig. 13-9c with HCl would yield emeraldine hydrochloride-. 

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