the facile synthesis of a wide range of substituted polyani-lines that are difficult to synthesize directly from their respective monomers.
Doped PAn • HA emeraldine salts are good electrical conductors, with conductivities typically in the range 110 S cm-1. It is generally agreed that polarons are the charge carriers responsible for this high conductivity. Evidence includes the observation of a strong electron spin resonance signal associated with the radical cation sites in structure (III) . It has also been shown that bipolaron states exist in polyaniline, but these are few in number and are not associated with the conducting regions of the polymer .
The attachment of functional groups to the aniline rings generally decreases the conductivity of polyaniline emeral-dine salts. This is attributed to steric crowding causing a marked twisting of the polymer backbone from planarity, leading to decreased conjugation along the chains. Structural defects such as those arising from undesirable ortho-coupling of aniline radical cations during the polymerization also lead to impaired electrical conductivity. There has been considerable recent interest in enhancing the conductivity of emeraldine salts via their exposure to "secondary dopants" such as m-cresol. This can cause an increase in conductivity of several orders of magnitude. The enhanced conductivity has been attributed to the adoption of an "extended coil" conformation by the polyaniline backbone, with the polarons delocalized along the chains .
The UV-visible near-infrared spectra of polyanilines are very sensitive to the polymer chain conformation. For example, when PAn • HCSA emeraldine salts are generated by acid doping the EB form with camphorsulfonic acid in organic solvents such as chloroform, DMSO, or NMP [Eq. (4); See Fig. 8], they typically exhibit three absorption bands in the visible region, as shown in Figure 6. The strong band observed in these cases at 750-850 nm has been assigned as a localized polaron band, while the two bands at ca. 430 and 340 nm are attributed to a second polaron band and a w-w* (bandgap) band, respectively . These spectral features and the moderate electrical conductivity of such salts (ca. 1 S cm-1) are considered diagnostic of a "compact coil" conformation for the polyaniline chains. In contrast, no long wavelength localized polaron band is observed in the visible region for PAn • HCSA salts generated by analogous
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