Figure 6.

400 500 600 700 800 900 1000 1100 Wavelength (nm)

UV spectra showing EB-ES. Reprinted with permission from Boonchu, Ph.D. Thesis, University of Wollongong, 2002.

acid doping in "secondary dopant" solvents such as m-cresol. These much more conducting salts (8 > 100 S cm-1) instead exhibit an intense free carrier tail in the near infrared region with an absorption maximum at ca. 2500 nm. These latter features are considered diagnostic of an "extended coil" conformation for the polyaniline backbone:

Circular dichroism (CD) spectroscopy provides a particularly powerful tool for probing the polymer chain conformation in related optically active emeraldine salts. For example, the "compact coil" form of PAn-(+)-HCSA in NMP solvent shows the characteristic bands seen in Figure 7. The strong bisignate CD bands at ca. 795 and 720 nm are associated with the localized polaron absorption at ca. 775 nm for this polymer in NMP (see Fig. 6) [72, 73]. Overlapping bisignate CD bands are also observed at lower wavelengths associated with the other two lower absorption bands seen for PAn-(+)-HCSA in Figure 6. The CD spectrum of "extended coil" PAn-(+)-HCSA (not shown) bears no similarity whatsoever to that seen in Figure 7 for the "compact coil" form [74].

Polyaniline contrasts with other ICPs in that it has three readily accessible oxidation states (fully reduced leu-coemeraldine, partly oxidized emeraldine, and fully oxidized pernigraniline), as shown in Figure 8 [75]. In addition, reversible protonation/deprotonation equilibria occur for two of these oxidation states. Thus, the emeraldine salt form (ES), which is the only electrically conducting form of polyaniline, is typically dedoped at pH > 4 to give nonconducting EB. Conversely, the reverse acid doping of EB with HA acids [Eq. (4)] provides a convenient route to conducting PAn • HA emeraldine salts with a wide range of acids. The pernigraniline oxidation state of PAn can similarly exist as both a protonated pernigraniline salt or as a neutral pern-igraniline base (Fig. 8). On the other hand, the fully reduced leucoemeraldine oxidation state appears to exist only in the neutral base form.

Polyaniline can be rapidly and reversibly switched between the various forms shown in Figure 8. The pH and redox switching is accompanied by marked changes in colour. For example, alkaline treatment of green PAn • HA emeraldine salts rapidly generates the blue color of EB. The associated UV-visible spectral changes caused by such alkaline dedoping are shown in Figure 8. Emeraldine base exhibits a characteristic exciton band at ca. 600 nm as well

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