Analysis of the structure of polyaniline absorption spectra based on spectroelectrochemical data

Using the results of spectroelectrochemical experiments (absorption spectra measured at different stationary potentials from 0.0 to 0.8 V versus Ag | AgCl) performed in aqueous acidic medium for polyaniline (PAN) films electrochemically deposited onto SnO2-glass substrates, individual absorption bands composing the spectra were analyzed. The key point of the method used for separation of the individual bands (Alentsev–Fock method) is that one need not have a preliminary notion about real shape, number and positions of individual components of the spectrum. The necessary conditions are the following: (1) in the most common case, the number of spectra used for separation should be equal to or exceed the number of possible components; (2) the spectra should contain different proportions of the individual bands. Both well-known (cation-radicals (near 435 nm), localized polarons (near 755 nm), quinoid structures (near 665 nm)) and some new absorption bands were found. Comparing these results with our earlier data for electrochemically deposited and vacuum evaporated PAN films in different acidic media, the newly revealed band near 570 nm may be attributed to donor–acceptor interaction of quinoid fragments of the polymer chain with counter anions. The band in the NIR region (>900 nm), which is often referred to as that of ‘free carriers’, apparently characterizes electron exchange between highly conductive areas of the film.