Vibrational spectra of polyaniline and its ¹⁵N- and ²H-substituted derivatives in as-polymerized, alkali-treated and reduced states

Raman spectra of polyaniline and its ¹⁵N- and ²H-substituted derivatives were studied in as-polymerized (conductive), alkali-treated (insulating) and electrochemically reduced (insulating) states. Emeraldine and N,N′-diphenyl-p-benzoquinone diimine were also studied. The CN stretching and some other vibrations were assigned on the basis of the isotopic shifts of the Raman wavenumbers. The presence of quinone diimine (Q) and p-disubstituted benzene (φ) structures was identified in alkali-treated samples (2A) by the 632.8 and 457.9 nm excited spectra in resonance and preresonance with the 630 and 330 nm absorptions, respectively. Together with the infrared NH stretching bands, these observations give experimental evidence of: (1) the head-to-tail polymerization and (2) the partial structure of 2A, -Q-φ-(NH-φ)_x-Q-φ-(NH-φ)_y-. The partial structure is the same for samples prepared electrochemically and chemically except that the composition (x, y, etc.) depends on the synthetic conditions in both cases. In the quinone diimine part (Q′) of as-polymerized samples (2S), electrons are more delocalized than in the corresponding part of 2A, since the CN stretching wavenumber is about 137 cm^-1 lower than that of 2A. In the course of reduction of 2S, such modified quinone diimine parts are changed into p-disubstituted benzene structure and the fully reduced samples (1S) are mostly composed of p-disubstituted benzene. The presence of the electron-delocalized quinone diimine parts in 2S is closely related to its high electrical conductivity.