Thomas C. Clarke

Abstract Oxidized and neutral films of polypyrrole have been prepared electrochemically in the absence of oxygen and water. The neutral films are insulating and can be readily oxidized by chemical oxidizing agents to give films of greater conductivity than can be achieved by electrochemical oxidation. Optical spectroscopy provides evidence for the similarity of the polymeric carbonium ion produced by both types of oxidation. NMR studies are consistent with the α,α’ bonding in these polymers; they also show the expected downfield shifts relative to the neutral polymer on both chemical and electrochemical oxidation. ESR studies of both the electrochemically oxidized and the neutral polymer suggest the presence of highly mobile spins.

The carbon-carbon bond lengths in polyacetylene have been directly measured with use of nutation NMR spectroscopy. 1.36 and 1.44 \AA{} were found for the double and single bonds, respectively, in trans-${(\mathrm{CH})}_{x}$, and 1.37 \AA{} was found for the double bond in the cis isomer. Over the temperature range 4.2-300 K, the dynamical single-double bond interchange predicted for chains in trans-${(\mathrm{CH})}_{x}$ containing a mobile soliton defect was not observed.

We report NMR experiments which imply that highly one-dimensional spin diffusion occurs in undoped trans-${(\mathrm{CH})}_{x}$ and trans-${(\mathrm{CH}{(\mathrm{As}{\mathrm{F}}_{5})}_{0.1})}_{x}$. The corresponding anisotropy in the diffusion coefficient would be about ${10}^{6}$.

Pristine 98% $^{13}\mathrm{C}$-enriched cis-polyacetylene has been investigated by electron-nuclear double-resonance spectroscopy at temperatures between 77 and 300 K. Spectra are characterized by two and only two $^{1}\mathrm{H}$ and $^{13}\mathrm{C}$ hyperfine tensors. The relative magnitudes and symmetry of the tensor elements established that the paramagnetic electron resides in a delocalized $\ensuremath{\pi}$ orbital. Experimental results indicate that the delocalized electron is constrained to a potential well of approximately 100 \AA{} width.

The optical transmittance spectra of polypyrrole perchlorate have been measured at various stages in the reduction to neutral polypyrrole at 300 and 120 K and at high pressure. Analysis of the results suggests that the polypyrrole chain is composed of conjugated segments of various lengths. The optical conductivity spectrum of polypyrrole perchlorate was obtained by the Kramers–Kronig transformation of the transmittance data from 0.025 to 6.25 eV. The spectrum is not consistent with Drude-like free carriers. The conduction mechanism probably involves hopping between the conjugated segments.