Mechanism of 3,4-diarylpyrrole electrooxidation

• The α,α'-coupling of pyrrole radical cations to σ-dimers is reversible and the deprotonation of the dimer is kinetically hindered.
• The addition of bases does not result in deprotonation of doubly charged σ–dimer in 2- and 2'-position.
• The slightly conducting poly(3,4-diarylpyrrole) is obtained via addition of neutral monomer to cation radical.
• The electrooxidation of 3,4-bis(3,4-dimethoxyphenyl)pyrroles is not produced expected poly(phenanthropyrrole) polymer.

The purpose of this study was to investigate 3,4-diarylpyrrole electropolymerization and simultaneous or subsequent intramolecular oxidative coupling leading to poly(phenanthropyrrole). Small amounts of π-conjugated products were obtained only under the conditions of increased concentration of the monomer and increased rate of potential sweep, which enabled both processes − monomer oxidation and deprotonation of the σ-dimer. The combined electrochemistry, UV-vis-NIR, ESR and TD-DFT results show, that the α,α'-coupling of 3,4-diarylpyrrole derivatives is inhibited by the stability of the σ-dimer dication, inhibition of the deprotonation and the reversal of the σ bond formation resulting in regeneration of the monomer. The addition of pyridine did not result in σ–dimer deprotonation in 2- and 2'-position.

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