Abstract
• Quantification of redox forms of polyaniline. • Octameric model for PANI transitions. • Back to aniline-black structures. The redox transitions of PANI in acidic medium have been monitored by a combination of cyclic voltammetry, in situ conductance and in situ FTIR spectroscopy. The results of the semiquantitative analysis strongly suggest that the classical tetrameric model of PANI does not satisfactorily describe the actual structures of the polymer at different redox states. An octameric model is revisited, with the inclusion of essential resonant structures, to provide an appropriate prediction of the relative IR intensity changes of the aromatic C C stretching (at around 1520 cm −1 ) and the quinoid C C stretching (at around 1590 cm −1 ) vibrations observed by FTIR, which are difficult to interpret by considering only 4 aniline rings. Particularly, it is found that the emeraldine state is better described as a resonance hybrid of the classical bipolaronic and semiquinoid (polaron lattice) structures, while most of the charge transferred at the onset of the second voltammetric peak comes from the additional oxidation of this hybrid, which becomes unstable in the electrochemical environment producing mineralization to CO 2 and release of soluble quinones.
Highlights
Polymers containing non-localized electrons in their backbones are often referred to as conducting polymers
The first studies on aniline-black come from the 19th century [10] but it was in the early 20th century that the works of Willstatter, Moore, Green and Woodhead defined the different oxidation states of an eight-unit molecule considered precursors of anilineblack [11,12]
A return from the current 4-ring structure of PANI to the classical octameric model has been proposed to rationalize the results of the semiquantitative spectroscopic analysis
Summary
Polymers containing non-localized electrons in their backbones are often referred to as conducting polymers. The progressive oxidation (or reduction) of the polymer backbone (doping process) increases the conductivity of the material by several orders of magnitude This behavior, and additional physicochemical properties, supported their applicability to non-linear optics, electrocatalysis, electrochemomechanical devices or charge storage systems, among other research fields [2]. Already at that time the complex redox structure of aniline-black was recognized and the terminology related to the different oxidation states of the octamer was assigned, the so-called leucoemeraldine, protoemeraldine, emeraldine, nigraniline and pernigraniline. These states correspond to 0, 25, 50, 75 and 100% of oxidized rings, respectively [12]. MacDiarmid’s seminal papers led to establishing that PANI presents a conductivity influenced by the pH of the synthesis medium and demonstrated that the conductive forms of PANI only appear at pH lower than 4 [13]
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