Preparation and characterization of neutral and oxidized polypyrrole films

Abstract

Oxidized and neutral films of polypyrrole have been prepared electrochemically in the absence of oxygen and water. The neutral rms 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-,a' 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. 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 a,a' 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.

Theoretical Modeling of the Doping Process in Polypyrrole by Calculating UV/Vis Absorption Spectra of Neutral and Charged Oligomers

“…14,98 At higher doping levels both bands shift to higher energy and two absorptions occur at 1.0 and 2.7 eV. 12 In contrast, Street et al 52 reported an absorption energy of 1.3 eV after oxidation with oxygen, Zotti et al 18 and Rapta et al 25 observed a band at 1.38 eV, and Fink et al 13 found two peaks at 1.5-2.0 eV and at 3.0 eV with electron energy loss spectroscopy at higher doping levels. Although there are significant differences in reported data and in the interpretation of experimental results, on combining polymer and oligomer data the following picture emerges: (1) since the absorption maximum of polypyrrole is close to that or slightly below that of 7P, conjugation length in PPy is likely in the range of seven to nine rings (as found also with Raman spectroscopy 99 ); (2) monocations and polarons give rise to two sub-band peaks at about 0.7-0.8 and 2.1-2.3 eV, while dications or bipolarons have a characteristic absorption in the range 1.0-1.4 eV.…”

“…[49][50][51] In the oligomer studies the issue of bipolaron formation was not addressed and the interpretation of polymer results remains controversial. 8,12,13,16,17,20,22,23,25,[52][53][54][55] Electronic structures of PPy and OPs have been investigated theoretically before; 20,[56][57][58][59][60][61][62][63][64][65][66][67][68][69][70] however, we are not aware of calculations of absorption spectra during the doping process of pyrrole oligomers or polymers. We have recently calculated such spectra for polyenes 71 and OTs.…”

The chemical and physical properties of pyrrole-based conducting polymers: The oxidation of neutral polypyrrole

“…An article on the preparation and characterization of neutral and oxidized polypyrrole films has been published previously 4 and contains comparisons of optical absorption, infrared transmission and 13 C NMR spectra in neutral and fully-oxidized as-grown polypyrrole. Preliminary electron paramagnetic resonance (EPR) 5 and X-ray photoelectron spectroscopy (XpS) 6 data on polypyrrole films have also been reported recently.…”

Characterization of Polypyrrole

“…All the films described in this work were prepared using the dry box techniques described in our previous paper. 4 This was done to simplifythe in tation of the c c ation data by avoiding contamination of the films by oxygen or moisture.…”

“…4 In order to resolve some of these problems and facilitate characterization, polymers have been synthesized in which both the P positions of the pyrrole ring are blocked to ensure exclusive a,a bonding of the polymer chain. In the case of the Pj, dimethyl pyrrole, this leads to an increased chain order which in turn results in significant improvement in the crystallinity of the polymer, enabling a model for the structure of polypyrrole to be described in this paper.…”