Redox reactions conjugated polymers

T. Yamamoto and H. Hayashi, ir-Conjugated soluble and fluorescent poly(thiophene-2,5-diyl)s with phenolic, hindered phenolic and /j-C6H4OCH3 substituents. Preparation, optical properties, and redox reaction, J. Polym. Set, Part A Polym. Chem., 35 463-474, 1997. 

A reaction mechanism in which the precursor polymer undergoes a redox reaction followed by loss of the bridge hydrogens is proposed. The resulting conjugated aromatic/quinonoid polymers generally have very small semiconductor band gaps in accord with predictions of recent theoretical calculations. 

Figure 4. General scheme of irreversible redox elimination reaction on the class of nonconjugated polymers containing alternating -C(R)H- and conjugated sections in the main chain yielding conjugated polymers.

In the area of ion sensing, cation recognition by electrodes containing functionalized redox-active polymers has been an area of considerable interest. Fabre and co-workers have reported the development of a boronate-functionalized polypyrrole as a fluoride anion-responsive electroactive polymer film. The electropolymerizable polypyrrole precursor (11) (Fig. 11) was synthesized by the hydroboration reaction of l-(phenylsulfonyl)-3-vinylpyrrole with diisopinocampheylborane followed by treatment with pinacol and the deprotection of the pyrrole ring.33 The same methodology was utilized for the production of several electropolymerizable aromatic compounds (of pyrrole (12) (Fig. 11), thiophene (13 and 14) (Fig. 11), and aniline) bearing boronic acid and boronate substituents as precursors of fluoride- and/or chloride-responsive conjugated polymer.34... 

First, we would like to address the question how sample quality influences the observed results. Synthesis and sample treatment influence the electronic properties of conjugated materials in a defined way [23]. We have already shown [31] that the shape and intensity of photoinduced absorption spectra in different representatives of the LPPPs may vary (see Fig. 9-16), indicating at least different trap densities but also different electronic properties of these traps, depending on the synthesis and subsequent treatment of the polymers. However, the electronic properties for this class of polymers can be imderstood in terms of effective conjugation length [23-25] charge transfer by photoexcitation or redox reactions [31] and also photo-oxidation upon intense visible irradiation under the influence of oxygen [23]. Therefore, by optical spectroscopy (absorption, photoluminescence, or photoinduced absorption) we can assess the quality of a sample. 

The monomers (213), which were ultimately converted into novel conjugated polymers, have been prepared in poor yield by double Wittig reaction of the dialdehyde (212). Bis-Wittig reactions of l,3-propanebis(triphenylphos-phorane) (214) with ferrocenyl aldehydes (215) have been used to prepare a series of bis(ferrocenyl)polyenes (216) containing a central methylene group. These compounds were converted into the corresponding polymethine cations which are prototype molecular wires with redox-active end groups. 

Since the discovery of the redox nature of conjugated polymers doping, various electrochemical techniques have been used, both to carry out the doping reaction and to characterize the doped polymer. 

The ion transport during the redox reactions of PPy has been attracting interest for two major reasons as an effort to explain charge transfer reactions and to apply the ion exchange properties to chemical analysis. More studies on this subject have been conducted on PPy compared to other 71-conjugated polymers thanks to its easy preparation in non-aqueous and aqueous media electrochemically or chemically, its reasonable stability in air, and simple methods of modification of the monomer, leading to a chemically functionalized polymer. On the other hand, the ion exchange properties of PTh have not been studied as extensively as for PPy since its electrochemistry in aqueous solutions is rather limited. We thus summarize recent advances in this area only on PPy. 

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