Polyacetylene oxidative doping

Alkali-doped polyacetylene is extremely air-sensitive and deteriorates much more quickly than oxidatively doped polyacetylene. But, whereas the latter degrades rapidly upon heating as a result of polymer-dopant reactions that alter the chain, alkali-doped polyacetylene is surprisingly stable thermally, up to 200°C. This can be exploited to anneal cis-rich doped samples, which leads to a considerable decrease of disorder and evolution towards the trans lattice [88]. Especially in K-doped samples, this leads to a strong conductivity increase. 

Early versions of ICPs, mostly based on oxidatively doped polyacetylenes (PAcs), faced several intrinsic obstacles that prevented their industrial commercialization. The material degrades readily in air, and no known good methods exist for making easily processable PAc polymers. These obstacles led... 

Another view has recently been proposed by Wegner.Naphthalene and other simple aromarics can be oxidize electrochemi-cally to form monomelic radial cat n salts (Ar. X ) which have conductivities of 10 to 10 s/cm. The crystal structures of these reveal that the aromatic moieties form stacks, along which the charges and the electrons are presumably delocalized. The structure is formally analogous to that deduced for oxidized (doped) polyacetylene in which the polyene chains are arranged in stacks. This leads to the idea that intermolecular delocalization is the important feature which leads to high conductivity. Other data are consistent with this rationale. Biphenyl and terphenyl radical cation salts have crystal structures very similar to that of oxidized (doped) poly(p-phenylene lO). In the older literature oligoanilines (26) are reported upon iodine treatment to yield conductivities up to 1 s/cm the aniline moieties are stacked in these materials as well. Poly(N-vinyl-carbazole) (27) forms radical cation structures by oxidation with... 

There are several approaches to the preparation of multicomponent materials, and the method utilized depends largely on the nature of the conductor used. In the case of polyacetylene blends, in situ polymerization of acetylene into a polymeric matrix has been a successful technique. A film of the matrix polymer is initially swelled in a solution of a typical Ziegler-Natta type initiator and, after washing, the impregnated swollen matrix is exposed to acetylene gas. Polymerization occurs as acetylene diffuses into the membrane. The composite material is then oxidatively doped to form a conductor. Low density polyethylene (136,137) and polybutadiene (138) have both been used in this manner. 

Figure 29 raises the question of how the energies of these two excited states evolve as one goes to longer polyene chains, in analogy to those found in polyacetylenes which become conductive upon oxidative doping (= ionization) or photoexcitation. 

Five aspects of the preparation of solids can be distinguished (i) preparation of a series of compounds in order to investigate a specific property, as exemplified by a series of perovskite oxides to examine their electrical properties or by a series of spinel ferrites to screen their magnetic properties (ii) preparation of unknown members of a structurally related class of solids to extend (or extrapolate) structure-property relations, as exemplified by the synthesis of layered chalcogenides and their intercalates or derivatives of TTF-TCNQ to study their superconductivity (iii) synthesis of a new class of compounds (e.g. sialons, (Si, Al)3(0, N)4, or doped polyacetylenes), with novel structural properties (iv) preparation of known solids of prescribed specifications (crystallinity, shape, purity, etc.) as in the case of crystals of Si, III-V compounds and... 

In their studies of effects of oxidation of polyacetylene on its dopability, Pochan et al.545 reported that iodine-doped polymer loses its conductivity in vacuum and concluded that the I3 counter-ions are able to react with the polymer chain, leading to iodination. Huq and Farrington 5561 found that bromine- and iodine-doped polyacetylenes lose conductivity rapidly at temperatures below 60 °C, whereas samples doped with AsFs are very much more stable. 

In summary, the oxidation of polyacetylene has the following features a) charge-transfer interaction with oxygen leads to doping, with an increase in conductivity,... 

If the principles, so far outlined, are valid then it is to be expected that n-type doping of polyacetylene would lead to a decrease in stability towards oxidation, and this is indeed so 578). However, the introduction of electrons into the chain can also give a new instability in that the oxidation potential can fall to the point where the polymer is able to reduce water and it becomes hydrolytically unstable. Thus n-type doped polyacetylene reacts rapidly with water and with alcohols, with partial hydrogenation of the chain and a rapid decrease in conductivity 579,580,581). Whitney and Wnek 582) have used the reaction of n-doped polyacetylene with alkyl halides and other reagents to prepare functionalized poly acetylene films. 

These quarternized polymers can be viewed as self-doped but exhibit relatively low intrinsic conductivities. The polymers can be oxidatively doped with iodine, or reductively doped with TTF, to give highly conducting polyacetylenes with conductivities of 10"4 and 10 1 S/cm, respectively. One additional attractive feature of this system is that, unlike PA, these quarternized PAs are very stable in air. 

The origin of the conduction mechanism has been a source of controversy ever since conducting polymers were first discovered. At first, doping was assumed to simply remove electrons from the top of the valence band (oxidation) or add electrons to the bottom of the conduction band (reduction). This model associates charge carriers with free spins (unpaired electrons). However, the measured conductivity in doped polyacetylene (and other conducting polymers such as polyphenylene and polypyrrole) is r greater than what can be accounted for on the basis of free spin alone. 

The properties of such apparently soluble materials differed greatly from those containing stabilized polyacetylene crystallites. Attempts to dope the soluble copolymers yielded materials with low conductivities, and chemistry typical of solution chemistry (bromination) was observed rather than the formation of a stable bromine-doped polyacetylene phase. A poly(isoprene-fo-acetylene) copolymer oxidized with iodine gave conductivities as high as 1-10 S/cm, but the characterization of the copolymer was insufiScient to unambiguously identify it as a soluble copolymer. On the basis of previously reported work, this material is likely to correspond to a stabilized suspension rather than a solution. 

Doping by FeCls is usually carried out in nitromethane solution. The anion species is identified by Fe Mossbauer spectroscopy and the extended X-ray absorption fine structure (EXAFS) di7 FeCU. The doping reaction involves dissociation of FeCb in nitromethane (16a) followed by oxidation of polyacetylene by FqC 2 (16b). Thus, the overall doping reaction can be written as equation (16c). 

Doping with these Group VA fluorides can be carried out electrochemically, using the polymer as the anode in a cell containing a solution of the dopant ion, e.g. tetrabutylammonium hexafluoroantimonate in propylene carbonate, or from the vapour phase. In the latter case the oxidative doping of polyacetylene is supposed to involve the following reaction of the dopant (for the case of AsFs) ... 

Most polymers (typified by polystyrene and polyethylene) are electrically insulating and have conductivities doped with iodine to become electrically conducting (values have now been reported up to olO Scm ) represented a pivotal discovery in polymer science that ultimately resulted in the award of the Nobel Prize for Chemistry in 2000 [4]. The study of electrically conducting polymers is now well advanced and two extremes in the continuum of transport mechanisms exist. If the charge carriers are present in delocalized orbitals that form a band structure along the polymer backbone, they conduct by a delocalization mechanism. In contrast, isolated groups in a polymer can function as acceptors or donors of electrons and can permit... 

Polyacetylene, which was originally prepared by a polymerization of acetylene by Shirak-awa with the aid of a Ziegler-Natta catalyst, is an insulator. It can be shaped into a silvery-looking film. Partial oxidation of the film, however, with iodine or other materials transforms it and increases its conductivity 10 -fold. The process of transforming a polymer to its conductive form through chemical oxidation or reduction is called doping. Polyacetylene, which can be cis or trans, is more thermodynamically stable in the trans form and converts from cis to trans when heated above 150 °C. 

The carotenes and other conjugated linear polyenes have been touted as "molecular wires" or as "molecular antennas" [21], and certainly will provide fast electronic access to single molecules. These polyenes are, unfortunately, very susceptible to air oxidation (as is the simplest conducting polymer, doped polyacetylene). 

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