Doped polyacetylene

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. 

Druy et al.562) showed that iodine- and perchlorate-doped samples lose conductivity quite rapidly in vacuum, due to reaction of the polymer with the counter-ion. Yang and Chien 56l) also observed the instability of these doped polymers and showed that the reaction of polyacetylene with perchlorate counter-ions can be explosive. They showed that doped samples are much more stable to oxygen than is the undoped material. Muller et al.565) also observed that the stability of polyacetylene in air depends on the extent of doping, as did Ohtsuka et al.566). Aldissi 5671 has suggested that iodine doped polyacetylene can be stabilized by phenolic antioxidants, although the effect was modest. 

In our own work on Durham polyacetylene 568) wfe find that the stability of doped polymers depends upon the extent of doping. Thus when AsF6 is the counter-ion, a polymer doped to low levels ( 1 mol %) shows very little change in conductivity over a period of days at room temperature in vacuum or dry air, whereas saturation doping (to about 17 mol%) produces a polymer whose conductivity decays rapidly, with ir evidence for the formation of C—F bonds in the polymer. 

Pron et al.569) looked at polyacetylene treated from the gas phase with H2S04 which leads to HS04 counter-ions. They found that the conductivity drops in air with the appearance of C=O bands in the ir, although the rate of decay is much lower than would be expected for undoped samples. The polymer was more rapidly degraded by exposure to water but could be redoped with further acid treatment. Pron et al.570) have also reported hydrolytic instability in polyacetylene with A1C14 as the counterion. In both cases the proposed mechanism involves addition of OH to the chain and keto-enol tautomerism to form carbonyl groups. 

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

Doping can be achieved from the vapour phase (if the vapour pressure is sufficiently high), or in solution, chemically as well as electrochemically. In the latter case the polymer is one of the electrodes in an electrochemical cell and the process can be carefully controlled. The incorporation of solvent molecules is not considered in any of the diffraction studies, although it is sometimes stated that it may occur for smaller, polar molecules (e.g. for NH3, but not, say, in the case of cyclohexane). 

Doping of polyacetylene has been studied extensively, especially n-type doping with alkali metals and p-type doping with iodine. Since 1990, however, papers on this topic have almost ceased to appear, presumably because interest has shifted to other polymers and other properties (fluorescence for instance, which is not shown by polyacetylene). The number of papers that appeared in the 1980 s is so large as to make it impracticable to discuss the contents of each. The development of the current knowledge will be sketched and relevant contributions to the field will be mentioned where this is appropriate. 

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Fig. 1. (a) Comparison of normalised electrical conductivity of individual MWCNTs (Langer 96 [17], Ebbesen [18]) and bundles of MWCNTs (Langer 94 [19], Song [20]). (b) Temperature dependence of resistivity of different forms (ropes and mats) of SWCNTs [21], and chemically doped conducting polymers, PAc (FeClj-doped polyacetylene [22]) and PAni (camphor sulfonic acid-doped polyaniline [2. ]) [24]. 

Heeger AJ, McDiarmld AG (1980) Conducting Organic Polymers Doped Polyacetylene. In Alcacer L (ed) The Physics and Chemistry of Low-Dimensional Solids. Reidel, Dordrecht, p 353... 

Although the conductivity of polyacetylene is generally discussed in terms of solitons, the question of the precise nature of the major charge-carriers continues to be a subject of debate, with conflicting evidence from different experiments. Spectro-electrochemical studies provide evidence that the charge in doped polyacetylene is stored in soliton-like species (although this is not the only possible interpretation [142, 143]), with absorptions in the optical spectra corresponding to transitions to states located at mid-gap [24,89, 119]. The intensity of the interband transitions... 

In TGA studies on the decomposition of iodine-doped polyacetylene, at high heating rates (30°C/min), decomposition becomes explosive at the m.p. of iodine, 113°C. This was attributed to exothermic reaction of liquid iodine with polyacety-lene. 

G A. Ozin, University of Toronto In view of the current interest in electrically conducting, undoped and doped polyacetylene thin films, do you envisage any possibility of initiating a controlled polymerization of a metal-metal triply bonded organometallie complex to produce a species of the form ... 

Chiang CK, Fincher CR Jr, Park YW, Fleeger AJ, Shirakawa H, Louis EJ (1977) Electrical conductivity in doped polyacetylene. Phys Rev Lett 39 1098-1101, erratum (1978) Phys Rev Lett 40 1472... 

When the 7r-systerns of two or more double bonds overlap, as in conjugated dienes and polyenes, the 7r-clccIrons will be delocalized. This has chemical consequences, which implies that the range of possible chemical reactions is vastly extended over that of the alkenes. Examples are various pericyclic reactions or charge transport in doped polyacetylenes. A detailed understanding of the electronic structure of polyenes is therefore of utmost importance for development within this field. We will first discuss the structure of dienes and polyenes based on theoretical studies. Thereafter the results from experimental studies are presented and discussed. 

CK Chiang, CR Fincher, YW Park, AJ Fleeger, FI Shirakawa, EJ Louis, SC Gau, and AG MacDiarmid, Electrical conductivity in doped polyacetylene, Phys. Rev. Lett., 39 1098-1101, 1977. 

M Ozaki, DL Peebles, BR Weinberg, CK Chiang, SC Gau, AJ Fleeger, and AG MacDiarmid, Junction formation with pure and doped polyacetylene, Appl. Phys. Lett., 35 83-85, 1979. 

FIGURE 57. Coupled soliton modes in n-doped polyacetylene. Reprinted with permission from Reference 66. Copyright (1990) American Chemical Society... 

Since the discovery of doped polyacetylene, a range of polymer-intense semiconductor devices have been studied including normal transistors and field-effect transistors (FETs), and photodiodes and light-emitting diodes (LEDs). Like conductive polymers, these materials obtain their properties due to their electronic nature, specifically the presence of conjugated pi-bonding systems. 

Doped polyphenylene sulfide Doped polythiophene Doped polyacetylene (Metallic) Conductors... 

In 1958, Natta and co-workers polymerized acetylene for the first time by using a Ti-based catalyst. This polymerization proceeds by the insertion mechanism like the polymerization of olefins. Because of the lack of processability and stability, early studies on polyacetylenes were motivated by only theoretical and spectroscopic interests. Thereafter, the discovery of the metallic conductivity of doped polyacetylene in 1977 stimulated research into the chemistry of polyacetylene, and now poly acetylene is recognized as one of the most important conjugated polymers. Many publications are now available about the chemistry and physics of polyacetylene itself. 

If an electron acceptor is added, it takes electrons from the lower n bonding band. The doped polyacetylene now has holes in its valence band and, like p-type semiconductors, has a higher conductivity than the undoped material. Electron donor dopants add electrons to the upper n band, making this partly full, and so producing an n-type semiconductor. 

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... 

Solitons are considered to be important defect states in these conjugated polymers (see Fig. 6.48). It has however been shown that correlation energy is the more important factor in giving rise to the energy gap in (CH) (Soos Ramasesha, 1983). Other polymers related to polyacetylene are polythiophene, polypyrrole, poly-phenylenesulphide, and polyparaphenylene (Section 3.3). Extensive measurements on doped polyacetylenes have been reported in the last five years and these materials, unlike other conducting polymers such as (SN), seem to have good technological potential. 

All three compounds (WF6 and MoF6 are best) will bring about a reaction (not a fluorination) that may have synthetic utility at 0CC in l,l,2-trichloro-l,2,2-trifluoroethane (Freon 113) or chloroform they will cleave N,TV-dimethyl- and N-tosylhydrazones and oximes back to the parent carbonyl compounds12,14 (UF6 converts any first-formed aldehydes into acid fluorides1213). All three hexafluorides will convert1215 tertiary amines into carbonyl compounds and carboxylic acids into acid fluorides.16 They also dope polyacetylene to the metallic regime.17... 

Polyacetylene and poly(p-phenylene) (PPP) are two polymeric hydrocarbons with a unique property both are conducting polymers on doping. Polyacetylene, a... 

Fig. 16a-c. Proposed crystal structures for a metal-ion doped polyacetylenes and fa metal-ion doped polyphenylene. Reproduced with permission from Ref. 4651... 

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. 

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