Doping trans-polyacetylene

F re 5-8. UPS valence band spectra of doped trans -polyacetylene for K-doped, neutral, and CIO4-doped systems, from top to bottom (adapted from Ref. [52]). 

FIGURE 15.48 Real and imaginary parts of the dielectric functions (solid lines) and loss function Imf-l/s) (dotted line) from Kramers-Kronig analyses of the reflectance spectra for light polarized parallel to the chain direction for highly oriented AsFs doped trans-polyacetylene. (From Leising, G., Phys. Rev. B, 38, 10313, 1988. Reprinted from the American Physical Society. With permission.)... 

Stafstrom, S., "Electronic Properties of Heavily Doped Trans-Polyacetylene", p.ll3 in Br6das, J.L. Silbey, R. (Eds.), Conjugated Polymers The Novel Science and technology of Highly Conducting and Nonlinear Optically Active Materials, Kluwer Academic Publishers, Norwell, MA, USA (1991). 

In Fig. 12 we compare the dc and microwave (9 GHz) conductivity of Iodine-doped trans-polyacetylene in the temperature range from 10 to 300 K /9/. (dc measurements were carried out hy a lock-in technique at 30 Hz to avoid ionic conductivity by migration of Iodine ions.)... 

ELECTRONIC PROPERTIES OF HEAVILY DOPED TRANS-POLYACETYLENE... 

Figure 6.48 (a) Effect of doping on the electrical conductivity (solid line) and thermopower (broken line) of polyacetylene. (Following Etemad et al, 1982.) (b) solitons in trans-polyacetylene (i) neutral, (ii) positive and (iii) negative solitons. Arrow marks the boundary between the two symmetric configurations. A, acceptor D, donor. (Following Subramanyam Naik, 1985.)... 

Several attempts to induce orientation by mechanical treatment have been reviewed 6). Trans-polyacetylene is not easily drawn but the m-rich material can be drawn to a draw ratio of above 3, with an increase in density to about 70% of the close-packed value. More recently Lugli et al. 377) reported a version of Shirakawa polyacetylene which can be drawn to a draw ratio of up to 8. The initial polymer is a m-rich material produced on a Ti-based catalyst of undisclosed composition and having an initial density of 0.9 g cm-3. On stretching, the density rises to 1.1 g cm-3 and optical and ir measurements show very high levels of dichroism. The (110) X-ray diffraction peak showed an azimuthal width of 11°. The unoriented material yields at 50 MPa while the oriented film breaks at a stress of 150 MPa. The oriented material, when iodine-doped, was 10 times as conductive (2000 S cm-1) as the unstretched film. By drawing polyacetylene as polymerized from solution in silicone oil, Basescu et al.15,16) were able to induce very high levels of orientation and a room temperature conductivity, after doping with iodine, of up to 1.5 x 10s S cm-1. 

The first conducting polymer was trans-polyacetylene which was doped with bromine and was produced at 1970s. Soon other conjugated polymers such as poly (p-phenylene), polypyrrole (PPy), polyethylene dioxythiophene (PEDOT) and polyaniline (PANi) and their derivatives which are stable and processable were synthesized. The molecular structures of a few ICPs are shown in Figurel. 

The report of the doping of polyacetylene (PAc) films to produce metallic levels of conductivity by Shirakawa et al. (1977) sparked the interest in electrically conductive polymers that has continued until today. While it was not the first example of a conductive polymer, the increase in conductivity, by a factor greater than 107, observed on exposing films of trans-PAc to arsenic pentafluoride and iodine, was dramatic, see Fig. 9.1. The impact of this result was immediate, and created an upsurge of interest in conjugated polymers and the possibility of rendering them conductive. 

Acetylene selectivity polymerizes in the presence of Ziegler catalysts whose components have low Lewis acidity [e.g., Ti(0-n-Bu)4—Et3Al(l 4)J. Cis-polyacetylene forms at low temperature, and trans-polyacetylene at high temperature (Eq. (1)). When doped, a polyacetylene film shows metallic conductivity, and hence the application of polyacetylene to polymer batteries and solar cells is now under intensive study 1-3). 

Figure 5.3. The reduction (doping) of trans-polyacetylene. In the ideal case, the mobility of the charged states allows the bipolaron to collapse into a pair...

The electronic structures of poiy(fluoroacetylene) and poly(difluoroacetylene) have been investigated previously using the ab initio Hartree-Fock crystal orbital method with a minimum basis set (42). Only the cis and trans isomers with assumed, planar geometries were studied. The trans isomer was calculated to be more stable in both cases, and the trans compounds were predicted to be better intrinsic semiconductors and more conductive upon reductive doping than trans polyacetylene. However, our results show that head-to-tail poly(fluoroacetylene) prefers the cis structure and that the trans structure for poly(difluoroacetylene) will not be stable. Thus the conclusions reached previously need to be re-evaluated based on our new structural information. Furthermore, as noted above, addition of electrons to these polymers may lead to structural deformations that could significantly change the conductive nature of the materials. 

Fronzoni G, Stener M, Decleva P (1999) Theoretical study of the excited and continuum states in the NEXAFS regions of Cl2. Phys Chem Chem Phys 1 1405-1414 Fujikawa T, Oizumi H, Oyanagi H, Tokumoto M, Kuroda H (1986) Short-range order full multiple scattering approach to die polarized -edge XANES of Br-doped in Trans-polyacetylene. J Phys Soc. Japan 55 4090-4102... 

This polymer can be considered a derivative of trans-polyacetylene (Figure 1,60) it has also been prepared with a Shirakawa catalyst. The electronic structure has a degenerate ground state, but there are three inequivalent positions for solitons on the backbone, Gibson et al. [356] have reported the polymer to be x-ray amorphous, in contrast to tran5-polyacetylene, and to be even more air-sensitive than the latter. Doping is feasible with acceptors such as I2 and AsFs, to a maximum conductivity of 1 S cm . ... 

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