Polyacetylene electronic properties

Baeriswyl, G. Harbeke, H. Kiess, and W. Meyer, Conducting polymers Polyacetylene, "Electronic Properties of Polymers," J. Mort and G. Pfister, eds., Wiley-Interscience, New York (1982). 

The examples of polyacetylenes whose main chain is directly bonded to heteroaromatic rings (e.g., silole, carbazole, imidazole, tetrathiafulvalene, ferrocene) are increasing in number. Such polymers are usually obtained by one of catalysts (W, Mo, and Rh). The formed polymers are expected to display interesting (opto)electronic properties such as electrochromism, cyclic voltammetry, electroluminescence, and so on. 

On the terminology of the band model c- bonds form the completely filled low band, and rc-bonds make the partially filled band, which defines the electronic properties of the polyacetylene. 

Active research of the electronic properties began in the early 1970s, when it was shown that polyacetylene may be synthesized as a flexible film with arbitrary and specially oriented fibrils. 

Two lower states of the frans-(CH) are energetically degenerated as follows from symmetry conditions. Theory shows that electron excitation invariably includes the lattice distortion leading to polaron or soliton formations. If polarons have analogs in the three dimensional (3D) semiconductors, the solitons are nonlinear excited states inherent only to ID systems. This excitation may travel as a solitary wave without dissipation of the energy. So the 1-D lattice defines the electronic properties of the polyacetylene and polyconjugated polymers. 

Though conductivity of polyaniline is not as high as that of some other polymers, it is emerging as the material of choice for many applications. It is stable in air and its electronic properties can be easily tailored. It is one of the oldest synthetic polymers, and probably it is the cheapest conducting polymer used in devices. It can be easily fabricated as thin films or patterned surfaces. Polyaniline will never replace the materials which have extremely high conductivity. However, it will be useful for certain specific applications. Andy Monkman has a program to extrude the polymer braids and lay the insulation of the coaxial cables in a single step. The work is supported by a cable company [3], Properties of polyacetylene are discussed in detail in Chapter 2. 

Table 2 Some one-electron properties of the investigated polyacetylene models IP (ionization potential), EA (electron affinity), fundamental gap (AE0)t valence band width (8EV), and conduction band width (8Ec), respectively, obtained from calculations up to the 28th neighbouring C2H2 unit (all values in eV)...

Ziessel et al. have explored extensively the possibility of controlling and modulating the electronic properties of a bridge by incorporating additional unit into the unsaturated polyacetylenic backbone [49]. An elegant example that illustrates this point is represented by a series of Ru-Os binuclear complexes where the polyacetylenic bridge contain a phenyl (13), a naphthyl (14), and an anthryl (15) group, respectively. 

In this contribution, we discussed effects of disorder on the electronic properties of quasi-one-dimensional Peierls systems, like the conjugated polymer trans-polyacetylene. Since polymer materials generally are rather disordered and the effect of disorder on any quasi-one-dimensional system is strong, a proper description of these materials requires consideration of such effects. 

The infrared and Raman spectra of polyacetylene undergo very large changes when the polymer is doped. This is because the electronic structure of the polymer has changed dramatically and infrared and Raman spectral intensities depend on the electronic properties. The metallic nature of the resulting materials also makes measuring the spectra difficult. All of these effects are irrelevant to the neutron so it... 

J.M. Andre and B. Champagne, in J.L. Bredas (Ed.), Nonlinear optical hyperpolarizabilities and electronic properties of oligomers and polymers the polyene story, from dyes to conducting and NLO systems, Conjugated oligomers, polymers, and dendrimers From polyacetylene to DNA, Bibliotheque Scientifique Francqui, De Boeck Universite, 1999, pp. 349-394. 

A significant difference between poly(RCOT)s and unsubstituted polyacetylene is that the former bear substituents which may perturb their electronic properties. The optical absorbances (Table 10-3) probe this to some extent, but electrochemical data are more sensitive. For example, whereas polyCs ec-butylCOT) and poly(Me3SiCOT) have similar absorption spectra, both the formal reduction and oxidation potentials of the silyl-substituted polymer are shifted positive of the alkyl-substituted polymer (Fig. 10-27). This is expected, based on the more electropositive nature of the silyl substituent [139]. Also, while the effects are not large, the substitution of either an electron-donating (para-methoxyphenyl) or an electron-withdrawing (para-trifluoromethylphenyl) substituent do perturb the polymer s electronic properties, with the latter material being harder to oxidize and easier to reduce than the former. 

Unfortunately, even for the simplest and most studied case, the polyacetylene film, there is not a homogeneous network [5]. The mixing of the amorphous and the crystalline part makes the average properties observed, much more difficult to interpret. Not only does the very complex structure of the conducting polymer films produce scattered data for the conductivity, but the spectroscopic data are often dependent on the packing and chain conformation. As a consequence, the electronic properties of conducting polymer films may vary from one sample to another. Therefore, a major difficulty arises in deciding whether or not the difference observed was as a result of the chosen chemical structure and polymerisation route or of the way the molecules were packed. 

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