Polyacetylenes electron delocalization

Extensive electron delocalization along the chain direction leads to an electronic transition energy for one-photon absorption (E0) of typically 15 000-16 000 cm-l for an unstrained backbone (.7). This value is very close to that of polyacetylene (or... 

As described in Chapter 6, Electric Properties of Polymers, there is a general relationship between the delocalization of electrons throughout a polymer chain or network and color so that the incidence of and darkness of color increases as electron delocalization increases. Thus polyethylene is colorless while polyacetylene is black. 

As explained in the introduction, the polysilanes (and related polygermanes and poly-stannanes) are different from all other high polymers, in that they exhibit sigma-electron delocalization. This phenomenon leads to special physical properties strong electronic absorption, conductivity, photoconductivity, photosensitivity, and so on, which are crucial for many of the technological applications of polysilanes. Other polymers, such as polyacetylene and polythiophene, display electron delocalization, but in these materials the delocalization involves pi-electrons. 

SoKtons produced in polyacetylene are delocalized over approximately 12 CH units, with the maximum charge density to the dopant counterion. Soliton formation results in the creation of a new localized electronic state which is in the middle of the energy gap. At a high level of doping the charged sohtons produce soliton bands that can merge to behave hke a metalhc conductor. 

It is well known that the rigid-rod polymer materials have good dielectric properties. However, it is interesting that because the extended 7r-conjugated structure provides extensive electron delocalization, the band gap calculated for PBZT in a coplanar conformation is 1.73 eV, which is close to that of polyacetylene (195,196). Therefore, although PBZT has low conductivity (about 10 S/cm)... 

AsFs compared to 400 S cm (y = 0.12) for the AsFs doped polyacetylene. Since the copolymers can be doped to the same level as polyacetylene and therefore should have a similar number of charge-transfer complexes, the dependence of conductivity on the number of the more flexible methylacetylene units would be due to restriction in carrier transported along the conjugated 7r-chains. It has been suggested that electron delocalization is sensitive to the loss of coplanarity in the conjugated 7i-system as the number of flexible methylacetylene units increased. 

Polyacetylene with n-alkyl substituents form another class, the tensile properties are markedly different from those of the aromatic substituted polyacetylenes the Young modulus being 600 to 800 MPa and 2500 MPa respectively (5), that is polyacetylene witii alkyl side chains exhibit a behavior similar to the one of the saturated polymers. On the other hand they are colorless in solution. For example, poly-2octyne in toluene has a maximum absorption located around 280 nm with no additional structure at longer wavelength. In term of electronic delocalization, the location of the maximum absoiption in the near U. V indicates a jc-electron localization at the mcaiomer unit scale. 

Contrary to naive expectation, an extended ir-electron system snch as that in the original silvery polyacetylene film does not imply perfect bond conjngation or perfectly like bonds, or conduction along the chain It only implies a degree of charge-density delocalization. Such a material has the electronic structure of a... 

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. 

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