Polyacetylenes doping with electron donors

The unique properties of polymers such as polyacetylene, whose backbones consist of an alternating succession of single and double bonds, and most of which show extraordinary electrical, optical and magnetic properties including electrical conductivity when "doped" with electron donors or acceptors [35], are also outside the scope of this work. Sophisticated quantum mechanical treatments are required to predict these properties of such polymers. 

The values of bandgap of polymers A-D are smaller than that of rmn -polyacetylene, and lead to the expectation of semiconductive behavior in these polymers. The predicted order of bandgap is Celectron affinity increase in the order of B < D < A < C while ionization potentials are in the order of Chighly conducting materials on doping with electron donors. 

Note 2 The electric conductivity of a conjugated polymer is markedly increased by doping it with an electron donor or acceptor, as in the case of polyacetylene doped with iodine. 

Most photoeonductive polymers can be used in solar batteries. The high resistivity of the polymers decreases the actual power of the devices. Possibilities may be connected with electron-donor doping of the polymers. As stated earlier some success has been achieved in this field for polyacetylenes and other conjugated polymers. 

POLYACETYLENE. A linear polymer of acetylene having alternate single and double bonds, developed in 1978. It is electrically conductive, but this property can be varied in either direction by appropriate doping either with electron acceptors (arsenic pentaflnoride or a halogen) or with electron donors (lithium, sodium). Thus, it can be made to have a wide range of conductivity from insulators to n- or >-type semiconductors to strongly conductive forms, Polyacetylene can be made in both cis and trans modifications in the form of fibers and thin films, the conductivity... 

As we can see from the Table all the three chains (and this is the case also for further two other polyacetylene and three polydiacetylene chains (7) which also have been calculated) have broad valence and conduction bands with widths between 4.4 and 6.5 eV-s. Comparing the band structure of the two polyacetylene chains we can find that the position of the bands and their widths is not very strongly influenced by the different geometries. This is again the case if we compare the here not described band structures of the further polyacetylene and polydiacetylene chains. On the other hand the position of the valence and conduction bands and the widths of the valence bands of the polydiacetylene chains is more different from those of the polyacetylene chains. To conclude we can say that due to the broad valence and conduction bands of these systems (which mean rather large hole and electron mobilities,respectively) one can expect that if doped with electron acceptors or donors these systems will become good conductors, which is, as it is experimentally estab-... 

Jons. In the present case we need to fill the entire n band with electrons (one extra filectron per site). The result is the structure of fibrous ilfur, selenium, and tellurium. These contain chains of atoms in which all the bond lengths are the same. (The in, however, has distorted so that it is not planar.) Polyacetylene itself may be made conducting by doping either with electron donors or acceptors. The removal of some electron density from the filled band (1330) or the addition of density to die empty band (13.31) leads to a conducting (metallic) situation (cf. 13.4). 

The conductivity of polyacetylene is also increased by dopants that are electron donors. For example, the polymer can be doped with alkali metals to give, for example, [Li5 (CH) ln. The wide range of conductivities produced by these two forms of doping is illustrated in Figure 6.3. 

Polyacetylene (PA), the simplest linear conjugated polymer, has been actively studied for two main reasons. First, the discovery of the direct synthesis method of PA films on the surface of a Ziegler-Natta catalyst solution [1]. Second, the discovery of a large increase in electronic conductivity, due to a synthetic metal by doping with small quantities of electron-attracting species such as iodine, AsFs, etc., or with an electron donor such as sodium. However, because of its high reactivity and poor solubility, it is difficult to obtain the experimental structural data of PA. 

Polyacetylene, becomes ionized after doping if the dopants are electron acceptors, or it receives extra electrons if the dopant represents an electron donor (symbolized by D+ in Fig. 9.12). The perfect polyacetylene exhibits the bond alternation discussed above, but it may be that we have a defect that is associated with a region of changing rhythm" (or phase ) from (— — = — =) to (— = — = —). Such a kink is sometimes described as a soliton wave (Fig. 9.12a,b) i.e., a solitary wave first observed in the 19th century in Scotland on a water channel, where it preserved its shape while moving over a distance of several kilometers. The soliton defects cause some new energy levels ( solitonic le >els ) to appear within the gap. These levels too form their own solitonic band. 

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, a one-dimensional, conjugated polymer represented as (CH) exhibits electrical conduction upon chemical doping with an electron acceptor or donor [1,2]. The chemical doping transforms the polyacetylene from insulator or semiconductor to conductor. Ordinary polyacetylene film is composed of fibrils that are bundles of polyene chains. Because the fibrils are randomly oriented, the inherent electrical conductivity of the polyacetylene chain is depressed owing to fibril contact resistance. This makes it difficult for polyacetylene to become a complete one-dimensional conductor at the macroscopic level. Today, the primary concern is how to align the fibrils of polyacetylene film in order to achieve higher electrical conductivity. 

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