Polyacetylene, electronic characteristic

A sequence may form and eventually meet a B sequence, as shown, but in doing so, a free radical, called a soliton, is produced. The soliton is a relatively stable electron with an unpaired spin and is located in a nonbonding state in the energy gap, midway between the conduction and valence bands. It is the presence of these neutral solitons which gives frany-polyacetylene the characteristics of an intrinsic semiconductor with conductivities of 10 to 10 (f2 cm) ... 

The momentum-space representation also proves particularly convenient for comparisons of the electron distributions of systems with different nuclear frameworks. Difference density plots in r-space are complicated by the different sets of nuclear positions. Such complications are absent in p-space and, in the case of polyenes [23], for example, momentum-space concepts have proved useful for examining the effects of bond alternation on the electron density - an important characteristic of such systems and of doped polyacetylene. 

It is certain that electrically conductive polymers have attracted much attention in the field of solid state science in recent years. They are expected to have convenient function in the production of useful electric or electronic devices such as the electrodes in rechargeable batteries, pn-junctions for use in integrated circuits (ICs) or in photovoltaic devices, and so on. In the normal sense, the organic polymers, even the 7r-conju-gated systems having mobile 7r electrons, are typical insulators of poor electrical conductivity and have been utilized as dielectric material. This is considered to be a result of the Peierls transition (Peierls, 1955), namely, a metallic-insulator transition, e.g., for polyacetylene, which is characteristic in the one-dimensional system. This situation is circumvented by the doping technique, in which the electron acceptors or donors... 

Both of these interpretations are clearly supported by scanning electron microscopy studies of extracted PA/PB blends. For example, Figure 1 shows a sequence of micrographs of a low polyacetylene content blend (<20% PA) extracted for different times with toluene. Figure 1(a) shows the surface texture of a typical unextracted PA/PB blend. The surface exhibits features characteristic of polybutadiene cast from a toluene solution. 

One of the most novel characteristics of the all-silicon backbone in polysilanes is that it possesses delocalized cr-electrons, a phenomenon that is virtually unknown in carbon chemistry." " " " This can be understood in terms of the nature of the molecular orbitals associated with the Si-Si cr-bonds. These are more diffuse than those associated with C-C cr-bonds as they are constructed from higher-energy 3s- and 3/)-atomic orbitals and silicon is less electronegative than carbon. This leads to significant interactions between the adjacent Si-Si cr-bonds along a polysilane chain, a situation analogous to that for the 7r-bonds in 7r-delocalized polymers such as polyacetylene. Thus, a band... 

In order to understand the physical properties of polyacetylene doped with divalent ions, it is important to consider the theory of conductivity of polyacetylene doped with monovalent ions. One of the most unusual characteristics of polyacetylene is that small amounts of dopant ions give rise to enormous increases in electrical conductivity without causing any increase in the number of unpaired electrons. In fact, the small level of paramagnetism observed in pristine polyacetylene actually decreases on doping (8). This is in contrast to what occurs in traditional semiconductors, such as silicon, where dopants increase both conductivity and paramagnetism. An explanation has been offered by the soliton theory of conductivity (9,10). 

This chemical n-doping procedure can readily be extended to the lanthanide ions Eu and Yb. Europium and ytterbium metals are known to dissolve in liquid ammonia (19). They form solvated divalent cations and solvated electrons in ammonia with the characteristic blue color. Upon immersion of polyacetylene the solvated electrons spontaneously reduce the polyacetylene chains to polycarbanions and the divalent lanthanide ions become countercations to maintain charge neutrality. 

ECPs appears in the course of its doping by counterions because of formation of delocalized n-electrons or holes and their transport under the action of electric field through the system of polyconjugated double bonds characteristic of any ECPs. ECPs include polyacetylene (Pac), polyaniline (PAni), poly(p-phenylene) (PPh), polythiophene (PT), polypyrrole (PPy), polyporphyrin (PP), and their derivatives. Eigure 28.3 shows structural formulas for some ECPs used in ECSCs. 

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