Trans -Polyacetylene

For the MINDO/2 calculations, the C-H bond length was increased to 1.19 A as recommended by Dewar et al. for molecular calculations. 

FIGURE 3.1. The unit-cell geometry and calculated net atomic charges for polyacetylene using the INDO and MINDO/2 methods. MINDO/2 charge values are in parentheses. 

FIGURE 3.2. Energy-band structure of polyacetylene computed with the aid of the INDO CO method. 

O Shea and Santry. The results, as expected, are very similar, considering the slight difference in geometrical parameters. We note that for overlapping bands in our methodology, the aln nature of a given band may change as k varies, and resolution into pure bands is necessary for direct comparison with their calculations. This is readily accomplished by inspection of the matrices C(k). 

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In the following Section we present results of the application of the method to two model prototype systems, namely molecular hydrogen chains and all-trans polyacetylene. 

The charge transport in a conjugated chain and the interchain hopping is explained in terms of conjugation defects (radical or ionic sites), called solitons and polarons. Several possible conjugation defects are demonstrated in Fig. 5.33 on the example of trans-polyacetylene. 

In real tran -polyacetylene, the structure is dimerized with two carbon atoms in the repeat unit. Thus the tt band is divided into occupied tt and unoccupied n bands. The bond-alternated structure of polyacetylene is characterishc of conjugated polymers. Consequently, since there are no partially filled bands, conjugated polymers are expected to be semiconductors, as pointed out earlier. However, for conducting polymers the interconnection of chemical and electronic structure is much more complex because of the relevance of non-linear excitations such as solitons (Heeger, 2001). 

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.)... 

Photoconduction and absorption spectra of trans-polyacetylene are presented in Fig. 16 [104],... 

These and other results on absorption and luminescence were the reason why the soliton model was proposed for the photoconduction process in trans-polyacetylene [103-110],... 

The process results in a cis-transoid structure. The formation of trans-polyacetylene is suggested to take place through isomerization of the new segment formed by cis insertion before it can crystallize.412... 

T. Kobayashi In the same context, I would like to point out that, in polymer systems with degenerate ground states such as trans-polyacetylene, it may occur that a fraction of the excited species (exciton) is split into a soliton and an antisoliton with opposite charges. These can be bound by Coulombic interaction and also by steric hindrance. 

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