Solitons 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.)... 

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],... 

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. 

Large third order susceptibilities have recently been observed for trans-polyacetylene, heteroaromatic polymers, and poly[p-phenylene vinylene] (4-11). Such nonlinear phenomena in electroactive polymers due to intense laser irradiation has been linked to the photogeneration of charged solitons on time scales of the order 10 s, and values of (3w-w+w+w)—4x 10 esu for... 

When two trans-polyacetylene chains with different phases are put together, an obvious disturbance occurs in the standard conjugation pattern. The bond alternation defect that appears is known as a neutral soliton (Fig. 1.7). This kind of quasi-particle has an unpaired electron but is electrically neutral and is isoenergetically mobile along the polymer chain in both directions. This soliton gives rise to a state in the middle of the otherwise empty energy gap that can be occupied by zero, one or two electrons (Fig. 1.8). 

The experiments showed clear differences of behavior between polyacetylene in the cis and trans forms. Namely, SEE and OE are obtained in the cis and trans forms, respectively. In other words, the electronic spins are fixed in ds-polyacetylene, and they become mobile in the trans form. This result is quite consistent with the soliton picture. In cw-polyacetylene, a bond alternation defect divides the chain into two parts ds-transoid and frans-cisoid, whose energies are different (Fig. 8a). Thus, to minimize energy, the spin defect will be trapped at one end of the chain (Fig. 8b). On the other hand, in trans-polyacetylene the chain is divided into two degenerate parts. The defect should therefore be free to move (Fig. 9). 

Questions that had been of fundamental importance to quantum chemistry for many decades were addressed. When the existence of bond alternation in trans-polyacetylene was been demonstrated [14,15], a fundamental issue that dates to the beginnings of quantum chemistry was resolved. The relative importance of the electron-electron and electron-lattice interactions in Ti-electron macromolecules quickly emerged as an issue and continues to be vigorously debated even today. Aspects of the theory of one-dimensional electronic structures were applied to these real systems. The important role of disorder on the electronic structure and properties of these low dimensional metals and semiconductors was immediately evident. The importance of structural relaxation in the excited state (solitons, polarons and bipolarons) quickly emerged. 

The work of Su Shrieffer and Heeger (11) focussed on conjugated polymers having two valence bond structures, say A and B. In the case of trans polyacetylene, where A and B are degenerate, they showed intrinsic defects can occur, in which A phase goes continuously over to B phase, forming a soliton kink. 

The Pariser-Parr-Pople Hamiltonian for the description of the 7i-electrons in trans-polyacetylene is reparametrized using ab initio Coupled Cluster Doubles calculations based on a Restricted Hartree Fock reference on trans-butadiene. To avoid the spin contaminations inherent in Unrestricted Hartree Fock (UHF) type calculations on polymethine chains in the doublet state the Annihilated Unrestricted Hartree Fock (AUHF) model is applied in our PPP calculations (tPA (CH) , polyenes H-(CH)2N-H, polymethines H-(CH)2N+1-H). In geometry optimizations on polymethine chains it is shown that in contrast to results from Hiickel type models the width of neutral solitons is strongly... 

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