Positive soliton

Polyacteylene turns out to be a special case when considering its neutral and doped forms. Comparison of the two neutral forms, shown in Scheme 3, reveals them to be structurally identical, and thus, their ground states are degenerate in energy. Two successive oxidations on one chain could yield radical cations that, upon radical coupling, become non-associated charges termed positive solitons . 

Fig. 4.2 A short segment of t rans -polyacetylene is shown with an abrupt (idealized) reversal of the bond alternation pattern (see text). Top- a neutral soliton with an unpaired spin and an energy state near the middle of the electron energy gap. Middle the addition of an electron results in the formation of a spinless negatively charged soliton. Bottom the extraction of an electron from the top results in the formation of a spinless positive soliton. The optical transitions associated with the charged solitons are indicated as arrows on the right.

Fig. 9.12 Energy level schemes of solitons and polarons in polyacetylene (a) soliton, (b) anti-soliton, (c) negative soliton, (d) positive soliton, (e) negative polaron and (f) positive polaron.

The low reactivity of the (CH+ )X Ion In these cases may be related to the fact that the positive charge Is believed to be delocalized over a positive soliton, consisting of approximately 15 CH units (6-7). Thus the carbon atoms In a CH unit would be less susceptible to nucleophilic attack by OH- or H20 than If the charge were localized on only one carbon atom. It seems not unlikely that the size of a positive soliton may vary with the size and polarizability of the counter anion. Moreover, the... 

In the original SSH model, the relaxation of the 1R state goes to a charged (S /S ) soliton pair. However, once electron correlation effect is taken into account, the negative and positive solitons appear to attract one another to form an exciton. Nevertheless, ex-... 

This process was accompanied by an increase in conductivity from circa 10 to circa 10 S/cm. If the polymer is stretch-oriented five- to sixfold before doping, conductivities parallel to the direction of stretching up to about 10 S/cm can be obtained [15, 19], Approximately 85% of the positive charge is delocalized over 15 CH units (depicted in Scheme 29.1 for simplicity over only five units) to give a positive soliton. 

FIGURE 5.20 Formation from (a) a neutral soliton, (b) a positive soliton and (c) a negative soliton by controlled addition of p- and -doping agents, respectively. 

The halogen doping produces positive polarons on the chains. At the heavy doping level, the density of positive polarons is so high, that the interaction between the polarons will change two polarons into the positive soliton pair(bipolaron), which is more stable than two polarons. 

Figure 4-3. Schematic structures of self-localized excitations in poly( p-phenylene) and trans-po y-acetylene. (a) Positive polaron (b) negative polaron (c) positive bipolaron (d) negative bipolaron (e) positive polaron (f) negative polaron (g) neutral soliton (h) positive soliton (i) negative soli-ton . D, donor A, acceptor -I-, positive charge negative charge , unpaired electron.

Figure 4-5. Schematic electronic band structures, (a) Neutral polymer (b) positive polaron ic) negative polaron (d) positive bipolaron (e) negative bipolaron (f) neutral soliton (g) positive soliton (h negative soliton. CB, conduction band VB, valence band , electron arrow, electronic transition.

When a soliton is formed, a nonbonding electronic level is formed at the center of the band gap (Figure 4-5f-h). For a neutral soliton, the nonbonding electronic level is occupied by one electron, while for a positive soliton and a negative soliton, the level is occupied by null and two electrons, respectively. Thus, all the neutral, positive, and negative solitons are expected to have only one intragap transition, ojs, as shown in Figure 4-5f-h. 

Fig. 11. Schematic representations of (a) neutral soliton, (b) negative soliton, and (c) positive soliton.

Fig. 28.13 Lattice polarization (net charge per CH unit) for (a) positive soliton, (b) negative soliton, (c) positive po-laron, (d) negative polaron in C40H42 from MNDO calculations. (From Ref. 114.)...

For poly (pyrrole), positively charged polarons and bipolarons have already been illustrated above, (Fig. 2-71. Negatively charged polarons and bipolarons can be visualized which are analogous to those of poly(p-phenylene), but in practice they do not exist, as poly(pyrrole) cannot be n-doped stably. For trans-poly(acetylene) (P(Ac)) (Fig. 2-13 ). one may have a neutral soliton and "antisoliton" possessing spin (a), and spinless positive and negative solitons (b) two adjacent, positive solitons would be the equivalent of a bipolaron, which for reasons cited earlier degenerates... 

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