Polyacetylene soliton

Double bonds can also be broken by excitation with light and thus solitons can be photogenerated. Photoexcitation is more likely to create charged solitons than neutral solitons and photoconductivity experiments might be a good way to test this concent. Several such experiments have been carried out(Fig.17).They show that photoconductivity is more pronounced in trans than in cis polyacetylene. In cis-polyacetylene solitons are unstable because of symmetry reasons (lattice forces will drive solitons to the chain ends (Fig. 25)) and so these experiments seem to support the soliton theory. 

An alternative interpretation for the activated behavior of the photocurrent and the PIA-decrease with temperature was proposed by Townsend et al. [35], They assigned their experimental results to a thermally activated interchuin-hoppmg mechanism for bipolaron-like charged soliton pairs, the experiments of which were carried out on Durham /ran.v-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. 

The electronic band structure of a neutral polyacetylene is characterized by an empty band gap, like in other intrinsic semiconductors. Defect sites (solitons, polarons, bipolarons) can be regarded as electronic states within the band gap. The conduction in low-doped poly acetylene is attributed mainly to the transport of solitons within and between chains, as described by the intersoliton-hopping model (IHM) . Polarons and bipolarons are important charge carriers at higher doping levels and with polymers other than polyacetylene. 

Fig. 1. Possible structures for polyacetylene chains showing the two degenerate trans-structures (a) and (b), and the two non-degenerate cis-structures, (c) cis-transoid and (d) trans-cisoid and (e), a soliton defect at a phase boundary between the two degenerate trans-phases of polyacetylene, where the bond alternation has been reversed.

Although the conductivity of polyacetylene is generally discussed in terms of solitons, the question of the precise nature of the major charge-carriers continues to be a subject of debate, with conflicting evidence from different experiments. Spectro-electrochemical studies provide evidence that the charge in doped polyacetylene is stored in soliton-like species (although this is not the only possible interpretation [142, 143]), with absorptions in the optical spectra corresponding to transitions to states located at mid-gap [24,89, 119]. The intensity of the interband transitions... 

FIGURE 57. Coupled soliton modes in n-doped polyacetylene. Reprinted with permission from Reference 66. Copyright (1990) American Chemical Society... 

The degeneracy of the ground state of polyacetylene influences its charge distribution. In fact, upon doping the charges, which in other polymers, such as the heterocyclics, would pair to form bipolarons, are here readily separated to form two positively charged solitons ... 

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 6.39 Isomerization of cis (A) and trans (B) sequences in polyacetylene that meet to form a soliton. Reprinted, by permission, from J. M. G. Cowie, Polymers Chemistry Physics of Modem Materials, 2nd ed., p. 417. Copyright 1991 by J. M. G. Cowie.

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) ... 

Many phenomena such as dislocations, electronic structures of polyacetylenes and other solids, Josephson junctions, spin dynamics and charge density waves in low-dimensional solids, fast ion conduction and phase transitions are being explained by invoking the concept of solitons. Solitons are exact analytical solutions of non-linear wave equations corresponding to bell-shaped or step-like changes in the variable (Ogurtani, 1983). They can move through a material with constant amplitude and velocity or remain stationary when two of them collide they are unmodified. The soliton concept has been employed in solid state chemistry to explain diverse phenomena. 

Solitons are considered to be important defect states in these conjugated polymers (see Fig. 6.48). It has however been shown that correlation energy is the more important factor in giving rise to the energy gap in (CH) (Soos Ramasesha, 1983). Other polymers related to polyacetylene are polythiophene, polypyrrole, poly-phenylenesulphide, and polyparaphenylene (Section 3.3). Extensive measurements on doped polyacetylenes have been reported in the last five years and these materials, unlike other conducting polymers such as (SN), seem to have good technological potential. 

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.)... 

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