N-polyacetylenes

Finally, theoretically insight into the mechanisms of charge transport should continue to improve. This will happen in concert with the development of better experimental probes of the solid state. Better understanding of the structural origins of the exceptional properties of N-polyacetylene is needed, and such work may point the way toward the syntheses of new families of materials. 

Another important advance was the modification of the polymerization conditions, e.g. using silicone oil or other viscous media. For instance, (CH) can be polymerized at room temperature to yield a new N-polyacetylene of at least the same quality as the standard S-(CH)jc obtained at — 78°C. Table 3.1 contains the results [18]. 

Fig, 11-17 TGA of (A) Z-N polyacetylene, (B) polyacetylene prepared using AsF, and (C) polyacetylene prepared using AsFj/AsFj. After Reference [405], reproduced with permission. 

The discovery of junipal focused the attention of Sorensen, who had been investigating the occurrence of polyacetylenes in Com-positae, on the possibility that these acetylenes were accompanied by thiophenes. From Coreopsis grandiflora Hogg ex sweet, 2-phenyl 5-(1-propynyl) thiophene (240) was isolated and its structure confirmed by synthesis of the tetrahydro compound, 2-phenyl-5-n-propyl-thiophene. From the root of tansy, the cis and trans isomers of methyl 5-(l-propynyl)-2-thienylacrylate (241) have been isolated. The total synthesis of trans (241) was achieved by reacting junipal with methylcarbethoxy triphenylphosphonium bromide (Wittig reaction) Several monosubstituted thiophenes, (242), (243), and... 

In the context of fra/u-polyacetylene cjia and c are, respectively, the creation and annihilation operators of an electron with spin projection a in the n-orbital of the nth carbon atom (n= l,...,N) that is perpendicular to the chain plane (see Fig. 3-3). Furthermore, u is the displacement along the chain of the nth CH unit from its position in the undimerized chain, P denotes the momentum of this unit, and M is its mass. 

Acetylene (ethyne), C2H2, can be polymerized, (a) Draw the Lewis structure for acetylene and draw a Lewis structure for the polymer that results when acetylene is polymerized. The polymer has formula (CH), where n is large, (b) Consider the polymers polyacetylene and polyethylene. The latter has the formula (CH2)W and is an insulating material (plastic wrap is made of polyethylene), whereas polyacetylene is a darkly colored material that can conduct electricity when properly treated. On the basis of your answer to part (a), suggest an explanation for the difference in the two polymers. 

Figure 1 Evolution as a function of the number of unit cells (N) of the longitudinal polarizability per unit cell, AoIl(N) = l(N) - 0[l(N-1) for prototypes of saturated (polyethylene) and unsaturated (polyacetylene) polymers.

Itoh, A., Sasaki, K., Mizukami, H., Ohashi, H., Sakurai, T. and Hiraoka, N. 1997. Geographical variation in the furanocoumarin and polyacetylenic compound compositions of wild Glehnia littoralis plants. Nat. Med. 51 50-55. 

Figure 13 shows the irreversible conversion of a nonconjugated poly (p-phenylene pentadienylene) to a lithiun-doped conjugated derivative which has a semiconducting level of conductivity (0.1 to 1.0 S/cm) (29). Obviously, the neutral conjugated derivative of poly (p-phenylene pentadienylene) can then be reversibly generated from the n-type doped material by electrochemical undoping or by p-type compensation. A very similar synthetic method for the conversion of poly(acetylene-co-1,3-butadiene) to polyacetylene has been reported (30), Figure 14. This synthesis of polyacetylene from a nonconjugated precursor polymer containing isolated CH2 units in an otherwise conjugated chain is to be contrasted with the early approach of Marvel et al (6) in which an all-sp3 carbon chain was employed.

The one-dimensional chain of hydrogen atoms is merely a model. Flowever, compounds do exist to which the same kind of considerations are applicable and have been confirmed experimentally. These include polyene chains such as poly acetylene. The p orbitals of the C atoms take the place of the lx functions of the H atoms they form one bonding and one antibonding n band. Due to the Peierls distortion the polyacetylene chain is only stable with alternate short and long C-C bonds, that is, in the sense of the valence bond formula with alternate single and double bonds ... 

Because the extension of the polaron in polyene radical cations is finite (10-20 double bonds depending on the type of calculation), its electronic structure is independent of the number of double bonds attached to either side of it, so that the two lines in Figure 29 must bend at some point to meet the abscissa horizontally, as indicated by the dashed curves. Apparently, the point of inflection has not been reached for n = 15, but it is of interest that the curve for the first excited state could well extrapolate to 0.35 eV, which happens to be where the absorption of a polaron in polyacetylene has been observed300. If this is true, a second, more intense absorption band should occur between 0.5 and 0.7 eV, but the corresponding experiments have not yet been carried out. 

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

A chemical reaction occurs above 1.5 GPa The sample turns black, new peaks develop in the Raman spectrum, and the absorption edge moves below 11,000cm. The recovered material has an optical band gap of 1.39eV, smaller than the band gap of polyacetylene. From the analysis of the Raman spectrum, it is seen that the C=C stretching mode completely disappears in the reaction product, while the C=N stretching band is present but at a different frequency than in cyanocetylene. In addition, the Raman bands of polyacetylene are observed with their characteristic frequency dependence on the wavelength... 

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