Trans -poly acetylene

FIGURE 9. Chemical structures of poly acetylene (a) trans-transoid (trans) (b) cis-transoid (cis)... 

The doped poly(acetylene) forms various junctions such as a) a p-n junction from p- and n- -f CH, b) a hetero-Schottky junction from the inorganic semiconductor and metalic -f CH-, and c) a heterojunction from the inorganic semiconductor and semiconducting The bandgaps of -(-CH (trans- 0.6 eV, cis- 0.9 eV)... 

The 2000 Nobel Prize in Chemistry was awarded for work on poly acetylenes. Acetylene can be polymerized using a Ziegler-Natta catalyst. The cis or trans stereochemistry of the products can be controlled by careful selection and preparation of the catalyst. The resulting polyacetylene is an electrical semiconductor with a metallic appearance. cw-Polyacetylene has a copper color, and frawi-polyacetylene is silver. 

Figure 7 Proton NMR signal enhancement as a function of the electronic pumping frequency for trans-rich (solid circle), and cw-rich (solid square) poly acetylene samples.

Figure 4.8-2 Optical absorption of conjugated polymers with a degenerate ground state (trans-poly(acetylene)) (a), according to Suzuki et ah, 1980 and a non-degenerate ground state (poly(thiophene)) (b), according to Danno et ah, 1993 in various doping states. Doping concentrations are indicated in % in (a) and by the applied potential in (b).

Figure 4.8-4 Raman spectra of trans-poly(acetylene) excited by different laser lines, according to (Itnhoff, 1983) (a), and a schematic representation of the dispersion effect in conjugated polymers. The continuous and the dashed arrows, respectively, refer to a red and a blue laser (b).

Molecular orbital calculations on fluorinated butadienes and hexatrienes were used to model the effects of fluorination on the properties of poly(acetylene). Like poly(acetylene), "head-to-head" poly(fluoro- acetylene), (-CH=CF-CF=CH-), is predicted to adopt a planar, all trans structure, but poly(difluoro-acetylene) favors a non-planar skewed chain conformation. "Head-to-tail" poly(fluoroacetylene), (-CH=CF-CH=CF-) is predicted to favor a nearly planar cis structure stabilized by intramolecular CF-HC hydrogen binding. Calculations on 2-fluoroethanol and on both 2-fluoroacetaldehyde enol and its alkali metal (Li, Na, K) enolates reveal moderately strong intramolecular CF—HO hydrogen bonds(1.9 and 3.2 kcal/mol, respectively) and even stronger intramolecular coordination of CF to alkali metal cations (9-12 kcal/mol). 

As shown in Table 3, cis- and trans-coni nU of poly acetylene prepared by the Ziegler-Natta catalysts depends strongly upon the polymerization temperature ss x ere are two possible explanations for this observation One is that the fundamental mechanism is the formation of cis double bonds by the cis insertion of acetylene monomer into the Ti—C bond of the catalyst. This fits the orbital interaction consideration for the role of the catalyst by Fukui and Inagaki, according to which the initially formed configuration of the double bond is cis as a result of the favorable orbital interaction between the inserting acetylene monomer and the active site of the catalyst. Because the cis double bond is thermodynami-... 

Fig. 5. Electronic energy vs. dimerisation coordinate u in trans-poly acetylene the two equal minima correspond to the A and B structures. The top of the barrier C would correspond to the undimerized chain C.

C/5-polyacetylene is very different from trans-poly-acetylene, as far as solitons are concerned. Certainly the band structure is more complicated because there are four atoms in an elementary cell along the chain instead of two. But the crucial difference is related to symmetry. In Figure 1.31 a soliton in trans-polyacety-lene is set in contrast to one in cw-polyacetylene. In rra 5-polyacetylene there is a mirror plane at the soliton, but not in cis. In cw-polyacetylene a soliton separates a cw-transoidal domain from a tran -cisoidal. The latter is richer in energy and the soliton is pushed... 

C/ -poly(acetylene) is relatively unstable and reverts to the thermodynamically stable tr<2/7 -poly(acetylene) via the metastable trans-cisoid form. 

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