Polyacetylenes Polyenes

in Recent Advances in the Quantum Theory of Polymers , ed. J.-M. Andre, J.-L. Bred as, J. Delhalle, J. Ladik, G. Leroy, and C. Moser, Springer-Verlag, Berlin, 1979, p. 271. 

The overwhelming majority of theoretical works on polyenes has been concentrated on the gap-problem. The majority of authors based their investigations on a one-electron picture (band theory), while a smaller group stressed the importance of many-electron (correlation) effects. It has been shown that, in fact, the two mechanisms can be active simultaneously (for a recent review of this field see ref. 15). 

We briefly summarize here the main results of ab initio investigations of five 

Comparable errors are also contained in calculations in which the interactions between distant cells are cut off too early since in this case the partial summation of the two long range interactions (which cancel each other accurately only for infinite distance) makes the results inaccurate. 

Finally, it must be mentioned that the above discussion refers only to pure polyacetylene samples. According to the results of calculations on halogen-doped polyacetylenes87 the presence of impurity atoms not only significantly changes the band structures but may completely reverse the conformations. 

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Hartree-Fock Calculations on c/s- and trans-Polyacetylenes (Polyenes)... 

Conveniently applied to obtain monodisperse block copolymers of polyenes, the second process, the so-called Durham route, uses presently several precursor monomers and catalysts [63, 64]. Diblocks and triblocks, each containing one or two sequences of poly-norbornene and one or two sequences of a precursor polymer that converts to polyacetylenes (polyenes)... 

A common example of the Peieds distortion is the linear polyene, polyacetylene. A simple molecular orbital approach would predict S hybddization at each carbon and metallic behavior as a result of a half-filled delocalized TT-orbital along the chain. Uniform bond lengths would be expected (as in benzene) as a result of the delocalization. However, a Peieds distortion leads to alternating single and double bonds (Fig. 3) and the opening up of a band gap. As a result, undoped polyacetylene is a semiconductor. 

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 ... 

The possibility that the results obtained for short polyenes can be extrapolated to longer polyenes and to polyacetylene has been discussed by several authors21,24,31 37. 

When the 7r-systerns of two or more double bonds overlap, as in conjugated dienes and polyenes, the 7r-clccIrons will be delocalized. This has chemical consequences, which implies that the range of possible chemical reactions is vastly extended over that of the alkenes. Examples are various pericyclic reactions or charge transport in doped polyacetylenes. A detailed understanding of the electronic structure of polyenes is therefore of utmost importance for development within this field. We will first discuss the structure of dienes and polyenes based on theoretical studies. Thereafter the results from experimental studies are presented and discussed. 

Figure 29 raises the question of how the energies of these two excited states evolve as one goes to longer polyene chains, in analogy to those found in polyacetylenes which become conductive upon oxidative doping (= ionization) or photoexcitation. 

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. 

Unlike polyacetylene, substituted polyacetylenes are amorphous, electrically insulator soluble polymers.413 They are highly stable and not sensitive to oxidation. Since the substituents exert a strong steric effect, the polyene backbone is not copla-nar, and as a result, only limited conjugation is possible. 

The films could be doped with I2, FeCl3 or BF3 to conductivities of the order of 10 -3 S cm 1, much lower than polyacetylene. Spectroscopy suggested that the average length of the polyene sequence was around 10C = C units and the authors suggest that this is due to the random initiation of an unzipping reaction at many points on... 

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