Polyacetylene chemical structure

Rg. 1 Pyrrole, thiophene, aniline and ftiran chemical structures (A), c/s-polyacetylene chemical structure and the two limiting mesomeric forms, aromatic and quinoid, of the heteroaromatic conjugated conducting polymers (B). 

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). 

The structure/property relationships that govern third-order NLO polarization are not well understood. Like second-order effects, third-order effects seem to scale with the linear polarizability. As a result, most research to date has been on highly polarizable molecules and materials such as polyacetylene, polythiophene and various semiconductors. To optimize third- order NLO response, a quartic, anharmonic term must be introduced into the electronic potential of the material. However, an understanding of the relationship between chemical structure and quartic anharmonicity must also be developed. Tutorials by P. Prasad and D. Eaton discuss some of the issues relating to third-order NLO materials. 

Fig. 4.1 The chemical structures of several relevant polymers are illustrated. There is a carbon atom at each vertex, and the hydrogen atoms are not shown. PE(CHf polyethylene, or PE, with only the C-H single bonds shown PA trans -polyacetylene PPV poly(p t -phenylenevinylene) and PPP poly(pita -phenylene). The lower three polymers are conjugated, according to the alternating single and double bond system.

In the case of a polymer with a saturated chemical structure, such as polyethylene, the strength of a-bonding is such that the band gap will be comparable to that in diamond. However, for a polymer with a conjugated structure, such as polyacetylene, the chemical binding of the re-electrons is much weaker and a gap of a few eV, comparable to those in inorganic semiconductors, is anticipated. 

Fig. 4.15 Chemical structures of trarcs-polyacetylene (a) with equal bond lengths, (b) bond alternation and (c) a bond alternation defect.

Fig. 9.8 Chemical structure of tra/w-polyacetylene with (a) equal bond lengths and (b) bond alternation.

The simplest polymer with a conjugated backbone is polyacetylene. Its structure is similar to that of the saturated polymer polyethylene, but has one of the hydrogen atoms removed from each carbon of the polyethylene chain. Each carbon atom in the polyacetylene chain thus has one excess electron which is not involved in the basic chemical binding. And if the separation of the carbon were constant, polyacetylene would conduct along the chain in other words it would behave like a metal in one dimension. But unfortunately this is not true as the free electrons tend to get localized in shorter double bonds. Conjugated polymers can at best be expected to display semiconducting properties. 

Conjugated polymers differ from saturated polymers in that each carbon of the main chain is bonded to only three other atoms. The classic example is polyacetylene, (CH) , in which each carbon is cr-bonded to only two neighboring carbons and one hydrogen atom. The chemical structures of polyacetylene and some of the other most commonly studied conjugated polymers are shown in Fig. 

Trans-polyacetylene, tra 5-(CH) was the first highly conducting organic polymer [1,2]. The simple chemical structure, -CH- units repeated (see Fig. IVB-la), would imply that each carbon contributes a single p electron to the tr-band. As a result, the rr-band would be half-filled. Thus, based upon this stmcture, an individual chain of neutral polyacetylene would be a metal since the electrons in this idealized metal could move only along the chain, polyacetylene would be a one-dimensional (Id) metal. However, experimental studies show clearly that neutral polyacetylene is a semiconductor with an energy gap greater than 1.5 eV. Rudolf Peierls [86] showed many years ago that Id metals are... 

F re 1 Chemical structure of tians-polyacetylene and a simplified orbital picture showing the p- orbitals sticking out of the plane formed by three a-bonds. Only two of these, namely those connecting neighbouring carbon atoms, are shown. There is an additional a-bondper carbon to a hydrogen atom (not shown)... 

Billaud et al. [99] have also recently proposed a distorted hexagonal structure (actually monoclinic) for highly oriented polyacetylene chemically doped with lithium to y = 0.16. 

Unfortunately, even for the simplest and most studied case, the polyacetylene film, there is not a homogeneous network [5]. The mixing of the amorphous and the crystalline part makes the average properties observed, much more difficult to interpret. Not only does the very complex structure of the conducting polymer films produce scattered data for the conductivity, but the spectroscopic data are often dependent on the packing and chain conformation. As a consequence, the electronic properties of conducting polymer films may vary from one sample to another. Therefore, a major difficulty arises in deciding whether or not the difference observed was as a result of the chosen chemical structure and polymerisation route or of the way the molecules were packed. 

Figure 6.26 shows the chemical structure of PPV. Block letters from A to F show the eight carbon sites in one PPV monomer unit. The principal axes of proton hyperfine coupling of an unpaired tt-electron, defined in Figure 6.18, arc also shown for two proton sites of inequivalent bond orientations. ESR spectra of stretch-oriented undoped PPV films have shown anisotropic lineshapes with an average g value of 2.003 and a spin concentration of about 1 spin/10 PPV monomer units. The observed anisotropic behaviour of the. g value and the linewidth, both larger for the stretch direction, are qualitatively similar to those found in stretch-oriented polyacetylene films mentioned in Section 3.2 and... 

The pristine conjugated polymers have been reported to contain electronic spins, presumably originating from inter-chain cross-linking in polyacetylene [25,26], formation of polynuclear structures in polypara-phenylene [27] and so on. The inter- and intra-chain reactions between these reactive sites can alter the chemical structure of conductive polymers even when they are pure, affecting their dopability and hence the electroactivity. 

Polyacetylene has a simple chemical structure, and the addition of small amounts of various dopants (both acceptors and donors) dramatically changes its electrical, optical, and magnetic behavior. The acetylene was first polymerized to a linear zr-conjugated polymer by Natta et al. using a Ti(0—Pr)4/Et3Al in 1958. ... 

A number of publications dealing with chemical, physical, and electrical properties of polyacetylene have appeared in the past 4 decades.But. the characterization of polyacetylene has not been fully investigated owing to its insolubility and infusibility, and the applications of polyacetylene have been restricted due to poor functionality peculiar to its simple chemical structure. 

The publication by Chiang et al. [1] led to a huge surge in interest in synthetic metals. In less than a decade, most of the monomer building blocks that we know today had been identified and many procedures for polymeric synthesis had been established. The chemical structures are illustrated in Figure 1.1. (In the nomenclature used in Figure 1.1, polyacetylene would be called polyvinylene. This is because some - common - names derive from the compound that is polymerized, while others, more correctly according to lUPAC conventions, use the monomeric unit in the product polymer.)... 

Figure 1.1 Chemical structures for monomers of the conjugated polymers discussed in this chapter (a) vinylene (the repeat unit of polyacetylene) (b) ethynylene (polydiacetylene is alternating vinylene-ethynylene) (c) phenylene (d) thiophene (e) leucoemeraldine form of polyaniline (f) pernigraniline form ofpolyaniline (the number of protons can vary between the...

Chart 2.2 Chemical structures of solitons formed in tra s-polyacetylene. 

Figure 8.1 Chemical structures of (a) polyacetylene, (b) polythiophene, (c) polypyrrole, and (d) polyanUine.

Thiophene A and thiophene A diol are the major polyacetylenes isolated from the hairy root of Ambrosia maritima L. (Asteraceae) [293]. Their chemical structures were determined by mass spectroscopy and using DEPT 135, HMQC (heteronuclear multiple quantum coherence), and HMBC (heteronuclear multiple bond coherence) NMR experiments (Figure 5.79). 

The many unusual properties of trans-polyacetylene have already been described in this book in the context of understanding the roles of electron-electron and electron-lattice interactions in 7r-electron models. These have been described in Chapters 4 and 7, in particular. In this chapter we describe some experimental observations and show how these are explained within a framework of correlated electrons with strong electron-lattice coupling. The chemical structure of trans-polyacetylene is illustrated in Fig. 1.1. 

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