Polyene backbone

Owing to their chemical structure, carotenes as polyterpenoids are hydrophobic in nature (Britton et al., 2004). Therefore, as it might be expected, the carotenes are bound within the hydrophobic core of the lipid membranes. Polar carotenoids, with the molecules terminated on one or two sides with the oxygen-bearing substitutes, also bind to the lipid bilayer in such a way that the chromophore, constituted by the polyene backbone is embedded in the hydrophobic core of the membrane. There are several lines of evidence for such a localization of carotenoids with respect to the lipid bilayers. 

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 photoisomerization of several indole derivatives has been studied while isomerization around the C=N double bonds of N-methoxy-l-(l-pyrene)meth-imine has been described. In certain cases, the reversibility of the isomerization step depends on the spin multiplicity of the reacting state." Several reports have been concerned with the photophysical properties of cyanine dyes promoted into upper excited states. Indeed, for a series of cyanine dyes having different polyene backbones it was possible to establish that isomerization takes place... 

Figure 28 shows the UV—visible spectra of monomer 72. poly-76, and copolymers 77—80. The monomer spectrum was taken in CHCI3. and the polymer spectra were taken in spin-coated films on a quartz substrate. Compound 75 shows the characteristic broad band at 540 nm due to the tt—jt electronic transition of the conjugated cyclic polyene backbone. Copolymers 77—82 containing a chromophore show two maximum absorption values around 390 and 550 nm due to a pendant chromophore and conjugated cyclic polyene backbone, respectively. 

The electronic structure of polyacetylenes [PA called also polyenes, (CH) t, (C2H2) c] has been a subject of interest for several decades in theoretical chemistry owing to the central role of the polyene backbone in... 

In the absence of metal cations (i.e. Ba " ), the pendants present conformational equilibria in which a certain form (synperiplanar 1, spl) predominates. This conformational preference of the pendants is transmitted to the polyene backbone, which adopts the most stable form [i.e. left-handed in the case of poly-(S )-l, Fig. 2]. 

Once prepared [14], the two enantiomeric MPA polymers, poly-(f )-2 and poly-(5)-2, showed null CD spectra in a number of solvents, suggesting the presence of analogous populations of both helical senses. Thus, despite the presence of stereogenic centres at the pendants, the resulting polymer was racemic in its axial chirality. NMR, Raman and differential scaiming calorimetry (DSC) [15-18] studies pointed to cis-cisoid configurations at their polyene backbones. 

The values obtained from the external crests of the single chains implied internal angles for the polyene backbone close to +75° and 75° for sp and ap conformations, respectively, and explained the P and M chirality of the corresponding backbones. 

AFM and MM studies showed that these polymers in CHCI3 presented identical handedness for the internal (polyene backbone) and the external (pendants) helices (3/1 helix), whereas in THF the internal and external helices (2/1 helix) presented opposite helical senses. DSC traces supported the cis-cisoidal and cis-transoidal helical structures associated with those structural features. 

Isomerism along the backbone of cw-poly(arylacetylene)s can arise from the different dihedral angles that are possible about the C-C bonds of the main chain. The transoid (s-trans) conformation is more extended, whereas the cisoid (s-cis) conformation is more compact and both are helical [126-129]. Degradation of the main chain structure occurs through cyclization reactions of the polyene backbone [126-129, 137-142], which require the polymer to adopt a cis-cisoidal conformation. Scheme 3 illustrates the stmctural transformations that degrade the polymer backbone. 6jr-Electrocyclization of triene segments in the backbone occurs under... 

Polyacetylene remains one of the most interesting conducting polymers. While this is in part due to the very high conductivities that can be achieved by lightly oxidizing stretched polyacetylene films with molecular iodine, this interest also reflects the expectation that, because of the simplicity of the polyene backbone, the link between electronic structure and transport can be most easily worked out for this system. It is in this sense that polyacetylene is a paradigm system for conducting polymers. 

Polyacetylene-based pendant polyradicals 14 were reported in late 1980s and early 1990s, as summarized in a recent review by Nishide and Kaneko (42). In polyacetylenes, connectivity of non-Kekule r-system compatible with ferromagnetic coupling may be obtained by the attachment of conjugated pendant radical at the every other carbon of the polyene backbone (Fig. 6). Molecular... 

A good example of a material that pyrolyses by unzipping is PVC. PVC first undergoes a loss of hydrogen chloride to form a conjugated polyene backbone. This unsaturated... 

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