Polyenes, electronic properties

As with carbocation-initiated polyene cyclizations, radical cyclizations can proceed through several successive steps if the steric and electronic properties of the reactant provide potential reaction sites. Cyclization may be followed by a second intramolecular step or by an intermolecular addition or alkylation. Intermediate radicals can be constructed so that hydrogen atom transfer can occur as part of the overall process. For example, 2-bromohexenes having radical stabilizing substituents at C(6) can undergo cyclization after a hydrogen atom transfer step.348... 

As with carbocation-inititated polyene cyclizations, radical cyclizations can proceed through several successive steps if the steric and electronic properties of the reactant 249 250 251 252... 

By introducing fluorine atoms to the polyenic system of retinal, the geometry, electronic properties, hydrophobicity, and absorption properties of the molecule will be modified. Thus, fluoro derivatives of retinal are useful tools to understand the interactions between retinal and opsin, especially on the level of charge and hydrophobic effects at the protein site. Moreover, fluorine atoms are probes in NMR and allow studies on model molecules of visual pigments Consequently, syntheses of mono-, di-, and trifluoro derivatives of retinal have been the subject of many investigations. 

Thus weak disorder may be the main source of localization and conjugation length limitation. The variations in electronic properties such as absorption will then not always follow laws such as those given in Section II.C N 1 variations are well verified for polyenes, but then would not give the correct extrapolation for N — °°. Weak disorder also localizes excitations for instance, photoexcited charge carriers. It will induce the presence of polarons even if they were not bound states of the perfect chain, or of bipolarons, modify the transport properties, and increase the lifetime of charge carriers against recombination [98]. 

We start our discussion with simple concepts from the band theory for solids, discuss what can break the symmetry of one-dimensional systems, introduce electrical conductivity and superconductivity, present the Mulliken charge transfer theory for solution complexes and its extension to solids, then discuss briefly the simple tt electron theory for long polyenes. Other articles in this volume review the detailed interplay between structure and electronic properties of conductors and superconductors [206], and electrical transport in conducting polymers [207],... 

J.M. Andre and B. Champagne, in J.L. Bredas (Ed.), Nonlinear optical hyperpolarizabilities and electronic properties of oligomers and polymers the polyene story, from dyes to conducting and NLO systems, Conjugated oligomers, polymers, and dendrimers From polyacetylene to DNA, Bibliotheque Scientifique Francqui, De Boeck Universite, 1999, pp. 349-394. 

Kohler BE (1990) A simple model for linearpolyene electronic structure. J Chem Phys 93 5838-5842 Kohler BE (1991) Electronic properties of linear polyenes. In Bredas JLand Silbey R(eds) Conjugated Polymers The novel science and technology of conducting and nonlinear optically active materials, pp 405-434. Kulver Press, Dordrecht Kohler BE(1993a) Octatetraene photoisomerization. ChemRev 93 41-54... 

Kohler BE (1991) Electronic properties of linear polyenes. In Bredas JL and Sdbey R, (eds) Conjugated Polymers, pp 405-434. Kluwer Academic Publishers, Dodrecht... 

The existence of alternancy symmetry leads to very strong predictions concerning the electronic properties of the system. For instance, it imposes that the lowest energy excitation is dipole allowed. However, in long polyenes, there is unambiguous experimental evidence for states below the lowest dipole-allowed excited state [17]. The alternancy symmetry also predicts that in polyene radicals (with an odd number of carbon atoms), the spin density should reside on only one sublattice and that the spin density on the other sublattice be strictly zero. However, electron spin resonance experiments indicate the presence of spin density on both sublattices, with the spin projections on the sites of one sublattice being opposite in direction to those of the sites belonging to the other sublattice [18]. 

As noted in the introduction of this chapter, free unsaturated hydrocarbons, such as olefins, polyenes, and arenes, typically react with electrophiles, not nucleophiles. However, coordination to an appropriate electron-deficient metal center alters their electronic properties, as shown in Figure 11.3. Coordination of an olefin to an electron-poor transition metal center causes the coordinated olefin to react with nucleophiles because this coordination leads to a net flow of electron density from the unsaturated hydrocarbon to the metal. 

McMurry coupling of a,j5-unsaturated aldehydes and ketones constitutes an easy route to conjugated polyenes (Figure 6.2), which exhibit interesting electronic properties. After yS-carotene [3], a series of minicarotenes such as 11 has been syn-... 

These findings demonstrate that the term "conjugation-length needs to be applied with some care, this has not always been true in the literature. The electronic properties of the material are influenced by the extent of the straight-chain (defect-free) polyene sequences present but these are n necessarily terminated by defects which "break-conjugation". This point is very important because, as mentioned above, chains terminated by symmetry breaking defects are expected to show different properties to chains which are terminated by defects which do not break bond-alternation symmetry. 

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