ELECTRONIC PROPERTIES OF LINEAR POLYENES

BRYAN E. KOHLER Chemistry Department University of California Riverside. CA 92521-0403 U.SA, 

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. 

In its undoped form polyacetylene is a disordered molecular material consisting of discrete linear polyene segments figure 1). It appears that although the molecular weight 

By the 1960 s it appeared that the electronic structure of the linear polyenes, arguably the simplest conjugated organic molecules, was well understood. In 1972 we reported the 

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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... 

ELECTRONIC PROPERTIES OF LINEAR POLYENES 7.4. RELAXATION ENERGY... 

Linear conjugated polyenes (LCP) and polycyclic aromatic hydrocarbons (PAH) as well as their ions are key reactants in many chemical processes or can be identified as transient species. They have rich and broad electronic spectra, ranging from the ultraviolet to the visible and near-infrared [139,140]. The electronic spectra of linear conjugated polyenes have served as tests for computational chemistry ever since the first semiempirical calculations were performed and have been discussed extensively in literature. However, it was not until recently that accurate, theoretical predictions of excited-state properties were attained by ab initio calculations [36,141]. 

Springborg and Kirtman have investigated the impact of donor/acceptor substitutions on the linear and nonlinear response properties of long polyenes and have demonstrated that, for sufficiently long chains, the responses per unit of a push-pull system becomes independent of the donor and acceptor groups. This conclusion, which concerns both the electronic and structural/geometrical relaxations, implies that, in that case, material properties cannot be improved upon substitution. Their predictions have been further illustrated and analyzed by adopting a Huckel like model. 

Firstly it provides a rather convenient mental picture of delocalized jr-electrons and the nodal properties of their wave functions. Secondly, it is a convenient way of remembering the number and position of the nodes in the 7r-orbitals of linear polyenes. The wave functions obtained by the HMO and FEM methods are shown in Fig. 1.31. 

In Chapter 3, the Hiickel model of linear and closed polyene chains is used to explain the origin of band structure in the one-dimensional crystal, outlining the importance of the nature of the electronic bands in determining the different properties of insulators, conductors, semiconductors and superconductors. 

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