Optical properties electrically active polymers

This section describes the use of oligomer films as active media and demonstrates the importance of the film morphology to both the electrical and the optical properties. This is a factor which has hardly received any attention until now, but one which certainly deserves consideration, for polymers as well. The crucial importance of the microslructurc is equally demonstrated by studies of the photovoltaic effect 1118J this topic will not be dealt with here. 

Conjugated polymers, including optically active polymers and dendronized polymers that are very useful in electrical and optical fields and asymmetric catalysis, will continue to attract interest from chemists and materials scientists. It is well anticipated that more and more polymers with interesting structures and properties will be synthesized from the transition metal coupling strategy. 

Considerable interest has been focused on both polymer-polymer mixtures and mixtures of polymers with low molecular weight solvents. Polymer-solvent systems have been extensively investigated in the solution range where the mixture contains an appreciable amount of solvent. Much less attention has been devoted to systems containing only a small amount of solvent which is then usually called additive. These systems are interesting from both an academic and industrial point of view. In fact, additives are widely used to affect the mechanical, thermal, electrical and optical properties of polymers. The mechanism by which additives become active is important not only because it enables us to understand the properties of the mixture, but also frequently because of its relevance to the pure polymer component. 

PAVs have emerged as one of the most important classes of conjugated polymers. Ironically, while their electrical properties, which were the original motivation for their study, have proved disappointing, their electro-optical properties are such that they comprise the active material in the first commercially available polymer-based LEDs, and they have been used in some of the best performing polymer-based solar cells yet tested. While considerable development remains before polymer-based electronic devices can represent more than a tiny share of the electronics market, there is every reason to believe that when they do, PAVs will form a significant fi action of the materials used. 

It was also observed that conjugated polymers that are also electrical conductors (see Chap. 10) exhibit optical activity that depends critically on their structural organization [78]. Thus, strong chiroptical properties can be obtained firom substituted polythiophene [79] (Chap. 10) with optically active side chains, especially when the monomers are coupled within the polymer in a regioregular head-to-tail fashion. Actually, optical activity of these materials is only found when the polymers are aggregated at low temperature, in poor solvent, or in solution cast films. This contrasts with other optically active polymers, like polypeptides, poly(l-alkynes) and polyisocyanates that show an optically active conformation of the main chain in the absence of supramolecular association. 

Most of the reported conjugated polymers with nonlinear optical or electrical activities are carbon-carbon conjugated systems, such as the extensively studied polyenes (e.g., polyacetylenes [15], polydiacetylenes [16,17], poly-/ -phenylenes) and heteroaromatic polymers (e.g., polypyrroles, polythiophenes, polycarbazoles, polyanilines) [18]. The synthetic methods, applications, and the structure-property relationships of these polymers have also been substantially investigated [19-21]. 

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