Polyphenylene-type emissive materials:: Poly(para-phenylene)s, polyfluorenes, and ladder polymers

This chapter reviews the synthesis of the various classes of polyphenylene-based materials that have been investigated as active materials in light-emitting applications. In particular, it is shown how the electronic properties may be controlled by synthetic design. Insoluble poly(para-phenylene) can be made by a variety of precursor routes. Attachment of solubilising side chains gives soluble polyphenylenes in which a high degree of torsion between adjacent benzene rings produced by steric interactions between the substituents strongly reduces their electronic interaction. The phenylene units can be made coplanar by bridging them with methine or other bridges to produce ladder-type polymers, which show excellent photophysical properties, but strong intermolecular interactions lead to problems in obtaining blue emission. Similar problems are seen for,stepladder' polymers such as polyfluorenes with only partial bridging of the phenylene rings. These interactions may be controlled by introduction of bulky substituents. The electroluminescence efficiency of these materials can also be enhanced by use of charge-transporting substituents. Copolymerisation with lower-band-gap units enables tuning of the emission colour across the entire visible range.