ONE advantage of using conjugated polymers in semiconductor applications is that they can be processed using techniques well established for conventional polymers. We reported recently that poly(p-phenylenevinylene) could be used as the active layer in a light-emitting diode1, producing yellow/green emission. We have now found that related copolymers, comprising a combination of different arylene units, can be chemically tuned to provide a range of materials with considerably improved properties for this and other applications. By incorporating two different leaving groups into a precursor copolymer, we can selectively eliminate one of these, to give a conjugated/non-conjugated copolymer, or both, to give a fully conjugated copolymer. This allows us to induce local variations in the Π-Π* electronic energy gap at both the molecular and supramolecular level. Variations at the molecular level can act to trap excitons, hindering their migration to quenching sites, and we find that these materials give strongly enhanced quantum yields for electroluminescence (by a factor of up to 30). They also allow control of the colour of emission. Variations at the supramolecular level, by patterning the films to control the progress of conversion, allow the production of structures suitable for multicolour displays. The ability to pattern the film also allows for fabrication of optical waveguides, as regions with different energy gaps have different refractive indices. © 1992 Nature Publishing Group.