Optical properties of cross-linkable fluorene copolymers

Three fluorene-based copolymers carrying reactive side groups (oxetanes) were synthesised and characterised. Their optical properties are essentially controlled by the introduction of either fluorene or lower energy gap (benzothiadiazole or terthiophene) comonomers, randomly distributed along the polymer chain. We show that, in the presence of a small amount of a photoacid dispersed in the polymer films, these are converted into insoluble polymeric networks upon UV irradiation and heating. Absorption and emission spectra of the polymeric networks are similar to those of the starting polymers, showing that the cross-linking, due to the polymerisation of the oxetane groups, does not interfere. This ability to induce their insolubility upon UV irradiation is used to fabricate multi-layer light-emitting diodes. Using a transmission electron microscope grid as a shadow mask we show that micrometer-size patterns can be created.