Enhancement of color purity in blue-emitting fluorene–pyridine-based copolymers by controlling the chain rigidity and effective conjugation length

A series of high molecular weight, readily soluble copolymers of 9,9-dioctylfluorene with pyridine (less than or equal to 50 mol%) are synthesized by Suzuki polycondensation. Copolymers emit blue light and exhibit high PL efficiency. PL efficiencies show the maximum at 3,5-pyridine content of around 30 mol% in the copolymer. With further the increase of meta-linkage contents, PL efficiencies decrease rapidly to15% for alternating copolymer. Cyclic voltammetry investigation reveals that LUMO levels of copolymers increase with the increase of pyridine content. The introduction of pyridine unit at 3,5-position into polyfluorene backbone significantly depresses the excimer formation. The intensity of excimer emission decreases with the increase of 3,5-pyridine contents. Narrow and pure blue EL emission is obtained for copolymer with pyridine content of 40 mol%. External quantum efficiency is moderately high (0.4–0.5%) for such a pure blue emitter. The threshold voltages of devices from copolymers with the pyridine content of less than 40 mol% are low in the range of 5–6 V based on the device configuration: ITO/PEDOT/polymer/Ba/Al. The results indicate that fluorene-co-3,5-pyridine copolymers are promising blue-emitting electroluminescent materials.