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Monolayer light emitting diodes from poly(1,4‐phenylenevinylene) (PPV) usually exhibit relatively low quantum efficiencies. So the external efficiency of an ITO/PPV/Ca LED is typically 0.01%. In order to increase the quantum yield in bilayer devices, oxadiazole polymers have been used. The syntheses of a number of novel polymethacrylates with pendant oxadiazole groups and some aromatic polyethers with oxadiazole units in the main chain are described. These polymers with the electron withdrawing oxadiazole units facilitate electron injection and transport in bilayer LEDs with PPV as hole transport layer. Thus an LED with a top layer of the polyether 3a exhibits a tenfold increase of the external quantum efficiency to 0.1%. Compared to conventional PPV LEDs, the improved bilayer devices show intense emission at low current densities.

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Detailed synthetic aspects of new aromatic polyethers containing oxadiazole units are described. They are obtained by the nucleophilic displacement reaction of 2,2-bis(4-hydroxypheny1)hexafluoropropane with 2,5-bis(4-fluoropheny1)-1,3,4-oxadiazole and 2,5-bis(4-fluoronaphthyl)-1,3,4-oxadiazole. The purity of the polyethers strongly depends on the reaction solvent. Best results were achieved with Ph2S02 as compared to Nmethyl-2-pyrrolidone (NMP) and 1,3-dimethyltetrahydroro-2( lH)-pyrimidinone (DMPU) in the temperature range from 190-200°C. All polyethers are fully soluble in common organic solvents and excellent films are obtained by spin-coating. High molecular weight polyethers without oligomers were only achieved, when Ph2S02 was used as solvent. All polyethers are characterized by high thermal stability and high glass transition temperatures. The substitution of bisphenyloxadiazole by bisnaphthyloxadiazole units in the polyethers leads to a bathochromic shift of the absorption from 299 nm to 345 nm. Polyethers with bisnaphthyloxadiazole units show a blue fluorescence with a maximum at 427 nm. The application of these novel polyethers as efficient electron transport and injection layer in bilayer light emitting diodes based on poly( 1,4-phenylenevinylene) is shown. The best results were obtained with oxadiazole polyethers containing bisnaphthyloxadiazole units.