Two fluorene-based copolymers (PF-33F and PF-50F) with p-difluorophenylene units in the backbone were synthesized. In comparison with the reference poly(9,9-dioctylfluorene) (PFO), the introduction of p-difluorophenylene units not only increased the fluorescent quantum yields, but also improved the spectra purity and stability of these deep blue emitting copolymers. The famous green emission band at 520 nm from fluorenone defects was never detected for these copolymers even after they were thermal annealed in air at 150 degrees C. Organic light-emitting diodes were fabricated using them as emitting layer and pure blue electroluminescence was obtained. It was observed that PF-33F based device exhibited much higher current density and brightness than PF-50F and PFO devices. A maximum external quantum efficiency of 1.14% (1.14 cd A(-1)) and the CIE (0.16, 0.13) were achieved for PF-33F device, which are among the best performance for polyfluorenes reported so far. (C) 2012 Elsevier Ltd. All rights reserved.