Two series of novel random polyfluorene copolymers containing quinoxaline units were prepared by stressing the coupling according to Yamamoto. The first series contains 2,3-bis-(4'-tert-butyl-biphenyl-4-yl)benzo[glquinoxaline and the second series 2,3-bis-(4-tert-butyl-biphenyl-4-yl)quinoxaline as energy accepting unit. The copolymers were identified by gel permeation chromatography, infrared spectroscopy, and nuclear magnetic resonance spectroscopy. Thermal properties were analyzed by thermal gravimetric analysis and differential scanning calorimetry revealing amorphous copolymers that are stable up to 430 degrees C. The morphology was investigated using atomic force microscopy. The optical properties in solutions and thin films were analyzed. Furthermore, the thin film electro-optical properties were determined in monolayer polymer light-emitting devices. Single layer devices were built with efficiencies ranging from 0.15 to 2.0 cd/A. For the random copolymers with 5 mol % benzoLglquinoxazoline in the polyfluorene backbone some threefold efficiency enhancement from 1.1 to 3.0 cd/A was achieved by utilizing an ultra thin interlayer of poly(9,9-di-n-octylfluorene-2,7-diyl)-alt-[1,4-phenylene-(4-see-butylphe nylimino)1,4-phenylenel between PEDOT:PSS and the emissive random copolymer layer. (c) 2007 Wiley Periodicals, Inc.