Linear polyfluorenes with low generation of side benzylether dendrons (PLG0, PLG1) and hyperbranched polyfluorenes with 1,3,5-benzene branch unit (PHG0 and PHG1) were prepared by the Suzuki coupling reaction to investigate the structural effect on optoelectronic properties. Their optical propel-ties, both in solution and film state, were investigated using absorption and photoluminescence (PL) spectra. The excimer emission of polyfluorene at about 530 nm, induced by thermal annealing, was completely suppressed by the hyperbranched structure, but the suppression was not obvious by the side benzylether dendrons. The optoelectronic performance of the EL devices (ITO/PEDOT:PSS/polymer/Ca/Al) was strongly dependent upon chemical structures of the emitting polyfluorenes. The hyperbranched PHG0 with zero generation of benzylether side groups revealed the best device performance, with maximal luminance and maximal luminance efficiency of 2350 cd/m(2) and 0.33 cd/A, respectively. The results suggest that incorporation of branch units with low generation of benzylether dendrons is an effective way to improve annealing stability and EL performance of the polyfluorenes. (c) 2008 Wiley Periodicals, Inc.