Using a new aromatic 1,2,4-triazole branching monomer (4.8-13.3 mol %), three hyperbranched polyfluorenes (P2-P4) were synthesized by the Suzuki coupling reaction to investigate the structural effect on optoelectronic properties. Poly(9,9-dihexylfluorene) (P1) was also prepared for comparative investigation. Their weight-average molecular weights and polydispersity indices are in the range of 1.16 X 10(4) to 5.9 X 10(4) and 1.49-2.25, respectively. Optical properties, both in solution and film state, were investigated using absorption and photoluminescence (PL) Spectra. In film state, the absorption and PL spectra peaked at 377-392 and 424-425 nm, respectively, blue-shift with increasing triazole concentration. Furthermore, a linear relationship between 1/lambda(max,abs) and 1/(1 - n(triazole)) is correlated (n: molar fraction), indicating a smooth decrease in conjugation length by incorporation of the branch unit. The P4 containing 13.3 mol % triazole reveals stable blue emission even at 150 degrees C (in air). The HOMO and LUMO levels of P2-P4, estimated from cyclic voltammograms, are -5.69, -5.73, -5.78 eV and -2.63, -2.64, -2.63 eV, respectively. The maximal brightness (current efficiency) of the electroluminescent devices (ITO/ PEDOT:PSS/P2-P4/Ca/Al) improves from 828 cd/m(2) (0.19 cd/A) to 2054 cd/m(2) (0.46 cd/A) with increasing triazole concentration. The results suggest that incorporation of aromatic 1,2,4-triazole branch units is an effective way to improve annealing stability and EL performance of polyfluorenes. (c) 2007 Wiley Periodicals, Inc.