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Hyperbranched luminescent polyfluorenes containing aromatic triazole branching units

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Abstract

Using a new aromatic 1,2,4-triazole branching monomer (4.8–13.3 mol %), three hyperbranched polyfluorenes (P2P4) 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 × 104 to 5.9 × 104 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/λmax,abs and 1/(1 − ntriazole) 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 °C (in air). The HOMO and LUMO levels of P2P4, 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/P2P4/Ca/Al) improves from 828 cd/m2 (0.19 cd/A) to 2054 cd/m2 (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. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 4465–4476, 2007

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