Synthesis of copolyfluorenes with high fluorenone contents and its application in electroluminescent device by simple blending

Poly(fluorene-co-fluorenone)s containing high contents of fluorenone chromophore were synthesized by the Suzuki coupling reaction to study their electroluminescent (EL) properties. The copolymers are thermally stable below 430°C (T_d) in nitrogen atmosphere. In film state, their absorption and photoluminescence spectra (peaked at 373-382 nm and 562-564 nm, respectively) are mainly originated from fluorenone units, because of its efficient energy transfer. Both LUMO and HOMO energy levels, estimated from their cyclic voltammograms, are lowered slightly (-3.17→-3.23 eV, -5.84→-5.89 eV) with increasing contents of electron-withdrawing fluorenone units. Double-layer EL devices, using the copolyfluorenes or their blends with poly(9,9-dihexylfluorene) (PF) as emitting layer, show exclusive emission originated from fluorenone chromophore (565 nm) when its content is high. Blending 0.02-5 wt % of PF-33 (fluorenone fraction: 0.37) with PF greatly enhances device performance (610 cd m^-2 → 4400 cd m^-2 and 0.45 cd A^-1 → 1.52 cd A^-1). Our results demonstrate that the copolyfluorenes are promising emitting materials for EL devices by simple blending.