Synthesis, characterization, and application of light-emitting copolyfluorenes slightly doped with distyrylbenzene derivatives

Copolyfluorenes PFG1∼PFG4 slightly doped with 0.006-0.5 mol % of 2,5-dihexyloxy-1,4-bis(2-phenyl-2-cyanovinyl)benzene (green chromophore) were synthesized by the Suzuki coupling reaction to be evaluated as hosts for white-light electroluminescent (EL) devices. Their optical, thermal and electrochemical properties were almost identical to those of polyfluorene (PF) due to minimal chromophore content. However, the electroluminescent (EL) spectra of the PFGn were very different from photoluminescence spectra in film state. Relative intensity of green emission (ca. 521 nm) in EL spectra are much stronger than those in PL spectra, which can be attributed to charges trapping in the chromophores due to its narrow band gap (E_g ≈2.56 eV). The performance of EL devices [ITO/PEDOT:PSS/polymer/Ca (50 nm)/Al (100 nm)] were improved with an increase in chromophore content. The PFG4 device revealed the best performance (6790 cd/m², 1.69 cd/A), and the PFG1 and PFG2 devices exhibited comparable intensity in blue and green emissions. Blend EL devices were fabricated by using the PFGn as the hosts and a red iridium complex [Ir(piq)₂(acac)] as dopant. By controlling the amount of the iridium complex, the white-light emitting device was achieved with PFG2, with maximum brightness and CIE coordinate being 4120 cd/m² and (0.31, 0.28), respectively.