Polyfluorenes containing pyrazine units: Synthesis, photophysics and electroluminescence

A series of conjugated copolymers of 9,9-dioctylfluorene and symmetrical pyrazine unit (BY) were synthesized by Suzuki copolymerization and were used as novel light-emitting materials in PLEDs. Efficient energy transfer was observed in both thin film and solution. Compared with the lowest occupied molecular orbital (LUMO) energy level of the polyfluorenes homopolymer (PFO), the lower LUMO energy levels of copolymers indicated that the introduction of the BY unit would be benefit to electron injection. The turn-on voltages of their single-layer electroluminescent (EL) devices (ITO/PEDOT/polymer/LiF/Al) were at 6.1-4.0 V, which were much lower than that of PFO (7.0 V). The maximum brightness, current efficiency, and external quantum efficiency of all PFBY copolymers were higher than those of the PFO homopolymer. The single-layer device of PFBY5 was the best one in the copolymers, with a maximum brightness of 485 cd/m(2), a current efficiency of 0.29 cd/A, and an external quantum efficiency of 0.10%. The introduction of PVK and TPBI for the multilayer device of PFBY5 increased the device efficiencies, which showed a maximum brightness of 3012 cd/m(2), a maximum current efficiency of 1.81 cd/A, and an external quantum efficiency of 0.66%.