Quinoxaline-containing polyfluorenes: Synthesis, photophysics, and stable blue electroluminescence

Six new copolymers of 9,9'-dioctylfluorene and 2,3-bis(p-phenylene)quinoxaline were synthesized, characterized, and used as blue-emitting materials in light-emitting diodes (LEDs). The glass transition temperature increased from 67 degreesC in poly(9,9'-dioctylfluorene) (PFO) to as high as 160 degreesC in the alternating copolymer. All the copolymers emitted blue light in dilute toluene solution (417423 nm), with photoluminescence (PL) quantum yields that decreased from 72% to 26% with increasing quinoxaline content, and as thin films (429-439 nm). The PL lifetimes of solutions and thin films were 308-617 ps. Blue electroluminescence with Commission Internationale de L'Eclairage coordinates of (0.16, 0.06) was achieved. Luminances of 240-520 cd/m(2) varied with the quinoxaline content. External quantum efficiencies of up to 1% at brightnesses of over 100 cd/m(2) were obtained. Enhancement in LED performance by factors of 3-6 compared to that of PFO was achieved in a copolymer containing 5 mol % 2,3-bis(p-phenylene)quinoxaline. Significant enhancements in LED brightness and external quantum efficiency by factors of up to 120 were observed in copolymers containing 15-50 mol % 2,3-bis(p-phenylene)quinoxaline when using bilayer LiF/Al cathodes instead of Al. These results demonstrate that composition can be used to optimize the light-emitting properties of electroluminescent copolymers and that the fluorene-quinoxaline copolymers are useful materials for developing stable blue LEDs.