Polyfluorenes containing dibenzo[a,c]phenazine segments:: Synthesis and efficient blue electroluminescence from intramolecular charge transfer states

Five new conjugated copolymers of 9,9'-dioctylfluorene and dibenzo[a,c]phenazine were synthesized by Suzuki copolymerization and were used to achieve efficient blue electroluminescence (EL) from intramolecular charge-transfer excited states. The copolymers had moderate molecular weights (M-w = 31 250-71 120, M-w/M-n = 2.06-3.48) and enhanced glass-transition temperature of 68-130 degrees C. Intramolecular charge transfer (ICT) between the fluorene and dibenzo[a,c]phenazine moieties dominated the ground and excited-state electronic structures of the copolymers, as evidenced by the new low-energy absorption band and the strong positive solvatochromism in photoluminescence (PL). The copolymers emitted blue light in dilute toluene solutions with PL quantum yields of 43-80% as the composition was varied from 2 to 20 mol % dibenzo[a,c]phenazine. Blue EL from ICT excited states of the copolymers was achieved in light-emitting diodes (LEDs) with luminances of 1260-4600 cd/m(2) and external quantum efficiencies of 0.47-2.89% (at > 100 cd/m(2)), depending on the copolymer composition. Besides their excellent stability as blue emitters, the fluorene-dibenzo[a,c]phenazine copolymer LEDs were substantially better than the polyfluorene homopolymer diodes. These results show that efficient blue electroluminescence can be achieved from the intramolecular charge-transfer emission of a conjugated donor-acceptor copolymer, leading to blue LEDs with enhanced performance compared to poly(9,9-dialkylfluorene)s.