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 (Mw = 31 250−71 120, Mw/Mn = 2.06−3.48) and enhanced glass-transition temperature of 68−130 °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/m2 and external quantum efficiencies of 0.47−2.89% (at >100 cd/m2), 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.