Bis(phenanthroimidazolyl)biphenyl derivatives as saturated blue emitters for electroluminescent devices

Abstract

Blue-emitting bis(phenanthroimidazolyl)biphenyl derivatives, 4,4′-bis(1-phenyl-1H-phenanthro[9,10-d]phenanthroimidazolyl-2-yl)biphenyl (PPIP), 4,4′-bis(1-p-tolyl-1H-phenanthro[9,10-d]phenanthroimidazolyl-2-yl)biphenyl (TPIP) and 4,4′-bis(1-p-anisyl-1H-phenanthro[9,10-d]phenanthroimidazolyl-2-yl)biphenyl (APIP) were effectively synthesized in high yields from commercially available starting materials through a simple two-step procedure without using expensive noble-metal catalysts. These compounds showed excellent thermal properties with a very high glass-transition temperature of 197–200 °C and emitted intense blue light in solution with emission peaks at ca. 428 and 446 nm. By using a different hole-transporting layer, we optimized the electroluminescent efficiencies of the PPIP-based devices. All the PPIP-based devices were turned on at very low applied voltages of <3.0 V and gave pure-blue light with a Commission Internationale d'Énclairage y-coordinate value (CIE_y) ≤ 0.15. Among them, device C using TCTA (4,4′,4″-tris(N-carbazolyl)triphenylamine) as the hole-transporting layer reached a very high external quantum efficiency of 6.31% and power efficiency of 7.30 lm/W. When using TPIP or APIP as the emissive layer to replace PPIP in the optimal device C, the resulting devices also exhibited very high external quantum efficiencies of more than 5%, and highly saturated blue light can be obtained from the TPIP-based device with CIE_y ≤ 0.10.