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

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 degrees C and emitted intense blue light in solution with using expensive noble-metal catalysts. These compounds showed excellent thermal properties with a very high glass-transition temperature of 197-200 degrees 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'Enclairage y-coordinate value (CIEy) <= 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 CIEy <= 0.10.