Versatile Benzimidazole/Triphenylamine Hybrids: Efficient Nondoped Deep-Blue Electroluminescence and Good Host Materials for Phosphorescent Emitters

Two new bipolar compounds, N,N,N',N'-tetraphenyl-5'-(1-phenyl-1H-benzimidazol- 2-yl)-1,1':3',1''-terphenyl-4,4''-diamine(1) and N,N,N',N'-tetraphenyl-5'-( 1-phenyl-1H-benzimidazol-2-yl)-1,1':3',1''-terphenyl-3,3''-diamine(2), were synthesized and characterized, and their thermal, photophysical, and electrochemical properties were investigated. Compounds 1 and 2 possess good thermal stability with high glass-transition temperatures of 109-129 degrees C and thermal decomposition temperatures of 501-531 degrees C. The fluorescence quantum yield of 1 (0.52) is higher than that of 2 (0.16), which could be attributed to greater pi conjugation be tween the donor and acceptor moieties. A nondoped deep-blue fluorescent organic light-emitting diode (OLED) using 1 as the blue emitter displays high performance, with a maximum current efficiency of 2.2 cdA(-1) and a maximum external efficiency of 2.9% at the CIE coordinates of (0.17, 0.07) that are very close to the National Television System Committee's blue standard (0.15, 0.07). Electrophosphorescent devices using the two compounds as host materials for green and red phosphor emitters show high efficiencies. The best performance of a green phosphorescent device was achieved using 2 as the host, with a maximum current efficiency of 64.3 cdA(-1) and a maximum power efficiency of 68.3 lm W-1; whereas the best performance of a red phosphorescent device was achieved using 1 as the host, with a maximum current efficiency of 11.5 cd A(-1), and a maximum power efficiency of 9.8 lm W-1. The relationship between the molecular structures and optoelectronic properties are discussed.