Construction of Highly Efficient Carbazol-9-yl-Substituted Benzimidazole Bipolar Hosts for Blue Phosphorescent Light-Emitting Diodes: Isomer and Device Performance Relationships

Four isomers of carbazol-9-yl-substituted
1,2-diphenylbenzimidazole
at 7, 6, 5, and 4 positions, named as 1-CbzBiz, 2-CbzBiz, 3-CbzBiz, and 4-CbzBiz, respectively, have been synthesized. Instead of having an identical
molecular weight, the CbzBiz’s have their glass
transition temperatures (Tg) spanning
a large range from 53 to 90 °C. Their Tg and melting point (Tm) basically
obey the Boyer–Kauzmann rule (Tg = g·Tm with g ≈ 0.7) on the absolute temperature scale (in kelvins).
However, while 1-CbzBiz and 4-CbzBiz demonstrate
a small g value of 0.66, which is significantly smaller
than other common organic glass molecules, 3-CbzBiz shows
an unexpectedly high g value of 0.73, implying higher
intermolecular interactions in the glass. These CbzBiz’s are suitable hosts for bis­[2-(4,6-difluorophenyl)­pyridinato-C2,N]­(picolinato)­iridium­(III)
(FIrpic) in phosphorescence organic light-emitting diodes (PhOLEDs). The optimized PhOLED of indium tin oxide/4,4′-cyclohexylidenebis­[N,N-bis­(4-methylphenyl)­benzenamine]/4Cbz/4-CbzBiz-FIrpic­(15%)/diphenylbis­(4-(pyridin-3-yl)­phenyl)­silane/LiF–Al
shows a maximum brightness of 18 760 cd/m2, a current
efficiency of 64.1 cd/A, and the external quantum efficiency of 30.9%.

四种以羰基苯并咪唑-9-基取代的1,2-二苯基苯并咪唑异构体，分别位于7、6、5和4位，命名为1-CbzBiz、2-CbzBiz、3-CbzBiz和4-CbzBiz，已成功合成。这些化合物虽然分子量不尽相同，但它们的玻璃化转变温度（Tg）却跨越了从53至90°C的宽广范围。它们的Tg和熔点（Tm）基本上遵循Boyer-Kauzmann规则（Tg = g·Tm，其中g ≈ 0.7）在绝对温度尺度（开尔文为单位）上。然而，尽管1-CbzBiz和4-CbzBiz表现出较小的g值0.66，这一值显著低于其他常见有机玻璃分子，3-CbzBiz却展现出意外的高g值0.73，这暗示了玻璃中的分子间作用力更强。这些CbzBiz化合物是磷光有机发光二极管（PhOLEDs）中bis-[2-(4,6-二氟苯基)吡啶亚胺-C2,N]-(吡啶甲酸酯)铱(III)（FIrpic）的理想宿主。优化后的In2O3/4,4'-环己亚甲基双-[N,N-双(4-甲基苯基)苯胺]/4Cbz/4-CbzBiz-FIrpic(15%)/二苯基双-(4-(吡啶-3-基)苯基)硅烷/LiF-Al磷光有机发光二极管展现了最高亮度达18760 cd/m²、电流效率为64.1 cd/A以及外部量子效率为30.9%的优异性能。