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%.
Keywords: Boyer−Kauzmann rule; PhOLEDs; benzimidazole; bipolar host materials; carbazole.