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 (T_g) spanning a large range from 53 to 90 °C. Their T_g and melting point (T_m) basically obey the Boyer–Kauzmann rule (T_g = g·T_m 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%.