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%.