Manipulation of electron deficiency of δ-carboline derivatives as bipolar hosts for blue phosphorescent organic light-emitting diodes with high efficiency at 1000 cd m−2

The effect of different linkages on thermal, electrochemical, photophysical, and optoelectronic properties was discussed in detail to illuminate the structure–property relationship of four δ-carboline derivatives, 5-(3′-(9H-carbazol-9-yl)-[1,1′-biphenyl]-3-yl)-5H-pyrido[3,2-b]indole (CzBPDCb), 3,3′-bis(5H-pyrido[3,2-b]indol-5-yl)-1,1′-biphenyl (BDCbBP), 5-(5-(3-(9H-carbazol-9-yl)phenyl)pyridin-3-yl)-5H-pyrido[3,2-b]indole (Cz35PyDCb), and 5-(6-(3-(9H-carbazol-9-yl)phenyl)pyridin-2-yl)-5H-pyrido[3,2-b]indole (Cz26PyDCb). Their triplet energies (ET = 2.78–2.96 eV) and the lowest unoccupied molecular orbital (LUMO) energy could be manipulated by the linkage and the number of δ-carboline units. These materials exhibited bipolar features with good hole- and electron-transporting properties. These host materials when doped with bis(3,5-difluoro-2-(2-pyridyl)phenyl-(2-carboxypyridyl)iridium(III)) (FIrpic) demonstrated efficient blue phosphorescent organic light emitting diode (OLED) with reduced driving voltage, high efficiency, and negligible roll-off. Among these four materials, Cz35PyDCb performed as a host material for blue OLED with ideal current and power efficiencies (CEmax at 44.7 cd A−1, and PEmax at 40.2 lm W−1), as well as good external quantum efficiency (EQEmax at 22.3%). Most importantly, at the normal 1000 cd m−2 operation brightness, the efficiencies remained at 42.6 cd A−1 for CE (95% of CEmax), 31.7 lm W−1 for PE (79% of PEmax), and 20.1% for EQE (90% of EQEmax), which indicated well-balanced carriers transporting and good confinement of the triplet excitons in the emitting layer, even at high current density. Moreover, introduction of the δ-carboline moiety could improve the thermal and morphological stabilities.