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 delta-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)-1,10-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 (E-T = 2.78-2.96 eV) and the lowest unoccupied molecular orbital (LUMO) energy could be manipulated by the linkage and the number of delta-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 (EQE(max) 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 EQE(max)), 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 delta-carboline moiety could improve the thermal and morphological stabilities.