Phosphorescent organic light-emitting diodes (PHOLEDs) continue to attract intense interest because they can, in theory, approach a 100% internal quantum efficiency by utilizing both singlet and triplet excitons.[1] To achieve highly efficient electrophosphorescence by reducing competitive factors such as concentration quenching and triplet–triplet annihilation, phosphorescent emitters of heavy-metal complexes are usually doped into a suitable host material.[2] Thus the synthesis of host materials and dopants are equally important for the formation of efficient PHOLEDs. It is desirable that the host materials have a large enough bandgap for effective energy transfer to the guest, good carrier transport properties for a balanced recombination of carriers in the emitting layer, and energy-level matching with neighboring layers for effective charge injection. Recently, bipolar hosts have aroused considerable interests in the area of organic light-emitting diodes (OLEDs) because they can provide more balance in electron and hole fluxes and simplify device structure.[3] However, a compromise is required between the bipolar transporting property and band gap of the material, because the electron-donating and electron-withdrawing moieties in bipolar molecules unavoidably lower the band gap of the material by intramolecular charge transfer, while the low triplet energy of the host can cause reverse energy transfer from the guest back to the host, which consequently decreases the efficiency of PHOLEDs. To address this issue, most recent molecular designs focus on the interruption of the π conjugation between electron-donating and electron-withdrawing moieties by the incorporation of steric groups [4] and/or meta linkages [2, 5] between the two moieties. Efficient blue (46 lm WÀ1, 24%),[2] green (27.3 cd AÀ1)[4b] and orange (22 cd AÀ1, 7.8%)[4a] electrophosphorescence from such small bipolar host molecules has been reported.

Carbazole derivatives can be used as host materials because of their high triplet energy and good hole-transporting ability.[6] For example, 4, 4’-N, N’-dicarbazolbiphenyl (CBP) is a popular host for triplet emitters. PHOLEDs that use CBP as a host material for various dopants have been reported to have peak efficiencies as high as 28 cdAÀ1 for green(fac-tris (2-phenylpyridinato-N, C2) iridium,[Ir-(ppy) 3]),[1a] 52 cdAÀ1 for green (tris [3, 6-bis (phenyl)-pyridazinato-N1, C2] iridium [Ir (BPPya) 3])[7] and 5.82 cdAÀ1 for deep red (a dendritic iridium complex).[8] Unfortunately, the CBP host is prone to crystallization, especially when the dopant concentration is too low.[8, 9] Furthermore, red PHOLEDs containing a CBP host usually need high driving voltages because the poor energy match between CBP and adjacent hole-and electron-transporting layers can result in insufficient and/or unbalanced injection of holes and electrons.[10] It is a worthwhile target to develop host materials with good thermal stability and matching energy levels to replace CBP. Oxadiazole derivatives have been proven to be very effective in improving the injection and transport of electrons. For example, 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1, 3, 4-oxadiazole (PBD)[11] and 1, 3-bis [4-tert-butylphenyl)-1, 3, 4-oxadiazolyl] phenylene (OXD7)[11b] are usually incorporated in OLEDs as electron-transport materials. Herein we report a novel carbazole/oxadiazole hybrid molecule o-CzOXD linked through the 9-position of carbazole with the ortho position of 2, 5-diphenyl-1, 3, 4-oxadiazole. The bipolar molecule o-CzOXD was easily prepared by an aromatic nucleophilic substitution reaction between carbazole and the fluoroarene,[12] which was activated by the electron …