Isomeric N-Linked Benzoimidazole Containing New Electron Acceptors for Exciplex Forming Hosts in Highly Efficient Blue Phosphorescent OLEDs

Three benzoimidazole containing new electron-accepting materials of 1,3,5-tris(2-(pyridin-2-yl)-1H-benzo[d]imidazol-1-yl)benzene (i TPyBIB), 1,1′,1′′-(pyridine-2,4,6-triyl)tris(2-phenyl-1H-benzo[d]imidazole) (i TPBIPy), and 1,1′,1′′-(pyridine-2,4,6-triyl)tris(2-(pyridin-2-yl)-1H-benzo[d]imidazole) (i TPyBIPy) are designed and synthesized. Compared to the commercial electron-transport 2,2,2-(1,3,5-phenylene)-tris(1-phenyl-1H-benzimidazole) (TPBI) with the C-atom in benzoimidazole linked to the central phenyl ring, the introduction of isomeric N-linkage to the central phenyl or pyridine ring in three compounds remarkably simplifies the synthesis to a one-step CN coupling reaction. iTPyBIB, iTPBIPy, and iTPyBIPy exhibit comparable highest occupied molecular orbital (≈6.0 eV) and lowest unoccupied molecular orbital (≈2.5 eV) energy levels, similar triplet energy levels of ≈2.65 eV with TPBI, whereas reduced electron-transport properties. All materials can be applied as electron acceptors to form universal exciplex with general hole-transport electron donors, such as N,N′-dicarbazolyl-3,5-benzene (mCP), 4,4′,4′′-tris(N-carbazolyl)-triphenylamine (TCTA), and 1,1-bis[4-[N,N-di(p-tolyl)-amino]phenyl]cyclohexane (TAPC) in the blended films with 1:1 molar ratio. A maximum external quantum efficiency of 17.0%, 19.3%, and 13.4% is achieved in mCP, mCP:iTPBIPy, and mCP:TPBI hosted blue phosphorescent organic light-emitting diodes, respectively, indicating the superior electron-acceptor property of the N-linked iTPBIPy for exciplex forming cohost.