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

Abstract

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.