Efficient donor–acceptor emitter based nonsymmetrical connection for organic emitting diodes with improving exciton utilization

A new strategy developed to construct blue emissive materials having an unsymmetrical connection with identical conjugated phenanthrimidazole groups results in the separation of the frontier orbitals and leads to donor–acceptor (D–A) architecture. The blue device with 2-(naphthalen-1-yl)-1-(4-(1-(naphthalen-1-yl)-1H-phenanthro[9,10-d]imidazol-2-yl)phenyl)-1H-phenanthro[9,10-d]imidazole (p-PPI)/2-(naphthalen-1-yl)-1-(3-(1-(naphthalen-1-yl)-1H-phenanthro[9,10-d]imidazol-2-yl)phenyl)-1H-phenanthro[9,10-d]imidazole (m-PPI) emissive layer (λ(EL) – 434/420 nm) shows high efficiencies: current efficiency (η(c)) – 5.83/3.56 cd A(−1); power efficiency (η(p)) – 5.73/3.48 lm W(−1); external quantum efficiency (η(ex)) – 8.98/6.48% at 3.0 V. Their cyano derivatives, p-CNPPI/m-CNPPI (λ(EL) – 422/406 nm) exhibit maximum efficiencies (η(c) – 6.28/4.38 cd A(−1); η(p) – 6.14/4.01 lm W(−1); η(ex) – 9.01/6.72%) at 2.9 V compared to the p-PPI/m-PPI devices. The weak charge transfer in the D–A emitters results in deep blue emission. The anisotropic structural characteristics of p-PPI, p-CNPPI, m-PPI and m-CNPPI induced horizontal dipole orientation in films and enhanced EL efficiency. These bipolar materials with suitable triplet energy can be used as hosts in green as well as red phosphorescent organic light emitting devices (PHOLEDs). The green/red device (λ(EL) – 504/618 nm) with p-CNPPI: Ir(ppy)(3)/Ir(MDQ)(2) (acac) exhibits a maximum L – 8823/38418 cd m(−2); η(ex) – 24.56/17.31%; η(c) – 84.30/18.09 cd A (−1); η(p) – 86.43/21.43 lm W (−1)with CIE (0.33, 0.61)/(0.65, 0.34).