Homoleptic Tris(Pyridyl Pyrazolate) Ir^III Complexes: En route to highly efficient phosphorescent OLEDs

Treatment of the metal re-agent IrCL₃-nH₂O with two equivalents of 2-pyridyl pyrazole (̂N^N)H (3-tert-butyl-5-(2-pyridyl) pyrazole, (bppz)H and 3-trifluoromethyl-5-(2-pyndyl) pyr-azole, (fppz)H), afforded the isomeric Ir^III metal complexes with a general formula cis-[Ir(OpPz)₂Cl₂]H (2a), trans-[Ir(bppz) ₂Cl₂]H (3a), cis-[Ir(fppz)₂-Cl₂]H (2b), and trans-[Ir(fppz)₂Cl₂]H (3b). Single-crystal X-ray diffraction studies on 2b and 3a revealed the co-existence of two pyrazolate chelates and two terminal chloride ligands on the coordination sphere. Subsequent reactivity studies confirmed their inter-mediacy to the preparation of homo-leptic mer-[Ir(bppz)₂] (1a) and mer-[Ir-(fppz)₃] (1b) that showed dual intrali-gand and ligand-to-ligand charge-transfer phosphorescence at room temperature. To attain bright, room-temperature phosphorescence further, we then synthesized two isoquinolinyl pyrazolate complexes, mer-[Ir(bipz)₃] (4a) and mer-[Ir(fipz)₃] (4b) ((bipz)H = 3-tert-butyl-5-(1-isoquinolyl) pyrazole and (fipz)H = 3-trifluorometh-yl-5-(1-isoquinolyl) pyrazole). Their or-ange luminescence is mainly attributed to the mixed MLCT/ππ transition, and the quantum yields were as high as 86 (4a) and 50% (4b) in degassed CH₂Cl₂ solution at RT. The organic light-emitting diodes (OLEDs) were then fabricated by using 4a as a dopant, giving orange luminescence with CIE_x,y = 0.55, 0.45 (CIE_x,y = the 1931 Commission Internationale de L'Eclairage (x,y) coordinates) and peak efficiencies of 14.6% photon/elec-tron, 34.8 cdA^-1, 26.11mW^-1. The device data were then compared with the previously reported heteroleptic complex [Ir(dfpz)₂(bipz)] (5) ((dfpz)H = l-(2, 4-difluorophenyl) pyra-zole), revealing the possible effect of the bipz chelate and phosphor design on the overall electrophosphorescent performance, which can be understood by the differences in the carrier-trans-port properties.