Homoleptic Tris(Pyridyl Pyrazolate) Ir-III Complexes: En Route to Highly Efficient Phosphorescent OLEDs

Treatment of the metal reagent IrCl3 center dot nH(2)O with two equivalents of 2-pyridyl pyrazole (N boolean AND N)H (3-tert-buty1-5-(2-pyridyl) pyrazole, (bppz)H and 3-trifluoromethy1-5-(2-pyridyl) pyrazole, (fppz)H), afforded the isomeric Ir-III metal complexes with a general formula cis-[Ir(bppz)(2)Cl-2](2a), trans-[Ir(bppz)(2)Cl-2]H (3a), cis-[Ir(fppz)(2)Cl-2]H (2b), and trans-[Ir(fppz)(2)Cl-2]H (3b). Single-crystal X-ray diffraction studies on 2b and 3a revealed the coexistence of two pyrazolate chelates and two terminal chloride ligands on the coordination sphere. Subsequent reactivity studies confirmed their intermediacy to the preparation of homoleptic mer-[Ir(bppz)(3)] (1a) and tner-Pr(fppz)(3)] (1b) that showed dual intraligand 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-isoquinoly1) pyrazole and (fipz)H=3-trifluoromethy1-5-(1-isoquinoly1) pyrazole). Their orange luminescence is mainly attributed to the mixed MLCT/am* transition, and the quantum yields were as high as 86 (4a) and 50% (41)) 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 CIEx,y =0.55, 0.45 (CIEx,y = the 1931 Commission Internationale de L'Eclairage (x,y) coordinates) and peak efficiencies of 14.6% photon/electron, 34.8 cd A(-1), 26.1 Im W-1. The device data were then compared with the previously reported heteroleptic complex [Ir(dfpz)(2)(bipz)] (5) ((dfpz)H=1-(2,4-difluorophenyl) pyrazole), 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-transport properties.