Homoleptic Tris-Cyclometalated Iridium Complexes with Substituted O-Carboranes: Green Phosphorescent Emitters for Highly Efficient Solution-Processed Organic Light-Emitting Diodes

Homoleptic tris-cyclometalated iridium complexes, fac-Ir[5-(2-RCB)ppy]3 (3a-3c; CB = o-carboran-1-yl; ppy = 2-phenylpyridinato-C(2),N; R = H (3a), Me (3b), (i)Bu (3c)) with 2-R-substituted o-carboranes at the 5-position of the ppy ligand, were prepared and characterized. X-ray diffraction analysis of 3a and 3c revealed that the three C^N ligands adopt a fac-arrangement around the Ir atom and that the carboranyl C-C bond distance increases with increasing steric effects of the 2-R substituent. The phosphorescence wavelengths of the complexes were apparently blue-shifted by ca. 20 nm (λem = 487-493 nm) compared to that of the parent fac-Ir(ppy)3 (4; λem = 508 nm). In particular, 3a-3c were highly emissive in toluene, and the phosphorescence quantum efficiencies of 3a and 3b (ΦPL = 0.95-0.98) were comparable to that of 4. Solution-processed electroluminescent devices incorporating 3a-3c as emitters displayed green light with high performance, and devices based on the 3c dopant showed the highest performance. In particular, the devices based on 3c exhibited performance more than double of that of the device based on 4 in terms of current efficiency (29.6 cd/A for 3c vs 15.8 cd/A for 4 at 4 wt % Ir and 1000 cd/m(2)), power efficiency (11.0 lm/W for 3c vs 6.3 lm/W for 4), and external quantum efficiency (10.2% for 3c vs 4.7% for 4) over a wide range of luminance. The higher PL quantum yields of doped host films with 3c than those with 4 at high dopant concentrations above 8 wt % suggested that along with high phosphorescence quantum efficiency, the steric bulkiness of the 2-(i)Bu-substituted o-carborane in 3c plays a crucial role in improving device performance.