Manipulation of phosphorescence efficiency of cyclometalated iridium complexes by substituted o-carboranes

A series of [(CN)₂Ir(acac)] complexes [{5-(2-RCB) ppy}₂Ir(acac)] (3a-3g ; acac = acetylacetonate, CB = o-car-boran-1-yl, ppy = 2-phenylpyridine; R = H (3a), Me (3b), iPr (3c), iBu (3d ), Ph (3e), CF₃C₆H₄ (3 f), C₆F₅ (3g)) with various 2-R-substituted o-carboranes at the 5-position in the phenyl ring of the ppy ligand were prepared. X-ray diffraction studies revealed that the carboranyl C-C bond length increases with increasing steric and electron-withdrawing effects from the 2-R substituents. Although the absorption and emission wavelengths of the complexes are almost invariant to the change of 2-R group, the phosphorescence quantum efficiency varies from highly emissive (&z.Theta;_PL ≈0.80 for R = H, alkyl) to poorly emissive (R = aryl) depending on the 2-R group and the polarity of the medium. Theoretical studies suggest that 1) the almost nonemissive nature of the 2-arylsubstituted complexes is mainly attributable to the large contribution to the LUMO in the S₁ excited state from an ocarborane unit and 2) the variation in the C-C bond length between the S₀ and T₁ state structures increases with increasing steric (2-alkyl) and electronic effects (2-aryl) of the 2-R substituent and the polarity of the solvent. The solutionprocessed electroluminescence (EL) devices that incorporated 3b and 3d as emitters displayed higher performance than the device based on the parent [(ppy)₂Ir(acac)] complex. Along with the high phosphorescence efficiency, the bulkiness of the 2-R-o-carborane unit is shown to play an important role in improving device performance.