Manipulation of Phosphorescence Efficiency of Cyclometalated Iridium Complexes by Substituted o-Carboranes

Abstract

A series of [(C<^>N)(2)Ir(acac)] complexes [{5-(2-R-CB)ppy}(2)Ir(acac)] (3a-3g; acac= acetylacetonate, CB=o-carboran-1-yl, ppy=2-phenylpyridine; R=H (3a), Me (3b), iPr (3c), iBu (3d), Ph (3e), CF3C6H4 (3f), 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 (Phi(PL) approximate to 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-aryl-substituted complexes is mainly attributable to the large contribution to the LUMO in the S-1 excited state from an o-carborane unit and 2) the variation in the C-C bond length between the S-0 and T-1 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 solution-processed 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.