Deep Red Phosphorescence of Cyclometalated Iridium Complexes by <i>o</i>‑Carborane Substitution

Heteroleptic (C^∧N)₂Ir­(acac) (C^∧N = 5-MeCBbtp (<b>5a</b>); 4-BuCBbtp (<b>5b</b>); 5-BuCBbtp (<b>5c</b>); 5-(<i>R</i>)­CBbtp = 2-(2′-benzothienyl)-5-(2-<i>R</i>-<i>ortho</i>-carboran-1-yl)-pyridinato-C²,N, R = Me and <i>n</i>-Bu; 4-BuCBbtp = 2-(2′-benzothienyl)-4-(2-<i>n</i>-Bu-<i>ortho</i>-carboran-1-yl)-pyridinato-C²,N, acac = acetylacetonate) complexes supported by <i>o</i>-carborane substituted C^∧N-chelating ligand were prepared, and the crystal structures of <b>5a</b> and <b>5b</b> were determined by X-ray diffraction. While <b>5a</b> and <b>5c</b> exhibit a deep red phosphorescence band centered at 644 nm, which is substantially red-shifted compared to that of unsubstituted (btp)₂Ir­(acac) (<b>6</b>) (λ_em = 612 nm), <b>5b</b> is nonemissive in THF solution at room temperature. In contrast, all complexes are emissive at 77 K and in the solid state. Electrochemical and theoretical studies suggest that the carborane substitution leads to the lowering of both the HOMO and LUMO levels, but has higher impact on the LUMO stabilization than the HOMO, resulting in the reduction of the triplet excited state energy. In particular, the LUMO stabilization in the 4-substituted <b>5b</b> is more contributed by carborane than that in the 5-substituted <b>5a</b>. The solution-processed electroluminescent device incorporating <b>5a</b> as an emitter displayed deep red phosphorescence (CIE coordinate: 0.693, 0.290) with moderate performance (max η_EQE = 3.8%) whereas the device incorporating <b>5b</b> showed poor performance, as well as weak luminance