Deep Red Phosphorescence of Cyclometalated Iridium Complexes by o‑Carborane Substitution

Heteroleptic (C∧N)2Ir(acac) (C∧N = 5-MeCBbtp (5a); 4-BuCBbtp (5b); 5-BuCBbtp (5c); 5-(R)CBbtp = 2-(2′-benzothienyl)-5-(2-R-ortho-carboran-1-yl)-pyridinato-C2,N, R = Me and n-Bu; 4-BuCBbtp = 2-(2′-benzothienyl)-4-(2-n-Bu-ortho-carboran-1-yl)-pyridinato-C2,N, acac = acetylacetonate) complexes supported by o-carborane substituted C∧N-chelating ligand were prepared, and the crystal structures of 5a and 5b were determined by X-ray diffraction. While 5a and 5c exhibit a deep red phosphorescence band centered at 644 nm, which is substantially red-shifted compared to that of unsubstituted (btp)2Ir(acac) (6) (λem = 612 nm), 5b 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 5b is more contributed by carborane than that in the 5-substituted 5a. The solution-processed electroluminescent device incorporating 5a as an emitter displayed deep red phosphorescence (CIE coordinate: 0.693, 0.290) with moderate performance (max ηEQE = 3.8%) whereas the device incorporating 5b showed poor performance, as well as weak luminance.