A Convenient Approach To Synthesize o-Carborane-Functionalized Phosphorescent Iridium(III) Complexes for Endocellular Hypoxia Imaging

The structure–property relationship of carborane-modified iridium(III) complexes was investigated. Firstly, an efficient approach for the synthesis of o-carborane-containing pyridine ligands a–f in high yields was developed by utilizing stable and cheap B₁₀H₁₀(Et₄N)₂as the starting material. By using these ligands, iridium(III) complexes I–VII were efficiently prepared. In combination with DFT calculations, the photophysical and electrochemical properties of these complexes were studied. The hydrophilic nido-o-carborane-based iridium(III) complex VII showed the highest phosphorescence efficiency (abs.ϕ_P=0.48) among known water-soluble homoleptic cyclometalated iridium(III) complexes and long emission lifetime (τ=1.24 μs) in aqueous solution. Both of them are sensitive to O₂, and thus endocellular hypoxia imaging of complex VII was realized by time-resolved luminescence imaging (TRLI). This is the first example of applying TRLI in endocellular oxygen detection with a water-soluble nido-carborane functionalized iridium(III) complex.