Mitochondria-localized iridium(III) complexes with anthraquinone groups as effective photosensitizers for photodynamic therapy under hypoxia

Photodynamic therapy (PDT) is a potential way for the tumor treatment. However, it notably suffers the limitation of hypoxia in solid tumors. Thus, it is significant to develop effective photosensitizers which can exhibit excellent therapeutic performance under both normoxia and hypoxia. Herein, we reported four ionic iridium(III) complexes (Ir1–Ir4) with anthraquinone groups which can regulate their excited state energy levels effectively. Among them, the energy gap of Ir1 was between 1.63 and 2.21 eV, which can match well with that of O₂, and the HOMO energy of Ir1 is less than −5.51 eV. Compared with Ir2–Ir4, the luminescent quantum efficiency of Ir1 was the highest. Particularly, Ir1 can specifically target the mitochondria of the tumor cells. Meanwhile, Ir1 showed high singlet oxygen quantum yields (Φ_Δ) in both solutions and living cells with low cytotoxicity. The results of PDT experiments revealed that Ir1, as a photosensitizer, exhibited excellent therapeutic effect not only in normoxia but also in hypoxia condition. We believe that this work is meaningful for developing excellent PDT agents based on cyclometalated Ir(III) complexes via rational ligand modification.