Fluorescein-Functionalized Iridium(III) Complexes as Dual-Mode Type I Photosensitizers for Hypoxia-Tolerant Photodynamic and X-ray-Induced Therapy

The development of photosensitizers that function effectively in hypoxic environments and enable deep-tissue treatment remains a significant challenge in photodynamic therapy (PDT). Here, we report two novel Ir(III) complexes functionalized with fluorescein designed as efficient Type I photosensitizers for both light-driven PDT and X-ray-induced PDT (X-PDT). By populating the triplet state of the fluorescein ligands, these complexes facilitate the generation of reactive oxygen species (ROS) through electron transfer, producing superoxide anion radicals (O₂^•-) and hydroxyl radicals (^•OH) under irradiation. The complexes exhibit pronounced phototoxicity against cancer cells, particularly under hypoxic conditions, where oxygen-dependent Type II photosensitizers are less effective. Remarkably, these complexes also demonstrate direct X-ray activation, offering a solution for deep-tissue cancer treatment. The lead complex, PS1, outperforms existing systems by efficiently generating both singlet oxygen O₂(¹Δ_g) and free radicals, enabling synergistic Type I and II PDT effects. This work represents a major advancement in the design of oxygen-independent PDT agents by using fluorescein’s triplet state, with potential applications in deep-tissue and hypoxic tumor environments.