Mitochondrial viscosity probes: iridium(III) complexes induce apoptosis in HeLa cells

Abstract Mitochondrial viscosity has emerged as a promising biomarker for diseases such as cancer and neurodegenerative disorders, yet accurately measuring viscosity at the subcellular level remains a significant challenge. In this study, we synthesized and characterized three cyclometalated iridium(III) complexes ( Ir1‐Ir3 ) containing 5‐fluorouracil derivatives as ligands. Among these, Ir1 selectively induced apoptosis in HeLa cells by increasing mitochondrial production of reactive oxygen species (ROS), which triggered a cascade of events leading to mitochondrial dysfunction. Additionally, the fluorescence lifetime of Ir1 demonstrated high sensitivity to intracellular viscosity changes, enabling real‐time fluorescence lifetime imaging microscopy (FLIM) of cellular micro‐viscosity during apoptosis. These findings underscore the potential of cyclometalated Ir(III) complexes for both therapeutic and diagnostic applications at the subcellular level.