Cyclometalated iridium(III) complexes containing an anthracene unit for sensing and imaging singlet oxygen in cellular mitochondria

Singlet oxygen (O-1(2)), as a highly reactive oxygen species, plays an important role in the physical, chemical and biomedical fields, especially during photodynamic therapy (PDT) process. In this work, two iridium(III) complexes containing an anthracene unit in their diimine ligand were designed and synthesized to monitor O-1(2) in living cells. The complexes were weakly emissive owing to the photoinduced electron transfer process, but exhibited intense luminescence upon capturing O-1(2), resulting from the formation of the corresponding endoperoxide analogues. The remarkable turn-on luminescence response was specific toward O-1(2) and in preference to other reactive oxygen species. The utilization of one of the complexes for imaging O-1(2) in living cells has also been demonstrated using three different cells lines. Cells incubated with the complexes were hardly emissive. Further light irradiation at 475 nm triggered intracellular emission turn on, indicative of the production of O-1(2) photochemically. The emissive pattern was well colocalized with commercially available MitoTracker, suggesting the potential applications of the complexes for imaging mitochondria O-1(2). The O-1(2) capturing properties rendered the complexes low photocytotoxicity since O-1(2)-caused oxidative damage toward cellular molecules and structures was inhibited.