Phosphorescent cyclometalated iridium(III) complexes incorporated into polynorbornene polymeric platform as potential probes for assessments of oxygen in cancer cells

Abstract

Phosphorescent transition metal complexes are considered as promising probes for oxygen sensing in living cells and tissues. Red light-emitting complexes are more valuable because the red irradiation better penetrates into biological tissues. In the present study, far-red light-emitting iridium(III) complexes PnIr1-PnIr4 on polyoxanorbornene platform were synthesized and their oxygen sensing properties were tested in water and in cells in vitro. Iridium(III) complexes incorporated into polymeric platform contained 1-(thien-2-yl)isoquinoline cyclometalating ligands and norbornene-substituted picolinate (PnIr1) and diimine (PnIr2-PnIr4) ancillary ligands. The quantum yields and phosphorescence lifetimes of the synthesized polymeric iridium probes in degassed water solutions were 1.5–2 times higher than in aerated solutions demonstrating oxygen-dependent quenching of phosphorescence. Of the four probes, PnIr1 easily penetrated into cultured cancer cells grown as monolayer and 3D spheroids and showed reliable response to hypoxia with increase of lifetime from 1.31 to 3.06 µs. Good water solubility, far-red oxygen-sensitive emission and low cytotoxicity make the new probe a promising tool for intracellular oxygen assessments in cancer research.