Anionic iridium(III) complexes and their conjugated polymer soft salts for time-resolved luminescent detection of intracellular oxygen levels

It is well known that hypoxia is a critical parameter with respect to several pathologies, like solid tumors. Hence, it is necessary to realize highly sensitive and selective monitoring of oxygen levels in biological systems. Phosphorescent iridium(III) complexes are an excellent class of oxygen probe through the energy transfer from triplet excitons to oxygen molecules. Current research mainly focuses on the neutral and cationic complexes, while the anionic ones received few attentions. Herein, we designed a series of anionic iridium(III) complexes with different C^N ligands and investigated their oxygen-sensing capability in solution and living cells. Importantly, based on the metathesis reactions of the anionic iridium(III) complexes with oxygen-insensitive cationic polyfluorene, water-soluble polymer soft salt P1 was synthesized successfully. Thus, ratiometric luminescent and reversible polymer hypoxia probe can be obtained because P1 can give dual emission with oxygen-sensitive phosphorescence from the iridium(III) complexes and reference fluorescence from polyfluorene, respectively. Furthermore, the photoluminescence lifetime imaging at different oxygen levels exhibited that P1 is an excellent oxygen probe utilizing the sensitive phosphorescent lifetime to oxygen level. To our knowledge, this is the first report about the application of anionic iridium(III) complexes and their conjugated polymer soft salts in biosensing and bioimaging fields.