Cyclometalated Iridium-Coumarin Ratiometric Oxygen Sensors: Improved Signal Resolution and Tunable Dynamic Ranges

In this work we introduce a new series of ratiometric oxygen sensors for hypoxic environments based on phosphorescent cyclometalated iridium centers partnered with organic coumarin fluorophores. Three different cyclometalating ligands and two different pyridyl-containing coumarin types were used to prepare six target complexes with tunable excited-state energies. Some of the complexes exhibit only phosphorescence originating from the cyclometalated Ir moiety, as a result of excited-state energy transfer from the coumarin to the Ir-centered excited states. Three of the complexes display dual emission, with fluorescence arising from the coumarin ligands and phosphorescence from the cyclometalated iridium synthons, and hence function as ratiometric oxygen sensors. Oxygen quenching experiments with these complexes demonstrate that the iridium centered phosphorescence is quenched under O₂ while fluorescence is unaffected. These sensors have good signal resolution, and the sensitivity and dynamic range, measured with Stern-Volmer analysis, span two orders of magnitude. This work demonstrates that this simple, modular approach for conjoining fluorescent and phosphorescent molecules can produce effective oxygen sensors with a wide range of attributes.