Cyclometalated Iridium(III) Ratiometric Oxygen Sensors and Supramolecular Constructs
Hypoxia, or a lack of oxygen, is a key contributor to a vast number of diseases such as cancer, diabetes and COPD. A wide variety of oxygen partial pressures (pO2) ranging from 0.01–100 mmHg needs to be accurately detected in these diseases. Ratiometric oxygen sensing, a technique which utilizes dual-emissive materials with oxygen-insensitive fluorescence as an internal standard and oxygen-sensitive phosphorescence as a measuring tool, has been applied in recent years to improve the sensitivity and dynamic range of oxygen sensing while reducing the cost of optics and need for instrument calibration. Despite being photochemically robust, very few examples of cyclometalated iridium(III) as ratiometric oxygen sensors exist in the literature and require complicated synthetic procedures to produce. The primary aim presented here was to synthesize cyclometalated iridium(III) compounds using simple methods, adding to the existing library of ratiometric sensors for detection of varying ranges of pO2. Expanding on the group’s previous work with iridium BODIPY (boron dipyrromethene) structures, supramolecular complexes linking two phosphorescent iridium centers via a BODIPY fluorophore were designed. However, BODIPY fluorescence dominated the emission spectra, eliminating dual-emission. Subsequently, coumarin dyes were used to give well-resolved blue fluorescence and red iridium-centered phosphorescence. This spectral separation enabled Stern-Volmer quenching to be performed and it was determined that several of our coumarin-isocyanide and coumarin-chloride iridium complexes were useful ratiometric oxygen sensors in detecting pO2 levels ranging from small (0.3–11 mmHg), to medium (8–46 mmHg) and to large (27–160 mmHg) amounts of oxygen. The chloride-terminated complexes have enhanced quantum yields over the isocyanide sensors and include a BODIPY structure which shows the possibility of visible excitation in this design.