Achieving simultaneous sensing of oxygen and temperature with metalloporphyrins featuring efficient thermally activated delayed fluorescence and phosphorescence

Abstract

Temperature is an essential parameter for reliable assessment of oxygen concentration by optical sensors. This can be done by a conventional sensor such as a resistance thermometer or by adding an optical temperature probe to the sensing material. A sensor that relies on a single emitter capable of simultaneous sensing of oxygen and temperature would be highly advantageous. Unfortunately, such materials are extremely rare and moreover poorly suitable for measurement at ambient temperatures and physiological conditions. To cover this gap, we systematically investigated 15 Pd(II) and 6 Pt(II) complexes of π-extended porphyrins that simultaneously emit thermally activated delayed fluorescence (TADF) and phosphorescence. The class of quinone-extended benzoporphyrins was found to feature especially high ratios of TADF to phosphorescence (1.4–2.7 and 0.10–0.21, for Pd(II) and Pt(II) complexes, respectively, at 25 °C) which represents approximately a sixteen-fold improvement compared to the benchmark octasulfone-substituted porphyrins. Interconversion of phosphorescence into TADF with increasing temperature enables ratiometric measurement of this parameter in the range from 0 to about 65 °C whereas the oxygen concentration is accessed through the decay time of either TADF or phosphorescence. Simultaneous sensing of both temperature and oxygen is demonstrated for a naphthoquinone-extended Pt(II) benzoporphyrin embedded in polystyrene that shows dual emission with maxima at 677 and 789 nm. The sensor is characterized for temperatures from 5° to 55 °C and oxygen partial pressure from 0 to 202 hPa, corresponding to the conditions found in the majority of applications. As an application example temperature compensated imaging of oxygen distribution is demonstrated.