Singlet Oxygen Quantum Yields: Comparing Chemical Probe and Time-Resolved Phosphorescence

Singlet oxygen (¹O₂) is an important reactive intermediate in aquatic photochemical reactions, altering the fate and transformation of pollutants. Past work has primarily quantified ¹O₂ generation in aquatic systems using the chemical probe furfuryl alcohol (FFA). An alternative method to quantify ¹O₂ is to directly observe ¹O₂ through its phosphorescence emission. While this method is receiving growing interest, there has yet to be a direct comparison of measured ¹O₂ generation efficiencies (i.e., quantum yields) between the FFA and ¹O₂ phosphorescence methods. In this study, we compared the ¹O₂ quantum yields (Φ_Δ) between these two methods for a broad set of samples, including small molecule sensitizers, dissolved organic matter (DOM) isolates, and pyrogenic DOM (PyDOM). FFA and ¹O₂ phosphorescence methods yielded significantly different Φ_Δ values for many small molecule sensitizers and DOM/PyDOM samples. For DOM/PyDOM, the ¹O₂ phosphorescence method yielded 1.4 - 3.8 times higher Φ_Δ. We additionally explored potential FFA-mediated triplet excited state quenching processes using laser spectroscopy. While we observed that FFA effectively quenched and/or chemically reacted with triplets of small molecule sensitizers, we observed minimal quenching of the triplet excited state DOM/PyDOM by FFA. This suggests that for DOM/PyDOM, FFA-triplet reactions are not major side reactions in FFA-based ¹O₂ measurements at 365 nm. Results from this study illuminate the potential issues of FFA interacting with sensitizer triplet excited states and advance the understanding of ¹O₂ phosphorescence-based Φ_Δ measurements.