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 (ΦΔ) 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 ΦΔ values for many small molecule sensitizers and DOM/PyDOM samples. For DOM/PyDOM, the 1O2 phosphorescence method yielded 1.4 – 3.8 times higher ΦΔ. 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 ΦΔ measurements.