Revisiting furfuryl alcohol as a probe for singlet oxygen formation mechanism in chlorine-hydrogen peroxide system

Singlet oxygen (¹O₂) is a key reactive species in environmental, chemical, and biological systems. For decades, furfuryl alcohol (FFA) has been used as a probe for quantifying ¹O₂ based on the assumption that it reacts selectively with ¹O₂ but is inert toward other oxidants. However, we demonstrate that this assumption is fundamentally flawed in the chlorine-hydrogen peroxide (H₂O₂) system-one of the most common contexts for ¹O₂ generation. Results demonstrate that chlorine alone can degrade FFA significantly, implying that previous studies may have systematically overestimated ¹O₂ yields. Specifically, raising pH facilitated the decay of chlorine but hindered FFA degradation, suggesting that ¹O₂ formation is unfavorable under alkaline conditions. Increasing the dosage of chlorine enhanced FFA degradation, but decreased the relative contribution of ¹O₂ to FFA degradation. More strangely, increasing the FFA dosage unexpectedly reduced the kinetics of FFA degradation, implying that the concentration of ¹O₂ was not constant under fixed chlorine-H₂O₂ condition. Maintaining low dissolved oxygen enhanced FFA decay, suggesting that superoxide is not an intermediate for ¹O₂ formation. Furthermore, pyranone was not formed during FFA degradation processes, invalidating it as a characteristic product. These evidences together challenge the accountability regarding FFA selectivity, pyranone formation, and ¹O₂ formation mechanisms before using FFA for ¹O₂ quantification in chlorine-involved systems.