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.