Sorbic Acid as a Triplet Probe: Triplet Energy and Reactivity with Triplet-State Dissolved Organic Matter via 1O2 Phosphorescence

Sorbic acid (2,4-hexadienoic acid; HDA) is commonly used as a probe and quencher for triplet-excited chromophoric dissolved organic matter (3CDOM*), an important transient species in natural waters, yet much remains unknown about its reactivity with 3CDOM* and its triplet energy. To better understand the quenching behavior of HDA, we measured HDA quenching rate constants for various humic substance isolates and whole waters with singlet oxygen (1O2) phosphorescence and determined the triplet energy of HDA. Low-temperature phosphorescence measurements determined the triplet energy of HDA to be 217 kJ mol–1, whereas a complementary method based on triplet quenching kinetics found a triplet energy of 184 ± 7 kJ mol–1. Time-resolved 1O2 phosphorescence measurements yielded different HDA quenching rate constants depending on the fitting method. Using an approach that considered the reactivity of the entire triplet pool produced values of (∼1–10) × 108 M–1 s–1, while an approach that considered only the reactivity of the high-energy triplets output higher rate constants ((∼7–30) × 108 M–1 s–1). In addition, the model based on high-energy triplet reactivity found that ∼30–60% of 3CDOM* is not quenched by HDA. Findings from this study provide a more comprehensive view on the use of HDA as a probe for 3CDOM*.