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

Abstract

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*.