Sorbic Acid as a Triplet Probe: Triplet Energy and Reactivity with Triplet-State Dissolved Organic Matter via ~1O_2 Phosphorescence

摘要

Sorbic acid (2,4-hexadienoic acid; HDA) is commonly used as a probe and quencher for triplet-excited chromophoric dissolved organic matter ((CDOM)-C-3*), an important transient species in natural waters, yet much remains unknown about its reactivity with (CDOM)-C-3* 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 (O-1(2)) 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 O-1(2) 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 (similar to 1-10) X 10(8) M-1 s(-1), while an approach that considered only the reactivity of the high-energy triplets output higher rate constants ((similar to 7-30) X 10(8) M-1 s(-1)). In addition, the model based on high-energy triplet reactivity found that similar to 30-60% of (CDOM)-C-3* is not quenched by HDA. Findings from this study provide a more comprehensive view on the use of HDA as a probe for (CDOM)-C-3*.