Determination of excited triplet states of dissolved organic matter using chemical probes: A comparative and mechanistic study

Dissolved organic matter (DOM) plays an important role in the biogeochemical cycle in natural waters. The determination and characterization of the excited triplet state of DOM (3DOM*) have attracted much attention recently. However, the underlying differences of determined 3DOM* through different pathways are not yet fully understood. In this study, the differences and underlying mechanisms of the determined 3DOM* using 2,4-hexadien-1-ol (HDO) through an energy transfer pathway and 2,4,6-trimethylphenol (TMP) through an electron transfer pathway, were investigated. The results showed that the determined quantum yields of 3DOM* (Φ3DOM*) for four commercial and four isolated local DOMs are different using HDO ((0.04 ± 0.00) × 10−2 to (2.9 ± 0.17) × 10−2)) and TMP ((0.08 ± 0.01) × 10−2 to (1.2 ± 0.17) × 10−2), respectively. For 17 DOM-analogs, significant differences were also observed with the quantum yields of their 3DOM* determined using HDO (ΦHDO) and the triplet-state quantum yield coefficients determined using TMP (fTMP). It indicates the different reactivity of TMP and HDO with the excited triplet of the chromophores with different structures within the isolated DOM. Based on the experimental and predicted values of fTMP and ΦHDO for different DOM-analogs, the impact of substituents on differences in 3DOM* values were further revealed. These results demonstrated that the levels of 3DOM* depended on the chemical functionalities present in the DOM-analogs.