Unraveling the Importance of Carboxyl Groups in Photochemically Excited Triplet Formation of Chromophoric Dissolved Organic Matter.

Carboxylation and decarboxylation are among the most ubiquitous processes in chromophoric dissolved organic matter (CDOM) transformation, occurring during humification, photooxidation, and microbial activity. However, their impacts on the formation of excited triplet-state CDOM (3CDOM*) remain poorly constrained. Using more than 20 model compounds representing common CDOM chromophores, we demonstrate that carboxylation enhances triplet quantum yields (Φ) by 1.1-69-fold relative to their parent compounds. Consistently, Φ values of CDOM isolates positively correlate with their carboxyl content. Theoretical calculations reveal that carboxylation shifts the lowest excited state from locally excited π-π* character toward mixed n-π*/charge-transfer states and increases spin-orbit coupling, thereby promoting intersystem crossing and triplet formation. Building on these insights, we developed quantitative models that predict Φ using thermodynamic and electronic descriptors and established empirical relationships between Φ and bulk CDOM properties, including molecular weight and total antioxidant capacity. Furthermore, solar irradiation of humic acid induced decarboxylation accompanied by a decline in Φ, whereas carboxyl introduction enhanced the overall photochemical reactivity of CDOM toward organic contaminant degradation. Together, these findings elucidate how carboxylation-decarboxylation modulates 3CDOM* formation, advancing mechanistic understanding of aquatic reactive species generation and natural attenuation of organic contaminants.