Quenching Kinetics of Triplet-State Photosensitizers in Environmental Photochemistry

Triplet-state photosensitizers (3Sens*) are central to carbon cycling and pollutant degradation in sunlit surface and atmospheric waters, but their environmental roles remain only partly understood. Although the photophysical and photochemical properties of 3Sens* have been investigated, most studies remain confined to simplified systems with limited environmental relevance. This review critically analyzes approaches for characterizing the kinetic behavior of 3Sens*, focusing on bimolecular quenching rate constants (kq, M-1 s-1), and evaluates the strengths and limitations of current methodologies for their determination. A curated dataset (n = 105), spanning 5 orders of magnitude (105 to 109 M-1 s-1), is then used to analyze how molecular structure (electron-donating groups, conjugation, steric hindrance) and environmental conditions (pH, solvent polarity, ionic strength, temperature) govern kq and alter underlying reaction mechanisms. By coupling kinetic isotope effects with spectroscopic evidence, the review distinguishes redox-driven pathways from energy transfer processes. These insights connect molecular-scale controls on kq with environmentally relevant conditions. However, key gaps remain in triplet-yield data, mixed-photosensitizer systems, and interfacial processes limiting our understanding of 3Sens*-mediated transformations in aqueous and atmospheric multiphase environments.