Electron Spin Resonance Study of Radicals Produced from Organic Triplet Excited States in Photolyzed Aqueous Mixtures

Photochemically produced triplet excited states of organic matter (³C*) play a crucial role in establishing free radical pools that drive oxidation chemistry in atmospheric and terrestrial water systems. Chemical characterization and quantitative speciation of these free radical mixtures are challenging due to their short lifetimes and the inherent complexity of the mixtures. In this study, we demonstrate the use of electron paramagnetic resonance (EPR) spectroscopy, assisted with the spin trap reagent 5,5-dimethyl-1-pyrroline-N-oxide (DMPO), to measure concentrations of free radicals initiated by ³C* formed through the photoexcitation of 3,4-dimethoxybenzaldehyde (DMB). We evaluate the background radical contributions from the photolysis of aqueous DMPO under various conditions of single-wavelength and broadband irradiation. Subsequently, we characterize and quantify the DMPO-trapped radical products produced in irradiated aqueous mixtures containing DMB, struvite microcrystal colloids, and a mixture of both DMB and struvite, serving as environmentally relevant proxies. The identified DMPO-trapped radical products include the spin adduct with hydroxyl (^·OH) radicals (DMPO-OH), the spin adduct with hydrogen (^·H) radicals (DMPO-H), the DMPO dimer radical, and the oxidation product DMPOX. Among these, DMPO-OH was found to exist at the highest concentration. The concentration of DMPO products increased linearly with ³C* concentration up to 50 μM, with DMPO-OH existing in the highest concentration. Struvite microcrystals were observed to influence the relative proportions of the DMPO products, altering the radical pool composition. Our experimental observations and computational studies of DMPO excitation energies in an aqueous environment revealed its photolytic lability under irradiation with wavelengths of 365 nm or shorter. These findings indicate the necessity of accounting for background photolysis reactions of DMPO to avoid overestimating radical concentrations during quantitative EPR studies. This work provides valuable insights into the mechanisms of free radical formation mediated by ³C* and evaluates the utility and limitations of DMPO as a tool for investigating complex aqueous photochemical radical systems.