Development of Novel Chemical Probes for Examining Triplet Natural Organic Matter under Solar Illumination

Excited triplet states of chromophoric dissolved organic matter (³CDOM∗) are critical transient species in environmental photochemistry. In the present study, sorbic amine (2,4-hexadien-1-amine) and sorbic alcohol were employed as new probe molecules for triplet measurements and compared to the results measured from sorbic acid under identical conditions. Unlike sorbic acid, sorbic amine and sorbic alcohol were not directly photolyzed under solar irradiation. Photosensitized isomerization of the probes with the conjugated diene structure could yield four geometrical isomers. The separation and quantitative determination of the geometrical isomers were accomplished using HPLC and high-resolution NMR analyses. When photoirradiated Suwannee River natural organic matter (SRNOM) was employed as a source of ³CDOM∗, significantly different photosensitized isomerization rates were observed for the diverse charged probes. The bimolecular reaction rate constants between ³SRNOM∗ and the probes were calculated as (0.42 ± 0.1) × 10⁹ M^-1 s^-1 for sorbic acid, (1.1 ± 0.1) × 10⁹ M^-1 s^-1 for sorbic alcohol, and (5.2 ± 0.4) × 10⁹ M^-1 s^-1 for sorbic amine, respectively. The average apparent ψ_triplet was (0.96 ± 0.03)% based on an irradiation range of 290 to 400 nm. We developed highly selective and efficient probes for triplet determination and elucidated the different reaction behaviors of these conjugated dienes containing different charged substituents within the photochemical energy transfer process.