Photodegradation of pollutants or chemicals can decrease their lifetime in the environment. It may occur via direct or indirect mechanisms. In the indirect mechanisms, a photosensitizer absorbs light and produces a series of reactive species that can degrade the target compound. In sunlit surface water, a heterogeneous mixture of naturally occurring organic molecules, known as chromophoric dissolved organic matter (CDOM), plays the role the photosensitizer. An important reactive species formed in this way is the triplet state of chromophoric dissolved organic matter (3CDOM*) itself. Investigation on the formation and decay of 3CDOM* is important for predicting triplet-state-dominated degradation kinetics and exploring the formation of other photochemically produced reactive intermediates. Unlike the study of the triplet states of well-defined compounds, CDOM is a very complex mixture that is still not well characterized. It is consisted of chromophores varying in their excited state energies, intersystem crossing quantum yields, triplet lifetimes and excited state redox potentials. For this reason, it is common to use single model sensitizers for CDOM, which produce triplet states mimicking 3CDOM* reactivity. The advantage of using a model sensitizer is that it is easier to explore the photophysical and photochemical properties of the triplet state of a known system in comparison to a mixture of unknown transient species of 3CDOM*. The goal of the present research is to study a set of model sensitizers and probe compounds that are widely used in the environmental photochemistry community to better understand their photochemical properties and quenching behavior. The chosen set of model sensitizers simulates the possible range of photoreactivity of various moieties present in CDOM and its behavior towards popular probe molecules for 3CDOM* characterization. In addition, the variation of the triplet state properties of the sensitizers used in this study also can allow one to estimate the fraction of the CDOM triplet pool captured by different probes. The dynamics of all these processes will be presented and discussed in detail.

Model sensitizers for investigating indirect photodegradation reactions

ACS Spring 2022

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