Excited triplet state chromophoric dissolved organic matter (3CDOM*), formed in sunlight surface waters, acts as a powerful oxidant and as a source of other reactive species (e.g., 1O2, OH●, H2O2). Thus, it is important for the degradation of organic pollutants and natural element cycling. Despite its centrality in aquatic photochemistry, there are many open questions about the nature and properties of 3CDOM●, mainly due to the fact that 3CDOM● is a complex mixture of mostly unknown molecules with a low average intersystem crossing quantum yield (QYISC). Thereby it has no clearly distinguishable signal in transient absorption spectra, which could be used to study its properties. Therefore, this study developed a new approach to investigate properties of 3CDOM● using transient absorption laser spectroscopy. Electron transfer is a dominant mode of reactivity for 3CDOM●. In this approach, the radical cation from N,N,N′,N′-tetramethyl-p-phenylenediamine (TMPD), formed through oxidation by 3CDOM●, was used to probe basic physical-chemical properties of CDOM: QYISC, triplet lifetime and triplet energy (distribution). Model sensitizers with a wide range of exited triplet state reduction potentials (0.8-2.4 V vs. SHE) oxidized TMPD at near diffusion-controlled rates, giving support to the idea that a large cross-section of triplet CDOM moieties are able to oxidize TMPD. By observing TMPD●+, the whole complex mixture of 3CDOM● was simplified to a single signal. Thereby it was possible to quantify triplet lifetime and QYISC for DOM isolates and natural water samples with kinetic modelling and comparison with model sensitizers with known properties.

Transient absorption-based approach to estimate the photophysical properties of triplet dissolved organic matter

253rd National Meeting (2017)

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