Abstract
Anilines have been shown to be especially susceptible to single-electron oxidation by excited triplet-state photosensitizers (3sens*), and thus, are good potential candidates to probe the oxidative properties of triplet-state chromophoric dissolved organic matter (3CDOM*). However, steady-state experiments tend to underestimate their rate of oxidation by 3CDOM* due to radical cation quenching (i.e., aniline•+ → aniline) by antioxidant moieties present in DOM. We envisioned the potential utility of N-cyclopropylaniline (CPA) to overcome this limitation, as it is known to undergo spontaneous, irreversible cyclopropyl ring-opening after an initial single-electron oxidation. To test this, first a set of CPA analogs was synthesized and then paired with a model sensitizer and antioxidant, or various DOM isolates, to examine their reactivity and susceptibility to antioxidant quenching during steady-state photolysis experiments. Next, time-resolved measurements of CPA and CPA analog oxidation were obtained by laser flash photolysis through direct observation of 3sens* and radical cations of CPA and CPA analogs. Finally, CPA photolysis products were isolated by semi-preparative high-performance liquid chromatography and identified by nuclear magnetic resonance spectroscopy. Outcomes of this work, including oxidation bimolecular rate constants of CPA and CPA analogs (∼9 × 108 to 4 × 109 M-1 s-1), radical cation lifetimes of CPA and its analogs (140-580 ns), and identified ring-opened products, support the usefulness of cyclopropylanilines as single-electron transfer probes in photosensitized aqueous solutions.
Highlights
In most natural aquatic systems, chromophoric dissolved organic matter is the dominate light absorber.[1−5] Upon absorption of light of sufficient energy, ground-state CDOM is initially promoted to its excited singlet-state and a small portion
10%) undergoes intersystem crossing to the excited tripletstate (3CDOM*).6 3CDOM* is a highly reactive species that plays a central role in sunlit natural waters through the generation of other reactive intermediates via energy transfer, and through the degradation of environmental contaminates via direct oxidation.[6,7]
It has been shown that 3CDOM* is capable of oxidizing electron-rich phenols to phenoxy radicals, and there is evidence that supports both two-step and single-step oxidation mechanisms.[7]
Summary
In most natural aquatic systems, chromophoric dissolved organic matter (chromophoric DOM or CDOM) is the dominate light absorber.[1−5] Upon absorption of light of sufficient energy, ground-state CDOM is initially promoted to its excited singlet-state and a small portion (i.e., ∼5−10%) undergoes intersystem crossing to the excited tripletstate (3CDOM*).6 3CDOM* is a highly reactive species that plays a central role in sunlit natural waters through the generation of other reactive intermediates (e.g., singlet oxygen, 1O2) via energy transfer, and through the degradation of environmental contaminates via direct oxidation.[6,7]There are numerous examples in the literature of small molecule, pollutant oxidation by 3CDOM*,8−11 of which, those pollutants containing aniline or phenol moieties are well represented.[7]. Rate constants obtained from DOM photolysis experiments were first corrected for light-screening by a previously described method.[27] Bimolecular reaction rate constants, or kCPA,2AN values, were estimated by calculating [1O2]ss and [32AN*]ss values derived from FFA depletion kinetics (see Text S1 discussion in the SI) during steady-state experiments.[7,28,29] Bimolecular quenching rate constants, or kq values, were estimated by direct observation of 32AN* lifetime as a function of CPA probe concentration during LFP
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