Development of N-Cyclopropylanilines to Probe the Oxidative Properties of Triplet-State Photosensitizers

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.