Singlet oxygen formation in non-aqueous oxygen redox chemistry: direct spectroscopic evidence for formation pathways and reliability of chemical probes

Abstract

Singlet oxygen (¹O₂) formation is now recognised as a key aspect of non-aqueous oxygen redox chemistry. For identifying ¹O₂, chemical trapping via 9,10-dimethylanthracene (DMA) to form the endoperoxide (DMA–O₂) has become the main method due to its sensitivity, selectivity, and ease of use. While DMA has been shown to be selective for ¹O₂, rather than forming DMA–O₂ with a wide variety of potentially reactive O-containing species, false positives might hypothetically be obtained in the presence of previously overlooked species. Here, we first provide unequivocal direct spectroscopic proof via the ¹O₂-specific near-infrared (NIR) emission at 1270 nm for the previously proposed ¹O₂ formation pathways, which centre around superoxide disproportionation. We then show that peroxocarbonates, common intermediates in metal–O₂ and metal carbonate electrochemistry, do not produce false-positive DMA–O₂. Moreover, we identify a previously unreported ¹O₂-forming pathway through the reaction of CO₂ with superoxide. Overall, we provide unequivocal proof for ¹O₂ formation in non-aqueous oxygen redox chemistry and show that chemical trapping with DMA is a reliable method to assess ¹O₂ formation.