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

Singlet oxygen ( 1 O 2 ) formation is now recognised as a key aspect of non-aqueous oxygen redox chemistry. For identifying 1 O 2 , chemical trapping via 9,10-dimethylanthracene (DMA) to form the endoperoxide (DMA-O 2 ) has become the main method due to its sensitivity, selectivity, and ease of use. While DMA has been shown to be selective for 1 O 2 , rather than forming DMA-O 2 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 1 O 2 -specific near-infrared (NIR) emission at 1270 nm for the previously proposed 1 O 2 formation pathways, which centre around superoxide disproportionation. We then show that peroxocarbonates, common intermediates in metal-O 2 and metal carbonate electrochemistry, do not produce false-positive DMA-O 2 . Moreover, we identify a previously unreported 1 O 2 -forming pathway through the reaction of CO 2 with superoxide. Overall, we provide unequivocal proof for 1 O 2 formation in non-aqueous oxygen redox chemistry and show that chemical trapping with DMA is a reliable method to assess 1 O 2 formation.