Reversible fluorenol photobases that perform CO capture and concentration from ambient air.

Leading strategies for the capture of CO from point sources and directly from the atmosphere face high energy costs for thermal sorbent regeneration. Photochemical processes, driven by sunlight as the sole external stimulus, offer a promising alternative. Despite many reported examples of light-induced pH swings using metastable photoacids, the complementary mode of operation, using photoswitchable bases, has not been extensively considered. This is due in part to the rarity of photobases that can support large, reversible pH jumps in water. Here we report the design of fluorenol-based Arrhenius photobases that take advantage of excited-state aromaticity and ground-state antiaromaticity to generate large basicity swings with high reversibility. The system is oxygen stable, can be driven by natural sunlight and captures/concentrates CO from ambient air. We elucidated the mechanism of C-O dissociation using transient absorption spectroscopy to understand the high efficiency of hydroxide release. This study provides a framework for the design of photoreversible aqueous bases and guiding principles for their use in solar-powered CO management.