Arrhenius Photobases and Their Application to CO2 Capture

Leading strategies for the capture of CO2 from point sources and directly from the atmosphere confront the challenge of high energy costs for thermal sorbent regeneration. In response, photochemical processes driven by sunlight as the sole external stimulus have recently been advanced as a promising alternative. Although many examples of light-induced pH swings using metastable photoacids have been reported, the complementary mode of operation, using photoswitchable bases, has not been extensively considered 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 (nearly 6 pH units) with high reversibility (ca. 1% degradation per cycle). The system is stable to oxygen, can be driven by natural sunlight, and is shown to concentrate CO2 from ambient air. To understand the high efficiency (>20% photochemical quantum yield) of hydroxide release, the mechanism of C–O dissociation was elucidated using transient-absorption spectroscopy. This study provides a general framework for the design of photoreversible aqueous bases and guiding principles for their usage in solar-powered CO2 management.