Reversible CO2 Capture and On-Demand Release by an Acidity-Matched Organic Photoswitch

Separation of carbon dioxide (CO2) from point sources or directly from the atmosphere can contribute crucially to climate-change mitigation plans for the coming decades. A fundamental practical limitation for current strategies is the considerable energy cost required to regenerate the sorbent and release the captured CO2 for storage or utilization. The feasibility of these approaches, including thermal stripping, pressure swing desorption, and electrochemical switching, can only be justified by the availability of affordable, storable, and widely distributable renewable energy. A photochemically driven system that demonstrates efficient passive capture and on-demand CO2 release triggered by sunlight as the sole external stimulus would provide an attractive alternative. However, little is known about the thermodynamic requirements for such a process, nor mechanisms for modulating changes in CO2 affinity with photoinduced metastable states. Here, we show that an organic photoswitchable molecule of precisely matched effective acidity can repeatedly capture and release a near-stoichiometric quantity of CO2 according to dark–light cycles. We show that the CO2-derived species rests as a solvent-separated ion pair, and key aspects of its excited-state dynamics that regulate photorelease efficiency are characterized by transient absorption spectroscopy. The thermodynamic and kinetic concepts established herein will serve as guiding principles for the development of viable solar-powered negative emissions technologies.