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

Separation of carbon dioxide (CO) from point sources or directly from the atmosphere can contribute crucially to climate change mitigation plans in the coming decades. A fundamental practical limitation for the current strategies is the considerable energy cost required to regenerate the sorbent and release the captured CO for storage or utilization. A directly photochemically driven system that demonstrates efficient passive capture and on-demand CO 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 or mechanisms for modulating the stability of CO-derived dissolved species by using photoinduced metastable states. Here, we show that an organic photoswitchable molecule of precisely tuned effective acidity can repeatedly capture and release a near-stoichiometric quantity of CO according to dark-light cycles. The CO-derived species rests as a solvent-separated ion pair, and key aspects of its excited-state dynamics that regulate the 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 emission technologies.