Carbon capture technology typically uses swings in temperature, pressure or voltage to move between the state capable of CO2 capture and the state where CO2 is released for utilization or storage. Performing these swings often takes considerable energy input. Therefore, using light energy — which could in principle be sunlight — to drive carbon capture presents an intriguing alternative approach. Now, Richard Liu and colleagues at Harvard University report that certain organic molecules can be used to capture gaseous CO2 and release it on-demand when exposed to visible light.
The researchers synthesize a series of photoresponsive spiropyran molecules that can exist in two forms with rather different chemical properties: one has an ‘open’ structure and the other has a ‘closed’ structure; the latter forming by cyclization. The two structures can interconvert, and the equilibrium is shifted towards the closed form on light irradiation. Due to its phenoxide group, the open form can act as a base, deprotonating water to form hydroxide ions, which can then trap CO2 as bicarbonate species in solution. On illumination, the protonated open structure is excited and converts to the unprotonated state and/or the closed structure. The proton that is released can ultimately protonate bicarbonate ions and release CO2. Liu and team’s careful exploration of the structure–activity relationships that allow CO2 capture–release cycles to proceed smoothly should assist future efforts to design improved systems for light-driven carbon capture.
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