Role of cavities created by azobenzene-modified UiO-66 in bulky ionic liquid for high photoresponsive CO2 uptake behavior

Abstract

Photoresponsive porous liquids (PLs) have been reported to capture CO₂ which is controlled by light rather than the traditionally employed temperature and pressure. However, the change in CO₂ uptake tailored by light is low (≤30%) over the reported photoresponsive PLs. Here, a new photoresponsive type III PL is prepared by dispersing a metal–organic framework (MOF, i.e. Azo-UiO-66 containing pendant azobenzene) in a polyethylene glycol-based dicationic bulky ionic liquid (IL). Intriguingly, 78% change in CO₂ uptake upon UV and visible light irradiation is achieved by the PL with 5% Azo-UiO-66, which is much higher than those with pristine Azo-UiO-66 (38%) and neat IL (1%) as well as all reported photoresponsive PLs (≤30%). We demonstrate that, in addition to the intrinsic cavity in Azo-UiO-66, extrinsic cavity is created by the IL packing around the framework with the pendant azobenzene. Upon UV-light irradiation, the bending of the azobenzene groups conceals the active sites and causes dense packing of the IL encircling the MOF, which dissipates the extrinsic cavity and lowers CO₂ adsorption. This is the first report on the generation of extrinsic cavity in PLs which is controllable by light irradiation, resulting in a different mechanism of light-regulated adsorption and a large change in CO₂ uptake.