Tuning the Capture of CO2 through Entropic Effect Induced by Reversible Trans–Cis Isomerization of Light-Responsive Ionic Liquids

Despite a great deal of gas capture strategies based on ionic liquids, reversible tuning of gas absorption by pure ionic liquids using light irradiation has never been reported. Herein, we demonstrate a novel strategy for tuning the capture of CO2 by light-responsive ionic liquids through reversible trans-cis isomerization. These light-responsive ionic liquids were constructed by tailoring the azobenzene group to the cationic moiety, which exhibited different CO2 absorption ability before and after ultraviolet (UV) irradiation. Through a combination of absorption experiments, NMR spectroscopy, differential scanning calorimetry analysis, viscosity measurement, and quantum chemical calculations, the results indicated that the significant difference in CO2 absorption capacity originated from the entropic effect, which was induced by the change in the aggregation state during trans-cis isomerization. This reversible isomerization of ionic liquids upon alternating irradiation of UV light and blue light shows the potential to control the capture and release of CO2 in an energy-saving way.