A series of basicity-tunable ionic liquids with carboxylate and sulfonate anion were designed, prepared, and applied in NO capture. Both high absorption capacity and low desorption residue were achieved through tuning the basicity of the anion, leading to a superhigh working capacity of more than 6 mol per mol ionic liquid, which is superior to that of traditional absorbents. Through a combination of absorption experiments, quantum chemical calculations, and spectroscopic investigations, the results indicated that such a high absorption capacity was originated from multiple sites interaction between oxygen atoms of carboxylate or sulfonate anion and NO, while the reduced interaction induced by the decreased basicity of the O-site ionic liquid led to excellent desorption. We show that the highly efficient and reversible NO capture by these tunable ionic liquids provides a new strategy for improving gas capture and utilization, which is also important for some other fields such as material, biology, and medicine.

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 CO 2 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 CO 2 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 CO 2 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 CO 2 in an energy-saving way.

A strategy to prepare liquid anion-functionalized pillar [5]arene by incorporating a large asymmetrical cation on the rims was proposed. This kind of pillar[5]arene-based ionic liquid showed high thermal stability, good CO 2 absorption capacity, and excellent reversibility. In contrast with the neutral pillar[5]arene and the noncyclic monomeric analogue, pillar[5]arene-based ionic liquid exhibited significantly enhanced CO 2 absorption capacity. Through a combination of absorption experiments, spectroscopic investigations, and quantum chemical calculations, the results indicated that the increased capacity originated from multiple-site interactions between CO 2 and the carboxylic anion, which was enhanced by the cavity of pillar [5]arene. This work provided an efficient methodology for preparing a liquid macrocyclic host as well as improving gas absorption.

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