Humidity-Responsive ON/OFF Switching of Gas Inclusion by Using Cooperative Opening/Closing of Heterogeneous Crystalline Cavities in a Peptide Ni^II Macrocycle

Humidity-responsive ON/OFF switching of CO₂ inclusion, associated with cooperative structural changes of heterogeneous cavities in crystalline peptide Ni^II macrocycles, 1⋅NO₃, is demonstrated. The structural transformation between an “open form” and a “closed form” was caused by reversible water uptake into one of the cavities. The adsorption behavior of CO₂ was studied with and without water vapor, combined with gas-composition analysis by using gas chromatography. The studies revealed that the open form of 1⋅NO₃ adsorbs CO₂ gas up to 34 g mol⁻¹, whereas the closed form adsorbs only a negligible amount of CO₂ gas. Also, structural studies (X-ray and IR spectroscopies) and investigation of water-content dependence of CO₂ adsorption revealed that the switching of gas inclusion occurred abruptly over a narrow range of water-vapor pressure and was caused by the cooperative opening of vacant space to give the open form due to the structural transformation by water uptake.