Storage of CO₂ into Porous Coordination Polymer Controlled by Molecular Rotor Dynamics

Design to store gas molecules, such as CO₂ , H₂ , and CH₄ , under low pressure is one of the most important challenges in chemistry and materials science. Herein, we describe the storage of CO₂ in the cavities of a porous coordination polymer (PCP) using molecular rotor dynamics. Owing to the narrow pore windows of PCP, CO₂ was not adsorbed at 195 K. As the temperature increased, the rotors exhibited rotational modes; such rotations dynamically expanded the size of the windows, leading to CO₂ adsorption. The rotational frequencies of the rotors (k≈10^-6  s) and correlation times of adsorbed CO₂ (τ≈10^-8  s) were elucidated via solid-state NMR studies, which suggest that the slow rotation of the rotors sterically restricts CO₂ diffusion in the pores. This restriction results in an unusually slow CO₂ mobility close to solid state (τ≥10^-8  s). Once adsorbed at room temperature, CO₂ is robustly stored in the PCP under vacuum at 195-233 K because of the steric hindrance of the rotors. We also demonstrate that this mechanism can be applied to the storage of CH₄ .