Unlocking Extra Flexibility of Soft Porous Crystals via Hydrogen Bonding Interaction for Multistep Acetylene Sorption

Flexible porous coordination polymers (PCPs) have garnered significant attention as promising materials for efficient gas storage due to their ability to exhibit large adsorption changes within narrow pressure ranges, although the chemical design principles necessary to control such behavior remain largely undeveloped. In this study, we demonstrate that minor chemical modifications to the ligand constituting PCPs can substantially change their flexibility and gas storage properties. By substituting the conventional isophthalate (ipa) ligand with pyridine-3,5-dicarboxylate (pyda) in a typical CID (coordination polymer with interdigitated framework)-type flexible PCPs, we achieved a significant enhancement in acetylene storage capacity and structural flexibility. The nitrogen atom on the pyda ligand acts as a hydrogen bond acceptor, facilitating strong interactions with acetylene molecules and activating latent structural flexibility. This modification enabled 276% pore volume expansion and a fivefold increase in acetylene adsorption compared to the ipa-based PCP, while also inducing unique multistep gate-opening behavior. Furthermore, the framework demonstrated selective adsorption for acetylene over ethylene and ethane, despite the minimal structural difference in the ligands. These findings highlight the critical role of hydrogen bonding in stabilizing and unlocking hidden framework phases, providing valuable insights into ligand design strategies for improving gas storage performance in flexible PCPs.