Channel adaptability in a permanently porous peptoid material

Porous materials have piqued the interest of material chemists for decades since they are useful for a variety of applications such as gas storage, catalysis, separation processes, and so on. Among the most studied porous materials metal-organic framewoks (MOF), covalent-organic frameworks (COF) and zeolites must be cited for their peculiar properties. Recently, it has been widely proven that cyclic peptoids are a viable building block for constructing porous molecular solids, thanks to the possibility to change the side chains nature and vary the ring size. In our research group, we observed and characterized the acetonitrile release and uptake capability of the compound cyclo-(Nme-Npa2)2 (1) through a single-crystal-to-single-crystal transformation (SCSC). During the uptake and release, the solid state assembly undergoes a dramatic reorganization, with two vertical propargyl side chains rotating by 113° and generating an unprecedented CH-π zipper that connects the peptoid columns in the desolvated crystal form. The conformational flexibility of 1 leads to several solvatomorphs showing different crystal packing. We have shown that it is possible to induce guest exchange in a methanol solvate crystal (form 1D) to generate an isomorphous hydrate form (form 1E), as well as a guest release to obtain a stable empty crystal form (form 1F). Given the presence of small voids in the crystal packing of the crystal form 1F, we decided to explore its sorption properties using in situ X-ray powder diffraction (XRPD) analysis using propyne, carbon dioxide and methane gas. Thanks to the combination of experimental techniques and computational studies, we were able to confirm the absorption of the different gases and precisely localize them into the unit cell. The crystal framework shows great adaptability to guest molecules and this ensures the guest exchange, release, and sorption. The volume of the cavities in can vary depending on the size of the guest molecule, ranging from 11.9 Å3 in hydrate form 1E and 14.6 Å3 in the empty crystal form 1F to 136.6 Å3 in the crystalline powder loaded with propine molecules.