Soft Hydrogen-Bonded Organic Frameworks Constructed Using a Flexible Organic Cage Hinge

Abstract

Soft porous crystals combine flexibility and porosity, allowing them to respond structurally to external physical and chemical environments. However, striking the right balance between flexibility and sufficient rigidity for porosity is challenging, particularly for molecular crystals formed by using weak intermolecular interactions. Here, we report a flexible oxygen-bridged prismatic organic cage molecule, <b>Cage-6-COOH</b>, which has three pillars that exhibit "hinge-like" rotational motion in the solid state. <b>Cage-6-COOH</b> can form a range of hydrogen-bonded organic frameworks (HOFs) where the "hinge" can accommodate a remarkable 67° dihedral angle range between neighboring units. This stems both from flexibility in the noncovalent hydrogen-bonding motifs in the HOFs and the molecular flexibility in the oxygen-linked cage hinge itself. The range of structures for <b>Cage-6-COOH</b> includes two topologically complex interpenetrated HOFs, <b>CageHOF-2α</b> and <b>CageHOF-2β</b>. <b>CageHOF-2α</b> is nonporous, while <b>CageHOF-2β</b> has permanent porosity and a surface area of 458 m² g^-1. The flexibility of <b>Cage-6-COOH</b> allows this molecule to rapidly transform from a low-crystallinity solid into the two crystalline interpenetrated HOFs, <b>CageHOF-2α</b> and <b>CageHOF-2β</b>, under mild conditions simply by using acetonitrile or ethanol vapor, respectively. This self-healing behavior was selective, with the <b>CageHOF-2β</b> structure exhibiting structural memory behavior.