Single-Crystal Dynamic\nCovalent Organic Frameworks\nfor Adaptive Guest Alignments

Dynamic 3D covalent organic frameworks (COFs) have shown\na concerted\nstructural transformation upon adaptive guest inclusion. However,\nthe origin of the conformational mobility and the host–guest\nadaptivity remain conjecture of the pedal motions of revolving imine\nlinkages, often without considering the steric hindrance from the\ninterwoven frameworks. Here, we present atomic-level observation of\nthe rotational and translational dynamics in single-crystal COF-300\nupon adaptive guest inclusion of various organic molecules, featuring\nmultiple rotamers of covalent linkages and switchable interframework\nnoncovalent interactions. Specifically, we developed a diffusion gradient\ntransimination protocol to facilitate the growth of COF single crystals,\nenabling a high-resolution X-ray diffraction structural analysis.\nWe uncovered metastable and low-symmetry intermediate phases from\ncontracted to expanded phases during structural evolution. We identified\ntorsion angles in the terephthalaldehyde diimine motifs that switch\nfrom <i>anti-periplanar</i> to <i>syn-periplanar</i>/<i>anticlinal</i> conformations. Moreover, the rotational\ndynamics of the imine linkage were concurrent with the translational\ndynamics of tetraphenylmethane units, which tend to form the translational\nquadruple phenyl embrace. Such conformational mobility allows the\nframeworks to adapt to various guest molecules, such as alcohols,\nesters, phenols, and diols, featuring double linear, herringbone,\nzigzag chains, triple helix, and tubular alignments. Quantitative\nenergy analyses revealed that such dynamic structure transformations\nare not arbitrary but follow specific pathways that resemble protein\nfolding. The work is paving the way to developing robust, dynamic,\nand crystalline molecular sponges for studying the condensed structure\nof liquids without the need for further crystallization.