Hydrogen Bonding Regulated Flexibility and Disorder in Hydrazone-Linked Covalent Organic Frameworks

Covalent organic framework (COF) chemistry is experiencing unprecedented development in recent decades. The current studies on COF chemistry are mainly focused on the discovery of novel covalent linkages, new topological structures, synthetic methodologies, and potential applications. However, despite the fact that noncovalent interactions are ubiquitous in COF chemistry, relatively little attention has been given to the role of noncovalent bonds on COF structures and their properties. In this work, a series of hydrazone-linked COFs involving noncovalent hydrogen bonds have been constructed, where the hydrogen-bonding interaction plays critical roles in the COF crystallinity and structures. The regulation of structural flexibility, the reversible transition between order and disorder, and the variety of host–guest interactions have been demonstrated in succession for the first time in COFs. The results obtained by the hydrogen-bonding-regulated strategy may also be extendable to other noncovalent interactions, such as π–π interactions, metal coordination interactions, Lewis acid–base interactions, etc. These findings will inspire future developments in the design, synthesis, structural regulation, and applications of COFs by manipulating noncovalent interactions

Hydrogelation and Crystallization of Sodium Deoxycholate Controlled by Organic Acids

The gelation and crystallization behavior of a biological surfactant, sodium deoxycholate (NaDC), mixed with l-taric acid (L-TA) in water is described in detail. With the variation of molar ratio of L-TA to NaDC (<i>r</i> = <i>n</i>_L‑TA/<i>n</i>_NaDC) and total concentration of the mixtures, the transition from sol to gel was observed. SEM images showed that the density of nanofibers gradually increases over the sol–gel transition. The microstructures of the hydrogels are three-dimensional networks of densely packed nanofibers with lengths extending to several micrometers. One week after preparation, regular crystallized nanospheres formed along the length of the nanofibers, and it was typical among the transparent hydrogels induced by organic acids with p<i>K</i>_a₁ value <3.4. Small-angle X-ray diffraction demonstrated differences in the molecular packing between transparent and turbid gels, indicating a variable hydrogen bond mode between NaDC molecules