(197i) Covalent Organic Framework Spheres, Hollow Fibers and Films with Pompon Structure

Covalent organic frameworks (COFs) are emergent nanoporous organic solids with pure honeycomb crystal structure and robust covalent linkage between building units. In particular, two-dimensional (2D) COF flakes with a high degree of crystallinity and versatile functionalities promise lightweight materials for diverse applications. Thus far, however, most reported COFs are in forms of sphere with a relatively low degree of crystallinity due to the equilibrium shape of crystals or particles can be determined by “Wulff construction”. For two-dimensional crystals, flakes are the most common shape. Here we report a facile dynamic combinatorial chemistry (DCC) strategy to synthesis 2D COF nanoflakes and their 3D assemblies with high crystallinities and controllable morphologies varying from nanosphere, nanofilm, to microfiber. Specifically, monofunctional amine and aldehyde are introduced to the synthesis system for increasing reversibility by reducing reaction energy. As a result, thermodynamic stabile morphology is preponderant bringing abundant secondary structure for COFs. Different shapes such as spheres, hollow fibers and films with pompon structure are obtained by our strategy, as well as improved crystallinity and surface area of 882 m²/g, which is highest for this species of COF.