Direct imaging of facet development during growth of single-crystal covalent organic frameworks

Single-crystal covalent organic frameworks (COFs) have received increasing interest due to their well-defined structures and exceptional performance. However, the reaction pathways of the facet development during growth of single-crystal COFs remains unclear, limiting the regulation of their shapes and functions. We report the dynamic imaging of the facet development during single-crystal COF-300 microcrystal growth using in situ dark-field optical microscopy. Our observations reveal that the concentration of terephthalaldehyde (BDA) linkers not only influences the morphology of the primary COF-300 crystal seeds but also governs the facet evolution, leading to the generation of differently shaped COF-300 microcrystals. Mechanistic studies reveal that the observed shape evolution of COF-300 single crystals arises from distinct kinetically or thermodynamically controlled growth pathways, depending on the BDA concentration. Moreover, we uncover the shape-dependent deformation capacity after solvent uptake, demonstrating the impact of COF-300 crystal morphology on its performance. These discoveries elucidate the COF facet transition mechanism and offer guidance for the future design of single-crystal COFs.