Imaging of coexisting classical and non-classical oriented attachment growth pathways in covalent organic framework microcrystals.

Three-dimensional covalent organic frameworks are promising multifunctional materials for applications in adsorption, separation, and catalysis. However, despite the continuous synthesis of an increasing number of three-dimensional covalent organic frameworks, little is known about the crystal growth pathways. Here, we report the real-time visual observation of the crystal growth process of COF-300 and LZU-79, two typical three-dimensional covalent organic frameworks, using in situ dark-field optical microscopy. Our dark-field optical microscopy imaging results reveal that two crystal-growth pathways are simultaneously operative during the liquid growth of COF-300 and LZU-79 microcrystals, including classical crystal growth modes and non-classical oriented attachment mechanisms. Specifically, detailed tracking of the trajectories between two rod-shaped single-crystal COF-300 pairs suggests that the oriented attachment process undergoes several distinct stages such as approach, alignment at (021) facets, tip-to-tip attachment, fusion, and shaping. Theoretical simulation results show that (021) facets of COF-300 microcrystals, which have a lower repulsive energy barrier due to steric solvation forces from intervening solvents, are energetically more favorable than (010) facets, inducing the oriented attachment between adjacent facets. This work enables a fundamental understanding of how three-dimensional covalent organic framework microcrystals grow dynamically, which can aid the further design of three-dimensional covalent organic frameworks with enhanced performances.