Polycrystalline Covalent Organic Frameworks: Multiscale Structural Insights from Morphological Diversity to Dynamic Lattice Evolution

Covalent organic frameworks are a class of crystalline porous materials with covalent skeletons and inherent pores. They are prepared through a topology-directed polymerization process and are usually obtained as polycrystalline materials. Despite rapid progress in materials synthesis and functional expansion, less is known about the structure of polycrystalline frameworks. This situation is ubiquitous in most cases, which complicates comparisons between frameworks, precludes the finding of intrinsic properties, and impedes discrete structure-property correlations. In this work, we report structures of polycrystalline frameworks by uncovering the coexistence of different morphologies and their static and dynamic behaviors, disclosing a realistic picture and multiple-scale insights into structures of the most frequently used polycrystalline frameworks. Visualization with high-resolution transmission electron microscopy revealed four distinct morphologies, i.e., nanoparticle aggregates, nanorod aggregates, ultrathin nanosheets, and parasitic fusion entities. Moreover, direct visualization revealed the existence of different polygons and channels and their contents together with their boundaries and unstructured complements. Our studies revealed new assembly mechanisms via covalent-bond-mediated nanoparticle fusion and amorphous-phase-interconnected crystalline domain formation, providing a foundation for synthesizing high-quality crystalline porous frameworks and exploring their potential properties and functions.