(614d) Controlling the Hierarchical Morphology and Crystallinity of 2D Imine-Based Covalent Organic Frameworks By Catalyst, Solvent, and Temperature Choreography

Two-dimensional covalent organic frameworks (2D COFs), synthesized via bottom-up assembly and covalent reticulation of molecular building blocks, offer exciting potential for scalable synthesis of functional materials with customizable pore size, topology, and morphology suited for tackling challenges in catalysis, separations, and other emerging industrial applications.^1,2 The uniformity and accessibility of COF pores are essential for achieving high molecular resolution and efficiency in these applications and are intrinsically linked to the in- and out-of-plane (i.e., stacking) crystallinity and structural hierarchy (i.e., reduced diffusion length scales), respectively, of COF materials. In this work, we elucidate synthesis-structure-function relations governing how room-temperature catalytic conditions can be exploited to tune covalent organic framework (COF) growth and thereby access unique hierarchical morphologies without the need to introduce secondary templates or structure directing molecules. Additionally, we uncover facile post-synthetic solvent and thermal processing strategies, enabling not only preservation but ultimate enhancement of the crystallinity of imine-based COF products.

Exploiting COF-LZU1 as a nominal imine-based COF framework, we establish a mechanistic picture of the unique dual role of scandium triflate, Sc(OTf)₃, as both an efficient synthetic catalyst as well as a growth modifier.³ The work offers a new paradigm for COF synthesis wherein framework defects and interfacial terminal amines of different binding strengths can be selectively titrated to facilitate anisotropic crystal growth. Comprised of interconnected, high-aspect-ratio crystalline porous sheets of only several unit cells in thickness, the resulting rosette-shaped COFs offer orders of magnitude reduction in diffusion length scales and several-fold increase in external surface area and pore accessibility (relative to acetic acid catalyzed COF-LZU1), enabling rapid uptake of bulky surrogate sorbates.

This new approach to controlling COF morphology exploits framework defects to direct growth from crystalline interfaces. Yet, ensuring high crystallinity of these and other COF products upon activation (i.e., removal of pore-occluded synthesis solvents) is critical for their broad application. While solvent exchange and/or rapid solvent removal can collapse pores and disrupt interlayer stacking,^4,5 here we describe facile methanol and water-based post-synthetic processing under controlled temperatures that leads to enhancement of 2D imine-based COF product crystallinity. Leveraging comprehensive characterization, including XRD, BET, FT-IR, XPS, and ICP-OES, we establish a mechanistic understanding of how such facile post-synthetic processing improves the in-plane crystallinity of COF-LZU1 synthesized under various catalytic conditions (e.g., acetic acid, Sc(OTf)₃). The result is a crystalline COF-LZU1 with a BET surface area of 2650 m²/g that is more than twice that reported in the literature.⁶ Taken together, this work establishes novel synthetic handles for controlling imine-based COF morphology and crystallinity. These advances promise impact across the broader class of imine-based COFs, offering unique design strategies for meeting demands of COF applications in separations, catalysis, and beyond.

References

(1) Huang, N.; Wang, P.; Jiang, D. Covalent Organic Frameworks: A Materials Platform for Structural and Functional Designs. Nature Reviews Materials 2016, 1, 16068.

(2) Diercks, C. S.; Yaghi, O. M. The Atom, the Molecule, and the Covalent Organic Framework. Science 2017, 355 (6328), eaal1585.

(3) Guo, H.; Cline, J. P.; Thorpe, R.; Kiely, C. J.; Rangarajan, S.; Snyder, M. A. Catalyst-Derived Hierarchy in 2D Imine-Based Covalent Organic Frameworks. Nanoscale 2025, 17, 6488-6504.

(4) Feriante, C.; Evans, A. M.; Jhulki, S.; Castano, I.; Strauss, M. J.; Barlow, S.; Dichtel, W. R.; Marder, S. R. New Mechanistic Insights into the Formation of Imine-Linked Two-Dimensional Covalent Organic Frameworks. J Am Chem Soc 2020, 142 (43), 18637–18644.

(5) Feriante, C. H.; Jhulki, S.; Evans, A. M.; Dasari, R. R.; Slicker, K.; Dichtel, W. R.; Marder, S. R. Rapid Synthesis of High Surface Area Imine-Linked 2D Covalent Organic Frameworks by Avoiding Pore Collapse During Isolation. Advanced Materials 2020, 32 (2).

(6) Emmerling, S. T.; Germann, L. S.; Julien, P. A.; Moudrakovski, I.; Etter, M.; Friščić, T.; Dinnebier, R. E.; Lotsch, B. V. In Situ Monitoring of Mechanochemical Covalent Organic Framework Formation Reveals Templating Effect of Liquid Additive. Chem 2021, 7 (6), 1639–1652.