(376h) Toward Synthesis-Structure-Function Relations of Hierarchically Structured 2D Covalent Organic Frameworks

The covalent linking of organic building blocks (BBs) into crystalline covalent organic frameworks (COFs) has emerged as an exciting route to materials with tunable porosity. Constructed through the dynamically reversible covalent assembly of organic molecules, COFs benefit from intrinsically tunable pore size, composition, topology, and functionality.^1,2 Yet, designing these materials to meet the demands of specific applications requires knowledge of how synthetic conditions offer control over specific structural hierarchy, spanning molecular and mesoscopic (i.e., pore size, topology) to macroscopic scales (i.e., morphology). Judicious choice of monomeric BBs and control of reaction conditions have been established as tools for controlling pore size and crystallinity.³ To tailor characteristic diffusion length scales of these materials, recent work has begun to explore how synthesis conditions (i.e., changes of monomers and kinetic factors) can be exploited for controlling COF morphology.^4,5

Here, we have leveraged COF-LZU1, the first reported 2D imine-based COF,⁶ and TAPB-PDA COF⁷ as test bed materials for elucidating synthesis-structure-function relations governing how factors such as catalyst type and composition influence kinetics of assembly, covalent linkage chemistry, crystal growth, and activation. Specifically, we demonstrate how scandium triflate, an efficient catalyst for critical transimination reactions involved in the synthesis of imine-based COFs, can be exploited for controlling COF mesostructure. Through systematic synthesis, comprehensive characterization, and computational insight, we have mapped a compositional pseudo-phase space in which globular COFs with sub-micron diffusion length scales evolve into unique rosette structures simply as a function of the synthetic catalyst content and type. The rosette-shaped COFs, comprised of interconnected, high-aspect ratio crystalline porous sheets of only several unit cells in thickness, offer orders of magnitude reduction in diffusion length scales and several fold increase in surface area relative to globular structures. Employing the uptake of bulky molecules of varying kinetic diameter and charge, we elucidate key synthesis-structure-function relations for the application of mesostructured COFs for water purification. More generally, the work establishes early synthesis-structure relations that, combined with building block-based design of microstructure, hold promise for realizing unique control over COF mesostructure, morphology, and function with implications for functional thin film (e.g., membranes, electrodes) and powder (e.g., catalysts, sorbents) processing.

References

[1] Ning Huang, et al., Nature Review Materials, 1 (2016), 1-19.

[2] Christian S. Diercks and Omar M. Yaghi, Science, 335 (2017), 923-931.

[3] Ioannina Castano, et al., ACS Cent. Sci., 5 (2019), 1892-1899.

[4] Yuanyuan He, et al., J. Mater. Chem. A, 10 (2022), 11030-11038.

[5] Song Wang, et al., JACS., 143 (2021), 5003-5010.

[6] San-Yuan Ding, et al., JACS., 133 (2011), 19816-19822.

[7] Michio Matsumoto, et al., JACS., 139 (2017), 4999-5002.