Nucleation of Two-Dimensional Boronate Ester-Linked Covalent Organic Framework Moieties in Solution

Nucleation of Two-Dimensional Boronate Ester-Linked Covalent Organic Framework Moieties in Solution

Brian Koo, Paulette Clancy

Chemical and Biomolecular Engineering, Cornell University

Covalent organic frameworks (COFs) are porous materials that are highly regarded for their potential applications in gas storage, catalysis, and optoelectronics. They rival metal-organic frameworks and zeolites in terms of surface area, weight, and porosity. Predictable assembly of COFs via covalent bond-forming reactions from simple building blocks has led to the discovery of many structurally diverse 2D and 3D networks, but the mechanisms governing their nucleation and growth are not understood. Although equilibrium stacking properties of many 2D COF sheets have been experimentally and computationally measured, the effect of solvent, temperature, and surface effects on the thermodynamic processes leading to the growth of thin-film and crystallite COFs has not been studied. COF-forming reactions are reversible in solution and thus lead to the formation of the most thermodynamically stable products.

Our computational approach explores the thermodynamics of nucleation of two COFs, COF-5 and Pc-PBBA, by following their nucleation from individual building blocks (octahydroxyphthalocyanine, hexahydroxytriphenylene, and 1,4-phenylenebis(boronic acid)) in a mesitylene and dioxane solution. We calculate the relative free energies of each boronate ester-linked Pc-PBBA and COF-5 moiety of increasing size to determine likely intermediate structures smaller than the critical nucleus. We then compare the differences between the free energy of growth in the lateral direction (forming larger 2D sheets), and growth in the stacking direction (forming thicker stacks of 2D sheets).

We combine thermodynamic integration and steered molecular dynamics to calculate binding free energies associated with stacking and use entropy calculations to estimate the reaction free energies associated with lateral growth. We follow reversible pathways from individual building blocks to COF fragments with multiple formula units and obtain free energy differences between intermediate products. These studies suggest that lateral growth of COF fragments proceeds more readily at first than vertical stacking, followed by a preference for vertical stacking once COF fragments reach sufficient size. From these results, an estimate of the critical nucleus size for COF growth in solution can be made. By performing free energy calculations of COF moieties we are able to resolve mechanisms of growth too rapid and species too small to be observed experimentally.