(214ae) 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.