Two-Dimensional Covalent Organic Framework Solid Solutions

Abstract

Covalent organic frameworks (COFs) generally leverage one or two monomers with specific sizes and shapes to access highly symmetric and periodic polymer networks. Almost all reported COFs employ the minimum sets of monomers needed for the polymerization (usually two, sometimes one) and crystallize in high-symmetry topologies. COFs synthesized from more than two monomers usually employ mixtures with different pendant functionalities to distribute these groups statistically throughout the structure, or monomers with different sizes in ratios targeting lower symmetry topologies. Here, we demonstrate that mixtures of monomers with different lengths generate single-phase, hexagonal two-dimensional covalent organic framework (2D COF) solid solutions at continuously variable feed ratios. X-ray diffraction measurements, Fourier-transform infrared spectroscopy, and Pawley refinement indicate that both monomers distribute randomly within the same lattice, and the lattice parameters continuously increase as more of the larger linker is incorporated. Furthermore, COF solid solutions are accessed directly by polymerizing a mixture of monomers but not via linker exchange from a preformed COF. As strain develops from the lattice accommodating monomers with different sizes, the nonlinear relationship between the monomer incorporation and the COF's lattice parameters suggests that bond-bending of the monomers plays a role in incorporating monomers of different lengths into the solid solutions. Solid solution formation represents a new strategy to design 2D COFs and increase their complexity. Specifically, varying the monomer composition of a given network enables many properties, such as the average pore size, to be continuously tuned between those of corresponding pure COFs.

Understanding fragility and engineering activation stability in two-dimensional covalent organic frameworks

“…This is consistent with the observation reported in prior studies that large pore substituents reduce the accessible pore volume, resulting in a decrease in the BET surface area. 48,49 This is also consistent with other studies of multivariant COFs, 50 in which the pore size changed continuously and became smaller with higher loadings of C8PDA feed ratios (Figure S61), due to occupation of the pore channels by long octyloxy side chains. The decreased BET surface areas after PFH activation (Figure S60) and the decreased pore size (Figure S61) therefore reflects the presence of both PDA and C8PDA in the multivariate COFs.…”

Multivariate Synthetic Strategy for Improving Crystallinity of Zwitterionic Squaraine‐Linked Covalent Organic Frameworks with Enhanced Photothermal Performance

“…The (100) peak position remained unchanged, which possibly resulted from the similar linker length between SQ and TPA building units. As early reported, [ 33 ] such a multivariate crystallization can generate single‐phase COF solid solutions with random distribution of different linkers in the same lattice. When X = 85% and 90%, the targeted PySQ‐COF was formed to afford the identical PXRD patterns, while (100) Bragg diffraction was further weakened.…”

Light-driven molecular motors embedded in covalent organic frameworks

“…1a , blue), also allowing integration of both molecular building blocks within the same framework. Co-condensation in a single framework 62 enhances the free space in which the motor can rotate. The methyl groups of the spacer further assist in increasing free volume for the motor rotation by favoring an antiparallel arrangement between neighboring fluorene units.…”