Competition between side-chain interactions dictates 2D polymer stacking order

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Controlling the two-dimensional polymerization processes of twodimensional covalent organic frameworks (2D COFs) is essential to fully realizing their distinct properties. Although most 2D COFs have been isolated as polycrystalline aggregates with only nanometer-scale crystalline domains, we have identified rapid, solvothermal conditions that provide micrometer-scale and larger single-crystal 2D polymers for a few 2D COFs. Yet it remains unclear why certain conditions produce far larger 2D polymers than others, which hinders generalizing these findings. The guiding principles for controlled two-dimensional polymerization in solution remain unclear. Here, we study the crystallization processes of both single-crystalline and polycrystalline 2D COFs using ultrasmallangle X-ray scattering (USAXS) for the first time, through which we characterized COF formation conditions with scattering data collected every few seconds. In situ USAXS experiments revealed distinct growth mechanisms between single-crystalline and polycrystalline COFs and suggested a nonclassical particle fusion-based growth model for single-crystalline COFs that results in faceted, hexagonal particles. These findings were corroborated by in situ wide-angle X-ray scattering (WAXS) experiments and scanning electron microscopy (SEM). In contrast, polymerizations that provide polycrystalline COFs evolve as spherical aggregates that do not fuse in the same way. These insights into how micrometer-sized, crystalline 2D polymers are formed in solution point a way forward to establishing a robust connection between the 2D polymer structure and designed properties by controlling their polymerization processes.

Section: ■ Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Nonclassical Crystallization Processes of Single-Crystalline Two-Dimensional Covalent Organic Frameworks

Natraj,

Landman,

Pelkowski

et al. 2024

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“…These results suggest that alkyloxy groups of these lengths might stabilize 2D COF structures, which is consistent with a recent finding that a hexyloxy-functionalized 2D COF retained its crystallinity at higher zwitterion loadings as compared to an analogous material containing methoxy groups. 61 These observations are important for understanding and controlling the COF formation, particularly for materials whose properties rely on achieving a specific pore size, long-range π-orbital overlap, or the incorporation of functional side chains.…”

Section: ■ Introductionmentioning

confidence: 99%

Tuning Crystallinity and Stacking of Two-Dimensional Covalent Organic Frameworks through Side-Chain Interactions

Pelkowski,

Natraj,

Malliakas

et al. 2023

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Two-dimensional covalent organic frameworks (2D COFs) form as layered 2D polymers whose sheets stack through high-surface-area, noncovalent interactions that can give rise to different interlayer arrangements. Manipulating the stacking of 2D COFs is crucial since it dictates the effective size and shape of the pores as well as the specific interactions between functional aromatic systems in adjacent layers, both of which will strongly influence the emergent properties of 2D COFs. However, principles for tuning layer stacking are not yet well understood, and many 2D COFs are disordered in the stacking direction. Here, we investigate effects of pendant chain length through a series of 2D imine-linked COFs functionalized with n-alkyloxy chains varying in length from one carbon (C 1 COF) to 11 carbons (C 11 COF). This series reveals previously unrecognized and unanticipated trends in both the stacking geometry and crystallinity. C 1 COF adopts an averaged eclipsed geometry with no apparent offset between layers. In contrast, all subsequent chain lengths lead to some degree of unidirectional slip stacking. As pendant chain length is increased, trends show average layer offset increasing to a maximum of 2.07 Å in C 5 COF and then decreasing as chain length is extended through C 11 COF. Counterintuitively, shorter chains (C 2 –C 4 ) give rise to lower yields of weakly crystalline materials, while longer chains (C 6 –C 9 ) produce greater yields of highly crystalline materials, as confirmed by powder X-ray diffraction and scanning electron microscopy. Molecular dynamics simulations corroborate these observations, suggesting that long alkyl chains can interact favorably to promote the self-assembly of sheets. In situ proton NMR spectroscopy provides insights into the reaction equilibrium as well as the relationship between low COF yields and low crystallinity. These results provide fundamental insights into principles of supramolecular assembly in 2D COFs, demonstrating an opportunity for harnessing favorable side-chain interactions to produce highly crystalline materials.

Photons, Excitons, and Electrons in Covalent Organic Frameworks

Blätte,

Ortmann,

Bein

2024

Covalent organic frameworks (COFs) are created by the condensation of molecular building blocks and nodes to form two-dimensional (2D) or three-dimensional (3D) crystalline frameworks. The diversity of molecular building blocks with different properties and functionalities and the large number of possible framework topologies open a vast space of possible welldefined porous architectures. Besides more classical applications of porous materials such as molecular absorption, separation, and catalytic conversions, interest in the optoelectronic properties of COFs has recently increased considerably. The electronic properties of both the molecular building blocks and their linkage chemistry can be controlled to tune photon absorption and emission, to create excitons and charge carriers, and to use these charge carriers in different applications such as photocatalysis, luminescence, chemical sensing, and photovoltaics. In this Perspective, we will discuss the relationship between the structural features of COFs and their optoelectronic properties, starting with the building blocks and their chemical connectivity, layer stacking in 2D COFs, control over defects and morphology including thin film synthesis, exploring the theoretical modeling of structural, electronic, and dynamic features of COFs, and discussing recent intriguing applications with a focus on photocatalysis and photoelectrochemistry. We conclude with some remarks about present challenges and future prospects of this powerful architectural paradigm.