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

Controlling the two-dimensional polymerization processes of two-dimensional 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 ultrasmall-angle 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.

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

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 (C1 COF) to 11 carbons (C11 COF). This series reveals previously unrecognized and unanticipated trends in both the stacking geometry and crystallinity. C1 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 C5 COF and then decreasing as chain length is extended through C11 COF. Counterintuitively, shorter chains (C2-C4) give rise to lower yields of weakly crystalline materials, while longer chains (C6-C9) 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.

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

Interrogating the stacking of two-dimensional polymers (2DPs) as a function of chemical composition is important to leverage their properties. We explore the dependence of 2DP crystallinity and porosity on variable amounts of zwitterions contained within the pores and find that high zwitterion loadings consistently diminish 2DP materials quality. A competition between disruptive zwitterion electrostatic forces and alkyl stabilization directs the stacking order of each 2DP and demonstrates the contrasting effects of side chain composition on 2DP crystallinity and porosity.

Synthesis, Hole Doping, and Electrical Properties of a Semiconducting Azatriangulene-Based Covalent Organic Framework.

Two-dimensional covalent organic frameworks (2D COFs) containing heterotriangulenes have been theoretically identified as semiconductors with tunable, Dirac-cone-like band structures, which are expected to afford high charge-carrier mobilities ideal for next-generation flexible electronics. However, few bulk syntheses of these materials have been reported, and existing synthetic methods provide limited control of network purity and morphology. Here, we report transimination reactions between benzophenone-imine-protected azatriangulenes (OTPA) and benzodithiophene dialdehydes (BDT), which afforded a new semiconducting COF network, OTPA-BDT. The COFs were prepared as both polycrystalline powders and thin films with controlled crystallite orientation. The azatriangulene nodes are readily oxidized to stable radical cations upon exposure to an appropriate p-type dopant, tris(4-bromophenyl)ammoniumyl hexachloroantimonate, after which the network's crystallinity and orientation are maintained. Oriented, hole-doped OTPA-BDT COF films exhibit electrical conductivities of up to 1.2 × 10-1 S cm-1, which are among the highest reported for imine-linked 2D COFs to date.