2D-to-3D transformations of a covalent organic framework via post-synthetic crosslinking

Abstract

Two-dimensional (2D) covalent organic frameworks (COFs) are easier to synthesize and functionalize than their three-dimensional (3D) counterparts, but the 2D frameworks lack stability due to weak non-covalent interactions that maintain the layered structure. Herein, we provide a post-synthetic strategy to covalently crosslink the independent sheets of 2D COFs while preserving the crystallinity and porosity of the materials. The crosslinked frameworks show greatly enhanced mechanical stability compared to the parent 2D frameworks, retaining more than 90% of the original Brunauer-Emmett-Teller (BET) surface area when subjected to extensive sonication or grinding. Further, crosslinking enables the reduction of the imine linkages with sodium borohydride while preserving crystallinity and porosity, which has yet to be shown for 2D COFs. Finally, the imine linkages on a crosslinked framework were first reduced and then reacted with an acyl chloride, establishing a general approach to framework functionalization. This post-synthetic crosslinking approach stabilizes 2D frameworks and opens access to amine linkages in these materials, thus increasing hydrolytic stability and potential functionalization as selective adsorbents.

Scheme 1. Synthesis and stability testing of COF-V (a and b), COF-S₂C₃ (c), and COF-S₁C₃ (d).

Fig. 1. (a) Simulated structure of COF-V. (b) Simulated structure of COF-S₂C₃. (c) Simulated structure of dithiol crosslinkers. (d) Chemical structures of reported materials. (e) ¹³C CP-MAS ssNMR spectra, (f) PXRD patterns, (g) BET surface areas, and (h–l) SEM images for the reported materials.

Fig. 2. (a) Chemical structure (n = 2, 3, or 4) for the crosslinked materials before and after imine reduction. (b) ¹⁵N CP-MAS ssNMR spectra, (c) FTIR spectra, (d) PXRD patterns, and (e–i) SEM images for COF-V-R, COF-S₂C₂-R, COF-S₂C₃-R, COF-S₂C₄-R, and COF-S₁C₃-R.

Fig. 3. (a) N₂ sorption isotherms at 77 K (filled circles show adsorption and open circles show desorption) for the reported materials after treatment with NaBH₄ and (b) retention of BET surface area (%) for after NaBH₄ treatment.

Fig. 4. (a) ¹³C CP-MAS ssNMR spectra and (b) ¹⁵N CP-MAS ssNMR spectra for COF-S₂C₃-R (blue) and COF-S₂C₃-Ac (orange).

Fig. 5. N₂ sorption isotherms of (a) COF-V, (b) COF-S₂C₃, and (c) COF-S₁C₃ before and after mechanical treatment. Filled circles show adsorption and open circles show desorption. (d) Retention of BET surface area (%) after mechanical stability tests for COF-V, COF-S₁C₃, and COF-S₂C₃.