Gradually Tuning the Flexibility of Two-Dimensional Covalent Organic Frameworks via Stepwise Structural Transformation and Their Flexibility-Dependent Properties

Flexible covalent organic frameworks (COFs) are intriguing for their dynamic properties distinctive from rigid counterparts but still suffer from limited accessibility. Especially, controlling flexibility of COFs is challenging and the impact of different flexibility on properties of COFs has rarely been unveiled. This article reports stepwise adjustment on flexibility of two-dimensional COFs, which is realized by the designed synthesis of rigid COF (R-COF), semi-flexible COF (SF-COF), and flexible COF (F-COF) through polymerization, linker exchange, and linkage conversion with a newly developed method for reduction of hydrazone, respectively. Significant difference in breathing behavior and self-adaptive capability of the three COFs are uncovered through vapor response and iodine capture experiments. Gas sorption experiments indicate that the porosity of F-COF could switch from close state in nitrogen to open state in carbon dioxide, which are not observed for R-COF and SF-COF. This study not only develops a strategy to adjust the flexibility of COFs by tuning their linkers and linkages, but also provides a deep insight into the impact of different flexibility on properties of COFs, which lays a foundation for the development of this new class of dynamic porous materials.

Automated summary

This article presents a designed synthesis method to adjust the flexibility of two-dimensional COFs, which includes the preparation of rigid COF (R-COF), semi-flexible COF (SF-COF), and flexible COF (F-COF) through polymerization, linker exchange, and linkage conversion. The study reveals significant differences in breathing behavior and self-adaptive capability among the three COFs through vapor response and iodine capture experiments. Gas sorption experiments show that the porosity of F-COF can switch from a close state in nitrogen to an open state in carbon dioxide, which is not observed for R-COF and SF-COF. This research provides insights into the impact of different flexibility on properties of COFs and offers a strategy to adjust their flexibility by tuning their linkers and linkages.