Dynamic Imaging of Solvent Exchange-Induced Interlayer Shifting in Single Two-Dimensional Covalent Organic Frameworks.

The interlayer shifting of two-dimensional (2D) covalent organic frameworks (COFs) is a critical issue affecting their sorption and catalytic properties. However, current analytical tools often measure static and averaged stacking properties from bulk samples, restricting a comprehensive understanding of the interlayer shifting process in individual 2D COFs. Here, we report real-time dynamic imaging of the solvent exchange-induced interlayer shifting in single 2D COFs using in situ dark-field optical microscopy (DFM). The increase in pore size from quasi-AB stacking to AA stacking in triphenylbenzene (TPB)-terephthaldehyde-COF and TPB-dimethoxyterephthaldehyde-COF in organic solvent and water facilitates the accommodation of more guest molecules, leading to a higher scattering intensity. By using the grayscale intensity of DFM images as a quantitative indicator, we visually observe that the solvent exchange-triggered interlayer shifting dynamics not only exhibits a significant particle-to-particle heterogeneity but also is sensitive to the solvent viscosity and interlayer interaction strength of 2D COFs. Moreover, theoretical simulations elucidate that this interlayer shifting process is governed by crystal stacking energies and follows a thermodynamically preferred pathway during the water-to-organic exchange process. This imaging methodology provides a useful way for direct observation of solvent exchange-induced interlayer shifting in single 2D COFs and understanding its dynamics.