Monomer Flexibility Enables the Regulation of Interlayer Stacking in Two-Dimensional Covalent Organic Frameworks

Abstract Precise control over interlayer stacking in two-dimensional covalent organic frameworks (2D COFs) remains far less developed than the design of in-plane topology. Here, rather than regulating stacking through external changes in the crystallization medium, we investigate the conformational compatibility of paired monomers as an internal molecular variable for directing the out-of-plane organization of 2D COFs. A topologically related series of eight imine-linked COFs was constructed using different combinations of flexible and rigid monomers. Within the present COF series, flexible-flexible monomer combinations were associated with AA′ stacking, whereas flexible-rigid combinations were associated with ABC stacking. These distinct stacking outcomes are associated with different conformational responses during framework formation. The resulting stacking-defined pore environments further give rise to different xylene adsorption and separation behaviors, with the AA′-stacked COFs showing enhanced discrimination among xylene isomers. This work identifies monomer flexibility matching as an important molecular design consideration for regulating interlayer stacking, extending 2D COF design from in-plane topology to out-of-plane structural control.