Partitioning the interlayer space of covalent organic frameworks by embedding pseudorotaxanes in their backbones

Mono- or few-layer sheets of covalent organic frameworks (COFs) represent an attractive platform of two-dimensional materials that hold promise for tailor-made functionality and pores, through judicious design of the COF building blocks. But although a wide variety of layered COFs have been synthesized, cleaving their interlayer stacking to obtain COF sheets of uniform thickness has remained challenging. Here, we have partitioned the interlayer space in COFs by incorporating pseudorotaxane units into their backbones. Macrocyclic hosts based on crown ethers were embedded into either a ditopic or a tetratopic acylhydrazide building block. Reaction with a tritopic aldehyde linker led to the formation of acylhydrazone-based layered COFs in which one basal plane is composed of either one layer, in the case of the ditopic macrocyclic component, or two adjacent layers covalently held together by its tetratopic counterpart. When a viologen threading unit is introduced, the formation of a host–guest complex facilitates the self-exfoliation of the COFs into crystalline monolayers or bilayers, respectively. Layered COFs are attractive precursors for two-dimensional materials but they are difficult to cleave into mono- or few-layer sheets. Pseudorotaxane moieties have now been embedded into layered COFs to facilitate their cleavage into sheets of uniform thickness. Crown-ether macrocycles within the COF backbone bind to ionic viologen guests, leading to electrostatic repulsion between layers.