Enhancing Structural Control in Covalent Organic Frameworks through Steric Interaction-Driven Linker Design

Covalent Organic Frameworks (COFs) exhibiting kagome (kgm) structures are promising crystalline porous materials with two distinct pores. However, the challenge arises from the potential formation of the polymorphic square-lattice (sql) structure, which is undesired in some cases. To this, we introduce a novel linker design strategy featuring bulky functional groups, enforcing the preferred kgm structure, while hindering sql network formation. Implementing this design, we synthesized a terphenyl core-based tetraaldehyde linker (4A2E) incorporating a bulky methoxycarbonyl-terminated phenyl group. By varying the diamine linker lengths, using phenylenediamine (PDA) and benzidine (Bz), the steric interaction was tuned leading to the formation of different topologies. Structural analysis revealed the formation of a kgm network formation with an unusual ABC stacking for the 4A2E-PDA-COF with the short PDA linker, in contrast to the sql network in 4A2E-Bz-COF with the longer benzidine. This steric interaction-driven design enhances control over COF structures, expanding the design toolbox, but also provides valuable insights into network formation and polymorphism.