Synthesis of a crystalline two-dimensional [c2]daisy chain honeycomb network

Abstract

Molecular daisy chains are mechanically bonded materials with unique properties and compelling structures. Despite the exploration of numerous daisy chain structures, the synthesis of a crystalline mechanically interlocked polymer comprising daisy chain units remains elusive because flexible linkers typically yield amorphous gels, while rigid structures lack processability. Here we combine supramolecular crystallization preorganization with post-insertion of mechanical bonds to address this limitation. We use a C3-symmetric tritopic monomer with ammonium moieties and oligoether arms to generate a preorganized supramolecular honeycomb-like crystalline network via complementary non-covalent interactions, in an aqueous environment. Subsequently, single-crystal-to-single-crystal transformation-directed thiol–ene click chemistry crosslinks terminal alkenes at the end of the oligoether arms using 1,2-ethanedithiol, covalently locking [c2]daisy chain linkages while preserving long-range order. This two-dimensional mechanically interlocked polymer can be exfoliated from its crystals to generate a multilayer counterpart exhibiting a 47-fold stiffness enhancement relative to its bulk parent. Moreover, the trilayer nanosheets preserve the structural integrity with the same hexagonal symmetry as the bulk parent. Our method enables the synthesis of a single-crystalline two-dimensional mechanically interlocked polymer from flexible monomers with precise synthetic control and unlocks the potential of developing mechanically interlocked materials. A purely organic crystalline two-dimensional mechanically interlocked polymer comprising [c2]daisy chain units forms via preorganized crystallization and thiol–ene click chemistry. This polymer network can be exfoliated to give nanosheets with a 47-fold stiffness enhancement relative to the bulk parent.