Chemical Construction of Molecular Truss Lattices with Tunable Topologies

Engineering connectivity at the nanoscale enables unprecedented mechanical metamaterials with exotic properties. However, nanomanufacturing 3D lattices with molecular connectivity and tunable topologies is challenging. Here, we select a supramolecular material named metal-organic framework (MOF) as the prototype, where molecules are employed as nodes and beams to construct nanosized truss lattices. An MOF named PCN-700 featuring a well-defined body-centered cubic structure is synthesized, of which the molecular connectivity, topology, and internal stress can be precisely tuned via postsynthetic installation of organic linkers with variable lengths. Herein, the topology is regulated with subnanometer resolution, affording lightweight materials with tunable elastic moduli (8.9-17.4 GPa) without apparent density changes, confirmed by atomic force microscopy indentation. The study of the compressive behaviors from nanonewton to millinewton regimes establishes a connection between the intrinsic chemical structures and the mechanical properties, where the molecular connectivity determines the lattice deformation mode. Raman spectra and ab initio calculations indicate that the PCN-700 can accommodate compressive deformation through the rotation of molecular planes within the organic ligands, contributing to the integral stiffness. The insights presented here will not only uncover MOFs' application potentials in mechanics but also inspire chemical design and precision engineering of mechanical metamaterials at the nanoscale.