Unidirectional compression and expansion of a crosslinked MOF crystal prepared via axis-dependent crosslinking and ligand exchange

The artificial construction of anisotropic deforming materials is one of the great challenges in materials and polymer chemistry. In this paper, we demonstrate a unidirectionally deformable material with reversibility. First, crystal crosslinking of a pillared-layer metal-organic framework (PLMOF) was accomplished, followed by the exchange of the pillar ligand with the monotopic ligand. The obtained crosslinked MOF crystal showed reversible unidirectional compression and expansion during cycles of drying and immersion in good solvents. The macroscopic unidirectional deformation was derived from microscopic variations in the layer distance in the MOF crystal. The polymerization between the organic ligand and the crosslinker effectively reinforced the MOF crystal, which had enough durability for reversible unidirectional deformation. Our strategy is a promising general method for the construction of anisotropic deforming materials, which are often seen in biological systems and mechanical devices.