One-dimensional Self-assembly of Metal-Organic Octahedra towards Mechanically Flexible Microporous Aerogels

One-dimensional Self-assembly of Metal-Organic Octahedra towards Mechanically Flexible Microporous Aerogels

ABSTRACT:

INTRODUCTION

The mechanical properties of materials intrinsically depend on the connectivity of their building blocks. A classic example is the difference between diamond and its low-dimensional allotropes, such as graphene and carbon nanotubes. Diamond possesses a three-dimensional sp three covalent network of carbon atoms, which provides significant stiffness. Due to the difficulty of the dislocation, diamond lacks plastic deformability, exhibits little capacity for stress dissipation, and consequently fractures when the applied stress exceeds a critical threshold. In contrast, graphene and carbon nanotubes are composed of sp two bonding networks of carbon atoms, forming two-dimensional sheets and one-dimensional tubular structures, respectively. Such topological anisotropy allows them to deform flexibly in response to applied stress. Thus, the dimensionality reduction of the bonding network increases the degrees of freedom for stress dissipation.

A similar concept can be applied beyond covalent linkages to molecular assemblies constructed through non-covalent interactions. Dense packing of organic molecules by non-covalent interactions forms molecular crystals with rigid yet brittle mechanical properties. Recent studies have shown that the control of the anisotropy of molecular arrangements within a crystal offers a new flexibility with directional deformation, so-called bending crystals. Ultimate reduction of the dimensionality of molecular assemblies through non-covalent interaction provides one-dimensional supramolecular polymers. Thanks to their fibrous structures self-assembled from pre-designed molecules, supramolecular polymers often exhibit notable mechanical flexibility. Large biological materials also follow a similar trend. For instance, proteins as large macromolecules can be assembled into three-dimensional periodic lattices and form brittle crystals. When proteins are assembled into fibrous structures like spider silk, they exhibit remarkable mechanical properties suitable for various practical applications.

Bridging the gap between small molecules and proteins, supramolecules have recently emerged as one of the essential building blocks for constructing superstructures. Supramolecules assembled from multiple molecular components typically range in size from one to ten nanometers, leading to the emergence of collective functions that are inaccessible to individual molecules alone. Metal-organic polyhedra are one of the representatives of supramolecules, which exhibit defined polyhedral shapes and an intrinsic cavity. Connecting metal-organic polyhedra to form solids via various bonding modes, including coordination or covalent bonds, provides a new porous material platform.

Very recently, we demonstrated that packing of metal-organic polyhedra solely with van der Waals interactions generates a series of three-dimensional diamond frameworks with high stability and porosity, consequently as rigid microporous crystals of van der Waals open frameworks: WaaFs. Similar to the above mentioned connectivity of atoms, molecules and proteins, the dimensionality control of supramolecular assembly can lead to a new material system with controllable mechanical properties; however, most of the reported metal-organic polyhedra assemblies

Here we report the one-dimensional self-assembly of octahedral metal-organic polyhedra through supramolecular polymerization, which leads to the formation of microporous aerogels with exceptional mechanical deformability. To achieve such supramolecular polymerization using metal-organic polyhedra as monomers, we tuned solvent conditions and found that poor solvents with high polarity and strong dispersive interaction induced the metal-organic polyhedra-metal-organic polyhedra interaction to form one-dimensional fibrils, followed by a self-supporting gel. Through the supercritical carbon dioxide drying process, the resulting gels were converted to the corresponding aerogels. Gas adsorption measurements demonstrated that these aerogels possess uniform microporosity, originating from the intrinsic pores of metal-organic polyhedra. Throughout the uniaxial compression tests, the aerogel withstood up to eighty-seven percent compressive strain without fracturing, demonstrating its high mechanical deformability. This result opens a venue for tuning the mechanical properties of supramolecular materials through the dimensional control of their assemblies.