Discovery of Porphyrinic Metal-Organic Frameworks as Advanced Materials Platform

Abstract

Recently, metal-organic frameworks (MOFs) have been growing in the field of solid-state material for the past three decades due to potential applications such as gas storage, sensing, and catalysis. MOFs consist of organic ligands and metal-based secondary building units (SBUs) via self-assembly. Numerous building blocks are possible to create about 80,000 MOFs based on reticular chemistry. Especially, several people want to embed active metal sites in the organic ligand, thus they introduced metalloligand such as porphyrins, ferrocenes, et al. Among them, we have been interested in the porphyrinic ligands as a building block of MOFs. In this study, various porphyrinic MOFs show fascinating behaviors such as abnormal phenomenon and sorption properties. The first theme is abnormal mechanical properties of 2D and 3D porphyrinic MOFs under external stimuli or guest inclusion. Mechanical metamaterials are currently in the spotlight and have unprecedented physical properties that are difficult to implement in existing materials. Examples include the materials with negative Poisson’s ratio (NPR). These unusual properties have been created by the development of 3D printing technology, suggesting rational designs for mechanical metamaterials on macroscales. However, it was an unexplored work to include movable building blocks inside microscales to allow manipulation of mechanical movements on a molecular scale. Herein, a new porphyrinic MOF, UPF-1 which consists of a novel Zn SBU and a porphyrinic ligand was reported. Especially, the Zn-based SBUs have movable parts as a hinge in a rotating model. The detailed structural analysis of UPF-1 and theoretical calculation demonstrated the NPR behavior of a mechanical metamaterial. In this work, we realized that the special geometric model of the structure with flexible parts is correlated to the unprecedented physical properties of metamaterials. Furthermore, we report a 2D porphyrinic MOF, PPF-301, self-assembled from Zn paddlewheel SBU and a flexible porphyrin linker, providing foldable movement based on an origami tessellation. The abnormal mechanical behavior of PPF-301 is demonstrated through the geometric analysis of the structure and theoretical calculation based on origami tessellation. The folding and unfolding movements of origami tessellation can create the NPR property of metamaterial. These works suggest that the development of meta-MOFs with abnormal mechanical properties can be predicted when special geometric models with NPR properties are introduced in the MOF system. In addition to proving the abnormal property of the metamaterial through structural analysis, 2D bilayer porphyrinic MOF, PPF-27D, shows abnormal guest-induced thermal expansion, associated with guest dynamics. The 2D MOF exhibits nonlinear and colossal thermal expansion at a specific temperature due to the rearrangement of the intercalated guest molecules. The plausible mechanism of solvent-mediated actuation is demonstrated through experimental and computational investigation. The porphyrinic MOFs could be a promising platform to demonstrate unprecedented properties. The second theme is MOFs as an adsorbent for adsorptive removal of emerging pollutants. Water is fundamental to human sustainability. Because of the lack of water resources, the issue of drinking water has been steadily debated by the private sectors and the government. Unfortunately, important amounts of emerging pollutants such as pharmaceuticals and personal care products (PPCPs) and organic dyes are released into the environment. Common water treatment forms are not effective for the pollutants under low concentration. MOFs with permanent porosity in a new material class are non-conventional and advanced adsorbents affording the structural tunability. We especially focus on the most recent advances in the MOF field for the adsorptive removal of PPCPs. Furthermore, we present adsorptive removal of representative organic dye, methyl orange (MO), using Zr porphyrinic MOFs, PCN-224 with the suitable pore aperture, porosity, and various adsorptive sites. PCN-224 exhibits the top performance among reported MOFs for MO adsorption. The MOFs could be an adsorptive platform for the removal of emerging pollutants.