Research Overview

Metal complexes are nanoscale inorganic–organic hybrid molecules composed of metal ions coordinated with ligands such as inorganic or organic molecules or ions. A key feature of metal complexes is the structural and functional diversity arising from the combination of metal centers and ligands.

In the context of molecular science, one of the key goals is to “control the spacing arrangement of individual molecules to create higher-order assemblies that exhibit cooperative, dynamic functions.” Organized molecular systems play essential roles in biological systems, such as electron transport chains in photosynthesis and molecular motors.

Metal complexes, which integrate the “exceptional single-function of inorganic compounds” with the “structural diversity and functional flexibility of organic molecules “, hold promise for creating novel physical properties and functions not achievable with conventional inorganic materials.

When these metal complex molecules are regularly connected to form multidimensional structures, the resulting materials are known as Coordination Polymers: CPs or Metal-Organic Frameworks: MOFs. These serve as foundational platforms for higher-order functional assemblies that exhibit enhanced, integrated properties derived from the individual metal complexes.

In our laboratory, we focus on the design and synthesis of MOFs with well-defined one-, two-, or three-dimensional structures, as well as Metal–Organic Cages (MOCs) with zero-dimensional hollow structures that resemble structural fragments of MOF. Our goal is to create unique, interactive spaces where diverse metal complexes and functional molecules are assembled into frameworks that dynamically interlink multiple functions and/or properties.

Our MOF-related research primarily involves metal cyanide complexes (e.g.,[M(CN)₄]^2-, [M(N)(CN)₄]^2-, [M(CN)₆]^n-etc ) to develop functional materials with the following capabilities:

• Low-pressure adsorption of CO₂ or NH₃ through the construction of functional porous spaces
• Development of chemical sensing and switching systems by integrating magnetic or luminescent properties into the MOF framework and coupling them with guest adsorption/desorption behavior
• Development of ferroelectrics based on new polar MOFs
• Emergence of ferroelectric ion conduction properties by the strong correlation between polarity within the framework and ion conduction
• Elucidation of the correlation between solid/liquid phase transition and physical properties
• Fabrication of M-MOFs with incorporated metal chains for tunable luminescent properties and investigation of the mechanistic basis of their emission behavior via solid-solution strategies
• Development of novel luminescent MOF based on the assembly of polynuclear complexes.

In parallel, our work on MOCs includes:

• Design of water-soluble and water-stable MOCs, and the development of simple and efficient enzyme immobilization methods using these MOCs
• Construction of functional frame via hybridization of structurally distinct MOCs
• Development of new MOCs designed for the selective adsorption of CO₂ and NH₃ by utilizing both intra- and intermolecular cavity spaces.

We also explore developments of new functional materials such as:

• High-sensitive electrochemical molecular sensing systems based on MOF/MOC–MXene hybrid materials
• Novel molecule-based quantum dots (Qubit) through precise control of the electronic structures, skeletal vibrations, and spatial configurations of mononuclear metal complexes.

These research efforts not only seek to establish rational molecular design principles and elucidate the mechanisms underlying novel properties and functions, but also contribute to green chemistry. Our work addresses socially relevant challenges such as carbon recycling, efficient separation, absorption and storage of next-generation fuels like ammonia, development of advanced functional materials, high-sensitivity sensors, and enhanced enzymatic activity and recyclability—efforts that are in line with the goals of the Sustainable Development Goals (SDGs).