RESEARCH

Charge Manipulation in MOFs

Challenges in Molecular Nanoscience and Molecular Spintronics

Metal complexes consist of transition metal ions surrounded by organic or inorganic molecules (known as “ligands”). Metal ions can flexibly change their oxidation state and spin state, but these states strongly depend on the surrounding ligands and the geometric structure around the metal (“ligand field theory”). Depending on the ligand, multiple metal ions can be bridged (i.e., “bridging ligands”), and this continuous bonding pattern forms an infinite lattice (Metal-organic frameworks; MOF), mediating magnetic interactions (magnetic correlations) and electron transfer (electron conjugation) between metal ions. In other words, by controlling the structural dimensionality of metal complexes at the atomic and molecular levels (the nanoscale) while simultaneously adjusting their electronic and spin states, it is possible to create molecular magnets and conductive materials. Meanwhile, the nanoscale “voids” formed by MOFs have also garnered attention in recent years as a stage for chemical reactions. Just as children climb into a jungle gym to play, small molecules can enter the “jungle gym” of MOFs (molecular frameworks). The chemical interactions between the inserted molecules and the MOF that arise upon molecular adsorption into the nanopores alter not only the inserted molecules but also the electronic and spin states, as well as the structure, of the MOF itself. Thus, it is anticipated that the electronic and spin states of MOFs can be freely transformed (physical response) based on the chemical perturbations and reactions associated with the adsorption and desorption of active molecules. Metal complexes, which combine diversity, flexibility, and high designability with controllable nano- and meso-scale properties, will create new molecular systems.

Platform for the Control of Physical Properties Based on Guest Sorption and Transports

A Hub for Multifaceted Analysis of Property Changes Induced by Molecular Adsorption

In-situ Measurement of Physical Properties under Gas Atmospheres

“Altering the electronic and spin properties of materials through molecular adsorption” is one of the key research themes in our laboratory. By utilizing “flexible” porous molecular lattice materials, we can control a wide range of phenomena through molecular adsorption. Examples include: 1) structural changes accompanying molecular adsorption, 2) chemical interactions between the inserted molecules and the MOF, and 3) the magnetic effects of paramagnetic guest molecules. In some cases, these phenomena occur simultaneously in a concerted manner. Naturally, this means we can expect not only changes in the host lattice but also the activation of the inserted guest molecules themselves. In this way, based on the physical structural changes and chemical perturbations or reactions (chemical signals) associated with molecular adsorption and desorption, we can modulate the electronic and spin states (physical signals) of the guest molecules and the MOF.

 On the other hand, determining how to capture these “changes” associated with molecular adsorption and desorption is a key challenge in conducting this research. Solving these issues requires technical expertise involving the setup of specialized equipment. Molecular adsorption depends on temperature and pressure (partial pressure). Furthermore, physical properties are related to external fields such as temperature, magnetic fields, electric fields, and light. Therefore, we aim to capture the aforementioned chemical and physical signals by using cryogenic equipment capable of varying these multiple parameters. In our laboratory, we can control the pressure and temperature of various gas molecules (N₂, O₂, CO₂, H₂, Ar, NO, CO, CH₄, C₂H₄, C₂H₂, etc.) and solvent vapors to perform the following measurements. With support from the Center for Advanced Energy Materials Research (E-IMR) and the Institute for Materials Research International Joint Research Program (GIMRT Program), we are conducting research on various molecularly adsorbed compounds and materials while collaborating with partners both domestically and internationally. At the same time, we have designated this suite of measurement technologies and equipment as the “Platform for the Control of Physical Properties Based on Guest Sorption and Transport" and are working to establish a hub for comprehensively characterizing property changes induced by molecular adsorption.

If you are interested in conducting measurements, please do not hesitate to contact us.

Project 1: World of Nanosized Molecular Magnets

The nanoscale (one-billionth of a meter) is the realm of atoms (ions) and small molecules—an “ultra-small” world governed by orbital characteristics and quantum states. By designing or observing molecules that utilize the intrinsic quantum characteristics of such atoms and ions (i.e., nano- and meso-scale control of molecules!), we can discover phenomena and mechanisms that are entirely different from those observed in bulk materials. This is precisely what “nanoscience” is, and controlling this nanoscale world is “nanotechnology.” We focus on “spin,” the origin of magnetism, and conduct research aimed at controlling the quantum spin of zero-dimensional isolated molecules (single-molecule magnets) and one-dimensional chain molecules (single-chain magnets) using external fields (temperature, magnetic fields, pressure, light, electric fields, etc.). The free control of quantum spins via external fields (free switching) is one of the key elements of nanotechnology in the near future.

Project 2: Rational Design of D/A–MOFs

The key lies in controlling “charge transfer and electron transfer.” Electron transfer is directly related to spin generation and electron transport (or hole transport), while a low-energy charge-transfer state facilitates intra-band electron transfer and strengthens magnetic interactions. For example, in a simple diamagnetic, neutral DA system consisting of an electron donor (D) and an electron acceptor (A), a single-electron transfer from D to A yields ionic D⁺ and A^– with spins of S = 1/2, respectively. Furthermore, in a D₂A system, a single-electron transfer results in a mixed valence state of D⁰ and D⁺. If these D/A systems could be directly extended to a multidimensional lattice, it is expected that electrons would hop across the lattice due to electron and charge transfer, resulting in a state where spins are ordered in multiple dimensions. Such systems would be the ultimate targets for research in molecular devices and molecular spintronics.

Project 3: Synergistic Control of Physical Properties in Chains

The transition from a neutral state to an ionic state (where charges are separated into ion pairs) within a lattice is referred to as a neutral-ionic transition (N–I transition). In a one-dimensional chain of DA units linked by covalent bonds (···DADADA···), if the neutral-ionic transition (···DADADA··· → ···D⁺A^–D⁺A^–D⁺A^–···) can be freely controlled by external fields (temperature, magnetic field, pressure, light, electric field, etc.), the changes in the spin ground state accompanying the transition, transient conductivity, dipole moment shifts due to charge transfer, and dielectric responses due to structural shifts can all be utilized simultaneously as outputs. In covalent chains in particular, since magnetic exchange interactions can be effectively achieved via cross-linking, there are high expectations for magnetic field control of the N-I transition. In 2011, we discovered the world’s first N-I transition in a covalent chain and elucidated its unique stepwise charge transfer.

Project 4: D/A-MOFs toward the Control of Physical Properties

Our research aims to design MOFs that exhibit redox activity and are expected to possess electrical conductivity and magnetic order. By inserting small molecules that are highly susceptible to electronic interactions, we seek to induce host-guest interactions and control the physical properties of the host lattice induced by these interactions. The electronic interactions between the inserted molecules and the host lattice give rise to fluctuations in the lattice charge and new magnetic orders. We interpret these charge fluctuations and electron transfers as output signals, such as changes in magnetic properties, electron transport capabilities, and structural changes. By utilizing the porous conductive magnets designed in Research Project 2 described above, we may be able to propose a new design method for high-phase-transition magnets. Furthermore, such systems may also allow the use of the interior of the pores as a redox reaction site. We are conducting cutting-edge research that bridges the fields of spatial chemistry and physical chemistry.

Project 5: Signals from Gated Adsorption Behavior

Structural flexibility is one of the defining characteristics of metal complexes. For example, if molecular orientation or substituent orientation exhibits uniaxial potential anisotropy, a dielectric response can be expected if the order or disorder of these orientations can be controlled by an external field. However, in general, systems in nature tend to settle into a neutral state (the para-dielectric phase) where moments cancel each other out. Therefore, what if we could lock the structural dynamics by opening and closing structural gates through the insertion and removal of molecules, or by utilizing steric hindrance between molecules? We are conducting research aimed at monitoring gas adsorption states using the dielectric properties of structural displacement.

Project 6: Gas Responsive Porous Magnets

In recent years, molecularly porous materials have been attracting attention. You may have heard of terms like Metal-Organic Frameworks (MOFs) and Porous Coordination Polymers (PCPs). These are lattices made of molecules that can adsorb small molecules into their nanoscale pores. While they may seem similar to conventional adsorbents, because they are lattices composed of “molecules” that can be modified in various ways, it is expected that these molecular modifications will allow us to select the guest molecules to be adsorbed or to impart additional functions. We are focusing on porous “magnets.” Although they are magnets, they adsorb common gas molecules such as O₂, N₂, and CO₂. We are developing “switchable magnets” whose magnetic properties change dramatically upon the adsorption of these gas molecules. In 2018, we developed the world’s first magnet whose magnetic phase changes upon the adsorption of oxygen.

Project 7: Creation of Ionic Energy

Redox-active coordination lattices (D/A-MOFs) composed of electron donor and acceptor units have the potential to create a unique electron-ion concerted system that not only provides diverse reaction sites for “electron transfer”—such as within the lattice and between the lattice and void guests—but also enables “ion transport” by utilizing the voids and surfaces enclosed by the lattice. Such systems can be applied to rechargeable secondary battery materials. By utilizing this system, which allows for precise control of the types and charge states of the D/A units within the framework, we are developing new battery materials.

Project 8: Science in Asymmetry

If we could actively and freely introduce molecular chirality and polarity into low-dimensional lattice materials using molecular chirality, what kind of physical properties might we be able to elicit? Focusing on organic-inorganic hybrid perovskite layered compounds and molecular materials, we are developing new materials that incorporate asymmetry and investigating the property-switching phenomena resulting from this asymmetry.

Project 9: Electronic Control in Honeycomb Layer D/A-MOFs

Two-dimensional honeycomb-structured compounds formed from tetraoxolene (X₂CA^n⁻) and transition metal ions in a 3:2 ratio allow for the regulation of charge transfer between metal ions and X₂CA^n⁻ within the layers, making them a group of compounds of interest for studying the dynamic electron transfer and related physical properties. Some of these compounds undergo charge transfer in response to changes in temperature or pressure. However, it has become clear that the state of charge transfer is significantly influenced not only by differences in the layered structure but also by the interlayer environment and the type and position of the counter cations between layers. While such findings have been observed in one-dimensional chains of neutral-to-ionic transition DA, this is the first systematic study of a series of two-dimensional layered compounds.

Project 10: Post-synthetic dimensional expansion of [Ru₂]

Waterwheel-type ruthenium dinuclear complexes ([Ru₂]) are functional molecules with excellent redox properties, and the axial sites of [Ru₂] can be utilized as catalytically active reaction sites. However, since the bridging ligands coordinating to the equatorial sites of most [Ru₂] complexes are substituent-inert, it is difficult to construct a porous lattice with a regular arrangement of [Ru₂] units while retaining the axial sites as reaction sites and maintaining the core structure. Therefore, we have developed a “post-synthesis molecular modification method” that involves reacting amine molecules with a novel [Ru₂] complex ( [Ru₂]-CHO) containing a formyl group (−CH=O). We are currently developing multidimensional imine-bridged functional materials ( [Ru₂]-MCOF) in which redox-active [Ru₂] complexes are covalently bridged, and exploring their catalytic functions.

Project 11: Emission Control in MOFs

We are focusing on metal-center luminescence in MOFs and aim to extract information about the adsorption and reactions of small molecules within the framework as a macroscopic luminescent output. We are also focusing on the specific adsorption and conversion of gas molecules and clean energy gases, which are attracting attention in the context of environmental and energy issues.