Name: Electron Spin Resonance (ESR) Spectra of Cu-THQ MOF
Uploaded: 2023-06-09T14:10:00
Description: Watch this Scientific Journal Video about Electron Spin Resonance ESR Spectra of Cu-THQ MOF at JoVE.com

electron spin resonance (esr) spectra of cu-thq mof

酸化還元活性MOFの磁気特性評価:固体電気化学

Since the term metal-organic framework (MOF) was introduced in the late 1990s, and especially in the 2010s, the most representative scientific concepts concerning MOFs have arisen from their structural porosity, including guest encapsulation, separation, catalytic properties, and molecule sensing1,2,3,4. Meanwhile, scientists were quick to realize that it is essential for MOFs to possess stimuli-responsive electronic properties in order to integrate them into modern smart devices. This idea triggered the spawning and flourishing of the conductive two-dimensional (2D) MOF family in the past 10 years, thereby opening the gate for MOFs to play key roles in electronics5 and, more attractively, in electrochemical energy storage devices6. These 2D MOFs have been incorporated as active materials in alkali metal batteries, aqueous batteries, pseudocapacitors, and supercapacitors7,8,9, and have exhibited tremendous capacity as well as excellent stability. However, to design better-performing 2D MOFs, it is crucial to understand their charge storage mechanisms in detail. Therefore, this article aims to provide a comprehensive understanding of the electrochemical mechanisms of MOFs, which can aid in the rational design of better-performing MOFs for energy storage applications.
In 2014, we first reported the solid-state electrochemical mechanisms of MOFs with redox-active sites on both metal cations and ligands10,11.&#160;These mechanisms were interpreted with the help of various in situ and ex situ spectroscopic techniques, such as X-ray photoelectron spectroscopy (XPS), X-ray absorption fine structure (XAFS), X-ray diffraction (XRD), and solid-state nuclear magnetic resonance (NMR). Since then, this research paradigm has become a trend in studies of the solid-state electrochemistry of molecular-based materials12. These methods work fine for identifying the redox events of conventional MOFs with carboxylate bridging ligands, as the molecular orbitals and energy levels of metal cluster building blocks and organic ligands are almost independent of each other in such MOFs12,13.
However, when encountering the strongly correlated 2D MOFs with significant &#960;-d conjugation, the limitations of these spectroscopic methods were exposed. One of these limitations is that the band levels of most aforementioned 2D MOFs cannot be considered as a simple combination of metal clusters and ligands, but are rather a hybridization of them, while most of the spectroscopic methods only provide averaged, qualitative information about the oxidation states14. The other limitation is that the interpretation of these data is always based on the assumption of localized atomic orbitals. Therefore, the intermediate states with metal-ligand hybridization and delocalized electronic states are usually overlooked and described incorrectly with only these spectroscopic methods15. It is necessary to develop new probes for the electronic states of these electrochemical intermediates of not only 2D MOFs, but also other materials with similar conjugated or strongly correlated electronic structures, such as covalent organic frameworks16, molecular conductors, and conjugated polymers17.
The most common and powerful tools for assessing the electronic structures of materials are electron spin resonance (ESR) and superconducting quantum interference device (SQUID) magnetic susceptibility measurements18,19. As both rely on unpaired electrons in the system, these tools can provide tentative information about the spin densities, spin distributions, and spin-spin interactions. ESR offers sensitive detection of unpaired electrons, while magnetic susceptibility measurement gives more quantitative signals for upper properties20. Unfortunately, both techniques unavoidably face great challenges when used to analyze the electrochemical intermediates. This is because target samples are not pure, but rather a mixture of target material, conductive additive, binder, and byproduct from the electrolyte, so the obtained data21,22 are the sum of contributions from both the material and the impurities. Meanwhile, most intermediates are sensitive to the environment, including air, water, certain electrolytes, or any other unpredictable perturbations; extra care is necessary while handling and measuring intermediates. Trial and error is normally necessary while dealing with a new combination of electrode material and electrolyte.
Here, we present a new paradigm, called electrochemical magnetometry, for analyzing the electronic states or spin states of 2D MOFs and similar materials using a series of techniques, utilizing electrochemistry and temperature-variable ex situ ESR spectroscopy as well as ex situ magnetic susceptibility measurements20. To demonstrate the effectiveness of this approach, we use Cu3THQ2 (THQ = 1,2,4,5-tetrahydroxybenzoquinone; referred to as Cu-THQ), a representative 2D MOF, as an example. We explain the selection of conductive additives and electrolytes, the fabrication of electrodes and electrochemical cells, as well as details on sample handling and measurement, including possible issues during measurement. By comparing with classic characterizations such as XRD and XAFS, electrochemical magnetometry can provide a comprehensive understanding of the electrochemical mechanism of most MOFs. This approach is capable of capturing unique intermediate states and avoiding incorrect assignment of redox events. The elucidation of energy storage mechanisms using electrochemical magnetometry can also contribute to a better understanding of the structure-function relationships in MOFs, leading to more intelligent synthetic strategies for MOFs and other conjugated materials.

著者スポットライト:酸化還元活性金属有機フレームワークの固体電気化学における中間体の磁気測定特性評価  

酸化還元活性金属-有機骨格の固体電気化学における中間体の磁気特性評価

Our research interest lies in understanding the electronic and spin states of 2D pi-d conjugated metal organic frameworks and their correlation with their electrochemical behavior of this MOFs in solid-state energy storage devices. In the last five years, many new 2D-conjugating MOF materials have been reported and studied for their use as active materials in electrochemical cells. However, the mechanism of their charge storage processes is still unclear.Spectroscopic methods, including X-ray diffraction, X-ray photoelectron spectroscopy, and the X-ray absorption fine structure are the mostly common used techniques in this field. Those techniques are crucial for categorizing the specific element's crystal structure and oxygen states. It is not possible to separate a 2D MOF from the device while analyzing its electrochemical intermediate state.Measurement must be taken on a mixture that includes conductive additives and binders. However, to accurately determine the MOF's electrochemical behavior, it is necessary to calibrate the contribution of these additives. Most spectroscopic measures assume well localized electrons in MOFs, but our protocol provides a physical view and reveals strongly-correlated phenomena in these materials.We will continue to better understand the electronic, magnetic, and quantum properties of 2D-conjugated MOFs and bridge the gap between coordination chemistry and solid-state physics through physical insights.

Watch this Scientific Journal Video about Electron Spin Resonance ESR Spectra of Cu-THQ MOF at JoVE.com

Electron Spin Resonance (ESR) Spectra of Cu-THQ MOF  

Cu-THQ MOFの電子スピン共鳴(ESR)スペクトル

サンプルチューブをゴムチューブに接続し、バルブでシールします。その後、真空下でサンプル管のシールヘッドを火炎化する。ESR分光計をセットアップします。次に、準備したサンプルチューブをマイクロ波キャビティに挿入し、サンプルを中央に配置します。次に、サンプルのESRを登録するには、マイクロ波電力、磁場掃引時間、中心磁場掃引幅、変調周波数と幅、チャネル振幅と時定数などの最適なパラメータを選択します。次に、磁場を掃引し、ESRスペクトルを記録します。マンガンマーカーの挿入量を800に調整し、前述のようにマンガンマーカーでESRスペクトルをキャプチャします。マンガン二価イオンの6本の超細線を使用して磁場を校正します。銅THQ MOFは幅広いESRラインを示しました。銅THQを1.5ボルトに還元すると、銅イオン信号が消失してラジカル信号が残り、イオンが銅に還元されたことを示しています。定量的ESR測定により、銅THQ MOFは常磁性状態で96%の銅イオンを有することが確認された。銅イオンのESR線幅は温度の低下とともに狭くなり、化学シフトは一定であり、有意な反強磁性相互作用を示した。ESRスピン磁化率には、イカ磁化率と比較して、温度に依存しない有意な常磁性項が観察されました。

Research

JoVE Journal

Chemistry

Electron Spin Resonance (ESR) Spectra of Cu-THQ MOF

Magnetometric Characterization of Intermediates in the Solid-State Electrochemistry of Redox-Active Metal-Organic Frameworks

Electrochemical energy storage has been a widely discussed application of redox-active metal-organic frameworks (MOFs) in the past 5 years. Although MOFs ...