This work was supported by the Department of Energy under Grant DE-SC0012446.

The authors declare no competing interests.

While the above results and protocol delineate general guidelines for minimizing scatter from MOFs in spectroscopic characterization, there is a wide variability in MOF particle size and structure that impacts spectroscopic results, and therefore blurs the methods of interpretation. To help clarify interpretation and ease the strain that comes with analyzing MOF spectroscopic data, finding a procedure to make the MOFs as small as possible is key. This is a limiting factor for most spectroscopy-related analyses of MOFs. B...

Metal-organic frameworks (MOFs) are composed of metal oxide nodes linked by organic molecules, that form hierarchical porous structures when their constituent parts react together under solvothermal conditions1. Permanently porous MOFs were first reported in the early 2000s, and since then, the burgeoning field has expanded to encompass a wide range of applications, given the unique tunability of their structural components2,3,4,5,6,7. During the growth of the field of MOFs, there have been a handful of researchers that have incorporated photoactive materials into the nodes, ligands, and pores of MOFs to harness their potential in light-driven processes, like photocatalysis8,9,10,11, upconversion12,13,14,15,16, and photoelectrochemistry17,18. A handful of the light-driven processes of MOFs revolve around energy and electron transfer between donors and acceptors17,19,20,21,22,23,24,25. The two most common techniques used to study energy and electron transfer in molecular systems are emission and transient absorption spectroscopy26,27.
A great deal of research on MOFs has focused on emission characterization, given the relative ease in preparing samples, performing measurements, and (relatively) straightforward analysis19,22,23,24,28. Energy transfer typically manifests itself as a loss in the donor emission intensity and lifetime and an increase in the emission intensity of the acceptor loaded into the MOF backbone19,23,28. Evidence of charge transfer in a MOF manifests as a decrease in emission quantum yield and lifetime of the chromophore in the MOF29,30. While emission spectroscopy is a powerful tool in the analysis of MOFs, it only addresses part of the necessary information to present a complete mechanistic understanding of MOF photochemistry. Transient absorption spectroscopy can not only provide support for the existence of energy and charge transfer, but the method can also detect spectral signatures associated with the non-emissive singlet and triplet excited state behaviors, making it one of the most versatile tools for characterization31,32,33.
The primary reason why more robust characterization techniques like transient absorption spectroscopy are seldom applied toward MOFs is due to the difficulty in preparing samples with minimal scatter, especially with suspensions34. In the few studies successfully performing transient absorption on MOFs, the MOFs are &#60;500 nm in size, with some exceptions, stressing the importance of reducing particle size to minimize scatter15,21,25,35,36,37. Other studies make use of MOF thin films17 or SURMOFs38,39,40 to circumvent the scatter issue; however, from an applicability stand-point, their use is quite limited. Additionally, some research groups have taken to making polymer films of MOFs with Nafion or polystyrene34, the former raising some concerns for stability given the highly acidic sulfonate groups on Nafion. Gaining inspiration from the preparation of colloidal semiconductor suspensions41,42, we have found great success using polymers to help suspend and stabilize MOF particles for spectroscopic measurements11. In this work, we establish widely applicable guidelines to follow when it comes to preparing MOF suspensions and characterizing them with emission, nanosecond (ns), and ultrafast (uf) transient absorption (TA) spectroscopy techniques.

<table border="1" fo:keep-together.within-page="1" fo:keep-with-next.within-page="always">
	<tbody>
		<tr>
			<td>1 cm cuvette sample mount (SM1)</td>
			<td>Edinburgh Instruments</td>
			<td>n/a</td>
			<td>Contact company</td>
		</tr>
		<tr>
			<td>1 mL disposable syringes</td>
			<td>EXELINT</td>
			<td>26044</td>
			<td></td>
		</tr>
		<tr>
			<td>10 mL disposable syringes</td>
			<td>EXELINT</td>
			<td>26252</td>
			<td></td>
		</tr>
		<tr>
			<td>1-dram vials</td>
			<td>FisherSci</td>
			<td>CG490001</td>
			<td></td>
		</tr>
		<tr>
			<td>20 nm syringe filters</td>
			<td>VWR</td>
			<td>28138-005</td>
			<td>The filters are made by Whatman/Cytiva, and their catalog number is 6809-1002</td>
		</tr>
		<tr>
			<td>200 nm syringe filters</td>
			<td>Cytiva, Whatman</td>
			<td>6784-1302</td>
			<td></td>
		</tr>
		<tr>
			<td>Absorption spectrophotometer</td>
			<td>Agilent&#160;</td>
			<td>Cary 5000 Spectrophotometer</td>
			<td>Contact company</td>
		</tr>
		<tr>
			<td>Acetronitrile (ACN)</td>
			<td>FisherSci</td>
			<td>AA36423</td>
			<td></td>
		</tr>
		<tr>
			<td>Ar gas tank</td>
			<td>Linde/PraxAir</td>
			<td>P-4563</td>
			<td></td>
		</tr>
		<tr>
			<td>bis amino-terminated polyethylene glycol (PNH2)</td>
			<td>Sigma-Aldrich</td>
			<td>452572</td>
			<td>MOF suspending agent</td>
		</tr>
		<tr>
			<td>Clamping sample mount for nsTA (SM2)</td>
			<td>Ultrafast Systems</td>
			<td>n/a</td>
			<td>Contact company</td>
		</tr>
		<tr>
			<td>Concave lens for telescope(CCL1)</td>
			<td>Thorlabs</td>
			<td>LD1613-A-ML</td>
			<td></td>
		</tr>
		<tr>
			<td>Convex lens for telescope (CVL1)</td>
			<td>Thorlabs</td>
			<td>LA1708-A-ML</td>
			<td></td>
		</tr>
		<tr>
			<td>Custom 1 cm optical cell with 24/40 outer joint</td>
			<td>QuarkGlass</td>
			<td>QSE-1Q10-2440 (Spectrosil Cat #1-Q-10</td>
			<td>We requested the 1 cm cell to have a joint</td>
		</tr>
		<tr>
			<td>Custom 2mm optical cell with 14/20 outer joint</td>
			<td>QuarkGlass</td>
			<td>QSE-1Q2-1420 (Spectrosil Cat # 1-Q-2)</td>
			<td>We requested the 2 mm cell to have a joint</td>
		</tr>
		<tr>
			<td>Dimethylformamide (DMF)</td>
			<td>FisherSci</td>
			<td>D119</td>
			<td></td>
		</tr>
		<tr>
			<td>Dye laser (Nd:YAG pumped) for 415 nm output</td>
			<td>Sirah</td>
			<td>CobraStretch</td>
			<td></td>
		</tr>
		<tr>
			<td>Dye laser dye, Exalite 417</td>
			<td>Luxottica</td>
			<td>4170</td>
			<td></td>
		</tr>
		<tr>
			<td>Femtosecond laser</td>
			<td>Coherent</td>
			<td>Astrella</td>
			<td></td>
		</tr>
		<tr>
			<td>Fluorimeter&#160;</td>
			<td>Photon Technology Inc. (Horiba)</td>
			<td>QuantaMaster QM-200-4E</td>
			<td></td>
		</tr>
		<tr>
			<td>Fluorimeter arc lamp, 75 W</td>
			<td>Newport</td>
			<td>6251NS</td>
			<td></td>
		</tr>
		<tr>
			<td>Fluorimeter PMT</td>
			<td>Hamamatsu</td>
			<td>1527</td>
			<td></td>
		</tr>
		<tr>
			<td>Fluorimeter Software</td>
			<td>PTI/Horiba</td>
			<td>FelixGX</td>
			<td></td>
		</tr>
		<tr>
			<td>Fluorimeter TCSPC Module</td>
			<td>Becker &#38; Hickl GmbH</td>
			<td>PMH-100</td>
			<td></td>
		</tr>
		<tr>
			<td>lens mounts for telescope</td>
			<td>Thorlabs</td>
			<td>LMR1</td>
			<td></td>
		</tr>
		<tr>
			<td>Long purging needles</td>
			<td>STERiJECT</td>
			<td>PRE-22100</td>
			<td></td>
		</tr>
		<tr>
			<td>Magnetic stirrer</td>
			<td>Ultrafast Systems</td>
			<td>n/a</td>
			<td>Contact company</td>
		</tr>
		<tr>
			<td>mirror 1 (MM1) 350-700 nm</td>
			<td>Newport</td>
			<td>10Q20BB.1</td>
			<td></td>
		</tr>
		<tr>
			<td>MM1 mount</td>
			<td>Thorlabs</td>
			<td>KM100</td>
			<td></td>
		</tr>
		<tr>
			<td>MM1 post</td>
			<td>Thorlabs</td>
			<td>TR2</td>
			<td></td>
		</tr>
		<tr>
			<td>MM1 post holder</td>
			<td>Thorlabs</td>
			<td>PH1.5</td>
			<td></td>
		</tr>
		<tr>
			<td>MM2 mount</td>
			<td>Thorlabs</td>
			<td>MFM05</td>
			<td></td>
		</tr>
		<tr>
			<td>MM2,3 mirrors</td>
			<td>thorlabs</td>
			<td>BB03-E02</td>
			<td></td>
		</tr>
		<tr>
			<td>MM2,3 post</td>
			<td>Thorlabs</td>
			<td>MS3R</td>
			<td></td>
		</tr>
		<tr>
			<td>MM2,3 post bases</td>
			<td>Thorlabs</td>
			<td>MBA1</td>
			<td></td>
		</tr>
		<tr>
			<td>MM2,3 post holders</td>
			<td>Thorlabs</td>
			<td>MPH50</td>
			<td></td>
		</tr>
		<tr>
			<td>MM3 mount</td>
			<td>Thorlabs</td>
			<td>MK05</td>
			<td></td>
		</tr>
		<tr>
			<td>mounting posts for telescope optics</td>
			<td>Thorlabs</td>
			<td>TR4</td>
			<td></td>
		</tr>
		<tr>
			<td>Nanosecond TA Nd:YAG lasers</td>
			<td>Spectra-Physics</td>
			<td>QuantaRay INDI Nd:YAG</td>
			<td></td>
		</tr>
		<tr>
			<td>Nanosecond TA spectrometer</td>
			<td>Edinburgh Instruments</td>
			<td>LP980</td>
			<td></td>
		</tr>
		<tr>
			<td>nsTA ICCD camera</td>
			<td>Oxford Instruments</td>
			<td>Andor iStar ICCD camera</td>
			<td>Contact company</td>
		</tr>
		<tr>
			<td>nsTA PMT&#160;</td>
			<td>Hamamatsu</td>
			<td>R928</td>
			<td></td>
		</tr>
		<tr>
			<td>Optical parametric amplifier</td>
			<td>Ultrafast Systems</td>
			<td>Apollo</td>
			<td></td>
		</tr>
		<tr>
			<td>Parafilm</td>
			<td>FisherSci</td>
			<td>S37440</td>
			<td></td>
		</tr>
		<tr>
			<td>Pinhole wheel</td>
			<td>Thorlabs</td>
			<td>PHW16</td>
			<td></td>
		</tr>
		<tr>
			<td>Pinhole wheel post base</td>
			<td>Thorlabs</td>
			<td>CF125C</td>
			<td></td>
		</tr>
		<tr>
			<td>Pinhole wheel post holder</td>
			<td>Thorlabs</td>
			<td>PH1.5</td>
			<td></td>
		</tr>
		<tr>
			<td>Pinhole wheel post/mount assembly</td>
			<td>Thorlabs</td>
			<td>NDC-PM</td>
			<td></td>
		</tr>
		<tr>
			<td>post bases for telescope optics</td>
			<td>Thorlabs</td>
			<td>CF125C</td>
			<td></td>
		</tr>
		<tr>
			<td>post holders for telescope optics</td>
			<td>Thorlabs</td>
			<td>PH4</td>
			<td></td>
		</tr>
		<tr>
			<td>Power detector for ns TA</td>
			<td>Thorlabs</td>
			<td>S310C</td>
			<td></td>
		</tr>
		<tr>
			<td>Prism assembly (P2,3)</td>
			<td>Edinburgh Instruments</td>
			<td>n/a</td>
			<td>Contact company</td>
		</tr>
		<tr>
			<td>Prism mount (P1)</td>
			<td>OWIS</td>
			<td>K50-FGS</td>
			<td></td>
		</tr>
		<tr>
			<td>Prism post (P1)</td>
			<td>Thorlabs</td>
			<td>TR4</td>
			<td></td>
		</tr>
		<tr>
			<td>Prism post base (P1)</td>
			<td>Thorlabs</td>
			<td>CF125C</td>
			<td></td>
		</tr>
		<tr>
			<td>Prism post holder (P1)</td>
			<td>Thorlabs</td>
			<td>PH4</td>
			<td></td>
		</tr>
		<tr>
			<td>Quartz prisms (P1-P3)</td>
			<td>Newport</td>
			<td>10SR20</td>
			<td></td>
		</tr>
		<tr>
			<td>Rubber outer joint septa (14/20)</td>
			<td>VWR</td>
			<td>89097-540</td>
			<td></td>
		</tr>
		<tr>
			<td>Rubber outer joint septa (24/40)</td>
			<td>ChemGlass</td>
			<td>CG-3022-24</td>
			<td></td>
		</tr>
		<tr>
			<td>Sonication tip</td>
			<td>Branson</td>
			<td>product discontinued</td>
			<td>Closest alternative is 1/8&#34; diam. tip from iUltrasonic</td>
		</tr>
		<tr>
			<td>Square ND filters</td>
			<td>Thorlabs</td>
			<td>NEK01S</td>
			<td></td>
		</tr>
		<tr>
			<td>Stir bars</td>
			<td>StarnaCells/FisherSci</td>
			<td>NC9126395</td>
			<td></td>
		</tr>
		<tr>
			<td>Thorlabs power detector for ufTA</td>
			<td>Thorlabs</td>
			<td>S401C</td>
			<td></td>
		</tr>
		<tr>
			<td>Thorlabs power meter</td>
			<td>Thorlabs</td>
			<td>PM100D</td>
			<td></td>
		</tr>
		<tr>
			<td>Tip sonicator</td>
			<td>Branson</td>
			<td>Digital Sonifer 450, product discontinued</td>
			<td>Closest alternative is SFX550 from iUltrasonic</td>
		</tr>
		<tr>
			<td>Tygon tubing</td>
			<td>Grainger</td>
			<td>8Y589</td>
			<td></td>
		</tr>
		<tr>
			<td>ufTA ND filter wheel</td>
			<td>Thorlabs</td>
			<td>NDC-25C-2-A</td>
			<td></td>
		</tr>
		<tr>
			<td>ufTA ND filter wheel mount</td>
			<td>Thorlabs</td>
			<td>NDC-PM</td>
			<td></td>
		</tr>
		<tr>
			<td>ufTA ND filter wheel post</td>
			<td>Thorlabs</td>
			<td>PH2</td>
			<td></td>
		</tr>
		<tr>
			<td>ufTA ND filter wheel post base</td>
			<td>Thorlabs</td>
			<td>CF125C</td>
			<td></td>
		</tr>
		<tr>
			<td>ufTA pump alignment mirror</td>
			<td>Thorlabs</td>
			<td>PF10-03-F01</td>
			<td></td>
		</tr>
		<tr>
			<td>Ultrafast TA telescope assembly</td>
			<td>Ultrafast Systems</td>
			<td>n/a</td>
			<td>Contact company</td>
		</tr>
		<tr>
			<td>Ultrafast transient absorption spectrometer</td>
			<td>Ultrafast Systems</td>
			<td>HeliosFire</td>
			<td></td>
		</tr>
		<tr>
			<td>Xe arc probe lamp</td>
			<td>OSRAM</td>
			<td>4050300508788</td>
			<td></td>
		</tr>
	</tbody>
</table>

a technical guide for performing spectroscopic measurements on metal-organic frameworks

Metal-organic frameworks (MOFs) offer a unique platform to understand light-driven processes in solid-state materials, given their high structural tunability. However, the progression of MOF-based photochemistry has been hindered by the difficulty in spectrally characterizing these materials. Given that MOFs are typically larger than 100 nm in size, they are prone to excessive light scatter, thereby rendering data from valuable analytical tools like transient absorption and emission spectroscopy nearly uninterpretable. To gain meaningful insights of MOF-based photo-chemical and physical processes, special consideration must be taken toward properly preparing MOFs for spectroscopic measurements, as well as the experimental setups that garner higher quality data. With these considerations in mind, the present guide provides a general approach and set of guidelines for the spectroscopic investigation of MOFs. The guide addresses the following key topics: (1) sample preparation methods, (2) spectroscopic techniques/measurements with MOFs, (3) experimental setups, (3) control experiments, and (4) post-run stability characterization. With appropriate sample preparation and experimental approaches, pioneering advancements toward the fundamental understanding of light-MOF interactions are significantly more attainable.

The electronic absorption spectra of PCN-222(fb) with and without PNH2 and filtering are shown in Figure 4. The MOF without PNH2 was just tip-sonicated and diluted. When comparing the two spectra, the biggest difference is the minimization of baseline scatter, which shows up as a broad upward absorption with decreasing wavelengths and also broadens the electronic transitions quite noticeably. For further comparison, the PCN-222(fb) ligand in solution, tetracarboxyphenyl...

Watch this Scientific Journal Video about A Technical Guide for Performing Spectroscopic Measurements on Metal-Organic Frameworks at JoVE.com

A Technical Guide for Performing Spectroscopic Measurements on Metal-Organic Frameworks

Here, we use a polymer stabilizer to prepare metal-organic framework (MOF) suspensions that exhibit markedly decreased scatter in their ground-state and transient absorption spectra. With these MOF suspensions, the protocol provides various guidelines to characterize the MOFs spectroscopically to yield interpretable data.

Metal-organic frameworks (MOFs) offer a unique platform to understand light-driven processes in solid-state materials, given their high structural ...

a-technical-guide-for-performing-spectroscopic-measurements-on-metal

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Chemistry

2.1K Views.  Virginia Polytechnic and State Institute. Here, we use a polymer stabilizer to prepare metal-organic framework (MOF) suspensions that exhibit markedly decreased scatter in their ground-state and transient absorption spectra. With these MOF suspensions, the protocol provides various guidelines to characterize the MOFs spectroscopically to yield interpretable data.

Video: A Technical Guide for Performing Spectroscopic Measurements on Metal-Organic Frameworks

Preparation of Hydrophobic Metal-Organic Frameworks via Plasma Enhanced Chemical Vapor Deposition of Perfluoroalkanes for the Removal of Ammonia

Plasma enhanced chemical vapor deposition (PECVD) of perfluoroalkanes has long been studied for tuning the wetting properties of surfaces. For high surface area microporous materials, such as metal-organic frameworks (MOFs), unique challenges present themselves for PECVD treatments. Herein the protocol for development of a MOF that was previously unstable to humid conditions is presented. The protocol describes the synthesis of Cu-BTC (also known as HKUST-1), the treatment of Cu-BTC with PECVD of perfluoroalkanes, the aging of materials under humid conditions, and the subsequent ammonia microbreakthrough experiments on milligram quantities of microporous materials. Cu-BTC has an extremely high surface area (~1,800 m2/g) when compared to most materials or surfaces that have been previously treated by PECVD methods. Parameters such as chamber pressure and treatment time are extremely important to ensure the perfluoroalkane plasma penetrates to and reacts with the inner MOF surfaces. Furthermore, the protocol for ammonia microbreakthrough experiments set forth here can be utilized for a variety of test gases and microporous materials.

Herein the procedures for plasma enhanced chemical vapor deposition of perfluoroalkanes on microporous materials such as metal-organic frameworks to enhance their stability and hydrophobicity are described. Furthermore, breakthrough testing of milligram quantities of samples is described in detail.

Plasma enhanced chemical vapor deposition (PECVD) of perfluoroalkanes has long been studied for tuning the wetting properties of surfaces. For high ...

Synthesis and Characterization of Functionalized Metal-organic Frameworks

Metal-organic frameworks have attracted extraordinary amounts of research attention, as they are attractive candidates for numerous industrial and technological applications. Their signature property is their ultrahigh porosity, which however imparts a series of challenges when it comes to both constructing them and working with them. Securing desired MOF chemical and physical functionality by linker/node assembly into a highly porous framework of choice can pose difficulties, as less porous and more thermodynamically stable congeners (e.g., other crystalline polymorphs, catenated analogues) are often preferentially obtained by conventional synthesis methods. Once the desired product is obtained, its characterization often requires specialized techniques that address complications potentially arising from, for example, guest-molecule loss or preferential orientation of microcrystallites. Finally, accessing the large voids inside the MOFs for use in applications that involve gases can be problematic, as frameworks may be subject to collapse during removal of solvent molecules (remnants of solvothermal synthesis). In this paper, we describe synthesis and characterization methods routinely utilized in our lab either to solve or circumvent these issues. The methods include solvent-assisted linker exchange, powder X-ray diffraction in capillaries, and materials activation (cavity evacuation) by supercritical CO2 drying. Finally, we provide a protocol for determining a suitable pressure region for applying the Brunauer-Emmett-Teller analysis to nitrogen isotherms, so as to estimate surface area of MOFs with good accuracy.

Synthesis, activation, and characterization of intentionally designed metal-organic framework materials is challenging, especially when building blocks are incompatible or unwanted polymorphs are thermodynamically favored over desired forms. We describe how applications of solvent-assisted linker exchange, powder X-ray diffraction in capillaries and activation via supercritical CO2 drying, can address some of these challenges.

Metal-organic frameworks have attracted extraordinary amounts of research attention, as they are attractive candidates for numerous industrial and ...

Synthesis of a Water-soluble Metal&#8211;Organic Complex Array

We demonstrate a method for the synthesis of a water-soluble multimetallic peptidic array containing a predetermined sequence of metal centers such as Ru(II), Pt(II), and Rh(III). The compound, named as a water-soluble metal-organic complex array (WSMOCA), is obtained through 1) the conventional solution-chemistry-based preparation of the corresponding metal complex monomers having a 9-fluorenylmethyloxycarbonyl (Fmoc)-protected amino acid moiety and 2) their sequential coupling together with other water-soluble organic building units on the surface-functionalized polymeric resin by following the procedures originally developed for the solid-phase synthesis of polypeptides, with proper modifications. Traces of reactions determined by mass spectrometric analysis at the representative coupling steps in stage 2 confirm the selective construction of a predetermined sequence of metal centers along with the peptide backbone. The WSMOCA cleaved from the resin at the end of stage 2 has a certain level of solubility in aqueous media dependent on the pH value and/or salt content, which is useful for the purification of the compound.

Synthesis of a Water-soluble Metal&amp;#8211;Organic Complex Array

A potential general method for the synthesis of water-soluble multimetallic peptidic arrays containing a predetermined sequence of metal centers is presented.

We demonstrate a method for the synthesis of a water-soluble multimetallic peptidic array containing a predetermined sequence of metal centers such as ...

Surface Functionalization of Metal-Organic Frameworks for Improved Moisture Resistance

Metal-organic frameworks (MOFs) are a class of porous inorganic materials with promising properties in gas storage and separation, catalysis and sensing. However, the main issue limiting their applicability is their poor stability in humid conditions. The common methods to overcome this problem involve the formation of strong metal-linker bonds by using highly charged metals, which is limited to a number of structures, the introduction of alkylic groups to the framework by post-synthetic modification (PSM) or chemical vapour deposition (CVD) to enhance overall hydrophobicity of the framework. These last two usually provoke a drastic reduction of the porosity of the material. These strategies do not permit to exploit the properties of the MOF already available and it is imperative to find new methods to enhance the stability of MOFs in water while keeping their properties intact. Herein, we report a novel method to enhance the water stability of MOF crystals featuring Cu2(O2C)4&#160;paddle-wheel units, such as HKUST (where HKUST stands for Hong Kong University of Science &#38; Technology), with the catechols functionalized with alkyl and fluoro-alkyl chains. By taking advantage of the unsaturated metal sites and the catalytic catecholase-like activity of CuII ions, we are able to create robust hydrophobic coatings through the oxidation and subsequent polymerization of the catechol units on the surface of the crystals under anaerobic and water-free conditions without disrupting the underlying structure of the framework. This approach not only affords the material with improved water stability but also provides control over the function of the protective coating, which enables the development of functional coatings for the adsorption and separations of volatile organic compounds. We are confident that this approach could also be extended to other unstable MOFs featuring open metal sites.

Robust functional catechol coatings were produced in one step by direct reaction of the material known as HKUST with synthetic catechols under anaerobic conditions. The formation of homogeneous coatings surrounding the entire crystal is ascribed to the biomimetic catalytic activity of Cu(II) dimers on the external surface of the crystals.

Metal-organic frameworks (MOFs) are a class of porous inorganic materials with promising properties in gas storage and separation, catalysis and sensing. ...

Development of Heterogeneous Enantioselective Catalysts using Chiral Metal-Organic Frameworks (MOFs)

Substrate size discrimination by the pore size and homogeneity of the chiral environment at the reaction sites are important issues in the validation of the reaction site in metal-organic framework (MOF)&#8211;based catalysts in an enantioselective catalytic reaction system. Therefore, a method of validating the reaction site of MOF-based catalysts is necessary to investigate this issue. Substrate size discrimination by pore size was accomplished by comparing the substrate size versus the reaction rate in two different types of carbonyl-ene reactions with two kinds of MOFs. The MOF catalysts were used to compare the performance of the two reaction types (Zn-mediated stoichiometric and Ti-catalyzed carbonyl-ene reactions) in two different media. Using the proposed method, it was observed that the entire MOF crystal participated in the reaction, and the interior of the crystal pore played an important role in exerting chiral control when the reaction was stoichiometric. Homogeneity of the chiral environment of MOF catalysts was established by the size control method for a particle used in the Zn-mediated stoichiometric reaction system. The protocol proposed for the catalytic reaction revealed that the reaction mainly occurred on the catalyst surface regardless of the substrate size, which reveals the actual reaction sites in MOF-based heterogeneous catalysts. This method for reaction site validation of MOF catalysts suggests various considerations for developing heterogeneous enantioselective MOF catalysts.

Development of Heterogeneous Enantioselective Catalysts using Chiral Metal-Organic Frameworks MOFs

Here, we present a protocol for active site validation of metal-organic framework catalysts by comparing stoichiometric and catalytic carbonyl-ene reactions to find out whether a reaction takes place on the inner or outer surface of metal-organic frameworks.

Substrate size discrimination by the pore size and homogeneity of the chiral environment at the reaction sites are important issues in the validation of ...

Synthesis of Single-Crystalline Core-Shell Metal-Organic Frameworks

Because of their designability and unprecedented synergistic effects, core-shell metal-organic frameworks (MOFs) have been actively examined recently. However, the synthesis of single-crystalline core-shell MOFs is very challenging, and thus a limited number of examples have been reported. Here, we suggest a method of synthesizing single-crystalline HKUST-1@MOF-5 core-shells, which is HKUST-1 at the center of MOF-5. Through the computational algorithm, this pair of MOFs was predicted to have the matched lattice parameters and chemical connection points at the interface. To construct the core-shell structure, we prepared the octahedral- and cubic-shaped HKUST-1 crystals as a core MOF, in which the (111) and (001) facets were mainly exposed, respectively. Via the sequential reaction, the MOF-5 shell was well-grown on the exposed surface, showing a seamless connect interface, which resulted in the successful synthesis of single-crystalline HKUST-1@MOF-5. Their pure phase formation was proved by optical microscopic images and powder X-ray diffraction (PXRD) patterns. This method presents the potential of and insights into the single-crystalline core-shell synthesis with different kinds of MOFs.

Here, we demonstrate a protocol for the two-step synthesis of single-crystalline core-shells using a non-isostructural metal-organic framework (MOF) pair, HKUST-1 and MOF-5, which have well-matched crystal lattices.

Because of their designability and unprecedented synergistic effects, core-shell metal-organic frameworks (MOFs) have been actively examined recently. ...

Synthesis of Low-Valent MOFs from Multitopic Phosphine Linkers

Experimental Approaches for the Synthesis of Low-Valent Metal-Organic Frameworks from Multitopic Phosphine Linkers

Metal-organic frameworks (MOFs) are the subject of intense research focus due to their potential applications in gas storage and separation, biomedicine, energy, and catalysis. Recently, low-valent MOFs (LVMOFs) have been explored for their potential use as heterogeneous catalysts, and multitopic phosphine linkers have been shown to be a useful building block for the formation of LVMOFs. However, the synthesis of LVMOFs using phosphine linkers requires conditions that are distinct from those in the majority of the MOF synthetic literature, including the exclusion of air and water and the use of unconventional modulators and solvents, making it somewhat more challenging to access these materials. This work serves as a general tutorial for the synthesis of LVMOFs with phosphine linkers, including information on the following: 1) the judicious choice of the metal precursor, modulator, and solvent; 2) the experimental procedures, air-free techniques, and required equipment; 3) the proper storage and handling of the resultant LVMOFs; and 4) useful characterization methods for these materials. The intention of this report is to lower the barrier to this new subfield of MOF research and facilitate advancements toward novel catalytic materials.

Author Spotlight: Experimental Approaches for the Synthesis of Low-Valent Metal-Organic Frameworks from Multitopic Phosphine Linkers  

Here, we describe a protocol for the synthesis of low-valent metal-organic frameworks (LVMOFs) from low-valent metals and multitopic phosphine linkers under air-free conditions. The resulting materials have potential applications as heterogeneous catalyst mimics of low-valent metal-based homogeneous catalysts.

Metal-organic frameworks (MOFs) are the subject of intense research focus due to their potential applications in gas storage and separation, biomedicine, ...

Magnetometric Characterization of Redox-Active MOFs Solid-State Electrochemistry

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 show outstanding performance in terms of gravimetric or areal capacitance and cyclic stability, unfortunately their electrochemical mechanisms are not well understood in most cases. Traditional spectroscopic techniques, such as X-ray photoelectron spectroscopy (XPS) and X-ray absorption fine structure (XAFS), have only provided vague and qualitative information about valence changes of certain elements, and the mechanisms proposed based on such information are often highly disputable. In this article, we report a series of standardized methods, including the fabrication of solid-state electrochemical cells, electrochemistry measurements, the disassembly of cells, the collection of MOF electrochemical intermediates, and physical measurements of the intermediates under the protection of inert gases. By using these methods for quantitatively clarifying the electronic and spin state evolution within a single electrochemical step of redox-active MOFs, one can provide clear insight into the nature of electrochemical energy storage mechanisms not only for MOFs, but also for all other materials with strongly correlated electronic structures.

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

Ex situ magnetic surveys can directly provide bulk and local information on a magnetic electrode to reveal its charge storage mechanism step by step. Herein, electron spin resonance (ESR) and magnetic susceptibility are demonstrated to monitor the evaluation of paramagnetic species and their concentration in a redox-active metal-organic framework (MOF).

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

Triazole and Tetrazole Functionalized Zr-MOFs Synthesis and Characterization

Synthesis of Triazole and Tetrazole-Functionalized Zr-Based Metal-Organic Frameworks Through Post-Synthetic Ligand Exchange

Metal-organic frameworks (MOFs) are a class of porous materials that are formed through coordination bonds between metal clusters and organic ligands. Given their coordinative nature, the organic ligands and strut framework can be readily removed from the MOF and/or exchanged with other coordinative molecules. By introducing target ligands to MOF-containing solutions, functionalized MOFs can be obtained with new chemical tags via a process called post-synthetic ligand exchange (PSE). PSE is a straightforward and practical approach that enables the preparation of a wide range of MOFs with new chemical tags via a solid-solution equilibrium process. Furthermore, PSE can be performed at room temperature, allowing the incorporation of thermally unstable ligands into MOFs. In this work, we demonstrate the practicality of PSE by using heterocyclic triazole- and tetrazole-containing ligands to functionalize a Zr-based MOF (UiO-66; UiO = University of Oslo). After digestion, the functionalized MOFs are characterized via various techniques, including powder X-ray diffraction and nuclear magnetic resonance spectroscopy.

Author Spotlight: Functionalizing Metal-Organic Frameworks: Advancements, Challenges, and the Power of Post-Synthetic Ligand Exchange 

Post-synthetic ligand exchange (PSE) is a versatile and powerful tool for installing functional groups into metal-organic frameworks (MOFs). Exposing MOFs to solutions containing triazole- and tetrazole-functionalized ligands can incorporate these heterocyclic moieties into Zr-MOFs through PSE processes.

Metal-organic frameworks (MOFs) are a class of porous materials that are formed through coordination bonds between metal clusters and organic ligands. ...

Using Nitrogen Porosimetry to Characterize Metal-Organic Frameworks

Determining Surface Areas and Pore Volumes of Metal-Organic Frameworks

The surface area and pore volume of a metal-organic framework (MOF) can provide insight into its structure and potential applications. Both parameters are commonly determined using the data from nitrogen sorption experiments; commercial instruments to perform these measurements are also widely available. These instruments will calculate structural parameters, but it is essential to understand how to select input data and when calculation methods apply to the sample MOF. This article outlines the use of the Brunauer-Emmett-Teller (BET) method and Barrett-Joyner-Halenda (BJH) method for the calculation of surface area and pore volume, respectively. Example calculations are performed on the representative MOF UiO-66. Although widely applicable to MOFs, sample materials and adsorption data must meet certain criteria for the calculated results to be considered accurate, in addition to proper sample preparation. The assumptions and limitations of these methods are also discussed, along with alternative and complementary techniques for the MOF pore space characterization.

Author Spotlight: Characterizing Porous Materials for Aiding the Development of Robust Metal-Organic Frameworks with Adsorption Behavior 

This article describes the use of nitrogen porosimetry to characterize metal-organic frameworks, using UiO-66 as a representative material.

The surface area and pore volume of a metal-organic framework (MOF) can provide insight into its structure and potential applications. Both parameters are ...