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><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&amp;nbsp;</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&amp;nbsp;</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 &amp;amp; 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&amp;nbsp;</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&quot; 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 ...

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Chemistry

2.3K Views.  Virginia Polytechnic and State Institute. Vaak is het meten van hoe licht interageert met metalen organische raamwerken, of MOF's, moeilijk vanwege hun sterk verstrooiende aard. Dit protocol is een eenvoudige en effectieve gids voor het voorbereiden van meetbare monsters voor zeer inzichtelijke spectroscopische technieken. De procedure is losjes gebaseerd op eerdere systemen met colloïdale halfgeleiders gestabiliseerd met polymeren.Het kan dus worden toegepast op verschillende systemen die de ophanging van materialen vereisen...

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 ...

Preparation of Highly Porous Coordination Polymer Coatings on Macroporous Polymer Monoliths for Enhanced Enrichment of Phosphopeptides

We describe a protocol for the preparation of hybrid materials based on highly porous coordination polymer coatings on the internal surface of macroporous polymer monoliths. The developed approach is based on the preparation of a macroporous polymer containing carboxylic acid functional groups and the subsequent step-by-step solution-based controlled growth of a layer of a porous coordination polymer on the surface of the pores of the polymer monolith. The prepared metal-organic polymer hybrid has a high specific micropore surface area. The amount of iron(III) sites is enhanced through metal-organic coordination on the surface of the pores of the functional polymer support. The increase of metal sites is related to the number of iterations of the coating process.
The developed preparation scheme is easily adapted to a capillary column format. The functional porous polymer is prepared as a self-contained single-block porous monolith within the capillary, yielding a flow-through separation device with excellent flow permeability and modest back-pressure. The metal-organic polymer hybrid column showed excellent performance for the enrichment of phosphopeptides from digested proteins and their subsequent detection using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry. The presented experimental protocol is highly versatile, and can be easily implemented to different organic polymer supports and coatings with a plethora of porous coordination polymers and metal-organic frameworks for multiple purification and/or separation applications.

A procedure for the preparation of porous hybrid separation media composed of a macroporous polymer monolith internally coated by a high surface area microporous coordination polymer is presented.

We describe a protocol for the preparation of hybrid materials based on highly porous coordination polymer coatings on the internal surface of macroporous ...

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. ...

In situ FTIR Spectroscopy as a Tool for Investigation of Gas/Solid Interaction: Water-Enhanced CO2 Adsorption in UiO-66 Metal-Organic Framework

In situ infrared spectroscopy is an inexpensive, highly sensitive, and selective valuable tool to investigate the interaction of polycrystalline solids with adsorbates. Vibrational spectra provide information on the chemical nature of adsorbed species and their structure. Thus, they are very useful for obtaining molecular level understanding of surface species. The IR spectrum of the sample itself gives some direct information about the material. General conclusions can be drawn concerning hydroxyl groups, some stable surface species and impurities. However, the spectrum of the sample is &#34;blind&#34; with respect to the presence of coordinatively unsaturated ions and gives rather poor information about the acidity of surface hydroxyls, species decisive for the adsorption and catalytic properties of the materials. Furthermore, no discrimination between bulk and surface species can be made. These problems are solved by the use of probe molecules, substances that interact specifically with the surface; the alteration of some spectral features of these molecules as a result of adsorption provides valuable information about the nature, properties, location, concentration, etc., of the surface sites.
The experimental protocol for in-situ IR studies of gas/sample interaction includes preparation of a sample pellet, activation of the material, initial spectral characterization through the analysis of the background spectra, characterization by probe molecules, and study of the interaction with a particular set of gas mixtures. In this paper we investigate a zirconium terephthalate metal organic framework, Zr6O4(OH)4(BDC)6 (BDC = benzene-1,4-dicarboxylate), namely UiO-66 (UiO refers to University of Oslo). The acid sites of the UiO-66 sample are determined by using CO and CD3CN as molecular probes. Furthermore, we have demonstrated that CO2 is adsorbed on basic sites exposed on dehydroxylated UiO-66. Introduction of water to the system produces hydroxyl groups acting as additional CO2 adsorption sites. As a result, CO2 adsorption capacity of the sample is strongly enhanced.

In situ FTIR Spectroscopy as a Tool for Investigation of Gas/Solid Interaction: Water-Enhanced CO2 Adsorption in UiO-66 Metal-Organic Framework

The use of FTIR spectroscopy for investigation of the surface properties of polycrystalline solids is described. Preparation of sample pellets, activation procedures, characterization with probe molecules and model studies of CO2 adsorption are discussed.

In situ infrared spectroscopy is an inexpensive, highly sensitive, and selective valuable tool to investigate the interaction of polycrystalline solids ...

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 ...

Synthesis and Discovery of Phosphonate-Based MOFs: A Solvothermal Approach

Discovery and Synthesis Optimization of Isoreticular Al(III) Phosphonate-Based Metal-Organic Framework Compounds Using High-Throughput Methods

High-throughput (HT) methods are an important tool for the fast and efficient screening of synthesis parameters and the discovery of new materials. This manuscript describes the synthesis of metal-organic frameworks (MOFs) from solution using an HT reactor system, resulting in the discovery of various phosphonate-based MOFs of the composition [Al2H12-x(PMP)3]Clx&#8729;6H2O (H4PMP = N,N &#39;-piperazine bis(methylenephosphonic acid)) for x = 4, 6, denoted as Al-CAU-60-xHCl, containing trivalent aluminum ions. This was accomplished under solvothermal reaction conditions by systematically screening the impact of the molar ratio of the linker to the metal and the pH of the reaction mixture on the product formation. The protocol for the HT investigation includes six steps: a) synthesis planning (DOE = design of&#160;experiment) within the HT methodology, b) dosing and working with in-house developed HT reactors, c) solvothermal synthesis, d) synthesis workup using in-house developed filtration blocks, e) characterization by HT powder X-ray diffraction, and f) evaluation of the data. The HT methodology was first used to study the influence of acidity on the product formation, leading to the discovery of Al-CAU-60&#8729;xHCl (x = 4 or 6).

Author Spotlight: Accelerating Discovery in Microporous Material Chemistry 

The targeted synthesis of new metal-organic frameworks (MOFs) is difficult, and their discovery depends on the knowledge and creativity of the chemist. High-throughput methods allow complex synthetic parameter fields to be explored quickly and efficiently, accelerating the process of finding crystalline compounds and identifying synthetic and structural trends.

High-throughput (HT) methods are an important tool for the fast and efficient screening of synthesis parameters and the discovery of new materials. This ...

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 ...

Self-Assembly of Polymer-Grafted Metal Organic Framework Monolayers

Synthesis and Characterization of Self-Assembled Metal-Organic Framework Monolayers Using Polymer-Coated Particles

Metal-organic frameworks (MOFs) are materials with potential applications in fields such as gas adsorption and separation, catalysis, and biomedicine. Attempts to enhance the utility of MOFs have involved the preparation of various composites, including polymer-grafted MOFs. By directly grafting polymers to the external surface of MOFs, issues of incompatibility between polymers and MOFs can be overcome. Polymer brushes grafted from the surface of MOFs can serve to stabilize the MOF while enabling particle assembly into self-assembled metal-organic framework monolayers (SAMMs) via polymer-polymer interactions.
Control over the chemical composition and molecular weight of the grafted polymer can allow for tuning of the SAMM characteristics. In this work, instructions are provided on how to immobilize a chain transfer agent (CTA) onto the surface of the MOF UiO-66 (UiO = Universitetet i Oslo). The CTA&#160;serves as initiation sites for the growth of polymers. Once polymer chains are grown from the MOF surface, the formation of SAMMs is achieved through self-assembly at an air-water interface. The resulting SAMMs are characterized and shown to be freestanding by scanning electron microscopy imaging. The methods presented in this paper are expected to make the preparation of SAMMs more accessible to the research community and thereby expand their potential use as a&#160;MOF-polymer composite.

Author Spotlight: Exploring Self-Assembled MOF-Polymer Composites

A protocol for the synthesis and characterization of self-assembled metal-organic framework monolayers is provided using polymer-grafted, metal-organic framework (MOF) crystals. The procedure shows that polymer-grafted MOF particles can be self-assembled at an air-water interface resulting in well-formed, free-standing, monolayer structures as evidenced by scanning electron microscopy imaging.

Metal-organic frameworks (MOFs) are materials with potential applications in fields such as gas adsorption and separation, catalysis, and biomedicine. ...

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

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