This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT (NRF-2022R1A2C1009706).

The reagents required to prepare MOFs and the ligands are listed in the Table of Materials.
1. Setting up the post-synthetic ligand exchange (PSE) process
<ol>
	<li>Completely dry the pre-synthesized UiO-66 MOFs under vacuum to remove any unreacted metal-salts and ligands in the pores, and remaining solvent residues overnight. 
	NOTE: See Supplementary File 1 for the synthesis procedure of UiO-66 MOFs.</li>
	<li>Prepare functionalized ligands, H2BDC-Triazole, and H2BDC-Tetrazole (see Supplementary File 1 for the preparation process; Supplementary Figure 1 and Supplementary Figure 2 for characterization) in an isolated state and fully dry under vacuum overnight.</li>
	<li>Prepare a 4% potassium hydroxide (KOH) aqueous solution by dissolving potassium hydroxide in deionized water.</li>
	<li>Perform the PSE process in 20 mL scintillation vials with polypropylene (PP) caps (see Table of Materials).</li>
	<li>Measure the H2BDC-Triazole (23.3 mg, 0.1 mmol) or H2BDC-Tetrazole (23.4 mg, 0.1 mmol) and place into the scintillation vial.</li>
	<li>Dissolve the H2BDC ligand in aqueous solution. Use a glass pipette to transfer 1.0 mL of the 4% KOH aqueous solution into the scintillation vial containing BDC-Triazole or BDC-Tetrazole. 
	CAUTION: The 4% KOH aqueous solution is highly basic. Avoid all forms of contact and wear personal protective equipment.</li>
	<li>Sonicate the mixture until all the solids have fully dissolved.</li>
	<li>Neutralize the solution to pH 7. Use a glass pipette to transfer 1 M hydrochloric acid (HCl) aqueous solution with stirring into the scintillation vial containing dicarboxylate until pH 7 is reached. Measure the pH either by pH paper (and its color) or a pH meter. 
	NOTE: It is crucial to maintain a pH of 7 during the exchange process, since MOFs are typically unstable under basic conditions (&#62;pH 7), and the BDC ligand is not soluble under acidic conditions (&#60;pH 7). Approximately 1.5 mL of 1 M HCl aqueous solution is required to neutralize BDC-Triazole or BDC-Tetrazole-containing solution.</li>
	<li>Add MOFs to the dicarboxylate solution and incubate. Add UiO-66 MOF (33 mg, 0.02 mmol) to the scintillation vial containing pH 7 solution. 
	NOTE: At pH 7, UiO-66 MOF particles are not soluble in water, resulting in their suspension in water.</li>
	<li>Incubate the scintillation vial containing the MOF and ligand in a shaker at 120 rpm and at room temperature for 24 h. 
	&#8203;NOTE: Stirring can be used as an alternative to shaker-incubation for the PSE process. However, caution should be taken, since physical contact between the magnetic stirring bar and MOFs can result in cracking or breaking of the MOF particles.</li>
</ol>
2. Isolating the exchanged MOF and washing process
<ol>
	<li>After incubation, isolate the solid MOF from the mixture by centrifugation (1,166 x g, 5 min, room temperature).</li>
	<li>Add fresh methanol (10 mL) to the obtained solid MOF and shake the mixture to form a heterogeneous mixture to dissolve the remaining unexchanged BDC ligands.</li>
	<li>Isolate the solid that was isolated by centrifugation (1,166 x g, 5 min, room temperature).</li>
	<li>Repeat steps 2.2-2.3 two more times for a total of three washing cycles.</li>
	<li>Fully dry the exchanged MOF solid under a vacuum for overnight after the last washing.</li>
</ol>
3. Characterization of the MOF by powder x-ray diffraction (PXRD)
<ol>
	<li>Transfer approximately 10 mg of the exchanged MOF solid to a PXRD sample holder (see Table of Materials).</li>
	<li>Place the sample holder in the diffractometer.</li>
	<li>Collect the PXRD pattern (Figure 2) in the 2&#952; range from 5&#176; to 30&#176;.</li>
	<li>Compare the obtained data to the parent UiO-66 MOF and the simulated pattern.</li>
</ol>
4. Characterization of the MOF by nuclear magnetic resonance (NMR) after digestion
<ol>
	<li>Transfer approximately 30 mg of the exchanged MOF solid to a fresh 4 mL vial.</li>
	<li>Use a micropipette to transfer 400 &#181;L of DMSO-d6 to the MOF sample.</li>
	<li>Use a micropipette to transfer 200 &#181;L of 4.14 M NH4F/D2O solution to a DMSO-d6 suspension of MOF powder. 
	NOTE: An aqueous solution of approximately 40% HF can be used instead of the NH4F/D2O solution. In this case, a significant H2O peak is observed in the NMR. However, clearer digestion is possible in the case of HF digestion. 
	CAUTION: HF is highly toxic to the body and central nervous system. All forms of contact should be avoided, work should be performed in a fume hood, and personal protective equipment should be worn.</li>
	<li>Sonicate the heterogeneous mixture for 30 min until the MOF is dissolved in the dimethyl sulfoxide (DMSO)-D2O mixed solvent after digestion.</li>
	<li>Remove any remaining insoluble solids by filtering the solution through a polyvinylidene difluoride (PVDF) syringe filter (&#934; 13 mm, 0.45 &#181;m pore; see Table of Materials) while transferring it from the 4 mL vial to an NMR tube.</li>
	<li>Place the NMR tube in the NMR machine. Collect the NMR data (Figure 3).</li>
</ol>

Triazole and Tetrazole Functionalized Zr-MOFs Synthesis and Characterization

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The authors have nothing to disclose.

The PSE process with functionalized BDC ligands toward Zr-based UiO-66 MOFs is a simple and versatile method to obtain MOFs with chemical tags. The PSE process is best conducted in aqueous media, requiring the initial step of solvating the ligand in an aqueous medium. When using pre-synthesized BDC with functional groups, direct dissolution in a basic solvent, such as a 4% KOH aqueous solution, is recommended. Alternatively, sodium or potassium salt of benzene-1,4-dicarboxylate may be used. Neutralization to pH 7 is crit...

Metal-organic frameworks (MOFs) are three-dimensional porous materials that are formed through coordination bonds between metal clusters and multi-topic organic ligands. MOFs have garnered significant attention due to their permanent porosity, low density, and ability to associate organic and inorganic components, which enables diverse applications1,2. Moreover, the vast range of metal nodes and strut organic linkers offer MOFs theoretically unlimited structural combinations. Even with identical framework structures, MOFs&#39; physical and chemical properties can be modified through ligand functionalization with chemical tags. This modification process offers a promising route to tailor the properties of MOFs for specific applications3,4,5,6,7,8,9.
Both the pre-functionalization of ligands prior to MOF synthesis and post-synthetic modification (PSM) of MOFs have been employed to introduce and/or modify functional groups in MOF ligands10,11. In particular, covalent PSMs have been extensively studied to introduce new functional groups and generate a range of MOFs with diverse functionalities12,13,14. For instance, UiO-66-NH2 can be converted to amide-functionalized UiO-66-AMs with different chain lengths (ranging from the shortest acetamide to the longest n-hexyl amide) through acylation reactions with appropriate acyl halides (such as acetyl chloride or n-hexanoyl chloride)15,16. This approach demonstrates the versatility of covalent PSMs to introduce specific functional groups onto MOF ligands, paving the way for a broad range of applications.
In addition to covalent PSMs, post-synthetic ligand exchange (PSE) is a promising strategy for modifying MOFs (Figure 1). Since MOFs are composed of coordination bonds between metals and ligands (such as carboxylates), these coordination bonds can be replaced with external ligands from a solution. Exposing MOFs to a solution containing the desired ligand with chemical tags can be incorporated into the MOFs via PSE17,18,19,20,21,22. Since the PSE process is accelerated by the existence of coordinative solvents, the phenomenon is also called solvent-assisted ligand exchange (SALE)23,24. This approach offers a flexible and facile method for functionalizing MOFs with a wide range of external ligands, enabling a broad spectrum of applications25,26,27,28,29.
<img alt="Figure 1" class="xfigimg" src="/files/ftp_upload/65619/65619fig01.jpg" /> 
Figure 1: Synthesis of triazole and tetrazole-functionalized H2BDC ligands and preparation of triazole- and tetrazole-functionalized UiO-66 MOF through PSE. <a href="https://www.jove.com/files/ftp_upload/65619/65619fig01large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
The progress of the PSE process can be controlled by adjusting the ligand ratio, exchange temperature, and time. Notably, room temperature PSE can be employed to obtain functionalized MOFs by exchanging ligands from a solution into MOF solids20. The PSE strategy is particularly useful for introducing both thermally unstable functional groups (such as azido groups) and coordinative functional groups (such as phenol groups) into MOF structures18. In addition, the PSE strategy has been applied to various MOFs with metal and coordination bond variations. This exchange is a universal process in the chemistry of MOFs30,31,32. In this study, we present a detailed protocol for PSE to obtain functionalized MOFs from pristine, non-functionalized MOFs, and we provide a characterization strategy to confirm successful functionalization of the MOFs. This method demonstrates the versatility and convenience of PSE for modifying MOFs with diverse functional groups.
Tetrazole-containing benzene-1,4-dicarboxylic acid (H2BDC-Tetrazole)33, and triazole-containing benzene-1,4-dicarboxylic acid (H2BDC-Triazole) are synthesized as target ligands and utilized in the PSE of UiO-66 MOFs to obtain novel, coordination-free, triazole-containing MOFs. Both triazoles and tetrazoles possess acidic N-H protons on their heterocyclic rings and can coordinate with metal cations, thus enabling their use in constructing MOFs34,35. However, there are limited studies on incorporating coordination-free tetrazoles and triazoles into MOFs and related structures. In case of triazole-functionalized Zr-MOFs, UiO-68 type MOFs were investigated to photophysical properties through direct solvothermal synthesis with benzotriazole functionalities36. For tetrazole-functionalized Zr-MOFs, the mixed direct synthesis was employed33. These heterocycle-functionalized MOFs could provide potential coordinating sites in MOF pores for catalysis, selective molecular uptake by binding affinity, and energy-related applications, such as proton conduction in fuel cells.

<table border="1" fo:keep-together.within-page="1" fo:keep-with-next.within-page="always">
	<tbody>
		<tr>
			<td>2-Bromoterephthalic acid</td>
			<td>BLD Pharm</td>
			<td>BD5695</td>
			<td>reagent for BDC-Triazole</td>
		</tr>
		<tr>
			<td>Azidotrimethylsilane</td>
			<td>Simga Aldrich</td>
			<td>155071</td>
			<td>reagent for BDC-Triazole</td>
		</tr>
		<tr>
			<td>Bis(triphenylphosphine)palladium(II) dichloride</td>
			<td>TCI</td>
			<td>B1667</td>
			<td>reagent for BDC-Triazole</td>
		</tr>
		<tr>
			<td>Copper(I) cyanide</td>
			<td>Alfa-Aesar</td>
			<td>12135</td>
			<td>reagent for BDC-Tetrazole</td>
		</tr>
		<tr>
			<td>Copper(I) iodide</td>
			<td>Acros organics</td>
			<td>20150</td>
			<td>reagent for BDC-Triazole</td>
		</tr>
		<tr>
			<td>Digital Orbital Shaker</td>
			<td>Daihan Scientific</td>
			<td>SHO-1D</td>
			<td>PSE</td>
		</tr>
		<tr>
			<td>Formic Acid</td>
			<td>Daejung chemical</td>
			<td>F0195</td>
			<td>reagent for BDC-Tetrazole</td>
		</tr>
		<tr>
			<td>Hybrid LC/Q-TOF system</td>
			<td>Bruker BioSciences</td>
			<td>maXis 4G</td>
			<td>HR-MS</td>
		</tr>
		<tr>
			<td>Lithum hydroxide monohydrate</td>
			<td>Daejung chemical</td>
			<td>5087-4405</td>
			<td>reagent for BDC-Triazole</td>
		</tr>
		<tr>
			<td>Magnesium sulfate</td>
			<td>Samchun chemical</td>
			<td>M1807</td>
			<td>reagent for BDC-Triazole</td>
		</tr>
		<tr>
			<td>Methyl alcohol</td>
			<td>Daejung chemical</td>
			<td>M0584</td>
			<td>reagent for BDC-Tetrazole</td>
		</tr>
		<tr>
			<td>N,N-Dimethylformamide</td>
			<td>Daejung chemical</td>
			<td>D0552</td>
			<td>reagent for BDC-Tetrazole</td>
		</tr>
		<tr>
			<td>Nuclear Magnetic Resonance Spectrometer-500 MHz</td>
			<td>Bruker</td>
			<td>AVANCE 500MHz</td>
			<td>NMR</td>
		</tr>
		<tr>
			<td>Polypropylene cap (22 mm, Cork-Backed Foil Lined)</td>
			<td>Sungho Korea</td>
			<td>22-200</td>
			<td>material for digestion</td>
		</tr>
		<tr>
			<td>Potassium cyanide</td>
			<td>Alfa-Aesar</td>
			<td>L13273</td>
			<td>reagent for BDC-Tetrazole</td>
		</tr>
		<tr>
			<td>PVDF Synringe filter (13 mm, 0.45 &#181;m)</td>
			<td>LK Lab Korea</td>
			<td>F14-61-363</td>
			<td>material for digestion</td>
		</tr>
		<tr>
			<td>Scintillation vial (20 mL, borosilicate glass)</td>
			<td>Sungho Korea</td>
			<td>74504-20</td>
			<td>material for digestion</td>
		</tr>
		<tr>
			<td>Sodium azide&#160;</td>
			<td>TCI</td>
			<td>S0489</td>
			<td>reagent for BDC-Tetrazole</td>
		</tr>
		<tr>
			<td>Sodium bicarbonate</td>
			<td>Samchun chemical</td>
			<td>S0343</td>
			<td>reagent for BDC-Triazole</td>
		</tr>
		<tr>
			<td>Tetrabutylammonium fluoride (1 M THF solution)</td>
			<td>Acros organics</td>
			<td>20195</td>
			<td>reagent for BDC-Triazole</td>
		</tr>
		<tr>
			<td>Triethylamine</td>
			<td>TCI</td>
			<td>T0424</td>
			<td>reagent for BDC-Triazole</td>
		</tr>
		<tr>
			<td>Triethylamine hydrochloride</td>
			<td>Daejung chemical</td>
			<td>8628-4405</td>
			<td>reagent for BDC-Tetrazole</td>
		</tr>
		<tr>
			<td>Trimethylsilyl-acetylene</td>
			<td>Alfa-Aesar</td>
			<td>A12856</td>
			<td>reagent for BDC-Triazole</td>
		</tr>
		<tr>
			<td>Triphenylphosphine</td>
			<td>TCI</td>
			<td>T0519</td>
			<td>reagent for BDC-Triazole</td>
		</tr>
		<tr>
			<td>X RAY DIFFRACTOMETER SYSTEM</td>
			<td>Rigaku</td>
			<td>MiniFlex 600</td>
			<td>PXRD</td>
		</tr>
		<tr>
			<td>Zirconium(IV) chloride</td>
			<td>Alfa-Aesar</td>
			<td>12104</td>
			<td>reagent for BDC-Tetrazole</td>
		</tr>
	</tbody>
</table>

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.

The successful synthesis of exchanged UiO-66 MOFs, UiO-66-Triazole, and UiO-66-Tetrazole produced colorless microcrystalline solids. Both H2BDC-Triazole and H2BDC-Tetrazole ligands also exhibited a colorless solid state. The standard method used to determine the success of the exchange involved measuring the PXRD patterns and comparing the crystallinity of the sample with pristine UiO-66 MOF. Figure 2 displays the PXRD patterns of exchanged UiO-66-Triazole and UiO-66-Te...

Watch this Scientific Journal Video about Functionalizing Metal-Organic Frameworks: Advancements, Challenges, and the Power of Post-Synthetic Ligand Exchange at JoVE.com

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.

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. ...

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2.6K Views.  Chungbuk National University. 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.

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

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 Ligand-free CdS Nanoparticles within a Sulfur Copolymer Matrix

Aliphatic ligands are typically used during the synthesis of nanoparticles to help mediate their growth in addition to operating as high-temperature solvents. These coordinating ligands help solubilize and stabilize the nanoparticles while in solution, and can influence the resulting size and reactivity of the nanoparticles during their formation. Despite the ubiquity of using ligands during synthesis, the presence of aliphatic ligands on the nanoparticle surface can result in a number of problems during the end use of the nanoparticles, necessitating further ligand stripping or ligand exchange procedures. We have developed a way to synthesize cadmium sulfide (CdS) nanoparticles using a unique sulfur copolymer. This sulfur copolymer is primarily composed of elemental sulfur, which is a cheap and abundant material. The sulfur copolymer has the advantages of operating both as a high temperature solvent and as a sulfur source, which can react with a cadmium precursor during nanoparticle synthesis, resulting in the generation of ligand free CdS. During the reaction, only some of the copolymer is consumed to produce CdS, while the rest remains in the polymeric state, thereby producing a nanocomposite material. Once the reaction is finished, the copolymer stabilizes the nanoparticles within a solid polymeric matrix. The copolymer can then be removed before the nanoparticles are used, which produces nanoparticles that do not have organic coordinating ligands. This nascent synthesis technique presents a method to produce metal-sulfide nanoparticles for a wide variety of applications where the presence of organic ligands is not desired.

Herein we present a method to synthesize ligand-free cadmium sulfide (CdS) nanoparticles based on a unique sulfur copolymer. The sulfur copolymer operates as a high temperature solvent and a sulfur source during the nanoparticle synthesis and stabilizes the nanoparticles after the reaction.

Aliphatic ligands are typically used during the synthesis of nanoparticles to help mediate their growth in addition to operating as high-temperature ...

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.

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. ...

Electrophoretic Crystallization of Ultrathin High-performance Metal-organic Framework Membranes

We report the synthesis of thin, highly intergrown, polycrystalline metal-organic framework (MOF) membranes on a wide range of unmodified porous and non-porous supports (polymer, ceramic, metal, carbon, and graphene). We developed a novel crystallization technique, which is termed the ENACT approach: the electrophoretic nuclei assembly for the crystallization of highly intergrown thin films (ENACT). This approach allows for a high density of heterogeneous nucleation of MOFs on a chosen substrate via the electrophoretic deposition (EPD) directly from the precursor sol. The growth of well-packed MOF nuclei leads to a highly intergrown polycrystalline MOF film. We show that this simple approach can be used for the synthesis of thin, intergrown zeolite imidazole framework (ZIF)-7 and ZIF-8 films. The resulting 500 nm-thick ZIF-8 membranes show a considerably high H2 permeance (8.3 x 10-6 mol m-2 s-1 Pa-1) and ideal gas selectivities (7.3 for H2/CO2, 15.5 for H2/N2, 16.2 for H2/CH4, and 2655 for H2/C3H8). An attractive performance for C3H6/C3H8 separation is also achieved (a C3H6 permeance of 9.9 x 10-8 mol m-2 s-1 Pa-1 and a C3H6/C3H8 ideal selectivity of 31.6 at 25 &#176;C). Overall, the ENACT process, owing to its simplicity, can be extended to synthesize intergrown thin films of a wide range of nanoporous crystalline materials.

A simple, reproducible, and versatile approach for the synthesis of intergrown, polycrystalline metal-organic framework membranes on a wide range of unmodified porous and non-porous supports is presented.

We report the synthesis of thin, highly intergrown, polycrystalline metal-organic framework (MOF) membranes on a wide range of unmodified porous and ...

Facile Synthesis of Colloidal Lead Halide Perovskite Nanoplatelets via Ligand-Assisted Reprecipitation

In this work, we demonstrate a facile method for colloidal lead halide perovskite nanoplatelet synthesis (Chemical formula: L2[ABX3]n-1BX4, L: butylammonium and octylammonium, A: methylammonium or formamidinium, B: lead, X: bromide and iodide, n: number of [BX6]4- octahedral layers in the direction of nanoplatelet thickness) via ligand-assisted reprecipitation. Individual perovskite precursor solutions are prepared by dissolving each nanoplatelet constituent salt in N,N-dimethylformamide (DMF), which is a polar organic solvent, and then mixing in specific ratios for targeted nanoplatelet thickness and composition. Once the mixed precursor solution is dropped into nonpolar toluene, the abrupt change in the solubility induces the instantaneous crystallization of nanoplatelets with surface-bound alkylammonium halide ligands providing colloidal stability. Photoluminescence and absorption spectra reveal emissive and strongly quantum-confined features. X-ray diffraction and transmission electron microscopy confirm the two-dimensional structure of the nanoplatelets. Furthermore, we demonstrate that the band gap of perovskite nanoplatelets can be continuously tuned in the visible range by varying the stoichiometry of the halide ion(s). Lastly, we demonstrate the flexibility of the ligand-assisted reprecipitation method by introducing multiple species as surface-capping ligands. This methodology represents a simple procedure for preparing dispersions of emissive 2D colloidal semiconductors.

This work demonstrates facile room-temperature synthesis of colloidal quantum-confined lead halide perovskite nanoplatelets by ligand-assisted reprecipitation method. Synthesized nanoplatelets show spectrally narrow optical features and continuous spectral tunability throughout the visible range by varying the composition and thicknesses.

In this work, we demonstrate a facile method for colloidal lead halide perovskite nanoplatelet synthesis (Chemical formula: L2[ABX3]n-1BX4, L: ...

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. ...

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.

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.