Physicochemical Methods for the Structuring and Assembly of MOF Crystals.

ConspectusMetal-organic frameworks (MOFs) are promising for various applications through the creation of innovative materials and assemblies. This potential stems from their modular nature, as diverse metal ions and organic linkers can be combined to produce MOFs with unique chemical properties and lattice structures. Following extensive research on the design and postsynthetic chemical modification of MOF lattices at the molecular level, increasing attention is now focused on the next hierarchical level: controlling the morphology of MOF crystals and their subsequent assembly and positioning to create functional composites.Beyond well-established methods to regulate crystal size and shape through nucleation and coordination modulation, physicochemical techniques leveraging wetting effects, interparticle interactions, and magnetic or electric fields offer attractive avenues for the hierarchical structuring and assembly of MOFs. These techniques facilitate crystal alignment and yield unique superstructures. While our research group primarily focuses on directing MOF crystal orientation and positioning using external stimuli such as magnetic and electric fields, we also explore hierarchical MOF synthesis and structuring using liquid interfaces and depletion force-assisted packing.This account highlights our journey and progress in developing methods to regulate the morphology, assembly, orientation, and positioning of MOF crystals, placed in the context of work by other groups. First, we examine commonly utilized structuring methods for MOF crystals that employ liquid-liquid and air-liquid interfaces to spatially confine reactions, allowing us to access unique morphologies such as mushroom-like crystals and Janus particles. We also discuss strategies for concentrating and packing MOF crystals into superstructures, utilizing fluid interfaces for spatial confinement of crystals, depletion forces, entropic effects, and crystal sedimentation.A particularly compelling challenge in expanding the applicability of MOF materials is how to manipulate free-standing MOF crystals. This issue is especially important because MOFs are typically produced as loose powders, and industrial material processing is generally more efficient when the material is fluidized. While extensive research has been conducted regarding MOF growth on substrates with both positional and orientational control, there is a clear need for similar precision with free-standing MOFs dispersed in a fluid matrix. Our group has thus focused on the relatively new, yet powerful approach of using electric and magnetic fields to manipulate MOF crystals, which offers unprecedented control over the orientation and positioning of dispersed MOF crystals, complementing the more well-established methods of MOF growth on substrates. In this Account, we provide foundational background and discussions on the interactions between these external fields and MOF crystals, including critical considerations for effective MOF manipulation using such techniques. We also discuss their unique advantages and applications, and briefly examine potential application areas, such as photonics, smart materials like soft robotics and absorbents, and sensing. This Account highlights the promising potential of well-organized and aligned MOF crystals over randomly oriented ones in various applications, owing to enhanced selectivity and performance. It underscores the importance of specialized assembly methods to advance materials science and engineering, encouraging the reader to explore such approaches.

Reactivity of Diamines in Acyclic Diamino Carbene Gold Complexes.

Acyclic diamino carbenes (ADCs) are interesting alternatives to their more widely studied N-heterocyclic carbene counterparts, particularly due to their greater synthetic accessibility and properties such as increased sigma donation and structural flexibility. ADC gold complexes are typically obtained through the reaction of equimolar amounts of primary/secondary amines on gold-coordinated isocyanide ligands. As such, the reaction of diamine nucleophiles to isocyanide gold complexes was expected to lead to bis-ADC gold compounds with potential applications in catalysis or as novel precursors for gold nanomaterials. However, the reaction of primary diamines with two equivalents of isocyanide gold chlorides resulted in only one of the amine groups reacting with the isocyanide carbon. The resulting ADC gold complexes bearing free amines dimerized via coordination of the amine to the partner gold atom, resulting in cyclic, dimeric gold complexes. In contrast, when secondary diamines were used, both amines reacted with an isocyanide carbon, leading to the expected bis-ADC gold complexes. Density functional theory calculations were performed to elucidate the differences in the reactivities between primary and secondary diamines. It was found that the primary amines were associated with higher reaction barriers than the secondary amines and hence slower reaction rates, with the formation of the second carbenes in the bis-ADC compounds being inhibitingly slow. It was also found that diamines have a unique reactivity due to the second amine serving as an internal proton shuttle.

Synthetically Versatile Nitrogen Acyclic Carbene Stabilized Gold Nanoparticles.

N-heterocyclic carbenes (NHCs) have received significant attention as gold nanoparticle stabilizers due to their strong binding affinity towards gold. However, their tunability is limited by the difficulty in obtaining nonsymmetric NHCs. In this regard, N-acyclic carbenes (NACs) are attractive alternatives due to their high synthetic versatility, allowing easy tuning of their steric and electronic properties towards specific applications. This work reports the first series of stable and monodisperse NAC-functionalized gold nanoparticles. These particles with sizes ranging 3.8 to 11.6 nm were characterized using NMR, UV/Vis and TEM. The nanoparticles display good stability at elevated temperatures and for extended periods both dried or dispersed in a medium, as well as in the presence of exogenous thiols. Importantly, these NAC-stabilized gold nanoparticles offer a promising and versatile alternative to NHC-stabilized gold nanoparticles.

An ICP-MS-based assay for characterization of gold nanoparticles with potential biomedical use.

Most of potential diagnostic and therapeutic nanoparticles fail to reach clinical trials because assessment of their 'drug-like' properties is often overlooked during the discovery stage. This compromises the results of cell culture and animal experiments, making them insufficient to evaluate the lead candidates for testing on patients. In this study, we demonstrate the potential of high-resolution inductively coupled plasma mass spectrometry (ICP-MS) as a nanoparticle qualification tool. Using novel gold nanoparticles stabilized by N-heterocyclic carbenes as test nanoparticles, it was shown that important prerequisites for biomedical applications, such as resistance to the action of human serum milieu or reactivity toward serum biomolecules, can be reliably assessed by recording the signals of gold or sulfur isotopes. Implemented during the screening stage, the method would provide benefits in shortening timelines and reducing cost for selection and initial testing of medicinal nanoparticle candidates.

Dynamic Electric Field Alignment of Metal-Organic Framework Microrods.

Alignment of metal-organic framework (MOF) crystals has previously been performed via careful control of oriented MOF growth on substrates, as well as by dynamic magnetic alignment. We show here that bromobenzene-suspended microrod crystals of the MOF NU-1000 can also be dynamically aligned via electric fields, giving rise to rapid electrooptical responses. This method of dynamic MOF alignment opens up new avenues of MOF control which are important for integration of MOFs into switchable electronic devices as well as in other applications such as reconfigurable sensors or optical systems.

Magnetic Control of MOF Crystal Orientation and Alignment.

Most metal-organic frameworks (MOFs) possess anisotropic properties, the full exploitation of which necessitates a general strategy for the controllable orientation of such MOF crystals. Current methods largely rely upon layer-by-layer MOF epitaxy or tuning of MOF crystal growth on appropriate substrates, yielding MOFs with fixed crystal orientations. Here, the dynamic magnetic alignment of different MOF crystals (NH2 -MIL-53(Al) and NU-1000) is shown. The MOFs were magnetized by electrostatic adsorption of iron oxide nanoparticles, dispersed in curable polymer resins (Formlabs 1+ clear resin/ Sylgard 184), magnetically oriented, and fixed by resin curing. The importance of crystal orientation on MOF functionality was demonstrated whereby magnetically aligned NU-1000/Sylgard 184 composite was excited with linearly polarized 405 nm light, affording an anisotropic fluorescence response dependent on the polarization angle of the excitation beam relative to NU-1000 crystal orientation.

Non-Close-Packed Breath Figures via Ion-Partitioning-Mediated Self-Assembly.

We report a one-step method of forming non-close-packed (NCP) pore arrays of micro- and sub-micropores using chloroform-based solutions of polystyrene acidified with hydrogen bromide for breath figure (BF) patterning. As BF patterning takes place, water vapor condenses onto the polystyrene solution, forming water droplets on the solution surface. Concurrently, preferential ion partitioning of hydrogen bromide leads to positively charged water droplets, which experience interdroplet electrostatic repulsion. Self-organization of charged water droplets because of surface flow and subsequent evaporation of the droplet templates result in ordered BF arrays with pore separation/diameter (L/D) ratios of up to 16.5. Evidence from surface potential scans show proof for preferential ion partitioning of HBr. Radial distribution functions and Voronoi polygon analysis of pore arrays show that they possess a high degree of conformational order. Past fabrication methods of NCP structures typically require multi-step processes. In contrast, we have established a new route for facile self-assembly of previously inaccessible patterns, which comprises of only a single operational step.

Photoresponsive liquid marbles and dry water.

Stimuli-responsive liquid marbles for controlled release typically rely on organic moieties that require lengthy syntheses. We report herein a facile, one-step synthesis of hydrophobic and oleophobic TiO2 nanoparticles that display photoresponsive wettability. Water liquid marbles stabilized by these photoresponsive TiO2 particles were found to be stable when shielded from ultraviolet (UV) radiation; however, they quickly collapsed after being irradiated with 302 nm UV light. Oil- and organic-solvent-based liquid marbles could also be fabricated using oleophobic TiO2 nanoparticles and show similar UV-induced collapse. Finally, we demonstrated the formation of the micronized form of water liquid marbles, also known as dry water, by homogenization of the TiO2 nanoparticles with water. The TiO2 dry water displayed a similar photoresponse, whereby the micronized liquid marbles collapsed after irradiation and the dry water turned from a free-flowing powder to a paste. Hence, by exploiting the photoresponsive wettability of TiO2, we fabricated liquid marbles and dry water that display photoresponse and studied the conditions required for their collapse.

Transparent PDMS Surfaces with Covalently Attached Lubricants for Enhanced Anti-adhesion Performance.

Liquid-like surfaces featuring slippery, omniphobic, covalently attached liquids (SOCALs) reduce unwanted adhesion by providing a molecularly smooth and slippery surface arising from the high mobility of the liquid chains. Such SOCALs are commonly prepared on hard substrates, such as glass, wafers, or metal oxides, despite the importance of nonpolar elastomeric substrates, such as polydimethylsiloxane (PDMS) in anti-fouling or nonstick applications. Compared to polar elastomers, hydrophobic PDMS elastomer activation and covalent functionalization are significantly more challenging, as PDMS tends to display fast hydrophobic recovery upon activation as well as superficial cracking. Through the extraction of excess PDMS oligomers and fine-tuning of plasma activation parameters, homogeneously functionalized PDMS with fluorinated polysiloxane brushes could be obtained while at the same time reducing crack formation. Polymer brush mobility was increased through the addition of a smaller molecular silane linker to exhibit enhanced dewetting properties and reduced substrate swelling compared to functionalizations featuring hydrocarbon functionalities. Linear polymer brushes were verified by thermogravimetric analysis. The optical properties of PDMS remained unaffected by the activation in high-frequency plasma but were impacted by low-frequency plasma. Drastic decreases in solid adhesion of not just complex contaminants but even ice could be shown in horizontal push tests, demonstrating the potential of SOCAL-functionalized PDMS surfaces for improved nonstick applications.

Fabrication of Oriented Polycrystalline MOF Superstructures.

The field of metal-organic frameworks (MOFs) has progressed beyond the design and exploration of powdery and single-crystalline materials. A current challenge is the fabrication of organized superstructures that can harness the directional properties of the individual constituent MOF crystals. To date, the progress in the fabrication methods of polycrystalline MOF superstructures has led to close-packed structures with defined crystalline orientation. By controlling the crystalline orientation, the MOF pore channels of the constituent crystals can be aligned along specific directions: these systems possess anisotropic properties including enhanced diffusion along specific directions, preferential orientation of guest species, and protection of functional guests. In this perspective, we discuss the current status of MOF research in the fabrication of oriented polycrystalline superstructures focusing on the specific crystalline directions of orientation. Three methods are examined in detail: the assembly from colloidal MOF solutions, the use of external fields for the alignment of MOF particles, and the heteroepitaxial ceramic-to-MOF growth. This perspective aims at promoting the progress of this field of research and inspiring the development of new protocols for the preparation of MOF systems with oriented pore channels, to enable advanced MOF-based devices with anisotropic properties.

Tuning omniphobicity via morphological control of metal-organic framework functionalized surfaces.

Fabrication of microstructures for imparting omniphobicity to a surface generally requires the use of lithographic techniques and specialized equipment. We report instead a simple strategy for the synthesis of microstructured surfaces via metal­organic framework (MOF) self-assembly. Our method allows us to localize epitaxial growth of MOF at the tips of needle crystals to create mushroom-shaped structures, thus conferring re-entrant textures to the MOF-functionalized surfaces. These structures synthesized via wet chemistry were found to have omniphobic properties due to the resulting re-entrant texture.

Hyper crosslinked polymer supported NHC stabilized gold nanoparticles with excellent catalytic performance in flow processes.

Highly active and selective heterogeneous catalysis driven by metallic nanoparticles relies on a high degree of stabilization of such nanomaterials facilitated by strong surface ligands or deposition on solid supports. In order to tackle these challenges, N-heterocyclic carbene stabilized gold nanoparticles (NHC@AuNPs) emerged as promising heterogeneous catalysts. Despite the high degree of stabilization obtained by NHCs as surface ligands, NHC@AuNPs still need to be loaded on support structures to obtain easily recyclable and reliable heterogeneous catalysts. Therefore, the combination of properties obtained by NHCs and support structures as NHC bearing "functional supports" for the stabilization of AuNPs is desirable. Here, we report the synthesis of hyper-crosslinked polymers containing benzimidazolium as NHC precursors to stabilize AuNPs. Following the successful synthesis of hyper-crosslinked polymers (HCP), a two-step procedure was developed to obtain HCP·NHC@AuNPs. Detailed characterization not only revealed the successful NHC formation but also proved that the NHC functions as a stabilizer to the AuNPs in the porous polymer network. Finally, HCP·NHC@AuNPs were evaluated in the catalytic decomposition of 4-nitrophenol. In batch reactions, a conversion of greater than 99% could be achieved in as little as 90 s. To further evaluate the catalytic capability of HCP·NHC@AuNP, the catalytic decomposition of 4-nitrophenol was also performed in a flow setup. Here the catalyst not only showed excellent catalytic conversion but also exceptional recyclability while maintaining the catalytic performance.

NHC stabilized copper nanoparticles via reduction of a copper NHC complex.

The bottom-up synthesis of plasmonic NHC@CuNPs from common starting reagents, via the formation of the synthetically accessible NHC-Cu(I)-Br complex and its reduction by NH3·BH3 is reported. The resulting NHC@CuNPs have been characterized in detail by XPS, TEM and NMR spectroscopy. The stability of NHC@CuNPs was investigated under both inert and ambient conditions using UV-Vis analysis. While the NHC@CuNPs are stable under inert conditions for an extended period of time, the NPs oxidize under air to form CuxO with concomitant release of the stabilizing NHC ligand.

Alignment of Breathing Metal-Organic Framework Particles for Enhanced Water-Driven Actuation.

As the majority of known metal-organic frameworks (MOFs) possess anisotropic crystal lattices and thus anisotropic physicochemical properties, a pressing practical challenge in MOF research is the establishment of robust and simple processing methods to fully harness the anisotropic properties of the MOFs in various applications. We address this challenge by applying an E-field to precisely align MIL-88A microcrystals and generate MIL-88A@polymer films. Thereafter, we demonstrate the impact of MOF crystal alignment on the actuation properties of the films as a proof of concept. We investigate how different anisotropies of the MIL-88A@polymer films, specifically, crystal anisotropy, particle alignment, and film composition, can lead to the synergetic enhancement of the film actuation upon water exposure. Moreover, we explore how the directionality in application of the external stimuli (dry/humid air stream, water/air interface) affects the direction and the extent of the MIL-88A@polymer film movement. Apart from the superior water-driven actuation properties of the developed films, we demonstrate by dynamometer measurements the higher degree of mechanical work performed by the aligned MIL-88A@polymer films with the preserved anisotropies compared to the unaligned films. The insights provided by this work into anisotropic properties displayed by aligned MIL-88A@polymer films promise to translate crystal performance benefits measured in laboratories into real-world applications. We anticipate that our work is a starting point to utilize the full potential of anisotropic properties of MOFs.

Fabrication of graded porous structure of hydroxypropyl cellulose hydrogels via temperature-induced phase separation.

A novel hydroxypropyl cellulose (gHPC) hydrogel with graded porosity has been fabricated, in which pore size, shape, and mechanical properties vary across the material. The graded porosity was achieved by cross-linking different parts of the hydrogel at temperatures below and above 42 °C, which was found to be the temperature of turbidity onset (lower critical solution temperature, LCST) for the HPC and divinylsulfone cross-linker mixture. Scanning electron microscopy imaging revealed a decreasing pore size along the cross-section of the HPC hydrogel from the top to the bottom layer. HPC hydrogels demonstrate graded mechanical properties whereby the top layer, Zone 1, cross-linked below LCST, can be compressed by about 50% before fracture, whereas the middle and bottom layers (Zone 2 and 3, respectively) cross-linked at 42 °C, can withstand 80% compression before failure. This work demonstrates a straightforward, yet novel, concept of exploiting a graded stimulus to incorporate a graded functionality into porous materials that can withstand mechanical stress and minor elastic deformations.

Effect of a Zr-Based Metal-Organic Framework Structure on the Properties of Its Composite with Polyaniline.

Composites of polyaniline (PANI) and Zr-based metal-organic frameworks (MOFs), UiO-66 and UiO-66-NH2, were synthesized by the oxidative polymerization of aniline in the presence of MOF templates with the MOF content in the resulting materials (78.2 and 86.7 wt %, respectively) close to the theoretical value (91.5 wt %). Scanning electron microscopy and transmission electron microscopy showed that the morphology of the composites was set by the morphology of the MOFs, whose structure was mostly preserved after the synthesis, based on the X-ray diffraction data. Vibrational and NMR spectroscopies pointed out that MOFs participate in the protonation of PANI and conducting polymer chains were grafted to amino groups of UiO-66-NH2. Unlike PANI-UiO-66, cyclic voltammograms of PANI-UiO-66-NH2 showed a well-resolved redox peak at around ≈0 V, pointing at the pseudocapacitive behavior. The gravimetric capacitance of PANI-UiO-66-NH2, normalized per mass of the active material, was also found to be higher compared to that of pristine PANI (79.8 and 50.5 F g-1, respectively, at 5 mV s-1). The introduction of MOFs into the composites with PANI significantly improved the cycling stability of the materials over 1000 cycles compared to the pristine conducting polymer, with the residual gravimetric capacitance being ≥100 and 77%, respectively. Thus, the electrochemical performance of the prepared PANI-MOF composites makes them attractive materials for application in energy storage.

Metal-Organic Framework superstructures with long-ranged orientational order via E-field assisted liquid crystal assembly.

Most MOFs are non-cubic, with functionality dependent upon crystallographic direction, and are largely prepared as microcrystalline powders. Therefore, general methods to orient and assemble free-standing MOF crystals are especially important and urgently needed. This is addressed here through the novel strategy of E-field assisted liquid crystal assembly, applied to MIL-53-NH2(Al), MIL-68(In) and NU-1000 MOF crystals, with aspect ratios ranging from 10 to 1.2, to form highly oriented MOF superstructures which were photopolymerized to fix their long-ranged order. This new strategy for controlling MOF orientation and packing side-steps the traditional requirements of particle monodispersity, shape homogeneity and high aspect ratios (>4.7) typical of colloidal and liquid crystal assembly, and is applicable even to polydispersed MOF crystals, thereby paving the way towards the development of highly oriented MOF composites with improved functionality.

CELLOPT: improved unit-cell parameters for electron diffraction data of small-molecule crystals.

Electron diffraction enables structure determination of organic small molecules using crystals that are too small for conventional X-ray crystallography. However, because of uncertainties in the experimental parameters, notably the detector distance, the unit-cell parameters and the geometry of the structural models are typically less accurate and precise compared with results obtained by X-ray diffraction. Here, an iterative procedure to optimize the unit-cell parameters obtained from electron diffraction using idealized restraints is proposed. The cell optimization routine has been implemented as part of the structure refinement, and a gradual improvement in lattice parameters and data quality is demonstrated. It is shown that cell optimization, optionally combined with geometrical corrections for any apparent detector distortions, benefits refinement of electron diffraction data in small-molecule crystallography and leads to more accurate structural models.

Protonation of the dinitrogen-reduction catalyst [HIPTN3N]Mo(III) investigated by ENDOR spectroscopy.

Dinitrogen is reduced to ammonia by the molybdenum complex of L = [HIPTN3N](3-) [Mo; HIPT = 3,5-(2,4,6-iPr3C6H2)2C6H3]. The mechanism by which this occurs involves the stepwise addition of proton/electron pairs, but how the first pair converts MoN2 to MoN ═ NH remains uncertain. The first proton of reduction might bind either at Nß of N2 or at one of the three amido nitrogen (N(am)) ligands. Treatment of MoCO with [2,4,6-Me3C5H3N]BAr'4 [Ar' = 2,3-(CF3)2C6H3] in the absence of reductant generates HMoCO(+), whose electron paramagnetic resonance spectrum has greatly reduced g anisotropy relative to MoCO. (2)H Mims pulsed electron nuclear double-resonance spectroscopy of (2)HMoCO(+) shows a signal that simulations show to have a hyperfine tensor with an isotropic coupling, aiso((2)H) = -0.22 MHz, and a roughly dipolar anisotropic interaction, T((2)H) = [-0.48, -0.93, 1.42] MHz. The simulations show that the deuteron is bound to N(am), near the Mo equatorial plane, not along the normal, and at a distance of 2.6 Å from Mo, which is nearly identical with the (Nam)(2)H(+)-Mo distance predicted by density functional theory computations.

Catalytically Active Gold Nanomaterials Stabilized by N-heterocyclic Carbenes.

Solid supported or ligand capped gold nanomaterials (AuNMs) emerged as versatile and recyclable heterogeneous catalysts for a broad variety of conversions in the ongoing catalytic 'gold rush'. Existing at the border of homogeneous and heterogeneous catalysis, AuNMs offer the potential to merge high catalytic activity with significant substrate selectivity. Owing to their strong binding towards the surface atoms of AuMNs, NHCs offer tunable activation of surface atoms while maintaining selectivity and stability of the NM even under challenging conditions. This work summarizes well-defined catalytically active NHC capped AuNMs including spherical nanoparticles and atom-precise nanoclusters as well as the important NHC design choices towards activity and (stereo-)selectivity enhancements.