Control over Catenation in Metal−Organic Frameworks via Rational Design of the Organic Building Block

Balancing volumetric and gravimetric capacity for hydrogen in supramolecular crystals

The storage of hydrogen is key to its applications. Developing adsorbent materials with high volumetric and gravimetric storage capacities, both of which are essential for the efficient use of hydrogen as a fuel, is challenging. Here we report a controlled catenation strategy in hydrogen-bonded organic frameworks (RP-H100 and RP-H101) that depends on multiple hydrogen bonds to guide catenation in a point-contact manner, resulting in high volumetric and gravimetric surface areas, robustness and ideal pore diameters (~1.2-1.9 nm) for hydrogen storage. This approach involves assembling nine imidazole-annulated triptycene hexaacids into a secondary hexagonal superstructure containing three open channels through which seven of the hexagons interpenetrate to form a seven-fold catenated superstructure. RP-H101 exhibits high deliverable volumetric (53.7 g l-1) and gravimetric (9.3 wt%) capacities for hydrogen under a combined temperature and pressure swing (77 K/100 bar → 160 K/5 bar). This work illustrates the virtues of supramolecular crystals as promising candidates for hydrogen storage.

Controlling the Degree of Interpenetration in Chiral Three-Dimensional Covalent Organic Frameworks via Steric Tuning

Network interpenetration plays a crucial role in functionalizing porous framework materials. However, controlling the degree of interpenetration in covalent organic frameworks (COFs) to influence their pore sizes, shapes, and functionalities still remains a significant challenge. Here, we demonstrate a steric tuning strategy to control the degree of COF interpenetration and modulate their physicochemical properties. By imine condensations of 1,1'-bi-2-naphthol-derived tetraaldehydes bearing different alkyl substituents with the monomer tetra(p-aminophenyl)-methane, we synthesized and characterized a family of two-component and three-component chiral COFs with different interpenetrated dia networks. The alkyl groups are periodically appended on the pore walls, and their types/contents that can be synthetically tuned control the interpenetration degree of COFs by minimizing repulsive interactions between the alkyl groups. Specifically, the COF with -OH groups adopts an interpenetrated dia-c5 topology, those with -OMe/-OEt groups take an interpenetrated dia-c4 topology, whereas those with the bulky -OnPr/-OnBu groups exhibit a noninterpenetrated dia-c1 topology. The multivariate COFs with both -OH and -OnBu groups display either a noninterpenetrated or dia-c5 topology, depending on the proportion of -OnBu groups. The extent of interpenetration in COFs significantly affects their porosity, thermal stability, and chemical stability, resulting in varying selective performances in the adsorption and separation of dyes and asymmetric catalysis. This work highlights the potential of using steric hindrance to tune and control interpenetration, porosity, stability, and functionalities of COFs materials, broadening the range of their applications.

Manipulating AIE ligands into layers of pillar-layered MOFs for enhanced emission

Four pillar-layered AIEgen-based MOFs exhibit higher thermal stability, tunable emission colors and improved QYs compared with that of non-pillar-layered AIEgen-based MOFs by confining the AIE ligands into layers. These results reveal that rationally manipulating AIE ligands into layers of pillar-layered MOFs is an effective strategy for the design and construction of tunable luminescent MOF systems.

Solvent-Driven Dynamics: Crafting Tailored Transformations of Cu(II)-Based MOFs

Metal-organic frameworks (MOFs), a sort of crystalline porous coordination polymers composed of metal ions and organic linkers, have been intensively studied for their ability to take up nonpolar gas-phase molecules such as ethane and ethylene. In this context, interpenetrated MOFs, where multiple framework nets are entwined, have been considered promising materials for capturing nonpolar molecules due to their relatively higher stability and smaller micropores. This study explores a solvent-assisted reversible strategy to interpenetrate and deinterpenetrate a Cu(II)-based MOF, namely, MOF-143 (noninterpenetrated form) and MOF-14 (doubly interpenetrated forms). Interpenetration was achieved using protic solvents with small molecular sizes such as water, methanol, and ethanol, while deinterpenetration was accomplished with a Lewis-basic solvent, pyridine. Additionally, this study investigates the adsorptive separation of ethane and ethylene, which is a significant application in the chemical industry. The results showed that interpenetrated MOF-14 exhibited higher ethane and ethylene uptakes compared to the noninterpenetrated MOF-143 due to narrower micropores. Furthermore, we demonstrate that pristine MOF-14 displayed higher ethane selectivity than transformed MOF-14 from MOF-143 by identifying the "fraction of micropore volume" as a key factor influencing ethane uptake. These findings highlight the potential of controlled transformations between interpenetrated and noninterpenetrated MOFs, anticipating that larger MOF crystals with narrower micropores and higher crystallinity will be more suitable for selective gas capture and separation applications.

Efficient CO2 Capture and Separation in MOFs: Effect from Isoreticular Double Interpenetration

Severe CO2 emissions has posed an increasingly alarming threat, motivating the development of efficient CO2 capture materials, one of the key parts of carbon capture, utilization, and storage (CCUS). In this study, a series of metal-organic frameworks (MOFs) named Sc-X (X = S, M, L) were constructed inspired by recorded MOFs, Zn-BPZ-SA and MFU-4l-Li. The corresponding isoreticular double-interpenetrating MOFs (Sc-X-IDI) were subsequently constructed via the introduction of isoreticular double interpenetration. Grand canonical Monte Carlo (GCMC) simulations were adopted at 298 K and 0.1-1.0 bar to comprehensively evaluate the CO2 capture and separation performances in Sc-X and Sc-X-IDI, with gas distribution, isothermal adsorption heat (Qst), and van der Waals (vdW)/Coulomb interactions. It is showed that isoreticular double interpenetration significantly improved the interactions between adsorbed gases and frameworks by precisely modulating pore sizes, particularly observed in Sc-M and Sc-M-IDI. Specifically, the Qst and Coulomb interactions exhibited a substantial increase, rising from 28.38 and 22.19 kJ mol-1 in Sc-M to 43.52 and 38.04 kJ mol-1 in Sc-M-IDI, respectively, at 298 K and 1.0 bar. Besides, the selectivity of CO2 over CH4/N2 was enhanced from 55.36/107.28 in Sc-M to 3308.61/7021.48 in Sc-M-IDI. However, the CO2 capture capacity is significantly influenced by the pore size. Sc-M, with a favorable pore size, exhibits the highest capture capacity of 15.86 mmol g-1 at 298 K and 1.0 bar. This study elucidated the impact of isoreticular double interpenetration on the CO2 capture performance in MOFs.

Isoreticular Expansion and Linker-Enabled Control of Interpenetration in Titanium-Organic Frameworks

Titanium-organic frameworks offer distinctive opportunities in the realm of metal-organic frameworks (MOFs) due to the integration of intrinsic photoactivity or redox versatility in porous architectures with ultrahigh stability. Unfortunately, the high polarizing power of Ti4+ cations makes them prone to hydrolysis, thus preventing the systematic design of these types of frameworks. We illustrate the use of heterobimetallic cluster Ti2Ca2 as a persistent building unit compatible with the isoreticular design of titanium frameworks. The MUV-12(X) and MUV-12(Y) series can be all synthesized as single crystals by using linkers of varying functionalization and size for the formation of the nets with tailorable porosity and degree of interpenetration. Following the generalization of this approach, we also gain rational control over interpenetration in these nets by designing linkers with varying degrees of steric hindrance to eliminate stacking interactions and access the highest gravimetric surface area reported for titanium(IV) MOFs (3000 m2 g-1).

Cooperative and Orthogonal Switching in the Solid State Enabled by Metal-Organic Framework Confinement Leading to a Thermo-Photochromic Platform

Cooperative behavior and orthogonal responses of two classes of coordinatively integrated photochromic molecules towards distinct external stimuli were demonstrated on the first example of a photo-thermo-responsive hierarchical platform. Synergetic and orthogonal responses to temperature and excitation wavelength are achieved by confining the stimuli-responsive moieties within a metal-organic framework (MOF), leading to the preparation of a novel photo-thermo-responsive spiropyran-diarylethene based material. Synergistic behavior of two photoswitches enables the study of stimuli-responsive resonance energy transfer as well as control of the photoinduced charge transfer processes, milestones required to advance optoelectronics development. Spectroscopic studies in combination with theoretical modeling revealed a nonlinear effect on the material electronic structure arising from the coordinative integration of photoresponsive molecules with distinct photoisomerization mechanisms. Thus, the reported work covers multivariable facets of not only fundamental aspects of photoswitch cooperativity, but also provides a pathway to modulate photophysics and electronics of multidimensional functional materials exhibiting thermo-photochromism.

Pillararene incorporated metal-organic frameworks for supramolecular recognition and selective separation

Crystalline frameworks containing incorporated flexible macrocycle units can afford new opportunities in molecular recognition and selective separation. However, such functionalized frameworks are difficult to prepare and challenging to characterize due to the flexible nature of macrocycles, which limits the development of macrocycle-based crystalline frameworks. Herein, we report the design and synthesis of a set of metal-organic frameworks (MOFs) containing pillar[5]arene units. The pillar[5]arene units were uniformly embedded in the periodic frameworks. Single crystal X-ray diffraction analysis revealed an interpenetrated network that appears to hinder the rotation of the pillar[5]arene repeating units in the frameworks, and it therefore resulted in the successful determination of the precise pillar[5]arene host structure in a MOF crystal. These MOFs can recognize paraquat and 1,2,4,5-tetracyanobenzene in solution and selectively remove trace pyridine from toluene with relative ease. The work presented here represents a critical step towards the synthesis of macrocycle-incorporated crystalline frameworks with well-defined structures and functional utility.

Photoresponsive Metal-Organic Frameworks as Adjustable Scaffolds in Reticular Chemistry

The easy and remote switching of light makes this stimulus an ideal candidate for a large number of applications, among which the preparation of photoresponsive materials stands out. The interest of several scientists in this area in order to achieve improved functionalities has increase parallel to the growth of the structural complexity of these materials. Thus, metal-organic frameworks (MOFs) turned out to be ideal scaffolds for light-responsive ligands. This review is focused on the integration of photoresponsive organic ligands inside MOF crystalline arrays to prepare enhanced functional materials. Besides the summary of the preparation, properties and applications of these materials, an overview of the future outlook of this research area is provided.

Supramolecular Interactions Lead to Remarkably High Thermal Conductivities in Interpenetrated Two-Dimensional Porous Crystals

The design of innovative porous crystals with high porosities and large surface areas has garnered a great deal of attention over the past few decades due to their remarkable potential for a variety of applications. However, heat dissipation is key to realizing their potential. We use systematic atomistic simulations to reveal that interpenetrated porous crystals formed from two-dimensional (2D) frameworks possess remarkable thermal conductivities at high porosities in comparison to their three-dimensional (3D) single framework and interpenetrated 3D framework counterparts. In contrast to conventional understanding, higher thermal conductivities are associated with lower atomic densities and higher porosities for porous crystals formed from interpenetrating 2D frameworks. We attribute this to lower phonon-phonon scattering and vibrational hardening from the supramolecular interactions that restrict atomic vibrational amplitudes, facilitating heat conduction. This marks a new regime of materials design combining ultralow mass densities and ultrahigh thermal conductivities in 2D interpenetrated porous crystals.

A Tetrathiafulvalene/Naphthalene Diimide-Containing Metal-Organic Framework with fsc Topology for Highly Efficient Near-Infrared Photothermal Conversion

Metal-organic frameworks (MOFs) provide broad prospects for the development of new photothermal conversion materials, while their design and synthesis remain challenging. A new Zn-MOF (1) containing both tetrathiafulvalene (TTF) as an electron donor and naphthalene diimide (NDI) as an electron acceptor was constructed by using a space limiting effect. The material exhibited wide absorption peaks in the near-infrared region, indicating that there was strong charge transfer interaction between the TTF and NDI units and providing the possibility of photothermal conversion. 1 shows efficient near-infrared photothermal conversion performance. Under 808 nm laser (0.4 W cm^-2) illumination, the temperature of 1 increased rapidly from room temperature to 250 °C, with good thermal stability and cycle durability. This work provides an efficient strategy for promising materials in photothermal therapy.

Two mesoporous anionic metal-organic frameworks for selective and efficient adsorption of a cationic organic dye

Anionic metal-organic frameworks (MOFs) are beginning to have a great impact in the field of absorption and separation of ionic organic molecules due to the enhanced electrostatic interactions between their anionic frameworks and counter-ionic guests. Herein, the rational design and synthesis of two mesoporous anionic MOFs, [Zn₃(ITTC)₃](Me₂ NH₂)₃·3DMF·H₂O (1) and [Cd₂(ITTC)₃](Me₂NH₂)₅·2DMF (2), where H₃ITTC = 4,4',4''-(1H-imidazole-2,4,5-triyl) tribenzoic acid, is reported. Structural analysis revealed that both materials are anionic MOFs with a 2-fold interpenetrating three dimensional (3D) framework. The cross sectional area of the open one-dimensional rectangular channels is 31.7 Å × 15.6 Å for 1, of which the architecture is indicative of an unprecedented (3,3,4,5)-connection topology. For 2, the diameter of the open one-dimensional regular hexagonal channel is about 34.1 Å, decorated with uncoordinated carboxyl O atoms, and the framework exhibits a (3,4)-connected fcu network. Due to their anionic frameworks and bulky pore window sizes, both MOFs can be employed for absorbing and separating the cationic organic dye methylene blue (MB). The results reveal that both MOFs have better dye adsorption selectivity for MB, than for MO and SDI, because of charge and size-matching effects, enabling them to be potential candidates for use in environmental cleaning. By comparison, 2 presents superior selectivity and adsorptivity for cationic MB which depends on the presence of a basic functionalized pore surface.

Interpenetrated Metal-Porphyrinic Framework for Enhanced Nonlinear Optical Limiting

Structural interpenetration in metal-organic frameworks (MOFs) significantly impacts on their properties and functionalities. However, understanding the interpenetration on third-order nonlinear optics (NLO) of MOFs have not been reported to date. Herein, we report two 3D porphyrinic MOFs, a 2-fold interpenetrated [Zn₂(TPyP)(AC)₂] (ZnTPyP-1) and a noninterpenetrated [Zn₃(TPyP)(H₂O)₂(C₂O₄)₂] (ZnTPyP-2), constructed from 5,10,15,20-tetra(4-pyridyl)porphyrin (TPyP(H₂)) and Zn(NO₃)₂ (AC = acetate, C₂O₄ = oxalate). ZnTPyP-1 achieves excellent optical limiting (OL) performance with a giant nonlinear absorption coefficient (3.61 × 10⁶ cm/GW) and large third-order susceptibility (7.73 × 10^-7 esu), which is much better than ZnTPyP-2 and other reported OL materials. The corresponding MOFs nanosheets are dispersed into a polydimethylsiloxane (PDMS) matrix to form highly transparent and flexible MOFs/PDMS glasses for practical OL application. In addition, the OL response optimized by adjusting the MOFs concentration in the PDMS matrix and the type of metalloporphyrin are discussed in the ZnTPyP-1 system. The theoretical calculation confirmed that the abundant π-π interaction from porphyrinic groups in the interpenetrated framework increased the electron delocalization/transfer and boosted the OL performance. This study opens a new avenue to enhance OL performance by the construction of interpenetrated structures and provides a new approach for the preparation of transparent and flexible MOF composites in nonlinear optical applications.

Photoresponsive frameworks: energy transfer in the spotlight

In this paper, spiropyran-containing metal- and covalent-organic frameworks (MOFs and COFs, respectively) are probed as platforms for fostering photochromic behavior in solid-state materials, while simultaneously promoting directional energy transfer (ET). In particular, Förster resonance energy transfer (FRET) between spiropyran and porphyrin derivatives integrated as linkers in the framework matrix is discussed. The photochromic spiropyran derivatives allow for control over material optoelectronic properties through alternation of excitation wavelengths. Photoinduced changes in the material electronic profile have also been probed through conductivity measurements. Time-resolved photoluminescence studies were employed to evaluate the effect of photochromic linkers on material photophysics. Furthermore, "forward" and "reverse" FRET processes occurring between two distinct chromophores were modeled, and the Förster critical radii and ET rates were estimated to support the experimentally observed changes in material photoluminescence.

High-Performance Water Harvester Framework for Triphasic and Synchronous Detection of Assorted Organotoxins with Site-Memory-Reliant Security Encryption via pH-Triggered Fluoroswitching

Atmospheric water harvesting, triphasic detection of water contaminants, and advanced antiforgery measures are among important global agendas, where metal-organic frameworks (MOFs), as an incipient class of multifaceted materials, can affect substantial development of individual properties at the interface of tailor-made fabrication. The chemically robust and microporous MOF, encompassing contrasting pore functionalization, exhibits an S-shaped water adsorption curve at 300 K with a steep pore-filling step near P/P₀ = 0.5 and shows reversible uptake-release performance. Density functional theory (DFT) studies provide atomistic-level snapshots of sequential insertion of H₂O molecules inside the porous channels and also portray H-bonding interactions with polar functional sites in the two-fold interpenetrated structure. The highly emissive attribute with an electron-pull system benefits the fast-responsive framework and highly regenerable detection of four classes of organic pollutants (2,4,6-trinitrophenol (TNP), dichloran, aniline, and nicotine) in water at a record-low sensitivity. In addition to solid-, liquid-, and vapor-phase sensing, host-guest-mediated reversible fluoroswitching is validated through repetitive paper-strip monitoring and image-based detection of food sample contamination. Structure-property synergism in the electron transfer route of sensing is justified from DFT calculations that describe the reshuffling of molecular orbital energy levels in an electron-rich network by each organotoxin, besides evidencing framework-analyte supramolecular interactions. The MOF further delineates the pH-responsive luminescence defect repair via site-specific emission modulation, wherein reversibly alternated "encrypted and decrypted" states are utilized as highly reusable anticounterfeiting labels over multiple platforms and conceptualized as artificial molecular switches. Aiming at self-calibrated, advanced security claims, a NOR-OR coupled logic gate is devised based on commensurate fluorescence-cum-real-time synchronous detection of organic and inorganic (HCl and NH₃) pollutants.

Factors Affecting Hydrogen Adsorption in Metal-Organic Frameworks: A Short Review

Metal–organic frameworks (MOFs) have significant potential for hydrogen storage. The main benefit of MOFs is their reversible and high-rate hydrogen adsorption process, whereas their biggest disadvantage is related to their operation at very low temperatures. In this study, we describe selected examples of MOF structures studied for hydrogen adsorption and different factors affecting hydrogen adsorption in MOFs. Approaches to improving hydrogen uptake are reviewed, including surface area and pore volume, in addition to the value of isosteric enthalpy of hydrogen adsorption. Nanoconfinement of metal hydrides inside MOFs is proposed as a new approach to hydrogen storage. Conclusions regarding MOFs with incorporated metal nanoparticles, which may be used as nanoscaffolds and/or H₂ sorbents, are summarized as prospects for the near future.

A rare 4-fold interpenetrated metal-organic framework constructed from an anionic indium-based node and a cationic dicopper linker

A unique 4-fold interpenetrated metal-organic framework, TIF-1, was synthesized by combining an anionic indium node with a cationic linker. This framework shows a rare type of 4-fold interpenetrated dia network, constructed from tessellation of biangular and tetragonal type metal-organic micropores. The porosity of TIF-1 is moderate due to four-fold interpenetration and charge-balancing anions. The cationic feature of this MOF may give good efficiency for selective small anion exchange or separation. In addition, the thermal stability and moderate CO2 adsorption property of the complex were studied.

Synthesis of dibenzosuberenone-based novel polycyclic π-conjugated dihydropyridazines, pyridazines and pyrroles

The synthesis of novel polycyclic π-conjugated dihydropyridazines, pyridazines, and pyrroles was studied. Dihydropyridazine dyes were synthesized by inverse electron-demand Diels–Alder cycloaddition reactions between a dibenzosuberenone and tetrazines that bear various substituents. The pyridazines were synthesized in high yields by oxidation of dihydropyridazine-appended dibenzosuberenones with PIFA or NO. p-Quinone derivatives of pyridazines were also obtained by H-shift isomerization following the inverse electron-demand Diels–Alder reaction of tetrazines with p-quinone dibenzosuberenone. Then these pyridazines were converted to the corresponding pyrroles by reductive treatment with zinc. It was observed that all the dihydropyridazines obtained gave absorbance and emission at long wavelengths.

Engineered Bifunctional Luminescent Pillared-Layer Frameworks for Adsorption of CO2 and Sensitive Detection of Nitrobenzene in Aqueous Media

Through a dual-ligand synthetic approach, five isoreticular primitive cubic (pcu)-type pillared-layer metal-organic frameworks (MOFs), [Zn₂ (dicarboxylate)₂ (NI-bpy-44)]⋅x DMF⋅y H₂ O, in which dicarboxylate=1,4-bdc (1), Br-1,4-bdc (2), NH₂ -1,4-bdc (3), 2,6-ndc (4), and bpdc (5), have been engineered. MOFs 1-5 feature twofold degrees of interpenetration and have open pores of 27.0, 33.6, 36.8, 52.5, and 62.1 %, respectively. Nitrogen adsorption isotherms of activated MOFs 1'-5' at 77 K all displayed type I adsorption behavior, suggesting their microporous nature. Although 1' and 3'-5' exhibited type I adsorption isotherms of CO₂ at 195 K, MOF 2' showed a two-step gate-opening sorption isotherm of CO₂ . Furthermore, MOF 3' also had a significant influence of amine functions on CO₂ uptake at high temperature due to the CO₂ -framework interactions. MOFs 1-5 revealed solvent-dependent fluorescence properties; their strong blue-light emissions in aqueous suspensions were efficiently quenched by trace amounts of nitrobenzene (NB), with limits of detection of 4.54, 5.73, 1.88, 2.30, and 2.26 μm, respectively, and Stern-Volmer quenching constants (K_sv ) of 2.93×10³ , 1.79×10³ , 3.78×10³ , 4.04×10³ , and 3.21×10³  m^-1 , respectively. Of particular note, the NB-included framework, NB@3, provided direct evidence of the binding sites, which showed strong host-guest π-π and hydrogen-bonding interactions beneficial for donor-acceptor electron transfer and resulting in fluorescence quenching.

Hierarchical Synthesis, Structure, and Photophysical Properties of Gallium- and Ruthenium-Porphyrins with Axially Bonded Azo Ligands

The hierarchical synthesis of three porphyrin and four bisporphyrin derivatives is presented. This strategy relies on the incorporation of linkers based on azo moieties appended with pyridyl and/or acetylenic groups that facilitate axial coordination to Ga- and Ru-metalloporphyrins. These porphyrinic systems allow for a quantitative analysis of the effects of diamagnetic anisotropy (DA) by using ¹ H NMR spectroscopic and X-ray crystallographic analyses. A simple power-law relationship between the proton chemical shift and the distance from the porphyrin core is experimentally outlined, which confirms previous theoretical predictions and shows that the limit of DA is about 2 nm. Photophysical properties of the azo-linked porphyrins are analyzed by UV/Vis spectroscopy, showing that significant cis-trans isomerization is not observed for azo ligands bound only to Ga-porphyrins. Incorporation of Ru-porphyrins to an azo ligand facilitates photoswitching behavior, but the process faces competition from decarbonylation of the Ru-porphyrin, and appreciable switching is only documented for GaL1Ru.

Porous and Nonporous Coordination Polymers Induced by Pseudohalide Ions for Luminescence and Gas Sorption

The three-dimensional (3D) coordination polymers [Cd(tpmd)(NCX)₂]_n [X = O (1), S (2), and BH₃ (3); tpmd = N,N,N',N'-tetrakis(pyridin-4-yl)methanediamine] have been determined to display their network structures through coordinated anionic ligands. Polymers 1 and 2 show nonporous structures, whereas polymer 3 shows a porous coordination framework. On the basis of the Cd(II) network structures, the 3D coordination polymer [Zn(tpmd)(NCBH₃)₂]_n·nMeOH (4) was self-assembled. In the cases of polymers 1 and 2, pseudohalide ions acted to form nonporous network structures; however, in polymers 3 and 4, NCBH₃^- helps to construct porous network structures. Polymers 1-4 show strong ultraviolet luminescence emissions, depending on the pseudohalide ions present, compared to the tpmd ligands. Interestingly, coordination polymers 3 and 4 that possess NCBH₃^- ions exhibit high porosities and gas sorption properties. The polymers appeared to absorb N₂, H₂, CO₂, and CH₄. In the case of polymer 4, the structure is almost identical with that of polymer 3, except for the Cd(II) ion. However, polymer 4 has a larger void volume and higher gas absorption ability for N₂ gas than polymer 3. For the sorption of gases, polymers 3 and 4 showed similar behaviors.

Visible-Light-Driven Rotation of Molecular Motors in a Dual-Function Metal-Organic Framework Enabled by Energy Transfer

The visible-light-driven rotation of an overcrowded alkene-based molecular motor strut in a dual-function metal-organic framework (MOF) is reported. Two types of functional linkers, a palladium-porphyrin photosensitizer and a bispyridine-derived molecular motor, were used to construct the framework capable of harvesting low-energy green light to power the rotary motion. The molecular motor was introduced in the framework using the postsynthetic solvent-assisted linker exchange (SALE) method, and the structure of the material was confirmed by powder (PXRD) and single-crystal X-ray (SC-XRD) diffraction. The large decrease in the phosphorescence lifetime and intensity of the porphyrin in the MOFs upon introduction of the molecular motor pillars confirms efficient triplet-to-triplet energy transfer between the porphyrin linkers and the molecular motor. Near-infrared Raman spectroscopy revealed that the visible light-driven rotation of the molecular motor proceeds in the solid state at rates similar to those observed in solution.

Interpenetrated Metal-Organic Frameworks with Topology and Versatile Functions

Through an "isoreticular expansion" strategy, a large number of highly porous zirconium-based metal-organic frameworks (Zr-MOFs) have been achieved using extended organic linkers in the past few years. However, interpenetrated Zr-MOFs with ftw topology have scarcely been reported, mainly owing to the used bulky tetratopic linkers that effectively prevent the network interpenetration. Here, we report a new family of zirconium and lanthanide (Ln) MOFs with ftw topology, constructed by hexanuclear Zr or Ln (Ln = Eu, Tb, Gd, Dy, Tm, Yb, Nd, and Er) clusters and a spirobifluorene-center tetracarboxylate linker. Our studies reveal that the isostructural Zr and Ln MOFs are all doubly interpenetrated with ultrahigh thermal and chemical stability. The observed unusual interpenetration can be attributed to the specific geometry of the spirobifluorene-center tetratopic linker. Gas adsorption studies show that the interpenetrated Zr-MOF is still highly porous and exhibits high performance for CO₂ storage, which can be attributed to the strong CO₂ binding environment contributed by the reduced pore size. In addition, the presented MOFs display strong characteristic luminescence in the UV-vis-NIR region. Moreover, the incorporation of the spiro-center linker into the framework can efficiently produce two-photon-excited photoluminescence with a large action cross-section value, which also benefited from the high packing density of the nonlinear optical chromophore linker in the interpenetrated structure.

1,2,4,5-Tetrazine based ligands and complexes

One of the most intriguing nitrogen based aromatic heterocycles is 1,2,4,5-tetrazine or s-tetrazine (TTZ) thanks to its electron acceptor character and fluorescence properties and the possibilities of functionalization in the 3 and 6 positions allowing access to various ligands. In this review we focus on the two main families of TTZ based ligands, i.e. ditopic symmetric and monotopic non-symmetric, together with their metal complexes, with a special emphasis on their solid state structures and physical properties. After a description of the most representative complexes containing unsubstituted TTZ as a ligand, symmetric TTZ ligands and complexes derived thereof are discussed in the order: 3,6-bis(2-pyridyl)-tetrazine, 3,6-bis(3-pyridyl)-tetrazine, 3,6-bis(4-pyridyl)-tetrazine, 3,6-bis(2-pyrimidyl)-tetrazine, 3,6-bis(2-pyrazinyl)-tetrazine, 3,6-bis(monopicolylamine)-tetrazine, 3,6-bis(vanillin-hydrazinyl)-tetrazine and TTZ containing carboxylic acids. Remarkable results have been obtained in recent years for metal-organic frameworks and magnetic compounds in which magnetic coupling is enhanced when the tetrazine bridge is reduced to radical anions. Non-symmetric ligands, such as dipicolylamine-TTZ and monopicolylamine-TTZ, are comparatively more recent than the symmetric ones. They allow in principle the preparation of mononuclear complexes in a controlled manner, although binuclear complexes have been isolated as well. Moreover, in the monopicolylamine-TTZ-Cl ligand, deprotonation of the amine, thanks to the electron acceptor character of TTZ, afforded a negatively charged ligand equivalent of a guanidinate.

Inorganic Molecular Complexes: Potential for Growth of a New Subject Area in Self-Assembly

The non-covalent assemblies among multiple non-identical metal complexes have scopes to develop a new subject area. There are infinite numbers of ways for different combinations among inorganic neutral or ionic complexes. Each partnering species of those molecular complexes would also have diversities by changing metal ions, ligands, oxidation states of metal ions, and coordination numbers. Keeping a view of the emergence of framework materials and self-assembled nano-structures of metal complexes, the non-covalently linked assemblies of inorganic molecular complexes would have scopes for new nano-dimensional materials. This account provides a systematic description of the different inorganic molecular complexes for a concerted effort to develop a new area that would have importance in applied materials.

Hydrogen-Bonding Linkers Yield a Large-Pore, Non-Catenated, Metal-Organic Framework with pcu Topology

Pillared paddle-wheel-based metal-organic framework (MOF) materials are an attractive target as they offer a reliable method for constructing well-defined, multifunctional materials. A drawback of these materials, which has limited their application, is their tendency to form catenated frameworks with little accessible volume. To eliminate this disadvantage, it is necessary to investigate strategies for constructing non-catenated pillared paddle-wheel MOFs. Hydrogen-bonding substituents on linkers have been postulated to prevent catenation in certain frameworks and, in this work, we present a new MOF to further bolster this theory. Using 2,2'-diamino-[1,1'-biphenyl]-4,4'-dicarboxylic acid, BPDC-(NH₂)₂, linkers and dipyridyl glycol, DPG, pillars, we assembled a MOF with pcu topology. The new material is non-catenated, exhibiting large accessible pores and low density. To the best of our knowledge, this material constitutes the pcu framework with the largest pore volume and lowest density. We attribute the lack of catenation to the presence of H-bonding substituents on both linkers.

Two zinc metal-organic framework isomers based on pyrazine tetracarboxylic acid and dipyridinylbenzene for adsorption and separation of CO2 and light hydrocarbons

Two highly porous metal-organic framework isomers Zn₂(TCPP)(DPB) (1 and 2, H₄TCPP = 2,3,5,6-tetrakis(4-carboxyphenyl)pyrazine and DPB = 1,4-di(pyridin-4-yl)benzene) were successfully synthesized using different solvents and acid species to adjust the topologies of these two MOFs. Both of them were constructed by paddlewheel Zn₂(COO)₄, TCPP^4-, and DPB ligands. In compound 1, the Zn₂(COO)₄ paddlewheel units were fitted together by the TCPP^4- ligands to form two-dimensional layers, which were further connected by DPB ligands as pillars to construct a two-fold catenated 3D framework. In compound 2, the cross-linkage of Zn₂(COO)₄ paddlewheel units and TCPP^4- ligands resulted in a three-dimensional framework of Zn-TCPP, in which DPB ligands coordinated to two axial vertical dinuclear Zn₂(COO)₄. Both activated MOFs exhibited permanent porosity with high BET areas (1324 m² g^-1 for 1 and 1247 m² g^-1 for 2) and possessed narrow pore size distributions (0.93 nm for 1 and 1.02 nm for 2). Moreover, the adsorption behaviors of the two activated MOFs for CO₂ and light hydrocarbons (C1, C2, and C3) at low pressure were evaluated and favorable selectivity was proven for C₃H₈/C₃H₆ over CH₄. These two MOF materials reported in this study for selective CO₂ and light hydrocarbon capture have immense potential applications for environmental protection.

The vital role of ditopic N–N bridging ligands with different lengths in the formation of new binuclear dioxomolybdenum(vi) complexes: synthesis, crystal structures, supramolecular framework and protein binding studies

Role of bis-(pyridyl) and bis-(imidazole) auxiliary ligands in the formation of supramolecular architectures and BSA binding with new binuclear dioxomolybdenum(vi) complexes.

Tuning the interpenetration of metal–organic frameworks through changing ligand functionality: effect on gas adsorption properties

Two interpenetrated MOFs are constructed with a 2-connected ligand, whereas one non-interpenetrated MOF is constructed by using a 3-connected ligand.

A historical overview of the activation and porosity of metal–organic frameworks

A historical overview of the activation and porosity of MOFs including strategies to design and preserve permanent porosity in MOFs.

Unusual adsorption behaviours and responsive structural dynamics via selective gate effects of an hourglass porous metal-organic framework

An hourglass porous metal-organic framework, LIFM-12, constructed on a T-shaped flexible ligand with Cu2+ paddle-wheel clusters, shows temperature and gas adsorption responsive structural dynamics upon reversible molecular guest binding. Temperature-dependent single crystal and powder X-ray diffraction experiments show that the open gate status of the framework with adaptive behaviours facilitates kinetic diffusion of gas molecules resulting in the sequential filling of pores of different sizes, thus creating a breathing behaviour reminiscent of the observation of several steps in adsorption isotherms. In addition, adsorption studies revealed that LIFM-12 performs exceptional adsorption selectivity of 10-25 for CO2 versus light gases N2, CH4, and CO and up to 200 for C3H6 versus CH4.

On the Use of MOFs and ALD Layers as Nanomembranes for the Enhancement of Gas Sensors Selectivity

Improving the selectivity of gas sensors is crucial for their further development. One effective route to enhance this key property of sensors is the use of selective nanomembrane materials. This work aims to present how metal-organic frameworks (MOFs) and thin films prepared by atomic layer deposition (ALD) can be applied as nanomembranes to separate different gases, and hence improve the selectivity of gas sensing devices. First, the fundamentals of the mechanisms and configuration of gas sensors will be given. A selected list of studies will then be presented to illustrate how MOFs and ALD materials can be implemented as nanomembranes and how they can be implemented to improve the operational performance of gas sensing devices. This review comprehensively shows the benefits of these novel selective nanomaterials and opens prospects for the sensing community.

Unidirectional rotary motion in a metal-organic framework

Overcrowded alkene-based light-driven molecular motors are able to perform large-amplitude repetitive unidirectional rotations. Their behaviour is well understood in solution. However, Brownian motion precludes the precise positioning at the nanoscale needed to harness cooperative action. Here, we demonstrate molecular motors organized in crystalline metal-organic frameworks (MOFs). The motor unit becomes a part of the organic linker (or strut), and its spatial arrangement is elucidated through powder and single-crystal X-ray analyses and polarized optical and Raman microscopies. We confirm that the light-driven unidirectional rotation of the motor units is retained in the MOF framework and that the motors can operate in the solid state with similar rotary speed (rate of thermal helix inversion) to that in solution. These 'moto-MOFs' could in the future be used to control dynamic function in crystalline materials.

A Zn(ii) metal–organic framework constructed by a mixed-ligand strategy for CO2 capture and gas separation

A microporous Zn(ii) metal–organic framework has been assembled using a mixed-ligand strategy, and it exhibits high capture ability for CO₂ and good selectivity for CO₂/CH₄, C₂H₆/CH₄ and C₃H₈/CH₄.

Influence of interpenetration on the flexibility of MUV-2

The crystal structure of an interpenetrated tetrathiafulvalene (TTF)-based metal–organic framework (MOF) is reported.

In Silico Screening of Metal-Organic Frameworks for Adsorption-Driven Heat Pumps and Chillers

A computational screening of 2930 experimentally synthesized metal-organic frameworks (MOFs) is carried out to find the best-performing structures for adsorption-driven cooling (AC) applications with methanol and ethanol as working fluids. The screening methodology consists of four subsequent screening steps for each adsorbate. At the end of each step, the most promising MOFs for AC application are selected for further investigation. In the first step, the structures are selected on the basis of physical properties (pore limiting diameter). In each following step, points of the adsorption isotherms of the selected structures are calculated from Monte Carlo simulations in the grand-canonical ensemble. The most promising MOFs are selected on the basis of the working capacity of the structures and the location of the adsorption step (if present), which can be related to the applicable operational conditions in AC. Because of the possibility of reversible pore condensation (first-order phase transition), the mid-density scheme is used to efficiently and accurately determine the location of the adsorption step. At the end of the screening procedure, six MOFs with high deliverable working capacities (∼0.6 mL working fluid in 1 mL structure) and diverse adsorption step locations are selected for both adsorbates from the original 2930 structures. Because the highest experimentally measured deliverable working capacity to date for MOFs with methanol is ca. 0.45 mL mL^-1, the selected six structures show the potential to improve the efficiency of ACs.