Quasi-Homogeneous Photocatalysis in Ultrastiff Microporous Polymer Aerogels.

The development of efficient and low-cost catalysts is essential for photocatalysis; however, the intrinsically low photocatalytic efficiency as well as the difficulty in using and recycling photocatalysts in powder morphology greatly limit their practical performance. Herein, we describe quasi-homogeneous photocatalysis to overcome these two limitations by constructing ultrastiff, hierarchically porous, and photoactive aerogels of conjugated microporous polymers (CMPs). The CMP aerogels exhibit low density but high stiffness beyond 105 m2 s-2, outperforming most low-density materials. Extraordinary stiffness ensures their use as robust scaffolds for scaled photocatalysis and recycling without damage at the macroscopic level. A challenging but desirable reaction for direct deaminative borylation is demonstrated using CMP aerogel-based quasi-homogeneous photocatalysis with gram-scale productivity and record-high efficiency under ambient conditions. Combined terahertz and transient absorption spectroscopic studies unveil the generation of high-mobility free carriers and long-lived excitonic species in the CMP aerogels, underlying the observed superior catalytic performance.

Precise Modulation of CO2 Sorption in Ti8Ce2-Oxo Clusters: Elucidating Lewis Acidity of the Ce Metal Sites and Structural Flexibility.

Investigating the structure-property correlation in porous materials is a fundamental and consistent focus in various scientific domains, especially within sorption research. Metal oxide clusters with capping ligands, characterized by intrinsic cavities formed through specific solid-state packing, demonstrate significant potential as versatile platforms for sorption investigations due to their precisely tunable atomic structures and inherent long-range order. This study presents a series of Ti8Ce2-oxo clusters with subtle variations in coordinated linkers and explores their sorption behavior. Notably, Ti8Ce2-BA (BA denotes benzoic acid) manifests a distinctive two-step profile during the CO2 adsorption, accompanied by a hysteresis loop. This observation marks a new instance within the metal oxide cluster field. Of intrigue, the presence of unsaturated Ce(IV) sites was found to be correlated with the stepped sorption property. Moreover, the introduction of an electrophilic fluorine atom, positioned ortho or para to the benzoic acid, facilitated precise control over gate pressure and stepped sorption quantities. Advanced in situ techniques systematically unraveled the underlying mechanism behind this unique sorption behavior. The findings elucidate that robust Lewis base-acid interactions are established between the CO2 molecules and Ce ions, consequently altering the conformation of coordinated linkers. Conversely, the F atoms primarily contribute to gate pressure variation by influencing the Lewis acidity of the Ce sites. This research advances the understanding in fabricating metal-oxo clusters with structural flexibility and provides profound insights into their host-guest interaction motifs. These insights hold substantial promise across diverse fields and offer valuable guidance for future adsorbent designs grounded in fundamental theories of structure-property relationships.

Diffusion-rate sieving of propylene and propane mixtures in a cooperatively dynamic porous crystal.

Selective molecular recognition is an important alternative to the energy-intensive industrial separation process. Porous coordination polymers (PCPs) offer designing platforms for gas separation because they possess precise controllability over structures at the molecular level. However, PCPs-based gas separations are dominantly achieved using strong adsorptive sites for thermodynamic recognition or pore-aperture control for size sieving, which suffer from insufficient selectivity or sluggish kinetics. Developing PCPs that work at high temperatures and feature both high uptake capacity and selectivity is urgently required but remains challenging. Herein, we report diffusion-rate sieving of propylene/propane (C3H6/C3H8) at 300 K by constructing a PCP material whose global and local dynamics cooperatively govern the adsorption process via the mechanisms of the gate opening for C3H6 and the diffusion regulation for C3H8, respectively, yielding substantial differences in both uptake capacity and adsorption kinetics. Dynamic separation of an equimolar C3H6/C3H8 mixture reveals outstanding sieving performance with a C3H6 purity of 99.7% and a separation factor of 318.

Modular Design of Highly Stable Semiconducting Porous Coordination Polymer for Efficient Electrosynthesis of Ammonia.

Developing highly stable porous coordination polymers (PCPs) with integrated electrical conductivity is crucial for advancing our understanding of electrocatalytic mechanisms and the structure-activity relationship of electrocatalysts. However, achieving this goal remains a formidable challenge because of the electrochemical instability observed in most PCPs. Herein, we develop a "modular design" strategy to construct electrochemically stable semiconducting PCP, namely, Fe-pyNDI, which incorporates a chain-type Fe-pyrazole metal cluster and π-stacking column with effective synergistic effects. The three-dimensional electron diffraction (3D ED) technique resolves the precise structure. Both theoretical and experimental investigation confirms that the π-stacking column in Fe-pyNDI can provide an efficient electron transport path and enhance the structural stability of the material. As a result, Fe-pyNDI can serve as an efficient model electrocatalyst for nitrate reduction reaction (NO3RR) to ammonia with a superior ammonia yield of 339.2 µmol h-1 cm-2 (14677 µg h-1 mgcat. -1) and a faradaic efficiency of 87 % at neutral electrolyte, which is comparable to state-of-the-art electrocatalysts. The in-situ X-ray absorption spectroscopy (XAS) reveals that during the reaction, the structure of Fe-pyNDI can be kept, while part of the Fe3+ in Fe-pyNDI was reduced in situ to Fe2+, which serves as the potential active species for NO3RR.

Switching molecular recognition selectivities by temperature in a diffusion-regulatory porous material.

Over the long history of evolution, nature has developed a variety of biological systems with switchable recognition functions, such as the ion transmissibility of biological membranes, which can switch their ion selectivities in response to diverse stimuli. However, developing a method in an artificial host-guest system for switchable recognition of specific guests upon the change of external stimuli is a fundamental challenge in chemistry because the order in the host-guest affinity of a given system hardly varies along with environmental conditions. Herein, we report temperature-responsive recognition of two similar gaseous guests, CO2 and C2H2, with selectivities switched by temperature change by a diffusion-regulatory mechanism, which is realized by a dynamic porous crystal featuring ultrasmall pore apertures with flip-flop locally-motive organic moiety. The dynamic local motion regulates the diffusion process of CO2 and C2H2 and amplifies their rate differences, allowing the crystal to selectively adsorb CO2 at low temperatures and C2H2 at high temperatures with separation factors of 498 (CO2/C2H2) and 181 (C2H2/CO2), respectively.

Metal cation substitution can tune CO2, H2O and CH4 switching pressure in transiently porous coordination networks.

Compared to rigid physisorbents, switching coordination networks that reversibly transform between closed (non-porous) and open (porous) phases offer promise for gas/vapour storage and separation owing to their improved working capacity and desirable thermal management properties. We recently introduced a coordination network, X-dmp-1-Co, which exhibits switching enabled by transient porosity. The resulting "open" phases are generated at threshold pressures even though they are conventionally non-porous. Herein, we report that X-dmp-1-Co is the parent member of a family of transiently porous coordination networks [X-dmp-1-M] (M = Co, Zn and Cd) and that each exhibits transient porosity but switching events occur at different threshold pressures for CO2 (0.8, 2.1 and 15 mbar, for Co, Zn and Cd, respectively, at 195 K), H2O (10, 70 and 75% RH, for Co, Zn and Cd, respectively, at 300 K) and CH4 (<2, 10 and 25 bar, for Co, Zn and Cd, respectively, at 298 K). Insight into the phase changes is provided through in situ SCXRD and in situ PXRD. We attribute the tuning of gate-opening pressure to differences and changes in the metal coordination spheres and how they impact dpt ligand rotation. X-dmp-1-Zn and X-dmp-1-Cd join a small number of coordination networks (<10) that exhibit reversible switching for CH4 between 5 and 35 bar, a key requirement for adsorbed natural gas storage.

MOF-Thermogel Composites for Differentiated and Sustained Dual Drug Delivery.

In recent years, multidrug therapy has gained increasing popularity due to the possibility of achieving synergistic drug action and sequential delivery of different medical payloads for enhanced treatment efficacy. While a number of composite material release platforms have been developed, few combine the bottom-up design versatility of metal-organic frameworks (MOFs) to tailor drug release behavior, with the convenience of temperature-responsive hydrogels (or thermogels) in their unique ease of administration and formulation. Yet, despite their potential, MOF-thermogel composites have been largely overlooked for simultaneous multidrug delivery. Herein, we report the first systematic study of common MOFs (UiO-66, MIL-53(Al), MIL-100(Fe), and MOF-808) with different pore sizes, geometries, and hydrophobicities for their ability to achieve simultaneous dual drug release when embedded within PEG-containing thermogel matrices. After establishing that MOFs exert small influences on the rheological properties of the thermogels despite the penetration of polymers into the MOF pores in solution, the release profiles of ibuprofen and caffeine as model hydrophobic and hydrophilic drugs, respectively, from MOF-thermogel composites were investigated. Through these studies, we elucidated the important role of hydrophobic matching between MOF pores and loaded drugs in order for the MOF component to distinctly influence drug release kinetics. These findings enabled us to identify a viable MOF-thermogel composite containing UiO-66 that showed vastly different release kinetics between ibuprofen and caffeine, enabling temporally differentiated yet sustained simultaneous drug release to be achieved. Finally, the MOF-thermogel composites were shown to be noncytotoxic in vitro, paving the way for these underexploited composite materials to find possible clinical applications for multidrug therapy.

Growth mechanisms and anisotropic softness-dependent conductivity of orientation-controllable metal-organic framework nanofilms.

Conductive metal-organic frameworks (cMOFs) manifest great potential in modern electrical devices due to their porous nature and the ability to conduct charges in a regular network. cMOFs applied in electrical devices normally hybridize with other materials, especially a substrate. Therefore, the precise control of the interface between cMOF and a substrate is particularly crucial. However, the unexplored interface chemistry of cMOFs makes the controlled synthesis and advanced characterization of high-quality thin films, particularly challenging. Herein, we report the development of a simplified synthesis method to grow "face-on" and "edge-on" cMOF nanofilms on substrates, and the establishment of operando characterization methodology using atomic force microscopy and X-ray, thereby demonstrating the relationship between the soft structure of surface-mounted oriented networks and their characteristic conductive functions. As a result, crystallinity of cMOF nanofilms with a thickness down to a few nanometers is obtained, the possible growth mechanisms are proposed, and the interesting anisotropic softness-dependent conducting properties (over 2 orders of magnitude change) of the cMOF are also illustrated.

A Nitro-Modified Luminescent Hydrogen-Bonded Organic Framework for Non-Contact and High-Contrast Sensing of Aromatic Amines.

The global demand for intelligent sensing of aromatic amines has consistently increased due to concerns about health and the environment. Efforts to improve material design and understand mechanisms have been made, but highly efficient non-contact sensing with host-guest structures remains a challenge. Herein, we report the first example of non-contact, high-contrast sensing of aromatic amines in a hydrogen-bonded organic framework (HOF) based on a nitro-modified stereo building block. Direct observation of binding interactions of trapped amines is achieved, leading to charge separation-induced emission quenching between host and guests. Non-contact sensing of aniline and diphenylamine is realized with quenching efficiencies up to 91.7 % and 97.0 %, which shows potential for versatile applications. This work provides an inspiring avenue to engineer multifunctional HOFs via co-crystal preparations, thus enriching applications of porous materials with explicit mechanisms.

Crossover Sorption of C2 H2 /CO2 and C2 H6 /C2 H4 in Soft Porous Coordination Networks.

Porous sorbents are materials that are used for various applications, including storage and separation. Typically, the uptake of a single gas by a sorbent decreases with temperature, but the relative affinity for two similar gases does not change. However, in this study, we report a rare example of "crossover sorption," in which the uptake capacity and apparent affinity for two similar gases reverse at different temperatures. We synthesized two soft porous coordination polymers (PCPs), [Zn2 (L1)(L2)2 ]n (PCP-1) and [Zn2 (L1)(L3)2 ]n (PCP-2) (L1= 1,4-bis(4-pyridyl)benzene, L2=5-methyl-1,3-di(4-carboxyphenyl)benzene, and L3=5-methoxy-1,3-di(4-carboxyphenyl)benzene). These PCPs exhibits structural changes upon gas sorption and show the crossover sorption for both C2 H2 /CO2 and C2 H6 /C2 H4 , in which the apparent affinity reverse with temperature. We used in situ gas-loading single-crystal X-ray diffraction (SCXRD) analysis to reveal the guest inclusion structures of PCP-1 for C2 H2 , CO2 , C2 H6 , and C2 H4 gases at various temperatures. Interestingly, we observed three-step single-crystal to single-crystal (sc-sc) transformations with the different loading phases under these gases, providing insight into guest binding positions, nature of host-guest or guest-guest interactions, and their phase transformations upon exposure to these gases. Combining with theoretical investigation, we have fully elucidated the crossover sorption in the flexible coordination networks, which involves a reversal of apparent affinity and uptake of similar gases at different temperatures. We discovered that this behaviour can be explained by the delicate balance between guest binding and host-guest and guest-guest interactions.

Soft corrugated channel with synergistic exclusive discrimination gating for CO2 recognition in gas mixture.

Developing artificial porous systems with high molecular recognition performance is critical but very challenging to achieve selective uptake of a particular component from a mixture of many similar species, regardless of the size and affinity of these competing species. A porous platform that integrates multiple recognition mechanisms working cooperatively for highly efficient guest identification is desired. Here, we designed a flexible porous coordination polymer (PCP) and realised a corrugated channel system that cooperatively responds to only target gas molecules by taking advantage of its stereochemical shape, location of binding sites, and structural softness. The binding sites and structural deformation act synergistically, exhibiting exclusive discrimination gating (EDG) effect for selective gate-opening adsorption of CO2 over nine similar gas molecules, including N2, CH4, CO, O2, H2, Ar, C2H6, and even higher-affinity gases such as C2H2 and C2H4. Combining in-situ crystallographic experiments with theoretical studies, it is clear that this unparalleled ability to decipher the CO2 molecule is achieved through the coordination of framework dynamics, guest diffusion, and interaction energetics. Furthermore, the gas co-adsorption and breakthrough separation performance render the obtained PCP an efficient adsorbent for CO2 capture from various gas mixtures.

Control over Phase Transformations in a Family of Flexible Double Diamondoid Coordination Networks through Linker Ligand Substitution.

In this work, we present the first metal-organic framework (MOF) platform with a self-penetrated double diamondoid (ddi) topology that exhibits switching between closed (nonporous) and open (porous) phases induced by exposure to gases. A crystal engineering strategy, linker ligand substitution, was used to control gas sorption properties for CO2 and C3 gases. Specifically, bimbz (1,4-bis(imidazol-1-yl)benzene) in the coordination network X-ddi-1-Ni ([Ni2(bimbz)2(bdc)2(H2O)]n, H2bdc = 1,4-benzenedicarboxylic acid) was replaced by bimpz (3,6-bis(imidazol-1-yl)pyridazine) in X-ddi-2-Ni ([Ni2(bimpz)2(bdc)2(H2O)]n). In addition, the 1:1 mixed crystal X-ddi-1,2-Ni ([Ni2(bimbz)(bimpz)(bdc)2(H2O)]n) was prepared and studied. All three variants form isostructural closed (ß) phases upon activation which each exhibited different reversible properties upon exposure to CO2 at 195 K and C3 gases at 273 K. For CO2, X-ddi-1-Ni revealed incomplete gate-opening, X-ddi-2-Ni exhibited a stepped isotherm with saturation uptake of 3.92 mol·mol-1, and X-ddi-1,2-Ni achieved up to 62% more gas uptake and a distinct isotherm shape vs the parent materials. Single-crystal X-ray diffraction (SCXRD) and in situ powder X-ray diffraction (PXRD) experiments provided insight into the mechanisms of phase transformation and revealed that the ß phases are nonporous with unit cell volumes 39.9, 40.8, and 41.0% lower than the corresponding as-synthesized α phases, X-ddi-1-Ni-α, X-ddi-2-Ni-α, and X-ddi-1,2-Ni-α, respectively. The results presented herein represent the first report of reversible switching between closed and open phases in ddi topology coordination networks and further highlight how ligand substitution can profoundly impact the gas sorption properties of switching sorbents.

Integrated Soft Porosity and Electrical Properties of Conductive-on-Insulating Metal-Organic Framework Nanocrystals.

A one-stone, two-bird method to integrate the soft porosity and electrical properties of distinct metal-organic frameworks (MOFs) into a single material involves the design of conductive-on-insulating MOF (cMOF-on-iMOF) heterostructures that allow for direct electrical control. Herein, we report the synthesis of cMOF-on-iMOF heterostructures using a seeded layer-by-layer method, in which the sorptive iMOF core is combined with chemiresistive cMOF shells. The resulting cMOF-on-iMOF heterostructures exhibit enhanced selective sorption of CO2 compared to the pristine iMOF (298 K, 1 bar, S CO 2 / H 2 ${{_{{\rm CO}{_{2}}/{\rm H}{_{2}}}}}$ from 15.4 of ZIF-7 to 43.2-152.8). This enhancement is attributed to the porous interface formed by the hybridization of both frameworks at the molecular level. Furthermore, owing to the flexible structure of the iMOF core, the cMOF-on-iMOF heterostructures with semiconductive soft porous interfaces demonstrated high flexibility in sensing and electrical "shape memory" toward acetone and CO2 . This behavior was observed through the guest-induced structural changes of the iMOF core, as revealed by the operando synchrotron grazing incidence wide-angle X-ray scattering measurements.

Self-Assembled Crystalline Bundles in Soluble Metal-Organic Nanotubes.

The use of nanotubes in the solution state is crucial not only for the exploration of physical and chemical behaviors at the molecular level but also for application such as thin-film fabrication. Surface modification is generally used to solubilize carbon nanotubes (CNTs) and various synthetic nanotubes; however, this method may affect the surface properties of the original nanotubes, and the detailed crystal structure obtained after modification is unclear. Here, we report the synthesis of a crystalline and soluble metal-organic nanotube consisting of a cationic tubular framework and an anion with a long alkyl chain. The nanotubular structures are formed not only in the solid state but also in the solution state, as confirmed by an X-ray structural analysis, optical measurements, and electron microscopy studies. This nanotube system is realized in different states without any surface modification, which is quite different from typical CNTs and synthetic nanotubes. In addition, self-assembled crystalline bundles are directly observed using transmission electron microscopy (TEM) for the first time in a metal-organic nanotube system. The bundle structures are also confirmed by atomic force microscopy (AFM) observations of thin nanotube films. We envisage a systematic design of such soluble metal-organic nanotubes that will enable direct observation of mass transport behavior in channels of bundles or a single nanotube, as well as a wide range of thin-film applications.

Selective sorption of oxygen and nitrous oxide by an electron donor-incorporated flexible coordination network.

Incorporating strong electron donor functionality into flexible coordination networks is intriguing for sorption applications due to a built-in mechanism for electron-withdrawing guests. Here we report a 2D flexible porous coordination network, [Ni2(4,4'-bipyridine)(VTTF)2]n(1) (where H2VTTF = 2,2'-[1,2-bis(4-benzoic acid)-1,2ethanediylidene]bis-1,3-benzodithiole), which exhibits large structural deformation from the as-synthesized or open phase (1α) into the closed phase (1ß) after guest removal, as demonstrated by X-ray and electron diffraction. Interestingly, upon exposure to electron-withdrawing species, 1ß reversibly undergoes guest accommodation transitions; 1α⊃O2 (90 K) and 1α⊃N2O (185 K). Moreover, the 1ß phase showed exclusive O2 sorption over other gases (N2, Ar, and CO) at 120 K. The phase transformations between the 1α and 1ß phases under these gases were carefully investigated by in-situ X-ray diffraction, in-situ spectroscopic studies, and DFT calculations, validating that the unusual sorption was attributed to the combination of flexible frameworks and VTTF (electron-donor) that induces strong interactions with electron-withdrawing species.

Post-Synthetic Cyano-ferrate(II) Functionalization of a Metal-Organic Framework, NU-1000.

Starting with ferrocyanide ions in acidic aqueous solution, cyano-ferrate(II) species are post-synthetically grafted to the nodes of a mesoporous zirconium-based MOF, NU-1000. As indicated by single-crystal X-ray crystallography, grafting occurs by substitution of cyanide ligands by node-based hydroxo and oxo ligands rather than by substitution of node aqua ligands by cyanide ligands as bridges between Fe(II) and Zr(IV). The installed moieties yield a broad absorption band that is tentatively ascribed to iron-to-zirconium charge transfer. Consistent with Fe(III/II) redox activity, a modest fraction of the installed iron complexes are directly electrochemically addressable.

Biomedically-relevant metal organic framework-hydrogel composites.

Metal organic frameworks (MOFs) are incredibly versatile three-dimensional porous materials with a wide range of applications that arise from their well-defined coordination structures, high surface areas and porosities, as well as ease of structural tunability due to diverse compositions achievable. In recent years, following advances in synthetic strategies, development of water-stable MOFs and surface functionalisation techniques, these porous materials have found increasing biomedical applications. In particular, the combination of MOFs with polymeric hydrogels creates a class of new composite materials that marries the high water content, tissue mimicry and biocompatibility of hydrogels with the inherent structural tunability of MOFs in various biomedical contexts. Additionally, the MOF-hydrogel composites can transcend each individual component such as by providing added stimuli-responsiveness, enhancing mechanical properties and improving the release profile of loaded drugs. In this review, we discuss the recent key advances in the design and applications of MOF-hydrogel composite materials. Following a summary of their synthetic methodologies and characterisation, we discuss the state-of-the-art in MOF-hydrogels for biomedical use - cases including drug delivery, sensing, wound treatment and biocatalysis. Through these examples, we aim to demonstrate the immense potential of MOF-hydrogel composites for biomedical applications, whilst inspiring further innovations in this exciting field.

Reversible transformations between the non-porous phases of a flexible coordination network enabled by transient porosity.

Flexible metal-organic materials that exhibit stimulus-responsive switching between closed (non-porous) and open (porous) structures induced by gas molecules are of potential utility in gas storage and separation. Such behaviour is currently limited to a few dozen physisorbents that typically switch through a breathing mechanism requiring structural contortions. Here we show a clathrate (non-porous) coordination network that undergoes gas-induced switching between multiple non-porous phases through transient porosity, which involves the diffusion of guests between discrete voids through intra-network distortions. This material is synthesized as a clathrate phase with solvent-filled cavities; evacuation affords a single-crystal to single-crystal transformation to a phase with smaller cavities. At 298 K, carbon dioxide, acetylene, ethylene and ethane induce reversible switching between guest-free and gas-loaded clathrate phases. For carbon dioxide and acetylene at cryogenic temperatures, phases showing progressively higher loadings were observed and characterized using in situ X-ray diffraction, and the mechanism of diffusion was computationally elucidated.

Fine Pore-Structure Engineering by Ligand Conformational Control of Naphthalene Diimide-Based Semiconducting Porous Coordination Polymers for Efficient Chemiresistive Gas Sensing.

Exploring new porous coordination polymers (PCPs) that have tunable structure and conductivity is attractive but remains challenging. Herein, fine pore structure engineering by ligand conformation control of naphthalene diimide (NDI)-based semiconducting PCPs with π stacking-dependent conductivity tunability is achieved. The π stacking distances and ligand conformation in these isoreticular PCPs were modulated by employing metal centers with different coordination geometries. As a result, three conjugated PCPs (Co-pyNDI, Ni-pyNDI, and Zn-pyNDI) with varying pore structure and conductivity were obtained. Their crystal structures were determined by three-dimensional electron diffraction. The through-space charge transfer and tunable pore structure in these PCPs result in modulated selectivity and sensitivity in gas sensing. Zn-pyNDI can serve as a room-temperature operable chemiresistive sensor selective to acetone.

Separating water isotopologues using diffusion-regulatory porous materials.

The discovery of a method to separate isotopologues, molecular entities that differ in only isotopic composition1, is fundamentally and technologically essential but remains challenging2,3. Water isotopologues, which are very important in biological processes, industry, medical care, etc. are among the most difficult isotopologue pairs to separate because of their very similar physicochemical properties and chemical exchange equilibrium. Herein, we report efficient separation of water isotopologues at room temperature by constructing two porous coordination polymers (PCPs, or metal-organic frameworks) in which flip-flop molecular motions within the frameworks provide diffusion-regulatory functionality. Guest traffic is regulated by the local motions of dynamic gates on contracted pore apertures, thereby amplifying the slight differences in the diffusion rates of water isotopologues. Significant temperature-responsive adsorption occurs on both PCPs: H2O vapour is preferentially adsorbed into the PCPs, with substantially increased uptake compared to that of D2O vapour, facilitating kinetics-based vapour separation of H2O/HDO/D2O ternary mixtures with high H2O separation factors of around 210 at room temperature.