Design Rules of Hydrogen-Bonded Organic Frameworks with High Chemical and Thermal Stabilities

Protein-driven interaction enhanced electrochemiluminescence biosensor of hydrogen-bonded biohybrid organic frameworks for sensitive immunoassay of disease markers

The oriented design of reticular materials as emitters can significantly enhance the sensitivity of electrochemiluminescence (ECL) sensing analysis for disease markers. However, due to the structural fragility of hydrogen bonds, relational research on hydrogen-bonded organic frameworks (HOFs) has not been thoroughly conducted. Additionally, the modulation of luminescence behavior through HOFs has been rarely reported. In view of this, hydrogen-bonded biohybrid organic frameworks (HBOFs) were synthesized and recruited for ECL immunoassay applications. HBOFs was easily prepared using 6,6',6″,6‴-(pyrene-1,3,6,8-tetrayl)tetrakis(2-naphthoic acid) as linkers via bovine serum albumin (BSA) activated hydrogen-bonded cross-linking. The material exhibited good fluorescence emission characteristics. And the highly ordered topological structure and molecular motion limitation mediated by BSA overcome aggregation-caused quenching and generate strong aggregation induced emission, expressing hydrogen-bond interaction enhanced ECL (HIE-ECL) activity with the participation of tri-n-propylamine. Furthermore, a sandwich immunosensor was constructed employing cobalt-based metal-phenolic network (CMPN) coated ferrocene nanoparticles (FNPs) as quenchers (CMPN@FNPs). Signal closure can be achieved by annihilating the excited state through electron transfer from both CMPN and FNPs. Using a universal disease marker, carcinoembryonic antigen, as the analysis model, the signal-off sensor obtained a detection limit of 0.47 pg/mL within the detection range of 1 pg/mL - 50 ng/mL. The synthesis and application of highly stable HBOFs triggered by proteins provide a reference for the development of new reticular ECL signal labels, and electron transfer model provides flexible solutions for more sensitive sensing analysis.

Hydrogen-bonded organic frameworks as stationary phase for open-tubular capillary electrochromatography

BACKGROUND

Capillary electrochromatography (CEC) stationary phases have always been the focus of attention. The selection of excellent stationary phases are the key to realize separate of different compounds. Hydrogen-bonded organic frameworks (HOFs) are porous materials connected by hydrogen bonds between molecules, which have the advantages of renewable, high specific surface area and mild synthesis conditions. At present, HOFs are used in gas adsorption and storage, catalysis and drug delivery. Because of its unique advantages, HOFs have a bright future as CEC stationary phases.

RESULTS

Using melamine (MA) and 1,3,6,8-tetra (4-carboxylphenyl)pyrene (H4TBAPy) as reaction monomers, a HOFs named MA/PFC-1 was synthesized by solvent evaporation at room temperature. The inner wall of the capillary column was coated with MA/PFC-1 by chemical bonding. Sulfonamides were used as the target analytes. The effects of pH, phosphate buffer solution concentration, organic additive content and applied voltage on sulfonamides separation were investigated. The MA/PFC-1-coated capillary column had good resolution (>1.5) and reproducibility. The intra-day, inter-day, column-to-column, and inter-batch precision of the retention times were 0.03%-0.09%, 0.04%-0.09%, 0.03%-0.14% and 0.06%-0.09%, respectively. The intra-day, inter-day, column-to-column, and inter-batch precision of the peak areas were 0.11%-0.25%, 0.13%-0.20%, 0.12%-0.15% and 0.08%-0.15%, respectively. The MA/PFC-1-coated capillary column was run 150 consecutive times, and the results showed no noticeable change, which proved that this method had good stability.

SIGNIFICANCE

This work applied HOFs to CEC. The results show the that MA/PFC-1-coated capillary column has good separation performance. The MA/PFC-1-coated capillary column has been successfully applied to the determination of sulfamethoxazole in tablets, which has practical application value. To open up the application of HOFs in CEC and provide a new idea for developing new CEC stationary phases.

Self-standing membranes for separation: Achievements and opportunities

Supported membranes and mixed matrix membranes have a limitation of harming the mass transfer due to the incompatibility between the support layer or the matrix and the active components of the membrane. Self-standing membranes, which could structurally abandon the support layer, altogether avoid the adverse effect, thus greatly facilitating the transmembrane mass transfer process. However, the abandonment of the support layer also reduces the membrane's mechanical properties and formability. In this review, our emphasis will be on self-standing membranes within the realm of materials and separation engineering. We will explore the materials employed in the fabrication of self-standing membranes, highlighting their ability to simultaneously enhance membrane performance and promote self-standing characteristics. Additionally, we will delve into the diverse techniques utilized for crafting self-standing membranes, encompassing interfacial polymerization, filtration, solvent casting, Langmuir-Blodgett & layer-by-layer assembly, electrospinning, compression, etc. Throughout the discussion, the merits and drawbacks associated with each of these preparation methods were elucidated. We also provide a brief overview of the applications of self-standing membranes, including water purification, gas separation, organic solvent nanofiltration, electrochemistry, and membrane reactor, as well as a brief description of the general strategies for performance enhancement of self-standing membranes. Finally, the current status of self-standing membranes and the challenges they may encounter were discussed.

Assembling Smallest Prussian Blue Analogs Using Chiral Hydrogen Bond-Donating Unit toward Complete Phase Transition

Exploring methods for assembling functional materials at the molecular level may yield functional expressions derived from the assembly method. This study developed novel switchable molecular assemblies characterized by abrupt, complete phase transitions promoted via hydrogen bonding with a chiral carboxylic acid. These assemblies were prepared by aggregating discrete molecules that are unresponsive to external stimuli. Furthermore, enantiopure hydrogen-bond donor (HBD) molecules provide switchable compounds with cooperative and abrupt phase transitions, whereas the racemic mixture of the HBD provides a hydrogen-bonded one-dimensional compound with a broad and incomplete phase transition when structural disordering is observed. This study presents a novel strategy for observing metal-to-metal electron-transfer-coupled spin transitions via hydrogen-bond formation.

Electro-Driven Multi-Enzymatic Cascade Conversion of CO2 to Ethylene Glycol in Nano-Reactor

Multi-enzymatic cascade reaction provides a new avenue for C─C coupling directly from CO2 under mild conditions. In this study, a new pathway with four enzymes including formate dehydrogenase (PaFDH), formaldehyde dehydrogenase (BmFADH), glycolaldehyde synthase (PpGALS), and alcohol dehydrogenase (GoADH) is developed for directly converting CO2 gas molecules to ethylene glycol (EG) in vitro. A rhodium-based NADH regeneration electrode is constructed to continuously provide the proton and electron of this multi-enzymatic cascade reaction. The prepared electrode can reach the Faradaic Efficiency (FE) of 82.9% at -0.6 V (vs. Ag/AgCl) and the NADH productivity of 0.737 mM h-1. Shortening the reaction path is crucial for multi-enzymatic cascade reactions. Here, a hydrogen-bonded organic framework (HOF) nano-reactor is successfully developed to immobilize four enzymes in one pot with a striking enzyme loading capacity (990 mg enzyme g-1 material). Through integrating and optimization of NADH electro-regeneration and enzymatic catalysis in one pot, 0.15 mM EG is achieved with an average conversion rate of 7.15 × 10-7 mmol CO2 min-1 mg-1 enzymes in 6 h. These results shed light on electro-driven multi-enzymatic cascade conversion of C─C coupling from CO2 in the nano-reactor.

Construction of Frustrated Lewis Pairs in Poly(heptazine Imide) Nanosheets via Hydrogen Bonds for Boosting CO2 Photoreduction

The creation of frustrated Lewis pairs on catalyst surface is an effective strategy for tuning CO2 activation. The critical step in the formation of frustrated Lewis pairs is the spatial effect of proximal Lewis acid-Lewis base pairs. Here, we demonstrate a facile surface functionalization methodology that enables hydrogen bonding between N and H atoms to mediate the construction of frustrated Lewis pairs in poly(heptazine imide), thereby increasing the propensity to activate CO2 molecules. Experimental and theoretical results show that the construction of active hydrogen bonding regions can facilitate the bending of CO2 molecules. Furthermore, the delocalization of electron clouds induced by the hydrogen bonding-mediated frustrated Lewis pairs can promote the heterolytic cleavage and photocatalytic conversion of CO2. This work highlights the potential of utilizing hydrogen bonding-mediated strategy in heterogeneously photocatalytic activation of CO2 over polymer materials.

Modulating the Microenvironments of Robust Metal Hydrogen-Bonded Organic Frameworks for Boosting Photocatalytic Hydrogen Evolution

Hydrogen-bonded organic frameworks (HOFs) are outstanding candidates for photocatalytic hydrogen evolution. However, most of reported HOFs suffer from poor stability and photocatalytic activity in the absence of Pt cocatalyst. Herein, a series of metal HOFs (Co2-HOF-X, X=COOMe, Br, tBu and OMe) have been rationally constructed based on dinuclear cobalt complexes, which exhibit exceptional stability in the presence of strong acid (12 M HCl) and strong base (5 M NaOH) for at least 10 days. More impressively, by varying the -X groups of the dinuclear cobalt complexes, the microenvironment of Co2-HOF-X can be modulated, giving rise to obviously different photocatalytic H2 production rates, following the -X group sequence of -COOMe>-Br>-tBu>-OMe. The optimized Co2-HOF-COOMe shows H2 generation rate up to 12.8 mmol g-1 h-1 in the absence of any additional noble-metal photosensitizers and cocatalysts, which is superior to most reported Pt-assisted photocatalytic systems. Experiments and theoretical calculations reveal that the -X groups grafted on Co2-HOF-X possess different electron-withdrawing ability, thus regulating the electronic structures of Co catalytic centres and proton activation barrier for H2 production, and leading to the distinctly different photocatalytic activity.

Halogen-Bonded Organic Frameworks (XOFs) Based on N⋅⋅⋅Br+⋅⋅⋅N Bonds: Robust Organic Networks Constructed by Fragile Bonds

Organic frameworks face a trade-off between the framework stability and the bond dynamics, which necessitates the development of innovative linkages that can generate stable frameworks without hindering efficient synthesis. Although iodine(I)-based halogen-bonded organic frameworks (XOFs) have been developed, constructing XOFs based on bromine(I) is desirable yet challenging due to the high sensitivity of bromine(I) species. In this work, we present the inaugural construction of stable bromine(I)-bridged two-dimensional (2D) halogen-bonded organic frameworks, XOF(Br)-TPy-BF4/OTf, based on sensitive [N⋅⋅⋅Br⋅⋅⋅N]+ halogen bonds. The formation of XOF(Br)-TPy-BF4/OTf was monitored by 1H NMR, XPS, IR, SEM, TEM, HR-TEM, SEAD. Their framework structures were established by the results from PXRD, theoretical simulations and SAXS. More importantly, XOF(Br) displayed excellent chemical and thermal stabilities. They exhibited stable two-dimensional framework structures in various organic solvents and aqueous media, even over a wide pH range (pH 3-12), while the corresponding model compounds BrPy2BF4/OTf decomposed quickly even in the presence of minimal water. Furthermore, the influence of the counterions were investigated by replacing BF4 with OTf, which improved the stability of XOF(Br). This characteristic enabled XOF(Br) to serve as an efficient oxidizing reagent in aqueous environments, in contrast with the sensitivity of BrPy2BF4/OTf, which performed well only in organic media. This study not only deepens our fundamental understanding of organic frameworks but also opens new avenues for the development and application of multifunctional XOFs.

An n-type semiconducting diazaporphyrin-based hydrogen-bonded organic framework

Significant effort has been devoted to the development of materials that combine high electrical conductivity and permanent porosity. This paper discloses a diazaporphyrin-based hydrogen-bonded organic framework (HOF) with porosity and n-type semiconductivity. A 5,15-diazaporphyrin Ni(ii) complex with carboxyphenyl groups at the meso positions afforded a HOF due to hydrogen-bonding interactions between the carboxy groups and meso-nitrogen atoms. The thermal and chemical stabilities of the HOF were examined using powder X-ray diffraction analysis, and the charge-carrier mobility was determined to be 2.0 × 10-7 m2 V-1 s-1 using the flash-photolysis time-resolved microwave conductivity (FP-TRMC) method. An analogous diazaporphyrin, which does not form a HOF, exhibited mobility that was 20 times lower. The results presented herein highlight the crucial role of hydrogen-bonding networks in achieving conductive pathways that can tolerate thermal perturbation.

Self-correcting mismatches in metastable hydrogen-bonded organic frameworks with an 11-fold interpenetrated array

The polymorphic self-correction from a metastable phase to a stable one often occurs and plays crucial roles in synthesizing robust hydrogen-bonded organic frameworks (HOFs). However, identifying metastable phases and understanding the self-correcting mechanisms is a challenging venture due to their intrinsic instability. Here, we for the first time introduce 1,8-naphtholactam (Np) as a hydrogen-bonding synthon positioned on the periphery of a bicarbazole to create a versatile molecular unit for 3D HOFs. The as-synthesized NCU-HOF1, analyzed by single-crystal X-ray diffraction (SCXRD), is found to be metastable. It exhibits an 11-fold interpenetrated dia topology with a quarter of the Np units exhibiting monomeric N-H⋯O interactions between adjacent Np link sites, which readily self-correct upon desolvation to form fully dimeric ones. Consequently, the resultant NCU-HOF1a becomes highly robust in polar solvents, strong acid or alkaline aqueous solutions, and has permanent porosity with contracted cavities for selective adsorption and efficient "turn-up" fluorescent sensing of C2H4 gas. This work not only debuts a new hydrogen-bonding synthon but offers more insights into investigating solid-state dynamics in metastable HOFs.

Microporous Materials in Polymer Electrolytes: The Merit of Order

Solid-state batteries (SSBs) have garnered significant attention in the critical field of sustainable energy storage due to their potential benefits in safety, energy density, and cycle life. The large-scale, cost-effective production of SSBs necessitates the development of high-performance solid-state electrolytes. However, the manufacturing of SSBs relies heavily on the advancement of suitable solid-state electrolytes. Composite polymer electrolytes (CPEs), which combine the advantages of ordered microporous materials (OMMs) and polymer electrolytes, meet the requirements for high ionic conductivity/transference number, stability with respect to electrodes, compatibility with established manufacturing processes, and cost-effectiveness, making them particularly well-suited for mass production of SSBs. This review delineates how structural ordering dictates the fundamental physicochemical properties of OMMs, including ion transport, thermal transfer, and mechanical stability. The applications of prominent OMMs are critically examined, such as metal-organic frameworks, covalent organic frameworks, and zeolites, in CPEs, highlighting how structural ordering facilitates the fulfillment of property requirements. Finally, an outlook on the field is provided, exploring how the properties of CPEs can be enhanced through the dimensional design of OMMs, and the importance of uncovering the underlying "feature-function" mechanisms of various CPE types is underscored.

Strategy to Efficient Photodynamic Therapy for Antibacterium: Donor-Acceptor Structure in Hydrogen-Bonded Organic Framework

While the construction of a donor-acceptor (D-A) structure has gained great attention across various scientific disciplines, such structures are seldomly reported within the field of hydrogen-bonded organic frameworks (HOFs). Herein, a D-A based HOF is synthesized, where the adjacent D-A pairs are connected by hydrogen bonds instead of the conventionally employed covalent bonds. This structural feature imparts material with a reduced energy gap between excited state and triplet state, thereby facilitating the intersystem crossing (ISC) and boosting the generation rate of single oxygen (quantum yield = 0.98). Consequently, the resulting material shows high performance for antimicrobial photodynamic therapy (PDT). The impact of D-A moiety is evident when comparing this finding to a parallel study conducted on an isoreticular HOF without a D-A structure. The study presented here provides in-depth insights into the photophysical properties of D-A pair in a hydrogen-bonded network, opening a new avenue to the design of innovative materials for efficient PDT.

Postsynthetic Modification of Hydrogen-Bonded Frameworks

Hydrogen-bonded frameworks have garnered significant attention due to their flexible structures with tailored porosity, making them a promising class of porous framework materials. However, the direct synthesis of hydrogen-bonded frameworks with specific functions is highly challenging due to the unpredictable formation of hydrogen-bonded frameworks. In response, postsynthetic modification has emerged as a potent strategy to imbue desired functions into hydrogen-bonded frameworks. Recent advances have demonstrated the effectiveness of postsynthetic modification in hydrogen-bonded frameworks for studying their mechanical, luminescent, electrochemical, and chiral properties. In this concept, we comprehensively summarize the methodologies and outcomes of postsynthetic modification to hydrogen-bonded frameworks, providing a highlight of this exciting research area.

A Biocompatible Hydrogen-Bonded Organic Framework (HOF) as Sonosensitizer and Artificial Enzyme for In-Depth Treatment of Alzheimer's Disease

Current phototherapeutic approaches for Alzheimer's disease (AD) exhibit restricted clinical outcomes due to the limited physical penetration and comprised brain microenvironment of noninvasive nanomedicine. Herein, a hydrogen-bonded organic framework (HOF) based sonosensitizer is designed and synthesized. Mn-TCPP, a planar molecule where Mn2+ ion is chelated in the core with a large p-conjugated system and 4 carboxylate acid groups, has been successfully used as building blocks to construct an ultrasound-sensitive HOF (USI-MHOF), which can go deep in the brain of AD animal models. The both in vitro and in vivo studies indicate that USI-MHOF can generate singlet oxygen (1O2) and oxidize β-amyloid (Aβ) to inhibit aggregation, consequently attenuating Aβ neurotoxicity. More intriguingly, USI-MHOF exhibits catalase (CAT)- and superoxide dismutase (SOD)-like activities, mitigating neuron oxidative stress and reprograming the brain microenvironment. For better crossing the blood-brain barrier (BBB), the peptide KLVFFAED (KD8) has been covalently grafted to USI-MHOF for improving BBB permeability and Aβ selectivity. Further, in vivo experiments demonstrate a significant reduction of the craniocerebral Aβ plaques and improvement of the cognition deficits in triple-transgenic AD (3×Tg-AD) mice models following deep-penetration ultrasound treatment. The work provides the first example of an ultrasound-responsive biocompatible HOF as non-invasive nanomedicine for in-depth treatment of AD.

Molecular Engineering of Hydrogen-Bonded Organic Framework for Enhanced Nitrate Electroreduction to Ammonia

Electrocatalytic nitrate reduction is an efficient way to produce ammonia sustainably. Herein, we rationally designed a copper metalloporphyrin-based hydrogen-bonded organic framework (HOF-Cu) through molecular engineering strategies for electrochemical nitrate reduction. As a result, the state-of-the-art HOF-Cu catalyst exhibits high NH3 Faradaic efficiency of 93.8%, and the NH3 production rate achieves a superior activity of 0.65 mmol h-1 cm-2. The in situ electrochemical spectroscopic combined with density functional theory calculations reveals that the dispersed Cu promotes the adsorption of NO3- and the mechanism is followed by deoxidation of NO3- to *NO and accompanied by deep hydrogenation. The generated *H participates in the deep hydrogenation of intermediate with fast kinetics as revealed by operando electrochemical impedance spectroscopy, and the competing hydrogen evolution reaction is suppressed. This research provides a promising approach to the conversion of nitrate to ammonia, maintaining the nitrogen balance in the atmosphere.

Self-Healing B ← N-Based Hydrogen-Bonded Organic Framework for Exclusive Recognition and Separation of Toluene from Methyl-Cyclohexane

The effective separation of aromatic and aliphatic hydrocarbons remains a notable challenge in the petrochemical industry. Herein, we report a self-healing three-dimensional B ← N-based hydrogen-bonded organic framework (HOF), BN-HOF-1, constructed from discrete B ← N inclusive dimers through weak C-H···F and C-H···N hydrogen-bonding interactions. To make use of the specific recognition of the B ← N inclusive dimers for the toluene molecules and the reversible ad/desorption nature of this novel HOF, BN-HOF-1 can exclusively recognize and separate toluene from the mixtures of toluene-methylcyclohexane, thus generating 99.6% pure toluene from its mixtures after gentle heating, the recorded value among any reported materials for toluene purification. After the toluene molecules were released from the framework, it becomes the condensed BN-HOF-1a, which can be further reused for the highly selective recognition and purification of toluene from its binary mixtures, through the reversible structural recovery back to BN-HOF-1. Single-crystal X-ray diffraction and molecular modeling studies reveal that the high specific toluene recognition is attributed to the complementary electrostatic potential between the host B ← N inclusive dimers and the guest toluene, while the self-healing and recovery nature of this HOF is attributed to weak intermolecular hydrogen-bonding interactions.

Double hydrogen bonding-induced compact H-type π-π stacking enhancing rapid carrier transfer in perylene diimide supramolecules achieving high oxygen evolution performance

Perylene diimides (PDI) are widely used in photocatalytic oxygen evolution due to their deep valence band potentials. Here, we report the synthesis of a unique supramolecular photocatalyst (designated s-PDI-P1) by introducing hydroxyl and carboxyl groups at the imide position of PDI. This modification allows the formation of intermolecular double hydrogen bond structures between the hydroxyl groups, oxygen atoms on the perylene cores and the carboxyl groups. The resulting double hydrogen bonding structures reduce lateral slip and promote the formation of supramolecular structures with H-type π-π stacking. In addition, the intermolecular hydrogen bonding interactions between the hydroxyl groups and the oxygen atoms on the perylene cores bring the PDI molecules closer together, enhancing the conjugation of the PDI supramolecules and facilitating the formation of ultrathin nanosheet-like structures. In this study, we successfully constructed ultrathin nanosheets of the supramolecular photocatalyst s-PDI-P1 with a compact H-type π-π stacking structure, which exhibited enhanced charge transfer capability, shorter charge migration distance, and achieved a high photocatalytic oxygen evolution rate of 3.23 mmolg-1h-1. These results highlight the potential of intermolecular double hydrogen bond structures to improve the separation and migration driving force of photogenerated charges, thus providing a novel design strategy for organic photocatalysts.

Ultrahigh proton conductivity of four ionic hydrogen-bonded organic frameworks based on functionalized terephthalates

Recently, the utilization of hydrogen-bonded organic frameworks (HOFs) with high crystallinity and inherent well-defined H-bonding networks in the field of proton conduction has received increasing attention, but obtaining HOFs with excellent water stability and prominent proton conductivity (σ) remains challenging. Herein, by employing functionalized terephthalic acids, 2,5-dihydroxyterephthalic acid, 2-hydroxyterephthalic acid, 2-nitro terephthalic acid, and terephthalic acid, respectively, four highly water-stable ionic HOFs (iHOFs), [(C8H5O6)(Me2NH2)]∙2H2O (iHOF 1), [(C8H5O5)(Me2NH2)] (iHOF 2), [(C8H4NO6)(Me2NH2)] (iHOF 3) and [(C8H5O4)(Me2NH2)] (iHOF 4) were efficiently prepared by a straightforward synthesis approach in DMF and H2O solutions. The alternating-current (AC) impedance testing in humid conditions revealed that all four iHOFs were temperature- and humidity-dependent σ, with the greatest value reaching 10-2 S·cm-1. As expected, the high density of free carboxylic acid groups, crystallization water, and protonated [Me2NH2]+ units offer adequate protons and hydrophilic environments for effective proton transport. Furthermore, the σ values of these iHOFs with different functional groups were compared. It was discovered that it dropped in the following order under 100 °C and 98 % relative humidity (RH): σ iHOF 1 (1.72 × 10-2 S·cm-1) > σ iHOF 2 (4.03 × 10-3 S·cm-1) > σ iHOF 3 (1.46 × 10-3 S·cm-1) > σ iHOF 4 (4.86 × 10-4 S·cm-1). Finally, we investigated the causes of the above differences and the proton transport mechanism inside the framework using crystal structure data, water contact angle tests, and activation energy values. This study provides new motivation to develop highly proton-conductive materials.

Advances and Challenges in Interfacial Binding Forces for Electrocatalysts

As a practical chemical energy conversion technology, electrocatalysis could be used in fields of energy conversion and environmental protection. In recent years, significant research efforts have been devoted to the design and development of high-performance electrocatalysts because the rational design of catalysts is crucial for enhancing electrocatalytic performance. Creating electrocatalysts by forming interactions between different components at the interface is an important means of controlling and improving performance. Therefore, several common interfacial binding forces used for synthesizing electrocatalysts was systematically summarized in this review for the first time. The discussion revolves around the crucial roles these binding forces play in various electrocatalytic reaction processes. Various characterization techniques capable of proving the existence of these interfacial binding forces was also involved in the review. Finally, some prospects and challenges for designing and researching materials through the utilization of interfacial binding forces were presented.

Crystalline Porous Organic Frameworks Based on Multiple Dynamic Linkages

A novel class of crystalline porous materials has been developed utilizing multilevel dynamic linkages, including covalent B-O, dative B←N and hydrogen bonds. Typically, boronic acids undergo in situ condensation to afford B3O3-based units, which further extend to molecular complexes or chains via B←N bonds. The obtained superstructures are subsequently interconnected via hydrogen bonds and π-π interactions, producing crystalline porous organic frameworks (CPOFs). The CPOFs display excellent solution processability, allowing dissolution and subsequent crystallization to their original structures, independent of recrystallization conditions, possibly due to the diverse bond energies of the involved interactions. Significantly, the CPOFs can be synthesized on a gram-scale using cost-effective monomers. In addition, the numerous acidic sites endow the CPOFs with high NH3 capacity, surpassing most porous organic materials and commercial materials.

Solid-State Self-Assembly: Exclusive Formation and Dynamic Interconversion of Discrete Cyclic Assemblies Based on Molecular Tweezers

In contrast to self-assembly in solution systems, the construction of well-defined assemblies in the solid state has long been identified as a challenging task. Herein, we report the formation of tweezers-shaped molecules into various assemblies through a solid-state self-assembly strategy. The relatively flexible molecular tweezers undergo exclusive and quantitative assembly into either cyclic hexamers or a porous network through classical recrystallization or the exposure of powders to solvent vapor, despite the fact that they form only dimers in solution. The cyclic hexamers have high thermal stability and exhibit moderate solid-state fluorescence. The formation of heterologous assemblies consisting of different tweezers allows for tuning these solid-state properties of the cyclic hexamer. Furthermore, (trimethylsilyl)ethynyl-substituted tweezers demonstrate solvent-vapor-induced dynamic interconversion between the cyclic hexamer and a pseudocyclic dimer in the solid state. This assembly behavior, which has been studied extensively in solution-based supramolecular chemistry, had not been accomplished in the solid state so far.

Flexible hydrogen-bonded organic frameworks (HOFs): opportunities and challenges

Flexible behavior is one of the most fascinating features of hydrogen-bonded organic frameworks (HOFs), which represent an emerging class of porous materials that are self-assembled via H-bonding between organic building units. Due to their unique flexibility, HOFs can undergo structural changes or transformations in response to various stimuli (physical or chemical). Taking advantage of this unique structural feature, flexible HOFs show potential in multifunctional applications such as gas storage/separation, molecular recognition, sensing, proton conductivity, biomedicine, etc. While some other flexible porous materials have been extensively studied, the dynamic behavior of HOFs remains relatively less explored. This perspective highlights the inherent flexible properties of HOFs, discusses their different flexible behaviors, including pore size/shape changes, interpenetration/stacking manner, H-bond breaking/reconstruction, and local dynamic behavior, and highlights their potential applications. We believe that this perspective will not only contribute to HOF chemistry and materials science, but will also facilitate the ongoing extensive research on dynamic porous materials.

Hydrogen-Bonded Mesoporous Frameworks with Tunable Pore Sizes and Architectures from Nanocluster Assembly Units

Construction of mesoporous frameworks by noncovalent bonding still remains a great challenge. Here, we report a micelle-directed nanocluster modular self-assembly approach to synthesize a novel type of two-dimensional (2-D) hydrogen-bonded mesoporous frameworks (HMFs) for the first time based on nanoscale cluster units (1.0-3.0 nm in size). In this 2-D structure, a mesoporous cluster plate with ∼100 nm in thickness and several micrometers in size can be stably formed into uniform hexagonal arrays. Meanwhile, such a porous plate consists of several (3-4) dozens of layers of ultrathin mesoporous cluster nanosheets. The size of the mesopores can be precisely controlled from 11.6 to 18.5 nm by utilizing the amphiphilic diblock copolymer micelles with tunable block lengths. Additionally, the pore configuration of the HMFs can be changed from spherical to cylindrical by manipulating the concentration of the micelles. As a general approach, various new HMFs have been achieved successfully via a modular self-assembly of nanoclusters with switchable configurations (nanoring, Keggin-type, and cubane-like) and components (titanium-oxo, polyoxometalate, and organometallic clusters). As a demonstration, the titanium-oxo cluster-based HMFs show efficient photocatalytic activity for hydrogen evolution (3.6 mmol g-1h-1), with a conversion rate about 2 times higher than that of the unassembled titanium-oxo clusters (1.5 mmol g-1h-1). This demonstrates that HMFs exhibited enhanced photocatalytic activity compared with unassembled titanium-oxo clusters units.

Drug carrier wonders: Synthetic strategies of zeolitic imidazolates frameworks (ZIFs) and their applications in drug delivery and anti-cancer activity

With the rapid advancement of nanotechnology, stimuli-responsive nanomaterials have emerged as a feasible choice for the designing of controlled drug delivery systems. Zeolitic imidazolates frameworks are a subclass of Metal-organic frameworks (MOFs) that are recognized by their excellent porosity, structural tunability and chemical modifications make them promising materials for loading targeted molecules and therapeutics agents. The biomedical industry uses these porous materials extensively as nano-carriers in drug delivery systems. These MOFs not only possess excellent targeted imaging ability but also cause the death of tumor cells drawing considerable attention in the current framework of anticancer drug delivery systems. In this review, the outline of stability, porosity, mechanism of encapsulation and release of anticancer drug have been reported extensively. In the end, we also discuss a brief outline of current challenges and future perspectives of ZIFs in the biomedical world.

Supramolecular Entanglement in a Hydrogen-Bonded Organic Framework Enables Flexible-Robust Porosity for Highly Efficient Purification of Natural Gas

The development of porous materials with flexible-robust characteristics shows some unique advantages to target high performance for gas separation, but remains a daunting challenge to achieve so far. Herein, we report a carboxyl-based hydrogen-bonded organic framework (ZJU-HOF-8a) with flexible-robust porosity for efficient purification of natural gas. ZJU-HOF-8a features a four-fold interpenetrated structure with dia topology, wherein abundant supramolecular entanglements are formed between the adjacent subnetworks through weak intermolecular hydrogen bonds. This structural configuration could not only stabilize the whole framework to establish the permanent porosity, but also enable the framework to show some flexibility due to its weak intermolecular interactions (so-called flexible-robust framework). The flexible-robust porosity of ZJU-HOF-8a was exclusively confirmed by gas sorption isotherms and single-crystal X-ray diffraction studies, showing that the flexible pore pockets can be opened by C3H8 and n-C4H10 molecules rather by C2H6 and CH4. This leads to notably higher C3H8 and n-C4H10 uptakes with enhanced selectivities than C2H6 over CH4 under ambient conditions, affording one of the highest n-C4H10/CH4 selectivities. The gas-loaded single-crystal structures coupled with theoretical simulations reveal that the loading of n-C4H10 can induce an obvious framework expansion along with pore pocket opening to improve n-C4H10 uptake and selectivity, while not for C2H6 adsorption. This work suggests an effective strategy of designing flexible-robust HOFs for improving gas separation properties.

Photo-Responsive Hydrogen-Bonded Molecular Networks Capable of Retaining Crystalline Periodicity after Isomerization

The molecular conformation, crystalline morphology, and properties of photochromic organic crystals can be controlled through photoirradiation, making them promising candidates for functional organic materials. However, photochromic porous molecular crystals with a networked framework structure are rare due to the difficulty in maintaining space that allows for photo-induced molecular motion in the crystalline state. This study describes a photo-responsive single crystal based on hydrogen-bonded (H-bonded) network of dihydrodimethylbenzo[e]pyrene derivative 4BDHP. A crystal composed of H-bonded undulate layers, 4BDHP-2, underwent photo-isomerization in the crystalline state due to loose stacking of the layers. Particularly, enantio-pure crystal (S,S)-4BDHP-2 allowed to reveal the structure of the photoisomerized crystal, in which the closed form (4BDHP) and open form (4CPD) were arranged alternately with keeping crystalline periodicity, although side reactions were also implied. The present proof-of-concept system of a photochromic framework that retains crystalline periodicity after photo-isomerization may provide new light-driven porous functional materials.

2D Porphyrin-Based Covalent-Organic Framework/PEG Composites: A Rational Strategy for Photocatalytic Hydrogen Evolution

Two-dimensional porphyrin-based covalent-organic frameworks (2D-por-COFs) have gained significant attention as attractive platforms for efficient solar light conversion into hydrogen production. Herein, it is found that introducing transition metal zinc and polyethylene glycol (PEG) into 2D-por-COFs can effectively improve the photocatalytic hydrogen evolution performance. The photocatalytic hydrogen evolution rate of ZnPor-COF is 2.82 times higher than that of H2Por-COF. Moreover, ZnPor-COF@PEG has the highest photocatalytic hydrogen evolution efficiency, which is 1.31 and 3.7 times that of pristine ZnPor-COF and H2Por-COF, respectively. The filling of PEG makes the layered structure of COFs more stable. PEG reduces the distortion and deformation of the carbon skeleton after the experiment of photocatalytic hydrogen evolution. The layered stacking and crystallization of 2D-por-COFs are also enhanced. Meanwhile, the presence of PEG also accelerates the transfer of excited electrons and enhances the photocatalytic hydrogen evolution activity. This strategy will provide valuable insights into the design of 2D-por-COFs as efficient solid photocatalysts for solar-driven hydrogen production.

Structural Design and Energy and Environmental Applications of Hydrogen-Bonded Organic Frameworks: A Systematic Review

Hydrogen-bonded organic frameworks (HOFs) are emerging porous materials that show high structural flexibility, mild synthetic conditions, good solution processability, easy healing and regeneration, and good recyclability. Although these properties give them many potential multifunctional applications, their frameworks are unstable due to the presence of only weak and reversible hydrogen bonds. In this work, the development history and synthesis methods of HOFs are reviewed, and categorize their structural design concepts and strategies to improve their stability. More importantly, due to the significant potential of the latest HOF-related research for addressing energy and environmental issues, this work discusses the latest advances in the methods of energy storage and conversion, energy substance generation and isolation, environmental detection and isolation, degradation and transformation, and biological applications. Furthermore, a discussion of the coupling orientation of HOF in the cross-cutting fields of energy and environment is presented for the first time. Finally, current challenges, opportunities, and strategies for the development of HOFs to advance their energy and environmental applications are discussed.

In Situ Transformation of an Amorphous Supramolecular Coating to a Hydrogen-Bonded Organic Framework Membrane to Trigger Selective Gas Permeation

We introduce a "solution-processing-transformation" strategy, deploying solvent vapor as scaffolds, to fabricate high-quality hydrogen-bonded organic framework (HOF) membranes. This strategy can overcome the mismatch in processing conditions and crystal growth thermodynamics faced during the facile solution processing of the membrane. The procedure includes the vapor-trigged in situ transformation of dense amorphous supramolecules to crystalline HOF-16, with HOF-11 as the transient state. The mechanism involves a vapor-activated dissolution-precipitation equilibrium shifting and hydrogen bonding-guided molecule rearrangement, elucidated through combined experimental and theoretical analysis. Upon removal of the molecular scaffolds, the resulting HOF-16 membranes showcase significant improvement in hydrogen separation performance over their amorphous counterparts and previously reported HOF membranes. The method's broad applicability is evidenced by successfully extending it to other substrates and HOF structures. This study provides a fundamental understanding of guest-induced ordered supramolecular assembly and paves the way for the advanced manufacture of high-performance HOF membranes for gas separation processes.

Protein-Integrated Hydrogen-Bonded Organic Frameworks: Chemistry and Biomedical Applications

Hydrogen-bonded organic frameworks (HOFs) are porous nanomaterials that offer exceptional biocompatibility and versatility for integrating proteins for biomedical applications. This minireview concisely discusses recent advancements in the chemistry and functionality of protein-HOF interfaces. It particularly focuses on strategic methodologies, such as the careful selection of building blocks and the genetic engineering of proteins, to facilitate protein-HOF interactions. We examine the role of enzyme encapsulation within HOFs, highlighting its capability to preserve enzyme function, a crucial aspect for applications in biosensing and disease diagnosis. Moreover, we discuss the emerging utility of nanoscale HOFs for intracellular protein delivery, illustrating their applicability as nanoreactors for intracellular catalysis and neuroprotective biorthogonal catalysis within cellular compartments. We highlight the significant advancement of designing biodegradable HOFs tailored for cytosolic protein delivery, underscoring their promising application in targeted cancer therapies. Finally, we provide a perspective viewpoint on the design of biocompatible protein-HOF assemblies, underlining their promising prospects in drug delivery, disease diagnosis, and broader biomedical applications.

Vitrification-enabled enhancement of proton conductivity in hydrogen-bonded organic frameworks

Hydrogen-bonded organic frameworks (HOFs) are versatile materials with potential applications in proton conduction. Traditional approaches involve incorporating humidity control to address grain boundary challenges for proton conduction. This study finds vitrification as an alternative strategy to eliminate grain boundary effect in HOFs by rapidly melt quenching the kinetically stable HOF-SXU-8 to glassy state HOF-g. Notably, a remarkable enhancement in proton conductivity without humidity was achieved after vitrification, from 1.31 × 10-7 S cm-1 to 5.62× 10-2 S cm-1 at 100 °C. Long term stability test showed negligible performance degradation, and even at 30 °C, the proton conductivity remained at high level of 1.2 × 10-2 S cm-1. Molecule dynamics (MD) simulations and X-ray total scattering experiments reveal the HOF-g system is consisted of three kinds of clusters, i.e., 1,5-Naphthalenedisulfonic acid (1,5-NSA) anion clusters, N,N-dimethylformamide (DMF) molecule clusters, and H+-H2O clusters. In which, the H+ plays an important role to bridge these clusters and the high conductivity is mainly related to the H+ on H3O+. These findings provide valuable insights for optimizing HOFs, enabling efficient proton conduction, and advancing energy conversion and storage devices.

Pore-Engineered Hydrogen-Bonded Supramolecular Fluorosensor for Ultrasensitive Determination of Copper Ions

The selective quantification of copper ions (Cu2+) in biosamples holds great importance for disease diagnosis, treatment, and prognosis since the Cu2+ level is closely associated with the physiological state of the human body. While it remains a long-term challenge due to the extremely low level of free Cu2+ and the potential interference by the complex matrices. Here, a pore-engineered hydrogen-bonded organic framework (HOF) fluorosensor is constructed enabling the ultrasensitive and highly selective detection of free Cu2+. Attributing to atomically precise functionalization of active amino "arm" within the HOF pores and the periodic π-conjugated skeleton, this porous HOF fluorosensor affords high affinity toward Cu2+ through double copper-nitrogen (Cu─N) coordination interactions, resulting in specific fluorescence quenching of the HOF as compared with a series of substances ranging from other metal ions, metabolites, amino acids to proteins. Such superior fluorescence quenching effect endows the Cu2+ quantification by this new HOF sensor with a wide linearity of 50-20 000 nm, a low detection limit of 10 nm, and good recoveries (89.5%-115%) in human serum matrices, outperforming most of the reported approaches. This work highlights the practicability of hydrogen-bonded supramolecular engineering for designing facile and ultrasensitive biosensors for clinical free Cu2+ determination.

Hypoxia-Responsive Hydrogen-Bonded Organic Framework-Mediated Protein Delivery for Cancer Therapy

The efficient delivery of therapeutic proteins to tumor sites is a promising cancer treatment modality. Hydrogen-bonded organic frameworks (HOFs) are successfully used for the protective encapsulation of proteins; however, easy precipitation and lack of controlled release of existing HOFs limit their further application for protein delivery in vivo. Here, a hypoxia-responsive HOF, self-assembled from azobenzenedicarboxylate/polyethylene glycol-conjugated azobenzenedicarboxylate and tetrakis(4-amidiniumphenyl)methane through charge-assisted hydrogen-bonding, is developed for systemic protein delivery to tumor cells. The newly generated HOF platform efficiently encapsulates representative cytochrome C, demonstrating good dispersibility under physiological conditions. Moreover, it can respond to overexpressed reductases in the cytoplasm under hypoxic conditions, inducing fast intracellular protein release to exert therapeutic effects. The strategy presented herein can be applied to other therapeutic proteins and can be expanded to encompass more intrinsic tumor microenvironment stimuli. This offers a novel avenue for utilizing HOFs in protein-based cancer therapy.

Peptide hydrogen-bonded organic frameworks

Hydrogen-bonded porous frameworks (HPFs) are versatile porous crystalline frameworks with diverse applications. However, designing chiral assemblies or biocompatible materials poses significant challenges. Peptide-based hydrogen-bonded porous frameworks (P-HPFs) are an exciting alternative to conventional HPFs due to their intrinsic chirality, tunability, biocompatibility, and structural diversity. Flexible, ultra-short peptide-based P-HPFs (composed of 3 or fewer amino acids) exhibit adaptable porous topologies that can accommodate a variety of guest molecules and capture hazardous greenhouse gases. Longer, folded peptides present challenges and opportunities in designing P-HPFs. This review highlights recent developments in P-HPFs using ultra-short peptides, folded peptides, and foldamers, showcasing their utility for gas storage, chiral recognition, chiral separation, and medical applications. It also addresses design challenges and future directions in the field.

Supramolecular polynuclear clusters sustained cubic hydrogen bonded frameworks with octahedral cages for reversible photochromism

Developing supramolecular porous crystalline frameworks with tailor-made architectures from advanced secondary building units (SBUs) remains a pivotal challenge in reticular chemistry. Particularly for hydrogen-bonded organic frameworks (HOFs), construction of geometrical cavities through secondary units has been rarely achieved. Herein, a body-centered cubic HOF (TCA_NH4) with octahedral cages was constructed by a C3-symmetric building block and NH4+ node-assembled cluster (NH4)4(COOH)8(H2O)2 that served as supramolecular secondary building units (SSBUs), akin to the polynuclear SBUs in reticular chemistry. Specifically, the octahedral cages could encapsulate four homogenous haloforms including CHCl3, CHBr3, and CHI3 with truncated octahedron configuration. Crystallographic evidence revealed the cages served as spatially-confined nanoreactors, enabling fast, broadband photochromic effect associated with the reversible photo/thermal transformation between encapsulated CHI3 and I2. Overall, this work provides a strategy by shaping SSBUs to expand the framework topology of HOFs and a prototype of hydrogen-bonded nanoreactors to accommodate reversible photochromic reactions.

Anionic Hydrogen-Bonded Frameworks Showing Tautomerism and Colorful Luminescence for the Ultrasensitive Detection of Acetone

Tautomers coexisting in an equilibrium system have significant potential for regulating luminescent properties because of their structural differences. However, separating and stabilizing tautomers at room temperature is a considerable challenge. In this study, it is found that hydrogen-bonded organic frameworks (HOFs) composed of Br- anions can effectively separate and stabilize two proton-transfer tautomers of triarylformamidinium bromide: namely, the nitrogen cation (BA-N) and carbon cation (BA-C). The BA-N crystal consisting of a dense anionic HOF and parallelly aligned organic cations exhibits green thermally activated delayed fluorescence and red room-temperature phosphorescence (RTP). The BA-C crystal contains acetone molecules that induce an antiparallel arrangement of the organic cations to form a loose HOF, producing blue prompt fluorescence and green RTP. Interestingly, switching of the HOFs between BA-N and BA-C can be achieved through the uptake and release of acetone, thereby dynamically adjusting multiple luminescent properties. Consequently, the HOF crystals can be used for the highly sensitive and specific sensing of acetone with a detection limit of 66.74 ppm. This study not only stabilizes tautomeric luminescent materials at room temperature, but also provides a new method for constructing smart HOFs with a sensitive response to a stimulus.

Charge-Assisted Ionic Hydrogen-Bonded Organic Frameworks: Designable and Stabilized Multifunctional Materials

Hydrogen-bonded organic frameworks (HOFs) are a class of crystalline framework materials assembled by hydrogen bonds. HOFs have the advantages of high crystallinity, mild reaction conditions, good solution processability, and reproducibility. Coupled with the reversibility and flexibility of hydrogen bonds, HOFs can be assembled into a wide diversity of crystalline structures. Since the bonding energy of hydrogen bonds is lower than that of ligand and covalent bonds, the framework of HOFs is prone to collapse after desolventisation and the stability is not high, which limits the development and application of HOFs. In recent years, numerous stable and functional HOFs have been developed by π-π stacking, highly interpenetrated networks, charge-assisted, ligand-bond-assisted, molecular weaving, and covalent cross-linking. Charge-assisted ionic HOFs introduce electrostatic attraction into HOFs to improve stability while enriching structural diversity and functionality. In this paper, we review the development, the principles of rational design and assembly of charge-assisted ionic HOFs, and introduces the different building block construction modes of charge-assisted ionic HOFs. Highlight the applications of charge-assisted ionic HOFs in gas adsorption and separation, proton conduction, biological applications, etc., and prospects for the diverse design of charge-assisted ionic HOFs structures and multifunctional applications.

Reticular synthesis of 8-connected carboxyl hydrogen-bonded organic frameworks for white-light-emission

The rational design and construction of hydrogen-bonded organic frameworks (HOFs) are crucial for enabling their practical applications, but controlling their structure and preparation as intended remains challenging. Inspired by reticular chemistry, two novel blue-emitting NKM-HOF-1 and NKM-HOF-2 were successfully constructed based on two judiciously designed peripherally extended pentiptycene carboxylic acids, namely H8PEP-OBu and H8PEP-OMe, respectively. The large pores within these two HOFs can adsorb fluorescent molecules such as diketopyrrolopyrrole (DPP) and 9-anthraldehyde (AnC) to form HOFs ⊃ DPP/AnC composites, subsequently used in the fabrication of white-light-emitting devices (WLEDs). Specifically, two WLEDs were assembled by coating NKM-HOF-1 ⊃ DPP-0.13/AnC-3.5 and NKM-HOF-2 ⊃ DPP-0.12/AnC-3 on a 330 nm ultraviolet LED bulb, respectively. The corresponding CIE coordinates were (0.29, 0.33) and (0.32, 0.34), along with corresponding color temperatures of 7815 K and 6073 K. This work effectively demonstrates the feasibility of employing reticular chemistry strategies to predict and design HOFs with specific topologies for targeted applications.

Orthogonal Postsynthetic Copolymerization of Hydrogen-Bonded Organic Frameworks into a PolyHOF Membrane

Hydrogen-bonded organic frameworks (HOFs) have shown promise in various fields; however, the construction of HOF/polymer hybrid membranes that can maintain both structural and functional integrity remains challenging. In this study, we here fabricated a new HOF (HOF-50) with reserved polymerizable allyl group via charge-assisted H-bonds between the carboxylate anion and amidinium, and subsequently copolymerized the HOF with monomers to construct a covalently bonded HOF/polymer hybrid (polyHOF) membrane. The resulting polyHOF membrane not only exhibits customizable mechanical properties and extreme stability, but also shows an exceptional ratiometric luminescent temperature-sensing function with very high sensitivity and visibility even when the lanthanide content is two orders of magnitude lower than that of the reported mixed-lanthanide metal-organic frameworks (MOFs) and lanthanide-doped covalent organic frameworks (COFs). This orthogonal postsynthesis copolymerization strategy may provide a general approach for preparing covalently connected HOF/polymer hybrid membranes for diverse applications.

Macrocycle-Based Hierarchically Porous Hydrogen-Bonded Organic Frameworks

Hydrogen-bonded organic frameworks (HOFs) are porous crystalline materials. The pores in HOFs are usually non-covalent extrinsic pores constructed through the formation of the framework. Supramolecular macrocycles with intrinsic pores in their structures are good candidates for constructing HOFs with intrinsic pores from the macrocycles themselves, thus leading to hierarchically porous structures. Combining the macrocycle and HOFs will endow these hierarchically porous materials with enhanced properties and special functionalities. This review summarizes recent advances in macrocycle-based HOFs, including the macrocycles used for constructing HOFs, the hierarchically porous structures of the HOFs, and the applications induced by the hierarchically HOFs porous structures. This review provides insights for future research on macrocycle-based hierarchically porous HOFs and the appropriate applications of the unique structures.

Rapid, high-capacity adsorption of iodine from aqueous environments with amide functionalized covalent organic frameworks

The uses and production of radionuclides in nuclear energy production and medical therapy are becoming more significant in today's world. While these applications have many benefits, they can produce harmful pollutants, such as radioactive iodine, that need to be sequestered. Effective capture and storage of radioactive iodine waste remains a major challenge for nuclear energy generation and nuclear medicine. Here we report the highly efficient capture of iodine in a series of mesoporous, two-dimensional (2D) covalent organic frameworks, called COFamides, which contain amide sidechains in their pores. COFamides are capable of rapidly removing iodine from aqueous solution at concentrations as low as 50 ppm, with total capacities greater than 650 wt%. In order to explain the high affinity of the COFamide series for iodine and iodide species in water, we performed a computational analysis of the interactions between the COFamide framework and iodine guests. These studies suggest that the origin of the large iodine capacity in these materials can be explained by the presence of multiple, cooperative, non-covalent interactions between the framework and both iodine, and iodide species.

Hydrogen-bonded organic frameworks for membrane separation

Hydrogen-bonded organic frameworks (HOFs) are a new class of crystalline porous materials that are formed through the interconnection of organic or metal-organic building units via intermolecular hydrogen bonds. The remarkable flexibility and reversibility of hydrogen bonds, coupled with the customizable nature of organic units, endow HOFs with mild synthesis conditions, high crystallinity, solvent processability, and facile self-healing and regeneration properties. Consequently, these features have garnered significant attention across various fields, particularly in the realm of membrane separation. Herein, we present an overview of the recent advances in HOF-based membranes, including their advanced fabrication strategies and fascinating applications in membrane separation. To attain the desired HOF-based membranes, careful consideration is dedicated to crucial factors such as pore size, stability, hydrophilicity/hydrophobicity, and surface charge of the HOFs. Additionally, diverse preparation methods for HOF-based membranes, including blending, in situ growth, solution-processing, and electrophoretic deposition, have been analyzed. Furthermore, applications of HOF-based membranes in gas separation, water treatment, fuel cells, and other emerging application areas are presented. Finally, the challenges and prospects of HOF-based membranes are critically pointed out.

Porous materials as effective chemiresistive gas sensors

Chemiresistive gas sensors (CGSs) have revolutionized the field of gas sensing by providing a low-power, low-cost, and highly sensitive means of detecting harmful gases. This technology works by measuring changes in the conductivity of materials when they interact with a testing gas. While semiconducting metal oxides and two-dimensional (2D) materials have been used for CGSs, they suffer from poor selectivity to specific analytes in the presence of interfering gases and require high operating temperatures, resulting in high signal-to-noise ratios. However, nanoporous materials have emerged as a promising alternative for CGSs due to their high specific surface area, unsaturated metal actives, and density of three-dimensional inter-connected conductive and pendant functional groups. Porous materials have demonstrated excellent response and recovery times, remarkable selectivity, and the ability to detect gases at extremely low concentrations. Herein, our central emphasis is on all aspects of CGSs, with a primary focus on the use of porous materials. Further, we discuss the basic sensing mechanisms and parameters, different types of popular sensing materials, and the critical explanations of various mechanisms involved throughout the sensing process. We have provided examples of remarkable performance demonstrated by sensors using these materials. In addition to this, we compare the performance of porous materials with traditional metal-oxide semiconductors (MOSs) and 2D materials. Finally, we discussed future aspects, shortcomings, and scope for improvement in sensing performance, including the use of metal-organic frameworks (MOFs), covalent-organic frameworks (COFs), and porous organic polymers (POPs), as well as their hybrid counterparts. Overall, CGSs using porous materials have the potential to address a wide range of applications, including monitoring water quality, detecting harmful chemicals, improving surveillance, preventing natural disasters, and improving healthcare.

Smart-Responsive HOF Heterostructures with Multiple Spatial-Resolved Emission Modes toward Photonic Security Platform

Luminescent hydrogen-bonded organic frameworks (HOFs) with the unique dynamics and versatile functional sites hold great potential application in information security, yet most of responsive HOFs focus on the single-component framework with restrained emission control, limiting further applications in advanced confidential information protection. Herein, the first smart-responsive HOF heterostructure with multiple spatial-resolved emission modes for covert photonic security platform is reported. The HOF heterostructures are prepared by integrating different HOFs into a single microwire based on a hydrogen-bond-assisted epitaxial growth method. The distinct responsive behaviors of HOFs permit the heterostructure to simultaneously display the thermochromism via the framework transformation and the acidichromism via the protonation effect, thus generating multiple emission modes. The dual stimuli-controlled spatial-resolved emission modes constitute the fingerprint of a heterostructure, and enable the establishment of the smart-responsive photonic barcode with multiple convert states, which further demonstrate the dynamic coding capability and enhanced security in anticounterfeiting label applications. These results offer a promising route to design function-oriented smart responsive HOF microdevices toward advanced anticounterfeiting applications.

Hierarchical Self-Assembly of Curved Aromatics: From Donor-Acceptor Porphyrins to Triply Periodic Minimal Surfaces

A saddle-shaped π-extended zinc porphyrin containing a peripheral pyridyl ligand undergoes quantitative self-assembly into a cyclic trimer. The trimer has a prismatic structure with negatively curved side walls, which promote the formation of supramolecular organic frameworks stabilized by dispersion interactions. The first framework type, UWr-1, has the npo topology, with a hexagonal structure analogous to the Schwartz H triply periodic minimal surface. Co-crystallization of the trimer with either C60 and C70 produces the isomorphous cubic UWr-2 and UWr-3 phases, characterized by the ctn network topology and a structural relationship to the Fischer-Koch minimal surface S. All three phases contain complex labyrinths of solvent-filled channels, corresponding to very large probe-accessible volumes (68 % to 76 %). The UWr-2 network could be partly desolvated while retaining its long range dimensional order, indicating remarkable strength of the dispersion interactions in the crystal. A theoretical analysis of noncovalent interactions shows the role of geometrical matching between the negatively curved porphyrin units and positively curved fullerenes.

Solution processing of crystalline porous material based membranes for CO2 separation

The carbon emission problem is a significant challenge in today's society, which has led to severe global climate issues. Membrane-based separation technology has gained considerable interest in CO2 separation due to its simplicity, environmental friendliness, and energy efficiency. Crystalline porous materials (CPMs), such as zeolites, metal-organic frameworks, covalent organic frameworks, hydrogen-bonded organic frameworks, and porous organic cages, hold great promise for advanced CO2 separation membranes because of their ordered and customizable pore structures. However, the preparation of defect-free and large-area crystalline porous material (CPM)-based membranes remains challenging, limiting their practical use in CO2 separation. To address this challenge, the solution-processing method, commonly employed in commercial polymer preparation, has been adapted for CPM membranes in recent years. Nanosheets, spheres, molecular cages, and even organic monomers, depending on the CPM type, are dissolved in suitable solvents and processed into continuous membranes for CO2 separation. This feature article provides an overview of the recent advancements in the solution processing of CPM membranes. It summarizes the differences among the solution-processing methods used for forming various CPM membranes, highlighting the key factors for achieving continuous membranes. The article also summarizes and discusses the CO2 separation performance of these membranes. Furthermore, it addresses the current issues and proposes future research directions in this field. Overall, this feature article aims to shed light on the development of solution-processing techniques for CPM membranes, facilitating their practical application in CO2 separation.

Hydrogen-bonded organic frameworks in solution enables continuous and high-crystalline membranes

Hydrogen-Bonded organic frameworks (HOFs) are a type of emerging porous materials. At present, little research has been conducted on their solution state. This work demonstrates that HOFs fragment into small particles while maintaining their original assemblies upon dispersing in solvents, as confirmed by Cryo-electron microscopy coupled with 3D electron diffraction technology. 1D and 2D-Nuclear Magnetic Resonance (NMR) and zeta potential analyses indicate the HOF-based colloid solution and the isolated molecular solution have significant differences in intermolecular interactions and aggregation behavior. Such unique solution processibility allows for fabricating diverse continuous HOF membranes with high crystallinity and porosity through solution-casting approach on various substrates. Among them, HOF-BTB@AAO membranes show high C3H6 permeance (1.979 × 10-7 mol·s-1·m-2·Pa-1) and excellent separation performance toward C3H6 and C3H8 (SF = 14). This continuous membrane presents a green, low-cost, and efficient separation technology with potential applications in petroleum cracking and purification.

Self-Healing Hydrogen-Bonded Organic Frameworks for Low-Concentration Ammonia Capture

The self-healing behavior has been extensively used in intelligent sensing systems capable of molecular recognition. However, most rigid crystalline frameworks, once collapsed under external stimuli like pressure, heat, or vacuum, could hardly recover to their crystalline phases under ambient conditions. Here, we report the self-healing of a new microporous hydrogen-bonded organic framework, FDU-HOF-3 (FDU = Fudan University), for ammonia (NH3) capture and compared it with the established mesoporous HOF-101. With the introduction of low-concentration NH3 into the pores, the HOFs became disordered but were then simply heated under a vacuum to return to their original crystalline states after NH3 removal. Close characterizations revealed that the repeatable self-healing behavior of these HOFs was achieved due to the COOH-NH3 acid-base interactions accompanied by the breaking and regeneration of complementary COOH-COOH hydrogen bonds. FDU-HOF-3 showed a record-capturing capability for low-concentration NH3 (8.13 mmol/g at 25 mbar) among all HOFs and displayed a quick photocurrent decrease after exposure to 250 ppm NH3 for less than 10 s. These self-healing HOFs were used to capture and release NH3 for over 10 cycles without any decrease in the adsorption capacities.

Multistate structures in a hydrogen-bonded polycatenation non-covalent organic framework with diverse resistive switching behaviors

The inherent structural flexibility and reversibility of non-covalent organic frameworks have enabled them to exhibit switchable multistate structures under external stimuli, providing great potential in the field of resistive switching (RS), but not well explored yet. Herein, we report the 0D+1D hydrogen-bonded polycatenation non-covalent organic framework (HOF-FJU-52), exhibiting diverse and reversible RS behaviors with the high performance. Triggered by the external stimulus of electrical field E at room temperature, HOF-FJU-52 has excellent resistive random-access memory (RRAM) behaviors, comparable to the state-of-the-art materials. When cooling down below 200 K, it was transferred to write-once-read-many-times memory (WORM) behaviors. The two memory behaviors exhibit reversibility on a single crystal device through the temperature changes. The RS mechanism of this non-covalent organic framework has been deciphered at the atomic level by the detailed single-crystal X-ray diffraction analyses, demonstrating that the structural dual-flexibility both in the asymmetric hydrogen bonded dimers within the 0D loops and in the infinite π-π stacking column between the loops and chains contribute to reversible structure transformations between multi-states and thus to its dual RS behaviors.

Photoinduced radical formation in hydrogen-bonded organic frameworks

Hydrogen-bonded organic frameworks (HOFs) constructed from naphthalene-diimide bearing tectons undergo photochromic changes whilst forming radical bearing species within the framework structure.