Molecular Tectonics: A Node‐and‐Linker Building Block Approach to a Family of Hydrogen‐Bonded Frameworks

A three-component hydrogen bonded framework

A porous three-component hydrogen bonded framework, 1⋅biphen⋅TP, was prepared from a tetra-amidinium component (14+) and two different dianions, benzene-1,4-dicarboxylate (terephthalate, TP2-) and biphenyl-4,4'-dicarboxylate (biphen2-). Interestingly, when the framework was prepared in ethanol/water, 1⋅biphen⋅TP forms even when an excess of either dicarboxylate is present. However, when only water is used as solvent, only two-component frameworks are formed.

Porous isoreticular non-metal organic frameworks

Metal-organic frameworks (MOFs) are useful synthetic materials that are built by the programmed assembly of metal nodes and organic linkers1. The success of MOFs results from the isoreticular principle2, which allows families of structurally analogous frameworks to be built in a predictable way. This relies on directional coordinate covalent bonding to define the framework geometry. However, isoreticular strategies do not translate to other common crystalline solids, such as organic salts3-5, in which the intermolecular ionic bonding is less directional. Here we show that chemical knowledge can be combined with computational crystal-structure prediction6 (CSP) to design porous organic ammonium halide salts that contain no metals. The nodes in these salt frameworks are tightly packed ionic clusters that direct the materials to crystallize in specific ways, as demonstrated by the presence of well-defined spikes of low-energy, low-density isoreticular structures on the predicted lattice energy landscapes7,8. These energy landscapes allow us to select combinations of cations and anions that will form thermodynamically stable, porous salt frameworks with channel sizes, functionalities and geometries that can be predicted a priori. Some of these porous salts adsorb molecular guests such as iodine in quantities that exceed those of most MOFs, and this could be useful for applications such as radio-iodine capture9-12. More generally, the synthesis of these salts is scalable, involving simple acid-base neutralization, and the strategy makes it possible to create a family of non-metal organic frameworks that combine high ionic charge density with permanent porosity.

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.

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.

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.

Crystalline porous organic salts

Crystalline porous organic salts (CPOSs), formed by the self-assembly of organic acids and organic bases through ionic bonding, possess definite structures and permanent porosity and have rapidly emerged as an important class of porous organic materials in recent years. By rationally designing and controlling tectons, acidity/basicity (pKa), and topology, stable CPOSs with permanent porosity can be efficiently constructed. The characteristics of ionic bonds, charge-separated highly polar nano-confined channels, and permanent porosity endow CPOSs with unique physicochemical properties, offering extensive research opportunities for exploring their functionalities and application scenarios. In this review, we systematically summarize the latest progress in CPOS research, describe the synthetic strategies for synthesizing CPOSs, delineate their structural characteristics, and highlight the differences between CPOSs and hydrogen-bonded organic frameworks (HOFs). Furthermore, we provide an overview of the potential applications of CPOSs in areas such as negative linear compression (NLC), proton conduction, rapid transport of CO2, selective and rapid transport of K+ ions, atmospheric water harvesting (AWH), gas sorption, molecular rotors, fluorescence modulation, room-temperature phosphorescence (RTP) and catalysis. Finally, the challenges and future perspectives of CPOSs are presented.

Hydrogen-Bonding Assembly Meets Anion Coordination Chemistry: Framework Shaping and Polarity Tuning for Xenon/Krypton Separation

Hybrid hydrogen-bonded (H-bonded) frameworks built from charged components or metallotectons offer diverse guest-framework interactions for target-specific separations. We present here a study to systematically explore the coordination chemistry of monovalent halide anions, i.e., F- , Cl- , Br- , and I- , with the aim to develop hybrid H-bond synthons that enable the controllable construction of microporous H-bonded frameworks exhibiting fine-tunable surface polarity within the adaptive cavities for realistic xenon/krypton (Xe/Kr) separation. The spherical halide anions, especially Cl- , Br- , and I- , are found to readily participate in the charge-assisted H-bonding assembly with well-defined coordination behaviors, resulting in robust frameworks bearing open halide anions within the distinctive 1D pore channels. The activated frameworks show preferential binding towards Xe (IAST Xe/Kr selectivity ca. 10.5) because of the enhanced polarizability and the pore confinement effect. Specifically, dynamic column Xe/Kr separation with a record-high separation factor (SF=7.0) among H-bonded frameworks was achieved, facilitating an efficient Xe/Kr separation in dilute, CO2 -containing gas streams exactly mimicking the off-gas of spent nuclear fuel (SNF) reprocessing.

Neuroprotective Bioorthogonal Catalysis in Mitochondria Using Protein-Integrated Hydrogen-Bonded Organic Frameworks

Mitochondria-targeted bioorthogonal catalysis holds promise for controlling cell function precisely, yet achieving selective and efficient chemical reactions within organelles is challenging. In this study, we introduce a new strategy using protein-integrated hydrogen-bonded organic frameworks (HOFs) to enable synergistic bioorthogonal chemical catalysis and enzymatic catalysis within mitochondria. Utilizing catalytically active tris(4,4'-dicarboxylicacid-2,2'-bipyridyl) ruthenium(II) to self-assemble with [1,1'-biphenyl]-4,4'-biscarboximidamide, we synthesized nanoscale RuB-HOFs that exhibit high photocatalytic reduction activity. Notably, RuB-HOFs efficiently enter cells and preferentially localize to mitochondria, where they facilitate bioorthogonal photoreduction reactions. Moreover, we show that RuB-HOFs encapsulating catalase can produce hydrogen sulfide (H2 S) in mitochondria through photocatalytic reduction of pro-H2 S and degrade hydrogen peroxide through enzymatic catalysis simultaneously, offering a significant neuroprotective effect against oxidative stress. Our findings not only introduce a versatile chemical toolset for mitochondria-targeted bioorthogonal catalysis for prodrug activation but also pave the way for potential therapeutic applications in treating diseases related to cellular oxidative stress.

Controlling the Crystallisation and Hydration State of Crystalline Porous Organic Salts

Crystalline porous organic salts (CPOS) are a subclass of molecular crystals. The low solubility of CPOS and their building blocks limits the choice of crystallisation solvents to water or polar alcohols, hindering the isolation, scale-up, and scope of the porous material. In this work, high throughput screening was used to expand the solvent scope, resulting in the identification of a new porous salt, CPOS-7, formed from tetrakis(4-sulfophenyl)methane (TSPM) and tetrakis(4-aminophenyl)methane (TAPM). CPOS-7 does not form with standard solvents for CPOS, rather a hydrated phase (Hydrate2920) previously reported is isolated. Initial attempts to translate the crystallisation to batch led to challenges with loss of crystallinity and Hydrate2920 forming favorably in the presence of excess water. Using acetic acid as a dehydrating agent hindered formation of Hydrate2920 and furthermore allowed for direct conversion to CPOS-7. To allow for direct formation of CPOS-7 in high crystallinity flow chemistry was used for the first time to circumvent the issues found in batch. CPOS-7 and Hydrate2920 were shown to have promise for water and CO2 capture, with CPOS-7 having a CO2 uptake of 4.3 mmol/g at 195 K, making it one of the most porous CPOS reported to date.

Hydrogen-bonded organic frameworks: new horizons in biomedical applications

Hydrogen-bonded organic frameworks (HOFs) are an emerging attractive class of highly crystalline porous materials characterized by significant biocompatibility, rich chemical functionalities and well-defined porosity. The unique advantages including metal-free nature and reversible binding manner significantly distinguish HOFs from other porous materials in the biotechnology and biomedical field. However, the relevant HOF studies still remain in their infancy despite the promising and remarkable results that have been presented in recent years. Due to the intricate and dynamic nature of physiological conditions, the major challenge lies in the stability and structural diversity of HOFs in vivo. In this Tutorial Review, we summarize the common building blocks for the construction of HOF-based functional biomaterials and the latest developments in the biological field. Moreover, we highlight current challenges regarding the stability and functionalization of HOFs along with the corresponding potential solutions. This Tutorial Review will have a profound effect in future years on the design and applications of HOF-based biomaterials.

Selective Removal of Sulfate from Water by Precipitation with a Rigid Bis-amidinium Compound

A simple, readily prepared biphenyl bis-amidinium compound (1⋅Cl₂ ) is able to selectively precipitate sulfate from water. The precipitant is effective at concentrations as low as 1 mM and shows complete selectivity against monovalent anions, and high selectivity even against CO₃ ^2- and HPO₄ ^2- . It is highly effective (>90 % sulfate removed) in both seawater and highly acidic conditions relevant to mining waste-streams. X-ray crystallography reveals that 1⋅SO₄ forms a tightly packed, anhydrous, structure where each sulfate anion receives eight hydrogen bonds from amidinium N-H hydrogen bond donors.

Hybrid Hydrogen-Bonded Organic Frameworks: Structures and Functional Applications

As a new class of porous crystalline materials, hydrogen-bonded organic frameworks (HOFs) assembled from building blocks by hydrogen bonds have gained increasing attention. HOFs benefit from advantages including mild synthesis, easy purification, and good recyclability. However, some HOFs transform into unstable frameworks after desolvation, which hinders their further applications. Nowadays, the main challenges of developing HOFs lie in stability improvement, porosity establishment, and functionalization. Recently, more and more stable and permanently porous HOFs have been reported. Of all these design strategies, stronger charge-assisted hydrogen bonds and coordination bonds have been proven to be effective for developing stable, porous, and functional solids called hybrid HOFs, including ionic and metallized HOFs. This Review discusses the rational design synthesis principles of hybrid HOFs and their cutting-edge applications in selective inclusion, proton conduction, gas separation, catalysis and so forth.

Correspondence on "Crystalline Porous Organic Salt for Ultrarapid Adsorption/Desorption-Based Atmospheric Water Harvesting by Dual Hydrogen Bond System"

In a recent Research Article, Ben and co-workers reported a hydrogen-bonded framework prepared from a 4⁺ tetra-amidinium component and a 4^- tetra-sulfonate component, termed CPOS-6. They showed that CPOS-6 could reversibly adsorb and desorb water over a narrow humidity window, and that this material offered potential for applications in atmospheric water harvesting. This conclusion was supported by experiments that showed the material was stable over 50 adsorption/desorption cycles and that the kinetics of these cycles were very rapid. In this Correspondence we present additional structural data regarding this framework in both its hydrated and dehydrated states and thus discern the mechanism of water binding. These data do not disagree with Ben and co-workers' findings: rather they emphasise how remarkable the cyclability and rapid kinetics of adsorption/desorption are, as these processes involve a complete crystal-to-crystal rearrangement of the framework.

Multifunctional Porous Hydrogen-Bonded Organic Frameworks: Current Status and Future Perspectives

Hydrogen-bonded organic frameworks (HOFs), self-assembled from organic or metalated organic building blocks (also termed as tectons) by hydrogen bonding, π-π stacking, and other intermolecular interactions, have become an emerging class of multifunctional porous materials. So far, a library of HOFs with high porosity has been synthesized based on versatile tectons and supramolecular synthons. Benefiting from the flexibility and reversibility of H-bonds, HOFs feature high structural flexibility, mild synthetic reaction, excellent solution processability, facile healing, easy regeneration, and good recyclability. However, the flexible and reversible nature of H-bonds makes most HOFs suffer from poor structural designability and low framework stability. In this Outlook, we first describe the development and structural features of HOFs and summarize the design principles of HOFs and strategies to enhance their stability. Second, we highlight the state-of-the-art development of HOFs for diverse applications, including gas storage and separation, heterogeneous catalysis, biological applications, sensing, proton conduction, and other applications. Finally, current challenges and future perspectives are discussed.

Modulation of the Sorption Characteristics for an H-bonded porous Architecture by Varying the Chemical Functionalization of the Channel Walls

Five isostructural microporous supramolecular architectures prepared by H-bonded assembly between the hexa-anionic complex [Zr₂ (Ox)₇ ]^6- (Ox=oxalate, (C₂ O₄ )^2- ) and tripodal cations (H₃ -TripCH₂ -R)³⁺ with R=H, CH₃ , OH and OBn (Bn=CH₂ Ph) are reported. The possibility to obtain the same structure using a mixture of tripodal cations with different R group (R=OH and R=CH₃ ) has also been successfully explored, providing a unique example of three-component H-bonded porous framework. The resulting SPA-1(R) materials feature 1D pores decorated by R groups, with apparent pore diameters ranging from 3.0 to 8.5 Å. Influence of R groups on the sorption properties of these materials is evidenced through CO₂ and H₂ O vapor sorption/desorption experiments, as well as with I₂ capture/release experiments in liquid media. This study is one of the first to demonstrate the possibility of tuning the porosity and exerting precise control over the chemical functionalization of the pores in a given H-bonded structure, without modifying the topology of the reference structure, and thus finely adjusting the sorption characteristics of the material.

Tetrazolyl Porphyrin-Based Hydrogen-Bonded Organic Frameworks: Active Sites-Mediated Host-Guest Synergy for Advanced Antimicrobial Applications

Hydrogen-bonded organic frameworks (HOFs) with multiple functions and permanent pores have received widespread attention due to their potential applications in gas adsorption/separation, drug delivery, photocatalysis, proton conduction, and other fields. Herein, we constructed a three-dimensional (3D) HOF with 1D square channels by utilizing a dual-functional tetrazolyl porphyrin ligand bearing an active center of the porphyrin core and open sites of nitrogen atoms through π-π stacking and hydrogen-bonding interaction self-assembly. The structure exhibits both solvent resistance and thermal stability, and especially, maintains these after being transformed into nanoparticles. Meanwhile, the active sites exposed on the inner wall of the pores can interact well with the photoactive cationic dye molecules to form an effective host-guest (H-G) system, which can realize boosted photosensitized singlet oxygen (1O2) production under red light irradiation and synergistic sterilization toward Staphylococcus aureus (S. aureus) with an inhibition ratio as high as 99.9%. This work provides a valuable design concept for HOF-related systems in pursuit of promoted photoactivity.

Water Sorption Controls Extreme Single-Crystal-to-Single-Crystal Molecular Reorganization in Hydrogen Bonded Organic Frameworks

As hydrogen bonded frameworks are held together by relatively weak interactions, they often form several different frameworks under slightly different synthesis conditions and respond dynamically to stimuli such as heat and vacuum. However, these dynamic restructuring processes are often poorly understood. In this work, three isoreticular hydrogen bonded organic frameworks assembled through charge-assisted amidinium⋅⋅⋅carboxylate hydrogen bonds (1^C/C , 1^Si/C and 1^Si/Si ) are studied. Three distinct phases for 1^C/C and four for 1^Si/C and 1^Si/Si are fully structurally characterized. The transitions between these phases involve extreme yet recoverable molecular-level framework reorganization. It is demonstrated that these transformations are related to water content and can be controlled by humidity, and that the non-porous anhydrous phase of 1^C/C shows reversible water sorption through single crystal to crystal restructuring. This mechanistic insight opens the way for the future use of the inherent dynamism present in hydrogen bonded frameworks.

Confining enzymes in porous organic frameworks: from synthetic strategy and characterization to healthcare applications

Enzymes are a class of natural catalysts with high efficiency, specificity, and selectivity unmatched by their synthetic counterparts and dictate a myriad of reactions that constitute various cascades in living cells. The development of suitable supports is significant for the immobilization of structurally flexible enzymes, enabling biomimetic transformation in the extracellular environment. Accordingly, porous organic frameworks, including metal organic frameworks (MOFs), covalent organic frameworks (COFs) and hydrogen-bonded organic frameworks (HOFs), have emerged as ideal supports for the immobilization of enzymes because of their structural features including ultrahigh surface area, tailorable porosity, and versatile framework compositions. Specially, organic framework-encased enzymes have shown significant enhancement in stability and reusability, and their tailorable pore opening provides a gatekeeper-like effect for guest sieving, which is beneficial for mimicking intracellular biocatalysis processes. This immobilization technique brings new insight into the development of next-generation enzyme materials and shows huge potential in healthcare applications, such as biomarker diagnosis, biostorage, and cancer and antibacterial therapies. In this review, we describe the state-of-the-art strategies for the structural immobilization of enzymes using the well-explored MOFs and burgeoning COFs and HOFs as scaffolds, with special emphasis on how these porous framework-confined technologies can provide a favorable microenvironment for mimicking natural biocatalysis. Subsequently, advanced characterization techniques for enzyme conformation, the effect of the confined microenvironment on the activity of enzymes, and the emerging healthcare applications will be surveyed.

Advances in the Structural Strategies of the Self-Assembly of Photoresponsive Supramolecular Systems

Photosensitive supramolecular systems have garnered attention due to their potential to catalyze highly specific tasks through structural changes triggered by a light stimulus. The tunability of their chemical structure and charge transfer properties provides opportunities for designing and developing smart materials for multidisciplinary applications. This review focuses on the approaches reported in the literature for tailoring properties of the photosensitive supramolecular systems, including MOFs, MOPs, and HOFs. We discuss relevant aspects regarding their chemical structure, action mechanisms, design principles, applications, and future perspectives.

Polyoxometalate-Incorporated Metal-Organic Network as a Heterogeneous Catalyst for Selective Oxidation of Aryl Alkenes

Selective oxidation of aryl alkenes is important for chemical synthesis reactions, in which the key lies in the rational design of efficient catalysts. Herein, four polyoxometalate (POM)-incorporated metal-organic networks, with the formulas of [Co(ttb)(H₂O)₃]₂[SiMo₁₂O₄₀]·2H₂O (1), [Co(ttb)(H₂O)₂]₂[SiW₁₂O₄₀]·8H₂O (2), [Zn(Httb)(H₂ttb)][BW₁₂O₄₀]·9H₂O (3) and {[Zn(H₂O)₃(ttb)]₄[Zn₃(H₂O)₆]}[H₃SiW_10.5Zn_1.5O₄₀]₂·24H₂O (4) (ttb = 1,3,5-tri(1,2,4-triazol-1-ylmethyl)-2,4,6-trimethylbenzene), were hydrothermally synthesized and structurally characterized. Structural analysis showed that compound 1 consists of a POM-encapsulated three-dimensional (3-D) supramolecular framework; compound 2 is composed of a POM-supported 3-D coordination network; and compounds 3-4 show POM-incorporated 3-D supramolecular networks. Using selective catalytic oxidation of styrene as the model reaction, compounds 1-4 as heterogeneous catalysts display excellent performance with the double advantages of high styrene conversion and benzaldehyde selectivity owing to the synergistic effect among POM anions and transition metal (TM) centers. Among them, compound 1 exhibits the highest performance with ca. 96% styrene conversion and ca. 99% benzaldehyde selectivity in 3 h. In addition, compound 1 also displays excellent substrate compatibility, good reusability, and structural stability. Thus, a plausible reaction pathway for the selective oxidation of styrene is proposed. This study on the structure-function relationship paves a way for the rational design of POM-based heterogeneous catalysts for important catalysis applications.

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

Hydrogen-bonded organic frameworks (HOFs), self-assembled from strategically pre-designed molecular tectons with complementary hydrogen-bonding patterns, are rapidly evolving into a novel and important class of porous materials. In addition to their common features shared with other functionalized porous materials constructed from modular building blocks, the intrinsically flexible and reversible H-bonding connections endow HOFs with straightforward purification procedures, high crystallinity, solution processability, and recyclability. These unique advantages of HOFs have attracted considerable attention across a broad range of fields, including gas adsorption and separation, catalysis, chemical sensing, and electrical and optical materials. However, the relatively weak H-bonding interactions within HOFs can potentially limit their stability and potential use in further applications. To that end, this Perspective highlights recent advances in the development of chemically and thermally robust HOF materials and systematically discusses relevant design rules and synthesis strategies to access highly stable HOFs.

Vibrational mode analysis of hydrogen-bonded organic frameworks (HOFs): synchrotron infrared studies

Hydrogen-bonded organic frameworks (HOFs) are a promising class of porous crystalline materials for gas sorption and gas separation technologies that can be constructed under mild synthetic conditions. In forming three-dimensional networks of flexible hydrogen bonds between donor/acceptor subunits, these materials have displayed high stability at elevated temperature and under vacuum. Although the structural properties of HOFs are commonly characterized by diffraction techniques, new complimentary methods to elucidate phase behaviour and host-guest interactions at the molecular level are sought, particularly those that can be applied under changing physical conditions or solvent environment. To this end, this study has applied synchrotron far-IR and mid-IR spectroscopy to probe the properties of two known and one new HOF system assembled from tetrahedral amidinium and carboxylate building blocks. All three frameworks produce feature-rich and resolved infrared profiles from 30 to 4000 cm^-1 that provide information on hydrogen-bonded water solvent networks and the HOF channel topography via lattice and torsional bands. Comparison of experimental peaks to frequencies and atomic displacements (eigenvectors) predicted by high-level periodic DFT calculations have allowed for the assignment of vibrational modes associated with the aforementioned physicochemical properties. Now compiled, the specific vibrational modes identified as common to charge-assisted hydrogen-bonding motifs, as well as low frequency lattice and torsional bands attributed to HOF pore morphology and water-of-hydration networks, can act as diagnostic features in future spectroscopic investigations of HOF properties, such as those toward the design and tuning of host-guest properties for targeted applications.

Combining a Genetically Engineered Oxidase with Hydrogen-Bonded Organic Frameworks (HOFs) for Highly Efficient Biocomposites

Enzymes incorporated into hydrogen‐bonded organic frameworks (HOFs) via bottom‐up synthesis are promising biocomposites for applications in catalysis and sensing. Here, we explored synthetic incorporation of d‐amino acid oxidase (DAAO) with the metal‐free tetraamidine/tetracarboxylate‐based BioHOF‐1 in water. N‐terminal enzyme fusion with the positively charged module Z_basic2 strongly boosted the loading (2.5‐fold; ≈500 mg enzyme g_material⁻¹) and the specific activity (6.5‐fold; 23 U mg⁻¹). The DAAO@BioHOF‐1 composites showed superior activity with respect to every reported carrier for the same enzyme and excellent stability during catalyst recycling. Further, extension to other enzymes, including cytochrome P450 BM3 (used in the production of high‐value oxyfunctionalized compounds), points to the versatility of genetic engineering as a strategy for the preparation of biohybrid systems with unprecedented properties.

Towards hydrogen and halogen bonded frameworks based on 3,5-bis(triazolyl)pyridinium motifs

Construction of supramolecular assemblies using hydrogen and halogen bonding between anions and the 3,5-bis(triazolyl)pyridinium motif was investigated.

Hybrid Nafion Membranes of Ionic Hydrogen-Bonded Organic Framework Materials for Proton Conduction and PEMFC Applications

As the high-power density and environmentally friendly energy resources, proton exchange membrane fuel cells (PEMFCs) have a promising future in portable power generation. Herein, the hybrid Nafion membranes of ionic hydrogen-bonded organic frameworks (iHOFs) for PEMFC applications are demonstrated. By adjusting the position of sulfonic groups on naphthalene disulfonic acid compounds, four iHOFs with different types of hydrogen bonds were synthesized successfully based on 1,1'-diamino-4,4'-bipyridylium and naphthalene disulfonic acid. The formation of hydrogen bond interactions between amino and sulfonate groups provides a rich hydrogen bond network, which makes such iHOFs have high conductivity, and the maximum value is 2.76 × 10^-3 S·cm^-1 at 100 °C and 98% RH. Besides, composite membrane materials were obtained by mixing Nafion and iHOFs, and the maximum proton conductivity values can achieve 1.13 × 10^-2 S·cm^-1 for 6%-iHOF-3/Nafion and 2.87 × 10^-3 S·cm^-1 for 6%-iHOF-4/Nafion membranes at 100 °C under 98% RH. Through the H₂/O₂ fuel cell performance test by using iHOF/Nafion as the solid electrolyte, the maximum power and current density values of hybrid membranes are 0.36 W·cm^-2 and 1.10 A·cm^-2 for 6%-iHOF-3/Nafion and 0.42 W·cm^-2 and 1.20 A·cm^-2 for 6%-iHOF-4/Nafion at 80 °C and 100% RH. This work provides a practicable approach for establishing high-performance proton exchange hybrid membranes by doping high proton-conducting iHOFs into the Nafion matrix.

A Hydrogen-Bonded Ravel Assembled by Anion Coordination

Anion-coordination-driven assembly (ACDA) is showing increasing power in the construction of anionic supramolecular architectures. Herein, by expanding the anion centers from oxoanion (phosphate or sulfate) to organic tris-carboxylates, an Archimedean solid (truncated tetrahedron) and a highly entangled, double-walled tetrahedron featuring a ravel topology have been assembled with tris-bis(urea) ligands. The results demonstrate the promising ability of tris-carboxylates as new anion coordination centers in constructing novel topologies with increasing complexity and diversity compared to phosphate or sulfate ions on account of the modifiable size and easy functionalization character of these organic anions.

Fluorescent Supramolecular Organic Frameworks Constructed by Amidinium-Carboxylate Salt Bridges

We report here a set of fluorescent supramolecular organic frameworks (SOFs) that incorporate aggregation-induced emission (AIE) units within their frameworks. The fluorescent SOFs of this study were constructed by linking the tetraphenylethylene (TPE)-based tetra(amidinium) cation TPE⁴⁺ and aromatic dicarboxylate anions through amidinium-carboxylate salt bridges. The resulting self-assembled structures are characterized by fluorescence quantum yields in the range of 4.6∼14 %. This emissive behavior is ascribed to a combination of electrostatic interactions and hydrogen bonds that operate in concert to impede motions that would otherwise lead to excited state energy dissipation. Single-crystal X-ray diffraction analyses revealed that the length of the dicarboxylate anion bridges has a considerable impact on the structural features of the resulting frameworks. Nevertheless, all SOFs prepared in the context of the present study were found to display emissive features characteristic of TPE-based AIE luminogens with only a modest dependence on the structural specifics being seen. The SOFs reported here could be reversibly "broken up" and "reformed" in response to acid/base stimuli. This reversible structural behavior is consistent with their SOF nature.

Amidinium⋯carboxylate frameworks: predictable, robust, water-stable hydrogen bonded materials

In the last few years, the amidinium⋯carboxylate interaction has emerged as a powerful tool for the relatively predictable construction of families of three dimensional hydrogen bonded organic frameworks. These frameworks can be prepared in water and are surprisingly stable, including to heating in polar organic solvents and water. This feature article describes the design and synthesis of these materials, discusses their structures and stability, and highlights their recent applications for enzyme encapsulation and as precursors for the synthesis of molecularly thin hydrogen bonded 2D nanosheets.

Room Temperature Hydrolysis of Benzamidines and Benzamidiniums in Weakly Basic Water

Benzamidinium compounds have found widespread use in both medicinal and supramolecular chemistry. In this work, we show that benzamidiniums hydrolyze at room temperature in aqueous base to give the corresponding primary amide. This reaction has a half-life of 300 days for unsubstituted benzamidinium at pH 9, but is relatively rapid at higher pH's (e.g., t_1/2 = 6 days at pH 11 and 15 h at pH 13). Quantum chemistry combined with first-principles kinetic modeling can reproduce these trends and explain them in terms of the dominant pathway being initiated by attack of HO^- on benzamidine. Incorporation of the amidinium motif into a hydrogen bonded framework offers a substantial protective effect against hydrolysis.

In-Situ Encapsulation of Protein into Nanoscale Hydrogen-Bonded Organic Frameworks for Intracellular Biocatalysis

Hydrogen-bonded organic frameworks (HOFs) are porous materials with great potential for biological applications. The self-assembly of HOFs and biomacromolecules, however, is challenging. We report herein the self-assembly of nanoscale HOFs (nHOFs) to encapsulate protein for intracellular biocatalysis. The self-assembly of tetrakis(4-amidiniumphenyl)methane and azobenzenedicarboxylate can encapsulate protein in situ to form protein@nHOFs under mild conditions. This strategy is applicable to proteins with different surface charge and molecular weight, showing a high protein encapsulation efficiency and minimal effect on protein activity. A cellular delivery study shows that the protein@TA-HOFs can efficiently enter cells and retain enzyme activity for biochemical catalysis in living cells for neuroprotection. Our strategy paves new avenues for interfacing nHOFs with biological settings and sheds light on expanding nHOFs as a platform for biomacromolecule delivery and disease treatment.

What's in an Atom? A Comparison of Carbon and Silicon-Centred Amidinium⋅⋅⋅Carboxylate Frameworks*

Despite their apparent similarity, framework materials based on tetraphenylmethane and tetraphenylsilane building blocks often have quite different structures and topologies. Herein, we describe a new silicon tetraamidinium compound and use it to prepare crystalline hydrogen bonded frameworks with carboxylate anions in water. The silicon-containing frameworks are compared with those prepared from the analogous carbon tetraamidinium: when biphenyldicarboxylate or tetrakis(4-carboxyphenyl)methane anions were used similar channel-containing networks are observed for both the silicon and carbon tetraamidinium. When terephthalate or bicarbonate anions were used, different products form. Insights into possible reasons for the different products are provided by a survey of the Cambridge Structural Database and quantum chemical calculations, both of which indicate that, contrary to expectations, tetraphenylsilane derivatives have less geometrical flexibility than tetraphenylmethane derivatives, that is, they are less able to distort away from ideal tetrahedral bond angles.

Monolayer nanosheets formed by liquid exfoliation of charge-assisted hydrogen-bonded frameworks

Hydrogen-bonded organic frameworks (HOFs) are a diverse and tunable class of materials, but their potential as free-standing two-dimensional nanomaterials has yet to be explored. Here we report the self-assembly of two layered hydrogen-bonded frameworks based on strong, charge-assisted hydrogen-bonding between carboxylate and amidinium groups. Ultrasound-assisted liquid exfoliation of both materials readily produces monolayer hydrogen-bonded organic nanosheets (HONs) with micron-sized lateral dimensions. The HONs show remarkable stability and maintain their extended crystallinity and monolayer structures even after being suspended in water at 80 °C for three days. These systems also exhibit efficient fluorescence quenching of an organic dye in organic solvents, superior to the quenching ability of the bulk frameworks. We anticipate that this approach will provide a route towards a diverse new family of molecular two-dimensional materials.

Direct enantioseparation of axially chiral 1,1′-biaryl-2,2′-diols using amidine-based resolving agents

Amidine-based optically active resolving agents for enantiomer separation of axially chiral 1,1′-biaryl-2,2′-diols have been developed. A strongly basic amidine bearing no substituents on its nitrogen atoms enables the formation of their diastereomeric salts upon being mixed with weakly acidic phenol derivatives. Enantiopure 1,1′-biaryl-2,2′-diols can be obtained in high yields after only one crystallization of their salts with the chiral amidine derived from dehydroabietic acid. X-ray crystallography revealed that the amidine moiety forms a salt with the phenol group and additional intermolecular NH/π interactions contribute to the efficient chiral recognition process.

Enantioseparation of atropisomeric biphenols using a chiral amidine derived from dehydroabietic acid was reported. Only one crystallization of their mixture gave pure diastereomeric salts of biphenols from racemate.

An Investigation of Five Component [3+2] Self-Assembled Cage Formation Using Amidinium···Carboxylate Hydrogen Bonds

The assembly of hydrogen bonded cages using amidinium···carboxylate hydrogen bonding interactions was investigated. A new tris-amidinium hydrogen bond donor tecton based on a tetraphenylmethane scaffold was prepared and its self-assembly with the terephthalate anion studied, and a new tricarboxylate hydrogen bond acceptor tecton was synthesised and its assembly with the 1,3-benzenebis(amidinium) hydrogen bond donor explored. In both cases, molecular modelling indicated that the formation of the cages was geometrically feasible and 1H NMR spectroscopic evidence was consistent with interactions between the components in competitive d6-DMSO solvent mixtures. DOSY NMR spectroscopy of both systems indicated that both components diffuse at the same rate as each other, and diffusion coefficients were consistent with cage formation, and with the formation of assemblies significantly larger than the individual components. An X-ray crystal structure showed that one of the assemblies did not have the desired cage structure in the solid state.

Crystalline supramolecular organic frameworks via hydrogen-bonding between nucleobases

We describe here the first crystalline hydrogen-bonded organic framework made from complementary guanine and cytosine nucleobases.

Crystalline Porous Organic Salts: From Micropore to Hierarchical Pores

Crystalline porous organic salts (CPOSs), as an emerging class of porous organic materials, combining the uniform microporous system and distinct polarized channels, have become a highly evolving field of important current interest. The unique ionic bond of a CPOS endows the confined channels with high polarity, making CPOSs distinct from other organic frameworks. CPOSs show many fascinating properties, such as proton conductivity and fast transport of polar molecules, which involve the interaction between highly polarized guest molecules and host frameworks. Substantial progress has been made in the synthesis and applications of CPOSs. Herein, an overview is provided to impart a comprehensive understanding of the link between the synthetic approaches and the resultant microporous structure, the structure-function correlation and the state-of-the-art applications of CPOSs. The enhanced mass-transport performance of hierarchically porous structure in combination with the intrinsic polarized channels of CPOSs is very promising to create new applications and contribute to a new research upsurge. The perspective to construct porous hierarchy within the crystalline porous organic salts is assessed and will open a new research avenue. In the conclusion, the current challenges on the synthesis, structural regulation, and applications of CPOSs and the future of hierarchically porous crystalline organic salts are discussed.

The charge-assisted hydrogen-bonded organic framework (CAHOF) self-assembled from the conjugated acid of tetrakis(4-aminophenyl)methane and 2,6-naphthalenedisulfonate as a new class of recyclable Brønsted acid catalysts

The acid-base neutralization reaction of commercially available disodium 2,6-naphthalenedisulfonate (NDS, 2 equivalents) and the tetrahydrochloride salt of tetrakis(4-aminophenyl)methane (TAPM, 1 equivalent) in water gave a novel three-dimensional charge-assisted hydrogen-bonded framework (CAHOF, F-1). The framework F-1 was characterized by X-ray diffraction, TGA, elemental analysis, and ¹H NMR spectroscopy. The framework was supported by hydrogen bonds between the sulfonate anions and the ammonium cations of NDS and protonated TAPM moieties, respectively. The CAHOF material functioned as a new type of catalytically active Brønsted acid in a series of reactions, including the ring opening of epoxides by water and alcohols. A Diels-Alder reaction between cyclopentadiene and methyl vinyl ketone was also catalyzed by F-1 in heptane. Depending on the polarity of the solvent mixture, the CAHOF F-1 could function as a purely heterogeneous catalyst or partly dissociate, providing some dissolved F-1 as the real catalyst. In all cases, the catalyst could easily be recovered and recycled.

Colossal Negative Linear Compressibility in Porous Organic Salts

Negative linear compressibility (NLC) is a common sense violation (that is, crystal phases expand in one or more directions under hydrostatic compression). The excellent NLC performance of crystal materials is intrinsically related to the geometric structure of its skeleton. Here, we discovered a crystalline porous organic salt (CPOS-1); high-pressure X-ray diffraction experiments reveal that the CPOS-1 shows colossal NLC (K_c = -90.7 T Pa^-1) behavior along the c axis. This incredible performance arises from the flexible "supramolecular spring" formed by the charge-enhanced N-H⁺···^-O-S hydrogen bond interaction between the anionic sulfonate and the cationic ammonium ion. Furthermore, we reveal the relationship between this rare NLC behavior and single crystal proton conductivity using high-pressure electrochemical impedance spectroscopy (EIS) method. We believe that NLC behavior research on such inexpensive and readily available porous organic materials is of great significance for accelerating the research and application of NLC materials, especially in organic system.

A three dimensional hydrogen bonded organic framework assembled through antielectrostatic hydrogen bonds

A 3D hydrogen bonded organic framework was assembled in water from tetraamidinium cations and antielectrostatically hydrogen bonded bicarbonate dimers.