Metalo Hydrogen-Bonded Organic Frameworks (MHOFs) as New Class of Crystalline Materials for Protonic Conduction

Harnessing Biomolecule-Infused 2D Multi-layered Luminescent Zn(II) Coordination Polymer for Electrochemical Energy Storage

Selecting the right functional linkers and metal centers is crucial for creating multifunctional crystalline coordination polymers, which show promise in energy storage applications. Herein, a new two-dimensional Zn(II)-based CP, named BPHCC-1, has been synthesized using solvothermal methods with 2-amino terephthalic acid (2ATA) and the biomolecule purine as key building blocks. Purine, which is relatively unexplored in CP synthesis, plays a crucial role in the distinct properties of CPs. BPHCC-1, obtained as a stable crystalline solid, was characterized through various analytical techniques including Fourier transform infrared spectroscopy, field emission scanning electron microscope, transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and Brunauer-Emmett-Teller analysis. The material's stability is attributed to extensive hydrogen bonding, π···π interactions, and coordination of the -NH2 group with the Zn(II) center. BPHCC-1 exhibits bright blue luminescence at 435 nm with a photoluminescence quantum yield of 29% in an aqueous dispersion. Furthermore, it demonstrates significant electrochemical energy storage performance, with a specific capacitance of 84 F g-1 at 3 A g-1 and retaining 64% of its original capacitance after 500 cycles. This study introduces a facile approach to designing multifunctional CPs, showcasing BPHCC-1's potential as a luminescent probe and pseudocapacitive supercapacitor. The findings highlight the versatility of BPHCC-1, suggesting broad opportunities for its use across diverse fields.

Proton Conduction via Water and Ammonia Coordinated Metal Cationic Species in MOF and MHOF Platforms

Although metal-organic frameworks (MOFs) and metalo hydrogen-bonded organic frameworks (MHOFs) are designed as promising solid-state proton conductors by incorporating various protonic species intrinsically or extrinsically, design and development of such materials by employing the concept of proton conduction through coordinated polar protic solvent is largely unexplored. Herein, we have constructed two proton-conducting materials having different solvent coordinated metal cationic species: In-H2O-MOF, ({[In(H2O)6][In3(Pzdc)6] ⋅ 15H2O}n; H2Pzdc: pyrazine-2,3-dicarboxylic acid) with coordinated water molecules from hexaaquaindium cationic species, and MHOF-4, ([{Co(NH3)6}2(2,6-NDS)2(H2O)2]n; 2,6-H2NDS: 2,6-naphthalenedisulfonic acid) with coordinated ammonia from hexaammoniacobalt cationic species. Interestingly, higher proton conductivity was achieved for In-H2O-MOF (1.5×10-5 S cm-1) than MHOF-4 (6.3×10-6 S cm-1) under the extreme conditions (80 °C and 95 % RH), which could be attributed to enhanced acidity of coordinated water molecules having much lower pKa value than that of coordinated ammonia. Greater charge polarization on hydrogen atoms of In3+-coordinated water molecules than that of Co2+-coordinated ammonia led to the high conductivity of In-H2O-MOF, as evident by quantum chemical studies. Such a comparative study on metal-coordinated protic polar solvents in achieving proton conduction in crystalline solids is yet to be made.

Metalo Hydrogen-Bonded Organic Frameworks Constructed by Coordinated Chains for Magnetic and Proton-Conductive Bifunctionality

Metalo hydrogen-bonded organic frameworks (MHOFs) have received growing interest in designing crystalline functional materials. However, reports on bifunctional MHOFs showing magnetic and proton-conductive properties are extremely limited and their design is challenging. Herein, we investigated the magnetic and proton-conductive properties of two sulfonated CoHOF and MnHOF, {M(H2O)2(abs)2}n (M = Co2+ and Mn2+, Habs = 4-aminoazobenzene-4'-sulfonic anion), constructed by coordination chains. The supramolecular frameworks sustained by H bonds between -SO3- and coordinated water show directional ladder-type H bonds with hydrophilic nanochannels, leading to high proton conduction with exceptionally high conductivity around 10-2 S cm-1 at 100 °C under 97% relative humidity. In particular, the maximum σ value of CoHOF, 2.11 × 10-2 S cm-1, recorded the highest value among the reported proton-conducting materials showing slow magnetic relaxation. Meanwhile, the molecular structure of organosulfonate enables the magnetic isolation of high-spin Co2+ and Mn2+ centers in the frameworks. Magnetic measurements indicated that the MHOFs show field-induced single-ion magnet (SIM) properties, making these compounds rare magnetic-proton-conductive MHOFs. The work provides not only two unique MHOFs with SIM behavior and high proton conduction performance but also avenues for designing stable bifunctional MHOFs via a coordination chain approach.

Rational Tuning the Proton Conductivity and Stability of Hydrogen-Bonded Organic Frameworks

In the development of proton conductors, it is crucial to regulate proton conduction pathways and enhance structural stability. In this study, we designed and constructed three hydrogen-bonded organic frameworks (HOFs), namely, NKM-HOF-9, NKM-HOF-10, and NKM-HOF-11, with different dimensional hydrogen-bonding pathways using 4,4'-sulfonyldibenzoic acid and various bases. They are cost-effective and easy to synthesize, allowing for their large-scale production at room temperature. By purposefully altering the ammonium ions, we achieved enhancements in the conductivity and stability of these HOFs. Proton conductivity studies at different humidities and temperatures revealed that at 85 °C and 98% relative humidity, the proton conductivity of NKM-HOF-10 reached 1.7 × 10-3 S cm-1, surpassing that of NKM-HOF-9 by 1 order of magnitude. This improvement was accomplished by increasing the number of proton donors from the base, which resulted in a transition of the hydrogen bond network from discontinuous to continuous, thereby enhancing the proton conduction performance. Moreover, stability tests showed that raising the base's pKa could improve the stability of these frameworks. NKM-HOF-11, which features the highest pKa, demonstrated superior stability by maintaining its structural integrity even at 450 °C.

Controlled crystallisation of porous crystals of luminescent platinum(II) complexes by electronic tuning of ancillary ligands

Porous molecular crystals (PMCs) have gained significant importance as next-generation functional porous materials. However, the selective crystallisation of the PMC phase remains a challenge. Herein, we have systematically controlled the stability of the luminescent PMC phase prepared using the luminescent Pt(II) complex [Pt(pbim)(N^O)] (pbim = 2-phenylbenzimidazolate, N^O = N-heteroaryl carboxylate) with Pt⋯Pt electronic interactions. The PMC phase formation varied significantly among the complexes depending on the heteroaryl group of the ancillary N^O ligand; the oxazolyl-bearing complex did not form a PMC phase, whereas the pyrazyl- and 5-fluoropyridyl-bearing complexes spontaneously formed a porous structure. This difference was rationalised by the π-stacking capability of the heteroaryl group of the ancillary ligand. Furthermore, owing to the presence of the one-dimensional Pt⋯Pt chains in this PMC phase, the photophysical properties of PMCs resulting from the Pt⋯Pt interactions were also significantly changed by the ancillary ligands.

Prominent Intrinsic Proton Conduction in Two Robust Zr/Hf Metal-Organic Frameworks Assembled by Bithiophene Dicarboxylate

To date, developing crystalline proton-conductive metal-organic frameworks (MOFs) with an inherent excellent proton-conducting ability and structural stability has been a critical priority in addressing the technologies required for sustainable development and energy storage. Bearing this in mind, a multifunctional organic ligand, 3,4-dimethylthiophene[2,3-b]thiophene-2,5-dicarboxylic acid (H2DTD), was employed to generate two exceptionally stable three-dimensional porous Zr/Hf MOFs, [Zr6O4(OH)4(DTD)6]·5DMF·H2O (Zr-DTD) and [Hf6O4(OH)4(DTD)6]·4DMF·H2O (Hf-DTD), using solvothermal means. The presence of Zr6 or Hf6 nodes, strong Zr/Hf-O bonds, the electrical influence of the methyl group, and the steric effect of the thiophene unit all contribute to their structural stability throughout a wide pH range as well as in water. Their proton conductivity was fully examined at various relative humidities (RHs) and temperatures. Creating intricate and rich H-bonded networks between the guest water molecules, coordination solvent molecules, thiophene-S, -COOH, and -OH units within the framework assisted proton transfer. As a result, both MOFs manifest the maximum proton conductivity of 0.67 × 10-2 and 4.85 × 10-3 S·cm-1 under 98% RH/100 °C, making them the top-performing proton-conductive Zr/Hf-MOFs. Finally, by combining structural characteristics and activation energies, potential proton conduction pathways for the two MOFs were identified.

High H2O-Assisted Proton Conduction in One Highly Stable Sr(II)-Organic Framework Constructed by Tetrazole-Based Imidazole Dicarboxylic Acid

Solid electrolyte materials with high structural stability and excellent proton conductivity (σ) have long been a popular and challenging research topic in the fuel cell field. This problem can be addressed because of the crystalline metal-organic frameworks' (MOFs') high structural stability, adjustable framework composition, and dense H-bonded networks. Herein, one highly stable Sr(II) MOF, {[Sr(H2tmidc)2(H2O)3]·4H2O}n (1) (H3tmidc = 2-(1H-tetrazolium-1-methylene)-1H-imidazole-4,5-dicarboxylic acid) was successfully fabricated, which was structurally characterized by single-crystal X-ray diffraction and electrochemically examined by the AC impedance determination. The results demonstrated that the σ of the compound manifested a positive dependence on temperature and humidity, and the optimal proton conductivity is as high as 1.22 × 10-2 S/cm under 100 °C and 98% relative humidity, which is at the forefront of reported MOFs with ultrahigh σ. The analysis of the proton conduction mechanism reveals that numerous tetrazolium groups, carboxyl groups, coordination, and crystallization water molecules in the framework are responsible for the high efficiency of proton transport. This work offers a fresh perspective on how to create novel crystalline proton conductive materials.

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.

Room-Temperature Superprotonic Conductivity beyond 10-1 S cm-1 in a Co(II) Coordination Polymer

An efficient design of crystalline solid-state proton conductors (SSPCs) is crucial for the progress of clean energy applications. Developing such materials to make them work at room temperature with a conductivity of ≥10-1 S cm-1 is of significant interest in terms of technical and commercial aspects. Utilizing the recently highlighted "coordinated-water-driven proton conduction" approach, herein, we have rationally synthesized two highly stable and scalable 1D Co(II) coordination polymers (CPs) as SSPCs, PCM-2 {[Co(bpy)(H2O)2(NO3)2]·H2O}n and PCM-3 {[Co2(bpy)2(SO4)2(H2O)6].4H2O}n, with distinct alignments in coordinated water and coordinated oxo-anions (nitrate and sulfate, respectively). The acidity of the metal-bound water molecules in PCM-2 is further enhanced through cooperative long-range continuous H bonds with coordinated Brønsted basic nitrates (proton acceptors), leading to ultrahigh superprotonic conductivities even at 25 °C (1.03 × 10-1 S cm-1 under 95% RH), and reached a maximum of 2.99 × 10-1 S cm-1 at 85 °C (95% RH). The conductivity at 25 °C is even higher than that of commercial Nafion 117 (6.74 × 10-2 S cm-1 at 100% RH). The absence of such an H-bonding interaction in PCM-3 (closed loops) resulted in a lesser conductivity of 5.87 × 10-5 S cm-1 (95% RH, 85 °C). PCM-2 represents the first example of SSPC exhibiting conductivity in the order 10-1 S cm-1 at ambient temperature (25 °C) with excellent recyclability.

Four Lanthanide(III) Metal-Organic Frameworks Fabricated by Bithiophene Dicarboxylate for High Inherent Proton Conduction

Currently, it is still a challenge to directly achieve highly stable metal-organic frameworks (MOFs) with superior proton conductivity solely through the exquisite design of ligands and the attentive selection of metal nodes. Inspired by this, we are intrigued by a multifunctional dicarboxylate ligand including dithiophene groups, 3,4-dimethylthieno[2,3-b]thiophene-2,5-dicarboxylic acid (H2DTD), and lanthanide ions with distinct coordination topologies. Successfully, four isostructural three-dimensional lanthanide(III)-based MOFs, [Ln2(DTD)3(DEF)4]·DEF·6H2O [LnIII = TbIII (Tb-MOF), EuIII (Eu-MOF), SmIII (Sm-MOF), and DyIII (Dy-MOF)], were solvothermally prepared, in which the effective proton transport will be provided by the coordinated or free solvent molecules, the crystalline water molecules, and the framework components, as well as a large number of highly electronegative S and O atoms. As expected, the four Ln-MOFs demonstrated the highest proton conductivities (σ) being 0.54 × 10-3, 3.75 × 10-3, 1.28 × 10-3, and 1.92 × 10-3 S·cm-1 for the four MOFs, respectively, at 100 °C/98% relative humidity (RH). Excitingly, Dy-MOF demonstrated an extraordinary ultrahigh σ of 1 × 10-3 S·cm-1 at 30 °C/98% RH. Additionally, the plausible proton transport mechanisms were emphasized.

From non-conductive MOF to proton-conducting metal-HOFs: a new class of reversible transformations induced by solvent-free mechanochemistry

Proton-conducting materials play an important role as solid electrolytes in electrochemical devices for energy storage and conversion, including proton exchange membrane fuel cells. Metal-organic frameworks (MOFs), covalent-organic frameworks (COFs) and more recently hydrogen-bonded organic frameworks (HOFs) have emerged as useful crystalline platforms for proton transport that provide high conductivity and enable insight into conduction pathways. Here, we present two new HOFs with high conductivity, reaching 2 × 10-2 S cm-1 at 60 °C and 75% relative humidity, obtained in reactions that represent a new class of reversible transformations of solids. The reactions are induced by solvent-free mechanochemistry and involve breaking of coordination linkages in a MOF and formation of extended hydrogen-bonded networks of metal-HOFs (MHOFs). This unprecedented class of MOF-to-MHOF transformations has been demonstrated using a non-conductive MOF (JUK-1) and formamidinium or methylammonium thiocyanates as solid reactants. Structural details of the solid-state reactions are revealed by powder X-ray diffraction and Rietveld refinements for the MHOF products. None of the attempts using conventional methods were successful in obtaining the MHOFs, emphasizing a unique role of mechanochemical stimuli in the reactivity of supramolecular polymer solids, including crystalline MOFs and HOFs. The reversible nature of non-covalent interactions in such materials may be utilized for the development of healable polymer systems.

Comparative Study on Proton Conductivity and Mechanism Analysis of Two Imidazole Modified Imine-Based Covalent Organic Frameworks

This work elucidates the potential impact of intramolecular H-bonds within the pore walls of covalent organic frameworks (COFs) on proton conductivity. Employing DaTta and TaTta as representative hosts, it was observed that their innate proton conductivities (σ) are both unsatisfactory and σ(DaTta)<σ(TaTta). Intriguingly, the performance of both imidazole-loaded products, Im@DaTta and Im@TaTta is greatly improved, and the σ of Im@DaTta (0.91×10-2  S cm-1 ) even surpasses that of Im@TaTta (3.73×10-3  S cm-1 ) under 100 °C and 98 % relative humidity. The structural analysis, gas adsorption tests, and activation energy calculations forecast the influence of imidazole on the H-bonded system within the framework, leading to observed changes in proton conductivity. It is hypothesized that intramolecular H-bonds within the COF framework impede efficient proton transmission. Nevertheless, the inclusion of an imidazole group disrupts these intramolecular bonds, leading to the formation of an abundance of intermolecular H-bonds within the pore channels, thus contributing to a dramatic increase in proton conductivity. The related calculation of Density Functional Theory (DFT) provides further evidence for this inference.

Dynamic Hydrogen-Bonded Zinc Adeninate Framework (ZAF) for Immobilization of Catalytic DNA

Effective immobilization and delivery of genetic materials is at the forefront of biological and medical research directed toward tackling scientific challenges such as gene therapy and cancer treatment. Herein we present a biologically inspired hydrogen-bonded zinc adeninate framework (ZAF) consisting of zinc adeninate macrocycles that self-assemble into a 3D framework through adenine-adenine interactions. ZAF can efficiently immobilize DNAzyme with full protection against enzyme degradation and physiological conditions until it is successfully delivered into the nucleus. As compared to zeolitic imidazolate frameworks (ZIFs), ZAFs are twofold more biocompatible with a significant loading efficiency of 96 %. Overall, our design paves the way for expanding functional hydrogen-bonding-based systems as potential platforms for the loading and delivery of biologics.

Synthesis and In Situ Monitoring of Mechanochemical Preparation of Highly Proton Conductive Hydrogen-Bonded Metal Phosphonates

Crystalline porous materials are recognized as promising proton conductors for the proton exchange membrane (PEM) in fuel cell technology owing to their tunable framework structure. However, it is still a challenging bulk synthesis for real-world applications of these materials. Herein, we report the mechanochemical gram-scale synthesis of two isostructural metal hydrogen-bonded organic frameworks (MHOFs) of Co(II) and Ni(II) based on 1-hydroxyethylidenediphosphonic acid (HEDPH₄) with 2,2'-bipyridine (2,2'-bipy): Co(HEDPH₃)₂(2,2'-bipy)·H₂O (1) and Ni(HEDPH₃)₂(2,2'-bipy)·H₂O (2). In situ monitoring of the mechanochemical synthesis using different synchrotron-based techniques revealed a one-step mechanism - the starting materials are directly converted to the product. With the existence of extensive hydrogen bonds with amphiprotic uncoordinated phosphonate hydroxyl and oxygen atoms, both frameworks exhibited proton conduction in the range of 10^-4 S cm^-1 at room temperature under humid conditions. This study demonstrates the potential of green mechanosynthesis for bulk material preparation of framework-based solid-state proton conductors.

Application of Hydrogen-Bonded Organic Frameworks in Environmental Remediation: Recent Advances and Future Trends

The hydrogen-bonded organic frameworks (HOFs) are a class of porous materials with crystalline frame structures, which are self-assembled from organic structures by hydrogen bonding in non-covalent bonds π-π packing and van der Waals force interaction. HOFs are widely used in environmental remediation due to their high specific surface area, ordered pore structure, pore modifiability, and post-synthesis adjustability of various physical and chemical forms. This work summarizes some rules for constructing stable HOFs and the synthesis of HOF-based materials (synthesis of HOFs, metallized HOFs, and HOF-derived materials). In addition, the applications of HOF-based materials in the field of environmental remediation are introduced, including adsorption and separation (NH3, CO2/CH4 and CO2/N2, C2H2/C2He and CeH6, C2H2/CO2, Xe/Kr, etc.), heavy metal and radioactive metal adsorption, organic dye and pesticide adsorption, energy conversion (producing H2 and CO2 reduced to CO), organic dye degradation and pollutant sensing (metal ion, aniline, antibiotic, explosive steam, etc.). Finally, the current challenges and further studies of HOFs (such as functional modification, molecular simulation, application extension as remediation of contaminated soil, and cost assessment) are discussed. It is hoped that this work will help develop widespread applications for HOFs in removing a variety of pollutants from the environment.

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.

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.

Proton Conduction at High Temperature in High-Symmetry Hydrogen-Bonded Molecular Crystals of RuIII Complexes with Six Imidazole-Imidazolate Ligands

A new H-bonded crystal [RuIII (Him)3 (Im)3 ] with three imidazole (Him) and three imidazolate (Im- ) groups was prepared to obtain a higher-temperature proton conductor than a Nafion membrane with water driving. The crystal is constructed by complementary N-H⋅⋅⋅N H-bonds between the RuIII complexes and has a rare Icy-c* cubic network topology with a twofold interpenetration without crystal anisotropy. The crystals show a proton conductivity of 3.08×10-5  S cm-1 at 450 K and a faster conductivity than those formed by only HIms. The high proton conductivity is attributed to not only molecular rotations and hopping motions of HIm frameworks that are activated at ∼113 K, but also isotropic whole-molecule rotation of [RuIII (Him)3 (Im)3 ] at temperatures greater than 420 K. The latter rotation was confirmed by solid-state 2 H NMR spectroscopy; probable proton conduction routes were predicted and theoretically considered.

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.

Hybrid Photochromic Lanthanide Phosphonate with Multiple Photoresponsive Functionalities

Hybrid photochromic materials (HPMs) with specific photoresponsive functionality have applications in many fields. The photoinduced electron-transfer (ET) strategy has been proved to be effective in the synthesis of HPMs with diverse photomodulated properties. The exploitation of new electron acceptors (EAs) is meaningful for promoting the development of HPMs. In this work, we introduced a rigid tetraimidazole derivative, 3,3,5,5-tetra(imidazol-1-yl)-1,1-biphenyl (TIBP) as a potential EA, into a metal-diphosphonate (1-hydroxyethylidene-1,1-diphosphonic acid, H₄-HEDP) system to explore HPMs and finally obtained a hybrid metal phosphonate (H₄-TIBP)_0.5·[Dy(H-HEDP) (H₂-HEDP)]·H₂O (1). 1 features anionic chains composed of diphosphonate and Dy³⁺ ions. The extra charge is balanced by protonated TIBP cations, which exist in the void of adjacent chains and form H-bonds with O_phosphonate (N-H···O). Upon photostimulation with a Xe lamp (300 W), the crystalline sample 1 exhibited coloration by changing from colorless to pale yellow because of the presence of photoinduced radicals that originated from the ET from O_phosphonate to N_TIBP. Along with the coloration, photomodulated fluorescence, magnetism, and proton conductivity were also detected in the photoactivated samples. Different from the reported HPMs based on polypyridine derivatives and photoactive species such as pyridinium and naphthalimide derivatives as EAs, our study provides a new category of EA units to yield HPMs with fascinating photoresponsive functionality via the assembly of polyimidazole derivatives and phosphonate-based supramolecular building blocks.

HOFs Built from Hexatopic Carboxylic Acids: Structure, Porosity, Stability, and Photophysics

Hydrogen-bonded organic frameworks (HOFs) have attracted renewed attention as another type of promising candidates for functional porous materials. In most cases of HOF preparation, the applied molecular design principle is based on molecules with rigid π-conjugated skeleton together with more than three H-bonding groups to achieve 2D- or 3D-networked structures. However, the design principle does not always work, but results in formation of unexpected structures, where subtle structural factors of which we are not aware dictate the entire structure of HOFs. In this contribution, we assess recent advances in HOFs, focusing on those composed of hexatopic building block molecules, which can provide robust frameworks with a wide range of topologies and properties. The HOFs described in this work are classified into three types, depending on their H-bonded structural motifs. Here in, we focus on: (1) the chemical aspects that govern their unique fundamental chemistry and structures; and (2) their photophysics at the ensemble and single-crystal levels. The work addresses and discusses how these aspects affect and orient their photonic applicability. We trust that this contribution will provide a deep awareness and will help scientists to build up a systematic series of porous materials with the aim to control both their structural and photodynamical assets.

A mixed strategy to fabricate two bifunctional ligand-based Ag-complexes with high proton conductivity

High proton conductivity materials BAg-1 and BAg-2 were obtained using a mixed strategy with the same main bifunctional ligand.

Multiple Strategies to Fabricate a Highly Stable 2D CuIICuI-Organic Framework with High Proton Conductivity

Using multifunctional organic ligands with multiple acidic groups (carboxylate and sulfonate groups) to synthesize metal-organic frameworks (MOFs) bearing effective H-bond networks is a promising strategy to obtain highly proton conductive materials. In this work, a highly stable two-dimensional MOF, [Cu^II₅Cu^I₂(μ₃-OH)₄(H₂O)₆(L)₂(H₂L)₂]·3H₂O (denoted as YCu161; H₃L = 6-sulfonaphthalene-1,4-dicarboxylic acid) containing mixed-valence [Cu^II₅Cu^I₂(μ₃-OH)₄]⁸⁺ subunits, was successfully prepared. It exhibited excellent stability and temperature- and humidity-dependent proton conduction properties. Its optimal proton conductivity reached 1.84 × 10^-3 S cm^-1 at 90 °C and 98% relative humidity. On the basis of a crystal structure analysis, water vapor adsorption test results, and activation energy calculations, we deduced the proton conduction pathway and mechanism. Apparently, uncoordinated sulfonic and carboxyl groups and a network of abundant H-bonds inside the framework were responsible for the efficient proton transfer. Therefore, the strategy of selecting suitable bifunctional ligands to construct two-dimensional Cu-cluster-based MOFs with excellent proton conductivity is feasible.

Multifunctional Viologen-Derived Supramolecular Network with Photo/Vapochromic and Proton Conduction Properties

A supramolecular network [H₄bdcbpy(NO₃)₂·H₂O] (H₄bdcbpy = 1,1′-Bis(3,5-dicarboxybenzyl)-4,4′-bipyridinium) (1) was prepared by a zwitterionic viologen carboxylate ligand in hydrothermal synthesis conditions. The as-synthesized (1) has been well characterized by means of single-crystal/powder X-ray diffraction, elemental analysis, thermogravimetric analysis and infrared and UV-vis spectroscopy. This compound possesses a three-dimensional supramolecular structure, formed by the hydrogen bond and π–π interaction between the organic ligands. This compound shows photochromic properties under UV light, as well as vapochromic behavior upon exposure to volatile amines and ammonia, in which the electron transfer from electron-rich parts to the electron-deficient viologen unit gives rise to colored radicals. Moreover, the intensive intermolecular H-bonding networks in 1 endows it with a proton conductivity of 1.06 × 10⁻³ S cm⁻¹ in water at 90 °C.

High and Tunable Proton Conduction in Six 3D-Substituted Imidazole Dicarboxylate-Based Lanthanide-Organic Frameworks

Six isostructural three-dimensional (3D) Ln(III)-organic frameworks, {[Ln₂(HMIDC)₂(μ₄-C₂O₄)(H₂O)₃]·4H₂O}_n [Ln^III = Gd^III (1), Eu^III (2), Sm^III (3), Nd^III (4), Pr^III (5), and Ce^III (6)], have been fabricated by using a multifunctional ligand of 2-methyl-1H-imidazole-4,5-dicarboxylic acid (H₃MIDC). Ln-metal-organic frameworks (MOFs) 1-6 present 3D structures and possess abundant H-bonded networks between imidazole-N atoms and coordinated and free water molecules. All the six Ln-MOFs demonstrate humidity- and temperature-dependent proton conductivity (σ) having the optimal values of 2.01 × 10^-3, 1.40 × 10^-3, 0.93 × 10^-3, 2.25 × 10^-4, 1.11 × 10^-4, and 0.96 × 10^-4 S·cm^-1 for 1-6, respectively, at 100 °C/98% relative humidity, in the order of Ce^III (6) < Pr^III (5) < Nd^III (4) < Sm^III (3) < Eu^III (2) < Gd^III (1). In particular, the σ for 1 is 1 order of magnitude higher than that for 6, and it enhances systematically according to the decreasing order of the ionic radius, indicating that the lanthanide-contraction tactics can effectively regulate the proton conductivity while retaining the proton conduction routes. This will offer valuable guidance for the acquisition of new proton-conducting materials. In addition, the outstanding water stability and electrochemical stability of such Ln-MOFs will afford a solid material basis for future applications.

Multifunctional ZnII–LnIII (Ln = Tb, Dy) complexes based on the amine-phenol ligand with field-induced slow magnetic relaxation, luminescence, and proton conduction

Two new isomorphic Zn^II–Ln^III–Zn^II trinuclear complexes based on the amine-phenol ligand have been prepared.

Stable hydrogen-bonded organic frameworks for selective fluorescence detection of Al3+ and Fe3+ ions

Two pairs of HOFs were prepared with H4TCPE ligand under different conditions, and 3 and 4 have high stability and exhibit fluorescence quenching and enhancement toward Fe3+ and Al3+ ions, respectively.

Structural, optical, dielectric and electrical transport properties of a [Mg(H2O)6]2+-templated proton conducting, semiconducting and photoresponsive 3D hydrogen bonded supramolecular framework

{[Co(2,5-Pdc)2,(H2O)2]2−·[Mg(H2O)6]2+·4(H2O)} (where 2,5-pdc = 2,5-pyridinedicarboxylate): a proton conducting semiconducting photoresponsive [Mg(H2O)6]2+ templated 3D hydrogen bonded supramolecular framework (HSF).

Metal–organic frameworks derived from a semi-rigid anthracene-based ligand and sulfonates: proton conductivity and dye degradation studies

The MOFs were constructed by ligand AHP and sulfonate analogues. MOF4 exhibits a high proton conductivity of 1.95 × 10⁻³ S cm⁻¹ at 95 °C and 98% relative humidity. MOFs 1–5 also serve as photocatalysts for methylene blue degradation.

A "Thermodynamically Stable" 2D Nickel Metal-Organic Framework over a Wide pH Range with Scalable Preparation for Efficient C2 s over C1 Hydrocarbon Separations

The design and construction of "thermodynamically stable" metal-organic frameworks (MOFs) that can survive in liquid water, boiling water, and acidic/basic solutions over a wide pH range is highly desirable for many practical applications, especially adsorption-based gas separations with obvious scalable preparations. Herein, a new thermodynamically stable Ni MOF, {[Ni(L)(1,4-NDC)(H₂ O)₂ ]}_n (IITKGP-20; L=4,4'-azobispyridine; 1,4-NDC=1,4-naphthalene dicarboxylic acid; IITKGP stands for the Indian Institute of Technology Kharagpur), has been designed that displays moderate porosity with a BET surface area of 218 m²  g^-1 and micropores along the [10-1] direction. As an alternative to a cost-intensive, cryogenic, high-pressure distillation process for the separation of hydrocarbons, MOFs have recently shown promise for such separations. Thus, towards an application standpoint, this MOF exhibits a higher uptake of C₂ hydrocarbons over that of C₁ hydrocarbon under ambient conditions, with one of the highest selectivities based on the ideal adsorbed solution theory (IAST) method. A combination of two strategies (the presence of stronger metal-N coordination of the spacer and the hydrophobicity of the aromatic moiety of the organic ligand) possibly makes the framework highly robust, even stable in boiling water and over a wide range of pH 2-10, and represents the first example of a thermodynamically stable MOF displaying a 2D structural network. Moreover, this material is easily scalable by heating the reaction mixture at reflux overnight. Because such separations are performed in the presence of water vapor and acidic gases, there is a great need to explore thermodynamically stable MOFs that retain not only structural integrity, but also the porosity of the frameworks.

Hydrogen-Bonded Organic Frameworks as a Tunable Platform for Functional Materials

As a novel class of porous crystalline materials, hydrogen-bonded organic frameworks (HOFs), self-assembled from organic or metal-organic building blocks through intermolecular hydrogen-bonding interactions, have attracted more and more attention. Over the past decade, a number of porous HOFs have been constructed through judicious selection of H-bonding motifs, which are further enforced by other weak intermolecular interactions such as π-π stacking and van der Waals forces and framework interpenetration. Since the H-bonds are weaker than coordinate and covalent bonds used for the construction of metal-organic frameworks (MOFs) and covalent organic frameworks (COFs), HOFs have some unique features such as mild synthesis condition, solution processability, easy healing, and regeneration. These features enable HOFs to be a tunable platform for the construction of functional materials. Here, we review the H-bonding motifs used for constructing porous HOFs and highlight some of their applications, including gas separation and storage, chiral separation and structure determination, fluorescent sensing, heterogeneous catalysis, biological applications, proton conduction, photoluminescent materials, and membrane-based applications.

Metal hydrogen-bonded organic frameworks: structure and performance

Although great progress has been made in the design, synthesis, and performance expansion of porous materials, new porous materials with stable structures still need to be explored further. In recent years, porous molecular crystals formed by intermolecular interactions have attracted wide attention from chemists, especially metal hydrogen-bonded organic frameworks (M-HOFs) formed by connecting metal complexes through hydrogen bonds. Metal complexes with specific properties (e.g., magnetism, luminescence, sensing, and catalysis) can expand and develop the application of M-HOFs further. However, the huge volume, irregular shape, complex coordination modes, and interference of coordination bonds pose certain challenges in the synthesis and performance expansion of M-HOFs. In this frontier, we summarize the latest progress in the use of 3d, 4d, and 4f metal complexes for the synthesis of M-HOFs, and briefly introduce the performance expansion of these M-HOFs, which is expected to help expand new porous materials with stable structures and specific functions.

Proton conduction in two hydrogen-bonded supramolecular lanthanide complexes

Two hydrogen-bonded supramolecular lanthanide complexes based on imidazole dicarboxylate show different proton conductivities.

Liquid–liquid interface-promoted formation of a porous molecular crystal based on a luminescent platinum(ii) complex

A Pt(ii)-based luminescent porous molecular crystal was selectively crystallised at the liquid–liquid interface, allowing control of porosity and luminescence.

Polyoxometalate-based hydrogen-bonded organic frameworks as a new class of proton conducting materials

To develop new types of crystalline proton conducting materials for fuel cells, a polyoxometalate-based hydrogen-bonded organic framework (PHOF) based on Keggin-type [PMo₁₂O₄₀]³⁻ and phenylimidazole (PHOF 1) has been prepared.

Proton conduction in two Cu/Zn dimer-based hydrogen-bonded supramolecular frameworks from imidazole multi-carboxylate

At 98% RH and 100 °C, two dimer-based HSFs show the best σ values of 3.13 × 10⁻⁴ S cm⁻¹ for 1 and 0.55 × 10⁻⁴ S cm⁻¹ for 2, which can be compared to the values of reported HSFs.