Atomic observation and structural evolution of covalent organic framework rotamers.

Dynamic 3D covalent organic frameworks (COFs) have shown concerted structural transformation and adaptive gas adsorption due to the conformational diversity of organic linkers. However, the isolation and observation of COF rotamers constitute undergoing challenges due to their comparable free energy and subtle rotational energy barrier. Here, we report the atomic-level observation and structural evolution of COF rotamers by cryo-3D electron diffraction and synchrotron powder X-ray diffraction. Specifically, we optimize the crystallinity and morphology of COF-320 to manifest its coherent dynamic responses upon adaptive inclusion of guest molecules. We observe a significant crystal expansion of 29 vol% upon hydration and a giant swelling with volume change up to 78 vol% upon solvation. We record the structural evolution from a non-porous contracted phase to two narrow-pore intermediate phases and the fully opened expanded phase using n-butane as a stabilizing probe at ambient conditions. We uncover the rotational freedom of biphenylene giving rise to significant conformational changes on the diimine motifs from synclinal to syn-periplanar and anticlinal rotamers. We illustrate the 10-fold increment of pore volumes and 100% enhancement of methane uptake capacity of COF-320 at 100 bar and 298 K. The present findings shed light on the design of smarter organic porous materials to maximize host-guest interaction and boost gas uptake capacity through progressive structural transformation.

Reticular Chemistry and In Situ "One-Pot" Strategy: A Dream Combination to Construct Metal-Organic Frameworks.

The establishment of reticular chemistry has significantly facilitated the development of porous materials, especially for metal-organic frameworks (MOFs). On the other hand, as an alternative approach, in situ "one-pot" strategy has been explored as a promising approach to constructing MOFs, in which the synthesis of organic linkers and the sequential construction of MOFs are integrated into one solvothermal condition. This strategy can efficiently avoid the limitations faced in the traditional construction method, such as time-consuming organic synthesis and multiple separation and purification. Herein, inspired by the reaction of aldehydes and o-phenylenediamine and deep structural analysis of UiO-68, a series of tetra-, hexa-, and octa-topic carboxylic acids are synthesized using 2',3'-diamino-[1,1':4',1'"-terphenyl]-4,4'"-dicarboxylic acid and di-, tri-, and tetra-topic aldehydes as precursor. Then nine multicarboxylate-based zirconium MOFs (Zr-MOFs) are successfully constructed via the combination of reticular chemistry and in situ "one-pot" strategy. The resultant Zr-MOFs can be regarded as the partial face decoration of UiO-68. More importantly, the emission properties of resultant Zr-MOFs can be well controlled using aldehydes with tunable electronic structures. This work provides a new path to rational design and construction of porous materials with specific structures guided by reticular chemistry and conducted using in situ "one-pot" strategy.

Reticular Chemistry Directed "One-Pot" Strategy to in situ Construct Organic Linkers and Zirconium-Organic Frameworks.

Zirconium-based metal-organic frameworks (Zr-MOFs) have emerged as one of the most studied MOFs due to the unlimited numbers of organic linkers and the varying Zr-oxo clusters. However, the synthesis of carboxylic acids, especially multitopic carboxylic acids, is always a great challenge for the discovery of new Zr-MOFs. As an alternative approach, the in situ "one-pot" strategy can address this limitation, where the generation of organic linkers from the corresponding precursors and the sequential construction of MOFs are integrated into one solvothermal condition. Herein, inspired by benzimidazole-contained compounds synthesized via reaction of aldehyde and o-phenylenediamine, tri-, tetra-, penta- and hexa-topic carboxylic acids and a series of corresponding Zr-MOFs can be prepared via the in situ "one-pot" method under the same solvothermal conditions. This strategy can be utilized not only to prepare reported Zr-MOFs constructed using benzimidazole-contained linkers, but also to rationally design, construct and realize functionalities of zirconium-pentacarboxylate frameworks guided by reticular chemistry. More importantly, in situ "one-pot" method can facilitate the discovery of new Zr-MOFs, such as zirconium-hexacarboxylate frameworks. The present study demonstrates the promising potential of benzimidazole-inspired in situ "one-pot" approach in the crystal engineering of structure- and property-specific Zr-MOFs, especially with the guidance of reticular chemistry.

Reticular Synthesis of Flexible Rare-Earth Metal-Organic Frameworks: Control of Structural Dynamics and Sorption Properties Through Ligand Functionalization.

An exciting direction in metal-organic frameworks involves the design and synthesis of flexible structures which can reversibly adapt their structure when triggered by external stimuli. Controlling the extent and nature of response in such solids is critical in order to develop custom dynamic materials for advanced applications. Towards this, it is highly important to expand the diversity of existing flexible MOFs, generating novel materials and gain an in-depth understanding of the associated dynamic phenomena, eventually unlocking key structure-property relationships. In the present work, we successfully utilized reticular chemistry for the construction of two novel series of highly crystalline, flexible rare-earth MOFs, RE-thc-MOF-2 and RE-teb-MOF-1. Extensive single-crystal to single-crystal structural analyses coupled with detailed gas and vapor sorption studies, shed light onto the unique responsive behavior. The development of these series is related to the reported RE-thc-MOF-1 solids which were found to display a unique continuous breathing and gas-trapping property. The synthesis of RE-thc-MOF-2 and RE-teb-MOF-1 materials represents an important milestone as they provide important insights into the key factors that control the responsive properties of this fascinating family of flexible materials and demonstrates that it is possible to control their dynamic behavior and the associated gas and vapor sorption properties.

Unprecedented POSS-Linked 3D Covalent Organic Frameworks with 2-Fold Interpenetrated scu or sqc Topology Regulated by Porphyrin Center for Photocatalytic CO2 Reduction.

The synthesis and characterization of porphyrin center regulated three-dimensional covalent organic frameworks (COFs) with 2-fold interpenetrated scu or sqc topology have been investigated. These COFs exhibit unique structural features and properties, making them promising candidates for photocatalytic applications in CO2 reduction and artemisinin synthesis. The porphyrin center serves as an anchor for metal ions, allowing precise control over structures and functions of the frameworks. Furthermore, the metal coordination within the framework imparts desirable catalytic properties, enabling their potential use in photocatalytic reactions. Overall, these porphyrin center regulated metal-controlled COFs offer exciting opportunities for the development of advanced materials with tailored functionalities.

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.

Precise Pore Engineering of Zirconium Metal-Organic Cages for One-Step Ethylene Purification from Ternary Mixtures.

One-step purification of ethylene from ternary mixtures (C2H2, C2H4, and C2H6) can greatly reduce the energy consumption of the separation process, but it is extremely challenging. Herein, we use crystal engineering and reticular chemistry to introduce unsaturated bonds (ethynyl and alkyne) into ligands, and successfully design and synthesized two novel Zr-MOCs (ZrT-1-ethenyl and ZrT-1-alkyne). The introduction of carbon-carbon unsaturated bonds provides abundant adsorption sites within the framework while modulating the pore window size. Comprehensive characterization techniques including single crystal and powder X-ray diffraction, as well as electrospray ionization time-of-flight mass spectrometry (ESI-TOF-MS) confirm that ZrT-1-ethenyl and ZrT-1-alkyne possess an isostructural framework with ZrT-1 and ZrT-1-Me, respectively. Adsorption isotherms and breakthrough experiments combined with theoretical calculations demonstrate that ZrT-1-ethenyl can effectively remove trace C2H2 and C2H6 in C2H4 and achieve separation of C2H2 from C2H4 and CO2. ZrT-1-ethenyl can also directly purify C2H4 in liquid solutions. This work provides a benchmark for MOCs that one-step purification of ethylene from ternary mixtures.

From Supramolecular Organic Cages to Porous Covalent Organic Frameworks for Enhancing Iodine Adsorption Capability by Fully Exposed Nitrogen-Rich Sites.

In order to overcome the limitations of supramolecular organic cages for their incomplete accessibility of active sites in the solid state and uneasy recyclability in liquid solution, herein a nitrogen-rich organic cage is rationally linked into framework systems and four isoreticular covalent organic frameworks (COFs), that is, Cage-TFB-COF, Cage-NTBA-COF, Cage-TFPB-COF, and Cage-TFPT-COF, are successfully synthesized. Structure determination reveals that they are all high-quality crystalline materials derived from the eclipsed packing of related isoreticular two-dimensional frameworks. Since the nitrogen-rich sites usually have a high affinity toward iodine species, iodine adsorption investigations are carried out and the results show that all of them display an enhancement in iodine adsorption capacities. Especially, Cage-NTBA-COF exhibits an iodine adsorption capacity of 304 wt%, 14-fold higher than the solid sample packed from the cage itself. The strong interactions between the nitrogen-rich sites and the adsorbed iodine species are revealed by spectral analyses. This work demonstrates that, utilizing the reticular chemistry strategy to extend the close-packed supramolecular organic cages into crystalline porous framework solids, their inherent properties can be greatly exploited for targeted applications.

Reticular Chemistry of the Fcu-Type Gd(III)-Doped Metal-Organic Framework for T1 -Weighted Magnetic Resonance Imaging.

Nanoscale metal-organic frameworks (nanoMOFs) are emerging as an important class of nanomaterials for the systematical investigation of biomedically relevant structure-property relationship (SPR) due to their highly tailorable features. In this work, the reticular chemistry approach is shown to explore the SPR of a fcu-type Zr(IV)-nanoMOF for T1 -weighted magnetic resonance imaging (MRI). Isoreticular replacement of the eight-coordinated square-antiprismatic Zr(IV) by nine-coordinated Gd(III) brings a stoichiometric water capped on the square-antiprismatic site, enabling the relaxation transfer in the inner-sphere, giving the r1 value of 4.55 mM-1 ·s-1 at the doping ratio of Gd : Zr = 1 : 1. Then, these isoreticular engineering studies provide feasible ways to facilitate the relaxation transfer in the second- and outer-sphere of the Gd(III)-doped Zr-oxo cluster for the relaxation respectively. Finally, these in vitro and in vivo MRI studies revealed that the Gd(III)-doped Zr-oxo cluster aggregated underlying the fcu-type framework surpasses its discrete molecular cluster for MRI. These results demonstrated that there is plenty of room inside MOFs for T1 -weighted MRI by reticular chemistry.

Single Crystalline, Non-stoichiometric Cocrystals of Hydrogen-Bonded Organic Frameworks.

Porous frameworks composed of non-stoichiometrically mixed multicomponent molecules attract much attention from a functional viewpoint. However, their designed preparation and precise structural characterization remain challenging. Herein, we demonstrate that cocrystallization of tetrakis(4-carboxyphenyl)hexahydropyrene and pyrene derivatives (CP-Hp and CP-Py, respectively) yields non-stoichiometric mixed frameworks through networking via hydrogen bonding. The composition ratio of CP-Hp and CP-Py in the framework was determined by single crystalline X-ray crystallographic analysis, indicating that the mixed frameworks were formed over a wide range of composition ratios. Furthermore, microscopic Raman spectroscopy on the single crystal indicates that the components are not uniformly distributed such as ideal solid solution, but are done gradationally or inhomogeneously.

Reticular Synthesis of One-Dimensional Covalent Organic Frameworks with 4-c sql Topology for Enhanced Fluorescence Emission.

Covalent organic frameworks (COFs) have been extensively investigated due to their unique structure, porosity, and functionality. However, at the topological level, COFs remain as two-dimensional (2D) or three-dimensional (3D) structures, while COFs with one-dimensional (1D) topology have not been systematically explored. In this work, we proposed a synthetic strategy for the construction of 1D-COFs based on non-linear edges and suitable high-symmetry vertices. Compared with their 2D-COFs counterparts, the 1D-COFs with AIEgens located at the vertex of the frame exhibited enhanced fluorescence. The density functional theory (DFT) calculations revealed that the dimensional-induced rotation restriction (DIRR) effect could spontaneously introduce additional non-covalent interactions between the strip frames, which could substantially diminish non-radiative transitions. This work also provides protocols for the design of 1D-COFs and a guidance scheme for the synthesis of emitting COFs.

Reticular Chemistry-Enabled Sonodynamic Activity of Covalent Organic Frameworks for Nanodynamic Cancer Therapy.

The development of covalent organic framework (COF) sonosensitizers with intrinsic sonodynamic effects is highly desirable. However, such COFs are generally constructed using small-molecule photosensitizers. Herein, we report that the reticular chemistry-based synthesis of COFs from two inert monomers yields a COF-based sonosensitizer (TPE-NN) with inherent sonodynamic activity. Subsequently, a nanoscale COF TPE-NN is fabricated and embedded with copper (Cu)-coordinated sites to obtain TPE-NN-Cu. Results show that Cu coordination can enhance the sonodynamic effect of TPE-NN, whereas ultrasound (US) irradiation for sonodynamic therapy can augment the chemodynamic efficacy of TPE-NN-Cu. Consequently, TPE-NN-Cu upon US irradiation shows high-performance anticancer effects based on mutually reinforced sono-/chemo-nanodynamic therapy. This study reveals the backbone-originated sonodynamic activity of COFs and proposes a paradigm of intrinsic COF sonosensitizers for nanodynamic therapy.

Tuning Pore Polarization to Boost Ethane/Ethylene Separation Performance in Hydrogen-Bonded Organic Frameworks.

Hydrogen-bonded organic frameworks (HOFs) show great potential in energy-saving C2 H6 /C2 H4 separation, but there are few examples of one-step acquisition of C2 H4 from C2 H6 /C2 H4 because it is still difficult to achieve the reverse-order adsorption of C2 H6 and C2 H4 . In this work, we boost the C2 H6 /C2 H4 separation performance in two graphene-sheet-like HOFs by tuning pore polarization. Upon heating, an in situ solid phase transformation can be observed from HOF-NBDA(DMA) (DMA=dimethylamine cation) to HOF-NBDA, accompanied with transformation of the electronegative skeleton into neutral one. As a result, the pore surface of HOF-NBDA has become nonpolar, which is beneficial to selectively adsorbing C2 H6 . The difference in the capacities for C2 H6 and C2 H4 is 23.4 cm3 g-1 for HOF-NBDA, and the C2 H6 /C2 H4 uptake ratio is 136 %, which are much higher than those for HOF-NBDA(DMA) (5.0 cm3 g-1 and 108 % respectively). Practical breakthrough experiments demonstrate HOF-NBDA could produce polymer-grade C2 H4 from C2 H6 /C2 H4 (1/99, v/v) mixture with a high productivity of 29.2 L kg-1 at 298 K, which is about five times as high as HOF-NBDA(DMA) (5.4 L kg-1 ). In addition, in situ breakthrough experiments and theoretical calculations indicate the pore surface of HOF-NBDA is beneficial to preferentially capture C2 H6 and thus boosts selective separation of C2 H6 /C2 H4 .

Olefin Metathesis in Confinement: Towards Covalent Organic Framework Scaffolds for Increased Macrocyclization Selectivity.

Covalent organic frameworks (COFs) offer vast structural and chemical diversity enabling a wide and growing range of applications. While COFs are well-established as heterogeneous catalysts, so far, their high and ordered porosity has scarcely been utilized to its full potential when it comes to spatially confined reactions in COF pores to alter the outcome of reactions. Here, we present a highly porous and crystalline, large-pore COF as catalytic support in α,ω-diene ring-closing metathesis reactions, leading to increased macrocyclization selectivity. COF pore-wall modification by immobilization of a Grubbs-Hoveyda-type catalyst via a mild silylation reaction provides a molecularly precise heterogeneous olefin metathesis catalyst. An increased macro(mono)cyclization (MMC) selectivity over oligomerization (O) for the heterogeneous COF-catalyst (MMC:O=1.35) of up to 51 % compared to the homogeneous catalyst (MMC:O=0.90) was observed along with a substrate-size dependency in selectivity, pointing to diffusion limitations induced by the pore confinement.

Photoresponsive Metal-Organic Frameworks as Adjustable Scaffolds in Reticular Chemistry.

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

Reticular Synthesis of Hydrogen-Bonded Organic Frameworks and Their Derivatives via Mechanochemistry.

Rational synthesis of hydrogen-bonded organic frameworks (HOFs) with predicted structure has been a long-term challenge. Herein, by using the efficient, simple, low-cost, and scalable mechanosynthesis, we demonstrate that reticular chemistry is applicable to HOF assemblies based on building blocks with different geometry, connectivity, and functionality. The obtained crystalline HOFs show uniform nano-sized morphology, which is challenging or unachievable for conventional solution-based methods. Furthermore, the one-pot mechanosynthesis generated a series of Pd@HOF composites with noticeably different CO oxidation activities. In situ DRIFTS studies indicate that the most efficient composite, counterintuitively, shows the weakest CO affinity to Pd sites while the strongest CO affinity to HOF matrix, revealing the vital role of porous matrix to the catalytic performance. This work paves a new avenue for rational synthesis of HOF and HOF-based composites for broad application potential.

Zeolite NPO-Type Azolate Frameworks.

Three-membered rings (3-rings) are an important structural motif in zeolite chemistry, but their formation remains serendipitous in reticular chemistry when designing zeolitic imidazolate frameworks (ZIFs). Herein, we report a design principle for constructing four new ZIFs, termed ZIF-1001 to -1004, from tetrahedral ZnII centers (T), benzotriazolate (bTZ), and different functionalized benzimidazolates (RbIM) that adopt a new zeolite NPO-type topology built from 3-rings. Two factors were critical for this discovery: i) incorporating the bTZ linker within the structures formed 3-rings due to a ∠(T-bTZ-T) angle of 120-130° reminiscent of the ∠(Ge-O-Ge) angle (130°) observed in germanate zeolite-type structures having 3-rings; and ii) RbIM guided the coordination chemistry of bTZ to bind preferentially in an imidazolate-type mode. This series' ability to selectively capture CO2 from high-humidity flue gas and trap ethane from tail gas during shale gas extraction was demonstrated.

The Current Status of MOF and COF Applications.

The amalgamation of different disciplines is at the heart of reticular chemistry and has broadened the boundaries of chemistry by opening up an infinite space of chemical composition, structure, and material properties. Reticular design has enabled the precise prediction of crystalline framework structures, tunability of chemical composition, incorporation of various functionalities onto the framework backbone, and as a consequence, fine-tuning of metal-organic framework (MOF) and covalent organic framework (COF) properties beyond that of any other material class. Leveraging the unique properties of reticular materials has resulted in significant advances from both a fundamental and an applied perspective. Here, we wish to review the milestones in MOF and COF research and give a critical view on progress in their real-world applications. Finally, we briefly discuss the major challenges in the field that need to be addressed to pave the way for industrial applications.

25 Years of Reticular Chemistry.

At its core, reticular chemistry has translated the precision and expertise of organic and inorganic synthesis to the solid state. While initial excitement over metal-organic frameworks (MOFs) and covalent organic frameworks (COFs) was undoubtedly fueled by their unprecedented porosity and surface areas, the most profound scientific innovation of the field has been the elaboration of design strategies for the synthesis of extended crystalline solids through strong directional bonds. In this contribution we highlight the different classes of reticular materials that have been developed, how these frameworks can be functionalized, and how complexity can be introduced into their backbones. Finally, we show how the structural control over these materials is being extended from the molecular scale to their crystal morphology and shape on the nanoscale, all the way to their shaping on the bulk scale.

A Glycosylated Covalent Organic Framework Equipped with BODIPY and CaCO3 for Synergistic Tumor Therapy.

Ca2+ , a ubiquitous but nuanced modulator of cellular physiology, is meticulously controlled intracellularly. However, intracellular Ca2+ regulation, such as mitochondrial Ca2+ buffering capacity, can be disrupted by 1 O2 . Thus, the intracellular Ca2+ overload, which is recognized as one of the important cell pro-death factors, can be logically achieved by the synergism of 1 O2 with exogenous Ca2+ delivery. Reported herein is a nanoscale covalent organic framework (NCOF)-based nanoagent, namely CaCO3 @COF-BODIPY-2I@GAG (4), which is embedded with CaCO3 nanoparticle (NP) and surface-decorated with BODIPY-2I as photosensitizer (PS) and glycosaminoglycan (GAG) targeting agent for CD44 receptors on digestive tract tumor cells. Under illumination, the light-triggered 1 O2 not only kills the tumor cells directly, but also leads to their mitochondrial dysfunction and Ca2+ overload. An enhanced antitumor efficiency is achieved via photodynamic therapy (PDT) and Ca2+ overload synergistic therapy.