Crystalline Porous Frameworks Research Articles

Construction of Porous Aromatic Frameworks with Specifically Designed Motifs for Charge Storage and Transport.

ConspectusPorous frameworks possess high porosity and adjustable functions. The two features conjointly create sufficient interfaces for matter exchange and energy transfer within the skeletons. For crystalline porous frameworks, including metal organic frameworks (MOFs) and covalent organic frameworks (COFs), their long-range ordered structures indeed play an important role in managing versatile physicochemical behaviors such as electron transfer or band gap engineering. It is now feasible to predict their functions based on the unveiled structures and structure-performance relationships. In contrast, porous organic frameworks (POFs) represent a member of the porous solid family with no long-range regularity. For the case of POFs, the randomly packed building units and their disordered connections hinder the electronic structural consistency throughout the entire networks. However, many investigations have demonstrated that the functions of POFs could also be designed and originated from their local motifs.In this Account, we will first provide an overview of the design and synthesis principles for porous aromatic frameworks (PAFs), which are a typical family of POFs with high porosity and exceptional stability. Specifically, the functions achieved by the specific design and synthesis of in-framework motifs will be demonstrated. This strategy is particularly intuitive to introduce desired functions to PAFs, owing to the exceptional tolerance of PAFs to harsh chemical treatments and synthetic conditions. The local structures can be either obtained by selecting suitable building units, sometimes with the aid of computational screening, or emerge as the product of coupling reactions during the synthetic process. Radical PAFs can be obtained by incorporating a persistent radical molecule as a building unit, and the rigid and porous framework may facilitate the formation of radical species by trapping spins in the organic network, which could avoid the delocalizing and recombining processes. Alternatively, radical motifs can also be formed during the formation of the framework linkages. The coupling reaction plays an important role in the construction of functional motifs like diacetylene. The highly porous, radical PAFs showed significant performance as anodes of lithium-ion batteries. To improve the charge transport within the framework, the building units and their connecting manner were cohesively considered, and the framework with a fully conjugated backbone was built up. In another case, the explicit product of the cross-coupling reaction ensured the precise assembly of two building units with electron donating and accepting abilities; therefore, the moving direction of photogenerated electrons was rationally controlled. Constructing a fully conjugated backbone or rationally designing a D-A system for charge transfer in porous frameworks introduced exciting properties for photovoltaic and photocatalysis, and their highly porous, stable frameworks improved their functional applications for perovskite solar cells and chemical productions. These investigations shed light on the designable combination of intrinsic functional motifs with highly porous organic frameworks for effective energy storage and conversion.

Modulating the D-π-A Interactions in Metal-Covalent Organic Frameworks for Efficient Electroreduction of CO2 into Formate.

Crystalline porous framework materials have attracted tremendous interest in electrocatalytic CO2 reduction owing to their ordered structures and high specific surface areas as well as rich designability, however, still suffer from a lack of accuracy in regulating the binding strength between the catalytic sites and intermediates, which is crucial for optimizing the electrocatalytic activity and expanding the product types. Herein, we report three new kinds of vinylene-linked metal-covalent organic frameworks (TMT-CH3-MCOF, TMP-CH3-MCOF and TMP-MCOF) with continuously tunable D-π-A interactions by adjusting the structure of the monomers at the molecular level for realizing efficient electroreduction of CO2 to formate for the first time. Interestingly, compared with TMT-CH3-MCOF and TMP-MCOF, the TMP-CH3-MCOF exhibited the highest HCOO- Faradaic efficiency (FEHCOO-) of 95.6 % at -1.0 V vs RHE and displayed the FEHCOO- above 90 % at the voltage range of -1.0 to -1.2 V vs. RHE, which is one of the highest among various kinds of reported electrocatalysts. Theoretical calculations further reveal that the catalytic sites in TMP-CH3-MCOF with unique moderate D-π-A interactions have suitable binding ability towards the reaction intermediate, which is beneficial for the formation of *HCOO and desorption of *HCOOH, thus effectively promoting the electroreduction of CO2 to formate.

Thickness-Driven Synthesis and Applications of Covalent Organic Framework Nanosheets.

Covalent organic frameworks (COFs) are two-dimensional, crystalline porous framework materials with numerous scopes for tunability, such as porosity, functionality, stability and aspect ratio (thickness to length ratio). The manipulation of π-stacking in COFs results in truly 2D materials, namely covalent organic nanosheets (CONs), adds advantages in many applications. In this Minireview, we have discussed both top-down (COFs→CONs) and bottom-up (molecules→CONs) approaches with precise information on thickness and lateral growth. We have showcased the research progress on CONs in a few selected applications, such as batteries, catalysis, sensing and biomedical applications. This Minireview specifically highlights the reports where the authors compare the performance of CONs with COFs by demonstrating the impact of the thickness and lateral growth of the nanosheets. We have also provided the possible scope of exploration of CONs research in terms of inter-dimensional conversion, such as graphene to carbon nanotube and future technologies.

Thickness‐Driven Synthesis and Applications of Covalent Organic Framework Nanosheets

AbstractCovalent organic frameworks (COFs) are two‐dimensional, crystalline porous framework materials with numerous scopes for tunability, such as porosity, functionality, stability and aspect ratio (thickness to length ratio). The manipulation of π‐stacking in COFs results in truly 2D materials, namely covalent organic nanosheets (CONs), adds advantages in many applications. In this Minireview, we have discussed both top‐down (COFs→CONs) and bottom‐up (molecules→CONs) approaches with precise information on thickness and lateral growth. We have showcased the research progress on CONs in a few selected applications, such as batteries, catalysis, sensing and biomedical applications. This Minireview specifically highlights the reports where the authors compare the performance of CONs with COFs by demonstrating the impact of the thickness and lateral growth of the nanosheets. We have also provided the possible scope of exploration of CONs research in terms of inter‐dimensional conversion, such as graphene to carbon nanotube and future technologies.

An Unusual Crystalline Porous Framework Constructed from Multitypes of Cages with Proton Conduction Property and Heterogeneous Catalytic Activity for Pyrazole Synthesis

An Unusual Crystalline Porous Framework Constructed from Multitypes of Cages with Proton Conduction Property and Heterogeneous Catalytic Activity for Pyrazole Synthesis

Photoresponsive Covalent Organic Frameworks: Visible‐Light Controlled Conversion of Porous Structures and Its Impacts

AbstractCovalent organic frameworks are a novel class of crystalline porous polymers that enable molecular design of extended polygonal skeletons to attain well‐defined porous structures. However, construction of a framework that allows remote control of pores remains a challenge. Here we report a strategy that merges covalent, noncovalent, and photo chemistries to design photoresponsive frameworks with reversibly and remotely controllable pores. We developed a topology‐guided multicomponent polycondensation system that integrates protruded tetrafluoroazobenzene units as photoresponsive sites on pore walls at predesigned densities, so that a series of crystalline porous frameworks with the same backbone can be constructed to develop a broad spectrum of pores ranging from mesopores to micropores. Distinct from conventional azobenzene‐based systems, the tetrafluoroazobenzene frameworks are highly sensitive to visible lights to undergo high‐rate isomerization. The photoisomerization exerts profound effects on pore size, shape, number, and environment, as well as molecular uptake and release, rendering the system able to convert and switch pores reversibly and remotely with visible lights. Our results open a way to a novel class of smart porous materials with pore structures and functions that are convertible and manageable with visible lights.

Study of pharmaceutical contaminant adsorption using HKUST-1 as metal-organic framework model

Tetracycline (TTC) and doxycycline (DXC) are antibiotics commonly used to treat infections; however, overuse of these antibiotics impacts human health and the environment. Metal-organic frameworks (MOFs) arose as a recent potential adsorbent has a coordinative crystalline porous framework constructed through metal cluster coordination bridged towards organic ligands exhibiting high surface area, adjustable pore size, and high stability. In this study, HKUST-1 synthesized at room temperature was used for the first time for the adsorption of TTC and DXC as single and binary components. The pseudo-first-order equation can well represent the adsorption kinetics of TTC and DXC data on HKUST-1, while the adsorption isotherm data for TTC and DXC are well represented by the Redlich-Paterson equation. The extended Langmuir, Sips, Toth, and Redlich-Patterson equations failed to represent binary component adsorption isotherm data in HKUST-1. By modifying the adsorption affinity parameter of the extended Redlich-Paterson equation, the adsorption isotherm data of the TTC and DXC binary components can be represented by this equation. Adsorbed TTC and DXC in the HKUST-1 framework could not be recovered through water regeneration. If organic solvents or highly concentrated base solutions were used as solvents, the structure of HKUST-1 would be disrupted.

Construction of Crystalline Porous Frameworks (COF/MOF) with Multi‐components or by Multi‐reactions

AbstractCovalent organic frameworks (COFs) and metal organic frameworks (MOFs) are crystalline porous materials with ordered framework structures. To create diverse framework structures, linkers with different geometries and sizes have been developed over the past decades. However, for more advanced applications, there is a need for continuous pore size control and property development. Researchers are exploring new synthesis methods, building blocks, and processing/fabrication protocols to meet these demands. Multi‐component or multiple ligands synthesis has been widely used in the construction of MOF structures, and this synthetic strategy has recently been adopted for COF synthesis and extended to versatile linker designing strategies. This review focuses on the recent development of the multi‐component or multi‐reaction strategy for COF synthesis, its application in new MOF synthetic strategies, and ligand development.

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-dimensionalframeworks. 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.

Superheterojunction covalent organic frameworks: Supramolecular synergetic charge transfer for highly efficient photocatalytic CO2 reduction

Photocatalytic conversion of CO2 into value–added chemicals under visible light irradiation is very appealing but challenging due to the easy recombination of hole–electron pairs in traditional photocatalysts. π–π stacking interactions can be favorable for efficient separation and transfer of photo–irradiated charges to promote photocatalysis, but are rarely studied in crystalline porous frameworks like covalent organic frameworks (COFs). Herein, we report a series of thienothiophenes–porphyrin based 2D COFs, denoted as TT–Por(M)–COF (M = 2 H/Co/Cu/Ni), which show enhanced intralayer and interlayer charge separation and migration due to the donor-acceptor heterojunctions and π–stacked columns. The TT–Por(Co)–COF achieves enhanced CO evolution rate than COF–366–Co without the heterojunctions and π–columns (10.05 mmol g–1 h–1 versus 7.10 mmol g–1 h–1) under visible light irradiation. This work not only shows the important role of π–π stacking interactions of COFs for CO2 conversion, but also provides insight into the mechanistic aspect for photocatalysis.

Engineering a molecular ruthenium catalyst into three-dimensional metal covalent organic frameworks for efficient water oxidation.

The water oxidation reaction plays an important role in clean energy conversion, utilization, and storage, but mimicking the oxygen-evolving complex of photosystem II for designing active and stable water oxidation catalysts (WOCs) is still an appealing challenge. Here, we innovatively engineered a molecular ruthenium WOC as a metal complex building unit to construct a series of three-dimensional metal covalent organic frameworks (3D MCOFs) for realizing efficient oxidation catalysis. The resultant MCOFs possessed rare 3D interlocking structures with inclined interpenetration of two-dimensional covalent rhombic nets, and the Ru sites were periodically arranged in the crystalline porous frameworks. Impressively, these MCOFs showed excellent performance towards water oxidation (the O2 evolution rate is as high as 2830 nmol g-1 s-1) via the water nucleophilic attack pathway. Besides, the MCOFs were also reactive for oxidizing organic substrates. This work highlights the potential of MCOFs as a designable platform in integrating molecular catalysts for various applications.

Targeted construction of a three-dimensional metal covalent organic framework with spn topology for photocatalytic hydrogen peroxide production

A 3D metal covalent organic framework (TiCOF-spn) with unreported spn topology was synthesized for photocatalytic H 2 O 2 production. This work expands the topology of the 3D COFs family and pioneers the application of metal-containing COFs in solar energy conversion. • Metal-containing units overcome the obstacle of limited polyhedral organic molecular for 3D COFs. • A 3D metal covalent organic framework (TiCOF-spn) possesses unreported spn topology. • TiCOF-spn pioneers the application of solar energy conversion with H 2 O 2 photocatalysis. To enrich the topology of three-dimensional covalent organic frameworks (3D COFs) is an appealing challenge. Till now, only 20 types of topologies have been achieved in 3D COFs because of the limited polyhedral building blocks. Herein, we report the first 3D titanium-based COF with spn topology (TiCOF-spn) through [6 + 3] imine condensation of a Ti(IV) complex with six aldehyde groups positioned in a trigonal antiprismatic arrangement and a planar triazine-based amine connector. Interestingly, TiCOF-spn contains two kinds of pores, that is, the linking of the tetrahedral cages by sharing vertices forms the truncated tetrahedral cages. By integrating the crystalline porous framework, photoactive titanium center and triazine component, TiCOF-spn shows strong visible light absorption and efficient photocatalytic activity for producing H 2 O 2 (489.94 μmol g −1 h −1 ). This work expands the topology of the 3D COFs family and pioneers the application of metal-containing COFs in solar energy conversion.

Enhanced CO2 Photoreduction through Spontaneous Charge Separation in End-Capping Assembly of Heterostructured Covalent-Organic Frameworks.

It is well known that charge separation is crucial for efficient photocatalytic solar conversion. Although some covalent-organic frameworks (COFs) exhibit visible-light harvest, the large exciton binding energies reduce their photocatalytic efficiencies. Herein, we developed a novel method to post-treat the olefin-linked COFs with end-capping polycyclic aromatic hydrocarbons (PAHs) for spontaneous charge separation. Interestingly, a type-II heterostructure is constructed in our perylene-modified COFs which displays drastically enhanced performance for photocatalytic CO2 reduction, with an efficiency of 8-fold higher than that of unmodified COF. A combination of electrochemical, steady-state, and time-resolved spectroscopic measurements indicates that such drastically enhanced performance should be attributed to photoinduced spontaneous charge separation in the heterostructure. These results illustrate the feasibility of engineering the charge-separation properties of crystalline porous frameworks at a molecular level for artificial photosynthesis.

Enhanced CO2 Photoreduction through Spontaneous Charge Separation in End‐Capping Assembly of Heterostructured Covalent‐Organic Frameworks

AbstractIt is well known that charge separation is crucial for efficient photocatalytic solar conversion. Although some covalent‐organic frameworks (COFs) exhibit visible‐light harvest, the large exciton binding energies reduce their photocatalytic efficiencies. Herein, we developed a novel method to post‐treat the olefin‐linked COFs with end‐capping polycyclic aromatic hydrocarbons (PAHs) for spontaneous charge separation. Interestingly, a type‐II heterostructure is constructed in our perylene‐modified COFs which displays drastically enhanced performance for photocatalytic CO2 reduction, with an efficiency of 8‐fold higher than that of unmodified COF. A combination of electrochemical, steady‐state, and time‐resolved spectroscopic measurements indicates that such drastically enhanced performance should be attributed to photoinduced spontaneous charge separation in the heterostructure. These results illustrate the feasibility of engineering the charge‐separation properties of crystalline porous frameworks at a molecular level for artificial photosynthesis.

Piperazine-Linked Metalphthalocyanine Frameworks for Highly Efficient Visible-Light-Driven H2O2 Photosynthesis.

Artificial photosynthesis of H2O2 from O2 reduction provides an energy-saving, safe, and green approach. However, it is still critical to develop highly active and selective 2e- oxygen reduction reaction photocatalysts for efficient H2O2 production owing to the unsatisfactory photosynthesis productivity. Herein, two new two-dimensional piperazine-linked CoPc-based covalent organic frameworks (COFs), namely, CoPc-BTM-COF and CoPc-DAB-COF, were afforded from the nucleophilic substitution reaction of hexadecafluorophthalocyaninato cobalt(II) (CoPcF16) with 1,2,4,5-benzenetetramine (BTM) or 3,3'-diaminobenzidine (DAB). Powder X-ray diffraction analysis in combination with electron microscopy and a series of spectroscopic technologies reveals their crystalline porous framework with a fully conjugated structure and eclipsed π-stacking model. Ultraviolet-visible diffuse reflectance absorption spectra unveil their excellent light absorption capacity in a wide range of 400-1000 nm. This, together with their enhanced photo-induced charge separation and transport efficiency as disclosed by photocurrent response and photoluminescence measurements, endows the as-prepared piperazine-linked CoPc-based COFs with superior photocatalytic activity toward O2-to-H2O2 conversion under visible-light irradiation (λ > 400 nm). In particular, CoPc-BTM-COF exhibits a record-high H2O2 yield of 2096 μmol h-1 g-1 among the COF-based photocatalysts and an impressive apparent quantum yield of 7.2% at 630 nm. The present result should be helpful for fabricating high-performance and low-cost photocatalysts for visible-light-driven H2O2 photosynthesis.

Research progress in AIE-based crystalline porous materials for biomedical applications

Crystalline porous materials (CPMs) not only present the precise integration of molecular building blocks into extensible structures with periodic frameworks and regular pores, but also provide limited molecular spaces for the interactions of guest molecules, electrons and photons. Incorporating aggregation-induced emission (AIE)-based units into crystalline porous frameworks can result in unique luminescent properties. AIE-based CPMs have widely tunable composition, high luminescent efficiency and good photo-stability, which make them useful for biomedical applications involving bio-sensing, bio-imaging and imaging-guided therapies. This review focused on structure design and luminescent property modulation of AIE-based CPMs with highlights on their applications in biomedical fields. The prospect and challenges in the development of AIE-based CPMs from chemistry, materials to biomedical applications were also discussed.

Light-Activated Biodegradable Covalent Organic Framework-Integrated Heterojunction for Photodynamic, Photothermal, and Gaseous Therapy of Chronic Wound Infection.

Chronic wound healing, impeded by bacterial infections and drug resistance, poses a threat to global human health. Antibacterial phototherapy is an effective way to fight microbial infection without causing drug resistance. Covalent organic frameworks (COFs) are a class of highly crystalline functional porous carbon-based materials composed of light atoms (e.g., carbon, nitrogen, oxygen, and borane), showing potential applications in the biomedical field. Herein, we constructed porphyrin-based COF nanosheets (TP-Por CON) for synergizing photodynamic and photothermal therapy under red light irradiation (e.g., 635 nm). Moreover, a nitric oxide (NO) donor molecule, BNN6, was encapsulated into the pore volume of the crystalline porous framework structure to moderately release NO triggered by red light irradiation for realizing gaseous therapy. Therefore, we successfully synthesized a novel TP-Por CON@BNN6-integrated heterojunction for thoroughly killing Gram-negative bacteria Escherichia coli and Gram-positive bacteria Staphylococcus aureus in vitro. Our research identified that TP-Por CON@BNN6 has favorable biocompatibility and biodegradability, low phototoxicity, anti-inflammatory properties, and excellent mice wound healing ability in vivo. This study indicates that the TP-Por CON@BNN6-integrated heterojunction with multifunctional properties provides a potential strategy for COF-based gaseous therapy and microorganism-infected chronic wound healing.

Hybrid Porous Crystalline Materials from Metal Organic Frameworks and Covalent Organic Frameworks.

Two frontier crystalline porous framework materials, namely, metal‐organic frameworks (MOFs) and covalent organic frameworks (COFs), have been widely explored owing to their outstanding physicochemical properties. While each type of framework has its own intrinsic advantages and shortcomings for specific applications, combining the complementary properties of the two materials allows the engineering of new classes of hybrid porous crystalline materials with properties superior to the individual components. Since the first report of MOF/COF hybrid in 2016, it has rapidly evolved as a novel platform for diverse applications. The state‐of‐art advances in the various synthetic approaches of MOF/COF hybrids are hereby summarized, together with their applications in different areas. Perspectives on the main challenges and future opportunities are also offered in order to inspire a multidisciplinary effort toward the further development of chemically diverse, multi‐functional hybrid porous crystalline materials.

Toward miniaturizing microelectronics using covalent organic framework dielectric

Toward miniaturizing microelectronics using covalent organic framework dielectric

Atomically precise metal chalcogenide supertetrahedral clusters: frameworks to molecules, and structure to function.

ABSTRACTMetal chalcogenide supertetrahedral clusters (MCSCs) are of significance for developing crystalline porous framework materials and atomically precise cluster chemistry. Early research interest focused on the synthetic and structural chemistry of MCSC-based porous semiconductor materials with different cluster sizes/compositions and their applications in adsorption-based separation and optoelectronics. More recently, focus has shifted to the cluster chemistry of MCSCs to establish atomically precise structure–composition–property relationships, which are critical for regulating the properties and expanding the applications of MCSCs. Importantly, MCSCs are similar to II–VI or I–III–VI semiconductor nanocrystals (also called quantum dots, QDs) but avoid their inherent size polydispersity and structural ambiguity. Thus, discrete MCSCs, especially those that are solution-processable, could provide models for understanding various issues that cannot be easily clarified using QDs. This review covers three decades of efforts on MCSCs, including advancements in MCSC-based open frameworks (reticular chemistry), the precise structure–property relationships of MCSCs (cluster chemistry), and the functionalization and applications of MCSC-based microcrystals. An outlook on remaining problems to be solved and future trends is also presented.