2D COFs Research Articles

Simulation of interlayer coupling for electroactive covalent organic framework design.

Porous, stacked two-dimensional covalent organic frameworks (2D COFs) bearing semiconducting linkers can support directional charge transfer across adjacent layers of the COF. To better inform the current and possible future design rules for enhancing electron and hole transport in such materials, an understanding of how linker selection and functionalization affects interlayer electronic couplings is essential. We report electronic structure simulation and analysis of electronic couplings across adjacent linker units and to encapsulated species in functionalized electroactive 2D COFs. The detailed dependence of these electronic couplings on interlayer interactions is examined through scans along key interlayer degrees of freedom and through configurational sampling from equilibrium molecular dynamics on semiempirical potential energy surfaces. Beyond affirming the sensitivity of the electronic coupling to interlayer distance and orientation, these studies offer guidance toward linker functionalization strategies for enhancing charge carrier transport in electroactive 2D COFs.

Chiral Covalent Organic Framework Films with Enhanced Photoelectrical Performances.

The desirable superimposed stacking of two-dimensional covalent organic frameworks (2D COFs) benefits out-of-plane charge transfer, whereas the actual stacking deviation cannot leverage the potential of 2D COFs for optoelectrical applications. Herein, we report a chirality-induced strategy to control the parallel AA-stacking sequence for the β-ketoenamine-linked COF film supported on a FTO substrate. The resulting chiral modules are periodically distributed at the framework node, ensuring identical mirrored configurations of layers for parallel stacking. Such unique architectonics exhibit the prolonged charge carrier lifetime, fast charge-transfer dynamics, and ultrahigh electron collection efficiency, thereby allowing for the excellent photocurrent response of 38 μA/cm2 at 0.25 V (vs RHE). The origin of superior performances lies in the intensified exciton gradient distribution and electron density for photoinduced electron-hole dissociation and charge transfer, in stark contrast to achiral analogues. This study highlights the stacking sequence regulated by chiral nanoarchitectonics and promises great potential of chiral COFs in photoelectrical catalysis.

Ultrahigh-Efficient N,O-Functionalized covalent organic framework towards thorium adsorption from uranium and rare earth elements

The development of novel porous materials as adsorbents for highly effective separation of thorium from ore or nuclear wastewater co-existing with uranium and rare earth elements is one of the most desirable methods but remains challenging. In this work, we prepare a N,O-functionalized two-dimensional covalent organic framework (2D COF), namely TAPT-DHTA, with a permanent porous structure and plenty of nitrogen and oxygen adsorption sites throughout the skeletons, which displays an ultrahigh Th(IV) saturated adsorption capacity of 1394 mg g−1 (at pH = 4.0), higher than all adsorbents reported so far. Moreover, the TAPT-DHTA COF has a large distribution coefficient (Kd) of up to 5.2 × 104 mL g−1, implying its extremely strong affinity for Th(IV) due to the collaborative coordination interactions between N/O atoms and Th(IV) as well as their specific binding mechanism. The distinguished adsorption behavior of the TAPT-DHTA COF affords an important platform for highly selective separation of Th(IV) from uranium and rare earth elements, thereby shedding some light on the design of novel adsorbents for thorium separation in the thorium fuel cycle.

Radiation-Assisted Assembly of a Highly Dispersed Nanomolybdenum-Functionalized Covalent Organic Framework.

Two-dimensional covalent organic frameworks (2D COFs), featuring a large surface area and 1D pore structure, serve as promising scaffolds for anchoring functional guest compounds, which can significantly enhance their performance and thus expand their potential applications. Postsynthetic strategy for COFs functionalization is versatile but challenging because of their tedious procedure with high time and energy consumption, generation of excess reaction waste, and damage to COF crystallinity. We report in this work a general strategy for the synthesis of inorganic nanocompound-functionalized COF composites in a one-pot way. Specifically, a high-crystallinity nanoscale molybdenum compound is successfully introduced into a COF skeleton with high dispersion in situ during the crystallization process of the COF induced by gamma ray radiation under ambient conditions. The obtained COF@Mo composites exhibit remarkable sorption performance for methylene blue and many other organic dyes in aqueous solution with the advantages of ultrarapid uptake dynamics and high removal efficiency.

On topological analysis of two-dimensional covalent organic frameworks via M-polynomial

Covalent organic frameworks (ZnP-COFs) made of zinc-porphyrin have become effective materials with a variety of uses, including gas storage and catalysis. To simulate the structural and electrical features of ZnP-COFs, this study goes into the computation of polynomials utilizing degree-based indices. We gave a methodical study of these polynomial computations using Excel, illustrating the complex interrelationships between the various indices. Degree-based indices provide valuable insights into the connectivity of vertices within a network. M-polynomials, on the other hand, offer a mathematical framework for representing and studying the properties of 2D COFs. By encoding structural information into a polynomial form, M-polynomials facilitate the calculation of various topological indices, including the Wiener index, Zagreb indices, and more. The different behavior of ZnP-COFs based on degree-based indices was illustrated graphically, and this comparison provided insightful information for prospective applications and the construction of innovative ZnP-COF structures. Moreover, we discuss the relevance of these techniques in the broader context of materials science and the design of functional covalent organic frameworks.

Modulated synthesis of stoichiometric and sub-stoichiometric two-dimensional covalent organic frameworks for enhanced ethylene purification

Controlled synthesis of two-dimensional covalent organic frameworks (2D COFs), including stoichiometric and sub-stoichiometric variations, is a topic of growing interest due to its potential in gas separation applications. In this study, we successfully synthesized three distinct 2D COFs by carefully adjusting solvent compositions and monomer ratios during the synthesis of [4 + 4] type COFs. These included a stoichiometric [4 + 4] type COF and two sub-stoichiometric [4 + 2] type COFs, featuring unreacted amino or formyl groups. The resulting COFs exhibit different gas adsorption and separation properties. Specifically, sub-stoichiometric COF-DA with residual amino groups shows comparable adsorption capacity for C2H2, C2H4, and CO2 to stoichiometric COF-DAPy. In contrast, sub-stoichiometric COF-Py with residual formyl groups displays enhanced adsorption selectivity for C2H2/C2H4 and C2H2/CO2 separation, with the C2H2/C2H4 selectivity being the highest among reported COFs, attributed to increased pore polarity resulting from the presence of formyl groups. This study not only offers an additional example of sub-stoichiometric COF synthesis but also advocates for further exploration of sub-stoichiometric COF materials, particularly in the field of gas adsorption and separation.

Self-Assembly of 2D Polyphthalocyanine in Lysosome Enables Multienzyme Activity Enhancement to Induce Tumor Ferroptosis.

Nanozymes show great potential in facilitating tumor ferroptosis by upregulation of reactive oxygen species (ROS) and downregulation of glutathione (GSH). However, mild acidity (pH 6.5-6.9) of tumor microenvironment severely restricts the activity of nanozymes. Although lysosomes as acidic organelles (pH = 3.5-5.5) are hopeful for improving enzyme-like activity, most reported nanozymes are not capable of effectively accumulating in the lysosomes. Herein, an acid-responsive self-assembly strategy based on iron phthalocyanine-rich covalent organic framework nanosheets (COFFePc NSs) is developed, which enables lysosomal targeting aggregation of COFFePc NSs due to the existence of abundant negative hydroxyl groups and rigid structure. Meanwhile, COFFePc NSs display exceptional multienzyme-mimic performance at lower pH to efficiently generate ROS to cause lysosome damage and apoptosis by synergistic photothermal effect. Subsequently, the released COFFePc with GSH oxidase-mimicking activity can consume GSH to promote ferroptosis. This is the first report of a 2D COF using its own properties to achieve lysosomal self-assembly. Overall, the work provides a new paradigm for the development of lysosome-targeted nanosystems.

Constructing 2D Phthalocyanine Covalent Organic Framework with Enhanced Stability and Conductivity via Interlayer Hydrogen Bonding as Electrocatalyst for CO2 Reduction.

Fabricating COFs-based electrocatalysts with high stability and conductivity still remains a great challenge. Herein, 2D polyimide-linked phthalocyanine COF (denoted as NiPc-OH-COF) is constructed via solvothermal reaction between tetraanhydrides of 2,3,9,10,16,17,23,24-octacarboxyphthalocyaninato nickel(II) and 2,5-diamino-1,4-benzenediol (DB) with other two analogous 2D COFs (denoted as NiPc-OMe-COF and NiPc-H-COF) synthesized for reference. In comparison with NiPc-OMe-COF and NiPc-H-COF, NiPc-OH-COF exhibits enhanced stability, particularly in strong NaOH solvent and high conductivity of 1.5×10-3Sm-1 due to the incorporation of additional strong interlayer hydrogen bonding interaction between the O-H of DB and the hydroxy "O" atom of DB in adjacent layers. This in turn endows the NiPc-OH-COF electrode with ultrahigh CO2-to-CO faradaic efficiency (almost 100%) in a wide potential range from -0.7 to -1.1V versus reversible hydrogen electrode (RHE), a large partial CO current density of -39.2mAcm-2 at -1.1V versus RHE, and high turnover number as well as turnover frequency, amounting to 45000 and 0.76S-1 at -0.80V versus RHE during 12h lasting measurement.

Benzotrithiophene-based covalent organic frameworks as efficient catalysts for artificial photosynthesis of H2O2 in pure water

Direct synthesis of hydrogen peroxide (H2O2) using oxygen and water through the photocatalytic route is a prospective approach for the on-site production of H2O2. However, the limited varieties of photocatalytic sites and ambiguous structure-active relationship impede the development of environmentally friendly technology. Herein, we report a active compound (benzotrithiophene, BTT) for the photosynthesis of H2O2 using oxygen and water, and design four benzotrithiophene-based 2D covalent organic frameworks (BTT-COFs) linked with aromatic amines linkers. Significant enhancement in photocatalytic activity has been observed via the introduction of BTT struct into 2D COFs. The high crystallinity of COFs increases the visible light absorption ability and photostability of BTT. The suitable distance of BTT struct in COFs and the construction of donor–acceptor (D-A) structure further promote the carrier generation and transfer. The COF-BTT-TAPT exhibited the highest H2O2 production rate of 620 μmol g−1 h−1. Mechanism investigation experiments have given the validation that BTT-COFs efficiently reduce oxygen in pure water to form H2O2 under visible-light irradiation. This work paves a revolutionary way for designing and fabricating high-performance metal-free photocatalysts for visible-light-driven H2O2 production.

Catalyst-free solid-state cross-linking of covalent organic frameworks in confined space

A “confined space” provides a unique environment to regulate the crystallization thermodynamics and kinetics by confining the reactants in the restricted space dimensions. Solid-state crystal-to-crystal transitions in confined space are controlled by the preassembly of molecules in a crystal lattice and occur inside the lattice. Herein, we report the first case of construction of crystalline cross-linked covalent organic frameworks (CL-COFs) through solid-state cross-linking of acetylenic groups-bridged 2D COFs in spatially limited systems. Specifically, this transformation is thermally induced, yielding CL-COFs with superlative properties, including outstanding enhancement in crystallinity, specific surface area, and stability. We further demonstrate the CL-COFs as high conductivity polymers after iodine doping. This work underscores the opportunity to use lattice-constrained solid-state cross-linking to develop more versatile and feature-rich polyacetylene networks.

Reversible modulation of interlayer stacking in 2D copper-organic frameworks for tailoring porosity and photocatalytic activity

The properties of two-dimensional covalent organic frameworks (2D COFs), including porosity, catalytic activity as well as electronic and optical properties, are greatly affected by their interlayer stacking structures. However, the precise control of their interlayer stacking mode, especially in a reversible fashion, is a long-standing and challenging pursuit. Herein, we prepare three 2D copper-organic frameworks, namely JNM-n (n = 7, 8, and 9). Interestingly, the reversible interlayer sliding between eclipsed AA stacking (i.e., JNM-7-AA and JNM-8-AA) and staggered ABC stacking (i.e., JNM-7-ABC and JNM-8-ABC) can be achieved through environmental stimulation, which endows reversible encapsulation and release of lipase. Importantly, JNM-7-AA and JNM-8-AA exhibit a broader light absorption range, higher charge-separation efficiency, and higher photocatalytic activity for sensitizing O2 to 1O2 and O2•− than their ABC stacking isostructures. Consequently, JNM-8-AA deliver significantly enhanced photocatalytic activities for oxidative cross-coupling reactions compared to JNM-8-ABC and other reported homogeneous and heterogeneous catalysts.

A wafer scale thin film of ultra-small Sc2O3 nanocrystals on a 2D COF with high rigidity.

Scandium oxide (Sc2O3) has a wide range of applications in metallurgy, chemical industry, electronics and many other high-tech fields. However, most Sc2O3 materials exist in the powder or bulk form, while nanostructured Sc2O3 has rarely been reported as there is a lack of a common method to control its dimensionality, hindering the understanding of new properties and potential applications of nano-Sc2O3 materials. In this paper, we establish a procedure to synthesize a two-dimensional (2D) Sc2O3-covalent organic framework (COF) composite film where the crystal size of Sc2O3 domains is as small as ∼3 nm. The composite film is prepared by a Schiff base condensation reaction at the sharp n-pentane/water interface using a combination of surfactant-monolayer-assisted interfacial synthesis and laminar assembly polymerization methods. Then the conditions of nucleation and uniform film formation of the 2D Sc2O3/COF are explored further. Meanwhile, an atomic force microscopy indentation test shows that the material has a high Young's modulus of 89.1 ± 3.8 GPa, which is much higher than those of the majority of reported 2D polymer materials. We further extended this synthesis method to the preparation of Yb2O3 (ytterbium oxide) and/or Er2O3 (erbium oxide)-incorporated 2D COF composite films, verifying the universality of this strategy. This work provides an opportunity to vary the dimensionality of many kinds of metal oxides and explore the potential applications of low-dimensional Sc2O3 materials.

In situ metallization of 2D COFs with acid/base site synergistic effects for boosting one-pot CO2 conversion

Metal-functionalized 2D COFs synthesized through an in situ metallization method were used as efficient heterogeneous catalysts in the one-pot conversion of olefins and CO2 into cyclic carbonates.

Effect of interlayer slipping on the geometric, thermal and adsorption properties of 2D covalent organic frameworks: a comprehensive review based on computational modelling studies.

Two-dimensional covalent organic frameworks (2D-COFs) are a class of crystalline porous organic polymers, consisting of 2D-planar sheets stacked together perpendicularly via noncovalent forces. Since their discovery, 2D-COFs have attracted extensive attention for optoelectronic and adsorption applications. Owing to the layer stacking nature of 2D COFs, various new slipped structures that are energetically favourable can be designed. These interlayer slipped structures are actively responsible for tuning (mostly enhancing) the optoelectronic properties, thermal properties, and mechanical strength of 2D COFs. This review summarizes the effect of interlayer slipping on the energetic stability, electronic behaviour and gas adsorption properties of 2D layered COFs, which is explained through computational modelling simulations. Since computational modelling offers a deep insight into electronic behaviour at the atomic scale, which is potentially impossible through experimental techniques, the introduction and role of computational techniques in such studies have also been described.

A pyrenetetrayl/phenanthroline-based one-dimensional covalent organic framework for metal-free photocatalytic organic conversion

A 1D COF, termed PP-COF, was reported with optimized eclipsed packing (AA) and excellent optoelectronic performance. Compared with 2D COF, PP-COF exhibited more efficient photocatalytic effects in bisarylation and oxidative coupling reactions.

Growth of two-dimensional covalent organic frameworks on substrates: insight from microsecond atomistic simulations.

While growing two-dimensional covalent organic frameworks (2D COFs) on substrates holds promise for producing functional monolayers, the presence of many defects in the resulting crystals often hinders their practical applications. Achieving structural order while suppressing defect formation necessitates a detailed atomic-level understanding. The key lies in understanding the polymerization process with high nano-scale accuracy, which presents significant challenges. Here, we perform microsecond atomistic molecular dynamics simulations to describe the deposition and polymerization of cyclohexa-m-phenylene on metal substrates, closely mimicking experimental conditions. Our improved approach highlights that 2D polymerization occurs through monomer addition and island coalescence, with a pre-bonding stage allowing monomers/oligomers to dynamically adjust their configurations to the expanding island structures. Our results elucidate the mechanisms underlying the formation of vacancy and dislocation defects during 2D polymerization as well as their healing processes. Overall, our findings underscore the significant roles that high surface mobility, effective monomer-substrate anchoring, high framework rigidity, moderate monomer coordination, and low bonding rate play in forming large, extended 2D crystals while suppressing vacancy and dislocation defects. We demonstrate how these factors can be tuned through substrate selection, deposition rate modulation, and temperature control, thereby offering valuable insight for strategically optimizing on-surface 2D polymerizations.

Rational design of dimensionally matched 2D/2D COF based photocatalysts for highly efficient noble-metal-free solar energy catalysis

Ultrathin non-precious metal co-catalysts (WS2) were combined with a 2D COF (TP1C) through a solvothermal method, achieving excellent photocatalytic hydrogen production performance.

An N-heterocyclic carbene functional covalent organic framework for Knoevenagel condensation reaction and γ-butyrolactone synthesis at room temperature

In this article, we have reported the conduction of an N-heterocyclic functional 2D COF, and the remarkable catalytic activity towards the Knoevenagel condensation reaction and the synthesis of γ-butyrolactones at room temperature.

Rapid, Mild, and Catalytic Synthesis of 2D and 3D COFs with Promising Supercapacitor Applications

Rapid, Mild, and Catalytic Synthesis of 2D and 3D COFs with Promising Supercapacitor Applications

OGRe: Optimal Grid Refinement Protocol for Accurate Free Energy Surfaces and Its Application in Proton Hopping in Zeolites and 2D COF Stacking.

While free energy surfaces are the crux of our understanding of many chemical and biological processes, their accuracy is generally unknown. Moreover, many developments to improve their accuracy are often complicated, limiting their general use. Luckily, several tools and guidelines are already in place to identify these shortcomings, but they are typically lacking in flexibility or fail to systematically determine how to improve the accuracy of the free energy calculation. To overcome these limitations, this work introduces OGRe, a Python package for optimal grid refinement in an arbitrary number of dimensions. OGRe is based on three metrics that gauge the confinement, consistency, and overlap of each simulation in a series of umbrella sampling (US) simulations, an enhanced sampling technique ubiquitously adopted to construct free energy surfaces for hindered processes. As these three metrics are fundamentally linked to the accuracy of the weighted histogram analysis method adopted to generate free energy surfaces from US simulations, they facilitate the systematic construction of accurate free energy profiles, where each metric is driven by a specific umbrella parameter. This allows for the derivation of a consistent and optimal collection of umbrellas for each simulation, largely independent of the initial values, thereby dramatically increasing the ease-of-use toward accurate free energy surfaces. As such, OGRe is particularly suited to determine complex free energy surfaces with large activation barriers and shallow minima, which underpin many physical and chemical transformations and hence to further our fundamental understanding of these processes.