Single-layer Covalent Organic Frameworks Research Articles

Ultra-Strong Janus Covalent Organic Framework Membrane with Smart Response to Organic Vapor.

Vapor-driven smart Janus materials have made significant advancements in intelligent monitoring, control, and interaction, etc. Nevertheless, the development of ultrafast response single-layer Janus membrane, along with a deep exploration of the smart response mechanisms, remains a long-term endeavor. Here, the successful synthesis of a high-crystallinity single-layer Covalent organic framework (COF) Janus membrane is reported by morphology control. This kind of membrane displays superior mechanical properties and specific surface area, along with excellent responsiveness to CH2Cl2 vapor. The analysis of the underlying mechanisms reveals that the vapor-induced breathing effect of the COF and the stress mismatch of the Janus structure play a crucial role in its smart deformation performance. It is believed that this COF Janus membrane holds promise for complex tasks in various fields.

Data-Driven Discovery of a Covalent Organic Framework Heterojunction as Efficient Photocatalysts for Overall Solar Water Splitting.

Searching for highly efficient visible-light photocatalysts is a high-cost and time-consuming process in the water splitting field. The integration of data-driven screening based on the database and density functional theory calculations represents a promising approach. In this study, we first present a topologically assembled single-layer covalent organic framework (COF) that is used to build a COF heterojunction database via AA stacking. Then we propose a systematic search procedure for COF heterojunctions as overall solar water splitting photocatalysts, including suitable band gap (screen 1), appropriate band edge position (screen 2), spontaneous catalytic reactions for water splitting (screen 3), and efficient separation of photogenerated electrons and holes (proof). Finally, we successfully identify 1 heterojunction from the pool of 222 items as an efficient photocatalyst for overall solar water splitting. Clearly, this kind of data-driven screening procedure, based on a COF heterojunction database, opens up new avenues and inspires the development of high-performance photocatalysts.

Precise regulation of CO2 packing pattern in s-block metal doped single-layer covalent organic frameworks for high-performance CO2 capture and separation

The development of high-performance CO2 capture and separation adsorbents is critical to alleviate the deteriorating environmental issues. Herein, six s-block metal doped single-layer polymeric tetraoxa[8]circulene (SBM-TOCs, SBM = Li, Na, K, Be, Mg, and Ca) were introduced as potential CO2 capture and separation adsorbents via precise regulation of CO2 packing patterns. Structural analyses showed that Li-TOC, Na-TOC, K-TOC, and Ca-TOC were screened with high cohesive energies ranging from 7.02 to 7.08 eV/atom and moderate formation energies ranging from –2.92 to –1.41 eV, guaranteeing their structural stabilities for CO2 capture and separation. Electronic structure analyses confirmed the large charge transfer and strong covalent bonding characters between the SBMs and direct-connected O atoms, thus constructing a favorable gas adsorption environment. Li-TOC exhibited an ultrahigh CO2 adsorption capacity of 10.78 mmol/g at 298 K and 1.0 bar, which was superior to previously reported Li-doped adsorbents. At 298 K and 1.0 bar, the selectivity of CO2 over N2/CH4 in Li-TOC reached up to ∼ 516 and ∼ 821, respectively. Polarity regulation analyses showed that SBMs had a more positive effect on the framework affinity for CO2 than N2/CH4 via both coulomb and van der Waals interactions. Space regulation analyses showed that strong interactions between the electropositive SBMs and the electronegative O atoms of CO2 facilitated vertical CO2 configurations and compact distributions, thereby rendering a multilayer CO2 adsorption and enhancing the CO2 adsorption capacity. Results of this work highlighted SBM-TOCs as ultrahigh-performance adsorbents for CO2 capture and separation over N2/CH4, and demonstrated that the precise regulation of CO2 packing pattern was an effective strategy to achieve high-performance gas capture and separation.

Exploring the similarity of single-layer covalent organic frameworks using electronic structure calculations.

Two-dimensional Covalent Organic Frameworks (2D COFs) have attracted considerable interest because of their potential for a broad range of applications. Different combinations of the monomeric units can lead to potentially novel materials with varying physico-chemical properties. In this study, we investigate the electronic properties of various 2D COFs with square lattice topology based on a tight-binding density functional theory approach. We first classify the 2D COFs into different classes according to the degree of π-conjugation. Interestingly, this classification is recovered by using a similarity measure based on specific features of the electronic band-structure of the COFs. Further, we study the effect of aromaticity on the electronic structure of fully-conjugated COFs. Our results show that the conjugation and aromaticity are keys in the electronic band-structure of COFs.

Enormous Promotion of Photocatalytic Activity through the Use of Near-Single Layer Covalent Organic Frameworks

Enormous Promotion of Photocatalytic Activity through the Use of Near-Single Layer Covalent Organic Frameworks

Predicting the bulk modulus of single-layer covalent organic frameworks with square-lattice topology from molecular building-block properties.

Two-dimensional Covalent Organic Frameworks (2D COFs) have attracted a lot of interest because of their potential for a broad range of applications. Different combinations of their molecular building blocks can lead to new materials with different physical and chemical properties. In this study, the elasticity of different single-layer tetrabenzoporphyrin (H2-TBPor) and phthalocyanine (H2-Pc) based 2D COFs is numerically investigated using a density-functional based tight-binding approach. Specifically, we calculate the 2D bulk modulus and the equivalent spring constants of the respective molecular building-blocks. Using a spring network model we are able to predict the 2D bulk modulus based on the properties of the isolated molecules. This provides a path to optimize elastic properties of 2D COFs.

Ultrathin porphyrin and tetra-indole covalent organic frameworks for organic electronics applications.

The electronic and charge transport properties of porphyrin and tetra-indole porphyrinoid single layer covalent organic frameworks (COFs) are investigated by means of density functional theory calculations. Ultrathin diacetylene-linked COFs based on oxidized tetra-indole cores are narrow gap 2D semiconductors, featuring a pronounced anisotropic electronic band structure due to the combination of dispersive and flat band characteristics, while registering high room temperature charge carrier mobilities. The capability of bandgap and charge carrier localization tuning via the careful selection of fourfold porphyrin and porphyrinoid cores and twofold articulated linkers is demonstrated, with the majority of systems exhibiting electronic gap values between 1.75 eV and 2.3 eV. Tetra-indoles are also capable of forming stable monolayers via non-articulated core fusing, resulting in 2D morphologies with extended π-conjugation and semi-metallic behavior.

Revealing the Local Electronic Structure of a Single-Layer Covalent Organic Framework through Electronic Decoupling.

Covalent organic frameworks (COFs) are molecule-based 2D and 3D materials that possess a wide range of mechanical and electronic properties. We have performed a joint experimental and theoretical study of the electronic structure of boroxine-linked COFs grown under ultrahigh vacuum conditions and characterized using scanning tunneling spectroscopy on Au(111) and hBN/Cu(111) substrates. Our results show that a single hBN layer electronically decouples the COF from the metallic substrate, thus suppressing substrate-induced broadening and revealing new features in the COF electronic local density of states (LDOS). The resulting sharpening of LDOS features allows us to experimentally determine the COF band gap, bandwidths, and the electronic hopping amplitude between adjacent COF bridge sites. These experimental parameters are consistent with the results of first-principles theoretical predictions.

Surface-confined single-layer covalent organic frameworks: design, synthesis and application

Two-dimensional (2D) nanomaterials, such as graphene and single layer covalent organic frameworks (sCOFs) are being widely studied due to their unusual structure/property relationships. sCOFs typically feature atomic thickness, intrinsic nanoscale porosity and a crystalline lattice. Compared to other organic 2D materials, sCOFs exhibit major advantages including topological designation and constitutional tunability. This review describes the state of the art of surface-confined sCOFs, emphasizing reticular design, synthesis approaches, and key challenges related to improving quality and exploring applications.

Insight into the Transimination Process in the Fabrication of Surface Schiff-Based Covalent Organic Frameworks.

The on-surface synthesis of single-layered covalent organic frameworks (sCOFs) has been investigated by employing a 3-fold symmetric monomer 1 carrying aldehyde groups and the ditopic diamine building block 2 on a highly oriented pyrolytic graphite surface. The self-assembly of molecule 1 is persistently observed at the stoichiometric ratio of the reactive groups. The growth of sCOF network is observed, however, only at the excess of diamine monomers. By investigating the growth process of the sCOF network, the role of excessive diamine monomers can be understood by two aspects. Increasing the molar ratio of diamine monomer provides the driving force for the structural transition from the monomer self-assembly to the formation of the sCOF network. On the other hand, the excess diamine monomers provide basic environment for the transimination reaction and promote the formation of highly ordered sCOFs. The present work provides molecular understanding of the role of transimination reaction in imine-based COF synthesis.

Local Electronic Structure of Molecular Heterojunctions in a Single-Layer 2D Covalent Organic Framework.

The synthesis of a single-layer covalent organic framework (COF) with spatially modulated internal potentials provides new opportunities for manipulating the electronic structure of molecularly defined materials. Here, the fabrication and electronic characterization of COF-420: a single-layer porphyrin-based square-lattice COF containing a periodic array of oriented, type II electronic heterojunctions is reported. In contrast to previous donor-acceptor COFs, COF-420 is constructed from building blocks that yield identical cores upon reticulation, but that are bridged by electrically asymmetric linkers supporting oriented electronic dipoles. Scanning tunneling spectroscopy reveals staggered gap (type II) band alignment between adjacent molecular cores in COF-420, in agreement with first-principles calculations. Hirshfeld charge analysis indicates that dipole fields from oriented imine linkages within COF-420 are the main cause of the staggered electronic structure in this square grid of atomically-precise heterojunctions.

Prediction of strain-controlled adhesion in a single-layer covalent organic framework

Recent research has revealed that strain can be used to control electronic properties, thermal conductivity, and permeability in two-dimensional (2D) materials. The use of deformation to tune physical properties is often termed strain engineering, and applications in photoelectronics and gas separation have been proposed. In the present contribution, we employ reactive molecular dynamics to show that strain can also be used to tune adhesion in a porous 2D covalent organic framework (COF-1). In our simulations, we apply uniaxial strain to single-layers of COF-1, and use a graphene monolayer to measure work of adhesion values. Our results show that strain forces COF-1 to assume a non-planar configuration, decreasing the effective contact area with graphene and, consequently, the adhesion – by more than 25%. This reduced ‘stickyness’ can be reversed, simply by removing the external strain. We then present a proof-of-concept simulation, where we apply strain-controlled adhesion to transfer a graphene membrane between substrates. Finally, we characterize the mechanical properties of COF-1 under uniaxial strain.

Design and synthesis of covalent organic frameworks

Covalent organic frameworks (COFs) materials are crystalline polymer networks constructed by organic building blocks. Their tunable structure, low density, well-defined pores, high surface areas and high thermal stability render them have promising applications in gas storage and separations, energy storage, catalysis, sensing and optoelectronic. This review focuses on the design principle of COF based on the structure and functionality, and summarizes the recent synthesize method of COF bulk powder, single layer COFs and COF films. The design principle of COFs is classified into the structural-oriented and functional-oriented protocols. For structural design, the basic principle is reticular chemistry. The geometry of the building units and the symmetry of the reaction group determine the dimension, topology, structure, and the pore size and shape of COFs. The reaction types of COF are usually based on thermodynamic controlled reversible reaction. Moreover, the functionality of COFs can be easily modulated via three strategies, including directly using functionalized building blocks to construct COFs, post-synthetic modification of the pre-prepared framework, and encapsulation of functional molecules into the channel of COFs. Along with the structural and functional design, various COF materials have been reported and greatly boosted the developments of this field. Then we summarized the synthetic methodologies of COFs. In general, the developed synthetic methodologies follow three aspects: (1) fast and efficient; (2) for large-scale industrial production; (3) for processing shaped-body of COF. Recently, there are several available methods to fabricate COF into free-standing films including the liquid-liquid interfacial polymerization, vapor-assisted conversion and baking the molecular precursors. These approaches indicate a way forward for accessing COF into films and enable applying to real industrial applications. Although there has been a significant expansion of COF chemistries over the past 15 years, yet this field is far from mature, the article ends with a perspective on the future developments in this growing field and discusses the challenges remain for these materials. To date, it is still hard to obtain single crystals, thus advanced characterization techniques and theoretical calculation methods are highly demanded to be developed. The mechanisms for the nucleus formation and growth are still needed to be deeply investigated. In addition, exploring novel topologies and linkages of COF are necessary from the chemistry perspective. Moreover, in order to precisely construct and tune the functions of COF materials, further studies on the structure-property correlation via the directly design functional building blocks and encapsulation of functional molecules into the pores are still required. In terms of the processing COF from the particle to shaped bodies and to devices would be critically important in many practical applications, like gas separation, catalysis and organic light-emitting diodes, thus further development towards the efficient synthesis of COF films or membranes with large area, high orientation, no defect, and uniform thickness would be of great interest. For COF materials, advances in the fast, efficient and economic fabrication methods and the exploration of new functionalization would make them become the commercialized material for industrial application in the fields of energy, health, environment and so on.

The impact of grafted surface defects on the on-surface Schiff-base chemistry at the solid-liquid interface.

We investigate the effect of covalently modified graphitic surfaces on the formation of single-layer covalent organic frameworks (sCOFs) at the solid-liquid interface. The impact of different densities of grafted aryl species was tested on the on-surface synthesis of three distinct imine-based 2D sCOFs. The grafted aryl species that act as defects provide steric barriers to the progress of the Schiff-base reaction, and can be locally removed to start the 2D polymer healing process. This unique strategy provides a general approach to study in situ this dynamic covalent on-surface chemistry.

Local Electronic Structure of a Single-Layer Porphyrin-Containing Covalent Organic Framework.

We have characterized the local electronic structure of a porphyrin-containing single-layer covalent organic framework (COF) exhibiting a square lattice. The COF monolayer was obtained by the deposition of 2,5-dimethoxybenzene-1,4-dicarboxaldehyde (DMA) and 5,10,15,20-tetrakis(4-aminophenyl) porphyrin (TAPP) onto a Au(111) surface in ultrahigh vacuum followed by annealing to facilitate Schiff-base condensations between monomers. Scanning tunneling spectroscopy (STS) experiments conducted on isolated TAPP precursor molecules and the covalently linked COF networks yield similar transport (HOMO-LUMO) gaps of 1.85 ± 0.05 eV and 1.98 ± 0.04 eV, respectively. The COF orbital energy alignment, however, undergoes a significant downward shift compared to isolated TAPP molecules due to the electron-withdrawing nature of the imine bond formed during COF synthesis. Direct imaging of the COF local density of states (LDOS) via dI/dV mapping reveals that the COF HOMO and LUMO states are localized mainly on the porphyrin cores and that the HOMO displays reduced symmetry. DFT calculations reproduce the imine-induced negative shift in orbital energies and reveal that the origin of the reduced COF wave function symmetry is a saddle-like structure adopted by the porphyrin macrocycle due to its interactions with the Au(111) substrate.

Concentration-Directed Polymorphic Surface Covalent Organic Frameworks: Rhombus, Parallelogram, and Kagome.

Polymorphic single-layered covalent organic frameworks (sCOFs) via on-surface synthesis have been investigated by employing the tetradentate monomer 1,3,6,8-tetrakis(p-formylphenyl)pyrene with D2h symmetry and ditopic linear diamine building blocks. Three kinds of well-ordered sCOFs, including rhombus, parallelogram, and Kagome networks, are observed on the graphite surface by scanning tunnel microscopy. The pore size and periodicity of sCOFs are tunable by employing diamine monomers with different lengths. Statistical analysis reveals that two types of quadrate networks are preferred at high concentration, whereas the occupancy of Kagome networks increases at low concentration. This trend can be understood by the differences in the network density of three kinds of networks. The reversibility and the self-sorting ability of the dynamic covalent reaction make it possible to control the polymorphic distribution similar to the principle demonstrated in supramolecular self-assembly.

Room-Temperature Synthesis of Covalent Organic Frameworks with a Boronic Ester Linkage at the Liquid/Solid Interface.

With various prospected applications in the field of nanoelectronics and catalysis, on-surface synthesis of single-layer covalent organic frameworks (surface COFs) with designable structures and properties have attracted enormous interest. Herein, we report on a scanning tunneling microscopic investigation of the surface-confined synthesis of a covalently bonded boronic ester network directly at the octanoic acid/ highly oriented pyrolytic graphite(HOPG) interface under room temperature. The dynamic reaction process was investigated in detail. STM results indicate that the surface networks undergo structural evolution from a hybrid covalent/noncovalent multiwall porous network to single-wall hexagonal COF with the decrease of monomer concentration. Further experimental observation disclosed that the boronic ester-linked system is sensitive to instantaneous voltage pulses and the stimulation of the STM tip. In addition, the 1 H NMR spectra has further confirmed that the surface and octanoic acid may play important roles in promoting the reaction between 4,4'-phenylazobenzoyl diboronic acid (ABBA) and 2,3,6,7,10,11-hexahydroxytriphenylene (HHTP) building units.

A Photoresponsive Surface Covalent Organic Framework: Surface-Confined Synthesis, Isomerization, and Controlled Guest Capture and Release.

By introducing an azobenzene group to the backbone of diboronic acid, we have obtained a surface-confined, photoresponsive single-layer covalent organic framework with long-range order and almost entire surface coverage. Scanning tunneling microscopic characterization indicates that though the covalent linkage provides a significant locking effect, isomerization can still happen under UV irradiation, which causes destruction of the surface COF. Furthermore, the decomposed surface COF can recover upon annealing. This photoinduced decomposition provides a facile approach for the controlled capture and release of targeted objects using these nanoporous surface COFs as a host, which has been demonstrated in this work using copper phthalocyanine as a model guest.

Construction of boronate ester based single-layered covalent organic frameworks.

Boronate ester based single-layered covalent organic frameworks (sCOFs) with large domain areas and uniform pore sizes have been fabricated on graphite under an ambient atmosphere. The phase separation to generate the boronate ester based sCOFs, boroxine based sCOFs and other nanostructures could be tuned using the molecular ratio of the two precursors, demonstrating a self-sorting process for on-surface dynamic covalent chemistry.

Substrate Orientation Effect in the On-Surface Synthesis of Tetrathiafulvalene-Integrated Single-Layer Covalent Organic Frameworks.

The on-surface reactions of tetrathiafulvalene equipped with four benzaldehyde groups (4ATTF) and ditopic diamine molecules are investigated. 4ATTF tends to form large-scale-ordered rhombus structures when reacted with p-phenylenediamine (PPDA). A longer ditopic diamine molecule, 1,1'-biphenyl-4,4'-diamine dihydrochloride (BPDA), causes the domain size of the regular rhombus structure to decrease and triangular and irregular rhombus structures to appear upon reaction with 4ATTF. However, in the rhombus structures formed by different-length ditopic diamine molecules, the single-layer covalent organic frameworks on the graphite surface preferentially orient in alignment with the underlying HOPG substrate lattice.