Post-synthetic Modification of Covalent Organic Frameworks through in situ Polymerization of Aniline for Enhanced Capacitive Energy Storage

Covalent organic frameworks (COFs) having layered architecture with open nanochannels and high specific surface area are promising candidates for energy storage. However, the low electrical conductivity of two-dimensional COFs often limits their scope in energy storage applications. The conductivity of COFs can be enhanced through post-synthetic modification with conducting polymers. Herein, we developed polyaniline (PANI) modified triazine-based COFs via in situ polymerization of aniline within the porous frameworks. The composite materials showed high conductivity of 1.4–1.9×10⁻² S cm⁻¹ at room temperature with a 20-fold enhancement of the specific capacitance than the pristine frameworks. The fabricated supercapacitor exhibited a high energy density of 24.4 W h kg⁻¹ and a power density of 200 W kg⁻¹ at 0.5 A g⁻¹ current density. Moreover, the device fabricated using the conducting polymer-triazine COF composite could light up a green light-emitting diode for 1 min after being charged for 10 s.     

A Thiophene Backbone Enables Two-Dimensional Poly(arylene vinylene)s with High Charge Carrier Mobility

Linear conjugated polymers have attracted significant attention in organic electronics in recent decades. However, despite intrachain π-delocalization, interchain hopping is their transport bottleneck. In contrast, two-dimensional (2D) conjugated polymers, as represented by 2D π-conjugated covalent organic frameworks (2D c-COFs), can provide multiple conjugated strands to enhance the delocalization of charge carriers in space. Herein, we demonstrate the first example of thiophene-based 2D poly(arylene vinylene)s (PAVs, 2DPAV-BDT-BT and 2DPAV-BDT-BP , BDT=benzodithiophene, BT=bithiophene, BP=biphenyl) via Knoevenagel polycondensation. Compared with 2DPAV-BDT-BP , the fully thiophene-based 2DPAV - BDT - BT exhibits enhanced planarity and π-delocalization with a small band gap (1.62 eV) and large electronic band dispersion, as revealed by the optical absorption and density functional calculations. Remarkably, temperature-dependent terahertz spectroscopy discloses a unique band-like transport and outstanding room-temperature charge mobility for 2DPAV-BDT-BT (65 cm² V⁻¹ s⁻¹), which far exceeds that of the linear PAVs, 2DPAV-BDT-BP , and the reported 2D c-COFs in the powder form. This work highlights the great potential of thiophene-based 2D PAVs as candidates for high-performance opto-electronics.     

Multicomponent Synthesis of Imidazole-Linked Fully Conjugated 3D Covalent Organic Framework for Efficient Electrochemical Hydrogen Peroxide Production

The semiconducting properties and applications of three dimensional (3D) covalent organic frameworks (COFs) are greatly hampered because of their long-ranged non-conjugated skeletons and relatively unstable linkages. Here, a robust imidazole-linked fully conjugated 3D covalent organic framework (BUCT-COF-7) is synthesized through the one-pot multicomponent Debus-Radziszewski reaction of the saddle-shaped aldehyde-substituted cyclooctatetrathiophene, pyrene-4,5,9,10-tetraone, and ammonium acetate. The semiconducting BUCT-COF-7, as a metal-free catalyst, shows excellent two electron oxygen reduction reaction (ORR) activity in alkaline medium with high hydrogen peroxide (H₂O₂) selectivity of 83.4 %. When the BUCT-COF-7 as cathode catalyst is assembled into the electrolyzer, the devices showed high electrochemical production rate of H₂O₂ up to 326.9 mmol g⁻¹ h⁻¹. The accumulative amount of H₂O₂ could totally degrade the dye methylene blue via Fenton reaction for wastewater treatment. This is the first report about intrinsic 3D COFs for efficient electrochemical synthesis of H₂O₂, revealing the promising applications of fully conjugated 3D COFs in the environment-related field.     

Two π-Conjugated Covalent Organic Frameworks with Long-Term Cyclability at High Current Density for Lithium Ion Battery

Organic lithium ion batteries (LIBs) are considered as one of the next-generation green electrochemical energy storage (EES) devices. However, obtaining both high capacity and long-term cyclability is still the bottleneck of organic electrode materials for LIBs because of weak structural and chemical stability and low conductivity. Covalent organic frameworks (COFs) show potential to overcome these problems owing to its good stability and high capacity. Herein, the synthesis and characterization of two π-conjugated COFs, derived from the Schiff-base reaction of 2,4,6-triaminopyrimidne (TM) respectively with 1,4-phthalaldehyde (PA) and 1,3,5-triformylbenzene (TB) by a mechanochemical process are presented. As anode materials for LIBs, the COFs exhibit favorable electrochemical performance with the highest reversible discharge capacities of up to 401.3 and 379.1 mAh g⁻¹ at a high current density (1 A g⁻¹), respectively, and excellent long-term cyclability with 74.8 and 72.7 % capacity retention after 2000 cycles compared to the initial discharge capacities.     

Superprotonic Conductivity in Flexible Porous Covalent Organic Framework Membranes

Poor mechanical stability of the polymer electrolyte membranes remains one of the bottlenecks towards improving the performance of the proton exchange membrane (PEM) fuel cells. The present work proposes a unique way to utilize crystalline covalent organic frameworks (COFs) as a self‐standing, highly flexible membrane to further boost the mechanical stability of the material without compromising its innate structural characteristics. The as‐synthesized p‐toluene sulfonic acid loaded COF membranes (COFMs) show the highest proton conductivity (as high as 7.8×10⁻² S cm⁻¹) amongst all crystalline porous organic polymeric materials reported to date, and were tested under real PEM operating conditions to ascertain their practical utilization as proton exchange membranes. Attainment of 24 mW cm⁻² power density, which is the highest among COFs and MOFs, highlights the possibility of using a COF membrane over the other state‐of‐the‐art crystalline porous polymeric materials reported to date.     

Nine-Electron Transfer of Binder Synergistic π-d Conjugated Coordination Polymers as High-Performance Lithium Storage Materials

To solve the problems such as the dissolution and the poor conductivity of organic small molecule electrode materials, we construct π-d conjugated coordination polymer Ni-DHBQ with multiple redox-active centers as lithium storage materials. It exhibits an ultra-high capacity of 9-electron transfers, while the π-d conjugation and the laminar structure inside the crystal ensure fast electron transport and lithium ion diffusion, resulting in excellent rate performance (505.6 mAh g⁻¹ at 1 A g⁻¹ after 300 cycles). The interaction of Ni-DHBQ with the binder CMC synergistically inhibits its dissolution and anchors the Ni atoms, thus exhibiting excellent cycling stability (650.7 mAh g⁻¹ at 0.1 A g⁻¹ after 100 cycles). This work provides insight into the mechanism of lithium storage in π-d conjugated coordination polymers and the synergistic effect of CMC, which will contribute to the molecular design and commercial application of organic electrode materials.     

Thin polymer films on semiconductors: From their protection to the realization of molecular electronic devices

When homogeneously grafted as thin films on small band-gap inorganic semiconductors (n-GaAs, n-CdS), organic conjugated polymers such as poly-methylthiophene afford interesting new structures. In their doped conducting state, these polymer films bring a long-term protection of these semiconductors against their photocorrosion in aqueous medium, and the subsequent inclusion of metallic aggregates in these films allows increased catalytic activity for achieving photoelectrochemical reactions. In their undoped semiconducting state, they lead to the realization of “organic-on-inorganic” p-n junctions which show very low leakage current and high allowed current density. As photovoltaic cell, an energy conversion efficiency of 17.5% has been obtained under 100 mW.cm⁻² irradiation with a p-PMeT/n-GaAs cell. The larger number of work already devoted to organic conjugated polymers such as polyacetylene, polypyrrole, polythiophene, has shown that these compounds can be switched between a doped oxidized state, with a nearly metallic conductivity, σ ∼ 10²-10³S.cm⁻¹, and an undoped neutral state which presents semiconducting properties, σ ∼ 10⁻⁷ -10⁻⁹ S. cm⁻¹. Either as free standing film or grafted on an electrode, these polymers exhibit interesting organic electrode properties which have been widely characterized. On the other hand much less is known on their behavior when deposited on inorganic semiconductors. In the following, this area will be discussed on two examples, the protection of narrow band-gap semiconductors against their photo-degradation in aqueous solution, and the new junction properties presented by “organic-on-inorganic” electronic devices.     

Two-Dimensional Conjugated Metal-Organic Frameworks with Large Pore Apertures and High Surface Areas for NO2 Selective Chemiresistive Sensing

The emergence of two-dimensional conjugated metal–organic frameworks (2D c-MOFs) with pronounced electrical properties (e.g., high conductivity) has provided a novel platform for efficient energy storage, sensing, and electrocatalysis. Nevertheless, the limited availability of suitable ligands restricts the number of available types of 2D c-MOFs, especially those with large pore apertures and high surface areas are rare. Herein, we develop two new 2D c-MOFs (HIOTP-M, M=Ni, Cu) employing a large p-π conjugated ligand of hexaamino-triphenyleno[2,3-b:6,7-b′:10,11-b′′]tris[1,4]benzodioxin (HAOTP). Among the reported 2D c-MOFs, HIOTP-Ni exhibits the largest pore size of 3.3 nm and one of the highest surface areas (up to 1300 m² g⁻¹). As an exemplary application, HIOTP-Ni has been used as a chemiresistive sensing material and displays high selective response (405 %) and a rapid response (1.69 min) towards 10 ppm NO₂ gas. This work demonstrates significant correlation linking the pore aperture of 2D c-MOFs to their sensing performance.     

Vinylene-Linked Covalent Organic Frameworks (COFs) with Symmetry-Tuned Polarity and Photocatalytic Activity

The polarity of a semiconducting molecule affects its intrinsic photophysical properties, which can be tuned by varying the molecular geometry. Herein, we developed a D_3h-symmetric tricyanomesitylene as a new monomer which could be reticulated into a vinylene-linked covalent organic framework (g-C₅₄N₆-COF) via Knoevenagel condensation with another D_3h-symmetric monomer 2,4,6-tris(4′-formyl-biphenyl-4-yl)-1,3,5-triazine. Replacing tricyanomesitylene with a C_2v-symmetric 3,5-dicyano-2,4,6-trimethylpyridine gave a less-symmetric vinylene-linked COF (g-C₅₂N₆-COF). The octupolar conjugated characters of g-C₅₄N₆-COF were reflected in its scarce solvatochromic effects either in ground or excited states, and endowed it with more promising semiconducting behavior as compared with g-C₅₂N₆-COF, such as enhanced light-harvesting and excellent photo-induced charge generation and separation. Along with the matched energy level, g-C₅₄N₆-COF enabled the two-half reactions of photocatalytic water splitting with an average O₂ evolution rate of 51.0 μmol h⁻¹ g⁻¹ and H₂ evolution rate of 2518.9 μmol h⁻¹ g⁻¹. Such values are among the highest of state-of-the-art COF photocatalysts.     

Synthesis of Robust MOFs@COFs Porous Hybrid Materials via an Aza-Diels–Alder Reaction: Towards High-Performance Supercapacitor Materials

Metal–organic frameworks (MOFs) and covalent organic frameworks (COFs) have attracted enormous attention in recent years. Recently, MOF@COF are emerging as hybrid architectures combining the unique features of the individual components to enable the generation of materials displaying novel physicochemical properties. Herein we report an unprecedented use of aza-Diels–Alder cycloaddition reaction as post-synthetic modification of MOF@COF-LZU1, to generate aza-MOFs@COFs hybrid porous materials with extended π-delocalization. A a proof-of-concept, the obtained aza-MOFs@COFs is used as electrode in supercapacitors displaying specific capacitance of 20.35 μF cm⁻² and high volumetric energy density of 1.16 F cm⁻³. Our approach of post-synthetic modification of MOFs@COFs hybrids implement rational design for the synthesis of functional porous materials and expands the plethora of promising application of MOFs@COFs hybrid porous materials in energy storage applications.     

One-Dimensional Covalent Organic Frameworks for the 2e− Oxygen Reduction Reaction

Two-dimensional covalent organic frameworks (2D COFs) are often employed for electrocatalytic systems because of their structural diversity. However, the efficiency of atom utilization is still in need of improvement, because the catalytic centers are located in the basal layers and it is difficult for the electrolytes to access them. Herein, we demonstrate the use of 1D COFs for the 2e⁻ oxygen reduction reaction (ORR). The use of different four-connectivity blocks resulted in the prepared 1D COFs displaying good crystallinity, high surface areas, and excellent chemical stability. The more exposed catalytic sites resulted in the 1D COFs showing large electrochemically active surface areas, 4.8-fold of that of a control 2D COF, and thus enabled catalysis of the ORR with a higher H₂O₂ selectivity of 85.8 % and activity, with a TOF value of 0.051 s⁻¹ at 0.2 V, than a 2D COF (72.9 % and 0.032 s⁻¹). This work paves the way for the development of COFs with low dimensions for electrocatalysis.     

Supramolecular Engineering of Cathode Materials for Aqueous Zinc-ion Energy Storage Devices: Novel Benzothiadiazole Functionalized Two-Dimensional Olefin-Linked COFs

Two-dimensional covalent organic frameworks (COFs) have emerged as promising materials for energy storage applications exhibiting enhanced electrochemical performance. While most of the reported organic cathode materials for zinc-ion batteries use carbonyl groups as electrochemically-active sites, their high hydrophilicity in aqueous electrolytes represents a critical drawback. Herein, we report a novel and structurally robust olefin-linked COF-TMT-BT synthesized via the aldol condensation between 2,4,6-trimethyl-1,3,5-triazine (TMT) and 4,4′-(benzothiadiazole-4,7-diyl)dibenzaldehyde (BT), where benzothiadiazole units are explored as novel electrochemically-active groups. Our COF-TMT-BT exhibits an outstanding Zn²⁺ storage capability, delivering a state-of-the-art capacity of 283.5 mAh g⁻¹ at 0.1 A g⁻¹. Computational and experimental analyses reveal that the charge-storage mechanism in COF-TMT-BT electrodes is based on the supramolecularly engineered and reversible Zn²⁺ coordination by the benzothiadiazole units.     

Designing Thiophene-Enriched Fully Conjugated 3D Covalent Organic Framework as Metal-Free Oxygen Reduction Catalyst for Hydrogen Fuel Cells

The application of three-dimensional (3D) covalent organic frameworks (COFs) in renewable energy fields is greatly limited due to their non-conjugated skeletons. Here, we design and successfully synthesize a thiophene-enriched fully conjugated 3D COF (BUCT-COF-11) through an all-thiophene-linked saddle-shaped building block (COThTh-CHO). The BUCT-COF-11 exhibits excellent semiconducting property with intrinsic metal-free oxygen reduction reaction (ORR) activity. Using the COF as cathode catalyst, the assembled anion-exchange membrane fuel cells (AEMFCs) exhibited a high peak power density up to 493 mW cm⁻². DFT calculations reveal that thiophene introduction in the COF not only improves the conductivity but also optimizes the electronic structure of the sample, which therefore boosts the ORR performance. This is the first report on the application of COFs as metal-free catalysts in fuel cells, demonstrating the great potential of fully conjugated 3D COFs as promising semiconductors in energy fields.     

Metal–Organic Frameworks and Other Crystalline Materials for Ultrahigh Superprotonic Conductivities of 10−2 S cm−1 or Higher

Proton-conducting materials in the solid state have received immense attention for their role as electrolytes in proton-exchange membrane fuel cells. Recently, crystalline materials—metal–organic frameworks (MOFs), hydrogen-bonded organic frameworks (HOFs), covalent organic frameworks (COFs), polyoxometalates (POMs), and porous organic crystals—have become an exciting research topic in the field of proton-conducting materials. For a better electrolyte, a high proton conductivity on the order of 10⁻² S cm⁻¹ or higher is preferred as efficient proton transport between the electrodes is ultimately necessary. With an emphasis on design principles, this Concept will focus on MOFs and other crystalline solid-based proton-conducting platforms that exhibit “ultrahigh superprotonic” conductivities with values in excess of 10⁻² S cm⁻¹. While only a handful of MOFs exhibit such an ultrahigh conductivity, this quality in other systems is even rarer. In addition to interpreting the structural–functional correlation by taking advantage of their crystalline nature, we address the challenges and promising directions for future research.     

Near IR Bandgap Semiconducting 2D Conjugated Metal-Organic Framework with Rhombic Lattice and High Mobility

Two-dimensional conjugated metal–organic frameworks (2D c-MOFs) are emerging as a unique class of electronic materials. However, 2D c-MOFs with band gaps in the Vis-NIR and high charge carrier mobility are rare. Most of the reported conducting 2D c-MOFs are metallic (i.e. gapless), which largely limits their use in logic devices. Herein, we design a phenanthrotriphenylene-based, D_2h-symmetric π-extended ligand ( OHPTP ), and synthesize the first rhombic 2D c-MOF single crystals ( Cu₂(OHPTP) ). The continuous rotation electron diffraction (cRED) analysis unveils the orthorhombic crystal structure at the atomic level with a unique slipped AA stacking. The Cu₂(OHPTP) is a p-type semiconductor with an indirect band gap of ≈0.50 eV and exhibits high electrical conductivity of 0.10 S cm⁻¹ and high charge carrier mobility of ≈10.0 cm² V⁻¹ s⁻¹. Theoretical calculations underline the predominant role of the out-of-plane charge transport in this semiquinone-based 2D c-MOF.     

Pore Geometry and Surface Engineering of Covalent Organic Frameworks for Anhydrous Proton Conduction

Developing new materials for anhydrous proton conduction under high-temperature conditions is significant and challenging. Herein, we create a series of highly crystalline covalent organic frameworks (COFs) via a pore engineering approach. We simultaneously engineer the pore geometry (generating concave dodecagonal nanopores) and pore surface (installing multiple functional groups such as −C=N−, −OH, −N=N− and −CF₃) to improve the utilization efficiency and host–guest interaction of proton carriers, hence benefiting the enhancement of anhydrous proton conduction. Upon loading with H₃PO₄, COFs can realize a proton conductivity of 2.33×10⁻² S cm⁻¹ under anhydrous conditions, among the highest values of all COF materials. These materials demonstrate good stability and maintain high proton conductivity over a wide temperature range (80–160 °C). This work paves a new way for designing COFs for anhydrous proton conduction applications, which shows great potential as high-temperature proton exchange membranes.     

Two-dimensional Covalent Organic Frameworks for Electrochromic Switching

The electrochromic materials have received immense attention for the fabrication of smart optoelectronic devices. The alteration of the redox states of the electroactive functionalities results in the color change in response to electrochemical potential. Even though transition metal oxides, redox-active small organic molecules, conducting polymers, and metallopolymers are known for electrochromism, advanced materials demonstrating multicolor switching with fast response time and high durability are of increasing demand. Recently, two-dimensional covalent organic frameworks (2D COFs) have been demonstrated as electrochromic materials due to their tunable redox functionalities with highly ordered structure and large specific surface area facilitating fast ion transport. Herein, we have discussed the mechanistic insights of electrochromism in 2D COFs and their structure-property relationship in electrochromic performance. Furthermore, the state-of-the-art knowledge for developing the electrochromic 2D COFs and their potential application in next-generation display devices are highlighted.     

Synthesizing Interpenetrated Triazine-based Covalent Organic Frameworks from CO2

Crystalline triazine-based covalent organic frameworks (COFs) are aromatic nitrogen-rich porous materials. COFs typically show high thermal/chemical stability, and are promising for energy applications, but often require harsh synthesis conditions and suffer from low crystallinity. In this work, we propose an environmentally friendly route for the synthesis of crystalline COFs from CO₂ molecules as a precursor. The mass ratio of CO₂ conversion into COFs formula unit reaches 46.3 %. The synthesis consists of two steps; preparation of 1,4-piperazinedicarboxaldehyde from CO₂ and piperazine, and condensation of the dicarboxaldehyde and melamine to construct the framework. The CO₂-derived COF has a 3-fold interpenetrated structure of 2D layers determined by powder X-ray diffraction, high-resolution transmission electron microscopy, and select-area electron diffraction. The structure shows a high Brunauer–Emmett–Teller surface area of 945 m² g⁻¹ and high stability against strong acid (6 M HCl), base (6 M NaOH), and boiling water over 24 hours. Post-modification of the framework with oxone has been demonstrated to modulate hydrophilicity, and it exhibits proton conductivity of 2.5×10⁻² S cm⁻¹ at 85 °C, 95 % of relative humidity.     

Hydrogen‐Bonded Organic Frameworks (HOFs): A New Class of Porous Crystalline Proton‐Conducting Materials

Two porous hydrogen‐bonded organic frameworks (HOFs) based on arene sulfonates and guanidinium ions are reported. As a result of the presence of ionic backbones appended with protonic source, the compounds exhibit ultra‐high proton conduction values (σ) 0.75× 10^?2 S cm^?1 and 1.8×10^?2 S cm^?1 under humidified conditions. Also, they have very low activation energy values and the highest proton conductivity at ambient conditions (low humidity and at moderate temperature) among porous crystalline materials, such as metal–organic frameworks (MOFs) and covalent organic frameworks (COFs). These values are not only comparable to the conventionally used proton exchange membranes, such as Nafion used in fuel cell technologies, but is also the highest value reported in organic‐based porous architectures. Notably, this report inaugurates the usage of crystalline hydrogen‐bonded porous organic frameworks as solid‐state proton conducting materials.     

Intermediate Formation of Macrocycles for Efficient Crystallization of 2D Covalent Organic Frameworks with Enhanced Photocatalytic Hydrogen Evolution

Covalent organic frameworks (COFs) have gained significant attention as key photocatalysts for efficient solar light conversion into hydrogen production. Unfortunately, the harsh synthetic conditions and intricate growth process required to obtain highly crystalline COFs greatly hinder their practical application. Herein, we report a simple strategy for the efficient crystallization of 2D COFs based on the intermediate formation of hexagonal macrocycles. Mechanistic investigation suggests that the use of 2,4,6-triformyl resorcinol (TFR) as the asymmetrical aldehyde build block allows the equilibration between irreversible enol-to-keto tautomerization and dynamic imine bonds to produce the hexagonal β-ketoenamine-linked macrocycles, the formation of which could provide COFs with high crystallinity in half hour. We show that COF-935 with 3 wt % Pt as cocatalyst exhibit a high hydrogen evolution rate of 67.55 mmol g⁻¹ h⁻¹ for water splitting when exposed to visible light. More importantly, COF-935 exhibits an average hydrogen evolution rate of 19.80 mmol g⁻¹ h⁻¹ even at a low loading of only 0.1 wt % Pt, which is a significant breakthrough in this field. This strategy would provide valuable insights into the design of highly crystalline COFs as efficient organic semiconductor photocatalysts.