Spectrally Switchable Photodetection with Near-Infrared-Absorbing Covalent Organic Frameworks

Semiconducting Covalent Organic Frameworks

Semiconductors form the foundational bedrock of modern electronics and numerous cutting-edge technologies. Particularly, semiconductors crafted from organic building blocks hold immense promise as next-generation pioneers, thanks to their vast array of chemical structures, customizable frontier orbital energy levels and bandgap structures, and easily adjustable π electronic properties. Over the past 50 years, advancements in chemistry and materials science have facilitated extensive investigations into small organic π compounds, oligomers, and polymers, resulting in a rich library of organic semiconductors. However, a longstanding challenge persists: how to organize π building units or chains into well-defined π structures, which are crucial for the performance of organic semiconductors. Consequently, the pursuit of methodologies capable of synthesizing and/or fabricating organic semiconductors with ordered structures has emerged as a frontier in organic and polymeric semiconductor research. In this context, covalent organic frameworks (COFs) stand out as unique platforms allowing for the covalent integration of organic π units into periodically ordered π structures, thus facilitating the development of semiconductors with extended yet precisely defined π architectures. Since their initial report in 2008, significant strides have been made in exploring various chemistries to develop semiconducting COFs, resulting in a rich library of structures, properties, functions, and applications. This review provides a comprehensive yet focused exploration of the general structural features of semiconducting COFs, outlining the basic principles of structural design, illustrating the linkage chemistry and synthetic strategies based on typical one-pot polymerization reactions to demonstrate the growth of bulk materials, nanosheets, films, and membranes. By elucidating the interactions between COFs and various entities such as photons, phonons, electrons, holes, ions, molecules, and spins, this review categorizes semiconducting COFs into nine distinct sections: semiconductors, photoconductors, light emitters, sensors, photocatalysts, photothermal conversion materials, electrocatalysts, energy storage electrodes, and radical spin materials, focusing on disclosing structure-originated properties and functions. Furthermore, this review scrutinizes structure-function correlations and highlights the unique features, breakthroughs, and challenges associated with semiconducting COFs. Furnished with foundational knowledges and state-of-the-art insights, this review predicts the fundamental issues to be addressed and outlines future directions for semiconducting COFs, offering a comprehensive overview of this rapidly evolving and remarkable field.

A critical review of COFs-based photocatalysis for environmental remediation

Covalent organic frameworks (COFs) are highly porous crystalline polymers formed through covalent bonding of molecular building blocks. Numerous fabrication strategies have been developed, including solvothermal, ionothermal, microwave, mechanochemical, and sonochemical methods, alongside ligand substitution and post-modification techniques, which allow for precise control over the structures and properties of COFs. The exceptional physicochemical stability, large specific surface area, broad visible light absorption, and extended π-conjugated systems have sparked significant interest in photocatalytic applications. Recently, COFs have shown remarkable efficacy in environmental remediation, demonstrating the ability to degrade a wide range of organic pollutants, including dyes, antibiotics, and drugs, as well as to reduce/oxidize heavy metals such as Cr(VI), U(VI), and As(III), in addition to targeting biological pollutants. This review comprehensively explores recent advancements in COFs-based photocatalysis, covering synthetic methods, COF types, modification method, theoretical calculations, environmental applications, and underlying mechanisms. Additionally, the challenges and opportunities for COFs as a robust, cost-effective technology in practical applications was discussed, and offering valuable insights for researchers in environmental remediation, materials science, and photocatalysis.

Intramolecular Charge Transfer-Regulated Isomeric Covalent Organic Frameworks for Multiple Solvent-Response

Stimulus-responsive covalent organic frameworks (COFs) own color-switching characteristics when exposed to external stimuli. However, the investigations on the multiple solvent-responsive COFs remain a challenge due to the synthetic difficulties and uncontrollable charge transfer process toward various solvents. In this contribution, two novel isomeric COFs with a regulated intramolecular charge transfer (ICT) process by modulating the distance between the donor/acceptor and the linkage are synthesized. The as-prepared two isomeric COFs exhibited significantly distinct solvatochromic behaviors in water, acid, and halogenated solvents, respectively. These multiple solvent-responsive functions are attributed to the various enhancement degrees of the ICT process by the hydrogen bond interactions, protonation interactions, and halogen/π interactions, respectively. In addition, the two isomeric COFs are employed as stimulation-responsive powder or ink, displaying excellent image and data encryption performances. The work can not only offer a novel viewpoint for the creation of multiple solvent-responsive COFs but also expand the COFs' potential applications in information encryption and anti-counterfeiting.

Viologen Covalent Organic Framework Mediates Near-Infrared Light-Induced Electron Transfer for Catalytic Oxidative Coupling Reactions

Near-infrared (NIR) light-driven photoreactions are advantageous over visible light-driven ones because NIR photons have lower energy and fewer side reactions, deeper penetration in reaction media, and high abundance in the solar spectrum. However, currently available covalent organic frameworks (COFs) absorb in the UV-vis region and catalyze photoreactions under blue or white light irradiation. Herein, we report a linker-to-linker charge transfer process in a viologen-linked porphyrin COF (Vio-COF), leading to a novel type of hyperporphyrin effect and extending the absorption into the NIR region with an absorption edge at 998 nm. Under NIR irradiation, photoinduced charge separation in Vio-COF generates a viologen radical that efficiently reduces oxygen to form superoxide radicals for catalytic oxidative coupling reactions. The proximity of porphyrin and viologen units within the framework significantly enhances the catalytic performance of Vio-COF, outperforming its homogeneous counterparts in aerobic oxidative amidation and amine coupling reactions. Vio-COF was readily recycled and used in six oxidative coupling reactions.

Rational Synthesis of Isomeric Graphdiyne Frameworks toward Single-Ruthenium Catalysts and High-Performance Nitrogen Reduction

Graphdiynes (GDYs), synthesized via direct coupling of arylacetylenes, have attracted great attention due to their unique electronic properties and structural diversity, typically forming 2D layered frameworks. However, crystalline GDY-like frameworks with 3D topology remain challenging to synthesize. Here, the study reports two highly crystalline, isomeric GDY-like frameworks with ThSi2 topology, constructed from 2,2'-binaphthalene and 6,6'-biazulene-based monomers. The azulene-based framework, due to its large dipole moment, exhibits a narrow bandgap of 1.15 eV, significantly lower than its naphthalene counterpart (2.33 eV). As ruthenium (Ru) single-atom supports, these frameworks enable strong Ru-diyne interactions, achieving an ammonia yield rate of 188.7 ± 1.6 µg h-1 mgcat -1 and a Faradaic efficiency of 37.4 ± 0.6%. Such bicontinuous channels and tunable electronic structures offer electrocatalysis field new opportunities. Moreover, the azulene-based framework, featuring a higher highest occupied molecular orbital  and lower lowest unoccupied molecular orbital  energy level, ensures superior electron mobility. These 3D crystalline frameworks introduce a new covalent organic framework (COF) family with diyne linkages and pure carbon skeletons, broadening the scope of COF materials. Their well-defined structures provide an ideal platform for tuning optoelectronic properties, enabling fundamental studies on structure-property relationships and opening new opportunities for catalytic and electronic applications.

Azatriangulene-Based Conductive C═C Linked Covalent Organic Frameworks with Near-Infrared Emission

Two near-infrared (NIR) emissive π-conjugated covalent organic frameworks (COFs) pTANG1 and pTANG2 are synthesized using Knoevenagel condensation of trioxaazatriangulenetricarbaldehyde (TATANG) with benzene- and biphenyldiacetonitriles, respectively. The morphology of the COFs is affected by the size of TATANG precursor crystals. Donor-acceptor interactions in these COFs result in small bandgaps (≈1.6 eV) and NIR emission (λmax = 789 nm for pTANG1). pTANG1 can absorb up to 9 molecules of water per unit cell, which is accompanied by a marked quenching of the NIR emission, suggesting applications as humidity sensors. p-Doping with magic blue significantly increases the electrical conductivities of the COFs by up to 8 orders of magnitude, with the room temperature conductivity of pTANG1 reaching 0.65 S cm-1, the highest among reported C═C linked COFs. 1H NMR relaxometry, temperature-dependent fluorescence spectroscopy, and DFT calculations reveal that the higher rigidity of the shorter phenylene linker is responsible for the more extended conjugation (red-shifted emission, higher electrical conductivity) of pTANG1 compared to pTANG2.

Photons, Excitons, and Electrons in Covalent Organic Frameworks

Covalent organic frameworks (COFs) are created by the condensation of molecular building blocks and nodes to form two-dimensional (2D) or three-dimensional (3D) crystalline frameworks. The diversity of molecular building blocks with different properties and functionalities and the large number of possible framework topologies open a vast space of possible well-defined porous architectures. Besides more classical applications of porous materials such as molecular absorption, separation, and catalytic conversions, interest in the optoelectronic properties of COFs has recently increased considerably. The electronic properties of both the molecular building blocks and their linkage chemistry can be controlled to tune photon absorption and emission, to create excitons and charge carriers, and to use these charge carriers in different applications such as photocatalysis, luminescence, chemical sensing, and photovoltaics. In this Perspective, we will discuss the relationship between the structural features of COFs and their optoelectronic properties, starting with the building blocks and their chemical connectivity, layer stacking in 2D COFs, control over defects and morphology including thin film synthesis, exploring the theoretical modeling of structural, electronic, and dynamic features of COFs, and discussing recent intriguing applications with a focus on photocatalysis and photoelectrochemistry. We conclude with some remarks about present challenges and future prospects of this powerful architectural paradigm.

High Performance Memristors Based on Imine-Linked Covalent Organic Frameworks Obtained Using a Protonation Modification Strategy

Imine-linked covalent organic frameworks (COFs) are garnering substantial interest in resistive random-access memory, attributed to their superior crystallinity, excellent chemical and thermal stability, and modifiable molecular structures. However, the development of high-performance COF-based memristors impeded by challenges such as low conjugation degree of imine bonds and poor electron delocalization ability. Herein, we report a protonation strategy to modify the imine bonds of donor-acceptor (D-A) type COFs. This modification significantly enhances the electron delocalization capability of imine bonds, lowers the energy barriers for electron injection from electrodes, and stabilizes the conductive charge transfer state, thus markedly improving device performance. The protonated COF-BTT-BPy and COF-BTT-TAPT thin films-based memristors show remarkable device performance with a high ON/OFF current ratio of 105, a low driving voltage, and outstanding endurance exceeding 600 and 1300 cycles, respectively, which is nearly twice the durability of analogous non-protonated COFs-based memristors. Notably, the protonated COF-BTT-TAPT-based memristor exhibit the highest number of cycles reported at present. This work not only unprecedentedly enhances the performance of COF-based memristors, but also provides a universal and promising approach for the molecular design and potential application of D-A type imine-linked COFs.

Dynamic Two-Dimensional Covalent Organic Frameworks with Large Solvent-Induced Lattice Expansion

Dynamic covalent organic frameworks (COFs) can switch reversibly between crystalline phases with different unit cell parameters and porosities upon physisorption of guest molecules. While impressive changes in unit cell volumes have been realized for three-dimensional frameworks, the solvent-induced volume changes of two-dimensional (2D) COFs have remained comparably small. We have now developed a series of 2D COFs where we systematically varied the length of the interconnecting bridge units. The optimized materials achieve solvent-induced expansions of up to 85% relative to the volume of the solvent-free contracted COFs. These structural changes are fully reversible and the frameworks fully retain their high degree of crystalline order. This study introduces a versatile design strategy for dynamic 2D COFs, which could enable future applications of these materials in gas separation and sensing.

Electrostatically Designing Materials and Interfaces

Collective electrostatic effects arise from the superposition of electrostatic potentials of periodically arranged (di)polar entities and are known to crucially impact the electronic structures of hybrid interfaces. Here, it is discussed, how they can be used outside the beaten paths of materials design for realizing systems with advanced and sometimes unprecedented properties. The versatility of the approach is demonstrated by applying electrostatic design not only to metal-organic interfaces and adsorbed (complex) monolayers, but also to inter-layer interfaces in van der Waals heterostructures, to polar metal-organic frameworks (MOFs), and to the cylindrical pores of covalent organic frameworks (COFs). The presented design ideas are straightforward to simulate and especially for metal-organic interfaces also their experimental implementation has been amply demonstrated. For van der Waals heterostructures, the needed building blocks are available, while the required assembly approaches are just being developed. Conversely, for MOFs the necessary growth techniques exist, but more work on advanced linker molecules is required. Finally, COF structures exist that contain pores decorated with polar groups, but the electrostatic impact of these groups has been largely ignored so far. All this suggest that the dawn of the age of electrostatic design is currently experienced with potential breakthroughs lying ahead.

Photothermal Conversion Porous Organic Polymers: Design, Synthesis, and Applications

Solar energy is a primary form of renewable energy, and photothermal conversion is a direct conversion process with tunable conversion efficiency. Among various kinds of photothermal conversion materials, porous organic polymers (POP) are widely investigated owing to their controllable molecular design, tailored porous structures, good absorption of solar light, and low thermal conductivity. A variety of POP, such as conjugated microporous polymers (CMP), covalent organic frameworks (COF), hyper-crosslinked porous polymers (HCP), polymers of intrinsic microporosity (PIM), porous ionic polymers (PIP), are developed and applied in photothermal conversion applications of seawater desalination, latent energy storage, and biomedical fields. In this review, a comprehensive overview of the recent advances in POP for photothermal conversion is provided. The micro molecular structure characteristics and macro morphology of POP are designed for applications such as seawater desalination, latent heat energy storage, phototherapy and photodynamic therapy, and drug delivery. Besides, a probe into the underlying mechanism of structural design for constructing POP with excellent photothermal conversion performance is methodicalized. Finally, the remaining challenges and prospective opportunities for the future development of POP for solar energy-driven photothermal conversion applications are elucidated.

Triphenylamine-Based Covalent Organic Framework Films for Dopamine-Responsive Electrofluorochromism

Covalent organic framework (COF) film with electrofluorochromic (EFC) and electrochromic (EC) properties has been synthesized by using triphenylamine-based monomers. The film exhibited a high maximum fluorescence contrast of 151 when subjected to a drive voltage of 0.75 V vs the Ag/AgCl electrode, causing the fluorescence to be quenched, which resulted in the EFC process's "fluorescence off" state. The switching times for the fluorescence on and off states were 0.51 and 7.79 s, respectively. Over the same voltage range, the COF film also displayed EC properties, achieving a contrast of 50.23% and a coloration efficiency of 297.4 cm2 C-1 at 532 nm, with switching times of 18.6 s for coloration and 0.7 s for bleaching. Notably, the quenched fluorescence of the COF film could be restored by adding dopamine as a reductant. This phenomenon enabled the implementation of a NAND logic gate using the applied potential as a physical input and dopamine addition as a chemical input. This study demonstrates the successful development of COF films with bifunctional EFC and EC properties, showcasing their potential for use in constructing advanced optoelectronic devices.

2D Conjugated Polymer Thin Films for Organic Electronics: Opportunities and Challenges

2D conjugated polymers (2DCPs) possess extended in-plane π-conjugated lattice and out-of-plane π-π stacking, which results in enhanced electronic performance and potentially unique band structures. These properties, along with predesignability, well-defined channels, easy postmodification, and order structure attract extensive attention from material science to organic electronics. In this review, the recent advance in the interfacial synthesis and conductivity tuning strategies of 2DCP thin films, as well as their application in organic electronics is summarized. Furthermore, it is shown that, by combining topology structure design and targeted conductivity adjustment, researchers have fabricated 2DCP thin films with predesigned active groups, highly ordered structures, and enhanced conductivity. These films exhibit great potential for various thin-film organic electronics, such as organic transistors, memristors, electrochromism, chemiresistors, and photodetectors. Finally, the future research directions and perspectives of 2DCPs are discussed in terms of the interfacial synthetic design and structure engineering for the fabrication of fully conjugated 2DCP thin films, as well as the functional manipulation of conductivity to advance their applications in future organic electronics.

Lewis Acid Regulation Strategy for Constructing D-A-A Covalent Organic Frameworks with Enhanced Photocatalytic Organic Conversion

Owing to their excellent photoelectric properties, donor-acceptor (D-A) type photocatalytic covalent organic frameworks (COFs) have attracted significant research interest in recent years. However, the limited D-A structural units of existing COFs restrict the development of novel and efficient photocatalytic COF materials. To solve this problem, we developed a series of D-A-A-type COFs utilizing a Lewis acid regulation strategy, in which Lewis acids act as the coordination centers, and pyridine and cyano groups act as ligands. Lewis acid sites in COFs serve as electron acceptors, facilitating the separation and transfer of photogenerated electron-hole pairs. This process is crucial for photocatalysis because it significantly increases the efficiency of the catalytic reaction by reducing the recombination rate of charge carriers. The developed Lewis acid-activated D-A-A COFs efficiently catalyzed the hydroxylation of various phenylboronic acid compounds under visible light. The developed catalysts are expected to contribute to increasing the fabrication efficiency of industrially important organic materials.

Dynamic two-dimensional covalent organic frameworks

Porous covalent organic frameworks (COFs) enable the realization of functional materials with molecular precision. Past research has typically focused on generating rigid frameworks where structural and optoelectronic properties are static. Here we report dynamic two-dimensional (2D) COFs that can open and close their pores upon uptake or removal of guests while retaining their crystalline long-range order. Constructing dynamic, yet crystalline and robust frameworks requires a well-controlled degree of flexibility. We have achieved this through a 'wine rack' design where rigid π-stacked columns of perylene diimides are interconnected by non-stacked, flexible bridges. The resulting COFs show stepwise phase transformations between their respective contracted-pore and open-pore conformations with up to 40% increase in unit-cell volume. This variable geometry provides a handle for introducing stimuli-responsive optoelectronic properties. We illustrate this by demonstrating switchable optical absorption and emission characteristics, which approximate 'null-aggregates' with monomer-like behaviour in the contracted COFs. This work provides a design strategy for dynamic 2D COFs that are potentially useful for realizing stimuli-responsive materials.

Covalent Organic Framework-Structured Raman Probes for Ultrasensitive In Vivo Bioimaging

Organic Raman probes, including polymers and small molecules, have attracted great attention in biomedical imaging owing to their excellent biocompatibility. However, the development of organic Raman probes is usually hindered by a mismatch between their absorption spectra and wavelength-fixed excitation, which makes it difficult to achieve resonance excitation necessary to obtain strong Raman signals. Herein, we introduce a covalent organic framework (COF) into the fine absorption spectrum regulation of organic Raman probes, resulting in their significant Raman signal enhancement. In representative examples, a polymer poly(diketopyrrolopyrrole-p-phenylenediamine) (DPP-PD) and a small molecule azobenzene are transformed into the corresponding COF-structured Raman probes. Their absorption peaks show an accurate match of less than 5 nm with the NIR excitation. As such, the COF-structured Raman probes acquire highly sensitive bioimaging capabilities compared to their precursors with negligible signals. By further mechanism studies, we discover that the crystallinity and size of COFs directly affect the π-conjugation degree of Raman probes, thus changing their bandgaps and absorption spectra. Our study offers a universal and flexible method for improving the signal performance of organic Raman probes without changing their structural units, making it more convenient to obtain the highly sensitive organic Raman probes for in vivo bioimaging.

An Appraisal for Providing Charge Transfer (CT) Through Synthetic Porous Frameworks for their Semiconductor Applications

In recent years, there has been considerable focus on the development of charge transfer (CT) complex formation as a means to modify the band gaps of organic materials. In particular, CT complexes alternate layers of aromatic molecules with donor (D) and acceptor (A) properties to provide inherent electrical conductivity. In particular, the synthetic porous frameworks as attractive D-A components have been extensively studied in recent years in comparison to existing D-A materials. Therefore, in this work, the synthetic porous frameworks are classified into conjugated microporous polymers (CMPs), covalent organic frameworks (COFs), and metal-organic frameworks (MOFs) and compare high-quality materials for CT in semiconductors. This work updates the overview of the above porous frameworks for CT, starting with their early history regarding their semiconductor applications, and lists CT concepts and selected key developments in their CT complexes and CT composites. In addition, the network formation methods and their functionalization are discussed to provide access to a variety of potential applications. Furthermore, several theoretical investigations, efficiency improvement techniques, and a discussion of the electrical conductivity of the porous frameworks are also highlighted. Finally, a perspective of synthetic porous framework studies on CT performance is provided along with some comparisons.

Templated 2D Polymer Heterojunctions for Improved Photocatalytic Hydrogen Production

2D polymers have emerged as one of the most promising classes of organic photocatalysts for solar fuel production due to their tunability, charge-transport properties, and robustness. They are however difficult to process and so there are limited studies into the formation of heterojunction materials incorporating these components. In this work, a novel templating approach is used to combine an imine-based donor polymer and an acceptor polymer formed through Knoevenagel condensation. Heterojunction formation is shown to be highly dependent on the topological match of the donor and acceptor polymers with the most active templated material found to be between three and nine times more active for photocatalysis than its constituent components. Transient absorption spectroscopy reveals that this improvement is due to faster charge separation and more efficient charge extraction in the templated heterojunction. The templated material shows a very high hydrogen evolution rate of >20 mmol h-1 m-2 with an ascorbic acid hole scavenger but also produces hydrogen in the presence of only water and a cobalt-based redox mediator. This suggests the improved charge-separation interface and reduced trapping accessed through this approach could be suitable for Z-scheme formation.

Phospholipid-inspired alkoxylation induces crystallization and cellular uptake of luminescent COF nanocarriers

The porous nature and structural variability of covalent organic frameworks (COFs) make them preferred for drug loading and delivery applications. However, most COF materials suffer from poor luminescent properties and inefficiency for cell uptake. Herein, we experimentally demonstrate the crucial role of long alkoxy chains in the synthesis of crystalline COF nanostructures with high cellular uptake efficiency. After luminescence integration through band engineering, the semiconducting COF exhibits an optical bandgap of 2.05 eV, an emission wavelength of 632 nm, a high quantum yield of 37 %, and excellent fluorescence stability (100 % at 3 h). Such excellent optical properties of the designed COF nanocarriers enable quantitative evaluations of cellular uptake and visual tracking of drug delivery. It was demonstrated that the cellular uptake efficiency was enhanced by orders of magnitude for the COF after the introduction of long n-octyloxy chains, which firstly delivered the anticancer camptothecin (CPT) to cell lysosomes, and then underwent "endo/lysosomal escape" to induce cell apoptosis. In vivo assay evidenced a significant enhancement in the therapeutic effect with a 96 % inhibition of tumor growth after 14 days of treatment. This progress sheds light on designing cutting-edge drug delivery nanosystems based on COF materials with integrated diagnostic and therapeutic functions.

Recent progress in emerging two-dimensional organic-inorganic van der Waals heterojunctions

Two-dimensional (2D) materials have attracted significant attention in recent decades due to their exceptional optoelectronic properties. Among them, to meet the growing demand for multifunctional applications, 2D organic-inorganic van der Waals (vdW) heterojunctions have become increasingly popular in the development of optoelectronic devices. These heterojunctions demonstrate impressive capability to synergistically combine the favourable characteristics of organic and inorganic materials, thereby offering a wide range of advantages. Also, they enable the creation of innovative device structures and introduce novel functionalities in existing 2D materials, avoiding the need for lattice matching in different material systems. Presently, researchers are actively working on improving the performance of devices based on 2D organic-inorganic vdW heterojunctions by focusing on enhancing the quality of 2D materials, precise stacking methods, energy band regulation, and material selection. Therefore, this review presents a thorough examination of the emerging 2D organic-inorganic vdW heterojunctions, including their classification, fabrication, and corresponding devices. Additionally, this review offers profound and comprehensive insight into the challenges in this field to inspire future research directions. It is expected to propel researchers to harness the extraordinary capabilities of 2D organic-inorganic vdW heterojunctions for a wider range of applications by further advancing the understanding of their fundamental properties, expanding the range of available materials, and exploring novel device architectures. The ongoing research and development in this field hold potential to unlock captivating advancements and foster practical applications across diverse industries.

A Stimuli-Responsive Dual-Emitting Covalent Organic Framework Shows Selective Sensing of Highly Corrosive Acidic Media via Fluorescence Turn-On Signal with White Light Emission

Luminescent covalent organic frameworks (LCOFs) have been employed as platforms for sensing analytes. Judicial incorporation of appropriate functional units inside the framework leads to the different electronic states in the presence of external stimuli, e.g., temperature, pH, etc. We report herein a new COF (TPEPy) as a solid-state acid sensor specific for the highly acidic environments that range from pH ∼0.5 to ∼3.0. This COF shows a protonation-induced reversible color change from bright yellow to deep red upon decreasing the pH from 3 to 0.5 and vice versa. No visual color change was, however, observed above pH 3.0. Photoluminescence (PL) studies show that the intrinsic emission peak of the TPEPy COF at 530 nm is shifted to 420 nm owing to the N-protonation of the imine nitrogen of COF within this pH range. Extensive studies demonstrate that the protonation behavior of the COF is counterion dependent. This was revealed when different acids, e.g., HCl, HNO3, HBr, and HI, were employed. The intensity of the proton-induced emission peak at 420 nm depends significantly upon the counterions with the order of HCl > HNO3 > HBr > HI. These anions interact with the protonated TPEPy COF by cation-anion and H-bonding interactions. Further, the pristine COF showed near white light emission at a particular pH of 2.5 (CIE coordinates 0.27, 0.32). From the PL spectrophotometric titrations, the deprotonation pKa was experimentally found to be 1.8 ± 0.02 for the TPEPy COF. The sensor reported herein is reversible, reusable, and regenerable and is useful for assessing pH fluctuations within a strongly acidic range via digital signaling.

A PEDOT enhanced covalent organic framework (COF) fluorescent probe for in vivo detection and imaging of Fe3

Simple and accurate in vivo monitoring of Fe3+ is essential for gaining a better understanding of its role in physiological and pathological processes. A novel fluorescent probe was synthesized via in situ solid-state polymerization of 3,4-ethylenedioxythiophene (PEDOT) in the pore channels of a covalent organic framework (COF). The PEDOT@COF fluorescent probe exhibited an absolute quantum yield (QY) 3 times higher than COF. In the presence of Fe3+ the PEDOT@COF 475 nm fluorescence emission, 365 nm excitation, is quenched within 180 s. Fluorescence quenching is linear with Fe3+ in the concentration range of 0-960 μM, with a detection limit of 0.82 μM. The fluorescence quenching mechanism was attributed to inner filter effect (IEF), photoinduced electron transfer (PET) and static quenching (SQE) between PEDOT@COF and Fe3+. A paper strip-based detector was designed to facilitate practical applicability, and the PEDOT@COF probe successfully applied to fluorescence imaging of Fe3+ levels in vivo. This work details a tool of great promise for enabling detailed investigations into the role of Fe3+ in physiological and pathological diseases.

Hydrogen Isotope Effect Endows a Breakthrough in Photoluminescent Covalent Organic Frameworks

Luminescent covalent organic frameworks (LCOFs) have emerged as indispensable candidates in various applications due to their greater tunable emitting properties and structural robustness compared to small molecule emitters. An unsolved issue in this area is developing highly luminescent LCOFs of which the nonradiative quenching pathways were suppressed as much as possible. Here, a robust aminal-linked COF (DD-COF) possessing perdeuterated light-emitting monomers was designed and synthesized. The solid-state photoluminescence quantum yield of the DD-COF reaches 81%, significantly outcompeting all state-of-the-art LCOFs reported so far. The exceptional luminescent efficiency is attributed to the inhibition of different pathways of nonradiative decay, especially from bond vibrations where only substitution by a heavier isotope with a lower zero-point vibration frequency works. Furthermore, the prepared deuterated COF not only boosts higher photostability under UV irradiation but also enables superior fluorescence sensing performance for iodine detection compared to nondeuterated COF.

Crystalline, Porous Helicene Covalent Organic Frameworks

Helicenes are a class of fascinating chiral helical molecules with rich chemistry developed continuously over the past 100 years. Their helical, conjugated, and twisted structures make them attractive for constructing molecular systems. However, studies over the past century are mainly focused on synthesizing helicenes with increased numbers of aromatic rings and complex heterostructures, while research on inorganic, organic, and polymeric helicene materials is still embryonic. Herein, we report the first examples of helicene covalent organic frameworks, i.e., [7]Helicene sp2 c-COF-1, by condensing [7]Helicene dialdehyde with trimethyl triazine via the C=C bond formation reaction under solvothermal conditions. The resultant [7]Helicene sp2 c-COF-1 exhibits prominent X-ray diffraction peaks and assumes a highly ordered 2D lattice structure originated from the twisted configuration of [7]Helicene unit. The C=C linked [7]Helicene sp2 c-COF-1 materials exhibited extended π conjugation and broadly tuned their absorption, emission, redox activity, photoconductivity, and light-emitting activity, demonstrating rich multifunctionalities and great potentials in developing various applications. This work opens a way to a new family of COFs as well as helicene materials, enabling the exploration of unprecedented π architectures and properties.

Isomeric Dual-Pore Two-Dimensional Covalent Organic Frameworks

Two-dimensional (2D) covalent organic frameworks (COFs) with hierarchical porosity have been increasingly recognized as promising materials in various fields. Besides, the 2D COFs with kagome (kgm) topology can exhibit unique optoelectronic features and have extensive applications. However, rational synthesis of the COFs with kgm topology remains challenging because of competition with a square-lattice topology. Herein, we report two isomeric dual-pore 2D COFs with kgm topology using a novel geometric strategy to reduce the symmetry of their building blocks, which are four-armed naphthalene-based and azulene-based isomeric monomers. Owing to the large dipole moment of azulene, as-prepared azulene-based COF (COF-Az) possesses a considerably narrow band gap of down to 1.37 eV, which is much narrower than the naphthalene-based 2D COF (COF-Nap: 2.28 eV) and is the lowest band gap among reported imine-linked dual-pore 2D COFs. Moreover, COF-Az was used as electrode material in a gas sensor and exhibits high selectivity for NO2, including a high response rate (58.7%) to NO2 (10 ppm), fast recovery (72 s), up to 10 weeks of stability, and resistance to 80% relative humidity, which are superior to those of reported COF-based NO2 gas sensors. The calculation and in situ experimental results indicate that the large dipole moment of azulene boosts the sensitivity of the imine linkages. The usage of isomeric building blocks not only enables the synthesis of 2D COFs with isometric kgm topology but also provides an azulene-based 2D platform for studying the structure-property correlations of COFs.

Saturated Linkers in Two-Dimensional Covalent Organic Frameworks Boost Their Luminescence

The development of highly luminescent two-dimensional covalent organic frameworks (COFs) for sensing applications remains challenging. To suppress commonly observed photoluminescence quenching of COFs, we propose a strategy involving interrupting the intralayer conjugation and interlayer interactions using cyclohexane as the linker unit. By variation of the building block structures, imine-bonded COFs with various topologies and porosities are obtained. Experimental and theoretical analyses of these COFs disclose high crystallinity and large interlayer distances, demonstrating enhanced emission with record-high photoluminescence quantum yields of up to 57% in the solid state. The resulting cyclohexane-linked COF also exhibits excellent sensing performance for the trace recognition of Fe³⁺ ions, explosive and toxic picric acid, and phenyl glyoxylic acid as metabolites. These findings inspire a facile and general strategy to develop highly emissive imine-bonded COFs for detecting various molecules.

Electrostatic Design of the Nanoscale Internal Surfaces of Porous Covalent Organic Frameworks

It is well established that the collective action of assemblies of dipoles determines the electronic structure of surfaces and interfaces. This raises the question, to what extent the controlled arrangement of polar units can be used to also tune the electronic properties of the inner surfaces of materials with nanoscale pores. In the present contribution, state-of-the-art density-functional theory calculations are used to show for the prototypical case of covalent organic frameworks (COFs) that this is indeed possible. Decorating pore walls with assemblies of polar entities bonded to the building blocks of the COF layers triggers a massive change of the electrostatic energy within the pores. This, inevitably, also changes the relative alignment between electronic states in the framework and in guest molecules and is expected to have significant impacts on charge separation in COF heterojunctions, on redox reactions in COFs-based electrodes, and on (photo)catalysis.

Photo-Enhanced Chemo-Transistor Platform for Ultrasensitive Assay of Small Molecules

Compared with traditional assay techniques, field-effect transistors (FETs) have advantages such as fast response, high sensitivity, being label-free, and point-of-care detection, while lacking generality to detect a wide range of small molecules since most of them are electrically neutral with a weak doping effect. Here, we demonstrate a photo-enhanced chemo-transistor platform based on a synergistic photo-chemical gating effect in order to overcome the aforementioned limitation. Under light irradiation, accumulated photoelectrons generated from covalent organic frameworks offer a photo-gating modulation, amplifying the response to small molecule adsorption including methylglyoxal, p-nitroaniline, nitrobenzene, aniline, and glyoxal when measuring the photocurrent. We perform testing in buffer, artificial urine, sweat, saliva, and diabetic mouse serum. The limit of detection is down to 10^-19 M methylglyoxal, about 5 orders of magnitude lower than existing assay technologies. This work develops a photo-enhanced FET platform to detect small molecules or other neutral species with enhanced sensitivity for applications in fields such as biochemical research, health monitoring, and disease diagnosis.

Construction of Monophosphine-Metal Complexes in Privileged Diphosphine-Based Covalent Organic Frameworks for Catalytic Asymmetric Hydrogenation

Privileged diphosphine ligands that chelate many transition metals to form stable chelation complexes are essential in a variety of catalytic processes. However, the exact identity of the catalytically active moieties remains ambiguous because the chelated metal catalysts may undergo rearrangement during catalysis to produce monophosphine-metal complexes, which are hard to isolate and evaluate the activities. By taking advantage of the isolation of two phosphorus atoms, we demonstrate here the successful construction of chiral monophosphine-Ir/Ru complexes of diphosphine ligands in covalent organic frameworks (COFs) for enantioselective hydrogenation. By condensation of the tetraaldehyde of enantiopure MeO-BIPHEP and linear aromatic diamines, we prepare two homochiral two-dimensional COFs with ABC stacking, in which the two P atoms of each diphosphine are separated and fixed far apart. Post-synthetic metalations of the COFs thus afford the single-site Ir/Ru-monophosphine catalysts, in contrast to the homogeneous chelated analogues, that demonstrated excellent catalytic and recyclable performance in the asymmetric hydrogenation of quinolines and β-ketoesters, affording up to 99.9% enantiomeric excess. Owing to the fact that the porous catalyst is capable of adsorbing and concentrating hydrogen, the catalytic reactions are promoted under ambient/medium pressure, which are typically performed under high pressure for homogeneous catalysis. This work not only shows that monophosphine-metal complexes of diphosphines can be catalytically active centers for asymmetric hydrogenation reactions but also provides a new strategy to prepare new types of privileged phosphine-based heterogeneous catalysts.

Targeted Synthesis of Isomeric Naphthalene-Based 2D Kagome Covalent Organic Frameworks

Targeted synthesis of kagome (kgm) topologic 2D covalent organic frameworks remains challenging, presumably due to the severe dependence on building units and synthetic conditions. Herein, two isomeric "two-in-one" monomers with different lengths of substituted arms based on naphthalene core (p-Naph and m-Naph) are elaborately designed and utilized for the defined synthesis of isomeric kgm Naph-COFs. The two isomeric frameworks exhibit splendid crystallinity and showcase the same chemical composition and topologic structure with, however, different pore channels. Interestingly, C₆₀ is able to uniformly be encapsulated into the triangle channels of m-Naph-COF via in situ incorporation method, while not the isomeric p-Naph-COF, likely due to the different pore structures of the two isomeric COFs. The resulting stable C₆₀ @m-Naph-COF composite exhibits much higher photoconductivity than the m-Naph-COF owing to charge transfer between the conjugated skeletons and C₆₀ guests.

Electrically Conductive Carbazole and Thienoisoindigo-Based COFs Showing Fast and Stable Electrochromism

Thienothiophene thienoisoindigo (ttTII)-based covalent organic frameworks (COFs) have been shown to offer low band gaps and intriguing optical and electrochromic properties. So far, only one tetragonal thienothiophene thienoisoindigo-based COF has been reported showing stable and fast electrochromism and good coloration efficiencies. We have developed two novel COFs using this versatile and nearly linear ttTII building block in a tetragonal and a hexagonal framework geometry to demonstrate their attractive features for optoelectronic applications of thienoisoindigo-based COFs. Both COFs exhibit good electrical conductivities, show promising optical absorption features, are redox-active, and exhibit a strong electrochromic behavior when applying an external electrical stimulus, shifting the optical absorption even farther into the NIR region of the electromagnetic spectrum and achieving absorbance changes of up to 2.5 OD. Cycle-stable cyclic voltammograms with distinct oxidation and reduction waves reveal excellent reversibility and electrochromic switching over 200 cycles and confirm the high stability of the frameworks. Furthermore, high coloration efficiencies in the NIR region and fast switching speeds for coloration/decoloration as fast as 0.75 s/0.37 s for the Cz-ttTII COF and 0.61 s/0.29 s for the TAPB-ttTII COF at 550 nm excitation were observed, outperforming many known electrochromic materials, and offering options for a great variety of applications, such as stimuli-responsive coatings, optical information processing, or thermal control.

One-pot cascade construction of nonsubstituted quinoline-bridged covalent organic frameworks

Irreversible locking of imine linkages into stable linkages represents a promising strategy to improve the robustness and functionality of covalent organic frameworks (COFs). We report, for the first time, a multi-component one-pot reaction (OPR) for imine annulation to construct highly stable nonsubstituted quinoline-bridged COFs (NQ-COFs), and that equilibrium regulation of reversible/irreversible cascade reactions by addition of MgSO₄ desiccant is crucial to achieve high conversion efficiency and crystallinity. The higher long-range order and surface area of NQ-COFs synthesized by this OPR than those of the reported two-step post-synthetic modification (PSM) facilitate charge carrier transfer and photogeneration ability of superoxide radicals (O₂˙^-), which makes these NQ-COFs more efficient photocatalysts for O₂˙^- mediated synthesis of 2-benzimidazole derivatives. The general applicability of this synthetic strategy is demonstrated by fabricating 12 other crystalline NQ-COFs with a diversity of topologies and functional groups.

Connecting the dots for fundamental understanding of structure-photophysics-property relationships of COFs, MOFs, and perovskites using a Multiparticle Holstein Formalism

Photoactive organic and hybrid organic-inorganic materials such as conjugated polymers, covalent organic frameworks (COFs), metal-organic frameworks (MOFs), and layered perovskites, display intriguing photophysical signatures upon interaction with light. Elucidating structure-photophysics-property relationships across a broad range of functional materials is nontrivial and requires our fundamental understanding of the intricate interplay among excitons (electron-hole pair), polarons (charges), bipolarons, phonons (vibrations), inter-layer stacking interactions, and different forms of structural and conformational defects. In parallel with electronic structure modeling and data-driven science that are actively pursued to successfully accelerate materials discovery, an accurate, computationally inexpensive, and physically-motivated theoretical model, which consistently makes quantitative connections with conceptually complicated experimental observations, is equally important. Within this context, the first part of this perspective highlights a unified theoretical framework in which the electronic coupling as well as the local coupling between the electronic and nuclear degrees of freedom can be efficiently described for a broad range of quasiparticles with similarly structured Holstein-style vibronic Hamiltonians. The second part of this perspective discusses excitonic and polaronic photophysical signatures in polymers, COFs, MOFs, and perovskites, and attempts to bridge the gap between different research fields using a common theoretical construct - the Multiparticle Holstein Formalism. We envision that the synergistic integration of state-of-the-art computational approaches with the Multiparticle Holstein Formalism will help identify and establish new, transformative design strategies that will guide the synthesis and characterization of next-generation energy materials optimized for a broad range of optoelectronic, spintronic, and photonic applications.

Porous Framework Materials for Bioimaging and Cancer Therapy

Cancer remains one of the most pressing diseases in the world. Traditional treatments, including surgery, chemotherapy, and radiotherapy still show certain limitations. Recently, numerous cancer treatments have been proposed in combination with novel materials, such as photothermal therapy, chemodynamic therapy, immunotherapy, and a combination of therapeutic approaches. These new methods have shown significant advantages in reducing side effects and synergistically enhancing anti-cancer efficacy. In addition to the above approaches, early diagnosis and in situ monitoring of lesion areas are also important for reducing side effects and improving the success rate of cancer therapy. This depends on the decent use of bioimaging technology. In this review, we mainly summarize the recent advances in porous framework materials for bioimaging and cancer therapy. In addition, we present future challenges relating to bioimaging and cancer therapy based on porous framework materials.

Covalent Organic Framework Thin-Film Photodetectors from Solution-Processable Porous Nanospheres

The synthesis of homogeneous covalent organic framework (COF) thin films on a desired substrate with decent crystallinity, porosity, and uniform thickness has great potential for optoelectronic applications. We have used a solution-processable sphere transmutation process to synthesize 300 ± 20 nm uniform COF thin films on a 2 × 2 cm² TiO₂-coated fluorine-doped tin oxide (FTO) surface. This process controls the nucleation of COF crystallites and molecular morphology that helps the nanospheres to arrange periodically to form homogeneous COF thin films. We have synthesized four COF thin films (TpDPP, TpEtBt, TpTab, and TpTta) with different functional backbones. In a close agreement between the experiment and density functional theory, the TpEtBr COF film showed the lowest optical band gap (2.26 eV) and highest excited-state lifetime (8.52 ns) among all four COF films. Hence, the TpEtBr COF film can participate in efficient charge generation and separation. We constructed optoelectronic devices having a glass/FTO/TiO₂/COF-film/Au architecture, which serves as a model system to study the optoelectronic charge transport properties of COF thin films under dark and illuminated conditions. Visible light with a calibrated intensity of 100 mW cm^-2 was used for the excitation of COF thin films. All of the COF thin films exhibit significant photocurrent after illumination with visible light in comparison to the dark. Hence, all of the COF films behave as good photoactive substrates with minimal pinhole defects. The fabricated out-of-plane photodetector device based on the TpEtBr COF thin film exhibits high photocurrent density (2.65 ± 0.24 mA cm^-2 at 0.5 V) and hole mobility (8.15 ± 0.64 ×10^-3 cm² V^-1 S^-1) compared to other as-synthesized films, indicating the best photoactive characteristics.

Photoresponse of CVD grown crystalline quantum dot-embedded covalent organic framework thin film

Covalent organic frameworks (COFs) are a new family of novel 2D materials which are highly sought after for integration into future sensors and other devices for their highly porous structures and large surface areas. However, low-temperature large-area growth of these semiconductive materials with a clean surface for direct device applications is still a challenging task. To provide an on-chip photonic device, a COF366-Quantum dot (COF366-QDs) thin-film-based device fabricated by in situ chemical vapor deposition (CVD) is presented. The high-resolution transmission electron microscopy (HRTEM) displays the formation of the periodic, crystalline and porous framework of the COF layer with mono-dispersed QDs of average particle size of ∼2.5-3 nm. The fabricated COF366-QD layer acts as a photoactive layer in the photonic device with an Au-COFQD-Au structure where a conduction path is formed between the metal electrodes through a network of COF layer with embedded QDs. The device shows photoactive response under 514 nm visible light with a very low dark current of 4.36 × 10-11 A with a minimum light detection capability of 160 nW and a responsivity of ∼3.42 A W-1. The photonic device was highly stable for successive switching cycles with very low attenuation. To our knowledge, this is the first report of a Quantum dot embedded COF366 thin-film by chemical vapor deposition. The proposed interfacing of COF366-QD thin-films on silicon substrate using in situ low-temperature CVD technique can be highly valuable for the development of transfer-free, clean, and low-cost preparation of industrial-scale organic electronics, optoelectronic device applications, and lab-on-chip based technologies for a wide range of future applications.

Cage-Based Covalent Organic Framework for the Effective and Efficient Removal of Malachite Green from Wastewater

A cage-covalent organic framework (COF)-TP {T = bis(tetraoxacalix[2]arene[2]triazine); P = piperazine}, a novel two-dimensional covalent organic skeleton substituted with a nucleophilic cyanuric chloride analogue, was synthesized by a simple polymerization process. Cage-COF-TP is advantageous owing to its good structural order, permanent porosity, and low preparation cost. This skeleton was employed as a cost-effective adsorbent for the intermittent adsorption of an organic dye from aqueous solutions. Adsorption experiments were carried out at different initial dye concentrations, contact times, and solution pH. The adsorption kinetics followed the pseudo-second order model, and the results of thermodynamic studies were consistent with the Langmuir isotherm model. The high degree of matching between the size and shape of malachite green (MG) and the shrunken channels present in Cage-COF-TP were responsible for the enhanced adsorption ability of this material. Furthermore, theoretical calculations indicated that the high adsorption of the studied adsorbent can be attributed to the presence of nitrogen-rich triazine units in the Cage-COF-TP, which are expected to strengthen its affinity to guest molecules. The obtained results showed that the developed adsorbent is an efficient adsorbent that is theoretically capable of stimulating the removal of ∼2000 mg/g MG from wastewater at ambient temperature. This study will therefore be expected to promote the development of new functional materials based on COFs.

Electroactive Covalent Organic Framework Enabling Photostimulus-Responsive Devices

Two-dimensional covalent organic frameworks (2D COFs) feature graphene-type 2D layered sheets but with a tunable structure, electroactivity, and high porosity. If these traits are well-combined, then 2D COFs can be applied in electronics to realize functions with a high degree of complexity. Here, a highly crystalline electroactive COF, BDFamide-Tp, was designed and synthesized. It shows regularly distributed pores with a width of 1.35 nm. Smooth and successive films of such a COF were fabricated and found to be able to increase the conductivity of an organic semiconductor by 10³ by interfacial doping. Upon encapsulation of a photoswitchable molecule (spiropyran) into the voids of the COF layer, the resulted devices respond differently to light of different wavelengths. Specifically, the current output ratio after UV vs Vis illumination reaches 100 times, thus effectively creating on and off states. The respective positive and negative feedbacks are memorized by the device and can be reprogrammed by UV/Vis illumination. The reversible photostimulus responsivity and reliable memory of the device are derived from the combination of electroactivity and porosity of the 2D COF. This work shows the capability of 2D COFs in higher-level electronic functions and extends their possible applications in information storage.

Designed synthesis of an sp2 carbon-conjugated fluorescent covalent organic framework for selective detection of Fe3

A new fully conjugated covalent organic framework material (COF_TFPPy-ThDAN) has been synthesized via the Knoevenagel condensation reaction using 1,3,6,8-tetra(4-formylphenyl)pyrene (TFPPy) as the chromogenic unit and 2,2'-([2,2'-bithiophene]-5,5'-diyl)diacetonitrile (ThDAN) as the linker. The COF_TFPPy-ThDAN has been successfully prepared under the optimized conditions and showed excellent crystallinity and chemical stability. Especially, it was able to maintain its crystallinity even when immersed under harsh conditions such as HCl (3 M) and NaOH (3 M). Meanwhile, COF_TFPPy-ThDAN exhibited good fluorescence properties, which could remain relatively stable under different pH conditions. Moreover, COF_TFPPy-ThDAN was further employed for the detection of Fe³⁺ with high sensitivity and selectivity with a limit of detection (LOD) of 1.26 μM. COF_TFPPy-ThDAN with high stability and fluorescence is a promising material for chemical sensing.

Recent Progress in External-Stimulus-Responsive 2D Covalent Organic Frameworks

Recently, smart 2D covalent organic frameworks (COFs), combining the advantages of both inherent structure features and functional building blocks, have been demonstrated to show reversible changes in conformation, color, and luminescence in response to external stimuli. This review provides a summary on the recent progress of 2D COFs that are responsive to external stimuli such as metal ions, gas molecules, pH values, temperature, electricity, light, etc. Moreover, the responsive mechanisms and design strategies, along with the applications of these stimulus-responsive 2D COFs in chemical sensors and photoelectronic devices are also discussed. It is believed that this review would provide some guidelines for designing novel single-/multistimulus-responsive 2D COFs with controllable responsive behaviors for advanced photoelectronic applications.

Controlled n-Doping of Naphthalene-Diimide-Based 2D Polymers

2D polymers (2DPs) are promising as structurally well-defined, permanently porous, organic semiconductors. However, 2DPs are nearly always isolated as closed shell organic species with limited charge carriers, which leads to low bulk conductivities. Here, the bulk conductivity of two naphthalene diimide (NDI)-containing 2DP semiconductors is enhanced by controllably n-doping the NDI units using cobaltocene (CoCp₂ ). Optical and transient microwave spectroscopy reveal that both as-prepared NDI-containing 2DPs are semiconducting with sub-2 eV optical bandgaps and photoexcited charge-carrier lifetimes of tens of nanoseconds. Following reduction with CoCp₂ , both 2DPs largely retain their periodic structures and exhibit optical and electron-spin resonance spectroscopic features consistent with the presence of NDI-radical anions. While the native NDI-based 2DPs are electronically insulating, maximum bulk conductivities of >10^-4  S cm^-1 are achieved by substoichiometric levels of n-doping. Density functional theory calculations show that the strongest electronic couplings in these 2DPs exist in the out-of-plane (π-stacking) crystallographic directions, which indicates that cross-plane electronic transport through NDI stacks is primarily responsible for the observed electronic conductivity. Taken together, the controlled molecular doping is a useful approach to access structurally well-defined, paramagnetic, 2DP n-type semiconductors with measurable bulk electronic conductivities of interest for electronic or spintronic devices.

An Efficient Route to 3,3'-Biindolinylidene-diones by Iron-Catalyzed Dimerization of Isatins

Iron-catalyzed dimerization of various isatin derivatives is described for the efficient synthesis of 3,3'-biindolinylidene-diones (isoindigos). The reaction provides easy access to self-coupled and cross-coupled 3,3'-indolinylidene-diones that have high relevance to biology and materials. This Fe(0)- or Fe(II)-catalyzed dimerization reaction tolerates a wide range of functionalities, such as fluoro, chloro, bromo, alkenyl, nitrile, ether, ester, pyrrolyl, indolyl and carbazolyl groups, including cyclic and acyclic alkyls as well as an alkyl-bearing fatty-alcohol moiety. Especially, the coupling between two distinct isatins provided excellent selectivity for the cross-dimerization with trace of self-couplings. The single-crystal X-ray diffraction study established the molecular structure of eight dimerized products. A preliminary mechanistic study of the Fe-catalyzed dimerization supported the radical pathway for the reaction.

Acridine-Functionalized Covalent Organic Frameworks (COFs) as Photocatalysts for Metallaphotocatalytic C-N Cross-Coupling

Covalent organic frameworks (COFs) are structurally tuneable, porous and crystalline polymers constructed through the covalent attachment of small organic building blocks as elementary units. Using the myriad of such building blocks, a broad spectrum of functionalities has been applied for COF syntheses for broad applications, including heterogeneous catalysis. Herein, we report the synthesis of a new family of porous and crystalline COFs using a novel acridine linker and benzene-1,3,5-tricarbaldehyde derivatives bearing a variable number of hydroxy groups. With the broad absorption in the visible light region, the COFs were applied as photocatalysts in metallaphotocatalytic C-N cross-coupling. The fully β-ketoenamine linked COF showed the highest activity, due to the increased charge separation upon irradiation. The COF showed good to excellent yields for several aryl bromides, good recyclability and even catalyzed the organic transformation in presence of green light as energy source.

Facile Preparation of a Thienoisoindigo-Based Nanoscale Covalent Organic Framework with Robust Photothermal Activity for Cancer Therapy

The covalent organic frameworks (COFs) so far are usually built with small aromatic subunits, which makes their absorption spectra mainly located in the high-energy part of the visible region. In this work, we have developed a COF with a low band gap by integrating electron-deficient thienoisoindigo and electron-rich triphenylamine. The intramolecular charge-transfer effect combining the extended length of the π-conjugated backbone of COF endow it with broad absorption even to the second near-infrared region. After optimizing the solvent, a uniform size and colloidal stable COF is obtained. Benefiting from the coplanar structure of the monomer, this COF achieves a considerable photothermal conversion efficiency (PCE) of 48.2%. With these advantages, it displays convincing cancer cell killing effect upon laser irradiation in vitro or in vivo. This work provides a simple and practical method to acquire promising a COF-based phototherapy reagent that is applied in biomedicine field.

Metal-organic frameworks and their composites for fuel and chemical production via CO2 conversion and water splitting

Increase in the global energy demand has been leading to major energy crises in recent years. The use of excess fossil fuels for energy production is causing severe global warming, as well as energy shortage. To overcome the global energy crisis, the design of various chemical structures as efficient models for the generation of renewable energy fuels is very much crucial, and will limit the use of fossil fuels. Current challenges involve the design of Metal-Organic Framework (MOF) materials for this purpose to diminish the energy shortage. The large surface area, tunable pore environment, unique structural property and semiconducting nature of the highly porous MOF materials enhance their potential applications towards the production of enhanced energy fuels. This review is focused on the architecture of MOFs and their composites for fuels and essential chemicals production like hydrogen, methane, ethanol, methanol, acetic acid, and carbon monoxide, which can be used as renewable fuel energy sources to limit the use of fossil fuels, thereby reducing global warming.