Anthracene-Resorcinol Derived Covalent Organic Framework as Flexible White Light Emitter

Anthracene-induced formation of highly twisted metallacycle and its crystal structure and tunable assembly behaviors

Polycyclic aromatic hydrocarbons (PAHs) continue to attract increasing interest with respect to their applications as luminescent materials. The ordered structure of the metal-organic complex facilitates the selective integration of PAHs that can be tuned to function cooperatively. Here, a unique highly twisted anthracene-based organoplatinum metallacycle was prepared via coordination-driven self-assembly. Single-crystal X-ray diffraction analysis revealed that the metallacycle was twisted through the cooperation of strong π···π stacking interactions and steric hindrance between two anthracene-based ligands. Notably, the intramolecular twist and aggregation behavior introduced restrictions to the conformational change of anthracenes, which resulted in increased emission intensity of the metallacycle in solution. The emission behaviors and suprastructures based on the highly twisted metallacycle can be modulated by the introduction of different solvents. This study demonstrates that this metallacycle with highly twisted structure is a promising candidate for sensing and bioimaging applications.

Emission Color Tuning and White Light Generation from a Trimolecular Cocktail in Cationic Micellar System with Promising Applicability in the Anticounterfeiting Technology

The development of photoluminescent (PL) systems, displaying multiple stimuli-responsive emission color tuning, has been the pressing priority in the recent times due to their huge role in contemporary lighting and anticounterfeiting technologies. Acknowledging this importance, we present a simple and eco-friendly PL system showing emission color tuning in response to different stimuli, that is, the composition of the system, pH, excitation wavelength, and the temperature with the plus point of getting significantly pure white light emission (WLE). The novel system is fabricated from the aqueous mixture of three organic fluorophores, umbelliferone (UMB), fluorescein (FLU), and Rhodamine-B (RB). By varying the fluorophore composition in the mixture at pH 12, nearly pure WLE with a Commission Internationale d'Eclairage (CIE) 1931 profile of (0.33, 0.33) was obtained at the excitation wavelength of 365 nm, the sustainability of which was ensured by employing the micellar self-assemblies of tetradecyltrimethylammonium bromide (TTAB) molecules. Similar WLE was obtained under mildly acidic conditions (pH 6) but at the excitation wavelength of 330 nm. By proper tuning of pH and the wavelengths of the system to use it as a fluorescent ink, we found a remarkable and highly applicable phenomenon observed for the first time, that is, triple-mode orthogonal emission color tuning with white light ON/OFF switching. We validate the vital applicability of this phenomenon in protecting the authenticity of the document with its hard-to-counterfeit property. The applicability of this phenomenon is also explored by synthesizing PVA-based fluorescent films from the tri-fluorophore mixture. Moreover, the emission color of the PL system was explored lucidly for its temperature dependence owing to the thermal responsiveness of RB emission, where the PL system proves to be a full-color RGB system.

Construction of Interlayer Conjugated Links in 2D Covalent Organic Frameworks via Topological Polymerization

Two-dimensional covalent organic frameworks (2D COFs) are well-defined polymeric sheets that usually stack in an eclipsed mode via van der Waals forces. Extensive efforts have been made to manipulate interlayer interactions, yet there still lack a way to construct conjugated connections between adjacent layers, which is important for (opto)electronic-related applications. Herein, we report an interlayer topological polymerization strategy to transform the well-organized diacetylene columnar arrays in three different 2D COFs (TAPFY-COF, TAPB-COF, and TAPP-COF) into conjugated enyne chains upon heating in the solid state. The resultant COFs (COF-P) with retained high crystallinity possess broadened absorption bands and narrowed band gaps. The newly formed conjugated chains provide extra charge carrier pathways through direct π-electron delocalization. As a proof-of-concept, after topological polymerization, the conductivity of the TAPFY-COF film achieves 2.8 × 10^-4 S/cm without doping, and the photothermal, photoacoustic, and oxygen reduction catalytic performance of TAPP-COF is significantly improved.

Silk fibroin-derived carbon aerogels embedded with copper nanoparticles for efficient electrocatalytic CO2-to-CO conversion

Metal-carbon matrix catalyst has attracted a great deal of interest in electrochemical carbon dioxide reduction reaction (CO₂RR) due to its excellent electrocatalytic performance. However, the design of highly active metal-carbon matrix catalyst towards CO₂RR using natural biomass and cheap chemical precursors is still under challenge. Herein, a self-assembly strategy, along with CO₂ gas as acidifying agent, to fabricate silk fibroin (SF) derived carbon aerogels (CA) combining trace copper nanoparticles (SF-Cu/CA) is developed. Zinc nitrate was introduced as a pore-forming agent to further optimize the pore structure of the as-prepared catalysts to form SF-Cu/CA-1. The rich mesoporous structure and unique constitute of SF-Cu/CA-1 is conducive to exposed numerous active sites, fast electron transfer rate, and the desorption of *CO intermediate, thus leading to the electrocatalytic CO₂RR of SF-Cu/CA-1 catalyst with an excellent current density of 29.4 mA cm^-2, Faraday efficiency of 83.06% towards carbon monoxide (CO), high the ratio value of CO/H₂ (19.58), and a long-term stability over a 10-hour period. This performance is superior to that of SF-Cu/CA catalyst (13.0 mA cm^-2, FE_CO=58.43%, CO/H₂ = 2.16). This work not only offers a novel strategy using natural biomass and cheap chemicals to build metal-carbon matrix catalyst for electrocatalytic CO₂-to-CO conversion, but also is expected to promote the industrial-scale implementations of CO₂ electroreduction.

Conjugation- and Aggregation-Directed Design of Covalent Organic Frameworks as White-Light-Emitting Diodes

2D covalent organic frameworks (COFs) have emerged as a promising class of organic luminescent materials due to their structural diversity, which allows the systematic tuning of organic building blocks to optimize emitting properties. However, a significant knowledge gap exists between the design strategy and the fundamental understanding of the key structural parameters that determine their photophysical properties. In this work, we report two highly emissive sp²-C-COFs and the direct correlation of the structure (conjugation and aggregation) with their light absorption/emission, charge transfer (CT), and exciton dynamics, the key properties that determine their function as luminescent materials. We show that white light can be obtained by simply coating COFs on an LED strip or mixing the two COFs. Using the combination of time-resolved absorption and emission spectroscopy as well as computational prediction, we show that the planarity, conjugation, orientation of the dipole moment, and interlayer aggregation not only determine the light-harvesting ability of COFs but also control the exciton relaxation pathway and photoluminescent quantum yield.

CuAg nanoparticle/carbon aerogel for electrochemical CO2 reduction

A small sized CuAg heterostructure anchored by a silk fibroin-derived carbon aerogel exhibits electrocatalytic CO2-to-CO conversion.

A cage-based covalent organic framework for drug delivery

A novel cage-based crystalline covalent organic framework, i.e. Cage-COF-TT (TT = triammonia–terephthalaldehyde), was prepared from a prism-like triammonia-containing molecular cage and terephthalaldehyde.

Optoelectronic processes in covalent organic frameworks

Covalent organic frameworks (COFs) are crystalline porous materials constructed from molecular building blocks using diverse linkage chemistries. The image illustrates electron transfer in a COF-based donor–acceptor system. Image by Nanosystems Initiative Munich.

Dual emission in purely organic materials for optoelectronic applications

Purely organic molecules, which emit light by dual emissive (DE) pathways, have received increased attention in the last decade. These materials are now being utilized in practical optoelectronic, sensing and biomedical applications. In order to further extend the application of the DE emitters, it is crucial to gain a fundamental understanding of the links between the molecular structure and the underlying photophysical processes. This review categorizes the types of DE according to the spin multiplicity and time range of the emission, with emphasis on recent experimental advances. The design rules towards novel DE molecular candidates, the most perspective types of DE and possible future applications are outlined. These exciting developments highlight the opportunities for new materials synthesis and pave the way for accelerated future innovation and developments in this area.

Aqueous Systems with Tunable Fluorescence Including White-Light Emission for Anti-Counterfeiting Fluorescent Inks and Hydrogels

White-light-emitting materials have attracted wide interest for potential applications in information displays and lighting. To date, the majority of reported white-light-emitting materials have been multicomponent systems that are typically processed with organic solvents. These features are associated with complex processing, poor white-light quality, and environmental pollution. Herein, a white-light-emitting aqueous system is realized by encapsulating a fluorophore, which has a vibration-induced emission effect, in Pluronic F127 micelles. Tunable multicolor fluorescence is achieved by changing the temperature, and the use of organic solvents is effectively avoided. Through this process, white-light emission with Commission Internationale de l'Eclairage coordinates of (0.3351, 0.3326) is obtained, which is very close to pure white light, and its color rendering index is as high as 89. The fluorescence color tunability of this system could be performed in a wide temperature range, rendering it a potential material in optical thermometry. Besides, the aqueous system also allows for the application of the material as a fluorescent ink and white-light-emitting hydrogels. Information could be embedded in paper-based materials and hydrogels through the fluorescence quenching effect of iron ions (Fe³⁺) on the fluorophore. Fluorescence could then be recovered upon removal of Fe³⁺ by adenosine 5'-triphosphate. Thus, fluorescent patterning and triple-mode anti-counterfeiting could be expected due to the temperature-sensitive emission, fluorescence quenching, and recovering properties.

Design of Fluorescent and Robust Covalent Organic Framework Host Matrices for Illuminating Mechanistic Insight into Solvatochromic Decoding

Two functional covalent organic frameworks (COFs), constructed from 3-connected triazine-based amine or hydrazine with linear dialdehyde, are decorated with molecular docking sites to showcase their solvatochromic decoding behavior toward volatile solvent molecules (VSMs). These luminescent and crystalline COFs, namely, COF-N and COF-NN, are characterized by numerous analytical techniques. After accommodation of different VSMs as guests, the inclusion compounds of COF-N and COF-NN display solvatochromism. More fascinatingly, the singlet energy, band gaps, and lifetime of these VSM-accommodated COF-N and COF-NN are linearly correlated with the properties of VSMs. Density functional theory (DFT) and Monte Carlo simulation studies further support the interaction of VSMs with COF-N and COF-NN. The presence of an extra amine functionality in COF-NN leads to the better interaction with VSMs and, therefore, results in different modes of interaction and correlation. Considering their inestimable chemical diversity, this study introduces a new path for finely tuned solvatochromic properties by COFs.

Research Progress in Covalent Organic Frameworks for Photoluminescent Materials

Covalent organic frameworks (COFs) are an emerging kind of crystalline porous polymers that present the precise integration of organic building blocks into extensible structures with regular pores and periodic skeletons. The diversity of organic units and covalent linkages makes COFs a rising materials platform for the design of structure and functionality. Herein, recent research progress in developing COFs for photoluminescent materials is summarised. Structural and functional design strategies are highlighted and fundamental problems that need to be solved are identified, in conjunction with potential applications from perspectives of photoluminescent materials.

Nanoscale covalent organic frameworks as theranostic platforms for oncotherapy: synthesis, functionalization, and applications

Cancer nanomedicine is one of the most promising domains that has emerged in the continuing search for cancer diagnosis and treatment. The rapid development of nanomaterials and nanotechnology provide a vast array of materials for use in cancer nanomedicine. Among the various nanomaterials, covalent organic frameworks (COFs) are becoming an attractive class of upstarts owing to their high crystallinity, structural regularity, inherent porosity, extensive functionality, design flexibility, and good biocompatibility. In this comprehensive review, recent developments and key achievements of COFs are provided, including their structural design, synthesis methods, nanocrystallization, and functionalization strategies. Subsequently, a systematic overview of the potential oncotherapy applications achieved till date in the fast-growing field of COFs is provided with the aim to inspire further contributions and developments to this nascent but promising field. Finally, development opportunities, critical challenges, and some personal perspectives for COF-based cancer therapeutics are presented.

Tuning the electronic energy level of covalent organic frameworks for crafting high-rate Na-ion battery anode

Crystalline Covalent Organic Frameworks (COFs) possess ordered accessible nano-channels. When these channels are decorated with redox-active functional groups, they can serve as the anode in metal ion batteries (LIB and SIB). Though sodium's superior relative abundance makes it a better choice over lithium, the energetically unfavourable intercalation of the larger sodium ion makes it incompatible with the commercial graphite anodes used in Li-ion batteries. Also, their sluggish movement inside the electrodes restricts the fast sodiation of SIB. Creating an electronic driving force at the electrodes via chemical manipulation can be a versatile approach to overcome this issue. Herein, we present anodes for SIB drawn on three isostructural COFs with nearly the same Highest Occupied Molecular Orbitals (HOMO) levels but with varying Lowest Unoccupied Molecular Orbitals (LUMO) energy levels. This variation in the LUMO levels has been deliberately obtained by the inclusion of electron-deficient centers (phenyl vs. tetrazine vs. bispyridine-tetrazine) substituents into the modules that make up the COF. With the reduction in the cell-potential, the electrons accumulate in the anti-bonding LUMO. Now, these electron-dosed LUMO levels become efficient anodes for attracting the otherwise sluggish sodium ions from the electrolyte. Also, the intrinsic porosity of the COF favors the lodging and diffusion of the Na⁺ ions. Cells made with these COFs achieve a high specific capacity (energy density) and rate performance (rapid charging-discharging), something that is not as easy for Na⁺ compared to the much smaller sized Li⁺. The bispyridine-tetrazine COF with the lowest LUMO energy shows a specific capacity of 340 mA h g^-1 at 1 A g^-1 and 128 mA h g^-1 at a high current density of 15 A g^-1. Only a 24% drop appears on increasing the current density from 0.1 to 1 A g^-1, which is the lowest among all the top-performing COF derived Na-ion battery anodes.

When Anthracene and Quinone Avoid Cycloaddition: Acid-Catalyzed Redox Neutral Functionalization of Anthracene to Aryl Ethers

Benzoquinone and 9-phenylanthracene barely undergo anticipated cycloaddition under acid catalysis. Instead, 9-anthracenyl aryl ethers are obtained as unexpected products. Mechanistic studies indicate that the reaction likely undergoes an ionic mechanism between protonated anthracene species and nucleophilic oxygen of 1,4-benzoquinone or 1,4-hydroquinone. A variety of 9-anthracenyl aryl ethers are constructed with this method. Produced anthracenyl aryl ethers are potential scaffolds for new fluorescent molecules.

White luminescent single-crystalline chlorinated graphene quantum dots

A new class of white luminescent materials, white-light-emitting graphene quantum dots (WGQDs), have attracted increasing attention because of their unique features and potential applications. Herein, we designed and synthesized a novel WGQDs via a solvothermal molecular fusion strategy. The modulation of chlorine doping amount and reaction temperature gives the WGQDs a single-crystalline structure and bright white fluorescence properties. In particular, the WGQDs also exhibit novel and robust white phosphorescence performance for the first time. An optimum fluorescence quantum yield of WGQDs is 34%, which exceeds the majority of reported WGQDs and other white luminescent materials. The WGQDs display broad-spectrum absorption within almost the entire visible light region, broad full width at half maximum and extend their phosphorescence emission to the entire white long-wavelength region. This unique dual-mode optical characteristic of the WGQDs originates from the synergistic effect of low-defect and high chlorine-doping in WGQDs and enlarges their applications in white light emission devices, cell nuclei imaging, and information encryption. Our finding provides us an opportunity to design and construct more advanced multifunctional white luminescent materials based on metal-free carbon nanomaterials.

MOF@COFs with Strong Multiemission for Differentiation and Ratiometric Fluorescence Detection

Aggregation-caused quenching (ACQ) is often observed in covalent organic frameworks (COFs) for their low emission. Here, we propose that limited COF layers form on UiO-66 to eliminate the ACQ by the formation of UiO@COF composites. UiO-66 is selected because this metal-organic framework (MOF) is easily prepared in nanosize with Zr⁴⁺ ion and 2-aminoterephthalic acid (BDC-NH₂). The high affinity of the Zr⁴⁺ ion to phosphate species improves sensing selectivity. The surface -NH₂ reacts with 2,4,6-triformylphloroglucinol (Tp) to integrate COF1 and COF2, which are prepared with Tp and phenylenediamine or tetraamino-tetraphenylethylene, respectively. The hydrogen bond formed between the hydroxyl group in Tp and imine nitrogen realizes excited-state intramolecular proton transfer; therefore, multiemission is observed from the enol and keto states of the COFs and UiO-66 at 360, 470, and 613 nm for UiO@COF1 and at 370, 470, and 572 nm for UiO@COF2. When phosphate ion is added in the composites, the emissions from the COFs keep stable, while that from UiO-66 is enhanced. However, adenosine-5'-triphosphate (ATP) improves the emissions from UiO-66 and COF's enol state, but that from the keto state keeps stable. The differentiation and ratiometric fluorescence detection of ATP and phosphate ion are therefore realized with the multiemission, the affinity of Zr⁴⁺ ions, and the structural selectivity of the COFs. Thus, UiO@COF is a novel strategy to integrate multiemission, affinity, and structural selectivity to improve the sensing performance for differentiation and ratiometric detection.

Porous Organic Polymers with Thiourea Linkages (POP-TUs): Effective and Recyclable Organocatalysts for the Michael Reaction

As novel porous organic polymers with thiourea linkages, POP-TUs were successfully synthesized with tris(4-aminophenyl) amine (TAA) and 1,4-phenylene diisothiocyanate (PDT) under different conditions. The as-synthesized POP-TUs possess distinctly different morphological characteristics and can effectively catalyze the Michael reaction of trans-β-nitrostyrenes to diethyl malonate. Particularly, the POP-TU-2-catalyzed Michael reaction can proceed smoothly even using an ultralow catalyst dosage of 0.03 mol %, whose turnover number (TON) and turnover frequency (TOF) can reach up to 2700 and 25 h^-1, respectively. Besides, POP-TU-2 also exhibits excellent recyclability and reusability. Only 2% decline in the isolated yield was found after five consecutive runs. This work shows a significant improvement over previously reported thiourea-based catalysts and can offer an effective strategy for developing highly efficient heterogeneous organocatalysts.

White-light emission from a pyrimidine-carbazole conjugate with tunable phosphorescence-fluorescence dual emission and multicolor emission switching

A metal-free organic carbazole-pyrimidine dye exhibiting phosphorescence-fluorescence dual emission was developed into a white-light emission-switching system. The two crystal polymorphs obtained by breaking the molecular symmetry responded to the external stimuli of heating, vapor-fuming, and mechanical grinding, resulting in a tricolor switching system that includes white-light emission.

Multiple-State Emissions from Neat, Single-Component Molecular Solids: Suppression of Kasha's Rule

Three rigid and structurally simple heterocyclic stilbene derivatives, (E)-3H,3'H-[1,1'-biisobenzofuranylidene]-3,3'-dione, (E)-3-(3-oxobenzo[c] thiophen-1(3H)-ylidene)isobenzofuran-1(3H)-one, and (E)-3H,3'H-[1,1'-bibenzo[c] thiophenylidene]-3,3'-dione, are found to fluoresce in their neat solid phases, from upper (S₂ ) and lowest (S₁ ) singlet excited states, even at room temperature in air. Photophysical studies, single-crystal structures, and theoretical calculations indicate that large energy gaps between S₂ and S₁ states (T₂ and T₁ states) as well as an abundance of intra and intermolecular hydrogen bonds suppress internal conversions of the upper excited states in the solids and make possible the fluorescence from S₂ excited states (phosphorescence from T₂ excited states). These results, including unprecedented fluorescence quantum yields (2.3-9.6 %) from the S₂ states in the neat solids, establish a unique molecular skeleton for achieving multi-colored emissions from upper excited states by "suppressing" Kasha's rule.

Rapid, Scalable Construction of Highly Crystalline Acylhydrazone Two-Dimensional Covalent Organic Frameworks via Dipole-Induced Antiparallel Stacking

Covalent organic frameworks are an emerging class of porous crystalline organic materials that can be designed and synthesized from the bottom up. Despite progress made in synthesizing COFs of diverse topologies, the synthesis methods are often tedious and unscalable, hampering practical applications. Herein, we demonstrate a scalable, robust method of producing highly crystalline acylhydrazone two-dimensional (2D) COFs with diversified structures (six examples) under open and stirred conditions, with growth typically completed in only 30 min. Our strategy involves selecting molecular building blocks that have bond dipole moments with spatial orientations that favor antiparallel stacking and whose structure allows the restriction of intramolecular bond rotation (RIR) via intra- and interlayer hydrogen bonding. This method is widely applicable for hydrazide linkers containing various side-chain functionalities and topicities. By this strategy, the gram-scale synthesis of two highly crystalline COFs (up to 1.4 g yield) was obtained in a one-pot reaction within 30 min.

Covalent Organic Frameworks: Design, Synthesis, and Functions

Covalent organic frameworks (COFs) are a class of crystalline porous organic polymers with permanent porosity and highly ordered structures. Unlike other polymers, a significant feature of COFs is that they are structurally predesignable, synthetically controllable, and functionally manageable. In principle, the topological design diagram offers geometric guidance for the structural tiling of extended porous polygons, and the polycondensation reactions provide synthetic ways to construct the predesigned primary and high-order structures. Progress over the past decade in the chemistry of these two aspects undoubtedly established the base of the COF field. By virtue of the availability of organic units and the diversity of topologies and linkages, COFs have emerged as a new field of organic materials that offer a powerful molecular platform for complex structural design and tailor-made functional development. Here we target a comprehensive review of the COF field, provide a historic overview of the chemistry of the COF field, survey the advances in the topology design and synthetic reactions, illustrate the structural features and diversities, scrutinize the development and potential of various functions through elucidating structure-function correlations based on interactions with photons, electrons, holes, spins, ions, and molecules, discuss the key fundamental and challenging issues that need to be addressed, and predict the future directions from chemistry, physics, and materials perspectives.

Nucleation-Elongation Dynamics of Two-Dimensional Covalent Organic Frameworks

Homogeneous two-dimensional (2D) polymerization is a poorly understood process in which topologically planar monomers react to form planar macromolecules, often termed 2D covalent organic frameworks (COFs). While these COFs have traditionally been limited to weakly crystalline aggregated powders, they were recently grown as micron-sized single crystals by temporally resolving the growth and nucleation processes. Here, we present a quantitative analysis of the nucleation and growth rates of 2D COFs via kinetic Monte Carlo (KMC) simulations using COF-5 as an example, which show that nucleation and growth have second-order and first-order dependences on monomer concentration, respectively. The computational results were confirmed experimentally by systematic measurements of COF nucleation and growth rates performed via in situ X-ray scattering, which validated the respective monomer concentration dependencies of the nucleation and elongation processes. A major consequence is that there exists a threshold monomer concentration below which growth dominates over nucleation. Our computational and experimental findings rationalize recent empirical observations that, in the formation of 2D COF single crystals, growth dominates over nucleation when monomers are added slowly, so as to limit their concentrations. This mechanistic understanding of the nucleation and growth processes will inform the rational control of polymerization in two dimensions and ultimately enable access to high-quality samples of designed two-dimensional polymers.

Dual emission N-doped carbon dot@benzotrithiophene tricarbaldehyde-terephthalic dihydrazide covalent organic framework

NCDs are encapsulated into COF_BTT–Th to enhance the COF_BTT–Th fluorescence based on RET and decrease the interlaminar accumulation and intramolecular rotation.

Function-oriented synthesis of two-dimensional (2D) covalent organic frameworks – from 3D solids to 2D sheets

This review provides guidelines for the function-oriented synthesis of 2D COFs from 3D solids to 2D sheets.

The luminescent and photophysical properties of covalent organic frameworks

This review highlights the luminescent and unique photophysical properties of covalent organic frameworks. Their potential use in applications related to chemical sensing, photocatalysis, and optoelectronics are discussed.

Rational synthesis of interpenetrated 3D covalent organic frameworks for asymmetric photocatalysis

Covalent organic frameworks (COFs) show great promise as heterogeneous photocatalysts, but they have not yet been explored for asymmetric photocatalysis, which is important for the sustainable production of pharmaceuticals and fine chemicals. We report here a pair of twofold interpenetrated 3D COFs adopting a rare (3,4)-connected ffc topology for photocatalytic asymmetric reactions by imine condensation of rectangular and trigonal building blocks. Both COFs containing a photoredox triphenylamine moiety are efficient photocatalysts for the cross-dehydrogenative coupling reactions and asymmetric α-alkylation of aldehydes integrated with a chiral imidazolidinone catalyst. Under visible-light irradiation, the targeted chiral products are produced in satisfactory yields with up to 94% enantiomeric excess, which are comparable to those of reported reactions using molecular metal complexes or organic dyes as photosensitizers. Whereas the COFs became amorphous after catalysis, they can be recrystallized through solvent-assisted linker exchange and reused without performance loss. This is the first report utilizing COFs as photocatalysts to promote enantioselective photochemical reactions.

Covalent Organic Frameworks: Chemical Approaches to Designer Structures and Built-In Functions

A new approach has been developed to design organic polymers using topology diagrams. This strategy enables covalent integration of organic units into ordered topologies and creates a new polymer form, that is, covalent organic frameworks. This is a breakthrough in chemistry because it sets a molecular platform for synthesizing polymers with predesignable primary and high-order structures, which has been a central aim for over a century but unattainable with traditional design principles. This new field has its own features that are distinct from conventional polymers. This Review summarizes the fundamentals as well as major progress by focusing on the chemistry used to design structures, including the principles, synthetic strategies, and control methods. We scrutinize built-in functions that are specific to the structures by revealing various interplays and mechanisms involved in the expression of function. We propose major fundamental issues to be addressed in chemistry as well as future directions from physics, materials, and application perspectives.

Strong dual emission in covalent organic frameworks induced by ESIPT

Here we reveal the effects of hydrogen bonds and alkyl groups on the structure and emission of covalent organic frameworks (COFs). Hydrogen bonds improve molecular rigidity leading to high crystallinity and restrict intramolecular rotation to enhance the emission of COFs. An excited-state intramolecular proton transfer (ESIPT) effect for dual emission is achieved via the intramolecular hydrogen bonds between hydroxyl groups and imine bonds. Alkyl groups increase interlayer spacing as a natural "scaffold" and achieve a staggered AB stacking mode to decrease aggregation-caused quenching. Based on the above guidance, COF-4-OH with strong emission is prepared with 2,4,6-triformylphloroglucinol (TFP) and 9,9-dibutyl-2,7-diaminofluorene (DDAF). Strong dual emission is observed and used to differentiate organic solvents with different polarities, to determine the water content in organic solvents, and to detect different pH levels. Our work serves as a guide for the rational design of functional monomers for the preparation of emissive COFs.

A Covalent Triazine-Based Framework Consisting of Donor-Acceptor Dyads for Visible-Light-Driven Photocatalytic CO2 Reduction

Photocatalytic conversion of CO₂ into value-added chemical fuels is a promising approach to address the depletion of fossil energy and environment-related concerns. Tailor-making the electronic properties and band structures of photocatalysts is pivotal to improve their efficiency and selectivity in photocatalytic CO₂ reduction. Herein, a covalent triazine-based framework was developed containing electron-donor triphenylamine and electron-acceptor triazine components (DA-CTF). The engineered π-conjugated electron donor-acceptor dyads in DA-CTF not only optimized the optical bandgap but also contributed to visible-light harvesting and migration of photoexcited charge carriers. The activity of photocatalytic CO₂ reduction under visible light was significantly improved compared with that of traditional g-C₃ N₄ and reported covalent triazine-based frameworks. This study provides molecular-level insights into the mechanism of photocatalytic CO₂ reduction.

Triggering White-Light Emission in a 2D Imine Covalent Organic Framework Through Lanthanide Augmentation

Recently, covalent organic frameworks (COFs) have emerged as an interesting class of porous materials, featuring tunable porosity and fluorescence properties based on reticular construction principles. Some COFs display highly emissive monocolored luminescence, but attaining white-light emission from COFs is difficult as it must account for a wide wavelength range. White-light emission is highly desired for solid-state lighting applications, and obtaining it usually demands the combination of red-, green-, and blue-light components. Hence, to achieve the targeted white-light emission, we report for the first time grafting of lanthanides (Eu³⁺/Tb³⁺) on a two-dimensional imine COF (TTA-DFP-COF). We studied the luminescence properties of the hybrid materials prepared by anchoring Eu³⁺ (red light) and Tb³⁺ (green light) β-diketonate complexes onto the TTA-DFP-COF. Reticular construction is exploited to design strong coordination of Eu³⁺ and Tb³⁺ ions into nitrogen-rich pockets of the imine COF. Mixed Eu³⁺/Tb³⁺ materials are then prepared to incorporate red and green components along with the inherent blue light from the organic moieties of the COF to produce white-light emission. We show that COFs have the potential for hosting Eu³⁺ and Tb³⁺ complexes, which can be tuned to obtain desired excitations for applications in the field of optoelectronics, microscopy, optical sensing, and bioassay.