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

Synthesis of shape-controlled covalent organic frameworks for light scattering detection of iron and chromium ions

Fabricating covalent organic frameworks with different morphologies based on the same structural motifs is both interesting and challenging. Here, a TTA-TFP-COF was synthesized by both solvothermal and room temperature methods, with 2,4,6-Tris(4-aminophenyl)-1,3,5-triazine (TTA) and 1,3,5-tris(4-formylphenyl)-benzene (TFP) as raw material. Using different synthesis conditions and adding aniline and benzaldehyde as regulators in the synthesis process, we found that these processes could slow down the reaction speed, increase the exchange and metathesis reactions of dynamic reversible reactions, and improve the reversibility of the reaction system. Thus, controllable synthesis of TTA-TFP-COF with different morphologies, including micro-particles, hollow tubes with controllable diameters, and micro-flowers was achieved. Our further study found that metal ions, Fe3+ and Cr3+ ions, could coordinate with N and O in TTA-TFP-COF and partially destroy the structure of TTA-TFP-COF. The particle size of the TTA-TFP-COF became smaller, thus resulting in the decrease of the light scattering intensity of the COF. An excellent linear relationship exists between the light scattering changes (ΔI) and metal ions concentration (c) from 2.0 to 350.0 μM for Fe3+ and 40.0-800.0 μM for Cr3+, respectively. Thus, rapid and selective analytical methods for detecting metal ions were developed by TTA-TFP-COF here.

Additive Mixing of Emissive Ligands in Covalent Organic Frameworks for White Light Emission

Emissive covalent organic frameworks (COFs) are a promising class of crystalline materials that have demonstrated applications for sensing and light-emitting diodes. However, white light emission from a single COF has not been achieved yet as it requires multicomponent organic chromophores that simultaneously emit blue, green, and red light. In this work, we report the successful synthesis of a single COF with efficient white light emission by utilizing tunable emission properties of 2,1,3-benzothiadazole after incorporating different functional groups on its core structure, which results in the formation of three ligands, i.e., 4',4-(benzothiadiazole-4,7-diyl)-dibenzaldehyde (BTD), 4,4'-(benzoselenadiazole-4,7-diyl)-dibenzaldehyde (BSD), and 4,4'-(naphtho[2,3-c][1,2,3] selenadiazole-4,9-diyl)-dibenzaldehyde (NSD), that emit in the blue, green, and red regions of the visible light spectrum. We show that white light emission can only occur when BTD, BSD, and NSD are assembled in a single COF structure due to the facilitated energy transfer process from BTD to BSD/NSD. This work demonstrates a unique approach to developing new white light-emitting materials based on the COF structure.

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.

Exciplex Formation by Complexation of an Electron-Accepting Guest in an Electron-Donating Pillar[5]arene Host Liquid

We present a novel system, a liquid-state pillar[5]arene decorated with tri(ethylene oxide) chains, that brings electron-donor and electron-acceptor molecules into proximity for efficient exciplex formation. The electron-accepting guests exhibit a blue-purple emission from a localized excited state upon excitation in common solvents. However, directly dissolving the guests in the electron-donating pillar[5]arene liquid (a bulk system) results in visible green emission from the formed exciplexes. In the bulk system, the guest molecules are always surrounded by excess pillar[5]arene molecules, resulting in the formation of mainly inclusion-type exciplexes. In the bulk system, energy migration occurs between the pillar[5]arene molecules. Excitation of the pillar[5]arenes results in a more intense green exciplex emission than that observed upon direct excitation of the guests. In summary, the pillar[5]arene liquid is a novel system for achieving efficient exciplex formation and energy migration that is different from typical solvent and solid systems.

Design and preparation of fluorescent covalent organic frameworks for biological sensing

Covalent organic frameworks (COFs) are a new class of functional solids featuring several fantastic structural characteristics, including a great diversity of building units and cross-linking patterns, precise integration of building blocks, and adjustable topology of porous architecture. In addition to the above features, some COF samples are constructed with high-density conjugated fragments, which have unique potential advantages in fluorescence imaging, and thus may have great potential applications in bioimaging. Herein, this article summarizes the recent progress in the design and preparation of fluorescent covalent organic frameworks. We investigate the systemic correlation between the structural qualities of COF networks and biological sensors. Finally, the significant advantages, major challenges, and future opportunities of fluorescent covalent organic frameworks are discussed for the development of next-generation porous materials for sensing applications.

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.

Linkages Make a Difference in the Photoluminescence of Covalent Organic Frameworks

Covalent organic frameworks (COFs) with structural designability and tunability of photophysical properties enable them to be a promising class of organic luminescent materials by incorporating well-designed fluorescent units directly into the periodic skeletons. The photophysical properties of COFs are mainly affected by the structural features, which determine the conjugation degree, charge delocalization ability, and exciton dynamics of COFs. To understand the relationship between COF structures and their photophysical properties, two COFs with the same pyrene chromophore units but different linkages (imine or vinylene) were designed and synthesized. Interestingly, different linkages endow COFs with huge differences in solid-state photoluminescence quantum yield (PLQY) for imine- and vinylene-linked pyrene-based COFs, which possess PLQY values of 0.34 % and 15.43 %, respectively. The femtosecond-transient absorption spectra and time-dependent density functional theory reveal the different charge-transfer pathways in imine- and vinylene-linked COFs, which influence the exciton relaxation way and fluorescence intensity. In addition, an effective white-light device was obtained by coating the vinylene-linked COF on a light-emitting diode strip.

The Keto-Switched Photocatalysis of Reconstructed Covalent Organic Frameworks for Efficient Hydrogen Evolution

The keto-switched photocatalysis of covalent organic frameworks (COFs) for efficient H2 evolution was reported for the first time by engineering, at a molecular level, the local structure and component of the skeletal building blocks. A series of imine-linked BT-COFs were synthesized by the Schiff-base reaction of 1, 3, 5-benzenetrialdehyde with diamines to demonstrate the structural reconstruction of enol to keto configurations by alkaline catalysis. The keto groups of the skeletal building blocks served as active injectors, where hot π-electrons were provided to Pt nanoparticles (NPs) across a polyvinylpyrrolidone (PVP) insulting layer. The characterization results, together with density functional theory calculations, indicated clearly that the formation of keto-injectors not only made the conduction band level more negative, but also led to an inhomogeneous charge distribution in the donor-acceptor molecular building blocks to form a strong intramolecular built-in electric field. As a result, visible-light photocatalysis of TP-COFs-1 with one keto group in the skeletal building blocks was successfully enabled and achieved an impressive H2 evolution rate as high as 0.96 mmol g-1  h-1 . Also, the photocatalytic H2 evolution rates of the reconstructed BT-COFs-2 and -3 with two and three keto-injectors were significantly enhanced by alkaline post-treatment.

Molecular Insertion: A Master Key to Unlock Smart Photoelectric Responses of Covalent Organic Frameworks

Covalent organic frameworks (COFs) show potentials in prominent photoelectric responses by judicious structural design. However, from the selections of monomers and condensation reactions to the synthesis procedures, the acquisition of photoelectric COFs has to meet overmuch high conditions, limiting the breakthrough and modulation in photoelectric responses. Herein, the study reports a creative "lock-key model" based on molecular insertion strategy. A COF with suitable cavity size, TP-TBDA, is used as the host to load guests. Merely through the volatilization of mixed solution, TP-TBDA and guests can be spontaneously assembled via non-covalent interactions (NCIs) to produce molecular-inserted COFs (MI-COFs). The NCIs between TP-TBDA and guests acted as a bridge to facilitate charge transfer in MI-COFs, unlocking the photoelectric responses of TP-TBDA. By exploiting the controllability of NCIs, the MI-COFs can realize the smart modulation of photoelectric responses by simply changing the guest molecule, thus avoiding the arduous selection of monomers and condensation reactions required by conventional COFs. The construction of molecular-inserted COFs circumvents complicated procedures for achieving performance improvement and modulation, providing a promising direction to construct late-model photoelectric responsive materials.

Direct visual observation of pedal motion-dependent flexibility of single covalent organic frameworks

Flexible covalent organic frameworks (COFs) have been studied for applications containing sorption, selective separation, and catalysis. How to correlate the microscopic structure with flexibility in COFs is a great challenge. Herein, we visually track the flexible deformation behaviors of single COF-300 and COF-300-AR particles in response to solvent vapour guests with dark-field microscopy (DFM) in an in operando manner. COF-300-AR with freely-rotating C-N single bonds are synthesized by the reduction of imine-based COF-300 consisting of rigid C=N double bonds without changing topological structure and crystallinity. Unexpectedly, we observe that the flexible deformation of COF-300 is extremely higher than that of COF-300-AR despite it bears many C-N single bonds, clearly illustrating the apparent flexibility decrease of COF-300 after reduction. The high spatiotemporal resolution of DFM enables the finding of inter-particle variations of the flexibility among COF-300 crystals. Experimental characterizations by variable-temperature X-ray diffraction and infrared spectroscopy as well as theoretical calculations demonstrate that the flexible deformation of COF-300 is ascribed to the pedal motion around rigid C=N double bonds. These observations provide new insights into COF flexibility.

Citric Acid-Based Intrinsic Band-Shifting Photoluminescent Materials

Citric acid, an important metabolite with abundant reactive groups, has been demonstrated as a promising starting material to synthesize diverse photoluminescent materials including small molecules, polymers, and carbon dots. The unique citrate chemistry enables the development of a series of citric acid-based molecules and nanomaterials with intriguing intrinsic band-shifting behavior, where the emission wavelength shifts as the excitation wavelength increases, ideal for chromatic imaging and many other applications. In this review, we discuss the concept of "intrinsic band-shifting photoluminescent materials", introduce the recent advances in citric acid-based intrinsic band-shifting materials, and discuss their potential applications such as chromatic imaging and multimodal sensing. It is our hope that the insightful and forward-thinking discussion in this review will spur the innovation and applications of the unique band-shifting photoluminescent materials.

Predicted Negative Ion Photoelectron Spectra of 1-, 2-, and 9-Cyanoanthracene Radical Anions and Computed Thermochemical Values of the Three Cyanoanthracene Isomers

In this work, the negative ion photoelectron spectra of 1-, 2-, and 9-cyanoanthracene (anthracenecarbonitrile, ACN) radical anions, obtained via the calculation of Franck-Condon (FC) factors based on a harmonic oscillator model, are reported. The FC calculations utilize harmonic vibrational frequencies and normal mode vectors derived from density functional theory using the B3LYP/6-311++G (2d,2p) basis set. The removal of an electron from the doublet anion allows for the computation of the negative ion photoelectron spectra that represents the neutral ground singlet state (S_o) and the lowest triplet state (T₁) in each of the three ACN molecules. The respective adiabatic electron affinity (EA) values for the S_o state in 1-, 2-, and 9-ACN isomers are calculated to be 1.353, 1.360, and 1.423 eV. The calculated EA of the 9-cyanoanthracene singlet isomer is in close agreement with the previously reported experimental value of 1.27 ± 0.1 eV. Calculations show that the T₁ states in 1-, 2-, and 9-ACN are located 1.656, 1.663, and 1.599 eV above the S_o state. The calculated T₁ negative ion spectra exhibit intense vibrational origins and weak FC activity beyond the origins, indicating little change in geometry following electron detachment from the doublet anionic state. Upon deprotonation, the EA values of the radical isomers increase by ∼400-700 meV, resulting in neutral deprotonated radicals with EAs between 1.740 and 2.220 eV. The calculated site-specific gas-phase acidity values of ACN isomers indicate that ACN molecules are more acidic than benzonitrile.

Porphyrin-Based Covalent Organic Frameworks: Design, Synthesis, Photoelectric Conversion Mechanism, and Applications

Photosynthesis occurs in high plants, and certain organisms show brilliant technology in converting solar light to chemical energy and producing carbohydrates from carbon dioxide (CO₂). Mimicking the mechanism of natural photosynthesis is receiving wide-ranging attention for the development of novel materials capable of photo-to-electric, photo-to-chemical, and photocatalytic transformations. Porphyrin, possessing a similar highly conjugated core ring structure to chlorophyll and flexible physical and chemical properties, has become one of the most investigated photosensitizers. Chemical modification and self-assembly of molecules as well as constructing porphyrin-based metal (covalent) organic frameworks are often used to improve its solar light utilization and electron transfer rate. Especially porphyrin-based covalent organic frameworks (COFs) in which porphyrin molecules are connected by covalent bonds combine the structural advantages of organic frameworks with light-capturing properties of porphyrins and exhibit great potential in light-responsive materials. Porphyrin-based COFs are expected to have high solar light utilization, fast charge separation/transfer performance, excellent structural stability, and novel steric selectivity by special molecular design. In this paper, we reviewed the research progress of porphyrin-based COFs in the design, synthesis, properties, and applications. We focused on the intrinsic relationship between the structure and properties, especially the photoelectric conversion properties and charge transfer mechanism of porphyrin-based COFs, and tried to provide more valuable information for the design of advanced photosensitizers. The applications of porphyrin-based COFs in photocatalysis and phototherapy were emphasized based on their special structure design and light-to-electric (or light-to-heat) conversion control.

A chiral fluorescent COF prepared by post-synthesis modification for optosensing of imazamox enantiomers

We report a post-synthesis modification for the preparation of a novel chiral fluorescent covalent organic framework (COF) for selective recognization of imazamox enantiomers. In this study, chiral COF was firstly synthesized via a Schiff-base reaction between 2,5-dihydroxyterephthalaldehyde (Dha) and 1,3,5-tris(4-aminophenyl)benzene (Tab) followed by a nucleophilic substitution using (1S)-(+)-10-camphorsulfonyl chloride as chiral modifier. The resulting regular spherical chiral COF Dha Tab not only presented the high optical efficiency, strong covalent bond structure, good crystallinity, large specific surface area but also showed the specific enantioselectivity and quick identification for imazamox enantiomers among five pesticide enantiomers (S/R-imazamox, acephate, acetochlor, propisochlor and metalaxyl). The detection limits for S- and R-imazamox were 4.20 μmol/L and 3.03 μmol/L, respectively. Meanwhile, the enantiomeric excess value (5.30 %) manifested that the chiral COF Dha Tab had the strong adsorption ability to imazamox enantiomers and more higher affinity for R-imazamox. This chiral fluorescent COF opened up a new way for the recognition of enantiomers.

Development of the new pH-driven carrier from alginate/carboxymethyl starch bio-coated co-drugs@COF-OH for controlled and concomitant colon cancer treatment

To develop a new more efficient colon cancer treatment bio-vehicle, in frontier research, for the first time, an attempt has been made to design a unique colon-targeted bio-carrier containing polysaccharides along with nanoporous materials. So, at first, an imine-based covalent organic framework (COF-OH) with respectively an average pore diameter and surface area at 8.5058 nm and 208.29 m²·g^-1 was fabricated. In the next step, about 41.68 % and 95.8 % of 5-fluorouracil (5-Fu) and curcumin (CUR) respectively were loaded on COF-OH, and 5-Fu + CUR@COF-OH was achieved. Due to the higher rate of drug releases in simulated stomach media, 5-Fu + CUR@COF-OH was coated with a mixture of alginate (Alg) and carboxymethyl starch (CMS) via the ionic crosslinking (Alg/CMS@(5-Fu + CUR@COF-OH)). Findings displayed that the use of polysaccharide coat reduce the drug releases in simulated gastric and improved it in simulated intestinal and colonic fluids. The beads swelled about 93.33 % under simulated gastrointestinal conditions, but this value was found higher in the simulated colonic environment and reached 326.67 %. The hemolysis rate lower than 5 %, as well as the cell viability higher than 80 %, were the main showing signs of system biocompatibility. Altogether, the results of the preliminary investigations can highlight the potential of the Alg/CMS@(5-Fu + CUR@COF-OH) for colon-specific drug delivery.

Highly Connected Three-Dimensional Covalent Organic Framework with Flu Topology for High-Performance Li-S Batteries

Lithium-sulfur batteries (LSBs) have been considered as a promising candidate for next-generation energy storage devices, which however still suffer from the shuttle effect of the intermediate lithium polysulfides (LiPSs). Covalent-organic frameworks (COFs) have exhibited great potential as sulfur hosts for LSBs to solve such a problem. Herein, a pentiptycene-based D_2h symmetrical octatopic polyaldehyde, 6,13-dimethoxy-2,3,9,10,18,19,24,25-octa(4'-formylphenyl)pentiptycene (DMOPTP), was prepared and utilized as a building block toward preparing COFs. Condensation of DMOPTP with 4-connected tetrakis(4-aminophenyl)methane affords an expanded [8 + 4] connected network 3D-flu-COF, with a flu topology. The non-interpenetrated nature of the flu topology endows 3D-flu-COF with a high Brunauer-Emmett-Teller surface area of 2860 m² g^-1, large octahedral cavities, and cross-linked tunnels in the framework, enabling a high loading capacity of sulfur (∼70 wt %), strong LiPS adsorption capability, and facile ion diffusion. Remarkably, when used as a sulfur host for LSBs, 3D-flu-COF delivers a high capacity of 1249 mA h g^-1 at 0.2 C (1.0 C = 1675 mA g^-1), outstanding rate capability (764 mA h g^-1 at 5.0 C), and excellent stability, representing one of the best results among the thus far reported COF-based sulfur host materials for LSBs and being competitive with the state-of-the-art inorganic host materials.

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.

Imine and imine-derived linkages in two-dimensional covalent organic frameworks

Covalent organic frameworks (COFs) are porous crystalline polymers that result from the formation of covalent bonds between precisely assembled organic units. Linkage chemistry is a crucial factor in the controllable synthesis and resulting physicochemical properties of COFs. Imine linkages are popular in the formation of polyfunctional two-dimensional (2D) COFs because they are formed easily with structural and functional diversity. There has been much recent interest in expanding beyond this to COFs with imine-derived linkages. This review highlights the development of chemistry to modify and prepare derivatives of imines within 2D COFs. We discuss the derivation of imine bonds via covalent and noncovalent bonding and the properties and potential applications of the resulting materials in order to provide a better understanding of the relationship between covalent linkages and overall performance for 2D COF materials.

Blue-Emitting Ligand-Mediated Assembly of Rare-Earth MOFs toward White-Light Emission, Sensing, Magnetic, and Catalytic Studies

New lanthanide carboxylate compounds with two- (2D) and three-dimensional (3D) structures have been prepared by employing 2,5-bis(prop-2-yn-1-yloxy)terephthalic acid (2,5-BPTA) as an organic linker. The compounds, [Ln(C₁₄H₈O₆)(C₇O₃H₄)·2H₂O]·4(H₂O), Ln = Y, Pr, Nd, Sm, Eu, Gd, Tb, Dy and [Ln(C₇O₃H₄)₃·(C₃H₇ON)·(H₂O)]·2(H₂O)(C₃H₇NO), Ln = La, Ce, Pr, have two- and three-dimensional structures, respectively. In all compounds, lanthanide ions are connected together, forming a dimer, which is connected by the 2,5-BPTA ligand. In the two-dimensional structure, there are two 2,5-BPTA moieties present, and in the three-dimensional structure, there are three 2,5-BPTA moieties present. The lanthanide centers are nine-coordinated, the 2D structure has a tricapped trigonal prismatic arrangement, and the 3D structure has a monocapped distorted square antiprismatic arrangement. The Pr compound forms in both 2D and 3D structures, whose formation depends on the time of the reaction (2 days─2D and 5-6 days─3D). The ligand emits in the blue region, and using the characteristic emission of Eu³⁺ (red) and Tb³⁺ (green) ions, we achieve white light emission in the (Y_0.96Tb_0.02Eu_0.02) compound. The overall quantum yield for the white light emission is 28%. The strong green luminescence of the Tb³⁺-containing compound was employed to selectively sense the Cr³⁺ and Fe³⁺ ions in aqueous solution with limits of detection (LODs) at 0.41 and 8.6 ppm, respectively. The Tb compound was found to be a good heterogeneous catalyst for the Ullman-type O-arylation reaction between phenol and bromoarene with yields of 95%. Magnetic studies on the Gd-, Tb-, and Dy-containing compounds showed weak exchange interactions within the dimeric Ln₂ units. The present work demonstrates the many utilities of the rare-earth-containing MOFs, especially toward white-light emission, metal-ion sensing, and heterogeneous catalysis.

Tale of COF-on-MOF Composites with Structural Regulation and Stepwise Luminescence Enhancement

Integrating metal-organic framework (MOF)-covalent organic framework (COF) allows versatile engineering of hybrid materials with properties superior to pristine components, especially COFs suffered from aggregation-caused quenching (ACQ), unlocking more possibilities to improve the luminescence of COFs. In this work, we prepared various MOF@COF composites with different COF layer thicknesses, in which stable UiO-66-NH₂ served as the inner substrate and 1,3,5-benzenetricarboxaldehyde (BT), and 3,3'-dihydroxybenzidine (DH) were used to construct a COF layer. In addition to the conventional preparation method, we increased the ratio of BT and DH to be 1:2.5, and impressively, the morphologies of acquired UC (1:2.5) materials were quite different from the previous reticular structure and gradually extended from the spherical structure to the prickly structure with the increase of COF monomers. Remarkably, all of the UC materials possessed better luminescence properties than individual COF due to the limited COF layers. Meanwhile, UC-1 materials with an optimal COF layer displayed the strongest emission. In comparison with a single COF, the quantum yields of UC-1 and UC-1 (1:2.5) were increased nearly 7 times and 5 times, respectively. Moreover, the fluorescence of UC-1 materials was progressively enhanced via selective F^- sensing. This work is expected to shed light on the potential hybridization of MOF-COF with structural adjustment, morphological design, and luminescence enhancement.

Combined experimental and TD-DFT/DMOl3 investigations, optical properties, and photoluminescence behavior of a thiazolopyrimidine derivative

We present here the FT-IR, DFT computation, XRD, optical, and photophysical characterization of a heterocyclic compound with thienopyrimidine and pyran moieties. TD-DFT/DMOl³ and TD-DFT/CASTEP computations were used to study the geometry of isolated and dimer molecules and their optical behavior. The indirect (3.93 eV) and direct (3.29 eV) optical energy bandgaps, HOMO-LUMO energy gap (3.02 eV), and wavelength of maximum absorption (353 nm) were determined in the gas phase with M062X/6-31+G (d, p). A thin film of the studied molecule was studied using XRD, FT-IR, and UV-Vis spectroscopy. The average crystallite size was found as 74.95 nm. Also, the photoluminescence spectroscopy revealed that the compound exhibited different emission bands at the visible range with different intensities depending on the degree of molecular aggregation. For instance, solutions with different concentrations emitted blue, cyan, and green light. On the other hand, the solid-state material produced a dual emission with comparable intensities at λ_max = 455, 505, and 621 nm to cover the entire visible range and produce white emission from a single material with CIE coordinates of (0.34, 0.32) that are very similar to the ideal pure white light. Consequently, these findings could lead to the development of more attractive new luminous materials.

Conjugated Three-Dimensional High-Connected Covalent Organic Frameworks for Lithium-Sulfur Batteries

Developing conjugated three-dimensional (3D) covalent organic frameworks (COFs) still remains an extremely difficult task due to the lack of enough conjugated 3D building blocks. Herein, condensation between an 8-connected pentiptycene-based D_2h building block (DMOPTP) and 4-connected square-planar linkers affords two 3D COFs (named 3D-scu-COF-1 and 3D-scu-COF-2). A combination of the 3D homoaromatic conjugated structure of the former building block with the 2D conjugated structure of the latter linking units enables the π-electron delocalization over the whole frameworks of both COFs, endowing them with excellent conductivities of 3.2-3.5 × 10^-5 S cm^-1. In particular, the 3D rigid quadrangular prism shape of DMOPTP guides the formation of a twofold interpenetrated scu 3D topology and high-connected permanent porosity with a large Brunauer-Emmett-Teller (BET) surface area of 2340 and 1602 m² g^-1 for 3D-scu-COF-1 and 3D-scu-COF-2, respectively, ensuring effective small molecule storage and mass transport characteristics. This, in combination with their good charge transport properties, renders them promising sulfur host materials for lithium-sulfur batteries (LSBs) with high capacities (1035-1155 mA h g^-1 at 0.2 C, 1 C = 1675 mA g^-1), excellent rate capabilities (713-757 mA h g^-1 at 5.0 C), and superior cycling stability (71-83% capacity retention at 2.0 C after 500 cycles), surpassing the most of organic LSB cathodes reported thus far.

A rigidity-flexibility balance strategy enabling highly photoluminescent two-dimensional covalent organic framework nanosheets

The fluorescence quenching phenomenon commonly found in two-dimensional COFs is due to either the strong interlayer π-π stacking or the non-radiative decay caused by intramolecular rotation. Here, we report a rigidity-flexibility balance strategy for constructing highly photoluminescent 2D COF nanosheets via the integration of rigid fluorescent molecular nodes with flexible non-planar building blocks. The prepared COF nanosheets, termed TPE-DBC-COF, achieve extremely high PLQY in common organic solvents, especially in tetrahydrofuran (43.5%). Besides, the prepared TPE-DBC-COF exhibits excellent sensitivity and selectivity to tetracycline hydrochloride and high cycling stability, so it can be used as a high-efficiency fluorescent sensor for the detection of tetracycline hydrochloride down to the ppm level.

Excellent Crystallinity and Stability Covalent-Organic Frameworks with High Emission and Anions Sensing

Covalent-organic frameworks (COFs) are a new class of porous crystalline frameworks with high π-conjugation and periodical skeletons. The highly ordered π-conjugation structures in some COFs allow exciton migration and energy transfer over the frameworks, which leads to good fluorescence probing ability. In this work, two COFs (TFHPB-TAPB-COF and TFHPB-TTA-COF) are successfully condensed via the Schiff base condensation reaction. The intramolecular hydrogen bonds between imine bonds and hydroxyl groups form the excited-state intramolecular proton transfer (ESIPT) strategy. Owing to intramolecular hydrogen bonds in the skeleton, the two COFs show high crystallinity, remarkable stability, and excellent luminescence. The COFs represent a good sensitivity and selectivity to fluoride anions via fluorescence turn-off. Other halogen anions (chloride, bromide, and iodine) and acid anions (nitrate and hydrogen carbonate) remain inactive. These results imply that only fluoride anion is capable of opening the hydrogen bond interaction and hence break the ESIPT strategy. The detection limit toward fluoride anion is down to nanomoles level, ranking the best performances among fluoride anion sensors systems.

Janus-Structural AIE Nanofiber with White Light Emission and Stimuli-Response

White-light emitting elastomers (WLEEs) based on stimuli-responsive aggregation-induced emission (AIE) and regulated Förster resonance energy transfer (FRET) have aroused increasing attention due to the demands for wearable optoelectronic devices. Herein, the blue and orange AIEgens with different environmental sensitivities are synthesized and then encapsulated on both sides of nanofibers via side-by-side electrospinning aiming to achieve the Janus WLEEs. After regulating the blue-orange AIEgens ratio, efficient and stable white light emission with a CIE coordinate of about (0.33, 0.31) is achieved at a blue-orange AIEgens mass ratio of 3:1. Besides, the Janus nanofibers (Janus-NFs) also present super stretchability with elongation at the break over 150% and tensile strength close to 7 MPa. The sensitivity of fluorescence for Janus-NFs to its stretching deformation is used to visualize the evolution of the microstructure of nanofibers during stretching. Moreover, the Janus-NFs are also sensitive to HCl and NH₃ , of which the fluorescence color would change under HCl and NH₃ fuming above 2 and 57 ppm in air, respectively. The promising applications of the white light Janus-NFs in smart fabrics, warning sensors, and anti-counterfeiting packaging are demonstrated. This finding provides an efficient strategy for achieving wearable WLEEs with multiple functionalities, promoting the development of wearable devices.

Nanopores of a Covalent Organic Framework: A Customizable Vessel for Organocatalysis

Covalent organic frameworks (COFs) as crystalline polymers possess ordered nanochannels. When their channels are adorned with catalytically active functional groups, their highly insoluble and fluffy powder texture makes them apt heterogeneous catalysts that can be dispersed in a range of solvents and heated to high temperatures (80-180 °C). This would mean very high catalyst density, facile active-site access, and easy separation leading to high isolated yields. Different approaches have been devised to anchor or disperse the catalytic sites into the nanospaces offered by the COF pores. Such engineered COFs have been investigated as catalysts for many organic transformation reactions. These range from Suzuki-Miyaura coupling, Heck coupling, Knoevenagel condensation, Michael addition, alkene epoxidation, CO2 utilization, and more complex biomimetic catalysis. Such catalysts employ COF as a "passive" support that merely docks catalytically active inorganic clusters, or in other cases, the COF itself participates as an "active" support by altering the electronics of the inorganic catalytic sites through the redox activity of its framework. Even more, catalytic organic pockets or metal complexes have been directly tethered to COF walls to make them behave like single-site organocatalysts. Here, we have listed most COF-based organic transformations by categorizing them as metal-free non-noble-metal@COF and noble-metal@COF. The initial part of this review highlights the advantages of COFs as a component of a heterogeneous catalyst, while the latter part discusses all of the current literature on this topic.

2D Covalent Organic Frameworks: From Synthetic Strategies to Advanced Optical-Electrical-Magnetic Functionalities

Covalent organic frameworks (COFs), an emerging class of organic crystalline polymers with highly oriented structures and permanent porosity, can adopt 2D or 3D architectures depending on the different topological diagrams of the monomers. Notably, 2D COFs have particularly gained much attention due to the extraordinary merits of their extended in-plane π-conjugation and topologically ordered columnar π-arrays. These properties together with high crystallinity, large surface area, and tunable porosity distinguish 2D COFs as an ideal candidate for the fabrication of functional materials. Herein, this review surveys the recent research advances in 2D COFs with special emphasis on the preparation of 2D COF powders, single crystals, and thin films, as well as their advanced optical, electrical, and magnetic functionalities. Some challenging issues and potential research outlook for 2D COFs are also provided for promoting their development in terms of structure, synthesis, and functionalities.

Chiral-Induced Ultrathin Covalent Organic Frameworks Nanosheets with Tunable Circularly Polarized Luminescence

Development of covalent organic frameworks (COFs) with circularly polarized luminescence (CPL) is still challenging. Here we first reported ultrathin COFs nanosheets (NS) based CPL materials using a chiral induced-synthesis strategy. Chiral amines served as chiral inducers to give COF TpBpy with chirality and participated in the modification of TpBpy, inhibiting the fluorescence quenching caused by π-π stacking to form ultrathin luminescent chiral COFs (chirCOFs) NS. The obtained chirCOFs R-/S-TpBpy NS had strong chirality and intense red CPL property with a |g_lum| of ∼0.02. Afterward, the carboxyl containing green and blue fluorescent dye molecules were postmodified onto the chirCOFs NS (chirCOFs/Dyes) to achieve color-adjustable CPL. Due to the chirality and energy transfer between chirCOFs and dye groups, the obtained chirCOFs/Dyes showed strong chirality and increased and tunable photoluminescence, exhibiting excellent, tunable, and amplified CPL performance with a maximum |g_lum| of ∼0.1, which was ∼5 times stronger than that of as-prepared chirCOFs NS. Moreover, the corresponding chirCOFs NS were dispersed into a polydimethylsiloxane (PDMS) matrix to form wafer size, highly transparent, and flexible COFs/PDMS films for practical CPL application. This study opens a new strategy to prepare ultrathin chirCOFs NS with strong and tunable CPL by chiral induction and provides a new approach for the preparation of transparent, large size, and flexible COFs composite films in chiral optical applications.

Ultrafast charge transfer dynamics in 2D covalent organic frameworks/Re-complex hybrid photocatalyst

Rhenium(I)-carbonyl-diimine complexes have emerged as promising photocatalysts for carbon dioxide reduction with covalent organic frameworks recognized as perfect sensitizers and scaffold support. Such Re complexes/covalent organic frameworks hybrid catalysts have demonstrated high carbon dioxide reduction activities but with strong excitation energy-dependence. In this paper, we rationalize this behavior by the excitation energy-dependent pathways of internal photo-induced charge transfer studied via transient optical spectroscopies and time-dependent density-functional theory calculation. Under band-edge excitation, the excited electrons are quickly injected from covalent organic frameworks moiety into catalytic Rhenium^I center within picosecond but followed by fast backward geminate recombination. While under excitation with high-energy photon, the injected electrons are located at high-energy levels in Rhenium^I centers with longer lifetime. Besides those injected electrons to Rhenium^I center, there still remain some long-lived electrons in covalent organic frameworks moiety which is transferred back from Rhenium^I. This facilitates the two-electron reaction of carbon dioxide conversion to carbon monoxide.

Re complexes within covalent organic frameworks have emerged as promising photocatalysts for CO₂ reduction. Here, authors identify a high-energy electron transfer pathway during CO₂ reduction that results in longer-lived excited states than a low-energy electron transfer pathway.

Rational Design of Covalent Heptazine Frameworks with Spatially Separated Redox Centers for High-Efficiency Photocatalytic Hydrogen Peroxide Production

The redox reaction centers in natural organisms conducting oxygenic photosynthesis are well arranged in a physically separated manner to convert sunlight into chemical energy efficiently. Mimicking natural photosynthesis via precisely constructing oxidative and reductive reaction centers within photocatalysts is ideal for enhancing catalytic performances in artificial photosynthesis. In this study, new covalent heptazine frameworks (CHFs) with spatially separated redox centers are rationally designed for photocatalytic production of H₂ O₂ from water and oxygen without using any sacrificial agents. Both experimental and computational investigations indicate that the two-electron oxygen reduction reaction occurs on the heptazine moiety, whereas the two-electron water oxidation reaction occurs on the acetylene or diacetylene bond in the CHFs. This unique spatial separation feature is critical for enhancing charge separation and achieving efficient H₂ O₂ production. Meanwhile, the measured exciton binding energy of the diacetylene-containing polymer is merely 24 meV. Under simulated solar irradiation, the rationally designed CHFs can achieve a solar-to-chemical conversion efficiency of 0.78%, surpassing previously reported photocatalytic materials. This study establishes a molecular engineering approach to construct periodically arranged and spatially separated redox centers in single-component polymer photocatalysts, representing a hallmark to create more exciting polymer structures for photocatalysis moving forward.

Nanotechnology-Assisted, Single-Chromophore-Based White-Light-Emitting Organic Materials with Bioimaging Properties

White-light-emitting (WLE) organic materials, especially small molecules comprising a single chromophoric unit, have received much attention due to their tremendous use in modern-day electronic devices and biomaterials. They can increase the efficiency and lifetime of devices compared to the currently used combination approach. Herein, we explored a small symmetric push-pull organic molecule Hexyl-TCBD with a single 1,1,4,4-tetracyanobuta-1,3-diene (TCBD) chromophoric unit containing urea as a key functional group on an acceptor-donor∼donor-acceptor (A-D∼D-A) backbone for its ability to show white-light emission in solution as well as in the solid state. The luminescence was absent in the solid state due to the H-bonding- and π-stacking-driven quenching processes, while emission behavior in solution was tunable with variable CIE chromaticity index values via hydrogen (H)-bonding-governed disaggregation phenomena. Translation of WLE from the Hexyl-TCBD solution to a solid state was demonstrated by utilizing nonemissive polystyrene (80 wt % with respect to the chromophore) as the matrix to obtain WLE nanofibers (made by the electrospun technique) via segregating the molecules. The optical microscopy study validated the WLE nanofibers. The presence of multicolor photoluminescence, including white light, could be fine-tuned through various excitation wavelengths, solvent polarities, and polystyrene matrices. Furthermore, the detailed photophysical studies, including lifetime measurements, indicated that the inherent intramolecular charge transfer (ICT) bands of Hexyl-TCBD exhibit better ICT state stabilization by space charge distribution through the modulation of H-bonding between urea groups. Finally, a cytotoxicity study was performed for Hexyl-TCBD on normal and cancer cell lines using the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT) assay to explore bioimaging applications in biosystems. MTT results revealed significant toxicity toward cancer cells, whereas normal cells exhibited good biocompatibility.

Robust and emissive covalent organic frameworks via intramolecular hydrogen bond interaction

Covalent organic frameworks (COFs) are a new class of crystalline porous polymers with periodic structure in the skeleton and pre-designable pore structure. COFs merge excellent crystallinity, porosity, stability, and emission,...

Luminescent Covalent Organic Frameworks for Biosensing and Bioimaging Applications

Luminescent covalent organic frameworks (LCOFs) have attracted significant attention due to their tunability of structures and photophysical properties at molecular level. LCOFs are built to highly ordered and periodic 2D or 3D framework structures through covalently assembling with various luminophore building blocks. Recently, the advantages of LCOFs including predesigned properties of structure, unique photoluminescence, hypotoxicity and good biocompatibility and tumor penetration, broaden their applications in biorelated fields, such as biosensing, bioimaging, and drug delivery. A specific review that analyses the advances of LCOFs in the field of biosensing and bioimaging is thus urged to emerge. Here the construction of LCOFs is reviewed first. The synthetic chemistry of LCOFs highlights the key role of chemical linkages, which not only concrete the building blocks but also affect the optical properties and even can act as the responsive sites for potential sensing applications. How to brighten LCOFs are clarified through description of structure managements. The ability to utilize the luminescence of LCOFs for applications in biosensing and bioimaging is discussed using state-of-the-art examples of varied practical goals. A prospect finally addresses opportunities and challenges the development of LCOFs facing from chemistry, physics to the applications, according to their current progress.

Excited-state intramolecular proton transfer based covalent organic framework for fluorescence anions sensing

An azine linked covalent organic framework, ACOF, has been constructed via hydrazine hydrate and aldehyde group building unit with hydroxyl group in situ under the solvothermal condition. ACOF possesses good...

Topology modulation of 2D covalent organic frameworks via a "two-in-one" strategy

Topology modulation of covalent organic frameworks (COFs) still remains barely explored, probably due to the lack of appropriate building blocks. A "two-in-one" strategy applies bifunctional monomers to endow ideal stoichiometry and has recently demonstrated great potential in the facile preparation of highly crystalline two-dimensional (2D) COFs with different topologies. Herein, we employ this approach to modulate the topology of 2D COFs by varying the solvents or the monomer concentrations. To our delight, 2D COFs featuring a Kagome (kgm) lattice with both hexagonal and triangular dual pores (DP) or featuring a rhombic square (sql) single pore (SP) structure can be selectively formed by varying the solvents. Furthermore, adjusting the monomer concentrations also successfully tuned the topology of the COFs. In addition, the highly porous dual-pore COF showed potential applications for controlled drug delivery.

Intrareticular charge transfer regulated electrochemiluminescence of donor-acceptor covalent organic frameworks

The control of charge transfer between radical anions and cations is a promising way for decoding the emission mechanism in electrochemiluminescence (ECL) systems. Herein, a type of donor-acceptor (D-A) covalent organic framework (COF) with triphenylamine and triazine units is designed as a highly efficient ECL emitter with tunable intrareticular charge transfer (IRCT). The D-A COF demonstrates 123 folds enhancement in ECL intensity compared with its benzene-based COF with small D-A contrast. Further, the COF's crystallinity- and protonation-modulated ECL behaviors confirm ECL dependence on intrareticular charge transfer between donor and acceptor units, which is rationalized by density functional theory. Significantly, dual-peaked ECL patterns of COFs are achieved through an IRCT mediated competitive oxidation mechanism: the coreactant-mediated oxidation at lower potential and the direct oxidation at higher potential. This work provides a new fundamental and approach to improve the ECL efficiency for designing next-generation ECL devices.

Core-Shell Fluorescent Polymeric Particles with Tunable White Light Emission Based on Aggregation Microenvironment Manipulation

White-light emitting polymers (WLEPs) based on aggregation microenvironment-sensitive aggregation-induced emission (AIE) and Förster resonance energy transfer (FRET) have aroused great interest in lighting and optoelectronic devices. Herein, we developed a novel strategy to construct WLEP particles via a stepwise self-stabilized precipitation polymerization of two emission-complementary AIEgens under core-shell engineering, where the AIE characteristics and FRET process of core-shell fluorescent polymeric particles (CS-FPPs) could be modulated by altering aggregation microenvironment under swelling and shrinking of polymers, facilitating the tunable white light emission of CS-FPPs. Furthermore, such tuning could be fast realized in the solid state, thus demonstrating the potential in anti-counterfeiting. This work proved the significance of aggregation microenvironment on emission of luminogens, guiding the development of high-efficiency emission-tunable materials.

Controllable Synthesis and Performance Modulation of 2D Covalent-Organic Frameworks

Covalent-organic frameworks (COFs) are especially interesting and unique as their highly ordered topological structures entirely built from plentiful π-conjugated units through covalent bonds. Arranging tailorable organic building blocks into periodically reticular skeleton bestows predictable lattices and various properties upon COFs in respect of topology diagrams, pore size, properties of channel wall interfaces, etc. Indeed, these peculiar features in terms of crystallinity, conjugation degree, and topology diagrams fundamentally decide the applications of COFs including heterogeneous catalysis, energy conversion, proton conduction, light emission, and optoelectronic devices. Additionally, this research field has attracted widespread attention and is of importance with a major breakthrough in recent year. However, this research field is running with the lack of summaries about tailorable construction of 2D COFs for targeted functionalities. This review first covers some crucial polymeric strategies of preparing COFs, containing boron ester condensation, amine-aldehyde condensation, Knoevenagel condensation, trimerization reaction, Suzuki CC coupling reaction, and hybrid polycondensation. Subsequently, a summary is made of some representative building blocks, and then underlines how the electronic and molecular structures of building blocks can strongly influence the functional performance of COFs. Finally, conclusion and perspectives on 2D COFs for further study are proposed.

Supramolecular Assembly with Aggregation-Induced Emission Property for Sensing and Detection

The fabrication of new supramolecular materials for real-time detection of analytes including ions, organic pollutants, gases, biomolecules, and drugs is of pivotal importance in industrial manufacture, clinical treatment, and environmental remediation. Incorporating fluorescent molecules with distinct aggregation-induced emission (AIE) effects into supramolecular assemblies has received much attention over the past two decades, owing to the remarkable performance of the AIE-active supramolecular materials in sensing and detection. In this minireview, we summarize the recent progress of superior detection systems on the basis of supramolecular assemblies accompanied with AIE features. We envision that this minireview will be helpful and timely for relevant researchers to stimulate new thinking for constructing new AIE-based supramolecular materials with advanced architectures for effective sensing and detection.

Deuterated Covalent Organic Frameworks with Significantly Enhanced Luminescence

Luminescent covalent organic frameworks (COFs) find promising applications in chemical sensing, photocatalysis, and optoelectronic devices, however, the majority of COFs are non or weakly emissive owing to the aggregation-caused quenching (ACQ) or the molecular thermal motion-based energy dissipation. Here, we report a previously unperceived approach to improve luminescence performance of COFs by introducing isotope effect, which is achieved through substitution of hydrogen from high-frequency oscillators X-H (X=O, N, C) by heavier isotope deuterium. Combining the "bottom-up" and in situ deuteration methods generates the first deuterated COF, which exhibits an impressively 19-fold enhancement in quantum yield over that of the non-deuterated counterpart. These results are interpreted by theoretical calculations as the consequence of slower C/N-D and OD⋅⋅⋅O vibrations that impede the nonradiative deactivation process. The proposed strategy is proved applicable to many other types of emissive COFs.

Editing Light Emission with Stable Crystalline Covalent Organic Frameworks via Wall Surface Perturbation

The ordered π skeletons of covalent organic frameworks make them viable light-emitting materials but their limited tunability has precluded further implementation. Here we report the synthesis of hydrazone-linked frameworks which are stable in water, acid, and base, and demonstrate their utility as a platform for light emission. The polygonal backbone is designed to be luminescent and partially π conjugated while the pore wall is docked with single atom or unit to induce resonance, hyperconjugation, and tautomerization effects. These effects can be transmitted to the backbone, so that the framework can emit three primary colors of light. The wall can be perturbated with multiple surface sites, rendering the material able to edit diverse emission colors in a predesignable and digital way. The systems show high activity, stability, tunability, and sensibility: a set of features attractive for light-emitting and sensing applications.