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Moisture-triggered reversible proton tautomerization of Sa-TAPA COF. (a) FT−IR spectra of dry Sa-TAPA COF and moisture adsorbed Sa-TAPA-H 2 O COF. (b) Solid-state CP/MAS 13 C NMR spectra of Sa-TAPA COF and Sa-TAPA-H 2 O COF.
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Two-dimensional (2D) covalent organic frameworks (COFs) are an emerging class of porous materials with potential for wide-ranging applications. Intense research efforts have been directed at tuning the structure and topology of COF, however the bandgap engineering of COF has received less attention, although it is a necessary step for developing th...
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... moisture-triggered reversible proton tautomerization of Sa-TAPA COF was monitored by Fourier Transform Infrared Spectroscopy (FT−IR) (Figure 4a).). The band at 1618−1612 cm −1 is mainly attributed to contributions from the CN stretching of the imine linkage with a minor component due to the CO stretching of the ketone group. ...
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... The COF bandgap will be determined by several factors, including the units that make up the COF backbone, the conformation or planarity of the COF, and also by interlayer interaction, in the case of 2D COFs. The bandgap can be tuned by increasing the conjugation length of the backbone [36], incorporating electron donating or withdrawing substituents [37,38], incorporating electron donor-acceptor dyads in the COF backbone [39], and through exfoliation of 2D COFs to produce nanosheets [40]. ...
Photocatalysis is an attractive, energy-efficient technology for organic transformations, polymer synthesis, and degradation of environmental pollutants. There is a need for new photocatalysts stable in different media and that can be tailored for specific applications. Covalent organic frameworks (COF) are crystalline, nanoporous materials with π-conjugated backbone monomers, representing versatile platforms as heterogeneous, metal-free photocatalysts. The backbone structure can be tailored to achieve desired photocatalytic properties, side-chains can mediate adsorption, and the nanoporous structure provides large surface area for molecular adsorption. While these properties make COFs attractive as photocatalysts, several fundamental questions remain regarding mechanisms for different photocatalytic transformations, reactant transport into porous COF structures, and both structural and chemical stability in various environments. In this perspective, we provide a brief overview of COF photocatalysts and identify challenges that should be addressed in future research seeking to employ COFs as photocatalysts. We close with an outlook and perspective on future research directions in the area of COF photocatalysts.
... A visible increase in intensity of the 13 C resonance peak at 123.2 ppm, which was attributed to the C* atom (c* denoted in Figure 3e) of C=C*À C=O block units, [15] was observed for the TP-COFs-1, along with a corresponding decrease in intensity of the 13 C resonance peak at 162.2 ppm belonging to the C atom (d denoted in Figure 3e) of the C-OH group. [16] For the BT-COFs-2 and TP-COFs-2, the strong characteristic peak attributed to the carbonyl (C=O) carbon (d* denoted in Figure 3e) indicated that the formed ketone configuration was dominated. The 13 C NMR peak at 115.3 ppm ascribed to the C* atom of the C=C*À C=N block unit was greatly intensified after the alkaline post-treatment, which offers solid evidence for the enol-keto tautomerism. ...
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.
... The narrower band gap of COF TPE-NN might arise from the extended π-conjugation lengths. [28] Accordingly, here, the band gap analysis was first performed to investigate the feasibility of COF TPE-NN as a sonosensitizer. The valence band (VB) positions of the COF TPE-NN and TPE-NN-Cu nanoparticles were determined via XPS, and the corresponding solid-state ultraviolet-visible (UV/ Vis) reflection spectra were measured to calculate the energy band gaps ( Figure S12-S15). ...
The development of covalent organic framework (COF) sonosensitizers with intrinsic sonodynamic effects is highly desirable. However, such COFs are generally constructed using small‐molecule photosensitizers. Herein, we report that the reticular chemistry‐based synthesis of COFs from two inert monomers yields a COF‐based sonosensitizer (TPE‐NN) with inherent sonodynamic activity. Subsequently, a nanoscale COF TPE‐NN is fabricated and embedded with copper (Cu)‐coordinated sites to obtain TPE‐NN‐Cu. Results show that Cu coordination can enhance the sonodynamic effect of TPE‐NN, whereas ultrasound (US) irradiation for sonodynamic therapy can augment the chemodynamic efficacy of TPE‐NN‐Cu. Consequently, TPE‐NN‐Cu upon US irradiation shows high‐performance anticancer effects based on mutually reinforced sono‐/chemo‐nanodynamic therapy. This study reveals the backbone‐originated sonodynamic activity of COFs and proposes a paradigm of intrinsic COF sonosensitizers for nanodynamic therapy.
... 147 COFs, on the other hand, do not normally exhibit sufficient electrical conductivity due to their high band gap. 148 Therefore, pairing MXenes with COFs makes up for the limited conductivity of COFs by acting as a binder-free current collector. Nevertheless, it is anticipated that augmenting the electrical conductivity of COF helps to widen its application range. ...
Covalent organic frameworks (COFs), a distinguished class of porous materials exhibiting precise modularity and crystallinity, and two-dimensional (2D) MXenes, a highly conductive, atomic layered transition metal carbides or nitrides or carbonitrides, are the two fascinating classes of advanced materials that have been intensively researched for energy storage recently. Thanks to the high surface area and porosity of COFs and high electrical conductivity coupled with highly redox active surfaces of MXenes, they have shown great potential in the energy storage applications such as batteries and supercapacitors. However, their electrochemical performance is limited by several inherent issues such as the restacking tendency of MXene sheets and low conductivity of COFs, when applied individually. Combining MXenes and COFs into heterostructures and their use as a single electrode helps in overcoming challenges for improving the energy storage capability. The current perspective intends to provide an overview of designing such COF/MXene heterostructures in the context of the energy storage applications. The research gaps that exist in designing COF/MXene heterostructures and the governing factors for improving the energy storage capability have also been highlighted as opportunities.
... The PXRD pattern demonstrates two broad diffraction peaks at around 5.1°and 20.5°that might be corresponding to the (100) or (110) and (001) planes, respectively (Figure 1f). 51 Yet, peaks are relatively broad showing the occurrence of low-range crystallinity due to the random displacement of the 2D layers (i.e., exfoliation). 52 These findings agree with the computationally predicted relatively low energy differences for isomers leading to a mixture of COP with slightly different geometries, that would severely reduce the crystallinity of the structure making less favorable the packing of isostructural layers. ...
A novel 2D covalent organic polymer (COP), based on conjugated quinoid-oligothiophene (QOT) and tris(aminophenyl) benzene (TAPB) moieties, is designed and synthesized (TAPB-QOT COP). Some DFT calculations are made to clarify the equilibrium between different QOT isomers and how they could affect the COP formation. Once synthetized, the polymer has been thoroughly characterized by spectroscopic (i.e., Raman, UV-vis), SSNMR and surface (e.g., SEM, BET) techniques, showing a modest surface area (113 m2 g-1) and micropore volume (0.014 cm3 g-1 with an averaged pore size of 5.6-8 Å). Notwithstanding this, TAPB-QOT COP shows a remarkably high iodine (I2) uptake capacity (464 %wt) comparable to or even higher than state-of-the-art porous organic polymers (POPs). These auspicious values are due to the thoughtful design of the polymer with embedded sulfur sites and a conjugated scaffold with the ability to counterbalance the relatively low pore volumes. Indeed, both morphological and Raman data, supported by computational analyses, prove the very high affinity between the S atom in our COP and the I2. As a result, TAPB-QOT COP shows the highest volumetric I2 uptake (i.e., the amount of I2 uptaken per volume unit) up to 331 g cm-3 coupled with a remarkably high reversibility (>80% after five cycles).
... The imine linkages were probed by FT-IR, solid-state NMR, and elemental analysis. The FT-IR spectra of the monomers and COFs-R were shown in Fig [40]. The imine bonds were further confirmed by the peaks in solid-state NMR spectra (Fig. 2b1-b4) between 148 and 151 ppm. ...
... The band gaps became narrower (2.07 and 2.38 eV) after introducing electron-donating substituents of COFs-OH and COFs-OMe accordingly. COFs-OH exhibited a narrower bandgap than COFs-OMe due to the influence of enol-keto tautomerization (Fig. S2) [40]. The electron-withdrawing substituent (-Br) further narrowed the bandgap of COFs-Br (2.49 eV) compared with COFs-H. ...
... Compared with COFs-H, COFs-OH showed a significantly decreased photocurrent, indicating the inhibited charge carrier separation after introducing electron-donating substituent -OH. The weakened photocurrent response might also be due to the enol-keto tautomerization after its exposure to moisture, in which C--C was formed [40] and the conjugated structure was broken (Fig. S2). Electrons of the C--C bond could only be excited by ultraviolet light, so the generation of charge carriers was hindered under visible light irradiation. ...
... The related peaks at ∼160 and ∼180 ppm can be ascribed to C−OH and C�O of the keto− Scheme 1. Illustration of the Interplay between Flexibility, Disorder, and Functionality in 2D Hydrazone-Linked COFs by Hydrogen-Bonding Regulation enol tautomer, respectively (Figures S17 and S18). 46 Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images for H-COFs indicate their morphologies of wormlike nanofibers with diameters of 100−200 nm (Figures S20−S24). The N content decreases successively from H-COF1 to H-COF5, while the carbon content first decreases and then increases based on the elemental analysis data (Table S1), which is consistent with the compositions of H-COFs. ...
Covalent organic framework (COF) chemistry is experiencing unprecedented development in recent decades. The current studies on COF chemistry are mainly focused on the discovery of novel covalent linkages, new topological structures, synthetic methodologies, and potential applications. However, despite the fact that noncovalent interactions are ubiquitous in COF chemistry, relatively little attention has been given to the role of noncovalent bonds on COF structures and their properties. In this work, a series of hydrazone-linked COFs involving noncovalent hydrogen bonds have been constructed, where the hydrogen-bonding interaction plays critical roles in the COF crystallinity and structures. The regulation of structural flexibility, the reversible transition between order and disorder, and the variety of host-guest interactions have been demonstrated in succession for the first time in COFs. The results obtained by the hydrogen-bonding-regulated strategy may also be extendable to other noncovalent interactions, such as π-π interactions, metal coordination interactions, Lewis acid-base interactions, etc. These findings will inspire future developments in the design, synthesis, structural regulation, and applications of COFs by manipulating noncovalent interactions.
... The disorder induced red shifting was also theoretically explained for COF due to the polaron/ bipolaron delocalization along the polymeric chain specifically at longer wavelength [1]. The red shift in the peak position indicates the facile transfer of charges and narrowed band gap for DAB-TFP Flow [59]. ...
Covalent organic frameworks (COF) are increasingly gaining attention as adsorbents for industrial pollutants due to their high surface area and ordered porosity. However, often they have low removal efficiency due to the limited access to their pores. Approachability of foreign molecules to active sites within the framework can be maximized by pore engineering and introduction of defects. Here, we report the synthesis of an imine-based COF (DAB-TFP) at ambient conditions with the incorporation of disorder using continuous flow for the first time. A reusable microreactor that was fabricated using a cost-effective scaffolding technique was used for the flow reactions. The disorder in the COF increased accessibility of its surface for foreign molecules. Moreover, the disorder within the COF resulted in accelerated adsorption of methylene blue (MB) in continuous flow. The removal efficiency of the COF for MB was three folds higher during in-situ adsorption in continuous flow than the traditional batch mode of adsorption. The COF could be regenerated and recycled in continuous flow without any significant loss in the activity. Further, almost 100 % of MB could be reclaimed. The COF was also able to efficiently adsorb a mixture of pollutants as well as a simulated textile waste. Hence, the reported method is a green and sustainable approach for industrial effluent treatment.
... Finally, the pore size distributions calculated using quenched solid density functional theory (QSDFT) were uniform and centered around 0.9 nm (Fig. S1 †), which were very close to that in prior studies of these materials. 42 Furthermore, the incremental pore volume (y-axis of pore size distribution plots, Fig. S1 †) showed similar values, indicating these samples shared similar pore volumes. ...
The sensitivity of covalent organic frameworks (COFs) to pore collapse during activation processes is generally termed activation stability, and activation stability is important for achieving and maintaining COF crystallinity and porosity which are relevant to a variety of applications. However, current understanding of COF stability during activation is insufficient, and prior studies have focused primarily on thermal stability or on the activation stability of other porous materials, such as metal-organic frameworks (MOFs). In this work, we demonstrate and implement a versatile experimental approach to quantify activation stability of COFs and use this to establish a number of relationships between their pore size, the type of pore substituents, pore architecture, and structural robustness. Additionally, density functional theory calculations reveal the impact on both inter-and intra-layer interactions, which govern activation stability, and we demonstrate that activation stability can be systematically tuned using a multivariate synthesis approach involving mixtures of functionalized and unfunctionalized COF building blocks. Our findings provide novel fundamental insights into the activation stability of COFs and offer guidance for the design of more robust COFs.
... Nitrogen-rich building blocks provide a framework with donor−acceptor charge transfer units, providing an opportunity to detect changes in the structure spectroscopically or even with the naked eye. 48,49 Carbazole units are famous for outstanding optoelectronic properties 50,51 and hence are excellent candidates for the design of materials with optical sensing functionality. ...
Isoreticular chemically stable two-dimensional imine covalent organic frameworks (COFs), further denoted as DUT-175 and DUT-176, are obtained in a reaction of 4,4′-bis(9H-carbazol-9-yl)biphenyl tetraaldehyde with phenyldiamine and benzidine. The crystal structures, solved and refined from the powder X-ray diffraction data and confirmed by high-resolution transmission electron microscopy, indicate AA-stacked layer structures. Both structures feature distorted hexagonal channel pores, assuring remarkable porosity (SBET = 1071 m² g–1 for DUT-175 and SBET = 1062 m² g–1 for DUT-176), as confirmed by adsorption of gases and vapors. The complex conjugated π system of the COFs involves electron-rich carbazole building units, which in combination with the imine groups allow reversible pH-dependent protonation of the frameworks, accompanied by charge transfer and shift of the absorption bands in the UV–vis spectrum. The sigmoidal shape of the water vapor adsorption and desorption isotherms with a steep adsorption step at p/p0 = 0.4–0.6 in combination with excellent stability over dozens of adsorption and desorption cycles ranks these COFs among the best materials for indoor humidity control applications.
