Covalent organic frameworks towards photocatalytic applications: Design principles, achievements, and opportunities

ConspectusPhotochromic diarylethenes featuring reversible regulation by external light irradiation have attracted increasing attention in versatile applications such as logic gates, supramolecular systems, liquid crystals, and super-resolution imaging because of their outstanding bistability and fatigue resistance. However, for typical diarylethene systems, there always exist three typical unsolved issues. The first is how to modulate the bistability between the open and closed forms from the viewpoint of ethene bridge aromaticity. The second is how to decrease and avoid the photoinactive parallel conformer in order to achieve a high quantum yield, since the open form possesses the photoactive antiparallel (ap) conformation and the photoinactive parallel (p) conformation. Because of the typical rapid rotation of the flexible side aryl groups, the two conformers cannot be separated efficiently, thereby resulting in a relatively low photocyclization quantum yield. The third is how to fulfill the enantiospecific transformation with reversibility to photomodulate the chirality. Stereochemically, the ap conformer with C2 symmetry can be further subdivided into a pair of enantiomers with P and M helicity originating from the central hexatriene moiety. Similarly, the rapid rotation can also lead to the loss of intrinsic chirality, restricting the development and application of light-driven chiroptical switches. Accordingly, it is desirable to construct a specific diarylethene system to break through these bottlenecks for real versatile applications.Our group has recently developed a unique sterically hindered diarylethene system based on benzobis(thiadiazole) as the ethene bridge for completely solving these issues. We introduce a low-aromaticity benzobis(thiadiazole) unit into the diarylethene as a central ethene bridge with incomparably high bistability. To block or freeze the rotation of flexible side aryls, we further incorporate a large bulky benzothiophene unit to induce a large steric hindrance, or rotation barrier, between the ethene bridge and side aryls, thereby successfully separating multiple conformers of the diarylethenes with high photocyclization quantum yields and enantiospecific photoreaction. Consequently, given such a fantastic building block, we enhance its performance by means of supramolecular self-assembly, thereby realizing unique conformer-dependent self-assembly as well as unprecedented concerted isomerization and enantiospecific photoreaction of photoresponsive metallacycles. In addition, decoration of the intrinsically chiral diarylethenes with mesogenic units can enable us to manipulate the helical superstructure of liquid crystals, thus achieving a multiple anticounterfeiting technique and a quadridimensional manipulable laser. We also unravel the dual aggregation-induced emission (AIE) behavior of the sterically hindered diarylethene, especially as applied in super-resolution imaging.

Sterically Hindered Diarylethenes with a Benzobis(thiadiazole) Bridge: Enantiospecific Transformation and Reversible Photosuperstructures.

ConspectusIn past decades, regular porous architectures have received a great amount of attention because of their versatile functions and applications derived from their efficient adsorption of various guests. However, most reported porous architectures exist only in the solid state. Therefore, their applications as biomaterials may face several challenges, such as phase separation, slow degradation, and long-term accumulation in the body. This Account summarizes our efforts with respect to the development and biomedical applications of water-soluble 3D diamondoid supramolecular organic frameworks (dSOFs), a family of supramolecular polymers that possess intrinsic regular nanoscale porosity.dSOFs have been constructed from tetratopic components and cucurbit[8]uril (CB[8]) through hydrophobically driven encapsulation by CB[8] for intermolecular dimers formed by peripheral aromatic subunits of the tetratopic components in water. All dSOFs exhibit porosity regularity or periodicity in aqueous solution, which is confirmed by solution-phase synchrotron SAXS and XRD experiments. Dynamic light scattering (DLS) reveals that their sizes range from 50 to 150 nm, depending on the concentrations of the components. As nonequilibrium supramolecular architectures, dSOFs can maintain their nanoscale sizes at micromolar concentrations for dozens of hours. Their diamondoid pores have aperture sizes ranging from 2.1 to 3.6 nm, whereas their water solubility and porosity regularity allow them to rapidly include discrete guests driven by ion-pair electrostatic attraction, hydrophobicity, or a combination of the two interactions. The guests may be small molecule or large macromolecular drugs, photodynamic agents (PDAs), or DNA.The rapid inclusion of bioactive guests into dSOFs has led to two important biofunctions. The first is to function as antidotes through including residual drugs. For heparins, the inclusion results in full neutralization of their anticoagulant activity. For clinically used porphyrin PDAs, the inclusion can alleviate their long-term posttreatment phototoxicity but does not reduce their photodynamic efficacy. The second is to function as in situ loading carriers for the intracellular delivery of antitumor drugs or DNA. Their nanoscale sizes bring out their ability to overcome the multidrug resistance of tumor cells, which leads to a remarkable enhancement of the bioactivity of the included drugs. By conjugating aldoxorubicin to tetrahedral components, albumin-mimicking prodrugs have also been constructed, which conspicuously improves the efficacy of aldoxorubicin toward multi-drug-resistant tumors through the delivery of the frameworks. As new supramolecular drugs and carriers, dSOFs are generally biocompatible. Thus, further efforts might lead to medical benefits in the future.

Supramolecular Organic Frameworks: Exploring Water-Soluble, Regular Nanopores for Biomedical Applications.

Controlling dynamics in extended molecular frameworks

The incorporation of molecular machines into the backbone of porous framework structures will facilitate nano actuation, enhanced molecular transport, and other out-of-equilibrium host–guest phenomena in well-defined 3D solid materials. In this work, we detail the synthesis of a diamine-based light-driven molecular motor and its incorporation into a series of imine-based polymers and covalent organic frameworks (COF). We study structural and dynamic properties of the molecular building blocks and derived self-assembled solids with a series of spectroscopic, diffraction, and theoretical methods. Using an acid-catalyzed synthesis approach, we are able to obtain the first crystalline 2D COF with stacked hexagonal layers that contains 20 mol% molecular motors. The COF features a specific pore volume and surface area of up to 0.45 cm3 g−1 and 604 m2 g−1, respectively. Given the molecular structure and bulkiness of the diamine motor, we study the supramolecular assembly of the COF layers and detail stacking disorders between adjacent layers. We finally probe the motor dynamics with in situ spectroscopic techniques revealing current limitations in the analysis of these new materials and derive important analysis and design criteria as well as synthetic access to new generations of motorized porous framework materials.

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Light-driven molecular motors embedded in covalent organic frameworks

Supramolecular organic frameworks improve the safety of clinically used porphyrin photodynamic agents and maintain their antitumor efficacy.

: Incorporation of molecular switches with light, heat, and electricity responsibility into arti ﬁ cial solids has been developed as a successful strategy to construct stimuli-responsive functional materials. However, precise manipulation of their molecular geometries and electronic structures to control the properties of macroscopic materials still remains a fundamental challenge. Herein, a photoresponsive covalent organic framework ( o -COF) with the square lattice was fabricated from the dynamic covalent chemistry reaction of ring-open dithienylethene − dialdehyde with 5,10,15,20-tetrakis(4-aminophenyl)porphyrin (H 2 TAPP). UV irradiation of the dithienyle-thene-based units in o -COF a ﬀ orded its reversible photoisomer ( c -COF) in the ring-closed form. In addition to a range of di ﬀ raction, microscopic, and gas physical sorption characterizations, spectroscopic investigations with the help of theoretical simulations revealed di ﬀ erent photocatalytic activities toward the evolution of singlet oxygen and corresponding photocatalytic oxidation of amines due to the di ﬀ erent energy transfer pathways from the porphyrin unit to BBTP photoisomers in these two COFs. Most interestingly, such di ﬀ erent photocatalytic behaviors for two COFs could be easily tuned in a reversible manner by adjusting the ring-closed/ open form of dithienylethene units by means of UV and visible light. mg) were added into 1.0 mL of CD 3 CN in a 5 mL quartz tube. The resultant mixture was stirred at room temperature upon irradiation with a 25 W blue LED light under air. Conversion of the product was determined by 1 H NMR in CD 3 CN. For the recycle test, the catalyst was separated by centrifugation, washed by CH 3 OH after every cycle, and dried at 25 ° C under vacuum for next use.

Photoresponsive Covalent Organic Frameworks with Diarylethene Switch for Tunable Singlet Oxygen Generation

Heparins are widely used anticoagulants for surgical procedures and extracorporeal therapies. However, all of them have bleeding risks. Protamine sulfate, the only clinically approved antidote for unfractionated heparin (UFH), has adverse effects. Moreover, protamine can only partially neutralize low‐molecular‐weight heparins (LMWHs) and is not effective for fondaparinux. Here, an inclusion–sequestration strategy for efficient neutralization of heparin anticoagulants by cationic porous supramolecular organic frameworks (SOFs) and porous organic polymers (POPs) is reported. Isothermal titration calorimetric and fluorescence experiments show strong binding affinities of these porous polymers toward heparins, whereas dynamic light scattering and zeta potential analysis confirm that the heparin sequences are adsorbed into the interior of the porous hosts. Activated partial thromboplastin time, anti‐FXa, and thromboelastography assays indicate that their neutralization efficacies are higher than or as high as that of protamine for UFH and generally superior to protamine for LMWHs and fondaparinux, which is further confirmed by tail‐transection model in mice and ex vivo aPTT or anti‐FXa analysis in rats. Acute toxicity evaluations reveal that one of the SOFs displays outstanding biocompatibility. This work suggests that porous polymers can supply safe and rapid reversal of clinically used heparins, as protamine surrogates, providing an improved approach for their neutralization.

/pdf/porous-polymers-as-universal-reversal-agents-for-heparin-3nvytgcz.pdf

Porous Polymers as Universal Reversal Agents for Heparin Anticoagulants through an Inclusion–Sequestration Mechanism

Photoelectrochemical (PEC) water splitting has attracted a great attention in the past several decades which holds great promise to address global energy and environmental issues by converting solar energy into hydrogen. However, its low solar-to-hydrogen (STH) conversion efficiency remains a bottleneck for practical application. Developing efficient photoelectrocatalysts with high stability and high STH conversion efficiency is one of the key challenges. As a typical n-type semiconductor, titanium dioxide (TiO 2 ) exhibits high PEC water splitting performance, especially high chemical and photo stability. But, TiO 2 has also disadvantages such as wide band gap and fast electron-hole recombination rate, which seriously hinder its PEC performance. This review focuses on recent development in TiO 2 -based photoanodes as well as some key fundamentals. The corresponding mechanisms and key factors for high STH, and controllable synthesis and modification strategies are highlighted in this review. We conclude finally with an outlook providing a critical perspective on future trends on TiO 2 -based photoanodes for PEC water splitting.

Recent Advances in TiO2-based Photoanodes for Photoelectrochemical Water Splitting.

Malignant proliferation and metastasis are the hallmarks of cancer cells. Aminated [70]fullerene exhibits notable antineoplastic effects, promoting it a candidate for multi‐targeted cancer drugs. It is an urgent need to reveal the structure–activity relationship for antineoplastic aminated fullerenes. Herein, three amphiphilic derivatives of [60]fullerene with clarified molecular structures are synthesized: TAPC‐4, TAPC‐3, and TCPC‐4. TAPC‐4 inhibits the proliferation of diverse tumor cells via G0/G1 cell cycle arrest, reverses the epithelial–mesenchymal transition, and abrogates the high mobility of tumor cells. TAPC‐4 can be excreted from the organism and achieves an in vivo inhibition index of 75.5% in tumor proliferation and 87.5% in metastatic melanoma with a wide safety margin. Molecular dynamics simulations reveal that the amphiphilic molecular structure and the ending amino groups promote the targeting of TAPC‐4 to heat shock protein Hsp90‐beta, vimentin, and myosin heavy chain 9 (MYH9), probably resulting in the alteration of cyclin D1 translation, vimentin expression, and MYH9 location, respectively. This work initially emphasizes the dominant role of the amphiphilic structure and the terminal amino moieties in the antineoplastic effects of aminated fullerenes, providing fundamental support for their anti‐tumor drug development.

https://onlinelibrary.wiley.com/doi/pdfdirect/10.1002/advs.202201541

Wei Zhou

Papers

Supramolecular organic frameworks improve the safety of clinically used porphyrin photodynamic agents and maintain their antitumor efficacy.

Photoresponsive Covalent Organic Frameworks with Diarylethene Switch for Tunable Singlet Oxygen Generation

Porous Polymers as Universal Reversal Agents for Heparin Anticoagulants through an Inclusion–Sequestration Mechanism

Recent Advances in TiO2-based Photoanodes for Photoelectrochemical Water Splitting.

Amphiphilic Aminated Derivatives of [60]Fullerene as Potent Inhibitors of Tumor Growth and Metastasis