Covalent Organic Frameworks Containing Dual O2 Reduction Centers for Overall Photosynthetic Hydrogen Peroxide Production.

Covalent organic frameworks (COFs) are highly desirable for achieving high-efficiency overall photosynthesis of hydrogen peroxide (H2 O2 ) via molecular design. However, precise construction of COFs toward overall photosynthetic H2 O2 remains a great challenge. Herein, we report the crystalline s-heptazine-based COFs (HEP-TAPT-COF and HEP-TAPB-COF) with separated redox centers for efficient H2 O2 production from O2 and pure water. The spatially and orderly separated active sites in HEP-COFs can efficiently promote charge separation and enhance photocatalytic H2 O2 production. Compared with HEP-TAPB-COF, HEP-TAPT-COF exhibits higher H2 O2 production efficiency for integrating dual O2 reduction active centers of s-heptazine and triazine moieties. Accordingly, HEP-TAPT-COF bearing dual O2 reduction centers exhibits a remarkable solar-to-chemical energy efficiency of 0.65 % with a high apparent quantum efficiency of 15.35 % at 420 nm, surpassing previously reported COF-based photocatalysts.

Direct Construction of 2D Conductive Metal-Organic Frameworks from a Nonplanar Ligand: In Situ Scholl Reaction and Topological Modulation.

Two-dimensional conductive metal-organic frameworks (2D c-MOFs) are an emerging class of promising porous materials with high crystallinity, tunable structures, and diverse functions. However, the limited topologies and difficulties in synthesizing suitable organic linkers remain a great challenge for 2D c-MOFs synthesis and applications. Herein, two layered 2D c-MOF polymorphs with either a rhombus structure (sql-TBA-MOF) or kagome structure (kgm-TBA-MOF) were directly constructed via in situ Scholl reaction and coordination chemistry from a flexible and nonplanar tetraphenylbenzene-based ligand (8OH-TPB) in a one-pot manner. Interestingly, the kgm-TBA-MOF comprising hexagonal and triangular dual pores exhibit higher conductivities of 1.65 × 10-3 S/cm at 298 K and 3.33 × 10-2 S/cm at 353 K than that of sql-TBA-MOF (4.48 × 10-4 and 2.90 × 10-3 S/cm, respectively). Moreover, the morphology and topology can be modulated via the addition of ammonium hydroxide as modulator. The present work provides a new pathway for design, synthesis, and topological regulation of 2D c-MOFs.

On-surface synthesis and characterization of nitrogen-doped covalent-organic frameworks on Ag(111) substrate.

Atomically precise fabrication of covalent-organic frameworks with well-defined heteroatom-dopant sites and further understanding of their electronic properties at the atomic level remain a challenge. Herein, we demonstrate the bottom-up synthesis of well-organized covalent-organic frameworks doped by nitrogen atoms on an Ag(111) substrate. Using high-resolution scanning tunneling microscopy and non-contact atomic force microscopy, the atomic structures of the intermediate metal-organic frameworks and the final covalent-organic frameworks are clearly identified. Scanning tunneling spectroscopy characterization reveals that the electronic bandgap of the as-formed N-doped covalent-organic framework is 2.45 eV, in qualitative agreement with the theoretical calculations. The calculated band structure together with the projected density of states analysis clearly unveils that the incorporation of nitrogen atoms into the covalent-organic framework backbone will remarkably tune the bandgap owing to the fact that the foreign nitrogen atom has one more electron than the carbon atom. Such covalent-organic frameworks may offer an atomic-scale understanding of the local electronic structure of heteroatom-doped covalent-organic frameworks and hold great promise for all relevant wide bandgap semiconductor technologies, for example, electronics, photonics, high-power and high-frequency devices, and solar energy conversion.

Superhydrophilic 2D Covalent Organic Frameworks as Broadband Absorbers for Efficient Solar Steam Generation.

The intrinsic hydrophobicity and limited light absorption especially in the near-infrared (NIR) region of porous organic polymers are two bottlenecks impeding their applications in solar steam generation (SSG). Herein, we develop a 1,4,5,8-tetrakis(phenylamino)anthracene-9,10-dione (TPAD)-based covalent organic framework (COF) (TPAD-COF) featuring both superhydrophilicity and broad light absorption covering from the entire UV/Vis to NIR regions for SSG. TPAD-COF serving as a highly efficient photothermal conversion material without any additives displays an excellent water evaporation of 1.42 kg m-2 h-1 and achieves a high energy conversion efficiency of 94 % under 1 sun irradiation. Further extension of the light absorption range of the TPAD-based COF is realized through post-synthetic modification by chelating BF2 moieties. Systematic control experiments and analysis confirm that the hydrophilicity of photothermal conversion materials plays a more dominant role in the current TPAD-based COFs for SSG.

Triangular Topological 2D Covalent Organic Frameworks Constructed via Symmetric or Asymmetric "Two-in-One" Type Monomers.

Most of the reported covalent organic frameworks (COFs) so far are prepared from highly symmetric building blocks, which to some extent limits the expansion of COF diversity and complexity. Low-symmetric building blocks can be designed through a desymmetrized vertex strategy, which might be used to construct new topological COFs. But reports of COFs constructed by asymmetric building blocks are thus far very rare. Here, a feasible strategy to design asymmetric building blocks for COF synthesis is introduced, by simply varying the positions of functional groups in the monomer. As a proof of concept, two isomeric hexaphenylbenzene-based "two-in-one" type monomers (1,2,4-HPB-NH2 and 1,3,5-HPB-NH2 ) are designed and synthesized. To the authors' surprise, self-polycondensation of the asymmetric 1,2,4-HPB-NH2 (i.e., the isomer of common C3 -symmetric 1,3,5-HPB-NH2 ) also affords highly crystalline COF (1,2,4-HPB-COF) similar to the symmetric 1,3,5-HPB-NH2 counterpart with identical topological structure. The triangular porous structures of both HPB-based COFs are well resolved by powder X-ray diffraction (PXRD), theoretical simulations, nitrogen sorption, and morphologies analysis. This work demonstrates the "two-in-one" type asymmetric building blocks can also produce highly crystalline frameworks and thus provides a new structural design strategy for reticular chemistry.

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.

Macrocycle-derived hierarchical porous organic polymers: synthesis and applications.

Porous organic polymers (POPs), as a new category of advanced porous materials, have received broad research interests owing to the advantages of light-weight, robust scaffolds, high specific surface areas and good functional tailorability. According to the long-range ordering of polymer skeletons, POPs can be either crystalline or amorphous. Macrocycles with inherent cavities can serve as receptors for recognizing or capturing specific guest molecules through host-guest interactions. Incorporating macrocycles in POP skeletons affords win-win merits, e.g. hierarchical porosity and novel physicochemical properties. In this review, we focus on the recent progress associated with new architectures of macrocycle-based POPs. Herein, these macrocycles are divided into two subclasses: non-planar (crown ether, calixarene, pillararene, cyclodextrin, cyclotricatechylene, etc.) and planar (arylene-ethynylene macrocycles). We summarize the synthetic methods of each macrocyclic POP in terms of the functions of versatile building blocks. Subsequently, we discuss the performance of macrocyclic POPs in environmental remediation, gas adsorption, heterogeneous catalysis, fluorescence sensing and ionic conduction. Although considerable examples are reported, the development of macrocyclic POPs is still in its infancy. Finally, we propose the underlying challenges and opportunities of macrocycle-based POPs.

Modulating Benzothiadiazole-Based Covalent Organic Frameworks via Halogenation for Enhanced Photocatalytic Water Splitting.

Two-dimensional covalent organic frameworks (2D COFs), an emerging class of crystalline porous polymers, have been recognized as a new platform for efficient solar-to-hydrogen energy conversion owing to their pre-designable structures and tailor-made functions. Herein, we demonstrate that slight modulation of the chemical structure of a typical photoactive 2D COF (Py-HTP-BT-COF) via chlorination (Py-ClTP-BT-COF) and fluorination (Py-FTP-BT-COF) can lead to dramatically enhanced photocatalytic H2 evolution rates (HER=177.50 µmol h-1 with a high apparent quantum efficiency (AQE) of 8.45 % for Py-ClTP-BT-COF). Halogen modulation at the photoactive benzothiadiazole moiety can efficiently suppress charge recombination and significantly reduce the energy barrier associated with the formation of H intermediate species (H*) on polymer surface. Our findings provide new prospects toward design and synthesis of highly active organic photocatalysts toward solar-to-chemical energy conversion.

New synthetic strategies toward covalent organic frameworks.

Covalent organic frameworks (COFs) enable precise reticulation of organic building units into extended 2D and 3D open networks using strong covalent bonds to constitute predesignable topologies and tunable pore structures, presenting an emerging class of crystalline porous polymers. Although rapid progress and substantial achievements in COF chemistry over the past 15 years have been realised, highly efficient strategies and reproducible procedures still play a central role in achieving high-quality COFs and serve as a major driving force for the further advancement of this promising field. In this review, we focused on the key progress in synthesising high-quality COF crystallites and films by highlighting their uniqueness from the viewpoints of synthetic strategies and procedures. We discussed representative synthetic methods including mechanochemical synthesis, microwave synthesis, multicomponent reaction, multistep synthesis and linker exchange strategies to compare their features in producing COFs. We scrutinised the recently developed "two-in-one" molecular design strategy to showcase advantages in optimising synthetic conditions such as catalyst, monomer feeding rate and tolerance to functional groups. We analysed interfacial polymerisation for fabricating various COF films by emphasising their scope and applicability. Moreover, we proposed key underlying challenges to be solved and predicted future frontiers from the perspectives of synthesising high quality crystallites and films that are key to practical applications.

Remotely Photocontrolled Microrobots based on Photomechanical Molecular Crystals.

Creating nano-to-macroscopic-sized artificial actuators in response to light has been a challenging issue. Herein, we describe the design, synthesis, and operation of a photomechanical molecular crystal (PMMC) that exhibits well-controlled multiple photo-driven motions, including translation, rotation, and jumping, by adjusting the irradiation sites. Theoretical calculation discloses that conversion of light energy into macroscopic motion occurs through a molecular conformation change between the excited and ground states mediated by ultrafast conical internal conversion, making the photomechanical/recovery responses a rapid cycle. Therefore, our PMMCs can complete the directional and continuous motions using only one laser beam. We also demonstrated the actuated rotation of a cross-shaped sample by rotating the polarization of the laser beam at a rate of >2 Hz, like a dancer under a spotlight. This finding could lead to remote-controlled micrometer-sized vehicles and valves on solid substrates.

An Upgraded "Two-in-One" Strategy toward Highly Crystalline Covalent Organic Frameworks.

A highly crystalline bicarbazole-based covalent organic framework (BCzP-COF) was synthesized via an upgraded "two-in-one" strategy by the self-polycondensation of A2 B2 monomer with two neopentyl acetal and two amine groups. Such a strategy is propitious to afford higher crystallinity, larger special surface areas and better morphology than that of using unprotected monomer with free aldehydes and amines. Additionally, the off-white powder of BCzP-COF could serve as acidichromism sensor with a significant color change. Intriguingly, the conductivity of the protonated BCzP-COF can improve by six orders of magnitude compared to that of the pristine samples. This work has the potential to lead to bicarbazole-functional materials for chemosensors and electronic devices.

2D covalent organic frameworks with built-in amide active sites for efficient heterogeneous catalysis.

Benzene-1,3,5-tricarboxamides (BTAs) are versatile building blocks for supramolecular assembly due to the strong intermolecular hydrogen bonding. Herein, a BTA based amine, N1,N3,N5-tris(4-aminophenyl)benzene-1,3,5-tricarboxamide (TABTA), was successfully applied to construct two new amide functionalized covalent organic frameworks (COFs) with apparent crystallinity, which were further applied as efficient catalysts for Knoevenagel condensation.

Enhanced optomechanical properties of mechanochemiluminescent poly(methyl acrylate) composites with granulated fluorescent conjugated microporous polymer fillers.

With the combination of mechanochemiluminescence from 1,2-dioxetane coupled polymers and granulated conjugated microporous polymer (CMP) nanosheets, a new kind of filling-type mechanolumninescent polymer composite was developed. Herein, polymeric 1,2-dioxetane performed as an autoluminescent probe of chain scission. Besides benefiting from their excellent optical properties and good interfacial compatibility with poly(methyl acrylate) (PMA) media, two stable and fluorescent CMP nanosheets were prepared and dispersed in crosslinked PMA, which can serve as effective energy acceptors and reinforcing nano-fillers. These polymer nanocomposites present both reinforced mechanical strength and mechanochemiluminescence, and offer exciting opportunities to study the failure process of polymer nanocomposites with unprecedented temporal and spatial resolution.

A New Biscarbazole-Based Metal-Organic Framework for Efficient Host-Guest Energy Transfer.

A new metal-organic framework (MOF), [Zn6 L4 (Me2 NH2 + )4 ⋅3 H2 O] (1) was constructed based on [9, 9'-biscarbazole]-3, 3', 6, 6'-tetracarboxylic acid (H4 L) and Zn2+ ions. The porous framework and intense blue fluorescence of the MOF based on the biscarbazole moiety of the ligand could facilitate efficient host to guest energy transfer, which makes it an ideal platform for the tuning of luminescence.

Tuning the Photophysical Properties of Symmetric Squarylium Dyes: Investigation on the Halogen Modulation Effects.

A series of symmetric squarylium dyes (SQDPA-X) with different halogen (X=F, Cl, Br, I) substituents have been developed. The photophysical properties could be facilely tuned by the halogen modulation effects. The strategy of incorporating different halogen substitutions into AIE active luminogens enables a facile approach for exploring new intriguing organic fluorescent dyes.