Benzoxazole-Linked Ultrastable Covalent Organic Frameworks for Photocatalysis

The structural uniqueness of covalent organic frameworks (COFs) has brought these new materials great potential for advanced applications. One of the key aspects yet to be developed is how to improve the robustness of covalently linked reticular frameworks. In order to make the best use of π-conjugated structures, we develop herein a "killing two birds with one stone" strategy and construct a series of ultrastable benzoxazole-based COFs (denoted as LZU-190, LZU-191, and LZU-192) as metal-free photocatalysts. Benefiting from the formation of benzoxazole rings through reversible/irreversible cascade reactions, the synthesized COFs exhibit permanent stability in the presence of strong acid (9 M HCl), strong base (9 M NaOH), and sunlight. Meanwhile, reticulation of the benzoxazole moiety into the π-conjugated COF frameworks decreases the optical band gap and therefore increases the capability for visible-light absorption. As a result, the excellent photoactivity and unprecedented recyclability of LZU-190 (for at least 20 catalytic runs, each with a product yield of 99%) have been illustrated in the visible-light-driven oxidative hydroxylation of arylboronic acids to phenols. This contribution represents the first report on the photocatalytic application of benzoxazole-based structures, which not only sheds new light on the exploration of robust organophotocatalysts from small molecules to extended frameworks but also offers in-depth understanding of the structure-activity relationship toward practical applications of COF materials.

Cd-Based metal-organic frameworks from solvothermal reactions involving in situ aldimine condensation and the highly sensitive detection of Fe3+ ions

Four Cd(ii)-based compounds (1-4) were synthesized from solvothermal reactions involving the in situ aldimine condensation of an o-diamino-functionalized precursor 3,6-di(4H-imidazol-4-yl)benzene-1,2-diamine (L), Cd(NO₃)₂·4H₂O and aldehyde. Two modes of cycloaddition ([4 + 1] cycloaddition and [4 + 2] cycloaddition) occurred during condensation, causing the in situ generation of two benzimidazole derivative ligands (L1 and L3) and a quinoxaline derivative ligand (L2). Furthermore, the chemical selectivity of the condensation was studied, where the condensation of o-diamino and the aldehyde is more stable and easy to operate. This strategy enriches the synthesis method of MOFs. Additionally, compound 2 containing uncoordinated quinoxaline N atoms showed excellent luminescent sensitivity for Fe³⁺ detection.

Coordination-supported organic polymers: mesoporous inorganic–organic materials with preferred stability

A simple and versatile strategy is developed for the synthesis of coordination-supported organic polymers(COPs) via coordination between Al³⁺ and 5-amino-8-hydroxyquinoline together with organic imine- or imide-based polycondensation.

Porphyrin-based porous polyimide polymer/Pd nanoparticle composites as efficient catalysts for Suzuki–Miyaura coupling reactions

Two porphyrin-based porous polyimide polymer/Pd nanoparticle composites were synthesized, which exhibit high stability and excellent catalytic efficiency as Suzuki–Miyaura coupling catalysts.

The geometry of periodic knots, polycatenanes and weaving from a chemical perspective: a library for reticular chemistry

The geometry of the most regular polycatenanes and weavings, as an extended family of discrete knots and catenanes, is described in terms of sticks and corners in their optimal embeddings.

A three-dimensional cubic halogen-bonded network

The rational, deliberate design of supramolecular architectures is of great importance for the discovery of complex materials.

From double-shelled grids to supramolecular frameworks

Double-shelled 4f and 3d–4f supramolecular grids were assembled by the combination of two different tritopic hydrazone ligands, and further stacked into supramolecular HOFs through sharing the hydrogen bonding donors of halide ions. This study provides the first lanthanide-based double-shelled supramolecular grids and functionalized HOFs.

Covalent Organic Frameworks: From Materials Design to Biomedical Application

Covalent organic frameworks (COFs) are newly emerged crystalline porous polymers with well-defined skeletons and nanopores mainly consisted of light-weight elements (H, B, C, N and O) linked by dynamic covalent bonds. Compared with conventional materials, COFs possess some unique and attractive features, such as large surface area, pre-designable pore geometry, excellent crystallinity, inherent adaptability and high flexibility in structural and functional design, thus exhibiting great potential for various applications. Especially, their large surface area and tunable porosity and π conjugation with unique photoelectric properties will enable COFs to serve as a promising platform for drug delivery, bioimaging, biosensing and theranostic applications. In this review, we trace the evolution of COFs in terms of linkages and highlight the important issues on synthetic method, structural design, morphological control and functionalization. And then we summarize the recent advances of COFs in the biomedical and pharmaceutical sectors and conclude with a discussion of the challenges and opportunities of COFs for biomedical purposes. Although currently still at its infancy stage, COFs as an innovative source have paved a new way to meet future challenges in human healthcare and disease theranostic.

Oligoprolines as Molecular Entities for Controlling Distance in Biological and Material Sciences

Nature utilizes large biomolecules to fulfill tasks that require spatially well-defined arrangements at the molecular level such as electron transfer, ligand-receptor interactions, or catalysis. The creation of synthetic molecules that enable precise control over spacing and functionalization provides opportunities across diverse disciplines. Key requirements of functionalizable oligomeric scaffolds include the specific control of their molecular properties where the correct balance of flexibility and rigidity must be maintained in addition to the prerequisite of defined length. These molecules must ideally be equally applicable in aqueous and organic environments, they must be easy to synthesize in a controlled stepwise fashion, and they must be easily modified with a palette of chemical appendages having diverse functionalities. Oligoproline, a peptidic polymer comprised of repeating units of the amino acid proline, is an ideal platform to meet such challenges. Oligoproline derives its characteristic rigidity and well-defined secondary structure from the innate features of proline. It is the only naturally occurring amino acid that has its side-chain cyclized to its α-amino group, generating often-populated trans and cis conformers around the tertiary amide bonds formed in proline oligomers. Oligoprolines are widely applied to define distance on the molecular level as they are capable of serving as both a "molecular ruler" with a defined length and as a "molecular scaffold" with precisely located and predictably oriented substitutions along the polymeric backbone. Our investigations focus on the use of oligoproline as a molecular scaffold. Toward this end, we have investigated the role of solvent upon helical structure of oligoproline, and the effect that substituents on the pyrrolidine ring and the oligomer termini have on the stability of the helix. We have also further explored the molecular characteristics of oligoproline through spectroscopic and crystallographic methods. All of these structural insights laid the basis for implementation of oligoproline in materials science and chemical biology. Within this Account, we highlight the value of oligoprolines for applications in distinctly different research areas. Toward materials chemistry, we have utilized oligoprolines for the size-controlled generation of noble metal nanoparticles, and to probe the role of spatial preorganization of π-systems for molecular self-assembly. Within the biological realm, we have applied oligoprolines to probe the role of distance on G-protein coupled receptor-mediated ligand uptake by cancerous cells and to investigate the effects of charge preorganization on the efficacy of cationic cell-penetrating peptides.

Covalent Organic Frameworks-Organic Chemistry Beyond the Molecule

The synthesis of organic molecules has at its core, purity, definitiveness of structure, and the ability to access specific atoms through chemical reactions. When considering extended organic structures, covalent organic frameworks (COFs) stand out as a true extension of molecular organic chemistry to the solid state, because these three fundamental attributes of molecular organic chemistry are preserved. The fact that COFs are porous provides confined space within which molecules can be further modified and controlled.

A Synthetic Route for Crystals of Woven Structures, Uniform Nanocrystals, and Thin Films of Imine Covalent Organic Frameworks

Developing synthetic methodology to crystallize extended covalent structures has been an important pursuit of reticular chemistry. Here, we report a homogeneous synthetic route for imine covalent organic frameworks (COFs) where crystallites emerge from clear solutions without forming amorphous polyimine precipitates. The key feature of this route is the utilization of tert-butyloxycarbonyl group protected amine building blocks, which are deprotected in situ and gradually nucleate the crystalline framework. We demonstrate the utility of this approach by crystallizing a woven covalent organic framework (COF-112), in which covalent organic threads are interlaced to form a three-dimensional woven framework. The homogeneous imine COF synthesis also enabled the control of nucleation and crystal growth leading to uniform nanocrystals, through microwave-assisted reactions, and facile preparation of oriented thin films.

Molecular Knots

The first synthetic molecular trefoil knot was prepared in the late 1980s. However, it is only in the last few years that more complex small-molecule knot topologies have been realized through chemical synthesis. The steric restrictions imposed on molecular strands by knotting can impart significant physical and chemical properties, including chirality, strong and selective ion binding, and catalytic activity. As the number and complexity of accessible molecular knot topologies increases, it will become increasingly useful for chemists to adopt the knot terminology employed by other disciplines. Here we give an overview of synthetic strategies towards molecular knots and outline the principles of knot, braid, and tangle theory appropriate to chemistry and molecular structure.

High-Crystallinity Covalent Organic Framework with Dual Fluorescence Emissions and Its Ratiometric Sensing Application

High crystallinity of covalent organic frameworks (COFs) with dual fluorescence emissions has not been reported so far. Here, we show the rational design and preparation of high-crystallinity COF TzDa via the synergetic interaction of docking sites and hydrogen bonds: 4,4',4″-(1,3,5-Triazine-2,4,6-triyl)trianiline (Tz) with the docking site and 2,5-dihydroxyterephthalaldehyde (Da) with the OH group are employed to synthesize the imine-linked two-dimensional high-crystallinity layered structure TzDa. The prepared mesoporous TzDa (ca. 36 Å) exhibits high thermal and chemical stability. The intramolecular charge transfer (ICT) and excited-state intramolecular proton transfer (ESIPT) effects bring TzDa two main fluorescence emissions around 500 and 590 nm. Water molecules can interfere with the ICT and ESIPT effects, allowing the development of a ratiometric fluorescent sensor for water in organic solvents. The proposed sensor shows high sensitivity to trace water in conventional organic solvents. The high stability of TzDa allows its recyclable uses for trace water detection. This work not only offers a platform for the construction of high-crystallinity COFs, but also provides a rational design of COFs with dual fluorescence emissions for ratiometric sensing applications.

A triaxial supramolecular weave

Despite recent advances in the synthesis of increasingly complex topologies at the molecular level, nano- and microscopic weaves have remained difficult to achieve. Only a few diaxial molecular weaves exist-these were achieved by templation with metals. Here, we present an extended triaxial supramolecular weave that consists of self-assembled organic threads. Each thread is formed by the self-assembly of a building block comprising a rigid oligoproline segment with two perylene-monoimide chromophores spaced at 18 Å. Upon π stacking of the chromophores, threads form that feature alternating up- and down-facing voids at regular distances. These voids accommodate incoming building blocks and establish crossing points through CH-π interactions on further assembly of the threads into a triaxial woven superstructure. The resulting micrometre-scale supramolecular weave proved to be more robust than non-woven self-assemblies of the same building block. The uniform hexagonal pores of the interwoven network were able to host iridium nanoparticles, which may be of interest for practical applications.

Electrochemical Stimuli-Driven Facile Metal-Free Hydrogen Evolution from Pyrene-Porphyrin-Based Crystalline Covalent Organic Framework

A [2 + 2] Schiff base type condensation between 5,10,15,20-tetrakis(4-aminophenyl)porphyrin (TAP) and 1,3,6,8-tetrakis (4-formylphenyl) pyrene (TFFPy) under solvothermal condition yields a crystalline, quasi-two-dimensional covalent organic framework (SB-PORPy-COF). The porphyrin and pyrene units are alternatively occupied in the vertex of 3D triclinic crystal having permanent microporosity with moderately high surface area (∼869 m² g^-1) and promising chemical stability. The AA stacking of the monolayers give a pyrene bridged conducting channel. SB-PORPy-COF has been exploited for metal free hydrogen production to understand the electrochemical behavior using the imine based docking site in acidic media. SB-PORPy-COF has shown the onset potential of 50 mV and the Tafel slope of 116 mV dec^-1. We expect that the addendum of the imine based COF would not only enrich the structural variety but also help to understand the electrochemical behavior of these class of materials.

Covalent Organic Frameworks as a Platform for Multidimensional Polymerization

The simultaneous polymerization and crystallization of monomers featuring directional bonding designs provides covalent organic frameworks (COFs), which are periodic polymer networks with robust covalent bonds arranged in two- or three-dimensional topologies. The range of properties characterized in COFs has rapidly expanded to include those of interest for heterogeneous catalysis, energy storage and photovoltaic devices, and proton-conducting membranes. Yet many of these applications will require materials quality, morphological control, and synthetic efficiency exceeding the capabilities of contemporary synthetic methods. This level of control will emerge from an improved fundamental understanding of COF nucleation and growth processes. More powerful characterization of structure and defects, improved syntheses guided by mechanistic understanding, and accessing diverse isolated forms, ranging from single crystals to thin films to colloidal suspensions, remain important frontier problems.

Molecular weaving via surface-templated epitaxy of crystalline coordination networks

One of the dream reactions in polymer chemistry is the bottom-up, self-assembled synthesis of polymer fabrics, with interwoven, one-dimensional fibres of monomolecular thickness forming planar pieces of textiles. We have made a major step towards realizing this goal by assembling sophisticated, quadritopic linkers into surface-mounted metal-organic frameworks. By sandwiching these quadritopic linkers between sacrificial metal-organic framework thin films, we obtained multi-heteroepitaxial, crystalline systems. In a next step, Glaser–Hay coupling of triple bonds in the quadritopic linkers yields linear, interwoven polymer chains. X-ray diffraction studies revealed that this topochemical reaction leaves the MOF backbone completely intact. After removing the metal ions, the textile sheets can be transferred onto different supports and imaged using scanning electron microscopy and atomic-force microscopy. The individual polymer strands forming the two-dimensional textiles have lengths on the order of 200 nm, as evidenced by atomic-force microscopy images recorded from the disassembled textiles.

The self-assembly of polymer threads into interwoven textiles is an important goal in polymer chemistry. Here the authors assemble interwoven polymer chains by cross-linking acetylene functionalized ligands in surface-mounted MOFs and subsequent removal of the metal ions affords 2D textile sheets.

Emerging ≈800 nm Excited Lanthanide-Doped Upconversion Nanoparticles

Lanthanide-doped upconversion nanoparticles can tune near-infrared light to visible or even ultra-violet light in emissions. Due to their unique photophysical and photochemical properties, as well as their promising bioapplications, there has been a great deal of enthusiastic research performed to study the properties of lanthanide-doped upconversion nanoparticles in the past few years. Despite the considerable progress in this area, numerous challenges associated with the nanoparticles, such as a low upconversion efficiency, limited host materials, and a confined excitation wavelength, still remain, thus hindering further development with respect to their applications and in fundamental science. Recently, innovative strategies that utilize alternative sensitizers have been designed in order to engineer the excitation wavelengths of upconversion nanoparticles. Here, focusing on the excitation wavelength at ≈800 nm, recent advances in the design, property tuning, and applications of ≈800 nm excited upconversion nanoparticles are summarized. Benefiting from the unique features of ≈800 nm light, including deep tissue penetration depth and low photothermal effect, the ≈800 nm excited upconversion nanoparticles exhibit superior potential for biosensing, bioimaging, drug delivery, therapy, and three dimensional displays. The critical aspects of such emerging nanoparticles with regards to meeting the ever-changing needs of future development are also discussed.

Using active colloids as machines to weave and braid on the micrometer scale

Controlling motion at the microscopic scale is a fundamental goal in the development of biologically inspired systems. We show that the motion of active, self-propelled colloids can be sufficiently controlled for use as a tool to assemble complex structures such as braids and weaves out of microscopic filaments. Unlike typical self-assembly paradigms, these structures are held together by geometric constraints rather than adhesive bonds. The out-of-equilibrium assembly that we propose involves precisely controlling the 2D motion of active colloids so that their path has a nontrivial topology. We demonstrate with proof-of-principle Brownian dynamics simulations that, when the colloids are attached to long semiflexible filaments, this motion causes the filaments to braid. The ability of the active particles to provide sufficient force necessary to bend the filaments into a braid depends on a number of factors, including the self-propulsion mechanism, the properties of the filament, and the maximum curvature in the braid. Our work demonstrates that nonequilibrium assembly pathways can be designed using active particles.

Facile synthesis of MOFs with uncoordinated carboxyl groups for selective CO2 capture via postsynthetic covalent modification

A facile strategy involving dual-acyl chloride has been developed to introduce uncoordinated carboxyl groups into amine containing metal–organic frameworks.

Spiers Memorial Lecture: : Progress and prospects of reticular chemistry

Reticular chemistry, the linking of molecular building units by strong bonds to make crystalline, extended structures such as metal–organic frameworks (MOFs), zeolitic imidazolate frameworks (ZIFs), and covalent organic frameworks (COFs), is currently one of the most rapidly expanding fields of science. In this contribution, we outline the origins of the field; the key intellectual and practical contributions, which have led to this expansion; and the new directions reticular chemistry is taking that are changing the way we think about making new materials and the manner with which we incorporate chemical information within structures to reach additional levels of functionality. This progress is described in the larger context of chemistry and unexplored, yet important, aspects of this field are presented.

Two dimensional covalent organic framework materials for chemical fixation of carbon dioxide: excellent repeatability and high selectivity

Two dimensional (2D) metalloporphyrin-based covalent organic framework (COF) composites were synthesized and employed to catalyze the coupling of CO₂ and epoxides to form cyclic carbonates.