Generation of Multicomponent Molecular Cages using Simultaneous Dynamic Covalent Reactions

Molecular vessels from preorganised natural building blocks

Supramolecular vessels emerged as tools to mimic and better understand compartmentalisation, a central aspect of living matter. However, many more applications that go beyond those initial goals have been documented in recent years, including new sensory systems, artificial transmembrane transporters, catalysis, and targeted drug or gene delivery. Peptides, carbohydrates, nucleobases, and steroids bear great potential as building blocks for the construction of supramolecular vessels, possessing complexity that is still difficult to attain with synthetic methods - they are rich in functional groups and well-defined stereogenic centers, ready for noncovalent interactions and further functions. One of the options to tame the functional and dynamic complexity of natural building blocks is to place them at spatially designed positions using synthetic scaffolds. In this review, we summarise the historical and recent advances in the construction of molecular-sized vessels by the strategy that couples synthetic predictability and durability of various scaffolds (cyclodextrins, porphyrins, crown ethers, calix[n]arenes, resorcin[n]arenes, pillar[n]arenes, cyclotriveratrylenes, coordination frameworks and multivalent high-symmetry molecules) with functionality originating from natural building blocks to obtain nanocontainers, cages, capsules, cavitands, carcerands or coordination cages by covalent chemistry, self-assembly, or dynamic covalent chemistry with the ultimate goal to apply them in sensing, transport, or catalysis.

pH-Responsive Self-Assembled Nanoparticles for Tumor-Targeted Drug Delivery

Recent advances in the field of drug delivery have opened new avenues for the development of novel nanodrug delivery systems (NDDS) in cancer therapy. Self-assembled nanoparticles (SANPs) based on tumor microenvironment have great advantages in improving antitumor effect, and pH-responsive SANPs prepared by the combination of pH-responsive nanomaterials and self-assembly technology can effectively improve the efficacy and reduce the systemic toxicity of antitumor drugs. In this review, we describe the characteristics of self-assembly and its driving force, the mechanism of pH-responsive NDDS, and the nanomaterials for pH-responsive SANPs type. A series of pH-responsive SANPs for tumor-targeted drug delivery are discussed, with an emphasis on the relation between structural features and theranostic performance.

Self-assembled porphyrin-peptide cages for photodynamic therapy

The development of photodynamic therapy requires access to smart photosensitizers which combine appropriate photophysical and biological properties. Interestingly, supramolecular and dynamic covalent chemistries have recently shown their ability to produce novel architectures and responsive systems through simple self-assembly approaches. Herein, we report the straightforward formation of porphyrin-peptide conjugates and cage compounds which feature on their surface chemical groups promoting cell uptake and specific organelle targeting. We show that they self-assemble, in aqueous media, into positively-charged nanoparticles which generate singlet oxygen upon green light irradiation, while also undergoing a chemically-controlled disassembly due to the presence of reversible covalent linkages. Finally, the biological evaluation in cells revealed that they act as effective photosensitizers and promote synergistic effects in combination with Doxorubicin.

Dynamic covalent synthesis

Dynamic covalent synthesis aims to precisely control the assembly of simple building blocks linked by reversible covalent bonds to generate a single, structurally complex, product. In recent years, considerable progress in the programmability of dynamic covalent systems has enabled easy access to a broad range of assemblies, including macrocycles, shape-persistent cages, unconventional foldamers and mechanically-interlocked species (catenanes, knots, etc.). The reversibility of the covalent linkages can be either switched off to yield stable, isolable products or activated by specific physico-chemical stimuli, allowing the assemblies to adapt and respond to environmental changes in a controlled manner. This activatable dynamic property makes dynamic covalent assemblies particularly attractive for the design of complex matter, smart chemical systems, out-of-equilibrium systems, and molecular devices.

Dual reactivity based dynamic covalent chemistry: mechanisms and applications

Dynamic covalent chemistry (DCC) focuses on the reversible formation, breakage, and exchange of covalent bonds and assemblies, setting a bridge between irreversible organic synthesis and supramolecular chemistry and finding wide utility. In order to enhance structural and functional diversity and complexity, different types of dynamic covalent reactions (DCRs) are placed in one vessel, encompassing orthogonal DCC without crosstalk and communicating DCC with a shared reactive functional group. As a means of adding tautomers, widespread in chemistry, to interconnected DCRs and combining the features of orthogonal and communicating DCRs, a concept of dual reactivity based DCC and underlying structural and mechanistic insights are summarized. The manipulation of the distinct reactivity of structurally diverse ring-chain tautomers allows selective activation and switching of reaction pathways and corresponding DCRs (C-N, C-O, and C-S) and assemblies. The coupling with photoswitches further enables light-mediated formation and scission of multiple types of reversible covalent bonds. To showcase the capability of dual reactivity based DCC, the versatile applications in dynamic polymers and luminescent materials are presented, paving the way for future functionalization studies.

Dynamic Cages-Towards Nanostructured Smart Materials

The interest in capsular assemblies such as dynamic organic and coordination cages has blossomed over the last decade. Given their chemical and structural variability, these systems have found applications in diverse fields of research, including energy conversion and storage, catalysis, separation, molecular recognition, and live-cell imaging. In the exploration of the potential of these discrete architectures, they are increasingly being employed in the formation of more complex systems and smart materials. This Review highlights the most promising pathways to overcome common drawbacks of cage systems (stability, recovery) and discusses the most promising strategies for their hybridization with systems featuring various dimensionalities. Following the description of the most recent advances in the fabrication of zero to three-dimensional cage-based systems, this Review will provide the reader with the structure-dependent relationship between the employed cages and the properties of the materials.

Construction of Covalent Organic Cages with Aggregation-Induced Emission Characteristics from Metallacages for Mimicking Light-Harvesting Antenna

A series of covalent organic cages built from fluorophores capable of aggregation-induced emission (AIE) were elegantly prepared through the reduction of preorganized M2 (LA )3 (LB )2 -type metallacages, simultaneously taking advantage of the synthetic accessibility and well-defined shapes and sizes of metallacages, the good chemical stability of the covalent cages as well as the bright emission of AIE fluorophores. Moreover, the covalent cages could be further post-synthetically modified into an amide-functionalized cage with a higher quantum yield. Furthermore, these presented covalent cages proved to be good energy donors and were used to construct light-harvesting systems employing Nile Red as an energy acceptor. These light-harvesting systems displayed efficient energy transfer and relatively high antenna effect, which enabled their use as efficient photocatalysts for a dehalogenation reaction. This research provides a new avenue for the development of luminescent covalent cages for light-harvesting and photocatalysis.

A Fluorescent Cage for Supramolecular Sensing of 3‐Nitrotyrosine in Human Blood Serum

3‐Nitrotyrosine (NT) is generated by the action of peroxynitrite and other reactive nitrogen species (RNS), and as a consequence it is accumulated in inflammation‐associated conditions. This is particularly relevant in kidney disease, where NT concentration in blood is considerably high. Therefore, NT is a crucial biomarker of renal damage, although it has been underestimated in clinical diagnosis due to the lack of an appropriate sensing method. Herein we report the first fluorescent supramolecular sensor for such a relevant compound: Fluorescence by rotational restriction of tetraphenylethenes (TPE) in a covalent cage is selectively quenched in human blood serum by 3‐nitrotyrosine (NT) that binds to the cage with high affinity, allowing a limit of detection within the reported physiological concentrations of NT in chronic kidney disease.

An Orthogonal Dynamic Covalent Chemistry Tool for Ring-Opening Polymerization of Cyclic Oligochalcogenides on Detachable Helical Peptide Templates

A model system is introduced as a general tool to elaborate on orthogonal templation of dynamic covalent ring‐opening polymerization (ODC‐TROP). The tool consists of 3₁₀ helical peptides as unprecedented templates and semicarbazones as orthogonal dynamic covalent linkers. With difficult‐to‐control 1,2‐dithiolanes, ODC‐TROP on the level of short model oligomers occurs with high templation efficiency, increasing and diminishing upon helix stabilization and denaturation, respectively. Further, an anti‐templated conjugate with mispositioned monomers gave reduced templation upon helix twisting. Even with the “unpolymerizable” 1,2‐diselenolanes, initial studies already afford mild templation efficiency. These proof‐of‐principle results promise that the here introduced tool, recyclable and enabling late‐stage side chain modification, will be useful to realize ODC‐TROP of intractable or unknown cyclic dynamic covalent monomers for dynamer materials as well as cellular uptake and signaling applications.

A fluorescent, chirality-responsive, and water-soluble cage as a multifunctional molecular container for drug delivery

In recent years, the rational design and construction of drug delivery systems (DDSs) via a supramolecular approach for improving chemical therapeutics have gained significant attention. Here, we report a host-guest DDS formed from a fluorescent, chirality-responsive, and water-soluble tetraphenylethene-based octacationic cage as a fluorescent/chiral probe, solubilizer, and molecular cargo, which can recognize chiral nucleoside drugs, enhance the solubility of insoluble drugs, and protect drugs from the outside environment by forming host-guest complexes in aqueous solution. Given the fluorescence properties and dynamically rotational conformation of tetraphenylethene (TPE) units, this fluorescent and chirality-responsive cage exhibits different responses including turn-on/turn-off fluorescence and negative/positive circular dichroism (CD) when binding with different chiral nucleoside drugs in water, resulting in multiple-responsive photophysical behaviors for these chiral drugs. Furthermore, this water-soluble cationic cage with a hydrophobic cavity can improve the water solubility of insoluble drugs (e.g., CPT) by forming host-guest complexes in water. More importantly, this multifunctional cage exhibits a low toxicity to both human colon and breast cancer cell lines in vitro, and drugs encapsulated by the cage are more effective in killing cancer cells than drugs alone. Finally, the on-off-on fluorescence responses in the formation and dissociation processes of the cage⊃drug complexes have been successfully used to monitor drug release and track drug delivery by fluorescence microscopy in vitro. Therefore, this fluorescent, chirality-responsive, and water-soluble cage as a multifunctional molecular container can be used to construct a smart drug delivery system with several functions of fluorescence and CD detection, water solubilization, real-time monitoring, and chemotherapy.

Expanding structural diversity in a library of disulfide macrocycles through in-situ imide hydrolysis

We describe here an unorthodox approach to dynamic covalent chemistry in which the initially-unexpected in-situ hydrolysis of a bis-imide is employed to control the composition of a library of structurally diverse macrocycles. A single building block is used to generate a library of numerous disulfide-based architectures in a one-pot single-step process. The dual-stimuli method is based on simultaneous changes in pH and DMSO concentration to expand the structural diversity of the macrocyclic products. Mechanistic details of this complex process are investigated by the kinetics analysis. We delivered a facile strategy for the synthesis of water-soluble, multicomponent and dynamic macrocycles equipped with number of different functional groups, thus giving a prospect of their application in guest-driven phase transfer.

New Cu(i) square grid-type and Ni(ii) triangle-type complexes: synthesis and characterization of effective binders of DNA and serum albumins

Metallosupramolecular square grid-type complex [Cu4L4]4+ and triangle-type complex [Ni3L3]6+ as a potential strategy for obtaining versatile metal-based DNA, Serum Albumin (SA) and DNA binders.

Hierarchical Self-Assembly and Multidynamic Responsiveness of Fluorescent Dynamic Covalent Networks Forming Organogels

Smart stimuli-responsive fluorescent materials are of interest in the context of sensing and imaging applications. In this project, we elaborated multidynamic fluorescent materials made of a tetraphenylethene fluorophore displaying aggregation-induced emission and short cysteine-rich C-hydrazide peptides. Specifically, we show that a hierarchical dynamic covalent self-assembly process, combining disulfide and acyl-hydrazone bond formation operating simultaneously in a one-pot reaction, yields cage compounds at low concentration (2 mM), while soluble fluorescent dynamic covalent networks and even chemically cross-linked fluorescent organogels are formed at higher concentrations. The number of cysteine residues in the peptide sequence impacts directly the mechanical properties of the resulting organogels, Young's moduli varying 2500-fold across the series. These materials underpinned by a nanofibrillar network display multidynamic responsiveness following concentration changes, chemical triggers, as well as light irradiation, all of which enable their controlled degradation with concomitant changes in spectroscopic outputs─self-assembly enhances fluorescence emission by ca. 100-fold and disassembly quenches fluorescence emission.

pH-Sensitive Biomaterials for Drug Delivery

The development of precise and personalized medicine requires novel formulation strategies to deliver the therapeutic payloads to the pathological tissues, producing enhanced therapeutic outcome and reduced side effects. As many diseased tissues are feathered with acidic characteristics microenvironment, pH-sensitive biomaterials for drug delivery present great promise for the purpose, which could protect the therapeutic payloads from metabolism and degradation during in vivo circulation and exhibit responsive release of the therapeutics triggered by the acidic pathological tissues, especially for cancer treatment. In the past decades, many methodologies, such as acidic cleavage linkage, have been applied for fabrication of pH-responsive materials for both in vitro and in vivo applications. In this review, we will summarize some pH-sensitive drug delivery system for medical application, mainly focusing on the pH-sensitive linkage bonds and pH-sensitive biomaterials.

Dynamic Covalent Chemistry as a Facile Route to Unusual Main-Group Thiolate Assemblies and Disulfide Hoops and Cages

Dynamic Covalent Chemistry (DCC) - combining the robustness of covalent bonds with the self-correcting nature of supramolecular chemistry - facilitates the modular synthesis of complex molecular assemblies in high yields. Although numerous reactions form covalent bonds, only a small set of chemical transformations affect covalent bond formation reversibly under suitable conditions for DCC. Further progress in this area still requires the identification of dynamic motifs and greater insights into their reversibility. We have fruitfully employed DCC of both thiolate coordination to main-group elements and disulfide formation for the facile self-assembly of: (1) metal/metalloid-thiolate assemblies, and (2) purely organic cyclic and caged disulfides, thioethers, and even hydrocarbons, many of which have remained elusive by traditional stepwise synthesis yet form readily through our methods. In this Minireview, we highlight the approaches to prepare these unusual compounds and the factors inducing structural transformations or favoring the formation of certain products over others, given a set of external stimuli or reaction conditions.

Strategy for the Construction of Diverse Poly-NHC-Derived Assemblies and Their Photoinduced Transformations

A series of supramolecular assemblies of types [Ag₈ (L)₄ ](PF₆ )₈ and [Ag₄ (L)₂ ](PF₆ )₄ , obtained from the tetraphenylethylene (TPE) bridged tetrakis(1,2,4-triazolium) salts H₄ -L(PF₆ )₄ and Ag^I ions, is described. The assembly type obtained dependends on the N-wingtip substituents of H₄ -L(PF₆ )₄ . Changes in the lengths of the N4-wingtip substituents enables controlled formation of assemblies with either [Ag₄ (L)₂ ](PF₆ )₄ or [Ag₈ (L)₄ ](PF₆ )₈ stoichiometry. The molecular structures of selected [Ag₈ (L)₄ ](PF₆ )₈ and [Ag₄ (L)₂ ](PF₆ )₄ assemblies were determined by X-ray diffraction analyses. While H₄ -L(PF₆ )₄ does not exhibit fluorescence in solution, their tetra-NHC (NHC=N-heterocyclic carbene) assemblies do upon NHC-metal coordination. Upon irradiation, all assemblies undergo a light-induced, supramolecule-to-supramolecule structural transformation by an oxidative photocyclization involving phenyl groups of the TPE core, resulting in a significant change of the luminescence properties.

Dynamic Covalent Switches and Communicating Networks for Tunable Multicolor Luminescent Systems and Vapor-Responsive Materials

Molecular switches are an intensive area of research, and in particular, the control of multistate switching is challenging. Herein we introduce a general and versatile strategy of dynamic covalent switches and communicating networks, wherein distinct states of reversible covalent systems can induce addressable fluorescence switching. The regulation of intramolecular ring/chain equilibrium, intermolecular dynamic covalent reactions (DCRs) with amines, and both permitted the activation of optical switches. The variation in electron-withdrawing competition between the fluorophore and 2-formylbenzenesulfonyl unit afforded diverse signaling patterns. The combination of switches in situ further enabled the creation of communicating networks for multistate color switching, including white emission, through the delicate control of DCRs in complex mixtures. Finally, reversible and recyclable multiresponsive luminescent materials were achieved with molecular networks on the solid support, allowing visualization of different types of vapors and quantification of primary amine vapors with high sensitivity and wide detection range. The results reported herein should be appealing for future studies of dynamic assemblies, molecular sensing, intelligent materials, and biological labeling.

Tuning the Solubility of Self-Assembled Fluorescent Aromatic Cages Using Functionalized Amino Acid Building Blocks

We previously reported novel fluorescent aromatic cages that are self-produced using a set of orthogonal dynamic covalent reactions, operating simultaneously in one-pot, to assemble up to 10 components through 12 reactions into a single cage-type structure. We now introduce N-functionalized amino acids as new building blocks that enable tuning the solubility and analysis of the resulting cages. A convenient divergent synthetic approach was developed to tether different side chains on the N-terminal of a cysteine-derived building block. Our studies show that this chemical functionalization does not prevent the subsequent self-assembly and effective formation of desired cages. While the originally described cages required 94% DMSO, the new ones bearing hydrophobic side chains were found soluble in organic solvents (up to 75% CHCl3), and those grafted with hydrophilic side chains were soluble in water (up to 75% H2O). Fluorescence studies confirmed that despite cage functionalization the aggregation-induced emission properties of those architectures are retained. Thus, this work significantly expands the range of solvents in which these self-assembled cage compounds can be generated, which in turn should enable new applications, possibly as fluorescent sensors.

Mutualistic benefit in the self-sorted co-aggregates of peri-naphthoindigo and a 4-amino-1,8-naphthalimide derivative

Photoluminescence enhancement for all the members of a self-sorted co-aggregate was observed for the first time by successfully amalgamating AIEE and social self-sorting. Intermolecular H-bonding and π-π stacking were utilised to prepare several co-aggregates of peri-naphthoindigo (PNI) and a 4-amino-1,8-naphthalimide derivative dye, NH₂-NMI. In the heteromeric aggregates, photoluminescence intensities were increased by 28% for the imide and more than 400% for PNI. Due to spectral overlap between the emission of the imide and the absorption of PNI, energy transfer took place from the former to the latter. The heteromeric aggregates are dual emissive and the relative intensities of the emissions can easily be tuned by varying the stoichiometry of the dyes.

Chirality transfer between hexaazamacrocycles in heterodinuclear rare earth complexes

Both the chiral hexaazamacrocyle L1 based on trans-1,2-diaminocyclohexane and the achiral hexaazamacrocyle L2 based on ethylenediamine form lanthanide(iii) dinuclear μ-hydroxo bridged complexes which have been characterized by NMR and CD spectroscopy. The homodinuclear complexes of the type [Ln₂(L1)₂(μ-OH)₂](NO₃)₄ (Ln = Nd^III, Eu^III, Tb^III and Yb^III) have been synthesized in the enantiopure form and the X-ray crystal structures of Nd^III, Eu^III and Yb^III derivatives have been determined. The heterodinuclear cationic complexes [Ln(L1)Ln'(L2)(μ-OH)₂X₂]ⁿ⁺ have been generated and characterized in solution by using the mononuclear complexes of L1 and L2 as substrates. While the formation of [LnLn'(L1)₂(μ-OH)₂X₂]ⁿ⁺ dinuclear complexes is accompanied by chiral narcissistic self-sorting, the formation of [Ln(L1)Ln'(L2)(μ-OH)₂X₂]ⁿ⁺ dinuclear complexes is accompanied by the sizable sociable self-sorting of macrocyclic units. The homodinuclear complexes [Y₂(L1)₂(μ-OH)₂X₂]ⁿ⁺ and [Ln₂(L2)₂(μ-OH)₂X₂]ⁿ⁺ (Ln = Dy^III, Pr^III and Nd^III) are CD silent in the visible region due to the lack of f-f transitions and the presence of an achiral ligand, respectively. In contrast, the heterodinuclear [Y(L1^S)Ln(L2)(μ-OH)₂X₂]ⁿ⁺ complexes give rise to CD signals arising from the f-f transitions because of the chirality transfer from the L1 macrocyclic unit to the L2 macrocyclic unit.

A tetrahedral molecular cage with a responsive vertex

Dynamic covalent chemistry (DCC) is a widely used method for the self-assembly of three-dimensional molecular architectures. The orthogonality of dynamic reactions is emerging as a versatile strategy for controlling product distributions in DCC, yet the application of this approach to the synthesis of 3D organic molecular cages is limited. We report the first system which employs the orthogonality of alkyne metathesis and dynamic imine exchange to prepare a molecular cage with a reversibly removable vertex. This study demonstrates the rational and controlled application of chemical orthogonality in DCC to prepare organic cages of expanded functionality which respond to chemical stimuli.

Growing Prospects of Dynamic Covalent Chemistry in Delivery Applications

Delivery remains a major obstacle restricting the potential action of small molecular drugs as well as novel biologics which cannot readily enter cells without the help of a vector. A successful active delivery process involves three steps: (a) tagging the drug with a vector, (b) effective trafficking of this [drug-vector] conjugate through biological barriers, and finally (c) controlled drug release. While covalent bond formation and/or supramolecular association is involved in the making of the [drug-vector] conjugate, the final step requires precisely a controlled dissociation in order to trigger drug release. Therefore, in pursuit of smart, effective, and nontoxic delivery systems, it has become widely recognized that control over dynamic self-assembly could unleash the efficacy of artificial vectors. In this Account, I discuss our endeavors, and those of colleagues, in the recent implementation of Dynamic Covalent Chemistry (DCvC) in delivery applications. DCvC exploits reversible covalent reactions to generate covalent systems that can self-fabricate, adapt, respond, and fall apart in a controlled fashion. A privileged set of reversible covalent reactions has emerged in the community working on delivery applications and is based on condensation reactions (imine, acylhydrazone, oxime), and disulfide and boronate ester formations. The latest developments making this chemistry particularly attractive for such a DCvC approach are discussed. The rational justifying the potential of DCvC in delivery is based on the principle that using such reversible covalent reactions afford transient [drug-vector] conjugates which form spontaneously and chemoselectively, then adapt and self-correct their structure during self-assembly and trafficking thanks to the dynamic nature of the reversible covalent bonds, and finally respond to physicochemical stimuli such as pH and redox changes, thereby enabling controlled dissociation and concomitant drug release. For these reasons, DCvC has recently emerged as a leverage tool with growing prospects for advancing toward smarter delivery systems. The implementation of DCvC can follow three approaches that are discussed herein: (1) dynamic covalent bioconjugates, involving the transient covalent conjugation with a vector, (2) dynamic covalent vectors, involving the controlled dynamic and adaptive assembly and disassembly of vectors that complex drugs through supramolecular association, and (3) dynamic covalent targeting, involving the transient chemoselective formation of covalent bonds with the constituents of cell membranes. While DCvC has already attracted interest in material sciences, the recent results described in this Account showcase the vast potential of DCvC in biological sciences, and in particular in delivery applications where self-fabricated, adaptive, and responsive devices are of utmost importance.

Self-templated synthesis of an orthoformate in,in-cryptand and its bridgehead inversion by dynamic covalent exchange

We report the template-free dynamic covalent self-assembly of a small orthoformate cryptand, which appears to be driven by the formation of two sets of intramolecular, four-centre hydrogen bonds.

Enhancing the kinetics of hydrazone exchange processes: an experimental and computational study

Hydrogen-bond acceptors (A) stabilize the transition state, lowering the energy barrier to rapid hydrazone exchange, without need for exogenous catalyst.

Dynamic polyimine macrobicyclic cryptands - self-sorting with component selection

Self-assembling macrobicyclic cryptand-type organic cages display remarkable self-sorting behavior with efficient component selection.

Self-assembling macrobicyclic cryptand-type organic cages display remarkable self-sorting behavior with efficient component selection. Making use of the dynamic covalent chemistry approach, eight different cages were synthesized by condensation of tris(2-aminopropyl)amine with structurally different dialdehydes. A series of self-sorting experiments were first carried out on simple dynamic covalent libraries. They reveal the influence of different structural features of the aldehyde components on the condensation with two triamine capping units. Subsequently, self-sorting experiments were performed on more complex systems involving several dialdehyde building blocks. Altogether, the results obtained describe the effect of the presence of a heteroatom, of electrostatic interactions, of delocalization and of the flexibility/stiffness of the propensity of a component to undergo formation of a macrobicyclic cage. In the presence of a catalytic amount of acid, the macrobicyclic structure undergoes dynamic component exchange.

Macrocycles and cages based on tetraphenylethylene with aggregation-induced emission effect

Syntheses, photophysical properties and applications of macrocycles and cages based on tetraphenylethylene with aggregation-induced emission (AIE) effect.