Light‐Triggered High‐Affinity Binding of Tetramethylammonium over Potassium Ions by [18]crown‐6 in a Tetrahedral Anion Cage

Photoswitching of Co(ii)-based coordination cages containing azobenzene backbones

Inclusion of photoswitchable azobenzene units as spacers into ditopic bridging ligands Lm and Lp, containing two chelating pyrazolyl-pyridine termini, allows formation of metal complex assemblies with Co(ii) that undergo a range of light-induced structural transformations. One notable result is the light-induced conversion of a Co2(Lp)3 dinuclear triple helicate (based on the E ligand isomer) to a C 3-symmetric Co4(Lp)6 assembly, assumed to be an edge-bridged tetrahedral cage, based on the Z ligand isomer. Another is the preparation of a series of Co4(Lm)6 complexes, of which Co4(E-Lm)6 was crystallographically characterised and consists of a pair of Co2(Lm)2 double helicates connected by an additional two bridging ligands which span the pair of helicate units, giving a cyclic Co4 array in which one and then two bridging ligands alternate around the periphery. A set of Co4(Lm)6 complexes could be prepared containing different ratios of Z : E ligand isomers (0 : 6, 2 : 4, 4 : 2 and 6 : 0) of which Co4(Z-Lm)2(E-Lm)4 was particularly stable and dominated the speciation behaviour, either during light-induced switching of the ligand geometry in pre-formed complexes, or when ligand isomers were combined in different proportions during the preparation. These examples of (i) interconversion between Co2L3 (helicate) and (ii) Co4L6 (cage) assemblies with Lp, and the interconversion between a series of Co4L6 assemblies Co4(Z-Lm)n(E-Lm)6-n with Lm, constitute significant advances in the field of photoswitchable supramolecular assemblies.

Biomimetic Charge-Neutral Anion Receptors for Reversible Binding and Release of Highly Hydrated Phosphate in Water

Control of phosphate capture and release is vital in environmental, biological, and pharmaceutical contexts. However, the binding of trivalent phosphate (PO4 3-) in water is exceptionally difficult due to its high hydration energy. Based on the anion coordination chemistry of phosphate, in this study, four charge-neutral tripodal hexaurea receptors (L1-L4), which were equipped with morpholine and polyethylene glycol terminal groups to enhance their solubility in water, were synthesized to enable the pH-triggered phosphate binding and release in aqueous solutions. Encouragingly, the receptors were found to bind PO4 3- anion in a 1 : 1 ratio via hydrogen bonds in 100 % water solutions, with L1 exhibiting the highest binding constant (1.2×103 M-1). These represent the first neutral anion ligands to bind phosphate in 100 % water and demonstrate the potential for phosphate capture and release in water through pH-triggered mechanisms, mimicking native phosphate binding proteins. Furthermore, L1 can also bind multiple bioavailable phosphate species, which may serve as model systems for probing and modulating phosphate homeostasis in biological and biomedical researches.

Self-assembly of conformation-adaptive dihydrophenazine-based coordination cages

In this study, we designed and synthesized three conformation-adaptive Pd2L4- and Pd3L6-type coordination cages based on three dihydrophenazine-based ligands with different lengths. Interestingly, the shorter ligands L1 and L2 self-assembled into Pd2L4-type coordination cages while the longer ligand L3 formed Pd3L6-type one, mainly driven by the anion template effect. All coordination cages were confirmed through single-crystal X-ray diffraction, and their structural conformations underwent great changes compared with those of their corresponding ligands. Moreover, the conformational changes also significantly affected their photophysical and electrochemical properties which were distinct from their parent ligands.

Strain-Induced Heteromorphosis Multi-Cavity Cages: Tension-Driven Self-Expansion Strategy for Controllable Enhancement of Complexity in Supramolecular Assembly

Coordinative supramolecular cages with adjustable cavities have found extensive applications in various fields, but the cavity modification strategies for multi-functional structures are still challenging. Here, we present a tension-driven self-expansion strategy for construction of multi-cavity cages with high structural complexity. Under the regulation of strain-induced capping ligands, unprecedented heteromorphosis triple-cavity cages S2 /S4 were obtained based on a metallo-organic ligand (MOL) scaffold. The heteromorphosis cages exhibited significant higher cavity diversity than the homomorphous double-cavity cages S1 /S3 ; all of the cages were thoroughly characterized through various analytical techniques including (1D and 2D) NMR, ESI-MS, TWIM-MS, AFM, and SAXS analyses. Furthermore, the encapsulation of porphyrin in the cavities of these multi-cavity cages were investigated. This research opens up new possibilities for the architecture of heteromorphosis supramolecular cages via precisely controlled "scaffold-capping" assembly with preorganized ligands, which could have potential applications in the development of multifunctional structures with higher complexity.

In Situ Photoisomerization of an Azobenzene-Based Triple Helicate with a Prolonged Thermal Relaxation Time

The phosphate-coordination triple helicates A2 L3 (A=anion) with azobenzene-spaced bis-bis(urea) ligands (L) have proven to undergo a rare in situ photoisomerization (without disassembly of the structure) rather than the typically known, stepwise "disassembly-isomerization-reassembly" process. This is enabled by the structural self-adaptability of the "aniono" assembly arising from multiple relatively weak and flexible hydrogen bonds between the phosphate anion and bis(urea) units. Notably, the Z→E thermal relaxation rate of the isomerized azobenzene unit is significantly decreased (up to 20-fold) for the triple helicates compared to the free ligands. Moreover, the binding of chiral guest cations inside the cavity of the Z-isomerized triple helicate can induce optically pure diastereomers, thus demonstrating a new strategy for making light-activated chiroptical materials.

Redox Triggers Guest Release and Uptake Across a Series of Azopyridine-Based Metal-Organic Capsules

Precise control over guest release and recapture using external stimuli is a valuable goal, potentially enabling new modes of chemical purification. Including redox moieties within the ligand cores of molecular capsules to trigger the release and uptake of guests has proved effective, but this technique is limited to certain capsules and guests. Herein, the construction of a series of novel metal-organic capsules from ditopic, tritopic, and tetratopic ligands is demonstrated, all of which contain redox-active azo groups coordinated to FeII centers. Compared to their iminopyridine-based analogs, this new class of azopyridine-based capsules possesses larger cavities, capable of encapsulating more voluminous guests. Upon reduction of the capsules, their guests are released and may then be re-encapsulated when the capsules are regenerated by oxidation. Since the redox centers are on the ligand arms, they are modular and can be attached to a variety of ligand cores to afford varying and predictable architectures. This method thus shows promise as a generalized approach for designing redox-controlled guest release and uptake systems.

Russian-Doll-Like Porous Carbon as Anode Materials for High-Performance Potassium-Ion Hybrid Capacitors

Pore-structure design with the sophisticated and pragmatic nanostructures still remains a great challenge. In this work, porous carbon with Russian-doll-like pores rather than traditional single modal is fabricated via a boiling carbonization approach, accompanied by K⁺ -pre-intercalation. The most important internal factor is that alkali can penetrate into the stereoscopic space of layered Malonic acid dihydrazide and the confinement effect leads to the in-depth development of different dimensional pore structures. The oxygenated and nitrogenated surface guarantees the K⁺ intercalation behavior. Benefiting from their open framework and enlarged interlayer spacing, K⁺ -pre-intercalated porous carbon with Russian-doll-like pores (denoted as KPCRPs) as anode material exhibits promising potassium storage performance. The assembled KPCRP//activated carbon potassium-ion hybrid supercapacitor in 30 m CH₃ COOK displays a high energy density of 157.29 Wh kg^-1 , an ultrahigh power output of 14 kW kg^-1 , and a long cycling life (99.58% capacity retention after 10000 cycles), highlighting the superiority of Russian-doll-like pore structure. This work sheds light on the designing of 3D pores structure, especially for multimodal pore architectures.

Solvato-Controlled Assembly and Structural Transformation of Emissive Poly-NHC-Based Organometallic Cages and Their Applications in Amino Acid Sensing and Fluorescence Imaging

Stimuli-induced structural transformation of supramolecular cages has drawn increasing attention because of their sensitive feature to external variations as model systems to simulate biological processes. However, combining structural transformation and useful functions has remained a difficult task. This study reports the solvato-controlled self-assembly of two unique topologies with different emission characteristics, a water-soluble Ag₈ L₄ cage (A) and an Ag₄ L₂ cage (B), produced from the same sulfonate-pendant tetraphenylethene (TPE) bridged tetrakis-(1,2,4-triazolium) ligand. Both cages show interesting solvent-responsive reversible structural transformation, and the change of fluorescence signals can efficiently track the process. Additionally, water-soluble cage A exhibits unique properties in thermochromism, thiol amino acid sensing, and subcellular imaging in aqueous media.

Metallo-Supramolecular Hexagonal Wreath with Four Switchable States Based on a pH-Responsive Tridentate Ligand

In biological systems, many biomacromolecules (e.g., heme proteins) are capable of switching their states reversibly in response to external stimuli, endowing these natural architectures with a high level of diversity and functionality. Although tremendous efforts have been made to advance the complexity of artificial supramolecules, it remains a challenge to construct metallo-supramolecular systems that can carry out reversible interconversion among multiple states. Here, a pH-responsive tridentate ligand, 2,6-di(1H-imidazole-2-yl)pyridine (H₂DAP), is incorporated into the multitopic building block for precise construction of giant metallo-supramolecular hexagonal wreaths with three metal ions, i.e., Fe(II), Co(II), and Ni(II), through coordination-driven self-assembly. In particular, a Co-linked wreath enables in situ reversible interconversion among four states in response to pH and oxidant/reductant with highly efficient conversion without losing structural integrity. During the state interconversion cycles, the physical properties of the assembled constructs are finely tuned, including the charge states of the backbone, valency of metal ions, and paramagnetic/diamagnetic features of complexes. Such discrete wreath structures with a charge-switchable backbone further facilitate layer-by-layer assembly of metallo-supramolecules on the substrate.

Anion-Coordination-Driven Assembly

The assembly of discrete architectures has been an important subject in supramolecular chemistry because of their elegant structures and fascinating properties. During the last several decades, supramolecular chemists have developed manifold strategies for hierarchical assembly, which are normally classified by two main types of driving force: covalent and noncovalent interactions. Typical noncovalent interactions include metal coordination, hydrogen bonding, and other weak forces. These approaches have achieved great progress in the construction of various supramolecular structures, such as macrocycles, cages, polyhedra, and interlocked systems. Among these methods, metal-coordination-driven assembly is attractive due to the well-defined coordination properties of metal ions. Indeed, in terms of supramolecular chemistry, the concept of "coordination" has been expanded beyond transition metals. In particular, anion coordination chemistry, which was first proposed by Lehn in 1978 [ Acc. Chem. Res. 1978, 11, 49] and then elucidated in detail by Bowman-James two decades later [ Acc. Chem. Res. 2005, 38, 671], has grown up to a subfield of supramolecular chemistry. It is noticeable that anions also show "dual valencies" like transition metals, wherein the "primary valence" is the charge balance for anions by countercations while the "secondary valence", i.e., the coordination, refers to hydrogen bonding interactions where the electron flow is from the electron-rich anion (the coordination center) to hydrogen bonding donors (the ligands). Thus, anions also display certain coordination numbers and specific coordination geometries. Although such features are far less regular than those of transition metals, they are sufficient to allow anion coordination to serve as the driving force for assembling discrete supramolecular architectures. In this Account, the anion-coordination-driven assembly (ACDA), a new assembling strategy established by us during the past decade, will be presented. We summarize our work in the construction of a series of "aniono" supramolecular structures, especially triple helicates and tetrahedral cages, based on the coordination between oligourea ligands and anions (mostly phosphate). In particular, we will detail the considerations in the design of ligands, the assembling process including structural transformation, and functionalization of the systems toward guest inclusion, supramolecular catalysis, photoswitches, and molecular devices. These results demonstrate the great potential of ACDA in fabricating novel anion-based systems. Although the design concept was originally loaned from traditional coordination chemistry of transition metals, and structures of anion complexes bear some resemblance to metal complexes, there are significant differences of the aniono supramolecular assemblies from the metallo analogues. For example, these metal-free systems are held together by multiple hydrogen bonds (dozens to nearly 100), thus facilitating assembly/disassembly under mild conditions and relatively flexible structures for adaptive guest inclusion. To this end, intriguing applications (supramolecular chirality, catalysis, energy storage, etc.) may be expected for aniono systems. We hope the current Account will attract more attention from researchers in supramolecular assembly and inspire more efforts in this fascinating area.

Anion-Coordination-Driven Assembly: From Discrete Supramolecular Self-Assemblies to Functional Soft Materials

Anion templated assembly of supramolecular systems has been extensively explored in previous reports, whereas anions serve only as an auxiliary and spectator role. With the development of anion coordination chemistry in recent years, anion coordination-driven assembly (ACDA) has emerged as a new strategy for the construction of supramolecular self-assemblies. Anions are proved to exist as the main actors in the construction of supramolecular architectures, i. e., serve as the coordination center. This Review will focus on the recent progress in anion-coordination-driven assembly of discrete supramolecular architectures, such as helicates, polyhedrons and polygons, and the various applications of 'aniono'-systems. At the end of this Review, we highlight current challenges and opportunities for future research of anion-coordination-driven self-assembly.