Assembled molecular face-rotating polyhedra to transfer chirality from two to three dimensions

Understanding the Adsorption and Diffusion Behaviors of PBUT in Biocompatible MOFs

With the increasing incidence of chronic kidney disease, the effective control of protein-bound uremic toxins (PBUTs), which are difficult to remove through dialysis, has become a priority. In this study, the adsorption and diffusion behaviors of several metal-organic frameworks (MOFs) for PBUTs (indoxyl sulfate and p-cresyl sulfate) were studied by molecular dynamics (MD) simulations and umbrella sampling. For the NU series of MOFs, good correlations between the Gibbs free energy (ΔG) and the experimental clearance rates of PBUTs are found. For the adsorption behaviors, in terms of ΔG, DAJWET exhibits the best adsorption effect for indoxyl sulfate (IS), whereas NU-1000 shows the best effect for p-cresyl sulfate (pCS). Similar trends observed in the radial distribution function and mean square displacement results suggest that the π-π stacking interactions play a crucial role in the adsorption and diffusion of PBUTs by MOFs. Furthermore, it can be concluded that MOFs with highly conjugated groups (porphyrin rings and pyrene groups) tend to generate more PBUT attraction, and provide design principles for potential MOF candidates in the removal of PBUTs.

Spheroid models to elaborate the broken symmetry and equivalent volume of molecules in crystalline phase

Dense packing of particles has provided powerful models to elaborate the important structural features of matter in various systems such as liquid, glassy, and crystalline phases. The simplest sphere packing models can represent and capture salient properties of the building blocks for covalent, metallic, and ionic crystals; it, however, becomes insufficient to reflect the broken symmetry of the commonly anisotropic molecules in molecular crystals. Here, we develop spheroid models with a minimal degree of anisotropy, which serve as a simple geometrical representation for a rich spectrum of molecules-including both isotropic and anisotropic, convex and concave ones-in crystalline phases. Our models are determined via an inverse packing approach: Given a molecular crystal, an optimal spheroid model is constructed using a contact diagram, which depicts the packing relationship between neighboring molecules within the crystal. The spheroid models are capable of accurately capturing the broken symmetry and characterizing the equivalent volume of molecules in the crystalline phases. Moreover, our model retrieves such molecular information from low-quality x-ray diffraction data with poorly resolved structures, and by using soft spheroids, it can also describe the packing behavior in cocrystals.

Molecular Face-Rotating Polyhedra: Chiral Cages Inspired by Mathematics

ConspectusMolecular polyhedral cages, notable for their enclosed inner cavities, can possess varying degrees of symmetry, spanning from regular Platonic polyhedra to lower symmetry forms that may display chirality. Crafting chiral molecular cages typically involves using building blocks containing stereogenic elements or arranging achiral components in a manner that lacks mirror and inversion symmetries. Achieving precise control over their chirality poses both significance and challenges.In this Account, we present an overview of our research endeavors in the realm of chiral molecular polyhedral cages, drawing inspiration from Buckminster Fuller's "Face-Rotating Polyhedra (FRP)". Mathematically, FRP introduce a unique form of chirality distinguished by a rotating pattern around the center of each face, setting it apart from regular polyhedra.Molecular FRP can be constructed using two types of facial building blocks. The first includes rigid, planar molecules such as truxene and triazatruxene, which exhibit either clockwise or counterclockwise rotations in two dimensions. The second category involves propeller-like molecules, e.g., tetraphenylethylene, 1,2,3,4,5-penta(4-phenylaldehyde)pyrrole, and tridurylborane, displaying dynamic stereochemistry.The synthesis of FRP may potentially yield a diverse array of stereoisomers. Achieving high stereoselectivity becomes feasible through the selection of building blocks with specific substitution patterns and rigidity. Prominent noncovalent repulsive forces within the resulting cages often play a pivotal role in the dynamic covalent assembly process, ultimately leading to the formation of thermodynamically stable FRP products.The capacity to generate a multitude of stereoisomers, combined with the integration of chiral vertices, has facilitated investigations into phenomena such as chiral self-sorting and the "sergeant and soldiers" chiral amplification effect in FRP. Even the inclusion of one chiral vertex significantly impacts the stereochemical configuration of the entire cage. While many facial building blocks establish a stable rotational pattern in FRP, other units, such as tridurylborane, can dynamically transition between P and M configurations within the cage structures. The kinetic characteristics of such stereolabile FRP can be elucidated through physicochemical investigations.Our research extends beyond the FRP concept to encompass mathematical analysis of these structures. Graph theory, particularly the coloring problem, sheds light on the intricate facial patterns exhibited by various FRP stereoisomers and serves as an efficient tool to facilitate the discovery of novel FRP structures. This approach offers a fresh paradigm for designing and analyzing chiral molecular polyhedral cages, showcasing in our work the synergy between mathematics and molecular design.

Enantioselective Self-Assembly of a Homochiral Tetrahedral Cage Comprising Only Achiral Precursors

How Nature synthesizes enantiomerically pure substances from achiral or racemic resources remains a mystery. In this study, we aimed to emulate this natural phenomenon by constructing chiral tetrahedral cages through self-assembly, achieved by condensing two achiral compounds-a trisamine and a trisaldehyde. The occurrence of intercomponent CH⋅⋅⋅π interactions among the phenyl building blocks within the cage frameworks results in twisted conformations, imparting planar chirality to the tetrahedrons. In instances where the trisaldehyde precursor features electron-withdrawing ester side chains, we observed that the intermolecular CH⋅⋅⋅π forces are strong enough to prevent racemization. To attain enantioselective self-assembly, a chiral amine was introduced during the imine formation process. The addition of three equivalents of chiral amino mediator to one equivalent of the achiral trisaldehyde precursor formed a trisimino intermediate. This chiral compound was subsequently combined with the achiral trisamino precursor, leading to an imine exchange reaction that releasing the chiral amino mediator and formation of the tetrahedral cage with an enantiomeric excess (ee) of up to 75 %, exclusively composed of achiral building blocks. This experimental observation aligns with theoretical calculations based on the free energies of related cage structures. Moreover, since the chiral amine was not consumed during the imine exchange cycle, it enabled the enantioselective self-assembly of the tetrahedral cage for multiple cycles when new batches of the achiral trisaldehyde and trisamino precursors were successively added.

Chiral Truxene-Based Self-Assembled Cages: Triple Interlocking and Supramolecular Chirogenesis

Incorporating chiral elements in host-guest systems currently attracts much attention because of the major impact such structures may have in a wide range of applications, from pharmaceuticals to materials science and beyond. Moreover, the development of multi-responsive and -functional systems is highly desirable since they offer numerous benefits. In this context, we describe herein the construction of a metal-driven self-assembled cage that associates a chiral truxene-based ligand and a bis-ruthenium complex. The maximum separation between both facing chiral units in the assembly is fixed by the intermetallic distance within the lateral bis-ruthenium complex (8.4 Å). The resulting chiral cavity was shown to encapsulate polyaromatic guest molecules, but also to afford a chiral triply interlocked [2]catenane structure. The formation of the latter occurs at high concentration, while its disassembly could be achieved by the addition of a planar achiral molecule. Interestingly the planar achiral molecule exhibits induced circular dichroism signature when trapped within the chiral cavity, thus demonstrating the ability of the cage to induce supramolecular chirogenesis.

Self-similar chiral organic molecular cages

The endeavor to enhance utility of organic molecular cages involves the evolution of them into higher-level chiral superstructures with self-similar, presenting a meaningful yet challenging. In this work, 2D tri-bladed propeller-shaped triphenylbenzene serves as building blocks to synthesize a racemic 3D tri-bladed propeller-shaped helical molecular cage. This cage, in turn, acts as a building block for a pair of higher-level 3D tri-bladed chiral helical molecular cages, featuring multilayer sandwich structures and displaying elegant characteristics with self-similarity in discrete superstructures at different levels. The evolutionary procession of higher-level cages reveals intramolecular self-shielding effects and exclusive chiral narcissistic self-sorting behaviors. Enantiomers higher-level cages can be interconverted by introducing an excess of corresponding chiral cyclohexanediamine. In the solid state, higher-level cages self-assemble into supramolecular architectures of L-helical or D-helical nanofibers, achieving the scale transformation of chiral characteristics from chiral atoms to microscopic and then to mesoscopic levels.

Tailored Water-Soluble Covalent Organic Cages for Encapsulation of Pyrene and Information Encryption

Forming pyridine salts to construct covalent organic cages is an effective strategy for constructing covalent cage compounds. Covalent organic cages based on pyridine salt structures are prone to form water-soluble supramolecular compounds. Herein, we designed and synthesized a triangular prism-shaped hexagonal cage with a larger cavity and relatively flexible conformation. The supramolecular cage structure was also applied to the encapsulation of pyrene and information encryption.

Guest Encapsulation Alters the Thermodynamic Landscape of a Coordination Host

The architecture of self-assembled host molecules can profoundly affect the properties of the encapsulated guests. For example, a rigid cage with small windows can efficiently protect its contents from the environment; in contrast, tube-shaped, flexible hosts with large openings and an easily accessible cavity are ideally suited for catalysis. Here, we report a "Janus" nature of a Pd6L4 coordination host previously reported to exist exclusively as a tube isomer (T). We show that upon encapsulating various tetrahedrally shaped guests, T can reconfigure into a cage-shaped host (C) in quantitative yield. Extracting the guest affords empty C, which is metastable and spontaneously relaxes to T, and the T⇄C interconversion can be repeated for multiple cycles. Reversible toggling between two vastly different isomers paves the way toward controlling functional properties of coordination hosts "on demand".

Photocontrolled Reversibly Chiral-Ordered Assembly Based on Cucurbituril

Chirality transfer and regulation, accompanied by morphology transformation, arouse widespread interest for application in materials and biological science. Here, a photocontrolled supramolecular chiral switch is fabricated from chiral diphenylalanine (l-Phe-l-Phe, FF) modified with naphthalene (2), achiral dithienylethene (DTE) photoswitch (1), and cucurbit[8]uril (CB[8]). Chirality transfer from the chiral FF moiety of 2 to a charge-transfer (CT) heterodimer consisting of achiral guest 1 and achiral naphthalene (NP) in 2 has been unprecedented achieved via the encapsulation of CB[8]. On the contrary, chirality transfer from chiral FF to NP cannot be conducted in only guest 2. Crucially, induced circular dichroism of the heterodimer can be further modulated by distinct light, attributing to reversible photoisomerization of the DTE. Meanwhile, topological nanostructures are changed from one-dimensional (1D) nanofibers to two-dimensional (2D) nanosheets in the orderly assembling process of the heterodimer, which further achieved reversible interconversion between 2D nanosheets and 1D nanorods with tunable-induced chirality stimulated by diverse light.

Chiral Molecular Cage with Tunable Stereoinversion Barriers

The manipulation of chirality in molecular entities that rapidly interconvert between enantiomeric forms is challenging, particularly at the supramolecular level. Advances in controlling such dynamic stereochemical systems offer opportunities to understand chiral symmetry breaking and homochirality. Herein, we report the synthesis of a face-rotating tetrahedron (FRT), an organic molecular cage composed of tridurylborane facial units that undergo stereomutations between enantiomeric trefoil propeller-like conformations. After resolution, we show that the racemization barrier of the enantiopure FRT can be regulated in situ through the reversible binding of fluoride anions onto the tridurylborane moieties. Furthermore, the addition of an enantiopure phenylethanol to the FRT can effectively induce chirality of the molecular cage by preferentially binding to one of its enantiomeric conformers. This study presents a new paradigm for controlling dynamic chirality in supramolecular systems, which may have implications for asymmetric synthesis and dynamic stereochemistry.

The sharp structural switch of covalent cages mediated by subtle variation of directing groups

It is considered a more formidable task to precisely control the self-assembled products containing purely covalent components, due to a lack of intrinsic templates such as transition metals to suppress entropy loss during self-assembly. Here, we attempt to tackle this challenge by using directing groups. That is, the self-assembly products of condensing a 1:2 mixture of a tetraformyl and a biamine can be precisely controlled by slightly changing the substituent groups in the aldehyde precursor. This is because different directing groups provide hydrogen bonds with different modes to the adjacent imine units, so that the building blocks are endowed with totally different conformations. Each conformation favors the formation of a specific product that is thus produced selectively, including chiral and achiral cages. These results of using a specific directing group to favor a target product pave the way for accomplishing atom economy in synthesizing purely covalent molecules without relying on toxic transition metal templates.

Solvent-Controlled Quadruple Catenation of Giant Chiral [8+12] Salicylimine Cubes Driven by Weak Hydrogen Bonding

Mechanically interlocked structures are fascinating synthetic targets and the topological complexity achieved through catenation offers numerous possibilities for the construction of new molecules with exciting properties. In the structural space of catenated organic cage molecules, only few examples have been realized so far, and control over the catenation process in solution is still barely achieved. Herein, we describe the formation of a quadruply interlocked catenane of giant chiral [8+12] salicylimine cubes. The formation could be controlled by the choice of solvent used in the reaction. The interlocked structure was unambiguously characterized by single crystal X-ray diffraction and weak hydrogen bonding was identified as a central driving force for the catenation. Furthermore, scrambling experiments using partially deuterated cages were performed, revealing that the catenane formation occurs through mechanical interlocking of preformed single cages.

Chiral emissive porous organic cages

A pair of chiral, emissive and porous tubular multi-functional organic molecular cages were synthesized easily by imine chemistry of 4,4',4'',4'''-(ethene-1,1,2,2-tetrayl)-tetrabenzaldehyde (ETTBA) with (R,R)- or (S,S)-diaminocyclohexane (CHDA). It was found that the chirality of CHDA was transferred and amplified to tetraphenylethylene (TPE) in the process of formation of cages, which further endowed the cages with circularly polarized luminescence (CPL) characteristics. As a result of the synergy of the chirality and porous structure in the solid state, both cages exhibited a good chiral adsorption enantioselectivity to a series of aromatic racemates.

Synthesis of a Tetrahedral Metal-Organic Supramolecular Cage with Dendritic Carbazole Arms

In recent years, incredible endeavors have been devoted to the design and self-assembly of discrete metal-organic cages (MOCs) with expanding intricacy and functionality. The controlled synthesis of metal-organic supramolecular cages with large branched chains remains an interesting and challenging work in supramolecular chemistry. Herein, a tetrahedral metal-organic supramolecular cage (ZnII4L4) containing 12 dendritic carbazole arms is unprecedentedly constructed through coordination-driven subcomponent self-assembly and characterized in different ways. Interestingly, tetrahedral supramolecular Cage-1 exhibited the potential for aggregation-induced emission (AIE) performance and stimulus-responsive luminescence features, and it achieved color-tunable photoluminescence due to the introduction of dendritic carbazole arms. Crucially, owing to the great photophysical properties of Cage-1 in solution, Cage-1 was enabled to act as a fluorescent ink for the vapor-responsive recording and wiping of information.

Real-time chirality transfer monitoring from statistically random to discrete homochiral nanotubes

Real time monitoring of chirality transfer processes is necessary to better understand their kinetic properties. Herein, we monitor an ideal chirality transfer process from a statistically random distribution to a diastereomerically pure assembly in real time. The chirality transfer is based on discrete trimeric tubular assemblies of planar chiral pillar[5]arenes, achieving the construction of diastereomerically pure trimers of pillar[5]arenes through synergistic effect of ion pairing between a racemic rim-differentiated pillar[5]arene pentaacid bearing five benzoic acids on one rim and five alkyl chains on the other, and an optically resolved pillar[5]arene decaamine bearing ten amines. When the decaamine is mixed with the pentaacid, the decaamine is sandwiched by two pentaacids through ten ion pairs, initially producing a statistically random mixture of a homochiral trimer and two heterochiral trimers. The heterochiral trimers gradually dissociate and reassemble into the homochiral trimers after unit flipping of the pentaacid, leading to chirality transfer from the decaamine and producing diastereomerically pure trimers.

Heterochiral Diastereomer-Discriminative Diphanes That Form Hierarchical Superstructures with Nonlinear Optical Properties

In order to study the emergence of homochirality during complex molecular systems, most works mainly concentrated on the resolution of a pair of enantiomers. However, the preference of homochiral over heterochiral isomers has been overlooked, with very limited examples focusing only on noncovalent interactions. We herein report on diastereomeric discrimination of twin-cavity cages (denoted as diphanes) against heterochiral tris-(2-aminopropyl)amine (TRPN) bearing triple stereocenters. This diastereomeric selectivity results from distinct spatial orientation of reactive secondary amines on TRPN. Homochiral TRPNs with all reactive moieties rotating in the same way facilitate the formation of homochiral and achiral meso diphanes with low strain energy, while heterochiral TRPNs with uneven orientation of secondary amines preclude the formation of cage-like entity, since the virtual diphanes exhibit considerably high strain. Moreover, homochiral diphanes self-assemble into an acentric superstructure composed of single-handed helices, which exhibits interesting nonlinear optical behavior. Such a property is a unique occurrence for organic cages, which thus showcases their potential to spawn novel materials with interesting properties and functions.

Controlling Chiral Self-Sorting in Truxene-Based Self-Assembled Cages

Coordination driven self-assembly of achiral components, i.e., hexa-alkylated truxene ligands (L) with bis-metallic complexes (M2), afforded three chiral face-rotating stereoisomer polyhedra (M6L2). By tuning the length of the alkyl chains as well as the distance between both ligands facing each other in the self-assemblies (M6L2), one can control the diastereomeric distribution between the expected homo- and hetero-chiral structures.

Chiral self-sorting and guest recognition of porous aromatic cages

The synthesis of ultra-stable chiral porous organic cages (POCs) and their controllable chiral self-sorting at the molecular and supramolecular level remains challening. Herein, we report the design and synthesis of a serial of axially chiral porous aromatic cages (PAC 1-S and 1-R) with high chemical stability. The theoretical and experimental studies on the chiral self-sorting reveal that the exclusive self-recognition on cage formation is an enthalpy-driven process while the chiral narcissistic and self-sorting on supramolecular assembly of racemic cages can be precisely regulated by π–π and C–H…π interactions from different solvents. Regarding the chemical stability, the crystallinity of PAC 1 is maintained in aqueous solvents, such as boiling water, high-concentrated acid and alkali; mixtures of solvents, such as 1 M H₂SO₄/MeOH/H₂O solution, are also tolerated. Investigations on the chiral sensing performance show that PAC 1 enables enantioselective recognition of axially chiral biaryl molecules.

The synthesis of stable chiral porous organic cages and the study of their chiral self-sorting properties is challenging. Here, the authors report axially chiral porous aromatic cages with high stability and solvent-controlled chiral self-sorting.

Supramolecular Proton Conductors Self-Assembled by Organic Cages

Proton conduction is vital for living systems to execute various physiological activities. The understanding of its mechanism is also essential for the development of state-of-the-art applications, including fuel-cell technology. We herein present a bottom-up strategy, that is, the self-assembly of Cage-1 and -2 with an identical chemical composition but distinct structural features to provide two different supramolecular conductors that are ideal for the mechanistic study. Cage-1 with a larger cavity size and more H-bonding anchors self-assembled into a crystalline phase with more proton hopping pathways formed by H-bonding networks, where the proton conduction proceeded via the Grotthuss mechanism. Small cavity-sized Cage-2 with less H-bonding anchors formed the crystalline phase with loose channels filled with discrete H-bonding clusters, therefore allowing for the translational diffusion of protons, that is, vehicle mechanism. As a result, the former exhibited a proton conductivity of 1.59 × 10^-4 S/cm at 303 K under a relative humidity of 48%, approximately 200-fold higher compared to that of the latter. Ab initio molecular dynamics simulations revealed distinct H-bonding dynamics in Cage-1 and -2, which provided further insights into potential proton diffusion mechanisms. This work therefore provides valuable guidelines for the rational design and search of novel proton-conducting materials.

Manipulating Reaction Energy Coordinate Landscape of Mechanochemical Diaza-Cope Rearrangement

Chiral vicinal diamines, a unique class of optically-active building blocks, play a crucial role in material design, pharmaceutical, and catalysis. Traditionally, their syntheses are all solvent-based approaches, which make organic solvent an indispensable part of their production. As part of our program aiming to develop chemical processes with reduced carbon footprints, we recently reported a highly practical and environmentally-friendly synthetic route to chiral vicinal diamines by solvent-free mechanochemical diaza-Cope rearrangement. We herein showed that a new protocol by co-milling with common laboratory solid additives, such as silica gel, can significantly enhance the efficiency of the reaction, compared to reactions in the absence of additives. One possible explanation is the Lewis acidic nature of additives that accelerates a key Schiff base formation step. Reaction monitoring experiments tracing all the reaction species, including reactants, intermediates, and product, suggested that the reaction profile is distinctly different from ball-milling reactions without additives. Collectively, this work demonstrated that additive effect is a powerful tool to manipulate a reaction pathway in mechanochemical diazo-Cope rearrangement pathway, and this is expected to find broad interest in organic synthesis using mechanical force as an energy input.

Enhanced Biological Imaging via Aggregation-Induced Emission Active Porous Organic Cages

Porous organic cages (POCs) have many advantages, including superior microenvironments, good monodispersity, and shape homogeneity, excellent molecular solubility, high chemical stability, and intriguing host-guest chemistry. These properties enable POCs to overcome the limitations of extended porous networks such as metal-organic frameworks (MOFs) and covalent organic frameworks (COFs). However, the applications of POCs in bioimaging remain limited due to the problems associated with their rigid and hydrophobic structures, thus leading to strong aggregation-caused quenching (ACQ) in aqueous biological media. To address this challenge, we report the preparation of aggregation-induced emission (AIE)-active POCs capable of stimuli responsiveness for enhanced bioimaging. We rationally design a hydrophilic, structurally flexible tetraphenylethylene (TPE)-based POC that is almost entirely soluble in aqueous solutions. This POC's conformationally flexible superstructure allows the dynamic rotation of the TPE-based phenyl rings, thus endowing impressive AIE characteristics for responses to environmental changes such as temperature and viscosity. We employ these notable features in the bioimaging of living cells and obtain good performance, demonstrating that the present AIE-active POCs are suitable candidates for further biological applications.

A class of organic cages featuring twin cavities

A variety of organic cages with different geometries have been developed during the last decade, most of them exhibiting a single cavity. In contrast, the number of organic cages featuring a pair of cavities remains scarce. These structures may pave the way towards novel porous materials with emergent properties and functions.We herein report on rational design of a three-dimensional hexaformyl precursor 1, which exhibits two types of conformers, i.e. Conformer-1 and -2, with different cleft positions and sizes. Aided by molecular dynamics simulations, we select two triamino conformation capturers (denoted CC). Small-sized CC-1 selectively capture Conformer-1 by matching its cleft size, while the large-sized CC-2 is able to match and capture both conformers. This strategy allows the formation of three compounds with twin cavities, which we coin diphane. The self-assembly of diphane units results in superstructures with tunable proton conductivity, which reaches up to 1.37×10^-5 S cm^-1.

The preparation of nanocages with unprecedented architectures may lead to new functions. Here the authors report the self-assembly of organic cages featuring twin cavities; the geometry and pocket size determine the molecular packing and the proton conductivity performance.

Stabilization of Dynamic Covalent Architectures by Multivalence

The formation of imine bond is reversible. This feature has been taken advantage of by chemists for accomplishing high yielding self-assembly. On the other hand, it also jeopardizes the intrinsic stability of these self-assembled products. However, some recent discoveries demonstrate that some of these imine bond containing molecules could be rather stable or kinetically inert. A deep investigation indicated that such enhanced stability results from, at least partially, multivalence. Such results also inspire chemists to use imine condensation for self-assembly in water, a solvent that is considered not compatible with imine bond for a long time.

Hollow and highly diastereoselective face-rotating polyhedra constructed through rationally engineered facial units

Molecular face-rotating polyhedra (FRP) exhibit complex stereochemistry, rendering it challenging to manipulate their assembly in a stereoselective manner. In our previous work, stereocontrolled FRP were gained at the cost of losing the confined inner space, which hampers their host–guest interactions and potential applications. Through a rational design approach, herein we demonstrate the successful construction of hollow FRP with high diastereoselectivity. Whereas the [4 + 4] imine condensation of meta-formyl substituted C_3h-symmetric TAT-m and C₃-symmetric Tri-NH2 led to the formation of all feasible FRP-12 diastereoisomers; the para-substituted constitutional isomer, TAT-p, exclusively assembled into a pair of homo-directional enantiomeric FRP-13-CCCC/AAAA with a cavity size larger than 600 ų. Detailed structural characterizations and theoretical investigations revealed the thermodynamic landscape of FRP assembly can be effectively shaped by modulating the van der Waals repulsive forces among the facial building blocks. Our work provided a novel strategy towards stereospecific assembly of pure organic cages, opening up new opportunities for further applications of these chiral materials.

The rationally engineered facial units, TAT-m and TAT-p, resulted in distinct diastereoselectivity of face-rotating polyhedra (FRP).

Self-assembly of chiral BINOL cages via imine condensation

Condensation of an (S)- or (R)-BINOL-derived dialdehyde and tris(2-aminoethyl)amine produced chiral [2+3] imine cages, which were further reduced to furnish more stable chiral amine cages and applied in the enantioselective recognition of (1R,2R)- and (1S,2S)-1,2-diaminocyclohexane.

Revealing the chirality origin and homochirality crystallization of Ag14 nanocluster at the molecular level

Although chirality is an ever-present characteristic in biology and some artificial molecules, controlling the chirality and demystifying the chirality origin of complex assemblies remain challenging. Herein, we report two homochiral Ag₁₄ nanoclusters with inherent chirality originated from identical rotation of six square faces on a Ag₈ cube driven by intra-cluster π···π stacking interaction between pntp⁻ (Hpntp = p-nitrothiophenol) ligands. The spontaneous resolution of the racemic (SD/rac-Ag14a) to homochiral nanoclusters (SD/L-Ag14 and SD/R-Ag14) can be realized by re-crystallizing SD/rac-Ag14a in acetonitrile, which promotes the homochiral crystallization in solid state by forming C–H···O/N hydrogen bonds with nitro oxygen atoms in pntp⁻ or aromatic hydrogen atoms in dpph (dpph = 1,6-bis(diphenylphosphino)hexane) on Ag₁₄ nanocluster. This work not only provides strategic guidance for the syntheses of chiral silver nanoclusters in an all-achiral environment, but also deciphers the origin of chirality at molecular level by identifying the special effects of intra- and inter-cluster supramolecular interactions.

The preparation of chiral monolayer-protected metal clusters is interesting for their potential applications in a variety of fields, including catalysis. Here, the authors synthesize chiral Ag₁₄ nanoclusters in an all-achiral environment, and decipher the origin of chirality at the molecular level; the solvent choice is key to achieve homochiral crystallization.

Cucurbitimines - imine cages with concave walls

The variety of shape-persistent organic cages by imine bond formation has tremendously enlarged in recent years by using different building blocks (aldehydes and amines) in the condensation reactions. Here, we describe the use of a kinked tetraldehyde to generate pumpkin-shaped cages with concave walls, similar to cucurbiturils.

Kinked tetraaldehyde building blocks lead in condensation reactions with diamines to pumpkin shaped cages – the cucurbitimines.

Soluble Congeners of Prior Insoluble Shape-Persistent Imine Cages

One of the most applied reaction types to synthesize shape‐persistent organic cage compounds is the imine condensation reaction and it is assumed that the formed cages are thermodynamically controlled products due to the reversibility of the imine condensation. However, most of the synthesized imine cages reported are formed as precipitate from the reaction mixture and therefore rather may be kinetically controlled products. There are even examples in literature, where resulting cages are not soluble at all in common organic solvents to characterize or study their formation by NMR spectroscopy in solution. Here, a triptycene triamine containing three solubilizing n‐hexyloxy chains has been used to synthesize soluble congeners of prior insoluble cages. This allowed us to study the formation as well as the reversibility of cage formation in solution by investigating exchange of building blocks between the cages and deuterated derivatives thereof.

Design, synthesis and photophysical properties of novel star-shaped truxene-based heterocycles utilizing ring-closing metathesis, Clauson-Kaas, Van Leusen and Ullmann-type reactions as key tools

Design, synthesis and properties of polycyclic aromatic hydrocarbons (PAHs) has historically attracted a considerable interdisciplinary interest from both fundamental as well as applied viewpoint on account of their wonderful optoelectronic properties. The scientific interest in two-dimensional star-shaped PAHs particularly in truxene architectures arises because of their high thermal stability, exceptional solubility and ease with which they can be constructed and modified. Therefore, bearing in mind a wide range of applications of truxene and its congeners, herein we reveal three novel distinctly different routes for the generation of C 3-symmetric pyrrole-based truxene architectures by means of cyclotrimerization, ring-closing metathesis (RCM), Clauson-Kaas and Ullmann-type coupling reactions as key steps. Moreover, we have also assembled some other interesting heterocyclic systems possessing oxazole, imidazole, benzimidazole, and benzoxazole in the framework of truxene. Additionally, the preliminary photophysical properties (absorption and emission) for these versatile systems has been revealed.

A Cavity-Tailored Metal-Organic Cage Entraps Gases Selectively in Solution and the Amorphous Solid State

Here we report the subcomponent self-assembly of a truxene-faced Zn4 L4 tetrahedron, which is capable of binding the smallest hydrocarbons in solution. By deliberately incorporating inward-facing ethyl groups on the truxene faces, the resulting partially-filled cage cavity was tailored to encapsulate methane, ethane, and ethene via van der Waals interactions at atmospheric pressure in acetonitrile, and also in the amorphous solid state. Interestingly, gas capture showed divergent selectivities in solution and the amorphous solid state. The selective binding may prove useful in designing new processes for the purification of methane and ethane as feedstocks for chemical synthesis.

Chiral Self-sorting of Giant Cubic [8+12] Salicylimine Cage Compounds

Chiral self-sorting is intricately connected to the complicated chiral processes observed in nature and no artificial systems of comparably complexity have been generated by chemists. However, only a few examples of purely organic molecules have been reported so far, where the self-sorting process could be controlled. Herein, we describe the chiral self-sorting of large cubic [8+12] salicylimine cage compounds based on a chiral TBTQ precursor. Out of 23 possible cage isomers only the enantiopure and a meso cage were observed to be formed, which have been unambiguously characterized by single crystal X-ray diffraction. Furthermore, by careful choice of solvent the formation of meso cage could be controlled. With internal diameters of din =3.3-3.5 nm these cages are among the largest organic cage compounds characterized and show very high specific surface areas up to approx. 1500 m2  g-1 after desolvation.

A Giant [8+12] Boronic Ester Cage with 48 Terminal Alkene Units in the Periphery for Postsynthetic Alkene Metathesis

Dynamic covalent chemistry (DCC) is a powerful synthetic tool to construct large defined molecules in one step from rather simple precursors. The advantage of the intrinsic dynamics of the applied reversible reaction steps is a self‐correction under the chosen conditions, to achieve high yields of the target compound. To date, only a few examples are known, in which DCC was used to build up a molecular defined but larger product that was chemically transferred to a more stable congener in a second (irreversible) step. Here, we present a nanometer‐sized [8+12] boronic ester cage containing 48 peripheral terminal alkene units which allows to put a hydrocarbon exoskeleton around the cage via alkene metathesis.

Truxene-based covalent organic polyhedrons constructed through alkyne metathesis

Dynamic alkyne metathesis has successfully been employed toward the synthesis of a truxene-based shape-persistent covalent organic polyhedron (COP) with high binding affinity for fullerenes.

Molecular Sensing with Host Systems for Hyperpolarized 129Xe

Hyperpolarized noble gases have been used early on in applications for sensitivity enhanced NMR. 129Xe has been explored for various applications because it can be used beyond the gas-driven examination of void spaces. Its solubility in aqueous solutions and its affinity for hydrophobic binding pockets allows "functionalization" through combination with host structures that bind one or multiple gas atoms. Moreover, the transient nature of gas binding in such hosts allows the combination with another signal enhancement technique, namely chemical exchange saturation transfer (CEST). Different systems have been investigated for implementing various types of so-called Xe biosensors where the gas binds to a targeted host to address molecular markers or to sense biophysical parameters. This review summarizes developments in biosensor design and synthesis for achieving molecular sensing with NMR at unprecedented sensitivity. Aspects regarding Xe exchange kinetics and chemical engineering of various classes of hosts for an efficient build-up of the CEST effect will also be discussed as well as the cavity design of host molecules to identify a pool of bound Xe. The concept is presented in the broader context of reporter design with insights from other modalities that are helpful for advancing the field of Xe biosensors.

Examination of the Dynamic Covalent Chemistry of [2 + 3]-Imine Cages

The synthesis of shape-persistent organic cage compounds by the formation of imine bonds opens the possibility to realize cages of different sizes, geometries, topologies, and functions. It is generally assumed that the imine bond is rather chemically labile allowing a self-correction mechanism until thermodynamic equilibrium is reached, which is often the case if a cage is formed. However, there are some contradictory experimental data to this assumption. To get a deeper insight into the imine bond dynamics of covalent organic cages, we studied the formation and exchange of both dialdehydes and triamines of two different [2 + 3] imine cages with the aid of a deuterated dialdehyde molecular building block.

Truncated Face-Rotating Polyhedra Constructed from Pentagonal Pentaphenylpyrrole through Graph Theory

Discovering novel families of molecular polyhedra through graph theory has attracted increasing interest. Nevertheless, the design principles of molecular polyhedra based on graph theory remain elusive, especially for those containing five-node units. Herein, we construct a series of chiral truncated face-rotating polyhedra (T-FRP) from pentagonal pentaphenylpyrrole (PPP) derivatives and chiral diamines. Graph theory is used to elucidate the geometry of these novel T-FRP, which represent a new family of molecular polyhedra. The phenyl flipping of PPP faces in these T-FRP is significantly restricted, thus making T-FRP chiral and strongly emissive in solution. In addition, T-FRP also generate circularly polarized luminescence. This study provides new insights into the rational design of novel molecular polyhedra through graph theory.

A family of diastereomeric dodecanuclear coordination cages based on inversion of chirality of individual triangular cyclic helicate faces

The dodecanuclear coordination cage [Cd12(Lnaph)12(Lmes)4](BF4)24 consists of a set of four triangular, trinuclear helical panels {Cd3(μ-Lnaph)3}6+ (based on ditopic bridging ligands Lnaph), which are connected by four tritopic ligands Lmes. The result is that the four triangular helical panels and the four Lmes-capped triangular faces of the cuboctahedral core form two alternating subsets of the eight triangular faces of the cuboctahedron. Crystallographic investigations revealed that the triangular helicate faces can have 'clockwise' (C) or 'anticlockwise' (A) helicity, and that the helicity of each face can vary independently of the others as they are mechanically separated. This generates a set of three diastereoisomers in which all four cyclic helicate faces in the cuboctahedron have the same chirality (AAAA/CCCC enantiomers with T symmetry; AAAC/CCCA enantiomers with C 3 symmetry; and achiral AACC with S 4 symmetry). This mirrors the known behaviour of many simpler M4L6 tetrahedral cages which can likewise exist as T, C 3 or S 4 isomers according to the sense of tris-chelate chirality around each individual metal centre: but here it is translated onto a much larger scale by the four chiral units being entire trinuclear helicate faces rather than single metal centres. 1H NMR spectroscopy confirms the presence of the three diastereoisomers with their different molecular symmetries in a ratio slightly different from what is expected on purely statistical grounds; and 1H NMR measurements on a non-equilibrium sample (enriched by manual crystal-picking before preparing the solution) showed that the distribution does not change over several weeks in solution, indicating the kinetic inertness of the cage assemblies.

Equi-size nesting of Platonic and Archimedean metal-organic polyhedra into a twin capsid

Inspired by the structures of virus capsids, chemists have long pursued the synthesis of their artificial molecular counterparts through self–assembly. Building nanoscale hierarchical structures to simulate double-shell virus capsids is believed to be a daunting challenge in supramolecular chemistry. Here, we report a double-shell cage wherein two independent metal–organic polyhedra featuring Platonic and Archimedean solids are nested together. The inner (3.2 nm) and outer (3.3 nm) shells do not follow the traditional “small vs. large” pattern, but are basically of the same size. Furthermore, the assembly of the inner and outer shells is based on supramolecular recognition, a behavior analogous to the assembly principle found in double-shell viruses. These two unique nested characteristics provide a new model for Matryoshka–type assemblies. The inner cage can be isolated individually and proves to be a potential molecular receptor to selectively trap guest molecules.

Supramolecular constructs that mimic complex biological assemblies are synthetically challenging. Here, the authors present a double-shell cage wherein two independent metal-organic polyhedra are nested together in a manner analogous to that found in double-shell virus capsids.