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

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.

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.

Cofacial porphyrin organic cages. Metals regulating excitation electron transfer and CO2 reduction electrocatalytic properties

Herein, we introduce a comprehensive study of the photophysical behaviors and CO2 reduction electrocatalytic properties of a series of cofacial porphyrin organic cages (CPOC-M, M = H2, Co(ii), Ni(ii), Cu(ii), Zn(ii)), which are constructed by the covalent-bonded self-assembly of 5,10,15,20-tetrakis(4-formylphenyl)porphyrin (TFPP) and chiral (2-aminocyclohexyl)-1,4,5,8-naphthalenetetraformyl diimide (ANDI), followed by post-synthetic metalation. Electronic coupling between the TFPP donor and naphthalene-1,4 : 5,8-bis(dicarboximide) (NDI) acceptor in the metal-free cage is revealed to be very weak by UV-vis spectroscopic, electrochemical, and theoretical investigations. Photoexcitation of CPOC-H2, as well as its post-synthetic Zn and Co counterparts, leads to fast energy transfer from the triplet state porphyrin to the NDI unit according to the femtosecond transient absorption spectroscopic results. In addition, CPOC-Co enables much better electrocatalytic activity for CO2 reduction reaction than the other metallic CPOC-M (M = Ni(ii), Cu(ii), Zn(ii)) and monomeric porphyrin cobalt compartment, supplying a partial current density of 18.0 mA cm-2 at -0.90 V with 90% faradaic efficiency of CO.

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.

A Diverse Array of Large Capsules Transform in Response to Stimuli

The allosteric regulation of biomolecules, such as enzymes, enables them to adapt and alter their conformation to fit specific substrates, expressing different functionalities in response to stimuli. Different stimuli can also trigger synthetic coordination cages to change their shape, size, and nuclearity by reconfiguring the dynamic metal-ligand bonds that hold them together. Here we demonstrate an abiological system consisting of different organic subcomponents and Zn^II metal ions, which can respond to simple stimuli in complex ways. A Zn^II₂₀L₁₂ dodecahedron transforms to give a larger Zn^II₃₀L₁₂ icosidodecahedron through subcomponent exchange, as an aldehyde that forms bidentate ligands is displaced in favor of one that forms tridentate ligands together with a penta-amine subcomponent. In the presence of a chiral template guest, the same system that produced the icosidodecahedron instead gives a Zn^II₁₅L₆ truncated rhombohedral architecture through enantioselective self-assembly. Under specific crystallization conditions, a guest induces a further reconfiguration of either the Zn^II₃₀L₁₂ or Zn^II₁₅L₆ cages to yield an unprecedented Zn^II₂₀L₈ pseudo-truncated octahedral structure. The transformation network of these cages shows how large synthetic hosts can undergo structural adaptation through the application of chemical stimuli, opening pathways to broader applications.

A Pair of Interconverting Cages Formed from Achiral Precursors Spontaneously Resolve into Homochiral Conformers

Without chiral induction the emergence of homochirality from achiral molecules is rather serendipitous, as the rationale is somewhat ambiguous. We herein provide a plausible solution. From achiral precursors are formed a pair of interconverting cage conformers that exhibit a C₃ -axis as the only symmetry element. When their interconversion is impeded with intramolecular H-bonding, each conformer self-sorts into a homochiral crystal, which is driven by a helical network of multivalent intermolecular interactions during the self-assembly of homochiral cage conformers. As no chiral induction is involved throughout, we believe our study could enlighten the rational design for the emergence of homochirality with several criteria: 1) formation of a molecule without inversion center or mirror plane; 2) suppression of the enantiomeric interconversion, and introduction of multivalent interactions along the helical trajectory of screw symmetry within the resulting superstructure.

Air-Stable Radical Organic Cages as Cascade Nanozymes for Enhanced Catalysis

The pursuit of single-assembled molecular cage reactors for complex tandem reactions is a long-standing target in biomimetic catalysis but still a grand challenge. Herein, nanozyme-like organic cages are reported by engineering air-stable radicals into the skeleton upon photoinduced electron transfer. The generation of radicals is accompanied by single-crystal structural transformation and exhibits superior stability over six months in air. Impressively, the radicals throughout the cage skeleton can mimic the peroxidase of natural enzymes to decompose H₂ O₂ into OH· and facilitate oxidation reactions. Furthermore, an integrated catalyst by encapsulating Au clusters (glucose oxidase mimics) into the cage has been developed, in which the dual active sites (Au cluster and radical) are spatially isolated and can work as cascade nanozymes to prominently promote the enzyme-like tandem reaction via a substrate channeling effect.

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.

Cage-Confinement Induced Emission Enhancement

As a proof-of-concept study, Imi-cage and Phos-cage organic molecular cages (OMCs) containing the triphenylphosphine (TPP) moiety, a nonclassic AIE luminogen (AIEgen), have been designed to demonstrate the cage-confinement induced emission enhancement (CCIEE). Thanks to the confinement effect of OMCs, the rigid Imi-cage exhibits much higher photoluminescence (PL) quantum yield (Φ_PL) than the open-shell Semicage and small molecule TPP in both solution and amorphous solid states. The emission of Phos-cage could be further enhanced in crystalline solid state with a remarkably high Φ_PL of 97.6% (vs 3.47% of crystalline TPP) benefiting from AIE enabled by the highly ordered molecular packing. The novel strategy of CCIEE via confining an AIEgen into an OMC to achieve a significant emission enhancement will shed light on the development of solid-state highly fluorescent materials. The fluorescent nature of Imi-cage was further exploited for the ultrahighly sensitive detection of the explosive picric acid.

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.

Understanding Metal-Organic Framework Nucleation from a Solution with Evolving Graphs

A mechanistic understanding of metal-organic framework (MOF) synthesis and scale-up remains underexplored due to the complex nature of the interactions of their building blocks. In this work, we investigate the collective assembly of building units at the early stages of MOF nucleation, using MIL-101(Cr) as a prototypical example. Using large-scale molecular dynamics simulations, we observe that the choice of solvent (water and N,N-dimethylformamide), the introduction of ions (Na⁺ and F^-) and the relative populations of MIL-101(Cr) half-secondary building unit (half-SBU) isomers have a strong influence on the cluster formation process. Additionally, the shape, size, nucleation and growth rates, crystallinity, and short and long-range order largely vary depending on the synthesis conditions. We evaluate these properties as they naturally emerge when interpreting the self-assembly of MOF nuclei as the time evolution of an undirected graph. Solution-induced conformational complexity and ionic concentration have a dramatic effect on the morphology of clusters emerging during assembly. While pure solvents lead to the rapid formation of a small number of large clusters, the presence of ions in aqueous solutions results in smaller clusters and slower nucleation. This diversity is captured by the key features of the graph representation. Principle component analysis on graph properties reveals that only a small number of molecular descriptors is needed to deconvolute MOF self-assembly. Descriptors such as the average coordination number between half-SBUs and fractal dimension are of particulalr interest as they can be can be followed experimentally by techniques like by time-resolved spectroscopy. Ultimately, graph theory emerges as an approach that can be used to understand complex processes revealing molecular descriptors accessible by both simulation and experiment.

Transformation of Porous Organic Cages and Covalent Organic Frameworks with Efficient Iodine Vapor Capture Performance

The reaction of 5,5'-([2,2'-bipyridine]-5,5'-diyl)diisophthalaldehyde (BPDDP) with cyclohexanediamine and [benzidine (BZ)/[2,2'-bipyridine]-5,5'-diamine (BPDA)], respectively, affords a nitrogen-rich porous organic cage BPPOC and two two-dimensional (2D) covalent organic frameworks (COFs), USTB-1 and USTB-2 (USTB = University of Science and Technology Beijing), under suitable conditions. Interestingly, BPPOC with a single-crystal X-ray diffraction structure is able to successfully transform into USTB-1 and USTB-2 (newly converted COFs denoted as USTB-1c and USTB-2c, respectively) upon exchange of the imine unit of cyclohexanediamine in the cage by BZ and BPDA. Such a transformation also enables the isolation of analogous COFs (USTB-3c and USTB-4c) on the basis of an isostructural organic cage, BTPOC, which is derived from 5,5'-([2,2'-bithiophene]-4,4'-diyl)diisophthalaldehyde (BTDDP) and cyclohexanediamine. However, the conventional solvothermal reaction between BTDDP and BPDA leads to an impure phase of USTB-4 containing incompletely converted aldehyde groups due to the limited solubility of the building block. The newly prepared COFs have been characterized by nuclear magnetic resonance spectroscopy, Fourier transform infrared spectroscopy, scanning electron microscopy, and transmission electron microscopy. In particular, BPPOC is able to absorb the iodine vapor with an uptake of 5.64 g g^-1, breaking the porous organic cage's (POC's) record value of 3.78 g g^-1. Nevertheless, the cage-derived COFs exhibit improved iodine vapor adsorption capability in comparison with the directly synthesized counterparts, with the highest uptake of 5.80 g g^-1 for USTB-1c. The mechanism investigation unveils the superiority of nitrogen atoms to sulfur atoms for POCs in iodine vapor capture with the assistance of definite crystal structures. This, in combination with porosity, synergistically influences the iodine vapor capture capacity of COFs.

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.

Circularly polarized luminescent porous crystalline nanomaterials

Circularly polarized luminescence (CPL)-active materials have attracted exclusive attention because of their wide potential applications in low-power-consumption displays, encrypted information storage, chiroptical sensors, and so on. However, there is always a trade-off between the luminescence dissymmetry factor (g_lum) and luminescence quantum yield, which are two critical parameters. Therefore, developing materials with both large g_lum values and high luminescence efficiency is a key issue for constructing high-efficiency CPL materials. To date, chiral porous crystalline nanomaterials (PCNMs) including metal-organic frameworks (MOFs), porous organic-cages (POCs), metal-organic cages (MOCs), and supramolecular organic frameworks (SOFs), have shown excellent potential for solving this problem and achieving functional CPL-active materials. In this review, we will summarize several approaches for fabricating CPL-active PCNMs, such as direct synthesis, chirality induction, and symmetry breaking. Furthermore, with flexibly tunable structures and comprehensive host-guest chemistry, modulation and amplification of CPL can be achieved in these PCNMs. We would like to provide insight and perspective that PCNMs can act as an efficient platform in the CPL research field.

Enantioselective assembly and recognition of heterochiral porous organic cages deduced from binary chiral components

Chiral recognition and discrimination is not only of significance in biological processes but also a powerful method to fabricate functional supramolecular materials. Herein, a pair of heterochiral porous organic cages (HPOC-1), out of four possible enantiomeric products, with mirror stereoisomeric crystal structures were cleanly prepared by condensation occurring in the exclusive combination of cyclohexanediamine and binaphthol-based tetraaldehyde enantiomers. Nuclear magnetic resonance and luminescence spectroscopy have been employed to monitor the assembly process of HPOC-1, revealing the clean formation of heterochiral organic cages due to the enantioselective recognition of (S,S)-binaphthol towards (R,R)-cyclohexanediamine derivatives and vice versa. Interestingly, HPOC-1 exhibits circularly polarized luminescence and enantioselective recognition of chiral substrates according to the circular dichroism spectral change. Theoretical simulations have been carried out, rationalizing both the enantioselective assembly and recognition of HPOC-1.

Heterochiral organic cages based on enantioselective self-assembly of binary chiral components have been prepared, exhibiting circularly polarized luminescence property and enantioselective recognition ability towards chiral substrates according to the circular dichroism spectral change.

Water-stable hydrazone-linked porous organic cages

Although porous organic cages (POCs), particularly imine-linked (C[double bond, length as m-dash]N) ones, have advanced significantly over the last few decades, the reversible nature of imine linkages makes them prone to hydrolysis and structural collapse, severely limiting their applications under moist or water conditions. Herein, seven water-stable hydrazone-linked (C[double bond, length as m-dash]N-N) POCs are prepared through a simple coupling of the same supramolecular tetraformylresorcin[4]arene cavitand with different dihydrazide linkers. Their structures are all determined by single-crystal X-ray crystallography, demonstrating rich structural diversity from the [2 + 4] lantern, [3 + 6] triangular prism, and unprecedented [4 + 8] square prism to the extra-large [6 + 12] octahedron. In addition, they respectively exhibit tunable window diameters and cavity volumes ranging from about 5.4 to 11.1 nm and 580 to 6800 ų. Moreover, their application in the water environment for pollutant removal was explored, indicating that they can effectively eliminate various types of contaminants from water, including radionuclide waste, toxic heavy metal ions, and organic micropollutants. This work demonstrates a convenient method for rationally constructing versatile robust POCs and presents their great application potentialities in water medium.

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).