High-Fidelity Stereochemical Memory in a FeII4L4 Tetrahedral Capsule

Electron Spin Polarization and Memory Effect in Supramolecular Gel Exclusively From Achiral Building Blocks

Chirality has been identified as a crucial component in achieving high spin selectivity in organic polymers and π-conjugated molecules. In particular, chiral polymers and supramolecular structures have emerged as promising candidates for spin filtering due to the chirality-induced spin selectivity (CISS) effect. However, the CISS effect in supramolecular systems has not been extensively investigated, despite its potential for applications in spintronics. In this work, for the first time, the potential applications of the CISS effect in supramolecular gel materials and shed light on its untapped possibilities have been successfully explored. The ability of supramolecular gel exclusively made from achiral building blocks to selectively filter electron's spin through the symmetry breaking has been demonstrated. Furthermore, this study shows that their spin filtering efficacy can be improved by using chiral solvents. More importantly, the CISS effect has been employed to explore a novel phenomenon referred to as the "spin memory effect", where the desired spin information is preserved by retaining the helicity even in the absence of the chiral solvent. These findings underscore the immense potential for spintronics applications that rely solely on achiral components, thereby paving the way for new possibilities in device design and functionality.

Algorithm-Driven Robotic Discovery of Polyoxometalate-Scaffolding Metal-Organic Frameworks

The experimental exploration of the chemical space of crystalline materials, especially metal-organic frameworks (MOFs), requires multiparameter control of a large set of reactions, which is unavoidably time-consuming and labor-intensive when performed manually. To accelerate the rate of material discovery while maintaining high reproducibility, we developed a machine learning algorithm integrated with a robotic synthesis platform for closed-loop exploration of the chemical space for polyoxometalate-scaffolding metal-organic frameworks (POMOFs). The eXtreme Gradient Boosting (XGBoost) model was optimized by using updating data obtained from the uncertainty feedback experiments and a multiclass classification extension based on the POMOF classification from their chemical constitution. The digital signatures for the robotic synthesis of POMOFs were represented by the universal chemical description language (χDL) to precisely record the synthetic steps and enhance the reproducibility. Nine novel POMOFs including one with mixed ligands derived from individual ligands through the imidization reaction of POM amine derivatives with various aldehydes have been discovered with a good repeatability. In addition, chemical space maps were plotted based on the XGBoost models whose F1 scores are above 0.8. Furthermore, the electrochemical properties of the synthesized POMOFs indicate superior electron transfer compared to the molecular POMs and the direct effect of the ratio of Zn, the type of ligands used, and the topology structures in POMOFs for modulating electron transfer abilities.

Subcomponent self-assembly construction of tetrahedral cage FeII4L4 for high-resolution gas chromatographic separation

Metal organic cages (MOCs), as an emerging discrete supramolecular compounds, have received widespread attention in separation, biomedicine, gas capture, catalysis, and molecular recognition due to their porosity, adjustability and stability. Herein, we present a new chiral MOC FeII4L4 coated capillary column prepared for gas chromatographic (GC) separation of different types of organic compounds, including n-alkanes, n-alcohols, alkylbenzenes, isomers, especially for racemic compounds. There are 20 different kinds of racemates (e.g., alcohols, ethers, epoxides, esters, alkenes, and aldehydes) were well resolved on the FeII4L4 chiral column and a maximum resolution value for 1-phenyl-1-propanol reaches 6.17. The FeII4L4 coated column exhibited high column efficiency (3100 plates m-1 for n-dodecane) and good enantiomeric resolution complementary to that of a commercial β-DEX 120 column and the previously reported chiral MOC [Fe4L6] (ClO4)8 coated column. The relative standard deviation (RSDs) of the peak area and retention time of glycidol and nitrotoluene were below 1.2 %. This study reveals that chiral MOCs have good application prospects in chromatographic separation.

Robust Imidazopyridinium Covalent Organic Framework as Efficient Iodine Capturing Materials in Gaseous and Aqueous Environment

The development of a high-performing adsorbent that can capture both iodine vapor from volatile nuclear waste and traces of iodine species from water is an important challenge, especially in industrially relevant process conditions. This study introduces novel imidazopyridinium-based covalent organic frameworks (COFs) through post-modification of a picolinaldehyde-based imine COF. These COFs demonstrate excellent iodine adsorption capacity, adsorption kinetics, and a high stability/recyclability in both vapor and water phases. Notably, one imidazopyridinium COF exhibits gaseous iodine uptake of 21 wt.% under dynamic adsorption conditions at 150 °C and a relative humidity of 50%, surpassing the performance of the currently used silver-based zeolite adsorbents (Ag@MOR (17wt.%)). Additionally, the same imidazopyridinium COFs can efficiently remove iodine species at a low concentration from aqueous solution. Seawater containing triiodide ions treated under dynamic flow-through conditions resulted in decreased concentrations down to the ppb level. The adsorption mechanisms for iodine and polyiodide species are elucidated for the imine COF and imidazopyridinium COFs; involving halogen bonding, hydrogen bonding, and charge-transfer complexes.

Metal Ions Trigger the Gelation of Cysteine-Containing Peptide-Appended Coordination Cages

We report a series of coordination cages that incorporate peptide chains at their vertices, prepared through subcomponent self-assembly. Three distinct heterochiral tripeptide subcomponents were incorporated, each exhibiting an L-D-L stereoconfiguration. Through this approach, we prepared and characterized three tetrahedral metal-peptide cages that incorporate thiol and methylthio groups. The gelation of these cages was probed through the binding of additional metal ions, with the metal-peptide cages acting as junctions, owing to the presence of sulfur atoms on the peripheral peptides. Gels were obtained with cages bearing cysteine at the C-terminus. Our strategy for developing functional metal-coordinated supramolecular gels with a modular design may result in the development of materials useful for chemical separations or drug delivery.

Supramolecular cage-mediated cargo transport

A steady stream of material transport based on carriers and channels in living systems plays an extremely important role in normal life activities. Inspired by nature, researchers have extensively applied supramolecular cages in cargo transport because of their unique three-dimensional structures and excellent physicochemical properties. In this review, we will focus on the development of supramolecular cages as carriers and channels for cargo transport in abiotic and biological systems over the past fifteen years. In addition, we will discuss future challenges and potential applications of supramolecular cages in substance transport.

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.

Stereocontrolled Self-Assembly of a Helicate-Bridged CuI12L4 Cage That Emits Circularly Polarized Light

Control over the stereochemistry of metal-organic cages can give rise to useful functions that are entwined with chirality, such as stereoselective guest binding and chiroptical applications. Here, we report a chiral CuI12L4 pseudo-octahedral cage that self-assembled from condensation of triaminotriptycene, aminoquinaldine, and diformylpyridine subcomponents around CuI templates. The corners of this cage consist of six head-to-tail dicopper(I) helicates whose helical chirality can be controlled by the addition of enantiopure 1,1'-bi-2-naphthol (BINOL) during the assembly process. Chiroptical and nuclear magnetic resonance (NMR) studies elucidated the process and mechanism of stereochemical information transfer from BINOL to the cage during the assembly process. Initially formed CuI(BINOL)2 thus underwent stereoselective ligand exchange during the formation of the chiral helicate corners of the cage, which determined the overall cage stereochemistry. The resulting dicopper(I) helicate corners of the cage were also shown to generate circularly polarized luminescence.

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.

Enantioselective fullerene functionalization through stereochemical information transfer from a self-assembled cage

The regioselective functionalization of C₆₀ remains challenging, while the enantioselective functionalization of C₆₀ is difficult to explore due to the need for complex chiral tethers or arduous chromatography. Metal-organic cages have served as masks to effect the regioselective functionalization of C₆₀. However, it is difficult to control the stereochemistry of the resulting fullerene adducts through this method. Here we report a means of defining up to six stereocentres on C₆₀, achieving enantioselective fullerene functionalization. This method involves the use of a metal-organic cage built from a chiral formylpyridine. Fullerenes hosted within the cavity of the cage can be converted into a series of C₆₀ adducts through chemo-, regio- and stereo-selective Diels-Alder reactions with the edges of the cage. The chiral formylpyridine ultimately dictates the stereochemistry of these chiral fullerene adducts without being incorporated into them. Such chiral fullerene adducts may become useful in devices requiring circularly polarized light manipulation.

Chirality-Driven Self-Assembly of Discrete, Homochiral FeII 2 L3 Cages

Coordination chemistry is a powerful method to synthesize supramolecular cages with distinct features that suit specific applications. This work demonstrates the synthesis of discrete, homochiral Fe^II ₂ L₃ cages via chirality-driven self-assembly. Specifically, the installation of chirality - at both the vertices and ligand backbones - allows the formation of discrete, homochiral Fe^II ₂ L₃ cages of different sizes via stereochemical control of the iron(II) centers. We observed that larger cages require multiple chiral centra (chiral ligands and vertices). In contrast, the formation of smaller cages is stereoselective with solely chiral ligands. The latter cages can also be formed from two chiral subcomponents, but only when they have matching chirality. Single-crystal X-ray diffraction of these smaller Fe^II ₂ L₃ cages revealed several non-covalent interactions as a driving force for narcissistic chiral self-sorting. This expected behavior was confirmed utilizing the shorter ligands in racemic form, yielding discrete, homochiral Fe^II ₂ L₃ cages formed in enantiomeric pairs.

Transformation networks of metal-organic cages controlled by chemical stimuli

The flexibility of biomolecules enables them to adapt and transform as a result of signals received from the external environment, expressing different functions in different contexts. In similar fashion, coordination cages can undergo stimuli-triggered transformations owing to the dynamic nature of the metal-ligand bonds that hold them together. Different types of stimuli can trigger dynamic reconfiguration of these metal-organic assemblies, to switch on or off desired functionalities. Such adaptable systems are of interest for applications in switchable catalysis, selective molecular recognition or as transformable materials. This review highlights recent advances in the transformation of cages using chemical stimuli, providing a catalogue of reported strategies to transform cages and thus allow the creation of new architectures. Firstly we focus on strategies for transformation through the introduction of new cage components, which trigger reconstitution of the initial set of components. Secondly we summarize conversions triggered by external stimuli such as guests, concentration, solvent or pH, highlighting the adaptation processes that coordination cages can undergo. Finally, systems capable of responding to multiple stimuli are described. Such systems constitute composite chemical networks with the potential for more complex behaviour. We aim to offer new perspectives on how to design transformation networks, in order to shed light on signal-driven transformation processes that lead to the preparation of new functional metal-organic architectures.

Guest-Driven Self-Assembly and Chiral Induction of Photofunctional Lanthanide Tetrahedral Cages

Chiral luminescent lanthanide-organic cages have many potential applications in enantioselective recognition, sensing, and asymmetric catalysis. However, due to the paucity of structures and their limited cavities, host-guest chemistry with lanthanide-organic cages has remained elusive so far. Herein, we report a guest-driven self-assembly and chiral induction approach for the construction of otherwise inaccessible Ln₄L₄-type (Ln = lanthanide ions, i.e., Eu^III, Tb^III; L = ligand) tetrahedral hosts. Single crystal analyses on a series of host-guest complexes reveal remarkable guest-adaptive cavity breathing on the tetrahedral cages, reflecting the advantage of the variation tolerance on coordination geometry of the f-elements. Meanwhile, noncovalent confinement of pyrene within the lanthanide cage not only leads to diminishment of its excimer emission but also facilitates guest to host energy transfer, opening up a new sensitization window for the lanthanide luminescence on the cage. Moreover, stereoselective self-assembly of either Λ₄- or Δ₄- type Eu₄L₄ cages has been realized via chiral induction with R/S-BINOL or R/S-SPOL templates, as confirmed by NMR, circular dichroism (CD), and circularly polarized luminescence (CPL) with high dissymmetry factors (g_lum) up to ±0.125.

The More the Slower: Self-Inhibition in Supramolecular Chirality Induction, Memory, Erasure, and Reversion

Self-inhibition has been observed widely in hierarchical biochemical processes but has yet to be demonstrated in pure molecular physical rather than chemical or biological processes. Herein, we report an unprecedented example of self-inhibition during the supramolecular chirality induction, memory, erasure, and inversion processes of pillar[5]arene (P[5]) derivatives. The addition of chiral alanine ethyl ester to bulky substituent-modified P[5]s led to time-dependent chirality induction due to the shift in the equilibrium of the S_P and R_P conformers P[5]. Intriguingly, more chiral inducers led to more intensive final chiroptical properties but lower chiral induction rates. Thus, the chiral inducer plays the role of both activator and inhibitor. Such self-inhibition essentially arises from kinetics manipulation of three tandem equilibria. Moreover, the chiroptical properties could be memorized by replacing the chiral inducer with an achiral competitive binder, and the chiroptical signal could be erased and reversed by an antipodal chiral inducer, which also showed the self-inhibition property.

Rational Synthesis of Imine-Linked Fluorescent Covalent Organic Frameworks with Different pKa for pH Sensing In Vitro and In Vivo

Precise modulation of pH in living cells plays a vital role in the study of many diseases, such as cancer and rheumatoid arthritis. Here, a series of imine-linked covalent organic frameworks (COFs) were rationally designed and developed for pH sensing in tumor cells and zebrafish. Four monomers were chosen to synthesize COFs (COF₁-COF₄) with different pK_a by a simple orthogonal combination through condensation reaction. The as-obtained COFs exhibited a sensitive pH-dependent fluorescence response compared to their building blocks. Among them, COF₂ possessed a high crystallinity, excellent fluorescence, and suitable pK_a for biosensing. For bioimaging applications, COF₂ was modified with poly-d-lysine (PDL) to improve its biocompatibility and endocytosis efficiency. After that, PDL-modified COF₂ (PDL@COF₂) was used as a novel fluorescence probe with a superior linear pH response over the range from 5.0 to 8.0 due to its fully reversible protonation and deprotonation. The fluorescent PDL@COF₂ was further employed as a good candidate for pH imaging in tumor cells and zebrafish. The as-constructed environment-sensitive fluorescent COFs have greatly expanded the applications of COFs in the biological area.

Metal-Stabilized Boronate Ester Cages

Molecular cages with arylboronate ester caps at the vertices are described. The cages were obtained by metal-templated polycondensation reactions of a tris(2-formylpyridine oxime) ligand with arylboronic acids. Suited templates are triflate or triflimide salts of Zn^II, Fe^II, Co^II, or Mn^II. In the products, the metal ions are coordinated internally to the pyridyl and oximato N atoms adjacent to the boronate ester, resulting in an improved hydrolytic stability of the latter. It is possible to decorate the cages with cyano or aldehyde groups using functionalized arylboronic acids. The aldehyde groups allow for a postsynthetic modification of the cages via an imine bond formation.

A Self-Assembled Cage for Wide-Scope Chiral Recognition in Water

Herein, we report the self-assembly of an anionic homochiral octahedral cage by condensing six Ga³⁺ cations and four trisacylhydrazone ligands. The robust nature of the hydrazone bond renders the cage stable in water, where it can take advantage of the hydrophobic effect for host-guest recognition. In addition to the internal binding site, namely, the inner cavity, the octahedral cage possesses four "windows", each of which represents an external binding site allowing peripheral complexation. These internal and external binding sites endow the cage with the capability to bind a broad range of guests whose sizes could either be smaller than or exceed the volume of the cage's inner cavity. Upon accommodation of a chiral guest, one of the two cage enantiomers becomes more favored than the other, producing circular-dichroism (CD) signals. The CD signal intensity of the cage is observed to be proportional to the ee value of the chiral guest, allowing a quantitative determination of the latter.

A Self-Assembled Homochiral Radical Cage with Paramagnetic Behaviors

Condensation of an inherently C₃ -symmetric polychlorotriphenylmethyl (PTM) radical trisaldehyde with tris(2-aminoethyl)amine (TREN) yields a [4+4] tetrahedral radical cage as a racemic pair of homochiral enantiomers in 75 % isolated yield. The structure was characterized by X-ray crystallography, confirming the homochirality of each cage framework. The homochirality results from intramolecular [CH⋅⋅⋅π] and hydrogen-bonding interactions within the cage framework. The four PTM radicals in a cage undergo weak through-space coupling. Magnetic measurements demonstrated that each cage bears 3.58 spins.

Self-Assembly of Lanthanide-Covalent Organic Polyhedra: Chameleonic Luminescence and Efficient Catalysis

A series of multinuclear lanthanide-covalent organic polyhedra (LnCOPs), including pillar-typed triangular prisms 1-Ln₃ and tetrahedra 2-Ln₄ (Ln = La^III, Sm^III, Eu^III), have been constructed for the first time, through either one-pot subcomponent self-assembly or postassembly metalation. In contrast to the known tetrahedral cages based on transition metals, the pillar-typed polyhedra were favored from the same organic components in the presence of lanthanides. Besides this, facile transmetalations between the 1-Ln₃ polyhedra endow cascade chameleonic luminescence. Meanwhile, the open metal sites and pendent amine groups on 1-Ln₃ enable these polyhedra to catalyze the Henry reaction efficiently.

Selective Formation of S4- and T-Symmetric Supramolecular Tetrahedral Cages and Helicates in Polar Media Assembled via Cooperative Action of Coordination and Hydrogen Bonds

We report on the synthesis and self-assembly study of novel supramolecular monomers encompassing quadruple hydrogen-bonding motifs and metal-coordinating 2,2'-bipyridine units. When mixed with metal ions such as Fe²⁺ or Zn²⁺, the tetrahedron cage complexes are formed in quantitative yields and full diastereoselectivity, even in highly polar acetonitrile or methanol solvents. The symmetry of the complexes obtained has been shown to depend critically on the flexibility of the ligand. Restriction of the rotation of the hydrogen-bonding unit with respect to the metal-coordinating site results in a T-symmetric cage, whereas introducing flexibility either through a methylene linker or rotating benzene ring allows the formation of S₄-symmetric cages with self-filled interior. In addition, the possibility to select between tetrahedral cages or helicates and to control the dimensions of the aggregate has been demonstrated with a three-component assembly using external hydrogen-bonding molecular inserts or by varying the radius of the metal ion (Hg²⁺ vs Fe²⁺). Self-sorting studies of individual Fe²⁺ complexes with ligands of different sizes revealed their inertness toward ligand scrambling.

Chirality transcription in the anion-coordination-driven assembly of tetrahedral cages

Enantiopure A₄L₄ tetrahedral cages were obtained through chirality transfer in the anion-coordination-driven assembly (ACDA) of chiral C₃-symmetric tris-bis(urea) ligands with phosphate.

A Highly Luminescent Chiral Tetrahedral Eu4L4(L')4 Cage: Chirality Induction, Chirality Memory, and Circularly Polarized Luminescence

Chiral lanthanide cages with circularly polarized luminescence (CPL) properties have found potential application in enantioselective guest recognition and sensing. However, it still remains a big challenge to develop a simple and robust method for the diastereoselective assembly of homochiral lanthanide cages in view of the large lability of the Ln(III) ions. Herein, we report the first example of the formation of a enantiopure lanthanide tetrahedral cage via a chiral ancillary ligand induction strategy. One such cage, (Eu₄L₄)(R/S-BINAPO)₄, is assembled by four achiral C₃-symmeric tris(β-diketones) (4,4',4″-tris(4,4,4-trifluoro-1,3-dioxobutyl)triphenylamine, L) as faces, four Eu(III) ions as vertices and four chiral R-/S-bis(diphenylphosphoryl)-1,1'-binaphthyl (R/S-BINAPO) as ancillary ligands. X-ray crystallography and NMR and CD spectra confirm the formation of a pair of enantiopure chiral topological tetrahedral cages, (Eu₄L₄)(R-BINAPO)₄ and (Eu₄L₄)(S-BINAPO)₄ (ΔΔΔΔ-1 and ΛΛΛΛ-1). As expected, the tetrahedral cages present strong CPL with |g_lum| values up to 0.20, while they unexpectedly give ultrahigh luminescent quantum yields (QYs) of up to 81%, the highest value reported in chiral Ln(III) complexes. More impressively, the chiral memory effect for a lanthanide-based assembly is observed for the first time. The chirality of the original cage 1 framework is retained after R/S-BINAPO is replaced by the achiral bis[2-(diphenylphosphino)phenyl] ether oxide (DPEPO), and thus another pair of enantiopure Eu(III) tetrahedral cages, ΔΔΔΔ- and ΛΛΛΛ-[(Eu₄L₄)(DPEPO)₄] (ΔΔΔΔ-2 and ΛΛΛΛ-2), have been isolated. Encouragingly, cage 2 also presents an impressive luminescence quantum yield (QY = 68%) and intense CPL (|g_lum| = 0.11). This study offers a simple and low-cost synthesis strategy for the preparation of lanthanide cages with CPL properties.

Self-Sorting Assembly of Molecular Trefoil Knots of Single Handedness

We report on the stereoselective synthesis of trefoil knots of single topological handedness in up to 90% yield (over two steps) through the formation of trimeric circular helicates from ligand strands containing either imine or, unexpectedly, amide chelating units and metal ion templates of the appropriate coordination character (zinc(II) for imines; cobalt(III) for amides). The coordination stereochemistry of the octahedral metal complexes is determined by asymmetric carbon centers in the strands, ultimately translating into trefoil knots that are a single enantiomer, both physically and in terms of their fundamental topology. Both the imine-zinc and amide-cobalt systems display self-sorting behavior, with racemic ligands forming knots that individually contain only building blocks of the same chirality. The knots and the corresponding trimeric circular helicate intermediates (Zn(II)₃ complex for the imine ligands; Co(III)₃ complex for the amide ligands) were characterized by nuclear magnetic resonance spectroscopy, mass spectrometry, and X-ray crystallography. The latter confirms the trefoil knots as 84-membered macrocycles, with each of the metal ions sited at a crossing point for three regions of the strand. The stereochemistry of the octahedral coordination centers imparts alternating crossings of the same handedness within each circular helicate. The expression of chirality of the knotted molecules was probed by circular dichroism: The topological handedness of the demetalated knots was found to have a greater effect on the CD response than the Euclidean chirality of an individual chiral center.

Substituent-controlled racemization of dissymmetric coordination capsules

We report the effect of substituents (methyl, isopropyl, methoxy, and methoxyphenyl) at the 6'-position of the 2,2'-bipyridyl arms on the racemization of dissymmetric coordination capsules 1a-d. When the capsules included (R)-4,4'-diacetoxy-2,2'-benzyloxycarboxyl-biphenyl ((R)-3), the (M)-helical conformer was enriched with a diastereomeric excess (de%) of >98% for 1a, 31% for 1b, 81% for 1c and 75% for 1d. The entrapped guests in 1a, 1c and 1d can be removed by washing the solid containing the host-guest complexes with diethyl ether. The rate of racemization in THF follows the order of 1c > 1d ≫ 1a. X-ray crystal structural analysis and density functional theory calculation of model complex 4c indicate a distorted tetrahedral coordination of the Cu(i) center, and UV-vis absorption spectroscopy indicates similar coordination environments in 1c and 4c. A series of experiments demonstrates that the racemization rate depends on the dihedral angles of the bipyridyl arms, and the angles are regulated by the substituents. The methoxy and methoxyphenyl substituents in 1c and 1d enlarge the dihedral angles of the bipyridyl arms. This facilitates the access of solvent molecules to the Cu(i) centers and promotes racemization. The slower racemization of 1d can be ascribed to the steric protection of the Cu(i) centers from incoming solvent molecules by the p-methoxyphenyl group.

Enantiopure [Cs+/Xe⊂Cryptophane]⊂FeII4L4 Hierarchical Superstructures

Hierarchically nested hosts offer new opportunities to control the guest binding of the inner host, functionalize the cavity of the outer host, and investigate communication between different layers. Here we report a self-assembled triazatruxene-based Fe^II₄L₄ capsule, which was able to encapsulate a covalent cage, cryptophane-111 (CRY). The resulting cage-in-cage complex was capable of accommodating a cesium cation or xenon atom with altered guest binding behavior compared to the CRY alone. A crystal structure of the Russian doll complex [Cs⁺⊂CRY]⊂Fe^II₄L₄ unambiguously demonstrated the unusual encapsulation of a cation within a capsule bearing a 8+ charge. Moreover, the binding of enantiopure CRY occurred with high enantioselectivity (530-fold) between the two enantiomers of the tetrahedron. This discrimination resulted in stereochemical information transfer from the inner covalent cage to the outer self-assembled capsule, leading to the formation of enantiopure [guest⊂cage]⊂cage complexes. The stereochemistry of the tetrahedron persisted even after displacement of CRY with an achiral guest.

Diastereoselective Control of Tetraphenylethene Reactivity by Metal Template Self-Assembly

The reaction of 4,4',4'',4'''-(ethene-1,1,2,2-tetrayl)tetraaniline with 2-pyridinecarboxaldehyde and iron(II) chloride resulted, after aqueous workup, in the diastereoselective formation of an [Fe₂ L₃ ]⁴⁺ triple-stranded helicate structure, irrespective of the stoichiometry employed. The helicate structure was characterized in solution by multinuclear NMR spectroscopy, and in the solid state by single-crystal X-ray crystallography. The reaction of iron(II) tetrafluoroborate or iron(II) bistriflimide with the tetraaniline and 2-pyridinecarboxaldehyde allowed the formation of an [Fe₈ L₆ ]¹⁶⁺ cube when the appropriate stoichiometry was used, but these structures were unstable with respect to hydrolysis. The pendant amine groups on the helicate can be functionalized by reaction with acid chlorides or anhydrides, and the resulting functionalized tetraphenylethene (TPE) units were isolated by the reaction of the helicate with tris(2-aminoethyl)amine. The emission properties of the TPE units were studied in THF/water mixtures, and they were found by dynamic light scattering to self-assemble into large (av. diameter 250 nm) structures.

Mechanistic Interplay between Light Switching and Guest Binding in Photochromic [Pd2Dithienylethene4] Coordination Cages

Photochromic [Pd₂L₄] coordination cages based on dithienylethene (DTE) ligands L allow triggering guest uptake and release by irradiation with light of different wavelengths. The process involves four consecutive electrocyclic reactions to convert all chromophores between their open and closed photoisomeric forms. So far, guest affinity of the fully switched species was elucidated, but mechanistic details concerning the intermediate steps remained elusive. Now, a new member of the DTE cage family allows unprecedented insight into the interplay between photoisomerization steps and guest location inside/outside the cavity. Therefore, the intrinsic chirality of the DTE backbones was used as reporter for monitoring the fate of a chiral guest. In its "open" photoisomeric form ( o-L, [Pd₂( o-L)₄] = o-C), the C₂-symmetric DTE chromophore quickly converts between energetically degenerate P and M helical conformations. After binding homochiral 1 R-( -) or 1 S-( +) camphor sulfonate ( R-CSA or S-CSA), guest-to-host chirality transfer was observed via a circular dichroism (CD) signal for the cage-centered absorption. Irradiating the R/S-CSA@ o-C host-guest complexes at 313 nm produced configurationally stable "closed" photoisomers, thus locking the induced chirality with an enantiomeric excess close to 25%. This value (corresponding to chiral induction for one out of four ligands), together with DOSY NMR, ion mobility mass spectrometry, and X-ray structure results, shows that closure of the first photoswitch is sufficient to expel the guest from the cavity.

Synthesis and Characterization of Self-Assembled Chiral FeII 2 L3 Cages

We present here the synthesis of chiral BINOL-derived (BINOL=1,1'-bi-2-naphthol) bisamine and bispyridine-aldehyde building blocks that can be used for the self-assembly of novel chiral FeII 2 L3 cages when mixed with an iron(II) precursor. The properties of a series of chiral cages were studied by NMR and circular dichroism (CD) spectroscopy, cold-spray ionization MS, and molecular modeling. Upon formation of the M2 L3 cages, the iron corners can adopt various isomeric forms: mer, fac-Δ, or fac-Λ. We found that the coordination geometry around the metal centers in R-Cages 1 and 2 were influenced by the chiral BINOL backbone only to a limited extent, as a mixture of cages was formed with fac and mer configurations at the iron corners. However, single cage species (fac-RR-Cage and fac-RS-Cage) that are enantiopure and highly symmetric were obtained by generating these chiral M2 L3 cages by using the bispyridine-aldehyde building blocks in combination with chiral amine moieties to form pyridylimine ligands for coordination to iron. Next to consistent NMR spectra, the CD spectra confirm the configurations fac-(Λ,Λ) and fac-(Δ,Δ) corresponding to RR- and RS-Cage, respectively.

Interconversion of molecular face-rotating polyhedra through turning inside out

We report the post-synthesis interconversion of two enantiomeric organic cages through turning inside out. By scrutinizing the thermodynamics and kinetics, we are able to control the racemization rate by various reaction conditions and reveal that the turning-inside-out interconversion is realized through a partial disassembly pathway. The kinetics investigation also provides insight into the dynamic essence of imine chemistry using different solvents and catalysts.

Elucidation of the origin of chiral amplification in discrete molecular polyhedra

Chiral amplification in molecular self-assembly has profound impact on the recognition and separation of chiroptical materials, biomolecules, and pharmaceuticals. An understanding of how to control this phenomenon is nonetheless restricted by the structural complexity in multicomponent self-assembling systems. Here, we create chiral octahedra incorporating a combination of chiral and achiral vertices and show that their discrete nature makes these octahedra an ideal platform for in-depth investigation of chiral transfer. Through the construction of dynamic combinatorial libraries, the unique possibility to separate and characterise each individual assembly type, density functional theory calculations, and a theoretical equilibrium model, we elucidate that a single chiral unit suffices to control all other units in an octahedron and how this local amplification combined with the distribution of distinct assembly types culminates in the observed overall chiral amplification in the system. Our combined experimental and theoretical strategy can be applied generally to quantify discrete multi-component self-assembling systems.