A Tetraferrocenyl‐Resorcinarene Cavitand as a Redox‐Switchable Host of Ammonium Salts

Tetraannulation of a resorcinarene‐octaamino cavitand with ferrocenecarboxaldehyde allows the preparation of a tetrabenzimidazole‐resorcinarene cavitand with four ferrocenyl moieties directly linked to the C2 atom of the imidazole units. Oxidation of the four ferrocenyl moieties produces important structural modifications of the molecule, as indicated by DFT calculations performed for the neutral and tetraoxidized forms of the cavitand. By means of ¹H NMR spectroscopic analysis, the encapsulating properties of the new tetraferrocenyl‐resorcinarene cavitand toward a series of ammonium salts were evaluated, and a clear cutoff point in binding affinity with respect to size was observed. Cyclic voltammetric studies allowed us to estimate the relative association constants for the neutral and oxidized forms of the cavitand, thus indicating that the guest was bound to the neutral (reduced) state of the cavitand and was released from the oxidized form. These redox‐addressable conformational and binding properties of the resorcinarene‐tetraferrocenyl cavitand constitute all the necessary features of a redox‐switchable molecular gripper. By means of mass‐spectrometric analysis, we could unambiguously confirm the molar stoichiometry of the host–guest complex (1:1) and assess the strong guest encapsulation, as indicated by triggering the covalent coupling between host and guest in the gas phase.     

Rim,Side Arms,and Cavity: Three Sites for the Recognition of Anions by Tetraazolium Resorcinarene Cavitands

Two tetrabenzoimidazolium‐resorcinarene cavitands were prepared and used for the recognition of chloride, bromide, iodide, cyanide, nitrate, perchlorate, hexanoate, benzenesulfonate, and p‐toluenesulfonate. Binding affinities of the two cavitands were determined by ¹H NMR titration and computational analysis. The observed spectral changes were related to specific interaction sites, which were supported by the computational studies. In the case of the C2?H tetrabenzoimidazolium‐resorcinarene, the recognition region of the inorganic anions and hexanoate was located at the rim of the cavitand, although chloride and bromide also interacted with the aromatic C?H bonds located between adjacent arms of the cavitand. By contrast, the recognition of the two anions with an aromatic ring (benzenesulfonate and p‐toluenesulfonate) results from encapsulation of the aromatic part of the anions inside the hydrophobic cavity of the host. In the case of the C2?Me tetrabenzoimizazolium‐resorcinarene receptor, the ability of the molecule to bind all inorganic anions and hexanoate was suppressed, but the receptor maintained its ability to strongly bind benzenesulfonate and p‐toluenesulfonate. This is interpreted in terms of suppression of the ability of the cavitand to form hydrogen bonds at the rim of the molecule due to replacement of the C2?H proton by a methyl group, while the hydrophobic pocket of the molecule maintains its binding abilities.     

Triptycene‐Derived Calix[6]arenes: Synthesis,Structures, and Their Complexation with Fullerenes C60 and C70

Two pairs of novel triptycene‐derived calix[6]arenes 4 a , b and 5 a , b have been efficiently synthesized through both one‐pot and two‐step fragment‐coupling strategies starting from 2,7‐bis(hydroxymethyl)‐1,8‐dimethoxytriptycene 1 . Subsequent demethylation of 4 a , b and 5 a , b with BBr₃ in dry dichloromethane gave the macrocyclic compounds 6 a , b and 7 a , b . Treatment of either 4 a or 6 a with AlCl₃ resulted in the same debutylated product 8 , while 9 was similarly obtained from either 5 a or 7 a . Structural studies revealed that all of the macrocycles have well‐defined structures with fixed conformations both in solution and in the solid state owing to the introduction of the triptycene moiety with a rigid three‐dimensional (3D) structure, making them very different from their classical calix[6]arene counterparts. As a consequence, it was found that all of these the triptycene‐derived calix[6]arenes could encapsulate small neutral molecules in their cavities in the solid state. Moreover, it was also found that the macrocycles 4 b and 5 b showed highly efficient complexation abilities toward fullerenes C₆₀ and C₇₀, forming 1:1 complexes with association constants ranging from (5.22±0.20)×10⁴ to (8.68±0.30)×10⁴ M ^?1.     

Ion‐Pair Complexation with a Cavitand Receptor

The capability of resorcinarenes to bind anions within the alkyl feet at the lower rim has been exploited as the starting point for developing a new cavitand able to engulf contact ion pairs of primary ammonium salts in chlorinated solvents with association constants (K_ass) in the range of 10³–10⁴ M ^?1. Methylene bridges were introduced into the upper rim to freeze the resorcinarene in the cone conformation with the four H_down protons converging in the lower pocket, thereby maximizing the CH–anion interactions responsible for the anion binding. Four additional phosphate moieties were introduced into the lower rim in close proximity to the anionic site to provide hydrogen‐bonding‐acceptor P?O groups and promote cation complexation at the bottom of the cavitand. The binding ability of the synthesized ligands was analyzed by ¹H NMR spectroscopy and, when possible, by isothermal titration calorimetry (ITC); the data were in agreement when complementary techniques were used.     

A Supramolecular Vesicle Based on the Complexation of p‐Sulfonatocalixarene with Protamine and its Trypsin‐Triggered Controllable‐Release Properties

Enzyme‐responsive assembly represents one of the increasingly significant topics in biomaterials research and finds feasible applications to the controlled release of therapeutic agents at specific sites at which the target enzymes are located. In this work, based on the concept of host–guest chemistry, a trypsin‐responsive supramolecular vesicle using p‐sulfonatocalix[4]arene as the macrocyclic host and natural serine protease trypsin‐cleavable cationic protein protamine as the guest molecule, is reported. The complexation of p‐sulfonatocalix[4]arene with protamine directs the formation of a supramolecular binary vesicle, which is dissipated by trypsin with high selectivity. Therefore, the present system represents a principle‐of‐concept to build a controlled‐release carrier at trypsin‐overexpressed sites.     

Triptycene‐Roofed Quinoxaline Cavitands for the Supramolecular Detection of BTEX in Air

Two novel triptycene quinoxaline cavitands ( DiTriptyQxCav and MonoTriptyQxCav ) have been designed, synthesized, and applied in the supramolecular detection of benzene, toluene, ethylbenzene, and xylenes (BTEX) in air. The complexation properties of the two cavitands towards aromatics in the solid state are strengthened by the presence of the triptycene moieties at the upper rim of the tetraquinoxaline walls, promoting the confinement of the aromatic hydrocarbons within the cavity. The two cavitands were used as fiber coatings for solid‐phase microextraction (SPME) BTEX monitoring in air. The best performances in terms of enrichment factors, selectivity, and LOD (limit of detection) values were obtained by using the DiTriptyQxCav coating. The corresponding SPME fiber was successfully tested under real urban monitoring conditions, outperforming the commercial divinylbenzene–Carboxen–polydimethylsiloxane (DVB–CAR–PDMS) fiber in BTEX adsorption.     

Tunable Nanosupramolecular Aggregates Mediated by Host–Guest Complexation

Nanosupramolecular assemblies with controlled topological features have inventive applications in fundamental studies and material manufacturing. Herein, a variety of morphologically interesting aggregates have been constructed using the supramolecular modulation with bipyridinium‐modified diphenylalanine derivative (BP‐FF). Benefiting from the high binding affinity of bipyridinium group with four different macrocyclic receptors, namely cucurbit[7]uril, cucurbit[8]uril, pillar[5]arene, and tetrasulfonated crown ether, we have succeeded in tuning the topological aggregates of BP‐FF from fine nanofibers to nanorods, octahedron‐like nanostructure, helical nanowires, and rectangular nanosheets without any tedious chemical modification. This supramolecular approach may provide us a powerful method to construct well‐defined nanostructures with different morphologies that can be conveniently controlled by facile host–guest interactions.     

A Boiling‐Water‐Stable,Tunable White‐Emitting Metal–Organic Framework from Soft‐Imprint Synthesis

A new avenue for making porous frameworks has been developed by borrowing an idea from molecularly imprinted polymers (MIPs). In lieu of the small molecules commonly used as templates in MIPs, soft metal components, such as CuI, are used to orient the molecular linker and to leverage the formation of the network. Specifically, a linear dicarboxylate linker with thioether side groups reacted simultaneously with Ln³⁺ ions and CuI, leading to a bimetallic net featuring strong, chemically hard Eu³⁺–carboxylate links, as well as soft, thioether‐bound Cu₂I₂ clusters. The CuI block imparts water stability to the host; with the tunable luminescence from the lanthanide ions, this creates the first white‐emitting MOF that is stable in boiling water. The Cu₂I₂ block also readily reacts with H₂S, and enables sensitive colorimetric detection while the host net remains intact.     

Supramolecular Complexation of Biological Phosphates with an Acyclic Triazolium‐Linked Anthracenyl‐1,3‐Diconjugate of Calix[4]Arene: Synthesis,Characterization, Spectroscopy,Microscopy, and Computational Studies

A triazolium‐anthracenyl calix[4]arene conjugate ( L ) was synthesized by methylating the precursor triazole derivative and then characterized. The potential of the cationic L to differentiate nucleoside triphosphates (NTPs) from their mono‐ and diphosphates was demonstrated. Due to its unique combination of arms with the calix‐platform, a fluorescence enhancement was observed for L with all the NTPs, whereas there is no report with such enhancement being exhibited in case of all the NTPs. This has been supported by the aggregation of L observed from microscopy. Selectivity of L towards NTPs over other phosphates was a result of specific weak interactions, namely, ion–ion, hydrogen bonding and π ??? π, present in the 1:2 complex of L and NTPs (based on ESI MS), which were absent in their congener‐phosphates as delineated by NMR and computational studies. Thus, L stands as a unique receptor for NTPs.     

Self‐Assembled Hexanuclear Organometallic Cages: Synthesis,Characterization, and Host–Guest Properties

A series of iridium‐ and rhodium‐based hexanuclear organometallic cages containing 2,5‐dichloro‐3,6‐dihydroxy‐1,4‐benzoquinone, 9,10‐dihydroxy‐1,4‐anthraquinone, and 6,11‐dihydroxynaphthacene‐5,12‐dione ligands were synthesized from the self‐assembly of the corresponding molecular “clips” and 2,4,6‐tri(4‐pyridyl)‐1,3,5‐triazine ligands in good yields. These organometallic cages can form inclusion systems with a wide variety of π‐donor substrates, including coronene, pyrene, [Pt(acac)₂], and hexamethoxytriphenylene. The 1:1 complexation of the resulting supramolecular assemblies was confirmed by ¹H NMR spectroscopy. Large complexation shifts (Δδ>1 ppm) were observed in the ¹H NMR spectra of guests in the presence of cage [Cp*₆M₆(μ‐DHNA)₃(tpt)₂](OTf)₆ ( 6a ; M=Ir, tpt=2,4,6‐tri(4‐pyridyl)‐1,3,5‐triazine). The formation of discrete 1:1 donor–acceptor complexes, pyrene ?6 b (M=Rh), coronene ?6 a , coronene ?6 b , and [Pt(acac)₂] ?6 a was confirmed by their single‐crystal X‐ray analyses. In these systems, the most important driving force for the formation of guest–host complexes is clearly the donor–acceptor π???π stacking interaction, including charge‐transfer interactions between the electron‐donating and electron‐accepting aromatic components. These structures provide compelling evidence for the existence of strong attractive forces between the electron‐deficient triazine core and electron‐rich guest. The results presented here may provide useful guidance for designing artificial receptors for functional biomolecules.     

Host–Guest Complexation of Amphiphilic Molecules at the Air–Water Interface Prevents Oxidation by Hydroxyl Radicals and Singlet Oxygen

The oxidation of antioxidants by oxidizers imposes great challenges to both living organisms and the food industry. Here we show that the host–guest complexation of the carefully designed, positively charged, amphiphilic guanidinocalix[5]arene pentadodecyl ether (GC5A‐12C) and negatively charged oleic acid (OA), a well‐known cell membrane antioxidant, prevents the oxidation of the complex monolayers at the air–water interface from two potent oxidizers hydroxyl radicals (OH) and singlet delta oxygen (SDO). OH is generated from the gas phase and attacks from the top of the monolayer, while SDO is generated inside the monolayer and attacks amphiphiles from a lateral direction. Field‐induced droplet ionization mass spectrometry results have demonstrated that the host–guest complexation achieves steric shielding and prevents both types of oxidation as a result of the tight and “sleeved in” physical arrangement, rather than the chemical reactivity, of the complexes.     

Metal–Organic Cyclohelicates as Optical Receptors for Glutathione: Syntheses,Structures, and Host–Guest Behaviors

Two trinuclear zinc‐based cyclohelicates, Zn–PDB (PDB=[5‐(dibenzylamino)‐N1′,N3′‐bis(pyridin‐2‐ylmethylene)isophthalohydrazide]) and Zn–PMB (PMB=[5‐(bodipy‐oxy)‐N1′,N3′‐bis(pyridin‐2‐ylmethylene)isophthalohydrazide]) containing dibenzylamino and BODIPY groups, respectively, were generated by incorporating two amide‐containing tridentate chelators into meta‐positions of a substituted phenyl ring. Single‐crystal structure analysis and related spectroscopic characterizations demonstrated the formation of macrocyclic helicals both in the solid state and in solution. The host–guest behavior of the cyclohelical hosts towards γ‐glutamyl‐cysteinyl‐glycine (GSH) and its component amino acids was investigated by spectroscopic titrations. UV/Vis absorption titration and NMR titrations of Zn–PDB and Zn–PMB upon addition of the above‐mentioned guests suggested that the Glu residue of GSH was positioned within the cavity. The COO⁻ groups interacted with metal ions through static interactions. The Cys moiety of GSH interacted with the amide groups sited in host molecules through hydrogen‐bonding interactions to produce measurable spectral changes. Fluorescent titrations of Zn–PMB upon the addition of GSH and ESI‐MS investigations of the titration solutions confirmed the host–guest interaction modes and revealed the possible 1:1 complexation stoichiometry. These results showed that the recognition of a substrate within the cavity of functionalized metal–organic cage‐like receptors could be a useful method to produce supramolecular sensors for biomolecules.     

Exploiting Cavities in Supramolecular Gels

Endowing supramolecular gelators with cavities opens up a number of opportunities not possible with other gel systems. The well‐established host–guest chemistry of cavitands can be utilized to build up and break down gel structures, introduce responsive functionalities, or enhance selectivity in applications such as catalysis and extraction. Cavity‐containing gelators provide an excellent case study for how different aspects of supramolecular chemistry can be used intelligently to create responsive materials.     

Self‐Assembled Boronic Ester Cavitand Capsules with Various Bis(catechol) Linkers: Cavity‐Expanded and Chiral Capsules

Two molecules of cavitand tetraboronic acid and four molecules of various bis(catechol) linkers self‐assemble into capsules through the formation of eight dynamic boronic ester bonds. Each capsule has a different cavity size depending on the linker used, and shows particular guest encapsulation selectivity. A chiral capsule made up of the cavitand and a chiral bis(catechol) linker was also constructed. This capsule induces supramolecular chirality with respect to a prochiral biphenyl guest by diastereomeric encapsulation through the asymmetric suppression of rotation around the axis of the prochiral biphenyl moiety.