Highly Efficient Chirality Transfer from Diamines Encapsulated within a Self‐Assembled Calixarene–Salen Host

A calix[4]arene host equipped with two bis‐[Zn(salphen)] complexes self‐assembles into a capsular complex in the presence of a chiral diamine guest with an unexpected 2:1 ratio between the host and the guest. Effective chirality transfer from the diamine to the calix–salen hybrid host is observed by circular dichroism (CD) spectroscopy, and a high stability constant K_2,1 of 1.59×10¹¹ M ^?2 for the assembled host–guest ensemble has been determined with a substantial cooperativity factor α of 6.4. Density functional calculations are used to investigate the origin of the stability of the host–guest system and the experimental CD spectrum compared with those calculated for both possible diastereoisomers showing that the M,M isomer is the one that is preferentially formed. The current system holds promise for the chirality determination of diamines, as evidenced by the investigated substrate scope and the linear relationship between the ee of the diamine and the amplitude of the observed Cotton effects.     

Thermodynamics of Hydrophobic Interactions: Entropic Recognition of a Hydrophobic Moiety by Poly(Ethylene Oxide)–Zinc Porphyrin Conjugates

The recognition of 4‐alkylpyridines by water‐soluble poly(ethylene oxide)–zinc porphyrin conjugates was studied with a focus on the thermodynamic parameters of binding. Microcalorimetric studies indicated that binding of the alkyl group of the guest in water is driven by the entropic term (δΔH⁰=ΔH⁰(4‐pentylpyridine)? ΔH⁰(4‐methylpyridine)=+1.7 kJ mol^?1, δTΔS⁰=TΔS⁰(4‐pentylpyridine)? TΔS⁰(4‐methylpyridine)=+11.8 kJ mol^?1 at 298 K), thus showing the significance of water reorganization during host–guest interaction. The enthalpy–entropy compensation temperature of binding of 4‐alkylpyridines was as low as 38 K; only below this temperature could the enthalpic term be a driving force. The binding affinity was modulated by the addition of cations and by varying the degree of polymerization of poly(ethylene oxide), which suggests that guest binding is coupled with polymer conformation.     

Host–Guest Interactions of Phosphorescent Molecular Tweezers Based on an Alkynylplatinum(II) Terpyridine System with Polyaromatic Hydrocarbons

Host–guest interactions of a molecular tweezer complex 1 with various planar organic molecules including polyaromatic hydrocarbons (PAHs) were investigated by 1D and 2D ¹H NMR spectroscopy, UV/Vis absorption and emission titration studies. 2D and DOSY NMR spectroscopies support the sandwiched binding mode based on 1:1 host–guest interactions. The binding constants (K_S) of complex 1 for various PAHs were determined by NMR titration studies and the values were found to span up to an order of 10⁴ M ^?1 for coronene to no observable interaction for benzene, indicating that the π‐surface area is important for such host–guest interactions. The substituent effect on the host–guest interaction based on the guest series of 9‐substituted anthracenes was also studied. In general, a stronger interaction was observed for the anthracene guest with electron‐donating groups, although steric and π‐conjugation factors cannot be completely excluded. The photophysical responses of complex 1 upon addition of various PAHs were measured by UV/Vis and emission titration studies. The UV/Vis absorption spectra were found to show a drop in absorbance of the metal‐to‐ligand charge‐transfer (MLCT) and ligand‐to‐ligand charge‐transfer (LLCT) admixture band upon addition of various guest molecules to 1 , whereas the emission behavior was found to change differently depending on the guest molecules, showing emission enhancement and/or quenching. It was found that emission quenching occurred either via energy transfer or electron transfer pathway or both, while emission enhancement was caused by the increase in rigidity of complex 1 as a result of host–guest interaction.     

The structural and dynamic properties of 1‐bromodecane in urea inclusion compounds investigated by solid‐state 1H, 13C and 2H NMR spectroscopy

For asymmetric guest molecules in urea, the end‐groups of two adjacent guest molecules may arrange in three different ways: head–head, head–tail and tail–tail. Solid‐state ¹H and ¹³C NMR spectroscopy is used to study the structural properties of 1‐bromodecane in urea. It is found that the end groups of the guest molecules are randomly arranged. The dynamic characteristics of 1‐bromodecane in urea inclusion compounds are probed by variable‐temperature solid‐state ²H NMR spectroscopy (line shapes, spin–spin relaxation: T₂, spin‐lattice relaxation: T_1Z and T_1Q) between 120 K and room temperature. The comparison between the simulation and experimental data shows that the dynamic properties of the guest molecules can be described in a quantitative way using a non‐degenerate three‐site jump process in the low‐temperature phase and a degenerate three‐site jump in the high‐temperature phase, in combination with the small‐angle wobbling motion. The kinetic parameters can be derived from the simulation. Copyright © 2011 John Wiley & Sons, Ltd.     

Double targeting of tumours with pyrenyl-modified dendrimers encapsulated in an arene-ruthenium metallaprism

The self‐assembly of 2,4,6‐tris(pyridin‐4‐yl)‐1,3,5‐triazine (tpt) triangular panels with p‐cymene–ruthenium building blocks and 5,8‐dioxido‐1,4‐naphthoquinonato (donq) bridges, in the presence of pyrenyl‐containing dendrimers of different generations (P₀, P₁ and P₂), affords the triangular prismatic host–guest compounds [P_n?Ru₆(p‐cymene)₆(tpt)₂(donq)₃]⁶⁺ ([P_n? 1 ]⁶⁺). The host–guest nature of these systems, with the pyrenyl moiety being encapsulated in the hydrophobic cavity of the cage and the dendritic functional group pointing outwards, was confirmed by NMR spectroscopy (¹H, 2D and DOSY). The host–guest properties of these systems were studied in solution by NMR and UV/Vis spectroscopic methods, allowing the determination of their affinity constants (K_a). Moreover, the ability of these water‐soluble host–guest systems to carry the pyrenyl‐containing dendrimers into cancer cells was evaluated on human ovarian cancer cells. The host–guest systems are all more cytotoxic than the empty cage [ 1 ][CF₃SO₃]₆ (IC₅₀≈4 μM ), with the most active compound, [P₀? 1 ][CF₃SO₃]₆, being an order of magnitude more cytotoxic.     

Evaluation of Weak Interactions in [2]Pseudorotaxanes

Viologens readily thread bis‐p‐phenylene crown ethers to form [2]pseudorotaxanes. However, the binding of sterically hindered 3,3′‐dimethylviologens is very weak. Density functional theory (DFT) calculations indicated that the additional energy cost of “flattening” is substantial, 55 kJ mol^?1, and prevents the formation of a stable host–guest complex. The structures of [2]pseudorotaxanes determined by X‐ray crystallography are in good agreement with the NMR characterisation and DFT results. Their association constants and thermodynamic parameters in solution were measured by using a dilution method and, for the first time, by host–guest nuclear Overhauser effect (NOE) correlations. The NOE approach was subsequently applied to study the sterically hindered analogues and it was shown that the binding in 3,3′‐dimethyl‐N,N‐dibenzyl [2]pseudorotaxane is by 8.5 kJ mol^?1 weaker than in its regular analogue. The proposed technique helps to quantify weak interactions in [2]pseudorotaxanes and can be applied to other host‐guest complexes.     

Cucurbituril‐Modulated Supramolecular Assemblies: From Cyclic Oligomers to Linear Polymers

Employing bis(p‐sulfonatocalix[4]arenes) (bisSC4A) and N′,N′′hexamethylenebis(1‐methyl‐4,4′‐bipyridinium) (HBV⁴⁺) as monomer building blocks, the assembly morphologies can be modulated by cucurbit[n]uril (CB[n]) (n=7, 8), achieving the interesting topological conversion from cyclic oligomers to linear polymers. The binary supramolecular assembly fabricated by HBV⁴⁺ and bisSC4A units, forms an oligomeric structure, which was characterized by NMR spectroscopy, atomic force microscopy (AFM), transmission electron microscopy (TEM), dynamic light scattering (DLS), isothermal titration calorimetry (ITC), and gel permeation chromatography (GPC) experiments. The ternary supramolecular polymer participated by CB[8] is constructed on the basis of host–guest interactions by bisSC4A and the [2]pseudorotaxane HBV⁴⁺@CB[8], which is characterized by means of AFM, DLS, NMR spectroscopy, thermogravimetric analysis (TGA), UV/Vis spectroscopy, and elemental analysis. CB[n] plays vital roles in rigidifying the conformation of HBV⁴⁺, and reinforcing the host–guest inclusion of bisSC4A with HBV⁴⁺, which prompts the formation of a linear polymer. Moreover, the CB[8]‐participated ternary assembly could disassemble into the molecular loop HBV²⁺@CB[8] and free bisSC4A after reduction of HBV⁴⁺ to HBV²⁺, whereas the CB[7]‐based assembly remained unchanged after the reduction. CB[8] not only controlled the topological conversion of the supramolecular assemblies, but also improved the redox‐responsive assembly/disassembly property practically.     

Synthesis of Doubly Ethyl‐Bridged Bis(p‐sulfonatocalix[4]arene) and Its Supramolecular Polymerization with Viologen Dimer

A water‐soluble supramolecular polymer with a high degree of polymerization and viscosity has been constructed based on the strong host–guest interaction between p‐sulfonatocalix[4]arenes (SC4As) and viologen. A homoditopic doubly ethyl‐bridged bis(p‐sulfonatocalix[4]arene) (d‐SC4A) was prepared and its binding behavior towards methyl viologen compared with the singly ethyl‐bridged bis(p‐sulfonatocalix[4]arene) (s‐SC4A) by NMR spectroscopy and isothermal titration calorimetry. By employing a viologen dimer (bisMV⁴⁺) as the homoditopic guest, two linear AA/BB‐type supramolecular polymers, d‐SC4A?bisMV⁴⁺ and s‐SC4A?bisMV⁴⁺, were successfully constructed. Compared with s‐SC4A?bisMV⁴⁺, d‐SC4A?bisMV⁴⁺ shows much higher solubility and viscosity, and has also been characterized by viscosity, diffusion‐ordered NMR spectroscopy, dynamic light scattering, and atomic force microscopy measurements. Furthermore, the polymer is responsive to electrostimulus as viologen is electroactive, which was studied by cyclic voltammetry. This study represents a proof‐of‐principle as the polymer can potentially be applied as a self‐healing and degradable polymeric material.     

Encapsulation Enhanced Dimerization of a Series of 4‐Aryl‐N‐Methylpyridinium Derivatives in Water: New Building Blocks for Self‐Assembly in Aqueous Media

The construction of supramolecular systems in aqueous media is still a great challenge owing to the limited sources of building blocks. In this study, a series of 4‐aryl‐N‐methylpyridinium derivatives have been synthesized. They formed very stable host–guest (1:2) complexes with CB[8] in water (binding constants up to 10¹⁴ M ^?2) with the two guest molecules arranged in a head‐to‐tail manner and the complexes showed high thermostability, which was revealed by ¹H NMR and UV/Vis spectroscopic studies, ITC, and crystallographic analysis.     

Inclusion of chlorophenols by 4,4′‐(cyclohexane‐1,1‐diyl)diphenol: structures and kinetics of decomposition

The structures of the inclusion compounds 4,4′‐(cyclohexane‐1,1‐diyl)diphenol–3‐chlorophenol (1/1) and 4,4′‐(cyclohexane‐1,1‐diyl)diphenol–4‐chlorophenol (1/1), both C₁₈H₂₀O₂·C₆H₅ClO, are isostructural with respect to the host molecule and are stabilized by extensive host–host, host–guest and guest–host hydrogen bonding. The packing is characterized by layers of host and guest molecules. The kinetics of thermal decomposition follow the R2 contracting‐area model, kt = [1 − (1 − α)^½], and yield activation energies of 105 (8) and 96 (8) kJ mol⁻¹, respectively.     

Distribution of Butane in the Host Water Cage of Structure II Clathrate Hydrates

To understand host–guest interactions of hydrocarbon clathrate hydrates, we investigated the crystal structure of simple and binary clathrate hydrates including butane (n‐C₄H₁₀ or iso‐C₄H₁₀) as the guest. Powder X‐ray diffraction (PXRD) analysis using the information on the conformation of C₄H₁₀ molecules obtained by molecular dynamics (MD) simulations was performed. It was shown that the guest n‐C₄H₁₀ molecule tends to change to the gauche conformation within host water cages. Any distortion of the large 5¹²6⁴ cage and empty 5¹² cage for the simple iso‐C₄H₁₀ hydrate was not detected, and it was revealed that dynamic disorder of iso‐C₄H₁₀ and gauche‐nC₄H₁₀ were spherically extended within the large 5¹²6⁴ cages. It was indicated that structural isomers of hydrocarbon molecules with different van der Waals diameters are enclathrated within water cages in the same way owing to conformational change and dynamic disorder of the molecules. Furthermore, these results show that the method reported herein is applicable to structure analysis of other host–guest materials including guest molecules that could change molecular conformations.     

Self‐Assembly and Host–Guest Chemistry of a 3.5‐nm Coordination Nanotube

Upon complexation with Pd^II ions, precisely designed strandlike ligands with two tris(3,5‐pyridine) units at both terminals were assembled, with the aid of a linear template molecule, into a discrete tubular complex with a length of 3.5 nm. The high stability and the well‐defined structure of the coordination nanotube were revealed by NMR spectroscopy, cold‐spray ionization MS, and single‐crystal X‐ray analysis. Guest lengths were discriminated by the tube: When the association of strandlike guest molecules, in which two biphenylene units are linked with an (OCH₂CH₂)_n linker, were compared, the tube selectively recognized an appropriate guest whose length was comparable to that of the tube. Tetrathiafulvalene (TTF)‐terminated linear guests were directly oxidized to TTF²⁺ in the tube, but reduced stepwise via TTF^+? outside the tube.     

A Study of the Interaction Between Cucurbit[8]uril and Alkyl‐Substituted 4‐Pyrrolidinopyridinium Salts

The interaction between cucuribit[8]uril (Q[8]) and a series of 4‐pyrrolidinopyridinium salts bearing aliphatic substituents at the pyridinium nitrogen, namely 4‐(C₄H₈N)C₅H₅NRBr, where R=Et (g1), n‐butyl (g2), n‐pentyl (g3), n‐hexyl (g4), n‐octyl (g5), n‐dodecyl (g6), has been studied in aqueous solution by ¹H NMR spectroscopy, electronic absorption spectroscopy, isothermal titration calorimetry and mass spectrometry. Single crystal X‐ray diffraction revealed the structure of the host–guest complexes for g1, g2, g3, and g5. In each case, the Q[8] contains two guest molecules in a centrosymmetric dimer. The orientation of the guest molecule changes as the alkyl chain increases in length. Interestingly, in the solid state, the inclusion complexes identified are different from those observed in solution, and furthermore, in the case of g3, Q[8] exhibits two different interactions with the guest. In solution, the length of the alkyl chain plays a significant role in determining the type of host–guest interaction present.     

Tuning Proton Conductivity by Interstitial Guest Change in Size‐Adjustable Nanopores of a CuI‐MOF: A Potential Platform for Versatile Proton Carriers

By exploiting the breathing behavior of nanopores, we have studied for the first time the dependency of the guest‐induced proton conductivity of an interpenetrated Cu^I metal–organic framework (Cu^I‐MOF, [ 1 ]) on various guest molecules. Proton conductivities of over 10^?3 S cm^?1 under humid conditions were induced by a series of guest molecules, namely N,N‐dimethylformamide, dimethyl sulfoxide, diethylamine, 1,4‐dinitrobenzene, nitrobenzene, pyridine, and 1H‐1,2,4‐triazole. A detailed investigation of the guest‐incorporated complexes revealed that low‐energy proton conduction occurs under humid conditions through the Grotthuss mechanism in [ 1 ?NB] and through the vehicle mechanism in the rest of the complexes. Single‐point energy computations revealed considerable stabilization upon guest encapsulation. To the best of our knowledge, [ 1 ] represents the first example in which considerably high protonic conductivity is triggered upon the facile incorporation of small molecules of such a variety. The investigation portrayed herein may be a stepping stone towards the rational design of proton‐conducting materials for practical applications.     

Voltammetric Characterization of Redox‐Inactive Guest Binding to LnIII[15‐Metallacrown‐5] Hosts Based on Competition with a Redox Probe

A novel competitive binding assay was implemented to monitor the binding of a redox inactive substrate to a redox inactive metallacrown host based on its competition with ferrocene carboxylate (FcC^?) using cyclic voltammetry (CV). First, the binding of FcC^? to Ln^III[15‐MC‐5] (LnMC) hosts was characterized by cyclic voltammetry. It was shown that the voltammetric half wave potentials, E_1/2, shifted to more positive potentials upon the addition of LnMC. The explicit dependence of E_1/2 with the concentration of LnMC was used to determine the association constants for the complex. The FcC^? binding strength decreased with larger central lanthanide metals in the LnMC hosts, and substantially weaker binding was observed with La^III. X‐ray crystallography revealed that the hydrophobic host cavity incompletely encapsulated FcC^? when the guest was bound to the nine‐coordinate La^III, suggesting the LnMC’s ligand side chains play a substantial role in guest recognition. With knowledge of the MC‐FcC^? solution thermodynamics, the binding affinity of a redox inactive guest was then assessed. Addition of sodium benzoate to a LnMC and FcC^? mixture resulted in E_1/2 shifting back to the value observed for FcC^? in the absence of LnMC. The association constants between benzoate and LnMC’s were calculated via the competitive binding approach. Comparison with literature values suggests this novel assay is a viable method for determining association constants for host–guest systems that exhibit the proper electrochemical behavior. Notably, this CV competitive binding approach does not require the preparation of a modified electrode or a tethered guest, and thus can be generalized to a number of host–guest systems.     

Simultaneous Determination of Nitrophenol Isomers Based on β‐Cyclodextrin Functionalized Reduced Graphene Oxide

β‐CD modified reduced graphene oxide (RGO) sheets have been prepared and characterized by TEM, AFM, IR, EIS and CVs. In comparison with bare glass carbon electrode (GCE) and RGO modified GCE, CD‐RGO/GCE showed much higher peak currents to the reduction of nitrophenol isomers (NPs), attributed to the larger specific surface area of RGO and high quantities of host–guest recognition sites. Three pairs of redox peaks are observed on the CVs of CD‐RGO for p‐NP (0.3 V), o‐NP (?0.2 V) and m‐NP (0.05 V), separating well with each other. Under the optimized condition, the anodic peak currents were linear over ranges around 1–10 mg dm^?3 for p‐NP, 1–9 mg dm^?3 for o‐NP and 1–6 mg dm^?3 for m‐NP, with the detection limits of 0.05 mg dm^?3, 0.02 mg dm^?3 and 0.1 mg dm^?3, respectively. Thus, the CD‐RGO is expected to be a promising sensor material for detecting trace NPs in waste water.     

Facile and Efficient Fabrication of Photoresponsive Microgels via Thiol–Michael Addition

A photoresponsive microgel is designed by the combination of a noncovalent assembly strategy with a covalent cross‐linking method. End‐functionalized poly(ethylene glycol) with azobenzene [(PEG‐(Azo)₂)] was mixed with acrylate‐modified β‐CD (β‐CD‐MAA) to form photoresponsive inclusion complex through host–guest interaction. The above photoresponsive complex was cross‐linked by thiol‐functionalized PEG (PEG‐dithiol) via Michael addition click reaction. The photoreversibility of resulted microgel was studied by TEM, UV–Vis spectroscopy, and ¹H NMR measurements. The characterization results indicated that the reversible size changes of the microgel could be achieved by alternative UV–Vis irradiations with good repeatability.     

Host–Guest Recognition and Fluorescence of a Tetraphenylethene‐Based Octacationic Cage

We report the synthesis and characterization of a three‐dimensional tetraphenylethene‐based octacationic cage that shows host–guest recognition of polycyclic aromatic hydrocarbons (e.g. coronene) in organic media and water‐soluble dyes (e.g. sulforhodamine 101) in aqueous media through CH???π, π–π, and/or electrostatic interactions. The cage?coronene exhibits a cuboid internal cavity with a size of approximately 17.2×11.0×6.96 ų and a “hamburger”‐type host–guest complex, which is hierarchically stacked into 1D nanotubes and a 3D supramolecular framework. The free cage possesses a similar cavity in the crystalline state. Furthermore, a host–guest complex formed between the octacationic cage and sulforhodamine 101 had a higher absolute quantum yield (Φ_F=28.5 %), larger excitation–emission gap (Δλ_ex‐em=211 nm), and longer emission lifetime (τ=7.0 ns) as compared to the guest (Φ_F=10.5 %; Δλ_ex‐em=11 nm; τ=4.9 ns), and purer emission (Δλ_FWHM=38 nm) as compared to the host (Δλ_FWHM=111 nm).     

Turn‐On Fluorescence and Unprecedented Encapsulation of Large Aromatic Molecules within a Manganese(II)–Triazole Metal–Organic Confined Space

For the purpose of investigating the coordination behavior of sterically congested alkenes and exploring the possibility of cofacial complexation in the polycyclic aromatic system for the formation of extended polymeric networks, a new tetradentate ligand, 1,1,2,2‐tetrakis[4‐(1H‐1,2,4‐triazol‐1‐yl)phenyl]ethylene (TTPE), has been designed and synthesized. By using TTPE as a building block with regard to the self‐assembly with MnCl₂ ? 4 H₂O, a novel two‐dimensional coordination framework {[Mn(TTPE)Cl₂] ? 4 CHCl₃}_n ( 1 ) can be isolated. Anion‐exchange and organic‐group‐functionalized aromatic guest TTPE‐loaded host–guest complex experimental results indicate that coordinated Cl^? anions in the 2D framework of 1 can be completely replaced with dissociative ClO₄^? groups in an irreversible single‐crystal‐to‐single‐crystal transformation fashion, as evidenced by the anion‐exchange products of {[Mn(TTPE)(H₂O)₂](ClO₄)₂ ? 0.5 TTPE ? 5.25 H₂O}_n ( 2 ). Interestingly, TTPE, acting as an organic template, was encapsulated in the confined space of the 2D grid of 2 . To the best of our knowledge, such large organic molecules encapsulated in the reactive organic‐group‐functionalized aromatic‐guest‐loaded host–guest complex are unprecedented up to now. Luminescence measurements illustrate that 1 and 2 represent novel examples of sensing materials based on triazole derivatives. Further, 2 has been demonstrated by tuning the fluorescence response of porous metal–organic frameworks as a function of adsorbed small analytes.     

Recognition of N‐Alkyl and N‐Aryl Acetamides by N‐Alkyl Ammonium Resorcinarene Chlorides

N‐Alkyl ammonium resorcinarene chlorides are stabilized by an intricate array of intra‐ and intermolecular hydrogen bonds that leads to cavitand‐like structures. Depending on the upper‐rim substituents, self‐inclusion was observed in solution and in the solid state. The self‐inclusion can be disrupted at higher temperatures, whereas in the presence of small guests the self‐included dimers spontaneously reorganize to 1:1 host–guest complexes. These host compounds show an interesting ability to bind a series of N‐alkyl acetamide guests through intermolecular hydrogen bonds involving the carbonyl oxygen (C?O) atoms and the amide (NH) groups of the guests, the chloride anions (Cl^?) and ammonium (NH₂⁺) cations of the hosts, and also through CH ??? π interactions between the hosts and guests. The self‐included and host–guest complexes were studied by single‐crystal X‐ray diffraction, NMR titration, and mass spectrometry.