A Combined Solution and Solid-State Study on the Tautomerism of an Azocalix[4]arene Chromoionophore

Azocalixarenes functionalized with cation binding sites are popular chromoionophores due to the ease of synthesis and the large complexation-induced shifts of their absorption band that originate from an azo-phenol-quinone-hydrazone tautomerism. Despite their extensive use, however, a thorough investigation of the structure of their metal complexes has not been reported. We describe herein the synthesis of a new azocalixarene ligand (2) and the study of its complexation properties with the Ca²⁺ cation. Through a combination of solution (¹H NMR and UV-vis spectroscopies) and solid-state (X-ray diffractometry) techniques, we demonstrate that metal complexation induces a shift of the tautomeric equilibration towards the quinone-hydrazone form, while deprotonation of the complex results in the reversion to the azo-phenol tautomer.

The Use of Anhydrous Barium Hydroxide for Selective Alkylation of Dialkyloxy-tert-butyl-calix[4]arenes

A practical method of selective alkylation of the third hydroxyl group of disubstituted tert-butyl-calix[4]arenes using anhydrous barium hydroxide as a base was developed in this study. The use of this method in the synthesis of inherently chiral derivatives is shown herein.

The Role of Triazole and Glucose Moieties in Alkali Metal Cation Complexation by Lower-Rim Tertiary-Amide Calix[4]arene Derivatives

The binding of alkali metal cations with two tertiary-amide lower-rim calix[4]arenes was studied in methanol, N,N-dimethylformamide, and acetonitrile in order to explore the role of triazole and glucose functionalities in the coordination reactions. The standard thermodynamic complexation parameters were determined microcalorimetrically and spectrophotometrically. On the basis of receptor dissolution enthalpies and the literature data, the enthalpies for transfer of reactants and products between the solvents were calculated. The solvent inclusion within a calixarene hydrophobic basket was explored by means of ¹H NMR spectroscopy. Classical molecular dynamics of the calixarene ligands and their complexes were carried out as well. The affinity of receptors for cations in methanol and N,N-dimethylformamide was quite similar, irrespective of whether they contained glucose subunits or not. This indicated that sugar moieties did not participate or influence the cation binding. All studied reactions were enthalpically controlled. The peak affinity of receptors for sodium cation was noticed in all complexation media. The complex stabilities were the highest in acetonitrile, followed by methanol and N,N-dimethylformamide. The solubilities of receptors were greatly affected by the presence of sugar subunits. The medium effect on the affinities of calixarene derivatives towards cations was thoroughly discussed regarding the structural properties and solvation abilities of the investigated solvents.

Self-Assembly of Bowlic Supramolecules on Graphene Imaged at the Individual Molecular Level using Heavy Atom Tagging

The self-assembly of bowlic supramolecules on graphene surface is studied with single molecular sensitivity. This is achieved by incorporating a heavy metal tag in the form of a single W atom into the tip of the molecular structure, which enables the direct imaging of molecular distribution using annular dark-field scanning transmission electron microscopy (ADF-STEM) along with graphene as an electron transparent support. The bowlic molecules have nonplanar geometry, and their orientations with respect to their graphene substrate and with each other result in various packing configurations. Statistical data on intermolecular distances is obtained from numerous measurements of the bright contrast from the single metal atom tags. The analysis shows that the bowlic molecules lie sideways on the graphene surface with favorable head-to-tail stacking, rather than sitting vertically with the bowl facing toward the graphene surface. In thicker film regions, nanoscale lamellar fringes are observed, demonstrating that large-scale aligned packing extends into 3D. Image simulations and various molecular packing schemes are discussed to help interpret the ADF-STEM images and the possible range of molecular interactions occurring. These results aid the understanding of nonplanar supramolecular assemblies on van der Waals surfaces for potential applications in molecular recognition by porous films.

Formation of the inclusion complex of water soluble fluorescent calix[4]arene and naringenin: solubility, cytotoxic effect and molecular modeling studies

Naringenin is considered as an important flavonoid in phytochemistry because of its important effect on cancer chemoprevention. Unfortunately its poor solubility has restricted its therapeutic applications. In this study, an efficient water-soluble fluorescent calix[4]arene (compound 5) was synthesized as host macromolecule to increase solubility and cytotoxicity in cancer cells of water-insoluble naringenin as well as to clarify localization of naringenin into the cells. Complex formed by host-guest interaction between compound 5 and naringenin was analyzed with UV-visible, fluorescence, FTIR spectroscopic techniques and molecular modeling studies. Stern-Volmer analysis showed binding constant value of K_sv 3.5 × 10⁷ M^-1 suggesting strong interaction between host and guest. Binding capacity shows 77% of naringenin was loaded on compound 5. Anticarcinogenic effects of naringenin complex were evaluated on human colorectal carcinoma cells (DLD-1) and it was found that 5-naringenin complex inhibits proliferation of DLD-1 cells 3.4-fold more compared to free naringenin. Fluorescence imaging studies show 5-naringenin complex was accumulated into the cytoplasm instead of the nucleus. Increased solubility and cytotoxicity of naringenin with fluorescent calix[4]arene makes it one of the potential candidates as a therapeutic enhancer. For deep understanding of host-guest interaction mechanisms, complementary multiscale molecular modeling studies were also carried out.Communicated by Ramaswamy H. Sarma.

Synthesis, in-Vitro and in Silico Studies of Azo-Based Calix[4]arenes as Antibacterial Agent and Neuraminidase Inhibitor: A New Look Into an Old Scaffold

Calixarene derivatives are reported as potential therapeutic agents. Azo derivatives of calixarenes have not been given much consideration to explore their biomedical applications. In the present study, some azo-based derivatives of calix[4]arene were synthesized and characterized and their antibacterial and antiviral potentials were studied. The mono azo products of sulphanilamide, sulfaguanidine and 2-methyl-4-aminobenzoic acid showed good activity against bacterial strains with minimum inhibition concentration values ranging from 0.97 to 62.5 μg/mL. For mono azo products, the diazotized salt was applied as a limiting reagent. The use of calix[4]arene and sodium acetate trihydrate in 1:3 (molar ratio) helped in partial substitution. Molecular docking was performed to see the interaction of the designed compounds with two bacterial and one viral (neuraminidase) receptor. Some of the derivatives showed good interaction with the active site of bacterial and neuraminidase enzymes through hydrogen, hydrophobic and pi-pi interactions, and could inhibit the activity of the selected enzymes.

Fixing the Conformation of Calix[4]arenes: When Are Three Carbons Not Enough?

Calix[4]arenes are unique macrocycles that through judicious functionalisation at the lower rim can be either fixed in one of four conformations or remain conformationally flexible. Introduction of propynyl or propenyl groups unexpectedly provides a new possibility; a unidirectional conformational switch, with the 1,3-alternate and 1,2-alternate conformers switching to the partial cone conformation, whilst the cone conformation is unchanged, under standard experimental conditions. Using ¹ H NMR kinetic studies, rates of switching have been shown to be dependent on the starting conformation, upper-rim substituent, where reduction in bulk enables faster switching, solvent and temperature with 1,2-alternate conformations switching fastest. Ab initio calculations (DFT) confirmed the relative stabilities of the conformations and point towards the partial cone conformer being the most stable of the four. The potential impact on synthesis through the "click" reaction has been investigated and found not to be significant.

Luminescence Properties of Self-Aggregating TbIII-DOTA-Functionalized Calix[4]arenes

Self-aggregating calix[4]arenes carrying four DOTA ligands on the upper rim for stable complexation of paramagnetic GdIII-ions have already been proposed as MRI probes. In this work, we investigate the luminescence properties of TbIII-DOTA-calix[4]arene-4OPr containing four propyl-groups and compare them with those of the analog substituted with a phthalimide chromophore (TbIII-DOTA-calix[4]arene-3OPr-OPhth). We show that, given its four aromatic rings, the calix[4]arene core acts as an effective sensitizer of Tb-centered luminescence. Substituents on the lower rim can modulate the aggregation behavior, which in turn determines the luminescence properties of the compounds. In solid state, the quantum yield of the phthalimide derivative is almost three times as high as that of the propyl-functionalized analog demonstrating a beneficial role of the chromophore on Tb-luminescence. In solution, however, the effect of the phthalimide group vanishes, which we attribute to the large distance between the chromophore and the lanthanide, situated on the opposite rims of the calix[4]arene. Both quantum yields and luminescence lifetimes show clear concentration dependence in solution, related to the strong impact of aggregation on the luminescence behavior. We also evidence the variability in the values of the critical micelle concentration depending on the experimental technique. Such luminescent calix[4]arene platforms accommodating stable lanthanide complexes can be considered valuable building blocks for the design of dual MR/optical imaging probes.

Two-Dimensional Calix[4]arene-based Metal-Organic Coordination Networks of Tunable Crystallinity

A flexible and versatile method to fabricate two-dimensional metal-organic coordination networks (MOCNs) by bottom-up self-assembly is described. 2D crystalline layers were formed at the air-water interface, coordinated by ions from the liquid phase, and transferred onto a solid substrate with their crystallinity preserved. By using an inherently three-dimensional amphiphile, namely 25,26,27,28-tetrapropoxycalix[4]arene-5,11,17,23-tetracarboxylic acid, and a copper metal node, large and monocrystalline dendritic MOCN domains were formed. The method described allows for the fabrication of monolayers of tunable crystallinity on liquid and solid substrates. It can be applied to a large range of differently functionalized organic building blocks, also beyond macrocycles, which can be interconnected by diverse metal nodes.

Ultraviolet Patterned Calixarene-Derived Supramolecular Gels and Films with Spatially Resolved Mechanical and Fluorescent Properties

Supramolecular assemblies have in the past been considered mechanically weak and in most cases unable to withstand their own weight. Calixarene-derived networks can, however, provide robust supramolecular gels. Incorporating a photoreactive stilbene moiety, we show that the aggregation state of the material can be tuned by heating and UV exposure in order to control the mechanical as well as the fluorescent properties. Regulating the extent of heating to control the proportion of H-aggregates and J-aggregates and further cross-linking of H-aggregates by control over UV exposure allows for adjustable photopatterning of the fluorescence as well as the material stiffness in the range from ∼100 to 450 kPa. We expect this straightforward supramolecular system will be suitable for advanced prototyping in applications where modulus and shape are important design criteria.

Synthesis and properties of calix[4]arene telluropodant ethers as Ag+ selective sensors and Ag+, Hg2+ extractants

Three novel phenyltelluroalkoxyl functionalized tweezer-like calix[4]arenes 6-8 and two monophenyltelluropropoxyl functionalized calix[4]arenes 10 (cone conformer) and 12 (partial cone conformer) were synthesized and characterized. They are good Ag(+)-selective ionophores in ion-selective electrodes evaluated by electromotive force measurements of polymer membrane electrodes. The tweezer-like ionophores 6-8 showed excellent extraction ability towards Ag(+) and Hg(2+).