Polythiacalixarene-Embedded Gold Nanoparticles for Visible-Light-Driven Photocatalytic CO2 Reduction.

Metal nanoparticles are potent reaction catalysts, but they tend to aggregate, thereby limiting their catalytic efficiency. Their coordination with specific functional groups within a porous structure prevents their aggregation and facilitates the mass flow of catalytic starting materials and products. Herein, we use a thiacalix[4]arene-based polymer as a porous support with abundant docking sites for Au nanoparticles. The sulfur atoms bridging the phenolic subunits of thiacalix[4]arene serve as Lewis basic sites that coordinate Au atoms. Therefore, this approach takes advantage of the functional groups inherent in the monomer and avoids laborious postsynthetic modifications of the polymer. The presented system was tested for visible-light-driven photocatalytic CO2 reduction, where it showed adequate ability to generate 6.74 µmol g-1 CO over the course of 4 h, while producing small amounts of the CH4 product. This study aims to stimulate interest in the design and development of synthetically simpler porous polymer supports for various metal nanoparticles in catalytic and other applications.

Viologen-cucurbituril host/guest chemistry - redox control of dimerization versus inclusion.

Two calix[4]arene systems, C234+ and C244+ - where 2 corresponds to the number of viologen units and 3-4 corresponds to the number of carbon atoms connecting the viologen units to the macrocyclic core - have been synthesized and led to the formation of [3]pseudorotaxanes when combined with either CB[7] or CB[8]. The [3]pseudorotaxanes spontaneously dissociate upon reduction of the bipyridinium units as the result of intramolecular dimerization of the two face-to-face viologen radical cations. CB[7] and CB[8]-based [2]pseudorotaxanes containing monomeric viologen guest model compounds, MC32+ and MC4+, do not undergo decomplexation and dimerization following electrochemical reduction of their bipyridinium units.

Rapid and Efficient Removal of Perfluorooctanoic Acid from Water with Fluorine-Rich Calixarene-Based Porous Polymers.

On account of its nonbiodegradable nature and persistence in the environment, perfluorooctanoic acid (PFOA) accumulates in water resources and poses serious environmental issues in many parts of the world. Here, we present the development of two fluorine-rich calix[4]arene-based porous polymers, FCX4-P and FCX4-BP, and demonstrate their utility for the efficient removal of PFOA from water. These materials featured Brunauer-Emmett-Teller (BET) surface areas of up to 450 m2 g-1, which is slightly lower than their nonfluorinated counterparts (up to 596 m2 g-1). FCX4-P removes PFOA at environmentally relevant concentrations with a high rate constant of 3.80 g mg-1 h-1 and reached an exceptional maximum PFOA uptake capacity of 188.7 mg g-1. In addition, it could be regenerated by simple methanol wash and reused without a significant decrease in performance.

Thioether-Crown-Rich Calix[4]arene Porous Polymer for Highly Efficient Removal of Mercury from Water.

A rational design of adsorbents with high uptake efficiency and fast kinetics for highly toxic pollutants is a key challenge in environmental remediation. Here, we report the design of a well-defined thioether-crown-rich porous calix[4]arene-based mesoporous polymer S-CX4P and its utility in removal of highly relevant toxic mercury (Hg2+) from water. The polymer shows an exceptional, record-high uptake efficiency of 1686 mg g-1 and the fastest initial adsorption rate of 278 mg g-1 min-1. Remarkably, S-CX4P can effectively remove Hg2+ from high concentration (5 ppm) to below the acceptable limit for drinking water (2 ppb) even in the presence of other competitive metals at high concentrations. In addition, the polymer can be easily regenerated at room temperature and reused multiple times with negligible loss in uptake rate and efficiency. The results demonstrate the potential of rationally designed thioether-crown-rich polymers for high performance mercury removal.

Calix[4]arene-Based Porous Organic Nanosheets.

Calixarenes are a common motif in supramolecular chemistry but have rarely been incorporated in structurally well-defined covalent 2D materials. Such a task is challenging, especially without a template, because of the nonplanar configuration and conformational flexibility of the calixarene ring. Here, we report the first-of-a-kind solvothermal synthesis of a calix[4]arene-based 2D polymer (CX4-NS) that is porous, covalent, and isolated as few-layer thick (3.52 nm) nanosheets. Experimental and theoretical characterization of the nanosheets is presented. Atomic force microscopy and transmission electron microscopy results are consistent with the calculated lowest energy state of the polymer. In the lowest energy state, parallel layers are tightly packed, and the calixarenes adopt the 1,2-alternate conformation, which gives rise to a two-dimensional pattern and a rhombic unit cell. We tested the material's ability to adsorb I2 vapor and observed a maximum capacity of 114 wt %. Molecular simulations extended to model I2 capture showed excellent agreement with experiments. Furthermore, the material was easily regenerated by mild ethanol washings and could be reused with minimal loss of efficiency.

Porous Polycalix[4]arenes for Fast and Efficient Removal of Organic Micropollutants from Water.

Organic micropollutants are hazards to the environment and human health. Conventional technologies are often inefficient at removing them from wastewater. For example, commercial activated carbon (AC) exhibits slow uptake rates, limited capacities, and is costly to regenerate. Here, we report the utility of porous calix[4]arene-based materials, CalPn (n = 2-4), for water purification. Calixarenes are a common motif in supramolecular chemistry but have rarely been incorporated into extended, porous networks such as organic polymers. CalPn exhibit pollutant uptake rates (kobs) and adsorption capacities (qmax) that are among the highest reported. For example, the kobs of CalP4 for bisphenol A (BPA) is 2.12 mg/g·min, which is significantly higher (16 to 240 times) than kobs for ACs and 1.4 times higher than that of the most efficient material previously reported; the qmax of CalP4 for BPA is 403 mg/g. The CalPn polymers can be regenerated several times, with performance levels left undiminished, by a simple wash procedure that is less energy intensive than that required for ACs. These findings demonstrate the potential of calixarene-based materials for organic micropollutant removal.

Sulphur-rich functionalized calix[4]arenes for selective complexation of Hg2+ over Cu2+, Zn2+ and Cd2.

The syntheses of two new ligands based on a calix[4]arene scaffold in the cone conformation functionalized on the phenolic positions 1 and 3 by diethylthiophosphonates (L1) or tetra(tri)thioethyleneglycol (L2) crowns are described. Together with ligand L3, the parent calix[4]arene substituted by a penta(tetra)thioethyleneglycol crown, the spectroscopic properties of the ligands were determined by means of UV-Vis absorption spectroscopy and steady-state and time-resolved fluorescence spectroscopy, showing that the ligands display modest but non-negligible intrinsic fluorescence properties (ϕfluo = 0.023; 0.026 and 0.029 for L1, L2 and L3 in CH2Cl2, respectively). The X-ray crystal structures of ligand L1, and of its synthetic precursor were determined and analyzed for their capacity to accommodate the incoming cationic species. The ligands were further investigated for their complexation properties of divalent cations such as Cu2+, Zn2+, Hg2+ and Cd2+ (under their nitrate salts) in 1 : 1 CH3CN/CH2Cl2 solutions (I = 0.01 M Et4NNO3, T = 25.0(2) °C), in which the additions of cations were monitored by absorption and steady-state fluorescence spectrophotometries. The stoichiometries of the corresponding complexes were assessed by ESI-MS, while insights into the structures of the complexes in solution were obtained with density functional theory (DFT) calculations. The influence of the sulphur and phenol coordinating moieties was addressed to show that the thiocrown compounds L2 and L3 displayed a marked affinity towards the soft mercuric cation (Δlog K ≥ 2), with no particular size selectivity effect, whereas ligand L1 can accommodate both the thio and phenol units to coordinate with Cu(ii). Altogether, these results point to the use of L3 as a selective fluoroionophore for detection of Hg2+.

Redox-Responsive Viologen-Mediated Self-Assembly of CB[7]-Modified Patchy Particles.

Sulfonated surface patches of poly(styrene)-based colloidal particles (CPs) were functionalized with cucurbit[7]uril (CB[7]). The macrocycles served as recognition units for diphenyl viologen (DPV(2+)), a rigid bridging ligand. The addition of DPV(2+) to aqueous suspensions of the particles triggered the self-assembly of short linear and branched chainlike structures. The self-assembly mechanism is based on hydrophobic/ion-charge interactions that are established between DPV(2+) and surface-adsorbed CB[7]. DPV(2+) guides the self-assembly of the CPs by forming a ternary DPV(2+)⊂(CB[7])2 complex in which the two CB[7] macrocycles are attached to two different particles. Viologen-driven particle assembly was found to be both directional and reversible. Whereas sodium chloride triggers irreversible particle disassembly, the one-electron reduction of DPV(2+) with sodium dithionite causes disassembly that can be reversed via air oxidation. Thus, this bottom-up synthetic supramolecular approach allowed for the reversible formation and directional alignment of a 2D colloidal material.

Thin-coated water soluble CdTeS alloyed quantum dots as energy donors for highly efficient FRET.

The synthesis of highly luminescent water soluble CdTe(x)S(y) quantum dots (QDs) is described and their elemental composition and optical properties are fully characterized. Glutathione (GSH)-capped nanocrystals were obtained from an aqueous solution of CdCl2, Na2TeO3 and GSH in the presence of NaBH4 upon heating at 100 °C. Spherical CdTe(x)S(y) alloyed nanoparticles with diameters ranging from 2 to 4 nm were formed, and characterized by X-ray powder diffraction and Transmission Electron Microscopy. Their elemental composition was determined from Inductively Coupled Plasma Atomic Emission Spectroscopy and CHN elemental analysis experiments. A model for the determination of their molecular formulas, molecular weights and extinction coefficients is proposed. Surface GSH molecules were involved in amide bond formation with fluorescent Nile-Red molecules, to be used as energy acceptor in Förster resonance energy transfer (FRET) experiments. FRET was observed from the CdTe(x)S(y) core (λ(ex) = 430 nm) to the Nile-Red dye (λ(em) = 648 nm) with an almost quantitative FRET efficiency (η(FRET) = 98%). A detailed analysis of the FRET is presented, revealing a core-dye distance of 24 Å, in very good agreement with the estimated radius of the core (13 Å) as measured by TEM. The QDs present excellent photophysical properties (QY up to 29%), easy synthesis and can be isolated as solids and redispersed in water without loss of their photoluminescence efficiency.

Radical-cation dimerization overwhelms inclusion in [N]pseudorotaxanes.

Suppression of the dimerization of the viologen radical cation by cucurbit[7]uril (CB7) in water is a well-known phenomenon. Herein, two counter-examples are presented. Two viologen-containing thread molecules were designed, synthesized, and thoroughly characterized by (1)H DOSY NMR spectrometry, UV/Vis absorption spectrophotometry, square-wave voltammetry, and chronocoulometry: BV(4+), which contains two viologen subunits, and HV(12+), which contains six. In both threads, the viologen subunits are covalently bonded to a hexavalent phosphazene core. The corresponding [3]- and [7]pseudorotaxanes that form on complexation with CB7, that is, BV(4+)⊂(CB7)2 and HV(12+)⊂(CB7)6, were also analyzed. The properties of two monomeric control threads, namely, methyl viologen (MV(2+)) and benzyl methyl viologen (BMV(2+)), as well as their [2]pseudorotaxane complexes with CB7 (MV(2+)⊂CB7 and BMV(2+)⊂CB7) were also investigated. As expected, the control pseudorotaxanes remained intact after one-electron reduction of their viologen-recognition stations. In contrast, analogous reduction of BV(4+)⊂(CB7)2 and HV(12+)⊂(CB7)6 led to host-guest decomplexation and release of the free threads BV(2(·+)) and HV(6(·+)), respectively. (1)H DOSY NMR spectrometric and chronocoulometric measurements showed that BV(2(·+)) and HV(6(·+)) have larger diffusion coefficients than the corresponding [3]- and [7]pseudorotaxanes, and UV/Vis absorption studies provided evidence for intramolecular radical-cation dimerization. These results demonstrate that radical-cation dimerization, a relatively weak interaction, can be used as a driving force in novel molecular switches.

Simultaneous self-assembly of a [2]catenane, a trefoil knot, and a Solomon link from a simple pair of ligands.

A topological triptych: Three molecular links, a [2]catenane, a trefoil knot, and a Solomon link, were obtained in one pot through the self-assembly of two simple ligands in the presence of Zn(II). The approach relied on dynamic covalent chemistry and metal templation.

Cation-pi interaction in complex formation between Tl+ ion and calix[4]crown-6 and some calix[4]biscrown-6 derivatives: thallium-203 NMR, proton NMR, and X-ray evidence.

Metallocapped complexation of the thallium(I) ion with calix[4]crown-6 (1) and three different calix[4]biscrown-6 derivatives (2-4) has been investigated by a combination of (203)Tl and (1)H NMR in a CD(3)CN/CDCl(3) (4:1 v/v) solution. The results clearly revealed the formation in solution of mono- and dithallium(I) complexes in which the metal cations are held in the ligands' cavities close to the calix[4]arene ring. In addition, a solid state mononuclear complex between 1,3-calix[4]bis-o-benzo-crown-6 (3) and TlClO(4) was prepared, and its X-ray crystal structure was determined. The results of the structural data in solution and the solid state suggested that the calix[4]crown-6 derivatives considered provide pi cavities as active sites for the complexation of thallium(I) ions.

Dynamic NMR study of the kinetics of complexation of Tl(+) ion with calix[4]crown-6.

The complexation and exchange kinetics and mechanism for the dissociation and conformational change of thallium ion complex of calix[4]crown-6 were studied in CD(3)CN/CDCl(3) (4:1 v/v) solution by dynamic (1)H NMR. The results show the formation of a 1:1 complex with cone conformation. From variable temperature dynamic NMR analysis in the range 223-293 K, two coalescence temperatures at 228 and 243 K were ascertained. The activation parameters for the dissociation process, E(a) (kJ/mol), DeltaS(++) (J/mol.K), and DeltaH(++) (kJ/mol) are 11.0, -133.2, and 10.1 for the bimolecular regime and 21.5, -112.8, and 20.6 for the unimolecular regime, respectively. In addition, the dynamic (1)H NMR spectroscopy shows that the exchange of Tl(+) between the two crown sides of the complexed ligand proceeds through an intramolecular tunneling. An Arrhenius convex curve was observed for intramolecular exchange. This phenomenon is explained in terms of two conformer state formations differentiated by hydrogen bond association.

Characterization of a self-assembled monolayer based on a calix[4]crown-5 derivate: fabrication of a chemical sensor sensitive to calcium.

The synthesis and self-assembled monolayer (SAM) formation of a calix[4]crown-5 derivative are reported. Several techniques, including electrochemistry, atomic force microscopy (AFM), Time-of-flight secondary ion mass spectrometry (ToF-SIMS) and contact angle measurements have been applied to characterise the monolayer film designed for chemical sensor applications. The recognition properties of this SAM for metal cations has been investigated using impedance spectroscopy (IS) showing an electrochemical response proportional to calcium ion concentration in the range from 10(-7) M to 10(-2) M. This response is related to microscopic changes at the gold surface induced by selective binding by the immobilised calixarene.

Highly sensitive and selective poly(vinyl chloride)-membrane potentiometric sensors based on a calix[4]arene derivative for 2-furaldehyde.

A 2-furaldehyde-selective PVC-membrane electrode is designed based on the host-guest interaction between tetrabenzyl ether Calix[4]arene, as an ionophore, and a lipophilic hydrazone derivative generated in situ from reaction of 2-furaldehyde and Girard's reagent T. At a pH of 9.2, the electrode exhibits a Nernstian response over the 2-furaldehyde concentration range of (5.0 x 10(-5))-(1.0 x 10(-1)) M. The electrode has found to be chemically inert and of adequate stability with a response time of 15 s with a good reproducibility (+/-0.2 mV) and can be used for a long working lifetime. In order to improve the minimum detectable concentration of 2-furaldehyde, further studies have been performed using a coated graphite electrode and coated platinum and gold disks. Some analytical aspects of adsorptive square wave voltammetry have also been presented in order to elucidate the adduct formation between 2-furaldehyde and Girard's reagent T. The interfering effects of some Na(+), K(+), NH(4)(+), formaldehyde, 5-hydroxymethyl 2-furaldehyde (HMF), excess of Girard's reagent T and organic solvents such as isopropyl alcohol and N,N-dimethylformamide on the sensor's response have been studied. The viability of using the electrode for the trace determination of 2-furaldehyde in several Iranian oil refinery wastewater samples is also demonstrated. The results obtained from the developed method for real samples are compared with those from UV-spectrophotometric and high-performance liquid chromatographic experiments.

Design and characterization of immunogens for raising antibodies directed towards chelated alkali metals.

The properties of immunogens synthesized from a calix[4]crown-6 were investigated with the aim of generating specific antibodies towards caesium chelates. Affinity capillary electrophoresis was successfully used to determine the stability parameters with caesium of both the hapten and the corresponding immunogens after coupling to bovine serum albumin (BSA). Unfortunately, the stability of the caesium chelate was shown to decrease drastically as the polarity of the medium increased. Nevertheless, the selectivity was proved to be rather stable, with a clear preference of caesium over potassium. The binding mechanism for the protein conjugates proved to be complex, as revealed by isotherms, whatever the number of calixarene groups coupled to BSA. Immunization of mice with caesium loaded immunogen resulted in the production of polyclonal antibodies able to distinguish between free calixarene and its complexes with either potassium or caesium.

Characterization of protein conjugates using capillary electrophoresis.

With the aim of generating antibodies, a calix[4]arene-crown-6 was coupled to bovine serum albumin. For that purpose, a complete procedure to optimize and characterize the coupling of hydrophobic haptens based on capillary electrophoresis (CE) was developed. We demonstrated the existence of a polynomial relationship between the electrophoretic mobility (mu(ep)) and the hapten density. This correlation was used not only to study the coupling reaction in terms of optimization and kinetics but also to determine the average coupling molar ratio of any given conjugate. An estimation of the heterogeneity of these conjugates by simulation of experimental peaks was also proposed.

Complexation of oxoanions and cationic metals by the biscatecholate siderophore azotochelin.

Azotochelin is a biscatecholate siderophore produced by the nitrogen-fixing soil bacterium Azotobacter vinelandii. The complexation properties of azotochelin with a series of oxoanions [Mo(VI), W(VI) and V(V)] and divalent cations [Cu(II), Zn(II), Co(II) and Mn(II)] were investigated by potentiometry, UV-vis and X-ray spectroscopy. Azotochelin forms a strong 1:1 complex with molybdate (log K=7.6+/-0.4) and with tungstate and vanadate; the stability of the complexes increases in the order Mo

Synthesis and characterization of immunogens based on calix[4]arene-crown-6 for the generation of antibodies directed towards cesium ions.

With the aim of generating antibodies directed towards cesium, a calix[4]arene-crown-6 in the 1,3-alternate conformation bearing an N-hydroxysuccinimide function was synthesized. The calixarene was coupled to bovine serum albumin using different calixarene/protein ratios, and cesium was loaded. Conjugates were characterized using trinitrobenzenesulfonic acid derivatization, matrix assisted laser desorption ionisation-mass spectrometry (MALDI-TOF-MS), and inductively coupled plasma-mass spectrometry (ICP-MS). Targeted lysines were determined after enzymatic cleavage followed by MALDI-TOF-MS, and a progressive targeting related to lysines accessibility was observed.

Synthesis, crystal structure and reactivity of uranium(IV) complexes with p-tert-butylcalix[4]arene ligands.

Reactions of UCl4 with 25,27-dimethoxy-5,11,17,23-tetra-tert-butylcalix[4]arene (H2Me2calix) in THF or pyridine at 80 degrees C gave [UCl2(Me2calix)L2] [L = THF (1) or pyridine (2)]. Similar treatment of U(acac)(4) (acac = MeCOCHCOMe) with H2Me2calix in THF or pyridine afforded [U(acac)2(Me2calix)] (3). The bis-calixarene compound [U(Me2calix)(H2calix)] (4) was obtained by reaction of U(OTf)4 or U(OTf)3 with H2Me2calix in pyridine at 110 degrees C. Treatment of UCl4 with H2Me2calix in pyridine at 110 degrees C gave [Mepy][UCl2(Hcalix)(py)2] (5) resulting from demethylation and acid cleavage of the methoxy groups of the calixarene ligand of 2. Adventitious traces of air were responsible for the formation of [Hpy][Mepy]4[{UCl(calix)}3(mu3-O)][UCl6] (6) during the reaction of UCl4 and H2Me2calix, and of [{U(Me2calix)(mu3-O)LiCl(THF)}2] (7) during the reaction of 2 with tBuLi. The X-ray crystal structures of 1.2THF, 2.2py, 3.0.25L (L = THF and py), 4.2py, 5, 6.3py and 7.THF have been determined.