Ionic interactions and multilayer structures on self-assembled surfaces of calix[4]resorcinarene

The ionization of a self-assembled monolayer of tetrathiol calix[4]resorcinarene, which is a synthetic host in guest-host recognition phenomena, was investigated. Using surface plasmon resonance measurements, it was found that ionization of the self-assembled surface occurred in water at a pH of approximately 11.5. These ionized surfaces of calix[4]resorcinarene were used to bind ionic guests such as metallic copper ions that were further exploited in the formation of supramolecular ionic assemblies of calix[4]resorcinarene. The structure of the molecular assemblies was characterized using polarization modulation infrared reflection absorption spectroscopy. The use of ionic interactions to attach molecules such as 11-mercapotundecanoic acid to the upper rim of the calix[4]resorcinarene host was also investigated as a method for simple and non-covalent modification of the host cavity.     

Organic nanoporous structures

Research on microporous materials, hollow solids with channels and cavities that include small guest molecules, has advanced in fundamental and applied aspects during 1999–2000. The retrosynthesis of crystal structures in terms of robust supramolecular synthons (recognition motifs) and functionalised organic molecules (building blocks) has led to the design of new porous architectures and modification in the properties of existing host materials. Even as conventional O–H⋯O and N–H⋯O hydrogen bonds continue to be used to attain these goals, weak hydrogen bonds and heteroatom interactions, such as C–H⋯O, halogen⋯halogen, strengthened by multi-point recognition and cooperativity effects, have emerged in new design strategies. A proper understanding of pseudopolymorphism, the phenomenon of solvent inclusion in crystals, will promote the next phase of host–guest research.     

In situ FT-IR measurements of competitive vapor adsorption into porous thin films containing silica nanoparticles

Vapor adsorption into porous ultrathin films on a gold surface is investigated with in situ surface plasmon resonance (SPR) and polarization-modulation infrared reflection-absorption spectroscopy (PM-IRRAS). The thin films are prepared by the electrostatic self-assembly of oppositely charged poly(L-lysine) (PL) and silica nanoparticles on a chemically modified gold surface. Characterization with ex situ SPR and PM-IRRAS demonstrates the buildup of multiple PL/SiO2 bilayers as well as an excellent correlation between the quantitative results from these two techniques. In situ vapor adsorption experiments with these thin films show evidence of porosity, reproducibility, and rapid reversibility. Exposure to acetone vapor (P/P0 = 0.032) causes the film to adsorb 9% acetone by volume, which corresponds to coverage of approximately one-half of the silica nanoparticle surface area. In situ PM-IRRAS provides much information about the molecular interactions occurring in the film upon adsorption or desorption of vapors. Dosing with a mixture of vapors leads to a competition for adsorption into the film, and PM-IRRAS results show that acetone slightly outcompetes nitromethane. These experiments with nanoparticle thin films demonstrate the advantages of using in situ PM-IRRAS for studying reversible adsorption in the presence of vapor mixtures.     

Removal efficiency of a calix[4]arene-based polymer for water-soluble carcinogenic direct azo dyes and aromatic amines

A Mannich base derivative of 5,11,17,23-tetrakis-[(1,4-dioxa-8-azaspiro-[4.5]decanyl)methyl]-25,26,27,28-tetrahydroxy calix[4]arene 3 was synthesized by the treatment of calix[4]arene with a cyclic secondary amine (1,4-dioxa-8-azaspiro-[4.5]decane) and formaldehyde. The compound 3 was treated with dibromoxylene to obtain a calix[4]arene-based copolymer 4. In batch sorption experiments of selected carcinogenic azo dyes and aromatic amines, the compounds 3 and 4 were found to be a better sorbent for azo dyes than for the aromatic amines. It was observed that the percentage of azo dye removal was 95-99% for compound 3 and 83-97% for 4 when the pH of the dye solution was in the range of 2.0-8.0. The sorption of azo dyes and aromatic amines by calix[4]arenes-based compounds indicates that amino groups play the major role for the formation of hydrogen bonds and electrostatic interactions.     

Evaluation of the performance of calix[8]arene derivatives as liquid phase extraction material for the removal of azo dyes

This paper presents a study on the removal of azo dyes (Reactive Black 5, Trapaeolin 000, Methyl Orange and Direct Violet 51) with calix[n]arene derivatives from aqueous solution into the organic phase in order to explore the potential use of calixarenes as low-cost efficient extractants for wastewater dye removal. The carboxylic acid derivative of calix[8]arene shows highest affinity towards the azo dyes. The influence of NaCl (present in the solution) on extraction process was also studied. The extent of the dye removal increased with the addition of NaCl. The proposed extraction mechanism involves several kinds of interactions: electrostatic repulsion between carboxylic acid groups of calix[8]arenes and sulfonate groups of azo dyes, hydrogen bonding and formation of an inclusion complex due to three dimensional cavity type calix[n]arene molecules through host-guest interactions.     

A model study of artificial linker system using self-assembled calix[4]arene derivative monolayers for protein immobilization

The attachment of biomolecules, in particular proteins, onto solid supports is fundamental in the development of advanced biosensors, biochips, bioreactors, and many diagnostic techniques. In addition, the effective investigation of biomolecular structure and function with chip-based modern instruments often requires effective attachment of the biomolecule to a substrate. For this reason, it is very important to construct well-characterized linker system that can immobilize protein efficiently. Here, we investigate the formation of self-assembled monolayers (SAMs) with calix[4]arene ethylester and carboxylic acid derivatives that can serve as a model system for protein immobilization at solid surfaces. The calix[4]arene derivative monolayers were formed on Au surface and carefully characterized by atomic force microscopy (AFM), Fourier transform infrared reflection absorption spectroscopy (FTIR-RAS) and surface plasmon resonance (SPR). Immobilization process of protein using bovine serum albumin (BSA) on the artificial linker layer was measured by SPR. The surface concentration of BSA was calculated by simulation of experimental SPR data. The surface concentration of BSA on the carboxylic acid form was higher than that of the ethylester. These results can help in modeling and understanding of protein immobilization on the linker layer.     

A functional zeolite analogue assembled from metalloporphyrins

The assembly of molecular building blocks with metal ions generating microporous network solids has been the focus of intense activity. Because of their potential applications associated with channels and cavities, such materials have been examined for size- and shape-selective catalysis, separations, sensors, molecular recognition and nanoscale reactors. Within this context, assemblies of robust and chemically versatile porphyrin and metalloporphyrin building blocks remain rare. Supramolecular architectures of porphyrin solids based on weak van der Waals interactions, hydrogen bonding and metal-ligand coordination networks have been reported. Although there are frequent allusions to zeolite-like microporosity from crystallography and loss of initial guest solvent molecules, evidence of functional microporous behaviour is scarce. We have demonstrated repeatable sorption-desorption with high selectivity on the basis of size, shape and functional group of the sorbate by a microporous metalloporphyrin solid in analogy to zeolites.     

25th Anniversary Article: Reversible and Adaptive Functional Supramolecular Materials: “Noncovalent Interaction” Matters

Supramolecular materials held together by noncovalent interactions, such as hydrogen bonding, host–guest interactions, and electrostatic interactions, have great potential in material science. The unique reversibility and adaptivity of noncovalent intreractions have brought about fascinating new functions that are not available by their covalent counterparts and have greatly enriched the realm of functional materials. This review article aims to highlight the very recent and important progresses in the area of functional supramoleuclar materials, focusing on adaptive mechanical materials, smart sensors with enhanced selectivity, soft luminescent and electronic nanomaterials, and biomimetic and biomedical materials with tailored structures and functions. We cannot write a complete account of all the interesting work in this area in one article, but we hope that it can in a way reflect the current situation and future trends in this prosperously developing area of functional supramolecular materials.     

A novel pyromellitic diimide derivative: Synthesis,gelation and spontaneous colorimetric sensing of dihydroxybenzene isomers

We report on the synthesis and self-assembly of a new hydrazide derivative N′, N′-bis[4-octadecyloxybenzamido]pyromellitic diimide (compound 1) which formed gels in several apolar organic solvents such as benzene and 1, 2-dichloroethane. ¹H NMR, and FT-IR spectroscopy studies confirmed that the intermolecular hydrogen bonding and van der Waals interactions were major driving forces for the formation of self-assembling gels. The gelator can form noncovalent interactions with dihydroxybenzenes, exhibiting different colors when it complexes with different positional isomers, and thus can be used to sense the positional isomers of dihydroxybenzenes by the naked eyes. This sensing property was further investigated by UV/Vis, ¹H NMR, and ¹H NMR NOESY spectroscopy which revealed that the charge–transfer interaction between hydroxyl groups of the dihydroxybenzene isomers (donor) and compound 1 (acceptor) accounted for this property.     

Modeling the structure of fulleranes and their endohedral complexes involving small molecules with nontrivial topological properties

'In' and 'out' isomers of perhydrogenated fullerenes and endohedral fullerene complexes have only recently been incorporated into the realm of topological chemistry. The 'in' isomers are, until now, purely hypothetical while for the latter group mostly studied are the complexes with metal ions that can be obtained during the fullerenes manufacturing. Much more difficult to obtain are the complexes with small molecules buried inside fullerene cages produced by laborious synthesis involving opening the cage, inserting the guest into it, and closing the cage chemically. This complicated procedure has only recently been accomplished for a hydrogen molecule put in the C60. Two H2 molecules inside the opened C70 cage and H2O in the opened C60 have been also reported recently. Model calculations, when carefully applied, allow one to predict the possibility of obtaining endohedral fullerene complexes with small molecules and 'in' isomers of perhydrogenated fullerenes. However, such systems are too large to be reliably handled by quantum calculations. Interestingly, such a simple method as molecular mechanics seems much more trustworthy.     

Intercalation in hydrotalcite-like layered Zn/Al-double hydroxides with naphthalenedisulphonates

Hydrotalcite-like layered Zn/Al-double hydroxides (Zn/Al-HTs) with one of three naphthalenedisulphonates (NijDSs) as an interlayer guest dianion, were obtained as precipitates from weak alkaline solutions of NijDS admixed with calcined host powder. Two values of interplanar spacing were observed in the powder X-ray diffraction patterns of NijDS-intercalated Zn/Al-HTs with two guest isomers. Intercalated materials have been characterized by means of UV-vis diffuse reflectance spectroscopy, differential thermal analysis/thermogravimetry and X-ray photoelectron spectroscopy. A model has been proposed in which the organic dianion at the interlayer gallery region bridges two Al³⁺ cations in neighbouring layers of double hydroxides. The guest molecules have an orientation perpendicular to the internal surface of the layers for all NijDS-intercalated materials. Another orientation has been suggested in which the molecules tilt from the perpendicular position for two isomers of NijDS.     

Ultrahigh and Selective SO2 Uptake in Inorganic Anion‐Pillared Hybrid Porous Materials

The efficient capture of SO₂ is of great significance in gas‐purification processes including flue‐gas desulfurization and natural‐gas purification, but the design of porous materials with high adsorption capacity and selectivity of SO₂ remains very challenging. Herein, the selective recognition and dense packing of SO₂ clusters through multiple synergistic host–guest and guest–guest interactions by controlling the pore chemistry and size in inorganic anion (SiF₆^2?, SIFSIX) pillared metal–organic frameworks is reported. The binding sites of anions and aromatic rings in SIFSIX materials grasp every atom of SO₂ firmly via S^δ+···F^δ? electrostatic interactions and O^δ?···H^δ+ dipole–dipole interactions, while the guest–guest interactions between SO₂ molecules further promote gas trapping within the pore space, which is elucidated by first‐principles density functional theory calculations and powder X‐ray diffraction experiments. These interactions afford new benchmarks for the highly efficient removal of SO₂ from other gases, even if at a very low SO₂ concentration. Exceptionally high SO₂ capacity of 11.01 mmol g^?1 is achieved at atmosphere pressure by SIFSIX‐1‐Cu, and unprecedented low‐pressure SO₂ capacity is obtained in SIFSIX‐2‐Cu‐i (4.16 mmol g^?1 SO₂ at 0.01 bar and 2.31 mmol g^?1 at 0.002 bar). More importantly, record SO₂/CO₂ selectivity (86–89) and excellent SO₂/N₂ selectivity (1285–3145) are also achieved. Experimental breakthrough curves further demonstrate the excellent performance of these hybrid porous materials in removing low‐concentration SO₂.     

Molecular recognition of protonated polyamines at calix[4]crown-5 self-assembled monolayer modified electrodes by impedance measurements

Molecular recognition of protonated aliphatic polyamines has been studied at calix[4]crown-5 self-assembled monolayer modified gold electrodes by electrochemical impedance spectroscopic (EIS) experiments. The energy of complex formation between the calix [4]crown-5 molecule and a series of alkyl ammonium ions was shown by molecular modeling and EIS experiments to depend on the number of amine groups in the alkyl chain as well as the number of methylene groups between the amine groups. The structures of complexes formed between the crown ether on the lower rim of calix[4]arene and protonated amines were determined by minimizing the complex formation energies. The adducts thus formed on the SAM rendered the electron transfer from the electrode to the probe (Fe(CN)63-/4- pair) easier or more difficult depending on the number of ammonium groups and their arrangement in linear alkyl chains. Analytical procedures have been developed to detect protonated spermidine (a recognized cancer marker) in simulated urine, blood, erythrocyte, and cerebrospinal fluids.     

Solvent extraction and stripping of silver ions in room-temperature ionic liquids containing calixarenes

We found that a calix[4]arene-bearing pyridine is soluble in a typical room-temperature ionic liquid (RTIL), 1-alkyl-3-methylimidazolium hexafluorophosphate. Pyridinocalix[4]arene showed a high extraction ability and selectivity for silver ions. The extraction performance of the calix[4]arene was greatly enhanced by dissolution in RTILs compared to in chloroform. In a competitive extraction test using five different metal ions (Ag+, Cu2+, Zn2+, Co2+, Ni2+), only silver ions were transferred by the calix[4]arene from the aqueous feed phase into the RTILs, through a cation-exchange mechanism. The pyridinocalix[4]arene was found to form a stable 1:1 complex with silver ions, both by slope analysis and by Job's method. Since it is easy to strip silver ions from RTILs by controlling the aqueous-phase pH, the extraction performance of calix[4]arene in RTILs was maintained after five repeated uses.     

Surface-enhanced micro-Raman detection and characterization of calix[4]arene-polycyclic aromatic hydrocarbon host-guest complexes

Surface-enhanced micro-Raman spectroscopy (micro-SERS) was used to detect traces of the hazardous pollutant polycyclic aromatic hydrocarbons (PAHs) pyrene and benzo[c]phenanthrene deposited onto a calix[4]arene-functionalized Ag colloidal surface. High spectral reproducibility and very low molecular detection limits (10(-8) M) were obtained by using 25,27-carboethoxy-26,28-hidroxy-p-tert-butylcalix[4]arene as host molecule. Films of immobilized aggregated Ag nanoparticles, obtained by chemical reduction with hydroxylamine, were prepared by direct adhesion on a glass surface. The influence of the aggregation degree of the initial Ag nanoparticles on the micro-SERS detection effectiveness was checked. Different relative concentrations of the host (calixarene receptor) and the guest (PAHs) were attempted in order to optimize detection of the pollutant. The obtained results indicated that the detection limit is much lower in the case of benzo[c]phenanthrene than in pyrene when exciting with the 785 nm line of a diode laser. A detailed interpretation of the Raman spectra was accomplished in order to obtain more information about the interaction mechanism of the host-guest complex, which could be useful in the future for the design of powerful detection systems.     

Interaction between calix[4]arene derivative bearing adenino units and complementary nucleosides at the air–water interface

The monolayers of the amphiphilic calix[4]arene derivatives at the lower rim with two adenino units on the surface of pure water, the aqueous subphases containing complementary nucleosides were studied by film balance measurement and relaxation experiments. LB films deposited from all subphases were investigated by UV spectra and FT-IR spectra. All the results indicate that the interaction between the adenino units in the headgroup of amphiphilic calix[4]arene derivatives and the complementary nucleosides in the subphase takes place through multiple complementary hydrogen bonding and the nucleosides in the subphases can be transferred to solid substrates along with their monolayers.     

Capillary Electrokinetic Chromatography Employing p-(Carboxyethyl)calix[n]arenes as Running Buffer Additives

Calixarenes, a class of macrocyclic phenolic compounds with a basket-like shape, are used as capillary electrophoresis reagents for separations of native and substituted polycyclic aromatic hydrocarbons. The p-(carboxyethyl)calix[n]arenes reported herein are a series of charged, moderately water soluble macrocyclic molecules that can form complexes with neutral molecules. Electrokinetic chromatographic separations are based on the differential distribution of molecules between a running buffer phase, which is transported by electroosmotic flow, and an electrophoretically mediated calixarene. The size of the calixarene influences separation performance, illustrating the importance of cavity size and geometry in the complexation process. p-(Carboxyethyl)calix[7]arene provides the best efficiency (>10(5) plates/m) and selectivity in these studies. The influences of pH, organic solvent, and field strength on elution range, capacity factors, efficiency, and selectivity are also reported. In general, capacity factors are rather low, but the high charge-to-mass ratios of certain calixarenes produce relatively wide elution ranges. Molecular modeling data and solubility data are used to interpret the observed selectivity.     

Comparison of sensing characteristics of ChemFEC devices based on spin-coated thin films of calix[n]arenes

ChemFECs (chemical field effect capacitors) or EIS (Electrolyte-Insulator-Semiconductor) devices were coated with calix[n]arene macrocycles for cation sensors development. Silicon nitride surface electrodes were spin-coated with p-tert-butylcalix[n]arene molecules for n = 8, 9 and 11. Electrochemical capacitance measurements were carried out to evaluate the sensitivity of the calix[n]arene layers towards specific cations as calcium and copper ions. Sensor responses were due to the “host-guest” specific interaction between the rigid cavity of calix[n]arene receptors (ligand) and chemical ion species. The developed chemical sensors have demonstrated good analytical characteristic in terms of: stability, selectivity and lifetime. In addition, experimental results were fitted using the site binding model in order to determine complex association constants, pK (1.9 ≤ pK ≤ 6.39) and surface density of ligands, N_L (9.10¹⁴ ≤ N_L ≤ 2.10¹⁶). All results obtained in this work were compared with those obtained using silicon dioxide EIS electrodes coated with the same calix[n]arene receptors using both spin-coating and thermal evaporation deposit processes.     

Complexation of higher fullerenes by calix[5]arene-based host molecules

The binding abilities of calix[5]arene-based host molecules for higher fullerenes (C76, C78, and C84) were investigated in organic solvents. The binding abilities of double-calix[5]arenes to higher fullerenes are high. In particular, their association constants to C76 and C78 in toluene reached at the order of 10(5) M(-1). In contrast, the binding abilities of simple calix[5]arenes decreased as increasing the size of the fullerenes. Accordingly, linking two calix[5]arenes with the covalent bonds creates the large cavities, complementary to higher fullerenes. The structure of the host-guest complex of double-calix[5]arene and C76 was discussed. The carbons of the polar end of C76 characteristically shifted upfield more than the other carbons in the host-guest complex, indicating that these polar carbons reside most deeply inside the cavity of the calix[5]arene.     

Comparative studies on the solvent extraction of transition metal cations by calixarene, phenol and ester derivatives

The ionophore solvent extraction of various alkali metal and transition metal cations from the aqueous phase to the organic phase was carried out by using diazo-coupling calix[n]arenes [p-(4-phenylazophenylazo)calix[4]arene (L1) and p-phenylazocalix[6]arene (L2)], phenol derivatives [2,6-dimethyl-3-phenylazophenol (L3), 2-(5-bromo-2-pyridylazo)-5-diethylamino phenol (L4), 2-chloro-4-nitro(phenylazo)-5-sec-butyl-2-phenol (L5) and 2-chloro-4-nitro(phenylazo)-5-tert-butyl-2-phenol (L6)], and ester derivatives [quinoline-8-benzoate (L7), phenyl-1,4-dibenzoate (L8), p-tolyltiobenzoate (L9)]. It was found that, all the compounds (L1-L9) examined showed selectivity for transition metal cations such as Ag+, Hg+ Hg2+, and poor efficiency for alkali metal cations (Na+ and K+). The best extraction efficiency was obtained with L1 and L4.