Oligoether‐Strapped Calix[4]pyrrole: An Ion‐Pair Receptor Displaying Cation‐Dependent Chloride Anion Transport

A ditopic ion‐pair receptor ( 1 ), which has tunable cation‐ and anion‐binding sites, has been synthesized and characterized. Spectroscopic analyses provide support for the conclusion that receptor 1 binds fluoride and chloride anions strongly and forms stable 1:1 complexes ([ 1? F]^? and [ 1? Cl]^?) with appropriately chosen salts of these anions in acetonitrile. When the anion complexes of 1 were treated with alkali metal ions (Li⁺, Na⁺, K⁺, Cs⁺, as their perchlorate salts), ion‐dependent interactions were observed that were found to depend on both the choice of added cation and the initially complexed anion. In the case of [ 1? F]^?, no appreciable interaction with the K⁺ ion was seen. On the other hand, when this complex was treated with Li⁺ or Na⁺ ions, decomplexation of the bound fluoride anion was observed. In contrast to what was seen with Li⁺, Na⁺, K⁺, treating [ 1?F ]^? with Cs⁺ ions gave rise to a stable, host‐separated ion‐pair complex, [F ?1? Cs], which contains the Cs⁺ ion bound in the cup‐like portion of the calix[4]pyrrole. Different complexation behavior was seen in the case of the chloride complex, [ 1? Cl]^?. Here, no appreciable interaction was observed with Na⁺ or K⁺. In contrast, treating with Li⁺ produces a tight ion‐pair complex, [ 1? Li ? Cl], in which the cation is bound to the crown moiety. In analogy to what was seen for [ 1? F]^?, treatment of [ 1? Cl]^? with Cs⁺ ions gives rise to a host‐separated ion‐pair complex, [Cl ?1? Cs], in which the cation is bound to the cup of the calix[4]pyrrole. As inferred from liposomal model membrane transport studies, system 1 can act as an effective carrier for several chloride anion salts of Group 1 cations, operating through both symport (chloride+cation co‐transport) and antiport (nitrate‐for‐chloride exchange) mechanisms. This transport behavior stands in contrast to what is seen for simple octamethylcalix[4]pyrrole, which acts as an effective carrier for cesium chloride but does not operates through a nitrate‐for‐chloride anion exchange mechanism.     

Ion‐Free and Ion‐Pairing Assemblies of Anion‐Responsive π‐Electronic Systems Possessing Directly Linked Alkyl Chains

Alkyl‐substituted pyrrole‐based anion‐responsive π‐electronic systems formed supramolecular gels and liquid crystals through effective π–π stacking and van der Waals interactions. The addition of chloride as a planar cation salt afforded ion‐pairing assemblies as soft materials comprising planar receptor‐Cl^? complexes and the cation.     

Ion‐Pairing Assemblies of Porphyrin–AuIII Complexes in Combination with π‐Electronic Receptor–Anion Complexes

Anion complexes of anion‐responsive π‐electronic molecules can behave as pseudo π‐electronic anions providing various ion pairs in combination with countercations. In this study, single crystals of ion‐pairing assemblies comprising porphyrin–Au^III complexes and Cl^? complexes of dipyrrolyldiketone BF₂ complexes were prepared from 1:1 mixtures of anion receptors and the Cl^? salts of cationic porphyrins in solution. In the solid state, the ion pairs formed characteristic assemblies, depending on the substituents of the anion receptors and porphyrin–Au^III complexes. Theoretical calculations on the ion pairs revealed that the stacking structures are stabilized by compensating positive and negative charges as well as π–π interactions.     

Ion‐Pairing Assemblies Based on Pentacyano‐Substituted Cyclopentadienide as a π‐Electronic Anion

Pentacyanocyclopentadienide (PCCp^?), a stable π‐electronic anion, provided various ion‐pairing assemblies in combination with various cations. PCCp^?‐based assemblies exist as single crystals and mesophases owing to interionic interactions with π‐electronic and aliphatic cations with a variety of geometries, substituents, and electronic structures. Single‐crystal X‐ray analysis revealed that PCCp^? formed cation‐dependent arrangements with contributions from charge‐by‐charge and charge‐segregated assembly modes for ion pairs with π‐electronic and aliphatic cations, respectively. Furthermore, some aliphatic cations gave dimension‐controlled organized structures with PCCp^?, as observed in the mesophases, for which synchrotron XRD analysis suggested the formation of charge‐segregated modes. Noncontact evaluation of conductivity for (C₁₂H₂₅)₃MeN⁺ ? PCCp^? films revealed potential hole‐transporting properties, yielding a local‐scale hole mobility of 0.4 cm² V^?1 s^?1 at semiconductor–insulator interfaces.     

Self‐Assembled Columnar Triazole Quartets: An Example of Synergistic Hydrogen‐Bonding/Anion–π Interactions

The self‐assembly of triazole amphiphiles was examined in solution, the solid state, and in bilayer membranes. Single‐crystal X‐ray diffraction experiments show that stacked protonated triazole quartets (T₄) are stabilized by multiple strong interactions with two anions. Hydrogen bonding/ion pairing of the anions are combined with anion–π recognition to produce columnar architectures. In bilayer membranes, low transport activity is observed when the T₄ channels are operated as H⁺/X^? translocators, but higher transport activity is observed for X^? in the presence of the K⁺‐carrier valinomycin. These self‐assembled superstructures, presenting intriguing structural behaviors such as directionality, and strong anion encapsulation by hydrogen bonding supported by vicinal anion–π interactions can serve as artificial supramolecular channels for transporting anions across lipid bilayer membranes.     

The Hofmeister Anion Effect on Ionophore‐based Ion‐selective Nanospheres Containing Solvatochromic Dyes

Recently reported ionophore‐based ion‐selective nanospheres contained pH‐independent and positively charged solvatochromic dyes. Here, we evaluate systematically the effect of anions to the fluorescence response of the nanospheres. The anion interference was found significant for anion concentrations above 10 mM. The sensor responses in the presence of various anion background was studied. While target ion (K⁺) causes the fluorescence of the nanospheres to decrease, increasing anion background also leads to lower fluorescence intensity. Lipophilic anions such as ClO₄^?, SCN^?, and I^? exhibited much more interference than hydrophilic anions (e. g., NO₃^?, Cl^?, F^?, SO₄^2?). The trend of the anion interference followed the Hofmeister series. A theoretical model was also demonstrated based on anion adsorption on the surface of the nanospheres.     

Voltage‐Switchable HCl Transport Enabled by Lipid Headgroup–Transporter Interactions

Synthetic anion transporters that facilitate transmembrane H⁺/Cl^? symport (cotransport) have anti‐cancer potential due to their ability to neutralize pH gradients and inhibit autophagy in cells. However, compared to the natural product prodigiosin, synthetic anion transporters have low‐to‐modest H⁺/Cl^? symport activity and their mechanism of action remains less well understood. We report a chloride‐selective tetraurea macrocycle that has a record‐high H⁺/Cl^? symport activity similar to that of prodigiosin and most importantly demonstrates unprecedented voltage‐switchable transport properties that are linked to the lack of uniport activity. By studying the anion binding affinity and transport mechanisms of four other anion transporters, we show that the lack of uniport and voltage‐dependent H⁺/Cl^? symport originate from strong binding to phospholipid headgroups that hampers the diffusion of the free transporters through the membrane, leading to an unusual H⁺/Cl^? symport mechanism that involves only charged species. Our work provides important mechanistic insights into different classes of anion transporters and a new approach to achieve voltage‐switchability in artificial membrane transport systems.     

Structure‐Driven Selection of Adaptive Transmembrane Na+ Carriers or K+ Channels

Self‐assembled alkyl‐ureido‐benzo‐15‐crown‐5‐ethers are selective ionophores for K⁺ cations, which are preferred to Na⁺ cations. The transport mechanism is determined by the optimal coordination rather than classical dimensional compatibility between the crown ether hole and the cation diameter. Herein, we demonstrate that systematic changes of the structure lead to unexpected modifications in the cation‐transport activity and suffice to produce adaptive selection. We show that the main contribution to performance arises from optimal constraints on the conformational freedom, which are determined by the binding macrocycles, the nature of the hydrogen‐bonding groups, and the hydrophobic tails. Simple changes to the flexible 15‐crown‐5‐ether lead to selective carriers for Na⁺. Hydrophobic stabilization of the channels through mutual interactions between lipids and variable hydrophobic tails appears to be an important cause of increased activity. Oppositely, restricted translocation is achieved when constrained hydrogen‐bonded macrocyclic relays are less dynamic in a pore superstructure.     

Investigation of Protein Binding With All Solid‐State Ion‐Selective Electrodes

A potentiometric sensor for studying charge based adsorption of proteins was created using a single‐piece polyaniline‐PVC ion‐selective electrode (ISE). Three different ISEs, two for Na⁺ and one for Cl^? ion determination, were studied. The Na⁺‐ISEs consisted of a neutral calixarene‐based ionophore and one with a charged carrier dinonylnapthalenesulfonic acid (DNNSA) whereas for the Cl^? ISE, an anion exchanger tridodecylmethylammonium chloride (TDDMA⁺Cl^?), was used. The Na⁺ ISE with DNNSA as the charged carrier was successfully able to discriminate the binding of two different proteins (bovine serum albumin and lysozyme) based on their intrinsic charge.     

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 Fluorescent Ditopic Rotaxane Ion‐Pair Host

We report a rotaxane based on a simple urea motif that binds Cl^? selectively as a separated ion pair with H⁺ and reports the anion binding event through a fluorescence switch‐on response. The host selectively binds Cl^? over more basic anions, which deprotonate the framework, and less basic anions, which bind more weakly. The mechanical bond also imparts size selectivity to the ditopic host.     

Raman Spectroscopic Study on Alkyl Chain Conformation in 1‐Butyl‐3‐methylimidazolium Ionic Liquids and their Aqueous Mixtures

Ionic liquids of 1‐butyl‐3‐methylimidazolium ([BMIM]) cation with different anions (Cl^?, Br^?, I^?, and BF₄^?), and their aqueous mixtures were investigated by using Raman spectroscopy and dispersion‐included density functional theory (DFT). The characteristic Raman bands at 600 and 624 cm^?1 for two isomers of the butyl chain in the imidazolium cation showed significant changes in intensity for different anions as well as in aqueous solutions. The area ratio of these two bands followed the order I^?>Br^?>Cl^?>BF₄^? (in terms of the anion X in [BMIM]X), indicating that the butyl chain of [BMIM]I tends to adopt the trans conformation. The butyl chain was found to adopt the gauche conformation upon dilution, irrespective of the anion type. The Raman bands in the butyl C?H stretch region for [BMIM]X (X=Cl^?, Br^?, and I^?) blueshifted significantly with the increase in the water concentration, whereas that for [BMIM]BF₄ changed very little upon dilution. The blueshift in the C?H stretch region upon dilution also followed the order: [BMIM]I>[BMIM]Br>[BMIM]Cl>[BMIM]BF₄, the same order as the above trans conformation preference of the butyl chain in pure imidazolium ionic liquids, which suggested that the cation‐anion interaction plays a role in determining the conformation of the chain.     

Modelling Ion‐Pair Geometries and Dynamics in a 1‐Ethyl‐1‐methylpyrrolidinium‐Based Ion‐Conductive Crystal

Full conformational and energy explorations are conducted on an organic ionic plastic crystal, 1‐ethyl‐1‐methylpyrrolidium tetrafluoroborate [C₂mpyr][BF₄]. The onsets of various stages of dynamic behaviour, which appear to account for low‐temperature solid–solid phase transitions, are investigated by using quantum‐chemical simulations. It is suggested that pseudorotation of the pyrrolidine ring occurs in the first instance; the partial rotation of the entire cation subsequently occurs and may be accompanied by reorientation of the ethyl chain as the temperature increases further. A cation–anion configuration, whereby BF₄^? interacts with the C₂mpy cation from the side of the ring, is the most likely structure in the low‐temperature phase IV region. These interpretations are supported by ¹³C nuclear magnetic resonance chemical‐shift analysis.     

Design of a Vitamin B1‐Selective Electrode Based on an Ion‐Pair and Its Application to Pharmaceutical Analysis

Vitamin B₁‐selective electrodes with PVC membrane were developed that contain ion associates of vitamin B₁ with an inorganic anion, BiI₄^?, and an organic anion, brilliant yellow, as electrode‐active substances. The linearity ranges of the electrode function are 1.0×10^?5–1.0×10^?2 and 1.0×10^?4–1.0×10^?2 M, the electrode function slopes are 33.0±1.0 and 33.1±1.1 mV decade^?1, the detection limits are 5.5×10^?6 and 8.3×10^?5 M for BiI₄^? and brilliant yellow respectively. The working range of pH is 5–12. The efficiency of the use of electrodes for the vitamin B₁ content control in multivitamin pharmaceutical preparations was shown by direct potentiometry and potentiometric titration methods.     

Bifunctional Ion‐Selective Electrode Based on C60‐Cryptand 22 for Silver and Iodine Ions

Fullerence C60‐cryptand 22 was prepared and successfully applied as the electric carrier in the PVC electrode membrane of a bifunctional ion‐selective electrode for cations, e.g., Ag⁺ ions as well as anions, e.g., I^? ions. The bifunctional ion‐selective electrode based on C60‐cryptand 22 can be applied as a Silver (Ag⁺) ion selective electrode with an internal electrode solution of 10^?3 M AgNO₃ in water (pH = 6.3), or as an Iodide (I^?) ion selective electrode with an acidic internal electrode solution of 10^?4 M KI_(aq) (pH = 2) in which the cryptand 22 is protonated, and the C60‐cryptand 22 is changed to C60‐Cryptand22–H⁺ and becomes an anionic electro‐carrier to absorb the I^? ion. The Ag⁺ ion selective electrode based on C60‐cryptand 22 gave a linear response with a near‐Nernstian slope (59.5 mV decade^?1) within the concentration range 10^?1‐10^?3 M Ag⁺_(aq). The Ag⁺ ion electrode exhibited comparatively good selectivity for silver ions, over other transition‐metal ions, alkali and alkaline earth metal ions. The Ag⁺ ion selective electrode with good stability and reproducibility was successfully used for the titration of Ag⁺_(aq) with Cl^? ions. The Iodide (I^?) Ion selective electrode based on protonated C60–cryptand22‐H⁺ also showed a linear response with a nearly Nernstian slope (58.5 mV decade^?1) within 10^?1 ‐ 10^?3 M I^? _(aq) and exhibited good selectivity for I^? ions and had small selectivity coefficients (10^?2–10^?3) for most of other anions, e.g., F^? , OH^?, CH₃COO^?, SO₄^2?, CO₃^2?, CrO₄^2?, Cr₂O₇^2? and PO₄^3? ions.     

Characteristics of Lariat Crown Ether‐Copper (II) Ion‐Selective Electrodes

An ion‐selective electrode using ionophore 2′‐picolyl sym‐dibenzo‐16‐crown‐5 ether as membrane carrier, sodium tetraphenylborate (NaTPB) as an anion excluder, and 2‐nitrophenyl‐octyl ether (NOPE) as the plasticing solvent mediator has been successfully developed. This electrode exhibits al in ear response with a slope of 42 mV/decade in concentration ranging from 10^?5 molL^?1 to 10^?1 molL^?1, slightly larger than the 30 mV expected from the one‐to‐one complex. The reason for the super‐Nernstain slope is the partial dimmer formation in side the membrane of the electrode, because this dimmer [Cu(C₂₅H₂₇NO₆)₂(H₂O)₂] 2ClO₄, has been isolated and confirmed by single crystal X‐ray crystallography. The detection limit for the cop per (II) ion was estimated to be 1 × 10^?6 molL^?1. Electrades composed of other plasticing solvent mediators such as tris(2‐ethylhexyl) phosphate (TOP), bis (2‐ethylhexyl) sebacate (DOS) and dibutyl phthalate (DBP) were also investigated. Stability constants (logKs) of the two to one and the one to one 2‐picolylsym‐dibenzo‐16‐crown‐5 ether‐Cu (II) complexes have been determined by potentiometric titration in methanol.     

Mass Distribution and Diffusion of [1‐Butyl‐3‐methylimidazolium][Y] Ionic Liquids Adsorbed on the Graphite Surface at 300–800 K

The structure and diffusion behavior of 1‐butyl‐3‐methylimidazolium ([bmim]⁺) ionic liquids with [Cl]^?, [PF₆]^?, and [Tf₂N]^? counterions near a hydrophobic graphite surface are investigated by molecular dynamics simulation over the temperature range of 300–800 K. Near the graphite surface the structure of the ionic liquid differs from that in the bulk and it forms a well‐ordered region extending over 30 Å from the surface. The bottom layer of the ionic liquid is stable over the investigated temperature range due to the inherent slow dynamics of the ionic liquid and the strong Coulombic interactions between cation and anion. In the bottom layer, diffusion is strongly anisotropic and predominantly occurs along the graphite surface. Diffusion perpendicular to the interface (interfacial mass transfer rate k_t) is very slow due to strong ion–substrate interaction. The diffusion behaviors of the three ionic liquids in the two directions all follow an Arrhenius relation, and the activation barrier increases with decreasing anion size. Such an Arrhenius relation is applied to surface‐adsorbed ionic liquids for the first time. The ion size and the surface electrical charge density of the anions are the major factors determining the diffusion behavior of the ionic liquid adjacent to the graphite surface.     

Micro Solid‐Contact Ion‐Selective Electrode Using a Carbon Nanotube Tower as Ion‐to‐Electron Transducer and Conductive Substrate

Solid contact (SC) ion‐selective electrodes (ISEs) have been recognized as the next generation of ISEs. In this work, the electrical conductivity and mechanical strength of a carbon nanotube (CNT) tower enable it to play the dual roles of transducer and substrate for micro SC‐ISEs. The electrode had a close to Nernstian slope of 35 mV/decade a_Ca2+, a linear range of four orders of magnitude of calcium ion activity (10^?5.6 to 10^?1.8 M), and a detection limit of 1.6×10^?6 M. The simplified fabrication by a one‐step drop casting makes miniaturizing SC‐ISEs and fabricating sensor arrays easier to achieve.     

Mass spectrometric investigations of α‐ and β‐cyclodextrin complexes with ortho‐, meta‐ and para‐coumaric acids by negative mode electrospray ionization

The mass spectrometric characterization of aqueous solutions of α‐ and β‐cyclodextrins (CDs) and o‐, m‐ and p‐coumaric acids (CAs) by negative ion electrospray ionization (ESI) indicates that the [CD+CA]^? ions were sourced from the inclusion complex present in solution and from the anion attached to CD molecules formed in the spray processes. The anion adducts formed in the spray process contribute significantly to the signal intensity of an ionized inclusion complex thus overestimating the calculated stability constant (K) of solution‐phase complexes by one to two orders of magnitude. The relative intensities of anion adducts in mass spectra depend on the concentration ratio of the anion and the CD in spray droplets, while the relative intensity of the ionized inclusion complex depends on CD and CA concentrations in solutions and the value of K. Ion Mobility Spectrometry Mass Spectrometry [IMS‐MS] measurements show that the collision cross‐section (Ω) values of the [CD+CA]^? or [(CD)₂₊CA]^2? and [CD+CA] complex ions are 5–6% larger than or equal to CD^? or [CD], respectively. Therefore, in the gas phase the anion adducts [CD+CA^?] on cyclodextrin molecules possess the same conformations as the ionized inclusion complexes [CD+CA]^?. Copyright © 2008 John Wiley & Sons, Ltd.     

Highly Thiocyanate‐Selective PVC Membrane Electrode Based on Lipophilic Ferrocene Derivative

A novel anion‐selective PVC membrane electrode based on bis‐[(3‐ferrocenyl)‐(2‐crotonic acid)] copper(II) complex [Cu(II)‐BFCA] as neutral carrier is described, which demonstrates excellent potentiometric response characteristics toward thiocyanate ion and anti‐Hofmeister selectivity sequence in following order: SCN^?>I^?>ClO >Sal^?>Br^?>NO >Cl^?≈NO >SO >SO . The electrode shows a near‐Nernstian response for thiocyanate ion in a wide range of 9.0×10^?7–1.0×10^?1 M with a detection limit 6.8×10^?7 M and a slope of ?59.1 mV/decade in pH 5.0 of phosphate buffer solution at 20 °C. The influences of lipophilic cationic and anionic additives on the response properties of the electrode were investigated. High sensitivity and wide linear dynamic range were observed for the electrode in the presence of hexadecyltrimethylammoniumborate (HTAB) as a lipophilic cationic additive. The electrode was successfully applied to the determination of thiocyanate ion in waste water and human saliva.