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The non-equilibrium liquid structure was achieved by interfacial jamming of pillar[5]arene carboxylic acid (P[5]AA) mediated by hydrogen bonding interactions. The assembly was reversibly modulated via jamming to unjamming transition thus dynamically shaping the liquid droplets. Interestingly, these supramolecular constructs showed pH-switchable gated diffusion of encapsulants, hence showcasing a next generation smart release system.

We report the fabrication of pillar[5]arene (P[5]A) stabilized MCs via the self-assembly and crosslinking of P[5]A nanoaggregates at the liquid-liquid interface. These MC microengines turn on fluid flow in the presence of paraquat (PQ) due to "host-guest" molecular recognition. The microengines could be useful in designing non-mechanical micropumps, powerless microfluidics, and diagnostic devices.

Distinct physicochemical properties observed in nanomaterials (NM) have attracted enormous technological and scientific interest as it enables innovative functional applications. These functional properties of NM are interdependent on their size, shape, crystal structure, chemical composition, oxidation states, surface composition, and atomic defects, etc. Hence, a comprehensive understanding of NM composition–structure–property relationships using appropriate characterization techniques is of the utmost importance for the rational design of NM, and to fine-tune its functional properties. Therefore, the selection of the right probing mechanism to study its functional properties will provide informed decisions to select, and optimize appropriate synthesis approaches as per the application demand. Hence, the main objective of this chapter is to summarize different state-of-the-art characterization techniques based on the information they can provide, and the complementarity of the information supplied by various methods. Furthermore, the fundamental probing mechanism of different characterization tools, their strengths, and weaknesses are discussed. Thus, it will serve as a guide for the selection of appropriate characterization techniques for structural, compositional, and physicochemical characterization of NM.

Macrocyclic host structures for generating transiently bound 129 Xe have been used in various ultra-sensitive NMR and MRI applications for molecular sensing of biochemical analytes. They are based on hyperpolarized nuclei chemical exchange saturation transfer (Hyper-CEST). Here, we tested a set of water-soluble pillar[5]arenes with different counterions in order to compare their potential contrast agent abilities with that of cryptophane-A (CrA), the most widely used host for such purposes. The exchange of Xe with such compounds was found to be sensitive to the type of ions present in solution and can be used for switchable magnetization transfer (MT) contrast that arises from off-resonant pre-saturation. We demonstrate that the adjustable MT magnitude depends on the interplay of saturation parameters and found that the optimum MT contrast surpasses the CrA CEST performance at moderate saturation power. Since modification of such water-soluble pillar[5]arenes is straightforward, these compounds can be considered a promising platform for designing various sensors that may complement the field of Xe HyperCEST-based biosensing MRI.

A quinoline functionalized pillar[5]arene, QPA has been prepared and its interaction with biologically relevant ions and molecules in aqueous solution has been demonstrated. The sensor molecule, QPA has shown selectivity towards Fe

A quinoline functionalized pillar[5]arene, QPA has been prepared and its interaction with biologically relevant ions and molecules in aqueous solution has been demonstrated. The sensor molecule, QPA has shown selectivity towards Fe3+ among eleven metal ions studied. The Fe3+ complex of QPA (FeQPA) selectively interacts with F- among halides by ∼4 fold fluorescence enhancement. Further, FeQPA has shown selectivity towards arginine and lysine among twenty naturally occurring amino acids. The binding of QPA with Fe3+ has been confirmed by MALDI-TOF and 1H NMR titrations.

Macrocyclic host structures for generating transiently bound

Cucurbit[n]urils (CB[n], n = 6-8) interact strongly with metal-bound 4'-substituted terpyridine ligands (M = Fe(II) and Ir(III)) via CH···O hydrogen bonding, despite significant separation between the positive metallic cation and the carbonylated rim of CB[n], and the location of the latter in the second coordination sphere of the metal ion. While water has been shown to mediate interactions between cations and CB[n]s in some assemblies, mediation by organic ligands is unprecedented. The recognition process is driven by the contrasted combination of extremely favorable binding enthalpies (up to 20.2 kcal/mol) and very unfavorable entropic components (as low as -10.2 kcal/mol). Dynamic oligomers were prepared in the presence of CB[8], which acts as a "soft", noncovalent linker between metal/terpyridine complexes, and interconnects two 4'-substituents inside its cavity. Social self-sorting between CB[8] and metal/terpyridine complexes bearing 4'-(2-naphthyl) and 4'-(2,3,5,6-tetrafluorophenyl) substituents was also observed, and could afford well-organized oligomers with alternating Fe(II) and Ir(III) cations.

The torsional barrier of biphenyls bearing a prochiral dimethylsulfonium group at their 3-position could be determined by variable-temperature 1H NMR spectroscopy only after encapsulation into cucurbit[7]- or -[8]uril, which triggered the splitting of the two methyl signals. Confinement of the biphenyl units into the macrocycles amplifies the dissymmetry caused by the various ortho- and ortho′-substituents and represents a new tool that can be used to access atropisomerization barriers of bi(hetero)aryl derivatives.

Biofilm formation, which frequently occurs in microbial infections and often reduces the efficacy of antibiotics, also perturbs many industrial and domestic processes. We found that a new class of water soluble pillar[5]arenes bearing phosphonium moieties (1, 2) and their respective ammonium analogues (3, 4) inhibit biofilm formation with IC50 values in the range of 0.67–1.66 μM. These compounds have no antimicrobial activity, do not damage red blood cell membranes, and do not affect mammalian cell viability in culture. Comparison of the antibiofilm activities of the phosphonium-decorated pillar[5]arene derivatives 1 and 2 with their respective ammonium counterparts 3 and 4 and their monomers 5 and 6, demonstrate that while positive charges, charge cooperativity and the pillararene platform are essential for the observed antibiofilm activity the nature of the charges is not.

It is estimated that up to 80% of bacterial infections are accompanied by biofilm formation. Since bacteria in biofilms are less susceptible to antibiotics than are bacteria in the planktonic state, biofilm-associated infections pose a major health threat, and there is a pressing need for antibiofilm agents. Here we report that water-soluble cationic pillararenes differing in the quaternary ammonium groups efficiently inhibited the formation of biofilms by clinically important Gram-positive pathogens. Biofilm inhibition did not result from antimicrobial activity; thus, the compounds should not inhibit growth of natural bacterial flora. Moreover, none of the cationic pillararenes caused detectable membrane damage to red blood cells or toxicity to human cells in culture. The results indicate that cationic pillararenes have potential for use in medical applications in which biofilm formation is a problem.

This review highlights the past six year advances in the blossoming field of cucurbit[n]uril chemistry. Because of their exceptional recognition properties in aqueous medium, these pumpkin-shaped macrocycles have been generating some tremendous interest in the supramolecular community. They have also become key units in various self-organizing and stimulus-controlled assemblies, as well as in advanced materials and drug carriers. The scope of this review is limited to the main family of cucurbit[n]urils (n = 5, 6, 7, 8, 10). The reader will find an overview of their preparation, their physicochemical and biological properties, as well as their recognition abilities towards various organic and inorganic guests. Detailed thermodynamic and kinetic considerations, as well as multiple applications including supramolecular catalysis are also discussed.

A calix[4]arene conjugate bearing salicylyl imine having dibenzyl moiety (L) has been synthesized and characterized, and its ability to recognize three most important essential elements of human system, viz., iron, copper, and zinc, has been addressed by colorimetry and fluorescence techniques. L acts as a sensor for Cu(2+) and Fe(2+) by exhibiting visual color change and for Zn(2+) based on fluorescence spectroscopy. L shows a minimum detection limit of 3.96 ± 0.42 and 4.51 ± 0.53 ppm and 45 ± 4 ppb, respectively, toward Fe(2+), Cu(2+), and Zn(2+). The in situ prepared [ZnL] exhibits phosphate sensing among 14 anions studied with a detection limit of 247 ± 25 ppb. The complexes of Zn(2+), Cu(2+), and Fe(2+) of L have been synthesized and characterized by different techniques. The crystalline nature of the zinc and copper complexes and the noncrystalline nature of simple L and its iron complex have been demonstrated by powder XRD. The structures of Cu(2+) and Zn(2+) complexes have been established by single crystal XRD wherein these were found to be 1:1 monomeric and 2:2 dimeric, respectively, using N(2)O(2) as binding core. The geometries exhibited by the Zn(2+) and the Cu(2+) complexes were found to be distorted tetrahedral and distorted square planar, respectively. The iron complex of L exists in 1:1 stoichiometry as evident from the mass spectrometry and elemental analysis.

A new molecular fluorescent sensor (L) for Cu 2+ has been synthesized by derivatizing the lower rim of calix[4]arene with benzothiazole moiety, through amide linkage to result in 1,3-di-derivative. The receptor molecule, L exhibited fluorescence quenching towards Cu 2+ among eleven divalent ions, viz., Mn 2+ , Fe 2+ , Co 2+ , Ni 2+ , Cu 2+ , Zn 2+ , Cd 2+ , Hg 2+ , Ca 2+ , Mg 2+ and Pb 2+ , studied. The 1:1 stoichiometry of the complex formed between L and Cu 2+ has been demonstrated by electronic absorption and ESI-MS. The role of calix[4]arene for the selective sensing of Cu 2+ has been established by comparing the data with that obtained for an appropriate control molecule. The minimum concentration at which L can detect Cu 2+ has been found to be 403 ppb. The computations carried out at DFT level have provided the coordination and structural features of the Cu 2+ complex of L as species of recognition. The Cu 2+ complex thus formed recognizes iodide by bringing change in the color, among the 14 anions studied.

A (1+1) macrocyclic calix[4]arene based Schiff base derivative has been synthesized and was subjected to complexation with different ions or ionic species, viz., VO2+, UO2 2 +, Fe3+, Ni2+, Cu2+ and Zn2+. Both the conjugate and the complexes were characterized using various spectral techniques, viz.; FTIR, 1H, 13C NMR and FAB mass. The complexes have been further characterized by UV-Vis, EPR and magnetic susceptibility, while the iron complex was studied further by Mossbauer spectroscopy. On the basis of all these studies, the VO2+ and UO2 2 + complexes were found to be mononuclear, whereas all the other complexes were found to be dinuclear. Based on the studies, the iron complex was found have a distorted octahedral high spin Fe(III) center with an antiferromagnetically coupled dinuclear core in the complex. © 2009 Elsevier Ltd. All rights reserved.

Amide linked lower rim 1,3-dibenzimidazole derivative of calix[4]arene, L has been shown to be sensitive and selective to Hg(2+) in aqueous acetonitrile solution based on fluorescence spectroscopy, and the stoichiometry of the complexed species has been found to be 1:1. The selectivity of L toward Hg(2+) has been shown among 11 M(2+) ions, viz., Mn(2+), Fe(2+), Co(2+), Ni(2+), Cu(2+), Zn(2+), Cd(2+), Hg(2+), Pb(2+), Ca(2+), and Mg(2+) studied, including those of the mercury group and none of these ions impede the recognition of Hg(2+) by L. Role of the solvent on the recognition of Hg(2+) has been demonstrated. The role of calix[4]arene platform and the benzimidazole moieties in the recognition of Hg(2+) by L has been delineated upon performing such studies with five different molecules of relevance as reference molecular systems. The binding cores formed by the receptor L and the reference compounds have been established based on the single crystal XRD structures, and the preferential metal ion binding cores have been discussed. The binding of Hg(2+) with L has been further established based on (1)H and (13)C NMR, ESI MS, absorption, and fluorescence lifetime measurements. Some of these techniques have been used to establish the stoichiometry of the species formed. The complex species formed between L and Hg(2+) have been isolated and characterized and found to be 1:1 species even in the isolated complex. Whereas transmission electron microscopy (TEM), atomic force microscopy (AFM), and scanning electron microscopy (SEM) provided the nanostructural behavior of L, the TEM and SEM demonstrated that the mercury complex has different characteristics when compared to L. The TEM, SEM, and powder XRD studies revealed that whereas L is crystalline, that of the mercury complex is not, perhaps a reason for not being able to obtain single crystals of the complex. Binding characteristics of Hg(2+) toward L have been established based on the DFT computational calculations.

The bis-{N-(2-hydroxynaphthyl-1-methylimine)} anchored 1,3-di-derivative of lower rim p-tert-butyl-calix[4]arene possessing a N2O2, N2O4 or N2O6 binding core was found to be selective for Zn(II) ions even at less-than-or-equals, slant60 ppb by eliciting fluorescence-on behaviour while the other ions, viz., Ti4+, VO2+, Cr3+, Mn2+, Fe2+, Co2+, Ni2+, Cu2+, Mg2+, Cd2+ and Hg2+ caused no change in the fluorescence. The reaction between 1 and Zn2+ was found to be stoichiometric with the formation of a 1:1 complex; while H+ quenched the fluorescence of the complex, OH− restored it. The studies of the 1:1 isolated complexes of Zn2+, Ni2+ and Cu2+ augmented the results., © Elsevier

The diastereoselectivity of Cucurbit[8]uril (CB[8]) binding was probed towards a series of rapidly interconverting diastereomers containing a Caryl–Caryl chiral axis and at least one other stereocenter. Relative binding affinities of up to 4.9 were determined when CB[8] interacted with ortho, meta, ortho′-substituted biphenyls bearing a chiral dialkylsulfonium substituent at their meta-position. Diastereoselectivities of up to 2.4-fold were obtained for ortho′-substituted 2-phenylpyridinium derivatives that bear a chiral myrtenyl N-substituent prone to CB[8] binding.

Biphenyl derivatives bearing a dimethylsulfonium group at position 3 and three different substituents at position 4 (H, F and CH3) have been prepared as probes to test the validity of the “supramolecular buttressing” concept. We define the latter as the alteration, by a neighboring unit, of a substituent effect on intermolecular recognition. In this case, the 4-substituents exert some pressure on the 3-dimethylsulfonium groups and control the ratio of their syn and anti conformations. As free species, biphenyls bearing 4-H and 4-F substituents are present as approximately equimolar mixtures of syn and anti-conformers, while the biphenyl scaffold with a 4-CH3 group adopts the anti-conformation exclusively. The 3-dimethylsulfonium substituents then interact with one of the carbonylated portals of Cucurbit[7]uril (CB[7]), and their conformations affect the position of the guests inside the cavity of the macrocycle, thereby validating our “supramolecular buttressing” model. Surprisingly however, binding affinities towards CB[7] are barely affected by the nature of the 4-substituents and the conformations of the neighboring sulfonium groups, despite very different electronic densities presented to the CB[7] portal in their syn or anti conformations. Solvation was found to dramatically smoothen host–guest Columbic interactions, although the latter remain important in the recognition process. Replacing the positively charged 3-dimethylsulfonium unit with an isopropyl substituent decreases the affinity of the biphenyl guest by 1000-fold.

This review highlights the past six year advances in the blossoming field of cucurbit[n]uril chemistry. Because of their exceptional recognition properties in aqueous medium, these pumpkin-shaped macrocycles have been generating some tremendous interest in the supramolecular community. They have also become key units in various self-organizing and stimulus-controlled assemblies, as well as in advanced materials and drug carriers. The scope of this review is limited to the main family of cucurbit[n]urils (n = 5, 6, 7, 8, 10). The reader will find an overview of their preparation, their physicochemical and biological properties, as well as their recognition abilities towards various organic and inorganic guests. Detailed thermodynamic and kinetic considerations, as well as multiple applications including supramolecular catalysis are also discussed.

A calix[4]arene conjugate bearing salicylyl imine having dibenzyl moiety (L) has been synthesized and characterized, and its ability to recognize three most important essential elements of human system, viz., iron, copper, and zinc, has been addressed by colorimetry and fluorescence techniques. L acts as a sensor for Cu(2+) and Fe(2+) by exhibiting visual color change and for Zn(2+) based on fluorescence spectroscopy. L shows a minimum detection limit of 3.96 ± 0.42 and 4.51 ± 0.53 ppm and 45 ± 4 ppb, respectively, toward Fe(2+), Cu(2+), and Zn(2+). The in situ prepared [ZnL] exhibits phosphate sensing among 14 anions studied with a detection limit of 247 ± 25 ppb. The complexes of Zn(2+), Cu(2+), and Fe(2+) of L have been synthesized and characterized by different techniques. The crystalline nature of the zinc and copper complexes and the noncrystalline nature of simple L and its iron complex have been demonstrated by powder XRD. The structures of Cu(2+) and Zn(2+) complexes have been established by single crystal XRD wherein these were found to be 1:1 monomeric and 2:2 dimeric, respectively, using N(2)O(2) as binding core. The geometries exhibited by the Zn(2+) and the Cu(2+) complexes were found to be distorted tetrahedral and distorted square planar, respectively. The iron complex of L exists in 1:1 stoichiometry as evident from the mass spectrometry and elemental analysis.

A new 1,3-diderivative of calix[4]arene appended with hydroxymethyl salicylyl imine has been synthesized and its ion recognition toward biologically relevant M(n+) ions studied. The receptor H(2)L showed selectivity toward Zn(2+) by switch-on fluorescence among the 12 metal ions studied with a detection limit of 192 ppb. The interaction of Zn(2+) with H(2)L has been further supported by absorption studies, and the stoichiometry of the complex formed (1:1) has been established on the basis of absorption and ESI MS. Competitive ion titrations carried out reveal that the Zn(2+) can be detected even in the presence of other metal ions of bioimportance. The mode of interaction of Zn(2+) with conjugate has been established by a fleet of computational calculations carried out in a cascade manner, either on the ligand or on the complex, wherein the final optimizations were carried out by the density functional theory (DFT) and found that the Zn(2+) and Cd(2+) indeed bind differently. In situ prepared [ZnL] complex responds to both inorganic phosphate as well as AMP, ADP, and ATP with a minimum detection limit of 426 ppb wherein the Zn(2+) from the complex is detached and recomplexed by the added phosphate moiety. It has been possible to build an INHIBIT logic gate for the conjugate using Zn(2+) and HPO(4)(2-) as inputs by monitoring the fluorescence emission band at 444 nm as output. The amino acid sensing abilities of [ZnL] have been explored by fluorescence and absorbance spectroscopy where it showed selectivity toward Cys, Asp, and His through the formation of the Zn(2+) complex of these amino acids by chelating through their side chain moieties. Thus, while H(2)L is selective for Zn(2+) among a number of cations, the [ZnL] is selective toward phosphate among a number of anions and also toward Asp, Cys, and His among the naturally occurring amino acids.

A structurally characterized lower rim 1,3-di{bis(2-picolyl)}amide derivative of calix[4]arene (L) exhibits high selectivity toward Ag(+) by Forming it 1:1 complex, among nine other biologically important metal ions, viz., Na(+), K(+), Mg(2+), Ca(2+), Mn(2+), Fe(2+), Co(2+), Ni(2+), and Zn(2+), its studied by fluorescence, absorption, and (1)H NMR spectroscopy. The 1:1 complex formed between L and Ag(+) has been further proven on the basis of ESI mass spectrometry and has been shown to have art association constant, K(a), of 11117 +/- 190 M(-1) based on fluorescence data. L acts as it primary ratiometric sensor toward Ag(+) by switch-on fluorescence and exhibits a lowest detectable concentration of 450 ppb. DFT computational studies carried out in mimicking thie formation of a 1:1 complex between L and Ag(+) resulted in it tetrahedral complex wherein the nitrogens of all four pyridyl moieties present on both arms are being coordinated. Whereas these pyridyls are located farther apart in the crystal structure, appropriate dihedral changes are induced in the arms in the presence of silver ion in order to form a coordination complex. Even the nanostructural features obtained in TEM clearly differentiates L from its Ag(+) complex. The in situ prepared silver complex of L detects Cys ratiometrically among the naturally occurring amino acids to a lowest concentration of 514 ppb by releasing L from the complex followed by formation of the cysteine complex of Ag(+). These were demonstrated on the basis of emission,absorption, (1)H NMR,and ESI mass spectra. The INH logic gate has also been generated by choosing Ag(+) and Cys as input and by monitoring the Output signal at 445 nm that originates from the excimer emission of L in the presence of Ag(+). Thus L is a potential primary sensor toward Ag(+) and is it secondary sensor toward Cys.

A highly selective fluorescence switch on sensor, L for detecting Cu2+ has been synthesized by introducing a bis-(2-picolyl)amine moiety at the lower rim of a calix[4]arene platform via amide linkage. Binding properties of L toward ten different biologically relevant Mn+ ions have been studied by fluorescence and absorption spectroscopy in methanol and aqueous methanol. L was found to detect Cu2+ selectively down to a concentration of 196 and 341 ppb, respectively, in methanol and 1:1 aqueous methanol even in the presence of other metal ions. The composition of the complex has been found to be 1:1 based on the Job plot and is further confirmed by ESI MS. The role of calix[4]arene platform as well as the pre-organized binding core in the selective recognition of Cu2+ has been demonstrated by studying appropriate reference molecules. The possible modes of binding of L with Cu2+ have been modeled by computational calculations. L and its Cu2+ complex could very well be differentiated based on the nano-structural features observed in SEM and AFM., © Elsevier

Single crystal XRD structure of the lower rim 1,3-di-amide-derivative of calix[4]arene possessing bis-{N-(2,2'-dipyridylamide)} pendants (L) exhibit two distinct binding cores, viz., N 4 and O 6 . L was found to be selective for Zn 2+ by switch-on and for Ni 2+ by switch-off fluorescence by forming 1:1 complexes. The binding and the composition of the complex formed have been addressed based on steady state and time-resolved fluorescence spectroscopy in addition to the absorption and ESI MS. As L can detect Zn 2+ and Ni 2+ to a concentration as low as 142 and 203 ppb, respectively, L can be a very sensitive molecular probe for these ions. The coordination details of the metal ion-bound complexes have been addressed based on ab initio calculations showing that the stabilization energies are commensurate with the coordination formed.

Interaction of heavy metal ions, viz., Pb2+, Hg2+ and Cd2+ with 1,3-bis(aminoethoxy)-calix[4]arene was studied using emission spectroscopy and the corresponding binding parameters were derived. Based on the changes observed in the fluorescence during titration with these metal ions, two types of species were proposed, viz., one in which the metal ion is bound at the lower rim, the other in which the metal ion interacts through the hydrophobic arene cavity. The extent to which these species were present depends upon the type of the metal ion as well as the number of metal ion equivalents added. The results were compared with unmodified calix[4]arene titration data. Based on these studies, Hg2+ can distinctly be recognized from the other heavy metal ions., © Elsevier