Ditopic Receptor Research Articles

Azadithiacrown ether based ditopic receptors capable of simultaneous multi-ionic recognition of Ag+ and Hg2+

Abstract Novel azadithiacrown ether containing bis(styryl)pyridinium dyes were synthesized and their complexation with different transition metal cations was investigated. It was found that the compounds give a selective ratiometric optical response in the presence of silver (I) and mercury (II) cations. Evidence was given for the occurrence of two stoichiometries: LM and LM 2 . Moreover, the formation of a mixed complex [Ag + · L ·Hg 2+ ] containing both Ag + and Hg 2+ cations simultaneously was demonstrated. Optical spectroscopy was used to estimate stability constant values of the complexes, NMR spectroscopy data allowed us to offer a model of spatial arrangement of the obtained species.

A new ion pair receptor fulfilling a dual function as a chromogenic molecular switch for F− and ratiometric selective recognition of HSO4−

A new ditopic ion pair receptor 1 containing two biindole moieties and a bis-benzocrown ether unit shows a remarkable color switching (ON- and -OFF) function induced by anion (F−) and cation (K+) recognition. The ditopic receptor 1 binds in a cooperative fashion to HSO4− in the presence of 1·K+ and acts as a selective ditopic receptor to recognize ion pairs with a wavelength-ratiometric manner.

Fluorescent receptor for dicarboxylates based on pyrrolecarboxamides

Neutral ditopic and fluorescent receptor 1 based on two 2,5-pyrrolecarboxamide units shows a strong association with dicarboxylate anions such as glutarate and adipate through multiple hydrogen bonds. Fluorescence and 1H NMR titrations display a 1:1 stoichiometry for the complexes, with glutarate as the most strongly bound guest. Receptor 1 exhibits PET-induced fluorescence quenching upon recognition due to the presence of a dansyl group. The synthetic methodology described here might provide access to closely related receptors by the introduction of different fluorophores in the last step of the synthesis.

Calix[4]arene-based ditopic receptors for simultaneous recognition of fluoride and cobalt(II) ions

A series of calix[4]arene based ditopic receptors possessing bipyridyl and hydrazone units have been synthesized and evaluated for ionic recognition. It has been observed that the synthesized derivatives function as allosteric receptors for simultaneous recognition of Co2+and F− ions through non-covalent interactions. Significant bathochromic shifts in the UV–visible spectrum with a profound colour change promise their use to engineer novel applications.

Ditopic Receptors based on Lower‐ and Upper‐Rim Substituted Hexahomotrioxacalix[3]arenes: Cation‐Controlled Hydrogen Bonding of Anion

Heteroditopic hexahomotrioxacalix[3]arene receptors that are capable of binding an anion and a cation simultaneously in a cooperative fashion were synthesized. The structure of one of the triamide derivatives was confirmed by single-crystal X-ray diffraction. The binding of alkali metals at the lower rim, and the binding of anions (chloride, bromide) at the upper rim, has been investigated by using (1)H NMR titration experiments. Alkali metal binding at the lower rim controls the calix cavity. Li(+)-ion binding to the lower rim can improve the binding ability of anions at the upper rim amide moiety by a factor of 15, thus suggesting a strong positive allosteric effect for anion recognition. However, when a Na(+) cation is bound to the ionophoric site on the lower rim, the calix cavity is changed from a "flattened cone" to a more-upright form, which is favored for intramolecular hydrogen bonding between the neighboring NH and C=O groups; this change can block the inclusion of anions onto the amide moiety at the upper rim, which strongly suggests a negative allosteric effect of Na(+)-ion binding, which controls the cooperative recognition system.

Synthesis, structural characterization, solvatochromism, and ion-binding studies of a ditopic receptor based on 2-(4-[2,2′: 6′,2′′]terpyridin-4′-yl-phenyl)-1H-phenanthro[9,10-d] imidazole (tpy-HImzphen) unit

A terpyridyl-imidazole based ditopic receptor 2-(4-[2,2′:6′,2′′]terpyridin-4′-yl-phenyl)-1H-phenanthro[9,10-d]imidazole (tpy-HImzphen) has been synthesized and characterized using standard analytical and spectroscopic techniques and also by single crystal X-ray crystallography. The receptor shows multi-channel sensing ability towards F− and Fe2+ ions. Both the anion and cation binding properties of the receptor were thoroughly investigated in dimethylformamide–acetonitrile (1 : 9) solution using absorption, emission, and 1H NMR spectral studies. In the presence of excess of F− ions, deprotonation of the imidazole N–H fragment occurs, an event which is signalled by the development of bright yellow color visible with the naked eye. Anion-induced lifetime enhancing makes the receptor a suitable lifetime-based sensor for F− ions. The absorption and emission titration of the receptor with Fe2+ ion indicates that single conversion of free tpy-HImzphen (colorless) to Fe2+ complex (deep violet color) of 2 tpy-HImzphen : 1 Fe2+ stoichiometry. The influence of different solvents on the absorption and emission spectral behavior and on the lifetime of the receptor and its Zn(II) complex have also been investigated in detail.

Fluorogenic sensing of CH3CO2− and H2PO4− by ditopic receptor through conformational change

Cyclo-bis-(urea-3,6-dichlorocarbazole) (1) forms a 1 : 2 complex with CH(3)CO(2)(-) and H(2)PO(4)(-) through hydrogen bonding with the two urea moieties, resulting in fluorescence enhancement via a combined photoinduced electron transfer (PET) and energy transfer mechanism. The binding mechanism involves a conformational change of the two urea receptors to a trans orientation after binding of the first anion, which facilitates the second interaction.

Kinetics of Zn2+ complexation by a ditopic phenanthroline-azamacrocyclic scorpiand-like receptor

Coordination of Zn(2+) to a ditopic phenanthroline-macrocycle receptor takes place in three steps, the first one being the coordination to the phenanthroline, followed by the slow movement of the metal to the polyamine macrocycle and a final re-arrangement to coordinate the pendent arm.

A Bisferrocene-Benzobisimidazole Triad as a Multichannel Ditopic Receptor for Selective Sensing of Hydrogen Sulfate and Mercury Ions

The bisferrocene-benzobisimidazole triad behaves as a selective redox and fluorescent chemosensor for HSO(4)(-) and Hg(2+) ions, exhibiting an easily detectable signal change in both the redox potential of the ferrocene/ferrocinium redox couple and in the emission band which is red-shifted (Δλ = 10-13 nm) and enhanced in intensity (Chelation Enhanced Fluorescence, CHEF = 486-225) upon complexation with these ions, in EtOH solutions.

Experimental and theoretical anion binding studies on coumarin linked thiourea and urea molecules

A series of coumarin linked thiourea and urea molecules 1– 5 have been designed and synthesized. Thiourea-based monotopic receptors 1– 4 showed selective sensing of F − and C 6H 5COO − by exhibiting significant change in emission as well as change in color. The ditopic receptor 5, on the other hand, fluorometrically distinguished isomeric aromatic dicarboxylates in polar solvent DMSO. Theoretical calculations have been carried out on systems 1–4 and their complexes with C 6H 5COO − and F − anions in the presence of CH 3CN solvent. Calculations have been carried out to determine the geometry and bonding of the complexes in the ground electronic state as well as on the excited electronic states relevant to the absorption spectra and emission spectra in an effort to obtain information on the spectral changes accompanying complexation of the receptors 1–4 with the two anions.

Beryllium(II) Recognition by Allosteric Effects in 1,2-Ethylenedioxybenzene Based Ditopic Receptors

Efficient ditopic receptor, uranyl(II) N,N'-(ethylenedioxy)benzenebis(salicylideneimine) (3) for beryllium ion has been obtained upon functionalization of 1,2-ethylenedioxybenzene (1) with a uranyl-salphen (salphen = N,N'-phenylenebis(salicylideneimine)) unit. Binding affinities of the receptor, 3 in AN-DMSO (v/v 95:5) solution have been measured for alkali and alkaline earth metal ions by conductometry comparing 1. The results showed that both monotopic 1 and ditopic receptor 3 were selective for ions over other cations, while especially 3 that can complex both with cations (coordinated to basic oxygen of ethylenedioxybenzene) and anions (coordinated to the Lewis acidic uranyl center) results in an increase of the stability constants by a factor of with respect to 1. Furthermore, the -3 interactions are demonstrated by NMR experiments in highly polar solvent medium, DMSO-. Higher selectivities were also observed for when the ditopic receptor, 3 was incorporated into PVC membranes and tested as ion selective electrodes at neutral pH.

Multimodal Metal Cation Sensing with Bis(macrocyclic) Dye

The synthesis, electrochemical, optical, and cation sensing properties of a bis(macrocyclic) dye 1, in which the benzo-15-crown-5 and phenylazathia-15-crown-5 subunits are linked through a styryl pyridinium moiety, are reported. In this new ditopic receptor, the benzo-15-crown-5 macrocycle acts as a highly selective binding site for alkaline earth metal cations (Mg(II) and Ba(II)), whereas the phenylazathia-15-crown-5 displays a strong binding affinity towards soft heavy-metal cations (Hg(II) and Ag(I)). The pronounced changes of the absorption and fluorescence spectra of this bichromophoric dye observed upon different metal cation addition make the dye suitable for dual-wavelength analysis and offer an enticing potential for multitasking sensors.

Synthesis, structure and inclusion properties of cone-tris{[(5′-methyl-2,2′-bipyridyl)-5-yl]oxycarbonylmethoxy}hexahomotrioxacalix[3]arene

Hexahomotrioxacalix[3]arene having [(5′-methyl-2,2′-bipyridyl)-5-yl]oxycarbonylmethoxy group with cone conformation was prepared, which shows strong Ag+ affinity and acts as a ditopic receptor for Ag+ and nBuNH3 +. A conformational change of 2,2′-bipyridyl moiety from the original outward orientation of the ring nitrogen to the inside orientation toward the thiacalixarene cavity was observed in the process of Ag+ complexation.

Discovery of Linear Receptors for Multiple Dihydrogen Phosphate Ions Using Dynamic Combinatorial Chemistry

We describe the use of dynamic combinatorial chemistry to discover a new series of linear hydrazone-based receptors that bind multiple dihydrogen phosphate ions. Through the use of a template-driven, selection-based approach to receptor synthesis, dynamic combinatorial chemistry allows for the identification of unexpected host structures and binding motifs. Notably, we observed the unprecedented selection of these linear receptors in preference to competing macrocyclic hosts. Furthermore, linear receptors containing up to nine building blocks and three different building blocks were amplified in the dynamic combinatorial library. The receptors were formed using a dihydrazide building block based on an amino acid-disubstituted ferrocene scaffold. A detailed study of the linear pentamer revealed that it forms a helical ditopic receptor that employs four acylhydrazone hydrogen-bond donor motifs to cooperatively bind two dihydrogen phosphate ions.

Lithium cation enhances anion binding in a tripodal phosphine oxide-based ditopic receptor.

A tripodal ditopic receptor presents H-bond donors and a phosphine oxide to potential guests. In the idealized binding conformation, an endohedral P=O functionality provides enhanced halide binding in the presence of lithium with the greatest ΔΔG° observed for bromide, while minimal changes in K(a) are observed in the presence of sodium.

A naked-eye selective detection of cyanide ion: studies on the effect of chromophores and spacers on ditopic receptors

A series of novel thiourea based receptors (1-12) with different signaling units and spacers have been synthesized for the detection of cyanide ion. Among these receptors, 4,4'-bis-[3-(4-cyanophenyl)thiourea]diphenyl sulfide 6 and 4,4'-bis-[3-(3-cyanophenyl)thiourea]diphenyl sulfide 9 exhibited selective detection of cyanide by the naked-eye as well as by UV-Vis spectroscopy. Furthermore, the chemosensor is insensitive to the presence of other environmentally relevant anions. The chromogenic transformation permits the detection of cyanide (TBA(+) and K(+) salts) in the natural environment.

Ion pair receptors based on anion–π interaction

A novel type of ditopic ion pair receptors based on anion-π interaction is reported. Oxacalix[2]arene[2]triazine azacrowns were synthesized efficiently from a one-pot reaction between 2,4-dichloro-4-methoxytriazine and 3,5-dihydroxybenzaldehyde followed by condensation with a diamine and reduction of bisimine; they acted as selective ditopic receptors to recognize ion pairs.

Ca2+-induced folding of a chiral ditopic receptor based on a Pybox ligand and enhancement of anion recognition

A chiral Pybox ligand bearing two urea units was developed for a Ca(2+)-induced folding ligand. 1:1 Ca(2+) complexation of the Pybox ligand afforded chiral foldamer formation with coordination of the urea carbonyls to Ca(2+). The halide-ion affinity of the foldamer was enhanced compared to Ca(2+)-free Pybox ligands.

1,3-Alternate Calix[4]arene Bifuntional Fluorescent Receptor Containing Urea and Crown Ether Moieties

Scheme 1. Synthetic routes to fluorescent chemosensor 1 Calix[4]arenes are important building blocks in supramolecular chemistry. The calixarene framework provides a unique possibility to organize several binding sites in an array complementary to a potential guest. A variety of calixarene-based receptors for cations, anions and neutral molecules have been synthesized in the past decade. Though anion recognition appears to be a relatively new area of research, both positively charged and neutral receptors for anionic species have been prepared in the last few years. As fluorogenic units, pyrenes are one of the most useful because of their relatively efficient excimer formation and emission. Since the intensity ratio of the excimer to the monomer emission (IE/IM) is sensitive to conformational changes of the pyrene-appended receptors, changes in IE/IM upon metal ion complexation can be an informative parameter in various sensing systems. The simultaneous binding of cationic and anionic guest species by ditopic receptors is a rapidly developing new field for ion pair recognition of environment and biological importance. By taking advantage of calixarene framework, a few neutral bifunctional receptors were developed and showed the simultaneous complexation properties of hydrophilic anions and cations in organic media. Those receptors are, however, only focused on the cone conformers of calix[4]arene. It has been known that 1,3-alternate conformation of calix[4]arene could provide the two excellent binding sites for guest molecules when the proper functionalization could be achieved. We have previously reported 'bifunctional calix[4]arene receptors' containing two urea groups and crown ether moieties at the calix[4]arene fixed in the 1,3-alternate conformation. It was shown the simultaneous binding properties of cation and anion guests with enhanced binding affinity of halide anions in the presence of potassium cation. Here we report a new bifunctional receptor 1 based on 1,3alternate conformer of calix[4]arene, which contains urea linking pyrene fluorophores instead of phenylurea and crown ether moieties at the opposite side of lower rim of calix[4]arene fixed in the 1,3-alternate conformation. Also, we studied binding affinity of various anions in the present of potassium ion.

Ethylene spacer-linked bis-acetamidopyridine for dicarboxylic acid recognition and polymeric new wave-like anti-perpendicular arrangement of a host–guest in the solid state

In this paper, 1,2-bis(2-acetamido-6-pyridyl)ethane, receptor 1, having an ethylene spacer is reported to recognise dicarboxylic acids. The binding study in the solution phase is carried out using 1H NMR (1:1) and UV–vis experiments and in the solid phase by single-crystal X-ray analysis. In 1H NMR, the downfield shifts of specific amide protons of receptor 1 in 1:1 complexes of receptor and guest diacids, and in the UV–vis experiment, the appearance of an isosbestic point as well as significant binding constants are observed, which thus unambiguously support the complexation of receptor 1 with dicarboxylic acids in solution. Receptor 2, simple 2-acetamido-6-methylpyridine, has lower binding constants than receptor 1 due to cooperative binding of two pyridine amide groups with two acid groups of diacids. In the solid phase, the ditopic receptor 1 shows a grid-like polymeric hydrogen-bonded network that changes to a polymeric wave-like 1:1 anti-perpendicular network instead of the syn–syn polymeric 1:1 (Goswami, S.; Dey, S.; Fun, H.-K.; Anjum, S.; Rahman, A.-U. Tetrahedron Lett.2005, 46, 7187–7191), anti–anti polymeric 1:1 (Goswami, S.; Jana, S.; Dey, S.; Razak, I.A.; Fun, H.-K. Supramol. Chem. 2006, 18, 571–574; Goswami, S.; Jana, S.; Fun, H.-K. Cryst. Eng. Comm. 2008, 10, 507–517; Goswami, S.; Jana, S.; Dey, S.; Sen, D.; Fun, H.-K.; Chantrapromma, S. Tetrahedron 2008, 64, 6426–6433), syn–syn 2:2 (Karle, I.L.; Ranganathan, D.; Haridas, V. J. Am. Chem. Soc. 1997, 119, 2777–2783) or top–bottom-bound 1:1 (Garcia-Tellado, F.; Goswami, S.; Chang, S.K.; Geib, S.J.; Hamilton, A.D. J. Am. Chem. Soc. 1990, 112, 7393–7394) co-crystals.