Selective fluorescence sensing of H2PO4- by the anion induced formation of self-assembled supramolecular polymers.

The utilization of anions to induce the formation of self-assembled supramolecular polymers in solution is an undeveloped area of host-guest chemistry. We report in this manuscript a comparative study of two tripodal anion receptors by hydrogen or halogen bonding interactions to form self-assembled supramolecular structures induced by the presence of anions. DOSY NMR and DLS experiments provided evidence for the formation of supramolecular structures in solution in both halogen and hydrogen bond donors with H2PO4- anions. The nucleation and elongation constants obtained using the thermodynamic model indicate that the polymers grow following an isodesmic mechanism. Emission studies demonstrate that only the formation of the supramolecular polymer between the halogen bond donor receptor and H2PO4- anions results in the appearance of the excimer emission band.

Triazole-Containing [FeFe] Hydrogenase Mimics: Synthesis and Electrocatalytic Behavior.

Through a Cu-catalyzed Huisgen cycloaddition between terminal alkynes and azides (CuAAC) reaction, azide [(µ-SCH2)2N(4-N3C6H4)Fe2(CO)6] has demonstrated to be a robust and versatile reagent able to incorporate the [(µ-SR)2Fe2(CO)6] fragment on a wide range of substrates, ranging from aromatic compounds to nucleosides, metallocenes, or redox and luminescent markers. The [FeIFeI]/[Fe0FeI] and [Fe0FeI]/[Fe0Fe0] reduction potentials of the triazole derivatives prepared are comparable to those of other aminodithiolate (adt) Fe-Fe hydrogenase mimics. The presence of the triazole linker influences the electrochemical behavior of these complexes depending on the strength of the acid employed.

Anion Recognition Strategies Based on Combined Noncovalent Interactions.

This review highlights the most significant examples of an emerging field in the design of highly selective anion receptors. To date, there has been remarkable progress in the binding and sensing of anions. This has been driven in part by the discovery of ways to construct effective anion binding receptors using the dominant N-H functional groups and neutral and cationic C-H hydrogen bond donors, as well as underexplored strong directional noncovalent interactions such as halogen-bonding and anion-π interactions. In this review, we will describe a new and promising strategy for constructing anion binding receptors with distinct advantages arising from their elaborate design, incorporating multiple binding sites able to interact cooperatively with anions through these different kinds of noncovalent interactions. Comparisons with control species or solely hydrogen-bonding analogues reveal unique characteristics in terms of strength, selectivity, and interaction geometry, representing important advances in the rising field of supramolecular chemistry.

Interlocked Supramolecular Polymers Created by Combination of Halogen- and Hydrogen-Bonding Interactions through Anion-Template Self-Assembly.

We present the synthesis and oxoanion-assembling properties of a monomer with a naphthalene ring as a central core decorated with two arms containing iodotriazolium rings as anion binding sites. Interactions with SO42-, H2PO4-, and HP2O73- anions, via a cooperative mechanism, afforded new supramolecular materials stabilized by a combination of halogen- and hydrogen-bonding interactions. 1H NMR experiments and solid-state structure provided evidence for the initial formation of a supramolecular linear chain, nucleation step, and then two different supramolecular chains are interpenetrated with each other, elongation steps, involving the formation of hydrogen bonds between two oxygens of the anion from one of the chains and the naphthalene inner protons from the other chain. Scanning electron microscopy studies revealed that the morphology of the crystals changed dramatically with the nature of the anion added.

Modulation of the Selectivity in Anions Recognition Processes by Combining Hydrogen- and Halogen-Bonding Interactions.

Most of the halogen bonding receptors for anions described use halogen bonding binding sites solely in the anion recognition process; only a few examples report the study of anion receptors in which the halogen bonding interaction has been used in combination with any other non-covalent interaction. With the aims to extend the knowledge in the behaviour of this kind of mixed receptors, we report here the synthesis and the anion recognition and sensing properties of a new halogen- and hydrogen- bonding receptor which binds anions by the cooperation of both non-covalent interactions. Fluorescence studies showed that the behaviour observed in the anion recognition sensing is similar to the one previously described for the halogen analogue and is quite different to the hydrogen one. On the other hand, the association constants obtained by ¹H-NMR data demonstrate that the mixed halogen- and hydrogen-bonding receptor is more selective for SO42- anion than the halogen or hydrogen analogues.

Host-Guest Chemistry: Oxoanion Recognition Based on Combined Charge-Assisted C-H or Halogen-Bonding Interactions and Anion⋠⋠⋠Anion Interactions Mediated by Hydrogen Bonds.

Several bis-triazolium-based receptors have been synthesized and their anion-recognition capabilities have been studied. The central chiral 1,1'-bi-2-naphthol (BINOL) core features either two aryl or ferrocenyl end-capped side arms with central halogen- or hydrogen-bonding triazolium receptors. NMR spectroscopic data indicate the simultaneous occurrence of several charge-assisted aliphatic and heteroaromatic C-H noncovalent interactions and combinations of C-H hydrogen and halogen bonding. The receptors are able to selectively interact with HP2 O7 (3-) , H2 PO4 (-) , and SO4 (2-) anions, and the value of the association constant follows the sequence: HP2 O7 (3-) >SO4 (2-) >H2 PO4 (-) . The ferrocenyl end-capped 7(2+) ⋅2 BF4 (-) receptor allows recognition and differentiation of H2 PO4 (-) and HP2 O7 (3-) anions by using different channels: H2 PO4 (-) is selectively detected through absorption and emission methods and HP2 O7 (3-) by using electrochemical techniques. Significant structural results are the observation of an anion⋅⋅⋅anion interaction in the solid state (2:2 complex, 6(2+) ⋅[H2 P2 O7 ](2-) ), and a short C-I⋅⋅⋅O contact is observed in the structure of the complex [8(2+) ][SO4 ]0.5 [BF4 ].

2,4,5-Trimethylimidazolium Scaffold for Anion Recognition Receptors Acting Through Charge-Assisted Aliphatic and Aromatic C-H Interactions.

A series of two-armed 2,4,5-trimethylimidazolium-based oxoanion receptors, which incorporate two end-capped photoactive anthracene rings, being the central core an aromatic or heteroaromatic ring, has been designed. In the presence of HP2O7(3-), H2PO4(-), and SO4(2-) anions, (1)H- and (31)P NMR spectroscopical data clearly indicate the simultaneous occurrence of several charge-assisted aliphatic and aromatic C-H noncovalent interactions, i.e., significant downfield shifts were observed for the imidazolium C(2)-CH3 protons, the methylene N-CH2 protons, and the inner aromatic proton or the outer heteroaromatic protons. Density functional theory calculations confirm the occurrence of these noncovalent interaction and suggest that the interaction between the anions and the receptors is mainly electrostatic in nature.

Comparative Study of Charge-Assisted Hydrogen- and Halogen-Bonding Capabilities in Solution of Two-Armed Imidazolium Receptors toward Oxoanions.

Two-armed imidazolium-based anion receptors have been prepared. The central 2,7-disubstituted naphthalene ring features two photoactive anthracene end-capped side arms with central 2-bromoimidazolium or hydrogen-bonding imidazolium receptors. Combined emission and (1)H and (31)P NMR studies carried out in the presence of a wide variety of anions reveal that only HP2O7(3-), H2PO4(-), SO4(2-), and F(-) anions promoted noticeable changes. The halogen receptor 6(2+)·2PF6(-) acts as a selective fluorescent molecular sensor for H2PO4(-) anions, since only this anion promotes the appearance of the anthracene excimer emission band, whereas it remains unchanged in the presence of the other tested anions. In addition this halogen receptor behaves as a chemodosimeter toward HP2O7(3-) anion, through its transformation into the corresponding bis-imidazolone after debromination by the action of the basic anion. The association constant values of the halogen-bonding complexes in a competitive solvent CD3CN/MeOD (8/2) mixture with H2PO4(-) and SO4(2-) anions are higher than those found for the hydrogen-bonding counterpart. In contrast, in the less competitive CH3CN solvent higher binding affinity for anions corresponds to the hydrogen-bonding receptor 7(2+)·2PF6(-). In addition, the receptor 6(2+)·2PF6(-) represents a useful alternative as an imaging agent in living cells in a wide range of emission wavelengths.

Dual Role of the 1,2,3-Triazolium Ring as a Hydrogen-Bond Donor and Anion-π Receptor in Anion-Recognition Processes.

Several bis(triazolium)-based receptors have been synthesized as chemosensors for anion recognition. The central naphthalene core features two aryltriazolium side-arms. NMR experiments revealed differences between the binding modes of the two triazolium rings: one triazolium ring acts as a hydrogen-bond donor, the other as an anion-π receptor. Receptors 9(2+)⋅2BF4(-) (C6H5), 11(2+)⋅2BF4(-) (4-NO2-C6H4), and 13(2+)⋅2BF(4-) (ferrocenyl) bind HP2O7(3-) anions in a mixed-binding mode that features a combination of hydrogen-bonding and anion-π interactions and results in strong binding. On the other hand, receptor 10(2+)⋅2 BF4(-) (4-CH3O-C6H4) only displays combined Csp2-H/anion-π interactions between the two arms of the receptors and the bound anion rather than triazolium (CH)(+)⋅⋅⋅anion hydrogen bonding. All receptors undergo a downfield shift of the triazolium protons, as well as the inner naphthalene protons, in the presence of H2PO4(-) anions. That suggests that only hydrogen-bonding interactions exist between the binding site and the bound anion, and involve a combination of cationic (triazolium) and neutral (naphthalene) C-H donor interactions. Theoretical calculations relate the electronic structure of the substituent on the aromatic group with the interaction energies and provide a minimum-energy conformation for all the complexes that explains their measured properties.

A case of oxoanion recognition based on combined cationic and neutral C-H hydrogen bond interactions.

A novel bidentate bis-(benzimidazolium) receptor containing pyrene as fluorescent signaling units has been synthesized. Fluorescence and NMR spectroscopy studies reveal that this receptor exclusively recognizes sulphate and hydrogenpyrophosphate in the competitive water-DMSO (1 : 9) medium; significant downfield shifts were observed for the C(2)-H protons of both the imidazolium groups, and appreciable downfield shifts were also observed for the inner naphthalene protons indicating their participation in hydrogen bonding with anions along with the C(2) imidazolium protons. The calculated association constants from (1)H NMR and fluorescence titrations demonstrate that the receptor binds sulphate stronger than hydrogenpyrophosphate anions.

Open bis(triazolium) structural motifs as a benchmark to study combined hydrogen- and halogen-bonding interactions in oxoanion recognition processes.

We have designed a series of triazolium-pyrene-based dyads to probe their potential as fluorescent chemosensors for anion recognition through combinations of hydrogen and halogen bonding. Cooperation between the two distinct noncovalent interactions leads to an unusual effect on receptor affinity, as a result of fundamental differences in the interactions of halogen and hydrogen bond donor groups with anions. Absorption, emission spectrophotometries and proton and phosphorus NMR spectroscopies indicate that the two interactions act in concert to achieve the selective binding of the hydrogen pyrophosphate anion, a conclusion supported by computational studies. Hence, as clearly demonstrated with respective halogen- and hydrogen-bonding triazolium receptors, the integration of a halogen atom into the anion receptor at the expense of one hydrogen-bonding receptor greatly influences the anion recognition affinity of the receptor. The association constant values of the halogen-bonding complexes are larger than the hydrogen-bonding counterpart. Thus, halogen bonding has been exploited for the selective fluorescent sensing of hydrogen pyrophosphate anion. Halogen bonding has been demonstrated to increase the strength of hydrogen pyrophosphate binding, as compared to the hydrogen-bonded analogue. Grimme's PBE-D functional, which adequately reproduces the pyrene stacking energies, has been successfully applied to model the affinity for anions, especially hydrogen pyrophosphate, of the new receptors.

Iodide-induced shuttling of a halogen- and hydrogen-bonding two-station rotaxane.

The first example of utilizing halogen-bonding anion recognition to facilitate molecular motion in an interlocked structure is described. A halogen-bonding and hydrogen-bonding bistable rotaxane is prepared and demonstrated to undergo shuttling of the macrocycle component from the hydrogen-bonding station to the halogen-bonding station upon iodide recognition. In contrast, chloride-anion binding reinforces the macrocycle to reside at the hydrogen-bonding station.

PDMS based photonic lab-on-a-chip for the selective optical detection of heavy metal ions.

The selective absorbance detection of mercury(II) (Hg(2+)) and lead(II) (Pb(2+)) ions using ferrocene-based colorimetric ligands and miniaturized multiple internal reflection (MIR) systems implemented in a low-cost photonic lab on a chip (PhLoC) is reported. The detection principle is based on the formation of selective stable complexes between the heavy metal ion and the corresponding ligand. This interaction modulates the ligand spectrum by giving rise to new absorbance bands or wavelength shifting of the existing ones. A comparative study for the detection of Hg(2+) was carried out with two MIR-based PhLoC systems showing optical path lengths (OPLs) of 0.64 cm and 1.42 cm as well as a standard cuvette (1.00 cm OPL). Acetonitrile solutions containing the corresponding ligand and increasing concentrations of the heavy metal ion were pumped inside the systems and the absorbance in the visible region of the spectra was recorded. The optical behaviour of all the tested systems followed the expected Beer-Lambert law. Thus, the best results were achieved with the one with the longest OPL, which showed a linear behaviour in a concentration range of 1 µM-90 µM Hg(2+), a sensitivity of 5.6 × 10(-3) A.U. µM(-1) and a LOD of 2.59 µM (0.49 ppm), this being 1.7 times lower than that recorded with a standard cuvette, and using a sample/reagent volume around 190 times smaller. This microsystem was also applied for the detection of Pb(2+) and a linear behaviour in a concentration range of 3-100 µM was obtained, and a sensitivity of 9.59 × 10(-4) A.U. µM(-1) and a LOD of 4.19 µM (0.868 ppm) were achieved. Such a simple analytical tool could be implemented in portable instruments for automatic in-field measurements and, considering the minute sample and reagent volume required, would enable the deployment of high throughput environmental analysis of these pollutants and other related hazardous species.

Fluorescent charge-assisted halogen-bonding macrocyclic halo-imidazolium receptors for anion recognition and sensing in aqueous media.

The synthesis and anion binding properties of a new family of fluorescent halogen bonding (XB) macrocyclic halo-imidazolium receptors are described. The receptors contain chloro-, bromo-, and iodo-imidazolium motifs incorporated into a cyclic structure using naphthalene spacer groups. The large size of the iodine atom substituents resulted in the isolation of anti and syn conformers of the iodo-imidazoliophane, whereas the chloro- and bromo-imidazoliophane analogues exhibit solution dynamic conformational behavior. The syn iodo-imidazoliophane isomer forms novel dimeric isostructural XB complexes of 2:2 stoichiometry with bromide and iodide anions in the solid state. Solution phase DOSY NMR experiments indicate iodide recognition takes place via cooperative convergent XB-iodide 1:1 stoichiometric binding in aqueous solvent mixtures. (1)H NMR and fluorescence spectroscopic titration experiments with a variety of anions in the competitive CD(3)OD/D(2)O (9:1) aqueous solvent mixture demonstrated the bromo- and syn iodo-imidazoliophane XB receptors to bind selectively iodide and bromide respectively, and sense these halide anions exclusively via a fluorescence response. The protic-, chloro-, and anti iodo-imidazoliophane receptors proved to be ineffectual anion complexants in this aqueous methanolic solvent mixture. Computational DFT and molecular dynamics simulations corroborate the experimental observations that bromo- and syn iodo-imidazoliophane XB receptors form stable cooperative convergent XB associations with bromide and iodide.

Host-guest complexes vs. supramolecular polymers in chalcogen bonding receptors: an experimental and theoretical study.

We describe here a comparative study between two tripodal anion receptors based on selenophene as the binding motif. The receptors use benzene or perfluorobenzene as a spacer. The presence of the electron-withdrawing ring activates the selenium atom for anion recognition inducing the formation of self-assembled supramolecular structures in the presence of chloride or bromide anions, which are bonded by the cooperative action of hydrogen and chalcogen bonding interactions. DOSY NMR and DLS experiments provided evidence for the formation of the supramolecular structures only in the presence of a perfluorobenzene based anion receptor while the analogous benzene one shows the classical anion/receptor complex without the participation of the selenium atom. The energetic and geometric features of the complexes of both receptors with the Cl and Br anions have been studied in solution. These results combined with the molecular electrostatic potential (MEP) surface plots allow us to rationalize the quite different behaviors of both receptors observed experimentally.

Ferrocene-substituted nitrogen-rich ring systems as multichannel molecular chemosensors for anions in aqueous environment.

The synthesis, electrochemical, optical, and anion sensing properties of ferrocene-fused imidazole dyads are presented. Ferrocene-benzobisimidazole dyad 1 behaves as a highly selective redox, chromogenic and fluorescent chemosensor molecule for AcO(-) anion in DMSO/H(2)O: the oxidation redox peak is cathodically shifted (DeltaE(1/2) = -170 mV), perturbation of the UV-vis spectrum, and the emission band is both red-shifted (Delta lambda = 13 nm) and increased (Chelation Enhanced Fluorescence, CHEF = 133) upon complexation with this anion. The related ferrocene-bisbenzimidazole dyad 2 has shown the ability for sensing both H(2)PO(4)(-) and HP(2)O(7)(3-) anions in the same medium. Upon complexation, it also displays a cathodic shift of the redox potential (DeltaE(1/2) = -90 to 80 mV), as well as a clear perturbation of the UV-vis spectrum and an increase in the intensity of the emission band (CHEF = 97-37). However, such magnitudes are smaller than those exhibited by 1. (1)H NMR studies have been carried out to obtain information about the molecular sites which are involved in the binding process.

A ferrocene-quinoxaline derivative as a highly selective probe for colorimetric and redox sensing of toxic mercury(II) cations.

A new chemosensor molecule 3 based on a ferrocene-quinoxaline dyad recognizes mercury (II) cations in acetonitrile solution. Upon recognition, an anodic shift of the ferrocene/ferrocenium oxidation peaks and a progressive red-shift (Δλ=140 nm) of the low-energy band, are observed in its absorption spectrum. This change in the absorption spectrum is accompanied by a colour change from orange to deep green, which can be used for a "naked-eye" detection of this metal cation.

Formation of self-assembled supramolecular polymers by anti-electrostatic anion-anion and halogen bonding interactions.

We report here the formation of self-assembled supramolecular polymers in which the cooperative action of anti-electrostatic anion-anion and halogen-bonding interactions serve as a powerful driving force for the formation of large supramolecular polymers. DOSY-NMR, DLS, TEM, SEM and X-ray experiments provide evidence of the formation of supramolecular structures in solution and solid state.

Imidazole-annelated ferrocene derivatives as highly selective and sensitive multichannel chemical probes for Pb(II) cations.

New ferrocenyl-containing imidazopyridine and imidazophenazine receptors 2-5 show high selective affinity for Pb(II) ions over a range of other metal ions examined through different channels. Imidazopyridine-ferrocene dyad 2 behaves as a highly selective redox, chromogenic, and fluorescent chemosensor molecule for Pb(2+) cations: the oxidation redox peak is anodically shifted (DeltaE(1/2) = 150 mV), and the low energy band of the absorption spectrum is red-shifted (Deltalambda = 44 nm) upon complexation with this metal cation. This change in the absorption spectrum is accompanied by a color change from colorless to orange, which allows the potential for "naked eye" detection. The emission spectrum undergoes an important chelation-enhanced fluorescence (CHEF) effect (CHEF = 620), with an unprecedented detection limit of 2.7 microg L(-1). The presence of Zn(2+) cations also induced a perturbation of the redox potential, absorption, and emission spectra although in less extension than those found with Pb(2+) cations. Imidazophenazine-ferrocene dyad 3 has also shown its ability for sensing Pb(2+) cations through redox (DeltaE(1/2) = 120 mV), absorption (Deltalambda = 23 nm), and emission (CHEF = 133) channels, whereas the presence of Zn(2+) only has a little effect on the emission spectrum (CHEF = 74). The electrochemical changes observed in the two-armed ferrocenes 4 and 5 upon complexation show that the potential shift is higher for Zn(2+) ions (DeltaE(1/2) = 190-170 mV) than for Pb(2+) ions (DeltaE(1/2) = 180-110 mV), which is in clear contrast to those observed for the monoarmed ferrocenes 2 and 3. The recognition properties of the two-armed imidazopyridine-ferrocene triad 4 are quite similar to those exhibited by the parent monosubstituted receptor 2, and the most salient features are a strong perturbation of the redox wave (DeltaE(1/2) = 180 mV), a dramatic increasing of the fluorescent quantum yield (Phi(complex)/Phi(ligand) = 890) in the presence of Pb(2+), while the optical responses toward Zn(2+) cations were silent. The two-armed imidazophenazine-ferrocene triad 5 senses Pb(2+) cations through perturbation of the oxidation potential of the Fe(II)/Fe(III) redox couple (DeltaE(1/2) = 110 mV), important blue shift (Deltalambda = 160 nm) of the high energy band in the absorption spectrum, and a remarkable increase of the emission band (CHEF = 220), whereas smaller changes were observed in the presence of Zn(2+) cations. (1)H NMR studies as well as DFT calculations have been carried out to get information about which molecular sites are involved in the binding event.

A selective redox and chromogenic probe for Hg(II) in aqueous environment based on a ferrocene-azaquinoxaline dyad.

A new chemosensor molecule 4 based on a ferrocene-azaquinoxaline dyad effectively recognizes Hg(2+) in an aqueous environment as well as Pb(2+) and Zn(2+) metal cations in CH(3)CN solution through three different channels. Upon recognition, an anodic shift of the ferrocene/ferrocenium oxidation peaks and a progressive red shift (Deltalambda = 112-40 nm) of the low energy band, in their absorption spectra, is produced. These changes in the absorption spectra are accompanied by color changes from orange to deep green, for Hg(2+), and to purple in the cases of Pb(2+) and Zn(2+). Remarkably, the redox and colorimetric responses toward Hg(2+) are preserved in the presence of water (CH(3)CN/H(2)O, 3/7). The emission spectrum of 4 in CH(3)CN (lambda(exc) = 270 nm) undergoes important chelation enhancement of fluorescence (CHEF) in the presence of Hg(2+) (CHEF = 204), Pb(2+) (CHEF = 90), and Zn(2+) (CHEF = 184) metal cations. Along with the spectroscopic data, the combined (1)H NMR data of the complexes and the theoretical calculation suggest the proposed bridging coordination modes.