A Conformationally Flexible, Urea-Based Tripodal Anion Receptor: Solid-State, Solution, and Theoretical Studies

Tripodal BODIPY-Tagged and Functional Molecular Probes: Synthesis, Computational Investigations and Explorations by Multiphoton Fluorescence Lifetime Imaging Microscopy

A range of novel BODIPY derivatives with a tripodal aromatic core was synthesized and characterized spectroscopically. These new fluorophores showed promising features as probes for in vitro assays in live cells and offer strategic routes for further functionalization towards hybrid nanomaterials. Incorporation of biotin tags facilitated proof-of-concept access to targeted bioconjugates as molecular probes. Computational explorations using DFT and TD-DFT calculations identified the most stable tripodal linker conformations and predicted their absorption and emission behavior. The uptake and speciation of these molecules in living prostate cancer cells was imaged by single- and two-photon excitation techniques coupled with two-photon fluorescence lifetime imaging (2P FLIM).

Solvent Acts as the Referee in a Match-Up Between Charged and Preorganized Receptors

The prevalence of anion-cation contacts in biomolecular recognition under aqueous conditions suggests that ionic interactions should dominate the binding of anions in solvents across both high and low polarities. Investigations of this idea using titrations in low polarity solvents are impaired by interferences from ion pairing that prevent a clear picture of binding. To address this limitation and test the impact of ion-ion interactions across multiple solvents, we quantified chloride binding to a cationic receptor after accounting for ion pairing. In these studies, we created a chelate receptor using aryl-triazole CH donors and a quinolinium unit that directs its cationic methyl inside the binding pocket. In low-polarity dichloromethane, the 1 : 1 complex (log K1 : 1 ~ 7.3) is more stable than neutral chelates, but fortuitously comparable to a preorganized macrocycle (log K1 : 1 ~ 6.9). Polar acetonitrile and DMSO diminish stabilities of the charged receptor (log K1 : 1 ~ 3.7 and 1.9) but surprisingly 100-fold more than the macrocycle. While both receptors lose stability by dielectric screening of electrostatic stability, the cationic receptor also pays additional costs of organization. Thus even though the charged receptor has stronger binding in apolar solvents, the uncharged receptor has more anion affinity in polar solvents.

Supramolecular Motifs in the Crystal Structures of Triethylbenzene Derivatives Bearing Pyridinium Subunits in Combination with Pyrimidinyl or Pyridinyl Groups

A series of mono- and dicationic 1,3,5-trisubstituted 2,4,6-triethylbenzenes containing pyridinium groups in combination with aminopyrimidine-/aminopyridine-based recognition units were synthesized and crystallographically studied. The combination of neutral and ionic building blocks represents a promising strategy for the development of effective and selective artificial receptors for anionic substrates. In the crystalline state, the investigated compounds show a tendency to bind the counterion PF6- in the cavity formed by the three functionalized side-arms. The intermolecular interactions with the PF6- ion comprise N-H∙∙∙F and C-H∙∙∙F bonds. Detailed analysis of various supramolecular motifs, including interactions with solvent molecules, provides deeper insights into the processes of molecular recognition. The information obtained is useful in the development of new receptor molecules for anions and in the selection of the most appropriate counterion.

Comparing the anion binding of 4-amido- with 4-amino-1,8-naphthalimides

The synthesis and evaluation of a new anion receptor based on the 4-amido-1,8-naphthalimide scaffold is described. The findings indicate that the amide N-H is an enhanced H-bond donor but is otherwise restricted in its ability to participate in the binding of simple anions.

Spectroscopic and Colorimetric Studies for Anions with a New Urea-Based Molecular Cleft

A new simple urea-based dipodal molecular cleft (L) has been synthesized and studied for its binding affinity for a variety of anions by 1H-NMR, UV-Vis and colorimetric techniques in DMSO-d6 and DMSO, respectively. The results from titration studies suggest that the receptor forms a 1:2 complex with each of the anions used via hydrogen bonding interactions and exhibits strong selectivity for fluoride among halides, showing the binding affinity in the order of fluoride > chloride > bromide > iodide; meanwhile, it displays moderate selectivity for acetate among oxoanions, showing the binding affinity in the order of acetate > dihydrogen phosphate > bicarbonate > hydrogen sulfate > nitrate. Colorimetric studies of L for anions in DMSO reveal that the receptor is capable of detecting fluoride, acetate, bicarbonate and dihydrogen phosphate, displaying a visible color change in the presence of the respective anions.

Highly Efficient, Tripodal Ion-Pair Receptors for Switching Selectivity between Acetates and Sulfates Using Solid-Liquid and Liquid-Liquid Extractions

A tripodal, squaramide-based ion-pair receptor 1 was synthesized in a modular fashion, and ¹H NMR and UV-vis studies revealed its ability to interact more efficiently with anions with the assistance of cations. The reference tripodal anion receptor 2, lacking a crown ether unit, was found to lose the enhancement in anion binding induced by presence of cations. Besides the ability to bind anions in enhanced manner by the “single armed” ion-pair receptor 3, the lack of multiple and prearranged binding sites resulted in its much lower affinity towards anions than in the case of tripodal receptors. Unlike with receptors 2 or 3, the high affinity of 1 towards salts opens up the possibility of extracting extremely hydrophilic sulfate anions from aqueous to organic phase. The disparity in receptor 1 binding modes towards monovalent anions and divalent sulfates assures its selectivity towards sulfates over other lipophilic salts upon liquid–liquid extraction (LLE) and enables the Hofmeister bias to be overcome. By changing the extraction conditions from LLE to SLE (solid–liquid extraction), a switch of selectivity from sulfates to acetates was achieved. X-ray measurements support the ability of anion binding by cooperation of the arms of receptor 1 together with simultaneous binding of cations.

Visual detection of fluoride ion based on ICT mechanism

A novel chemosensor (H₃NTS) has been designed for a fast and accurate detection of F^- in acetonitrile medium. The UV-Vis, ¹H, ¹³C NMR experiments and computational calculations demonstrate that H₃NTS probe offers a potential colorimetric sensor for discriminating F^- among other relevant anions. The detection limit is as low as 6 μM and a reversible colorimetric switch could be devised by sequential addition of F^-/Ca²⁺. The promising application of H₃NTS is also confirmed in real examples where the concentration of F^- is a critical parameter, e.g., commercial toothpaste, paper strip and gel form analysis of samples.

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.

Systematic size mediated trapping of anions of varied dimensionality within a dimeric capsular assembly of a flexible neutral bis-urea platform

The systematic and consistent size mediated recognition of spherical, planar and tetrahedral anions is observed within a neutral cation-sealed dimeric capsular assembly of a meta-difunctionalized organic bis-urea receptor in solid and solution states.

Fluoride Anion Recognition by a Multifunctional Urea Derivative: An Experimental and Theoretical Study

In this work we demonstrate the ability of a multifaceted N,N'-disubstituted urea to selectively recognize fluoride anion (F(-)) among other halides. This additional function is now added to its already reported organocatalytic and organogelator properties. The signaling mechanism relies on the formation of a charge-transfer (CT) complex between the urea-based sensor and F¯ in the ground state with a high association constant as demonstrated by absorption and fluorescence spectroscopy. The nature of the hydrogen bonding interaction between the sensor and F¯ was established by ¹H-NMR studies and theoretical calculations. Moreover, the recovery of the sensor was achieved by addition of methanol.

Anion coordinated capsules and pseudocapsules of tripodal amide, urea and thiourea scaffolds

This review provides a detailed and comparative account of the solid- and solution-states anion (halides and oxyanions) binding affinities of hydrogen bonding tripodal scaffolds.

Enhancing binding affinity and selectivity through preorganization and cooperative enhancement of the receptor

When direct host–guest binding interactions are weakened by unfavorable solvent competition, guest-triggered intrareceptor interactions could be used to augment the binding.

Insights into the electrooptical anion sensing properties of a new organic receptor: solvent dependent chromogenic response and DFT studies

AcO⁻ ion mediated color transition properties of an organic receptor in solvents of varying polarity highlighting the influence of dipole moment in stabilizing the excited state.

Solid-State Examination of Conformationally Diverse Sulfonamide Receptors Based on Bis(2-anilinoethynyl)pyridine, -Bipyridine, and -Thiophene

Utilizing an induced-fit model and taking advantage of rotatable acetylenic C(sp)-C(sp²) bonds, we disclose the synthesis and solid-state structures of a series of conformationally diverse bis-sulfonamide arylethynyl receptors using either pyridine, 2,2'-bipyridine, or thiophene as the core aryl group. Whereas the bipyridine and thiophene structures do not appear to bind guests in the solid state, the pyridine receptors form 2 + 2 dimers with water molecules, two halides, or one of each, depending on the protonation state of the pyridine nitrogen atom. Isolation of a related bis-sulfonimide derivative demonstrates the importance of the sulfonamide N-H hydrogen bonds in dimer formation. The pyridine receptors form monomeric structures with larger guests such as BF₄^- or HSO₄^-, where the sulfonamide arms rotate to the side opposite the pyridine N atom.

Synthesis and anion binding studies of tris(3-aminopropyl)amine-based tripodal urea and thiourea receptors: Proton transfer-induced selectivity for hydrogen sulfate over sulfate

Tris(3-aminopropyl)amine-based tripodal urea and thiourea receptors, tris([(4-cyanophenyl)amino]propyl)urea (L1) and tris([(4-cyanophenyl)amino]propyl)thiourea (L2), have been synthesized and their anion binding properties have been investigated for halides and oxoanions. As investigated by 1H NMR titrations, each receptor binds an anion with a 1:1 stoichiometry via hydrogen-bonding interactions (NH⋯anion), showing the binding trend in the order of F- > H2PO4- > HCO3- > HSO4- > CH3COO- > SO42- > Cl- > Br- > I in DMSO-d6 . The interactions of the receptors were further studied by 2D NOESY, showing the loss of NOESY contacts of two NH resonances for the complexes of F-, H2PO4-, HCO3-, HSO4- or CH3COO- due to the strong NH⋯anion interactions. The observed higher binding affinity for HSO4- than SO42- is attributed to the proton transfer from HSO4- to the central nitrogen of L1 or L2 which was also supported by the DFT calculations, leading to the secondary acid-base interactions. The thiourea receptor L2 has a general trend to show a higher affinity for an anion as compared to the urea receptor L1 for the corresponding anion in DMSO-d6 . In addition, the compound L2 has been exploited for its extraction properties for fluoride in water using a liquid-liquid extraction technique, and the results indicate that the receptor effectively extracts fluoride from water showing ca. 99% efficiency (based on L2).

Experimental and theoretical studies on halide binding with a p-xylyl-based azamacrocycle

A p-xylyl-based macrocycle L has been synthesized and its binding properties with halides have been investigated by (1)H NMR titrations, single crystal X-ray diffraction analysis, and density functional theory (DFT) calculations. As investigated by (1)H NMR titrations, the ligand preferentially binds a halide in a 1:2 binding mode, with the association constants (in log K2) of 2.82, 2.70, 2.28, and 2.20 for fluoride, chloride, bromide, and iodide, respectively. The overall binding trend was found to be in the order of fluoride > chloride > bromide > iodide, reflecting that the binding strength correlates with the relative basicity and size of the respective halide. Crystallographic studies indicate that the ligand forms 1:2 complexes with chloride, bromide and iodide. In the chloride complex, the ligand is hexaprotonated and each chloride is held via three NH···Cl(-) bonds. The ligand is tetraprotonated for the other complexes, where each halide is H-bonded to two secondary ammonium NH(+) groups via NH···X(-) bonds. The results of DFT calculations performed on [H6L](6+) at M062x/6-311G (d,p) level in both gas and solvent phases, suggest that the ligand binds halides with the binding energy in the order of F(-) > Cl(-) > Br(-) > I(-), supporting the experimental data obtained from (1)H NMR studies. Results from DFT calculations further indicate that a 1:2 binding is energetically more favorable than a 1:1 binding of the ligand.

Propeller-shaped molecules with a thiazole hub: structural landscape and hydrazone cap mediated tunable host behavior in 4-hydrazino-1,3-thiazoles

Propeller-shaped molecules with 2,4,5-trisubstituted-1,3-thiazole as the hub and tunable blades (B₁–B₃) were synthesized as trivariant scaffolds.

A fluorescent probe based on hydroxylnaphthalene 2-cyanoacrylate: fluoride ion detection and its bio-imaging in live cells

A novel ratiometric fluorescent probe based on hydroxylnaphthalene 2-cyanoacrylate shows a high selectivity for fluoride ions and can be used for intracellular imaging.

Progress of 3-aminopyridinium-based synthetic receptors in anion recognition

This review describes the overall development on synthetic receptors built on the pyridinium motif with different functionalities at the 3-position in anion recognition.

1, 5-Bis (2-hydroxyacetophenone)thiocarbohydrazone: a novel colorimetric and fluorescent dual-mode chemosensor for the recognition of fluoride

1,5-Bis (2-hydroxyacetophenone)thiocarbohydrazone (H4L) has been synthesized and characterized by means of spectroscopic and single crystal X-ray diffraction methods. Interactions of the H4L with a variety of anions were investigated using a combination of UV-visible and fluorescence spectroscopic methods in a biological competing solvent DMSO. The H4L has a high degree of selectivity for fluoride over other anions. (1)H NMR titration experiments indicate that a deprotonation process is involved in the chemo sensing process.