Recent advances in selective recognition of fluoride with macrocyclic receptors

Probing Host-Guest Interactions of the Dual Anion Receptor Harmane with Halide and HSO4- Anions

Harmane is a polycyclic amine that can recognize F- and HSO4- via the ═N-H or ≡N binding site. The active binding site depends on whether the solvent is protic or aprotic, but the underlying molecular mechanism remains unclear. As a first step toward obtaining such mechanisms in solutions, we investigated the interactions of harmane with halide anions (F-, Cl-, Br-, and I-) and HSO4- in the gas phase using negative ion photoelectron spectroscopy combined with theoretical calculations. The adiabatic/vertical detachment energies for deprotonated harmane and harmane·X- (X = F, Cl, Br, I, and HSO4) were determined to be 2.72/2.79, 3.25/3.38, 4.19/4.43, 4.35/4.40, 3.93/3.99, and 4.49/4.75 eV, respectively, with an uncertainty of ±0.05 eV. All the X- anions were found to form hydrogen bonds with harmane through the ═N-H site. A nearly complete proton transfer was observed within the harmane·F- complex anion. Larger halide anions in other harmane-halide complexes remain relatively intact. Four closely lying isomers of harmane·HSO4- were identified. The photodetachment locations of the harmane complex anions were also revealed by electronic state calculations and molecular orbital analyses. The current work lays out a foundation for future work on microsolvated clusters to probe how solvent molecules influence the harmane-anion binding motif.

Proline-based tripodal cages with guest-adaptive features for capturing hydrophilic and amphiphilic fluoride substances

Proteins exhibit remarkable molecular recognition by dynamically adjusting their conformations to selectively interact with ligands at specialized binding sites. To bind hydrated ligands, proteins leverage amino acid residues with similar water affinities as the substrate, minimizing the energy required to strip water molecules from the hydrophilic substrates. In synthetic receptor design, replicating this sophisticated adaptability remains a challenge, as most artificial receptors are optimized to bind desolvated substances. Here, we show that proline-based synthetic receptors can mimic the conformational dynamics of proteins to achieve selective binding of hydrophilic and amphiphilic fluoride substances in aqueous environments. This finding highlights the critical role of receptor flexibility and strategic hydrophilicity in enhancing ligand recognition and affinity in water. Moreover, it establishes a new framework for designing versatile synthetic receptors with tunable hydrophobicity and hydrophilicity profiles.

A comprehensive review of the imidazole, benzimidazole and imidazo[1,2-a]pyridine-based sensors for the detection of fluoride ion

Imidazole-based chemicals exhibit significant potential in various scientific fields, mainly in the chemical and pharmaceutical sciences. The imidazole ring is a five-membered aromatic heterocycle found in several natural and synthetic substances. Its distinctive structural property, which includes a desirable electron-rich characteristic, allows imidazole derivatives to readily bond with a wide range of anions, cations, and neutral organic molecules. This review aims to assemble the sensing qualities of the most recently reported imidazole derivatives and analyse their potential as sensors. Among all other ions, fluoride sensing is primarily targeted for this context, because fluoride ions have garnered a lot of attention in recent decades due to their distinctive physiochemical properties and essential roles in many biological, chemical, pharmaceutical, and environmental processes. Fluoride ion detection is a broad field, and several fluorescent probes are continuously introduced to bind fluoride ions in aqueous and organic media. A few reviews have been published, emphasizing macrocycle cages, nanomaterial probes, bio-material sensors, and large organic molecule chemosensors for F- detection. A special review focusing solely on fluoride sensing by the imidazole-based moiety has not yet been addressed. Imidazole compounds have surged in prominence over the last few years, making them particularly desirable for developing efficient, sensitive, and selective fluoride detection methods. The present review concisely represents the contribution of a wide variety of imidazole fluorophores for fluoride ion detection.

A 2,6-diamidopyridine-based macrocyclic aromatic amide receptor with cascade ion pair recognition

Ion-pair receptors constitute an important class of synthetic receptors within the realm of host-guest and supramolecular chemistry. Their unique ability to simultaneously recognize and accommodate both cations and anions has rendered them invaluable across various applications. In this study, we have synthesized a cascade macrocyclic ion-pair receptor, composed of three 2,6-amidopyridine building blocks bridged by aromatic spacers. Notably, the diamide binding sites of this receptor exhibit a high degree of selectivity for fluoride ions. Furthermore, despite lacking any dedicated cation-binding sites within its macrocyclic structure, this receptor is capable of selectively binding tetraethylammonium cations through a series of cascade electrostatic interactions facilitated by the bound flouride ions.

Fluorescent calix[4]triazole for selective fluoride anion sensing

Fluoride ions (F-) play an important role in preventing cavities and treating osteoporosis, but excessive exposure can lead to serious health problems such as fluorosis and kidney damage. These dual characteristics highlight the need for selective and sensitive methods to detect fluoride ions for health monitoring. Accordingly, in this study, we investigated the anion-binding ability of Py-CT4, a fluorescent chemosensor in which pyrene is linked to calix[4]triazole via an ester linker. Notably, Py-CT4 exhibited significant fluorescence quenching for F- compared to other anions, and its fluorescence intensity gradually decreased with increasing F- concentration. This phenomenon is driven by electron transfer from calix[4]triazole to pyrene, initiated by hydrogen bonding with F- and followed by F--induced deprotonation of calix[4]triazole. The selectivity of Py-CT4 for F- appears to stem from its relatively flexible structure and low acidity compared to the previously reported Py-CT4+. Py-CT4 thus represents the first macrocyclic receptor based on charge-neutral 1,2,3-triazole that selectively recognizes F- through fluorescence quenching. Compared to traditional detection methods, Py-CT4 utilizes the advantages of fluorescent detection, such as higher sensitivity, faster response times, and ease of use, for fluoride ion detection. Py-CT4 also demonstrates excellent selectivity for F- even in the presence of competing anions. These features make Py-CT4 a promising tool for monitoring fluoride ions in biological and environmental systems, providing valuable insights into public health and safety.

Fluorenone-naphthyl encapsulated dual sensor for recognition of F- and Hg2+: Syngenetic effect with drug sobisis and molecular docking studies

A novel fluorenone-naphthyl pendant sensor (FTU) possessing thiourea functionality has been synthesized via a simple condensation method and utilized for the recognition of F- and Hg2+ ions in the solution of CH3CN. The addition of F- and Hg2+ ions to the FTU solution led to the appearance of red-shifted absorption bands at 340 and 315 nm, respectively. On the other hand, in the fluorescence spectrum, the two-fold decrease in fluorescence intensity of probe FTU was observed with F- ions; while complete quenching of the fluorescence intensity was noticed with Hg2+ ions at 423 nm. The limit of detection values of F- and Hg2+ ions were found to be 1.02 & 29.1 nM, respectively, measured by UV-vis studies and 0.0185 & 0.81 nM, respectively, measured by fluorescence studies, which are less than recommended by WHO. DFT computational assessments and 1H NMR titration experiments pointed to F- induced deprotonation of thiourea NH signals. However, the chelation-enhanced quenching effect (CHEQ) was held responsible for fluorescence quenching with Hg2+ addition. Moreover, the in-situ formed FTU + F- complex was utilized for secondary sensing of drug sobisis. Furthermore, the real-world applicability of sensor FTU has been successfully scrutinized for the recognition of F- ions in the toothpaste samples. In addition, molecular docking studies revealed that FTU exhibited excellent antibacterial potency towards different gram-positive as well as negative strains.

Selective Turn-On Luminescent Recognition of Perchlorate Ion Using Pyridyl-Benzimidazole-Based Probe

Anions play a crucial role in various environmental, chemical, and biological processes. Among various anions, the production of perchlorate (ClO4 -) ion is expected to rise in upcoming years, and thus, an efficient method for the detection of perchlorate ion is highly desirable. In this effort, a pyridyl-benzimidazole-based luminescent probe (RSB1) containing two N-H donor sites has been synthesized for selective detection of perchlorate ion. Different spectral techniques such as FT-IR, NMR, ESI-mass, UV-Vis, and fluorescence analyses have been used to characterize this probe. High selectivity of RSB1 for ClO4 - was realized even in presence of strongly interfering species in aqueous-acetonitrile (CH3CN-H2O; 4:1, v/v) solution. Notably, RSB1 served as a "turn-on" perchlorate-responsive probe and exhibited an emission enhancement at 363 nm when excited at 300 nm. The detection limit (LoD) and the binding constant (Kb) values were depicted to be 0.121 μM and 2.6 × 105 M-1, respectively, while the binding mechanism for RSB1-ClO4 - was validated via Job's plot, NMR, and DFT analyses. Furthermore, this probe was successfully employed to trace perchlorate in real samples such as tap water, distilled water, and soil samples with good to excellent recovery values.

Highly selective recognition of fluoride using a trapezoidal cage

By using a trapezoidal cage (1), highly selective recognition of F- (S > 1000) was achieved in highly competitive environments composed of a large excess of various common anions. Through NMR, fluorescence, UV-vis and HRMS studies, the binding behavior and binding affinity (β2 = ∼107 M-2) of 1 + F- are shown.

An ESIPT-active orange-emissive 2-(2'-hydroxyphenyl)imidazo[1,2-a]pyridine-derived chemodosimeter for turn-on detection of fluoride ions via desilylation

Fluoride is an essential element for oral health with an optimum concentration of 0.7-1.2 ppm in drinking water, but it is detrimental at higher concentrations, causing fluorosis, acute gastric ulcer, urolithiasis, and kidney infection, which adds immense significance to its detection in water sources. In the current study, a new chemodosimeter (HIPS-Br) is designed by protecting a 2-(2'-hydroxyphenyl)imidazo[1,2-a]pyridine derivative (HIP-Br) with a fluoride recognizable tert-butyldiphenylsilane moiety and utilized for the selective detection of F- ions by an excited-state intramolecular proton transfer (ESIPT)-based fluorimetric response. The probe HIPS-Br exhibits blue fluorescence in solution, and upon the incremental addition of F- ions, it exhibits a turn-on response, exhibiting a strong orange emission at 598 nm by spontaneous cleavage of the tert-butyldiphenylsilane group to release fluorescent HIP-Br in the working solution. HIPS-Br displayed no or insignificant response towards numerous common anions, cations and small molecules, affirming its selectivity to F- ions and offered a low limit of detection (LOD) of 1.2 ppb (6.6 × 10-8 M). The real sample analysis by spiking fluorides in water and toothpaste samples showed excellent percent recoveries. The chemodosimeter was successfully utilized in the solid-phase detection of F- ions on silica-coated TLC plates and analyzed by ImageJ analysis, marking its utility in on-site quantitation purposes.

Fluorinated benzothiadiazole fluorescent probe based on ICT mechanism for highly selectivity and sensitive detection of fluoride ion

Excessive fluoride ion (F-) in the environment can affect health and even endanger life when ingested by the human body. However, most fluoride probes have the disadvantages of low sensitivity and long detection time. Herein, fluorescent probe 3a is successfully synthesized by linking two acetylenyltrimethylsilyl groups at both ends of the fluorinated benzothiadiazole core. After the addition of F- to 3a, the emission at 436 nm is significantly quenched and slightly blue-shifted. It is confirmed by electrospray ionization high-resolution mass spectrometry (ESI-HRMS) and density functional theory calculations (DFT) that these changes are due to the F- triggered Si-C bond cleavage and the subsequent inactivation of intramolecular charge transfer (ICT). The detection limit and response time of probe 3a for F- are 10-8 mol/L and 25 s, respectively. Importantly, fluorescent material 3a can be processed into portable test tools for the visual detection of fluoride ion.

Macrocyclic receptors for anion recognition

Macrocyclic receptors have emerged as versatile and efficient molecular tools for the recognition and sensing of anions, playing a pivotal role in molecular recognition and supramolecular chemistry. The following review provides an overview of the recent advances in the design, synthesis, and applications of macrocyclic receptors specifically tailored for anion recognition. The unique structural features of macrocycles, such as their well-defined structures and pre-organised binding sites, contribute to their exceptional anion-binding capabilities that have led to their application across a broad range of the chemical and biological sciences.

Naphthalene Diimide and Bis-Heteroleptic Ru(II) Complex-Based Hybrid Molecule with 3-in-1 Functionalities

Multipurpose applications of a newly developed homobimetallic Ru(II) complex, Ru-NDI[PF6]4, which incorporates 1,10-phenanthroline and triazole-pyridine ligands and linked via a (-CH2-)3 spacer to the reputed anion-π interacting NDI system, are described. Solution-state studies of the bimetallic complex, including EPR, PL, UV-vis, and NMR experiments, reveal two sequential one-electron transfers to the NDI unit, generating NDI⋅- and NDI2- in the presence of F- selectively. This process inhibits the primary electron transfer from Ru(II) to the NDI unit, thereby allowing the 3MLCT-based emission of the complex to be recovered, resulting in a corresponding ten-fold increase in luminescence intensity. DFT and TD-DFT computational studies further elucidate the experimentally observed absorption spectra of the complex. Secondly, CT-DNA binding studies with the complex are performed using various spectroscopic analyses such as UV-vis, PL, and CD. Comparative DNA binding studies employing EB and molecular docking reveal that the binding with CT-DNA occurs through both intercalative and groove binding modalities. Thirdly, the photocatalytic activities of the complex towards C-C, C-N, and C-O bond formation in organic cross-coupling reactions, including the amidation of α-keto acids to amines and the oxidation of alcohol to aldehydes, are also demonstrated.

Cyclic Azahelicene Dimers Showing Bright Circularly Polarized Luminescence and Selective Fluoride Recognition

Although helicenes are promising molecules, the synthetic difficulty and tediousness have often been problems, and only small amounts of optically pure helicenes have been obtained by using chiral HPLC in most cases. Herein, aza[7]helicenes or closed-aza[7]helicenes with (1R)-menthyl substituents were selectively synthesized via the intramolecular Scholl reaction, and the diastereomeric pairs were separated by silica gel column chromatography. The optically pure helicenes were further transformed into the corresponding cyclic dimers, and the chiroptical properties were investigated. The rigid π-frameworks of the dimers led to the high molar extinction coefficients and fluorescence quantum yields, while the twisted helicene moieties induced clear Cotton effects and CPL in the visible region, and the high CPL brightness (BCPL) was achieved. Furthermore, the cyclic dimers were found to have the macrocyclic cavity with the two NH groups suitable for the selective binding of a fluoride anion, which induced significantly redshifted fluorescence and CPL in the red region.

Sensing Volatile Pollutants with Spin-Coated Films Made of Pillar[5]arene Derivatives and Data Validation via Artificial Neural Networks

Different types of solvents, aromatic and aliphatic, are used in many industrial sectors, and long-term exposure to these solvents can lead to many occupational diseases. Therefore, it is of great importance to detect volatile organic compounds (VOCs) using economic and ergonomic techniques. In this study, two macromolecules based on pillar[5]arene, named P[5]-1 and P[5]-2, were synthesized and applied to the detection of six different environmentally volatile pollutants in industry and laboratories. The thin films of the synthesized macrocycles were coated by using the spin coating technique on a suitable substrate under optimum conditions. All compounds and the prepared thin film surfaces were characterized by NMR, Fourier transform infrared (FT-IR), elemental analysis, atomic force microscopy (AFM), scanning electron microscopy (SEM), and contact angle measurements. All vapor sensing measurements were performed via the surface plasmon resonance (SPR) optical technique, and the responses of the P[5]-1 and P[5]-2 thin-film sensors were calculated with ΔI/Io × 100. The responses of the P[5]-1 and P[5]-2 thin-film sensors to dichloromethane vapor were determined to be 7.17 and 4.11, respectively, while the responses to chloroform vapor were calculated to be 5.24 and 2.8, respectively. As a result, these thin-film sensors showed a higher response to dichloromethane and chloroform vapors than to other harmful vapors. The SPR kinetic data for vapors validated that a nonlinear autoregressive neural network was performed with exogenous input for the best molecular modeling by using normalized reflected light intensity values. It can be clearly seen from the correlation coefficient values that the nonlinear autoregressive with exogenous input artificial neural network (NARX-ANN) model for dichloromethane converged more successfully to the experimental data compared to other gases. The correlation coefficient values of the dichloromethane modeling results were approximately 0.99 and 0.98 for P[5]-1 and P[5]-2 thin-film sensors, respectively.

Anion Recognition in Solution: Insights from Thermodynamics and Ultrafast Structural Dynamics

Anion recognition through noncovalent interactions stands as an emerging field in supramolecular chemistry, exerting a profound influence on the regulation of biological functions. Herein, the thermodynamics of complexation between sodium cyanate (NaOCN) and calix[4]pyrrole was systematically investigated by linear and nonlinear IR spectroscopy, highlighting enthalpy changes as the dominant driving force. The overall orientational relaxation of bound anion can be described by an Arrhenius-type activated process, yielding an activation energy of 15.0 ± 1.0 kJ mol-1. The structural dynamics of contact ion pairs (CIPs) formed between Na+ and OCN- in solution showed a negligible temperature effect, suggesting entropy changes as the principal governing factor. Further analysis revealed that anion recognition in solution is mediated by conformational changes of the receptor and collective rearrangement of hydrogen bond dynamics. This study, framed within the paradigms of thermodynamics and ultrafast structural dynamics, substantially advances our comprehension of the microscopic mechanisms underlying anion recognition in the realm of supramolecular chemistry.

Vitamin B6-based fluorescence chemosensor for selective detection of F- ions: design, synthesis, and characterization

The present paper reports on the synthesis and characterization of a new chemosensor for fluoride ions, a hydrazone derived from pyridoxal 5'-phosphate and benzothiazole. The structure of the chemosensor was confirmed using 1H and 13C NMR, FT-IR and mass spectroscopy. The conformational diversity of the chemosensor influencing the sensor activity was studied by the quantum chemistry methods on the B3LYP/6-311++G(d, p) (H, C, N, O, P, S) level, and the optimal structure of the chemosensor was chosen. The selective capability of detecting F- in the aqueous solution, which also contains Cl-, Br-, I-, NCS-, ClO4-, HSO4-, and NO3- was demonstrated. The detection limit (LOD) for fluoride ions was 0.22 µM as determined by the 3σ method. The turn-on effect in the presence of fluoride ions is based on the deprotonation of the chemosensor and its subsequent aggregation in DMSO. In addition, the chemosensor was used for the detection and estimation of F- in real samples using fluorescence spectroscopy.

Photo-controllable binding and release of HP2O73- using an azobenzene based smart macrocycle

Herein, we describe the design and synthesis of an unusual azobenzene-bearing macrocycle 1, whose trans isomer was found able to 100% transform into its cis configuration under photoirradiation, for selectively recognizing HP2O73- with reversibly photo-controllable binding and release properties.

Fluorescent and chromogenic organic probes to detect group 10 metal ions: design strategies and sensing applications

Group 10 metals including Ni, Pd and Pt have been extensively applied in various essential aspects of human social life, material science, industrial manufactures, medicines and biology. The ionic forms of these metals are involved in several biologically important processes due to their strong binding capability towards different biomolecules. However, the mishandling or overuse of such metals has been linked to serious contamination of our ecological system, more specifically in soil and water bodies with acute consequences. Therefore, the detection of group 10 metal ions in biological as well as environmental samples is of huge significance from the human health point of view. Related to this, considerable efforts are underway to develop adequately efficient and facile methods to achieve their selective detection. Optical sensing of metal ions has gained increasing attention of researchers, particularly in the environmental and biological settings. Innovatively designed optical probes (fluorescent or colorimetric) are usually comprised of three basic components: an explicitly tailored receptor unit, a signalling unit and a clearly defined reporter unit. This review deals with the recent progress in the design and fabrication of fluorescent or colorimetric organic sensors for the detection of group 10 metal ions (Ni(II), Pd(II) and Pt(II)), with attention to the general aspects for design of such sensors.

Synthesis of Imidazolium Cations Linked to Para-t-Butylcalix[4]arene Frameworks and Their Use as Synthons for Nickel-NHC Complexes Tethered to Calix[4]arenes

A series of cationic p-tert-butylcalix[4]arenes, with side-arms that are functionalized with imidazolium groups, have been synthesized in good yields. The parent tetrahydroxy para-t-butyl-calix[4]arene was dialkylated at the phenolic hydrogen atoms using α,ω-dibromo-alkanes to yield bis(mono-brominated) alkoxy-chains of variable length. The brominated side-arms in these compounds were then further alkylated with substituted imidazoles (N-methylimidazole, N-(2,4,6-trimethyl-phenyl)imidazole, or N-(2,6-di-isopropylphenyl)imidazole) to yield a series of dicationic calixarenes with two imidazolium groups tethered, via different numbers of methylene spacers (n = 2-4), to the calixarene moiety. Related tetracationic compounds, which contain four imidazolium units linked to the calix[4]arene backbone, were also prepared. In all of these compounds, the NMR data show that the calixarenes adopted a cone configuration. All molecules were characterized by NMR spectroscopy and by MS studies. Single crystal X-ray diffraction studies were attempted on many mono-crystals of these cations, but significant disorder problems, partly caused by occluded solvent in the lattice, and lack of crystallinity resulting from partial solvent loss, precluded the good resolution of most X-ray structures. Eventually, good structural data were obtained from an unusually disordered single crystal of 5a, (1,3)-Cone-5,11,17,23-tetra-t-butyl-25,27-di-hydroxy-26,28-di-[2-(N-2,6-diisopropylphenyl-imidazolium)ethoxy]calix[4]arene dibromide and its presumed structure was confirmed. The structure revealed the presence of H-bonded interactions and some evidence of π-stacking. Some of these imidazolium salts were reacted with nickelocene to form the nickel N-heterocyclic carbene (NHC) complexes 7a-7d. A bis-carbene nickel complex 8 was also isolated and its structure was established by single crystal X-ray diffraction studies. The structure was disordered and not of high quality, but the structural data corroborated the spectroscopic data.

Iron-doped cerium/nucleotide coordination polymer as highly efficient peroxidase mimic for colorimetric detection of fluoride ion

A new coordination polymer (Ce-Fe-GMP) with excellent catalytic activity was prepared by a facile route, which was further applied to the detection of F^- with high sensitivity and selectivity. The simple doping of Fe³⁺ into the coordination network can easily modulate the mixing ratio of Ce³⁺ and Ce⁴⁺ in the presence of H₂O₂, which can extremely improve the catalytic ability of Ce-Fe-GMP. Based on the synergistic effect, the Ce-Fe-GMP with dual-active sites shows better peroxidase activity than that of Ce-GMP. In addition, we found that F^- can inhibit the peroxidase activity of Ce-Fe-GMP because of the coordination structure fragmentation and the regulation of Ce³⁺/Ce⁴⁺ ratio. Therefore, different concentrations of F^- can be detected by the colorimetric reaction based on this mechanism. The absorption at 652 nm displays a good linear relationship versus the concentration of F^- over the range 2.0 to 100.0 μM. Furthermore, F^- in real mineral-mixed samples can be measured with satisfactory results. The colorimetric strategy based on the peroxidase activity of Ce-Fe-GMP is simple and low-cost, which shows the potential applications in the field of on-site environment measurement.

Recent developments in calix[4]pyrrole (C4P)-based supramolecular functional systems

Recent advances with calix[4]pyrrole-based supramolecular functional entities in the fields of molecular recognition (receptors, sensors, and metal ion caged systems), self-assembly (polymers), photo/pH-responsive molecular switches and catalysis are reviewed.