Recognition of Anions, Monoamine Neurotransmitter and Trace Amine Hydrochlorides by Ureido-Hexahomotrioxacalix[3]arene Ditopic Receptors

A facile and sensitive hexahomotrioxacalix[3]arene-based fluorescent sensor for the detection of trace amounts of 2,4,6-trinitrophenol

Nitroaromatic compounds are common explosives and toxic pollutants, the selective and sensitive detection of which is of great importance. Herein, a facile and sensitive fluorescent sensor L was constructed for the sensing of TNP based on the hexahomotrioxacalix[3]arene skeleton. The fluorescence emission of L was drastically quenched in the presence of 2,4,6-trinitrophenol (TNP), while other tested NACs, metal ions, and anions induced negligible changes. Under the optimized conditions, the spectroscopic studies revealed that L exhibited extremely sensitive and selective TNP recognition, with a detection limit of 9.17 × 10-7 M and a quenching constant of 2.44 × 104 M-1. The sensitivity of sensor L for TNP was attributed to the formation of a ground-state charge-transfer complex and an inner filter effect, which also contributed to the special selectivity of the sensor among the various nitroaromatic analogues. Compared with previous reports, L can serve as a highly efficient sensor for the sensing of TNP and can be employed over a wide pH range of 2 to 12. Sensor L was effectively used to quantify TNP in real water and soil samples. Additionally, fluorescent test strips were also developed for visual and rapid detection of TNP in both the solution and vapour phases.

Anion Binding by Fluorescent Ureido-Hexahomotrioxacalix[3]arene Receptors: An NMR, Absorption and Emission Spectroscopic Study

Fluorescent receptors (4a-4c) based on (thio)ureido-functionalized hexahomotrioxacalix[3]arenes were synthesised and obtained in the partial cone conformation in solution. Naphthyl or pyrenyl fluorogenic units were introduced at the lower rim of the calixarene skeleton via a butyl spacer. The binding of biologically and environmentally relevant anions was studied with NMR, UV-vis absorption, and fluorescence titrations. Fluorescence of the pyrenyl receptor 4c displays both monomer and excimer fluorescence. The thermodynamics of complexation was determined in acetonitrile and was entropy-driven. Computational studies were also performed to bring further insight into the binding process. The data showed that association constants increase with the anion basicity, and AcO-, BzO- and F- were the best bound anions for all receptors. Pyrenylurea 4c is a slightly better receptor than naphthylurea 4a, and both are more efficient than naphthyl thiourea 4b. In addition, ureas 4a and 4c were also tested as ditopic receptors in the recognition of alkylammonium salts.

Dihomooxacalix[4]arene-Based Fluorescent Receptors for Anion and Organic Ion Pair Recognition

Fluorescent dihomooxacalix[4]arene-based receptors 5a-5c, bearing two naphthyl(thio)ureido groups at the lower rim via a butyl spacer, were synthesised and obtained in the cone conformation in solution. The X-ray crystal structures of 1,3- (5a) and 3,4-dinaphthylurea (5b) derivatives are reported. Their binding properties towards several anions of different geometries were assessed by ¹H-NMR, UV-Vis absorption and fluorescence titrations. Structural and energetic insights of the naphthylurea 5a and 5b complexes were also obtained using quantum mechanical calculations. The data showed that all receptors follow the same trend, the association constants increase with the anion basicity, and the strongest complexes were obtained with F^-, followed by the oxoanions AcO^- and BzO^-. Proximal urea 5b is a better anion receptor compared to distal urea 5a, and both are more efficient than thiourea 5c. Compounds 5a and 5b were also investigated as heteroditopic receptors for biologically relevant alkylammonium salts, such as the neurotransmitter γ-aminobutyric acid (GABA·HCl) and the betaine deoxycarnitine·HCl. Chiral recognition towards the guest sec-butylamine·HCl was also tested, and a 5:2 selectivity for (R)-sec-BuNH₃⁺·Cl^- towards (P) or (M) enantiomers of the inherently chiral receptor 5a was shown. Based on DFT calculations, the complex [(S)-sec-BuNH₃⁺·Cl^-/(M)-5a] was indicated as the more stable.

Calix[3]arene-Analogous Metacyclophanes: Synthesis, Structures and Properties with Infinite Potential

Calixarene-analogous metacyclophanes (CAMs) are a special class of cyclophanes that are cyclic polyaromatic hydrocarbons containing three or more aromatic rings linked by one or more methylene bridging groups. They can be considered to be analogues of calixarenes, since, in both types of molecules, the component aromatic rings are linked by methylene groups, which are meta to each other. Since the prototype or classical calix[4]arene consists of four benzene rings each linked by methylene bridges, which are also meta to each other, it can be considered to be an example of a functionalized [1.1.1.1]metacyclophane. A metacyclophane (MCP) that consists of three individual hydroxyl-group functionalized aromatic rings linked by methylene groups, e.g., a trihydroxy[1.1.1]MCP may therefore, by analogy, be termed in the broadest sense as a "calix[3]arene" or a "calix[3]arene-analogous metacyclophane". Most of the CAMs reported have been synthesized by fragment coupling approaches. The design, synthesis and development of functionalized CAMs, MCPs, calixarenes and calixarene analogues has been an area of great activity in the past few decades, due their potential applications as molecular receptors, sensors and ligands for metal binding, and for theoretical studies, etc. In this review article, we focus mainly on the synthesis, structure and conformational properties of [1.1.1]CAMs, i.e., "calix[3]arenes" and their analogues, which contain three functionalized aromatic rings and which provide new scaffolds for further explorations in supramolecular and sensor chemistry.

Specific Binding of Primary Ammonium Ions and Lysine-Containing Peptides in Protic Solvents by Hexahomotrioxacalix[3]arenes

The binding of ammonium ions by two homooxacalix[3]arene-based receptors was studied using NMR spectroscopy and in silico methods. Both receptors are shown to endocomplex, even in a protic environment, a large variety of primary ammonium ions, including biomolecules. The binding mode is similar for all guests with the ammonium ion deeply inserted into the polyaromatic cavity and its NH₃⁺ head nearly in the plane defined by the three oxygen atoms of the 18-crown-3 moiety, thus enabling it to establish three H-bonds with the ethereal macrocycle. The remarkable electronic, size, and shape complementarity between primary ammonium ions and the two cavity-based receptors leads to an unprecedented specificity for primary ammonium ions over secondary, tertiary, and quaternary ones. These binding properties were exploited for the selective liquid-liquid extraction of primary ammonium salts from water and for the selective recognition of lysine-containing peptides, opening new perspectives in the field of peptide sensing.