New Chromogenic and Fluorescent Probes for Anion Detection: Formation of a [2 + 2] Supramolecular Complex on Addition of Fluoride with Positive Homotropic Cooperativity

13-8-13-Membered Tricyclic Ladder-Type Siloxanes Hybridized with BINOLs: Synthesis, Characterization, and Fluorescence Sensing of Fluorides

Developing fluorescent chemosensors with sensitivity and high specificity for recognizing fluorides is still challenging. Herein, four innovative compounds based on 13-8-13-membered tricyclic ladder-type siloxanes hybridized with BINOLs (abbreviated as TLS-BINOLs) were prepared through the B(C6F5)3-catalyzed Piers-Rubinsztajn reaction. The well-defined ladder-type structure of the TLS-BINOLs was determined by X-ray crystallographic analysis. Additionally, the fluorescent sensing ability of the TLS-BINOLs toward anions was studied. Our finding revealed that all four ladder-type compounds (TLS-BINOLs) exhibited high specificity in recognizing fluorides through fluoride-triggered structural decomposition. The detection limits for fluorides were determined to be 0.37, 0.35, 0.39, and 0.48 μM for the respective TLS-BINOLs. The nonemissive product induced by the fluorides was also determined using single-crystal X-ray diffraction analysis.

Enantio- and Diastereoselective Nucleophilic Addition of N-tert-Butylhydrazones to Isoquinolinium Ions through Anion-Binding Catalysis

A highly enantio- and diastereoselective thiourea-catalyzed dearomatization of isoquinolines employing N-tert-butylhydrazones as neutral α-azo carbanions and masked acyl anion equivalents has been developed. Experimental and computational data supports the generation of highly ordered complexes wherein the chloride behaves as a template for the catalyst, the hydrazone reagent, and the isoquinolinium cation, providing excellent stereocontrol in the formation of two contiguous stereogenic centers. The ensuing selective and high-yielding transformations provide appealing dihydroisoquinoline derivatives.

Azo-Label Heterometal-Organic Rhomboids Exhibiting Photoswitchable NIR Luminescence in Crystalline State

Photochromism is an important strategy for realizing reversible light-controllable fluorescence switching. In spite of several reports on fluorescence switching via a photochromic process, the success of photochromic multimetallic complexes reversibly showing fluorescence switching in the solid or crystalline state has been limited for their application importance. Here, we report a photoswitchable near-infrared (NIR) fluorescence based on photochromism in the azo-label 3d/4f heterometal-organic rhomboids, azo-Zn₂Ln₂ (Ln = Eu (1), Yb (2), and Er (3)), in the crystalline state. An individual metallorhomboid contains up to four azobenzene fragments, which is prepared via the three-component assembly of a trans-azobenzene-grafted multifunctional ligand, and 3d and 4f metal ions. The photoisomerization quantum yields of azo-Zn₂Ln₂ complexes can be retained or even higher when compared to the free ligand due to the modification of electronic structure. The impressive crystalline-state NIR luminescence is observed for the complexes of azo-Zn₂Yb₂ (2) and azo-Zn₂Er₂ (3) at room temperature. Intriguingly, the switchable NIR luminescence can be effectively regulated by photochromism in the crystalline state. These features endow the self-assembly of the 3d/4f metallorhomboid with synergetic multifunctional behavior between photochromism and NIR luminescence.

Functionalized Quinoxaline for Chromogenic and Fluorogenic Anion Sensing

This Review article provides a comprehensive analysis of recent examples reported in the field of quinoxaline-based chromogenic and fluorogenic chemosensors for inorganic anions such as fluoride, cyanide, acetate, and phosphate, as well as their utility in biomolecular science. It commences with a discussion of the various structural motifs such as quinoxaline-based oligopyrroles, polymers, sulfonamides, cationic receptors, and miscellaneous receptors bearing mixed recognition sites in the same receptor. Advances are discussed in depth, where the focus of this review is to tackle mainly solution state anion sensing utilizing quinoxaline-based receptors using different spectroscopic techniques with reference to anion selectivity by colorimetric and fluorescence response. The various examples discussed in this Review illustrate how the integration of anion binding elements with the quinoxaline chromophore could result in anion responsive chemosensors. Over the years, it has been observed that structural modification of the quinoxaline moiety with different sets of signaling unit and recognition sites has resulted in a few anion specific chemosensors.

A highly selective ratiometric visual and red-emitting fluorescent dual-channel probe for imaging fluoride anions in living cells

Recently, growing attention has been paid to the accurate determination of fluoride anion (F(-)) in the environment and living systems for its toxicity and biological function investigation. In this paper, we developed a ratiometric visual and red-emitting fluorescent dual-channel probe (1) employed Si-O bond as a highly selective recognition receptor for imaging F(-) in living cells. Probe 1 possesses a potential internal charge transfer (ICT) structure, and displays a large (158 nm) red-shifted absorption spectrum and the color changes from yellow to blue upon addition of F(-) in the aqueous solution. In addition, probe 1 can be used to detect F(-) quantitatively by the ratiometric absorption and turn-on fluorescence spectroscopy methods with excellent sensitivity. Finally, the results of its application to bioimaging of F(-) in living cells show that probe 1 would be of great benefit to biomedical researchers for investigating the effects of fluoride in biological systems.

Recognition, encapsulation, and selective fluorescence sensing of nitrate anion by neutral C3-symmetric tripodal podands bearing amide functionality

A series of neutral C(3)-symmetric acyclic artificial receptors incorporating amide functionality has been designed, synthesized, and fully characterized. Upon protonation, these conformationally flexible N-bridged tripodal podands 1-5 form in situ cone shape conformation through hydrogen bonding and C-H···π interactions. The protonation-induced interior preorganized cavity is capable of entrapping nitrate anions through the amide N-H bonds to form discrete nitrate complexes (1a-5a), which were fully characterized by NMR, HRESI mass spectra, and single crystal structures. By incorporating suitable fluorophores at each branch of the tripod receptor, the resulting fluorescent receptor 5 selectively recognized nitrate anions by fluorescent quenching in a DMSO solution and displayed one of the highest binding affinities for nitrate anions reported so far in polar media. Receptor 5 represents a unique example of a neutral receptor for the recognition of nitrate anions in polar solvent media by its zwitterionic form. The possible mechanism of proton-induced preorganization of these flexible, acyclic receptors in a convergent cone conformation followed by nitrate complexation has been proposed to rationalize the effective nitrate recognition.

First theophylline-based ratiometric fluorescent synthetic receptor for selective recognition of dihydrogenphosphate and biological phosphate ions

We have developed a new ratiometric fluorescent chemosensor 1 based on xanthine alkaloid theophylline moiety for the detection of dihydrogen phosphate and ATP. The chemosensor 1 selectively recognizes tetrabutylammonium dihydrogen phosphate in CH(3)CN/H(2)O (9:1) by exhibiting a significant decrease in the emission of naphthalene and its sensing properties regarding ATP and other related phosphate species were evaluated. The anion binding properties of 1 were evaluated by (1)H NMR, UV-vis, and fluorescence spectroscopic methods.

Interaction of protonated merocyanine dyes with amines in organic solvents

2,6-Diphenyl-4-(2,4,6-triphenylpyridinium-1-yl)phenolate (1a) and 4-[(1-methyl-4(1H)-pyridinylidene)-ethylidene]-2,5-cyclohexadien-1-one (2a) were protonated in organic solvents (dichloromethane, acetonitrile, and DMSO) to form 1b and 2b, respectively. The appearance of the solvatochromic bands of 1a and 2a was studied UV-vis spectrophotometrically by deprotonation of 1b and 2b in solution in the presence of the following amines: aniline (AN), N-methylaniline (NMAN), N,N-dimethylaniline (NDAN), n-butylamine (BA), diethylamine (DEA), and triethylamine (TEA). Titrations of 1b and 2b with the amines were carried out and the binding constants were determined from the titration curves in each solvent, using a mathematical model adapted from the literature which considers the simultaneous participation of two dye: amine stoichiometries, 1:1 and 1:2. The data obtained showed the following base order for the two compounds in DMSO: BA>DEA>TEA, while aromatic amines did not cause any effect. In dichloromethane, the following base order for 1b was verified: TEA>DEA>BA≫NDAN, while for 2b the order was: TEA>DEA>BA, suggesting that 1b is more acidic than 2b. The data in acetonitrile indicated for 1b and 2b the following order for the amines: DEA>TEA>BA. The diversity of the experimental data were explained based on a model that considers the level of interaction of the protonated dyes with the amines to be dependent on three aspects: (a) the basicity of the amine, which varies according to their molecular structure and the solvent in which it is dissolved, (b) the molecular structure of the dye, and (c) the solvent used to study the system.

Anion-responsive covalently linked and metal-bridged oligomers

Ethynyl-substituted acyclic anion receptors were synthesized for discrete covalently linked and metal-bridged dimers, which form various anionic complexes including double helical structures as the first examples of Cl(-)-bridged [2+2]-type complexes that are stable in the solution state.

A highly selective colorimetric and "off-on-off" fluorescent probe for fluoride ions

In this paper, a highly selective and sensitive probe for fluoride ions (F(-)), containing a phenylpyridylvinylene derivative reporter and a Si-O bond receptor, was designed and characterized. The reaction mechanism is based on the intramolecular charge transfer (ICT) mechanism. Upon addition of F(-), probe 1 showed a remarkable red-shift (183 nm) in the absorption spectra accompanying with the color changes from colorless to purple, so probe 1 could serve as a "naked-eye" probe for F(-). The absorbance of probe 1 at 545 nm increased linearly with the concentration of F(-) from 20 to 150 µM. The detection limit was calculated to be 0.1 µM. Besides, "off-on-off" fluorescence intensity changes were also observed in the fluorescence spectra. The present results may provide a useful approach for the development of highly selective dual-channel probes for F(-).