Through bond energy transfer (TBET)-operated fluoride ion sensing via spirolactam ring opening of a coumarin–fluorescein bichromophoric dyad

The detection of fluoride ions in a competitive environment often poses several challenges. In this work, we have designed and synthesized a coumarin functionalized fluorescein dyad (R3) which represents an ideal through bond energy transfer (TBET) fluorophore with the coumarin unit as donor and fluorescein unit as acceptor. The bichromophoric dyad demonstrates the detection of fluoride ions in the parts per billion (ppb) concentration level (22.8 ppb) with high selectivity via a TBET emission signal at 548 nm with a diagnostic bright yellow colour fluorescence output. Based on UV-visible, fluorescence, ¹H NMR and DFT studies, it is shown that the fluoride ion induces the opening of the spirolactam ring of the fluorescein moiety and provides a π-conjugation link between the donor and acceptor units enabling a TBET phenomenon with a larger pseudo-Stokes shift of 172 nm. To the best of our knowledge, this is the first report where the fluoride ion is detected via a TBET signal between the coumarin and fluorescein units in a bichromophoric dyad.

A coumarin–fluorescein based bichromophoric dyad detects fluoride ions in parts per billion concentration level via a TBET emission signal at 548 nm with a diagnostic bright yellow fluorescence.

β-Carboline–imidazopyridine hybrids: selective and sensitive optical sensors for copper and fluoride ions

Two rationally designed β-carboline–imidazopyridine hybrid chromofluorescent sensors S1 and S2 have been successfully synthesized and evaluated for the selective sensing of metal ions and anions.

Application of Heterocyclic Polymers in the Ratiometric Spectrophotometric Determination of Fluoride

Herein we report the use of heterocyclic functional polymers in the ratiometric spectrophotometric determination of fluoride (F^-). Polymers incorporating benzo[d][1,2,3]triazole moieties linked to the polymer backbone via urea links are demonstrated to have utility for the ratiometric detection of the F^- ion, with a detection limit in the order of ∼2 μM. The hydrogen-bonding recognition between the benzo[d][1,2,3]triazole moiety and F^- ion was investigated using UV-vis spectrophotometry and NMR analysis. The importance of the urea linkage was elucidated by investigating a second benzo[d][1,2,3]triazole functional monomer wherein the heterocyclic group is attached to the polymerizable group via a carbamate linkage. The replacement of the urea link with a carbamate group led to significantly reduced F^- sensitivity. Moreover, by examining an analogous benzo[d]imidazole monomer it was demonstrated that having a nitrogen atom in the 2-position of the heterocycle was important for maximizing the sensitivity of the assay. Taken together, these results demonstrated that the urea-substituted benzo[d][1,2,3]triazole motif greatly enhances F^- ion detection. Importantly, the F^- ion sensing capability of the monomer is retained after incorporating into a diblock copolymer using reversible addition-fragmentation chain transfer (RAFT) polymerization.

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(-).

Nitro-substituted 3,3'-bis(indolyl)methane derivatives as anion receptors: electron-withdrawing effect and tunability of anion binding properties

A series of nitro-substituted 3,3'-bis-indolyl phenylmethane derivatives were synthesized and their anion binding properties were investigated in detail. The introduction of the electron-withdrawing nitro group into indole unit and/or meso-phenyl ring, which leads to the increased acidity of indole NH and meso-position CH proton, has a positive effect on anion binding. The nitro-substituted bis(indolyl)methane receptors exhibited selective colorimetric sensing of F- anion, as revealed by the notable color and spectral changes, rationally due to the deprotonation of the indole NH of the receptor. Meanwhile, the additive introduction of the nitro substituents on the meso-phenyl ring of bis(indolyl)methane can lead to the deprotonation of the meso-position CH and further induce an irreversible oxidation process obtaining bis(indolyl)methene product in the F- anion sensing system.

Reaction-based sensing of fluoride ions using built-in triggers for intramolecular charge transfer and photoinduced electron transfer

Two Bodipy derivatives with silyl-protected phenolic functionalities signal fluoride concentrations both in solution and in a poly(methyl methacrylate) matrix. The exact location of the "nascent" phenolate group is important. If it is at the meso position, photoinduced electron transfer is triggered; however, if it is in full conjugation via a styryl moiety to the Bodipy core, strong intramolecular charge transfer is triggered, resulting in a large red shift in the absorbance peak. In either case, a selective methodology for fluoride sensing is the invariable result.

A simple and efficient anionic chromogenic chemosensor based on 2,4-dinitrodiphenylamine in dimethyl sulfoxide and in dimethyl sulfoxide-water mixtures

Solutions of 2,4-dinitrodiphenylamine (1) in dimethylsulfoxide (DMSO) are colorless but upon deprotonation they become red. Addition of various anionic species (HSO(4)(-), H(2)PO(4)(-), NO(3)(-), CN(-), CH(3)COO(-), F(-), Cl(-), Br(-), and I(-)) to solutions of 1 revealed that only CN(-), F(-), CH(3)COO(-), and H(2)PO(4)(-) led to the appearance of the red color in solution. The presence of increasing amounts of water in solutions containing 1 made it progressively selective toward CN(-) and the system with the addition of 4.3% (v/v) of water was highly selective for CN(-) among all anions studied. The experimental data collected indicated that proton transfer from 1 to the anion occurs, and a model was used to explain the experimental results, which considers two 1:anion stoichiometries, 1:1 and 1:2. For the latter, the data suggest that the anion forms firstly a hydrogen-bonded complex with a second anion equivalent necessary for the abstraction of the proton, with the formation of a [HA(2)](-) complex. The study performed here demonstrates the important role of the environment of the anion and 1 for the efficiency of the chromogenic chemosensor. Besides the different affinities of each anion for water, the solvation of both the anion and 1 is responsible for reducing the interaction between these species. In small amounts, water or hydrogen-bonded DMSO-water complexes are able to stabilize the conjugated base of 1 through hydrogen bonding, making 1 more acidic, which explains the change from 1:1 and 1:2 toward 1:1 1:anion stoichiometry upon addition of water. In addition, water is able to solvate the anion and also 1, which hinders the formation of 1:1 hydrogen-bonded 1:anion complexes prior to the abstraction of the proton.

Synthesis and binding characteristics of novel calix[4]arene(amidocrown) diquinones

A series of new calix[4]arene(amidocrown) diquinones (3a–3d) have been synthesized, characterized, and evaluated for cation recognition. It has been observed that 3b interacts with alkali metal ions (Li+, Na+, and K+) and ammonium ions to induce an unprecedented downfield shift in the NH proton resonance, which can be attributed to polarization of the amidocrown ring of the calix[4]arene diquinone receptor. The observation has been confirmed by a significant anodic shift (Li+ > NH4+ > Na+> K+) of the corresponding amidocrown-diquinone redox couple in cyclic and square wave voltammetric experiments. Both NMR and electrochemical studies of the binding characteristics of 3b with alkali metal cations and ammonium ions revealed a 1:1 binding stoichiometry for all and exhibited the highest association constant for lithium ions. This indicated that the receptor 3b selectively binds the lithium ion in preference to other alkali metal cations and ammonium ions.