Ramalingam Manivannan

Tuning of the H-bonding ability of imidazole N–H towards the colorimetric sensing of fluoride and cyanide ions as their sodium salts in water

Imidazole functionalized receptors, 2-R-1H-naphtho[2,3-d]imidazole-4,9-dione (2a–h), containing naphthoquinone as a chromogenic signalling unit have been synthesized from the reaction of 2,3-diaminonaphthoquinone and different aldehydes. These receptors showed a color change upon addition of fluoride and cyanide ions in DMSO with a bathochromic shift of the characteristic intramolecular charge transfer (ICT) transition band. No color change was observed upon addition of other anions such as Cl−, Br−, I−, NO3−, AcO− and H2PO4−. 1H NMR and electrochemical studies revealed that these receptors sense fluoride and cyanide ion via the formation of H-bond with the imidazole N–H moiety. Electronic and spectrofluorimetric studies indicated that the binding constants of these receptors with F− and CN− ions were in the order of ∼106. The results of the spectral studies indicated that, by changing the R group in the receptor, the acidity of the imidazole N–H can be varied from δH 13.70 (for isopropyl) to 14.94 ppm (for thiophene). Theoretical calculations based on Density Functional Theory showed that the HOMO–LUMO energy gap for the ICT transition corroborate the results of the spectral studies. Receptor 2f (R = thiophene) was also able to detect fluoride and cyanide ions as their sodium salts in aqueous solution with a visual color change.

Anion induced azo-hydrazone tautomerism for the selective colorimetric sensing of fluoride ion

The design, synthesis, characterization and their anion sensing properties of two receptors capable of exhibiting azo-hydrazone tautomerism are reported. The anion sensing properties have been investigated using electronic, fluorescence and nuclear magnetic spectral studies in addition to electrochemical and visual detection experiments. Both the receptors selectively bind fluoride ion with >100 nm red-shift in the electronic spectrum and the color changes from yellow to red. The results of the spectral studies revealed that the sensing mechanism involves fluoride ion induced change of chromophore from C=N (hydrazone form) to N=N (azo form) in these receptors leading to the visible color change. Density Functional Theory calculations were conducted to rationalize the optical response of the receptors.

Benzoquinone–imidazole hybrids as selective colorimetric sensors for cyanide in aqueous, solid and gas phases

Five new chemosensors (R1–R5), possessing benzoquinone as the signaling unit and imidazole as the H-bond donor unit, for cyanide sensing have been rationally designed, synthesized and characterized by NMR and mass spectroscopy. The structure of R5 was confirmed by single crystal XRD studies. These receptors exhibited a prominent visual colour change toward the cyanide ion over other common anions in an aqueous HEPES buffer–DMF (9 : 1 v/v) medium. The complexation of receptor–CN− has been addressed by UV-Vis, fluorescence and 1H NMR spectra and was supported by electrochemical and DFT studies. The mechanism of sensing involves formation of H-bonds between imidazole N–H and CN− ions. The stoichiometry of the receptor–CN− complexes was found to be 1 : 2 (receptor–CN−) and the detection limit was observed to be in the range of 1.1–3 nM. The test strips based on R5 were fabricated and could act as convenient and efficient CN− test kits. Notably, the novelty of the present investigation is that the receptor R5 selectively senses CN− ions in solid, aqueous and gas phases i.e. ‘a complete receptor’.

Design, synthesis and characterization of indole based anion sensing receptors

The design and synthesis of six new receptors (R1–R6) and their anion sensing properties through multiple channels are reported. These receptors are constructed in such a way that they possess indole groups as the binding sites and different acceptors units of varying electron acceptor strengths. Receptors R1, R3 and R5 could recognize fluoride ions visually and spectroscopically with high selectivity over other anions in DMF, which was demonstrated by a visual detection experiment and UV-Vis, fluorescence and 1H NMR spectral studies. The remaining three receptors (R2, R4 and R6) exhibited colour changes with both fluoride and cyanide ions. The binding constants for fluoride binding by these receptors were determined to be in the order of 104 to 106 M−1 and found to depend on the electron accepting property of the acceptor unit in the intra molecular charge transfer (ICT) transition existing with the indole donor units. 1H NMR titration experiments not only provide evidence for the existence of H-bonding interactions between the indolic N–H groups of these receptors and F−, but also offer key insight into the strengths of the receptor–anion complexes of stoichiometry 1:2. The higher fluoride binding ability of the receptor containing the naphthoquinone signalling unit has been interpreted in terms of the greater electron deficiency of the acceptor unit (quinone) and enhanced H-bond donating character of the indole N–H group. The results of the electrochemical and DFT computation studies corroborate well with the spectroscopic studies.

Structure–reactivity correlation in selective colorimetric detection of cyanide in solid, organic and aqueous phases using quinone based chemodosimeters

Quinone based colorimetric chemodosimeters, which could instantly detect CN− in solid, organic and aqueous phases with selectivity and sensitivity are described. The results of UV-Vis, fluorescence, 1H and 13C NMR spectroscopy, electrochemical studies and product analysis confirmed that the mechanism of sensing is through nucleophilic addition of CN− to the CC bond of the indole ring. DFT calculations were conducted to rationalize the sensing mechanism of these sensors. Though the sensors show a different color change with F− in organic medium via H-bond formation, they are highly selective towards CN− as shown in the relay experiment as well as when they are added together. A novel attempt is made to explain the effect of substituents on the sensing behavior by carrying out “Structure–Reactivity Correlation” using Hammetts substituent constants (σp).

A diquinone–imidazole ensemble for selective colorimetric sensing of cyanide in aqueous medium via anion induced NIR absorption

A diquinone–imidazole ensemble (R) is rationally designed, synthesized and employed as a selective and sensitive colorimetric sensor for cyanide ions in DMSO : H2O (1 : 4 v/v). 1H NMR titration indicates that the mechanism of sensing involves the formation of a H-bond between CN− and imidazole N–H. As rationalized, the R–CN− complex exhibited intense NIR-absorption (913 nm; log e 4.3) with a 428 nm red-shift from the absorbance maximum of free R. DFT calculations indicate that the R–CN− complex is perfectly planar with a HOMO–LUMO energy gap of 0.9826 eV which is optimal for NIR-absorption.

An ICT-based chemodosimeter for selective dual channel sensing of cyanide in an aqueous solution

Herein, using intramolecular charge transfer (ICT) transition as a signaling mechanism, a quinoline–benzothiazolium hybrid was designed, synthesized, and developed as a dual channel (colorimetric/turn-on fluorometric) sensor for cyanide in aq. HEPES buffer/DMF (70 : 30 v/v; pH 7.2). This receptor can detect cyanide with very high selectivity and instantaneous response. The mechanism of sensing involves nucleophilic addition of CN− to the C atom of the CN bond of benzothiazolium moiety. This irreversible addition blocks the π-conjugation and ICT transition between quinoline and benzothiazolium moieties and consequently alters the colour and spectral properties. The results of electronic, fluorescence, and 1H & 13C NMR spectral studies corroborated well with those of the DFT calculations.