Kuppanagounder P. Elango

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.

Spectroscopic studies on the interaction of cimetidine drug with biologically significant σ- and π-acceptors

Spectroscopic studies revealed that the interaction of cimetidine drug with electron acceptors iodine and 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) resulted through the initial formation of ionic intermediate to charge transfer (CT) complex. The CT-complexes of the interactions have been characterized using UV-vis, (1)H NMR, FT-IR and GC-MS techniques. The formation of triiodide ion, I(3)(-), is further confirmed by the observation of the characteristic bands in the far IR spectrum for non-linear I(3)(-) ion with C(s) symmetry at 156 and 131cm(-1) assigned to nu(as)(I-I) and nu(s)(I-I) of the I-I bond and at 73cm(-1) due to bending delta(I(3)(-)). The rate of formation of the CT-complexes has been measured and discussed as a function of relative permittivity of solvent and temperature. The influence of relative permittivity of the medium on the rate indicated that the intermediate is more polar than the reactants and this observation was further supported by spectral studies. Based on the spectroscopic results plausible mechanisms for the interaction of the drug with the chosen acceptors were proposed and discussed and the point of attachment of the multifunctional cimetidine drug with these acceptors during the formation of CT-complex has been established.

Electronic, Raman and FT-IR spectral investigations of the charge transfer interactions between ketoconazole and povidone drugs with iodine

The interaction between ketoconazole and povidone drugs with iodine was found to proceed through initial formation of a charge transfer (CT) complex as an intermediate. The stoichiometry of the complex was found to be 1:1 in the case of povidone-iodine system and 1:2 in the case of ketoconazole-iodine system and the same was confirmed by thermal (TGA/DSC) studies. The formation of I(3)(-) species was confirmed by electronic and laser Raman spectra. The three peaks appeared in Raman spectra, of the isolated adducts corresponds to nu(as)(I-I), nu(s)(I-I) and delta(I(3)(-)), confirmed the presence of asymmetric I(3)(-) ion. The rate of the interaction has been measured as a function of time and solvent. The pseudo-first-order rate constants at various temperatures for the interactions were measured and the activation parameters (DeltaG(#), DeltaS(#) and DeltaH(#)) were computed. Based on the spectral and spectrokinetic evidences a mechanism involving the formation of a polar intermediate and its conversion to the final product has been proposed and discussed.

Spectroscopic and spectrofluorimetric studies on the interaction of irbesartan with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone and iodine.

Raman, UV-vis, 1H NMR, FT-IR, mass and fluorescence spectral techniques were employed to investigate the mechanism of interaction of irbesartan (IRB) drug with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) and iodine. Interaction of IRB with iodine yields triiodide ion and its formation was confirmed by electronic and Raman spectra. The peaks appeared in Raman spectrum of the isolated product at 143, 113 and 76 cm(-1) are assigned to νas(I-I), νs(I-I) and δ(I3-) respectively, confirmed the presence of I3- ion. The interaction of DDQ with irbesartan was found to proceed through the formation of outer complex and its conversion to the CT complex. Formation constant (K), molar extinction coefficient (ɛ) and thermodynamic properties ΔH#, ΔS# and ΔG# were determined and discussed. Fluorescence quenching studies indicated that the interaction between the IRB and the acceptors are spontaneous and the IRB-DDQ interaction is found to be stronger than that the other system. Solvent variation studies indicated that the binding constant increased with an increase in polarity of the medium.

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’.

Spectrophotometric and spectrofluorimetric studies on the selective sensing of fluoride ions by Co(II) and Ni(II) complexes of naphthoquinone derivative possessing enhanced H-bonding property.

A novel colorimetric chemosensor based on aminonaphthoquinone (L) bearing an N-H receptor unit directly attached to quinone signaling unit has been designed, synthesized and demonstrated. The ligand showed a highly selective colorimetric response to fluoride ions based on H-bond formation with the receptor unit. The binding constants of the L and its square planar [Co(L)Cl(2)]·3H(2)O and [Ni(L)Cl(2)]·4H(2)O complexes, computed using fluorescent enhancement data, were found to be 0.6, 1.5 and 0.9×10(8)M(-1), respectively, indicating enhancement of H-bond donor ability of the receptor unit, as a result of complexation with metal ions, towards fluoride ion sensing. Also, these sensors had high selectivity for fluoride ion detection over other common anions, such as Cl(-), Br(-), I(-), AcO(-), NO(3)(-), H(2)PO(4)(-) and CN(-) in acetonitrile.

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.

Highly selective colorimetric sensing of Cu(II) ions in aqueous solution via modulation of intramolecular charge transfer transition of aminonaphthoquinone chemosensor

An aminonaphthoquinone based colorimetric chemosensor has been developed and demonstrated for the highly selective detection of Cu(II) ions in aqueous solution. The intramolecular charge transfer (ICT) transition exits in amine moiety directly attached to the quinone ring is modulated by the d-d transition of a square planar Cu(II)-receptor complex resulting in a change of color from yellow to blue. No significant color change was observed upon addition of other selected metal ions. The sensing property has been investigated using various spectral techniques (UV-Vis, fluorescence) and product analysis (Elemental analysis, magnetic moment, UV-Vis, FT-IR, EPR).

Synthesis and spectral characterization of lanthanide complexes with sulfamethoxazole and their antibacterial activity

A series of nonelectrolytic lanthanide(III) complexes, [ML2Cl3]·2H2O, where M is lanthanum(III), praseodymium(III), neodymium(III), samarium(III), gadolinium(III), terbium(III), dysprosium(III), and yttrium(III), containing sulfamethoxazole ligand (L) are prepared. The structure and bonding of the ligand are studied by elemental analysis, magnetic susceptibility measurements, IR, 1HNMR, TG/DTA, X-ray diffraction studies, and electronic spectra of the complexes. The stereochemistry around the metal ions is a monocapped trigonal prism in which four of the coordination sites are occupied by two each from two chelating ligands, sulfonyl oxygen, and nitrogen of the amide group and the remaining three positions are occupied by three chlorines. The ligand and the new complexes were tested in vitro to evaluate their activity against the bacteria Escherichia coli and Staphylococcus aureus.