Fluorescent Lower Rim 1,3-Dibenzooxadiazole Conjugate of Calix[4]arene in Selective Sensing of Fluoride in Solution and in Biological Cells Using Confocal Microscopy.

p- tert-Butyl-calix[4]arene was derivatized by integrating a benzooxadiazole fluorescent tag into its 1,3-arms at the lower rim to result in L and was characterized. L was titrated with 17 anions in THF and found selective for F- ions with lowest detection limit of 109 ppb. L and F- form a 1:1 complex. L self-assembles in THF to result in sheet like structures which converts into smaller spherical particles upon addition of F-. The site of interaction of F- was deduced based on 1H NMR spectroscopy and the coordination features by density functional theory (DFT) computations wherein six noncovalent interactions of the type X-H···F (where X = O, N, or C) were noticed. The sensing of F- is reversible when titrated with Ca2+, and the reversibility was demonstrated for 10 cycles without losing sensitivity. The study has been extended to the biological cells using fluorescence and confocal microscopy. While L shows strong fluorescence in HeLa cells, increasing concentrations of F- exhibited greater fluorescence quenching. Thus, L acts as a good sensor for F- in solution as well as in biological cells, a rare and unique combination for a calixarene conjugate to exhibit such sensing behavior in dual media.

Physicochemical and Ion-Sensing Properties of Benzofurazan-Appended Calix[4]arene in Solution and on Gold Nanoparticles: Spectroscopy, Microscopy, and DFT Computations in Support of the Species of Recognition.

A calix[4]arene conjugate (L) functionalized at the lower rim with a benzofurazan fluorophore (NBD) and at the upper rim with a thioether moiety has been synthesized and characterized by 1H NMR, 13C NMR, and mass spectrometry techniques. Both the absorption and emission spectral data for L in different solvents exhibited progressive changes with an increase in polarity. Ion recognition studies were performed by absorption and fluorescence spectroscopy using 10 different metal ions. Among these, Hg2+ exhibited greater changes in these spectra, whereas Cu2+ showed only significant changes and all other ions showed no change in the spectral features. Although the Hg2+ has dominant influence on the spectral features and provides a detection limit of 56.0 ± 0.6 ppb, the selectivity was hampered because of the presence of the derivatizations present on both the rims of L for ion interaction in solution. Therefore, L was immobilized onto gold nanoparticles (AuNPL's) so that the upper rim derivatizations anchor onto the gold surface through Au-S interactions, and this leaves out only the lower rim NBD derivatization for interaction with ions selectively. The AuNPL's were characterized by transmission electron microscopy, scanning electron microscopy, energy-dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy (XPS) analyses. The surface characteristics were analyzed by contact angle measurements. The AuNPL's exhibit greater selectivity and enhanced sensitivity for Hg2+ ions with a lowest detection limit of 48.0 ± 0.8 ppb. The immobilization of L onto AuNPs was reflected in the corresponding fluorescence lifetime values, and the addition of Hg2+ to either L or AuNPL showed fluorescence quenching. The reversible recognition of Hg2+ by L was demonstrated by titrating L or AuNPL with Hg2+ followed by tetra-butyl ammonium iodide for several cycles. The structural features of Hg2+-bound species were demonstrated by density functional theory computations and were supported by the XPS data. The Hg2+ induces aggregated fibrillar morphology into supramolecular L, as demonstrated by microscopy when Hg2+ was added either to L or to AuNPL, supporting aggregation-caused quenching.

Buckybowls as adsorbents for CO2, CH4, and C2H2: Binding and structural insights from computational study.

Noncovalent functionalization of buckybowls sumanene (S), corannulene (R), and coronene (C) with greenhouse gases (GGs) such as CO2 , CH4 (M), and C2 H2 (A) has been studied using hybrid density functional theory. The propensity and preferences of these small molecules to interact with the concave and convex surfaces of the buckybowls has been quantitatively estimated. The results indicate that curvature plays a significant role in the adsorption of these small molecules on the π surface and it is observed that buckybowls have higher binding energies (BEs) compared with their planar counterpart coronene. The concave surface of the buckybowl is found to be more feasible for adsorption of small molecules. BEs of small molecules towards π systems is CO2 > A > M and the BEs of π systems toward small molecules is S > R > C. Obviously, the binding preference is dictated by the way in which various noncovalent interactions, such as π···π, lone pair···π, and CH···π manifest themselves on carbaneous surfaces. To delineate the intricate details of the interactions, we have employed Bader's quantum theory of atoms in molecule and localized molecular orbital energy decomposition analysis (LMO-EDA). LMO-EDA, which measures the contribution of various components and traces the physical origin of the interactions, indicates that the complexes are stabilized largely by dispersion interactions.

Estimating the binding ability of onium ions with CO2 and π systems: a computational investigation.

Density functional theory (DFT) calculations have been employed on 165 complexes of onium ions (NH4(+), PH4(+), OH3(+), SH3(+)) and methylated onium ions with CO2, aromatic (C6H6) and heteroaromatic (C5H5X, X = N, P; C4H5Y, Y = N, P; C4H4Z, Z = O, S) systems. The stability of CO2···onium, CO2···π and onium···π complexes was shown to be mediated through various noncovalent interactions such as hydrogen bonding, NH-π, PH-π, OH-π, SH-π, CH-π and π-π. We have discussed 17 complexes wherein the proton transfer occurs between the onium ion and the heteroaromatic system. The binding energy is found to decrease with increasing methyl substitution of the complexes containing onium ions. Binding energy components of all the noncovalent complexes were explored using localized molecular orbital energy decomposition analysis (LMO-EDA). The CO2···π complexes were primarily stabilized by the dispersion term followed by contributions from electrostatic and polarization components. In general, for onium ion complexes with CO2 or π systems, the electrostatic and polarization terms primarily contribute to stabilize the complex. As the number of methyl groups increases on the onium ion, the dispersion term is seen to have a key role in the stabilization of the complex. Quantum theory of atoms in molecules (QTAIM) analysis and charges based on natural population analysis (NPA) in various complexes have also been reported in order to determine the nature of noncovalent interactions in different complexes.

Evaluating the efficacy of amino acids as CO2 capturing agents: a first principles investigation.

Comprehension of the basic concepts for the design of systems for CO2 adsorption is imperative for increasing interest in technology for CO2 capture from the effluents. The efficacy of 20 naturally occurring amino acids (AAs) is demonstrated as the most potent CO2 capturing agents in the process of chemical absorption and physisorption through a systematic computational study using highly parametrized M05-2X/6-311+G(d,p) method. The ability of AAs to bind CO2 both in the noncovalent and covalent fashion and presence of multiple adsorption sites with varying magnitude of binding strengths in all 20 AAs makes them as most promising materials in the process of physisorption. The binding energies (BEs) estimating the strength of noncovalent interaction of AAs and CO2 are calculated and results are interpreted in terms of the nature and strength of the various types of cooperative interactions which are present. The study underlines the possibility to engineer the porous solid materials with extended networks by judiciously employing AA chains as linkers which can substantially augment their efficacy. Results show that a significant increase in the CO2···AA affinity is achieved in the case of AAs with polar neutral side chains. Furthermore, the study proposes AAs as effective alternatives to alkanolamines in chemical dissolution of CO2.