Design And Synthesis Of New Receptors For Anion Recognition

Molecular Recognition is an important area of supramolecular chemistry. And supramolecular chemistry includes sensing of neutral molecular, cations and anions. The recognition and sensing of anions have received considerable attention for their important roles in biological, industrial, and environmental process. In this thesis, we constructed chromophores with ground-state charge transfer. Several highly selective and sensitive sensors for anions were developed. And naked-eye detection came true. The thesis consists of four chapters.In Chapter 1, the development of anion recognition in the past four years was briefly reviewed which contained: some basic concepts such as supramolecular chemistry, anion recognition, the colorimetric and fluorescent sensors for anions.In Chapter 2, a novel ditopic chromogenic receptor, N-[5'-(8'-hydroxy) quinoline]-N'-(4'-nitro-phenyl) thiourea (1), was synthesized. Metal complexes such as Cu²⁺, Hg²⁺, Co²⁺, Ni²⁺, Zn²⁺ and Pb²⁺ between 1 and different metal ions were investigated by absorption spectroscopy. The results implied that 1 was a good ligand for Cu²⁺ and Hg²⁺. The metal complex 1-Hg²⁺ showed sensitive and highly selective responses to F^-, the spectrum of 1-Hg²⁺ only underwent dramatic change for F^-. A new absorption peak at long wavelength of 490 nm appeared and the solution turned from colorless to red. Other anions such as CH₃CO₂^-, H₂PO₄^-, HSO₄^- and so on induced no significant change in both spectral profile and solution color. It was explained that Hg²⁺ bound to the thiourea moiety and the hydroxyquinoline moiety of 1 simultaneously. Meanwhile, the binding ability of thiourea moiety with Hg²⁺ was stronger than that of 8-hydroxy quinoline moiety. At the beginning, HgF₂ was formed with Hg²⁺ displaced from 8-hydroxy quinoline moiety of 1-Hg²⁺. However, with higher concentration of F^-, it formed hydrogen bond with 1-Hg²⁺ to the hydroxy moiety. Hg²⁺ bound with thiourea moiety acting as electron acceptor and F^- as electron donor, 1-Hg²⁺-F^- complex formed, which promoted the intramolecular charge transfer and led to dramatic spectral and color change. Thus, a colorimetric assay of F^- was developed in acetonitrile by naked-eye detection. F^- behaved linearly in the 8.0×10^-6 to 2.0×10^-5 mol L^-1 concentration range with LOD as 1.4×10^-6 mol L^-1.In Chapter 3, a ditopic receptor, N-(4'-phenyl-15-Crown-5) -N'-(4'-nitrophenyl) thiourea (4), and reference compounds were obtained. The interactions between receptors and anions and cations were studied via absorption spectroscopy. The presence of anions such as F^-, AcO^-, H₂PO₄^- and Cl^- resulted in spectral change of 4. The affinity between 4 and anions followed the order of F^-＞AcO^-＞H₂PO₄^-＞Cl^-＞＞ClO₄^-, NO₃^-, Br^-. The binding constant of 4-F^- was more than 10⁷ mol^-1 L which showed high sensitivity. The addition of F^- would produced the maximal absorbance wavelength of 4 at 334 ran red shifted to 434 nm and the solution color change from colorless to yellow. However, upon adding cations such as Na⁺, Li and K⁺ to 4-F^-solution, the spectrum recovered to be 4's spectrum along with color change from yellow to colorless. According to the absorbance change, the binding ability of 4 with cation was followed as Na⁺＞Li⁺＞K⁺, the binding constants of 4 and Na⁺ was 3.01×10⁵ mol^-1 L. In the mixture of 4 and Na⁺, the concentration of F^- was more than 4 equiv. of receptor 4, the new peak at 434 nm appeared along with color change from colorless to yellow. It was assumed that cation bonded with crown ether moiety and the presence of metal ions restrained the binding action between receptor and anion.In Chapter 4, the binding properties of curcumin with anions in acetonitrile were examined for the first time by the UV-Vis absorption and fluorescence spectroscopy. The curcumin shows highly selectivity towards F" with little interference for the tested anions such as AcO^-, H₂PO₄^- and Cl^- because of anionic complex formation via hydrogen bond. Upon addition of F^-, the Curcumin shows red shift in the UV-Vis spectra and its fluorescence is quenched, which accompany with color change from yellow to purple which was detected by naked eyes. The addition of other anions such as AcO^-, H₂PO₄^-, HSO₄^-, NO₃^-, Cl^- and Br^- didn't result in observable spectral change and solution color change. The binding constant between curcumin and F^- was 2.0×10⁵ mol^-1 L and the recognizing mechanism was discussed as well.