Anions Recognition And Multi-responsive System Based On Schiff Base Derivatives

In recent years, it has cause a great attention to design and synthesize the smallmolecular ion probes and multiple stimuli responsive gels. Both ion probes and stimuliresponsive gel are based on selective response system，therefore, the design ofappropriate recognition sites, which can be integrated on the ion probes and stimuliresponsive gel become the research focus. Though the metal ion probes have beendeveloped quickly, the design of anion probes is relatively slow. Thus, the developing ofmultiple stimuli responsive gels with new molecular structures and properties is quitechallenging.Schiff bases, as excellent stimuli responsive molecules, are good candidates fordesign and synthesize the new anion probes and multiple stimuli responsive gels.Therefore, we synthesized several anion probes and a new ALS type multiple stimuliresponsive gel based on schiff bases and conducted thoroughly examination on theselectivity, sensitivity, and mechanism of them. The main content contained fouraspects:Firstly, based on the salicylic aldehyde hydrazone molecules, we studied the groupeffect on the anions sensing. By changing different substituents, including theelectron-donating groups, electron-withdrawing groups, the conjugate effect, we foundthe substituents had a great effect on the selectivity and sensitivity for anion recognition.For example, electron-donating groups could reduce the combination ability of probewith anion, but enhance the selectivity of anion recognition; in contrast, the conjugatedgroup and electron-withdrawing group could enhance the capacity for combining anions,but reduce the selectivity of probe. Furthermore, we found the mechanism of thesensing process was a deprotonation process.Secondly, based on the phenol schiff bases, we had developed a dual channel probewhich could detect fluoride ions and cyanide ions in different channel respectively. Inthe UV-vis absorption spectra, the probe could response to fluoride ions with goodselectivity. It could be used as a naked eye probe to fluoride ions; while in the fluorescence spectra, the probe only responsed to cyanide ions with high selectivity.Furthermore, we proved that the colorimetric response to fluoride anions was based on adeprotonation process and the fluorescent response to cyanide anions (fluorescencequantum yield reached to0.9095,606-fold enhancement) was based on a cyclizationprocess. Moreover, it is the the first time to report cyclization of phenolic Schiff basesinduced by cyanide, and this mechanism could be used as a new way to synthesize2-substituted benzoxazoles.Thirdly, the pyrene phenol–hydrazone has been selected as a hypochlorite probe. Thisprobe displayed a color bleaching and fluorescence turning on in the presence ofhypochlorite. The fluorescence quantum yield reached to0.2259(42.6-foldenhancement) after sensing hypochlorite. The probe also exhibited a high selectivity andsensitivity to hypochlorite. Furthermore, the structure of C=N NH plays a key role inthe sensing process. As the phenol–hydrazone structure is easily synthesized; thisstrategy has great potential application in designing simple and efficient probes forhypochlorite.Fourthly, a new multi-response gelator based on salicylidene Schiff base has beensynthesized and demonstrated. The gelator can gelate many solvents by self-cooling orultrasonic processing. Scanning electron microscopy reveals that the xerogels of thestable gels have fibrillar microstructure. The gel exhibits an enhanced fluorescenceemission which is ascribed to the combination of inhibition of the intramolecularrotation and the formation of J-aggregates. Thermochromism is observed, which can beexplained by the equilibrium between the enol form and the keto form. Furthermore, thegel shows a selective fluorescent response to Zn2+in protic solvent ethanol and anexcellent colorimetric response to F in the aprotic solvent acetonitrile.