A Combined Experiemental And Theretical Study Of1,3-diamide Squaraines For The Fluoride Recognition

In this paper,1,2-squaramide (L1H2) and1,3-dimide squaramide (L2H2, L3H2,L4H2, L5H2) have been synthesized at different reaction conditions by reactingseveral kinds of arylamine with squara acid or diethyl squarate. The products havebeen characterized by infrared spectroscopy, NMR, elemental analysis,High-Rosolution mass spectrometry, X-ray single crystal diffraction. Investigated inDMSO by absorption, emission,1H NMR spectroscopies and theoretical calculation,the recognition performance and mechanism of receptor L1H2，L3H2for detecting thefive anions (F-、AcO-、Cl-、Br-、I-) have also been explored. The results are listed asbelow:1. Comparing with the squaramides L1H2, the isomer L3H2is more sensitive forF-. The color of the DMSO solution of L3H2gradually changed from dark yellow(λmax=399nm) to pulple (λmax=550nm) after increasing the strong base F-ion, anda strong fluorescence emission peak appeared at613nm wavelength as well. However,there is not any obvious change for AcO-, Cl-, Br-, I-.2. combined theoretical calculation and laboratory finding, the detectivemechanism for anion of the1,3-isomer L3H2is obviously different from that of thesquaramides L1H2. The former undergo a stepwise deprotonation in comparison to thelatter encounters deprotonation after complexation. The energy barrier ofdeprotonation of L3H2is gradually reduced increasing the basicity of anion, and thereaction is exothermic.3. Due to the introduction of strong electron-withdrawing nitro group, thechemical shift of N-H proton of isomers (L4H2, L5H2) shifted to the lower field (about12ppm), which means that the acidity of these is more enhanced, the two stepsdeprotonation of receptor L4H2、L5H2is more clear when adding F-anion, verifiedthe reliability of the mechanism proposed above.