Design, Synthesis And Multi-ion Recognition Of Novel C₃-symmetric Schiff Base Derivatives

C3-symmetric ligand is a special kind of compound which is constructed of atoms such as C, N, P or some cyclic structures in the center. Such a high symmetric molecular structure is not only beautiful but also shows excellent performance and potential applications in many fields of science, such as asymmetric catalysis,molecular recognition, molecular self-assembly and so on. The design and synthesis of molecules possessing C3-symmetry is of great significance for the development of functional materials especially in the area of ions recognition by using Schiff based C3-symmetric chemosensors.For the purpose of multi-ion detection by one sensor, two classes of C3-symmetric Schiff bases were designed and synthesized in this work, including C3-symmetric Schiff bases derived from triphenylamine(L1 and L2)and C3-symmetric Schiff bases derived from 1,3,5-three benzamide(L3, L4 and L5). The target products yield were all above 80% and their structures were confirmed by IR,1H-NMR,13C-NMR, MS and X-ray single crystal diffraction. Through UV-Vis and fluorescence channels, the multi-ion selective and sensitive detection was obtained.Fluorescence emission spectrum test results of these C3-symmetric Schiff bases showed that the Schiff bases L2, L4 and L5 have good fluorescence emission, while Schiff bases L1 and L3 have no obvious fluorescence emission peaks because of their carboxylic substituents, and the ion recognition of L2, L4 and L5 was further studied.The results showed that Schiff base L2 can be used as a fluorescent bifunctional probe for Zn2+ and CN-. The recognition of Schiff base L2 for Zn2+ and CN- is highly selective and can realize quantitative detection for them. By using NMR titration and Job’s plot method, the sensing mechanism of L2 to target ions was further investigated. Job’s plot confirmed the 1:3 binding ratio between fluorescent probe L2 and Zn2+, as same as CN-. 1H-NMR titration suggested that Zn2+ had a coordination with L2 which could prevent the isomerization of C=N and excited the ESIPT process as well,and this finally lead to the fluorescence enhancement of L2. On the other side,the strong nucleophilic CN- combined with the C atom of C=N which ultimately destroyed the conjugation of probe L2 and lead to the fluorescence quenching.Similarly, Schiff base L4 can be used as a bifunctional probe for Zn2+, Mg2+, CN-å'Œ F- following the same sensing mechanism with the case of L2. By the method of UV absorption spectroscopy and fluorescence emission spectroscopy, Schiff base L5 can be used as dual anion probe of F- and CN- by proposed sensing mechanism of deprotonation of phenol hydroxide.