Syntheses,Crystal Structures And Exploration Of Luminescent Properties Of Schiff Bases

Schiff base fluorescent sensors H₂L¹（oxime-based）and H₂L² for Zn²⁺ based on the combination of ICT and CHEF mechanisms were designed and synthesized.Sensor H₂L¹ is a tetradentate bifunctional ligand,and the complexes formed by this compound are tetranuclear complexes.Sensors H₂L¹ and H₂L² exhibit lower background fluorescence due to intramolecular PET,however,a “turn-on” fluorescence emission was observed upon sequential addition of Zn²⁺and thus the emission increases with proportional increase in Zn²⁺concentration.The results of investigation show that the sensors have excellent fluorescence selectivity for Zn²⁺ over many other important metal ions including the rival,Cd²⁺,in particular.Emphatically,sensor H₂L¹ could be useful in discriminating between Cd²⁺ and Zn²⁺which have very similar chemical properties and have been reported to exhibit interference.Likewise,sensor H₂L² also could be applied to sense the presence of Cu²⁺.The crystal structures of reported complexes vividly showed that sensor H₂L¹ forms excellent,unique and uniform coordination with each metal ion and the design of the ligand molecule made it suitable for the four ligands with the double-functionalized chelating moieties（ONON）to coordinate with four metal ions,which was further proved by the four isosbestic points observed in the UV-Vis spectra,indicating a 1:1 stoichiometry.Both zinc and nickel ions showed same 1:1 stoichiometry and are both five-coordinated.The results of fluorescence and UV-Vis spectra as well as X-ray single crystal structures of Zn²⁺ and Ni²⁺complexes evidently act as infallible proofs for the 1:1 binding stoichiometry with sensor H₂L¹.Owing to the fundamental importance of Stokes shift to the sensitivity of fluorescence techniques,an observed large value of Stokes shift for sensor H₂L¹ would make the detection of fluorescent species(Zn²⁺)easier.Finally,both sensors are chemically reversible and these selective and sensitive results may lead to the potential applications for the detection of trace amounts of Zn²⁺ and Cu²⁺ in both biological and environmental systems.[H₂L¹]? C₆H₃N₃O₇: Empirical formula,C22H19N5O10,Molecular weight,513.42,Triclinic,Space group: P-1,Cell parameters: a = 7.214 ?,b = 14.102 ?,c = 14.279 ?,α = 114.563°,β= 91.226°,γ = 99.421°,Z = 2,V = 1296.9 ?3,R1 = 0.1437,wR2 = 0.2706.[Zn₄（L₄）]: Empirical formula,C₆₄H₅₆N₈O₁₂Zn₄,Molecular weight,1390.73,Tetragonal,Space group: I 41/a,Cell parameters: a = 21.9438（3）?,b = 21.9438（3）?,c = 12.4417（3）?,α = 90°,β = 90°,γ = 90°,Z = 4,V = 5991.1（2）?3,R1 = 0.0525,wR2 = 0.1497.[Ni₄（L₄）]: Empirical formula,C₆₄H₅₆N₈O₁₂Ni₄,Molecular weight,1363.93,Triclinic,Space group: P-1,Cell parameters: a = 13.2898（8）?,b = 14.7012（8）?,c = 16.7421（9）?,α= 81.534（5）°,β = 67.214（5）°,γ = 72.519（5）°,Z = 2,V = 2874.7（3）?3,R1 = 0.0759,wR2 =00.1220.[H₂L²]: Empirical formula,C₁₅H₁₃NO₃,Molecular weight,255.26,Monoclinic,Space group: P 21/c,Cell parameters: a = 7.5158 ?,b = 6.3940 ?,c = 25.881 ?,α = 90°,β =93.243°,γ = 90°,Z = 4,V = 1241.8 ?3,R1 = 0.0547,wR2 = 0.1350.