Synthesis Of Cu²⁺/Zn²⁺ Ion Dual-recognition Probe Based On Coumarin Schiff Base

Coumarins have large conjugate structures with the advantages of large Stokes shifts,high fluorescence quantum yields,and excellent stability and are widely used in various fluorescent materials.Recently,many fluorescent probes have been reported for detecting copper and zinc ions.However,most probes can only sense one of them.Therefore,the probes with dual sensing properties can greatly simplify the detection process.In this study,4-diethylaminosalicylic aldehyde was used as a starting material to be condensed with ethyl nitroacetate,and then the intemidiate was further reduced to give the product of 3-amino-7-diethylaminocoumarin?A2?.Compond A2 was coupled with three o-hydroxy aromatic aldehydes to give Schiff base probes 7-?Diethylamino?-3-[?2'-hydroxybenzylidene?amino] coumarin?X1?,7-?Diethylamino?-3-[?2'-hydroxy-4-dimethylaminobenzylidene?amino] coumarin?X2?,and 7-?diethylamino?-3-[?2'-hydroxy-1'-naphthalenemethylene?amino] coumarin?X3?,respectively.The intermidates and probes were characterized by NMR,HRMS and IR.Through the analyses by UV-vis absorption and fluorescence emission spectra,the results are showed as following:Probe X1 shows dual recognitions to Cu²⁺ and Zn²⁺ in acetonitrile/HEPES?9:1,v/v?.Cu²⁺ ions quench the fluorescence of X1 and Zn²⁺ ions significantly enhance the fluorescence of X1.The limit detections of X1 are 6.78 ?M to Cu²⁺ with the complex constant of Ka = 3.26×10⁵ M and 7.55 ?M to Zn²⁺ with Ka = 2.67×10⁵ M.Probe X2 has dual recognitions to Cu²⁺ and Zn²⁺ in THF/H2O?4:6,v/v?.Cu²⁺ ions quench the fluorescence of X2 and Zn²⁺ ions cause a significant red shift?70 nm?of the emission wavelength with a change from green to orange yellow under UV light.The detection limit of X2 is 2.02 ?M to Cu²⁺ with Ka = 1.51×10⁵ M and 6.90 ?M to Zn²⁺ with Ka = 1.302×10⁵ M,respectively.Probe X3 has specific sensing ability to Cu²⁺ in THF/HEPES?6:4,v/v?,and the fluorescence is quenched.After Cu²⁺ was added,the solution quickly changed from yellow to pink.The detection limit of X3 is 6.31 ?M to Cu²⁺ with Ka = 3.23×10⁵ M.Probes X1 and X3 show good stability in the p H range of 3-11 and can realize naked-eye recognitions.Density functional calculations show that LUMO is mainly concentrated on the metal ions and the bonding atoms after probes X1-X3 are complexed with Cu²⁺,which cause the ligand-to-metal charge transfer?LMCT?effect to quench the fluorescence.In contrast,when probes X1-X3 combine with Zn²⁺,LUMO is concentrated to the coumarin part of the fluorophore,resulting in the enhancement in fluorescence.Moreover,X2 with Zn²⁺ shows a strong intramolecular charge transfer effect,causing a fluorescent red-shift.