Fluorescence Characteristics Of Rare Earth-Salicylate Complexes And The Application For Sensor

The salicylate derivative with ortho-hydroxybenzoyl as a basic structural unit is a kind of weakly coordinated organic intermediate and produce keto-type tautomers through hydrogen-bonded proton transfer（ESIPT）in the excited state.It is widely used in medicine,food and cosmetics and other fields,but also as an intermediate for organic synthesis.This paper studied the fluorescence characteristics of salicylate rare earth complexes in solids and solution and their applications in ion and small molecule sensing.Methyl salicylate and ten rare earth ions were synthesized by solvothermal method.IR and powder diffraction data showed that they have the same crystal structure.The single crystal X-ray diffraction structure represented by the yttrium complex proved that the complex crystallizes in a monoclinic system,P2₁/n space group,a（?）=24.3594,b（?）=21.4212,c（?）=32.1562,α（°）=90,β（°）=109.533,γ（°）=90.The asymmetric structural unit is composed of 9 yttrium ions and 16 methyl salicylate molecules.The middle yttrium is linked through the eight bridgedμ₃-OH oxygen atoms to the interlaced plane formed by the upper and lower groups of four yttrium.There are two bridgedμ₄-OH in the center of the two square planes.All of the yttrium ions are surrounded by eight oxygen atoms,and yttrium of the square planes are respectively coordinated with two salicylate ligands.The fluorescence spectra of the solid complexes and their solvents in ethanol and DMF show that different rare earth element complexes have two types of fluorescence emission and the emission intensity will be different.The solid complexes of Tb,Eu,and Er exhibit only the characteristic fluorescence of rare earth ions,demonstrating that the energy transfer between the ligand and the rare earth ions is effective.While the fluorescence of the solid complexes of Tm,Dy,Sm,Ho,and Yb show rare earth ions and ligands fluorescence,indicating that only part of the energy is effectively transmitted;the solid complexes of Lu and Y are pure ligand fluorescence.In solution,in addition to Tb and Er,other rare-earth complexes can be observed the fluorescence emission of the ligand,which proved that the effect of the solvent molecules and the complexes will lead to the decrease of the energy transfer efficiency of the ligand to the rare earth ions.It is believed that the difference between the emission excited state energy of the rare earth ions and the lowest triplet energy level of the ligand and the strength of the coordination bond are the main factors affecting the luminous efficiency of rare earth ions.The fluorescence spectra of methyl salicylate and four ester derivatives of salicylic acid with rare earth ions in solution were compared.The composition and stability constants of the rare earth complexes were determined.The results show that the composition of the complex is 1:1.The decrease of stability constant of the complex will increase the energy reversal of the metal ion to the ligand,making the solvent molecule more easily interact with the rare earth and reducing the energy transfer efficiency.Therefore,only Sm,Dy,Ho and Tb exhibit the characteristic fluoresence of the rare earth ion.The emission peaks of other rare-earth ions（Gd,Yb,La,Lu,Tm,and Y）are around 417 nm and belong to the fluorescence emission of ligands.The excitation peaks are located at 345-350nm.Compared with the excitation（324 nm）and emission（442 nm）peaks of methyl salicylate,the peak wavelength of the excitation red shifts,while the emission peak shifts blue and stokes shift decreases.Based on the linear relationship between the quenching of the fluorescence of salicylic acid triethylene glycol diester and lanthanum ion complexes by magnesium ions,the use of magnesium ions can specifically quench the fluorescence of lanthanum ions at 546 nm to achieve the purpose of detection.High sensitivity and low detection limit(1.74×10^-5μmol/L)detection of magnesium ions were realized,and the mechanism of fluorescence quenching was explored by hdrogen spectrum.It was considered that the addition of magnesium ions did not damage the structure of complexes.Interaction with the oxygen atom of the polyether ester chain of salicylic acid,magnesium added reduces the coordination strength of the ligand and the terbium ion,reduces the energy transfer efficiency and quenches the characteristics of the terbium ion.Based on the fluorescence emission of methyl salicylate yttrium,the fluorescence of yttrium complex at 415 nm was quenched by nitric oxide.The appropriate ratios and concentrations of yttrium and methyl salicylate were tested.As the same time the selectivity of nitric oxide at the 640 nm UV absorption peak is getted.By comparing with the structure of zinc and methyl salicylate,fluorescence quenching is not caused by structural differences.Hydrogen spectrum and infrared spectroscopy under different conditions prove that it is a small molecule nitric oxide that interacts with metal ions in solution,weakens the antenna effect of the ligand and quenches the coordination fluorescence.