Synthesis And Properties Of Schiff Base And Coumarin Fluorescent Small Molecules And Their Europium Complexes

The unique spectroscopy produced by the lanthanide ion can be attributed to the4f valence orbital,which is buried in the core of the ion,and the filled 5s and 5p orbitals shield the coordination environment.Since the s and p orbitals shield the 4f orbit,the spectral characteristics of the lanthanide ions are least disturbed by the external field generated by the ligand or counter ion.This leads to a steep f-f absorption band compared to the wider d-d absorption band of the d-block transition metal.Therefore,the absorption intensity and emission intensity of these transitions are weak,the molar absorption rate is usually less than 10 M^-1cm^-1,and the radiation lifetime is 0.1 to 1.0 ms.Therefore,it is necessary to design an organic chromophore with high energy absorption efficiency to coordinate with the rare earth ion to form a rare earth complex.The chromophore absorbs incident photon energy through the antenna effect,and then transmits the energy to the rare earth ion,so that the rare earth ion emits photons.In this paper,coumarin derivative ligands and Schiff base derivative ligands are used as organic chromophores,which are verified by methods such as 1H NMR,13C NMR,MS,UV-Vis,and FT-IR spectroscopy.The fluorescent properties of Schiff base derivatives and coumarin derivative ligands,such as AIE properties and PH response,were studied.Moreover,the corresponding standard lanthanide complexes were synthesized and confirmed by UV-Vis spectroscopy,FT-IR spectroscopy,TG analysis,molar conductivity,and other analytical characterization methods.The fluorescent properties,thermal stability and electrochemical properties of standard lanthanide complexes were studied.The main contents of this article are as follows:（1）A series of new coumarin derivatives were synthesized using Mieic acid,salicylaldehyde with different substituents,and ethylenediamine diacetic acid as the main raw materials.First,Mieic acid and salicylaldehyde with different substituents are reacted to form 2-Oxo-2H-methylene-3-carbonyl chloride with different substituents,then 2-Oxo-2H-methylene with different substituents A series of new coumarin derivatives were obtained by the reaction of yl-3-carbonyl chloride and ethylenediiminodiacetic acid.Then it is prepared as a standard europium complex with europium nitrate hexahydrate as raw materials.The PH response performance of the coumarin derivative ligand L^1-2and the target complex AIE performance were studied.The results show that the ligand has ICT properties,and under acidic conditions,the solution fluorescence intensity increases sharply as the p H value of the solution decreases,and the fluorescence intensity changes linearly.As the concentration of the target europium complex increases or the proportion of the poor solvent increases,the fluorescence intensity of the target europium complex increases to show excellent AIE performance.The effects of the thermal and electrochemical properties of the target complex and the effects of different substituents on the fluorescence intensity and quantum yield of the target complex were further studied.The standard europium complex emits red fluorescence,which is the characteristic fluorescence of europium ions,and its monochromaticity and color purity are very good.Studies have shown that the introduction of a methyl group（-CH₃）on the ligand can make the energy gap between S₁and T₁smaller,allowing the energy transfer efficiency of the ligand to the standard complex to increase,thereby allowing the standard quantum fluorescence to produce The rate and fluorescence intensity increase.The introduction of nitro group（-NO₂）on the ligand makes the energy gap between the excited state and the triplet state larger,reducing the fluorescence quantum yield and fluorescence intensity of the standard complex.The standard complex[Eu（NO₃）₃L^1-2]·2H₂O has better thermal stability.The results of electrochemical studies show that the methyl group（-CH₃）increases the HOMO energy level of the standard europium complex and decreases the LOMO energy level,while the introduction of electron-withdrawing groups（-NO₂）decreases the HOMO energy level and Increase the LOMO energy level.（2）Two Schiff base derivative ligands L^1-2were prepared from raw materials such as 4-diphenylaminobenzaldehyde with different substituents,hydrazine hydrate and 5-chloro-2-hydroxybenzaldehyde.First,5-chloro-2-hydroxybenzaldehyde is reacted with hydrazine hydrate to synthesize 4-chloro-2-hydrazinomethylphenol,then 4-chloro-2-hydrazinomethylphenol and 4-diphenyl with different substituents A series of new Schiff base derivatives are obtained by aminobenzaldehyde reaction.The Europium nitrate hexahydrate solution was prepared into a standard lanthanide complex.The AIE performance and PH response performance of the target ligand L^1-2were studied.The research results show that the ligand L^1-2increases with the solution PH value,the solution fluorescence intensity continues to increase,and when the solution p H value reaches 10.5,the solution fluorescence intensity shows a sharp increase.Ligand L^1-2has only very weak fluorescence in organic solvents.When the concentration remains unchanged,as the proportion of water increases,the fluorescence intensity of the solution continues to increase,showing good AIE performance.The standard europium complex[Eu（NO₃）₃L^1-2]·2H₂O made of Schiff base chromophore can emit Eu³⁺characteristic red fluorescence with good monochromaticity and very good color purity.And Eu³⁺is in the asymmetric center in the standard complex.The fluorescence intensity and quantum yield of the standard europium complex are greatly affected by the introduction of different substituents,which can be sorted as methyl（-CH₃）>hydrogen（-H）.Due to the introduction of the methyl group,the energy transfer efficiency of the ligand to the lanthanide ion is increased,and the energy gap between S₁and T₁is reduced,resulting in an increase in the fluorescence intensity of the standard complex.This shows that the prepared Schiff base fluorescent small molecule and its corresponding europium complex can be applied to fluorescent probes,optical materials,and other fields.