Syntheses And Luminescence Properties Of A New Class Of Difunctional Ligands And Their Europium Complexes

In this paper, a series of novel difunctional ligands, which have both carboxyl andβ-diketones as functional group for coordination, were designed. Five ligands,6-benzoylacetyl-2-pyridinecarboxylic acid （H₂L₁）,6-（4-methylbenzoylacetyl）-2-pyridine-carboxylic acid （H₂L₂）,6-（4-methoxylbenzoylacetyl）-2-pyridinecarboxylic acid （H₂L₃）,6-（4-chlorobenzoylacetyl）-2-pyridinecarboxylic acid （H₂L₄） and6-（4-bromobenzoylacetyl）-2-pyridine-carboxylic acid （H₂L₅）, were synthesized through the classical Claisen keto-estercondensation reactions between dimethyl2,6-pyridinedicarboxylate and the correspondingsubstituted acetophenone, of which the compounds H₂L₂, H₂L₃, H₂L₄and H₂L₅are newcompounds. The structures of these ligands were determined by IR spectroscopy,1H-NMRspectroscopy, and MS. The results showed that all compounds have enol-keto tautomer, butthe enol isomers are prevailing. Eexperiment results indicated that the reaction activity ofsubstances with different substituents in the Claisen keto-ester condensation reactions are asfollows: OCH₃> CH₃> H> Cl> Br.Five binary Eu（Ⅲ） complexes with H₂L₁to H₂L₅and five ternary Eu（Ⅲ） complexeswith H₂L₁to H₂L₅as the first ligands and1,10-phenanthroline or2,2’-bipyridine as thesecond ligand were prepared. All complexes were characterized by IR spectroscopy, TG-DTAand elementary analysis and the luminescent properties of the complexes were investigated byfluorescence spectroscopy. The results showed that all Eu（Ⅲ） complexes emit strong redfluorescence, all the positions of fluorescence emission are about615nm. Because the secondligands have strong UV absorption and replace the coordination water molecules, whichreduce the energe loss due to the vibration of the water molecules, the fluorescence intensitiesof the ternary complexes are much stronger than those of the binary complexes. These ligandsare excellent sensitizers for Eu（Ⅲ） and do not change the positions of characteristic Eu（Ⅲ）fluorescence emission. The different substituents have different effects on the luminescentproperties of the complexes, and there is no obvious relationship between the complexes andthe luminescent properties. These phenomena are well explained by the coordinationenvironment and energy-matching mechanism of the ligands. At last, we synthesized complexes doped with non-fluorescent rare earth ions (Gd³⁺andY³⁺) or divalent metal ions (Cu²⁺, Zn²⁺, Mg²⁺). The studies on their fluorescence propertiesshowed that there were not remarkable relationship between the fluorescence intensity and theamount of non-fluorescent rare earth ions. The introduce of divalent metal ions (Cu²⁺，Zn²⁺，Mg²⁺) quench the fluorescence of complexes.