Tunable Emission And Luminescence Sensing Activities Of Ln-doped MOFs Materials Based On Aromatic Carboxylic Acid Ligands

Metal-Organic Frameworks（MOFs）are a kind of crystalline porous materials with network structure,which are formed by self-assembly of organic ligands and metal ions or clusters through coordination bonds.Ln MOFs can combine the unique luminescent characteristics of Ln³⁺ions with the inherent pores of MOFs,which may have potential application value in the fields of luminescence sensing,white light materials,temperature sensing and so on.In this dissertation,O/S functionalized aromatic carboxylic acid ligands were selected as organic building units to react with transition metal salts and rare earth metal salts under hydrothermal conditions to synthesize novel MOFs.The luminescence sensing properties of these MOFs toward metal ions and organic pollutants were investigated in details.Furthermore,the luminescence properties of MOFs could be regulated by doping rare earth elements within the framework or by post synthesis modification.This dissertation mainly includes the following three aspects:1.Two Ln-MOFs,namely[Eu（L1）（DMF）（H₂O）]_n（1）and[Tb（L1）（DMF）（H₂O）]_n（2）were synthesized by the reaction of 3,5-bis（2-carboxyp Henoxy）benzoic acid（H₃L1）with rare earth metal salts.Complexes 1 and 2 have three-dimensional network structure,showing characteristic emission peaks of Eu³⁺and Tb³⁺respectively.The luminescence results show that complex 2 can be used as a multifunctional luminescence sensing material in DMF and H₂O:（1）it exhibits obvious luminescence quenching in DMF of acetone and acetylacetone with detection limits of 0.023 vol%and 1.075μM,respectively.It can be used as a luminescence sensing material for carbonyl containing organic compounds.（2）Fe³⁺and benzothiazole compounds 2-Mercaptobenzothiazole（MBT）,2-chlorobenzothiazole（CBT）and 2-amino-6-methoxybenzothiazole（AMOBT）can be effectively recognized by luminescence quenching in DMF and H₂O;（3）obvious luminescence quenching effect can also be observed in aqueous solutions of HPO₄^2-,H₂PO₄^-and Cr₂O₇^2-.The mechanism investigation shows that the quenching is due to the energy competitive absorption and weak interaction between the analyte and complex 2.In view of the luminescent characteristics of rare earth metal ions,we also obtained a series of isomorphic doped complexes Eu_xTb_1-xfrom red light to yellow light and green light by changing the ratio of Eu to Tb in raw materials.Among them,the complex Eu_0.6Tb_0.4emits white light under 330 nm excitation light with CIE coordinates of（0.3253,0.318）,which is close to the CIE coordinates of ideal white light point（0.333,0.333）.Furthermore,Al³⁺can be detected by the color change from white light to blue light in H₂O.2.Two Ln-MOFs,namely{[Eu（L2）_1.5（H₂O）₂]·DEF·2.5H₂O}_n（3）and{[Tb（L2）_1.5（H₂O）₂]·DEF·2.5H₂O}_n（4）,were synthesized by the reaction of 3,4-dimethylthiopheno[2,3-b]thiophene-2,5-dicarboxylic acid（H₂L2）with rare earth metal salts.Complexes 3and 4 with three-dimensional pore structure exhibit characteristic emission of Eu³⁺and Tb³⁺,respectively.The two complexes possess high stability in water,and show luminescence quenching behavior toward a variety of ions:（1）Fe³⁺and Cr₂O₇^2-could be detected by luminescence quenching of complexes 3 and 4;（2）Al³⁺is detected by fluorescence enhancement of complex 3,and complex 4 has high sensitivity toward Cu²⁺,C₂O₄^2-and HPO₄^2-in H₂O,with detection limits of 0.273,0.548 and 0.953μM,respectively.（3）Complexes 3 and 4 can effectively detect 2-Mercaptobenzothiazole（MBT）by luminescence quenching,and the detection limit of complex 3 is as low as0.854μM.By changing the ratio of Eu and Tb in the framework,red to yellow and green luminescence properties of the doped complexes could be controlled.The complex Eu_0.3Tb_0.7emits white light at the excitation of 400 nm,and the CIE coordinate is（0.307,0.2805）.The doped complex has excellent luminescence sensing properties and high selectivity for organic solvent NB.The luminescence intensity of the complex Eu_0.3Tb_0.7increased slightly with the increase of temperature.3.One Cd complex,[Cd（L3）（dmbpy）]_n（5）（dmbpy=5,5’-dimethyl-2,2’-bipyridine）,was synthesized from 2,4-bis（4-carboxyl-benzyloxy）benzoic acid（H₃L3）.The carboxylic acid ligands adopts（κ¹-κ¹）-（κ¹-κ¹）-（κ¹-κ⁰）-μ₃coordination mode to connect adjacent Cd²⁺ions to form a binuclear unit{Cd₂（CO₂）₂}.These binuclear units are connected by carboxyl groups from different benzene rings to form a one-dimensional chain structure along the b axis.Luminescence measurements showed that complex 5 could specifically recognize Fe³⁺and Cr₂O₇^2-in DMF and H₂O by luminescence quenching,while Zn²⁺in H₂O through fluorescence enhancement.Considering the presence of uncoordinated carboxyl groups within the structure,we modified complex 5 by using rare earth metal salts Eu（NO₃）₃·6H₂O and Tb（NO₃）₃·6H₂O,respectively.Two Ln³⁺-doped complexes,Eu³⁺@complex 5 and Tb³⁺@complex 5,were obtained.In addition to the recognition of Fe³⁺and Cr₂O₇^2-,by luminescence quenching,the modified compounds can also detect Zn²⁺in aqueous solutions through color change.The above research shows that complex 5 and its Ln³⁺-post modification products may have potential application as multifunctional luminescence sensors.