Syntheses And Fluorescent Properties Of Metal-Organic Frameworks Based On Tricarboxylic Acid Ligands

Metal-Organic Frameworks?MOFs?have attracted the attention of researchers as a new type of porous inorganic-organic hybrid materials.They have various structures and rich porosity,so they can be used as a host to encapsulate some functional nanoparticles to improve their chemical properties.In recent years,due to luminescent MOFs good luminescence properties and unique porous structure,they not only have been widely used in the field of tunable luminescence and white light emission,but also exhibit ultra-high sensitivity and superior selectivity in fluorescence sensing.Compared with traditional detection methods,luminescent MOFs have the advantages of high efficiency,low cost and fast response.At present,the rational design and synthesis of MOFs with expected functions is still an attractive and challenging research topic.It is well known that many factors such as organic ligand type,pH value,temperature,solvent,reaction time,etc.directly affect the self-assembly crystallization process of MOFs.Among them,organic ligands play a key role in the formation of the final structure of MOFs,and small changes in shape,rigidity/flexibility and symmetry can cause the changes in the structure and properties of MOFs.Among different organic ligands,tricarboxylic acid ligands have different coordination modes,which are beneficial to construct MOFs with rich structure and diverse pores,and have attracted close attention.In this work,selecting two tricarboxylic acid with strong coordination ability,3-?3,5-dicarboxylatobenzyloxy?benzoic acid?H₃L₁?and 4-?5-carboxypyridin-2-yl?isophthalic acid?H₃L₂?as organic ligands self-assembled with lanthanide metal ions and Zn²⁺,Cd²⁺metal ions.Under the solvothermal conditions,nine new luminescent MOFs were synthesized and their structural analysis and characterization were carried out.Their fluorescence sensing and luminescence properties.The main research results as follows:1.Three novel isomorphous Ln-MOFs[Ln?L₁??H₂O?]·1.5H₂O?Ln=Eu 1,Tb 2,Gd 3?were successfully synthesized based on H₃L₁ ligands,exbiting three-dimensional?3D?framework with tetragonal one-dimensional?1D?channels.It is worth noting that the activated product of compound 1?1A?can be used as a multifunctional fluorescent sensor for detecting acetone,Fe³⁺,Cr₂O₇^2-and4-nitrophenol?4-NP?in aqueous solution with detection limits of 0.0704 vol%,0.87?mol L^-1,1.25?mol L^-1 and 0.92?mol L^-1,respectively.1A can also be regenerated at least five times for sensing Fe³⁺,Cr₂O₇^2-and 4-NP.In addition,1A possesses pH-dependent luminescence response ability in a wide pH range.Owing to compounds 1-3 are isomorphic,a series of novel Eu/Tb or Eu/Gd bimetallic and Eu/Tb/Gd trimetallic doped Ln-MOFs were designed and synthesized by adjusting the molar ratio of three different lanthanide ions.At the same excitation wavelength,the luminescent color of the doped Eu_x/Tb_1-x-MOFs could be tuned from green,yellow,and orange to red.The compound Eu_0.033Gd_0.967-MOF-4 and the compound Eu_0.035Tb_0.005Gd_0.96-MOF-5 showed white light emission with CIE chromaticity coordinates of?0.34,0.31?and?0.32,0.34?,respectively,which are very close to ideal white light?0.33,0.33?.2.Using 4,4'-bipy as an auxiliary ligand,a 2D layered compound[Zn₂?L₁?₂?4,4'-bipy?₂]·0.8H₂O?6?was assembled from H₃L₁ ligand and Zn²⁺ions.Without adding 4,4'-bipy,another 2D layered compound[Zn₄??₃-OH?₂?L₁?₂?H₂O?₂]?7?was obtained.Both compounds 6 and 7 could be further linked to form a 3D supramolecular structure by?-?interaction between the adjacent layers.And they all showed better luminescence properties.Moreover,compound 6 exhibited good fluorescence response to Fe³⁺,MnO₄^-,Cr₂O₇^2-and nitro explosives,and compound 7displayed good Fe³⁺ion detection ability.3.By using H₃L₂ ligand and Cd²⁺ions,two fluorescent MOFs[Cd?HL₂?]·0.8H₂O?8?and[Cd₃?L₂?₂]·3H₂O?9?were synthesized.Compound 8 was a 2D layered structure and the adjacent layers were connected by hydrogen bonding to form a 3D supramolecular structure.Compound 9 possessed a?8,8?-connected 3D network.We used an effective strategy to encapsulate carbon dots?CDs?with the excellent optical properties into 8 and 9.The as-prepared CDs@8 and CDs+9 not only retained the strong fluorescent activity of CDs,but also embeded Eu³⁺ions in the hybrid to form dual-emitting Eu³⁺/CDs@8 and Eu³⁺/CDs+9.These novel composites exhibited good fluorescence sensing capabilities for Fe³⁺ions,p-nitrotoluene?p-NT?and 2,4-dinitrophenol?2,4-DNP?,indicating that Eu³⁺/CDs@8and Eu³⁺/CDs+9 can be regarded as the potential fluorescent sensors with superior sensitivity and excellent selectivity.