Investigation Of Fluorescence Sensing For Ions And Small Molecules Based On The Lanthanide Organic Frameworks

Because of the unique mesoporous structure, unsaturated coordination and highly sensitive fluorescence for the coordination environment, lanthanide organic frameworks sensing for the ions and small molecules in the field of organisms and the environment, has recently aroused great concern. Taking advantage of rare earth ions and multidentate organic ligand, we synthesized frameworks with porous three-dimensional network structure. Through changing the organic ligand's size and functional groups we can control the aperture size and geometry of the three-dimensional frameworks to probe the different kinds of molecules or ions. When the outside ions (or molecules) came into the porous rare earth organic framework, such ions (or molecules) would coordinate with the unsaturated rare earth ions or the organic ligand, which would change the chemical environment of rare earth ions and the energy transfer efficiency of the entire system and also alter the luminescence of rare-earth ions, in order to achieve the aim of probing the anions, cations and small organic molecules.The frameworks of Eu(PDC)_1.5(DMF)_1.5(H₂O)_0.5(PDC = pyridine - 3, 5 -dicarboxylate, DMF = dimethyl formamide) and Tb(BTC)·(DMF)·(H₂O)_0.5(BTC = 1,3,5 - benzenetricarboxylate) were synthesized by the method of solvothermal synthesis. The derived frameworks were desolvated at 140℃in the vacuum conditions to form Eu(PDC)_1.5DMF and Tb(BTC)·(DMF)_0.6, respectively. And the fluorescence sensing of cations, anions and organic solvent molecules fluorescence was carried out.The framework EuPDC has a good structural stability in several organic solvents and high thermal stability, and is investigated for the cation sensing (Na⁺,Cu²⁺,Co²⁺,Al³⁺,Fe³⁺). After activated in different concentrations of cation solutions, the fluorescence intensity all decreased regularly to some degree. Especially EuPDC is sensitive to the particular cation, such as Fe³⁺ and Cu²⁺, and its fluorescence intensity has decreased rapidly. The mechanism for the sensing of cations demonstrated by the results is mainly due to the electrostatic interaction between cations and organic ligand of framework EuPDC, which results in transferring part of photo-induced electrons to the cation, thereby reducing the energy transmission efficiency between triplet state of organic lignad and rare-earth ions which decrease the fluorescence intensity accordingly. At the same time because of different number of the outer layers, especially the outmost electron layer configurations.electronic unsaturated level of the different metal decides the ability to attract electrons, this experiment show that EuPDC is specially sensitive to Cu²⁺ and Fe³⁺ ions.The framework TbBTC is also investigated for sensing of the anion and organic solvent molecules, respectively. Fluorescence data of anion (F^-,Cl^-,Br^-,NO₃^-,CO₃^2-,SO₄^2-) detection show that after being activated in the anion solution from the 10^-5 to 10^-2 mol/L, the fluorescence intensity of TbBTC is all increased to some degree. TbBTC is more sensitive to F^- than other anions in the fluorescence experiment. The mechanism for sensing the special anion is that the organic ligand do not have enough coordination spots for the lanthanide Tb and several solvent molecules would coordinate with the lanthanide Tb unavoidably. Such solvent molecules always possessing fluorescence quenchers -OH, would quench the fluorescence intensity of lanthanide Tb. Bringing the anions into the holes of the frameworks, hydrogen bonding would form between anions and solvent molecules, thereby limiting the vibration of quenchers, which enhance fluorescence intensity of the lanthanide Tb. In addition anions have the different hydrogen bonding intensity with solvent molecules. From this experiment it can be seen that the F^- ion and CH₃OH formed a relatively strong hydrogen bonding allowing fluorescence to increase about four orders compared with the original intensity.Furthermore, the fluorescence study of framewrok TbBTC for sensing DMF, THF, acetone and tetrahydrofuran is also carried out in the solvent of n-propanol. By adding various different solvent molecules, frameworks Tb(BTC)·(DMF)_0.6 fluorescence spectroscopy shows that the fluorescence intensity has changed regularly, particularly the acetone which greatly weaken the fluorescence intensity of the Tb(BTC). The sensing mechanism of such frameworks for the solvent molecules is decided by the different polarity and the characteristic absorption band of the solvent molecules which causes the energy exchange between organic lignads and solvent molecules.Based on the results, by changing the organic ligand and lanthanide ion to control the further system structure of organic framework, it is expected to synthesize a new class of specific framework sensors for ions and small organic molecules.