Synthesis And Luminescence Properties Of Metal Organic Frameworks Constructed By 4f/5f Metal Ions

Lanthanides(Ln)and actinides(An)elements are in the group ?B of the periodic table.These elements also named as 4f and 5f metallic elements.Both these two series of elements are less abundant in the earth's crust than transition metals.However,they play an irreplaceable role in many fields of modern society,especially in high-tech fields such as electronics production,energy generation,aviation and so forth.These elements with large ion radius and complex electron configuration which are generally feathered with abundant physical and chemical properties.Lanthanide metal ions gift with characteristic line-like fluorescence spectra,and their emission wavelength covers an extremely wide spectrum from ultraviolet to infrared region.Due to its rich optical properties,lanthanides have been widely used in light emitting diodes(LEDs),imaging materials,scintillators,laser crystals and many other important fields.In contrast,since actinides are all radioactive,the difficulty in operation makes it arduous to receive a wide investigation as lanthanides have.As the heaviest naturally occurring radioactive metal,uranium,is considered as the most important nuclide in actinide series.One of the uranium isotope 235U is available for fission reactions which accompanied by huge energy release.It is therefore that most efforts of the human beings have been directed at extracting nuclear energy from this unique element in order to configure it into either destructive weapons or clean energies.However,one of the important properties of uranium,bright green luminescence from uranyl(UO22+),suffers from a long-term ignorance.Although these two kinds of elements have rich physical and chemical properties,the current applications of these elements are mainly focused on inorganic materials or alloy materials.In order to obtain suitable material systems for the development of new applications of the two types of elements,the study of their luminescent properties in different material systems is considered being of prime important.These investigations can not only expand the application scope of the two series of elements,but also play an important role in deepening the understanding of their basic physical and chemical properties.In this work,the 4f elements(Eu2+/Eu3+and Tb3+)and 5f-element(UO22+)were used as metal centers for the construction of a series of luminescent metal-organic frameworks(MOFs).The crystal structures of the as-synthesized materials are firstly investigated and the corresponding stability test including thermostability,hydrolytic stability and irradiation stability were evaluated for determining the possibility ot the real application.The luminescence characteristics of the materials and the applications as luninescent thermometer,white-emitting material,chromate sensor,radioactive uranyl sensor and UV radiation probe were systematically investigated.The possible mechanisms were also discussed and confirmed by a combined experimental methods and theoretical simulations.The detailed research contents are as follows:Chapter 2.We have developed a mild synthetic strategy for producing mixed-valent Eu2+/Eu3+-doped MOFs.Eighteen materials with different Eu2+/Eu3+moler ratio were successfully developed which exhibit multichannel photoluminescence properties.Compared with high temperature reduction process,the presented strategy is more ideal for a continuous control the moler ratio between Eu2+and Eu3+.It is demonstrated that Eu2+and Eu3+ can be stabilized in the MoF lattice with precise ratio control(0-0.77).A long-lived charge transfer emission peak seated at 450 nm-600 nm with lifetime of 36 ms was observed by time-delayed emission spectrum.The dynamics of Eu2+in the excited 4f 5d/and charge transfer states predominately determines the luminescence color of the series materials.Of the particular interest is the energy transfer between Eu2+ and Eu3+ is highly temperature dependent.This phenomenon directly leading to a lumineacence color change from blue at room temperature to pink at low temperature of material 1.A nearly linear correlation between temperature and emission intensity ratio of Eu2+/Eu3+ is obtained which even can be used under the extremely low temperature region(9K).On the other hand,due to the tunable Eu2+/Eu3+ moler ratio of the materials 1-17,the luminescence color could be continuously tuned from blue to red.By incorporating Tb3+ into the crystal lattice,white light emission with a quantum yield of 4.17%was achieved.Considering the formate ligand is not capable of efficiently sensitize the emission intensity of lanthanide metal ions,it is still of great potential for further improving the sensitivity of the luminescence thennometer as well as internal quantum efficiency of white light emitting materials in other strontium-based materials as new host lattices.Chapter 3 and 4.A highly hydrolytic stable cationic 3D luminescent europium(?)based MOF(EuOF-CrP-1)and a mesoporous terbium(?)based MOF(TbOF-UP-1)were successfully synthesized through solvothermal method.Both of these two materials are with high porosity and therefore leading to a great accessibility to chrome ions and uranyl ions respectively.EuOF-CrP-1 is of excellent stability in aqueous solution which even could sustain its integrity in seawater for 24 hours.Due to the cationic nature of EuOF-CrP-1,the material could selectively enrich anion ions from aqueous solution.Because absorption peak ot EuOF-CrP-1 is partially overlapped by the absorption of Cr(?)at the excitation light region(280 nm),the material is successfully developed as a highly sensitive and selective chromate probe.The chromate concentration dependent luminescence spectra,selectivity,detection limit was than systematically investigated in aseries of natural water incuding lake water and seawater.At last,the selective quenching mechanism was confirmed by combined spectra and batch experiments.As for TbOF-UP-1,the abundant lewis basic sites laced in the 1D channel exhibits selective and efficient accessibility toward uranyl ions.High selectivity over competing environmentally and nuclear-fission relevant metal ions,together with the high sensitivity of TbOF-UP-1,indicate that this porous material can perform as a highly desired uranyl fluorescence sensor.Hence,the detection capbihty including the luminescence selectivity,concentration denpendet luminescence spectra and detection limit were investigated in three water systems(deionized water,Dushu lake water and seawater).In order to further confirm the real application of TbOF-UP-1 as uranyl sensor in natural water systems,a comparation was earried out between ICP-MS and TbOF-UP-1 for the determination of the uranyl concentration in Bohai seawater.At last,the batch experiments,EXAFS analysis and DFT caculations were employed for confirming the luminescence quenching mechanism and finding the sorption sites at molecular level.In these two cases,we firstly systematically investigated the correlation between the luminescence response and adsorption property,and confirmed that the strong enrichment and high quantum yield of MOFs material are greatly benifical for improving the sensitibity of the MOF based sensors.By taking advantage of this strategy,we obtain two luminescence MOFs with high sensitivity and selectivity for detecting of chromate and uranyl ions in aqueous solution.All the detection limits of the materials toward Cr(?)and uranium are much lower than the standard in drinking water defined by the United States EPA.Chapter 5.In this part,a highly radiation stable uranium-organic framework(UOF-UVP-1)was successfully synthesized through solvothermal method which exhibits characteristic transition of uranyl.The intrinsic luminescence of UOF-UVP-1 could be effectively quenched by UV light which makes it suitable for monitoring UV radiation.The UV radiation dose dependent luminescence spectra were collected for the determining of the correlation between the quench ratio and the UV radiation dose.The generation of DMF radical was confirmed by EPR analysis and X-ray crystallography study.Additionally,the radical-induced quenching mechanism was confirmed by density of states(DOS)and DFT calculations.The impurity energy levels induced by DMF radicals is directly responsible for the luminescence degradation when UOF-UVP-1 recieves UV radiation.At last,powder of UOF-UVP-1 was also developed into a membrane material without any loss in luminescence intensity or character which further confirms the applicability of this material to UV detection.This work provides us new opportunity for searching powerful UV respondsive materials by taking advantage of efficient UV light asbsorber(uranyl)as metal center.