Design And Construction Of Novel Lanthanide Metal-Organic Frameworks And Their Applications In Fluorescent Sensors

Metal-organic frameworks（MOFs）are crystalline and porous materials formed by organic linkers and metal nodes via coordination,which have been widely applied in abundant fields,such as gas storage and separation,drug delivery,catalysis,chemical sensing and so on.MOFs possess the characteristics including controlled topological structures,morphologies,pore sizes as well as confined space can be well engineered through judiciously selecting the metal nodes and organic linkers.Lanthanide endowed with high quantum yield,long fluorescence lifetime,sharp characteristic emission peaks,high color purity,large stokes shifts and near infrared excitation or emission,which are great candidates as metal nodes to fabricate lanthanide-based metal-organic frameworks（Ln-MOFs）.Recent years,Ln-MOFs with multiple fluorescent centers have been successfully fabricated,which provides an alternative strategy to fabricate the ratiometric fluorescence sensors.However,there is huge limitation to fabricate Ln-MOFs with muti-luminescent centers by just converting lanthanide ions because the limited number of lanthanide ions.Therefore,rational design and synthesis of photoresponsive organic ligands is the promising way to prepare Ln-MOFs with high fluorescent yields and multi-luminescent centers.In this thesis,three kinds of novel luminescent Ln-MOFs were fabricated by rational design and synthesis of several organic ligands with photoresponsive properties.The structure and intermolecular energy transfer process of these materials have been systematically studied.Finally,the Ln-MOFs were applied to the construction of ratiometric sensors.This thesis is mainly presented as the following five parts:Chapter 1:The research background and luminescence mechanism of Ln-MOFs were firstly summarized.Then the strategies to design and synthesize Ln-MOFs were emphatically introduced.Furthermore,their state-of-the-art applications in fluorescence sensing were enumerated.Finally,the research strategies and contents of this thesis were put forward.Chapter 2:A mixed-lanthanide MOF(Eu_2.2Tb_97.8-TCA)with white-light-emitting was fabricated based on triphenylamine-derived carboxylic acid ligand（H₃TCA）.The material could not only serve as a ratiometric luminescent sensor for trace water detecting in ethanol,but also achieve a wide-range temperature sensing.Eu_2.2Tb_97.8-TCA was formed by Eu³⁺and Tb³⁺,featuring with long fluorescence lifetime and high color purity,as the lanthanide luminescent centers,and H₃TCA as sensitized antenna.Two lanthanides and organic linker formed multi-luminescent centers,which exhibited different sensitivities to water and temperature.Subsequently,the ability for fluorescent detection for trace water and the detecting mechanism of Eu_2.2Tb_97.8-TCA were investigated.Consequently,this material showed a fast（less than20 s）and relatively ultrasensitive（LOD=0.016%v/v）response to trace water in ethanol,and achieved the sensitive visual determination of water in ethanol with multiple readout methods.Interestingly,according to the ratio of green（G）to red（R）values combined with a smartphone color picker application（APP）,the visual and quantitative detection of water with a LOD of 0.035%v/v was achieved.We also found that the sensor could detect temperature in the range of 300-380 K with good linear relationship,high accuracy and excellent recyclability.Finally,Eu_2.2Tb_97.8-TCA could also be acted as white light emitting material,which was used as a single-phase UV phosphor.Chapter 3:On the basis of work in Chapter 2,tetraphenylpyrazine-derived ligand（H₄TCPP）was designed and synthesized to improve the chemical stability of Ln-MOFs.A series of novel and chemically stable anionic Ln-MOFs i.e.,Ln-TCPP（Ln=Y,Sm,Eu,Gd,Tb,Dy,Ho,Er,Tm,Yb,Lu）,were constructed by solvothermal method.Firstly,the microstructure and morphology of Ln-TCPP were systematically studied.The crystallographic data revealed that Ln-TCPP belonged to the monoclinic space group I2/a,where the Ln³⁺as metal node was in regular six-coordinate mode.The single crystal structure showed that the charge-balancing dimethylammonium cations（[（CH₃）₂NH₂]⁺）generated from the decomposition of DMF stably existed in the channel of Ln-TCPP through hydrogen bonds with the unbonded oxygen atoms in the ligand.Secondly,we found that TCPP^4-could only sensitize Eu³⁺,and the dual emission of ligand and Eu³⁺could be achieved by introducing Tb³⁺through co-doping method.By comparing the performance,Eu_0.52Tb_0.48-TCPP was selected for subsequent research.Then,based on the strategy of cation exchange,the phenomenon and mechanism of lanthanide-luminescence in Ln-MOFs modulated by Ag⁺were investigated.Consequently,upon adding of excess Ag⁺into the aqueous solution of Eu_0.52Tb_0.48-TCPP,the luminescent intensity of ligand decreased to 4.2 times,while that of Eu³⁺increased to about 2 times,and the luminescent intensity ratio(I₆₁₆/I₄₁₆)of them increased from 1.44 to 12.1.The mechanism of regulating luminescence by Ag⁺and the reason that the ligand in Tb-TCPP couldn’t sensitize Tb³⁺under the premise of energy level matching were explained by theoretical calculation.On the other hand,the fluorescence sensing performance and mechanism of ratiometric sensor Eu_0.52Tb_0.48-TCPP for picric acid were also studied.Chapter 4:Considering the results of Chapter 3,a new tetraphenylpyrazine-derived“X”type ligand（H₄L）was synthesized to improve the chemical stability and pore size of Ln-MOFs.In addition,a series of novel and stable Ln-MOFs,i.e.,Ln-L（Ln=Sm,Eu,Gd,Tb）,were constructed by regulating the coordinated mode of ligand and Ln³⁺.The crystallographic data revealed that Ln-L belonged to the tetragonal space group P42₁2.The single crystal structure showed that Ln-L had 3D structural network formed by interpenetration of 2D structure,in which the adjacent Ln³⁺were connected through L^4-to form double-stranded helical chains,with the right-and left-handed structure,extending along the b-axis.Therefore,the whole crystal was mesomeric.In order to study the growth process of Eu-L,the structure and morphology changes of Eu-L under different reaction times were investigated.Finally,the sensing properties and mechanism of Eu-L toward tetracycline in aqueous solution were also investigated.As a consequence,the tetracycline and its analogues could be quantitatively detected with a fast response less than 30 s and a low detection limit of 0.43,0.92 and 0.81μM for tetracycline,oxytetracycline and chlortetracycline,respectively.Furthermore,the material also exhibited a good reusability and an excellent selectivity.Chapter 5:Summaries and perspectives of this thesis.