Sensing Properties Of Luminescent Metal-Organic Frameworks

Metal-organic frameworks?MOFs?are unparalleled in their degree of tenability and structural diversity as well as their range of chemical and physical properties.They have been extensively studied for their potential applications in numerous areas,including but not limited to,optoelectronics,magnetics,gas storage/separation,heterogeneous catalysis,chemical sensing.Luminescent MOFs?LMOFs?is certainly very promising as a multifunctional luminescent material,because both the inorganic and the organic moieties can provide the platforms to generate luminescence,while metal-ligand charge transfer related luminescence within MOFs can add another dimensional luminescent functionalities.In addition,guest molecules can also emit and/or induce luminescence.Naturally,these properties of LMOFs can potentially be used for real-world applications.To date,most of the research works focused on sensing properties of one Ln-MOF sensor.However,the luminescent properties of LMOFs are very sensitive to and dependent on their structural characteristics,coordination environment of metal ions,nature of organic ligand,and their interactions with guest species through coordination bonds,and?-?interations and hydrogen bonding etc.,which have provided the solid rationale to develop luminescent sensing MOFs.Considering all the aspects stated above,the work focused on the influence factors of sensing properties of LMOFs,such as centre metal,substituent group of organic ligand and guest molecule.1.Two isostructural LMOFs,[H2N?CH3?2][Ln?cppa?2?H2O?2]?Ln = Tb/Eu CTGU-1Tb/-1Eu,H2cppa = 5-?4-carboxyphenyl?picolinic acid,CTGU = China Three Gorges University?,were selected to investigate the effects of the different metal centre on the sensing properties.They are three-dimensional?3-D?supramolecular framework containing one-dimensional?1-D?channel of 11.6 x 21.32 A.CTGU-lEu can detect small organic molecule acetone and nitroaromatic explosives in water,whereas CTGU-1Tb can only detect explosive nitrophenol in water.The results reveal that the sensing properties of Ln-MOFs can be adjusted by the different centre metal.2.Three isostructural LMOFs,Zn?L??ip?·?DMF??H20?1.5?1?,Zn?L??aip?·?H2O??2?,Zn?L??HBTC??H2O?2?3??L = N4,N4'-di?pyridin-4-yl?biphenyl-4,4'-dicarboxamide,H2ip ?isophthalic acid,H2aip = 5-aminoisophthalic acid and H3BTC = 1,3,5-benzenetricarboxylic acid?,were selected to investigate the effects of the different substituent group on the sensing properties.The three functionalized LMOFs 1-3 with highly ordered amide groups and different dubstituent group?-H,-NH2 and-COOH?in the channels show notable interaction with the guest molecules.Ligand L is selected as a controlled study.In water,the photoluminescence spectra of ligand L shows a fluorescence emission at 404 nm and a shoulder peak at 474 nm,attributed to two kinds of interactions:between solvent molecules and pyridine group and amide group.Compounds 1 and 3 exhibit selective water-dependent fluorescence red-shift even color change for compound 1 and blue-shift for compound 3,respectively.Compounds 1-3 show potential applicaton for the luminescence sensing of small organic molecules.Furthermore,the intensities of compounds 1-3 in the photoluminescence spectra strongly depend on the nitroaromatic explosives.Nitrophenol exhibit the most significant quenching effect with fluorescence red-shift even color change even the concentration is very low.It is noted that different compounds in the same nitrophenol solution show different luminescence behavior.This is due to they have different substituents,leading to the materials with different energy level structure.3.Two new LMOFs,[Cd?Tipa?2]₂ClO4-xSolvent?4,Tipa =tris?4-?1H-imidazol-1-yl?phenyl?amine,Solvent = methanol,N,N'-dimethyl formamide and water?and[Cd?Tipa??HCOO?2]·3H2O?5?,based on a nitrogen centre tridentate rigid ligand tri?4-?imidazole-1-yl?phenyl?amine were successfully synthesized.Compound 4 is a two-dimensional cationic structure.Compound 5 is a three-dimensional hcb array comprised of?6,3?layers giving inclined interpenetration.Compound 4 exhibits selective adsorption ability toward anionic dyes methyl orange,giving rise to a new complex MO@4.Both complexes 4 and MO@4 have recognition site,basic nitrogen atoms,for selective detection of the nitroaromatic explosive nitrophenol in the aqueous phase.Both the two complexes can sensitively detect 2,4,6-trinitrophenol by the quenching responses with color change from blue to pale blue to green.While MO@4 has a higher fluorescence response?I0/I,where I0 and I are the maximum fluorescent intensity of MO@4 before and after the addition of analyte,respectively?for 2,4,6-trinitrophenol.Additionally,complex MO@4 also shows a highly sensitive ability to detect 4-nitrophenol by fluorescence quenching.The results reveal that the guest molecules could significantly improve the fluorescent sensing properties of materials.