Structural Transformation, Seperation And Fluorescence Properties For Porous Metal-organic Frameworks

Since the early 1990 s, because of the widely application in many prospects, metal-organic frameworks(MOFs) have attracted widespread attention among the chemistry researchers. After twenty years development, the research of MOFs has been turned to function-guided, reasonable design of the structures and controllable preparation to obtain required properties of the materials. Under the external stimuli, structures of the flexible metal-organic frameworks may change reversibly or irreversibly. In light of the structure-property relationships of materials, the researches on the structural transformation and concomitant with property changes of MOFs have become one of the most rapid development branches in this area during the past few years. The main studies of this paper are the structural convertions of the flexible MOFs in aqueous solution and lanthanide-based complex detect metal ions and explosives as a fluorescent sensor.This paper includes four parts as follows:1. A CuII-based porous metal-organic framework, which possesses a Kagomé lattice, is solvothermally synthesized based on the ligand 5-(2-acetoxy-propionylamino)-isophthalic acid(H2La). Triggered by water, the 2D Kagomé lattice(complex 1) primarily undergoes a reversible single-crystal to single-crystal(SCSC) structural transformation to a distorted Kagomé intermediate(complex 2), which is followed by a dissolution recrystallization structural transformation(DRST) process to form a 3D NbO framework(complex 3) in situ. To the best of our knowledge, this is the first demonstration of a mixed SCSC and DRST transformation process. More interestingly, the adsorption isotherms of water, methanol, and ethanol for the activated materials are distinctly different from one another. Complex 2? can uptake all three vapors with different adsorption capacities; however, the 3D transformed material complex 3′ only significantly absorbs water, which is concomitant with an amorphous-to-crystalline transformation, leading to the selective extraction of water from alcohol.2. A layered microporous structure(complex 4) with the 1D honeycomb channels was synthesized based on the ligand [2-(2-acetoxy-propionylamino)-terephthalic acid(H2Lb) in the presence of auxiliary ligand 1,4-diazabicyclo[2.2.2]octane(dabco), which can be dissolved in water under the condition of ultrasound. New crystals with various structures will be recrystallized from the solution at different temperatures. Complex 5 with a simple one-dimensional chain structure can be formed at room temperature, while a two-dimensional grid-like structure(complex 6) can be generated at 120 oC. However, with the temperature increased, complex 7 which possess a three-dimensional framework structure crystallized at 140 oC. We also found that dabco which dissolved in water will not coordinated with cadmium again. Besides, the ligand has various coordination modes in these structures. Based on the microporous structure, we studied the gas adsorption and multi-step energy transfer process between complex 4 and organic dyes. The energy match between absorption and emission spectra provides the possibility of continuous multi-step energy transfer process in the(AO + RhB)@4 system [complex 4 and acridine orange(AO), AO and Rhodamine B(RhB)].3. A water-stable luminescent terbium-based metal-organic framework(MOF), {[Tb(L2)1.5(H2O)]?3H2O}n(Tb-MOF, 8), with rod-shaped secondary building units(SBUs) and honeycomb-type channels has been synthesized. The microporous nature and the high green emission intensity of the complex 8 indicate that it can potentially be used as a luminescent sensor. In this work, we show that complex 8 can selectively sense Fe3+ and Al3+ ions from mixed metal ions in aqueous solution through different detection mechanisms. In addition, complex 8 also exhibits high sensitivity for 2,4,6-trinitrophenol(TNP) in the presence of other nitro aromatic compounds in water by luminescence quenching experiments.4. We have construced five microporous metal organic frameworks(complex 9-13) by use of a biphenyl carboxylic acid derivative ligand 3-(2-acetoxy-propionylamino)-biphenyl-4,4’-dicarboxylic acid(H2Lc) reacted with copper salts. Complex 9 is an unprecedented 2D + 3D interpenetrated array of 4-connected sql and nbo topologies.The researches indicate that porous metal-organic coordination polymers have structural diversity, and reversible or irreversible structural transformation tends to proceed among the flexible frameworks under the external stimulus. Our work not only enriches the porous metal organic materials, but also provides the theoretical basis for exploring the structural conversion. Besides, the lanthanides-based complexes with excellent luminescent properties can be used for the detection of metal ions and explosives.