Synthesis And Properties Of Metal-organic Frameworks Based On Dicarboxylic Acid And N-heterocyclic Organic Ligands

As a hot research material in recent years,metal-organic frameworks(MOF s)play an important role in materials chemistry because of its diversity of structure and wide application prospects.Compared with traditional porous materials,MOFs can be used as a potential gas adsorption material.Flexible metal-organic frameworks(FMOFs),as a branch of MOFs,exhibit reversible singl-crystal to single-crytal(SCSC)or solid-state structural transformations under the stimulation of external effects(guest molecules,light,pressure,temperature,etc.),which induces special properties such as "gateopening","S-shaped",multi-step adsorption isotherms.These special adsorption behaviours can lead to high adsorption capacity and separation selectivity.This paper designed a serial of MOFs based on dicarboxylic and N-heterocyclic organic ligands,and study dynamic structural transformations,selective gas adsorption and fluorescence recognition for these MOFs.The specific research contents are as follows:1.A two-dimensional porous metal-organic framework[Co3(ndc)3(bimb)]·3DMF·2.5H2O(1α)with 1D porous channels were synthesized under solvothermal conditions by using a dicarboxylic acid(naphthalene-2,6dicarboxylic acid,H2ndc)and bipheny-bridged diimidazole-type organic ligand[(4,4’-bis((imidazol-1-yl)methyl)biphenyl,bimb)].Upon removal of guests by heating,1α exhibits solid-state structural transformations and transformed to 1β with 0D voids.The structural transformations between 1α and 1β are reversible and accompanied with a shift in the metal and carboxylate coordination modes.Gas adsorption experiments show that 1β diplays a two-step CO2 adsorption isotherm,and close-toopen(type F-Ⅳ)isotherms for C2H2,C2H4 and C2H6 at 195 K.However,1β displays typical type Ⅰ adsorption isotherms for these gases,and show selective uptake of C2H2 over CH4 and CO2 at room temperature.2.Three novel three-dimensional metal-organic frameworks[(CH3)2NH2]2[Zn2(DMTDC)2(dmpypz)2]·DMF·2H2O(2α),[Zn2(DMTDC)(dmpypz)2]·8DMF 2H2O(3),[(CH3)2NH2][Zn3(DMTDC)3(dmpypz)]·3DMF·4H2O(4)were synthesized by a dicarboxylic H2DMTDC(3,4-dimethylthieno[2,3-b]thiophene-2,5dicarboxylic acid)and dimethyl pryazolyl pyridine organic ligand(4-(3,5-dimethyl1H-pyrazol-4-yl)pyridine,dmpypz)under different conditions.Single-crystal X-ray diffraction results show that 2α exhibits solid-state structural transformation upon guest removal and transformed to 2β.The structural transformation between 2α and 2β are reversible.Gas adsorption experimental results show that 2β exhibits type I adsorption isotherms of N2 at 77 K,and also type Ⅰ adsorption isotherms of C2H2,C2H4 and C2H6 at 195 K,but 2β diplays a two-step(open to more open)CO2 adsorption isotherm.However,2β displays a temperature-dependent gate-opening isotherms of C2H2,C2H4 and C2H6 at 273 K,283 K and 298 K.Upon removal of guests,the activated sample 3a and 4a hardly absorb N2 at 77 K,but exhibit type I adsorption isotherms for CO2 at 195 K and C1 and C2 gases at room temperature.According to Ideal Adsorption Solution Theory(IAST),the selectivity of 2β for C2H2/CH4,C2H2/C2H6(v/v=1:1)were calculate to 707.9,96.2.The selectivity of 3a for C2H2/CH4,CO2/CH4(v/v=1:1)were calculate to 390166.8,216.5 and the selectivity of 4a for C2H2/CH4(v/v=1:1)were calculate to26.2 at 298 K.3.A new three-dimensional metal-organic framework[Zn(DMTDC)0.5(pypz)]·3DMF·3.5H2O(5a)were synthesized under solvothermal conditions by using thiophene-containing dicarboxylic H2DMTDC and another pryazolyl pyridine organic ligand 4-(1H-pyrazol-4-yl)pyridine(pypz).Gas adsorption experiment results showed that activated sample 5a’ exhibits type I adsorption isotherms of N2,CO2,C2 and C1 at 77 K,195 K and 298 K,respectively.According to IAST,the selectivity of 5a’ for C2H2/CH4,C2H2/C2H6(v/v=1:1)were calculate to 62.6,26.3.5a can recognize nitrobenzene and N-methylaniline with the naked eye under natural light when soaked in different aromatic solvent molecules and displays guestdependent fluorescence properties.The photoluminescent measurements reveal that 5a formed a compound 5a@DMA with strong blue-green emission when soaked in N,Ndimethylaniline(DMA).Moreover,5a@DMA is stable in water,and can recognize Fe3+ by fluorescence quenching when dispersed in water.