Synthesis,Structure And Properties Of Aromatic Tetrakis (4-Carboxyphenyl) Diamine MOFs

Metal organic frameworks（MOFs）are a new class of porous crystalline materials with large specific surface area,high porosity,adjustable pore size,and easy functionalization of pore surfaces.Carboxyl MOFs are considered to be very promising porous materials,which is due to the large negative charge density on the carboxyl group,the strong coordination ability of the oxygen atom on the carboxyl group with the metal,and the metal ions can form a variety of coordination modes with them,etc.The ligand of aromatic tetrakis（4-carboxyphenyl）diamine has unique molecular structures:at least two benzene rings around the two nitrogen atoms,the arms are left-handed and right-handed propellers,and the center is highly designable.Therefore,this type of MOFs has broad application prospects.However,there are still many unexplored areas in the elucidation of the relationship between the different structures of tetrakis（4-carboxyphenyl）aryldiamine MOFs.To this end,we first obtained two cases of flexible MOFs with different porosity but equal grids by adjusting size,and explored their different properties due to different pore sizes.Then,four methyl groups were added to benzidine through external modification,and a novel rigid organic small molecule ligand 4Me-BDTA（N,N,N’,N’-tetrakis（4-carboxyphenyl）-3,3’,5,5’-tetramethylbenzidine）was synthesized.A highly stable MOF with partial electrostatic force was constructed by a steric hindrance strategy,and the effects of flexible and rigid ligands on structure and properties were investigated.Finally,Zr-MOF with photoluminescence behavior was constructed by 4Me-BDTA ligand,and the relationship between its structure and luminescence behavior was explored.The details are as follows:Firstly,two flexible MOFs with the same topological type（FJU-119 and FJU-117）were constructed by the two ligands:DTA（N,N,N’,N’-tetrakis（4-carboxyphenyl）-1,4-phenylenediamine）and BDTA（N,N,N’,N’-tetrakis（4-carboxyphenyl）-benzidine）,with indium ions under the adjust of HBF₄,respectively.Due to the difference size between DTA and BDTA,the center of DTA was a planar monophenyl,while BDTA was a biphenyl with a certain twist angle.The increase of the central benzene ring has allowed them to obtain their own unique pore size structure.Their tunable porosity has found different applicability in selective cationic dyes.And it was further found that FJU-119 can be used for electrochemical reduction of CO₂ with the highest FE_HCOOH of 79.1%.Then,we used tetramethyl to modify BDTA to obtain a novel ligand 4Me-BDTA（N,N,N’,N’-tetrakis（4-carboxyphenyl）-3,3’,5,5’-tetramethylbenzidine）and then FJU-118was prepared.The four methyl groups have clearly constrained the rotation of the two C-N bonds of BDTA.Compared with the 8-coordinated ions in FJU-117,the steric hindrance effect of 4Me-BDTA has led to the 7-coordinated indium ions in FJU-118.The partially residual charge indium ions in FJU-118 can provide electrostatic forces to anchor small molecules including lattice water and gases.Interestingly,electrostatic forces help keep the lattice water in FJU-118 up to 320°C.Among the reported In-MOFs,activated FJU-118a exhibited the third highest BET surface area/Langmuir surface area（1860/2106 m²/g）.The single-crystal results show that part of the residual charges on the indium sites have electrostatic forces on the-C≡C-bonds of C₂H₂ molecules,which are beneficial for the selective separation of C₂H₂/CO₂ gas mixtures.Last,the transition metal zirconium ion with 4d⁰ configuration and the ligand 4Me-BDTA were selected to prepare FJU-130 under the regulation of acetic acid.The zirconium nodes effectively separated and immobilized 4Me-BDTA,which not only reduced the aggregation quenching effect,but also weakened the molecular vibration and non-radiative loss,thus enabling FJU-130 to generate strong blue fluorescence.The material shows fluorescence quenching effect on pyridine,and has good selectivity and sensitivity.The quenching constant(1 was calculated to be 1026.83 L mol^-1 by the Stern-Volmer equation from the fluorescence experiments of FJU-130 at different pyridine concentrations.Therefore,FJU-130 can be used as a potential material for the detection of pyridine.