Molecular Design On Luminescent Metal-organic Frameworks To Recognize Formaldehyde And Nitro-explosives

Luminescent Metal-organic frameworks (LMOFs) have distinctive advantages as the fluorescence sensing materials. This paper mainly discuss the essence and mechanism of LMOFs for the recognition of formaldehyde and nitro-explosives. By using density functional theory (DFT) and time-dependent density functional theory (TDDFT) to study the hydrogen bonding between LMOFs and formaldehyde (nitro-explosives). We select the hydrogen-bonded complex structure fragment based on the existing single crystal, investigate the behavior of various hydrogen bonds in the excited state through study the structure fragment, closely combined with experimental to study the hydrogen bond behavior and then delineate the essence of the hydrogen bond in the excited state, study the effect of hydrogen bond on the luminescence mechanism of MOF and study the electron transfer in the excited state, explore the competition between radiative transition and the radiative transition, and then get the interaction between the hydrogen bonding and luminescence properties. By study the molecular recognition and chemical sensing mechanism of LMOFs, then provide the guidance for the experimenters to design, synthesis and develop more efficient and stable LMOFs materials in theory.The hydrogen bonding between LMOF [Zn2(H2L)(2,2’-bpy)2(H2O)]n and formaldehyde was studied. By the analysis of frontier molecular orbitals and electronic configuration of the hydrogen-bonded complex which is formed from the fragment and formaldehyde, we conclude that the type of luminescent mechanism is ligand to ligand charge transfer, with π’-π and π*-n characteristics. We demonstrate that the hydrogen bond in the excited state is strengthened by exploring the bond length, the vibration frequencies and 1H NMR chemical shift three aspects of the hydrogen bond. In addition, we illustrate that the enhanced of hydrogen bond in the excited state would lead to a luminescence decreasing of [Zn2(H2L)(2,2’-bpy)2(H2O)]n, the quantitative calculation results also demonstrate that the fluorescence rate coefficient of the MOF is decreased with the enhanced of hydrogen bond, which demonstrate that MOF [Zn2(H2L)(2,2’-bpy)2(H2O)]n is a potential fluorescence sensing material for the recognition of formaldehyde.The hydrogen bonding between LMOF Zn(L2)2·(CH2Cl2)·(CH30H)0.67 and nitrobenzene (the molecular model of nitro-explosives) was studied. By analyzing the frontier molecular orbitals and electronic configuration of the hydrogen-bonded complex which is formed from the fragment and nitrobenzene, we conclude that the type of luminescent mechanism is guest molecular to ligand charge transfer, and also has the charge transfer from the guest molecular to metal Zn. We also demonstrate that the hydrogen bond in the excited state is strengthened by exploring the behavior of the hydrogen bond from three aspects, the bond length, the vibration frequencies and 1H NMR chemical shift. And demonstrate that the strengthened of hydrogen bond in the excited state would lead to a luminescence quenching phenomenon of Zn(L2)2·(CH2Cl2)·(CH30H)0.67 qualitatively, which ndicate that MOF Zn(L2)2·(CH2Cl2)·(CH30H)0.67 could used as a potential fluorescence sensing material for the recognition of nitro-explosives.