Theoretical Investigation On The Gas Adsorption And Molecular Recognition Of Metal-Organic Frameworks

Metal-organic frameworks (MOFs) represent a novel class of hybrid organic-inorganic supramolecular materials comprised of ordered networks formed from organic-linkers and metal-cations (or metal-clusters), and are a new development on the interface between coordination chemistry and materials science. Due to the outstanding stability, large surface areas, tunable pore sizes and excellent optical properties, MOFs have enjoyed extensive explorations in the field of gas capture and separation, energy storage and conversion, molecular recognition and chemical sensing.Gas capture and storage based on MOFs is the most highlighted issue, in recent years, the capture of harmful gas (SOx、NOx、CO etc.) by MOFs have received increasing attentions with the deepening of environment deterioration and atmosphere pollution. We have conducted a detailed investigation of carbon monoxide capture by IRMOFs with grand canonical Monte Carlo (GCMC) method, explored the effects of density, pore diameter, surface area, free volume and topology on the carbon monoxide uptake, and analyzed the influence factors of gas uptake in the different pressure ranges. At low-pressure, pore diameter is the main influencing factor of gas uptake, the polarity of linker ligand also has effect on the gas uptake. At high-pressure, surface area and free volume are the main influencing factors of gas uptake; structure topology has no obvious effect on the gas uptake. These IRMOFs show higher adsorption capacity of CO than the activated carbon at high pressure. It indicates that MOFs may be promising materials for CO capture and storage in the practical applications. Besides, the density has a significant impact on gas uptake, low-density materials have large gravimetric uptake, while high-density materials have large volumetric uptake.The combination of metal-cations and organic-ligands provides excellent luminescent properties for MOFs. The luminescent MOFs are sensitive to structural characteristics:the coordination environment of metal ions, the pore surface, and interactions with guest species through coordination bonds,π-π interactions, and hydrogen bondings. MOF-based fluorescence probes could be used for molecular recognition and chemical sensing. We have investigated the excited-state hydrogen bondings between the LMOF [Zn(sfdb)(bpy)(H2O)]n and nitrobenzene with Density Functional Theory (DFT) and Time-dependent Density Functional Theory (TDDFT) methods, explored the excited-state hydrogen bonding behavior effect on the luminescent process. The analysis of frontier molecular orbitals reveals the luminescent mechanism of [Zn(sfdb)(bpy)(H2O)]n is ligand-to-ligand charge transfer, and the charge transfer mechanism between MOF and nitrobenzene is ligand-to-guest. We demonstrated the hydrogen bonding is strengthened in the excited-state by comparing the bond lengths, binding energies and IR spectra in the ground and excited states. The strengthening of the hydrogen bond facilitates the intermolecular charge transfer and leads to the fluorescence quenching. It theoretically suggests that the LMOF [Zn(sfdb)(bpy)(H2O)]n could be used for explosive detection.