Metal-organic frameworks (MOFs) are hybrid crystalline materials made up of metal ions and organic ligands. MOFs have found a variety of potential applications, including gas storage, separation, catalysis, and drug delivery. A subset of MOFs has recently gained attention because of their guest-induced structural flexibility, which is highly relevant for gas separation applications (i.e., carbon dioxide capture). However, it has remained a challenge to modulate the flexible behavior in MOFs. This thesis focuses on the development of flexible MOFs based on specific functional group approach.;A systematic overview of the flexible behavior of Azo-MOFs is presented to highlight the versatility of the azo-functional group approach to modulate the flexible behavior in MOFs. A series of azo-containing MOFs are synthesized from the one-pot solvothermal reaction of zinc nitrate, 1,4-benzene dicarboxylic acid (1,4-BDC) and 4,4'-azobipyridine (azpy) in different solvent systems such as N,N-dimethylformamide (DMF), N,N-diethylformamide (DEF), or N,N-dimethylacetamide (DMA) etc. These MOFs are fully characterized by single crystal X-ray crystallography, powder X-ray diffraction, thermo gravimetric analysis, and gas adsorption analysis. The crystal structure of Azo-MOFs is illustrated by a three-dimensional (3D) pillared lattice. They are constructed from rectangular or paddle wheel secondary building units that are equatorially co-ordinated by 1,4-BDC dianions to form two-dimensional (2D) square grids. 4,4'-Azobipyridine acts as a pillar ligand to extend the two-dimensional layers into a 3D structure. These MOFs incorporate the azo-group as part of the backbone of the MOF lattice. The flexible behavior of Azo-MOFs has been validated by stepwise adsorption isotherms and PXRD data of the dried samples. We have demonstrated that flexible behavior of Azo-MOFs is highly influenced by structural variation, interpenetration, solvent, carboxylate and azobipyridine linkers.;Most intriguingly, we have synthesized other Azo-MOFs based on different carboxylic acid system to validate the significance of carboxylate linker in flexible behavior. The gas adsorption studies of fumaric Azo-MOFs clearly exemplify the significance of carboxylate linker and the cavity size in flexible behavior.;Given the success with the identification of the azo-functionalized MOFs and their intriguing flexible behavior (vide infra), we have decided to evaluate the gas adsorption behavior of structurally related MOFs that lack azo functional groups. We have used 1,2-di(4-pyridyl)ethylene(bpe) ligand system to construct structurally related MOFs(Bpe-MOF). The comparative gas adsorption analysis of Azo-MOFs with that of Bpe-MOF further reveals the significance of the azo-group in modulating the flexibility of the MOFs.;These Azo-MOFs show selective carbon dioxide adsorption over nitrogen, due to their guest-induced flexible feature. These findings suggest that CO2 selectivity can be systematically manipulated through different strategies including suitable ligand design. Thus, Azo-MOFs are promising materials for efficient and cost effective carbon dioxide capture applications. We anticipate these novel findings will open up a new path for the design of future generations of stimuli-responsive porous materials.
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