Stimuli-Responsive 2D Porphyrin Metal-Organic Frameworks

Abstract

Many researchers have focused on developing smart materials, responsive to external stimuli.Metal-Organic Frameworks (MOFs) have been spotlighted as one type of smart materials response to temperature, pressure, guest, and light. MOFs have advantages that they have tunable flexibility by designing flexible compartment in the frameworks, such as metal nodes, organic ligand, type of framework, and interlayer space. Especially, interlayer space is the unique and invaluable feature of 2D MOFs. Utilizing the interlayer space, 2D MOFs have been widely studied having anomalous responsive behaviors mainly to guest molecules. We studied temperature-, and guest- responsive properties of 2D porphyrin paddlewheel frameworks (PPFs) based on 2D porphyrinic grid layers. First, PPF-1, the very beginning PPF, has AB stacking of the layers. In this thesis, its anisotropic thermal behavior is studied with its ordered structures of PPF-1, which was published as a disordered model. Second, PPF-27, which has bilayer structure pillared to 2D grid layers, has colossal thermal expansion at a certain temperature range. By single crystal X-ray diffraction data, we successfully visualized the original part of the exceptional thermal behavior. Rather than bilayer, interlayer distance highly increases when colossal thermal expansion appeared. The reason that the interlayer space increases is related to the host-guest interaction. Also, the amount of the high increase in interlayer distance is tunable by guest molecules. Lastly, by solvent exchange from N,N’-diethylformamide (DEF) to N,N’-dimethylformamide (DMF), PPF-1 has shear movement rapidly in 10 minutes to be PPF-1_DMF. Compared to other 2D MOFs, which response to guest molecules, PPF-1 transforms quickly and PPF-1 is sensitive to change of the guest molecules. PPF-1_DMF has the same structure of BNAS-11, synthesized by one-pot in DMF. BNAS-11 has bilayer-like structure that the coordinated solvents in two near layers aligned in the same interlayer space. Combined with structural transformation, its fluorescence intensity is enhanced. Compared to PPF-1, PPF-27, which has no shear movement in the same solvents, is rarely changed in its optical property. Therefore, we speculate that the fluorescence intensity change is caused by the shear movement.