Metal–organic frameworks (MOFs) are crystalline and porous solid materials formed by an extended network of metal ions (or clusters) coordinated to multidentate organic molecules. This new class of porous materials began to gain increasing interest in the 1990s, and now MOF chemistry is advancing at an extraordinary pace with an explosion of papers in the chemical literature. In the line of MOF research, it has been known that MOF structures have inherent transformability, and their dynamic properties are not only scientifically interesting, but also practically applicable in various forms, such as advanced non-composite materials and stimuli-responsive smart materials .
In this sense, I demonstrated the use of MOFs as precursors that transform into various functional nanomaterials with a special emphasis on understanding the relationship between the intrinsic nature of parent MOFs and daughter nanomaterials. For example, in Section 2.2 I synthesized nanoporous manganese oxides by thermal conversion of Mn-based MOFs by introducing a second aliphatic ligand. Furthermore, the oxidation states of manganese oxides (i.e., MnO, Mn3O4, Mn5O8, and Mn2O3) could be controlled by optimizing the sequential conversion reactions between annealing in N2 and calcination with atmospheric oxygen.
Importantly, the resultant MnO, Mn3O4, and Mn5O8 exhibited nearly identical textural properties, including their morphologies, surface areas, pore volumes, and sizes of the nanocrystals that make up the nanoporous frameworks. Another trending topic in the MOF field is imparting flexibility to MOF frameworks and revealing their structural dynamics so that external stimuli such as guest exchange, temperature, pressure, oxidation, and light can induce structural rearrangement to change properties. physico-chemical of MOFs.
Rigidity and flexibility of MOFs
The history of MOFs goes back thirty years, when Robson reported an “infinite polymer framework,”12 after which Yaghi coined the term “metal-organic framework” in 1995.13 At the time, a wide variety of microporous coordination compounds existed, typically hosting guests . solvent molecules in the pores were known. Unfortunately, however, most compounds irreversibly lost their crystallinity and porosity upon the removal of the guest molecules. Therefore, a major research effort was devoted to the synthesis of MOFs that could withstand the removal/reintroduction of guests without the change of their porous structures.
Based on this trend, Kitagawa in 1998 classified MOFs into three categories: 1st, 2nd and 3rd generation MOFs (illustrated in Figure 1.2a).14,15 Interestingly, he defined the 3rd generation compounds as flexible porous frameworks that respond to external stimuli, and predicted their appearance and occurrence in the immediate future. The illustrations in the box on the right represent the behavior of dynamic structures upon adsorption/desorption of gas molecules; (b) induced-fit-type pores, (c) breathing-type pores, (d) gas-exchange deformation-type pores, and (e) healing-type pores.19,20. According to common sense, one can designate a MOF as a flexible MOF when a significant yet reversible change in the unit cell is observed upon a change in the external environment.
However, in terms of the structural dynamics, the range of flexible movements will be very wide, whether at the microscopic or macroscopic level. This occurs for example in various forms such as the rearrangement of the metal carboxylate (M-COO) cluster that forms the secondary building unit (SBU), the rotation of the benzene ring of the organic ligand, and the expansion/contraction of the cell volume .
Thermal decomposition and transformation of MOFs into nanomaterials
Thermal Conversion into Functional Nanomaterials
- Transformative route to nanoporous metal oxides with identical properties
- Thermal conversion of a Li- and Si-containing MOF for preparation of Li 4 SiO 4 ceramic
- Gradual decomposition of a tailored MOF toward CdS photocatalyst with S-doped carbon
- Facile synthesis of nanostructured transition metal/ceria solid solutions via bimetallic MOF …. 65
Pore size distribution of the nanoporous manganese oxides obtained by the BJH method from the desorption branch of the N2 physisorption isotherm. Nyquist plots of the manganese oxide samples obtained by impedance spectra at a fixed potential of 0.7 V (vs. RHE) in O2-saturated 0.1 M KOH. XRPD patterns of LiTCS: (a) measured pattern of as-synthesized LiTCS and (b) simulated pattern from single crystal X-ray diffraction data.
The result for Li4SiO4 shows a 70% weight loss up to 550℃, which corresponds to the decomposition of residual carbon and further progress of the incomplete reaction. Schematic of the nanoporous structure np-TMxCe1-xO2-δ (TM = Mn, Ni, Co and Fe). XRPD patterns for as-prepared solid solutions of np-MnxCe1-xO2-δ and np-CeO2, together with a standard card of cubic CeO2 (JCPDS file no. 81-0792).
NLDFT pore size distribution curves of np-MnxCe1-xO2-δ solid solutions as well as np-CeO2. a) Raman spectra of the np-MnxCe1-xO2-δ series and np-CeO2. XRPD patterns for the imp-CeO2-Mn materials with cubic CeO2 standard map (JCPDS file no. 81-0792).
Stimul-Responsive Transformation of Flexible MOFs
Direct detection of explosive nitroaromatic compounds in a luminescent Li-based MOF
The luminescent MOF was screened as a detector of toxic and explosive aromatic compounds containing nitro groups as an explosophore by changing its visible color and exhibiting solid-state luminescence quenching. Single crystal X-ray diffraction results for 1⊃nitrobenzene clearly showed strong 𝜋-𝜋 interactions between nitrobenzene and benzene rings of CPMA2- in 1 and induction of CH∙∙∙𝜋 interactions between neighboring CPMA2- ligands in the framework. The white precipitate suspended in the solution was kept at room temperature for 4 hours and acetic acid was added to the solution until the precipitate was completely dissolved.
In Figure 3.9a and b, the XRPD pattern of the as-synthesized 1 is compared with the simulated pattern based on the. In the TGA trace of as-synthesized 1 (Figure 3.10), the gas solvent molecules occupied in channel A of 1 can be removed in the range from room temperature to ca. The disappearance of (200) peak at the range of low angles shows the collapse of channels in Li-MOF, leading to the different structure of the as-synthesized.
The UV–vis absorption spectrum of 1 in the solid state (Figure 3.16) shows two absorption peaks, at 248 and 352 nm, indicating no significant color in the visible region. Such a visualization of the uptake of harmful molecules in the solid state offers great opportunities to utilize this MOF as a sensor. UV-visible spectra of 1 (black) and 1⊃nitrobenzene (red) in the solid state. a) Photographs of 1, 1⊃nitrobenzene and 1 regenerated by heating.
The emission in 1 is derived from a strong intraligand charge transfer (ILCT) from the donor N-methylamino group to the acceptor carboxylate groups of the CPMA2 molecule. In Figure 3.9c and d, the XRPD pattern of 1⊃nitrobenzene coincides with the simulated pattern based on X-ray single crystal diffraction data showing the homogeneous absorptions of nitrobenzene in 1. The close-up view of interaction between nitrobenzene and one of CPMA2- incorporated in 1 is shown in Figure 3.20b.
Moreover, the π-π interactions, as evidenced by the superimposed X-ray structures of 1 and 1⊃nitrobenzene (Figure 3.20c), induce the molecular dynamics of the flexible ligand CPMA2-; Meanwhile, the methyl group of CPMA2- approaches the neighboring benzene ring of CPMA2- to induce C-H∙∙∙π interactions in the framework. This interligand interaction also contributes to the change in the electronic structure of 1, quenching its fluorescence. A luminescent Li-based MOF selectively detected explosive nitroaromatic compounds by exhibiting intense color change as well as luminescence quenching in the solid state.
Due to the SC-SC transformation after guest exchange, the effects of host-guest interactions on the motions of the molecular components in the coordination framework were directly observed. Since no change was observed, the possibility of dissolution of 1 in the solvent, followed by crystallization or renucleation on the surface, and the growth of a new phase was ruled out. The crystal was pushed from the solvent layer into the void space using a very thin glass fiber and most of the solvent was removed, leaving a small amount of solvent in the capillary.
During the guest exchange, the possibility of dissolution and recrystallization of 1 in the new solvents was excluded by photographs obtained with an optical microscope during the immersion of the crystals, which also indicated the retention of the single crystallinity of 1 during the exchange process (Figure 3.30). SCD analysis revealed the dynamic movement of the interpenetrated nets (Figure 3.31) during the guest exchange.30-33 In the structure of 1, the adjacent interpenetrated nets created two interesting spaces, which were composed of two phenyl rings of each net (Figure 3.31 ). 3.31a). Meanwhile, the dihedral angle between two phenyl rings of the framework showed small changes, namely 66.36o.
This was sufficient for the guest molecules to form the strongest π−π interactions without a significant change in the host framework, because edge-to-edge π−π interactions are more stable than face-to-face interactions.34-36 In 1hexane, the incorporated n-hexane molecules formed CH−π interactions with two phenyl rings of CPMA2- ligands at B sites (shortest C···C distances, 3.919 and 4.016 Å) (Figure 3.31c). The TGA data of the guest-exchanged compounds also reflected the strength of the host-guest interaction depending on the guest molecules. The changes in the cell parameters were attributed to the compression of the individual PC meshes and the sliding motion between the intermediate framework along the c-axis, as shown in the figure below Table 1.
Since each edge of the pcu network corresponded to the flexible CPMA ligand connecting the oxo groups (Figure 3.21b), the new interactions changed the degree of frame compression, which can be expressed by the distances between the oxo centers in the intraframe (Table 3.1). In 1MeOH, the main changes were in the reduction of a and b parameters, which were due to strong host-host interactions. The peak positions of the measured XRPD patterns for 1 and the guest-exchanged products coincided with those of the simulated patterns derived from X-ray single-crystal data, except for the XRPD pattern of 1MeOH.
The lattice plane compression of 1hexane and 1MeOH was confirmed by the shift of the XRPD peaks to the higher angle region than that of 1 or 1benzene. SCD analysis revealed that the structural transformations associated with sliding motions of the interpenetrating networks and dynamic motions of the molecular components were induced by the host–guest interactions. The π−π interactions in 1benzene and CH−π interactions in 1hexane between the introduced solvent molecules and the phenyl rings of the CPMA2–.