Introduction
Flexible Metal Organic Framework MOF play a significant role in the study of physical and chemical properties, as well as external stimuli. Many renowned scientists throughout the past century concentrated their study on the dynamics of these porous materials. The name MOF dates to the late 1990s, when (Kaur & Kaur, 2021) ignited a quantum leap in knowledge, accelerating the production of such hybrid materials. These scholars’ pioneering work and remarkable discoveries in MOFs are well admired. As many books and review articles testify, their investigations and explanations of their integration and characterization techniques (or art) have resulted in an extensive literature. This work aims to study specialized literature and collect data analysis.
Furthermore, new research has allowed access to a considerably broader variety of investigative and synthesizing methods. Thousands of MOFs were successfully synthesized using structural formulae of varying degrees of an unlimited number of combinations and variations between metal complexes with either a tetrahedrotetrahedronsedral network of molecular rod-like linking units (Aljammal, et al., 2019). Flexible structures with dynamic characteristics are few among these recently found scaffolding-like substances, and only a handful have been extensively studied. Some of these porous materials’ elastic properties are interesting for so-called “smart” materials because they may structurally react to an external stimulus. The adaptability of the MOF(s) may significantly influence the interface architecture of the MOF(s), and therefore in their compatibility. The basic knowledge of MOFs flexibility sparks an interest in interpreting the phenomenon by examining the characteristics of particular organic linkers and metal nodes of unique MOFs. To that aim, literature has developed several methods for classifying and visualizing their content, size, and form. Full characterization and sufficient explanation should be saved for MOFs with intriguing flexible behavior. This study aims to provide as much information and as many examples as possible to get a basic and complete knowledge of MOF flexibility, origin, and control mechanisms.
Three major themes will be explored in-depth: I the adaptability behavior and the reliance of the chemical and physiological body of the structure on its organic ligands and metal endpoints, (ii) the occurrence of adaptable MOFs, how to regulate it, and its implementation; this theme is based on the types of adsorbate molecules on the flexible approach (Thornton et al., 2016). To investigate the adaptability of MOFs, assistance from scientists in related fields such as reticular chemistry and collaborative experimental simulation is required. This study will provide a literature analysis of the significant developments in flexible MOFs up to 2021, including synthetic methods, characterization, and in-depth implementation.
The Basis of MOF Versatility
The selection of scaffolding components like metal terminals and organic ligands influences the design and production of flexible scaffold-like materials. It is worth noting that the choice of metal complexes will be affected in part by the structure construction method that will be utilized. Organic linkers with organic compounds, on the other hand, are gaining popularity and play an essential role in adaptability, such as stiff MOFs with elastic ligands. As a result, each of the three potential sources contributes to MOF flexibility. In this regard, two hypotheses may be advanced: (I) the basis of the versatility and (ii) how the flexible MOF responds to external stimulation. The structure and functioning are determined by the intentional way the two components are constructed and assembled and the kind of connection that links them. The logical and logical design of the connecting ligand and metallic nodes has piqued the curiosity and interest of many researchers to synthesize a diverse variety of scaffolding-like materials with unique and beneficial characteristics. Fundamentally, most studies have focused on connecting scaffold-like molecules utilizing essential supramolecular components or secondary building components and molecular connectors. MOFs with a high degree of stiffness is often produced using fixed organic critical ingredients. Such frameworks often exhibit relatively good mechanical and thermal durability and the capacity to retain porosity after eliminating guest solutions. This should be unsurprising that the characteristics of MOFs with adjustable ligands cannot indicate whether the whole structure is flexible. Since adaptable ligands may be utilized to create both flexible and dynamic MOFs, the character of the resultant scaffolds cannot be defined only by the suppleness or stiffness of the dimers. However, given the preceding observations about the nature of flexibility, researchers must be careful when selecting the source of flexibility. There is a clear distinction between flexible MOFs and those with variable compounds. The latter correlates to MOFs in the structure with one adaptable element (Ehrling et al., 2021). The stiffness or versatility of the organic linker does not relate to the nature of the structure.
What is not well known is the likely significance of the connectivity of metal vertices and organic linkers and the functioning of an organic ligand as a link in the optimal design of structural parts. Flexible MOFs are those that can react to environmental stimuli, have a varied framework, and their response manifests as conformation mobility coupled with coherence choices (Li et al., 1999). This reaction is determined by the metal composition used and whether the linkers may spin, bend, or flex. This one-of-a-kind characteristic substantially enhances the MOF’s activity in various functions such as retention, isolation, sensors, and others. In contrary to the advent of inflexible MOFs, the misguided perception persists in other domains of flexible MOF structure, synthesizing, and usage. This study will offer critical instances from various laboratories to understand the structure and connectivity of individual components and the characteristics that govern characterizations and use.
Linker Flexibility/Functionalized Linker
Linkers heavily influence the ultimate nature of this type of porous structure. Linkers, also called ligands, are organic bridging components that link aggregates or metallic centers to form extended framework structures. The shape, length, functioning, and connection can guide the framework’s final design. Adjusting the linker connection while keeping the cornerstone shape the same, for instance, may result in a completely different structure with very distinct traits. Many analyses have shown that such linkers, which binding strengths may represent, may enable structural diversity due to their potential energies. Their reaction to external stimuli may take many forms, including spinning the host architecture in a metal ion and a chemical ligand, rotation across single bonds in an organic linker, and shifting sub-networks. Various linkers offer a flexible framework with dynamic characteristics: Aliphatic carbon chains, for example, are relevant in producing adaptable MOFs due to their capacity to reposition themselves in response to environmental conditions. The longer the width of the chain, the more readjustment possibilities. Aromatic rings can rotate or shift the organic ligands’ hanging side chains. Ligands have a flexible structure. The creation of linkers with the central atom M is required for this kind of ligand. The reduction indentation provides the ligand with numerous degrees of rotating flexibility encircling the inorganic component. The linker ratios, and the collective impact of metal hubs and organic linkers on framework adaptability, are major-league variables that influence the degree of flexibility. The interfaces of linkers may also be utilized as connection points for adding extra functions to adjust the scaffolding flexibility through substituent interactions at the linker. This kind of functionalization yields multimodal MOFs.
The techniques used to adjust the framework’s core network flexibility can be divided into two categories: (a) the utilization of organic synthetic methods with a specialized classification of the electron-donating nature of the “contributing” workable dimers, and (b) the integration of extensive cationic aromatic processes to the linker(s). Additionally, the form, size, and common orientation of the substituent placed onto the primary linker all contribute to determining the MOF’s architecture and its entire system topology. Asymmetrical linkers also help to tune the network’s topographical adaptability. Given the almost limitless ways a linker(s) may express itself, a standard category of MOFs centered on carboxylic fusions or synthesized and characterized 1,4-benzene dicarboxylate linkers has been intensively investigated so far. Currently, these forms of linkers have dominated the area of MOF flexibility identification.
Isomerism Characteristics of Carboxylate Linkers
Various researchers have suggested many practical guidelines for the effect of carboxylate fusions on MOF versatility. For instance, Férey & Serre (2009) offer various guidelines as indicated. According to the first criterion, diatopic carboxylate molecules coupled to two metal complexes or SBUs are advantageous for creating adaptable MOFs. As a result, the basic synthetic tenets, which share a reflection center with the carboxyl group linkers, can allow structure swelling; MIL-88 is an excellent illustration of this rule in action. (2) The second principle concerns the proportion of carbons enclosing the cluster to the number of metallic atoms in the cluster (C/M); multiple studies have shown that if this proportion is greater or equal to two, the brick may allow inflammation. Mill-88, with a C/M = 2, exemplifies this idea. This principle, though, must be defined by several other MOFs. (3) The third specification states that tri- or tetra-topic carboxyl group linkers prevent MOF respiration, unlike diatopic carboxyl group linkers. Ostensibly, the preceding rules constitute the main structural capacity for the occurrence of flexibility. Ligand formation has the potential to alter conformational changes. The use of synthesized linker monomers to decorate the pores may add intricacy to their characteristics. For example, introducing a group that may interact through h – bonding, such as OH, –NH2, –and COOH, would result in a greater degree of dissociation and a larger density of multiple metal sites, increasing the M/L proportion. MIL-88 and MIL-53 are two third-generation composite materials that are extensible MOFs. Their intriguing characteristics piqued the interest of researchers doing theoretically and experimentally linker functionalization studies. Ahnfeldt et al. explored the incorporation of organic linkers, including amines, into MIL-53. The composition included AlO4(OH)2-octahedra connections with 1,4-benzene dicarboxylicbenzene dicarboxylic acetone, which led toto a small increase of the molar volume. Monitoring this activity has given crucial clues to comprehend the significance of derivatization, allowing the statement that the kind of functionalization would be highly dependent on the form of the intended application.
Mixed-Linker
Research is being focused on enhancing the design of linkers by blending different natural linkers. A mixture of this kind may be more adaptable. Mowat et al. (2012) used this process to develop 18 one-phase MTVs of MOF-5, beginning with eight different functionalized 1,4-BDC organic ligands. The resultant MOF series has various functions. The authors provided another well-studied mixed-linker topology, focusing on pillared-layer MOFs (Mowat et al., 2012). This kind of MOF is distinguished by structural flexibility, layer interconnection, and elasticity. A simple adjustment to the dipyridyl linker may vary the channel size and functioning and the level of flexibility and pore volume of the system while preserving its pillared structure. Zhao, Tsang, & Fairen-Jimenez (2021). have also shown that utilizing a mixed ligand framework may improve the functional properties of aromatic carboxylate fusions. Due to the resultant MOFs’ sustained elasticity, for example, Cd(II) coordinate polymers were synthesized under solvothermal conditions utilizing a semi-flexible carboxylase acid receptor H3L pyridyl-based co-ligand. The heterocyclic polycarboxylate co-ligands significantly influence the structure, elasticity, and interaction modes of Cdbmb polymers. Some of these instances include the fact that the elasticity of MOFs is primarily due to a configuration shift in the flexible organic ligands.
The Metal Nodes’ Flexibility
The artificial block, also known as metal connectors, describes the overall application’s spatial patterns. These organic hubs, which may be metal clusters or trace metals, are where elasticity and change can begin since many metallic nanoparticles are not irrevocably labile but are in a condition of transient stability until exposed to external events. At this point, they assume various forms. In many instances, the metal terminals of the structure do not adhere to the overarching structure’s uniformity. Numerous studies have identified this problem and the alterations that happen when external factors trigger these structures. As a result, the architecture at the metal nodes will be an essentialessential element in regulating the framework’s suppleness. TheThe altered coordinating environment of metallic ions is one criterioncriterion for differentiating flexibility MOFs from inflexible MOFs (Krause et al., 2016). Other parameters, such as (ii) the distorted layout of additional building units incorporating metal ions because of reduction or binding of coactive components, have been suggested.
Moreover, experimental investigations revealed that the structure flexibility scales inversely with the intensity of the metal-ligand connection. Certain delocalized electrons in the metallic center chains may further restrict crinkles’ dynamic characteristics and alter the overall compression or extension distances between neighboring metal complexes. MIL-53(M) may be used as a model to illustrate the function of metal nodes in ultimate framework adaptability.
By changing the metal M, you may create a variety of constructions. The advantageous interaction with the metal core selection will result in different, flexible activity, with MIL-53(Cr) exhibiting an enhanced capacity for pore opening following dehydration. When a mercurial intrusion–extrusion assessment was conducted on MIL-53(Cr) and MIL-53(Al) employing hydrostatic stress less than 500 MPa, a shift from enlarged pores to narrower pores was observed; mercury absorption occurred inside pores bigger than 3 nm. The results of this test showed that MIL-53(Al) solid exhibits irreversible compression, while MIL-53(Cr) solid experiences a changeable structural change under identical circumstances, as illustrated in figure 1 below. Such remarkable behavior can only be described by corner-sharing networks of AlO4(OH)2 tetrahedrally, which result in structure stiffness when pressure is applied.
It is insufficient to add metallic nods/ions, connectors, or their functions to build adaptable MOFs; there must also be an intelligent technique for the preparation and delicate interaction between them in terms of concentration, position, and proximity specific framework components. As a result, some parallelism is anticipated between the actions of metal hubs and organic ligands and how the entire structure responds to temperature fluctuations, stress, or guest compounds.
Elements of Structural Flexibility in General
Regarding framework flexibility, many aspects have been explored. Each occurrence requires a thorough and profound understanding of the structure and connection of individual MOFs’ elements. Furthermore, a review of their structural transition after exposure to environmental stimuli to determine which variables may be enhanced during an implementation regarding seemingly abnormal actions. Flexible MOFs respond to stimuli amazingly. When discussing the dynamic characteristics of adaptable MOFs in response to environmental conditions, three design levels of reaction should be kept in mind: the host organizational framework (cage), (ii) the crystalline structure, and (iii) the framework of the overall particle. External stimuli, including light, heat, or pressure, are required to mimic the stereo-dynamic activity of third-generation MOFs. The kind of stimulus does not affect the response received from the intended framework (McPherson et al., 2019). Several significant repercussions ensue. The degree of responsiveness will be determined by an architectural framework’s ability to reorganize itself in response to stimuli-dependence actions. Such reactions may be shown by gate opening, phase shift, and changes in cell characteristics. Several environmental stimuli, such as heat, were utilized to aggravate the structural change. When MOFs are subjected to various temperatures, the structure of the linker changes.
Thermo-Responsivity
The temperature has a significant impact on the transition state of elastic MOFs. Some of the elastic MOFs undergo a dramatic transition state from narrower pore (NP) architectures at low temperatures to large pore (LP) structures once the cutoff temperature is reached. It is worth noting that expanding the carbonyl group length and orientations reduces the transformation temperature in such a responsive manner. On the converse, the higher the temperature range, since the rotation of the pillars ultimately results in the expansion of the pore space. Returning to the famous example, when a high temperature is applied to MIL-53(Cr), the rotation of the phenyl group increases, resulting in a conversion to the open state. As a result, altering the structure of the aromatic spacers has a significant effect on the flexibility of these solids.
Elasticity and mechanical characteristics
Pressure is essential in a variety of practical uses. Since a result, researching its impact has piqued the attention of many academics, as applying pressure to particular frameworks imparts the elasticity to enable dramatic changes in their structural features. To date, only a few pressure-induced characteristics of elastic MOFs have been studied. However, the amount of MOFs subjected to mechanical stress is insignificant compared to the vast number of MOFs generated. (Choi et al., 2018) demonstrated the importance of applying modest pressures on crystals with porosity systems to investigate the structural changes in structure reaction. Their research showed that pressure provides a unique means to fully explore the mostly structure-function interactions of flexible MOFs since it was apparent that their porous nature adds an extra dimension when exposed to high pressure. When metallic polyhedra structures were subjected to high pressure, a remarkable flexible pattern was observed. Polyhedral metal is regarded as a key indicator of extremely flexible and high elastic MOFs since it may relax, rotate, and alter the unit cell volume when subjected to external compression.
Photo-Responsiveness
Various proof-of-concept study investigations indicate a different characteristic of MOFs when they interact with light, laying the groundwork for discovering structure-function connections of photo-switches occurrences. There are 3 types of photo-responsive platforms. The first phase of MOFs has dangling groups that are thermally or visually useful to the overall framework. These groups function as responsive troops within these settings, changing their conformation in response to light exposure. Because of their photochromic components that may undergo efficient, rapid, and externally reversible photoisomerization, these MOFs are often produced utilizing azobenzene and its variants. Stock and his colleagues discovered this feature while studying porous doubly interpenetrated MOFs with an azo activity that protrudes into the crevices. Exposure of this MOF to UV-light with a frequency of 365 nm causes the linker to undergo cis-trans conversion, while reverse flipping has been seen when irradiation with a frequency of 440 nm, as illustrated in figure 2 below.
The cis-trans transformation and guest photo interesterification may result in significant alterations in adsorptive characteristics. The MOFs with azobenzene-functionalized linkers often get the most attention. Brown and colleagues investigated a photo-switched property of an azobenzene-functionalized linkage mediated by the IRMOF-74-III structure [Mg2] (C26H16O6N2). When this MOF was stimulated with light, the scientists observed a transition from the trans-conformer to the cis conformer. In addition, a distance increase between the para-carbon ions in the azobenzene monomer 10.3 from 8.3 was observed. In more recent research, Park and his colleagues investigated the reversible change of CO2 uptake in response to light exposure or heat treatment. It was discovered that changeable azobenzene modification substantially impacted the framework’s ability for CO2 absorption. The structure substantially reduces CO2 absorption when the azobenzene is in the cis form compared to the same framework when the binder is genderfluid. Other research groups investigated and expanded on this idea with a different host context; for example, specific host frameworks prohibit and impede the cis-trans transition, as in the case of azobenzene in MIL-53 (Al). Shifting is severely hampered in this generation because the azo compounds are essential linker structures. As a result, this kind has yet to be disclosed.
Conclusion
This review aims to create a guide through the research on the forms, structure, regulation, and characterization of adaptable MOFs. This study defined the significant characteristics and described the approach used in their design to discover the desired path for each application to develop certain fundamental principles. Many reviews and publications highlight advancements in theoretical thought and skilled experimental studies on inflexible MOFs. However, theoretically, and experimentally, research on flexible MOFs is sparse, and no effort has been made to explain the changes in their architecture caused by external stimuli for industrial uses. This review provides a brief but comprehensive overview of the existing literature on flexible MOFs and advice the audience on where the difficulties in their design and, therefore, their reactive behavior lie. Finally, clever management of metal-ligand interaction, along with their broad responsive stimuli, may result in an excellent 3rd generation porous layer with a broad array of applications.
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