Conspectus Metal–organic frameworks (MOFs) and porous coordination polymers (PCPs) have been well-recognized as emerging porous materials that afford highly tailorable and well-defined nanoporous ...structures with three-dimensional lattices. Because of their microporous nature, MOFs can accommodate small molecules in their lattice structure, thus discriminating them on the basis of their size and physical properties and enabling their separation even in the gas phase. Such characteristics of MOFs have attracted significant attention in recent years for diverse applications and have ignited a worldwide race toward their development in both academic and industrial fields. Most recently, new challenges in porous materials science demand processable liquid, melt, and amorphous forms of MOFs. This trend will provide a new fundamental class of microporous materials for further widespread applications in many fields. In particular, the application of flexible membranes for gas separation is expected as an efficient solution to tackle current energy-intensive issues. To date, amorphous MOFs have been prepared in a top-down approach by the introduction of disorder into the parent frameworks. However, this new paradigm is still in its infancy with respect to the rational design principles that need to be developed for any approach that may include bottom-up synthesis of porous soft materials. Herein we describe recent progress in bottom-up “modular” approaches for the synthesis of porous, processable MOF-based materials, wherein metal–organic cages (MOCs), alternatively called metal–organic polyhedra (MOPs), are used as “modular cavities” to build porous soft materials. The outer periphery of a MOP is decorated with polymeric and dendritic side chains to obtain a polymer-grafted MOP, imparting both solution and thermal processability to the MOP cages, which have an inherent nanocavity along with high tailorability analogous to MOFs. Well-ordered MOP assemblies can be designed to obtain phases ranging from crystals to liquid crystals, allowing the fabrication of flexible free-standing sheets with preservation of the long-range ordering of MOPs. Furthermore, future prospects of the modular design for porous soft materials are provided with the anticipation that the bottom-up design will combine porous materials and soft matter sciences, leading to the discovery and development of many unexplored new materials and devices such as MOF-based self-healing membranes possessing well-defined nanochannels. The macroscopic alignment of channels can be controlled by external factors, including electric and magnetic fields, external forces, and modified surfaces (templating and patterning), which are conventionally used for engineering of soft materials.
In this Minireview, we discuss the fundamental chemistry of soft porous crystals (SPCs) by characterizing their common structural features and the resulting structural softness and transitions. In ...particular, we focus on the recently emerging properties based on metastable transitions and those arising from local dynamics. By comparing the resulting adsorption properties to those of commonly applied rigid adsorbents, we highlight the potential of SPCs to revolutionize adsorption‐based technologies, considering our current understanding of the thermodynamic and kinetic aspects. We provide brief outlines for the experimental and computational characterization of such phenomena and offer an outlook toward next‐generation SPCs likely to be discovered in the next decade.
Soft porous crystals (SPCs) can undergo cooperative stimuli‐responsive structural transitions that strongly impact the physical properties and pore structure of the crystal. Hysteretic behavior and counterintuitive adsorption phenomena are a consequence of the complex energy landscapes and cooperativity of SPCs with regard to adsorption/desorption‐induced deformation.
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•Factors of governing water resistance of porous coordination polymers were surveyed and discussed.•Representative studies were given with emphasis on adsorptive-based gas separations ...by water resistant porous coordination polymers.Representative studies were given with emphasis on membrane-based gas separations by water resistant porous coordination polymers.
Porous coordination polymer (PCP) chemistry has a promising future because of the tunable structures and excellent properties of polymers. However, the strategy for designing and preparing water-resistant PCPs is a considerable challenge. This review surveys and investigates the factors governing water resistance in a hierarchy sequence. Subsequently, representative studies are provided with an emphasis on their adsorptive- and membrane-based gas separations. This review is intended to be useful for researchers who are interested in designing water-resistant PCPs and exploring promising applications for gas separation.
Ion conduction and transport in solids are both interesting and useful and are found in widely distinct materials, from those in battery-related technologies to those in biological systems. ...Scientists have approached the synthesis of ion-conductive compounds in a variety of ways, in the areas of organic and inorganic chemistry. Recently, based on their ion-conducting behavior, porous coordination polymers (PCPs) and metal–organic frameworks (MOFs) have been recognized for their easy design and the dynamic behavior of the ionic components in the structures. These PCP/MOFs consist of metal ions (or clusters) and organic ligands structured via coordination bonds. They could have highly concentrated mobile ions with dynamic behavior, and their characteristics have inspired the design of a new class of ion conductors and transporters. In this Account, we describe the state-of-the-art of studies of ion conductivity by PCP/MOFs and nonporous coordination polymers (CPs) and offer future perspectives. PCP/MOF structures tend to have high hydrophilicity and guest-accessible voids, and scientists have reported many water-mediated proton (H+) conductivities. Chemical modification of organic ligands can change the hydrated H+ conductivity over a wide range. On the other hand, the designable structures also permit water-free (anhydrous) H+ conductivity. The incorporation of protic guests such as imidazole and 1,2,4-triazole into the microchannels of PCP/MOFs promotes the dynamic motion of guest molecules, resulting in high H+ conduction without water. Not only the host–guest systems, but the embedding of protic organic groups on CPs also results in inherent H+ conductivity. We have observed high H+ conductivities under anhydrous conditions and in the intermediate temperature region of organic and inorganic conductors. The keys to successful construction are highly mobile ionic species and appropriate intervals of ion-hopping sites in the structures. Lithium (Li+) and other ions can also be transported. If we can optimize the crystal structures, this could offer further improvements in terms of both conductivity and the working temperature range. Another useful characteristic of PCP/MOFs is their wide application to materials fabrication. We can easily prepare heterodomain crystal systems, such as core–shell or solid solution. Other anisotropic morphologies (thin film, nanocrystal, nanorod, etc.,) are also possible, with retention of the ion conductivity. The flexible nature also lets us design morphology-dependent ion-conduction behaviors that we cannot observe in the bulk state. We propose (1) multivalent ion and anion conductions with the aid of redox activity and defects in structures, (2) control of ion transport behavior by applying external stimuli, (3) anomalous conductivity at the hetero-solid–solid interface, and (4) unidirectional ion transport as in the ion channels in membrane proteins. In the future, scientists may use coordination polymers not only to achieve higher conductivity but also to control ion behavior, which will open new avenues in solid-state ionics.
Design of the gas-diffusion process in a porous material is challenging because a contracted pore aperture is a prerequisite, whereas the channel traffic of guest molecules is regulated by the ...flexible and dynamic motions of nanochannels. Here, we present the rational design of a diffusion-regulatory system in a porous coordination polymer (PCP) in which flip-flop molecular motions within the framework structure provide kinetic gate functions that enable efficient gas separation and storage. The PCP shows substantial temperature-responsive adsorption in which the adsorbate molecules are differentiated by each gate-admission temperature, facilitating kinetics-based gas separations of oxygen/argon and ethylene/ethane with high selectivities of ~350 and ~75, respectively. Additionally, we demonstrate the long-lasting physical encapsulation of ethylene at ambient conditions, owing to strongly impeded diffusion in distinctive nanochannels.
Recently, syntheses of epoch-making space materials using coordination compounds are underway by various groups. The rational synthetic strategy of the space of coordination compounds provides unique ...structures and functions, which have not been found in traditional materials. Now, we intend to focus on this scientific field from a new viewpoint. We define
coordination space as the space where the coordination bond plays an important role in the formation of the spatial structures and where various physical properties are exhibited. In this article, we focus on the
coordination spaces provided by porous coordination polymers, and their uniqueness is illustrated with current representative results.
Porous coordination polymers (PCPs) have attracted vast interest in recent years because of their possibility for rational design of crystal structures and functional properties. An emerging trend in ...PCP research is hybridization, which is the subject of this review. We have divided hybrid PCPs into three broad classes: class I, isomorphous mixed metal/ligand PCPs; class II, core@shell PCPs; class III, PCP⊃guest or PCPs with accommodated guests. In this Review, we examine 62 representative hybrid PCPs in the area of gas adsorption/separation/storage, luminescence, catalysis, drug delivery, and ionic conductivity with representative examples of each class to understand the underlying principles of hybridization for PCPs and their advantages over conventional PCPs. The future directions and applications of hybrid PCPs are postulated.
The discovery of a method to separate isotopologues, molecular entities that differ in only isotopic composition
, is fundamentally and technologically essential but remains challenging
. Water ...isotopologues, which are very important in biological processes, industry, medical care, etc. are among the most difficult isotopologue pairs to separate because of their very similar physicochemical properties and chemical exchange equilibrium. Herein, we report efficient separation of water isotopologues at room temperature by constructing two porous coordination polymers (PCPs, or metal-organic frameworks) in which flip-flop molecular motions within the frameworks provide diffusion-regulatory functionality. Guest traffic is regulated by the local motions of dynamic gates on contracted pore apertures, thereby amplifying the slight differences in the diffusion rates of water isotopologues. Significant temperature-responsive adsorption occurs on both PCPs: H
O vapour is preferentially adsorbed into the PCPs, with substantially increased uptake compared to that of D
O vapour, facilitating kinetics-based vapour separation of H
O/HDO/D
O ternary mixtures with high H
O separation factors of around 210 at room temperature.
There are two categories of coordination polymers (CPs): inorganic CPs (i‐CPs) and organic ligand bridged CPs (o‐CPs). Based on the successful crystal engineering of CPs, we here propose ...noncrystalline states and functionalities as a new research direction for CPs. Control over the liquid or glassy states in materials is essential to obtain specific properties and functions. Several studies suggest the feasibility of obtaining liquid/glassy states in o‐CPs by design principles. The combination of metal ions and organic bridging ligands, together with the liquid/glass phase transformation, offer the possibility to transform o‐CPs into ionic liquids and other ionic soft materials. Synchrotron measurements and computational approaches contribute to elucidating the structures and dynamics of the liquid/glassy states of o‐CPs. This offers the opportunity to tune the porosity, conductivity, transparency, and other material properties. The unique energy landscape of liquid/glass o‐CPs offers opportunities for properties and functions that are complementary to those of the crystalline state.
Heart of glass: The liquid/glass states of coordination polymers (CPs) and metal‐organic frameworks (MOFs) constitute a new class of amorphous materials and are related to ionic liquids and other ionic soft materials. The unique energy landscape and dynamics of liquid/glass CPs/MOFs with coordination networks offer additional opportunities for properties and functions that are complementary to those of the crystalline state.
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