Porous isoreticular non-metal organic frameworks

Metal-organic frameworks (MOFs) are useful synthetic materials that are built by the programmed assembly of metal nodes and organic linkers1. The success of MOFs results from the isoreticular principle2, which allows families of structurally analogous frameworks to be built in a predictable way. This relies on directional coordinate covalent bonding to define the framework geometry. However, isoreticular strategies do not translate to other common crystalline solids, such as organic salts3-5, in which the intermolecular ionic bonding is less directional. Here we show that chemical knowledge can be combined with computational crystal-structure prediction6 (CSP) to design porous organic ammonium halide salts that contain no metals. The nodes in these salt frameworks are tightly packed ionic clusters that direct the materials to crystallize in specific ways, as demonstrated by the presence of well-defined spikes of low-energy, low-density isoreticular structures on the predicted lattice energy landscapes7,8. These energy landscapes allow us to select combinations of cations and anions that will form thermodynamically stable, porous salt frameworks with channel sizes, functionalities and geometries that can be predicted a priori. Some of these porous salts adsorb molecular guests such as iodine in quantities that exceed those of most MOFs, and this could be useful for applications such as radio-iodine capture9-12. More generally, the synthesis of these salts is scalable, involving simple acid-base neutralization, and the strategy makes it possible to create a family of non-metal organic frameworks that combine high ionic charge density with permanent porosity.

Crystalline porous organic salts

Crystalline porous organic salts (CPOSs), formed by the self-assembly of organic acids and organic bases through ionic bonding, possess definite structures and permanent porosity and have rapidly emerged as an important class of porous organic materials in recent years. By rationally designing and controlling tectons, acidity/basicity (pKa), and topology, stable CPOSs with permanent porosity can be efficiently constructed. The characteristics of ionic bonds, charge-separated highly polar nano-confined channels, and permanent porosity endow CPOSs with unique physicochemical properties, offering extensive research opportunities for exploring their functionalities and application scenarios. In this review, we systematically summarize the latest progress in CPOS research, describe the synthetic strategies for synthesizing CPOSs, delineate their structural characteristics, and highlight the differences between CPOSs and hydrogen-bonded organic frameworks (HOFs). Furthermore, we provide an overview of the potential applications of CPOSs in areas such as negative linear compression (NLC), proton conduction, rapid transport of CO2, selective and rapid transport of K+ ions, atmospheric water harvesting (AWH), gas sorption, molecular rotors, fluorescence modulation, room-temperature phosphorescence (RTP) and catalysis. Finally, the challenges and future perspectives of CPOSs are presented.

Fluorescent Supramolecular Organic Frameworks Constructed by Amidinium-Carboxylate Salt Bridges

We report here a set of fluorescent supramolecular organic frameworks (SOFs) that incorporate aggregation-induced emission (AIE) units within their frameworks. The fluorescent SOFs of this study were constructed by linking the tetraphenylethylene (TPE)-based tetra(amidinium) cation TPE⁴⁺ and aromatic dicarboxylate anions through amidinium-carboxylate salt bridges. The resulting self-assembled structures are characterized by fluorescence quantum yields in the range of 4.6∼14 %. This emissive behavior is ascribed to a combination of electrostatic interactions and hydrogen bonds that operate in concert to impede motions that would otherwise lead to excited state energy dissipation. Single-crystal X-ray diffraction analyses revealed that the length of the dicarboxylate anion bridges has a considerable impact on the structural features of the resulting frameworks. Nevertheless, all SOFs prepared in the context of the present study were found to display emissive features characteristic of TPE-based AIE luminogens with only a modest dependence on the structural specifics being seen. The SOFs reported here could be reversibly "broken up" and "reformed" in response to acid/base stimuli. This reversible structural behavior is consistent with their SOF nature.

Amidinium⋯carboxylate frameworks: predictable, robust, water-stable hydrogen bonded materials

In the last few years, the amidinium⋯carboxylate interaction has emerged as a powerful tool for the relatively predictable construction of families of three dimensional hydrogen bonded organic frameworks. These frameworks can be prepared in water and are surprisingly stable, including to heating in polar organic solvents and water. This feature article describes the design and synthesis of these materials, discusses their structures and stability, and highlights their recent applications for enzyme encapsulation and as precursors for the synthesis of molecularly thin hydrogen bonded 2D nanosheets.

Crystalline Porous Organic Salts: From Micropore to Hierarchical Pores

Crystalline porous organic salts (CPOSs), as an emerging class of porous organic materials, combining the uniform microporous system and distinct polarized channels, have become a highly evolving field of important current interest. The unique ionic bond of a CPOS endows the confined channels with high polarity, making CPOSs distinct from other organic frameworks. CPOSs show many fascinating properties, such as proton conductivity and fast transport of polar molecules, which involve the interaction between highly polarized guest molecules and host frameworks. Substantial progress has been made in the synthesis and applications of CPOSs. Herein, an overview is provided to impart a comprehensive understanding of the link between the synthetic approaches and the resultant microporous structure, the structure-function correlation and the state-of-the-art applications of CPOSs. The enhanced mass-transport performance of hierarchically porous structure in combination with the intrinsic polarized channels of CPOSs is very promising to create new applications and contribute to a new research upsurge. The perspective to construct porous hierarchy within the crystalline porous organic salts is assessed and will open a new research avenue. In the conclusion, the current challenges on the synthesis, structural regulation, and applications of CPOSs and the future of hierarchically porous crystalline organic salts are discussed.

A three dimensional hydrogen bonded organic framework assembled through antielectrostatic hydrogen bonds

A 3D hydrogen bonded organic framework was assembled in water from tetraamidinium cations and antielectrostatically hydrogen bonded bicarbonate dimers.

Recent advances in self-assembled amidinium and guanidinium frameworks

Recent advances in amidinium and guanidinium-containing hydrogen-bonded framework materials are highlighted.

Supramolecular frameworks based on 5,10,15,20-tetra(4-carboxyphenyl)porphyrins

Hydrogen bonding is used to prepare porphyrin-containing supramolecular frameworks.