Atoms and the void: modular construction of ordered porous solids

Porous Organic Nanotubes: Chemistry of One-Dimensional Space

ConspectusOne-dimensional organic nanotubes feature unique properties, such as confined chemical environments and transport channels, which are highly desirable for many applications. Advances in synthetic methods have enabled the creation of different types of organic nanotubes, including supramolecular, hydrogen-bonded, and carbon nanotube analogues. However, challenges associated with chemical and mechanical stability along with difficulties in controlling aspect ratios remain a significant bottleneck. The fascination with structured porous materials has paved the way for the emergence of reticular solids such as metal-organic frameworks (MOFs), covalent organic frameworks (COFs), and organic cages. Reticular materials with tubular morphology promise architectural stability with the additional benefit of permeant porosity. Despite this, the current synthetic approaches to these reticular nanotubes focus more on structural design resulting in less reliable morphological uniformity. This Account, highlights the design motivation behind various classes of organic nanotubes, emphasizing their porous interior space. We explore the strategic assembly of organic nanotubes based on their bonding characteristics, from weak supramolecular to robust covalent interactions. Special attention is given to reticular nanotubes, which have gained prominence over the past two decades due to their distinctive micro and mesoporous structures. We examine the synergy of covalent and noncovalent interactions in constructing assembly of these nanotube structures.This Account furnishes a comprehensive overview of our efforts and advancements in developing porous covalent organic nanotubes (CONTs). We describe a general synthetic approach for creating robust imine-linked nanotubes based on the reticular chemistry principles. The use of spatially oriented tetratopic triptycene-based amine and linear ditopic aldehyde building blocks facilitates one-dimensional nanotube growth. The interplay between directional covalent bonds and solvophobic interactions is crucial for forming uniform, well-defined, and high aspect ratio nanotubes. The nanotubes derive their permeant porosity and thermal and chemical stability from their covalent architecture. We also highlight the adaptability of our synthetic methodology to guide the transformation of one-dimensional nanotubes to toroidal superstructures and two-dimensional thin fabrics. Such morphological transformation can be directed by tuning the reaction time or incorporating additional intermolecular interactions to control the intertwining behavior of individual nanotubes. The cohesion of covalent and noncovalent interactions in the tubular nanostructures manifests superior viscoelastic mechanical properties in the assembled CONT fabrics. We establish a strong correlation between structural framework design and nanostructures by translating reticular synthesis to morphological space and gaining insights into the assembly processes. We anticipate that the present Account will lay the foundation for exploring new designs and chemistry of organic nanotubes for many application platforms.

Network topology diversification of porous organic salts

Hydrogen-bonded organic frameworks (HOFs) are porous organic materials constructed via hydrogen bonds. HOFs have solubility in specific high-polar organic solvents. Therefore, HOFs can be returned to their components and can be reconstructed, which indicates their high recyclability. Network topologies, which are the frameworks of porous structures, control the pore sizes and shapes of HOFs. Therefore, they strongly affect the functions of porous materials. However, hydrogen bonds are usually weak interactions, and the design of the intended network topology in HOFs from their components has been challenging. Porous organic salts (POSs) are an important class of HOFs, are hierarchically constructed via strong charge-assisted hydrogen bonds between sulfonic acids and amines, and therefore are expected to have high designability of the porous structure. However, the network topology of POSs has been limited to only dia-topology. Here, we combined tetrasulfonic acid with the adamantane core (4,4',4'',4'''-(adamantane-1,3,5,7-tetrayl)tetrabenzenesulfonic acid; AdPS) and triphenylmethylamines with modified substituents in para-positions of benzene rings (TPMA-X, X = F, methyl (Me), Cl, Br, I). We changed the steric hindrance between the adamantane and substituents (X) in TPMA-X and obtained not only the common dia-topology for POSs but also rare sod-topology, and lon- and uni-topologies that are formed for the first time in HOFs. Changing template molecules under preparation helped in successfully isolating the porous structures of AdPS/TPMA-Me with dia-, lon-, and sod-topologies which exhibited different gas adsorption properties. Therefore, for the first time, we demonstrated that the steric design of HOF components facilitated the formation, diversification, and control of the network topologies and functions of HOFs.

Charge-Assisted Ionic Hydrogen-Bonded Organic Frameworks: Designable and Stabilized Multifunctional Materials

Hydrogen-bonded organic frameworks (HOFs) are a class of crystalline framework materials assembled by hydrogen bonds. HOFs have the advantages of high crystallinity, mild reaction conditions, good solution processability, and reproducibility. Coupled with the reversibility and flexibility of hydrogen bonds, HOFs can be assembled into a wide diversity of crystalline structures. Since the bonding energy of hydrogen bonds is lower than that of ligand and covalent bonds, the framework of HOFs is prone to collapse after desolventisation and the stability is not high, which limits the development and application of HOFs. In recent years, numerous stable and functional HOFs have been developed by π-π stacking, highly interpenetrated networks, charge-assisted, ligand-bond-assisted, molecular weaving, and covalent cross-linking. Charge-assisted ionic HOFs introduce electrostatic attraction into HOFs to improve stability while enriching structural diversity and functionality. In this paper, we review the development, the principles of rational design and assembly of charge-assisted ionic HOFs, and introduces the different building block construction modes of charge-assisted ionic HOFs. Highlight the applications of charge-assisted ionic HOFs in gas adsorption and separation, proton conduction, biological applications, etc., and prospects for the diverse design of charge-assisted ionic HOFs structures and multifunctional applications.

Anisotropic functionalized platelets: percolation, porosity and network properties

Anisotropic functionalized platelets are able to model the assembly behaviour of molecular systems in two dimensions thanks to the unique combination of steric and bonding constraints. The assembly scenarios can vary from open to close-packed crystals, finite clusters and chains, according to the features of the imposed constraints. In this work, we focus on the assembly of equilibrium networks. These networks can be seen as disordered, porous monolayers and can be of interest for instance in nano-filtration and optical applications. We investigate the formation and properties of two dimensional networks from shape anisotropic colloids functionalized with four patches. We characterize the connectivity properties, the typical local bonding motives, as well as the geometric features of the emerging networks for a large variety of different systems. Our results show that networks of shape anisotropic colloids assemble into highly versatile network topologies, that may be utilized for applications at the nanoscale.

Isoreticular Chemistry and Applications of Supramolecularly Assembled Copper-Adenine Porous Materials

The useful concepts of reticular chemistry, rigid and predictable metal nodes together with strong and manageable covalent interactions between metal centers and organic linkers, have made the so-called metal-organic frameworks (MOFs) a flourishing area of enormous applicability. In this work, the extension of similar strategies to supramolecularly assembled metal-organic materials has allowed us to obtain a family of isoreticular compounds of the general formula [Cu7(μ-adeninato-κN3:κN9)6(μ3-OH)6(μ-OH2)6](OOC-R-COO)·nH2O (R: ethylene-, acetylene-, naphthalene-, or biphenyl-group) in which the rigid copper-adeninato entities and the organic dicarboxylate anions are held together not by covalent interactions but by a robust and flexible network of synergic hydrogen bonds and π-π stacking interactions based on well-known supramolecular synthons (SMOFs). All compounds are isoreticular, highly insoluble, and water-stable and show a porous crystalline structure with a pcu topology containing a two-dimensional (2D) network of channels, whose dimensions and degree of porosity of the supramolecular network are tailored by the length of the dicarboxylate anion. The partial loss of the crystallization water molecules upon removal from the mother liquor produces a shrinkage of the unit cell and porosity, which leads to a color change of the compounds (from blue to olive green) if complete dehydration is achieved by means of gentle heating or vacuuming. However, the supramolecular network of noncovalent interactions is robust and flexible enough to reverse to the expanded unit cell and color after exposure to a humid atmosphere. This humidity-driven breathing behavior has been used to design a sensor in which the electrical resistance varies reversibly with the degree of humidity, very similar to the water vapor adsorption isotherm of the SMOF. The in-solution adsorption properties were explored for the uptake and release of the widely employed 5-fluorouracil, 4-aminosalycilic acid, 5-aminosalycilic acid, and allopurinol drugs. In addition, cytotoxicity activity assays were completed for the pristine and 5-fluorouracil-loaded samples.

Rigid-Flexible Hybrid Porous Molecular Crystals with Guest-Induced Reversible Crystallinity

A weak CH/O hydrogen-bonded organic framework (HOF) with both rigidity and flexibility that could easily and reversibly switch from a non-crystalline to a crystalline phase was constructed. The specific solvent molecule acts as a "key" to control the crystallinity, while the highly rigid triangle macrocycle as the building block is the "lock". The introduction and removal of the "key" could influence the local flexibility of the whole framework and lead to switchable crystallinity. Furthermore, the obtained HOF exhibits excellent separation efficiency for benzene and cyclohexane (94.4 %).

Thermally Crosslinked Hydrogen-Bonded Organic Framework Membranes for Highly Selective Ion Separation

The weak bonding energy and flexibility of hydrogen bonds can hinder the long-term use of hydrogen-bonded organic framework (HOF) materials under harsh conditions. Here we invented a thermal-crosslinking method to form polymer materials based on a diamino triazine (DAT) HOF (FDU-HOF-1), containing high-density hydrogen bonding of N-H⋯N. With the increase of temperature to 648 K, the formation of -NH- bonds between neighboring HOF tectons by releasing NH₃ was observed based on the disappearance of the characteristic peaks of amino groups on FDU-HOF-1 in the Fourier transform infrared (FTIR) and solid-state nuclear magnetic resonance (ss-NMR). The variable temperature PXRD indicated the formation of a new peak at 13.2° in addition to the preservation of the original diffraction peaks of FDU-HOF-1. The water adsorption, acid-base stability (12 M HCl to 20 M NaOH) and solubility experiments concluded that the thermally crosslinked HOFs (TC-HOFs) are highly stable. The membranes fabricated by TC-HOF demonstrate the permeation rate of K⁺ ions as high as 270 mmol m^-2 h^-1 as well as high selectivity of K⁺/Mg²⁺ (50) and Na⁺/Mg²⁺ (40), which was comparable to Nafion membranes. This study provides guidance for the future design of highly stable crystalline polymer materials based on HOFs.

Design Rules of Hydrogen-Bonded Organic Frameworks with High Chemical and Thermal Stabilities

Hydrogen-bonded organic frameworks (HOFs), self-assembled from strategically pre-designed molecular tectons with complementary hydrogen-bonding patterns, are rapidly evolving into a novel and important class of porous materials. In addition to their common features shared with other functionalized porous materials constructed from modular building blocks, the intrinsically flexible and reversible H-bonding connections endow HOFs with straightforward purification procedures, high crystallinity, solution processability, and recyclability. These unique advantages of HOFs have attracted considerable attention across a broad range of fields, including gas adsorption and separation, catalysis, chemical sensing, and electrical and optical materials. However, the relatively weak H-bonding interactions within HOFs can potentially limit their stability and potential use in further applications. To that end, this Perspective highlights recent advances in the development of chemically and thermally robust HOF materials and systematically discusses relevant design rules and synthesis strategies to access highly stable HOFs.

Photosensitizer-based metal-organic frameworks for highly effective photodynamic therapy

Photodynamic therapy (PDT) uses a photosensitizer, molecular oxygen, and visible light as an alternative clinical protocol against located malignant tumors and other diseases. More recently, PDT has been combined to immunotherapy as a promising option to treat metastatic cancer. However, previous generations of photosensitizers (PSs) revealed clinical difficulties such as long-term skin photosensitivity (first generation), the need for drug delivery vehicles (second generation), and intracellular self-aggregation (third generation), which have generated a somewhat confusing scenario in PDT approaches and evolution. Recently, metal-organic frameworks (MOFs) with exceptionally high PS loading as a building unit of MOF framework have emerged as fourth-generation PS and presented outstanding outcomes under pre-clinical studies. For PS-based MOFs, the inorganic building unit (metal ions/clusters) plays an important role as a coadjuvant in PDT to alleviate hypoxia, to decrease antioxidant species, to yield ROS, or to act as a contrast agent for imaging-guided therapy. In this review, we intend to carry out a broad update on the recent history and the characteristics of PS-based MOFs from basic chemistry to the structure relationship with biological application in PDT. The details and variables that result in different photophysics, size, and morphology, are discussed. Also, we present an overview of the achievements on the pre-clinical assays in combination with other strategies, including alleviating hypoxia in solid tumors, chemotherapy, and the most recent immunotherapy for cancer.

Quasiparticle electronic structure of two-dimensional heterotriangulene-based covalent organic frameworks adsorbed on Au(111)

The modular nature and unique electronic properties of two-dimensional (2D) covalent organic frameworks (COFs) make them an attractive option for applications in catalysis, optoelectronics, and spintronics. The fabrications of such devices often involve interfaces formed between COFs and substrates. In this work, we employ the first-principlesGWapproach to accurately determine the quasiparticle electronic structure of three 2D carbonyl bridged heterotriangulene-based COFs featuring honeycomb-kagome lattice, with their properties ranging from a semi-metal to a wide-gap semiconductor. Moreover, we study the adsorption of these COFs on Au(111) surface and characterize the quasiparticle electronic structure at the heterogeneous COF/Au(111) interfaces. To reduce the computational cost, we apply the recently developed dielectric embeddingGWapproach and show that our results agree with existing experimental measurement on the interfacial energy level alignment. Our calculations illustrate how the many-body dielectric screening at the interface modulates the energies and shapes of the Dirac bands, the effective masses of semiconducting COFs, as well as the Fermi velocity of the semi-metallic COF.