Porous Molecular Crystals Derived from Cofacial Porphyrin/Phthalocyanine Heterodimers

Porphyrin-based porous materials are of growing interest as heterogeneous catalysts especially for reactions that are of importance to sustainability. Here we demonstrate that porous molecular crystals can be prepared by the simple co-crystallisation of tetraphenylporphyrin (TPP) with octa(2',6'-di-iso-propylphenoxy)phthalocyanine or some of its metal complexes [(dipPhO)8PcM; M=H2, Al-OH, Ti=O, Mn-Cl, Fe-Cl, Co, Ni, Cu, Zn, Ga-Cl, Ag, In-Cl or Au-Cl]. This process is facilitated by the efficient formation of the supramolecular heterodimer between TPP and (dipPhO)8PcM, which is driven by the complementary shape and symmetry of the two macrocycles. The (dipPhO)8PcM component directs the crystal structure of the heterodimers to form Phthalocyanine Nanoporous Crystals (PNCs) of similar structure to those formed by (dipPhO)8PcM alone. The incorporation of TPP appears to partially stabilise the PNCs towards the removal of included solvent and for cocrystals containing (dipPhO)8PcCo stability can be enhanced further by the insitu addition of 4,4-bipyridyl to act as a "molecular wall tie". These stabilised PNC/TPP cocrystals have a Brunauer-Emmett-Teller surface area (SABET) of 454 m2 g-1 and a micropore volume (Vmp) of 0.22 mL g-1. The reactivity of both macrocycles within the PNC/TPP co-crystals is demonstrated by insitu metal insertion.

Tandem Solid-Solution Phase Post-Synthetic Modification of Porous Molecular Crystals for In-Situ Generation of Fluorophores

Conventional synthetic methods of organic luminescent molecules often involve labor-intensive solution-phase organic synthesis, which violate the principles of atom-economic transformation. Post-synthetic modification (PSM) offers a promising alternative, allowing direct transformation from one fluorophore to another. Although PSM is commonly implemented in extended frameworks, its application in porous molecular crystals remains challenging. Herein, we focus on utilizing porous molecular crystals, specifically tetraphenylethylene-cored frameworks, as versatile platforms for tandem PSM reactions to customize organic fluorophores. The tailored skeleton design ensures both the formation of porous structures and the occurrence of tandem solid-solution phase reactions while maintaining the solid state of reactants and products in each step. The inherent non-covalent bonding nature of the frameworks facilitates processing and characterization, offering unparalleled advantages for porous networks. The accompanying solid-state fluorescence transition from green to blue and then to green (or yellow) enables real-time monitoring of tandem reactions and provides intuitive mechanistic insights. This phenomenon is exploited for the facile construction of a dynamic information encryption system using fluorescent quick response codes.

Ordered Carbonaceous Framework Synthesized from Hexaazatrinaphthylene with Enediyne Groups via Solid-State Bergman Cyclization Reaction

Porous materials synthesized through bottom-up approaches, such as metal-organic frameworks and covalent organic frameworks, have attracted attention owing to their design flexibility for functional materials. However, achieving the chemical and thermal stability of these materials for various applications is challenging considering the reversible coordination bonds and irreversible covalent bonds in their frameworks. Thus, ordered carbonaceous frameworks (OCFs) emerge as a promising class of bottom-up materials with good periodicity, thermal and chemical stability, and electrical conductivity. However, a few OCFs have been reported owing to the limited range of precursor molecules. Herein, we designed a hexaazatrinaphthylene-based molecule with enediyne groups as a precursor molecule for synthesizing an OCF. The solid-state Bergman cyclization of enediyne groups at a low temperature formed a microporous polymer and an OCF, exhibiting redox activity and demonstrating their potential for electrochemical applications. The microporous polymer was used as an active material in sodium-ion batteries, while the OCF was used as an electrochemical capacitor. These findings illustrate the utility of the Bergman cyclization reaction for synthesizing microporous polymers and OCFs with a customizable functionality for broad applications.

Stimuli-Responsive, Dynamic Supramolecular Organic Frameworks

Supramolecular organic frameworks (SOFs) are a class of three-dimensional, potentially porous materials obtained by the self-assembly of organic building blocks held together by weak interactions such as hydrogen bonds, halogen bonds, π⋅⋅⋅π stacking and dispersion forces. SOFs are being extensively studied for their potential applications in gas storage and separation, catalysis, guest encapsulation and sensing. The supramolecular forces that guide their self-assembly endow them with an attractive combination of crystallinity and flexibility, providing intelligent dynamic materials that can respond to external stimuli in a reversible way. The present review article will focus on SOFs showing dynamic behaviour when exposed to different stimuli, highlighting fundamental aspects such as the combination of tectons and supramolecular interactions involved in the framework formation, structure-property relationship and their potential applications.

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.

Tuning the Selectivity between C2H2 and CO2 in Molecular Porous Materials

A combined experimental and theoretical study of C₂H₂ and CO₂ adsorption and separation was performed in two isostructural molecular porous materials (MPMs): MPM-1-Cl ([Cu₂(adenine)₄Cl₂]Cl₂) and MPM-1-TIFSIX ([Cu₂(adenine)₄(TiF₆)₂]). It was revealed that MPM-1-Cl displayed higher low-pressure uptake, isosteric heat of adsorption (Q_st), and selectivity for C₂H₂ than CO₂, whereas the opposite was observed for MPM-1-TIFSIX. While MPM-1-Cl contains only one type of accessible channel, which has a greater preference toward C₂H₂, MPM-1-TIFSIX contains three distinct accessible channels, one of which is a confined region between two large channels that represents the primary binding site for both adsorbates. According to molecular simulations, the initial adsorption site in MPM-1-TIFSIX interacts more strongly with CO₂ than C₂H₂, thus explaining the inversion of adsorbate selectivity relative to MPM-1-Cl.

Solvophobicity-directed assembly of microporous molecular crystals

Dense packing is a universal tendency of organic molecules in the solid state. Typical porous crystals utilize reticular strong intermolecular bonding networks to overcome this principle. Here, we report a solvophobicity-based methodology for assembling discrete molecules into a porous form and succeed in synthesizing isostructural porous polymorphs of an amphiphilic aromatic molecule Py6Mes. A computational analysis of the crystal structure reveals the major contribution of dispersion interaction as the driving force for assembling Py6Mes into a columnar stacking while the columns are sterically salient and form nanopores between them. The porous packing is facilitated particularly in solvents with weak dispersion interaction due to the solvophobic effect. Conversely, solvents with strong dispersion interaction intercalate between Py6Mes due to the solvophilic effect and provide non-porous inclusion crystals. The solvophobicity-directed polymorphism is further corroborated by the polymorphs of Py6Mes-analogues, m-Py6Mes and Ph6Mes.

Creating porosity in a trianglimine macrocycle by heterochiral pairing

Macrocycles are usually non-porous or barely porous in the solid-state because of their small intrinsic cavity sizes and tendency to close-pack. Here, we use a heterochiral pairing strategy to introduce porosity in a trianglimine macrocycle, by co-crystallising two macrocycles with opposing chiralities. The stable racemic trianglimine crystal contains an interconnected pore network that has a Brunauer-Emmett-Teller (BET) surface area of 355 m2 g-1.

Solvatomorphism Influence of Porous Organic Cage on C2H2/CO2 Separation

Porous organic molecular (POM) materials can exhibit solvatomorphs via altering their crystallographic packing in the solid state, but investigating real gas mixture separation by porous materials with such a behavior is still very rare. Herein, we report that a lantern-shaped calix[4]resorcinarene-based porous organic cage (POC, namely, CPOC-101) can exhibit eight distinct solid-state solvatomorphs via crystallization in different solvents. This POC solvatomorphism has a significant influence on their gas sorption capacities as well as separation abilities. Specifically, the apparent Brunauer-Emmett-Teller (BET) surface area determined by nitrogen gas sorption at 77 K for CPOC-101α crystallized from toluene/chloroform is up to 406 m² g^-1, which is much higher than the rest of CPOC-101 solvatomorphs with BET values less than 40 m² g^-1. More interestingly, C₂H₂ and CO₂ adsorbed capacities, in addition to the C₂H₂/CO₂ separation ability at room temperature for CPOC-101α, are superior to those of CPOC-101β crystalized from nitrobenzene, the representative of POC solvatomorphs with low BET surface areas. These results indicate the possibility of adjusting gas sorption and separation properties of POC materials by controlling their solvatomorphs.

Inclusion of Amine Isomers with Open-Chain Hosts Having a Partial Structure of p-tert-Butylthiacalixarene

Porous materials, which can capture a specific compound from a hard-to-separate molecular mixture, are strongly desired for practical separation and purification processes. Aiming to develop such materials, we have investigated the performance of our original host compounds, [3,3'-thiobis(5-tert-butyl-2-hydroxybenzene)-1,1'-diyl]diacetic acid (2) and its monopropyl ester (3), in discriminating among regio- or stereoisomers of three groups of amines, 2-, 3-, and 4-methylpyridine, 2-, 6-, and 8-methylquinoline, and cis- and trans-4-cyclohexanamine. Diacid 2 selectively included 4-methylpyridine in hexane and 3-methylpyridine in toluene in competitive inclusion among the three regioisomers. Mechanistic studies revealed that the inclusions of 3- and 4-methylpyridine are favored under kinetic and thermodynamic control, respectively. Solvent-dependent switching in guest selectivity was also observed in competitive inclusion among the methylquinoline isomers with diacid 2, whereas trans-4-methylcyclohexanamine was selectively included over the cis-isomer by monoester 3, as well as diacid 2, regardless of the solvent employed. X-ray crystallographic analysis of the resulting inclusion crystals suggests that the wide guest scope of the host compounds originates from their flexible ability to form complexes with amines.

Anionic Polymerization in Porous Organic Frameworks: A Strategy to Fabricate Anchored Polymers and Copolymers

An anionic mechanism is used to create polymers and copolymers as confined to, or anchored to, high-surface-area porous nanoparticles. Linear polymers with soft and glassy chains, such as polyisoprene and polymethylmethacrylate, were produced by confined anionic polymerization in 3D networks of porous aromatic frameworks. Alternatively, multiple anions were generated on the designed frameworks which bear removal protons at selected positions, and initiate chain propagation, resulting in chains covalently connected to the 3D network. Such growth can continue outside the pores to produce polymer-matrix nanoparticles coated with anchored chains. Sequential reactions were promoted by the living character of this anionic propagation, yielding nanoparticles that were covered by a second polymer anchored by anionic block copolymerization. The intimacy of the matrix and the grown-in polymers was demonstrated by magnetization transfer across the interfaces in 2D ¹ H-¹³ C-HETCOR NMR spectra.

Robust Supramolecular Nano-Tunnels Built from Molecular Bricks*

Herein we report a linear ionic molecule that assembles into a supramolecular nano-tunnel structure through synergy of trident-type ionic interactions and π-π stacking interactions. The nano-tunnel crystal exhibits anisotropic guest adsorption behavior. The material shows good thermal stability and undergoes multi-stage single-crystal-to-single-crystal phase transformations to a nonporous structure on heating. The material exhibits a remarkable chemical stability under both acidic and basic conditions, which is rarely observed in supramolecular organic frameworks and is often related to structures with designed hydrogen-bonding interactions. Because of the high polarity of the tunnels, this molecular crystal also shows a large CO₂ -adsorption capacity while excluding other gases at ambient temperature, leading to high CO₂ /CH₄ selectivity. Aggregation-induced emission of the molecules gives the bulk crystals vapochromic properties.

Ternary Cocrystals with Large Soft Cavities: A 1,4-diiodotetrafluorobenzene (DITFB)⋅4-Biphenylpyridine N-oxide (BPNO) Host Assembled by Inclusion of Planar Aromatic Guests

A large soft-cavity host composed of 1,4-diiodotetrafluorobenzene (DITFB) and 4-biphenylpyridine N-oxide (BPNO) is assembled under the mediation of a planar aromatic guest molecule (pyrene or perylene) through C-I⋅⋅⋅^- O-N⁺ halogen bonds and π-hole⋅⋅⋅π bonds. Single-crystal X-ray diffraction reveals that guest molecules can be completely encapsulated in the four-layer host cavity to assemble ternary host-guest cocrystals; namely, Pyr@DITFB ⋅ BPNO and Per@DITFB ⋅ BPNO. The luminescence of these ternary cocrystals originates from their discrete guest molecules, which exhibit pure-blue and yellow emissions, respectively, that are localized at 425 nm and in the range of 485 to 578 nm, respectively. In addition, the contribution of different fragments to the stabilization of the crystal structure is estimated by computational chemistry. These cocrystals have significant potential for use in optical applications or materials, such as photonics or organic light-emitting diodes, respectively, that require to avoid the aggregation between luminophores.

DCM self-trapping by the host deformation in flexible host–guest molecules

The desolvated structure can self-trap the DCM molecules to return to the 1·DCM state via ligand deformation even under weak host–guest interactions. The capture behavior of DCM is mostly due to the flexibility of the ligand.

Modulation of Crystal Packing via the Tuning of Peripheral Functionality for a Family of Dinuclear Mesocates

A family of four novel pyrazinyl-hydrazone based ligands have been synthesized with differing functionality at the 5-position of the central aromatic ring. Previous work has shown such ligands to form dinuclear triple mesocates which pack to form hexagonal channels capable of gas sorption. The effect of the peripheral functionality of the ligand on the crystal packing was investigated by synthesizing complexes 1 to 4 which feature amino, bromo, iodo and methoxy substituents respectively. Complexes 1 to 3 crystallized in the same hexagonal space group P6₃ /m and featured 1D channels. However, on closer inspection while the packing of 1 is mediated by hydrogen bonding interactions, the packing of complexes 2 and 3 are not, due to a subtlety different π-π stacking interaction enforced by the halogen substituent. The more bulky nature of the methoxy substituent of 4 results in the complex crystallizing in the triclinic space group P-1, featuring an entirely different crystal packing.

Sigmoidally hydrochromic molecular porous crystal with rotatable dendrons

Vapochromic behaviour of porous crystals is beneficial for facile and rapid detection of gaseous molecules without electricity. Toward this end, tailored molecular designs have been established for metal-organic, covalent-bonded and hydrogen-bonded frameworks. Here, we explore the hydrochromic chemistry of a van der Waals (VDW) porous crystal. The VDW porous crystal VPC-1 is formed from a novel aromatic dendrimer having a dibenzophenazine core and multibranched carbazole dendrons. Although the constituent molecules are connected via VDW forces, VPC-1 maintains its structural integrity even after desolvation. VPC-1 exhibits reversible colour changes upon uptake/release of water molecules due to the charge transfer character of the constituent dendrimer. Detailed structural analyses reveal that the outermost carbazole units alone are mobile in the crystal and twist simultaneously in response to water vapour. Thermodynamic analysis suggests that the sigmoidal water sorption is induced by the affinity alternation of the pore surface from hydrophobic to hydrophilic.

Supramolecular-Macrocycle-Based Crystalline Organic Materials

Supramolecular macrocycles are well known as guest receptors in supramolecular chemistry, especially host-guest chemistry. In addition to their wide applications in host-guest chemistry and related areas, macrocycles have also been employed to construct crystalline organic materials (COMs) owing to their particular structures that combine both rigidity and adaptivity. There are two main types of supramolecular-macrocycle-based COMs: those constructed from macrocycles themselves and those prepared from macrocycles with other organic linkers. This review summarizes recent developments in supramolecular-macrocycle-based COMs, which are categorized by various types of macrocycles, including cyclodextrins, calixarenes, resorcinarenes, pyrogalloarenes, cucurbiturils, pillararenes, and others. Effort is made to focus on the structures of supramolecular-macrocycle-based COMs and their structure-function relationships. In addition, the application of supramolecular-macrocycle-based COMs in gas storage or separation, molecular separation, solid-state electrolytes, proton conduction, iodine capture, water or environmental treatment, etc., are also presented. Finally, perspectives and future challenges in the field of supramolecular-macrocycle-based COMs are discussed.

Self-Assembled Two-Dimensional Nanoporous Crystals as Molecular Sieves: Molecular Dynamics Studies of 1,3,5-Tristyrilbenzene-Cn Superstructures

Due to their unique geometry complex, self-assembled nanoporous 2D molecular crystals offer a broad landscape of potential applications, ranging from adsorption and catalysis to optoelectronics, substrate processes, and future nanomachine applications. Here we report and discuss the results of extensive all-atom Molecular Dynamics (MD) investigations of self-assembled organic monolayers (SAOM) of interdigitated 1,3,5-tristyrilbenzene (TSB) molecules terminated by alkoxy peripheral chains Cn containing n carbon atoms (TSB3,5-Cn) deposited onto highly ordered pyrolytic graphite (HOPG). In vacuo structural and electronic properties of the TSB3,5-Cn molecules were initially determined using ab initio second order Møller-Plesset (MP2) calculations. The MD simulations were then used to analyze the behavior of the self-assembled superlattices, including relaxed lattice geometry (in good agreement with experimental results) and stability at ambient temperatures. We show that the intermolecular disordering of the TSB3,5-Cn monolayers arises from competition between decreased rigidity of the alkoxy chains (loss of intramolecular order) and increased stabilization with increasing chain length (afforded by interdigitation). We show that the inclusion of guest organic molecules (e.g., benzene, pyrene, coronene, hexabenzocoronene) into the nanopores (voids formed by interdigitated alkoxy chains) of the TSB3,5-Cn superlattices stabilizes the superstructure, and we highlight the importance of alkoxy chain mobility and available pore space in the dynamics of the systems and their potential application in selective adsorption.

From Frustrated Packing to Tecton-Driven Porous Molecular Solids

Structurally divergent molecules containing bulky substituents tend to produce porous materials via frustrated packing. Two rigid tetrahedral cores, tetraphenylmethane and 1,3,5,7-tetraphenyladamantane, grafted peripherally with four (trimethylsilyl)ethynyl moieties, were found to have only isolated voids in their crystal structures. Hence, they were modified into tecton-like entities, tetrakis(4-(iodoethynyl)phenyl)methane [I4TEPM] and 1,3,5,7-tetrakis(4-(iodoethynyl)phenyl)adamantane [I4TEPA], in order to deliberately use the motif-forming characteristics of iodoethynyl units to enhance crystal porosity. I4TEPM not only holds increased free volume compared to its precursor, but also forms one-dimensional channels. Furthermore, it readily co-crystallizes with Lewis basic solvents to afford two-component porous crystals.

Aurophilicity-Mediated Construction of Emissive Porous Molecular Crystals as Versatile Hosts for Liquid and Solid Guests

The first examples of porous molecular crystals that are assembled through Au⋅⋅⋅Au interactions of gold complex 1 are here reported along with their exchange properties with respect to their guest components. Single-crystal X-ray diffraction (XRD) analyses indicate that the crystal structure of 1/CH₂ Cl₂ ⋅pentane is based on cyclic hexamers of 1, which are formed through six Au⋅⋅⋅Au interactions. The packing of these cyclic hexamers affords a porous architecture, in which the one-dimensional channel segment contains CH₂ Cl₂ and pentane as guests. These guests can be exchanged through operationally simple methods under retention of the host framework of 1, which furnished 1/guest complexes with 26 different guests. A single-crystal XRD analysis of 1/eicosane, which contains the long linear alkane eicosane (n-C₂₀ H₄₂ ), successfully provided its accurately modeled structure within the porous material. These host-guest complexes show chromic luminescence with both blue- and redshifted emissions. Moreover, this porous organometallic material can exhibit luminescent mechanochromism through release of guests.

Hydrogen-bonded porous frameworks constructed by rigid π-conjugated molecules with carboxy groups

This review covers construction and properties of porous molecular crystals (PMCs) constructed through hydrogen-bonding of C3-symmetric, rigid, π-conjugated molecular building blocks possessing carboxyaryl groups, which was reported in the last 5 years by the author’s group. PMCs with well-defined, self-standing pores have been attracted attention due to various functionalities provided by selective and reversible inclusion of certain chemical species into the pores. However, it has been recognized for long time that construction of PMCs with permanent porosity is not easy due to weakness of noncovalent intermolecular interactions. Systematic construction of PMCs have been limited so far. To overcome this problem, the author has proposed a unique molecular design concept based on C3-symmetric π-conjugated molecules (C3PIs) possessing o-bis(4-carboxyphenyl)benzene moieties in their periphery and demonstrated that C3PIs systematically yielded hydrogen-bonded organic frameworks (HOFs) composed of H-bonded 2D hexagonal networks (H-HexNets) or interpenetrated 3D pcu-networks, which exhibit permanent porosity, significant thermal stability, polar solvent durability, robustness/flexibility, and/or multifunctionality.

Hysteresis in the gas sorption isotherms of metal–organic cages accompanied by subtle changes in molecular packing

Cooperative gas uptake in metal–organic cages is tuned using supramolecular chemistry.

Computer modeling of 2D supramolecular nanoporous monolayers self-assembled on graphite

Nano-porous two-dimensional molecular crystals, self-assembled on atomically flat host surfaces offer a broad range of possible applications, from molecular electronics to future nano-machines. Computer-assisted designing of such complex structures requires numerically intensive modeling methods. Here we present the results of extensive, fully atomistic simulations of self-assembled monolayers of interdigitated molecules of 1,3,5-tristyrilbenzene substituted by C6 alkoxy peripheral chains (TSB3,5-C6), deposited onto highly-ordered pyrolytic graphite. Structural and electronic properties of the TSB3,5-C6 molecules were determined from ab initio calculations, then used in Molecular Dynamics simulations to analyze the mechanism of formation, epitaxy, and stability of the TSB3,5-C6 nanoporous superlattice. We show that the monolayer disordering results from the competition between flexibility of the C6 chains and their stabilization by interdigitation. The inclusion of guest molecules (benzene and pyrene) into superlattice nanopores stabilizes the monolayer. The alkoxy chain mobility and available pore space defines the systems dynamics, essential for potential application.

Unraveling the Sorption Mechanism of CO2 in a Molecular Crystal without Intrinsic Porosity

The facile uptake of CO₂ gas in a nonporous molecular crystal constituted by long molecules with carbazole and ethynylphenyl moieties was reported in experiments recently. Herein, the mechanism of gas uptake by this crystal is elucidated using atomistic molecular simulations. The uptake of CO₂ is shown to be facilitated by (i) the capacity of the crystal to expand in volume because of weak intermolecular interactions, (ii) the parallel orientation of the long molecules in the crystal, and (iii) the ability of the molecule to marginally bend, yet not lose crystallinity because of the anchoring of the terminal carbazole groups. The retention of crystallinity upon sorption and desorption cycles is also demonstrated. At high enough pressures, near-neighbor CO₂ molecules sorbed in the crystal are found to be oriented parallel to each other.

Highly stable fullerene-based porous molecular crystals with open metal sites

The synthesis of conventional porous crystals involves building a framework using reversible chemical bond formation, which can result in hydrolytic instability. In contrast, porous molecular crystals assemble using only weak intermolecular interactions, which generally do not provide the same environmental stability. Here, we report that the simple co-crystallization of a phthalocyanine derivative and a fullerene (C₆₀ or C₇₀) forms porous molecular crystals with environmental stability towards high temperature and hot aqueous base or acid. Moreover, by using diamond anvil cells and synchrotron single-crystal measurements, stability towards extreme pressure (>4 GPa) is demonstrated, with the stabilizing fullerene held between two phthalocyanines and the hold tightening at high pressure. Access to open metal centres within the porous molecular co-crystal is demonstrated by in situ crystallographic analysis of the chemisorption of pyridine, oxygen and carbon monoxide. This suggests strategies for the formation of highly stable and potentially functional porous materials using only weak van der Waals intermolecular interactions.

Soft Porous Crystal Based upon Organic Cages That Exhibit Guest-Induced Breathing and Selective Gas Separation

Soft porous crystals (SPCs) that exhibit stimuli-responsive dynamic sorption behavior are attracting interest for gas storage/separation applications. However, the design and synthesis of SPCs is challenging. Herein, we report a new type of SPC based on a [2 + 3] imide-based organic cage (NKPOC-1) and find that it exhibits guest-induced breathing behavior. Various gases were found to induce activated NKPOC-1 crystals to reversibly switch from a "closed" nonporous phase (α) to two porous "open" phases (β and γ). The net effect is gate-opening behavior induced by CO₂ and C3 hydrocarbons. Interestingly, NKPOC-1-α selectively adsorbs propyne over propylene and propane under ambient conditions. Thus, NKPOC-1-α has the potential to separate binary and ternary C3 hydrocarbon mixtures, and the performance was subsequently verified by fixed bed column breakthrough experiments. In addition, molecular dynamics calculations and in situ X-ray diffraction experiments indicate that the gate-opening effect is accompanied by reversible structural transformations. The adsorption energies from molecular dynamics simulations aid are consistent with the experimentally observed selective adsorption phenomena. The understanding gained from this study of NKPOC-1 supports the further development of SPCs for applications in gas separation/storage because SPCs do not inherently suffer from the recyclability problems often encountered with rigid materials.

Postsynthetic Metalation of a Robust Hydrogen-Bonded Organic Framework for Heterogeneous Catalysis

Hydrogen-bonded organic framework (HOF)-based catalysts still remain unreported thus far due to their relatively weak stability. In the present work, a robust porous HOF (HOF-19) with a Brunauer-Emmett-Teller surface area of 685 m² g^-1 was reticulated from a cagelike building block, amino-substituted bis(tetraoxacalix[2]arene[2]triazine), depending on the hydrogen bonding with the help of π-π interactions. The postsynthetic metalation of HOF-19 with palladium acetate afforded a palladium(II)-containing heterogeneous catalyst with porous hydrogen-bonded structure retained, which exhibits excellent catalytic performance for the Suzuki-Miyaura coupling reaction with the high isolation yields (96-98%), prominent stability, and good selectivity. More importantly, by simple recrystallization, the catalytic activity of deactivated species can be recovered from the isolation yield 46% to 92% for 4-bromobenzonitrile conversion at the same conditions, revealing the great application potentials of HOF-based catalysts.

Understanding the effect of host flexibility on the adsorption of CH4, CO2 and SF6 in porous organic cages

Molecular simulations for gas adsorption in microporous materials with flexible host structures is challenging and, hence, relatively rare. To date, most gas adsorption simulations have been carried out using the grand-canonical Monte Carlo (GCMC) method, which fundamentally does not allow the structural flexibility of the host to be accounted for. As a result, GCMC simulations preclude investigation into the effect of host flexibility on gas adsorption. On the other hand, approaches such as molecular dynamics (MD) that simulate the dynamic evolution of a system almost always require a fixed number of particles in the simulation box. Here we use a hybrid GCMC/MD scheme to include host flexibility in gas adsorption simulations. We study the adsorption of three gases – CH4, CO2 and SF6 – in the crystal of a porous organic cage (POC) molecule, CC3-R, whose structural flexibility is known by experiment to play an important role in adsorption of large guest molecules [L. Chen, P. S. Reiss, S. Y. Chong, D. Holden, K. E. Jelfs, T. Hasell, M. A. Little, A. Kewley, M. E. Briggs, A. Stephenson, K. Mark Thomas, J. A. Armstrong, J. Bell, J. Busto, R. Noel, J. Liu, D. M. Strachan, P. K. Thallapally, A. I. Cooper, Separation of rare gases and chiral molecules by selective binding in porous organic cages. Nat. Mater. 2014, 13, 954, D. Holden, S. Y. Chong, L. Chen, K. E. Jelfs, T. Hasell, A. I. Cooper, Understanding static, dynamic and cooperative porosity in molecular materials. Chem. Sci. 2016, 7, 4875]. The results suggest that hybrid GCMC/MD simulations can reproduce experimental adsorption results, without the need to adjust the host–guest interactions in an ad hoc way. Negligible errors in adsorption capacity and isosteric heat are observed with the rigid-host assumption for small gas molecules such as CH4 and CO2 in CC3-R, but the adsorption capacity of the larger SF6 molecule in CC3-R is hugely underestimated if flexibility is ignored. By contrast, hybrid GCMC/MD adsorption simulations of SF6 in CC3-R can accurately reproduce experiment. This work also provides a molecular level understanding of the cooperative adsorption mechanism of SF6 in the CC3-R molecular crystal.

A double helix of opposite charges to form channels with unique CO2 selectivity and dynamics

Porous molecular materials represent a new front in the endeavor to achieve high-performance sorptive properties and gas transport. Self-assembly of polyfunctional molecules containing multiple charges, namely, tetrahedral tetra-sulfonate anions and bifunctional linear cations, resulted in a permanently porous crystalline material exhibiting tailored sub-nanometer channels with double helices of electrostatic charges that governed the association and transport of CO2 molecules. The charged channels were consolidated by robust hydrogen bonds. Guest recognition by electrostatic interactions remind us of the role played by the dipolar helical channels in regulatory biological membranes. The systematic electrostatic sites provided the perfectly fitting loci of complementary charges in the channels that proved to be extremely selective with respect to N2 (S = 690), a benchmark in the field of porous molecular materials. The unique screwing dynamics of CO2 travelling along the ultramicropores with a step-wise reorientation mechanism was driven by specific host-guest interactions encountered along the helical track. The unusual dynamics with a single-file transport rate of more than 106 steps per second and an energy barrier for the jump to the next site as low as 2.9 kcal mol-1 was revealed unconventionally by complementing in situ 13C NMR anisotropic line-shape analysis with DFT modelling of CO2 diffusing in the crystal channels. The peculiar sorption performances and the extraordinary thermal stability up to 450 °C, combined with the ease of preparation and regeneration, highlight the perspective of applying these materials for selective removal of CO2 from other gases.

Enantioseparations by Gas Chromatography Using Porous Organic Cages as Stationary Phase

The resolution of chiral compounds into optically pure enantiomers is very important in various fields, such as pharmaceutical, chemical, agricultural, and food industries. Chiral gas chromatography (GC) is one of the efficient methods for enantioseparations of volatile compounds. In recent years, porous materials as stationary phases for chromatographic separations have achieved increasing attention. Porous organic cages (POCs) represent an emerging class of porous materials, which are assembled by discrete organic molecules with shape-persistent and permanent cavities through weak intermolecular forces. This chapter describes several chiral POCs as chiral stationary phases for GC enantioseparations of racemic compounds.

Eigencages: Learning a Latent Space of Porous Cage Molecules

Porous organic cage molecules harbor nanosized cavities that can selectively adsorb gas molecules, lending them applications in separations and sensing. The geometry of the cavity strongly influences their adsorptive selectivity. For comparing cages and predicting their adsorption properties, we embed/encode a set of 74 porous organic cage molecules into a low-dimensional, latent "cage space" on the basis of their intrinsic porosity. We first computationally scan each cage to generate a three-dimensional (3D) image of its porosity. Leveraging the singular value decomposition, in an unsupervised manner, we then learn across all cages an approximate, lower-dimensional subspace in which the 3D porosity images congregate. The "eigencages" are the set of orthogonal, characteristic 3D porosity images that span this lower-dimensional subspace, ordered in terms of importance. A latent representation/encoding of each cage follows by approximately expressing it as a combination of the eigencages. We show that the learned encoding captures salient features of the cavities of porous cages and is predictive of properties of the cages that arise from cavity shape. Our methods could be applied to learn latent representations of cavities within other classes of porous materials and of shapes of molecules in general.

Self-assembly of lattices with high structural complexity from a geometrically simple molecule

Here we report an anomalous porous molecular crystal built of C-H···N-bonded double-layered roof-floor components and wall components of a segregatively interdigitated architecture. This complicated porous structure consists of only one type of fully aromatic multijoint molecule carrying three identical dipyridylphenyl wedges. Despite its high symmetry, this molecule accomplishes difficult tasks by using two of its three wedges for roof-floor formation and using its other wedge for wall formation. Although a C-H···N bond is extremely labile, the porous crystal maintains its porosity until thermal breakdown of the C-H···N bonds at 202°C occurs, affording a nonporous polymorph. Though this nonporous crystal survives even at 325°C, it can retrieve the parent porosity under acetonitrile vapor. These findings show how one can translate simplicity into ultrahigh complexity.