Integrating Levels of Hierarchical Organization in Porous Organic Molecular Materials

Porous organic molecular materials(POMMs)are an emergent class of molecular-based materials characterized by the formation of extended porous frameworks,mainly held by non-covalent interactions.POMMs represent a variety of chemical families,such as hydrogen-bonded organic frameworks,porous organic salts,porous organic cages,C-H···πmicroporous crystals,supramolecular organic frameworks,π-organic frameworks,halogen-bonded organic framework,and intrinsically porous molecular materials.In some porous materials such as zeolites and metal organic frameworks,the integration of multiscale has been adopted to build materials with multifunctionality and optimized properties.Therefore,considering the significant role of hierarchy in porous materials and the growing importance of POMMs in the realm of synthetic porous materials,we consider it appropriate to dedicate for the first time a critical review covering both topics.Herein,we will provide a summary of literature examples showcasing hierarchical POMMs,with a focus on their main synthetic approaches,applications,and the advantages brought forth by introducing hierarchy.

Highly Stable sp^(2) Carbon-Conjugated Porous Organic Cages

The high-yield synthesis of ultrastable porous organic cages(POCs)by facile methods is highly desirable but challenging.Inspired by highly stable sp^(2) carbon-conjugated(C=C)covalent organic frameworks,we used rational design and synthesized the first family of sp^(2) carbon-linked POCs(sp^(2)c-POCs)with triangular prism shapes by a one-step Knoevenagel reaction that is a high yield[2+3]condensation reaction.The stability of sp^(2)c-POC was demonstrated under strenuous conditions involving for example concentrated HCl or saturated NaOH solution.sp^(2)c-POC was used as a robust adsorbent for efficient separation of CO_(2)/CH_(4) or CO_(2)/N_(2) mixtures.In view of their facile synthesis and unique properties,we expect widespread use of these sp^(2) C=C linkages in the development of POC materials.

Eco-Friendly Encapsulation of Metal Clusters in Porous Organic Cages for Engineerable Microenvironment and Enhanced Catalysis

Developing artificial catalysts that mimic the functionality of enzymes and adapt to the surrounding microenvironment to achieve specific activity and selectivity is a fascinating research area yet remains a great challenge.In this work,we present a meticulously designed strategy for the successful encapsulation of ultrasmall metal clusters(MCs)within an amine-type porous organic cage(POC)through electrostatic complexation,phase transfer,and alcohol reduction processes.The amine cage showcases an intriguing and customizable feature that allows for the regulation of the surrounding microenvironment of the confined MCs through a feasible postmodifi-cation approach.This functionalization of cage skeleton further facilitates precise adjustment to the surface electronic state of Pd cluster,thereby influencing the adsorption behavior of substrate.Consequently,this controlled regulation leads to modified activity and chemoselectivity in the catalytic hydrogenation of halogenated nitrobenzene.Importantly,the investigation of the correlation between the surrounding microenvironment,substrate adsorption,and catalytic performance in the POC-immobilized MCs system has not been previously reported.We anticipate that our research will provide valuable insights in this field.

Porous organic cages as a novel platform for second harmonic generation

Second harmonic generation(SHG) is one of the most extensively applied nonlinear optical phenomena. Compared to traditional inorganic SHG materials, organic SHG materials, including non-centrosymmetric small molecular crystals and organic polymers, have been extensively explored for SHG optics due to their advantageous properties, including high light-induced damage threshold, ultrafast response speed, high polarization ratio, wide transparency range, and elevated dielectric constant. However,in organic SHG materials, most small molecular crystals often show poor thermal stability(<200 ℃) due to their low molecular weight, which is an important obstacle for practical application under high temperatures. On the other hand, the difficulty in the growth of large-size crystals as well as the disorderly array of chromospheres on polymer chains associated with organic polymers have hindered their widespread application in SHG due to the requirements of processability and homogeneity.Therefore, it is significantly necessary to develop novel organic SHG material with high thermal stability, large-size crystals,excellent processability, high homogeneity, and a well SHG effect. In the contribution, we developed herein porous organic cages(POCs) as a novel platform for SHG optics. Thanks to its inherent macromolecular composition and zero-dimensional discrete structures, POCs can not only solve the instability problem of small molecule crystals but also overcome the drawbacks in growing large-size crystals for organic polymer. The obtained two POCs with chiral asymmetric centers(CPOC-R-5 and CPOCS-7) display advantageous features for SHG optics including well SHG performance, large-size crystal(5 mm×3 mm×1 mm),high thermal stability(≥250 ℃), wide transparency window(750–2,000 nm) and high polarization ratio(up to 94.5%). This work thus develops POCs as a new platform toward the construction of organic NLO materials with the potential application in sorts of SHG optics.

Robust Giant Octahedral[6+8]Porous Organic Cages for Efficient Ethylene/Ethane Separation

The exploration of ethane(C_(2)H_(6))-selective porous materials for the direct production of polymer-grade ethylene(C_(2)H_(4))from a C_(2)H_(6)/C_(2)H_(4) mixture in a single energy-saving adsorption step is of utmost importance but remains a significant challenge.Thus,developing robust C_(2)H_(6)-selective adsorbents with high C_(2)H_(6) capacity and C_(2)H_(6)/C_(2)H_(4) selectivity is urgently needed for industrial applications.In this study,we have successfully designed and synthesized two novel calix[4]resorcinarene-based porous organic cages(POCs)named CPOC-501 and CPOC-502.The POCs were formed via a Schiff-base reaction involving face-directed[6+8]condensation between a bowlshaped tetratopic tetraformylcalix[4]resorcinarene and triangular tritopic amine synthons.Analysis using single crystal X-ray crystallography revealed that both cages possess large truncated octahedral cavities with a volume of approximately 6500Å3 and 12 accessible rhombic windows with a side length of approximately 10.5Å.Furthermore,the cages exhibited excellent chemical stability under neutral,acidic,and basic conditions and high Brunauer–Emmett–Teller specific surface areas of up to 2175 m^(2) g^(−1) after desolvation.Both POCs demonstrated superior adsorption capabilities for C_(2)H_(6) over C_(2)H_(4).Notably,CPOC-502 exhibited a C_(2)H_(6) capacity and C_(2)H_(6)/C_(2)H_(4) selectivity of 83 cm^(3) g^(−1) and 2.83,respectively,surpassing most of the best-performing C_(2)H_(6)-selective porous organic materials reported to date.Moreover,breakthrough experiments confirmed that both cages efficiently produced polymer-grade C_(2)H_(4)(>99.9%)directly from the C_(2)H_(6)/C_(2)H_(4) mixture,highlighting their outstanding recyclability.

Enhancing the activity,selectivity,and recyclability of Rh/PPh_(3) system‐catalyzed hydroformylation reactions through the development of a PPh_(3)‐derived quasi‐porous organic cage as a ligand

In contrast to heterogeneous network frameworks(e.g.,covalent organic frameworks and metal‐organic frameworks)and porous organic polymers,porous organic cages(POCs)are soluble molecules in common organic solvents that provide significant potential for homogeneous catalysis.Herein,we report a triphenylphosphine‐derived quasi‐porous organic cage(denoted as POC‐DICP)as an efficient organic molecular cage ligand for Rh/PPh_(3) system‐catalyzed homogeneous hydroformylation reactions.POC‐DICP not only displays enhanced hydroformylation selectivity(aldehyde selectivity as high as 97%and a linear‐to‐branch ratio as high as 1.89)but can also be recovered and reused via a simple precipitation method in homogeneous reaction systems.We speculate that the reason for the high activity and good selectivity is the favorable geometry(cone angle=123.88°)and electronic effect(P site is relatively electron‐deficient)of POC‐DICP,which were also demonstrated by density functional theory calculations and X‐ray absorption fine‐structure characterization.

Multiscale design and biomechanical evaluation of porous spinal fusion cage to realize specified mechanical properties

Backgrou nd Dense titanium(Ti)fusion cages have been commonly used in transforaminal lumbar interbody fusion.However,the stiffness mismatch between cages and adjacent bone endplates increases the risk of stress shielding and cage subsidence.Methods The current study presents a multiscale optimization approach for porous Ti fusion cage development,including microscale topology optimization based on homogenization theory that obtains a unit cell with prescribed mechanical properties,and macroscale topology optimization that determines the layout of framework structure over the porous cage while maintaining the desired stiffness.The biomechanical performance of the designed porous cage is assessed using numerical simulations of fusion surgery.Selective laser melting is employed to assists with fabricating the designed porous structure and porous cage.Results The simulations demonstrate that the designed porous cage increases the strain energy density of bone grafts and decreases the peak stress on bone endplates.The mechanical and morphological discrepancies between the as-designed and fabricated porous structures are also described.Conclusion From the perspective of biomechanics,it is demonstrated that the designed porous cage contributes to reducing the risk of stress shielding and cage subsidence.The optimization of processing parameters and post-treatments are required to fabricate the designed porous cage.The present multiscale optimization approach can be extended to the development of cages with other shapes or materials and further types of orthopedic implants.

Assembling ionic liquid into porous molecular filler of mixed matrix membrane to trigger high gas permeability,selectivity,and stability for CO_(2)/CH_(4) separation

As an emerging zero-dimensional nano crystalline porous material,porous organic cages(POCs)with soluble properties in organic solvents,are promising candidates as molecular fillers in mixed matrix membranes(MMMs).The pore structure of POCs should be adjusted to trigger efficient gas separation performance,and the interaction between filler and matrix should be optimized.In this work,ionic liquid(IL)was introduced into the molecular fillers of CC3,to construct the IL@CC3/PIM-1 membrane to effectively separate CO_(2) from CH_(4).The advantages of doping IL include:(1)narrowing the cavity size of POCs from 4.4 to 3.9Åto enhance the diffusion selectivity,(2)strengthening the CO_(2) solubility to heighten the gas permeability,and(3)improving the compatibility between filler and matrix to upgrade membrane stability.After the optimization of the membrane composite,the IL@CC3/PIM-1-10%membrane possesses the CO_(2) permeability of 7868 Barrer and the CO_(2)/CH_(4) selectivity of 73.4,which compared to the CC3/PIM-1-10%membrane,improved by 15.9%and 106.2%,respectively.Furthermore,the membrane has maintained a stable separation performance at varied temperatures and pressures during the long-term test.The proposed method offers an efficient way to improve the performance of POCs-based MMMs in gas separation.

A tetraaldehyde-derived porous organic cage and covalent organic frameworks:Syntheses,structures,and iodine vapor capture

Dynamic covalent imine reactions between 2,3-dimethoxy-[1,1:4,1-terphenyl]-3,3,5,5-tetracarbaldehyde(DMTT)and cyclohexanediamine,p-phenylenediamine,and benzidine,respectively,generate a porous organic cage(DMPOC)and two covalent organic frameworks(COFs),USTB-29,and USTB-30.DMPOC shows a[3+6]topological cage-like structure according to single crystal X-ray diffraction result.In contrast,both microcrystalline USTB-29 and USTB-30 exhibit two-dimensional monoporous structures in an eclipsed AA stacking style based on powder X-ray diffraction and theoretical simulations.In addition,DMPOC is capable of efficiently absorbing the iodine vapor with an outstanding uptake of 5.10 g/g,much higher than that of USTB-29(3.07 g/g)and USTB-30(3.16 g/g).Cage to COFs transformations have been realized from DMPOC to USTB-29 and USTB-30 via the imine bond exchange with slightly increased iodine vapor uptake.Mechanism investigations uncover that both nitrogen and oxygen atoms of POC and COFs contribute to iodine vapor capture due to the formation of charge transfer matter,and loose interaction introducing adaptive expanding voids of DMPOC is suggested to capture more iodine vapor than that of COFs with strongπ-πinteractions.

Phosphine-Built-In Porous Organic Cage Supported Ultrafine Pd Nanoclusters Enable Highly Efficient and Regioselective Hydrogenation of Epoxides

Selective hydrogenation of epoxides has been regarded as an atom-economical and straightforward method for the synthesis of alcohols.However,it remains a big challenge in the precise control of regioselectivity.To date,the reaction enabled by a reusable and high-performance heterogeneous catalyst with excellent regioselectivity is very scarce.Herein,we develop highly loaded and ultrafine Pd nanoclusters(NCs)encapsulated on a phosphinebuilt-in porous organic cage(FPPOC)for the catalytic hydrogenation of epoxides by molecular hydrogen.Benefiting from the unique characters of uniform dispersion and strong interaction between Pd NCs and FPPOC,the resultant Pd NCs exhibited superior catalytic activity and excellent regioselectivity for the hydrogenation of epoxides under milder conditions.A diverse set of terminal and internal epoxides was efficiently reduced to the corresponding linear or branched alcohols in an extremely regioselective manner,well tolerating diverse functional groups.Remarkably,the catalyst demonstrates high stability and could be reused up to 10 times with marginal decay in activity and regioselectivity.Furthermore,the catalyst is applicable for scale-up synthesis with a record turnover number as high as 16,111,to the best of our knowledge,outperforming those previous state-of-the-art catalysts.Control experiments and characterizations in combination with density functional theory calculations provide insight into the superior activity and excellent regioselectivity.

Porous Organic Cage‑Based Quasi‑Solid‑State Electrolyte with Cavity‑Induced Anion‑Trapping Effect for Long‑Life Lithium Metal Batteries

Porous organic cages(POCs)with permanent porosity and excellent host–guest property hold great potentials in regulating ion transport behavior,yet their feasibility as solid-state electrolytes has never been testified in a practical battery.Herein,we design and fabricate a quasi-solid-state electrolyte(QSSE)based on a POC to enable the stable operation of Li-metal batteries(LMBs).Benefiting from the ordered channels and cavity-induced anion-trapping effect of POC,the resulting POC-based QSSE exhibits a high Li+transference number of 0.67 and a high ionic conductivity of 1.25×10^(−4) S cm^(−1) with a low activation energy of 0.17 eV.These allow for homogeneous Li deposition and highly reversible Li plating/stripping for over 2000 h.As a proof of concept,the LMB assembled with POC-based QSSE demonstrates extremely stable cycling performance with 85%capacity retention after 1000 cycles.Therefore,our work demonstrates the practical applicability of POC as SSEs for LMBs and could be extended to other energy-storage systems,such as Na and K batteries.

Dimeric Calix[4]resorcinarene-based Porous Organic Cages for CO_(2)/CH_(4) Separation

Investigating gas separation by emerging porous organic cage(POC) solids is still on its initial stage. In this work, two novel [2+4] organic cages with distinguished structures have been prepared based on the Schiff-based condensation reaction between tetraformyl-functionalized calix[4]resorcinarene building blocks and xylylenediamine(XDA) isomers. Specifically, the use of para-position XDA affords lantern-shaped cage(CPOC-105) with a medium cavity of ca. 0.526 nm^(3), while the meta-position produces peanut-shaped structure(CPOC-106) with two small cavities of ca. 0.181 nm^(3). Both CPOC-105 and CPOC-106 exhibit high selectivity capture of CO_(2) over CH4 with calculated selectivity coefficients of 4.5 and 3.1, respectively, under ambient conditions, and are capable of separating CO_(2)/CH_(4) mixtures by fixed-bed column breakthrough experiments.

Lactonization of dyes promoted by a coordination cage facilitates cavity-induced chromism as ion mobile detector

Dye molecules often change colors(so-called “chromism”) according to the environment variation. However, they are rarely induced by a catalytic amount of cavity. Through encapsulation in the cavity of a porous coordination cage(PCC-2), Rhodamine B(1) is transformed from red quinonoid form(1q) into colorless lactone form(1l) in aprotic polar solutions. The μ4-OH groups in the cavity of PCC-2 are shown to stabilize the uncommon zwitterion intermediate(1z), followed by converting to 1l, thus accelerating the equilibrium. The chromism is catalyzed by 0.25 mol% of PCC-2, and the reaction rate is improved by 80,400times. 1@PCC-2 can be further fabricated to a sol-gel that exhibits ion recognition properties. The resulting encapsulation and stabilization of an unconventional intermediate by a catalytic amount of the coordination cage provides fundamental insights into molecular isomerization and has potential use in chemical sensing.

Porous Pyrene Organic Cage with Unusual Absorption Bathochromic-Shift Enables Visible Light Photocatalysis

We have constructed a novel porous pyrene-based organic cage,PyTC1,through the condensation reaction of cyclohexanediamine with 5,5′-(pyrene-1,6-diyl)diisophthalaldehyde.Single-crystal X-ray diffraction analysis reveals the effective intercage C–H...πinteraction between cyclohexanediimine and pyrene segments.Such a soft intercage C–H...πinteraction,rather than a classic J-aggregate with slippedπ–π-stacking configuration,induced an unusual bathochromic shift of pyrene-based chromophore absorption from an ultraviolet region of PyTC1 in solution to the visible light region of PyTC1 in solid-state.

Post-synthetic metalation of organic cage for enhanced porosity and catalytic performance

Porous organic cages(POCs)have shown great potential in many applications,and post-synthetic modification(PSM)has been confirmed to be an effective strategy to tailor their structures and related functionalities.However,it is extremely challenging to develop a general platform for simple-to-make functional POCs for advanced applications by PSM method.Herein,we reported that octahedral calix[4]resorcinarene-based hydrazone-linked porous organic cage(HPOC-401)provides an excellent platform for post-synthetic metalation by various transition metal(TM)ions under mild conditions due to the abundance of coordination sites in its skeleton.Such metalated products(HPOC-401-TM)exhibit Brunauer-Emmett-Teller(BET)surface area up to1,456 m^(2)g^(-1),much higher than that of the pristine HPOC-401,which has a BET value of 474 m^(2)g^(-1).Moreover,the metalation and porosity increases further influence their gas capture,separation,as well as catalytic performance.For instance,HPOC-401-TM products exhibit higher CO_(2),H_(2),and C2 hydrocarbon gas uptake,as well as higher C_(2)H_(6)/C_(2)H_(4) selectivity than HPOC-401.Moreover,the HPOC-401-TM also shows better catalytic performance in the cycloaddition of CO_(2) with epoxides compared to HPOC-401.These findings uncover a simple yet effective approach for modifying the porosity characteristics of organic cages,which will undoubtedly expand their future implementations.

An Allochroic Molecular Cage Switch for Sensing and Capturing Organic Pollutants

The adsorption method is considered to be one of the most promising organic pollutants emission reduction strategies.The design and synthesis of high-performance porous adsorbents are one of the most important but challenging works.In this work,we constructed a new class of porous molecular cage switches by a simple reaction using phenolphthalein as the raw material.The molecular cage switches displayed interesting on-off behavior towards organic guests,which is highly responding to organic pollutants with rapid color change and is also able to adsorb these organic pollutants through an open-to-close pathway.This molecular cage switch also has excellent regenerative cycling properties and water resistance,which is expected to be employed in the handling of organic pollutants in the future.