### Recent Submissions

• #### Optimizing Host–Guest Selectivity for Ethylbenzene Capture Toward Superior Styrene Purification

(Chemistry of Materials, American Chemical Society (ACS), 2021-12-08) [Article]
The separation of ethylbenzene (EB) and styrene (ST) mixtures to obtain pure ST has been an enduring challenge for the petrochemical industry. So far, adsorptive separation using porous materials has mainly focused on capturing ST rather than EB, where high temperatures are needed to reactivate the sieving materials and collect the product. Here, we tuned the host–guest interactions in thienothiophene-based trianglimine (ThT-TI) macrocycles to selectively adsorb the unreacted EB over ST, after a dehydrogenation reaction, to readily provide pure ST without the need for further thermal treatments. This is the first report on the selective adsorptive separation of EB over ST using macrocycles as molecular hosts. Both crystalline and amorphous ThT-TI can be used to separate EB with 96% uptake capacity. Single-crystal and powder X-ray diffraction patterns suggest that this selective adsorption arises from a guest-induced structural reordering and involvement of the sulfur atoms in host/guest C–H···π interactions. We believe that this work paves the way for a new generation of molecular sieves that are designed to afford high-purity products by in situ capturing of the unreacted starting materials.
• #### Molecular engineering of intrinsically microporous polybenzimidazole for energy-efficient gas separation

(Applied Materials Today, Elsevier BV, 2021-12-04) [Article]
Polybenzimidazole (PBI) is a high-performance polymer that exhibits high thermal and chemical stability. However, it suffers from low porosity and low fractional free volume, which hinder its application as separation material. Herein, we demonstrate the molecular engineering of gas separation materials by manipulating a PBI backbone possessing kinked moieties. PBI was selected as it contains NH groups which increase the affinity towards CO$_2$, increase sorption capacity, and favors CO$_2$ over other gasses. We have designed and synthesized an intrinsically microporous polybenzimidazole (iPBI) featuring a spirobisindane structure. Introducing a kinked moiety in conjunction with crosslinking enhanced the polymer properties, markedly increasing the gas separation performance. In particular, the BET surface area of PBI increased 30-fold by replacing a flat benzene ring with a kinked structure. iPBI displayed a good CO$_2$ uptake of 1.4 mmol g$^{−1}$ at 1 bar and 3.6 mmol g$^{−1}$ at 10 bar. Gas sorption uptake and breakthrough experiments were conducted using mixtures of CO$_2$/CH$_4$ (50%/50%) and CO$_2$/N$_2$ (50%/50%), which revealed the high selectivity of CO$_2$ over both CH$_4$ and N$_2$. The obtained CO$_2$/N$_2$ selectivity is attractive for power plant flue gas application requiring CO$_2$ capturing materials. Energy and process simulations of biogas CO$_2$ removal demonstrated that up to 70% of the capture energy could be saved when iPBI was used rather than the current amine technology (methyl diethanolamine [MDEA]). Similarly, the combination of iPBI and MDEA in a hybrid system exhibited the highest CO$_2$ capture yield (99%), resulting in nearly 50% energy saving. The concept of enhancing the porosity of PBI using kinked moieties provides new scope for designing highly porous polybenzimidazoles for various separation processes.
• #### Nearly 100% energy transfer at the interface of metal-organic frameworks for X-ray imaging scintillators

(Matter, Elsevier BV, 2021-12) [Article]
In this work, we describe a highly efficient and reabsorption-free X-ray-harvesting system using luminescent metal-organic framework (MOF)-fluorescence chromophore composite films. The ultrafast time-resolved experiments and density functional theory calculations demonstrate that a nearly 100% energy transfer from a luminescent MOF with a high atomic number to an organic chromophore with thermally activated delayed fluorescence (TADF) character can be achieved. Such an unprecedented efficiency of interfacial energy transfer and the direct harnessing of singlet and triplet excitons of the TADF chromophore led to remarkable enhancement of radioluminescence upon X-ray radiation. A low detection limit of 256 nGy/s of the fabricated X-ray imaging scintillator was achieved, about 60 times lower than the MOF and 7 times lower than the organic chromophore counterparts. More importantly, this detection limit is about 22 times lower than the standard dosage for a medical examination, making it an excellent candidate for X-ray radiography.
• #### Ultrafast Aggregation-Induced Tunable Emission Enhancement in a Benzothiadiazole-Based Fluorescent Metal–Organic Framework Linker

(The Journal of Physical Chemistry B, American Chemical Society (ACS), 2021-11-30) [Article]
Aggregation-induced emission enhancement (AIEE) is a process recently exploited in solid-state materials and organic luminophores, and it is explained by tight-molecular packaging. However, solution-phase AIEE and its formation mechanism have not been widely explored. This work investigated AIEE phenomena in two donor–acceptor–donor-type benzodiazole-based molecules (the organic building block in metal–organic frameworks) with an acetylene and phenyl π-conjugated backbone tapered with a carboxylic acid group at either end. This was done using time-resolved electronic and vibrational spectroscopy in conjunction with time-dependent density functional theory (TD-DFT) calculations. Fluorescence up-conversion spectroscopy and time-correlated single-photon counting conclusively showed an intramolecular charge transfer-driven aggregate emission enhancement. This is shown by a red spectral shift of the emission spectra as well as an increase in the fluorescence lifetime from 746 ps at 1.0 × 10–11 to 2.48 ns at 2.0 × 10–3 M. The TD-DFT calculations showed that a restricted intramolecular rotation mechanism is responsible for the enhanced emission. The femtosecond infrared (IR) transient absorption results directly revealed the structural dynamics of aggregate formation, as evident from the evolution of the C≡C vibrational marker mode of the acetylene unit upon photoexcitation. Moreover, the IR data clearly indicated that the aggregation process occurred over a time scale of 10 ps, which is consistent with the fluorescence up-conversion results. Interestingly, time-resolved results and DFT calculations clearly demonstrated that both acetylene bonds and the sulfur atom are the key requirements to achieve such a controllable aggregation-induced fluorescence enhancement. The finding of the work not only shows how slight changes in the chemical structure of fluorescent chromophores could make a tremendous change in their optical behavior but also prompts a surge of research into a profound understanding of the mechanistic origins of this phenomenon. This may lead to the discovery of new chemical strategies that aim to synthesize novel chromophores with excellent optical properties for light-harvesting applications.
• #### Perovskite-Nanosheet Sensitizer for Highly Efficient Organic X-ray Imaging Scintillator

(ACS Energy Letters, American Chemical Society (ACS), 2021-11-27) [Article]
The weak X-ray capture capability of organic scintillators always leads to poor imaging resolution and detection sensitivity. Here, we realize an efficient and reabsorption-free organic scintillator at the interface of perovskite nanosheets using a very efficient energy transfer strategy. Our steady-state and ultrafast time-resolved experiments supported by density functional theory calculations demonstrate that an efficient interfacial energy transfer from the perovskite nanosheet to the organic chromophore with thermally activated delayed fluorescence (TADF) character can be achieved. Interestingly, we found that the direct harnessing of both singlet and triplet excitons of the TADF chromophores also contributed greatly to its remarkably enhanced radioluminescence intensity and X-ray sensitivity. A high X-ray imaging resolution of 135 μm and a low detection limit of 38.7 nGy/s were achieved in the fabricated X-ray imaging scintillator.
• #### Toward Liquid Phase Processable Metal Organic Frameworks: Dream or Reality?

(Accounts of Materials Research, American Chemical Society (ACS), 2021-11-11) [Article]
• #### High-Capacity NH4+ Charge Storage in Covalent Organic Frameworks

(Journal of the American Chemical Society, American Chemical Society (ACS), 2021-11-05) [Article]
Ammonium ions (NH4+), as non-metallic charge carriers, have spurred great research interest in the realm of aqueous batteries. Unfortunately, most inorganic host materials used in these batteries are still limited by the sluggish diffusion kinetics. Here, we report a unique hydrogen bond chemistry to employ covalent organic frameworks (COFs) for NH4+ ion storage, which achieves a high capacity of 220.4 mAh g–1 at a current density of 0.5 A g–1. Combining the theoretical simulation and materials analysis, a universal mechanism for the reaction of nitrogen and oxygen bridged by hydrogen bonds is revealed. In addition, we explain the solvation behavior of NH4+, leading to a relationship between redox potential and desolvation energy barrier. This work provides a new insight into NH4+ ion storage in host materials based on hydrogen bond chemistry. This mechanism can be leveraged to design and develop COFs for electrochemical energy storage.
• #### Metal-organic frameworks characterization via inverse pulse gas chromatography

(Applied Sciences, MDPI AG, 2021-11-01) [Article]
The desire to customize the properties of a material through complete control over both its chemical and architectural structure has created a constant and persistent need for efficient and convenient characterization techniques. Inverse gas chromatography (IGC) is considered a useful characterization method for probing the material’s surface properties, like its enthalpies of adsorption, which are the key stimulus components for their adsorption performance. Here, we conclusively review the significance of a less common application of the IGC technique for the physicochemical characterization of metal-organic frameworks (MOFs), which are an innovative subclass of porous materials with matchless properties in terms of structure design and properties. This review focuses on the fundamental theory and instrumentation of IGC as well as its most significant applications in the field of MOF characterization to shed more light on this unique technique.
• #### Electrochemical synthesis of continuous metal–organic framework membranes for separation of hydrocarbons

(Nature Energy, Springer Science and Business Media LLC, 2021-08-09) [Article]
Membrane-based approaches can offer energy-efficient and cost-effective methods for various separation processes. Practical membranes must have high permselectivity at industrially relevant high pressures and under aggressive conditions, and be manufacturable in a scalable and robust fashion. We report a versatile electrochemical directed-assembly strategy to fabricate polycrystalline metal–organic framework membranes for separation of hydrocarbons. We fabricate a series of face-centred cubic metal–organic framework membranes based on 12-connected rare-earth or zirconium hexanuclear clusters with distinct ligands. In particular, the resultant fumarate-based membranes containing contracted triangular apertures as sole entrances to the pore system enable molecular-sieving separation of propylene/propane and butane/isobutane mixtures. Prominently, increasing the feed pressure to the industrially practical value of 7 atm promoted a desired enhancement in both the total flux and separation selectivity. Process design analysis demonstrates that, for propylene/propane separation, the deployment of such face-centred cubic Zr-fumarate-based metal–organic framework membranes in a hybrid membrane–distillation system offers the potential to decrease the energy input by nearly 90% relative to a conventional single distillation process.
• #### Molecular Engineering of Covalent Organic Framework Cathodes for Enhanced Zinc-Ion Batteries

Covalent organic frameworks (COFs) are potentially promising electrode materials for electrochemical charge storage applications thanks to their pre-designable reticular chemistry with atomic precision, allowing precise control of pore size, redox-active functional moieties, and stable covalent frameworks. However, studies on the mechanistic and practical aspects of their zinc-ion storage behavior are still limited. In this study, a strategy to enhance the electrochemical performance of COF cathodes in zinc-ion batteries (ZIBs) by introducing the quinone group into 1,4,5,8,9,12-hexaazatriphenylene-based COFs is reported. Electrochemical characterization demonstrates that the introduction of the quinone groups in the COF significantly pushes up the Zn2+ storage capability against H+ and elevates the average (dis-)charge potential in aqueous ZIBs. Computational and experimental analysis further reveals the favorable redox-active sites that host Zn2+/H+ in COF electrodes and the root cause for the enhanced electrochemical performance. This work demonstrates that molecular engineering of the COF structure is an effective approach to achieve practical charge storage performance.
• #### 25 years of Reticular Chemistry

(Angewandte Chemie, Wiley, 2021-07-09) [Article]
At its core, reticular chemistry has translated the precision and expertise of organic and inorganic synthesis to the solid state. While initial excitement over metal-organic frameworks (MOFs) and covalent organic frameworks (COFs) was undoubtedly fueled by their unprecedented porosity and surface areas, the most profound scientific innovation of the field has been the elaboration of design strategies for the synthesis of extended crystalline solids through strong directional bonds. In this contribution we highlight the different classes of reticular materials that have been developed, how these frameworks can be functionalized and how complexity can be introduced into their backbones. Finally, we show how the structural control over these materials is being extended from the molecular scale to their crystal morphology and shape on the nanoscale, all the way to their shaping on the bulk scale.
• #### The Importance of Highly Connected Building Units in Reticular Chemistry: Thoughtful Design of Metal–Organic Frameworks

(Accounts of Chemical Research, American Chemical Society (ACS), 2021-07-06) [Article]
The prediction of crystal structures assembled in three dimensions has been considered for a long time, simultaneously as a chemical wasteland and a certain growth point of the chemistry of the future. Less than 30 years after Roald Hoffmann’s statement, we can categorically affirm that the elevation of reticular chemistry and the introduction of metal−organic frameworks (MOFs) significantly tackled this tridimensional assembly issue. MOFs result from the assembly of organic polytopic organic ligands bridging metal nodes, clusters, chains, or layers together into mostly three-periodic open frameworks. They can exhibit extremely high porosity and offer great potential as revolutionary catalysts, drug carrier systems, sensors, smart materials, and, of course, separation agents. Overall, the progressive development of reticular chemistry has been a game changer in materials chemistry during the last 25 years.Such diverse properties often result not only from the selected organic and inorganic molecular building blocks (MBBs) but also from their distribution within the framework. Indeed, the size and shape of the porous system, as well as the location of active sites influence the overall properties. Therefore, in the continuity of achieving the crystallization of three-periodic structures, chemists and crystal engineers faced the next challenge, as summarized by John Maddox: “it remains in general impossible to predict the structure of even the simplest crystallographic solids from knowledge of their chemical composition”. This is where rational design takes place. In this Account, we detail three specific approaches developed by our group to facilitate the design and assembly of finely tuned MOFs. All are based on careful geometrical consideration and a deep study and understanding of the existing nets and topologies. We recognized that highly connected nets, if possible, edge-transitive, are ideal blueprints because their number is limited in contrast to nets with lower connectivity. Therefore, we embarked on taking advantage of existing highly connected MBBs, or, in parallel, promoting their formation to meet our requirements. This is achieved by utilizing externally decorated metal−organic polyhedra as supermolecular building blocks (SBBs), serving as a net-coding building unit, comprising the requisite connectivity and directional information coding for the chosen nets. The SBB approach allowed the synthesis of several families of SBB-based MOFs, including fcu, rht, and gea-MOFs, that are detailed here. The second strategy is directly inherited from the success of the SBB approach. In seeking highly connected building units, our group naturally expanded its research focus to nets that can be deconstructed into layers, pillared in various ways. In the supermolecular building layer (SBL) approach, the layers have an almost infinite connectivity, and the framework backbone is fixed in two dimensions while the third is free for pillar expansion and functionalization. The cases of trigonal pillaring leading to rtl, eea, and apo MOFs as well as the quadrangular pillaring leading to a family of tbo-MOFs are discussed here, along with recent cases of highly connected pillars in pek and aea-MOFs. Finally, our experience with highly coordinated MBBs led us to develop a novel way to use them as secondary building units of lower connectivity and unlock the possibility of assembling a novel class of zeolite-like MOFs (ZMOFs). The case of the Zr-sod-ZMOFs designed through a cantellation strategy is described as a future leading direction of MOF design.
• #### Insights into the Enhancement of MOF/Polymer Adhesion in Mixed-Matrix Membranes via Polymer Functionalization

(ACS Applied Materials & Interfaces, American Chemical Society (ACS), 2021-06-09) [Article]
MOF-based mixed-matrix membranes (MMMs) prepared using standard routes often exhibit poor adhesion between polymers and MOFs. Herein, we report an unprecedented systematic exploration on polymer functionalization as the key to achieving defect-free MMMs. As a case study, we explored computationally MMMs based on the combination of the prototypical UiO-66(Zr) MOF with polymer of intrinsic porosity-1 (PIM-1) functionalized with various groups including amidoxime, tetrazole, and N-((2-ethanolamino)ethyl)carboxamide. Distinctly, the amidoxime-derivative PIM-1/UiO-66(Zr) MMM was predicted to express the desired enhanced MOF/polymer interfacial interactions and thus subsequently prepared and evaluated experimentally. Prominently, high-resolution transmission electron microscopy confirmed optimal adhesion between the two components in contrast to the nanometer-sized voids/defects shown by the pristine PIM-1/UiO-66(Zr) MMM. Notably, single-gas permeation measurements further corroborated the need of optimal MOF/polymer adhesion in order to effectively enable the MOF to play a role in the gas transport of the resulting MMM.
• #### Operando Elucidation on the Working State of Immobilized Fluorinated Iron Porphyrin for Selective Aqueous Electroreduction of CO2 to CO

(ACS Catalysis, American Chemical Society (ACS), 2021-05-19) [Article]
Iron porphyrin-based molecular catalysts can electrocatalyze CO2 reduction to CO at nearly 100% selectivity in water. Nevertheless, the associated active sites and reaction mechanisms remain debatable, impeding the establishment of design guidelines for effective catalysts. This study reports coupling in operando experiments and theoretical calculations for immobilized 5,10,15,20-tetrakis(pentafluorophenyl) porphyrin Fe(III) chloride (FeF20TPP) for electrocatalytic CO2 reduction in an aqueous phase. In operando UV–vis and X-ray absorption near-edge structure spectra indicated the persisting presence of Fe(II) species during the cathodic reaction, acting as catalytic sites that accommodate CO as Fe(II)–CO adducts. Consistently, the density functional calculations pointed out that the ligand-reduced state with oxidized Fe, namely, [Fe(II)F20(TPP•)]−, prevails in the catalytic cycle prior to the rate-controlling step. This work provides the conclusive representation related to the working states of Fe-based molecular catalysts under reaction conditions.
• #### Directional Exciton Migration in Benzoimidazole-Based Metal–Organic Frameworks

(The Journal of Physical Chemistry Letters, American Chemical Society (ACS), 2021-05-19) [Article]
Highly luminescent metal-organic frameworks (MOFs) have recently received great attention due to their potential applications as sensors and light-emitting devices. In these MOFs, the highly ordered fluorescent organic linkers positioning prevents excited-state self-quenching and rotational motion, enhancing their light-harvesting properties. Here, the exciton migration between the organic linkers with the same chemical structure but different protonation degrees in Zr-based MOFs was explored and deciphered using ultrafast laser spectroscopy and density functional theory calculations. First, we clearly demonstrate how hydrogen-bonding interactions between free linkers and solvents affect the twisting changes, internal conversion processes, and luminescent behavior of a benzoimidazole-based linker. Second, we provide clear evidence of an ultrafast energy transfer between well-aligned adjacent linkers with different protonation states inside the MOF. These findings provide a new fundamental photophysical insight into the exciton migration dynamics between linkers with different protonation states coexisting at different locations in MOFs and serve as a benchmark for improving light-harvesting MOF architectures.
• #### 2D Covalent-Organic Framework Electrodes for Supercapacitors and Rechargeable Metal-Ion Batteries

(Advanced Energy Materials, Wiley, 2021-05-05) [Article]
Covalent-organic frameworks (COFs) represent a new frontier of crystalline porous organic materials with framework structures in 2D or 3D domains, which make them promising for many applications. Herein, the fundamental structural design aspects of 2D-COFs are reviewed, which position them as suitable electrodes for electrochemical energy storage. The ordered π–π stacked arrangement of the organic building blocks in juxtaposed layers provides a pathway for efficient electronic charge transport; the 2D structure provides a pathway for enhanced ionic diffusion, which enhances ionic transport. Importantly, the tunable pore size enables 2D-COFs to accommodate mobile ions with different sizes and charges, positioning them as prospect materials for various types of batteries. Distinctively, the ability to functionalize their pore system with a periodic array of redox active species, enriching their potential redox chemistry, provides a pathway to control the redox and capacitive contributions to the charge storage mechanism. The strong covalently linked framework backbone of COFs is an additional merit for achieving long cycle life, and stability against the “leaching out” problem of active molecules in strong electrolytes as observed in other organic materials applied in energy storage devices.
• #### [Cu 15 (PPh 3 ) 6 (PET) 13 ] 2+ : a Copper Nanocluster with Crystallization Enhanced Photoluminescence

(Small, Wiley, 2021-03-19) [Article]
Due to their atomically precise structure, photoluminescent copper nanoclusters (Cu NCs) have emerged as promising materials in both fundamental studies and technological applications, such as bio-imaging, cell labeling, phototherapy, and photo-activated catalysis. In this work, a facile strategy is reported for the synthesis of a novel Cu NCs coprotected by thiolate and phosphine ligands, formulated as [Cu<sub>15</sub> (PPh<sub>3</sub> )<sub>6</sub> (PET)<sub>13</sub> ]<sup>2+</sup> , which exhibits bright emission in the near-infrared (NIR) region (≈720 nm) and crystallization-induced emission enhancement (CIEE) phenomenon. Single crystal X-ray crystallography shows that the NC possesses an extraordinary distorted trigonal antiprismatic Cu<sub>6</sub> core and a, unique among metal clusters, "tri-blade fan"-like structure. An in-depth structural investigation of the ligand shell combined with density functional theory calculations reveal that the extended CH···π and π-π intermolecular ligand interactions significantly restrict the intramolecular rotations and vibrations and, thus, are a major reason for the CIEE phenomena. This study provides a strategy for the controllable synthesis of structurally defined Cu NCs with NIR luminescence, which enables essential insights into the origins of their optical properties.
• #### [Ag9(1,2-BDT)6]3–: How Square-Pyramidal Building Blocks Self-Assemble into the Smallest Silver Nanocluster

(Inorganic Chemistry, American Chemical Society (ACS), 2021-03-17) [Article]
The emerging promise of few-atom metal catalysts has driven the need for developing metal nanoclusters (NCs) with ultrasmall core size. However, the preparation of metal NCs with single-digit metallic atoms and atomic precision is a major challenge for materials chemists, particularly for Ag, where the structure of such NCs remains unknown. In this study, we developed a shape-controlled synthesis strategy based on an isomeric dithiol ligand to yield the smallest crystallized Ag NC to date: [Ag<sub>9</sub>(1,2-BDT)<sub>6</sub>]<sup>3-</sup> (1,2-BDT = 1,2-benzenedithiolate). The NC's crystal structure reveals the self-assembly of two Ag square pyramids through preferential pyramidal vertex sharing of a single metallic Ag atom, while all other Ag atoms are incorporated in a motif with thiolate ligands, resulting in an elongated body-centered Ag<sub>9</sub> skeleton. Steric hindrance and arrangement of the dithiolated ligands on the surface favor the formation of an anisotropic shape. Time-dependent density functional theory based calculations reproduce the experimental optical absorption features and identify the molecular orbitals responsible for the electronic transitions. Our findings will open new avenues for the design of novel single-digit metal NCs with directional self-assembled building blocks.