2D framework materials for energy applications

In recent years a massive increase in publications on conventional 2D materials (graphene, h-BN, MoS₂) is documented, accompanied by the transfer of the 2D concept to porous (crystalline) materials, such as ordered 2D layered polymers, covalent-organic frameworks, and metal–organic frameworks. Over the years, the 3D frameworks have gained a lot of attention for use in applications, ranging from electronic devices to catalysis, and from information to separation technologies, mostly due to the modular construction concept and exceptionally high porosity. A key challenge lies in the implementation of these materials into devices arising from the deliberate manipulation of properties upon delamination of their layered counterparts, including an increase in surface area, higher diffusivity, better access to surface sites and a change in the band structure. Within this minireview, we would like to highlight recent achievements in the synthesis of 2D framework materials and their advantages for certain applications, and give some future perspectives.

In recent years the 2D concept has been transferred from conventional 2D materials to porous 2D framework materials. This minireview takes a closer look onto the preparation of 2D framework materials and their merits for energy applications.     

Direct air capture of CO2via crystal engineering

This article presents a perspective view of the topic of direct air capture (DAC) of carbon dioxide and its role in mitigating climate change, focusing on a promising approach to DAC involving crystal engineering of metal–organic and hydrogen-bonded frameworks. The structures of these crystalline materials can be easily elucidated using X-ray and neutron diffraction methods, thereby allowing for systematic structure–property relationships studies, and precise tuning of their DAC performance.

A perspective view of direct air capture (DAC) of CO₂ and its role in mitigating climate change is presented. The article focuses on a promising approach to DAC involving crystal engineering of metal–organic and hydrogen-bonded frameworks.     

Two-dimensional lanthanide coordination polymer nanosheets for detection of FOX-7

Despite the recent surge of interest in two-dimensional (2D) inorganic nanosheets derived from photoactive coordination polymers of lanthanide ions having interesting optical properties, research in this area is still in its infancy. Luminescent lanthanide ions, Eu(iii) or/and Tb(iii), as well as a bis-terpyridine ligand (L), were used in this study as the building blocks for the synthesis of the archetypical layered structure of coordination polymers (CPs) (L·Eu/L·Tb). 2D-nanosheets were obtained through exfoliation of the layered precursor of CPs in a suitable solvent system following a sonication-assisted strategy. These nanosheets exhibit lateral sizes on the micrometer scale (0.3–1 μm) and an ultrathin thickness of 2–6.5 nm. 1,1-Diamino-2,2-dinitroethene or FOX-7 is an insensitive high explosive; in a binder mixture, it exhibits a slightly superior detonation velocity of 8870 m s⁻¹ in comparison to RDX. The insensitive nature of FOX-7 makes it a key component for the development of low vulnerable high explosive compositions for further application in weaponry. The growing demand for FOX-7, for use as a suitable replacement of conventional explosives, is of serious concern to human security. Achieving rapid and efficient detection of this unexplored explosive is a challenging task. In the present study, the developed luminescent nanosheets were used for the first time for micromolar level detection of FOX-7 both in solution and in the solid state. A visually distinct color change of the nanosheets from red (L·Eu) and green (L·Tb) to colorless was witnessed upon UV light irradiation during the detection process. Notably, the solid-state detection technique could be exploited for developing a commercial spray kit for quick onsite screening of this important explosive.

A new class of luminescent lanthanide 2D nanosheets for detection of FOX-7.     

Aperiodic metal–organic frameworks

Metal–organic frameworks (MOFs) represent one of the most diverse structural classes among solid state materials, yet few of them exhibit aperiodicity, or the existence of long-range order in the absence of translational symmetry. From this apparent conflict, a paradox has emerged: even though aperiodicity frequently arises in materials that contain the same bonding motifs as MOFs, aperiodic structures and MOFs appear to be nearly disjoint classes. In this perspective, we highlight a subset of the known aperiodic coordination polymers, including both incommensurate and quasicrystalline structures. We further comment upon possible reasons for the absence of such structures and propose routes to potentially access aperiodic MOFs.

This perspective discusses progress and future directions in metal–organic frameworks with aperiodic structures. Reported quasicrystalline and incommensurate materials are presented, and pathways towards designing new such materials are provided.     

Understanding the geometric diversity of inorganic and hybrid frameworks through structural coarse-graining

Much of our understanding of complex structures is based on simplification: for example, metal–organic frameworks are often discussed in the context of “nodes” and “linkers”, allowing for a qualitative comparison with simpler inorganic structures. Here we show how such an understanding can be obtained in a systematic and quantitative framework, combining atom-density based similarity (kernel) functions and unsupervised machine learning with the long-standing idea of “coarse-graining” atomic structure. We demonstrate how the latter enables a comparison of vastly different chemical systems, and we use it to create a unified, two-dimensional structure map of experimentally known tetrahedral AB₂ networks – including clathrate hydrates, zeolitic imidazolate frameworks (ZIFs), and diverse inorganic phases. The structural relationships that emerge can then be linked to microscopic properties of interest, which we exemplify for structural heterogeneity and tetrahedral density.

A coarse-graining approach enables structural comparisons across vastly different chemical spaces, from inorganic polymorphs to hybrid framework materials.     

In vivo oral insulin delivery via covalent organic frameworks

With diabetes being the 7th leading cause of death worldwide, overcoming issues limiting the oral administration of insulin is of global significance. The development of imine-linked-covalent organic framework (nCOF) nanoparticles for oral insulin delivery to overcome these delivery barriers is herein reported. A gastro-resistant nCOF was prepared from layered nanosheets with insulin loaded between the nanosheet layers. The insulin-loaded nCOF exhibited insulin protection in digestive fluids in vitro as well as glucose-responsive release, and this hyperglycemia-induced release was confirmed in vivo in diabetic rats without noticeable toxic effects. This is strong evidence that nCOF-based oral insulin delivery systems could replace traditional subcutaneous injections easing insulin therapy.

We report the successful use of a gastro-resistant covalent organic framework for in vivo oral delivery of insulin.     

Tetrathiafulvalene-based covalent organic frameworks for ultrahigh iodine capture

To safeguard the development of nuclear energy, practical techniques for capture and storage of radioiodine are of critical importance but remain a significant challenge. Here we report the synergistic effect of physical and chemical adsorption of iodine in tetrathiafulvalene-based covalent organic frameworks (COFs), which can markedly improve both iodine adsorption capacity and adsorption kinetics due to their strong interaction. These functionalized architectures are designed to have high specific surface areas (up to 2359 m² g⁻¹) for efficient physisorption of iodine, and abundant tetrathiafulvalene functional groups for strong chemisorption of iodine. We demonstrate that these frameworks achieve excellent iodine adsorption capacity (up to 8.19 g g⁻¹), which is much higher than those of other materials reported so far, including silver-doped adsorbents, inorganic porous materials, metal–organic frameworks, porous organic frameworks, and other COFs. Furthermore, a combined theoretical and experimental study, including DFT calculations, electron paramagnetic resonance spectroscopy, X-ray photoelectron spectroscopy, and Raman spectroscopy, reveals the strong chemical interaction between iodine and the frameworks of the materials. Our study thus opens an avenue to construct functional COFs for a critical environment-related application.

The synergistic effect of physical and chemical adsorption of iodine in tetrathiafulvalene-based covalent organic frameworks (COFs) has been explored. The iodine adsorption capacity of these materials is higher than other materials reported so far.     

Construction of isostructural hydrogen-bonded organic frameworks: limitations and possibilities of pore expansion

The library of isostructural porous frameworks enables a systematic survey to optimize the structure and functionality of porous materials. In contrary to metal–organic frameworks (MOFs) and covalent organic frameworks (COFs), a handful of isostructural frameworks have been reported for hydrogen-bonded organic frameworks (HOFs) due to the weakness of the bonds. Herein, we provide a rule-of-thumb to develop isostructural HOFs, where we demonstrate the construction of the third and fourth generation of isostructural HAT-based HOFs (TolHAT-1 and ThiaHAT-1) by considering three important structural factors, that are (1) directional H-bonding, (2) shape-fitted docking of the HAT core, and (3) modulation of peripheral moieties. Their structural and photo-physical properties including HCl vapor detection are presented. Moreover, TolHAT-1, ThiaHAT-1, and other isostructural HOFs (CPHAT-1 and CBPHAT-1) were thoroughly compared from the viewpoints of structures and properties. Importantly, molecular dynamics (MD) simulation proves to be rationally capable of evaluating the stability of isostructural HOFs. These results can accelerate the development of various isostructural molecular porous materials.

The library of isostructural porous frameworks enables a systematic survey to optimize the structure and functionality of porous materials.     

Acid-triggered interlayer sliding of two-dimensional copper(i)–organic frameworks: more metal sites for catalysis

The interlay sliding of two-dimensional (2D) metal–organic and covalent–organic frameworks (MOFs and COFs) affects not only the layout features of the structures, but also the functional output of the materials. However, the control of interlay stacking is the major hurdle that needs to be overcome to construct new functional layer materials. Herein, we report the preparation of a pair of isostructural 2D copper(i) organic frameworks with an eclipsed AA stacking structure, namely JNM-3-AA, and a staggered ABC stacking topology, denoted JNM-3-ABC, by combining the chemistry of MOFs and COFs. The variation of interlayer stacking largely influences their functionality, including porosity (BET surface areas of 695.61 and 34.22 m² g⁻¹ for JNM-3-AA and JNM-3-ABC, respectively), chemical stability, and catalytic activities (less than 10% or ∼86% yield using JNM-3-AA or JNM-3-ABC as catalysts for click reaction, respectively). More interestingly, the structure transformation from JNM-3-ABC to JNM-3-AA is readily achieved by simple addition of trifluoroacetic acid accompanied by the extension of porosities from BET surface areas of 34.22 to 441.22 m² g⁻¹, resulting in in situ acceleration of the adoption rate (removal efficiency increases from ∼10 to 99.9%), which is rarely observed in 2D MOFs and COFs.

The addition of TFA can trigger the interlay sliding of 2D copper(i) organic frameworks prepared by combing the chemistry of MOFs and COFs. The variation of interlay stacking largely affected the porosity, chemical stability and catalytic activities.     

Self-healing and shape memory functions exhibited by supramolecular liquid-crystalline networks formed by combination of hydrogen bonding interactions and coordination bonding

We here report a new approach to develop self-healing shape memory supramolecular liquid-crystalline (LC) networks through self-assembly of molecular building blocks via combination of hydrogen bonding and coordination bonding. We have designed and synthesized supramolecular LC polymers and networks based on the complexation of a forklike mesogenic ligand with Ag⁺ ions and carboxylic acids. Unidirectionally aligned fibers and free-standing films forming layered LC nanostructures have been obtained for the supramolecular LC networks. We have found that hybrid supramolecular LC networks formed through metal–ligand interactions and hydrogen bonding exhibit both self-healing properties and shape memory functions, while hydrogen-bonded LC networks only show self-healing properties. The combination of hydrogen bonds and metal–ligand interactions allows the tuning of intermolecular interactions and self-assembled structures, leading to the formation of the dynamic supramolecular LC materials. The new material design presented here has potential for the development of smart LC materials and functional LC membranes with tunable responsiveness.

New supramolecular hybrid liquid-crystalline networks exhibiting self-healing and shape memory properties are developed by self-assembly of small components through hydrogen bonding interactions and coordination bonding.     

Switchable electrical conductivity in a three-dimensional metal–organic framework via reversible ligand n-doping

Redox-active metal–organic frameworks (MOFs) are promising materials for a number of next-generation technologies, and recent work has shown that redox manipulation can dramatically enhance electrical conductivity in MOFs. However, ligand-based strategies for controlling conductivity remain under-developed, particularly those that make use of reversible redox processes. Here we report the first use of ligand n-doping to engender electrical conductivity in a porous 3D MOF, leading to tunable conductivity values that span over six orders of magnitude. Moreover, this work represents the first example of redox switching leading to reversible conductivity changes in a 3D MOF.

Redox-active ligands are used to reversibly tune electrical conductivity in a porous 3D metal–organic framework (MOF).     

Cyanine-based near infra-red organic photoredox catalysis

Direct metal-free near infra-red photoredox catalysis is applied to organic oxidation, photosensitization and reduction, involving cyanines as photocatalysts. This photocatalyst is competitive with conventional reactions catalyzed under visible light. Kinetic and quenching experiments are also reported. Interestingly, these systems are compatible with water media, opening perspective for various applications.

Direct metal-free near infra-red photoredox catalysis is applied to oxidation, reduction and photosensitization, involving cyanines as photocatalysts. Mechanistic insights through kinetic and quenching experiments are also reported.     

Towards photoswitchable quadruple hydrogen bonds via a reversible “photolocking” strategy for photocontrolled self-assembly

Developing new photoswitchable noncovalent interaction motifs with controllable bonding affinity is crucial for the construction of photoresponsive supramolecular systems and materials. Here we describe a unique “photolocking” strategy for realizing photoswitchable control of quadruple hydrogen-bonding interactions on the basis of modifying the ureidopyrimidinone (UPy) module with an ortho-ester substituted azobenzene unit as the “photo-lock”. Upon light irradiation, the obtained Azo-UPy motif is capable of unlocking/locking the partial H-bonding sites of the UPy unit, leading to photoswitching between homo- and heteroquadruple hydrogen-bonded dimers, which has been further applied for the fabrication of novel tunable hydrogen bonded supramolecular systems. This “photolocking” strategy appears to be broadly applicable in the rational design and construction of other H-bonding motifs with sufficiently photoswitchable noncovalent interactions.

A photolocking strategy is described to achieve the construction of effectively photoswitchable quadruple hydrogen bonds featuring with photoregulable H-bonding affinities, which is further applied in the photocontrollable H-bonded self-assemblies.     

Beyond structural motifs: the frontier of actinide-containing metal–organic frameworks

In this perspective, we feature recent advances in the field of actinide-containing metal–organic frameworks (An-MOFs) with a main focus on their electronic, catalytic, photophysical, and sorption properties. This discussion deviates from a strictly crystallographic analysis of An-MOFs, reported in several reviews, or synthesis of novel structural motifs, and instead delves into the remarkable potential of An-MOFs for evolving the nuclear waste administration sector. Currently, the An-MOF field is dominated by thorium- and uranium-containing structures, with only a few reports on transuranic frameworks. However, some of the reported properties in the field of An-MOFs foreshadow potential implementation of these materials and are the main focus of this report. Thus, this perspective intends to provide a glimpse into the challenges, triumphs, and future directions of An-MOFs in sectors ranging from the traditional realm of gas sorption and separation to recently emerging areas such as electronics and photophysics.

This perspective deviates from exclusively focusing on structural features of actinide-containing metal-organic frameworks and pivots towards their prospect as avant-garde materials with an emphasis on their physicochemical properties.     

Using steric hindrance to manipulate and stabilize metal halide perovskites for optoelectronics

The chemical instability of metal halide perovskite materials can be ascribed to their unique properties of softness, in which the chemical bonding between metal halide octahedral frameworks and cations is the weak ionic and hydrogen bonding as in most perovskite structures. Therefore, various strategies have been developed to stabilize the cations and metal halide frameworks, which include incorporating additives, developing two-dimensional perovskites and perovskite nanocrystals, etc. Recently, the important role of utilizing steric hindrance for stabilizing and passivating perovskites has been demonstrated. In this perspective, we summarize the applications of steric hindrance in manipulating and stabilizing perovskites. We will also discuss how steric hindrance influences the fundamental kinetics of perovskite crystallization and film formation processes. The similarities and differences of the steric hindrance between perovskite solar cells and perovskite light emission diodes are also discussed. In all, utilizing steric hindrance is a promising strategy to manipulate and stabilize metal halide perovskites for optoelectronics.

Manipulation on steric hindrance can influence the fundamental kinetics of perovskite crystallization and film formation, therefore stabilizing and passivating perovskite structures, and promoting the commercialization of stable perovskite devices.     

Electrical conductivity in a non-covalent two-dimensional porous organic material with high crystallinity

Electroactive macrocycle building blocks are a promising route to new types of functional two-dimensional porous organic frameworks. Our strategy uses conjugated macrocycles that organize into two dimensional porous sheets via non-covalent van der Waals interactions, to make ultrathin films that are just one molecule thick. In bulk, these two-dimensional (2D) sheets stack into a three-dimensional van der Waals crystal, where relatively weak alkyl–alkyl interactions constitute the interface between these sheets. With the liquid-phase exfoliation, we are able to obtain films as thin as two molecular layers. Further using a combination of liquid-phase and mechanical exfoliation, we are able to create non-covalent sheets over a large area (>100 μm²). The ultrathin porous films maintain the single crystal packing from the macrocyclic structure and are electrically conductive. We demonstrate that this new type of 2D non-covalent porous organic framework can be used as the active layer in a field effect transistor device with graphene source and drain contacts along with hexagonal boron nitride as the gate dielectric interface.

Ultrathin porous films held together by non-covalent van der Waals interactions was obtained by a top-down approach, which is then utilized as channel material in a two-dimensional planar field-effect transistor device through easy stamp transfer.

We describe a new type of two-dimensional (2D), molecularly-thin porous organic framework that is formed from macrocyclic building blocks that assemble, through non-covalent interactions, into a porous two-dimensional plane. Covalent organic frameworks (COFs) are promising in applications due to their ability to host other functional molecules in the voids.^1–7 Many porous frameworks have been demonstrated to be useful in energy storage,⁸ catalysis,^9–11 separation,^12,13 optoelectronics^4,14 and sensing.^15,16 In order to construct nanodevices with porous channels, ultrathin films of porous frameworks has been prepared with bottom-up^4,17–20 and top-down^1,21 approaches. The top-down approaches to these materials are enabled by strong covalent bonds in the two-dimensional plane and weak van der Waals interactions between them, similar to what is seen in two-dimensional materials such as graphene and TMDs.^22–27 For porous ultrathin films, the electrical conductance has not been extensively investigated.^2,7,20,28,29Here, we explore making molecularly thin layers in which conjugated macrocycles are used as building blocks and non-covalent van der Waals interactions are the adhesive that assembles these molecules into rigid, porous layers. By adjusting the relative strengths of the interactions that direct the assembly within the plane and those holding the two-dimensional layers with respect to each other, we can exfoliate these non-covalent porous frameworks using the same means employed for traditional two-dimensional van der Waals materials.³⁰ Using liquid-phase and mechanical exfoliation, we create porous films that are as thin as two-layers of molecules. These new results are exciting and useful because previously we were not able to obtain such high-ordered thin porous film directly from its bulk crystal and were limited to investigating the electronic properties of this hollow organic capsules in spin-coated films. These ultrathin porous films are ordered over large areas and maintain the single crystal packing from the macrocyclic building blocks. To demonstrate the utility of this new type of ultrathin material, we fabricated 2D field effect transistor (FET) devices in which graphene is the source/drain contacts, hexagonal boron nitride is the gate dielectric interface, and the exfoliated molecular sheet is the active layer. These ultrathin self-assembled materials are efficacious at transporting electrons and will find utility in gas sensing and applications similar to traditional two-dimensional materials. Fig. 1 displays the molecular building block (1). Characterization is contained in the ESI^† and a previous report.³¹1 has several important molecular features in its solid-state assembly. It is a rigid and shape persistent macrocycle that has an interior and an exterior (Fig. 1a), and in bulk, has a pore of ∼11.4 Å in diameter and a surface area of 20 m² g⁻¹ from BET measurements.³¹ When it assembles in the crystalline state, it forms two-dimensional porous sheets with two types of cavities (Fig. 1b), one molecule thick, that are held together by relatively strong π–π contacts and Br–PDI interaction between the bromine atoms on the thiophenes and adjacent PDI molecules (Fig. 1c), which plays a crucial role in the self-assembly of the films. The close proximity of the molecules in the 2D plane together with the conjugation within the macrocycle facilitate charge transport of electrons in the 2D plane. These electrically conductive porous sheets then stack into a three-dimensional crystal in which adjacent sheets are separated from one another by the alkane sidechains of the perylene diimide (Fig. 1d). It is in this alkane gallery that we see an opportunity for exfoliation to yield ultrathin 2D sheets.Open in a separate windowFig. 1Structure and packing of molecule 1. (a) Chemical structure, side view and top view of molecular structure of 1. C, N, O, S, Br are colored in grey, blue, red, yellow and green, respectively. The vertical distance of one macrocycle is about 1.5 nm. (b) Face-on view and edge-on view of one layer of 1. The internal cavity of 1 and the cavity formed by the packing of 1 are labeled as i and i′, respectively. (c) Interactions binding two adjacent macrocycles from neighboring brominated thiophene rings. (d) View of packing of 1 along the c axis through the interaction of alkane sidechains. Exfoliation takes place at the alkyl–alkyl interface. One layer of 1 is about 2 nm in thickness.We isolated crystals of this material that were grown from solution and then tested whether they can be exfoliated. Fig. 2a displays a representative micrograph of one of the crystals. The crystal has a pseudo-hexagonal packing of the molecular building blocks in the two-dimensional plane, and this symmetry is mirrored in the hexagonally-shaped crystals. The simplest method for exfoliation is mechanical exfoliation, which is most commonly performed using scotch-tape.^32,33 We place the single crystals onto clean scotch-tape and repeat the mechanical exfoliation process for a few repetitions, and then we transfer the exfoliated crystals onto a clean silicon wafer. Fig. 2b displays an atomic force microscopy (AFM) micrograph of the typical non-covalent porous 2D sheet of 1 we obtained from this method. The porous sheets are flat and smooth and a few micrometers in diameter with a thickness of ∼8 nm; this thickness corresponds to a stack of five molecular layers of 1. This result demonstrates that non-covalent interactions are strong enough to hold molecules together to form ultrathin porous materials. Just as with traditional two-dimensional materials, the non-covalent porous organic 2D sheets of 1 are flexible as evidenced by the wrinkles and folds in the micrograph in Fig. 2b and S2.^†Open in a separate windowFig. 2Mechanical exfoliation of 1. (a) Optical microscopy and scanning electron microscope (SEM) (inset) images of a single crystal of macrocycle 1. (b) AFM and optical microscope (inset) images of the ultrathin non-covalent porous sheet of 1 on a silicon wafer obtained from mechanical exfoliation.We were unable to obtain porous films as thin as a single layer and also large-area samples using mechanical exfoliation, and thus we next explored if liquid-phase exfoliation^34,35 could produce them. Because the halogen bonds that hold the sheets together should be most robust in solvents with a low-dielectric constant that lack heteroatoms, and because the groups holding the sheets together are the alkane sidechains, we chose saturated hydrocarbons (hexane or heptane) as the solvents for exfoliation. Fig. 3a shows the process we follow for the liquid-phase exfoliation. We suspend single crystals of 1 in heptane and sonicate the mixture for five minutes. We drop cast the supernatant solution on silicon wafer and examine them with AFM. Remarkably, we are able to obtain non-covalent porous organic frameworks as thin as only two molecular layers (Fig. 3b).³⁶ Nevertheless, the lateral size of the porous 2D sheets of 1 we could obtain using this method are quite small, making it difficult to fabricate devices from them.Open in a separate windowFig. 3Liquid-phase exfoliation and combination of liquid-phase and mechanical exfoliation. (a) Schematic showing the liquid-phase exfoliation process. (b) AFM image of the ultrathin sheet of 1 on a silicon wafer obtained from liquid-phase exfoliation method. The sheets in this micrograph are two molecular layers (left) and three molecular layers (right) in thickness. (c) AFM image of the ultrathin sheet of 1 on a silicon wafer obtained from combination of liquid-phase and mechanical exfoliation, inset: optical microscope image of the ultrathin sheet 1. (d) AFM image showing the height change across the ultrathin sheet of 1 on silicon wafer obtained with combination methods, inset: optical microscope image of the ultrathin sheet of 1.To get large-area films characteristic of the mechanical exfoliation and thin films characteristic of the liquid-phase exfoliation, we combined the two methods. We first immerse the crystal of 1 in heptane for a few minutes to let the solvent seep into the gallery between the sheets and weaken the interlayer interactions. Then, we use mechanical exfoliation to isolate the ultrathin films. With this method, we obtained sheets of 1 with a lateral size of over ten micrometers, as shown in Fig. 3c. By carefully examining the exfoliated non-covalent porous 2D sheets of 1, we were also able to observe the height change across the sheet (Fig. 3d) with integer values of the layer thickness after the exfoliation steps. As marked red in Fig. 3d, we could identify a single layer of 1 with a height difference between these two surfaces of about 1.5 nm, corresponding to monolayer of molecule 1.We conducted transmission electron microscopy (TEM) studies to characterize the crystallinity of the as-prepared non-covalent porous 2D sheets of 1. As shown in Fig. 4a (inset), the 2D sheets exhibit a layered structure after liquid-phase exfoliation. The selected area electron diffraction (SAED) in the area (marked by the red circle) reveals a hexagonal diffraction pattern, with the bright reflections corresponding to the (2 1(−) 0) plane, with a spacing of 11.3 Å. This diffraction pattern confirms that the non-covalent, porous 2D sheets of 1 retain the single crystal packing and are stable to the liquid-phase processing.Open in a separate windowFig. 4TEM characterization and device fabrication. (a) SEAD pattern and TEM image (inset) of the non-covalent porous ultrathin sheet 1 obtained by liquid-phase exfoliation. (b) Schematic showing the structure of the hBN/Graphene/1 device based on the non-covalent porous ultrathin sheet 1 with graphene as electrodes and hBN as the dielectric layer. (c) Optical microscope image showing the as-fabricated hBN/Graphene/1 device. (d) Transfer curve of the hBN/Graphene/1 device.We next sought to determine the ability of these non-covalent porous ultrathin layers to transport charge. Because these films are van der Waals materials, we sought to make devices with van der Waals interfaces. [The ESI^† contains the current/voltage curves for 1 in a more traditional organic FET with Au contacts and trichloro(octadecyl)silane coated SiO₂ as the gate dielectric.] The source-drain contacts were fabricated from graphene and the dielectric interface was hexagonal boron nitride (hBN). A schematic of the device is shown in Fig. 4b. To create this device, we first exfoliate graphene and hBN onto a silicon substrate. We then follow a published procedure³⁷ to first pick up hBN and then graphene to make the hBN/graphene stack. We transfer this hBN/graphene stack onto another clean silicon substrate. Then the graphene was cut using electron beam lithography and an oxygen plasma to open a 300 nm gap between graphene electrodes (see Fig. S4^† for the AFM details of graphene electrodes). In order to transfer the non-covalent porous ultrathin sheets of 1 onto the graphene electrodes, we exploit the combined liquid/mechanical exfoliation method above to obtained 2D sheet of 1 with polydimethylsiloxane (PDMS) polymer as the substrate, which was then used for the stamp transferring. In this manner, we were able to transfer the ultrathin sheets (∼20 nm) onto the graphene electrodes. Fig. 4c displays the optical microscope image of the device, and Fig. 4d displays the FET transfer curves revealing that the material is an efficacious, n-type transistor. Several features of the device are noteworthy. The electron mobility in the linear regime was estimated to be 1.6 × 10⁻⁴ cm² V⁻¹ s⁻¹ from the transfer curve. As expected, it is somewhat lower than the electron mobility estimated from the saturation regime of the traditional OFET shown in Fig. S3.^†38 Despite the small size and the nanoscale thickness, the material exhibits over 3 orders of magnitude difference in current between the off and on state of the device. The threshold voltage is about 39 V, implying that the device turns on at relatively high voltage. We surmise that the contact, through the alkane sidechains is an impediment to more efficient charge transport.     

Effect of modulator connectivity on promoting defectivity in titanium–organic frameworks

The recognition of defect chemistry as a true synthetic tool for targeted creation of defects and controllable performance remains limited by the pool of frameworks explored. The value of defect engineering in controlling the properties of defective frameworks has been beautifully exemplified and largely demonstrated with UiO-type materials based on Zr(iv) nodes. However, titanium–organic frameworks remain largely unexplored in this context arguably due to the complex chemistry in solution of Ti(iv) and the difficulties in growing crystalline solids. We report a systematic study on the ability of mono- and dicarboxylic modulators (benzoic and isophthalic acid) to promote defect creation in the heterometallic Ti-MOF of the MUV-10 family. Our results indicate that both acids behave as capping modulators at high concentrations, but isophthalic acid is a more efficient defect promoter, yielding defective phases with nearly 40% of missing linkers. Our computational results suggest that this difference cannot be solely ascribed to relative changes in acidity but to the ability of this bidentate linker in compensating the structural distortion and energy penalty imposed by breaking the connectivity of the underlying framework.

The connectivity of mono- and dicarboxylic modulators controls their ability to promote defects in the titanium framework MUV-10.     

The structures of ordered defects in thiocyanate analogues of Prussian Blue

We report the structures of six new divalent transition metal hexathiocyanatobismuthate frameworks with the generic formula , M = Mn, Co, Ni and Zn. These frameworks are defective analogues of the perovskite-derived trivalent transition metal hexathiocyanatobismuthates M^III[Bi(SCN)₆]. The defects in these new thiocyanate frameworks order and produce complex superstructures due to the low symmetry of the parent structure, in contrast to the related and more well-studied cyanide Prussian Blue analogues. Despite the close similarities in the chemistries of these four transition metal cations, we find that each framework contains a different mechanism for accommodating the lowered transition metal charge, making use of some combination of Bi(SCN)₆³⁻ vacancies, M_Bi antisite defects, water substitution for thiocyanate, adventitious extra-framework cations and reduced metal coordination number. These materials provide an unusually clear view of defects in molecular framework materials and their variety suggests that similar richness may be waiting to be uncovered in other hybrid perovskite frameworks.

We report the structures of six new divalent transition metal hexathiocyanatobismuthate Prussian Blue analogues frameworks, which contain complex ordered defect structures.     

Universal quenching of common fluorescent probes by water and alcohols

Although biological imaging is mostly performed in aqueous media, it is hardly ever considered that water acts as a classic fluorescence quencher for organic fluorophores. By investigating the fluorescence properties of 42 common organic fluorophores recommended for biological labelling, we demonstrate that H₂O reduces their fluorescence quantum yield and lifetime by up to threefold and uncover the underlying fluorescence quenching mechanism. We show that the quenching efficiency is significantly larger for red-emitting probes and follows an energy gap law. The fluorescence quenching finds its origin in high-energy vibrations of the solvent (OH groups), as methanol and other linear alcohols are also found to quench the emission, whereas it is restored in deuterated solvents. Our observations are consistent with a mechanism by which the electronic excitation of the fluorophore is resonantly transferred to overtones and combination transitions of high-frequency vibrational stretching modes of the solvent through space and not through hydrogen bonds. Insight into this solvent-assisted quenching mechanism opens the door to the rational design of brighter fluorescent probes by offering a justification for protecting organic fluorophores from the solvent via encapsulation.

Overtones and combinations of O–H vibrations in the solvent efficiently quench red-emitting fluorophores by resonant energy transfer.     

Selective single-atom electrocatalysts: a review with a focus on metal-doped covalent triazine frameworks

Single-atom electrocatalysts (SACs), which comprise singly isolated metal sites supported on heterogeneous substrates, have attracted considerable recent attention as next-generation electrocatalysts for various key reactions from the viewpoint of the environment and energy. Not only electrocatalytic activity but also selectivity can be precisely tuned via the construction of SACs with a defined coordination structure, such as homogeneous organometallics. Covalent organic frameworks (COFs) are promising supports for single-atom sites with designed coordination environments due to their unique physicochemical properties, which include porous structures, robustness, a wide range of possible designs, and abundant heteroatoms to coordinate single-metal sites. The rigid frameworks of COFs can hold unstable single-metal atoms, such as coordinatively unsaturated sites or easily aggregated Pt-group metals, which exhibit unique electrocatalytic selectivity. This minireview summarizes recent advances in the selective reactions catalysed by SACs, mainly those supported on triazine-based COFs.

Single-atom electrocatalysts (SACs) have attracted considerable attention as selective electrocatalysts. Metal-doped covalent triazine frameworks will be a novel platform for selective SACs to solve energy and environmental issues.