A porous, electrically conductive hexa-zirconium(iv) metal-organic framework

Microporous Zr-metal-organic frameworks based-nanocomposites for thermoelectric applications

In the area of energy storage and conversion, Metal-Organic Frameworks (MOFs) are receiving more and more attention. They combine organic nature with long-range order and low thermal conductivity, giving them qualities to be potentially attractive for thermoelectric applications. To make the framework electrically conductive so far, thermoelectricity in this class of materials requires infiltration by outside conductive guest molecules. In this study, an in-situ polymerization of conductive polyaniline inside the porous structure of MOF-801 was conducted to synthesize PANi@MOF-801 nanocomposites for thermoelectrical applications. The growth of polyaniline chains of different loadings inside the host MOF matrix generally enhanced bulk electrical conductivity by about 6 orders of magnitude, leading to Seebeck coefficient value of -141 µVK-1 and improved thermal stability. The unusual increase in electrical conductivity was attributed to the formation of highly oriented conductive PANi chains inside the MOF pores, besides host-guest physical interaction, while the Seebeck coefficient enhancement was because of the energy filtering effect of the developed structure. Modulating the composition of PANi@MOF-801 composites by varying the aniline: MOF-801 ratio in the synthesis bath from 2:1 and 1:1 to 1:2 leads to a change in the semiconductor properties from p-type semiconductor to n-type. Among the examined composites with n-type semiconducting properties exhibited the highest ZT value, 0.015, and lowest thermal conductivity, 0.24 Wm-1 K-1. The synthesized composites have better performance than those recently reported for a similar category of thermoelectric materials related to MOF-based composites.

Transformation of SBUs and Synergy of MOF Host-Guest in Single Crystalline State: Ingenious Strategies for Modulating Third-Order NLO Signals

Central metal exchange can innovatively open the cavity of metal-organic frameworks (MOFs) by alternating the framework topology. Here, the single-crystal-to-single-crystal (SC-SC) transformation is reported from a Co-based MOF {[Co1.25 (HL)0.5 (Pz-NH2 )0.25 (µ3 -O)0.25 (µ2 -OH)0.25 (H2 O)]·0.125 Co·0.125 L·10.25H2 O}n (Co-MOF, L = 5,5'-(1H-2,3,5-triazole-1,4-diyl)diisophthalic acid) into two novel MOF materials, {[Cu1.75 L0.75 (Pz-NH2 )0.125 (µ3 -O)0.125 (µ2 -OH)0.25 (H2 O)0.375 ]•3CH3 CN}n (Cu-MOF) and {[Zn1.75 L0.625 (Pz-NH2 )0.25 (µ3 -O)0.25 (µ2 -O)0.25 (H2 O)1.25 ]•4CH3 CN}n (Zn-MOF), through exchanging the Co2+ in the MOF into Cu2+ or Zn2+ , respectively. The free Co2+ and L4- in the Co-MOF channels fuse with the skeleton during the Co→Cu and Co→Zn exchange processes, leading to the expansion of the channel space and the transformation of the secondary building units (SBUs) to form an adjustable skeleton. The nonlinear optical response results show that the MOFs generated by the exchange of the central metal exhibit different saturable absorption and the self-focusing effect. In addition, loading polypyrrole (PPy) into the MOFs can not only improve the stability of the MOFs but also further optimize the nonlinear optical behavior. This work suggests that SC-SC central metal exchange and the introduction of polymer molecules can tune the nonlinear optical response, which provides a new perspective for the future study of nonlinear optical materials.

On-off conduction photoswitching in modelled spiropyran-based metal-organic frameworks

Materials with photoswitchable electronic properties and conductance values that can be reversibly changed over many orders of magnitude are highly desirable. Metal-organic framework (MOF) films functionalized with photoresponsive spiropyran molecules demonstrated the general possibility to switch the conduction by light with potentially large on-off-ratios. However, the fabrication of MOF materials in a trial-and-error approach is cumbersome and would benefit significantly from in silico molecular design. Based on the previous proof-of-principle investigation, here, we design photoswitchable MOFs which incorporate spiropyran photoswitches at controlled positions with defined intermolecular distances and orientations. Using multiscale modelling and automated workflow protocols, four MOF candidates are characterized and their potential for photoswitching the conductivity is explored. Using ab initio calculations of the electronic coupling between the molecules in the MOF, we show that lattice distances and vibrational flexibility tremendously modulate the possible conduction photoswitching between spiropyran- and merocyanine-based MOFs upon light absorption, resulting in average on-off ratios higher than 530 and 4200 for p- and n-conduction switching, respectively. Further functionalization of the photoswitches with electron-donating/-withdrawing groups is demonstrated to shift the energy levels of the frontier orbitals, permitting a guided design of new spiropyran-based photoswitches towards controlled modification between electron and hole conduction in a MOF.

From a Collapse-Prone, Insulating Ni-MOF-74 Analogue to Crystalline, Porous, and Electrically Conducting PEDOT@MOF Composites

Electrically conductive porous metal-organic frameworks (MOFs) show great promise in helping advance electronics and clean energy technologies. However, large porosity usually hinders long-range charge transport, an essential criterion of electrical conductivity, underscoring the need for new strategies to combine these two opposing features and realize their diverse potentials. All previous strategies to boost the conductivity of porous MOFs by introducing redox-complementary guest molecules, conducting polymers, and metal nanoparticles have led to a significant loss of frameworks' porosity and surface areas, which could be otherwise exploited to capture additional guests in electrocatalysis and chemiresistive sensing applications. Herein, we demonstrate for the first time that the in situ oxidative polymerization of preloaded 3,4-ethylenedioxythiophene (EDOT) monomers into the polyethylenedioxythiophene (PEDOT) polymer inside the hexagonal cavities of an intrinsically insulating Ni2(NDISA) MOF-74 analogue (NDISA = naphthalenediimide N,N-disalicylate), which easily collapses and becomes amorphous upon drying, simultaneously enhanced the crystallinity, porosity, and electrical conductivity of the resulting PEDOT@Ni2(NDISA) composites. At lower PEDOT loading (∼22 wt %), not only did the Brunauer-Emmett-Teller surface area of the PEDOT@Ni2(NDISA) composite (926 m2/g) more than double from that of evacuated pristine Ni2(NDISA) (387 m2/g), but also its electrical conductivity (1.1 × 10-5 S/cm) soared 105 times from that of the pristine MOF, demonstrating unprecedented dual benefits of our strategy. At higher PEDOT loading (≥33 wt %), the electrical conductivity of Ni2(NDISA)⊃PEDOT composites further increased modestly (10-4 S/cm), but their porosity dropped precipitously as large amounts of PEDOT filled up the hexagonal MOF channels. Thus, our work presents a simple new strategy to simultaneously boost the structural stability, porosity, and electrical conductivity of intrinsically insulating and collapse-prone MOFs by introducing small amounts of conducting polymers that can not only reinforce the MOF scaffolds and prevent them from collapsing but also help create a much coveted non-native property by providing charge carriers and charge transport pathways.

Unique Octupolar 2D-Polymer Frameworks as Mixed Conductors and Metal-Free Catalysts for Dual-Promoted Li and S Electrochemistry: Multi-regulation Role of Ethoxylation Chemistry

Here, we for the first time introduce ethoxylation chemistry to develop a new octupolar cyano-vinylene-linked 2D polymer framework (Cyano-OCF-EO) capable of acting as efficient mixed electron/ion conductors and metal-free sulfur evolution catalysts for dual-promoted Li and S electrochemistry. Our strategy creates a unique interconnected network of strongly-coupled donor 3-(acceptor-core) octupoles in Cyano-OCF-EO, affording enhanced intramolecular charge transfer, substantial active sites and crowded open channels. This enables Cyano-OCF-EO as a new versatile separator modifier, which endows the modified separator with superior catalytic activity for sulfur conversion and rapid Li ion conduction with the high Li+ transference number up to 0.94. Thus, the incorporation of Cyano-OCF-EO can concurrently regulate sulfur redox reactions and Li-ion flux in Li-S cells, attaining boosted bidirectional redox kinetics, inhibited polysulfide shuttle and dendrite-free Li anodes. The Cyano-OCF-EO-involved Li-S cell is endowed with excellent overall electrochemical performance especially large areal capacity of 7.5 mAh cm-2 at high sulfur loading of 8.7 mg cm-2 . Mechanistic studies unveil the dominant multi-promoting effect of the triethoxylation on electron and ion conduction, polysulfide adsorption and catalytic conversion as well as previously-unexplored -CN/C-O dual-site synergistic effect for enhanced polysulfide adsorption and reduced energy barrier toward Li2 S conversion.

Enhancing Built-in Electric Fields for Efficient Photocatalytic Hydrogen Evolution by Encapsulating C60 Fullerene into Zirconium-Based Metal-Organic Frameworks

High-efficiency photocatalysts based on metal-organic frameworks (MOFs) are often limited by poor charge separation and slow charge-transfer kinetics. Herein, a novel MOF photocatalyst is successfully constructed by encapsulating C₆₀ into a nano-sized zirconium-based MOF, NU-901. By virtue of host-guest interactions and uneven charge distribution, a substantial electrostatic potential difference is set-up in C₆₀ @NU-901. The direct consequence is a robust built-in electric field, which tends to be 10.7 times higher in C₆₀ @NU-901 than that found in NU-901. In the catalyst, photogenerated charge carriers are efficiently separated and transported to the surface. For example, photocatalytic hydrogen evolution reaches 22.3 mmol g^-1  h^-1 for C₆₀ @NU-901, which is among the highest values for MOFs. Our concept of enhancing charge separation by harnessing host-guest interactions constitutes a promising strategy to design photocatalysts for efficient solar-to-chemical energy conversion.

Targeted Synthesis of Isomeric Naphthalene-Based 2D Kagome Covalent Organic Frameworks

Targeted synthesis of kagome (kgm) topologic 2D covalent organic frameworks remains challenging, presumably due to the severe dependence on building units and synthetic conditions. Herein, two isomeric "two-in-one" monomers with different lengths of substituted arms based on naphthalene core (p-Naph and m-Naph) are elaborately designed and utilized for the defined synthesis of isomeric kgm Naph-COFs. The two isomeric frameworks exhibit splendid crystallinity and showcase the same chemical composition and topologic structure with, however, different pore channels. Interestingly, C₆₀ is able to uniformly be encapsulated into the triangle channels of m-Naph-COF via in situ incorporation method, while not the isomeric p-Naph-COF, likely due to the different pore structures of the two isomeric COFs. The resulting stable C₆₀ @m-Naph-COF composite exhibits much higher photoconductivity than the m-Naph-COF owing to charge transfer between the conjugated skeletons and C₆₀ guests.

Charge transfer in metal-organic frameworks

Metal-organic frameworks (MOFs, also known as porous coordination polymers or PCPs) are a novel class of crystalline porous material. The tailorable porous structure, in terms of size, geometry and function, has attracted the attention of researchers across all disciplines of materials science. One of the many exciting aspects of MOFs is that through directional and reversible coordination bonding, organic linkers (chromophores with metal-coordinating functional groups) and metal ions (and clusters) can be spatially organized in a preconceived geometry. The well-defined spatial geometry of the metals and linkers is very advantageous for optoelectronic functions (solar cells, light-emitting diodes, photocatalysts) of the materials. This feature article evaluates the scope of charge transfer (CT) interactions in MOFs, involving the organic linkers and metal ion or cluster components. Irrespective of the type (size, shape, electronic property) of organic chromophores involved, MOFs provide an insightful path to design and make the CT process efficient. The selected examples of MOFs with CT characteristics do not only illustrate the design principles but render a pathway towards understanding the complex photophysical processes and implementing those for future optoelectronic and catalytic applications.

Theoretical exploration of noncovalent interactions in Sc2C2@C2n (n = 40, 41, and 42)⊂[12]CPP, PF[12]CPP

The encapsulation of fullerenes by carbon nanorings has gained increasing attention because of the unique molecular structure and special properties of the formed complexes. The host-guest interactions between the fullerenes and the carbon nanorings can influence the metal ion orientation and the molecular electronic structure. In this study, we hooped a series of carbide cluster metallofullerenes, namely Sc₂C₂@C_2v(5)-C₈₀, Sc₂C₂@C_3v(8)-C₈₂, and Sc₂C₂@D_2d(23)-C₈₄, with molecular carbon nanorings of [12]cycloparaphenylene ([12]CPP) and perfluoro[12]cycloparaphenylene (PF[12]CPP). The formed complexes were computationally studied via dispersion-corrected density functional theory calculations. The results showed that the deformation rate of PF[12]CPP after the formation of the fullerene-containing complexes was significantly smaller than that of [12]CPP. The binding energy and thermodynamic information showed that PF[12]CPP was more suitable for fullerene encapsulation. Moreover, charge population analysis showed that PF[12]CPP transferred more electrons to Sc₂C₂@C_2n (n = 40, 41, and 42) compared with [12]CPP. Energy decomposition and real-space function analyses of host-guest interactions revealed the characteristics and nature of the noncovalent interactions in the supramolecules. These results provide theoretical support for the study of host-guest systems based on metallofullerenes.

Trinitroaromatic Salts as High-Energy-Density Organic Cathode Materials for Li-Ion Batteries

Even though organic molecules with designed structures can be assembled into high-capacity electrode materials, only limited functional groups such as -C═O and -C═N- could be designed as high-voltage cathode materials with enough high capacity. Here, we propose a common chemical raw material, trinitroaromatic salt, to have promising potential to develop organic cathode materials with high discharge voltage and capacity through a strong delocalization effect between -NO₂ and aromatic ring. Our first-principles calculations show that electrochemical reactions of trinitroaromatic potassium salt C₆H₂(NO₂)₃OK are a 6-electron charge-transfer process, providing a high discharge capacity of 606 mAh g^-1 and two voltage plateaus of 2.40 and 1.97 V. Electronic structure analysis indicates that the discharge process from C₆H₂(NO₂)₃OK to C₆H₂(NO₂Li₂)₃OK stabilizes oxidized [C₆]ⁿ⁺ to achieve a stable conjugated structure through electron delocalization from -NO₂ to [C₆]ⁿ⁺. The ordered layer structure C₆H₂(NO₂)₃OK can provide large spatial pore channels for Li-ion transport, achieving a high ion diffusion coefficient of 3.41 × 10^-6 cm² s^-1.

Metal-organic frameworks and their derivatives for metal-ion (Li, Na, K and Zn) hybrid capacitors

Metal-ion hybrid capacitors (MIHCs) hold particular promise for next-generation energy storage technologies, which bridge the gap between the high energy density of conventional batteries and the high power density and long lifespan of supercapacitors (SCs). However, the achieved electrochemical performance of available MIHCs is still far from practical requirements. This is primarily attributed to the mismatch in capacity and reaction kinetics between the cathode and anode. In this regard, metal-organic frameworks (MOFs) and their derivatives offer great opportunities for high-performance MIHCs due to their high specific surface area, high porosity, topological diversity, and designable functional sites. In this review, instead of simply enumerating, we critically summarize the recent progress of MOFs and their derivatives in MIHCs (Li, Na, K, and Zn), while emphasizing the relationship between the structure/composition and electrochemical performance. In addition, existing issues and some representative design strategies are highlighted to inspire breaking through existing limitations. Finally, a brief conclusion and outlook are presented, along with current challenges and future opportunities for MOFs and their derivatives in MIHCs.

Fine-Tuning the Electrocatalytic Regeneration of NADH Cofactor Using [Rh(Cp*)(bpy)Cl]+-Functionalized Metal-Organic Framework Films

Electrochemical regeneration of the reduced form of the nicotinamide adenine dinucleotide (NADH) cofactor catalyzed by immobilized [Rh(Cp*)(bpy)Cl]⁺ is a promising approach for the enzymatic synthesis of many valuable chemicals with NAD-dependent dehydrogenases. However, rational control of the efficiency is often limited by the irregular structure of the electrode/electrolyte interface and the accessibility of the molecular catalyst. Here, we propose an electrochemical system for NADH cofactor regeneration, based on highly ordered three- dimensional (3D) metal-organic framework (NU-1000) films. [Rh(Cp*)(bpy)Cl]⁺ is incorporated at the zirconium nodes of NU-1000 via solvent-assisted ligand incorporation (SALI), leading to a diffusion-controlled behavior, associated with an electron hopping mechanism. Varying the ratio of redox-active [Rh(Cp*)(bpy)Cl]⁺ and inactive postgrafting agents enables the elaboration of functional electrodes with tunable electrocatalytic activity for NADH regeneration. The exceptionally high faradic efficiency of 97%, associated with a very high turnover frequency (TOF) of ∼1400 h^-1 for NADH regeneration, and the total turnover number (TTN) of over 20000 for the enzymatic conversion from pyruvate to l-lactate, when coupled with l-lactate dehydrogenases (LDH) as a model reaction, open up promising perspectives for employing these electrodes in various alternative bioelectrosynthesis approaches.

Tetrazine Linkers as Plug-and-Play Tags for General Metal-Organic Framework Functionalization and C60 Conjugation

The value of covalent post-synthetic modification in expanding the chemistry and pore versatility of reticular solids is well documented. Here we use mesoporous crystals of the metal-organic framework (MOF) UiO-68-TZDC to demonstrate the value of tetrazine connectors for all-purpose inverse electron-demand Diels-Alder ligation chemistry. Our results suggest a positive effect of tetrazine reticulation over its reactivity for quantitative one-step functionalization with a broad scope of alkene or alkyne dienophiles into pyridazine and dihydropyridazine frameworks. This permits generating multiple pore environments with diverse chemical functionalities and the expected accessible porosities, that is also extended to the synthesis of crystalline fulleretic materials by covalent conjugation of fullerene molecules.

Approaches to Enhancing Electrical Conductivity of Pristine Metal-Organic Frameworks for Supercapacitor Applications

Metal-organic frameworks (MOFs), known as porous coordination polymers, have attracted intense interest as electrode materials for supercapacitors (SCs) owing to their advantageous features including high surface area, tunable porous structure, structural diversity, etc. However, the insulating nature of most MOFs has impeded their further electrochemical applications. A common solution for this issue is to transform pristine MOFs into more stable and conductive metal compounds/porous carbon materials through pyrolysis, which however losses the inherent merits of MOFs. To find a consummate solution, recently a surge of research devoted to improving the electrical conductivity of pristine MOFs for SCs has been carried out. In this review, the most related research work on pristine MOF-based materials is reviewed and three effective strategies (chemical structure design of conductive MOFs (c-MOFs), composite design, and binder-free structure design) which can significantly increase their conductivity and consequently the electrochemical performance in SCs are proposed. The conductivity enhancement mechanism in each approach is well analyzed. The representative research works on using pristine MOFs for SCs are also critically discussed. It is hoped that the new insights can provide guidance for developing high-performance electrode materials based on pristine MOFs with high conductivity for SCs in the future.

Two-Dimensional Metal-Organic Framework Nanosheets: Synthesis and Applications in Electrocatalysis and Photocatalysis

Two-dimensional metal-organic nanosheets (2D MONs) are an emerging class of ultrathin, porous, and crystalline materials. The organic/inorganic hybrid nature offers MONs distinct advantages over other inorganic nanosheets in terms of diversity of organic ligands and metal notes. Compared to bulk three-dimensional metal-organic frameworks, 2D MONs possess merits of high density and readily accessible catalytic sites, reduced diffusion pathways for reactants/products, and fast electron transport. These features endow MONs with enhanced physical/chemical properties and are ideal for heterogeneous catalysis. In this Review, state-of-the-art synthetic methods for the fabrication of 2D MONs were summarized. The advances of 2D MONs-based materials for electrocatalysis and photocatalysis, including hydrogen evolution reaction (HER), oxygen evolution reaction (OER), oxygen reduction reaction (ORR), carbon dioxide reduction reaction (CO₂ RR), and electro-/photocatalytic organic transformations were systematically discussed. Finally, the challenges and perspectives regarding future design and synthesis of 2D MONs for high-performance electrocatalysis and photocatalysis were provided.

Gas sensing based on metal-organic frameworks: Concepts, functions, and developments

Effective detection of pollutant gases is vital for protection of natural environment and human health. There is an increasing demand for sensing devices that are equipped with high sensitivity, fast response/recovery speed, and remarkable selectivity. Particularly, attention is given to the designability of sensing materials with porous structures. Among diverse kinds of porous materials, metal-organic frameworks (MOFs) exhibit high porosity, high degree of crystallinity and exceptional chemical activity. Their strong host-guest interactions with guest molecules facilitate the application of MOFs in adsorption, catalysis and sensing systems. In particular, the tailorable framework/composition and potential for post-synthetic modification of MOFs endow them with widely promising application in gas sensing devices. In this review, we outlined the fundamental aspects and applications of MOFs for gas sensors, and discussed various techniques of monitoring gases based on MOFs as functional materials. Insights and perspectives for further challenges faced by MOFs are discussed in the end.

Copper(II) Frameworks with Varied Active Site Distribution for Modulating Selectivity of Carbon Dioxide Electroreduction

Metal-organic frameworks (MOFs) can be utilized as electrocatalysts for CO₂ reduction reaction (CO₂RR) due to their well dispersed metal centers. However, the influence of metal node distribution on electrochemical CO₂RR was rarely explored. Here, three Cu-MOFs with different copper(II) site distribution were employed for CO₂ electroreduction. The Cu-MOFs [Cu(L)SO₄]·H₂O (Cu1), [Cu(L)₂(H₂O)₂](CH₃COO)₂·H₂O (Cu2), and [Cu(L)₂(H₂O)₂](ClO₄)₂ (Cu3) were achieved by using the same ligand 1,3,5-tris(1-imidazolyl)benzene (L) but different Cu(II) salts. The results show that the Faraday efficiency of CO (FE_CO) for Cu1 is 4 times that of the FE_H2, while the FE_CO of Cu2 is twice that of the FE_H2. As for Cu3, there is not much difference between FE_CO and FE_H2. Such difference may arise from the distinct electrochemical active surface area and charge transfer kinetics caused by different copper site distribution. Furthermore, the different framework structures also affect the activity of the copper sites, which was supported by the theoretically calculated Gibbs free energy and electron density, contributing to the selectivity of CO₂RR. This study provides a strategy for modulating the selectivity of CO₂RR by tuning the distribution of the active centers in MOFs.

Homogeneously Mixing Different Metal-Organic Framework Structures in Single Nanocrystals through Forming Solid Solutions

Pore engineering plays a significant role in the applications of porous materials, especially in the area of separation and catalysis. Here, we demonstrated a metal-organic framework (MOF) solid solution (MOSS) strategy to homogeneously and controllably mix NU-1000 and NU-901 structures inside single MOF nanocrystals. The key for the homogeneous mixing and forming of MOSS was the bidentate modulator, which was designed to have a slightly longer distance between two carboxylate groups than the original tetratopic ligand. All of the MOSS nanocrystals showed a uniform pore size distribution with a well-tuned ratio of mesopores to micropores. Because of the appropriate pore ratio, MOSS nanocrystals can balance the thermodynamic interactions and kinetic diffusion of the substrates, thus showing exceedingly higher separation abilities and a unique elution sequence. Our work proposes a rational strategy to design mixed-porous MOFs with controlled pore ratios and provides a new direction to design homogeneously mixed MOFs with a high separation ability and unique separation selectivity.

Redox-Hopping-Based Charge Transport Mediated by Ru(II)-Polypyridyl Species Immobilized in a Mesoporous Metal-Organic Framework

Electronic charge transport through crystalline metal-organic frameworks (MOFs) can be accomplished by site-to-site electron (or hole) hopping, provided that redox-active sites, such as easily reducible or oxidizable MOF linkers, are present. If the framework itself is redox-inert, solvent-assisted ligand incorporation of redox-active moieties can serve to enable hopping-based charge transport. Here we have studied the redox hopping process within Ru-bpy@NU-1008, where Ru-bpy is a carboxylate-functionalized derivative, i.e., a node-ligating derivative, of the well-known chromophore Ru(2,2′-bipyridine)32+, and NU-1008 is a redox-inert MOF featuring hierarchical porosity and csq topology. Chronoamperometry experiments with electrode-supported thin films of Ru-bpy@NU-1008 show that charge transport is feasible through portions of the MOF, with other portions being inaccessible. Possible confounding features are the undersized c-pores that cross-connect 1D mesoporous channels, as ingress and egress of charge-compensating anions is believed to accompany the net oxidation of Ru(II) to Ru(III) and the reduction of Ru(III) to Ru(II). Phenomenologically, transport through the electroactive portion of the films is diffusion-like, with the magnitude of the apparent diffusion coefficient being 6 × 10−12 cm2/s.

Multifunctional Anthracene-Based Ni-MOF with Encapsulated Fullerenes: Polarized Fluorescence Emission and Selective Separation of C70 from C60

We report an anthracene-based Ni-MOF [Ni(II) metal-organic framework, {[Ni(μ₂-L)₂Cl₂]·x(C₆H₆)·y(MeOH)}_n (1), L = anthracene-9,10-diylbis(methylene)diisonicotinate] whose crystal structure reveals the presence of hexagonal channels with a pore size of 1.4 nm that can accommodate guests such as C₆₀ and C₇₀. Both confocal fluorescence and Raman microscopy results are in agreement with a homogeneous distribution of fullerenes throughout the single crystals of 1. Efficient energy transfer from 1 to the fullerenes was observed, which emitted partially polarized fluorescence emission. Stronger binding between 1 and C₇₀ versus C₆₀ was confirmed from HPLC analysis of the dissolved material and provides a basis for the selective retention of C₇₀ in liquid chromatography columns packed with 1.

Conductive metal and covalent organic frameworks for electrocatalysis: design principles, recent progress and perspective

Metal and covalent organic frameworks (MOFs/COFs) are emerging promising candidates in the field of catalysts due to their porous nature, chemically well-defined active sites and structural diversity. However, they are typically provided with poor electrical conductivity, which is insufficient for them to work as satisfying electrocatalysts. Designing and fabricating MOFs/COFs with high conductivity presents a new avenue towards special electrochemical reactions. This minireview firstly highlighted the origin and design principles of conductive MOFs/COFs for electrocatalysis on the basis of typical charge transfer mechanisms, that is "through space", "extended conjugation" and "through bond". An overview of conductive MOFs/COFs used in the electrocatalytic carbon dioxide reduction reaction (CO₂RR), water splitting and the oxygen reduction reaction (ORR) was then made to track the very recent progress. In the final remarks, the present challenges and perspectives for the use of conductive MOFs/COFs as electrocatalysts including their structural optimization, feasible applications and structure-activity correlation are proposed.

Donor–acceptor systems in metal–organic frameworks: design, construction, and properties

In this highlight, the development of donor acceptor (D–A) MOF was briefly reviewed and summarized in the aspects of design, construction, and properties. Also, an outlook about the research and potential application of D–A MOF has been presented.

Proton-Conductive Cerium-Based Metal-Organic Frameworks

In this study, proton-conducting behaviors of a cerium-based metal-organic framework (MOF), Ce-MOF-808, its zirconium-based isostructural MOF, and bimetallic MOFs with various Zr-to-Ce ratios are investigated. The significantly increased proton conductivity (σ) and decreased activation energy (E_a) are obtained by substituting Zr with Ce in the nodes of MOF-808. Ce-MOF-808 achieves a σ of 4.4 × 10^-3 S/cm at 25 °C under 99% relative humidity and an E_a of 0.14 eV; this value is among the lowest-reported E_a of proton-conductive MOFs. Density functional theory calculations are utilized to probe the proton affinities of these MOFs. As the first study reporting the proton conduction in cerium-based MOFs, the finding here suggests that cerium-based MOFs should be a better platform for the design of proton conductors compared to the commonly reported zirconium-based MOFs in future studies on energy-related applications.

Group 4 Metal-Based Metal—Organic Frameworks for Chemical Sensors

Metal-organic frameworks (MOFs) have attracted great attention for their applications in chemical sensors mainly due to their high porosity resulting in high density of spatially accessible active sites, which can interact with the aimed analyte. Among various MOFs, frameworks constructed from group 4 metal-based (e.g., zirconium, titanium, hafnium, and cerium) MOFs, have become especially of interest for the sensors requiring the operations in aqueous media owing to their remarkable chemical stability in water. Research efforts have been made to utilize these group 4 metal-based MOFs in chemosensors such as luminescent sensors, colorimetric sensors, electrochemical sensors, and resistive sensors for a range of analytes since 2013. Though several studies in this subfield have been published especially over the past 3–5 years, some challenges and concerns are still there and sometimes they might be overlooked. In this review, we aim to highlight the recent progress in the use of group 4 metal-based MOFs in chemical sensors, and focus on the challenges, potential concerns, and opportunities in future studies regarding the developments of such chemically robust MOFs for sensing applications.

Self-Assembled Metal-Organic Framework Stabilized Organic Cocrystals for Biological Phototherapy

Organic self-assembled co-crystals have garnered considerable attention due to their facile synthesis and intriguing properties, but supramolecular interactions restrict their stability in aqueous solution, which is especially important for biological applications. Herein, we report on the first biological application of aqueous dispersible self-assembled organic co-crystals via the construction of metal-organic framework (MOF) -stabilized co-crystals. In particular, we built an electron-deficient MOF with naphthalene diimide (NDI) as the ligand and biocompatible Ca²⁺ as the metal nodes. An electron donor molecule, pyrene, was encapsulated to form the host-guest MOF self-assembled co-crystal. We observed that such MOF structure leads to uniquely high-density ordered arrangement and the close intermolecular distance (3.47 Å) of the charge transfer pairs. Hence, the concomitant superior charge transfer interaction between pyrene/NDI can be attained and the resultant photothermal conversion efficiency of Py@Ca-NDI in aqueous solution can thus reach up to 41.8 %, which, to the best of our knowledge, is the highest value among the reported organic co-crystal materials; it is also much higher than that of the FDA approved photothermal agent ICG as well as most of the reported MOFs. Based on this realization, as a proof of concept, we demonstrated that such a self-assembled organic co-crystal platform can be used in biological applications that are exemplified via highly effective long wavelength light photothermal therapy.

Effects of intervalence charge transfer interaction between π-stacked mixed valent tetrathiafulvalene ligands on the electrical conductivity of 3D metal-organic frameworks

Achieving a molecular-level understanding of how the structures and compositions of metal–organic frameworks (MOFs) influence their charge carrier concentration and charge transport mechanism—the two key parameters of electrical conductivity—is essential for the successful development of electrically conducting MOFs, which have recently emerged as one of the most coveted functional materials due to their diverse potential applications in advanced electronics and energy technologies. Herein, we have constructed four new alkali metal (Na, K, Rb, and Cs) frameworks based on an electron-rich tetrathiafulvalene tetracarboxylate (TTFTC) ligand, which formed continuous π-stacks, albeit with different π–π-stacking and S⋯S distances (d_π–π and d_S⋯S). These MOFs also contained different amounts of aerobically oxidized TTFTC˙⁺ radical cations that were quantified by electron spin resonance (ESR) spectroscopy. Density functional theory calculations and diffuse reflectance spectroscopy demonstrated that depending on the π–π-interaction and TTFTC˙⁺ population, these MOFs enjoyed varying degrees of TTFTC/TTFTC˙⁺ intervalence charge transfer (IVCT) interactions, which commensurately affected their electronic and optical band gaps and electrical conductivity. Having the shortest d_π–π (3.39 Å) and the largest initial TTFTC˙⁺ population (∼23%), the oxidized Na-MOF 1-ox displayed the narrowest band gap (1.33 eV) and the highest room temperature electrical conductivity (3.6 × 10⁻⁵ S cm⁻¹), whereas owing to its longest d_π–π (3.68 Å) and a negligible TTFTC˙⁺ population, neutral Cs-MOF 4 exhibited the widest band gap (2.15 eV) and the lowest electrical conductivity (1.8 × 10⁻⁷ S cm⁻¹). The freshly prepared but not optimally oxidized K-MOF 2 and Rb-MOF 3 initially displayed intermediate band gaps and conductivity, however, upon prolonged aerobic oxidation, which raised the TTFTC˙⁺ population to saturation levels (∼25 and 10%, respectively), the resulting 2-ox and 3-ox displayed much narrower band gaps (∼1.35 eV) and higher electrical conductivity (6.6 × 10⁻⁵ and 4.7 × 10⁻⁵ S cm⁻¹, respectively). The computational studies indicated that charge movement in these MOFs occurred predominantly through the π-stacked ligands, while the experimental results displayed the combined effects of π–π-interactions, TTFTC˙⁺ population, and TTFTC/TTFTC˙⁺ IVCT interaction on their electronic and optical properties, demonstrating that IVCT interactions between the mixed-valent ligands could be exploited as an effective design strategy to develop electrically conducting MOFs.

Through-space charge movement enabled by intervalence charge transfer interactions between π-stacked mixed-valent tetrathiafulvalene ligands creates electrical conductivity in three-dimensional metal–organic frameworks.

Molecular structures and magnetic properties of endohedral metallofullerenes

Endohedral metallofullerenes have fascinating core-shell structures, with metal atoms or metal clusters encaged in fullerene cages, and they display various chemical, optical and magnetic properties derived from different types of fullerene cages and metal moieties. Fullerene cages can act as carriers to stabilize unusual cluster moieties. Many bizarre species that are hard to produce via synthetic methods survive well under the protection of a fullerene cage, making metallofullerenes ideal platforms for generating new clusters and bonds. Fullerene cages can also be carriers to hold active unpaired electrons. Some metallofullerenes possess electron spin and show intriguing magnetic properties, making them applicable for use in quantum computing, high density information storage and magnetoreception systems. The exploration of new metallofullerenes is still ongoing, while function-oriented studies are also promoted for the future application of metallofullerenes. Herein, we highlight the recent progress in the synthesis, electron spin characteristics and magnetic properties of metallofullerenes. Discussions and an outlook on the future development of metallofullerenes are also stated.

Metal-Organic Frameworks Derived Catalyst for High-Performance Vanadium Redox Flow Batteries

Vanadium redox flow battery (VRFB) is one of the most promising technologies for grid-scale energy storage applications because of its numerous attractive features. In this study, metal-organic frameworks (MOF)-derived catalysts (MDC) are fabricated using carbonization techniques at different sintering temperatures. Zirconium-based MOF-derived catalyst annealed at 900 °C exhibits the best electrochemical activity toward VO2+/VO2+ redox couple among all samples. Furthermore, the charge-discharge test confirms that the energy efficiency (EE) of the VRFB assembled with MOF-derived catalyst modified graphite felt (MDC-GF-900) is 3.9% more efficient than the VRFB using the pristine graphite felt at 100 mA cm−2. Moreover, MDC-GF-900 reveals 31% and 107% higher capacity than the pristine GF at 80 and 100 mA cm−2, respectively. The excellent performance of MDC-GF-900 results from the existence of oxygen-containing groups active sites, graphite structure with high conductivity embedded with zirconium oxide, and high specific surface area, which are critical points for promoting the vanadium redox reactions. Because of these advantages, MDC-GF-900 also possesses superior stability performance, which shows no decline of EE even after 100 cycles at 100 mA cm−2.

Synthesis, Structures, and Sorption Properties of Two New Metal-Organic Frameworks Constructed by the Polycarboxylate Ligand Derived from Cyclotriphosphazene

Solvothermal reactions of hexakis(4-carboxyphenoxy)cyclotriphospazene (H₆L1) with copper ions in DMF/H₂O produced one complex, {[Cu₆(L1)₂(OH)(H₂O)₃]·guest} _n (1), but with copper ions and auxiliary rigid 4,4-bipyridine (bpy) produced another new complex, namely, {[Cu₃(L1)(bpy)(H₂O)₆]·guest} _n (2). These complexes had been characterized by IR spectroscopy, elemental analysis, and X-ray structural determination. 1 exhibits a 3D anionic structure with the binodal 4,8-connected network with Schläfli symbol {4⁶}₂{4⁹·6¹⁸·8}, consisting of Cu₆ clusters and L1 ligands. In contrast, complex 2 possesses a different 3D network with trinodal 3,4,6-c topology with Schläfli symbol {4·6²}₂{4²·6⁶·8⁵·10²}{6⁴·8·10}. In these two complexes, the semirigid hexacarboxylate ligands adopt distinct conformations to connect metal ions/clusters, which must be ascribed to the addition of the auxiliary rigid ligand in reaction systems. In addition, gas absorption properties of 1 and 2 including CO₂ and N₂ were further investigated.

Conductive Metal-Organic Frameworks: Electronic Structure and Electrochemical Applications

Smarter and minimization of devices are consistently substantial to shape the energy landscape. Significant amounts of endeavours have come forward as promising steps to surmount this formidable challenge. It is undeniable that material scientists were contemplating smarter material beyond purely inorganic or organic materials. To our delight, metal-organic frameworks (MOFs), an inorganic-organic hybrid scaffold with unprecedented tunability and smart functionalities, have recently started their journey as an alternative. In this review, we focus on such propitious potential of MOFs that was untapped over a long time. We cover the synthetic strategies and (or) post-synthetic modifications towards the formation of conductive MOFs and their underlying concepts of charge transfer with structural aspects. We addressed theoretical calculations with the experimental outcomes and spectroelectrochemistry, which will trigger vigorous impetus about intrinsic electronic behaviour of the conductive frameworks. Finally, we discussed electrocatalysts and energy storage devices stemming from conductive MOFs to meet energy demand in the near future.

Hydrophobic Metal-Organic Frameworks and Derived Composites for Microelectronics Applications

The extraordinary characteristic features of metal-organic frameworks (MOFs) make them applicable for use in a variety of fields but their conductivity in microelectronics over a wide relative humidity (RH) range has not been extensively explored. To achieve good performance, MOFs must be stable in water, i. e., under humid conditions. However, the design of ultrastable hydrophobic MOFs with high conductivity for use in microelectronics as conducting and dielectric materials remains a challenge. In this Review, we discuss applications of an emerging class of hydrophobic MOFs with respect to their use as active sensor coatings, tunable low-κ dielectrics and conductivity, which provide high-level roadmap for stimulating the next steps toward the development and implementation of hydrophobic MOFs for use in microelectronic devices. Several methodologies including the incorporation of long alkyl chain and fluorinated linkers, doping of redox-active 7,7,8,8-tetracyanoquinodimethane (TCNQ), the use of guest molecules, and conducting polymers or carbon materials in the pores or surface of MOFs have been utilized to produce hydrophobic MOFs. The contact angle of a water droplet and a coating can be used to evaluate the degree of hydrophobicity of the surface of a MOF. These unique advantages enable hydrophobic MOFs to be used as a highly versatile platform for exploring multifunctional porous materials. Classic representative examples of each category are discussed in terms of coordination structures, types of hydrophobic design, and potential microelectronic applications. Lastly, a summary and outlook as concluding remarks in this field are presented. We envision that future research in the area of hydrophobic MOFs promise to provide important breakthroughs in microelectronics applications.

Physical properties of porphyrin-based crystalline metal‒organic frameworks

Metal‒organic frameworks (MOFs) are widely studied molecular assemblies that have demonstrated promise for a range of potential applications. Given the unique and well-established photophysical and electrochemical properties of porphyrins, porphyrin-based MOFs are emerging as promising candidates for energy harvesting and conversion applications. Here we discuss the physical properties of porphyrin-based MOFs, highlighting the evolution of various optical and electronic features as a function of their modular framework structures and compositional variations.

Pyrene-based metal organic frameworks: from synthesis to applications

Pyrene is one of the most widely investigated aromatic hydrocarbons given to its unique optical and electronic properties. Hence, pyrene-based ligands have been attractive for the synthesis of metal-organic frameworks (MOFs) in the last few years. In this review, we will focus on the most important characteristics of pyrene, in addition to the development and synthesis of pyrene-based molecules as bridging ligands to be used in MOF structures. We will summarize the synthesis attempts, as well as the post-synthetic modifications of pyrene-based MOFs by the incorporation of metals or ligands in the structure. The discussion of promising results of such MOFs in several applications; including luminescence, photocatalysis, adsorption and separation, heterogeneous catalysis, electrochemical applications and bio-medical applications will be highlighted. Finally, some insights and future prospects will be given based on the studies discussed in the review. This review will pave the way for the researchers in the field for the design and development of novel pyrene-based structures and their utilization for different applications.

Conductive, Large-Area, and Continuous 7,7,8,8-Tetracyanoquinodimethane@HKUST-1 Thin Films Fabricated Using Solution Shearing

Most metal-organic frameworks (MOFs) have an insulating nature due to their porosity and redox-inactive organic components. The electrical conductivity of the prototypical MOF, HKUST-1, can be tuned by infiltrating a small-molecule organic semiconductor, 7,7,8,8-tetracyanoquinodimethane (TCNQ), into the HKUST-1 pores, creating TCNQ@HKUST-1. However, current processes of creating TCNQ@HKUST-1 films have many roadblocks such as slow crystallization rates, which limit high throughput production, and the formation of Cu(TCNQ) as a byproduct, which affects the electrical conductivity and degrades the chemical structure of HKUST-1. In this work, we show that HKUST-1 films can be rapidly synthesized over large areas with consistent thickness and no pinholes via a meniscus-guided coating technique called solution shearing. The subsequent pore activation process and TCNQ impregnation can be completed via solvent exchange to minimize the formation of the Cu(TCNQ) byproduct, and we obtain an increase in electrical conductivity of the solution-sheared TCNQ@HKUST-1 thin films of over 7 orders of magnitude, reaching a maximum value of 2.42 × 10^-2 S m^-1 when TCNQ is incorporated for 10 days. The conductivity of solution-sheared TCNQ@HKUST-1 is higher compared to films formed by high-pressure pelletization of TCNQ@HKUST-1. We show that solution shearing can produce large-area thin films rapidly and reduce the formation of grain boundaries better than pelletization, allowing for large-area electronics with both charge transport and porosity for applications as sensors and electronics.

Electrically Conductive Coordination Polymers for Electronic and Optoelectronic Device Applications

Electrically conductive coordination polymers (generally known as metal-organic frameworks, MOFs) are a class of crystalline hybrid materials produced by the reasonable self-assembly of metal nodes and organic linkers. The unique and intriguing combination of inorganic and organic components endows coordination polymers with superior optical and electrical properties, which have recently aroused much attention in several electronic and optoelectronic technological applications. However, there are many challenging obstacles and issues that need to be addressed in this burgeoning field. In this Perspective, we first provide a fundamental understanding about the electronic design strategies that provide better guidance for realizing high conductivities and good mobilities in coordination polymers. We then examine the current established synthetic approaches to construct high-quality working samples of electrically conductive coordination polymers for device integration. This is followed by a discussion of the current state-of-the-art progress toward the preliminary achievements in (opto)electronic devices spanning chemiresistive sensors, field-effect transistors, organic photovoltaics, photodetectors, etc. Finally, we conclude this Perspective with the existing hurdles and limitations in this area, along with the critical directions and opportunities for future research.

Selective Formation of Polyaniline Confined in the Nanopores of a Metal-Organic Framework for Supercapacitors

In this study, a strategy that can result in the polyaniline (PANI) solely confined within the nanopores of a metal-organic framework (MOF) without forming obvious bulk PANI between MOF crystals is developed. A water-stable zirconium-based MOF, UiO-66-NH₂ , is selected as the MOF material. The polymerization of aniline is initiated in the acidic suspension of UiO-66-NH₂ nanocrystals in the presence of excess poly(sodium 4-styrenesulfonate) (PSS). Since the pore size of UiO-66-NH₂ is too small to enable the insertion of the bulky PSS, the quick formation of pore-confined solid PANI and the slower formation of well dispersed PANI:PSS occur within the MOF crystals and in the bulk solution, respectively. By taking advantage of the resulting homogeneous PANI:PSS polymer solution, the bulk PANI:PSS can be removed from the PANI/UiO-66-NH₂ solid by successive washing the sample with fresh acidic solutions through centrifugation. As this is the first time reporting the PANI solely confined in the pores of a MOF, as a demonstration, the obtained PANI/UiO-66-NH₂ composite material is applied as the electrode material for supercapacitors. The PANI/UiO-66-NH₂ thin films exhibit a pseudocapacitive electrochemical characteristic, and their resulting electrochemical activity and charge-storage capacities are remarkably higher than those of the bulk PANI thin films.