Topotactic transformation of zeolitic imidazolate frameworks into high-performance battery type electrodes for supercapattery application

Supercapacitors (SCs) are well recognized for their excessive power output and cycling stability, but they often suffer from limited energy density. A promising solution to this challenge is the hybrid supercapattery (HSC) concept, which integrates two different electrodes with disparate charge-storage systems to provide energy and power. In this work, transition-metal phosphides (TMPs), specifically a Cu-doped cobalt phosphide wrapped with an N-doped porous carbon network (CCP-NPC), were used as positive electrode materials in HSCs. With a specific capacitance of 5.99 F cm-2 and a capacitance retention of 87% after 10 000 cycles, the extremely active CCP-5-NPC (5% Cu-doped cobalt phosphide wrapped with an N-doped porous carbon network) exhibits numerous redox sites. The unique structure of CCP-5-NPC, characterized by its cubical shape, coarse surface, and porous structure, greatly enhances the electrochemically active sites (EAS) and specific surface areas (SSA) of the electrode material, facilitating efficient charge transfer kinetics for ions and electrons in HSCs. The potential hybrid supercapattery (CCP-5-NPC||r-GO device) also demonstrated a higher energy density of 0.563 mW h cm-2 at a power density of 4.8 mW cm-2 at 3 mA cm-2 and a cyclic stability of 87.7% after 10 000 cycles. This work provides a basis for the development of highly efficient HSCs in the future by topotactically converting extremely porous materials into energy storage devices.

Reversible Co(II)-Co(III) Transformation in a Family of Metal-Dipyrazolate Frameworks

Transformation between oxidation states is widespread in transition metal coordination chemistry and biochemistry, typically occurring in solution. However, air-induced oxidation in porous crystalline solids with retention of crystallinity is rare due to the dearth of materials with high structural stability that are inherently redox active. Herein, we report a new family of such materials, four isostructural cobalt-pyrazolate frameworks of face-centered cubic, fcu, topology, fcu-L-Co, that are sustained by Co8 molecular building blocks (MBBs) and dipyrazolate ligands, L. fcu-L-Co were observed to spontaneously transform from Co(II)8 to Co(III)8 MBBs in air with retention of crystallinity, marking the first such instance in metal-organic frameworks (MOFs). This transformation can also be achieved through water vapor sorption cycling, heating, or chemical oxidation. The reverse reactions were conducted by exposure of fcu-L-Co(III) to aqueous hydrazine. fcu-L-Co(II) exhibited high gravimetric water vapor uptakes of 0.55-0.68 g g-1 at 30% relative humidity (RH), while in fcu-L-Co(III) the inflection point shifted to lower RH and framework stability improved. Insight into the transformation between fcu-L-Co(II) and fcu-L-Co(III) was gained from single crystal X-ray diffraction and in situ spectroscopy. Overall, the crystal engineering approach we adopted has afforded a new family of MOFs that exhibit cobalt redox chemistry in a confined space coupled with high porosity.

The Molecular Nature of Redox-Conductive Metal-Organic Frameworks

ConspectusRedox-conductive metal-organic frameworks (RC-MOFs) are a class of porous materials that exhibit electrical conductivity through a chain of self-exchange reactions between molecularly defined, neighboring redox-active units of differing oxidation states. To maintain electroneutrality, this electron hopping transport is coupled to the translocation of charge balancing counterions. Owing to the molecular nature of the redox active components, RC-MOFs have received increasing attention for potential applications in energy storage, electrocatalysis, reconfigurable electronics, etc. While our understanding of fundamental aspects that govern electron hopping transport in RC-MOFs has improved during the past decade, certain fundamental aspects such as questions that arise from the coupling between electron hopping and diffusion migration of charge balancing counterions are still not fully understood.In this Account, we summarize and discuss our group's efforts to answer some of these fundamental questions while also demonstrating the applicability of RC-MOFs in energy-related applications. First, we introduce general design strategies for RC-MOFs, fundamentals that govern their charge transport properties, and experimental diagnostics that allow for their identification. Selected examples with redox-active organic linkers or metallo-linkers are discussed to demonstrate how the molecular characteristics of the redox-active units inside RC-MOFs are retained. Second, we summarize experimental techniques that can be used to characterize charge transport properties in a RC-MOF. The apparent electron diffusion coefficient, Deapp, that is frequently determined in the field and obtained in large perturbation, transient experiments will be discussed and related to redox conductivity, σ, that is obtained in a steady state setup. It will be shown that both MOF-intrinsic (topology, pore size, and apertures) and experimental (nature of electrolyte, solvent) factors can have noticeable impact on electrical conductivity through RC-MOFs. Lastly, we summarize our progress in utilizing RC-MOFs as electrochromic materials, materials for harvesting minority carriers from illuminated semiconductors and within electrocatalysis. In the latter case, recent work on multivariate RC-MOFs in which redox active linkers are used to "wire" redox catalysts in the crystal interiors will be presented, offering opportunities to independently optimize charge transport and catalytic function.The ambition of this Account is to inspire the design of new RC-MOF systems, to aid their identification, to provide mechanistic insights into the governing ion-coupled electron hopping transport mode of conductivity, and ultimately to promote their applications in existing and emerging areas. With basically unlimited possibilities of molecular engineering tools, together with research in both fundamental and applied fields, we believe that RC-MOFs will attract even more attention in the future to unlock their full potential.

Extrinsically conducting MOFs: guest-promoted enhancement of electrical conductivity, thin film fabrication and applications

Conductive metal-organic frameworks are of current interest in chemical science because of their applications in chemiresistive sensing, electrochemical energy storage, electrocatalysis, etc. Different strategies have been employed to design conductive frameworks. In this review, we discuss the influence of different types of guest species incorporated within the pores or channels of metal-organic frameworks (MOFs) and porous coordination polymers (PCPs) to generate charge transfer pathways and modulate their electrical conductivity. We have classified dopants or guest species into three different categories: (i) metal-based dopants, (ii) molecule and molecular entities and (iii) organic conducting polymers. Different types of metal ions, metal nano-clusters and metal oxides have been used to enhance electrical conductivity in MOFs. Metal ions and metal nano-clusters depend on the hopping process for efficient charge transfer whereas metal-oxides show charge transport through the metal-oxygen pathway. Several types of molecules or molecular entities ranging from neutral TCNQ, I2, and fullerene to ionic methyl viologen, organometallic like nickelcarborane, etc. have been used. In these cases, the charge transfer process varies with the guest species. When organic conducting polymers are the guest, the charge transport occurs through the polymer chains, mostly based on extended π-conjugation. Here we provide a comprehensive and critical review of these strategies to add electrical conductivity to the, in most cases, otherwise insulating MOFs and PCPs. We point out the guest encapsulation process, the geometry and structure of the resulting host-guest complex, the host-guest interactions and the charge transport mechanism for each case. We also present the methods for thin film fabrication of conducting MOFs (both, liquid-phase and gas-phase based methods) and their most relevant applications like electrocatalysis, sensing, charge storage, photoconductivity, photocatalysis,… We end this review with the main obstacles and challenges to be faced and the appealing perspectives of these 21st century materials.

Metal-organic frameworks for biomedical applications: A review

Metal-organic frameworks (MOFs) are emergent materials in diverse prospective biomedical uses, owing to their inherent features such as adjustable pore dimension and volume, well-defined active sites, high surface area, and hybrid structures. The multifunctionality and unique chemical and biological characteristics of MOFs allow them as ideal platforms for sensing numerous emergent biomolecules with real-time monitoring towards the point-of-care applications. This review objects to deliver key insights on the topical developments of MOFs for biomedical applications. The rational design, preparation of stable MOF architectures, chemical and biological properties, biocompatibility, enzyme-mimicking materials, fabrication of biosensor platforms, and the exploration in diagnostic and therapeutic systems are compiled. The state-of-the-art, major challenges, and the imminent perspectives to improve the progressions convoluted outside the proof-of-concept, especially for biosensor platforms, imaging, and photodynamic therapy in biomedical research are also described. The present review may excite the interdisciplinary studies at the juncture of MOFs and biomedicine.

Chromium(II)-isophthalate 2D MOF with Redox-Tailorable Gas Adsorption Selectivity

Redox-active metal-organic frameworks (MOFs) are very promising materials due to their potential capabilities for postsynthetic modification aimed at tailoring their application properties. However, the research field related to redox-active MOFs is still relatively underdeveloped, which limits their practical application. We investigated the self-assembly process of Cr(II) ions and isophthalate (m-bdc) linkers, which have been previously demonstrated to yield 0D metal-organic polyhedra. However, using the diffusion-controlled synthetic approach, we demonstrate the selective preparation of a 2D-layered Cr(II)-based MOF material [Cr(m-bdc)]·H2O (1·H2O). Remarkably, the controlled oxidation of the developed 2D MOF using nitric oxide or dry oxygen resulted in modified porous materials with excellent H2/N2 adsorption selectivities.

Emerging Mixed-Valence Porous Materials

The development and applications of porous materials have been revolutionized over the past decades. To fine-tune their fascinating behaviors such as charge-transport, magnetic, and catalytic properties, one of the most effective strategies is incorporating mixed valency into the structures. This Concept provides recent progress on emerging crystalline porous materials, namely, metal-organic frameworks, metal-organic polyhedra, covalent organic frameworks, and hydrogen-bonded organic frameworks, where their mixed valency is either intrinsic or postsynthetically induced via an external stimulus. We further highlight the investigation of mixed valency and potential applications, which sheds light on the design and exploitation of mixed-valence crystalline porous materials.

Zirconium-based metal-organic frameworks and their roles in electrocatalysis

Due to their exceptional chemical stability in water and high structural tunability, zirconium(IV)-based MOFs (Zr-MOFs) have been considered attractive materials in the broad fields of electrocatalysis. Numerous studies published since 2015 have attempted to utilise Zr-MOFs in electrocatalysis, with the porous framework serving as either the active electrocatalyst or the scaffold or surface coating to further enhance the performance of the actual electrocatalyst. Herein, the roles of Zr-MOFs in electrocatalytic processes are discussed, and some selected examples reporting the applications of Zr-MOFs in various electrocatalytic reactions, including several studies from our group, are overviewed. Challenges, limitations and opportunities in using Zr-MOFs in electrocatalysis in future studies are discussed.

Porous Crystalline Materials Based on Tetrathiafulvalene and Its Analogues: Assembly, Charge Transfer, and Applications

ConspectusThe directed synthesis and functionalization of porous crystalline materials pose significant challenges for chemists. The synergistic integration of different functionalities within an ordered molecular material holds great significance for expanding its applications as functional materials. The presence of coordination bonds connected by inorganic and organic components in molecular materials can not only increase the structural diversity of materials but also modulate the electronic structure and band gap, which further regulates the physical and chemical properties of molecular materials. In fact, porous crystalline materials with coordination bonds, which inherit the merits of both organic and inorganic materials, already showcase their superior advantages in optical, electrical, and magnetic applications. In addition to the inorganic components that provide structural rigidity, organic ligands of various types serve as crucial connectors in the construction of functional porous crystalline materials. In addition, redox activity can endow organic linkers with electrochemical activity, thereby making them a perfect platform for the study of charge transfer with atom-resolved single-crystal structures, and they can additionally serve as stimuli-responsive sites in sensor devices and smart materials.In this Account, we introduce the synthesis, structural characteristics, and applications of porous crystalline materials based on the famous redox-active units, tetrathiafulvalene (TTF) and its analogues, by primarily focusing on metal-organic frameworks (MOFs) and covalent organic frameworks (COFs). TTF, a sulfur-rich conjugated molecule with two reversible and easily accessible oxidation states (i.e., radical TTF•+ cation and TTF2+ dication), and its analogues boast special electrical characteristics that enable them to display switchable redox activity and stimuli-responsive properties. These inherent properties contribute to the enhancement of the optical, electrical, and magnetic characteristics of the resultant porous crystalline materials. Moreover, delving into the charge transfer phenomena, which is key for the electrochemical process within these materials, uncovers a myriad of potential functional applications. The Account is organized into five main sections that correspond to the different properties and applications of these materials: optical, electrical, and magnetic functionalities; energy storage and conversion; and catalysis. Each section provides detailed discussions of synthetic methods, structural characteristics, the physical and chemical properties, and the functional performances of highlighted examples. The Account also discusses future directions by emphasizing the exploration of novel organic units, the transformation between radical cation TTF•+ and dication TTF2+, and the integration of multifunctionalities within these frameworks to foster the development of smart materials for enhanced performance across diverse applications. Through this Account, we aim to highlight the massive potential of TTF and its analogues-based porous crystals in chemistry and material science.

Aliovalent Substitution Tunes Physical Properties in a Conductive Bis(dithiolene) Two-Dimensional Metal-Organic Framework

Two-dimensional conductive metal-organic frameworks have emerged as promising electronic materials for applications in (opto)electronic, thermoelectric, magnetic, electrocatalytic, and energy storage devices. Many bottom-up or postsynthetic protocols have been developed to isolate these materials or further modulate their electronic properties. However, some methodologies commonly used in classic semiconductors, notably, aliovalent substitution, are conspicuously absent. Here, we demonstrate how aliovalent Fe(III) to Ni(II) substitution enables the isolation of a Ni bis(dithiolene) material from a previously reported Fe analogue. Detailed characterization supports the idea that aliovalent substitution of Fe(III) to Ni(II) results in an in situ oxidation of the organic dithiolene linker. This substitution-induced redox tuning modulates the electronic properties in the system, leading to higher electrical conductivity and Hall mobility but slightly lower carrier densities and weaker antiferromagnetic interactions. Moreover, this aliovalent substitution improves the material's electrochemical stability and thus enables pseudocapacitive behavior in the Ni material. These results demonstrate how classic aliovalent substitution strategies in semiconductors can also be leveraged in conductive MOFs and add further support to this class of compounds as emerging electronic materials.

Diffusional Electron Transport Coupled to Thermodynamically Driven Electron Transfers in Redox-Conductive Multivariate Metal-Organic Frameworks

The development of redox-conductive metal-organic frameworks (MOFs) and the fundamental understanding of charge propagation through these materials are central to their applications in energy storage, electronics, and catalysis. To answer some unresolved questions about diffusional electron hopping transport and redox conductivity, mixed-linker MOFs were constructed from two statistically distributed redox-active linkers, pyromellitic diimide bis-pyrazolate (PMDI) and naphthalene diimide bis-pyrazolate (NDI), and grown as crystalline thin films on conductive fluorine-doped tin oxide (FTO). Owing to the distinct redox properties of the linkers, four well-separated and reversible redox events are resolved by cyclic voltammetry, and the mixed-linker MOFs can exist in five discrete redox states. Each state is characterized by a unique spectroscopic signature, and the interconversions between the states can be followed spectroscopically under operando conditions. With the help of pulsed step-potential spectrochronoamperometry, two modes of electron propagation through the mixed-linker MOF are identified: diffusional electron hopping transport between linkers of the same type and a second channel that arises from thermodynamically driven electron transfers between linkers of different types. Corresponding to the four redox events of the mixed-linker MOFs, four distinct bell-shaped redox conductivity profiles are observed at a steady state. The magnitude of the maximum redox conductivity is evidenced to be dependent on the distance between redox hopping sites, analogous to the situation for apparent electron diffusion coefficients, Deapp, that are obtained in transient experiments. The design of mixed-linker redox-conductive MOFs and detailed studies of their charge transport properties present new opportunities for future applications of MOFs, in particular, within electrocatalysis.

Radical-Driven Crystal-Amorphous-Crystal Transition of a Metal-Organic Framework

Self-assembly-based structural transition has been explored for various applications, including molecular machines, sensors, and drug delivery. In this study, we developed new redox-active metal-organic frameworks (MOFs) called DGIST-10 series that comprise π-acidic 1,4,5,8-naphthalenediimide (NDI)-based ligands and Ni2+ ions, aiming to boost ligand-self-assembly-driven structural transition and study the involved mechanism. Notably, during the synthesis of the MOFs, a single-crystal-amorphous-single-crystal structural transition occurred within the MOFs upon radical formation, which was ascribed to the fact that radicals prefer spin-pairing or through-space electron delocalization by π-orbital overlap. The radical-formation-induced structural transitions were further confirmed by the postsynthetic solvothermal treatment of isolated nonradical MOF crystals. Notably, the transient amorphous phase without morphological disintegration was clearly observed, contributing to the seminal structural change of the MOF. We believe that this unprecedented structural transition triggered by the ligand self-assembly magnifies the structural flexibility and diversity of MOFs, which is one of the pivotal aspects of MOFs.

Alloxazine-Based Ligands Appended with Coordinating Groups: Synthesis, Electrochemical Studies, and Formation of Coordination Polymers

Three new ligands based on the alloxazine core appended with pyridyl coordinating groups have been designed, synthesized, and characterized. The ligands are revealed to be redox-active in DMF solution, as attested to by CV and combined CV/EPR studies. The spin of the reduced species appears to be delocalized on the alloxazine core, as attested to by DFT calculations. The coordination abilities of one of the ligands toward Cu2+ or Ni2+ 3d cations revealed the formation of the first alloxazine-based 3D coordination polymers, presenting strong π-π stacking and substantial cavities. Preliminarily charge/discharge experiments in Li batteries evidence Li+ insertion in such systems.

Metal Complexes of Redox Non-Innocent Ligand N,N'-Bis(3,5-di-tertbutyl-2-hydroxy-phenyl)-1,2-phenylenediamine

Redox non-innocent ligands react with metal precursors to form complexes where the oxidation states of the ligand and thus the metal atom cannot be easily defined. A well-known example of such ligands is bis(o-aminophenol) N,N'-bis(3,5-di-tertbutyl-2-hydroxy-phenyl)-1,2-phenylenediamine, previously developed by the Wieghardt group, which has a potentially tetradentate coordination mode and four distinct protonation states, whereas its electrochemical behavior allows for five distinct oxidation states. This rich redox chemistry, as well as the ability to coordinate to various transition metals, has been utilized in the syntheses of metal complexes with M2L, ML and ML2 stoichiometries, sometimes supported with other ligands. Different oxidation states of the ligand can adopt different coordination modes. For example, in the fully oxidized form, two N donors are sp2-hybridized, which makes the ligand planar, whereas in the fully reduced form, the sp3-hybridized N donors allow the formation of more flexible chelate structures. In general, the metal can be reduced during complexation, but redox processes of the isolated complexes typically occur on the ligand. Combination of this non-innocent ligand with redox-active transition metals may lead to complexes with interesting magnetic, electrochemical, photonic and catalytic properties.

Efficient Direct X-ray Detection and Imaging Based on a Lead-Free Electron Donor-Acceptor MOF

Metal-organic frameworks (MOFs) have recently gained extensive attention as potential materials for direct radiation detection due to their strong radiation absorption, long-range order, and chemical tunability. However, it remains challenging to develop a practical MOF-based X-ray direct detector that possesses high X-ray detection efficiency, radiation stability, and environmental friendliness. The integration of donor-acceptor (D-A) pairs into crystalline MOFs is a powerful strategy for the precise fabrication of multifunctional materials with unique optoelectronic properties. Herein, a new lead-free MOF, Cu2I2(TPPA) (CuI-TPPA, TPPA = tris[4-(pyridine-4-yl)phenyl]amine), with a 6-fold interpenetrated structure is designed and synthesized based on the electron donor-acceptor strategy. CuI-TPPA has a large mobility-lifetime (μτ) product of 5.8 × 10-4 cm2 V-1 and a high detection sensitivity of 73.1 μC Gyair-1 cm-2, surpassing that of commercial α-Se detectors. Moreover, the detector remains fairly stable with only a 2% reduction in photocurrent under continuous bias irradiation conditions with a total dose of over 42.83 Gyair. The CuI-TPPA/poly(vinylidene fluoride) flexible composite X-ray detector films are successfully manufactured with different thicknesses. Through multifaceted assessments, the optimal thickness is found with a high detection sensitivity of up to 143.6 μC Gyair-1 cm-2. As proof-of-concept, 11 × 9 pixelated X-ray detectors are fabricated on the same composite film to realize X-ray direct imaging. This work opens up potential applications of MOFs in environmentally friendly and wearable devices for direct X-ray detection and imaging.

Rational Design of Stimuli-Responsive Inorganic 2D Materials via Molecular Engineering: Toward Molecule-Programmable Nanoelectronics

The ability of electronic devices to act as switches makes digital information processing possible. Succeeding graphene, emerging inorganic 2D materials (i2DMs) have been identified as alternative 2D materials to harbor a variety of active molecular components to move the current silicon-based semiconductor technology forward to a post-Moore era focused on molecule-based information processing components. In this regard, i2DMs benefits are not only for their prominent physiochemical properties (e.g., the existence of bandgap), but also for their high surface-to-volume ratio rich in reactive sites. Nonetheless, since this field is still in an early stage, having knowledge of both i) the different strategies for molecularly functionalizing the current library of i2DMs, and ii) the different types of active molecular components is a sine qua non condition for a rational design of stimuli-responsive i2DMs capable of performing logical operations at the molecular level. Consequently, this Review provides a comprehensive tutorial for covalently anchoring ad hoc molecular components-as active units triggered by different external inputs-onto pivotal i2DMs to assess their role in the expanding field of molecule-programmable nanoelectronics for electrically monitoring bistable molecular switches. Limitations, challenges, and future perspectives of this emerging field which crosses materials chemistry with computation are critically discussed.

Peculiar Differences between Two Copper Complexes Containing Similar Redox-Active Ligands: Density Functional and Multiconfigurational Calculations

Transition metal complexes featuring redox-active ligands often exhibit multiple redox states, influenced by the interplay between the metal center and the ligand. This study delves into the electronic structures of two mononuclear complexes of copper with two similar redox-active urea azine ligands. The ligands differ by the replacement of an NCH3 moiety by an S atom in the ligand backbone. Experimental analysis yields pronounced electronic structural disparities between these complexes, observable in both the solution and solid phases. Conventional quantum chemical methods, such as density functional theory using different functionals (B3LYP, TPSSh, and CAM-B3LYP), remain inadequate to rationalize the observed spectroscopic anomalies. However, a multiconfigurational approach elucidates the disparate behaviors of these complexes. Multireference perturbation theory, based on complete active space self-consistent field computations, identifies Cu(I) in the case of the complex with the NCH3 containing ligands and a state with substantial Cu(II) contributions in the case of the complex with the S atom containing ligands. In contrast, DFT indicates Cu(I) in both scenarios.

Synthesis and Characterization of Two Novel Naphthalenediimide/Zinc Phosphonate Crystalline Materials Precipitated from Different Solvents

Hybrid naphthalenediimide/zinc phosphonate materials (NDI/Zn) were prepared by mixing solutions of N,N'-bis(2-phosphonoethyl)-1,4,5,8-naphthalenediimide (PNDI) and zinc nitrate, resulting in the precipitation of the desired compounds. Samples precipitated from water and N,N-dimethylformamide (DMF) were produced. The obtained samples had the expected elemental composition, and the presence of naphthalenediimides (NDI) was ascertained by infrared and UV-visible spectroscopy. All the samples were crystalline, according to powder X-ray diffraction. Nitrogen adsorption isotherms showed the presence of porosity in the NDI/Zn samples. Mesopores with a diameter = 4.1 nm were present in the sample from DMF, with total pore volume reaching 0.13 cm3/g.

Mixed-Valence CuI /CuIII Metal-Organic Frameworks with Non-innocent Ligand for Multielectron Transfer

We report two novel three-dimensional copper-benzoquinoid metal-organic frameworks (MOFs), [Cu4 L3 ]n and [Cu4 L3  ⋅ Cu(iq)3 ]n (LH4 =1,4-dicyano-2,3,5,6-tetrahydroxybenzene, iq=isoquinoline). Spectroscopic techniques and computational studies reveal the unprecedented mixed valency in MOFs, formal Cu(I)/Cu(III). This is the first time that formally Cu(III) species are witnessed in metal-organic extended solids. The coordination between the mixed-valence metal and redox-non-innocent ligand L, which promotes through-bond charge transfer between Cu metal sites, allows better metal-ligand orbital overlap of the d-π conjugation, leading to strong long-range delocalization and semiconducting behavior. Our findings highlight the significance of the unique mixed valency between formal Cu(I) and highly-covalent Cu(III), non-innocent ligand, and pore environments of these bench stable Cu(III)-containing frameworks on multielectron transfer and electrochemical properties.

Exploring redox properties of a 3D Co-based framework with bis(triarylamine) terphenyl as a redox-active linker

A 3D Co-based metal-organic framework has been prepared, which contains a bis(triarylamine) with terphenyl units as a redox-active linker. Manipulation of the redox events via the electrochemical method confirmed that charge hopping is dominant within the 3D framework. Investigation of the in situ spectroelectrochemical properties within the structure leads to the formation of mono and dual radical cations obtained reversibly in two-steps due to the presence of two redox-active sites.

Stable Dication Diradicals of Triply Fused Metallo Chlorin-Porphyrin Heterodimers: Impact of the Bridge on the Control of Spin Coupling to Reactivity

We report an unexpected rearrangement, controlled by the nature of the bridge, leading to the formation of novel, remarkably stable triply fused dinickel(II)/dicopper(II) chlorin-porphyrin dication diradical heterodimers in excellent yields. Here, a dipyrromethene bridge gets completely fused between two porphyrin macrocycles with two new C-C and one C-N bonds. The two macrocycles exhibit extensive π-conjugation through the bridge, which results in an antiferromagnetic coupling between the two π-cation radicals. In addition, the macrocyclic distortion also favours a rare intramolecular ferromagnetic interaction between the CuII and π-cation radical spins to form a triplet state. The structural and electronic perturbation in the unconjugated dication diradical possibly enables the bridging pyrrolic nitrogen to undergo a nucleophilic attack at the nearby β-carbon of the porphyrin π-cation radical with a computed free energy barrier of >20 kcal mol-1 which was supplied in the form of reflux condition to initiate such a rearrangement process. UV-vis, EPR and ESI-MS spectroscopies were used to monitor the rearrangement process in situ in order to identify the key reactive intermediates leading to such an unusual transformation.

Electrochromism in Isoreticular Metal-Organic Framework Thin Films with Record High Coloration Efficiency

The power of isoreticular chemistry has been widely exploited to engineer metal-organic frameworks (MOFs) with fascinating molecular sieving and storage properties but is underexplored for designing MOFs with tunable optoelectronic properties. Herein, three dipyrazole-terminated XDIs (X = PM (pyromellitic), N (naphthalene), or P (perylene); DI = diimide) with different lengths and electronic properties are prepared and employed as linkers for the construction of an isoreticular series of Zn-XDI MOFs with distinct electrochromism. The MOFs are grown on fluorine-doped tin oxide (FTO) as high-quality crystalline thin films and characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Due to the constituting electronically isolated XDI linkers, each member of the isoreticular thin film series exhibits two reversible one-electron redox events, each at a distinct electrochemical potential. The orientation of the MOFs as thin films as well as their isoreticular nature results in identical cation-coupled electron hopping transport rates in all three materials, as demonstrated by comparable apparent electron diffusion coefficients, Deapp. Upon electrochemical reduction to either the [XDI]•- or [XDI]2- state, each MOF undergoes characteristic changes in its optical properties as a function of linker length and redox state of the linker. Operando spectroelectrochemistry measurements reveal that Zn-PDI@FTO (PDI = perylene diimide) thin films exhibit a record high coloration efficiency of 941 cm2 C-1 at 746 nm, which is attributed to the maximized Faradaic transformations at each electronically isolated PDI unit. The electrochromic response of the thin film is retained to more than 99% over 100 reduction-oxidation cycles, demonstrating the applicability of the presented materials.

Catalyst Development for High-Temperature Polymer Electrolyte Membrane Fuel Cell (HT-PEMFC) Applications

A constant increase in global emission standard is causing fuel cell (FC) technology to gain importance. Over the last two decades, a great deal of research has been focused on developing more active catalysts to boost the performance of high-temperature polymer electrolyte membrane fuel cells (HT-PEMFC), as well as their durability. Due to material degradation at high-temperature conditions, catalyst design becomes challenging. Two main approaches are suggested: (i) alloying platinum (Pt) with low-cost transition metals to reduce Pt usage, and (ii) developing novel catalyst support that anchor metal particles more efficiently while inhibiting corrosion phenomena. In this comprehensive review, the most recent platinum group metal (PGM) and platinum group metal free (PGM-free) catalyst development is detailed, as well as the development of alternative carbon (C) supports for HT-PEMFCs.

Exploring the Impact of Successive Redox Events in Thin Films of Metal-Organic Frameworks: An Absorptiometric Approach

Metal-organic frameworks (MOFs) featuring redox activity are highly appealing for electrocatalytic or charge accumulation applications. An important aspect in this field is the ability to address as many redox centers as possible in the material by an efficient diffusion of charges. Herein, we investigate for the first time the charge diffusion processes occurring upon two sequential one-electron reductions in an MOF thin film. Two pyrazolate-zinc(II)-based MOFs including highly electro-deficient perylene diimide (PDI) ligands were grown on conducting substrates, affording thin films with double n-type electrochromic properties as characterized by spectroelectrochemical analysis. In depth electrochemical and chronoabsorptiometric investigations were carried out to probe the charge diffusion in the MOF layers and highlighted significant differences in terms of diffusion kinetics and material stability between the first and second successive reduction of the redox-active PDI linkers. Our results show that MOFs based on multiredox centers are more sensitive to encumbrance-related issues than their monoredox analogues in the context of electrochemical applications, an observation that further underlines the fundamental aspect of careful pore dimensions toward efficient and fast ion diffusion.

Experimental manifestation of redox-conductivity in metal-organic frameworks and its implication for semiconductor/insulator switching

Electric conductivity in metal-organic frameworks (MOFs) follows either a band-like or a redox-hopping charge transport mechanism. While conductivity by the band-like mechanism is theoretically and experimentally well established, the field has struggled to experimentally demonstrate redox conductivity that is promoted by the electron hopping mechanism. Such redox conductivity is predicted to maximize at the mid-point potential of the redox-active units in the MOF, and decline rapidly when deviating from this situation. Herein, we present direct experimental evidence for redox conductivity in fluorine-doped tin oxide surface-grown thin films of Zn(pyrazol-NDI) (pyrazol-NDI = 1,4-bis[(3,5-dimethyl)-pyrazol-4-yl]naphthalenediimide). Following Nernstian behavior, the proportion of reduced and oxidized NDI linkers can be adjusted by the applied potential. Through a series of conductivity measurements, it is demonstrated that the MOF exhibits minimal electric resistance at the mid-point potentials of the NDI linker, and conductivity is enhanced by more than 10000-fold compared to that of either the neutral or completely reduced films. The generality of redox conductivity is demonstrated in MOFs with different linkers and secondary building units, and its implication for applications that require switching between insulating and semiconducting regimes is discussed.

Fundamental Perspectives on the Electrochemical Water Applications of Metal-Organic Frameworks

HIGHLIGHTS

The recent development and implementation of metal-organic frameworks (MOFs) and MOF-based materials in electrochemical water applications are reviewed. The critical factors that affect the performances of MOFs in the electrochemical reactions, sensing, and separations are highlighted. Advanced tools, such as pair distribution function analysis, are playing critical roles in unraveling the functioning mechanisms, including local structures and nanoconfined interactions. Metal-organic frameworks (MOFs), a family of highly porous materials possessing huge surface areas and feasible chemical tunability, are emerging as critical functional materials to solve the growing challenges associated with energy-water systems, such as water scarcity issues. In this contribution, the roles of MOFs are highlighted in electrochemical-based water applications (i.e., reactions, sensing, and separations), where MOF-based functional materials exhibit outstanding performances in detecting/removing pollutants, recovering resources, and harvesting energies from different water sources. Compared with the pristine MOFs, the efficiency and/or selectivity can be further enhanced via rational structural modulation of MOFs (e.g., partial metal substitution) or integration of MOFs with other functional materials (e.g., metal clusters and reduced graphene oxide). Several key factors/properties that affect the performances of MOF-based materials are also reviewed, including electronic structures, nanoconfined effects, stability, conductivity, and atomic structures. The advancement in the fundamental understanding of these key factors is expected to shed light on the functioning mechanisms of MOFs (e.g., charge transfer pathways and guest-host interactions), which will subsequently accelerate the integration of precisely designed MOFs into electrochemical architectures to achieve highly effective water remediation with optimized selectivity and long-term stability.

Quinoid-Based Three-Dimensional Metal-Organic Framework Fe2(dhbq)3: Porosity, Electrical Conductivity, and Solid-State Redox Properties

We report the synthesis, characterization, and electronic properties of the quinoid-based three-dimensional metal-organic framework [Fe₂(dhbq)₃]. The MOF was synthesized without using cations as a template, unlike other reported X₂dhbq₃-based coordination polymers, and the crystal structure was determined by using single-crystal X-ray diffraction. The crystal structure was entirely different from the other reported [Fe₂(X₂dhbq₃)]^2-; three independent 3D polymers were interpenetrated to give the overall structure. The absence of cations led to a microporous structure, investigated by N₂ adsorption isotherms. Temperature dependence of electrical conductivity data revealed that it exhibited a relatively high electrical conductivity of 1.2 × 10^-2 S cm^-1 (E_a = 212 meV) due to extended d-π conjugation in a three-dimensional network. Thermoelectromotive force measurement revealed that it is an n-type semiconductor with electrons as the majority of charge carriers. Structural characterization and spectroscopic analyses, including SXRD, Mössbauer, UV-vis-NIR, IR, and XANES measurements, evidenced the occurrence of no mixed valency based on the metal and the ligand. [Fe₂(dhbq)₃] upon incorporating as a cathode material for lithium-ion batteries engendered an initial discharge capacity of 322 mAh/g.

Broad Electronic Modulation of Two-Dimensional Metal-Organic Frameworks over Four Distinct Redox States

Two-dimensional (2D) inorganic materials have emerged as exciting platforms for (opto)electronic, thermoelectric, magnetic, and energy storage applications. However, electronic redox tuning of these materials can be difficult. Instead, 2D metal-organic frameworks (MOFs) offer the possibility of electronic tuning through stoichiometric redox changes, with several examples featuring one to two redox events per formula unit. Here, we demonstrate that this principle can be extended over a far greater span with the isolation of four discrete redox states in the 2D MOFs Li_xFe₃(THT)₂ (x = 0-3, THT = triphenylenehexathiol). This redox modulation results in 10,000-fold greater conductivity, p- to n-type carrier switching, and modulation of antiferromagnetic coupling. Physical characterization suggests that changes in carrier density drive these trends with relatively constant charge transport activation energies and mobilities. This series illustrates that 2D MOFs are uniquely redox flexible, making them an ideal materials platform for tunable and switchable applications.

Redox-Active Two-Dimensional Tetrathiafulvalene-Copper Metal-Organic Framework with Boosted Electrochemical Performances for Supercapatteries

Metal-organic frameworks (MOFs) have attracted noticeable attention as promising candidates for electrochemical energy storage. However, the lack of electrical conductivity and the weak stability of most MOFs result in poor electrochemical performances. Here, a tetrathiafulvalene (TTF)-based complex, formulated as [(CuCN)₂(TTF(py)₄)] (1) (TTF-(py)₄ = tetra(4-pyridyl)-TTF), is assembled by in situ generation of coordinated CN^- from a nontoxic source. Single-crystal X-ray diffraction analysis reveals that compound 1 possesses a two-dimensional layered planar structure, which is further stacked in parallel to form a three-dimensional supramolecular framework. The planar coordination environment of 1 is the first example of a TTF-based MOF. Attributed to the unique structure and redox TTF ligand, the electrical conductivity of 1 is significantly increased by 5 orders of magnitude upon iodine treatment. The iodine-treated 1 (1-ox) electrode displays typical battery-type behavior through electrochemical characterizations. The supercapattery based on the 1-ox positrode and AC negatrode presents a high specific capacity of 266.5 C g^-1 at a specific current of 1 A g^-1 with a remarkable specific energy of 62.9 Wh kg^-1 at a specific power of 1.1 kW kg^-1. The excellent electrochemical performance of 1-ox is one of the best among those reported supercapatteries, demonstrating a new strategy for developing MOF-based electrode materials.

High performance Li-, Na-, and K-ion storage in electrically conducting coordination polymers

Coordination polymers (CPs) made of redox-active organic moieties and metal ions emerge as an important class of electroactive materials for battery applications. However, the design and synthesis of high voltage alkali-cation reservoir anionic CPs remains challenging, hindering their practical applications. Herein, we report a family of electrically conducting alkali-cation reservoir CPs with the general formula of A2-TM-PTtSA (wherein A = Li+, Na+, or K+; TM = Fe2+, Co2+, or Mn2+; and PTtSA = benzene-1,2,4,5-tetra-methylsulfonamide). The incorporation of transition metal centers not only enables intrinsic high electrical conductivity, but also shows an impressive redox potential increase of as high as 1 V as compared to A4-PTtSA analogues, resulting in a class of organometallic cathode materials with a high average redox potential of 2.95-3.25 V for Li-, Na- and K-ion batteries. A detailed structure - composition - physicochemical properties - performance correlation study is provided relying on experimental and computational analysis. The best performing candidate shows excellent rate capability (86% of the nominal capacity retained at 10C rate), remarkable cycling stability (96.5% after 1000 cycles), outstanding tolerance to low carbon content (5 wt%), high mass loading (50 mg cm-2), and extreme utilisation conditions of low earth orbit space environment tests. The significance of the disclosed alkali-ion reservoir cathodes is further emphasized by utilizing conventional Li-host graphite anode for full cell assembly, attaining a record voltage of 3 V in an organic cathode Li-ion proof-of-concept cell.

Multi-Redox Responsive Behavior in a Mixed-Valence Semiconducting Framework Based on Bis-[1,2,5]-thiadiazolo-tetracyanoquinodimethane

The two-dimensional (2-D) framework, [Cu(BTDAT)(MeOH)] {BTDAT = bis-[1,2,5]-thiadiazolo-tetracyanoquinodimethane}, possesses remarkable multi-step redox properties, with electrochemical studies revealing six quasi-stable redox states in the solid state. In situ electron paramagnetic resonance and visible-near infrared spectroelectrochemistry elucidated the mechanism for these multi-step redox processes, as well as the optical and electrochromic behavior of the BTDAT ligand and framework. In studying the structural, spectroscopic, and electronic properties of [Cu(BTDAT)(MeOH)], the as-synthesized framework was found to exist in a mixed-valence state with thermally-activated semiconducting behavior. In addition to pressed pellet conductivity measurements, single-crystal conductivity measurements using a pre-patterned polydimethylsiloxane layer on a silicon substrate provide important insights into the anisotropic conduction pathways. As an avenue to further understand the electronic state of [Cu(BTDAT)(MeOH)], computational band structure calculations predicted delocalized electronic transport in the framework. On the balance of probabilities, we propose that [Cu(BTDAT)(MeOH)] is a Mott insulator (i.e., electron correlations cause a metal-insulator transition). This implies that the conductivity is incoherent. However, we are unable to distinguish between activated transport due to Coulombically bound electron-hole pairs and a hopping mechanism. The combined electrochemical, electronic, and optical properties of [Cu(BTDAT)(MeOH)] shine a new light on the experimental and theoretical challenges for electroactive framework materials, which are implicated as the basis of advanced optoelectronic and electrochromic devices.

Redox-Active Metal-Organic Frameworks with Three-Dimensional Lattice Containing the m-Tetrathiafulvalene-Tetrabenzoate

Metal-organic frameworks (MOFs) constructed by tetrathiafulvalene-tetrabenzoate (H₄TTFTB) have been widely studied in porous materials, while the studies of other TTFTB derivatives are rare. Herein, the meta derivative of the frequently used p-H₄TTFTB ligand, m-H₄TTFTB, and lanthanide (Ln) metal ions (Tb³⁺, Er³⁺, and Gd³⁺) were assembled into three novel MOFs. Compared with the reported porous Ln-TTFTB, the resulted three-dimensional frameworks, Ln-m-TTFTB ([Ln₂(m-TTFTB)(m-H₂TTFTB)_0.5(HCOO)(DMF)]·2DMF·3H₂O), possess a more dense stacking which leads to scarce porosity. The solid-state cyclic voltammetry studies revealed that these MOFs show similar redox activity with two reversible one-electron processes at 0.21 and 0.48 V (vs. Fc/Fc⁺). The results of magnetic properties suggested Dy-m-TTFTB and Er-m-TTFTB exhibit slow relaxation of the magnetization. Porosity was not found in these materials, which is probably due to the meta-configuration of the m-TTFTB ligand that seems to hinder the formation of pores. However, the m-TTFTB ligand has shown to be promising to construct redox-active or electrically conductive MOFs in future work.

Highly Oxidized Cobalt Porphyrin Dimer: Control of Spin Coupling via a Bridge

A cobalt porphyrin dimer is constructed in which two Co(II)porphyrins are connected covalently through a redox-active diethylpyrrole moiety via a flexible but "nonconjugated" methylene bridge. Upon oxidation with even a mild oxidant such as iodine, each cobalt(II) center and porphyrin ring undergo 1e^- oxidation, leading to the formation of a 4e^--oxidized cobalt(III)porphyrin dication diradical complex. Other oxidants such as Cl₂ and Br₂ also produce similar results. To stabilize such highly oxidized dication diradicals, the "nonconjugated" methylene spacer undergoes a facile and spontaneous oxidation to form a methine group with a drastic structural change, thereby making the bridge fully π-conjugated and enabling through-bond communication. This results in a strong spin coupling between two π-cation radicals which stabilizes the singlet state. The experimental observations are also strongly supported by extensive density functional theory calculations. The present study highlights the crucial role played by the nature of the bridge in the long-range electronic communication.

Size-Dependent Properties of Solution-Processable Conductive MOF Nanocrystals

The diverse optical, magnetic, and electronic behaviors of most colloidal semiconductor nanocrystals emerge from materials with limited structural and elemental compositions. Conductive metal-organic frameworks (MOFs) possess rich compositions with complex architectures but remain unexplored as nanocrystals, hindering their incorporation into scalable devices. Here, we report the controllable synthesis of conductive MOF nanoparticles based on Fe(1,2,3-triazolate)₂. Sizes can be tuned to as small as 5.5 nm, ensuring indefinite colloidal stability. These solution-processable MOFs can be analyzed by solution-state spectroscopy and electrochemistry and cast into conductive thin films with excellent uniformity. This unprecedented analysis of MOF materials reveals a strong size dependence in optical and electronic behaviors sensitive to the intrinsic porosity and guest-host interactions of MOFs. These results provide a radical departure from typical MOF characterization, enabling insights into physical properties otherwise impossible with bulk analogues while offering a roadmap for the future of MOF nanoparticle synthesis and device fabrication.

Long-Range Intramolecular Spin Coupling through a Redox-Active Bridge upon Stepwise Oxidations: Control and Effect of Metal Ions

Dinickel(II) and dicopper(II) porphyrin dimers have been constructed in which two metalloporphyrin units are widely separated by a long unconjugated dipyrrole bridge. Two macrocycles are aligned somewhat orthogonally to each other, while oxidation of the bridge generates a fully π-conjugated butterfly-like structure, which, in turn, upon stepwise oxidations by stronger oxidants result in the formation of the corresponding one- and two-electron-oxidized species exhibiting unusual long-range charge/radical delocalization to produce intense absorptions in the near-infrared (NIR) region and electron paramagnetic resonance (EPR) signals of a triplet state due to interaction between the unpaired spins on the Cu(II) ions. Although the two metal centers have a large physical separation through the bridge (more than 16 Å), they share electrons efficiently between them, behaving as a single unit rather than two independent centers. Detailed UV-vis-NIR, electrospray ionization mass spectrometry, IR, variable-temperature magnetic study, and EPR spectroscopic investigations along with X-ray structure determination of unconjugated, conjugated, and one electron-oxidized complexes have been exploited to demonstrate the long-range electronic communication through the bridge. The experimental observations are also supported by density functional theory (DFT) and time-dependent DFT calculations. The present study highlights the crucial roles played by a redox-active bridge and metal in controlling the long-range electronic communication.

Microscopic Insights into Cation-Coupled Electron Hopping Transport in a Metal-Organic Framework

Electron transport through metal-organic frameworks by a hopping mechanism between discrete redox active sites is coupled to diffusion-migration of charge-balancing counter cations. Experimentally determined apparent diffusion coefficients, D_e^app, that characterize this form of charge transport thus contain contributions from both processes. While this is well established for MOFs, microscopic descriptions of this process are largely lacking. Herein, we systematically lay out different scenarios for cation-coupled electron transfer processes that are at the heart of charge diffusion through MOFs. Through systematic variations of solvents and electrolyte cations, it is shown that the D_e^app for charge migration through a PIZOF-type MOF, Zr(dcphOH-NDI) that is composed of redox-active naphthalenediimide (NDI) linkers, spans over 2 orders of magnitude. More importantly, however, the microscopic mechanisms for cation-coupled electron propagation are contingent on differing factors depending on the size of the cation and its propensity to engage in ion pairs with reduced linkers, either non-specifically or in defined structural arrangements. Based on computations and in agreement with experimental results, we show that ion pairing generally has an adverse effect on cation transport, thereby slowing down charge transport. In Zr(dcphOH-NDI), however, specific cation-linker interactions can open pathways for concerted cation-coupled electron transfer processes that can outcompete limitations from reduced cation flux.

Glycols in the Synthesis of Zinc-Anilato Coordination Polymers

We report the synthesis, structural investigation, and thermal behavior for three zinc-based 1D-coordination polymers with 3,6-di-tert-butyl-2,5-dihydroxy-p-benzoquinone, which were synthesized in the presence of different glycols. The interaction of zinc nitrate with glycols, followed by using the resulting solution in solvothermal synthesis with the anilate ligand in DMF, makes it possible to obtain linear polymer structures with 1,2-ethylene or 1,2-propylene glycols coordinated to the metal. The reaction involving 1,3-propylene glycol under similar conditions gives a crystal structure that does not contain a diol. The crystal and molecular structures of the synthesized compounds were determined using single crystal by X-ray structural analysis. The influence of glycol molecules coordinated to the metal on the thermal destruction of synthesized compounds is shown.

Coordination polymers derived from alkali metal complexes of redox-active ligands

Treatment solutions of (dpp-bian)M (dpp-bian = 1,2-bis[(2,6-diisopropylphenyl)imino]acenaphthene; M = Li, Na, K) with 4,4′-bipyridine (4,4′-bipy) affords coordination polymers [(dpp-bian)M(4,4′-bipy)m]n, (M = Li or K, m = 1; M = Na, m = 2).

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.