Graphite electrodes offer remarkable electrochemical properties, emerging as a viable alternative to glassy carbon (GCE) and other carbon-based electrodes for fundamental electrochemistry research. We report the fabrication and characterization of high-purity graphite disk electrodes (GDEs), made from cost-effective materials and a solvent-free methodology employing readily available laboratory equipment. Analysis of their physical properties via SEM, EDX and XPS reveals no metallic interferences and a notably high porosity, emphasizing their potential. The electrochemical performances of GDEs were found to be comparable to those of GCE. Immobilization of peptides and enzymes, both via covalent coupling and surface adsorption, was used to explore potential applications of GDEs in bioelectrochemistry. Enzyme activity could be addressed both via direct electron transfer and mediated electron transfer mechanism. These results highlight the interesting properties of our GDEs and make them a low-cost alternative to other carbon-based electrodes, with potential for future real-world applications.

Electroactive covalent organic frameworks (COFs) are attracting intense research interest due to their diverse applications in energy and environmental technologies. However, the underlying electron transport mechanism is not fully understood yet. Herein, we present experimental evidence that electron transport through electroactive COFs can operate through a redox-conductivity mechanism, i.e., by electron hopping between neighboring redox-active units that differ in oxidation states. Electroactive naphthalene diimide (NDI)-based COF thin films are prepared as they display two distinct and reversible one-electron redox waves, correlating to the [NDI]0/•- and [NDI]•-/2- redox pairs, respectively. Utilizing an operando UV-vis spectroelectrochemistry technique, the potential-dependent absorption of the COF film can be dynamically followed and unambiguously assigned. Therefore, the redox composition of the film can be precisely controlled by just varying the electrode potential. This enables a clear experimental demonstration of the potential- or redox composition-dependent bell-shaped conductivity distribution in the COF film, corresponding to the electron-hopping transport nature that has been observed in redox-active polymers and metal-organic frameworks (MOFs). Importantly, the COF film displays reversible and stable insulator-to-semiconductor transitions upon electrochemical modulation for 100 cycles, setting the foundation for many practical applications. Further, we demonstrate that both the apparent electron diffusion coefficient, Deapp, and redox conductivities are cation-dependent, highlighting the cation-coupled electron transport nature. Finally, this hopping transport mechanism is found to be operative in two more NDI-based electroactive COFs, demonstrating its generality.

Abstract Electrochemical CO2 reduction (eCO2R) is a sustainable strategy for converting CO2 into value-added chemicals. Here, we report the Cu-azolate metal−organic framework (MOF) CuBBTA (Cu2Cl2-bbta, MAF-X29) as a stable electrocatalyst for eCO2R in a zero-gap electrolyzer. CuBBTA was spray-coated onto carbon paper using either PTFE or Nafion binders to form gas diffusion electrodes that achieved Faradaic efficiencies of up to 50% for carbon-containing products (CH4, C2H4, and CO) at current densities as high as 100 mA cm−2 under a flow of CO2. Post-electrolysis studies confirmed retention of the MOF structure. DFT calculations reveal that CO2 reduction proceeds via COOH* and CO* intermediates, with a thermodynamically favourable and kinetically accessible CO*−CO* dimerization pathway for ethylene formation. Importantly, polymer binder selection significantly influences product selectivity. While PTFE favours methane formation, Nafion shifts selectivity toward CO, highlighting the critical yet underexplored role of electrode environment in MOF-based eCO2R catalysis.

Unidirectional electron flow is essential for applications in electron storage and reconfigurable electronics, and traditionally realized in semiconductor junctions and even single-molecule concepts. Combining aspects from both technologies, redox-conductive metal-organic frameworks (RC-MOFs) exhibit molecule-like behavior in a crystalline, porous matrix. Herein, we show that bilayer RC-MOF electrodes composed of sequentially deposited Zn(PMDI) and Zn(NDI) on fluorine-doped tin oxide (FTO) function as chemical free-energy-based rectifying junctions. Unidirectional electron flow arises from thermodynamically allowed, and spatially organized redox reactions at the Zn(PMDI)|Zn(NDI) interface. Showcasing the rectifying function, electrons that reach the outer Zn(NDI) layer in the FTO|Zn(PMDI)|Zn(NDI) configuration are trapped as NDI•− and cannot be recovered by applying an oxidative bias. Introduction of [Co(bpy)3]3+ to the electrolyte creates a source–drain situation that reveals the potential-dependent directional electron flow across the bilayer. These results position RC-MOF bilayers as programmable electrochemical diodes, with rectification governed by layer sequence and redox accessibility.

Surface modification is an effective method to realize high performance photoelectrodes. While current investigations mostly aim to leverage surface layers for improved charge carrier kinetics during charge separation, interfacial charge transfer, and decreased recombination, carrier transport within the surface layer is largely unattended. Herein, we explore this charge transport process on a model photocathode consisting of p-Si and GaP semiconductors (SCs) that are coated with a redox-active Zn-NDI (NDI = naphthalene diimide bis-pyrazolate) metal-organic framework (MOF) surface layer. The MOF layer is able to accept photogenerated electrons and support a large photovoltage of the underlying SC. In addition to well-established carrier generation and interfacial transfer processes that are frequently considered to control photocurrents, experimental photoelectrochemical data of the MOF@SC electrodes expose limitations that arise from electron transport in the surface layer coating. The transport-limited regime becomes relevant when the illumination intensity is gradually increased and is sensitive to the nature of the underlying semiconductor as well as the electrolyte. The phenomenon reported in this work is likely present in other surface-modified photoelectrodes with thick cocatalysts or redox-active polymer coatings but can easily be overlooked. In the MOF@SC construct, the transition between different limiting regimes can be visualized owing to the well-behaved cation-coupled photoelectron hopping transport in the MOF layer. These findings support the design and realization of efficient photoelectrodes.

Hemithioindigo (HTI) photoswitches exhibit robust photoisomerization under visible light and relatively high thermal bistability. In this work, we report various modifications of the HTI core, namely the introduction of aldehydes and carboxylic acids at the para position of the stilbene fragment with different oxidation states of the sulfur center, and the incorporation of a Schiff base moiety. These modifications allowed tuning of the absorption properties, quantum yields of isomerization, and thermal stability of the metastable E-isomers. Notably, the formyl- and carboxyl-substituted HTI switches achieved high yields of isomerization under visible light in various solvents, while sulfur oxidation enhanced quantum yields but reduced photochromism. Schiff base formation led to red-shifted absorption and increased thermal stability. Finally, by leveraging the carboxyl substituents, we incorporated an HTI chromophore into the NU-1000 metal-organic framework (MOF), and demonstrated solid-state photoisomerization. These findings highlight key structural modifications that expand the applicability of HTI photoswitches for molecular switching in solution and solid-state environments.

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.

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.

The modularity and synthetic flexibility of metal–organic frameworks (MOFs) have provoked analogies with enzymes, and even the term MOFzymes has been coined. In this review, we focus on molecular catalysis of energy relevance in MOFs, more specifically water oxidation, oxygen and carbon dioxide reduction, as well as hydrogen evolution in context of the MOF–enzyme analogy. Similar to enzymes, catalyst encapsulation in MOFs leads to structural stabilization under turnover conditions, while catalyst motifs that are synthetically out of reach in a homogeneous solution phase may be attainable as secondary building units in MOFs. Exploring the unique synthetic possibilities in MOFs, specific groups in the second and third coordination sphere around the catalytic active site have been incorporated to facilitate catalysis. A key difference between enzymes and MOFs is the fact that active site concentrations in the latter are often considerably higher, leading to charge and mass transport limitations in MOFs that are more severe than those in enzymes. High catalyst concentrations also put a limit on the distance between catalysts, and thus the available space for higher coordination sphere engineering. As transport is important for MOF-borne catalysis, a system perspective is chosen to highlight concepts that address the issue. A detailed section on transport and light-driven reactivity sets the stage for a concise review of the currently available literature on utilizing principles from Nature and system design for the preparation of catalytic MOF-based materials.

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.

Lateral intermolecular charge transfer between photosensitizers on metal oxide substrates is important for the understanding on the overall working principles of dye-sensitized systems. Such studies usually concentrate on either hole or electron transfer separately and are conducted in solvents with a high dielectric constant (εs) that are known, however, to show a drastic decrease of the local dielectric constant close to the metal oxide surface. In the present study, both hole and electron hopping between organic donor-acceptor photosensitizers was experimentally investigated on PB6 dye-sensitized mesoporous ZrO2 films. The donor (close to the surface) and acceptor (away from surface) subunit of the PB6 dye were observed to be involved in hole and electron hopping, respectively. Hole and electron transfer kinetics were found to differ remarkably in high-εs solvents, but similar in solvents with εs < 12. This finding indicates that low-εs solvents maintain similar local dielectric constant values close to, and further away from, the semiconductor surface, which is different from the previously observed behavior of high dielectric constant solvents at a metal oxide interface.

C-Alkylations of nine different classes of methyl-substituted N-heteroarenes, including quinolines, quinoxalines, benzimidazoles, benzoxazoles, pyrazines, pyrimidines, pyridazines, pyridines, and triazines are disclosed. A bench stable earth-abundant Mn(i)-complex catalyzed the chemoselective hydrogen-transfer reaction utilizing a diverse range of primary alcohols as the non-fossil fuel-derived carbon source. The diversified N-heteroarenes (41 examples) were isolated in high yields and selectivities. Water is produced as the sole byproduct, making the protocol environmentally benign.

Bidentate NN-ligands have been derived from the reaction between aldehydes and 2-(aminomethyl)pyridine. The treatment of these ligands with Mn(CO)(5)Br gave complexes that are highly bench stable. The complexes were characterized by various analytical and spectral methods. Single-crystal XRD of complex Mn-2 was performed, which indicates an octahedral geometry around the metal center. The complexes efficiently catalyze the N-alkylation of anilines with alcohols under optimized reaction conditions.