Cobalt Metal-Organic Frameworks Incorporating Redox-Active Tetrathiafulvalene Ligand: Structures and Effect of LLCT within the MOF on Photoelectrochemical Properties

Rational design of redox active metal organic frameworks for mediated electron transfer of enzymes

The efficient immobilization of redox mediators remains a major challenge in the design of mediated enzyme electrode platforms. In addition to stability, the ability of the redox-active material to mediate electron transfer from the active-site buried enzymes, such as flavin adenine dinucleotide-dependent glucose dehydrogenase (FADGDH) and lactate oxidase (LOx), is also crucial. Conventional immobilization techniques can be synthetically challenging, and immobilized mediators often exhibit limited durability, particularly in continuous operation. Here, we design a novel redox-active cobalt-based metal-organic framework (raMOF) obtained via the partial ligand substitution of 2-methylimidazole (MeIm) with a 1,2-naphthoquinone-4-sulfonate (NQSO) redox probe, as a promising platform for high-performance enzyme electrodes. This nanostructured raMOF, combined with multi-walled carbon nanotubes (CNTs), provided a high current density of up to 2.06 mA cm-2 during enzymatic reactions and maintained remarkable operational stability, retaining 100% of its current over 54 hours. This stability far exceeded that of adsorbed NQSO on CNTs, which experienced a complete loss of the initial current, highlighting the significant advantage of the raMOF-based platform for high-performance enzyme electrodes.

A metal-organic framework with mixed electron donor and electron acceptor ligands for efficient lithium-ion storage

Electron donor tetrathiafulvalene (TTF) and electron acceptor naphthalene diimide (NDI) derivatives were used to synthesize a 3D Zn-TTF/NDI-MOF. Multiple redox active sites and charge transfer endow the pristine MOF anode with excellent rate behavior and long term cycling performance (with an average specific capacity of 956 mA h g-1 at 1 A g-1 over 600 cycles). This study highlights the great potential of elaborately-designed MOFs for developing efficient anode materials.

The mechanism governing the formation of intermolecular charge transfer bands: a series of polyoxomolybdates as a case study

A series of proof-of-concept models of polyoxomolybdates with different protonated disubstituted aniline counterions and the same β-Mo8O26 polyanion were synthesized to study the mechanism governing the formation of the intermolecular charge transfer (inter-CT) band.

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.

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.

A Tetrathiafulvalene/Naphthalene Diimide-Containing Metal-Organic Framework with fsc Topology for Highly Efficient Near-Infrared Photothermal Conversion

Metal-organic frameworks (MOFs) provide broad prospects for the development of new photothermal conversion materials, while their design and synthesis remain challenging. A new Zn-MOF (1) containing both tetrathiafulvalene (TTF) as an electron donor and naphthalene diimide (NDI) as an electron acceptor was constructed by using a space limiting effect. The material exhibited wide absorption peaks in the near-infrared region, indicating that there was strong charge transfer interaction between the TTF and NDI units and providing the possibility of photothermal conversion. 1 shows efficient near-infrared photothermal conversion performance. Under 808 nm laser (0.4 W cm^-2) illumination, the temperature of 1 increased rapidly from room temperature to 250 °C, with good thermal stability and cycle durability. This work provides an efficient strategy for promising materials in photothermal therapy.

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.

Benzotrithiophene-based MOFs: interchromophoric interactions affected Ln(III) crystallization selectivity and optoelectronic properties

Metal-organic frameworks (MOFs) provide an ideal platform for the assembly of chromophores and thus show wide potential applications in optoelectronic devices. The spatial arrangement and interaction of the incorporated chromophores play a key role in the generation of coherent optical and electronic properties. In this work, two series of benzo-(1,2;3,4;5,6)-tristhiophene (BTT) based Ln-MOFs (Ln-1s and Ln-2s) were synthesized. These two series of MOFs present different assembly states of BTT chromophores, that is, BTT-containing ligands exist as separated monomers in Ln-1s but gather as dimers in Ln-2s. From the comparison between these two series of MOFs and theoretical calculations, we show for the first time that this chromophore assembly state difference could affect the crystallization selectivity of MOFs towards different Ln³⁺ ions. In addition, the interaction between BTT chromophores in the dimer also leads to the red-shifted photoluminescence and enhanced photocurrent of Ln-2s relative to those of Ln-1s. The results of this work demonstrate the multiple functions of interchromophoric interactions in the structures and optoelectronic properties of MOFs.

Tetrathiafulvalene-Cobalt Metal-Organic Frameworks for Lithium-Ion Batteries with Superb Rate Capability

Although pristine metal-organic framework (MOF) anodes for lithium-ion batteries (LIBs) show moderate activities and relatively stable cycling, the poor rate capability of the MOF anodes limited their applications in the development of a new generation of energy storage. Herein, the electric active Co^II ion is selected to coordinate with redox-active S-rich tetrathiafulvalene (TTF) derivatives to create two TTF-Co-MOFs, formulated as [Co₂(py-TTF-py)₂(BDC)₂]·2DMF·H₂O (TTF-Co-MOF 1) and [Co₂(py-TTF-py)₂(BPDC)₂]·3DMF·3H₂O (TTF-Co-MOF 2), where py-TTF-py = 2,6-bis(4'-pyridyl)tetrathiafulvalene, H₂BDC = terephthalic acid, H₂BPDC = biphenyl-4,4'-dicarboxylic acid, and DMF = N,N-dimethylformamide. Crystallographic characterization indicated that the two MOFs possess similar 2-fold-interpenetrating 3D frameworks but with two different pore sizes. The pore-size-dependent performances of the TTF-Co-MOFs were explored to optimize the MOFs as the anode materials for LIBs. TTF-Co-MOF 1 presents a high reversible specific capacity of 1186.6 mAh g^-1 at 200 mA g^-1 after 287 cycles. The rate capability is greatly enhanced by the introduction of Co^II into TTF-based MOFs with specific capacities of 1028.6 mAh g^-1 at 5 A g^-1 and 966.5 mAh g^-1 at 10 A g^-1. On the basis of the series analysis of theoretical calculations, electrochemical impedance spectroscopy, and crystal structures, it is found that the Co^II metal centers play a bridging role in charge transport within the MOF framework, which is beneficial for the transportation of Li ions. The competitive performances of TTF-Co-MOF 1 are attributed to the synergistic effect of the Co^II metal centers and S-rich TTF ligand as well as suitable porosity. The study shed some light for the fabrication of advanced energy storage devices through the rational design of MOF-based anode materials.

Copper-bipyridine grid frameworks incorporating redox-active tetrathiafulvalene: structures and supercapacitance

Redox active tetrathiafulvalene (TTF) and its derivatives when used as electrode additives have exhibited improved energy efficiency and sustainability in batteries. However, the structure-property relationship has not been investigated in detail until very recently. In this work, three redox-active TTF compounds were synthesized, and formulated as [Cu(HL)₂(bpa)₂]_n (1), [Cu(bpe)₂(H₂O)₂]_n·2n(HL)·nMeOH·nH₂O (2), and [Cu(bpp)₂(H₂O)₂]_n·2n(HL) (3) (L = dimethylthio-tetrathiafulvalene-bicarboxylate) for this work. The effects of conjugated state and spacer length of the linkers on structural assembly and band gap as well as the interactions of TTF-TTF/TTF-bpy are discussed. Compound 1 is a bpa and HL co-coordinated 1D Cu(ii) polymer. Compounds 2 and 3 are 2D Cu(ii)-bipyridine (4,4) MOFs incorporating HL (1^-) as free anion columns. The photocurrent density of 2 is larger than those of 1 and 3 due to a strong charge transfer from TTF to bpe in compound 2. The supercapacitance performances of these compounds were evaluated by cyclic voltammetry (CV) and galvanostatic charge-discharge (GCD) techniques. The results revealed that the 2D MOF structures of 2 and 3 are beneficial for good specific capacitance values (C_sp). This work revealed the structure-property relationships of TTF derivatives for use as electrode active materials in energy transfer and storage.

Tuning the photoelectric response of pyrene-based coordination polymers by optimizing charge transfer

Three π–π stacked CPs were designed and synthesized for application of photoelectric response. The effect of charge transfer on the photoelectric properties is explored by adjusting the composition and π-stacking fashion of the CPs.