Functional coordination polymers based on redox-active tetrathiafulvalene and its derivatives

Dynamic Interchain Motion in 1D Tetrathiafulvalene-Based Coordination Polymers for Highly Sensitive Molecular Recognition

The application of electrically conductive 1D coordination polymers (1D CPs) in nanoelectronic molecular recognition is theoretically promising yet rarely explored due to the challenges in their synthesis and optimization of electrical properties. In this regard, two tetrathiafulvalene-based 1D CPs, namely [Co(m-H2TTFTB)(DMF)2(H2O)]n (Co-m-TTFTB), and {[Ni(m-H2TTFTB)(CH3CH2OH)1.5(H2O)1.5]·(H2O)0.5}n (Ni-m-TTFTB) are successfully constructed. The shorter S···S contacts between the [M(solvent)3(m-H2TTFTB)]n chains contribute to a significant improvement in their electrical conductivities. The powder X-ray diffraction (PXRD) under different organic solvents reveals the flexible and dynamic structural characteristic of M-m-TTFTB, which, combined with the 1D morphology, lead to their excellent performance for sensitive detection of volatile organic compounds. Co-m-TTFTB achieves a limit of detection for ethanol vapor down to 0.5 ppm, which is superior to the state-of-the-art chemiresistive sensors based on metal-organic frameworks or organic polymers at room temperature. In situ diffuse reflectance infrared Fourier transform spectroscopy, PXRD measurements and density functional theory calculations reveal the molecular insertion sensing mechanism and the corresponding structure-function relationship. This work expands the applicable scenario of 1D CPs and opens a new realm of 1D CP-based nanoelectronic sensors for highly sensitive room temperature gas detection.

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.

Binucleating Jäger-type {(N2O2)2}4- ligands: magnetic and electronic interactions of Fe(II), Ni(II) and Cu(II) across an in-plane TTF-bridge

The simultaneous presence of different electrophores provides an interesting playground for responsive materials. Herein, we present the incorporation of a twice-reversibly oxidizable tetrathiafulvalene (TTF) unit into a binucleating ligand, bridging two metal centers in a fully conjugated plane. A two-step synthesis scheme gave the D2h symmetric Schiff base-like ligand H4L in moderate yields from which the corresponding copper(II) [Cu2L], nickel(II) [Ni2L], [Ni2L(py)4] and iron(II) complexes [Fe2L(py)4], [Fe2L(dmap)4] and [Fe2L(bpee)2]·1 Tol could be obtained. Characterization was performed through 1H-NMR, IR, UV-vis and 57Fe-Mössbauer spectroscopy, SQUID magnetometry and cyclic voltammetry, supported by density functional theory (DFT) calculations. Single crystal X-ray analysis of [Ni2L(py)4] revealed six-coordinate paramagnetic centers, whereas [Ni2L] underwent gradual coordination induced spin state switching (CISSS) in solution. The magnetic independence of both metal centers is echoed by close-to-ideal Curie-plots of the [Cu2L] system and the gradual spin crossover of all iron(II) compounds. By contrast, cyclic voltammetry measurements in solution indicated oxidation-dependent TTF-metal interactions, as well as metal-metal interactions. The reversible TTF-borne events in H4L and [Ni2L] are overlaid with metal-borne events in the case of [Fe2L(py)4], as is corroborated by an analysis of the frontier orbital landscapes and through diagnostic spectral features upon chemical oxidation.

Topotactic Conversion of Titanium-Oxo Clusters to a Stable TOC-Based Metal-Organic Framework with the Selective Adsorption of Cationic Dyes

Titanium-oxo cluster (TOC)-based metal-organic frameworks (MOFs) have received considerable attention in recent years due to their ability to expand the application of TOCs to fields that require highly stable frameworks. Herein, a new cyclic TOC formulated as [Ti6O6(OiPr)8(TTFTC)(phen)2]2 (1, where TTFTC = tetrathiafulvalene tetracarboxylate and phen = phenanthroline) was crystallographically characterized. TOC 1 takes a rectangular ring structure with two phen-modified Ti6 clusters as the width and two TTFTC ligands as the length. An intracluster ligand-to-ligand (TTF-to-phen) charge transfer in 1 was found for TOCs for the first time. Compound 1 undergoes topotactic conversion to generate stable TOC-MOF P1, in which the rectangular framework in 1 formed by a TOC core and ligands is retained, as verified by comprehensive characterization. P1 shows an efficient and rapid selective adsorption capacity for cationic dyes. The experimental adsorption capacity (qex) of P1 reaches a value of up to 789.2 mg/g at 298 K for the crystal violet dye, which is the highest among those of various adsorbents. The calculated models are first used to reveal the structure-property relationship of the cyclic host to different guest dyes. The results further confirmed the host MOF structure of P1.

Redox-Active Mixed-Linker Metal-Organic Frameworks with Switchable Semiconductive Characteristics for Tailorable Chemiresistive Sensing

Metal-organic frameworks (MOFs), with diverse metal nodes and designable organic linkers, offer unique opportunities for the rational engineering of semiconducting properties. In this work, we report a mixed-linker conductive MOF system with both tetrathiafulvalene and Ni-bis(dithiolene) moieties, which allows the fine-tuning of electronic structures and semiconductive characteristics. By continuously increasing the molar ratio between tetrathiafulvalene and Ni-bis(dithiolene), the switching of the semiconducting behaviors from n-type to p-type was observed along with an increase in electrical conductivity by 3 orders of magnitude (from 2.88×10^-7  S m^-1 to 9.26×10^-5  S m^-1 ). Furthermore, mixed-linker MOFs were applied for the chemiresistive detection of volatile organic compounds (VOCs), where the sensing performance was modulated by the corresponding linker ratios, showing synergistic and nonlinear modulation effects.

Photocatalytic nitrogen fixation under an ambient atmosphere using a porous coordination polymer with bridging dinitrogen anions

The design of highly electron-active and stable heterogeneous catalysts for the ambient nitrogen reduction reaction is challenging due to the inertness of the N₂ molecule. Here, we report the synthesis of a zinc-based coordination polymer that features bridging dinitrogen anionic ligands, {[Zn(L)(N₂)_0.5(TCNQ-TCNQ)_0.5]·(TCNQ)_0.5}_n (L is tetra(isoquinolin-6-yl)tetrathiafulvalene and TCNQ is tetracyanoquinodimethane), and show that it is an efficient photocatalyst for nitrogen fixation under an ambient atmosphere. It exhibits an ammonia conversion rate of 140 μmol g^-1 h^-1 and functions well also with unpurified air as the feeding gas. Experimental and theoretical studies show that the active [Zn²⁺-(N≡N)^--Zn²⁺] sites can promote the formation of NH₃ and the detachment of the NH₃ formed creates unsaturated [Zn²⁺···Zn⁺] intermediates, which in turn can be refilled by external N₂ sequestration and fast intermolecular electron migration. The [Zn²⁺···Zn⁺] intermediates stabilized by the sandwiched cage-like donor-acceptor-donor framework can sustain continuous catalytic cycles. This work presents an example of a molecular active site embedded within a coordination polymer for nitrogen fixation under mild conditions.

Bright, Modular, and Switchable Near-Infrared II Emission from Compact Tetrathiafulvalene-Based Diradicaloid Complexes

Near-infrared (NIR)-emitting molecules are promising candidates for biological sensing and imaging applications; however, many NIR dyes are large conjugated systems which frequently have issues with stability, solubility, and tunability. Here, we report a novel class of compact and tunable fluorescent diradicaloid complexes which are air-, water-, light-, and temperature-stable. These properties arise from a compressed π manifold which promotes an intense ligand-centered π-π transition in the NIR II (1000-1700 nm) region and which subsequently emits at ∼1200 nm. This emission is among the brightest known for monomolecular lumiphores with deep NIR II (>1100 nm) emission, nearly an order of magnitude brighter than the commercially available NIR II dye IR 26. Furthermore, this fluorescence is electrochemically sensitive, with efficient switching upon addition of redox agents. The brightness, stability, and modularity of this system distinguish it as a promising candidate for the development of new technologies built around NIR emission.

Two-dimensional dysprosium(III) coordination polymer: Structure, single-molecule magnetic behavior, proton conduction, and luminescence

A new dysprosium (III) coordination polymer [Dy(Hm-dobdc) (H₂O)₂]·H₂O (Dy-CP), was hydrothermal synthesized based on 4,6-dioxido-1,3-benzenedicarboxylate (H₄m-dobdc) ligand containing carboxyl and phenolic hydroxyl groups. The Dy(III) center adopts an octa-coordinated [DyO₈] geometry, which can be described as a twisted square antiprism (D_4d symmetry). Neighboring Dy(III) ions are interconnected by deprotonated Hm-dobdc³⁻ ligand to form the two-dimensional infinite layers, which are further linked to generate three-dimensional structure through abundant hydrogen bonds mediated primarily by coordinated and lattice H₂O molecules. Magnetic studies demonstrates that Dy-CP shows the field-induced slow relaxation of magnetization and the energy barrier U_eff/k_B and relaxation time τ₀ are 35.3 K and 1.31 × 10^–6 s, respectively. Following the vehicular mechanism, Dy-CP displays proton conductivity with σ equal to 7.77 × 10^–8 S cm⁻¹ at 353 K and 30%RH. Moreover, luminescence spectra reveal that H₄m-dobdc can sensitize characteristic luminescence of Dy(III) ion. Herein, good magnetism, proton conduction, and luminescence are simultaneously achieved, and thus, Dy-CP is a potential multifunctional coordination polymer material.

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.

Enhanced Photocatalytic Hydrogen Production Activity by Constructing a Robust Organic-Inorganic Hybrid Material Based Fulvalene and TiO2

A novel redox-active organic-inorganic hybrid material (denoted as H₄TTFTB-TiO₂) based on tetrathiafulvalene derivatives and titanium dioxide with a micro/mesoporous nanomaterial structure has been synthesized via a facile sol-gel method. In this study, tetrathiafulvalene-3,4,5,6-tetrakis(4-benzoic acid) (H₄TTFTB) is an ideal electron-rich organic material and has been introduced into TiO₂ for promoting photocatalytic H₂ production under visible light irradiation. Notably, the optimized composites demonstrate remarkably enhanced photocatalytic H₂ evolution performance with a maximum H₂ evolution rate of 1452 μmol g⁻¹ h⁻¹, which is much higher than the prototypical counterparts, the common dye-sensitized sample (denoted as H₄TTFTB-5.0/TiO₂) (390.8 μmol g⁻¹ h⁻¹) and pure TiO₂ (18.87 μmol g⁻¹ h⁻¹). Moreover, the composites perform with excellent stability even after being used for seven time cycles. A series of characterizations of the morphological structure, the photoelectric physics performance and the photocatalytic activity of the hybrid reveal that the donor-acceptor structural H₄TTFTB and TiO₂ have been combined robustly by covalent titanium ester during the synthesis process, which improves the stability of the hybrid nanomaterials, extends visible-light adsorption range and stimulates the separation of photogenerated charges. This work provides new insight for regulating precisely the structure of the fulvalene-based composite at the molecule level and enhances our in-depth fundamental understanding of the photocatalytic mechanism.

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.

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.

Enhancing the photothermal conversion of tetrathiafulvalene-based MOFs by redox doping and plasmon resonance

Near-infrared (NIR) photothermal materials hold great promise for use in several applications, particularly in photothermal therapy, diagnosis, and imaging. However, current NIR responsive materials often show narrow absorption bands and low absorption efficiency, and have long response times. Herein, we demonstrate that the NIR absorption of tetrathiafulvalene-based metal–organic frameworks (MOFs) can be tuned by redox doping and using plasmonic nanoparticles. In this work, a MOF containing redox-active tetrathiafulvalene (TTF) units and Dy-carboxylate chains was constructed, Dy-m-TTFTB. The NIR absorption of the as-synthesized Dy-m-TTFTB was further enhanced by Ag⁺ or I₂ oxidation, transforming the neutral TTF into a TTF˙⁺ radical state. Interestingly, treatment with Ag⁺ not only generated TTF˙⁺ radicals, but it also formed Ag nanoparticles (NPs) in situ within the MOF pores. With both TTF˙⁺ radicals and Ag NPs, Ag NPs@Dy-m-TTFTB was shown to exhibit a wide range of absorption wavelengths (200–1000 nm) and also a high NIR photothermal conversion. When the system was irradiated with an 808 nm laser (energy power of 0.7 W cm⁻²), Ag NPs@Dy-m-TTFTB showed a sharp temperature increase of 239.8 °C. This increase was higher than that of pristine Dy-m-TTFTB (90.1 °C) or I₂ treated I₃⁻@Dy-m-TTFTB (213.0 °C).

The photo-response of the redox-active metal–organic framework has been systematically tuned by incorporating plasmonic Ag nanoparticles and tetrathiafulvalene radicals, resulting in efficient near-infrared photothermal conversion materials.

A family of lanthanide metal-organic frameworks based on a redox-active tetrathiafulvalene-dicarboxylate ligand showing slow relaxation of magnetisation and electronic conductivity

The reaction of the redox-active tetrathiafulvalene ligand and lanthanide ions is an important approach to prepare photo-electro-magnetic multifunctional metal-organic framework materials. A series of isostructural lanthanide metal-organic frameworks (Ln-MOFs) based on the in situ generated tetrathiafulvalene dicarboxylate (TTF-DC) ligand, {[Ln₄(TTF-DC)₆(DMF)₄(H₂O)₂]·4DMF}_n (Ln = Gd (1-Gd), Tb (1-Tb), Dy (1-Dy) and Er (1-Er)), was synthesized and characterized. These Ln-MOFs display tunable redox-active properties and semiconductor performance, and their electronic conductivities have been significantly improved after oxidation. All MOFs except 2-Tb exhibit slow magnetic relaxation under an applied dc field. 1-Dy and 2-Dy show field-induced single-molecule magnet (SMM) behaviour with energy barriers (U_eff) of 30.77 K (τ₀ = 5.23 × 10^-8) and 26.41 K (1.04 × 10^-8 s), respectively.

A New Electrically Conducting Metal-Organic Framework Featuring U-Shaped cis-Dipyridyl Tetrathiafulvalene Ligands

A new electrically conducting 3D metal-organic framework (MOF) with a unique architecture was synthesized using 1,2,4,5-tetrakis-(4-carboxyphenyl)benzene (TCPB) a redox-active cis-dipyridyl-tetrathiafulvalene (Z-DPTTF) ligand. While TCPB formed Zn₂(COO)₄ secondary building units (SBUs), instead of connecting the Zn₂-paddlewheel SBUs located in different planes and forming a traditional pillared paddlewheel MOF, the U-shaped Z-DPTTF ligands bridged the neighboring SBUs formed by the same TCPB ligand like a sine-curve along the b axis that created a new sine-MOF architecture. The pristine sine-MOF displayed an intrinsic electrical conductivity of 1 × 10⁻⁸ S/m, which surged to 5 × 10⁻⁷ S/m after I₂ doping due to partial oxidation of electron rich Z-DPTTF ligands that raised the charge-carrier concentration inside the framework. However, the conductivities of the pristine and I₂-treated sine-MOFs were modest possibly because of large spatial distances between the ligands that prevented π-donor/acceptor charge-transfer interactions needed for effective through-space charge movement in 3D MOFs that lack through coordination-bond charge transport pathways.

In(III) Metal-Organic Framework Incorporated with Enzyme-Mimicking Nickel Bis(dithiolene) Ligand for Highly Selective CO2 Electroreduction

Inspired by the exciting physical/chemical properties in metal-organic frameworks (MOFs) of the redox-active tetrathiafulvalene (TTF) ligands, nickel bis(dithiolene-dibenzoic acid), [Ni(C₂S₂(C₆H₄COOH)₂)₂], has been designed and developed as an inorganic analogue of the corresponding TTF-type donors (such as tetrathiafulvalene-tetrabenzoate, TTFTB), where a metal site (Ni) replaces the central C═C bond. In this work, [Ni(C₂S₂(C₆H₄COOH)₂)₂] and In³⁺ have been successfully assembled into a three-dimensional MOF, (Me₂NH₂⁺){In^III-[Ni(C₂S₂(C₆H₄COO)₂)₂]}·3DMF·1.5H₂O (1, DMF = N,N-dimethylformamide), with satisfying chemical and thermal stabilities. With the combination of reversible redox activity and unsaturated metal sites originated from [Ni(C₂S₂(C₆H₄COOH)₂)₂], 1 showed a significantly enhanced performance in electrocatalytic CO₂ reduction compared with the isomorphic MOF, (Me₂NH₂⁺)[In^III-(TTFTB)]·0.7C₂H₅OH·DMF (2, with TTFTB ligand). More importantly, by mimicking the active [NiS₄] sites of formate dehydrogenase and CO-dehydrogenase, a prominently higher conversion rate and Faradaic efficiency (FE), with FE_HCOO^- increasing from 54.7% to 89.6% (at -1.3 V vs RHE, j_HCOO^- = 36.0 mA cm^-2), were achieved in 1. Mechanistic investigations further confirm that [NiS₄] can serve as a CO₂ binding site and efficient catalytic center. This unprecedented effect of redox-active nickel dithiolene-based MOF catalysts on the performance of electroreduction of CO₂ provides an important strategy for designing stable and efficient crystalline enzyme-mimicking catalysts for the conversion of CO₂ into high-value chemical stocks.

Iron(II) Spin Crossover Coordination Polymers Derived From a Redox Active Equatorial Tetrathiafulvalene Schiff-Base Ligand

Two polymorphic Fe^II coordination polymers [Fe^IIL (TPPE)_0.5] 1) and [(Fe^II₃L₃ (TPPE)_1.5)] 2), were obtained from a redox-active tetrathiafulvalene (TTF) functionalized ligand [H₂L = 2,2’-(((2-(4,5-bis-(methylthio)-1,3-dithiol-2-ylidene)benzo(d) (1,3) dithiole-5,6-diyl)bis-(azanediyl))bis-(meth anylylidene)) (2E,2E')-bis(3-oxobutanoate)] and a highly luminescent connector {TPPE = 1,1,2,2-tetrakis[4-(pyridine-4-yl)phenyl]-ethene}. Complex 1 has a layered structure where the TPPE uses its four diverging pyridines from the TPPE ligand are coordinated by the trans positions to the flat TTF Schiff-base ligand, and complex 2 has an unprecedented catenation of layers within two interpenetrated frameworks. These coordination polymers reserved the redox activity of the TTF unit. Complex 1 shows gradual spin transition behavior without hysteresis. And the fluorescence intensity of TPPE in 1 changes in tandem with the spin crossover (SCO) transition indicating a possible interplay between fluorescence and SCO behavior.

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.

Redox Activity as a Powerful Strategy to Tune Magnetic and/or Conducting Properties in Benzoquinone-Based Metal-Organic Frameworks

Multifunctional molecular materials have attracted material scientists for several years as they are promising materials for the future generation of electronic devices. Careful selection of their molecular building blocks allows for the combination and/or even interplay of different physical properties in the same crystal lattice. Incorporation of redox activity in these networks is one of the most appealing and recent synthetic strategies used to enhance magnetic and/or conducting and/or optical properties. Quinone derivatives are excellent redox-active linkers, widely used for various applications such as electrode materials, flow batteries, pseudo-capacitors, etc. Quinones undergo a reversible two-electron redox reaction to form hydroquinone dianions via intermediate semiquinone radical formation. Moreover, the possibility to functionalize the six-membered ring of the quinone by various substituents/functional groups make them excellent molecular building blocks for the construction of multifunctional tunable metal-organic frameworks (MOFs). An overview of the recent advances on benzoquinone-based MOFs, with a particular focus on key examples where magnetic and/or conducting properties are tuned/switched, even simultaneously, by playing with redox activity, is herein envisioned.

Tetrathiafulvalene-based double metal lead iodides: structures and electrical properties

Introducing electronically active organic components into lower dimensional metal halide compounds is an effective strategy to improve the electronic properties of hybrid metal halide materials. We have previously used this strategy to explore hybrid halides with tetrathiafulvalenes (TTFs) and a series of lead iodides and bismuth halides were isolated. The electronic properties were improved notably using this modification. In this work, we expand the study of TTF based main-group metal halides to double metal halides with mixed lead and copper transition metals. Two hybrid TTF-lead-cuprous iodides, formulated as [TTF]5[Pb2Cu2I10]·H2O (1) and [TTF]2[PbCu2I6] (2), and two monometal analogues of [TTF]2[Cu4I6]·H2O (3) and [TTF]2[Ag4I6] (4) were crystallographically characterized. The anion of 1 is a 0D cluster, while that of the others is a 1D chain structure. The anion structures of 1-4 are novel and are reported for the first time. The TTF moieties are stacked to form a 2D framework in 1 and 1D columns in 2-4. We found that the semiconductor properties of the hybrids are related to electron donation from an anion to a cation. The electronic state of the TTF cations is another significant factor that affects the electronic properties of the materials. More notably, this work proved that the conductivity and photoconductivity of the mixed metal iodides are superior to those of the monometal iodides.

Charge Transfer Metal-Organic Framework Containing Redox-Active TTF/NDI Units for Highly Efficient Near-Infrared Photothermal Conversion

Metal-organic frameworks (MOFs), as a class of new inorganic-organic hybrid crystal materials, could have important applications in near-infrared (NIR) photothermal conversion. Herein, a new charge-transfer MOF (Co-MOF) with mixed ligands of H₄ TTFTB and bpmNDI incorporating redox-active tetrathiafulvalene/naphthalene diimide (TTF/NDI) units into one system is reported. Due to the presence of TTF/NDI oxidative and reductive couples, stable radicals can be observed in the MOF. In addition, charge transfer from the electron donor (TTF) to the acceptor (NDI) results in a broad absorption in the NIR region. The Co-MOF exhibited an efficient photothermal effect induced by irradiation with a NIR laser. Under the 808 nm laser (0.7 W cm^-2 ) illumination, the temperature of the Co-MOF increased from room temperature to 201 °C in only 10 s. Furthermore, a series of polydimethylsiloxane (PDMS) films doped with trace amounts of Co-MOF showed efficient NIR photothermal conversion. When a Co-MOF@PDMS (0.6 wt %) film is irradiated by 808 nm laser with power of 0.5 W cm^-2 , it's temperature can reach a plateau at 62 °C from 20 °C within 100 s. Our experimental results from the Co-MOF@PDMS film demonstrate that the effectiveness and feasibility of the material is promising for photothermal applications.

Rare-Earth Metal Tetrathiafulvalene Carboxylate Frameworks as Redox-Switchable Single-Molecule Magnets

Using the redox-active tetrathiafulvalene tetrabenzoate (TTFTB^4- ) as the linker, a series of stable and porous rare-earth metal-organic frameworks (RE-MOFs), [RE₉ (μ₃ -OH)₁₃ (μ₃ -O)(H₂ O)₉ (TTFTB)₃ ] (1-RE, where RE=Y, Sm, Gd, Tb, Dy, Ho, and Er) were constructed. The RE₉ (μ₃ -OH)₁₃ (μ₃ -O) (H₂ O)₉ ](CO₂ )₁₂ clusters within 1-RE act as segregated single-molecule magnets (SMMs) displaying slow relaxation. Interestingly, upon oxidation by I₂ , the S=0 TTFTB^4- linkers of 1-RE were converted into S= 1 / 2 TTFTB^.3- radical linkers which introduced exchange-coupling between SMMs and modulated the relaxation. Furthermore, the SMM property can be restored by reduction in N,N-dimethylformamide. These results highlight the advantage of MOFs in the construction of redox-switchable SMMs.

Donor/acceptor substituted dithiafulvenes and tetrathiafulvalene vinylogues: electronic absorption, crystallographic, and computational analyses

The structural, electronic, and crystallographic properties of nitrophenyl and methoxyphenyl-substituted dithiafulvenes and tetrathiafulvalene vinylogues were systematically investigated by experimental and computational approaches.

A Chiral Metal-Organic 1D-Coordination Polymer Upon Complexation of Phenylene-Bridged Bipyrrole and Palladium (II) Ion

Metal-organic 1D-coordination polymers, having unique electronic and optical properties, are expected to be a novel advanced functional material capable of fabricating smart plastics, films, and fibers. In this study, we have synthesized a novel metal-organic 1D-coordination polymer composed of a phenylene-bridged bipyrrole bearing N-alkylimino groups (BPI) and palladium(II) ion. The BPI and Pd(II) form square planar bis(bidentate) complex to form a metal coordinated π-conjugation polymer (Poly-BPI/Pd). It is stable in solutions at room temperature, and allowed measurement of its average molecular weight in SEC (M w = 106,000 and M n = 18,000, M w/M n = 5.88). It also provided a reversible multi redox profile in cyclic voltammetry, most likely originating from strong π-electronic interactions between the BPI components via Pd ion. A variety of substituent groups can be attached to the imino-nitrogens of BPI. A coordination polymer composed of a BPI derivative bearing chiral alkyl chains and Pd(II) showed strong circular dichroism (CD) in the solution due to the unidirectional chiral conformation of the BPI components in the polymer backbone.

Tetrathiafulvalene-Based Metal-Organic Framework as a High-Performance Anode for Lithium-Ion Batteries

Metal-organic frameworks (MOFs) have aroused great interest as lithium-ion battery (LIB) electrode materials. In this work, we first report that a pristine three-dimensional tetrathiafulvalene derivatives (TTFs)-based zinc MOF, formulated [Zn₂(py-TTF-py)₂(BDC)₂]·2DMF·H₂O (1) (py-TTF-py = 2,6-bis(4'-pyridyl)tetrathiafulvalene and H₂BDC = terephthalic acid), can work as a high-performance electrode material for rechargeable LIBs. The TTFs-Zn-MOF 1 electrode displayed a high discharge specific capacity of 1117.4 mA h g^-1 at a current density of 200 mA g^-1 after 150 cycles along with good reversibility. After undergoing elevated discharging/charging rates, the electrode showed superior lithium storage performance in the extreme case of 20 A g^-1 and could finally recover the capability when the current rate was back to 200 mA g^-1. Particularly, specific capacities of 884.2, 513.8, and 327.8 mA h g^-1 were reached at high current densities of 5, 10, and 20 A g^-1 after 180, 175, and 300 cycles along with good reversibility, respectively. Such an excellent performance is first reported for the LIB anode materials. TTFs-Zn-MOF 2, namely, [Zn₂(py-TTF-py) (BDC)₂]·DMF·2H₂O (2), was prepared as a contrast to explore the relationship between the structures of the electrode materials and the electrochemical properties. Based on the structural analysis of 1 and 2 and ex situ X-ray photoelectron spectroscopy, the TTF moiety and the twofold TTF pillar play a key role in the excellent electrochemical performance. The full cell of MOF 1 with NMC 622 delivered the capacity of 131.9 mA h g^-1 at 100 mA g^-1 with the Coulombic efficiency of 99.45% after 70 cycles and exhibited the tolerance to high-current operation.

A Metal-Organic Framework Based on a Nickel Bis(dithiolene) Connector: Synthesis, Crystal Structure, and Application as an Electrochemical Glucose Sensor

Functionalizing the redox-active tetrathiafulvalene (TTF) core with groups capable of coordination to metals provides new perspectives on the modulation of architectures and electronic properties of organic-inorganic hybrid materials. With a view to extending this concept, we have now synthesized nickel bis(dithiolene-dibenzoic acid), [Ni(C2S2(C6H4COOH)2)2], which can be considered as the inorganic analogue of the organic tetrathiafulvalene-tetrabenzoic acid (H4TTFTB). Likewise, [Ni(C2S2(C6H4COOH)2)2] is a redox-active linker for new functional metal-organic frameworks, as demonstrated here with the synthesis of [Mn2{Ni(C2S2(C6H4COO)2)2}(H2O)2]·2DMF, (1, DMF = N,N-dimethylformamide). 1 is isomorphic to the reported [Mn2(TTFTB)(H2O)2] (2) but is a better electrochemical glucose sensor due to the multiple oxidation-reduction states of the [NiS4] core, which allow glucose to be oxidized to glucolactone by the high oxidation state [NiS4] center. As a non-enzymatic glucose sensor, 1 on Cu foam (CF), 1-CF, was synthesized by a one-step hydrothermal method and exhibited an excellent electrochemical performance. The fabricated 1-CF electrode offers a high sensitivity of 27.9 A M-1 cm-2, with a wide linear detection range from 2.0 × 10-6 to 2.0 × 10-3 M, a low detection limit of 1.0 × 10-7 M (signal/noise = 3), and satisfactory stability and reproducibility.

Spin crossover modulation in a coordination polymer with the redox-active bis-pyridyltetrathiafulvalene (py2TTF) ligand

A one-dimensional Fe^II coordination polymer (CP) has been formed which includes the redox-active ligand bis-pyridyltetrathiafulvalene (py₂TTF) and a Schiff base-like N₂O₂ ligand. This CP is both spin crossover (SCO) and redox-active in the solid-state, and chemical oxidation results in SCO modification.

Reversible single crystal-to-single crystal double [2+2] cycloaddition induces multifunctional photo-mechano-electrochemical properties in framework materials

Reversible structural transformations of porous coordination frameworks in response to external stimuli such as light, electrical potential, guest inclusion or pressure, amongst others, have been the subject of intense interest for applications in sensing, switching and molecular separations. Here we report a coordination framework based on an electroactive tetrathiafulvalene exhibiting a reversible single crystal-to-single crystal double [2 + 2] photocyclisation, leading to profound differences in the electrochemical, optical and mechanical properties of the material upon light irradiation. Electrochemical and in situ spectroelectrochemical measurements, in combination with in situ light-irradiated Raman spectroscopy and atomic force microscopy, revealed the variable mechanical properties of the framework that were supported using Density Functional Theory calculations. The reversible structural transformation points towards a plethora of potential applications for coordination frameworks in photo-mechanical and photoelectrochemical devices, such as light-driven actuators and photo-valves for targeted drug delivery.

Porous coordination frameworks that undergo reversible structural transformations are promising for sensing, switching and separations. Here, the authors report an electroactive framework that exhibits a reversible single crystal-to-single crystal double [2+2] photocyclisation, leading to property changes.

Complexes of Cobalt(II) Iodide with Pyridine and Redox Active 1,2-Bis(arylimino)acenaphthene: Synthesis, Structure, Electrochemical, and Single Ion Magnet Properties

Complexes [(dpp-BIAN)⁰Co^III₂]·MeCN (I) and [(Py)₂CoI₂] (II) were synthesized by the reaction between cobalt(II) iodide and 1,2-bis(2,6-diisopropylphenylimino)acenaphthene (dpp-BIAN) or pyridine (Py), respectively. The molecular structures of the complexes were determined by X-ray diffraction. The Co(II) ions in both compounds are in a distorted tetrahedral environment (CoN₂I₂). The electrochemical behavior of complex I was studied by cyclic voltammetry. Magnetochemical measurements revealed that when an external magnetic field is applied, both compounds exhibit the properties of field-induced single ion magnets.

Electrically Conductive Metal-Organic Frameworks

Metal–organic frameworks (MOFs) are intrinsically porous extended solids formed by coordination bonding between organic ligands and metal ions or clusters. High electrical conductivity is rare in MOFs, yet it allows for diverse applications in electrocatalysis, charge storage, and chemiresistive sensing, among others. In this Review, we discuss the efforts undertaken so far to achieve efficient charge transport in MOFs. We focus on four common strategies that have been harnessed toward high conductivities. In the “through-bond” approach, continuous chains of coordination bonds between the metal centers and ligands’ functional groups create charge transport pathways. In the “extended conjugation” approach, the metals and entire ligands form large delocalized systems. The “through-space” approach harnesses the π–π stacking interactions between organic moieties. The “guest-promoted” approach utilizes the inherent porosity of MOFs and host–guest interactions. Studies utilizing less defined transport pathways are also evaluated. For each approach, we give a systematic overview of the structures and transport properties of relevant materials. We consider the benefits and limitations of strategies developed thus far and provide an overview of outstanding challenges in conductive MOFs.

A Series of Tetrathiafulvalene Bismuth Chlorides: Effects of Oxidation States of Cations on Structures and Electric Properties

Most large organic cations in the low-dimensional hybrid halide perovskites deteriorate the photoelectric conversion efficiency of the cells. Integrating electronically active organic components into hybrid metal halides is an effective method to improve their photoelectric properties. In this work, a series of compounds obtained by hybridizing redox-active tetrakis(methylthio)tetrathiafulvalene (TMT-TTF) with bismuth chloride, formulated as [TMT-TTF]₄[Bi₆Cl₂₂] (1 and 1'), [TMT-TTF]₃[Bi₄Cl₁₆] (2), [TMT-TTF]₂[Bi₃Cl₁₃] (3), [TMT-TTF]₂[Bi₂Cl₁₀] (4), and {[TMT-TTF][Bi₂Cl₈]}_n (5), were crystallographically characterized. These hybrids exhibit changeable oxidation states of the TTF moiety. The radical cation TTF^•+ exists in 1 and 1', while a mixed-valence TTF^•+/TTF²⁺ appears in 2 that has never been documented in any compounds and the dication TTF²⁺ exists in 3-5 that has never been introduced into hybrid organic-inorganic materials. The different charged states of the TTF cations lead to various degrees of connectivity of metal chloride anions, which exert a significant effect on the cation-anion arrangement and result in different supramolecular interactions between TMT-TTF and between cations and anions. The changeable oxidation states of the TTF moiety and varying degrees of metal chloride connectivity provide a good comparison among these hybridized bismuth chlorides. The order of conductivity is 2 > 1 > 1' > 3 ≈ 4 ≫ 5, which results from the synergistic effect of different oxidation states, the packing of TMT-TTF cations, and back charge transfer from the Bi-Cl anion to the TMT-TTF cation. Notably, the electrical conductivity and carrier mobility can be modulated with the fact that compound 2 has the highest performances in the dark, while in light, these properties of 1 and 1' are in turn higher than that of 2. The order of the photocurrent densities is in accordance with the increase of carrier mobility under irradiation of light. This work is the first systematic study of hybrid metal halides with various oxidation states of TTFs and presents a clear structure-property relationship and offers a fresh view on the design of new perovskite materials at the molecular level.

Interdigitated conducting tetrathiafulvalene-based coordination networks

Assembly of a novel ethylenedithio-tetrathiafulvalene (EDT-TTF) derivative bearing two adjacent 4-thiopyridyl groups with M(NCS)2 nodes (M = Fe, Co) leads to two isostructural 1D coordination polymers showing an enhancement of their electronic conductivity by six orders of magnitude (10-6vs. 10-12 S cm-1), upon surface oxidation by iodine and subsequent generation of EDT-TTF-based radicals.

Zirconium metal-organic frameworks incorporating tetrathiafulvalene linkers: robust and redox-active matrices for in situ confinement of metal nanoparticles

Redox-active metal-organic frameworks (MOFs), with highly ordered porous structures and redox tunability, have attracted research interest in the fields of catalysis, energy storage, and electrochemical sensing. However, the chemical lability has limited the application scope of many redox-active MOFs. Herein, we selected stable Zr₆ inorganic nodes and redox-active tetrathiafulvalene (TTF)-based linkers to construct two robust, redox-active MOFs, namely compounds 1 ([Zr₆(TTFTB)₂O₈(OH₂)₈]) and 2 ([Zr₆(Me-TTFTB)_1.5O₄(OH)₄(C₆H₅COO)₆]) (TTFTB = tetrathiafulvalene tetrabenzoate; Me-TTFTB = tetrathiafulvalene tetramethylbenzoate). The structure and topology of the MOFs were controlled by tuning the linker conformation through steric effects, resulting in a variety of pore structures from microporous channels (compound 1) to hierarchically micro/mesoporous cages (compound 2). Compound 2 shows high porosity with a BET surface area of 1932 m² g^-1 and strong chemical stability in aqueous solutions with pH ranging from 1 to 12. Furthermore, the reductive TTF moieties allow for in situ generation and stabilization of ultra-small noble metal (Ag, Pd, and Au) nanoparticles by incubating MOFs in the respective metal salt solution. Single crystal structures, TEM images, and pore size distribution data from N₂ adsorption measurements indicated that the metal nanoparticles were mostly placed in the small cubic cavities of hierarchically porous compound 2, leaving the large cages open for substrate diffusion. As a proof of concept, Pd NPs@compound 2 was utilized as a heterogeneous catalyst for aerobic oxidation of alcohols, showing noteworthy activity and recyclability.

Stabilization and isolation of radical cation and dication salts of a tetrathiafulvalene derivative functionalized with amino groups

The radical cation and dication salts of a tetrathiafulvalene derivative functionalized with amino groups have been stabilized and isolated by chemical oxidation. Comprehensive research on their structure–property relationship was fully performed.

Electrical conductivity and magnetic bistability in metal–organic frameworks and coordination polymers: charge transport and spin crossover at the nanoscale

This review aims to reassess the progress, issues and opportunities in the path towards integrating conductive and magnetically bistable coordination polymers and metal–organic frameworks as active components in electronic devices.

Redox, transmetalation, and stacking properties of tetrathiafulvalene-2,3,6,7-tetrathiolate bridged tin, nickel, and palladium compounds

Here we report that capping the molecule TTFtt (TTFtt = tetrathiafulvalene-2,3,6,7-tetrathiolate) with dialkyl tin groups enables the isolation of a stable series of redox congeners and facile transmetalation to Ni and Pd. TTFtt has been proposed as an attractive building block for molecular materials for two decades as it combines the redox chemistry of TTF and dithiolene units. TTFttH₄, however, is inherently unstable and the incorporation of TTFtt units into complexes or materials typically proceeds through the in situ generation of the tetraanion TTFtt^4-. Capping of TTFtt^4- with Bu₂Sn²⁺ units dramatically improves the stability of the TTFtt moiety and furthermore enables the isolation of a redox series where the TTF core carries the formal charges of 0, +1, and +2. All of these redox congeners show efficient and clean transmetalation to Ni and Pd resulting in an analogous series of bimetallic complexes capped by 1,2-bis(diphenylphosphino)ethane (dppe) ligands. Furthermore, by using the same transmetalation method, we synthesized analogous palladium complexes capped by 1,1'-bis(diphenylphosphino)ferrocene (dppf) which had been previously reported. All of these species have been thoroughly characterized through a systematic survey of chemical and electronic properties by techniques including cyclic voltammetry (CV), ultraviolet-visible-near infrared spectroscopy (UV-vis-NIR), electron paramagnetic resonance spectroscopy (EPR), nuclear magnetic resonance spectroscopy (NMR) and X-ray diffraction (XRD). These detailed synthetic and spectroscopic studies highlight important differences between the transmetalation strategy presented here and previously reported synthetic methods for the installation of TTFtt. In addition, the utility of this stabilization strategy can be illustrated by the observation of unusual TTF radical-radical packing in the solid state and dimerization in the solution state. Theoretical calculations based on variational 2-electron reduced density matrix methods have been used to investigate these unusual interactions and illustrate fundamentally different levels of covalency and overlap depending on the orientations of the TTF cores. Taken together, this work demonstrates that tin-capped TTFtt units are ideal reagents for the installation of redox-tunable TTFtt ligands enabling the generation of entirely new geometric and electronic structures.

A Potential Hybrid Hole-Transport Material Incorporating a Redox-Active Tetrathiafulvalene Derivative with CuSCN

Inorganic CuSCN and organic tetrathiafulvalene derivatives (TTFs) have been exploited as hole-transport materials (HTM) in hybrid perovskite solar cells. To develop new HTM, we herein report two hybrid materials incorporating redox-active TTFs with CuSCN framework (TTFs-CuSCN). Single-crystal analysis showed that compound [Cu₂(py-TTF-py)(SCN)₂] (1) is three-dimensional (3D) and compound [Cu(py-TTF-py)(SCN)] (2) is two-dimensional (2D) (py-TTF-py = 2,6-bis(4'-pyridyl)tetrathiafulvalene). There are covalent coordination interactions between CuSCN and py-TTF-py and short S···S contacts between the py-TTF-py ligands for both compounds. Besides, C···S contacts exist between py-TTF-py ligands of the neighboring 2D networks in 2, which facilitate the charge transfer and supply efficient multidimensional pathways for carrier migration. As a result, 2 presented better semiconductor performance in comparison with that of 1. The performance of 2 related to the HTMs could be significantly improved by modulating the electronic state of the TTFs-CuSCN framework via oxidative doping. The iodine-doped 2D material (2-I₂) gives the most excellent conductivity and carrier mobility, which might be a potential new HTM.