Redox-induced solid-solid state transformation of tetrathiafulvalene (TTF) microcrystals into mixed-valence and π-dimers in the presence of nitrate anions

Electrical Conductivity-Relay between Organic Charge-Transfer and Radical Salts toward Conductive Additive-Free Rechargeable Battery

In recent years, organic electrode materials have been strongly considered for use in sustainable batteries. However, most organic electrode materials have low electrical conductivity and require a lot of conductive additives, which decrease the effective capacity based on the entire electrode weight/volume. In this study, we propose a novel electrical conductivity-relay system that imparts electrical conductivity to organic small molecular electrodes without any conductive additive throughout the charge/discharge cycles. It consists of the combination of the charge-transfer phenomenon in a pristine state and the formation of organic radical salts in redox states. Herein, we demonstrate this electrical conductivity-relay system using a simply mixed molecular crystal couple of tetrathiafulvalene (TTF) and tetracyanoquinodimethane (TCNQ) as a cathode without any conductive additive and aqueous sodium bromide as an electrolyte. During charge/discharge, the electrical conductivity of the cathode is supported by charge-transfer at the TTF/TCNQ interface and (TTF)Br_n (0.7 ≤ n ≤ 0.8) and NaTCNQ radical salts, and the cathode exhibits a specific capacity of 112 mAh g^-1 and a retention rate of 80.7% at the 30th cycle. Furthermore, the molecular crystal couple electrode of TTF and TCNQ shows better charge/discharge performance than the pure charge-transfer complex electrode, indicating that this system expands candidates for organic electrode materials to various pairs and mixing ratios of small molecules that do not form charge-transfer complexes.

Metal-Organic Framework Co-MOF-74-Based Host-Guest Composites for Resistive Gas Sensing

Increasing demands in the field of sensing, especially for gas detection applications, require new approaches to chemical sensors. Metal-organic frameworks (MOFs) can play a decisive role owing to their outstanding performances regarding gas selectivity and sensitivity. The tetrathiafulvalene (TTF)-infiltrated MOF, Co-MOF-74, has been prepared following the host-guest concept and evaluated in resistive gas sensing. The Co-MOF-74-TTF crystal morphology has been characterized via X-ray diffraction and scanning electron microscopy, while the successful incorporation of TTF into the MOF has been validated via X-ray photoemission spectroscopy, thermogravimetric analysis, UV/vis, infrared (IR), and Raman investigations. We demonstrate a reduced yet ample uptake of CO₂ in the pores of the new material by IR imaging and adsorption isotherms. The nanocomposite Co-MOF-74-TTF exhibits an increased electrical conductivity in comparison to Co-MOF-74 which can be influenced by gas adsorption from a surrounding atmosphere. This effect could be used for gas sensing.

Confinement Effects of Metal-Organic Framework on the Formation of Charge-Transfer Tetrathiafulvalene Dimers

Three transition metal coordination polymers (CPs) based on the redox-active dimethylthio-tetrathiafulvalene-bicarboxylate (L) and 1,3-bi(4-pyridyl)propane (bpp) ligands, formulated as [MnL(bpp)]_n (1), [CdL(bpp)]_n (2), and [Cd₂L(bpp)₂(H₂O)(C₂O₄)_0.5]_n·n(ClO₄)·n(H₂O) (3), are crystallographically characterized. Complexes 1 and 2 are isostructural 2-D polymers, and 3 features an unusual 3-D metal-organic framework (MOF). The 3-D MOF is constructed from tetranuclear cluster nodes built through the μ₂-O bridge of the TTF ligand, which is first found for TTF coordination polymers. It is found that the channel generated by the 3-D MOF exerts a confinement effect on the formation of TTF dimers. The TTF dimers show strong intradimer interaction with partial electron transfer or charge transfer, and hence, the Cd compound 3 has relatively good photocurrent response property in comparison with that of 2-D Cd compound 2.

Exploiting redox activity in metal–organic frameworks: concepts, trends and perspectives

This feature article highlights latest developments in experimental, theoretical and computational concepts relevant to redox-active metal–organic Frameworks.