Electrochemically Directed Synthesis of Cu2I(TCNQF4II–)(MeCN)2 (TCNQF4 = 2,3,5,6-Tetrafluoro-7,7,8,8-tetracyanoquinodimethane): Voltammetry, Simulations, Bulk Electrolysis, Spectroscopy, Photoactivity, and X-ray Crystal Structure of the Cu2I(TCNQF4II–)(EtCN)2 Analogue

TCNQ and Its Derivatives as Electrode Materials in Electrochemical Investigations-Achievement and Prospects: A Review

7,7',8,8'-tetracyanoquinodimethane (TCNQ) is one of the most widely used effective surface electron acceptors in organic electronics and sensors, which opens up a very interesting field in electrochemical applications. In this review article, we outline the historical context of electrochemically stable selective electrode materials based on TCNQ and its derivatives and their development, their electrochemical characteristics, and the experimental aspects of their electrochemical applications. TCNQ-modified electrodes are characterized by long-term stability, reproducibility, and a low detection limit compared to other sensors; thus, their use can increase determination speed and flexibility and reduce investigation costs. TCNQ and its derivatives can also be successfully combined with other detector materials for cancer-related clinical diagnostic testing. Examples of simple, rapid, and sensitive detection procedures for various analytes are provided. Applications of new electrochemically stable TCNQ-based metal/covalent-organic hybrid frameworks, with exceptionally large surface areas, tunable pore sizes, diverse functionality, and high electrical conductivity, are also presented. As a result, they also offer enormous potential as revolutionary catalysts, drug carrier systems, and smart materials, as well as for use in gas storage. The use of TCNQ compounds as promising active electrode materials in high-power organic batteries/energy storage devices is discussed. We hope that the information featured in this review will provide readers with a good understanding of the chemistry of TCNQ and, more importantly, help to find good ways to prepare new micro-/nanoelectrode materials for rational sensor design.

An Old Crystallization Technique as a Fast, Facile, and Adaptable Method for Obtaining Single Crystals of Unstable "Li2TCNQF4" and New Compounds of TCNQ or TCNQF4: Syntheses, Crystal Structures, and Magnetic Properties

Detailed structural information is essential for understanding the properties of TCNQ and TCNQF₄ compounds (TCNQ = 7,7,8,8-tetracyanoquinodimethane; TCNQF₄ = 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane). The ineludible requirement of obtaining crystals of a size and quality sufficient to yield a successful X-ray diffraction analysis has been challenging to satisfy because of the instability of many of these compounds in solution. Crystals of two new complexes of TCNQ, [trans-M(2ampy)₂(TCNQ)₂] [M = Ni (1), Zn (2); 2ampy = 2-aminomethylpyridine], as well as unstable [Li₂(TCNQF₄)(CH₃CN)₄]·CH₃CN (3), can be prepared in minutes by a horizontal diffusion technique and can be harvested easily for X-ray structural studies. Compound 3, previously described as "Li₂TCNQF₄," forms a one-dimensional (1D) ribbon. Compounds 1 and 2 can also be obtained as microcrystalline solids from methanolic solutions of MCl₂/LiTCNQ/2ampy. Their variable-temperature magnetic studies confirmed a contribution of strongly antiferromagnetically coupled pairs of TCNQ^•- anion radicals at higher temperatures with exchange coupling J/k_B = -1206 K and J/k_B = -1369 K for 1 and 2, respectively, estimated using a spin dimer model. The presence of magnetically active anisotropic Ni(II) atoms with S = 1 in 1 was confirmed, and the magnetic behavior of 1, representing an infinite chain of alternating S = 1 sites and S = 1/2 dimers, was described by a spin-ring model suggesting ferromagnetic exchange coupling between Ni(II) sites and anion radicals.

Charge transport of F4TCNQ with different electronic states in single-molecule junctions

The molecular conductance of 2,3,5,6-tetrafluoro-7,7,8,8,-tetracyano-quinodimethane (F4TCNQ) with different electronic states (neutral, radical anion, and dianion) was investigated by the scanning tunneling microscope break junction (STM-BJ) technique. These electronic states have distinct conductance, and the conductance decreases in the order of neutral > radical anion > dianion. Surprisingly, the molecular conductance of the neutral F4TCNQ junction reaches 10^-1.17G₀, attributed to its LUMO energy level being close to the Fermi level of the gold electrode. Moreover, we found that neutral F4TCNQ can be gradually reduced to radical anions under a relatively low bias voltage of 100 mV. These results will advance the development of organic optoelectronic devices and molecule electronics.

Tetraruthenium Macrocycles with Laterally Extended Bis(alkenyl)quinoxaline Ligands and Their F4TCNQ•− Salts

We report on the tetraruthenium macrocycles Ru4-5 and -6 with a π-conjugated pyrene-appended 5,8-divinylquinoxaline ligand and either isophthalate or thiophenyl-2,5-dicarboxylate linkers and their charge-transfer salts formed by oxidation with two equivalents of F4TCNQ. Both macrocyclic complexes were characterized by NMR spectroscopy, mass spectrometry, cyclic and square-wave voltammetry, and by IR, UV–vis–NIR, and EPR spectroscopy in their various oxidation states.

Electron-Rich Diruthenium Complexes with π-Extended Alkenyl Ligands and Their F4 TCNQ Charge-Transfer Salts

The synthesis of dinuclear ruthenium alkenyl complexes with {Ru(CO)(Pi Pr3 )2 (L)} entities (L=Cl- in complexes Ru2 -3 and Ru2 -7; L=acetylacetonate (acac- ) in complexes Ru2 -4 and Ru2 -8) and with π-conjugated 2,7-divinylphenanthrenediyl (Ru2 -3, Ru2 -4) or 5,8-divinylquinoxalinediyl (Ru2 -7, Ru2 -8) as bridging ligands are reported. The bridging ligands are laterally π-extended by anellating a pyrene (Ru2 -7, Ru2 -8) or a 6,7-benzoquinoxaline (Ru2 -3, Ru2 -4) π-perimeter. This was done with the hope that the open π-faces of the electron-rich complexes will foster association with planar electron acceptors via π-stacking. The dinuclear complexes were subjected to cyclic and square-wave voltammetry and were characterized in all accessible redox states by IR, UV/Vis/NIR and, where applicable, by EPR spectroscopy. These studies signified the one-electron oxidized forms of divinylphenylene-bridged complexes Ru2 -7, Ru2 -8 as intrinsically delocalized mixed-valent species, and those of complexes Ru2 -3 and Ru2 -4 with the longer divinylphenanthrenediyl linker as partially localized on the IR, yet delocalized on the EPR timescale. The more electron-rich acac- congeners formed non-conductive 1 : 1 charge-transfer (CT) salts on treatment with the F4 TCNQ electron acceptor. All spectroscopic techniques confirmed the presence of pairs of complex radical cations and F4 TCNQ.- radical anions in these CT salts, but produced no firm evidence for the relevance of π-stacking to their formation and properties.

Increased Crystallization of CuTCNQ in Water/DMSO Bisolvent for Enhanced Redox Catalysis

Controlling the kinetics of CuTCNQ (TCNQ = 7,7,8,8-tetracyanoquinodimethane) crystallization has been a major challenge, as CuTCNQ crystallizing on Cu foil during synthesis in conventional solvents such as acetonitrile simultaneously dissolves into the reaction medium. In this work, we address this challenge by using water as a universal co-solvent to control the kinetics of crystallization and growth of phase I CuTCNQ. Water increases the dielectric constant of the reaction medium, shifting the equilibrium toward CuTCNQ crystallization while concomitantly decreasing the dissolution of CuTCNQ. This allows more CuTCNQ to be controllably crystallized on the surface of the Cu foil. Different sizes of CuTCNQ crystals formed on Cu foil under different water/DMSO admixtures influence the solvophilicity of these materials. This has important implications in their catalytic performance, as water-induced changes in the surface properties of these materials can make them highly hydrophilic, which allows the CuTCNQ to act as an efficient catalyst as it brings the aqueous reactants in close vicinity of the catalyst. Evidently, the CuTCNQ synthesized in 30% (v/v) water/DMSO showed superior catalytic activity for ferricyanide reduction with 95% completion achieved within a few minutes in contrast to CuTCNQ synthesized in DMSO that took over 92 min.

FTIR Spectroelectrochemistry of F4TCNQ Reduction Products and Their Protonated Forms

The tetrafluorinated derivative of 7,7,8,8-tetracyanoquinodimethane (TCNQ), 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ), is of interest for charge transfer complex formation and as a p-dopant in organic electronic materials. Fourier transform infrared (FTIR) spectroscopy is commonly employed to understand the redox properties of F4TCNQ in the matrix of interest; specifically, the ν(C≡N) region of the F4TCNQ spectrum is exquisitely sensitive to the nature of the charge transfer between F4TCNQ and its matrix. However, little work has been done to understand how these vibrational modes change in the presence of possible acid/base chemistry. Here, FTIR spectroelectrochemistry is coupled with density functional theory spectral simulation for study of the electrochemically generated F4TCNQ radical anion and dianion species and their protonation products with acids. Vibrational modes of HF4TCNQ^-, formed by proton-coupled electron transfer, are identified, and we demonstrate that this species is readily formed by strong acids, such as trifluoroacetic acid, and to a lesser extent, by weak acids, such as water. The implications of this chemistry for use of F4TCNQ as a p-dopant in organic electronic materials is discussed.

Electrochemically Directed Synthesis of Cobalt(II) and Nickel(II) TCNQF21–/2– Coordination Polymers: Solubility and Substituent Effects in the TCNQFn (n=0, 1, 2, 4) Series of Complexes

The reversible diffusion controlled cyclic voltammetry for the reduction of TCNQF­n0/1–/2– (where n=0, 1, 2, 4) changes significantly on addition of Co2+ and Ni2+ transition metal ions (M2+) because the kinetics associated with electrocrystallisation of the resulting coordination polymers [M(TCNQF2)2(H2O)2] and [M(TCNQF2)] are rapid on the voltammetric time scale. The voltammetry of solutions containing M2+ and TCNQF­2 was undertaken in acetonitrile (0.1M Bu4NPF6) at both GC and ITO electrodes. New one electron reduced TCNQF2 materials prepared via electrochemically directed synthesis were shown to have the formula [M(TCNQF2)2(H2O)2], assessed by vibrational (IR and Raman) spectroscopy, elemental analysis and thermogravimetric analysis. The solubility of [Ni(TCNQF2)2(H2O)2] (Ksp=8.29×10−11 M3) was significantly higher than the [Co(TCNQF2)2(H2O)2] (Ksp=1.43×10−11M3). Cyclic voltammetric data suggest the electrocrystallisation of two phases of [Ni(TCNQF2)2(H2O)2] occurs, which is not evident for [Co(TCNQF2)2(H2O)2]. Electrocrystallisation of the highly insoluble [M(TCNQF2)] was achieved at low M2+ and TCNQF2 concentrations. A comparison with published data on the voltammetry of TCNQF­n (n=0, 1, 2 and 4) for the series of TCNQF­n (n=0, 1, 2 and 4) containing M2+ is provided. An assessment of the electronic impact of the fluorine substituent of the underlying redox reactions also is established. Predictions are made for the voltammetric behaviour expected for the other transition metal cations with reduced TCNQFn derivatives.

X4TCNQ2− dianions: versatile building blocks for supramolecular systems

A new synthetic approach has led to the incorporation of TCNQ and F₄TCNQ dianions into a wide variety of structures.

Nanostructured charge transfer complex of CuTCNQF4 for efficient photo-removal of hexavalent chromium

The fabrication of a nanostructured CuTCNQF₄ organic charge transfer complex on copper foil by employing a facile redox reaction in acetonitrile and its ability to promote catalytic reduction of toxic Cr⁶⁺ to its non-toxic Cr³⁺ counterpart.