Synthetic Precursors for TCNQF42– Compounds: Synthesis, Characterization, and Electrochemical Studies of (Pr4N)2TCNQF4 and Li2TCNQF4

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.

Thermally Induced Formation of HF4TCNQ- in F4TCNQ-Doped Regioregular P3HT

The prototypical system for understanding doping in solution-processed organic electronics has been poly(3-hexylthiophene) (P3HT) p-doped with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F₄TCNQ). Multiple charge-transfer states, defined by the fraction of electron transfer to F₄TCNQ, are known to coexist and are dependent on polymer molecular weight, crystallinity, and processing. Less well-understood is the loss of conductivity after thermal annealing of these materials. Specifically, in thermoelectrics, F₄TCNQ-doped regioregular (rr) P3HT exhibits significant conductivity losses at temperatures lower than other thiophene-based polymers. Through detailed spectroscopic investigation of progressively heated P3HT films coprocessed with F₄TCNQ, we demonstrate that this diminished conductivity is due to formation of the nonchromophoric, weak dopant HF₄TCNQ^-. This species is likely formed through hydrogen abstraction from the α aliphatic carbon of the hexyl chain at the 3-position of thiophene rings of rr-P3HT. This reaction is eliminated for polymers with ethylene glycol-containing side chains, which retain conductivity at higher operating temperatures. In total, these results provide a critical materials design guideline for organic electronics.

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.

Double doping of conjugated polymers with monomer molecular dopants

Molecular doping is a crucial tool for controlling the charge-carrier concentration in organic semiconductors. Each dopant molecule is commonly thought to give rise to only one polaron, leading to a maximum of one donor:acceptor charge-transfer complex and hence an ionization efficiency of 100%. However, this theoretical limit is rarely achieved because of incomplete charge transfer and the presence of unreacted dopant. Here, we establish that common p-dopants can in fact accept two electrons per molecule from conjugated polymers with a low ionization energy. Each dopant molecule participates in two charge-transfer events, leading to the formation of dopant dianions and an ionization efficiency of up to 200%. Furthermore, we show that the resulting integer charge-transfer complex can dissociate with an efficiency of up to 170%. The concept of double doping introduced here may allow the dopant fraction required to optimize charge conduction to be halved.

[FeII(L•)2][TCNQF4•−]2: A Redox-Active Double Radical Salt

The reaction of [FeII(L•)2][BF4]2 with LiTCNQF4 results in the formation of [FeII(L•)2][TCNQF4•−]2·2CH3CN (1) (L• is the neutral aminoxyl radical ligand 4,4-dimethyl-2,2-di(2-pyridyl)oxazolidine-N-oxide; TCNQF4 is 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane). Single-crystal X-ray diffraction; Raman, Fourier-transform infrared (FTIR) and ultraviolet–visible spectroscopies; and electrochemical studies are all consistent with the presence of a low-spin FeII ion, the neutral radical form (L•) of the ligand, and the radical anion TCNQF4•−. 1 is largely diamagnetic and the electrochemistry shows five well-resolved, diffusion-controlled, reversible one-electron processes.

Use of the TCNQF4 2- Dianion in the Spontaneous Redox Formation of [FeIII (L- )2 ][TCNQF4 ⋅- ]

The reaction of [Fe^II (L^. )₂ ](BF₄ )₂ with Li₂ TCNQF₄ results in the formation of [Fe^III (L^- )₂ ][TCNQF₄ ^{. -} ] (1) where L^. is the radical ligand, 4,4-dimethyl-2,2-di(2-pyridyl)oxazolidine-N-oxide and TCNQF₄ is 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane. This has been characterised by X-ray diffraction, Raman and Fourier transform infrared (FTIR) spectroscopy, variable-temperature magnetic susceptibility, Mössbauer spectroscopy and electrochemistry. X-ray diffraction studies, magnetic susceptibility measurements and Raman and FTIR spectroscopy suggest the presence of low-spin Fe^III ions, the anionic form (L^- ) of the ligand and the anionic radical form of TCNQF₄ ; viz. TCNQF₄ ^{. -} . Li₂ TCNQF₄ reduces the [Fe^II (L^. )₂ ]²⁺ dication, which undergoes a reductively induced oxidation to form the [Fe^III (L^- )₂ ]⁺ monocation resulting in the formation of [Fe^III (L^- )₂ ][TCNQF₄ ^{. -} ] (1), the electrochemistry of which revealed four well-separated, diffusion-controlled, one-electron, reversible processes. Mössbauer spectroscopy and electrochemical measurements suggest the presence of a minor second species, likely to be [Fe^II (L^. )₂ ][TCNQF₄ ^2- ].

Solvent-, Cation- and Anion-Induced Structure Variations in Manganese-Based TCNQF4 Complexes: Synthesis, Crystal Structures, Electrochemistry and Their Catalytic Properties

The reaction of Mn(BF₄ )₂ ⋅x H₂ O with (Pr₄ N)₂ TCNQF₄ (TCNQF₄ =2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane) in a mixture of CH₃ OH/CH₂ Cl₂ gives a 2:3 stoichiometric complex of (Pr₄ N)₂ [Mn₂ (TCNQF₄ )₃ (CH₃ OH)₂ ] (1). If the solvent system used for the crystallisation of 1 is changed to CH₃ OH/DMF, then a different product, [Mn(TCNQF₄ )(DMF)₂ ]⋅(CH₃ OH)₂ (2), is obtained. The use of Li₂ TCNQF₄ instead of (Pr₄ N)₂ TCNQF₄ leads to the generation of [Mn₂ (TCNQF₄ )₂ (DMF)₄ ]⋅3 DMF (3). An unexpected mixed oxidation state network with a composition of [Mn^II ₄ Mn^III ₁₆ O₁₀ (OH)₆ (OCH₃ )₂₄ (TCNQF₄ )₂ ](NO₃ )₂ ⋅24 CH₃ OH (4), is formed if Mn(NO₃ )₂ ⋅x H₂ O is used in place of Mn(BF₄ )₂ ⋅x H₂ O in the reaction that leads to the formation of 3. Compounds 1-3 have been characterised by X-ray crystallography; FTIR, Raman and UV/Vis spectroscopy; and electrochemistry. Compound 4 has only been analysed by X-ray crystallography and vibrational spectroscopy (Raman, FTIR), owing to rapid deterioration of the compound upon exposure to air. These results indicate that relatively minor changes in reaction conditions have the potential to yield products with vastly different structures. Compound 1 adopts an anionic 2D network with unusual π-stacked dimers of the TCNQF₄ ^2- dianion, whereas 2 and 3 are composed of similar neutral sheets of [Mn(TCNQF₄ )(DMF)₂ ]. Interestingly, the solvent has a significant influence on the stacking of the sheets in the structures of 2 and 3. In compound 4, clusters with a composition of [Mn^II ₄ Mn^III ₁₆ O₁₀ (OH)₆ (OCH₃ )₂₄ (CH₃ OH)₄ ]⁶⁺ serve as eight-connecting nodes, whereas TCNQF₄ ^2- ligands act as four-connecting nodes in a 3D network that has the same topology as fluorite. Compound 3 exhibits an exceptionally high super-catalytic activity for the electron-transfer reaction between ferricyanide and thiosulfate ions in aqueous media.

Structural, Spectroscopic, and Electrochemical Characterization of Semi-Conducting, Solvated [Pt(NH3)4](TCNQ)2·(DMF)2 and Non-Solvated [Pt(NH3)4](TCNQ)2

The demand for catalysts that are highly active and stable for electron-transfer reactions has been boosted by the discovery that [Pt(NH3)4](TCNQF4)2 (TCNQF4 = 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane) is an efficient catalyst. In this work, we prepare and characterize the two related [Pt(NH3)4]2+ complexes, [Pt(NH3)4](TCNQ)2·(DMF)2 (1) and [Pt(NH3)4](TCNQ)2 (2). Reaction of [Pt(NH3)4](NO3)2 with LiTCNQ in a mixed solvent (methanol/dimethylformamide, 4 : 1 v/v) gives [Pt(NH3)4](TCNQ)2·(DMF)2 (1), whereas the same reaction in water affords [Pt(NH3)4](TCNQ)2 (2). 2 has been previously reported. Both 1 and 2 have now been characterized by single-crystal X-ray crystallography, Fourier-transform (FT)IR, Raman and UV-vis spectroscopy, and electrochemistry. Structurally, in 1, the TCNQ1− anions form infinite stacks with a separation between adjacent anions within the stack alternating between 3.12 and 3.42 Å. The solvated structure 1 differs from the non-solvated form 2 in that pairs of TCNQ1− anions are clearly displaced from each other. The conductivities of pressed pellets of 1 and 2 are both in the semi-conducting range at room temperature. 2 can be electrochemically synthesized by reduction of a TCNQ-modified electrode in contact with an aqueous solution of [Pt(NH3)4](NO3)2 via a nucleation growth mechanism. Interestingly, we discovered that 1 and 2 are not catalysts for the ferricyanide and thiosulfate reaction. Li+ and tetraalkylammonium salts of TCNQ1−/2− and TCNQF41−/2− were tested for potential catalytic activity towards ferricyanide and thiosulfate. Only TCNQF41−/2− salts were active, suggesting that the dianion redox level needs to be accessible for efficient catalytic activity and explaining why 1 and 2 are not good catalysts. Importantly, the origin of the catalytic activity of the highly active [Pt(NH3)4](TCNQF4)2 catalyst is now understood, enabling other families of catalysts to be developed for important electron-transfer reactions.