Xiaohu Qu

(Pro2H+)2(TCNQ.−)2⋅TCNQ: An Amino Acid Derived Semiconductor

Materials based on TCNQ (tetracyanoquinodimethane) derivatives are of particular interest as they offer promise as biodegradable components for the semiconductor industry. TCNQ itself is a classical electron acceptor with an electron affinity of 2.88 eV so that the anionic radical, TCNQC is readily formed by chemical reduction, photoreduction, or electrochemical methods. In the presence of electron donors, the resulting charge-transfer (CT) complexes with TCNQC are characterized by an extensive range of electronic and optical properties. To date, TCNQC materials have been formed in combination with many cations, including metal ions (Na, Mg, Cu, Gd), organometallic complexes (e.g., ferrocene, [Ru(bpy)3] ), as well as some organic cations (Me4N , NMP, TTF). Characterization of these materials yields a wide range of 1) stoichiometries, 9] including fractional charge transfer ratios, 2) morphologies, even with the same cation, although the radical anion consistently forms laminar p-stacked columns, and 3) phases, induced by electrochemical, photochemical, or thermal methods. Interestingly, even though TCNQ itself is a semiconductor, the band gap is greater than 2 eV 12] so that the conductivity of pure TCNQ crystals is quite low. TCNQC based CT complexes generally have a much higher conductivity, approaching even metallic conductivity 12] or superconductivity under certain conditions. Amino acids are key building blocks for polymeric macromolecules, especially proteins. The side-chain functionality includes a variety of polar, non-polar, aromatic, and heteroatoms. Proline (Scheme 1) is unique insofar as it does not contain a primary amino group, instead it has a secondary amine thereby raising the pKa from 9 to > 10.5. [15] Unlike the typical cationic TCNQ complexes, amino acids are not obvious candidates for TCNQ CT complexes. However, there is one report whereby solid TCNQ was ground with four different amino acids with the formation of a CT product indicated by IR spectroscopy. Here we report the preparation and characterization of a novel bioorganic TCNQ material (Pro2H )2(TCNQC )2·TCNQ (ProTCNQ), formed as a CT compound between neutral l-proline and TCNQ. Water was found to be necessary to provide the proton and the redox balance achieved through the oxidation of water (see Supporting Information, Section S3). Subsequently, rational methods of synthesis were introduced. An extensive range of physicochemical methods has been employed to characterize this new biomaterial. This is the first member of a new class of CT biomaterials derived from amino acids with TCNQ. Synthesis of the ProTCNQ complex was achieved using four different synthetic routes involving either TCNQ or LiTCNQ as a starting material (Section S1). Astonishingly, all methods resulted in the same product, which suggests that the stoichiometry found for ProTCNQ is thermodynamically favored. Dark blue, single crystals of ProTCNQ were obtained by diffusion of diethyl ether into a methanol solution of l-prolineH·BF4 [13a] and LiTCNQ. The asymmetric unit contains two crystallographically independent proline residues and three halves of TCNQ species (Figure 1a, namely TCNQ-A, TCNQ-B, and TCNQ-C, respectively). From the analysis of the mean bond lengths for each TCNQ (Table S1), TCNQ-A and TCNQ-C are regarded as TCNQC radical anions, whereas TCNQ-B is a neutral TCNQ molecule. The structure consists of alternating layers of proline cations and TCNQ moieties (Figure 1b). In each case, the planar TCNQ molecules form three separate 1D chains defined by weak H-bonding interactions between CN and H groups of each TCNQ (Figure 1c). These TCNQ chains run parallel to the b axis (the TCNQ molecules themselves lie parallel to the ab plane), and stack along the c axis to create a 2D layer (Figure 1b). There are strong p–p interactions between the chains containing the TCNQ-A and TCNQ-C Scheme 1. Molecular structure of proline (left) and TCNQ (right).

Detailed Electrochemical Analysis of the Redox Chemistry of Tetrafluorotetracyanoquinodimethane TCNQF4, the Radical Anion [TCNQF4]•–, and the Dianion [TCNQF4]2– in the Presence of Trifluoroacetic Acid

The electrochemistry of 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (TCNQF(4)), [TCNQF(4)](•-), and [TCNQF(4)](2-) have been studied in acetonitrile (0.1 M [Bu(4)N][ClO(4)]). Transient and steady-state voltammetric techniques have been utilized to monitor the generation of [TCNQF(4)](•-) and [TCNQF(4)](2-) anions as well as their reactions with trifluoroacetic acid (TFA). In the absence of TFA, the reduction of TCNQF(4) occurs via two, diffusion controlled, chemically and electrochemically reversible, one-electron processes where the reversible formal potentials are 0.31 and -0.22 V vs Ag/Ag(+). Unlike the TCNQ analogues, both [TCNQF(4)](•-) and [TCNQF(4)](2-) are persistent when generated via bulk electrolysis even under aerobic conditions. Voltammetric and UV-vis data revealed that although the parent TCNQF(4) does not react with TFA, the electrochemically generated radical anion and dianion undergo facile protonation to yield [HTCNQF(4)](•), [HTCNQF(4)](-) and H(2)TCNQF(4) respectively. The voltammetry can be simulated to give a complete thermodynamic and kinetic description of the complex, coupled redox and acid-base chemistry. The data indicate dramatically different equilibrium and rate constants for the protonation of [TCNQF(4)](•-) (K(eq) = 3.9 × 10(-6), k(f) = 1.0 × 10(-3) M(-1) s(-1)) and [TCNQF(4)](2-) (K(eq) = 3.0 × 10(3), k(f) = 1.0 × 10(10) M(-1) s(-1)) in the presence of TFA.

Synthesis and Structural Characterization of a TCNQ Based Organic Semi-Conducting Material with a 2:5 Stoichiometry

The tetrabutylammonium complex with a 2:5 stoichiometry, (n-Bu(4)N)(2)(TCNQ)(5), has been prepared and structurally characterized by X-ray crystallography. Diagnostic bands in the Raman spectrum and signature features in the electrochemistry confirm that the TCNQ moieties are partially charged in the solid state. EPR, magnetic susceptibility, and electrical conductivity measurements are all consistent with (n-Bu(4)N)(2)(TCNQ)(5) behaving as a quasi-one-dimensional organic semiconductor.

Redox and Acid–Base Chemistry of 7,7,8,8-Tetracyanoquinodimethane, 7,7,8,8-Tetracyanoquinodimethane Radical Anion, 7,7,8,8-Tetracyanoquinodimethane Dianion, and Dihydro-7,7,8,8-Tetracyanoquinodimethane in Acetonitrile

The chemistry and electrochemistry of TCNQ (7,7,8,8-tetracyanoquinodimethane), TCNQ(•-), TCNQ(2-), and H(2)TCNQ in acetonitrile (0.1 M Bu(4)NPF(6)) solution containing trifluoroacetic acid (TFA) has been studied by transient and steady-state voltammetric methods with the interrelationship between the redox and the acid-base chemistry being supported by simulations of the cyclic voltammograms. In the absence of acid, TCNQ and its anions undergo two electrochemically and chemically reversible one-electron processes. However, in the presence of TFA, the voltammetry is considerably more complex. The TCNQ(2-) dianion is protonated to form HTCNQ(-), which is oxidized to HTCNQ(•), and H(2)TCNQ which is electroinactive over the potential range of -1.0 to +1.0 V versus Ag/Ag(+). The monoreduced TCNQ(•-) radical anion is weakly protonated to give HTCNQ(•), which disproportionates to TCNQ and H(2)TCNQ. In acetonitrile, H(2)TCNQ deprotonates slowly, whereas in N,N-dimethylformamide or tetrahydrofuran, rapid deprotonation occurs to yield HTCNQ(-) as the major species. H(2)TCNQ is fully deprotonated to the TCNQ(2-) dianion in the presence of an excess concentration of the weak base, CH(3)COOLi. Differences in the redox and acid-base chemistry relative to the fluorinated derivative TCNQF(4) are discussed in terms of structural and electronic factors.

Two-Step Electrochemically Directed Synthesis of Pr4N(TCNQ)n (n=1, 2): Preparation, Structure, and Properties of a Magnetically Isolated Dimer and a Quasi-One-Dimensional Chain†

Solid-state electrochemistry of a tetracyanoquinodimethane (TCNQ)-modified electrode in contact with a tetrapropylammonium cation (Pr(4)N(+)) electrolyte showed two electron-transfer steps to give Pr(4)N(TCNQ)(2) (1) and Pr(4)N(TCNQ) (2) rather than the traditional one-electron step to directly give Pr(4)N(TCNQ). Two thermodynamically stable Pr(4)N(+)-TCNQ stoichiometries, 1 and 2, were synthesized and characterized. The degree of charge transfer (ρ) calculated from the crystal structure is -0.5 for the TCNQ moieties in 1 and -1.0 for those in 2. Raman spectra for Pr(4)N(TCNQ)(2) show only one resonance for the extracyclic C=C stretching at 1423 cm(-1), which lies approximately midway between that of TCNQ at 1454 cm(-1) and TCNQ(-) at 1380 cm(-1). Both the magnetic susceptibility and EPR spectra are temperature-dependent, with a magnetic moment close to that for one unpaired electron per (TCNQ)(2) unit in 1, whereas 2 is almost diamagnetic. Pressed discs of both complexes show conductivity (1-2×10(-5) S cm(-1)) in the semiconductor range. For 1, the position of zero current for the steady-state voltammograms implies 50% of TCNQ(-) and 50% TCNQ(0) is present in solution, thereby supporting a dissociation of (TCNQ)(2)(-) in solution, but is indicative of only TCNQ(-) being present for 2.

Underpotential and overpotential electrocrystallization of semiconducting silver-tetracyanoquinodimethane onto gold substrates from an ionic liquid

Electrocrystallization of nanoneedles and nanorods of silver-tetracyanoquinodimethane (AgTCNQ) onto a gold substrate has been achieved from the ionic liquid, 1-n-butyl-3-methylimidazolium tetrafluoroborate (BMIMBF4), containing dissolved TCNQ and Ag+. In ionic liquid media, underpotential deposition (UPD) and overpotential deposition (OPD) of metallic Ag at a gold electrode occur at more positive potentials than that for reduction of TCNQ to TCNQ−. In contrast, the reduction of TCNQ and Ag+ occurs at almost the same potential in MeCN. The different thermodynamics that apply in the ionic liquid environment enables controlled electrocrystallization of AgTCNQvia potential-dependent mechanisms. Nanoneedles AgTCNQ could be obtained at 0.3 V vs.Fc0/+ (Fc = ferrocene), while nanorods could be formed at −0.2 V vs.Fc0/+. Raman, IR and X-ray diffraction data imply that the formation of highly pure and crystalline phase of AgTCNQ on gold, and that AgTCNQ electrocrystallized under UPD or OPD conditions only differ in morphology and not in phase. The study highlights the capability of the electrocrystallization method to precisely control the morphology of nanomaterials, and also using ionic liquids as media for preparation of technologically important metal-TCNQ charge transfer complexes.

Synthesis, Physical Properties, Structural, and Electrochemical Characterization of Methimidazolium and Imidazolium-based Tetracyanoquinodimethane Anion Radical Salts

Two methimazolium and two imidazolium-based salts derived from combination with the tetracyanoquinodimethane (TCNQ) radical anion have been synthesized (1–4). The 1:1 (cation:anion) stoichiometry of the chemically synthesized materials is fully supported by steady-state voltammetric measurements at a microdisc electrode in acetonitrile. The methimazolium TCNQ salts (1 and 2), which contain an acidic proton on the cation, exhibit a protonation step coupled to the TCNQ1–/2– charge-transfer process. Solid–solid transformations at a TCNQ-modified electrode also lead to electrochemical synthesis of 1–4, but also indicate that other cation:anion stoichiometries are accessible. Atomic force microscopy for electrochemically synthesized samples exhibit rod-like morphology. Conductivity measurements on chemically and electrochemically prepared salts are in the semiconducting range. Scanning electrochemical microscopy approach curve data support the substantial conductivity of these solids. Extensive physicochemical characterization of these materials is in complete accordance with the X-ray crystal structure of 1-acetonitrile-3-methylimidazolium tetracyanoquinodimethane, [AMim+][TCNQ1–], 4.

Novel Semiconducting Biomaterials Derived from a Proline Ester and Tetracyanoquinodimethane Identified by Handpicked Selection of Individual Crystals

Complex mixtures of cation : anion stoichometries often result from the syntheses of tetracyanoquinodimethane (TCNQ) salts, and often these cannot be easily separated. In this study, the reaction of N,N-dimethyl-d-proline-methylester (Pro(CH3)3+) with LiTCNQ resulted in a mixture of crystals. Hand selection and characterisation of each crystal type by X-ray, infrared, Raman and electrochemistry has provided two stoichometries, 1 : 1 [Pro(CH3)3TCNQ] and 2 : 3 ([(Pro(CH3)3)2(TCNQ)3]). A detailed comparison of these structures is provided. The electrochemical method provides an exceptionally sensitive method of distinguishing differences in stoichiometry. The room temperature conductivity of the mixture is 3.1 × 10–2 S cm–1, which lies in the semiconducting range.