Chemical stability and electroconductivity of 7,7,8,8-tetracyanoquinodimethane salts containing polycation polymers

The chemical and electrical stabilities of 7,7,8,8-tetracyanoquinodimethane (TCNQ) salts composed of neutral TCNQ (TCNQ^?), anion radicals of TCNQ(TCNQ^?·), and polycation polymers were studied by measuring their electronic spectra and resistivities (ρ). The results of spectral and chemical analyses confirmed that TCNQ^?· in TCNQ salts was decomposed to α,α-dicyano-p-toluoylcyanide (DTC^?) as the final product by the intermediate formation of TCNQ^? and p-phenylenediamalononitrile (H₂TCNQ) and that H₂O played an important part in the reaction. From these results it was concluded that TCNQ salts are decomposed by two reaction processes: The resistivity of TCNQ salts increases with the decomposition of TCNQ^?·. Studies on electroconductivity of TCNQ salts assume that the change in resistivity arises from the loss of unpaired electrons which become conduction carriers and also from the disintegration of the TCNQ^? and TCNQ^?· complex which forms the conduction path.     

Crystal structure,magnetic property and DFT analysis for a bis(tetracyanoquinodimethane)zinc(II) complex

A bis(tetracyanoquinodimethane)zinc(II) complex (1) was structurally characterized, in which the Zn²⁺ ion occupies at an inversion centre and is bonded to two tetracyanoquinodimethane radical anions (TCNQ^?), two H₂O and two DMF molecules to form almost perfectly octahedral coordination geometry. The strong H-bonding interactions are observed between H₂O molecules as well as between H₂O molecule and TCNQ^? radical anion of the neighboring complex molecules, additionally, there exist π?π stack interactions between TCNQ^? radical anions. The intermolecular H-bonding and π?π stack interactions lead to a supramolecular network sheet forming on the crystallographic ac-plane, and the neighboring supramolecular network sheets further extend into three-dimensional supramolecular structure via weak van der Waals forces. Symmetry-broken approach in the theoretical formwork of DFT for magnetic exchange constants analysis disclosed that the ground state of 1 is singlet state, the excited triplet state is much closed to the nonmagnetic ground state with a small energy gap of 1.25 × 10^?5 eV, and there exist AFM interaction in the TCNQ π-stacks, and these predictions are in agreement with magnetic susceptibility measurements.     

All-valence-electron band structures of infinite stacked poly(TCNQ) and poly(TTF) chains

We have computed the energy band structures of the inifite poly(TCNQ), poly(TCNQ^2?), poly(TTF) and poly(TTF²⁺) chains using the CNDO/2 and where possible the MINDO/2 crystal orbital approximation schemes. The results show a broad conduction band for poly(TCNQ) and a broad valence band for poly(TTF). The bandwidths within the MINDO/2 CO scheme are found to be smaller by roughly a factor of $\text{1}\text{2}$ as compared with those found within the CNDO/2 CO scheme. Our findings are in agreement with a bandwidth of 0.4–0.5 eV for the conduction band of TCNQ—TTF crystals as found by experiments. A brief discussion is given of the k-dependence of the physically interesting bands which is not always simple.     

Spectroelectrochemistry of tetracyanoquinodimethane modified electrodes

Thin films of a tetracyanoquinodimethane (TCNQ) oligomer adsorbed on optically transparent platinum electrodes have been studied using spectroelectrochemical techniques. In contact with aqueous electrolytes the electron aceptor sites in these modified electrodes are reduced to mixtures of the radical aion, TCNQ^?, the dimer radical anion, TCNQ^2?₂, the dimer dianion, TCNQ₂^2?, and the dianion, TCNQ^2∮. Electrolyte effects on the reduction process and the stability of the reduced films were studied. Both monovalent and divalent ions exhibited Nernstian potential dependence, and in the presence of Ca²⁺ ions the dianion state was stabilized. Counterion effects are suggested as the origin of variations in the wave shape in multcycle voltammograms.     

Synthesis and characterization of microstructured sheets of semiconducting Ca[TCNQ]2 via redox-driven solid-solid phase transformation of TCNQ microcrystals

Microstructured sheets of semiconducting Ca[TCNQ]₂ (TCNQ = 7,7,8,8-tetracyanoquinodimethane) have been synthesized via electrochemically driven (TCNQ)/Ca[TCNQ]₂ solid-solid phase transformation that occurs upon one-electron reduction of solid TCNQ, mechanically attached to an electrode surface, in the presence of an aqueous Ca²⁺ _(aq) electrolyte solution. Voltammetric probing of the electrochemically irreversible TCNQ/Ca[TCNQ]₂ interconversion revealed that it is highly dependent on scan rate and Ca²⁺ _(aq) electrolyte concentration. This voltammetric behavior, supported by double potential-step chronoamperometric evidence, clearly attests that formation of Ca[TCNQ]₂ takes place via a rate-determining nucleation/growth process, which involves ingress of Ca²⁺ _(aq) cations into the TCNQ^·? crystal lattice at the triple phase TCNQ/TCNQ^·? _(s)│GC_(s)│Ca²⁺ _(aq) electrolyte junction. The overall redox process associated with this chemically reversible solid-solid transformation can be described by the equation: TCNQ⁰ _(S)?+?2e^??+?Ca²⁺ _(aq) ? {Ca[TCNQ]₂}_(S). SEM characterization of the morphology of the generated Ca[TCNQ]₂ material showed the formation of microstructured sheets, which are substantially different from those of parent TCNQ crystals and the needle-shaped crystals of group I cations (M⁺?=?Li, Na, K, Rb, and Cs). The kinetic and thermodynamic implications of the ΔE _p and E _m values as a function of scan rate are discussed in terms of nucleation–growth and their relevance to those reported for the conceptually related group I cations and binary M[TCNQ]₂ (M²⁺?=?Mn, Fe, Co, and Ni)-based coordination polymers.     

Photomagnetic Response in Highly Conductive Iron(II) Spin‐Crossover Complexes with TCNQ Radicals

Co‐crystallization of a cationic Fe^II complex with a partially charged TCNQ^.δ? (7,7′,8,8′‐tetracyanoquinodimethane) radical anion has afforded molecular materials that behave as narrow band‐gap semiconductors, [Fe(tpma)(xbim)](X)(TCNQ)_1.5?DMF (X=ClO₄^? or BF₄^?; tpma=tris(2‐pyridylmethyl)amine, xbim=1,1′‐(α,α′‐o‐xylyl)‐2,2′‐bisimidazole). Remarkably, these complexes also exhibit temperature‐and light‐driven spin crossover at the Fe^II center, and are thus the first structurally defined magnetically bistable semiconductors assembled with the TCNQ^.δ? radical anion. Transport measurements reveal the conductivity of 0.2 S cm^?1 at 300 K, with the low activation energy of 0.11 eV.     

The Influence of TCNQ0 Content in TCNQ Salts on Polycations with Sulfonium Groups in the Main Chain

The influence of the TCNQ⁰ content defined as x = [TCNQ⁰]/[TCNQ^?] on the properties of TCNQ complex salts containing S⁺ in the main chains are presented and discussed. Both the specific resistivity and the thermal activation energy of the conductivity had a minimum for x close to 0.9. The decrease of resistivity with an increase in the x ratio is explained by a decrease in the dimer concentration. The increase of resistivity for x increasing above 0.9 is explained by the formation of crystalline TCNQ⁰ domains. The discussion is based on ESR, optic absorption, and crystallinity data.     

Poly(3-octylthiophene) as solid contact for ion-selective electrodes: contradictions and possibilities

The hydrophobic conductive polymer, poly(3-octylthiophene) (POT), is considered as uniquely suited to be used as an ion-to-electron transducer in solid contact (SC) ion-selective electrodes (ISEs). However, the reports on the performance characteristics of POT-based SC ISEs are quite conflicting. In this study, the potential sources of the contradicting results on the ambiguous drift and poor potential reproducibility of POT-based ISEs are compiled, and different approaches to minimize the drift and the differences in the standard potentials of POT-based SC ISEs are shown. To set the potential of the POT film, it has been loaded with a 7,7,8,8-tetracyanoquinodimethane (TCNQ/TCNQ^·?) redox couple. An approximately 1:1 TCNQ/TCNQ^·?ratio in the POT film has been achieved through potentiostatic control of the potential of the redox couple-loaded conductive polymer. It is hypothesized that once the POT film has a stable, highly reproducible redox potential, it will provide similarly stable and reproducible interfacial potentials between the POT film and the electron-conducting substrate and result in SC ISEs with excellent reproducibility and potential stability. Towards this goal, the potentials of Au, GC, and Pt electrodes with drop-cast POT film coatings were recorded in KCl solutions as a function of time. Some of the POT films were loaded with TCNQ and coated with a K⁺-selective membrane. The improvement in the potential stabilities and sensor-to-sensor reproducibility as a consequence of the incorporation of TCNQ in the POT film and the potentiostatic control of the TCNQ/TCNQ^·?ratio is reported.     

“Homodoping” strategy for building conducting Langmuir-Blodgett films

A strategy leading to the formation of conducting LB films without any doping treatment is investigated. The method consists of mixing, within the same layer, semi-amphiphilic and amphiphilic derivatives of the same electroactive nucleus (here Tetracyanoquinodimethane (TCNQ)). LB films obtained in this way with the sulphonium TCNQ salt as the semi-amphiphilic molecule and the octadecyl TCNQ as the amphiphilic one in the molecular ratio 1 to 2, are of high optical quality. Such films show a d.c. conductivity which is almost constant with variation in the number of layers, down to one bilayer. Preliminary results obtained by changing the nature of the counterion in the semi-amphiphilic molecule are in good agreement with those of the sulphonium TCNQ-octadecyl TCNQ⁰.     

Probing TCNQ‐mediated Metal Reduction Reactions at Liquid‐liquid Interface with SECM

Metal reduction at the interface between two immiscible electrolyte solutions (ITIES) has been studied with scanning electrochemical microscopy (SECM). Metal cations in the aqueous phase are reduced by 7,7,8,8‐tetracyanoquinodimethane anion (TCNQ^?) residing in the oil phase, methyl isobutyl ketone (MIBK). TCNQ^? is formed at the SECM tip by reducing TCNQ, which results in a positive feedback loop between the tip and the ITIES when an electron is donated to a metal cation. The effect of the Galvani potential difference on the rate of the interfacial electron transfer was investigated, establishing the potential difference either by an additional substrate electrode in the aqueous phase or by an a common ion in both phases. It is shown that the Galvani potential difference as a driving force does enable TCNQ^? mediated Cu²⁺ reduction. Finite element method (FEM) simulations were run to provide information on the reaction kinetics and stoichiometry.     

Synthesis and Chemical Properties of Diacetylenes with Pyridinium and 4,4′‐Bipyridinium Groups

Diacetylenes (DAs) having a dipolar D‐π‐A structure (D=donor: amino group; π=π‐conjugation core; A=acceptor: pyridinium (Py) and bipyridinium (BPy) groups), i.e., 4 (APBPyDA) and 5 (APPyPyDA), or an A‐π‐A structure, i.e., 7 (DBPyDA) and 8 (PyDA(Cl)), were obtained by 1 : 1 and 1 : 2 reactions of 4,4′‐(buta‐1,3‐diyne‐1,4‐diyl)bis[benzenamine] (APDA; 3 ) with 1‐(2,4‐dinitrophenyl)‐1′‐hexyl‐4,4′‐bipyridinium bromide chloride (1 : 1 : 1) ( 1 ), 1‐(2,4‐dinitrophenyl)‐4‐(pyridin‐4‐yl)pyridinium chloride ( 2 ), or 1‐(2,4‐dinitrophenyl)pyridinium chloride ( 6 ) (Schemes 1 and 2). The anion‐exchange reactions of 8 with NaI and Li(TCNQ) (TCNQ^?=2,2′‐(cyclohexa‐2,5‐diene‐1,4‐diylidene)bis[propanedinitrile] radical ion (1?)) yielded the corresponding I^? and TCNQ^? salts 9 (PyDA(I)) and 10 (PyDA(TCNQ)). Compounds 10 and 4 exhibited a UV/VIS absorption due to a charge transfer between the TCNQ^? and the pyridinium groups and a strong solute–solvent interaction of a dipolar solute molecule in the polar environment, respectively. Compounds 8 – 10 exhibited photoluminescence in solution, whereas 4 and 7 did not because of the presence of the 4,4′‐bipyridinium quenching groups. Differential‐scanning‐calorimetry (DSC) measurements suggested that the DAs obtained in this study can be converted into poly(diacetylenes) by thermal polymerization.     

Amperometric enzyme electrodes: Part II. Conducting salts as electrode materials for the oxidation of glucose oxidase

Results are reported for the direct oxidation of the enzyme glucose oxidase on 7 different one-dimensional conducting donor acceptor salts. Experiments conducted with the enzyme in bulk solution are shown to be in good agreement with theory. Three salts, made of the cations tetrathiafulvalinium (TTF⁺), N-methylphenazinium or quinolinium with the anion tetracyanoquinodimethanide (TCNQ^?) had the lowest background currents and were used to make membrane sensors for glucose. Analysis of the variation of current with glucose concentration identified the rate limiting processes as transport of gluycose through the membrane and electrochemical kinetics under unsaturated and saturated conditions respectively. The electrochemical rate constants for these three materials were all greater than 10^?2 cm s^?1. TTF⁺TCNQ^? is the material of choice and linear calibration plots were obtained for glucose concentrations between 50 μmol dm^?3 and 10 mmol dm^?3.     

Organic semiconductors: optical magnetic and electrical properties of 1-methyl-3-propylimidazolium 7,7,8,8-tetracyano-p-quinodimethane

Electrical conductivity and spectral properties (UV VIS absorption and especially the temperature dependence of IR absorption) are studied for the simple salt 1-methyl-3-propyl-imidazolium 7,7,8,8-tetracyano-p-quinodimethane (MPI⁺ TCNQ⁻. The temperature dependence of the IR absorption coefficient for the bands due to activation of the mode A_g is discussed in terms of changes in energy level population, geometry, and electron interactions. The temperature dependence of the absolute number of the TCNQ dimers in the triplet state is measured independently using the EPR method. The temperature dependences of the ground (singlet) state population of the TCNQ dimers and of the singlet—triplet energy separation J(T) = [(0.18 ± 0.01)-(2.5 ± 0.5) × 10⁻⁴ T] eV were determined from this measurement. These values lead to an unrealistically large value of the linear temperature expansion coefficient within the framework of the simple isolated model of dimers. Therefore, the weakly interacting model of dimers should be preferred. The activation energy of the triplet state exchange frequency. E_W = 0.27 eV, and the room temperature jumping frequency of the exciton along the TCNQ stacks, ν_j = 5 × 10¹⁰ s⁻¹, were determined from EPR measurements of the width of triplet lines.     

Excited stateπ-electron polarizabilities for some condensed aromatic molecules

Excited state π-electron polarizabilities are calculated with Hückel molecular orbitals where the β parameter is calibrated to electrochromic effect data for some condensed aromatic systems. The excited state parameter is found to beβ_c=3.6 eV as compared to the ground state value of β_g=7.5 eV. Our results are about the same as calculations with PPP orbitals and agreement with experimental data is generally good.     

Determination and characterization of a transition structure for water–formaldehyde addition

Quantum mechanical calculations of the geometric, energetic, electronic, and vibrational features of a transition structure for gas-phase water–formaldehyde addition (FW1?) are described, and a new transition-structure search algorithm is presented. Basis-set-dependent effects are assessed by comparisons of computed properties obtained from self-consistent field (SCF) molecular orbital (MO) calculations with STO-3G, 4-31G, and 6-31G^** basis sets in the absence of electron correlation. The results obtained suggest that STO-3G-level calculations may be sufficiently reliable for the prediction of the transition structure of FW1? and for the transition structures of related carbonyl addition reactions. Moreover, the calculated activation energy for formation of FW1? from water and formaldehyde (?44 kcal mol^?1) is very similar in all three basis sets. However, the energy of formaldehyde hydration predicted by STO-3G (? ?45 kcal mol^?1) is about three times larger than that predicted by the other two basis sets, with the activation energy for dihydroxymethane dehydration also being too large in STO-3G. Calculated force constants in all three basis sets are generally too large, leading to vibrational frequencies that are also too large. However, uniformly scaled force constants (in internal coordinates) give much better agreement with experimental frequencies, scaled 4-31G force constants being slightly superior to scaled STO-3G force constants.     

SCF-Xα MO studies of the x-ray spectra of CuO and ZnO

SCF-Xα scattered wave cluster MO calculations for the oxyanions CuO^?6₄ (D_4h symmetry) and ZnO^?6₄ (Td symmetry) yield results in good agreement with the X-ray photoelectron and X-ray emission spectra of CuO and ZnO, respectively. Agreement of the calculations with optical data is fair. Calculations of the valence electron and core electron hole states of these oxyanions support the assignment of photoelectron shakeup satellites to valence band to conduction band transitions. Calculated shakeup energies for the Cu2p core spectrum in CuO are 7.4 and 9.9 eV (cf. experimental values of 7.5 and 10.0 eV) while shakeup peaks in the valence region spectrum are predicted at 6.1 and 8.0 eV. (Cf. a broad peak with maximum at 8.1 eV observed experimentally.) The absence of intense low energy satellites in the spectra of ZnO is explained by the small amount of electron reorganization in the outer valence levels attendant upon hole formation.     

DFT and kinetics study of O/O2 mixtures reacting over a graphite (0001) basal surface

Spin-polarized density functional calculations were used to investigate the interaction of atomic and molecular oxygen on the basal graphite surface at several atomic coverages. Two carbon layers were enough to represent the graphite surface. Oxygen atoms bind mainly over C?CC bridge sites forming an epoxide-like structure with a two carbon puckering and with adsorption energies in the 0.95?C1.28?eV range, depending on the atomic coverage. Molecular oxygen only shows a very weak physisorption. Atomic adsorption and diffusion along with atomic recombination via Eley?CRideal and Langmuir?CHinshelwood mechanisms were studied. All surfaces processes were activated with energy barriers that decreased for lower atomic coverages. Relaxation effects were non-negligible. A microkinetic model with six elementary surface processes was proposed to see the overall behaviour of several initial O/O₂ mixtures flowing over a graphite surface at 300?C1,000?K. Thermal rate constants were derived from Density Functional Theory data and standard Transition State Theory. A very low steady-state atomic coverage (??_???<?0.5%) was predicted, and also very low atomic recombination coefficients were observed (??_O?<?5?×?10^?4). The Eley?CRideal together with the adsorption and desorption processes was much more important than the Langmuir?CHinshelwood reaction.     

Supermolecular CNDO/S calculation of parameters for extended hubbard hamiltonian of TCNQ dimers

Based on the CNDO/S-Cl method, the parameters of the extended Hubbard hamiltonian for unpaired electrons on a TCNQ perfect sandwich dimer were determined in wide range of intermolecular separations. Using the exact solution of the Hubbard hamiltonian the low-lying energy states of the isolated (2TCNQ)^2? dimer were calculated. Necessity of inclusion of doubly excited supermolecular states in the CNDO/S-CI energy spectra calculations on (2TCNQ)^2? dimers was emphasized.     

Studies on organotin compounds using the Del Re method : VIII. Mössbauer spectra or organotin compounds

A method for calculating the electron density at the nucleus of the tin atom in a tetravalent compound has been developed using the quantities available from the Del Re calculations. The calculated electron densities show a fair correlation with the experimental Mössbauer isomer shifts, and a value of +3.2 × 10^?4 is obtained for (ΔR/R), in excellent agreement with the value +3.30 × 10^?4 obtained from an independent method using internal electron conversion measurements. By defining an assymmetry parameter, A, which is a measure of the deviation of the calculated p-electron distribution from the hypothetical spherically-symmetric distribution, in which the p-electron would be distributed equally between the three p-orbitals, the experimental quadrupole splittings have been correlated and interpreted. Factors governing the magnitude of quadrupole splitting are also discussed.     

Encapsulation of TCNQ and the Acridinium Ion within a Bisporphyrin Cavity: Synthesis,Structure, and Photophysical and HOMO–LUMO‐Gap‐Mediated Electron‐Transfer Properties

The encapsulation of tetracyanoquinodimethane (TCNQ) and fluorescent probe acridinium ions (AcH⁺) by diethylpyrrole‐bridged bisporphyrin (H₄DEP) was used to investigate the structural and spectroscopic changes within the bisporphyrin cavity upon substrate binding. X‐ray diffraction studies of the bisporphyrin host (H₄DEP) and the encapsulated host–guest complexes (H₄DEP ? TCNQ and [H₄DEP ? AcH]ClO₄) are reported. Negative and positive shifts of the reduction and oxidation potentials, respectively, indicated that it was difficult to reduce/oxidize the encapsulated complexes. The emission intensities of bisporphyrin, upon excitation at 560 nm, were quenched by about 65 % and 95 % in H₄DEP ? TCNQ and [H₄DEP ? AcH]ClO₄, respectively, owing to photoinduced electron transfer from the excited state of the bisporphyrin to TCNQ/AcH⁺; this result was also supported by DFT calculations. Moreover, the fluorescence intensity of encapsulated AcH⁺ (excited at 340 nm) was also remarkably quenched compared to the free ions, owing to photoinduced singlet‐to‐singlet energy transfer from AcH⁺ to bisporphyrin. Thus, AcH⁺ acted as both an acceptor and a donor, depending on which part of the chromophore was excited in the host–guest complex. The electrochemically evaluated HOMO–LUMO gap was 0.71 and 1.42 eV in H₄DEP ? TCNQ and [H₄DEP ? AcH]ClO₄, respectively, whilst the gap was 2.12 eV in H₄DEP. The extremely low HOMO–LUMO gap in H₄DEP ? TCNQ led to facile electron transfer from the host to the guest, which was manifested in the lowering of the CN stretching frequency (in the solid state) in the IR spectra, a strong radical signal in the EPR spectra at 77 K, and also the presence of low‐energy bands in the UV/Vis spectra (in the solution phase). Such an efficient transfer was only possible when the donor and acceptor moieties were in close proximity to one another.