Tetracyanoquinodimethane Molecules Research Articles

Crystal Structures and Electrical Conductivities of TXCn-TTF (X=Sulfur, Selenium, Tellurium; n=2,3)·TCNQ Charge-Transfer Complexes

Abstract The crystal structures and the electrical conductivities have been studied for tetrakis(alkylchalcogeno)tetrathiafulvalene (TXCn-TTF (X=sulfur, selenium, tellurium; n=2,3)) complexes with tetracyanoquinodimethane (TCNQ). All the complexes with a composition of 1 : 1 had the same crystal system (triclinic) and the same space group (P\bar1). The donor and acceptor molecules were alternately stacked in their crystals. (TSeC2-TTF)TCNQ was obtained as plate and needle crystals. The molecular structure of TSeC2-TTF and the overlapping mode between TSeC2-TTF and TCNQ molecules of the two complexes were subtly different. All the complexes were semiconductors because of their mixed-stack structures. (TTeC2-TTF)TCNQ showed the lowest resistivity (5 Ω, cm at room temperature). Its good conductivity was brought about by the effective degree of charge-transfer and the favorable overlap between TTeC2-TTF and TCNQ molecules. An overall discussion on the structural and physical properties of a series of TCNQ complexes with TXCn-TTF (X=sulfur, selenium, tellurium; n=1,2,3) is presented.

Crystal structure and magnetic properties of TCNQ salts with metallocene cations

Conducting organometallic salts based on tetracyanoquinodimethane (TCNQ) and metallocenes of iron, (Cp)Fe(Ar), where the metal is complexed by two rings: cyclopentadienyl (Cp) and substituted benzen ring (Ar), have been synthesized with the following arene ligands: Ar = C6H6, C6H5Me, C6H5Cl, C6H3Me3, C6H3(tBu)3, C6Me6 and C6Et6. The magnetic behaviour is characteristic of linear chains of S = 12 spins (the metallocene cations are diamagnetic). A steric effect, due to the substituents carried by the benzen ring, is observed, which induces changes in intrastack distances between TCNQ molecules and, therefore, in the antiferromagnetic exchange coupling: the larger the substituent size, the lower the AF exchange coupling constant J.

Atomic scale surface studies of conductive organic compounds 2. Scanning tunneling microscopy and crystal structure of the charge transfer complex of 4-ethylpyridine-TCNQ 2 (4-EP-TCNQ 2)

The scanning tunneling microscopy (STM) studies of the conductive charge transfer complex of 4-ethylpyridine (4EP) with tetracyanoquinodimethane (TCNQ) were combined with X-ray diffraction measurements. The existence of two different crystalline modifications of the complex was revealed. The block-type monocrystals have a triclinic crystallographic structure with the following parameters: a=7.112 2 A ̊ , b=7.862 7 A ̊ , c=13.954 4 A ̊ , a=72.95 4°, β=78.40 2° and γ=69.92 3° . The existence of a columnar structure of TCNQ molecules with diadic repeat unit is shown. The interplanar distances between neighboring TCNQ molecules in stack is 3.21 Å. The STM images of one of the surfaces of such monocrystals correspond well with the distribution of TCNQ atoms in the surface layer constructed from the crystallographic ( a, b) plane. The quality of needle-shaped monocrystals does not permit detailed crystallographic measurements. Only the triclinic crystal system and the lattice constants were determined: a=3.865 2 A ̊ , b=6.658 9 A ̊ , c=26.386 7 A ̊ , α=92.63 5°, β=94.12 3° and γ=106.80 8° . STM images provide additional structural information about this crystal modification. The formation of stacks of TCNQ molecules with the average parameters of 4.1 Å (periodicity in stack) and 13.1 Å (distance between neighboring stacks) seems to be the characteristic feature of this structure. Observed structures are compared with the known crystallographic data of other charge transfer systems. The peculiarities of STM imaging in charge transfer systems are discussed in due course.

Structure analysis of conductive polymer systems: Poly-4-vinylpyridine and poly(butadiene-b-4-vinylpyridine) with 7,7′,8,8′-tetracyanoquinodimethane

The structure analysis of two conductive polymer systems-poly-4-vinylpyridine and poly(butadiene-b-4-vinylpyridine) with 7,7′,8,8′-tetracyanoquinodimethane (TCNQ)-was done by X-ray diffraction, scanning tunneling microscopy (STM) and FTIR. The charge transfer complex formed between the pyridine group and the known electron acceptor, TCNQ, is supposed to be the conductive element in these systems. In order to understand the structure of this complex, a model compound, the complex of 4-ethylpyridine (4EP) with TCNQ, 4EP/TCNQ2, was studied by the mentioned methods. It appears that there are two crystalline modifications of the model compound with different type of stacks of the TCNQ molecules. In polymer systems only one type of the complex is dominant as revealed by joint analysis of X-ray diffraction diagrams, STM and FTIR data. In the STM image of the polymer surface one can distinguish that molecular stacks with periodicities of 4.1 A in a row are separated (12.5A) from each other. Such organization is similiar to the one observed in conductive charge transfer complexes such as tetrathiofulvalene (TTF) with TCNQ. The ordered molecular domains are scattered on the polymer surface and take part in the formation of the conductive network.

Spectroscopic, photoelectric and electrical conductivity measurements for some anthracene charge transfer complexes doped in polystyrene

The photoconductivity of polystyrene films, doped with anthracene as donor and 7.7.8.8. tetracyanoquinodimethane (TCNQ), tetracyanoethylene (TCNE) and chloranil as acceptors, was investigated. It was found that polystyrene film containing (6:4) TCNQ to anthracene has the greatest photosensitivity (8.2). The electrical resistivities of the compounds used were found to be: anthracene 1.52 × 1014Ωcm, TCNQ 1.6 × 108Ωcm, TCNE 4.9 × 105Ωcm and chloranil 1.24 × 1014Ωcm. The activation energies of these compounds were also measured as 0.66, 0.51, 0.41 and 0.17 eV, respectively. The electronic absorption spectra of TCNQ-Anth., TCNQ and anthracene in polystyrene were measured in the region from 250 to 950 nm, in addition to their IR spectra. The charge transfer was found to be mainly dependent on the ionized TCNQ molecules formed.

The structure and physical properties of N- docosylpyridinium-bistetracyanoquinodimethane Langmuir-Blodgett films

The structure of Langmuir-Blodgett (LB) films of N-docosylpyridinium- tetracyanoquinodimethane (TCNQ) 1:2 charge transfer salt was studied. The polarized spectra of the film in the UV-visible region showed no in-plane anisotropy, which is consistent with the results of direct observation of the film by polarized microscopy and scanning electron microscopy. The absorption bands owing to the TCNQ appear in the UV-visible region and showed the distinct blue shift compared with that of the powder sample. This fact was explained well by the difference between the energy of three-dimensional solids and the quasi-two- dimensional LB system using a Hubbard-like model. The striking feature of the spectra of the LB film was the strong absorption band which appeared at 32×10 3 cm −1. The transition direction of this band was the direction of the long axis of the TCNQ molecular plane and proved to be normal to the thin surface. The TCNQ molecules had their short axis parallel and long axis perpendicular to the film plane. The polarized IR spectra were also consistent with the UV-visible spectra.

Structural trends in potential organic conductors based on (donor-CH2)2 tetracyanoquinodimethane molecules

Structural trends in potential organic conductors based on (donor-CH2)2 tetracyanoquinodimethane molecules

Quenching of fluorescence of pyrene-substituted lecithin by tetracyanoquinodimethane in liposomes

In this work we have applied a kinetic scheme derived from fluorescence kinetics of pyrene-labeled phosphatidylcholine in phosphatidylcholine membrane to explain the fluorescence quenching of 1-palmitoyl-2-(10-[pyrenl-yl]-sn-glycerol-3-phosphatidylchol ine (PPDPC) liposomes by tetracyanoquinodimethane (TCNQ). The scheme was also found to be applicable to neat PPDPC and the effect of the quencher could be attributed to certain steps of the proposed mechanism. The TCNQ molecules influence the fluorescence of pyrene moieties in PPDPC liposome in two ways. Firstly, an interaction between the quencher molecule and the pyrene monomer in the excited state quenches monomer fluorescence and effectively prevents the diffusional formation of the excimer. Secondly, an interaction between the quencher molecule and the excited dimer quenches the excimer fluorescence. The TCNQ molecule does not prevent the formation of the excimer in pyrene moieties aggregated in such a way that they require only a small rotational motion to attain excimer configuration. The diffusional quenching rate constant is calculated to be 1.0 x 10(8) M-1 s-1 for the pyrene monomer quenching and 1.3 x 10(7) M-1 s-1 for the pyrene excimer quenching. The diffusion constant of TCNQ is 1.5 x 10(-7) cm2 s-1 for the interaction radii of 0.8-0.9 nm. The TCNQ molecules are practically totally partitioned in the membrane phase.

Electron spin resonance study of Langmuir-Blodgett films of N-docosylpyridinium- di(tetracyanoquinodimethane)

An electron spin resonance (ESR) investigation is performed on Langmuir-Blodgett films consisting of a surface-active 1:2 complex of N-docosylpyridinium and tetracyanoquinodimethane (TCNQ), whose temperature-dependent spin susceptibility has been so far found to be consistent with the random-exchange Heisenberg antiferromagnetic chain (REHAC) model of Soos and Bondeson. An out-of-plane anisotropy in the ESR spectra shows that the long axes of TCNQ molecules are preferentially oriented along the plane normal to the sample, where the negligible in- plane anisotropy indicates the random distribution of the short and the pπ orbital axes in the film plane. A heat treatment applied to the samples results in an increase in the concentration of disorder, showing the possibility of controlling it in the present REHAC system.

Electrochromic properties of vacuum evaporated organic thin films: Part I. Electrochromic thin films of the three primary colors

Characteristics of newly developed electrochromic organic thin films prepared by the vacuum evaporation technique are described. ECDs (Electrochromic Displays) of the three primary colors (RGB) were obtained with 2,4,5,7-tetranitro-9-fluorenone (TetraNF) for red, 2,4,7-trinitro-9-fluorenylidene malononitrile (TNF-MN) for green and tetracyanoquinodimethane (TCNQ) for blue. Use of a co-evaporated film of TCNQ and tetrathiofulvalene (TTF)-TCNQ complex was found to increase stability, because of the dispersion of TCNQ in the matrix of the TTF-TCNQ complexes and the number of TCNQ molecules which take part in the redox reaction was also found to be greater in these films. It seems that since the TTF-TCNQ complex is an electro-conductive material, the apparent electrode area is extended. The process for making a RGB pattern by the vacuum evaporation technique is shown. By that process, a full-color ECD was found to be feasible.

Two-step transfer of conducting Langmuir films from a glycerol subphase

A new method for building up conducting Langmuir-Blodgett (LB) films is described. The film is initially formed and compressed on a glycerol subphase from a semi-amphiphilic mixed-valence conducting tetracyanoquinodimethane (TCNQ) salt, namely octadecylpyridinium + ∮(TCNQ1,45) −. We show that the conducting properties are retained on this subphase for surface pressures up to 18 mN m −1, which corresponds to the elasticity limit of the film. The film is then shifted onto a water subphase, contained in a dipping compartment, which has been previously incorporated in the main Langmuir trough, the water level being set identical to that of the glycerol. The shift onto the water allows the transfer onto a solid substrate with the classical dipping-withdrawing method. The propertie films are investigated by analysing their UV-visible and IR spectra and their linear dichroism. The IR and UV-visible spectra are reminiscent of those of mixed-valence TCNQ complexes with only a poor conductivity. This behaviour is verified bya d.c. conductivity measurement at room temperature, which provides a stable value of 10 −4μ −1 cm −1. Furthermore, the high degree of order in the film is evidenced by its strong dichroic behaviour. It turns out that TCNQ molecules stand edge on with their molecular planes roughly perpendicular to the plane of the substrate and their long axis close to being perpendicular to it. In addition, the influence of the subphase in the fibrication of LB conducting films is pointed out.

A new family of Langmuir–Blodgett films of tetracyanoquinodimethane charge transfer salts: Pristine and iodine doped conducting films

A new family of semiamphiphilic tetracyanoquinodimethane (TCNQ) ionic salts of nonconjugated cations, namely octadecyl-dimethylsulfonium, octadecyl-methyl-ethyl-sulfonium, and octadecyl-trimethylphosphonium labeled a, b, and c, respectively, has been synthetized. Langmuir–Blodgett (LB) films are built up from these three compounds, which are mixed with octadecylurea in a molar ratio 1/1, in order to improve the stability of the film at the water surface. The structural properties of the transferred films are thoroughly investigated by linear dichroism in the IR and UV–visible ranges, together with ESR spectroscopy. In the films of the three compounds, the polar sheets contain (TCNQ− • )2 dimers, which are the largely prevalent species. The molecular planes of these dimers are found to be roughly parallel to the substrate. Moreover, a strong in-plane orientation of the dimers is found in films of compound c. This phenomenon is thought to be related to an overall flow orientation of rod-shaped crystallites in the floating film, during the high speed downstroke of the substrate across the film. Advantage is taken from the high and well-controlled thinness of LB films to make iodine vapor diffuse into the films and convert the precursor films into conducting ones. The iodination process leads for the three salts to a stable ternary conducting compound located in the polar planes of the LB assembly. Correlatively, it induces a strong molecular reorganization in the films: the TCNQ molecules stand on edge with their long molecular axis perpendicular to the substrate. In spite of this considerable rearrangement arising from iodination, we notice that the strong in-plane anisotropy observed in pristine films of c is retained in conducting films of the iodinated c compound. Beside these structural results, the IR spectra of conducting films are reminiscent of these of highly conducting, but not metallic-like, organic conductors. The conductivity of the films is estimated from their IR spectra and found to be 20–50 Ω−1 cm−1. However, dc conductivity measurements provide a value at least ten times lower than the one estimated from IR spectra. This behavior is likely related to the polycrystalline nature of the films whose macroscopic inhomogeneities are evidenced by scanning electron microscopy.

Electron energy-loss spectroscopy of tetracyanoquinodimethane, TCNQ, tetrathiafulvalene, TTF, and the salt TTF-TCNQ

Energy loss spectra of low energy electrons (ELS) have been obtained for thin solid films of tetracyanoquinodimethane (TCNQ), tetrathiafulvalene (TTF), and the charge transfer salt TTF-TCNQ as a function of incident energy (20 to 100 eV). The incident energy dependence of energy loss features allows the identification of triplet and symmetry forbidden excitations in TCNQ and TTF. The ELS spectrum of TTF-TCNQ is compared with high energy electron loss data to show that the surface of evaporated films of TTF-TCNQ contains 20–50% neutral TCNQ molecules.

Simple model for nonconducting tetracyanoquinodimethane (TCNQ) salts with donor to acceptor ratio 2:1

We propose a simple model for those nonconducting tetracyanoquinodimethane (TCNQ) salts in which the ratio of the numbers of donor and acceptor chains is 2:1. We assume that each TCNQ chain can be subdivided into electrically neutral boxes containing two TCNQ molecules and one electron. Electron transfer between boxes is neglected. The dominant Coulomb and exchange interactions are identified. The free energy is calculated exactly in two limits, vanishing and infinite intrabox hopping. Depending on the signs and relative magnitude of the Coulomb interactions, we find in the first case singlet, or paramagnetic doublet, or triplet ground states. In the second case, we find ferromagnetic or antiferromagnetic ground states. We discuss also the intermediate cases.

Inelastic electron tunneling spectroscopy of single ring compounds adsorbed on alumina

Studies of inelastic tunneling spectra of nitro-substituted single aromatic ring compounds have shown a correlation between tunneling mode intensity and orientation of the ring relative to the tunnel junction interface. Details of the adsorption of substituent groups on the alumina barrier can also be deduced from the tunneling spectra. In a series of halogen-substituted benzoic acids where the ring orientation is relatively constant intensity changes for specific modes can be interpreted in terms of changes in the bond character with substitution. Tunneling spectra of tetracyanoquinodimethane (TCNQ), tetrathiafulvalene-tetracyanoquinodimethane (TTF-TCNQ), ${\mathrm{Li}}^{+}$ ${(\mathrm{TCNQ})}^{\ensuremath{-}}$, and TCNQ-${\mathrm{d}}_{4}$ have shown that for all derivatives the ionized form of the molecule is the species adsorbed on the alumina barrier. Analysis of the mode frequencies and intensities suggests that the Raman type of interaction makes dominant contributions to the tunneling intensity in TCNQ derivatives. Comparison of the tunneling intensities to calculated linear electron-phonon coupling constants for the TCNQ molecule shows that the magnitudes of tunneling intensity and coupling constant are correlated and can be identified with modes of the same symmetry.

Electronic spectra of heptafulvalene tetracyanoquinodimethane (TCNQ) in acetonitrile

Absorption spectra of heptafulvalene (HF) tetracyanoquinodimethane (TCNQ) in acetonitrile are reported. The spectrum of HF—TCNQ is a superposition of the spectra of HF and TCNQ, with a 1:1 ratio. The fraction of the TCNQ molecules which are ionized in acetonitrile is about 3%.

Electron escape depth, surface composition, and charge transfer in tetrathiafulvalene tetracyanoquinodimethane (TTF-TCNQ) and related compounds: Photoemission studies

Ultraviolet photoemission spectroscopy (UPS) has been used to study in-situ-prepared tetracyanoquinodimethane (TCNQ), ${\mathrm{Cs}}_{2}$${(\mathrm{TCNQ})}_{3}$, and tetrathiafulvalene-TCNQ (TTF-TCNQ) thin films. The measurments on ${\mathrm{Cs}}_{2}$${(\mathrm{TCNQ})}_{3}$ were made during the charge-transfer reaction between Cs vapor and TCNQ solid, which allows an unambiguous determination of the energy levels near ${E}_{F}$ of the TCN${\mathrm{Q}}^{\ensuremath{-}}$ anion. Our results demonstrate that the excess charge of TCN${\mathrm{Q}}^{\ensuremath{-}}$ is localized to a molecular scale and that the TCNQ molecule in solids will normally exist in either the neutral or -1 charge state, and not in a state of shared fractional charge. Quantitative comparison of the UPS spectra between ${\mathrm{Cs}}_{2}$${(\mathrm{TCNQ})}_{3}$ and TTF-TCNQ permits a rather complete assignment of the structure in the TTF-TCNQ data. The structures near -1 and -2 eV are assigned to TCN${\mathrm{Q}}^{\ensuremath{-}}$, those near -3 eV to TT${\mathrm{F}}^{+}$ and those near -4 eV to both TCN${\mathrm{Q}}^{0}$ and TCN${\mathrm{Q}}^{\ensuremath{-}}$. In addition, the data suggest the presence of TT${\mathrm{F}}^{+}$ states immediately below ${E}_{F}$. From studies of ultrathin films, the electron escape depth is estimated to be less than 10\AA{} in these solids, which indicates a strong representation of the surface electronic structure in our data. TTF-TCNQ deposited at 77\ifmmode^\circ\else\textdegree\fi{}K is found to undergo irreversible changes in both its UPS spectra and its bulk thermal properties upon annealing to room temperature. A model is presented which accounts for these changes in terms of molecular rearrangements upon annealing which are accompanied by altered surface composition of the films and increased ionization of the TTF-TCNQ complex. In room-temperature TTF-TCNQ there is evidence for significant charge transfer, for the presence of some TCN${\mathrm{Q}}^{0}$ at the surface, and evidence against any significant surface concentration of TT${\mathrm{F}}^{0}$. In view of this work we suggest (i) that prior uv-photoemission work may have sampled principally the surface and (ii) that the surface of room-temperature TTF-TCNQ films may have a different composition from the bulk.

Epitaxial growth of an organic semiconductor from the vapor phase — TCNQ on potassium chloride

Epitaxial films of 7,7′,8,8′-tetracyanoquinodimethane (TCNQ), a strong electron acceptor were formed on a KCl cleavage face by condensation from the vapor phase. Electron optical studies revealed that the films assume two different configurations depending upon the condition of the surface treatment, although they are both composed of small crystallites of ac-orientation and take multiple positioning with certain regularities characteristic of the individual types. It was deduced from specific features of the mutual orientation of the deposit and substrate crystals that epitaxial nucleation is governed primarily by the oriented adsorption of TCNQ molecules on the ionic lattice of KCl and secondarily by the sporadic matching of lattice points at the interface of both crystals.

Crystal structure of the 1 : 1 molecular complex of chrysene and 7,7,8,8-tetracyanoquinodimethane

The crystal and molecular structure of the title complex has been determined by three-dimensional X-ray diffraction methods. Crystals are triclinic, a= 7·255, b= 7·995, c= 9·210 Å, α= 99·8, β= 89·7, γ= 92·9°, space group P, Z= 1. The structure was determined from 1288 diffractometer data by Patterson and Fourier methods and refined by least-squares methods to R 0·104. In the crystal, alternate chrysene and tetracyanoquinodimethane molecules form stacks along the a axis. The mean separation of the molecular planes is 3·37 Å and the interplanar angle is 3·4°. The interplanar separations in a series of related tetracyanoquinodimethane complexes are shown to be dependent on crystal system as well as orbital energies. The relative orientation of the two molecules is not ideal for maximum overlap of highest-filled-donor and lowest-vacant-acceptor orbitals, and suggests that dipoldipole interactions and interactions involving other donor and acceptor orbitals are also important in determining orientation. The dimensions of both molecules are in good agreement with known values, and no disorder is apparent.

Molecular complexes. Part XVI. Crystal structure of the 1 : 1 molecular complex of pyrene and 7,7,8,8-tetracyanoquinodimethane

The crystal and molecular structure of the title π complex has been determined by three-dimensional X-ray diffraction methods. Crystals are monoclinic, a= 7·14, b= 14·73, c= 10·01 A, γ= 102·5°, space group P21/b, Z= 2. Two determinations of the structure were made: from 1084 visually estimated photographic data by the symbolic addition method, and from 1068 diffractometer data by Patterson and Fourier methods; each was refined by least-squares methods to R 0·136 and 0·137, respectively. In the crystal alternate pyrene and tetracyanoquinodi-methane molecules form stacks along the a axis. The mean separation of the molecular planes, which are inclined at an angle of 6·3°, is 3·50 A. The relative orientation of the two molecules is not that predicted by MO calculations of charge-transfer interactions, and reflects the influence of lattice packing requirements and dipole–dipole interactions. In both determinations the molecular dimensions found for tetracyanoquinodimethane are in good agreement with known values, whereas those found for pyrene are not. Thermal vibration parameters suggest that the pyrene molecule is disordered.