Model for conduction in TCNQ salts

We have extended our model for conductivity, σ, and its temperature, T, dependence to a group of molecular conductors including (Qn) (TCNQ)₂, (Adz) (TCNQ)₂ and (Adn) (TCNQ)₂. We have parametrically fit and then quantitatively calculated σ(T) for each of these materials as a product of an activated carrier concentration (600K, 450K, and 350K respectively) and a strongly T-dependent mobility determined by known electron-phonon coupling to the molecular vibrations of TCNQ.     

Optical absorption in complex TCNQ salts

Optical absorption spectra of complex TCNQ salts (Qn, 2-2' BIP, 4-4' BIP, NMePy and MEM) compressed into KBr were measured. It is demonstrated that the low energy transition around 0.3 eV appears in all cases, but is not simply related to the transport properties. Powder optical spectra can not be used to determine the strength of Coulomb interaction in these materials.     

Some properties of TCNQ complex salts films

Films of triethylammonium and quinolinium complex salts of TCNQ were made by two methods: cathode sputtering in an argon discharge and vacuum deposition. The films consisted of two phases. Both the electrical and the optical properties of the films changed during aging. In the present paper, the following topics are mainly dealt with: the temperature dependence of resistance and thermoelectric voltage within the temperature interval approximately from 150 °K or 200 °K, to 300 °K, and the changes in these properties during aging of the samples. Our results concerning the triethylammonium complex salt of TCNQ agree in principle with the findings of other authors on crystals (except for the changes during aging). This is not true for the quinolinium complex salt films. The results are interpreted only qualitatively in terms of LeBlanc's model.     

Raman spectra of conducting TCNQ salts; Estimation of the degree of charge transfer from vibrational frequencies

The linear dependency of TCNQ ν₄ frequencies on the formal charge was observed and was attributed to the motional narrowing of the Raman band by hopping of conduction electrons. The formal charges of TTF-TCNQ (0.59) and NMP-TCNQ (0.62) estimated from this dependency agreed well with those from other experiments.     

Spectroscopic investigation of pressure-induced phase transitions in TCNQ complex salts

In most of the TCNQ complex salts, conduction electrons are localized on specific TCNQ sites, so that these salts show nonmetalic behavior. The caesium salt, Cs₂(TCNQ)₃, is one of the 2:3 complex salts. In the crystal, TCNQ molecules form trimeric units, which consist of two TCNQ radical anion sandwiching a neutral TCNQ along the column. The rubidium salt, Rb₂(TCNQ)₃, also has a similar crystal structure to Cs₂(TCNQ)₃. We measured infrared absorption (IR) and Raman spectra for these salts under high pressure by using a diamond anvil cell. In the case of IR spectra, Cs₂(TCNQ)₃ showed a spectral change probably due to a pressure-induced phase transition. Similar feature was not clearly observed in the Rb₂(TCNQ)₃. On the other hand, the Raman spectra, Cs₂(TCNQ)₃ showed two phase transition at 2.5 and 4.1 GPa in the compression stage. The change from localization phase to delocalization phase occurred at latter transition with large hysteresis. Similar phase transition occurred at 3.2 GPa in the Rb₂(TCNQ)₃. The reason for the difference in transition pressure is that the ion radius of Rb⁺ is smaller than that of Cs⁺, because a small ion radius of the counter ion probably favors the charge localization-delocalization transition of the TCNQ column.     

The effect of the temperature and magnetic field on TCNQ anion radical salts in dimer approximation

Some of organic charge transfer crystals can have triplet excitons with very low excitation energy. A class of such crystals which can be treated in dimer approximation is discussed here. The effects of lattice distortion, the temperature and magnetic field are discussed. A possibility of getting high concentrations of triplet excitons close to saturated concentration of triplet excitons is emphasized.     

Pressure effect on the surface enhanced resonance Raman effect

Abstract

Surface enhanced resonance Raman spectra are presented for iron phenanthroline and ferricytochrome c up to 3 kbar. The frequency shifts are similar to those of the free species in solution. High pressure induces an irreversible loss of the scattering intensity.     

Charge — Transfer enhanced infrared absorption in simple alkaline TCNQ salts

The intensities of the infrared light absorption spectra of the simple Alkaline TCNQ salts approximated by a collection of noninteracting dimers of TCNQ^– ions are studied theoretically in the frame of the previously used model. The charge transfer interaction between the two molecules of the dimer is shown to enhance strongly some vibrational transitions into the states of the dimer with two molecular vibrational quanta. The effect of a certain kind of impurities is also discussed on the infrared spectra of these salts.     

Thermoelectric power,Coulomb correlation and charge transfer in TCNQ salts

We show how the amount of charge transfer for a large class of TCNQ systems can be simply deduced from a measurement of the high temperature saturation value of the thermoelectric power. Of particular interest is the value - 60 μV/°K characteristic of a quarter-filled correlated band, and found in almost all 2 : 1 TCNQ salts.     

Charge-transfer singlet exciton in 1:1 high conductivity TCNQ ion-radical salts

A question of the existence of a certain class of charge transfer excitons in the high conductivity salts of the tetracyanoquinodimethane is considered. Interactions which can be rather strong in real crystals and can possibly stabilize the excitonic motion are discussed.     

Optical properties of charge transfer salts of tetracyanoquinodimethane (TCNQ)

A unified interpretation is given for the infrared and optical spectra of many TCNQ compounds which exhibit a wide variety of physical properties. Inter- and intra-molecular transitions are distinguished by polarization measurements on single crystals of several TCNQ salts. It is suggested that there is incomplete charge transfer in both tetrathiofulvalinium (TTG)- and N-methylphenazinium (NMP)-TCNQ and that the electron-electron interactions are not appreciably screened by the excitonic polarizability of the cations.     

Raman study of the mechanism of electrical switching in Cu TCNQ films

The mechanism of electrical switching in Cu/Cu TNCQ films has been studied in situ by Raman spectroscopy. The initial film is Cu⁺TCNQ^-., while neutral TCNQ was found after electrical switching in amounts increasing with time of application of the electric field.     

On the magnetic properties of TCNQ anion radical salts (2)

By introducing the alternant site energy ε₀, a Schottky type behavior of the magnetic susceptibility is also obtained in the case of a 4-site 2-electron ring Hubbard model which is the model for TCNQ complex salts.     

Mixed-valence states in TCNQ salts of monoalkylbiferrocenes and their mixed crystals with dialkyl derivative

TCNQ salts of 1-monoalkylbiferrocenes showed a temperature-independent trapped-valence state as expected for the unsymmetrical structure of cations. On the other hand, mixed crystals of 1-monobutyl- and 1.1-dibutylbiferrocenium⁺(TCNQ) ₃ ^– showed a temperature dependence of the mixed-valence states, showing that the lattice affects the mixed-valence states.     

Specific heat and magnetic susceptibility at low temperature of two conductive TCNQ salts

The specific heat measurements between 1.4 and 4.4 K of acridinium (TCNQ)₂ and quinolinium (TCNQ)₂ salts show up a linear component; moreover, in presence of a strong magnetic field (H = 40kG), an artificial Schottky anomaly is revealed. Magnetic susceptibility experiments confirm the simultaneous existence of charge carriers, in a partially filled energy band, and localized paramagnetic centers. A standard energy band model is proposed to interpret these two properties.     

Magnetic properties of TCNQ complex salts with polycations containing sulfur atoms in the main chain

The magnetic properties of TCNQ anion salts with polycations containing sulfur atoms in the main chain were investigated using ESR and magnetostatic methods. For the majority of these complex salts asymmetric ESR lines were recorded and characterized by g_⊥, g_∥, and ΔH_max values as well as by an asymmetry parameter δ′. An analysis of symmetrical ESR lines was carried out by the linear anamorphose technique. The temperature dependence of magnetic susceptibility χ_p was studied for the K_s [x, y] complex salts obtained. A correlation was found between the magnetic properties (ΔH_max, δ′, and χ_p) and the electrical properties ([sgrave]₂₅, E_a) studied in our previous works.     

Effect of the electron-exciton and the electron-vibronic interactions in the absorption spectra of TCNQ salts

The energies and the relative intensities of the lines of the optical absorption spectra of a certain class of TCNQ salts are calculated within a strong coupling approximation. The unpaired electrons are supposed to interact strongly with the intramolecular excitons and with several modes of intramolecular vibrations. The agreement of the theory and experiment is satisfactory. The ways of possible improvements of the theory and the interpretation of the experimental data are discussed.