Intrinsic and extrinsic magnetic susceptibilities of some TCNQ complexes

The magnetic susceptibilities (χ) of quinolinium·(TCNQ)₂, N-methyl phenazinium·TCNQ and Li·TCNQ were measured from 2 to 300 K and are discussed in connection with the low-temperature specific heats (C) measured by other authors, χ is decomposed into three parts: χ_d the temperature-independent part, χ_c, Curie-Weiss type paramagnetism, and χ_p, the remainder. Correspondingly, C is composed of three terms, γT, H/T² and αT³. The electronic state of these substances is discussed in terms of each type of susceptibility.The model, on which the above separation of χ and C is based, defines two types of electrons: localized electrons associated with a magnetic moment and band electrons. Though this model is useful phenomenologically, it is shown that the analysis of χ on the basis of this model indicates less band electrons and more localized electrons or stronger magnetic interactions than does that of C.     

Direct evidence for controlled band filling in the mixed crystal NMPxPhen1−xTCNQ

Raman spectra of the segregated stack mixed crystal NMP_xPhen_1?xTNCQ and the mixed stack compound Phen TCNQ are presented. By means of the relation between charge transfer and line shift, we give for the first time a direct proof for controlled band filling in the mixed crystal series, whereas for Phen TCNQ a zero charge transfer is obtained. Raman spectra of Phenazine and the related NMP⁺ have also been measured and used for comparison.     

Some magnetic properties of Mn(TCNQ)2

The EPR spectra of polycrystalline Mn(TCNQ)₂·3H₂O and Mn(TCNQ?d₄)₂ have been studied as a function of temperature from 1.5 K to 375 K. At very low temperatures the line width indicates an exchange interaction similar to that of other manganese salts. At 77 K and above the line is narrowed and shifted most likely through interaction with the electronic motion. The bulk susceptibility was measured at room temperature. The observed μ_eff=4.66 implies an antiferromagnetic coupling of the manganese ions.     

Determination of electron affinity of electron accepting molecules

The unoccupied π ^* states of the solid film of electron accepting organic molecules, 7,7,8,8-tetracyanoquinodimethane (TCNQ), fluorinated TCNQ derivatives, 11,11,12,12-tetracyanonaphtho-2,6-quinodimethane (TNAP), C₆₀, and 6,6-phenyl-C₆₁-butyric acid methyl ester (PCBM) have been studied by inverse photoemission spectroscopy. The assignment of the π ^* affinity levels of these typical electron accepting molecules provides the basic information for the organic electronics and the new electronic functional molecular design. The comparison with density functional theory calculations enables understanding how the electron affinity evolves in terms of molecular orbitals. The correlation between the film morphology and the irradiation damage on the TCNQ derivative samples by electron impact during the inverse photoemission measurements is also discussed.     

Low temperature EPR study of the “impurity” resonances in (TCNQ)- and (TCNQ)-2 salts

The so-called “impurity” resonances in polycrystalline Li⁺ (TCNQ)^-, Cu⁺ (TCNQ)^- and Cu⁺ (TCNQ)^-₂ have been studied at liquid helium-temperatures. The Li⁺ (TCNQ)^- resonance is partially resolved at low microwave powers with g_⊥ = 2.0025 ± 0.0005 and g_∥ = 2.008 ± 0.001. Itis found that the (TCNQ) and (TCNQ)^-₂ complexes give characteristics spectra. The possible origin of these resonances is discussed.     

Temperature and frequency dependence of the nuclear relaxation rate in Qn(TCNQ)2

The magnitude, frequency dependence and temperature dependence of the proton relaxation rates in Qn(TCNQ)₂ are explained in terms of a theory which treats the short wavelength response as coherent and one-dimensional, while the long wavelength response is dominated by anisotropic (intrachain vs interchain) diffusion. The diffusive character of the response near q = 0 is consistent with recent attempts to understand the electronic properties of Qn(TCNQ)₂ in terms of weak localization of states in one-dimension. Analysis of the data leads to the conclusion that the effective screened Coulomb interaction is relatively weak.     

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.     

Pulsed ESR on the organic ion radical salt: 3,3′-diethyl-4,4′-dimethyl-2,2′-thiazolocyanine-[TCNQ]2

We present firstT _1e ^?1 andT _2e ^?1 measurements on the organic ion radical salt 3,3′-diethyl-4,4′-dimethyl-2,2′-thiazolocyanine-(TCNQ)₂ as function of temperature and of orientation. The electronic spin diffusion constant could be determined directly by the electron spin echo field gradient technique:D (300 K)=0.03±0.02 cm²/sec. Pulsed ESR experiments have — in comparison to conventional cw-ESR — the advantage to monitor viaT _1e ^?1 andT _2e ^?1 the spectral density of dynamical processes at different frequencies. This is shown in a general manner on 3,3′-diethyl-4,4′-dimethyl-2,2′-thiazolocyanine-(TCNQ)₂. Between 300 and 60 K,T _1e ^?1 andT _2e ^?1 are close in amplitude and have a similar temperature dependence. At 60 K their degeneracy is lifted, yielding a quantitative value for the effective spin exchange between localized spinsτ _ex ^?1 sec^?1 and via the absolute value of the relaxation an average distance of the localized centers of about 12 Å. The dynamical data as evaluated above cannot be correlated with the conductivity, clearly indicating that the conduction electrons are a minority, not being monitored by the ESR-experiments.     

On the trimer approximation to the theory of Cs2(TCNQ)3 electronic structure

The electronic states of the crystalline Cs₂(TCNQ)₃ are studied under an assumption that the crystal consists of isolated trimers of TCNQ molecules occupied by two unpaired electrons. The Hamiltonian is a modified Hubbard Hamiltonian. The theory is related to the experimental data. A direction of necessary extension of the theory is discussed.     

Observation of quasi-universal magnetic behavior in a random exchange Heisenberg antiferromagnetic chain: Neutron irradiated quinolinium (TCNQ)2

Low field electron spin resonance measurements of the magnetic susceptibility (χ) and absorption linewidth over the temperature range 0.04 – 300 K are reported for quinolinium (TCNQ)₂ into which increased amounts of disorder have been introduced by fast neutron irradiation. It is found that below 20 K, χ = AT^-α; A increases linearly with the irradiation dose, but $\text{α (? 0.8)}$ is almost independent of it, in agreement with the quasi universal behavior predicted by recent renormalization calculations for a random exchange Heisenberg antiferromagnetic chain. Measurements of the g-shift at 4.2 K range indicate that all of χ is associated with TCNQ chains. These results are discussed in terms of the renormalization calculation of Soos and Bondeson.     

Spin resonance of tetramethyltetrathiafulvalene tetracyanoquinodimethane: a comparison with tetrathiafulvalene tetracyanoquinodimethane

Electron spin resonance spectra and linewidth studies of two different phases of tetramethyltetrathiafulvalene (TMTTF) tetracyanoquinodimethane (TCNO) are presented. The linewidth of the solution grown 1:1 phase (TMTTF)(TCNQ), which is the structural analog to tetrathiafulvalene- tetracyanoquinodimethane (TTF) (TCNQ), is shown to have a similar temperature dependence to that of (TTF) (TCNQ) in contrast with earlier reports. The vapor grown phase, identified by X-ray studies as (TMTTF)_1.66 (TCNQ)₂, is shown to have different magnetic and electronic properties. These results are discussed and compared with earlier spin resonance reports on (TMTTF) (TCNQ).     

Angular variation of ESR linewidth in organic ionic radical salt: 3,3′-diethyl-2,2′-selenacyanine-[TCNQ]2

Angular dependence of ESR linewidth of 3,3′-diethyl-2,2′-selenacyanine-[TCNQ]₂ single crystal reveals the two-dimensional magnetic nature of the system. The dependence is represented well by ΔH(θ)=α(3 cos²θ?1)²+β sin⁴θ where α and β are temperature-dependent parameters.     

Static dielectric constant of disordered organic quasi one-dimensional conductors: Localization by defects

We have measured the low temperature dielectric constant ? of two similar quasi one-dimensional organic conductors, N-Me-iso Qn(TCNQ)₂ and Qn(TCNQ)₂. For N-Me-iso Qn(TCNQ)₂ below 10 K, ? is independent of temperature and is frequency independent in the range 5 × 10⁵ Hz to 9 × 10⁹ Hz, within the 50% experimental uncertainty. Thus we believe the low temperature microwave dielectric constant to be a good approximation of the static value in this salt. For Qn(TCNQ)₂ at low temperatures, the relation ? ∝ (c+c₀)^-2 holds, where c is the defect concentration and c₀ is an effective defect concentration of the nominally pure material. This relation is predicted by the model of interrupted metallic strands with energy spacings larger than kT, and it indicates that electrons are strongly localized by defects along the conducting chains.     

Thermoelectric power of ETPP (TCNQ)2 and DEPE (TCNQ)4

Results of measurements of thermoelectric power of two complexes of TCNQ, namely, ethyltriphenylphosphonium (TCNQ)₂ and 1,2 Di (N-ethyl-4-pyridinium) ethylene (TCNQ)₄ in the temperature range 100–370 K are presented. Over a certain temperature range, thermoelectric power remains independent of temperature suggesting that the most likely mechanism of charge transfer is hopping.     

ESR studies on ferromagnetic-spin dynamics in a nearly two-dimensional organic ionic radical salt: 1-methyl-3'-ethyl-2, 2'-quinoselenacy anine-[TCNQ]2

The angular dependence at 77 K of the ESR linewidth of 1-methyl-3'-ethyl-2, 2'-quinoselenacyanine-[TCNQ]₂ single crystal is well expressed by ΔH(θ) = 2.2(3 cos²θ-1)²Oe. The results reveal the two-dimensional ferromagnetic nature of the system.     

Nuclear relaxation in a one-dimensional triplet-exciton system

The proton spin-lattice relaxation time has been measured in (?₃AsCH₃)⁺ (TCNQ)^-₂ at 145 K as a function of the frequency. The results are interpreted in terms of interrupted one-dimensional random walk of the excitons along the TCNQ chains. The interruption is discussed in terms of the exciton-exciton collisions.     

Photophysical parameters and fluorescence quenching of 7-diethylaminocoumarin (DEAC) laser dye

The optical properties including electronic absorption spectrum, emission spectrum, fluorescence quantum yield, and dipole moment of electronic transition of 7-diethylaminocoumarin (DEAC) laser dye have been measured in different solvents. Both electronic absorption and fluorescence spectra are red shifted as the polarity of the medium increases, indicating that the dipole moment of molecule increases on excitation. The fluorescence quantum yield of DEAC decreases as the polarity of solvent increases, a result of the role of solvent polarity in stabilization of the twisting of the intramolecular charge transfer (TICT) in excited state, which is a non-emissive state, as well as hydrogen bonding with the hetero-atom of dye. The emission spectrum of DEAC has also been measured in cationic (CTAC) and anionic (SDS) micelles, the intensity increases as the concentration of surfactant increases, and an abrupt change in emission intensity is observed at critical micelle concentration (CMC) of surfactant. 2×10⁻³ mol dm⁻³ of DEAC gives laser emission in the blue region on pumping with nitrogen laser (λ_ex=337.1 nm). The laser parameters such as tuning range, gain coefficient (α), emission cross section (σ_e), and half-life energy have been calculated in different solvents, namely acetone, dioxane , ethanol, and dimethyforamide (DMF). The photoreactivity of DEAC has been studied in CCl₄ at a wavelength of 366 nm. The values of photochemical yield (?_c) and rate constant (k) are determined. The interaction of organic acceptors such as picric acid (PA), tetracyanoethylene (TCNE), and 7,7,8,8-tetracynoquinonedimethane (TCNQ) with DEAC is also studied using fluorescence measurements in acetonitrile (CH₃CN); from fluorescence quenching study we assume the possible electron transfer from excited donor DEAC to organic acceptor forming non-emissive exciplex.     

Isotope effect on the Peierls transition temperature of TTF-TCNQ

Conductivity measurements on isotopically substituted single crystals of TTF-TCNQ show a relatively large shift in the phase transition temperatures at 52.5 K (T₁) and 37.7 K (T₃). Deuteration of both TTF and TCNQ chains increases T₁ by 0.75 ± 0.15 K, N¹⁵ substitution increases T₁ by 0.6±0.1 K while deuteration of the TTF chains apparently has no significant effect. Different possible interpretations of these results are briefly discussed.     

Isotope effect on the Peierls transition temperature of TTF-TCNQ

Conductivity measurements on isotopically substituted single crystals of TTF-TCNQ show a relatively large shift in the phase transition temperatures at 52.5 K (T₁) and 37.7 K (T₃). Deuteration of both TTF and TCNQ chains increases T₁ by 0.75 ± 0.15 K, N¹⁵ substitution increases T₁ by 0.6 ± 0.1 K while deuteration of the TTF chains apparently has no significant effect. Different possible interpretations of these results are briefly discussed.     

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.