TCNQ stacks

SBSBSBS-TCNQ 2/c 29.6 17.9 17.9 Helicene and TCNQ stacked alternatively 154)... 

Donor-acccptor it complex alternating parallel molecules of 2890 Ni(tmp) and TCNQ stacking in columns. Intrastack spacing about 330 pm conductivity < 10 Scm 1 Ni—N, 195 Partially oxidized complex stacks of Ni(tmp) molecules (Ni—Ni 2891... 

The transition-metal component of [Pt(bipy)2][TCNQ]3 is also diamagnetic. The structure86 consists of stacks of trimers, [T CNQ], surrounded by non-interacting Pt(bipy)2+ cations. Adjacent trimers are closely spaced with an interplanar separation of only 3.33 A as compared with 3.23 A within the trimer. The magnetic properties of this system have been studied much less extensively than those of [TMPD]2[Ni(mnt)2]. Single crystal EPR spectra do show, however, that spins are interacting along the TCNQ stacks. Structurally, this compound is very different from the 1 2 compound [Pt(bipy)2][TCNQ]258> (Sect. 2) which forms Da stacks with the two TCNQ moieties <7-bonded to each other.. ... 

At high temperature, TTF TCNQ is metallic, with a(T) oc T-2 3 since TTF TCNQ has a fairly high coefficient of thermal expansion, a more meaningful quantity to consider is the conductivity at constant volume phonon scattering processes are dominant. A CDW starts at about 160K on the TCNQ stacks at 54 K, CDW s on different TCNQ chains couple at 49 K a CDW starts on the TTF stacks, and by 38 K a full Peierls transition is seen. At TP the TTF molecules slip by only about 0.034 A along their long molecular axis. 

An important distinction must be made between the two-stack systems (like TTF TCNQ, where electrons travel on the TCNQ stacks while holes live on the TTF stacks) and the one-stack systems, or ion-radical salts (or radical ion salts), such as the (TMTSF)2X salts, the alkali TCNQ salts M (TCNQ), and the (ET)2X salts The holes are localized on the TMTSF or ET sublattice, whereas the electrons are on the TCNQ sublattice. 

Figure 3 (a) Structure of TMA.TCNQ.I in the monoclinic C2/m symmetry as described in Refs. 24 and 27 (b) how iodine columns and zigzag TCNQ stacks are arranged in the crystal. 

More specific to low-dimensional organic conductors is the approach of Chasseau [69]. The conductivity was assumed to be directly related to the efficiency of the molecular overlap between adjacent moieties within TCNQ stacks. This overlap efficiency was quantized on the basis of the total intersection volume between van der Waals spheres attached to each atom. This method, although very approximate, gave good results for ammonium... 

Figure 14 Polarized reflectance of MTPP(TCNQ)2 for different crystal faces. The electric vector is parallel to the TCNQ stacks direction in the case of faces rf(100) and /(Oil) (a) and is parallel to the TCNQ long axis for /(010) (b). (From Ref. 69.)...

Two chain compounds, such as TTF-TCNQ and its derivatives, where both donor (TTF) stacks and acceptor (TCNQ) stacks give comparable contributions to the electrical conductivity. Although these compounds... 

Organic semiconductors such as K-TCNQ and TEA(TCNQ)2 have anomalous infrared (IR) absorption lines at frequencies corresponding the totally symmetric (ag) vibration modes of the TCNQ molecule. Normally, these modes would not be IR active, but because they modulate the electron overlap integrals, they induce charge oscillations along the (nonuniform) TCNQ stacks [108]. This is perhaps the clearest experimental evidence for... 

In the case of the TTF and TCNQ stacks, it is interesting to note that the valence band of TTF (from which in the mixed crystal the charge transfer occurs) and the conduction band of TCNQ (to which the charge is transferred) are comparatively broad ( a.0.3 eV and % 1.2 eV, respectively), while the conduction band of TTF and the valence band of TCNQ are very narrow. The position of the... 

In the diffuse X-ray measurements of TTF-TCNQ the superlattice reflection was found with the wave number 4kp = 0.59 b [56]. It is observed even at room temperature and suggests the absence of the interchain correlation above 49 K. A set of superlattice reflection was found below 49 K suggesting the formation of an ordered structure of three-dimension. This superstructure is ascribed to the molecular displacement caused by the Wigner crystal of electrons through the electron-lattice interaction [67]. The 4 p structure is considered to be formed predominantly on the TTF stacks. The 2kp superstructure is rather ascribed to TCNQ stacks. This is suggested [68] by detailed analyses of the results of X-ray, neutron, EPR and NMR measurements. 

Figure 2 Room-temperature crystal structure of TTF TCNQ [188]. The upper diagram is projection along [ 100], with the unit cell axes a vertical and c horizontal, which shows the stacks of TTF (open circles for the atom positions) and TCNQ (filled circles for atom positions). The lower diagram shows the intermolecular overlap, projected normal to the least-squares molecular planes, along the TTF stack (left), and the TCNQ stack...

M2P-TCNQ has probably received more attention than any phenazine complex except MP-TCNQ. First prepared by Melby its structure was determined by Goldberg and Shmueli The mixed alternating M2P-TCNQ stack is shown in Fig. 11. The bent M2P was interpreted as favoring a neutral ground state, although the interesting remark that epr absorbtion is obtained only when the crystals are powdered suggests facile formation of MjP and TCNQ" ion radicals. 

The M2P-TCNQF4 complex could be crystallized, however, and its structure resembles that of TMPD-TCNQ rather than the dimerized M2P-TCNQ stacks in Fig. 11. As shown in Fig. 13, M2P-TCNQF4 forms mixed regular stacks containing planar M2P ions. Inversion centers at each D and A" site ensure equal interaction with both neighbors. The susceptibility, x(T), in Fig. 14 follows the Bonner-Fisher... 

In spite of the great number of measurements of the properties of this compound, including conductivity, dielectric constant, magnetic susceptibility, electron and nuclear spin resonance, specific heat, thermoelectric power, etc., many over wide temperature and frequency ranges, there is still no consensus as to how all the various pieces of the puzzle fit together. Even such a basic question as to whether most of the high temperature conduction is along the TTF or the TCNQ stacks or in hybridized orbitals of both remains open. 

The TCNQ molecular orbitals corresponding to the conduction band go over into localized states because of the disorder. These states are doubly occupied, singly occupied, or empty. The singly occupied states correspond to localized spins which are randomly positioned along the TCNQ stack. There is a probability p <, 1 that a localized state be singly occupied, p turns out- to be about 1/3 when the theory is fitted to the PC data of di Salvo. 

The interaction between ccdw s on adjacent like stacks (e.g. TCNQ stacks separated by aQ=12.3S in the crystallographic a direction) is given by... 

What may seem surprising is that a TSeF impurity, while presumably inducing only a short-range charge density wave on its TTF stack is able to have as large an effect as it does on the TCNQ stacks. That such an effect should occur is consistent with the notion, which will be pursued... 

Perturbations in the TCNQ stacks have big effects on the TTF stacks, comparable with the effects of perturbations directly in the TTF stacks. 

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