TTF+-TCNQ

Since TCNQ forms one-dimensional stacks, it may be reasonable to suggest that Peierls distortion occurs along the stack and that this increases the activation barrier. In compounds with considerably less than complete ET from the other atoms or molecules, Peierls distortion should not occur. Here, the resistivity is, in fact, very small, suggesting that delocalization has occurred. 

In 1973, it was discovered that solutions of TTF and TCNQ mix, resulting in a complex called TTF-TCNQ. Separate stacks of TTF and TCNQ are formed, adjacent to each other. A sharp conductivity peak appears at about 60 K. At this temperature, TTF-TCNQ conducts as a metal. 

For elevated temperatures, the conductivity decreases slowly. The conductivity is thus activated with conductivity equal to zero at T = 0, but with a very small activation energy. The activation may be related to the formation of electrons and holes. 

FIGURE 18.9 Acceptor tetracyanoquinodimethane, TCNQ (a), and donor tetrathiafulvalene, TTF (b). 

TTF-TCNQ does not show any sign of Peierls dimerization, which may depend on the fact that the band is much less than half filled, but with charge transfer occurring already in the ground state. The very small activation energy seems to be associated with ET within the stacks. 

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The highly conductive class of soHds based on TTF—TCNQ have less than complete charge transfer (- 0.6 electrons/unit for TTF—TCNQ) and display metallic behavior above a certain temperature. However, these soHds undergo a metal-to-insulator transition and behave as organic semiconductors at lower temperatures. The change from a metallic to semiconducting state in these chain-like one-dimensional (ID) systems is a result of a Peieds instabihty. Although for tme one-dimensional systems this transition should take place at 0 Kelvin, interchain interactions lead to effective non-ID behavior and inhibit the onset of the transition (6). 

The temperature of the metal-to-insulator transition in TTF—TCNQ is 53 K. For systems with increased interchain coupling, the transition temperature for the onset of metallic conduction increases roughly as the square of the interaction between the chains. This behavior is tme as long as the coupling between chains remains relatively weak. For compounds with strong interactions between stacks, the material loses its quasi-ID behavior. Thus, the Peieds distortion does not occur even at low temperatures, and the materials remain conductive. 

This model, which is sometimes referred to as the Fluctuating Gap Model (FGM) [42], has been used to study various aspects of quasi-one-dimensional systems. Examples arc the thermodynamic properties of quasi-one-dimensional organic compounds (NMP-TCNQ, TTF-TCNQ) [271, the effect of disorder on the Peierls transition [43, 44, and the effect of quantum lattice fluctuations on the optical spectrum of Peierls materials [41, 45, 46]. 

Since the discovery of the first organic conductors based on TTF, [TTF]C1 in 1972 [38] and TTF - TCNQ in 1973 [39], TTF has been the elementary building block of hundreds of conducting salts [40] (1) charge-transfer salts if an electron acceptor such as TCNQ is used, and (2) cation radical salts when an innocent anion is introduced by electrocrystallization [41]. In both cases, a mixed-valence state of the TTF is required to allow for a metallic conductivity (Scheme 5), as the fully oxidized salts of TTF+ cation radicals most often either behave as Mott insulators (weakly interacting spins) or associate into... 

Figure 1 (a) A view normal to the ac plane of the crystal packing in TTF-TCNQ. The... 

We saw in Section 12.2.3.1 that the presence of additional chalcogen atoms in BEDT-TTF/TCNQ promotes interstack interactions, suppressing the Peierls distortion and imparting upon the salt increased dimensionality compared to TTF/TCNQ. The result of including a different chalcogen into the TTF/TCNQ structure is shown in Table 2. Despite losing donor efficiency compared to TTF (Table 1) the TCNQ complexes of m/trans-diselenadithiafulvalene (DSDTF, 55/56) and TSF show an improvement in conductivity when two or four selenium atoms are incorporated. The reduced metal-insulator transition suggests that this effect is also caused by a suppression of the Peierls distortion. Increased Se-Se interstack contacts add dimensionality to the structure and limit the co-facial dimerisation typical of Peierls distortion. Wider conduction bands are afforded from the improved overlap of diffuse orbitals. 

The synthesis of the bis-l,3-dithiolium radical cation (TTF+) in 1970 [1] enabled dramatic growth in the field of molecular conductors in the decades thereafter. TTF and several of its homologues are depicted in Scheme 1. The field of low dimensional molecular metals was further motivated by the discovery of the TTF-TCNQ charge-transfer complex in 1973 [2, 3]. Seven years later, superconductivity was induced in the cation-radical salt, (TMTSF)2PF6, upon application of 12 kbar pressure [4]. Shortly thereafter, superconductivity below 1.4 K was observed at ambient pressure in the perchlorate analog [5]. 

Chemical research of molecular metals was activated by the discovery of the metallic charge transfer salt TTF-TCNQ in 1973 [40]. Two basic molecular architectures have been studied intensively. One is based on organic molecules with the... 

The erroneous yet over-publicized claim of almost superconductivity in the salt TTF TCNQ (Heeger, 1973). 

This requirement to planarity is important but not very strict. The salt [(TTF)+ (TCNQ) ] is an example. It possesses metallic conductivity. However, its fulvalene rings are not strictly coplanar and the cyano groups are, to a certain degree, bent in respect of the quinone ring. Moreover, one organic metal has been prepared on the basis of the principally nonplanar chiral ion-radical (TMET)2 PFg . Its conductivity is equal to 5 cm at ambient conditions (Wallis et al. 1986). 

X-ray Charge Densities and Chemical Bonding table 6.2 Results of Charge Integration of the TTF-TCNQ Data... 

The organic conductor properties of tetrathiaflulvalenetetracyanoquino-dimethane (TTF-TCNQ) as a material for constructing electrodes, viz. its catalytic response and resistance to passivation, are of special interest for the determination of biological compounds, which usually have slow electrode kinetics and a low sensitivity, and tend to foul electrode surfaces. The response of a TTF-TCNQ microarray sensor inserted in a flow system for... 

As will be discussed later (Section 1.5), molecules containing no metallic elements are able to combine and form materials exhibiting metallic character, e.g., HMTSF-TCNQ, TTF-TCNQ, etc., or even lose any electrical resistance below a given temperature and thus become superconductors, e.g., (TMTSF)2C104. Metal-free molecules can also, in the solid state, show magnetic order, such as / -NPNN and /7-NC-C6F4-CNSSN, where in the absence of -electrons the magnetic properties are related to unpaired -electrons. 

Dlh TTF, TCNQ, TMTTF, TMTSF, BEDT-TTF, PTCDA, Ni(dmit)2, BEDT-TSF, BDA-TTP, pentacene... 

Figure 1.7. Force plot obtained with a silicon microfabricated cantilever with a force constant of 25 N and an ultrasharp tip of radius R < 10 nm on a highly aft-oriented thin TTF-TCNQ trim.

Figure 1.15. Crystal structure of TTF-TCNQ perspective view along the stacking -axis. P2ilc, a = 1.230 nm, b = 0.382 nm, c = 1.847 nm, fi = 104.46°. C, S and N atoms are represented by black, medium grey and light grey balls, respectively. H atoms are not represented for clarity. Crystallographic data from Kistenmacher et al, 1974.

Materials with the general formula (TMTCF)2X, where C stands for chalcogens sulfur and selenium and X for monovalent anions, are known as Bechgaard-Fabre salts (BFS). Because of the extremely high quality of the BFS that can be achieved, these are, together with TTF-TCNQ and BEDT-TTF salts, the most extensively studied crystalline MOMs and a matter of intensive research. 

One way of experimentally exploring the electronic structure of solids is by means of photoemission spectroscopies such as UPS and X-ray photoelectron spectroscopy (XPS), where photoexcited electrons are analyzed dispersively as a function of their kinetic energy. The electronic structure of the reference material TTF-TCNQ will be extensively discussed in Section 6.1. Figure 1.31 shows the XPS spectra of the S2p core line for (TMTTF)2PF6 (black dots) and BEDT-TTF (grey dots). 

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