TMTTF-TCNQ

Let us assume for this estimation that the susceptibilities of both chains of the compounds are equal, though we know that the experiments give 0.3 s (T Q), < o, 1+ /14, 57, 58/. We feel justified in doing so since we are interested, for the moment, in factors 5 to 10 and not in 20% effects or so. The values of the experimental susceptibility are summarized in Table 2, either per molecule of TMTTF /TCNQ/ or per electron at P=0 kbar and P= 8kbar, assuming a 0.55 elec/mole-cule charge transfer and i -0.23 eV. Assuming that X (S 0Kj ( T °), we derive for / ]a =0.55 from... 

Pressure dependence proton relaxation rates at 300K /24 MHz/ in TTF-TCNQ , TMTTF-TCNQ ... 

Fig. 11 Pressure dependence of the effective Coulomb repulsion in TMTTF-TCNQ or /TTF-TCNQ/...

The preparation of TTF-TCNQ and TMTTF-TCNQ is the matter of a report P. Calas, J.M. Fabre, M. Khalife-El-Saleh,... 

The most recent work has dealt with modifications of the donor in the prototype TTF-TCNQ /l/, including the notable successes TMTTF-TCNQ /2/, TSF-TCNQ /3/, TMTSF-TCNQ /4/ and HMTSF-TCNQ /5/. 

Q- TTF-TCNQ (above 58 K) TMTSF-TCNQ (black form) NMP-TCNQ TSF-TCNQ (above 40 K) HMTSF-TNAP TMTTF-TCNQ HMTTF-TCNQ... 

Some of the early Hopkins conductivity data for the series TMTTF-TCNQ, TTF-TCNQ (for whom the conductivity anisotropy is 500 5 <1), TMTSF-TCNQ, and HMTSF-TCNQ are shown in Fig. 10 [25, 26, 112-115]. From other (mainly magnetic) data it is surmised that... 

Fig. 10. Temperature-dependent resistivities of TTF-TCNQ, TMTTF-TCNQ, TMTSF-TCNQ, and HMTSF-TCNQ [26].

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

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). 

XRD patterns of TTF-TCNQ hlms grown by CVD on Si(lOO) substrates also show this kind of extra reflections (de Caro et al, 2000a). This is perhaps the first evidence, albeit incomplete and thus questionable, of a new crystallographic phase of TTF-TCNQ. The conclusive observation of a new phase of TTF-TCNQ, e.g., with mixed-stacked structure as for the red phase of TMTSF-TCNQ, would be extremely interesting. The known and newly observed structures for both TMTTF and TTF-TCNQ might be tentatively ascribed to the thermodynamical and kinematical phases, respectively. 

Monolayers prepared from TMTTF octadecyl TCNQ and TTF-octadecyl TCNQ on glycerin subphase Surface-pressure/surface-area measurements and in situ conductivity Lateral d.c. conductivity was of the order of 1 Scm"1 741... 

LB films prepared from a mixture of TMTTF and octadecyl-TCNQ acceptor and transferred to solid substrates Absorption and infrared spectroscopy... 

LB films prepared from a 3 2 charge-transfer complex of TMTTF and TCNQ and transferred to cadmium-arachidate (five layers) precoated sheets of polyethylene terephthalate ESR... 

LB films prepared from hexadecyl-TCNQ TMTTF and (heptadecyl-dimethyltetrathia-fulvalene)2 TCNQ and from their mixtures Electron microscopy and electron diffraction Molecular packing determined conductivities best lateral d.c. conductivity was 0.5 S cm -1 765... 

Chemical alloying is simply replacing parts of the molecules by isostructural and isoelectronic parts. For example, alloys such as (TTF) . .(TSeF TCNQ [163], (TMTSF), t(TMTTF),C104 [164], and (TMTSF)2(C104), (ReO,), [165] have been obtained. 

There are two groups of organic conductors which, contrary to onedimensional TCNQ salts, show considerable interchain coupling. They are both of single-stack type, with donor molecules of tetramethyltetrathia-fulvalene (TMTTF) or tetramethyltetraselenafulvalene (TMTSF) and BEDT-... 

NMR relaxation experiments have been performed in many organic conductors and have usually been interpreted according to the mechanisms described above. Expression (39), for example, has been used for TTF-TCNQ(D4). Tf1 in (TMTTF)2PF6 [41] and TTF[Ni(dmit)2]2 [42] behave as one-dimensional conductors with deviations to Korringa law (Korringa law T 1 oc T). 

Finally, it may be remarked that the two salts MEM(TCNQ)2 and TEA(TCNQ)2 bear several points of resemblance in their physical properties with the most interesting salts of the (TMTTF)2X series (TMTTF = tetramethyltetrathiafulvalene) [14]. They are all organic conductors with a quasi-one-dimensional character, with p = 5 (or ), and with a dominant electron-electron interaction. They exhibit comparable modest values of the electrical conductivity at high temperature, which indicate that the electrons are not very delocalized in the materials, and in all of them an underlying 4kF dimerization is also present, due to the cations. These common features, which are thought to be at the origin of sizable Umklapp scattering effects in the salts of the (TMTTF)2 series [14], could also be able to produce the same kinds of effects in MEM(TCNQ)2 and TEA(TCNQ)2 (see also Chapter 2). 

The spin-Peierls transition has been the object of several recent papers. Examples of the first case, with p = 1, are provided by the insulating alkali metal simple salts [47] (see below), and examples of the second case, with p = 3 or I, are given by the salt MEM(TCNQ)2 [17-19,46], by the salts of the (TMTTF)2X series [46,48], or by the salts of the (BCPTTF)2X series (BCPTTF = benzocyclopentyltetrathiafulvalene) [46,49]. 

Qn(TCNQ)2, (DMDCNQI)2Ag, and (TMTTF)2PF6 The metal-like semiconductor model of Epstein et al. (see Section II.C) has been found to fit rather nicely the electrical conductivity data a(T) of the quinolinium salt Qn(TCNQ)2, for instance [16]. In this case, the experimental values of the fitting parameters were A0 = 600 K, a = 2.7, and Tm = Aq/u = 240 K. 

ESR spectra of the LB film of TMTTF-C18TCNQ show a single line without structures, which together with the observed g value indicates that there is strong coupling between TMTTF radical spins and TCNQ radical spins [59] as in the TTF-TCNQ crystal [60]. The temperature dependence of the spin susceptibility of the film suggests the presence of spin species due to conduction electrons. 

Anomalous behavior was observed in the ESR spectra of the TMTTF-C14TCNQ LB film shown in Fig. 8 [61,62] a decrease in spin susceptibility with decreasing temperature, a prominent maximum linewidth, and a maximum g value. These features are similar to those observed in TTF-TCNQ crystals around the Peierls transition temperature [60,63,64]. The results can be reconciled if the system is an organic metal the observed anomalies... 

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