TCNQ and TTF Stacks

It is well known that the mixed three-dimensional molecular crystal built up from alternating 7,7, 8,8 -tetracyanoquinodimethane (TCNQ) and tetrathiofulvalene (TTF) stacks exhibits metallic conductivity owing to charge transfer of about 0.6 from a TTF molecule to a TCNQ unit. For the band-structure calculations of the two stacks we have applied the CNDO/2 method, and in the case of the TCNQ stack also the MINDO/2 method. (No MINDO/2 calculations could be conducted for the TTF chains owing to the lack of a parametrization scheme for sulfur at that time.) All band-structure calculations were performed in the first-neighbor-interaction approximation. This approximation is well justified in view of the large distances between neighboring molecules in the chain. 

The following SCF criteria were applied during the course of calculations  

In order to calculate the elements of matrices P(4 ), a numerical integration over k in the interval (0, nta) is required. This was performed using Simpson s rule. 

For each TCNQ and TTF chain the eigenvalue problem of a complex Hermitian matrix of order 68 and 64, respectively, had to be solved 80-150 times. The calculations were accelerated by diagonalizing the Hermitian complex matrices with the aid of a complex matrix eigenvalue program based on the QR algorithm. The resulting computing time was only 6-9 min for one chain on the IBM 360/91 computer. 

Second lowest unfilled band Conduction band Valence band Second highest filled band Lx)west filled band 

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Figure 4 Room-temperature crystal packing of TTF-TCNQ. (a) View normal to the (ac) plane. (From Ref. 35.) (b) Schematic representation of the structure (shaded rectangles TCNQ white rectangles TTF). The molecular planes are tilted about the a axis there are two identical molecules with opposite tilts for one repeat unit along c, and TCNQ and TTF stacks alternate in the a direction.

The Valence and Conduction Bands of TCNQ and TTF Stacks and of the Four Homopolynucleotides (in eV-s)... 

The method has been applied to polyene, polyacetylene and polydiacetylene chains, to formamide chains (both hydrogen-bonded and stacked). Applications have been done also to TCNQ and TTF stacks, to (SN) and to periodic DNA models (the four homopolynucleotides), to the sugar phosphate chain of DNA and to different periodic protein models (homopolypeptides). All these systems have relatively broad valence and conduction bands (bandwidths around or larger than 0.5eV) according to our results. 

We propose in this paper that the observed Peierls instability in TTF-TCNQ and the richness of structural transitions arises from the interaction of the tt electron and hole system on separate TCNQ and TTF chains, respectively, with orientational modes (librons) of the solid. We suggest, that the chiral charge density waves (CCDW), which result from the orientational distortion of the TCNQ and TTF stacks account for the observed continuous increase in the unit cell dimension from a = 2a at 54° to a = 4a at 38°K. 

The second chapter examines applications of crystal-orbital theory presented earlier. These applications include polymers widely used in the production of plastics, primarily polyethylene and its fluorine derivatives. Other examples are from the field of highly conducting polymers, such as the different polyacetylenes, (SN) , and TCNQ and TTF stacks. Applications to nucleotide base stacks and to periodic polynucleotides and periodic polypeptides conclude this part of the book. 

This chapter presents some examples of the application of the ab initio crystal-orbital method described in Chapter 1. Though these applications range from the field of plastics (polyethylene and its fluoro derivatives) through highly conducting polymers [polyacetylenes and polydiacetylenes, (SN) c, TCNQ and TTF stacks] to biopolymers (homopolynucleotides and homopolypeptides), they are only illustrative. No attempt has been made to review the numerous other applications performed by the Namur group and by other researchers, as this would increase unduly the size of this book. 

Ab initio SCF LCAO CO calculations on the infinite neutral TCNQ and TTF stacks were performed using a TCNQ (see Figure 2.5a) or TTF (Figure 2.5b) molecule as unit cell. These ab initio Hartree-Fock band structures can serve as a starting point for further improvements, such as the treatment of charge transfer between the stacks and interac-... 

It should be pointed out here that in the case of polymers with a larger molecule as unit cell (like TCNQ and TTF stacks, nucleotide base stacks or homopolypeptides with a larger amino acid residue as unit cell) it is necessary to localize further the WF s or Bloch orbitals into different parts of the molecule which is repeated in the periodic polymer. For that purpose one has to investigate how is possible to combine the Fourier transformation leading to WF s with the usual localization... 

A correspondence between the Covalon model and the electrical conductivity characteristics observed in TR-A, B, and C temperature ranges can be made as follows According to the crystal structure investigation [43] the closest intermolecular distance in [TCNQ-TTF] crystals is between TCNQ and TTF, i.e., the distance between TCNQ or TTF molecules themselves are further apart. Thus, the physical path of electron conduction through the crystals is not through a stack of TCNQ or TTF but rather through -(TCNQ)- (TTF)- (TCNQ)- alternation. Therefore, stepwise charge transfer is [see Fig. 14(c)] as follows ... 

Infinite Stacks of TCNQ and TTF Molecules.—The quasi one-dimensional charge-transfer molecular crystal TCNQ (7,7, 8,8 -tetracyanoquinodimethane)-TTF (tetrathiofulvalene) has received considerable attention in the past decade because of its interesting solid-state physical properties. In recent publications81... 

In Table 3.3 the charge distributions in the stack obtained for a TCNQ and TTF molecule, respectively, are shown. They clearly indicate strong charge transfer (CT) between the two molecules. 

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. 

A very important advantage of TTF-TCNQ is that its band structure can be compared to those of neutral TTF and TCNQ because both molecules exist as neutral and charged species (see Section 2.4). Figure 6.4 shows the band structure and the partial density of states (PDQS) of TTF-TCNQ and Fig. 6.5 the band structures and PDOS of neutral TTF and neutral TCNQ. Note that there are two molecules per repeat unit of the crystal structure for neutral TTF (a = 0.736 nm, b = 0.402 nm, c = 1.392 nm, ft = 101.42° (Cooper et al, 1971)) but four for neutral TCNQ (a = 0.891 nm, b = 0.706 nm, c = 1.639 nm, = 98.54° (Long et al, 1965)). TTF contains ID stacks along the ( -direction where every successive molecule slides along the long molecular axis. These stacks are similar to those found in TTF-TCNQ. 

In contrast, neutral TCNQ does not really contain ID stacks as in TTF-TCNQ but does contain 2D stacks in the afc-planes. While the TTF HQMO bands are practically degenerate in TTF-TCNQ, they are not in neutral TTF, e.g., the interchain interactions are larger in the neutral compound. The dispersion of these bands along the chain direction is not very different in TTF and TTF-TCNQ. In contrast, the structural origin of the spreading of the TCNQ LUMQ levels is very different in the neutral and the salt compounds, and as a consequence the appearance of the TCNQ LUMO bands is very different in both cases. 

FIGURE 6.7 Structures of (a) TTF and TCNQ and (b) solid TTF-TCNQ, showing alternate stacks of TTF and TCNQ molecules. 

Figure 1 Relationship of donor and acceptor molecules in charge transfer complexes (a) mixed stacks of alternating donor and acceptor molecules in a normal charge transfer complex (b) segregated stacks of donor and acceptor molecules in (TTF)(TCNQ) and related materials...

Figure 2 Relationship of (a) TCNQ in acceptor stack, (b) TTF in donor stack and (c) stacking arrangement of TCNQ anions in (TTF)(TCNQ) showing plane of symmetry uniting molecules in a stack...

Figure 3 Relationship of donor and acceptor stacks in (TTF)(TCNQ) showing misalignment of symmetry planes for individual stacks...

The relative orientations of the donor and acceptor stacks in (TTF)(TCNQ) give no reason to assume the existence of low energy orbital overlap between the stacks, in agreement with the observed difference of two orders of magnitude between the conductivity along... 

The structure of TTF/TNAP like that of TTF/TCNQ is composed of stacks of TTF and TNAP ions arranged in alternating layers with similar interionic distances in the two compounds (cation cation = 0.34 nm anion anion = 0.31 nm) (see Fig. 26). Flowever, the structure within the layers is quite different with the TTF ions in the TNAP salt all tilted in the same direction whereas in TTF/TCNQ the TTF ions in adjacent columns are inclined in... 

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. 

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