Mixed-stack structures

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. 

Charge-transfer adducts are formed with the gold(I) trimers [Au3(MeN=COR)3] (R = Me, Et), which act as electron donors, and organic electron acceptors as nitro-9-fluorenes. The structures of these adducts involve mixed stacks in which the gold trimers and the planar nitro-fluorenes are interleaved.3130 No luminescence has been observed from these solid charge-transfer adducts, which is not surprising since the luminescence of [Au3(MeN=COR)3] is a property that is associated with the supramolecular organization in the solid. 

Structural Characterization of Mixed-Stack Charge Transfer... 

The X-ray diffraction pattern of an 11-layer film of the mixed-stack CT complex of octadecyl-TCNQ doped with (Me)2P revealed that the film has a well-ordered layered structure in which the D and A are highly oriented [28], A d value of 3.3 nm indicates that each layer in the CT film contains biomole-cular layers of alternately stacked D and A. A comparison of the IR transmission and RA spectra of the one-layer mixed-stack CT films suggested that both chromophoric planes of D and A and their long molecular axes are preferentially perpendicular to the substrate surface, while the hydrocarbon chains are parallel to it [28]. In the case of the multi-layer CT films, the D and A planes were found to be slightly tilted with respect to the surface normal and the... 

Fig. 11(a) shows the AFM image of an 11-layer mixed-stack CT film of octadecyl-TCNQ and (Me)2P scanned at room temperature with a scan area of 2x2 pm2 [29]. It can be seen from the image that the CT film consists of platelet microcrystal domains of a few micrometers in size in which a multi-layered structure with many steps is observed. An analysis of the cross-sectional profile revealed that the layered platelet microcrystal domains have a step of 3.3 nm thickness [29]. This is in good agreement with the d value measured by the X-ray diffraction method [28]. Therefore, it seemed that the X-ray diffraction peaks originate from the multi-layered structure inside the domains. Each layer in the domains apparently consists of biomolecular layers of octadecyl-TCNQ and (Me)2P because the layer thickness of 3.3 nm is larger than the molecular length (3.0nm) of octadecyl-TCNQ. The biomolecular layer structure also supports that the CT film is in a mixed-stack pattern. 

Mixed-metal enolate has also been found. The core of the lithium sodium enolate of pinacolone forms an open stack structure in which two edges of the face-sharing cubes are absent. 

This Structural feature has been shown to be a necessary condition for electrical conductivity in these materials, although the mixed mode of stacking is generally considered to be the thermodynamically preferred one (Shaik 1982). Proof of the relative stability of the mixed and segregated stack motifs, and a recipe for obtaining crystals of the latter, came with the discovery of a pair of polymorphic 1 1 complexes of 6-II with 6-III (Bechgaard etal. 1977 Kistenmacher etal. 1982). The red, transparent, mixed-stack form of the complex is a semiconductor, while the black, opaque structure with segregated stacks is a conductor (Fig. 6.2). 

Fig. 6.2 Views of the two polymorphic structures of 6-II 6-III. In both cases the view is on the plane of the tetracyanoquinodimethane molecule 6-II. (a) The red, transparent, mixed-stack complex, a semiconductor (b) the black opaque, segregated stack complex, a conductor. (From Bernstein 99 b, with permission.)...

Fig. 6.4 Packing diagrams of the two forms of the complex formed by 6-V and 6-VI. (a) a form. The vertical mixed stacks are along the vertical c crystallographic axis, (b) /3 form. The mixed donor/acceptor stacks are along the diagonal indicated. The structure also exhibits donor/donor and acceptor/acceptor interactions in a direction approximately perpendicular to that of the mixed stacks. Nitrogen, oxygen and chlorine atoms are identified in representative molecules. (After Bernstein 1999, with permission.)...

Le Cointe, M., Lemee-Cailleau, M. H., Cailleau, H. and Toudic, B. (1996). Structural aspects of the neutral-to-ionic transition in mixed stack charge-transfer complexes. 

The 5-thioxo-l,2-dithiole-3-thiolate anion forms charge-transfer complexes with 1,2-dithiolylium ions.293-294 The structure of the complex 183 has been reported. The low conductivity (10 12 ohm1) was explained by donor and acceptor molecules existing in mixed stacks. The conductivity of a series of analogous complexes was found to be much lower. 

The mixed stacks in pure EP-TCNQ are energetically not very different from a lattice with segregated stacks. The possibility of nearly equivalent minima for widely different structures is suggested naturally by the structural diversity of RP systems. 

PP-TCNQ could not be obtained as crystals suitable for a full structure but unit cell parameters are listed in Table 4. The structure of BP-TCNQ was determined and related to the TSE spectra With the exception of the folded butyl group and a slight torsion (2.3° and 4.3° respectively) of the C = (CN) groups, all the molecules are planar and there is effective it-it overlap of the TCNQ ions at 3.15(3) A. The complete structure and single crystal epr was also obtained for BP-TCNQF4, whose novel mixed stack in Fig. 9 clearly shows the face-to-face TCNQ"" dimers. 

The 5.10-dihydro-5-methyl-10-ethylphenazine (MEP) complex of TCNQ has been prepared but not characterized in detail. We anticipate the MEP-TCNQ structure and physical properties to resemble MjP-TCNQ and E2P-TCNQ. The 5.10-dihydro-5.10-diethylphenazine (E2P)-TCNQ complex s structure has been determined. The E2P moeities are folded along the N-N axis, as shown in Fig. 11 for MjP-TCNQ the two benzene halves have a dihedral angle of 12.7°. The mixed stack is again dimerized. Both ethyls are bent to one side of the average phenazine plane, a configuration that reinforces the dimerization or is reinforced by the dimerization. 

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