Mixed stacks

A number of other donor—acceptor molecular soHds such as TMPD -TCNQ or TMPD chloraniF crystalline as mixed stacks of alternating D and A molecules. These compounds typically have much higher resistivities than the segregated salts because the alternating -DADA- sequence leaves no continuous channel for conduction. 

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. 

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. 

Some semiconducting organic CT complexes of mixed-stack architecture exhibit the rather unusual neutral-to-ionic (N-I) phase transition upon variation of an external variable of parameter hyperspace, such as P or r. The transition manifests itself by a change of q and a dimerization distortion with the formation of donor-acceptor dimers along the stacking axis in the I-phase. 

TTF-CA more ionic, increasing q up to 0.7. The space group of the I-phase is Pn with two equivalent donor-acceptor dimers related by a glide plane with a ferroelectric arrangement (see Fig. 6.33(b)). Further examples of mixed-stack organic CT materials exhibiting N-I transitions are tetramethylbenzidine-TCNQ (Tn-i — 205 K) (Iwasa et al, 1990) and DMTTF-CA (Tn-i 65 K) (Aoki et al, 1993). 

Iwasa Y, Koda T, Koshihara S, Tokura Y, Iwasawa N, Saito G (1989) Intrinsic negative-resistance effect in mixed-stack charge-transfer crystals. Phys Rev B39 10441-10444... 

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...

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. 

The adduct consists of quasi-hexagonal-packed mixed stacks of alternating donor and acceptor molecules3. 

The absolute values of the Madelung energies Em for naphthalene TCNE and hexamethylbenzene p-chloranil are smaller than the cost of ionizing the lattice Iu Aa, so they are predicted by Eq. (12.2.8) to be complexes of almost neutral constituents, in agreement with experiment Em for TMPD TCNQ and TMPD chloranil are larger than fD — Aa, so they are predicted by Eq. (12.2.7) to be complexes of almost fully ionic constituents, in agreement with experiment these results are for mixed-stack crystals, where the D and A species are stacked atop each other, with large intermo-lecular overlap [13]. 

Planar molecules D or A with delocalized n molecular orbitals are best. Molecular components should be of appropriate or compatible size. Onedimensional metals need segregated stacks of radicals, and not mixed stacks. 

Fig. 6.1 Schematic diagram of the mixed-stack and segregated stack motifs for packing of n molecular charge-transfer complexes. (From Bernstein 99 b, with permission.)...

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.)...

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