Dimerized donor-acceptor crystals

In this paper, illustrations of some peculiar aspects of the spectroscopy of CT crystals have been given with reference to simple dimeric models. Quantitative applications of such models are only appropriate for systems with localized electron states, such as insulating ion radical salts and donor-acceptor crystals. Analysis of spectroscopic data pertaining to these systems allows an accurate determination of some fundamental interaction parameters. 

In another theoretical study on metal-metal interactions in indium(I) and thallium(I) cyclopentadienyls (178), the authors interpret the short M—M contacts in the above dimers as weak donor-acceptor interactions, similar to the Sn—Sn bond in dimeric stannylenes (12) (163,175). However, they also attribute the general structural arrangement as being determined by crystal packing forces. ... 

The most efficient organic conductor material consists of co-crystals of tetracyano-Jt-quinodimethane, an electron-poor quinone analog, and tetrathia-fulvalen, an extremely potent electron donor. The crystals are green and have a conductivity of o = 1.5 x 10 Siemens cm Vat 66 K as compared to metallic copper with a a = 6 x 10 Siemens cm at 298 K. In order to obtain such high conductivity, organic charge transfer complexes must not appear as face-to-face dimers in crystals. In such cases, the acceptor takes up an electron and... 

For dilute solutions of essentially independent donor and acceptor molecules the Forster or resonance interaction is quite important in molecular aggregrates and in molecular crystals exciton interactions are likely to be important. When the interaction is strong the excitation is not localized on the donor or acceptor but is spread over both. If larger aggregrates are involved, the excitation can be spread over many molecules/33-3 This can easily be seen for the case of a dimer where the donor and acceptor are... 

The tautomer 82c of 3-methylimidazole, however, was found in the 1 1 complex with rac-17. X-ray structure analysis of the above inclusion complex showed that molecules of 82c act as hydrogen-bond donors and acceptors between two dimeric assemblies of binaphthyl molecules (Scheme 2). Methyl groups are located in the vicinity of the dimeric host. However, steric hindrance of this methyl group is less important for the energetics of crystal construction than formation of two hydrogen bonds. 

Most of the early organic conductors were based on the TCNQ acceptor molecule [1]. Many of these are radical-ion salts having nonuniform stacks composed of dimers, trimers, or tetramers and are thus semiconductors at room temperature (see, e.g., Ref. 8 or Chapter 8 in this volume). From the crystal structures and molecular arrangements [8], it is clear that for the vast majority of these salts, this nonuniformity is driven by Coulomb attraction between the positive donor ions and the ir-electron density on the TCNQ molecules, which is larger in the regions where they are more closely spaced. A few materials with uniform stacks were known before 1972, such as Qn(TCNQ)2 and TTF Br0 7, and these were the best organic conductors known until the discovery of TTF-TCNQ, but their conductivity was limited by intrinsic disorder. 

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