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Crystal structures for the electron donor DBTTF and its charge-transfer salts with the acceptors TCNQ and 2,5-TCNQF2 are reported. Crystal data for the three systems are as follows: (a) neutral DBTTF; monoclinic, space group P21/c, a=12.082(2) Å, b=3.955(1) Å, c=14.553(2) Å, β=114.36(1)0, V=633.6 Å3; (b) DBTTF-TCNQ; triclinic, space group P1, a=9.215(3) Å, b=10.644(4) Å, c=7.734(2) Å, α=113.32(3)0, β=122.28(2)0, γ=67.66(3)0, V=574.6 Å3; (c) DBTTF-2,5-TCNQF2; triclinic, space group P1, a=8.838(2) Å, b=9.276(2) Å, c=7.794(1) Å, α=101.29(2)0, β=100.89(1)0, γ =107.42(2)0, V=576.6 Å3. The crystal structure of the neutral DBTTF donor is dominated by columns of DBTTF molecules as found in the similar structure of the donor TTF. The crystalline motifs adopted by the TCNQ and 2,5-TCNQF2 salts of DBTTF are composed of columns in which the donor and acceptor molecules alternate. These … DADADA … stacked columns are tied into sheets through weaker donor-donor overlapping. While the structures of the TCNQ and 2,5-TCNQF2 salts are very similar, they differ in the way sheets of donor-coupled … DADADA … stacks are arranged relative to each other. Madelung energies for both salts have been computed from charge distributions obtained from LCAO-MO calculations. Degrees of charge transfer are evaluated from the molecular geometries of both the donor and acceptor molecules and from the cyano stretching frequencies of the acceptors.

The crystal structure of the asymmetric electron acceptor TCNQF has been determined by X-ray diffraction techniques. The crystals are monoclinic, space group P21/n, with the following primary crystallographic data: a = 7.596(3)A, b = 8.204(4)A, c = 8.428(2)A, β = 90.90(3)d`, V = 525.3(3)A3, Z = 2, Dmeasd = 1.42(2) g cm-3, Dcalcd = 1.405 g cm-3. The presence of two molecules per cell requires 1(C1) molecular symmetry, inconsistent with the molecular structure of TCNQF. A centrosymmetrical twin model has been developed and refined by the full-matrix least-squares method based on counter-collected intensities. The dominant intermolecular interaction in the crystal is the one-dimension coupling of TCNQF molecules through their electric dipole moments. An estimation from an LCAO-MO calculation yields a molecular dipole moment of 2.0 D for TCNQF.

The twinned crystal structure of the charge-transfer complex TTF-2,5-TCNQF2 has been determined by single-crystal X-ray diffraction techniques. From intensity measurement on one twin component, a full structural model has been developed. The model encompasses segregated stacks of TTF donors and 2,5-TCNQF2 acceptors, with each chain exhibiting significant dimerization. The alternate interplanar spacings in the TTF stack are 3.34A and 3.60A, while those in the 2,5-TCNQF2 stack are 3.23A and 3.55A. Nearly eclipsed molecular overlap patterns are found within each of the dimerized pairs. The Madelung energy of TTF-2,5-TCNQF2 has been calculated at - 2.65 eV per donor/acceptor pair for full charge transfer, utilizing partial atomic charges obtained from CNDO/INDO molecular orbital calculations. Full charge transfer is considered appropriate for this complex based primarily on the observed component molecular geometries. The dimerized structural motif observed at room temperature for the TTF-2,5-TCNQF2 system may reflect a phase transition at a higher temperature, owing to either an electronic-or a spin-Peierls instability. Diffraction data to temperatures on the order of 390 K do not indicate a solid-state phase transformation before crystal decomposition.

The crystal structure of the electron acceptor 2,5-difluoro-7,7,8,8-tetracyano-p-quinodimethane, 2,5-TCNQF2, has been determined by single-crystal X-ray diffraction methods. The layered crystal structure is dominated by the antiparallel coupling of cyano bond moments of symmetry-related molecules. The molecular and electronic structure (INDO approximation) of 2,5-TCNQF2 are extensively compared to that of its parent TCNQ and the perfluoro analog TCNQF4. Crystal data for 2,5-TCNQF2 are as follow: monoclinic, space group C2/m, a=10.208(4) Å, b=6.026(2) Å, c=8.836(3) Å,β=106.64(3)d`, V=520.8 Å3. The site symmetry (2/m;C2h) in the crystal is identical to that for the free 2,5-TCNQF2 molecule

The development of the organic metallic state is traced from our original synthesis and study of TTF-TCNQ to the new conductor HMTSF-TCNQ, the first organic substance which remains metallic as T → 0.