9-(Dicyanomethylidene)fluorene–tetrathiafulvalene (1/1)

The title compound, C16H8N2·C6H4S4, crystallizes with the fluorene derivative placed in a general position and two half tetrathiafulvalene (TTF) molecules, each completed to a whole molecule through an inversion center. The fluorene ring system is virtually planar (r.m.s. deviation from the mean plane = 0.027 Å) and the dicyano group is twisted from the fluorene plane by only 3.85 (12)°. The TTF molecules are also planar, and their central C=C bond lengths [1.351 (8) and 1.324 (7) Å] compare well with the same bond length in neutral TTF (ca 1.35 Å). These features indicate that no charge transfer occurs between molecules in the crystal; the compound should thus be considered a cocrystal rather than an organic complex. This is confirmed by the crystal structure, in which no significant stacking interactions are observed between molecules.

The title compound, C 16 H 8 N 2 ÁC 6 H 4 S 4 , crystallizes with the fluorene derivative placed in a general position and two half tetrathiafulvalene (TTF) molecules, each completed to a whole molecule through an inversion center. The fluorene ring system is virtually planar (r.m.s. deviation from the mean plane = 0.027 Å ) and the dicyano group is twisted from the fluorene plane by only 3.85 (12) . The TTF molecules are also planar, and their central C C bond lengths [1.351 (8) and 1.324 (7) Å ] compare well with the same bond length in neutral TTF (ca 1.35 Å ). These features indicate that no charge transfer occurs between molecules in the crystal; the compound should thus be considered a cocrystal rather than an organic complex. This is confirmed by the crystal structure, in which no significant stacking interactions are observed between molecules.

9-(Dicyanomethylidene)fluorene-tetrathiafulvalene (1/1) Amparo Salmerón-Valverde and Sylvain Bernès Comment
There is a vast literature dealing with the organic charge-transfer complexes based on the emblematic π-donor tetrathiafulvalene (TTF) and TTF derivatives. Generally, research in this field is carried out with the hope of obtaining organic materials exhibiting metallic conductivity. It is now known that two essential conditions are required for obtaining such conductivity: i) partial oxidation and reduction of the donor and acceptor molecules, respectively. The difference between the redox potentials of the molecules should be less than ca. 0.34 V (Saito & Ferraris, 1980); ii) molecules must be stacked in the solid state, forming one-dimensional or pseudo one-dimensional crystal structures. The mode of stacking and distances separating molecules along a stack must be suitable for charge-transfer (Wright, 1995). The title compound was formed by mixing TTF and a potential π * -acceptor molecule derived from fluorene, namely 9-(dicyanomethylene)fluorene (DCF hereafter). The X-ray structure of the resulting compound, TTF.DCF, shows that condition ii) is not present in the structure.
The asymmetric unit includes one DCF molecule, placed in a general position, and two half-TTF molecules, each close to an inversion center, generating the TTF.DCF chemical composition (Fig. 1). The DCF moiety is almost planar, with a r.m.s. deviation of 0.027 Å for the mean plane of the fluorene ring (13 C atoms). The dicyanomethylene plane is twisted by 3.85 (12)° from the fluorene ring, and the C═C bond length in this group, 1.352 (5) Å, is similar to those found in other dicyanomethylene derivatives (e.g. Andrew et al., 2010). The same is observed for TTF molecules, giving r.m.s. deviations of 0.037 and 0.020 Å for TTF-1 (S15···C19 and symmetry related atoms) and TTF-2 (S20···C24 and symmetry related atoms), respectively. The central C═C bond lengths are 1.351 (8) and 1.324 (7) Å, no longer that the same bond in neutral TTF, ca. 1.35 Å (Batsanov, 2006). These features indicate that molecules are not involved in charge-transfer in the solid state. This is fully confirmed with the crystal structure ( Fig. 2). TTF and DCF are segregated in different layers parallel to the (001) plane (Fig. 2,inset), the separation between planes being c/2 = 7.1 Å. In the TTF layers, molecules are arranged in a herringbone pattern, avoiding π-π interactions. In the DCF layers, two molecules related by inversion are parallel and the separation between mean-planes for each molecule is relatively short, 3.401 Å. However, DCF molecules are slipped along the stack, and the distance between the centroids of two inversion-related DCF is 3.834 (1) Å. Such an arrangement does not favor π-π interactions for this component.
Spectroscopic data (Salmerón-Valverde, 2008) are consistent with the observed crystal structure. In the solid state, the IR vibration of the cyano groups in TTF.DCF is not shifted with respect to the same vibration in pure DCF (2224 cm -1 ), while a significant shift is expected for an actual charge-transfer complex (Salmerón-Valverde et al., 2003). In the same way, the central C═C bond in TTF, which is known to be sensitive to charge-transfer, is also unaffected when the cocrystal TTF.DCF is formed (ν C═C : 1527 cm -1 ). In solution, no charge-transfer band is observed in the visible region for TTF.DCF, at any dilution in CH 3 CN.

Experimental
Solutions of DCF (7.8 mg, 0.034 mmol) in hot CH 3 CN (2.5 ml) and TTF (7 mg, 0.034 mmol) in CH 3 CN (1.8 ml) were mixed and transferred in a test tube (12 × 1.5 cm). Solvent was slowly evaporated in the dark, over 10 days. After all solvent had evaporated, most of the crystals collected on the wall of the test tube were starting components, which present characteristic colors: yellow for TTF and orange for DCF. However, few green crystals of TTF.DCF were produced, with an approximate yield of 25%.