Crystal structure of 4,4′-bis(4-bromophenyl)-1,1′,3,3′-tetrathiafulvalene

The molecule of the title compound has a C-shape, with C s molecular symmetry. The dihedral angle between the planes of the dithiol and phenyl rings is 8.35 (9)°. In the crystal, molecules form helical chains along [001], the shortest interactions being π⋯S contacts within the helices.


Structural commentary
The molecular structure of the title compound is illustrated in Fig. 1. The molecule has a C-shape with C s molecular symmetry and resides on the mirror plane passing through the central C1 C1(x, Ày + 3/2, z) bond [1.343 (7) Å ]. The C-S distances in the TTF moiety are in the range 1.729 (4)-1.778 (4) Å and correspond to reported values (CSD version 5.40, last update November 2018;Groom et al., 2016). The dihedral angle between the dithiol and phenyl rings is 8.35 (9) .

Hirshfeld surface analysis
CrystalExplorer17.5 (Wolff et al., 2012, Mackenzie et al., 2017 was used to generate the molecular Hirshfeld surface. The total d norm surface of the title compound is shown in Fig. 3 where the red spots correspond to the most significant interactions in the crystal. In the studied molecule, they include only weak C-HÁ Á Á interactions at distances that are slightly higher than the sum of van der Waals radii.

Frontier molecular orbital calculations
The highest occupied molecular orbital (HOMO) acts as an electron donor and the lowest unoccupied molecular orbital 1196 Rigin and Fonari C 18 H 10 Br 2 S 4 Acta Cryst. (2019). E75, 1195-1198 research communications Figure 1 A view of the molecular structure of the title compound with the atom labelling. Displacement ellipsoids are drawn at the 50% probability level. Suffix a corresponds to the symmetry operation x, Ày + 3 2 , z.

Figure 3
Hirshfeld surface mapped over d norm for the title compound in the range À0.1138 to 1.1257 a.u.

Figure 2
The crystal packing of the title compound.

Figure 5
Molecular orbital energy levels of the trans isomer of the title compound.

Figure 4
Molecular orbital energy levels of the title compound (cis isomer).
(LUMO) acts as an electron acceptor. A small HOMO-LUMO energy gap indicates a highly polarizable molecule and high chemical reactivity. Molecular orbital energy levels for the title compound were calculated with Gaussian 16W software (Frisch et al., 2016) using density functional theory (DFT) at the B3LYP/6-311+G(d,p) level of theory. The frontier orbitals of the title compound and its trans-isomer are shown in Figs. 4 and 5, respectively. The energy gap determines chemical hardness, chemical potential, electronegativity and the electrophilicity index. The orbital energy values for the title compound, its trans-isomer and unsubstituted TTF are summarized in Table 1. The conformation energy difference between the cis-and trans isomers is 1.6331 kJ mol À1 . For both isomers the energy gap is large; hence both molecules are considered to be hard materials and would be difficult to polarize. As seen from Table 1, the bromophenyl substituents reduce the HOMO-LUMO energy gap and therefore the unsubstituted TTF molecule would be even more difficult to polarize.

Crystallization
The single crystals of the title compound were obtained in attempt to co-crystallize it with tetracyanoquinodimethane (TCNQ) in a 1:1 molar ratio. A saturated solution of 4,4 0 -bis(4bromophenyl)-1,1 0 ,3,3 0 -tetrathiafulvalene (2 mg, Aldrich) in chloroform was mixed with a saturated solution of TCNQ (1 mg, Aldrich) in acetonitrile and left at room temperature. Red prismatic crystals suitable for the X-ray diffraction analysis were obtained after a week of slow evaporation.

Computing details
Data collection: APEX2 (Bruker, 2016); cell refinement: SAINT (Bruker, 2016); data reduction: SAINT (Bruker, 2016); program(s) used to solve structure: SHELXT (Sheldrick, 2015a); program(s) used to refine structure: SHELXL2018 (Sheldrick, 2015b); molecular graphics: OLEX2 (Dolomanov et al., 2009); software used to prepare material for publication: OLEX2 (Dolomanov et al., 2009). Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.