Crystal structure and Hirshfeld surface analysis of (2E,2′E)-3,3′-(1,4-phenylene)bis[1-(2,4-difluorophenyl)prop-2-en-1-one]

The asymmetric unit of the title compound consists of one and a half bischalcone molecules. In the crystal, molecules are linked into a three-dimensional network by C—H⋯F and C—H⋯O hydrogen bonds, some of the C—H⋯F links being unusually short (< 2.20 Å). Hirshfeld surface analyses are presented and discussed.


Chemical context
Chalcones, considered to be the precursors of flavonoids and isoflavonoids, are abundant in edible plants. They consist of two aromatic rings joined by a three-carbon-atom unsaturated carbonyl system (-CH CH-CO-). Bischalcones with the general formula Ar-CH CH-CO-CH CH-Ar (Baeyer & Villiger, 1902) are an important class of compounds that are widely used in many fields such as organic solid-state photochemistry, and display anti-oxidative and anti-inflammatory activities, cytotoxicity, non-linear optical activity (Uchida et al., 1998) and fluorescence and luminescent properties (Tay et al., 2016). Several crystal structures of this type of compound have been reported (Fun et al., 2010;Park et al., 2013;Ruanwas et al., 2014;Sim et al., 2017). As part of our studies in this area, we report herein the syntheses and structure of the title compound, C 24 H 14 F 4 O 2 , (I), and a Hirshfeld analysis of its intermolecular interactions. ISSN 2056-9890

Supramolecular features
In the crystal of (I), the C11B-H11BÁ Á ÁO1A hydrogen bonds (Table 1) generate R 2 2 (12) and R 2 3 (23) graph-set motifs with the C5A-H5AÁ Á ÁO1B and C2B-H2BÁ Á ÁF3A hydrogen bonds ( Table 2). As the central benzene ring of molecule B is located about an inversion center, pairs of these hydrogen bonds link the molecules into a centrosymmetric trimer (Fig. 2, Table 2). Atom F2A acts as double acceptor and links the trimers into a three-dimensional network via C2A-H2AÁ Á ÁF2A and C23A-H23AÁ Á ÁF2A hydrogen bonds, as shown in Fig. 3. Table 1 Selected torsion and dihedral angles ( ) for the title compound.
The dihedral angle is between the mean planes of the terminal 2,4-difluorophenyl rings and the central benzene ring.

Figure 3
The packing of (I) shown in projection down the a axis.

Figure 2
The partial packing of (I), showing a centrosymmetric trimer.

Hirshfeld surface analysis
The Hirshfeld surface analyses (McKinnon et al., 2004) of (I) were generated by CrystalExplorer 3.1 (Wolff et al., 2012), and can be summarized by fingerprint plots mapped over d norm .
The contact distances to the closest atom inside (d i ) and outside (d e ) of the Hirshfeld surface analyze the intermolecular interaction via the mapping of d norm . In a d norm surface, any intermolecular interactions will appear as a red spot.
Dark-red spots that are close to atoms O1B, H11B and H2BA in the d norm surface mapping are the result of C-HÁ Á ÁO and C-HÁ Á ÁF hydrogen bonds (Fig. 4a). Similarly, the C-HÁ Á ÁF interactions are identified by red spots near the F2A atom in molecule A (Fig. 4b). As illustrated in Fig. 5, the corresponding fingerprint plots (FP) for Hirshfeld surfaces of the title compound are shown with characteristic pseudosymmetry wings in the d e and d i diagonal axes represent the overall two-dimensional FP and those delineated into FÁ Á ÁH/ HÁ Á ÁF, HÁ Á ÁH and OÁ Á ÁH/HÁ Á ÁO contacts, respectively. The most significant intermolecular interactions are the reciprocal FÁ Á ÁH/HÁ Á ÁF interactions (30.1%), which appear as two sharp symmetric spikes in FP maps with a prominent long spike at d e + d i ' 2.3 Å (Fig. 5b). The HÁ Á ÁH interactions appear in the central region of the FP with d e = d i ' 2.4 Å and contribute 29.0% to the Hirshfeld surface ( Fig. 5c) whereas two symmetrical narrow pointed wings corresponding to the OÁ Á ÁH/HÁ Á ÁO interactions with 12.7% contribution appear at diagonal axes of d e + d i ' 2.4 Å (Fig. 5d). The percentage contributions for other intermolecular contacts are less than 10% in the Hirshfeld surface mapping (Fig. 6).  The two-dimensional fingerprint plots for (I) showing contributions from different contacts; views on the right highlight the relevant surface patches associated with the specific contacts. Table 2 Hydrogen-bond geometry (Å , ). Symmetry codes: (i) x; y À 1; z; (ii) x; y þ 1; z; (iii) Àx þ 1; y þ 1 2 ; Àz þ 3 2 ; (iv) Àx þ 1; Ày; Àz þ 1; (v) x À 1; Ày þ 3 2 ; z À 1 2 .

Synthesis and crystallization
A solution of terephthaldialdehyde (0.01 mol) in methanol (20 ml) was mixed with 2,4-difluoroacetophenone (0.02 mol) in methanol (20 ml) in the presence of NaOH. The reaction mixtures were stirred for about 5-6 h at room temperature. The resultant crude products were filtered, washed successively with distilled water and recrystallized from ethanol solution to get the title compound. Yellow blocks of (I) were obtained by slow evaporation using acetone as solvent. (

(2E,2′E)-3,3′-(1,4-Phenylene)bis[1-(2,4-difluorophenyl)prop-2-en-1-one]
Crystal data 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.