Crystal structure, Hirshfeld surface analysis and DFT studies of (E)-1-(4-bromophenyl)-3-(3-fluorophenyl)prop-2-en-1-one

The title halogenated organic chalcone was prepared by a Claisen–Schmidt condensation reaction. A Hirshfeld surface analysis was carried out to reveal the percentage contributions of the intermolecular interactions. A theoretical study was performed using the density functional theory (DFT) at B3LYP with the 6–311 G++(d,p) basis set level to compare with the experimental results of the X-ray analysis and UV–vis absorption analysis in term of the geometrical parameters, HOMO-LUMO energy gap and charge distributions.

The asymmetric unit of the title halogenated chalcone derivative, C 15 H 10 BrFO, contains two independent molecules, both adopting an s-cis configuration with respect to the C O and C C bonds. In the crystal, centrosymmetrically related molecules are linked into dimers via intermolecular hydrogen bonds, forming rings with R 1 2 (6), R 2 2 (10) and R 2 2 (14) graph-set motifs. The dimers are further connected by C-HÁ Á ÁO interactions into chains parallel to [001]. A Hirshfeld surface analysis suggests that the most significant contribution to the crystal packing is by HÁ Á ÁH contacts (26.3%). Calculations performed on the optimized structure obtained using density functional theory (DFT) at B3LYP with the 6-311 G++(d,p) basis set reveal that the HOMO-LUMO energy gap is 4.12 eV, indicating the suitability of this crystal for optoelectronic and biological applications. The nucleophilic and electrophilic binding site regions are elucidated using the molecular electrostatic potential (MEP).

Chemical context
Chalcones are natural or synthetic compounds belonging to the flavonoid family (Di Carlo et al., 1999), consisting of openchain flavonoids in which the aromatic rings are linked by a three-carbon ,-unsaturated carbonyl system (Thanigaimani et al., 2015). Chalcone derivatives have attracted significant interest in the field of non-linear optics due to their excellent blue-light transmittance, good crystal stability, large nonlinear optical coefficients and relatively short cut-off wavelengths of transmittance (Goto et al., 1991;Patil et al., 2006a,b;Zhao et al., 2000). The presence of halogen substitutions results in alterations of the physicochemical properties and biological activities of organic compounds, without introducing much major steric change. As a result of this, many researchers have worked intensively on fluorine substitution to develop a wide range of biologically active materials (O'Hagan et al., 2008). As part of our studies in this area, fluoro and bromo substituents were introduced in the title compound and the resulting organic molecular crystal is reported herein in term of its structural stability, the percentage contributions of the various interactions to the crystal packing, and electronic charge transfer within the molecule.

Structural commentary
The asymmetric unit of the title compound [ Fig. 1(a)] contains two independent molecules (A and B) with different conformations: the fluorobenzene group in molecule A is rotated by ISSN 2056-9890 approximately 180 about the C9-C10 bond with respect to molecule B, the C9Á Á ÁC11-C12-F1 torsion angle formed by non-bonded atoms being 178.4 (3) and À177.0 (3) in molecules A and B, respectively. The optimized structure of the title compound was performed with the Gaussian 09W software package (Frisch et al., 2009) using the DFT method at the B3LYP/6-311 G++(d,p) level to provide information about the molecular geometry.

Supramolecular features
In the crystal packing of the compound, the B molecules are centrosymmetrically connected via intermolecular C15B-H15BÁ Á ÁO1B interactions, forming a ring with an R 2 2 (14) graph-set motif [ Table 1 (a) The molecular structure of the title compound with displacement ellipsoids drawn at the 50% probability level and (b) the optimized molecular structure of the title compound generated using the DFT method at the B3LYP/6-311 G++(d,p) level.

Figure 2
Crystal packing of the title compound showing C-HÁ Á ÁO hydrogen bonds (dotted lines). H atoms not involved in hydrogen bonding are omitted. The insets show the formation of (a) R 2 2 (14) ring motifs and (b) R 2 1 (6) and R 2 ecules into inversion dimers, forming two R 2 1 (6) and one R 2 2 (10) ring motifs. Finally, the C13A-H13AÁ Á ÁO1B interactions act as a bridge, linking the dimers into chains extending parallel to the c axis (Fig. 3).

Hirshfeld Surface analysis
Hirshfeld surface analysis provides the percentage contribution of the intermolecular interactions inside the unit-cell packing. The surface and the related two-dimensional fingerprint plots were generated with CrystalExplorer3.1 (Wolff et al., 2012). The d norm and d e surfaces are presented in Fig. 4(a) and Fig. 4(b), respectively. All C-HÁ Á ÁO contacts are recognized in the d norm mapped surface as deep-red depression areas showing the interaction between the neighbouring molecules [ Fig. 4(a)]. Further existence of these contacts can be visualized under the d e surfaces. The side view I (Fig. 4) shows that the A molecules may interact through C9A-H9AAÁ Á ÁO1A and C11A-H11AÁ Á ÁO1A interactions, resulting in the formation of three ring motifs. Meanwhile, side view II (Fig. 4) indicates that for B molecules only one ring motif is achieved through C15B-H15BÁ Á ÁO1B interactions. Two-dimensional fingerprint plots provide information about the major and minor percentage contribution of interatomic contacts in the compound. The blue colour refers to the frequency of occurrence of the (d i , d e ) pair and the grey colour is the outline of the full fingerprint (Ternavisk et al., 2014). The fingerprint plots (Fig. 5) show that the HÁ Á ÁH contacts clearly make the most significant contribution to the Hirshfeld surface (26.3%): there is one distinct spike with a d e + d i value approximately less than the sum of Van der Waals radii (2.4 Å ). In addition, CÁ Á ÁH/HÁ Á ÁC and OÁ Á ÁH/HÁ Á ÁO contacts contribute 21.2% and 8.3%, respectively, to the Hirshfeld surface. In particular, the OÁ Á ÁH/HÁ Á ÁO contacts indicate the Partial crystal packing of the title compound viewed approximately down the a axis showing the formation of a molecular chain parallel to the c axis by C-HÁ Á ÁO interactions (dotted lines).

Figure 4
Hirshfeld surfaces of the title compound mapped over d norm and d e . presence of intermolecular C-HÁ Á ÁO interactions where the distance is shorter than the sum of d e + d i ($2.32 Å ).

Frontier molecular orbital and UV-vis Analyses
Frontier molecular orbital analysis is an important tool in quantum chemistry for studying the molecular electronic charge mobility from the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). The HOMO-LUMO separation confirms the energy gap of the compound where it is responsible for the ICT (intramolecular charge transfer) from the end-capping electron-donor groups to the efficient electron-acceptor groups through the -conjugated path. The electron-density plots of the HOMO and LUMO for the title compound were calculated using density functional theory (DFT) at the B3LYP/6-311 G++(d,p) level. As seen from the orbital plots ( Fig. 6), both HOMO and LUMO extend mainly over the entire molecule, but the molecular orbital localization differs. This can be seen specifically at the enone moiety where the orbital accumulates around the carbon-carbon double bond at the HOMO state whereas it is localized at the carbon-carbon single bond at the LUMO state, indicating conjugation within the molecule. The calculated energy gap, E LUMO -E HOMO , is 4.12 eV. The experimental UV-vis absorption spectrum consists of one major band (Fig. 7) occurring in the visible region at 304 nm which was assigned to the -* transition. This sharp peak was expected to arise from the carbonyl group of the chalcone . From the UV-vis absorption edge, the calculated energy band-gap value is 3.60 eV, which is similar to that found in a previous study of a related chalcone (Zaini et al., 2018).

Molecular electrostatic potential
The molecular electrostatic potential (MEP) is useful in depicting the molecular size and shape as well as in visualizing the charge distributions of molecules. The MEP map (Fig. 8) of the title compounds was calculated theoretically at the DFT/B3LYP/6-311 G++(d,p) level of theory. The colour grading in the plot represents the electrostatic potential regions in which the red-coloured region is nucleophile and electron rich, the blue colour indicates the electron-poor electrophile region and the white region indicates neutral atoms. These sites provide information about where the intermolecular interactions are involved within the molecule (Gunasekaran et al., 2008). The reactive sites are found near the carbonyl group: the region is represented in red and possesses the most negative potential spots. This nucleophile site (negative potential value of À0.04713 a.u.) is distributed around the oxygen atom due to the intermolecular C-HÁ Á ÁO interactions; in the molecular structure it indicates the strongest repulsion site (electrophilic attack), whereas the strongest attraction regions (nucleophilic attack) portrayed by the blue spots are localized on the hydrogen atoms. The molecular electrostatic potential surface of the title compound calculated at the DFT/B3LYP/6-311 G++(d,p) level.

Figure 6
Molecular orbitals showing the HOMO-LUMO electronic transitions in the title compound.

Figure 7
The UV-vis absorption spectrum of the title compound.

Synthesis and crystallization
The title compound was prepared by a standard Claisen-Schmidt condensation reaction at room temperature. A mixture of 4-bromoacetophenone (0.5 mmol) and 3-fluorobenzaldehyde (0.5 mmol) was dissolved in methanol (20 ml) and the solution stirred continuously. A catalytic amount of NaOH (5 ml, 20%) was added to the solution dropwise until a precipitate formed and the reaction was stirred continuously for about 5 h. After stirring, the solution was poured into 60 ml of ice-cold distilled water. The resultant crude product was filtered and washed successively with distilled water until the filtrate turned colourless. The dried precipitate was further recrystallized to obtain the desired chalcone. Crystals suitable for X-ray diffraction analysis were formed by slow evaporation of an acetone solution.

Refinement
Details of the crystal data collection and structure refinement are summarized in Table 2. All C-bound H atoms were positioned geometrically (C-H = 0.930 Å ) and refined using a riding model with U iso (H) = 1.2U eq (C). One outlier (311) was omitted in the last cycles of refinement.  Computer programs: APEX2 and SAINT (Bruker, 2009) and SHELXTL (Sheldrick, 2008) and PLATON (Spek, 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. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > 2sigma(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.