Crystal structures of three 1-[4-(4-bromobutoxy)phenyl] chalcone derivatives: (E)-1-[4-(4-bromobutoxy)phenyl]-3-phenylprop-2-en-1-one, (E)-1-[4-(4-bromobutoxy)phenyl]-3-(4-methoxyphenyl)prop-2-en-1-one and (E)-1-[4-(4-bromobutoxy)phenyl]-3-(3,4-dimethoxyphenyl)prop-2-en-1-one

Molecules (I) and (II) are nearly planar, while molecule (III) is not planar. In compounds (I) and (II), molecules are linked into chain by C—H⋯π interactions. In compound (III), molecules are linked by a pair of C—H⋯O hydrogen bonds, forming inversion dimers. Weak C—Br⋯π interactions are present in (III).


Chemical context
Chalcones are 1,3-diphenyl-2-propene-1-one derivatives, in which two aromatic rings are linked by a three carbon ,unsaturated carbonyl system. In these materials, the C O bond acts as an electron-withdrawing group, and electron-rich substituents in the aromatic rings serve as electron-donating groups, forming a so-called D-Á Á ÁA type molecule. When the electron-rich groups are located on the 4 and/or 4 0 positions, the electron flow follows a Ã-shaped path, and therefore the molecule is called a Ã-shaped molecule (Devia et al., 1999).
Chalcone compounds are widely used in organic solid photochemistry (Goud et al., 1995). Chalcone derivatives show non-linear optical (NLO) properties with excellent blue light transmittance and good crystallizability (Shettigar et al., 2006). The substitution of bromine to o-nitro aniline increases its SHG conversion efficiency substantially and is matter of interest in research (Bappaliage et al., 2010). In chalcones, the presence of a bromo substituent is useful to obtain good quality single crystals (Prabhu et al., 2013). The transparency and the thermal stability of the materials can be improved when the compounds are substituted with a bromo group (Zhao et al., 2000). Chalcone derivatives with p-methoxyphenyl groups possess first order hyperpolarizability and good optical transparency for non-linear optical applications (Muhammad et al., 2016). In view of the importance of methoxy-and bromo-substituted butoxy side chains in chalcones, the crystal structures of the three title chalcones were determined and analysed.

Structural commentary
The molecular structures of the title compounds (I), (II) and (III) are shown in Figs. 1, 2 and 3, respectively. All three molecules contain a chalcone unit consisting of two phenyl rings (ring A: C5-C10; ring B: C14-C19) connected by an enone moiety with a bromobutoxy side chain attached at the 4-position of one of the phenyl rings. In molecule (I), no other substitution is present, in molecule (II) a methoxy side chain is attached to ring B at the 4-position and in molecule (III), two methoxy side chains are attached at the 3-and 4-positions of The molecular structure of compound (I), with displacement ellipsoids drawn at the 50% probability level. H atoms are shown as small sphere of arbitrary radius.

Figure 2
The molecular structure of compound (II), with displacement ellipsoids drawn at the 50% probability level. H atoms are shown as small sphere of arbitrary radius.
In compounds (I) and (II), the shortest distances between parallel C C double bonds are 4.2059 (16) and 4.2881 (18) Å , which are close to the reference value of 4.2 Å for a photo-reactive crystal (Turowska-Tyrk et al., 2003). In compound (III), the shortest distance between neighbouring ethylenic double bonds is 4.6818 (16) Å , indicating that these crystals might be photo inert.

Supramolecular features
The packing for molecules (I), (II) and (III) is shown in Figs. 4, 5 and 6, respectively. In the absence of strong hydrogen-bond donors in compounds (I) and (II), the crystal packing is stabilized by weak intermolecular interactions (Nishio et al., 1995). The involvement of the benzene rings, which are a reservoir of charges in the C-HÁ Á Á interaction, leads to intermolecular conjugation (Patil et al., 2013) and plays an important role in controlling the stereoselectivity of the organic reactions (Nishio et al., 2005). The C-HÁ Á Á interaction in compound (I) involves the C2 carbon atom via atom H2A of ring A and the centroid of ring B of a symmetryrelated molecule (Table 1), forming chains parallel to the c axis. In compound (II), molecules are linked into chains parallel to the c axis by two C-HÁ Á Á interactions involving the C2 and C3 carbon atoms via atoms H2B and H3A of ring A and the centroid of ring B of two symmetry-related molecules ( Table 2).
In compound (III), inversion-related molecules are linked into dimers through pairs of intermolecular hydrogen bonds involving the C10 carbon atom of ring A via atom H10 and the O3 oxygen atom (Table 3). In addition, a non-covalent C-  Crystal packing of the compound (I), viewed down the a axis.

Figure 5
Crystal packing of the compound (II), viewed down the a axis.

Figure 6
Crystal packing of the compound (III), viewed down the c axis. Hydrogen atoms not involved in hydrogen bonding (dashed lines) are omitted. Table 1 Hydrogen-bond geometry (Å , ) for (I).
Cg is the centroid of the C14-C19 ring.

Database survey
A search of the Cambridge Structural Database (Version 5.36, last update May 2015; Groom et al., 2016) revealed that the number of compounds based on the chemical unit of chalcone yielded 2168 hits. This involved some compounds with ring closure at the C C bridge. Avoiding these, the search for the basic unit with two phenyl rings joined by an enone moiety of the title compounds yielded 604 hits. The search for a methoxy substitution on one of the phenyl rings of the basic unit gave 124 hits. Extending the search to bromomethoxy, bromoethoxy, bromopropiloxy and bromobutoxy side chains on the other phenyl ring at the 4-position yielded no hits.
Mixtures of chalcone (1 equiv.), 1,4-dibromobutane (1.2 equiv.) and anhydrous potassium carbonate (2 equiv.) in dry acetone (40 mL) were then stirred at 333 K for 12 h. After completion of reactions, the solvents were evaporated under reduced pressure and the residues extracted with CH 2 Cl 2 (3 Â 100 ml). The organic layers were separated, washed with brine (1 Â 150 ml), dried over anhydrous Na 2 SO 4 and evaporated to give the crude bromo compounds, which were purified by column chromatography (SiO 2 ) using a mixture of hexane/ CHCl 3 (9:2 v/v) as eluent to afford yellow solids. The compounds were recrystallized by slow evaporation of chloroform solutions.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 4. For all compounds, H atoms were localized in difference-Fourier maps and were constrained geometrically with C-H = 0.93, 0.96 and 0.97 Å for aryl, methyl and methylene H atoms, respectively. The U iso (H) values were set to 1.2U eq (C) or 1.5U eq (C) for methyl H atoms.  Computer programs: APEX2 and SAINT (Bruker, 2004), SHELXS97 (Sheldrick, 2008), SHELXL97 (Sheldrick, 2008), ORTEP-3 for Windows (Farrugia, 2012), PLATON (Spek, 2009) and publCIF (Westrip, 2010  For all structures, data collection: APEX2 (Bruker, 2004); cell refinement: SAINT (Bruker, 2004); data reduction: SAINT (Bruker, 2004); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXS97 (Sheldrick, 2008); molecular graphics: ORTEP-3 for Windows (Farrugia, 2012) and PLATON (Spek, 2009); software used to prepare material for publication: publCIF (Westrip, 2010). 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.

(E)-1-[4-(4-Bromobutoxy)phenyl]-3-(4-methoxyphenyl)prop-2-en-1-one (II)
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.

Hydrogen-bond geometry (Å, º)
Cg is the centroid of the C14-C19 ring.  where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.002 Δρ max = 0.25 e Å −3 Δρ min = −0.60 e Å −3 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.