Synthesis, spectroscopic and Hirshfeld surface analysis and fluorescence studies of (2E,2′E)-3,3′-(1,4-phenylene)bis[1-(4-hydroxyphenyl)prop-2-en-1-one] N,N-dimethylformamide disolvate

In the bischalcone molecule, the central benzene and terminal hydroxyphenyl rings form a dihedral angle of 14.28 (11)° and the central C=C double bond adopts a trans conformation. In the crystal, the title molecule and solvate are linked by O—H⋯O hydrogen bonds.


Chemical context
The development of new fluorescent probes has attracted much attention because of their applications in a wide range of electronic and optoelectronic devices related to telecommunications, optical computing, optical storage and optical information processing. Fluorescence generally occurs when a fluorescent probe (fluorophore) resonantly absorbs electromagnetic radiation that promotes it to an excited electronic state; subsequent relaxation of the excited state results in the emission of light, in which a portion of the excitation energy is lost through heat or vibration, and the rest is emitted at longer wavelengths compared to the excitation radiation. For a given fluorophore, the fluorescence intensity is directly proportional to the intensity of the radiation received. Fluorophores can be identified and quantified on the basis of their excitation and emission properties. Different materials may exhibit different colours and intensities of fluorescence despite seeming identical when observed in daylight conditions. In recent years, chalcones have been used in the field of material science as non-linear optical devices (Raghavendra et al., 2017;Chandra Shekhara Shetty et al., 2017), photorefractive polymers (Sun et al., 1999), optical limiting (Shettigar et al., 2006a;Chandra Shekhara Shetty et al., 2016) and electrochemical sensing agents (Delavaux-Nicot et al., 2007). The ,-unsaturated ketone (C C-C O) moiety in the chalcone skeleton plays a vital role in its biological activities (Kumar et al., 2013a,b). Apart from these biological activities, the photophysical properties of chalcone derivatives have also attracted considerable attention from both chemists and physicists. In view of the above and as a part of our ongoing work on such molecules (Shettigar et al., 2006b;Tejkiran et al., 2016;Pramodh et al., 2018;Naveen et al., 2017), we herein report the synthesis, structure determination, Hirshfeld surface analysis and fluorescence properties of (2E,2 0 E)-3,3 0 -(1,4-phenylene)bis [1-(4-hydroxyphenyl)prop-2-en-1-one] N,N-dimethylformamide disolvate.

Structural commentary
The asymmetric unit of the title compound comprises of half of the bischalcone molecule, completed by inversion (symmetry operation 1 À x, 2 À y, Àz) and a DMF molecule (Fig. 1). The title compound crystallizes in the triclinic system with Z = 1 in space group P1. The bischalcone molecule is constructed from two individually planar rings (central benzene and terminal hydroxyphenyl rings) and a C C-C( O)-C enone bridge with the central C C double bond in a trans configuration. The hydroxyphenyl (C1-C6) and benzene (C10-C12/C10A-C12A) rings are almost parallel to each other, subtending a dihedral angle of 14.28 (11) . The enone fragment and its attached benzene ring are slightly twisted, as indicated by the torsion angles O1-C7-C8-C9 = À5.6 (4) and C1-C6-C7-O1 = 1.7 (4) . All bond lengths and angles of the titled compound are in normal ranges (Allen et al., 2002).

Supramolecular features
In the crystal, the components are linked by O2-H2BÁ Á ÁO3 i hydrogen bonds, which connect the DMF solvate molecules to both terminal 4-hydroxyphenyl rings of the main molecules ( Fig. 2, Table 1).

Figure 1
The molecular structure of the title compound, showing the atomlabelling scheme, with 40% probability displacement ellipsoids. Atoms labelled with the suffix A are generated by the symmetry operation 1 À x, 2 À y, Àz. (Wolff et al., 2012). The red spots on the d norm surface arise as a result of the short interatomic contact; the positive electrostatic potential (blue regions) over the surface indicate hydrogen-donor potential, whereas the hydrogen-bond acceptors are represented by negative electrostatic potential (red regions). The d norm surface plots and electrostatic potential of the title compound are shown in Fig. 3. The surface shows a red spot on the hydroxyl and carbonyl groups of the main molecule and solvate, respectively. This is a result of the O2-H2BÁ Á ÁO3 hydrogen bonds present in the structure (Fig. 4a). These observations are further confirmed by the respective electrostatic potential map in which the atoms involved in the formation of hydrogen bonds are seen as blue (hydrogen-bond donor) and red (hydrogen-bond acceptor) spots (Fig. 4b). The corresponding fingerprint plots (FP) for Hirshfeld surfaces show characteristic pseudosymmetry wings in the d e and d i diagonal axes in the overall 2D FP (Fig. 5a). HÁ Á ÁH contacts (i.e. dispersive forces) make the greatest percentage contribution to the Hirshfeld surface, followed by OÁ Á ÁH/HÁ Á ÁO and CÁ Á ÁH/HÁ Á ÁC contacts (Fig. 6). The HÁ Á ÁH contacts appear as the largest region on the fingerprint plot with a high concentration in the middle region, at d e = d i $ 1.2 Å with an overall contribution to the Hirshfeld surface of 54.0% (Fig. 5b). The reciprocal OÁ Á ÁH/HÁ Á ÁO interaction (26.4%) appears as two sharp symmetric spikes in the FP plot, which is characteristic of a strong hydrogenbonding interaction, at d e + d i ' 1.7 Å (Fig. 5c). Two symmetrical broad blunted wings corresponding to the CÁ Á ÁH/ HÁ Á ÁC interaction (with a 9.8% contribution) appear at d e + d i ' 3.0 Å (Fig. 5d). Analysis of the close contact on the d norm surface plot suggests that the CÁ Á ÁH/HÁ Á ÁC interaction might arise from weak C-HÁ Á Á and C-HÁ Á Áalkene interactions between the solvate and main molecules (Fig. 7).   The two-dimensional fingerprint plots for the title compound showing contributions from different contacts; the views on the right highlight the relevant surface patches associated with the specific contacts. d norm and electrostatic potential mapped on Hirshfeld surfaces to visualize the intermolecular contacts in the title compound. The molecule in the ball-and-stick model is in the same orientation as for the Hirshfeld surface and electrostatic potential plots.

Figure 6
Percentage contributions of the various intermolecular contacts contributing to the Hirshfeld surfaces of the title compound.

Solid-state fluorescence studies
A powder sample of the subject compound (0.72 mol) was heaped in the tray, covered with a quartz plate and was then fixed in the fluorescence spectrometer. The solid-state fluorescence properties were measured at the excitation wavelength ( ex ) of 4400 Å , which was selected from the absorption spectrum of the compound. The difference in the relative intensities of reflections between the sample and MgO powder was calibrated using diffusion reflections in a non-absorbed wavelength, in the present case this was 6500 Å . Finally, the fluorescence quantum yield (F f ) was determined by Wrighton's method and calculated according to the È f = j f /(Çj o À j) (Wrighton et al., 1974) where, j f is the fluorescence intensity of the sample, Ç the calibration factor, j 0 the back-scattered intensity of excitation light from a blank (here MgO) and j the back-scattered intensity of a loaded sample. The solid-state excitation and emission spectrum of the title compound ( ex at 4400 Å ) is shown in Fig. 8. The emission wavelength (blue line) appears at 5510 Å , which corresponds to yellow light. The solid-state fluorescence quantum yield (F f ) of the title compound is 0.18.

Synthesis and crystallization
A mixture of corresponding 4-hydroxyacetophenone 0.02 mol) and terephthaldialdehyde (0.01 mol) was dissolved in methanol (20 mL). A catalytic amount of NaOH was added to the solution dropwise with vigorous stirring. The reaction mixture was stirred for about 5-6 h at room temperature. The resultant crude product was filtered, washed successively with distilled water and recrystallized from acetone solution. Crystals suitable for X-ray diffraction studies were obtained by the slow evaporation technique using DMF as solvent. Yield: 85%, m.p. = 544-546 K. FT

Figure 7
d norm mapped on Hirshfeld surfaces to visualize the weak intermolecular C-HÁ Á Á and C-HÁ Á Áalkene interactions in the title compound.

Figure 8
Solid-state excitation and emission spectrum for the title compound and refined using a riding model with U iso (H) = 1.5U eq (Cmethyl) and 1.2U eq (C) for other H atoms. program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL2013 (Sheldrick, 2015); molecular graphics: SHELXL2013 (Sheldrick, 2015) and Mercury (Macrae et al., 2006); software used to prepare material for publication: SHELXL2013 (Sheldrick, 2015) 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.