(E)-1-(Benzo[d][1,3]dioxol-5-yl)-3-([2,2′-bithiophen]-5-yl)prop-2-en-1-one: crystal structure, UV–Vis analysis and theoretical studies of a new π-conjugated chalcone

The essentially planar chalcone unit adopts an s-cis configuration with respect to the carbonyl group within the ethylenic bridge. In the crystal, weak C—H⋯π interactions connect the molecules into zigzag chains along the a-axis direction.


Chemical context
Chalcones are organic compounds composed of open-chain flavonoids in which the two aromatic rings are joined by a three-carbon ,-unsaturated carbonyl system (Zingales et al., 2016). Compounds with the chalcone backbone are becoming important in the design of new materials, employing donor-acceptor (D--A) bridge systems to further enhance their future development for optoelectronic applications. In principle, the intermolecular charge-transfer (ICT), HOMO-LUMO gap and optical properties can be tailored by attaching electron donors and acceptors of various electronic nature, assuring efficient DÁ Á ÁA interactions and planarization of the entire molecule (Bureš, 2014). The presence of long -conjugated systems in chalcones have been shown to turn them into chromophores whereby certain colours can be displayed as a result of absorbing light in the visible region . Electron-donating and accepting groups containing these chromophores have been examined for their applications in the field of material science. Additionally, the substitution of a phenyl group into a polythiophene compound stabilizes the conjugated -bond system and forms a smaller band-gap material for supercapacitor applications (Mei-Rong et al., 2014). As part of our ongoing studies utilizing thiophene-ring substituents with chalcone derivatives (Zainuri et al., 2017), we hereby report the synthesis, structural, UV-Vis, Hirshfeld surface and DFT analyses of the title compound, (I). ISSN 2056-9890

(3)
, whereas the corresponding DFT value is 179.6 . The slight difference is the result of the optimization being carried out in an isolated gaseous state whereas the experimental molecular structure could easily be affected by its normal environment (Zaini et al., 2018).
For the theoretical geometry optimization calculation, the starting geometries of the compound were taken from the single-crystal X-ray refinement data. The optimization of the molecular geometries leading to energy minima was achieved using DFT [Becke's non-local three parameter exchange and Lee-Yang-Parr's correlation functional (B3LYP)] with the 6-311++G(d,p) basis set as implemented in the Gaussian09W software package (Frisch et al., 2009). Selected bond lengths and angles for the experimental and theoretical (DFT) studies are compared in the supporting information; all values are within normal ranges (Allen et al., 1987).

Supramolecular features
In the crystal, molecules are linked in a head-to-tail manner via C3-H3AÁ Á ÁCg(Àx + 1, y À 1 2 , Àz + 1 2 ) interactions involving ring C (Table 1, Fig. 2), forming zigzag chains along the baxis direction. In the absence of any classical hydrogen bonds, this interaction stabilizes the crystal structure. The chains stack along the c-axis direction.

Hirshfeld surface analysis
Hirshfeld surface analysis was undertaken using Crystal Explorer 3.1 (Wolff et al., 2012) to investigate the molecular packing. HÁ Á ÁH interactions are the most important, contri-buting 31.1% to the overall crystal packing. In the fingerprint plot ( Fig. 3) they are seen as widely scattered points of high density due to the large hydrogen-atom content of the molecule, with d e + d i = 2.50 Å (d i and d e are the distances to the nearest atom inside and outside the surface; Shit et al., 2016). CÁ Á ÁH/HÁ Á ÁC contacts (16.7%) are indicated by a pair of peaks at d e + d i = 2.75 Å , while the HÁ Á ÁO/OÁ Á ÁH contacts (19.4%) are represented by a pair of short spikes at d e + d i = 2.60 Å . The significant contributions by HÁ Á ÁH, HÁ Á ÁC/CÁ Á ÁH and HÁ Á ÁO/OÁ Á ÁH interactions suggest that weak hydrogen bonding and van der Waals interactions do play relevant roles in the crystal packing (Hathwar et al., 2015). The surface mapped over shape-index reveals small changes in the surface shape, indicating the C-HÁ Á Á (Fig. 4)    Hirshfeld surface mapped over shape-index. spots indicate ring atoms of the molecule inside the surface (Chkirate et al., 2018).

UV-Vis and frontier molecular orbital analyses
The experimental UV-Vis absorption spectrum consists of one major band that lies in the visible region at 400 nm ( Fig. 5a), while the simulated value is observed at 422 nm ( Fig. 5b). The absorption maximum is assigned to the -* transitions that arise from the carbonyl group (C O) of the compound. The slight difference in wavelength is due to the fact that the experimental study is conducted in solution whereas the theoretical study is performed for a gaseous environment (Zainuri et al., 2018a). The strong cut-off wavelength for the experimental study is 455 nm (Fig. 5a) with an energy band gap of 2.73 eV.
The highest occupied molecular orbital (HOMO) acts as an electron donor and represents the ability to donate electrons while the lowest unoccupied molecular orbital (LUMO) acts as the electron acceptor, representing the ability to accept electrons (Balasubramani et al., 2018). The HOMO and LUMO electron-density plots were computed using the DFT/ B3LYP/6-311 G++(d,p) basis set. The E HOMO -E LUMO gap is calculated to be 3.18 eV. Generally, the value of the energy gap characterizes the chemical stability of the molecule (Zainuri et al., 2018b). As shown in Fig. 6, the charge densities are accumulated over the entire molecule for the HOMO and LUMO states. A large HOMO-LUMO energy gap defines it as a 'hard' molecule while a small one defines a 'soft' molecule (Bayar et al., 2018). Hard molecules are less polarizable than the soft ones as there is a need of higher energy for excitation (Balasubramani et al., 2018). The energy gap value in the title compound indicates good stability and a high chemical hardness.

Molecular electrostatic potentials
Molecular electrostatic potentials (MEP) are useful in investigating the relationship between the molecular structure and its physicochemical properties, visualizing the molecular size and shape, along with the charge distributions in molecules in terms of colour grading (Zainuri et al., 2018b). The MEP map (Fig. 7) was calculated at the B3LYP/6-311G++ (d,p) level of theory. The red-and blue-coloured regions indicate nucleophiles that are electron rich, and electrophile regions that are electron poor, respectively. The remaining white regions indicate neutral atoms. Information about intermolecular interactions within the compound can be obtained from these regions (Gunasekaran et al., 2008). In the title molecule, the reactive site, localized in the carbonyl group, is shown in red. It possesses the most negative potential and is thus the strongest repulsion site (electrophilic attack). The blue spots indicate the strongest attraction regions, which are occupied mostly by hydrogen atoms (Zaini et al., 2019).  Theoretical molecular electrostatic potential surface calculated at the DFT/B3LYP/6-311G++ (d,p) basis set level.

Figure 5
The (a) experimental and (b) calculated UV-Vis absorption spectra.

Synthesis and crystallization
A mixture of 3 0 ,4 0 -(methylenedioxy)acetophenone (0.5 mmol) and 2,2 0 -bithiophene-5-carboxaldehyde (0.5 mmol) was dissolved in methanol. A catalytic amount of NaOH was added dropwise with vigorous stirring. The reaction mixture was stirred for about 5 h at room temperature and then poured into ice-cold water. The resulting crude solid was collected by filtration. Single crystals were grown from an acetone solution by slow evaporation.