Crystal structure and optical spectroscopic analyses of (E)-3-(1H-indol-2-yl)-1-(4-nitrophenyl)prop-2-en-1-one hemihydrate

The title molecule adopts an s-cis configuration with respect to the C=O and C=C bonds. The dihedral angle between the indole ring system and the nitro-substituted benzene ring is 37.64 (16)°. In the crystal, molecules are linked by O—-H⋯O and N—H⋯O hydrogen bonds, forming chains along [010]. In addition, weak C—H⋯O, C—H⋯π and π–π interactions further link the structure into a three-dimensional network.

The asymmetric unit of the title compound, 2C 17 H 12 N 2 O 3 ÁH 2 O comprises two molecules of (E)-3-(1H-indol-2-yl)-1-(4-nitrophenyl)prop-2-en-1-one and a water molecule. The main molecule adopts an s-cis configuration with respect to the C O and C C bonds. The dihedral angle between the indole ring system and the nitro-substituted benzene ring is 37.64 (16) . In the crystal, molecules are linked by O--HÁ Á ÁO and N-HÁ Á ÁO hydrogen bonds, forming chains along [010]. In addition, weak C-HÁ Á ÁO, C-HÁ Á Á andinteractions further link the structure into a three-dimensional network. The optimized structure was generated theoretically via a density functional theory (DFT) approach at the B3LYP/6-311 G++(d,p) basis level and the HOMO-LUMO behaviour was elucidated to determine the energy gap. The obtained values of 2.70 eV (experimental) and 2.80 eV (DFT) are desirable for optoelectronic applications. The intermolecular interactions were quantified and analysed using Hirshfeld surface analysis.

Chemical context
Chalcone compounds consist of open-chain flavanoids in which two aromatic rings are joined by a three carbon ,unsaturated carbonyl system (Thanigaimani et al., 2015). The design of the chalcone system such as donor--acceptor (D--A) plays a significant role in intramolecular charge-transfer transitions (ICT) in which optical excitation leads to the movement of charge from the donor group to the acceptor group. In addition, the chalcone bridge consists of two different double bonds, C C and C O, which contribute to the conjugation of charge transfer, leading to their excellent structural and spectroscopic properties (de Toledo et al., 2018). Furthermore, the non-linear optical (NLO) properties of chalcone molecules originate mainly from a strong donoracceptor intramolecular interaction and delocalization of the -electrons (Prabhu et al., 2015). Many researchers are currently investigating the nitro (NO 2 ) group as an acceptor group because the decrease of the resonance effect leads to substantial changes in -electron delocalization in the ring (Dobrowolski et al., 2009). In this work, the title chalcone compound was successfully synthesized and its crystal structure is reported herein.

Structural commentary
The molecular structure of the title compound is shown in Fig. 1a. The structure was optimized with the Gaussian09W software package using the DFT method at the B3LYP/6-311G++(d,p) level, providing information about the geometry of the molecule. The optimized structure is shown in Fig. 1b. The geometrical parameters are mostly within normal ranges, the slight deviations from the experimental values are due to the fact that the optimization is performed in isolated conditions, whereas the crystal environment and hydrogen-bonding interactions affect the results of the X-ray structure (Zainuri et al., 2017).
In the title compound, the enone group (O1/C9-C11) adopts an s-cis configuration with respect to the C11 O1 [1.209 (4) Å ] and C9 C10 [1.310 (5) Å ] bonds. The compound is twisted about the C10-C11 bond with C9-C10-C11-O1 torsion angle of À21.9 (6) . The corresponding torsion angle obtained from the DFT study is 0.08 . In addition, the molecule is twisted about the C11-C12 bond with an O1-C11-C12-C13 torsion angle of 167.7 (4) (calculated value 179.4 ). The differences between the experimental and calculated values show that the intermolecular hydrogen bond involving the water molecule does not affect the planarity of the compound. A previous study (Zheng et al., 2016) reported that the intermolecular hydrogen bond present in the optimized structure stabilizes both the main molecule and the water molecule, which is why we claim that the hydrogen bond does affect the planar conformation in our optimized structure. In the experimental structure, a weak intermolecular hydrogen bond involving an O atom of the nitro group (Table 1) may be responsible for the distortion from planarity of the molecule. Furthermore, the twisted nature of this part of the molecule might also be expected because of the steric effects between the carbonyl group and the nitro-substituted benzene ring (Kozlowski et al., 2007).

Supramolecular features
In the crystal, four symmetry-related molecules are connected to each other via O-HÁ Á ÁO and N-HÁ Á ÁO hydrogen bonds involving the solvent water molecule. The water molecule is connected to the carbonyl group and indole ring system by intermolecular O1W-H1OWÁ Á ÁO1 i and N1-H1AÁ Á ÁO1W hydrogen bonds (Table 1), forming chains extending along the b-axis direction (Fig. 2). In addition, weak C4-H4AÁ Á ÁO2 ii interactions (  Table 1 Hydrogen-bond geometry (Å , ).

Hirshfeld surface analysis
Analysis of the Hirshfeld surfaces provides a three-dimensional representation of intermolecular interactions. The Hirshfeld surfaces and related two-dimensional fingerprint (FP) plots were generated with CrystalExplorer3.1 (Wolff et al., 2012). In the FP plots, d i and d e are the distances from the Hirshfeld surface to the nearest atoms outside and inside the surface. The blue colour represents a low frequency of occurrence of a (d i , d e ) pair and the full fingerprint is outlined in grey (Ternavisk et al., 2014). The water molecule and HÁ Á ÁO interactions are visualized as bright-red spots on the Hirshfeld surface mapped over d norm with neighbouring molecules connected by O1W-H1OWÁ Á ÁO1 and N1-H1AÁ Á ÁO1 hydrogen bonds (Fig. 4). The fingerprint plots indicate the percentage contributions of the various intermolecular contacts (Fig. 5). The HÁ Á ÁH contacts clearly make the most significant contribution (36.6%), whereas OÁ Á ÁH/HÁ Á ÁO and CÁ Á ÁH/HÁ Á ÁC contacts make contributions of 29.9 and 12.5%, respectively, to the Hirshfeld surface. The presence of OÁ Á ÁH/ HÁ Á ÁO interactions is indicated by two symmetrical narrow spikes with d i + d e $1.7 Å arise specifically due to hydrogenbonding interactions between the water H atom and the carbonyl oxygen. Furthermore, the existence of CÁ Á ÁH/HÁ Á ÁC interactions is shown by the pair of characteristics wings with the edge at d i + d e $2.9 Å , which is due to the contribution of C-HÁ Á Á interaction. The 11.5% contribution of the CÁ Á ÁC interactions arises from theinteraction, where the sum of d i and d e obtained is quite similar at 3.5 Å . Interestingly, the NÁ Á ÁH contacts showed a 2.6% contribution elucidated by a butterfly fingerprint plot resulting from the N1-H1AÁ Á ÁO1 interaction.
The presence of the C-HÁ Á Á interactions can be seen in the pale-orange spot inside the circle of black arrows on the Hirshfeld surface mapped over d e in (Fig. 6a) Hirshfeld surface of the title compound mapped over d norm .    indexed mapping, the C-HÁ Á Á interactions can be observed as a bright-red spot identified with black arrows in Fig. 6b. The blue spots near the ring represent the reciprocal C-HÁ Á Á interactions.

Frontier molecular orbital and UV-vis studies
Frontier molecular orbital analysis is a vital tool in the development of molecular electronic properties. The energy gap (E g ) between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) is a crucial factor in elucidating the molecular electrical transport properties. In the present study, the HOMO and LUMO were computed at the DFT/B3LYP/6-311G++(d,p) theoretical level and the respective plots of the frontier molecular orbital are illustrated in Fig. 7. At a specific separation between donor and acceptor, charge transfer may occur in the ground state if the HOMO of the donor lies energetically above the LUMO of the acceptor (Caruso et al., 2014). As can be seen from Fig. 7, the charge at the HOMO state is more localized at the indole group and enone moiety while charge is accumulated entirely at the nitro-substituted phenyl ring and the enone moiety in the LUMO state. The results reveal that the intramolecular charge transfer (ICT) occurred from the electron-donor groups to the electron-acceptor groups through the enone moiety. The carbon-carbon double bond connecting the donor and acceptor groups is responsible for the charge movement through -conjugation, triggering elec-tronic delocalization within the molecule (Prabhu et al., 2015). The energy gap of 2.80 eV obtained from the DFT calculations indicates strong chemical reactivity and weaker kinetic stability, which increase the polarizability and NLO properties (Maidur et al., 2018).
The absorption spectrum of the title compound was carried out in acetonitrile with a concentration of 10 À4 M. The absorption spectrum comprises of four major bands (Fig. 8). The strongest band occurs in the region of 396 nm, which was assigned to -* transition. This sharp peak is suspected to arise from the indole ring and carbonyl group (C O). The second strong UV-vis band is observed at 269 nm and is mainly attributed to the electron-withdrawing substituent of   The UV-vis absorption spectrum of the title compound.

Figure 6
Graphical view of the Hirshfeld surfaces for the title compound (a) mapped over d e with a pale-orange spot and (b) mapped over shapeindex with a bright-red spot, both inside the black arrows, signifying the involvement of the C-HÁ Á Á interactions. the nitro group (Pavia et al., 2001). The energy gap of the title compound was calculated from the UV-vis absorption edge at 461 nm (Fig. 8), giving an energy band gap value of 2.70 eV, comparable with the HOMO-LUMO energy gap obtained from the DFT study. This band gap is similar to those in reported studies (D'silva et al., 2011) and within the energygap range for semiconducting materials (Emmanuel et al., 2002).

Synthesis and crystallization
The title compound was synthesized via a Claisen-Schmidt condensation reaction. A mixture of 1-(4-nitrophenyl)ethan-one (0.5 mmol) and indole-2-carboxaldehyde (0.5 mmol) was dissolved in methanol (20 mL). Sodium hydroxide (NaOH) solution was then added dropwise under vigorous stirring. The reaction mixture was stirred for 5-6 h at room temperature. The final precipitate was filtered, washed with distilled water and recrystallized by slow evaporation from acetone solution to obtain orange plate-shaped crystals.

Refinement
Crystal data collection and structure refinement details are summarized in Table 2. All C-bound H atoms were positioned geometrically (C-H = 0.93 Å ) and refined using a riding model with U iso (H) = 1.2U eq (C). The water O atom was refined with half-occupancy. The O-and N-bound H atoms were located from difference-Fourier maps and refined freely.  program(s) used to solve structure: SHELXS97 (Sheldrick 2008); program(s) used to refine structure: SHELXL2013 (Sheldrick, 2015); molecular graphics: DIAMOND (Brandenburg, 2009) and Mercury (Macrae et al., 2008); software used to prepare material for publication: PLATON (Spek, 2009).

(E)-3-(1H-indol-2-yl)-1-(4-nitrophenyl)prop-2-en-1-one hemihydrate
Crystal data Special details Experimental. The following wavelength and cell were deduced by SADABS from the direction cosines etc. They are given here for emergency use only: CELL 0.71150 6.604 8.276 28.665 93.349 89.966 113.537 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.