Structural, Hirshfeld and DFT studies of conjugated D–π–A carbazole chalcone crystal

The title conjugated carbazole chalcone compound, synthesized using a Claisen–Schmidt condensation reaction, adopts an s-cis conformation with respect to the ethylenic double bonds (C=O and C=C).


Chemical context
Chalcone is a privileged structure comprising two aromatic rings that are linked by a three-carbon ,-unsaturated carbonyl system. Chalcones demonstrate wide-ranging biological activities such as anti-inflammatory and anticancer (Cui et al., 2008;Srinivasan et al., 2009;Wang et al., 2013) and have applications in non-linear optics . They are currently attracting considerable attention because they offer an excellent -conjugated system within the double bond at the ethylenic bridge (Teo et al., 2017). Furthermore, the conjugated chalcone could be enhanced with appropriate electron-pulling and electron-pushing functional groups on the benzene rings (Zhuang et al., 2017). The increased involvement of donor and acceptor interactions in the molecule improves the molecular charge transfer and degree of non-linearity (Davanagere et al., 2019). The high planarity and presence of stable E isomer in the solid state stabilizes the crystal structure (Custodio et al., 2020).
In a continuation of our studies (Zaini et al., 2018;, we report herein the synthesis and structural properties of the conjugated carbazole chalcone system of (E)-3-[4-(9,9a-dihydro-8aH-carbazol-9-yl)phenyl]-1-(4nitrophenyl)prop-2-en-1-one (CPNC). The experimental and theoretical studies and chemical reactivity analysis are discussed. ISSN 2056-9890 2. Structural commentary CPNC is composed of 9-phenylcarbazole and nitrobenzene moieties, which represent donor and acceptor groups, connected by an ethylenic bridge. The molecular and optimized structures of the CPNC with assigned atom-numbering scheme are illustrated in Fig. 1. The geometrical optimization of CPNC was computed with the Gaussian09W software package (Frisch et al., 2009) using the DFT method and the B3LYP/6-311G++(d,p) basis set without enforcing any molecular symmetry constraints. There is good agreement between the experimental and optimized structures (see the table in the supporting information), indicating that the basis set used was appropriate in both isolated conditions and the solid-state phase.
The 9H-carbazole unit and the C13-C18 phenyl ring subtend a dihedral angle of 53.26 (10) , which is similar to the dihedral angle of 53.8 (3) between the bridge aromatic ring and the 9H-carbazole unit in the related compound 2-[4-(9Hcarbazol-9-yl)benzylidene]-2,3-dihydroinden-1-one (Kim et al., 2011). The 9H-carbazole moiety is nearly co-planar with the nitrobenzene unit, making a dihedral angle of 5.19 (7) (Fig. 1c). This planar nature is possibly due to steric repulsion by the hydrogen atoms of the aromatic rings, leading to a small -electron delocalization. However, the phenyl ring of the 9-phenylcarbazole moiety subtends a large dihedral angle to the nitrobenzene group of 56.74 (10) (Fig. 1d), which tends to suppress the extension of the conjugation effect through the enone moiety.

Supramolecular features
The crystal structure of CPNC is built up in a cluster pattern style where the molecules are linked to each other along the baxis direction via C15-H15AÁ Á ÁO1 interactions (Table 1), as shown in Fig. 2a. The tilted distortion of 9-phenylcarbazole ring system is the results of the C18-H18AÁ Á ÁO2 interaction involving the nitro group, which links the molecules in a headto-tail arrangement, propagating diagonally along the ac direction. Weak C9-H9AÁ Á ÁCg4 interactions involving the C13-C18 phenyl ring and a carbazole hydrogen of carbazole moiety link the molecules into infinite chains, as depicted in

Hirshfeld surface analysis
Hirshfeld surface analysis is used to gain a clear understanding of the molecular structure interaction and visualize them graphically. The Hirshfeld surface and related two-dimensional fingerprint plots were generated using Crystal Explorer3.1 (Wolff et al., 2012). In the d norm surface (Fig. 3), the bright-red spots indicate the involvement of intermolecular C-HÁ Á ÁO interactions. The fingerprint plots (Ternavisk et al., 2014) (Fig. 4) indicate the percentage contribution of the HÁ Á ÁH, CÁ Á ÁH/HÁ Á ÁC, OÁ Á ÁH/HÁ Á ÁO and CÁ Á ÁC contacts. The HÁ Á ÁH contacts make the largest contribution to the Hirshfeld surface (38.4%) followed by the CÁ Á ÁH/HÁ Á ÁC contacts (28.2%), which are represented as a pair of characteristic wings. The OÁ Á ÁH/HÁ Á ÁO (19.1%) contacts display two symmetrical narrow spikes, which confirm the existence of C-HÁ Á ÁO interactions. In addition, the presence of weak intermolecular C-HÁ Á Á interactions can be seen as an orange region marked with black arrows in the shape-index surface (Fig. 5).

Molecular electrostatic potential (MEP) analysis
The reactive sites of a molecule can be investigated using molecular electrostatic potential (MEP) analysis (Barakat et  The packing of CPNC showing (a) C-HÁ Á ÁO and C-HÁ Á Á interactions (dashed lines) and (b) C-HÁ Á Á interactions forming an infinite chain along the ac-plane direction. Table 1 Hydrogen-bond geometry (Å , ).
Cg4 is the centroid of the C13-C18 ring.

Figure 3
The d norm surfaces showing the intermolecular interactions in CPNC: (a) front and (b) back.

Figure 4
Quantification of different types of contacts and respective fingerprints plots.

Figure 5
Representation of the C-HÁ Á Á interactions (indicated by black arrows).
al., 2015). In this study, DFT with the B3LYP/6-311G++(d,p) basis set was utilized to predict the possible location of the nucleophilic and electrophilic attacks. The MEP surface with a colour code from red (À0.04728 a.u) to blue (0.04728 a.u) is depicted in Fig. 6a. The carbonyl and nitro groups are nucleophilic (electron-rich) sites in the red-coloured region, while the blue colour indicates the electrophilic (electrondeficient) site localized on the hydrogen atom. These reactive sites are responsible for intermolecular interactions where the red and blue spots suggest the strongest repulsion site (electrophilic attack) and strongest attraction site (nucleophilic attack), respectively. The MEP results are further supported by the electrostatic potential contour map showing the isosurface lines shown in Fig. 6b where the red lines refer to the strong electron-withdrawing atoms such as in carbonyl and nitro substituents.  Kim et al., 2011) in which the 9phenylcarbazole unit is attached to a 2,3-dihydro-1H-inden-1one moiety. The two crystals were grown by different methods, ZIJPUG by slow evaporation from acetone solution and ISADOW by solvent diffusion using dichloromethane and hexane. The reported molecular structures of ZIJPUG and ISADOW exhibit a -bridge linker of an enone moiety and the aromatic ring of 9-phenylcarbazole, respectively. Furthermore, the C16-C17-C18-C19 torsion angle in ZIJPUG [À16.4 (3) ] indicates a slight twist, which is which comparable to that in ISADOW [C8-C10-C11-C12 = 178.6 (2) ].

Synthesis and crystallization
4 0 -Nitroacetophenone (5 mmol) and N-(4-formylphenyl)carbazole (5 mmol) were dissolved in 20 mL of methanol and then a catalytic amount of sodium hydroxide solution (5 mL, 20%) was added dropwise under continuous stirring for about 5-6 h at room temperature until a precipitate formed. This was filtered off, washed successively with distilled water and recrystallized from acetone solution, yielding orange blockshaped crystals suitable for X-ray diffraction analysis.

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 0.71074 8.313 9.985 13.418 68.212 88.424 85.648 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.