Crystal structure, Hirshfeld surface and photophysical analysis of 2-nitro-3-phenyl-9H-carbazole

The title compound was synthesized from a dinitrobiphenylbenzene derivative using a novel modification of the Cadogan reaction. The reaction has several possible ring-closed products and the title compound was separated as the major product. It crystallizes in the monoclinic P space group and possesses a single closed Cadogan ring.

The title compound, C 18 H 12 N 2 O 2 , was synthesized from a dinitrobiphenylbenzene derivative using a novel modification of the Cadogan reaction. The reaction has several possible ring-closed products and the title compound was separated as the major product. The X-ray crystallographic study revealed that the carbazole compound crystallizes in the monoclinic P1 space group and possesses a single closed Cadogan ring. There are two independent molecules in the asymmetric unit. In the crystal, the molecules are linked by N-HÁ Á ÁO hydrogen bonding.

Chemical context
Carbazole consists of two benzene ring fused on either side of a central pyrrole ring and is also known as dibenzopyrrole or diphenylenimine. This N-containing heterocyclic compound was discovered by Graebe and Glaser in 1872 (Collin et al., 2006). Carbazoles represent an important class of heterocycles with several advantages. By the introduction of substituents in the carbazole fragment at the nitrogen atom and the aromatic framework at positions 3 and 6, the photophysical properties can be modified (Srivastava & Chakrabarti, 2017;Sun et al., 2015). The high stability and redox potential property of carbazole-based polymers compared with other conducting polymers has attracted a great attention (Nandy et al., 2014;Bashir et al., 2015;Sutanto et al., 2021;Niu et al., 2021). Carbazole-based ligands exhibit high hole-transporting mobility and strong absorption in the UV-visible spectroscopic region, and therefore show good electro-and photoactive properties (Yavuz et al., 2001). Polycyclic compounds containing two pyrrole rings have become widely used because of their good charge-transfer properties and the feasibility of tuning the electronic levels in the compound for different types of applications (Wakim et al., 2008;Reig et al., 2015;Xiang et al., 2018;Zhang et al., 2018;Szafraniec-Gorol et al., 2021), These types of compounds are therefore excellent candidates for applications such as OLEDs (organic lightemitting diodes; Svetlichnyi et al., 2010;Oda et al., 2021;Zhou et al., 2021;Bao et al., 2020), DSSCs (dye-sensitized solar cells; Zhang et al., 2009;Li et al., 2018;Lokhande et al., 2019), OPV (organic photovoltaics; Chan et al., 2013;Yang et al., 2020) and OFETs (organic field-effect transistors; Reig et al., 2015;Chen et al., 2020;Koli et al., 2020).
The title compound was isolated as an intermediate in the middle of the synthetic route for the synthesis of double Cadogan-fused carbazoles. The reaction between 1,3-dinitrodiphenylbenzene and triphenylphosphine using the solvent

Photophysical study
The absorption and emission spectra of compound 1 were measured in dilute CH 2 Cl 2 solution at room temperature, as shown in Fig. 4. Compound 1 exhibits an absorption band at 260 nm to 410 nm, which can be assigned to the carbazole moieties. The broad absorption bands at the lower energy peak around 350 nm suggest the formation of the carbazole dimer excimer from the carbazole groups. The PL spectrum of compound 1 excited at 350 nm shows a dominant blue-violet broad peak at 400 nm associated with the emission from the carbazole excimer.

Synthesis and crystallization
The synthesis of the title compound is shown in Fig. 5. The reaction yielded single and double Cadogan ring-closure products. First we prepared dinitro compound a by a nitration reaction and then we synthesized terphenyl compound b by performing double Suzuki-coupling reaction on 1,5-dibromo-2,4-dinitrobenzene and benzeneboronic acid. A two-necked flask fitted with a condenser was charged with 1,3-dinitro-4,6diphenyl benzene (b) (0.320 g, 1 mmol) and 0.655 g (2.5 mmol) of triphenylphosphine. 8 mL of the solvent o-dichlorobenzene were added o the reaction mixture. The resulting reaction mixture was stirred at 473 K under nitrogen for 24 h. The solvent was removed under reduced pressure at 333 K and the crude product was purified by column chromatography (silica gel, 10% EA in hexanes as eluent) to provide 0.230 g of the title product as a beige solid (yield: 86%). 1

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2 The Hirshfeld surface of the title compound mapped over d norm to visualize the intermolecular interactions.

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.