Crystal structure and fluorescence of 1-[8-phenyl-9-(phenylethynyl)-4H-benzo[def]carbazol-4-yl]ethan-1-one

A dual emissive fluorescent substituted benzo[def]carbazole was obtained through C—H bond activation catalysed by Pd(OAc)2. It crystallizes in the monoclinic space group P21/n.

The photophysical properties of 4H-benzo[def]carbazole have been studied over the past few decades (Bender et al., 1964;Zander et al., 1966;Favini et al., 1971;Horaguchi et al., 1980). The spectra show that the wavelengths of absorption and emission maxima are in the ranges 230-410 nm and 345-520 nm, respectively, at different temperatures and for different solvents. The effect of the solvent on absorption and fluorescence bands as well as comparisons with theoretical expectations have been used to estimate the dipole moment of the first excited state. Geng et al. (2016) reported the optimized geometry, electron-density distributions, and HOMO and LUMO of carbazole and 4H-benzo [def]carbazole. Changes in the HOMO-LUMO gap (Eg) and the design of molecules for material applications can be realized by comparing frontier molecular orbitals, HOMO and LUMO energy levels, and exploring their electron-density maps.
In order to obtain the benzo[def]carbazole 2 efficiently, we utilized the pathway through the conversion of diphenylphenanthrene 1 to N-acetyl benzo[def]carbazole 2. We obtained N-acetyl carbazole 2 in quantitative yield utilizing Buchwalds' method by treatment of diphenylphenanthrene 1 as a substrate in the presence of Pd(OAc) 2 (10 mol %), NaOAc (1.0 equiv.), Cu(OAc) 2 (2.0 equiv.) and powdered molecular sieves in toluene under oxygen at 393 K for 24 h. Single crystals of 2 were grown from the a mixture of hexanes and DCM (v/v = 1:1) at room temperature by slow thermal evaporation.

Structural commentary
Compound 2 crystallizes in the monoclinic space group P2 1 /n with two independent molecules in the asymmetric unit. The atomic labelling scheme is shown in Fig. 1. The C-C bond lengths are within the expected values known for aromatic systems (Allen et al., 1987).
In the structure of 2, both independent conformers occupy their own coordinates in the asymmetric unit, but are in the same configuration. On the other hand, owing to the space group of the title compound, P2 1 /n, which is centrosymmetric, both molecules will produce two identical configurations that are 180 inverted from each other. In the stereoscopic view, we can observe that the phenyl group and the phenyl alkynyl moiety in the two independent conformers have different dihedral angles with respect to the benzo[def]carbazole, which are 22.2 (1), 25.7 (2) and 50.8 (2), 59.7 (2) , respectively.
Specifically, the crystal is stabilized by the phenyl groups of the alkynyl moiety, which interacts weakly with each other (Fig. 3b, red dashed lines) throughstacking. Furthermore, the phenyl group also interacts with another neighboring phenyl moiety and with the phenyl alkynyl moiety through C-HÁ Á Á interactions (Table 1). In addition,stacking and carbonylcarbonyl interactions with C + and O À between the two acetyl groups are observed. The molecules are ordered into infinite ribbons extending along the [001] direction through alternating intermolecular C-HÁ Á Á andstacking interactions.

Database survey
A search of the Cambridge Structural Database (WebCSD accessed 21 April 2022; Groom et al., 2016) results in over a thousand carbazole derivatives of which 45 are derivatives of benzo [def]carbazole. Most of the compounds are cyclized with the benzo moiety to the skeleton of benzo[def]carbazole. Of these, 31 molecular structures are derivatives of the main structure of phenanthro [1,10,9,8-cdefg]carbazole that is commonly used to design functional molecules, such as organic transistors or n-doped thermoelectric devices [KUTZUX ; ZAJMUW (Martell et al., 2021)], white-light emissive material (ILIGIW; Chatsirisupachai et al., 2021), N-annulated perylene diimide for stable organic materials with unique optical, electronic, magnetic properties ( The packing of 2. Table 1 Hydrogen-bond geometry (Å , ).

Figure 1
The molecular structure of 2, showing the atom-labelling scheme and displacement ellipsoids at the 50% probability level.

Synthesis and crystallization
To a dried reaction tube, phenanthrene 1 (0.1 mmol), Pd(OAc) 2 (2.25 mg, 0.01 mmol), Cu(OAc) 2 (36.3 mg, 0.2 mmol), NaOAc (16.4 mg, 0.2 mmol) and powdered molecular sieves (40 mg, activated 3 Å ) were added under air and covered with a septum. The tube was evacuated and refilled with N 2 . Under a positive N 2 pressure, toluene (2 mL) was added via a syringe followed by degassing under a weak vacuum to this tube, and it was refilled with O 2 three times. The reaction mixture was sealed and stirred at 293 K for 24 h under an O 2 atmosphere. After completion of the reaction, the solution was cooled to room temperature and diluted with ethyl acetate followed by filtration through a thin pad of Celite. The crude product was purified by flash chromatography (hexanes/EtOAc) on silica gel to afford N-acetyl benzo[def]carbazole 2. Crystals of the title compound were obtained by thermal evaporation of the pure compound from a 1:1 solution of dichloromethane and hexanes.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. C-bound H atoms were positioned geometrically (C-H = 0.95-0.98 Å ) and refined using a riding model, with U iso (H) = 1.2 or 1.5U eq (C).

Funding information
This work was supported financially by the Ministry of Science and Technology of Taiwan  program(s) used to solve structure: SHELXD (Sheldrick, 2008); program(s) used to refine structure: SHELXL2014/6 (Sheldrick, 2015); molecular graphics: SHELXTL (Sheldrick, 2008); software used to prepare material for publication: SHELXTL (Sheldrick, 2008).  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.