6,6′-Diheptyl-3,3′-bis[(pyridin-3-yl)ethynyl]-5H,5′H-dipyrrolo[1,2-b:1′,2′-g][2,6]naphthyridine-5,5′-dione

The complete molecule of the title compound is generated by a crystallographic centre of symmetry. The pendant heptyl chains adopt extended conformations and the dihedral angle between the pyrrole and pyridine rings is 8.18 (15)°·In the crystal, the molecules are arranged in columnar stacks propagating in the [010] direction via slipped aromatic π–π stacking interactions.

The complete molecule of the title compound, C 42 H 42 N 4 O 2 , is generated by a crystallographic centre of symmetry. The pendant heptyl chains adopt extended conformations and the dihedral angle between the pyrrole and pyridine rings is 8.18 (15) . In the crystal, the molecules are arranged in columnar stacks propagating in the [010] direction via slipped aromaticstacking interactions.

Structure description
5H,11H-dipyrrolo[1,2-b:1 0 ,2 0 -g][2,6]naphthyridine-5,11-dione (C 18 H 16 N 2 O 2 ; DPND) is a cross-conjugated dye that has attracted significant attention since it was first reported by Grzybowski et al. (2016). Such a skeleton is composed of electron-rich pyrrole rings and electron-poor carbonyl groups. Several studies have shown that it has interesting electrochemical and photophysical properties and it is widely used as a fluorescent dye (Sadowski et al., 2017;Sadowski, Loebnitz, et al., 2018;Sadowski, Rode, et al., 2018). It also has become a potential candidate in singlet fission for enhancing the performance of photo-voltaic devices (Wang et al., 2020), two-photon absorption materials (Sadowski et al., 2017) and photodynamic therapy agents (Morgan, Yun, Jamhawi, et al., 2023). In order to explore the luminescence properties of such molecules in the near infrared region, the strategy of expanding the DPND conjugated system by introducing a pendant pyridine unit was adapted and we synthesized the title compound C 42 H 42 N 4 O 2 , named DPND-3Py, and we now describe its structure and spectroscopic properties. The complete molecule is generated by a crystallographic centre of symmetry ( Fig. 1) and the central chromophore is almost planar (r.m.s. deviation for 16 atoms = 0.028 Å ). The pyridine unit is connected to the pyrrole ring of the DPND core by an alkyne bond, which enhances the rigidity of the molecule: the dihedral angle between the N1/C1-C4 data reports and N2/C10-C14 rings is 8.18 (15) . The pendant heptyl chains adopt extended conformations.
UV-vis spectra were recorded on a TU-1810DPC spectrometer using dichloromethane (DCM) as solvent and a concentration of 2.5 Â 10 À6 mol l À1 . As shown in Fig. 3, the title compound has three distinct absorption peaks in the range 250 to 800 nm, with a maximum absorption peak of 582 nm. The spectrum features strong absorption in the 500-600 nm region ascribed to an optically allowed S 0 ! S 1 transition.
Photoluminescence spectra were recorded on a F-320 spectrometer or HORIB Fluorolog-3. Figs. 4 and 5 show the photoluminescence spectra both in solution (1.0 Â 10 À5 mol l À1 in dichloromethane) and the solid state. The solution spectrum displays two peaks (maximum emission wavelength 625 nm) in the range 550 nm to 800 nm. As shown in Fig. 5, the solid-state fluorescence spectrum exhibits a strong emission peak at 767 nm, a shift of over 100 nm compared with solution, indicating strong intermolecular interactions.

Figure 2
The packing arrangement of the title compound, which shows a slip-stack pattern with adistance of 3.402 Å between the closest planes of these two molecules.

Figure 5
Solid-state emission spectrum of the title compound at an excitation wavelength of 470 nm.

Figure 1
The molecular structure of the title compound with displacement ellipsoids drawn at the 50% probability level. Symmetry code for the primed atoms: 1 À x, 2 À y, 1 À z.

Synthesis and crystallization
In a reaction flask containing a magnetic stirring bar was placed: 3,3 0 -dibromo-6,6 0 -diheptyl-5H,5 0 H-dipyrrolo[1,2b:1 0 ,2 0 -g][2,6]naphthyridine-5,5 0 -dione (59.04 mg, 0.100 mmol), CuI (1.9 mg, 0.01 mmol), Pd(PPh 3 ) 4 (5.78 mg, 0.005 mmol) and 3-pyridine-acetylene (30.94 mg, 0.300 mmol). The vessel was evacuated and backfilled with argon (three times) and anhydrous, degassed tetrahydrofuran (THF) was added (3 ml) followed by dry triethylamine (56 ml, 0.40 mmol). The vessel was tightly closed and again carefully evacuated (until the mixture started to boil) and backfilled with argon (3 times). The content of the flask was stirred for 20 h at 70 C (above the boiling point), and it was cooled to room temperature. Dichloromethane (DCM) was added to dilute the reaction solution, which was washed three times with water and dried over sodium sulfate. The solvent was evaporated and the product was purified using column chromatography (silica, petroleum ether: ethyl acetate = 5:1), and recrystallized from mixed solvents of DCM and methanol to obtain a dark-purple solid (38.5 mg, yield of 35%) (Grzybowski et al., 2016). Figure  S1 in the supporting information shows the 1 H NMR spectrum of the title compound. The title compound dissolved in methylene chloride and methanol solution grew dark-red crystals suitable for crystallographic studies by slowly volatilizing the solvents.

Refinement
Crystal data, data collection, and structure refinement details are summarized in Table 1.

Funding information
We gratefully acknowledge support by the Guizhou Provincial Science and Technology Projects (grant No. ZK[2021]1Y 048).   where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.001 Δρ max = 0.23 e Å −3 Δρ min = −0.34 e Å −3 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.