Synthesis, optical properties and crystal structure of (E,E)-1,3-(3,4:9,10-dibenzododeca-1,11-diene-5,7-diyne-1,12-diyl)benzene

A dehydrobenzannulene, (E,E)-1,3-(3,4:9,10-dibenzododeca-1,11-diene-5,7-diyne-1,12-diyl)benzene, was stereoselectively synthesized, and its crystal structure and UV-Vis absorption and photoluminescence optical properties were determined.


Chemical context
Dehydrobenzannulenes (DBAs) attract intensive attention because they often show new functionality for -expanded compounds, such as a novelinteraction mode in fluoroarylene-DBA (Karki et al., 2022), guest-dependent structuretransformative DBA inclusion crystals (Shigemitsu et al., 2012), and a synthetic intermediate of [6.8] 3 cyclacene (Esser et al., 2008). In the syntheses of DBAs, ethenylene and ethynylene arrays are often used to connect aromatic rings to one another. For example, 1,3-(3,4:9,10-dibenzododeca-1,11diene-5,7-diyne-1,12-diyl)benzene, C 26 H 16 , (1), is composed of three phenyl rings, a single butadienylene and a couple of ethenylene arrays. The synthesis of 1 was accomplished in 1985 (Ojima et al., 1985). The synthetic route of 1 reported by Ojima was rather straightforward, and the desired dehydrobenzannulene 1 were successfully obtained. However, while the formation of (E,E)-1 was spectroscopically confirmed, X-ray single crystallographic analysis has not yet been performed because of a poor chemical yield of (E,E)-1 in Ojima's route. Recently we established an (E)-stereoselective synthesis of diarylethene via photocatalyst-assisted reductive desulfonylation of the corresponding diarylethenyl sulfone under irradiation by visible light (Watanabe et al., 2020(Watanabe et al., , 2021. It was found out that this protocol could produce (E,E)-1 efficiently in a pure form. This work reports the synthesis of the dehydrobenzannulene (E,E)-1 and its single-crystal X-ray structure together with UV absorption and photoluminescence optical properties of (E,E)-1 in CHCl 3 solution and in the solid state.

Supramolecular features
In the crystal, (E,E)-1 molecules form columnar structures that extend along the a-axis direction in which the interlayer distance is 3.3639 (9) Å (calculated as the perpendicular distance from the mid-point of the 15-membered ring to the mean plane through the corresponding ring of an adjacent molecule in the stack), indicating an efficient intermolecular attractive interaction throughstacking (Fig. 2). The columns in which the (E,E)-1 molecules are stacked are densely packed by van der Waals interactions.

Figure 1
The molecular structure of (E,E)-1 with displacement ellipsoids drawn at the 50% probability level. ethenyl)benzene fragment in analogous DBA is also common, with more than ten examples reported including the close relative of metacyclophanetrienes (GOBJIR and GOGMAR; Esser et al., 2008).

Synthesis and crystallization
The dehydrobenzannulene 1 was synthesized from 2 in five steps (Fig. 3). The starting disulfone 2 and -expanded pyrene photocatalyst 7 were prepared according to the literature (Orita et al., 2006;Watanabe et al., 2021, respectively). A consecutive treatment of 2 with BuLi, 2-bromobenzaldehyde, and acetic anhydride gave 3 in 94% yield as a diastereomeric mixture. The diacetate 3 was successfully converted to 4 in a 94% yield by treatment with t-BuOK, and the resulting dibromobis(sulfonylethenyl)benzene 4 was transformed to 5 with a 69% yield via Sonogashira-Hagihara coupling with trimethylsilylethyne (Watanabe et al., 2020). Subsequently our original photocatalyst-assisted reductive desulfonylation was applied to bis(1-phenylsulfonylethenyl)benzene 5 (Watanabe et al., 2021). When blue light (447 nm, 30 W) was irradiated on a THF/MeCN solution of 5 in the presence of 5 mol% of pyrene photocatalyst 7 (2.5 mol% per sulfonylethene moiety) and i-Pr 2 NEt as sacrificial reductant at 323 K for 9 h, the stereoselective reductive desulfonylation proceeded smoothly to produce (E,E)-6 in 78% yield. In contrast, during greenlight irradiation (514 nm, 30 W), this desulfonylation proceeded only sluggishly. When an ether/pyridine solution of 6 was treated with a THF solution of TBAF (tetrabutyl-ammonium fluoride), desilylation occurred rapidly to give terminal ethyne 8. After the completion of the desilylation was confirmed by thin-layer chromatography (TLC) analysis, the final step, oxidative cyclization of the resulting terminal bisyne 8, was carried out in the presence of Cu(OAc) 2 in air at 323 K for 3 h. The desired dehydrobenzannulene 1 was obtained as yellow powder after column chromatography on silica gel. The spectroscopic data ( 1 H NMR) were identical to that reported by Ojima et al. (1985).
Synthetic procedure from (E,E)-6 to (E,E)-1 To an ether (3.3 mL) and pyridine (1.1 mL) solution of 6 (47.5 mg, 0.10 mmol) was added a THF solution of TBAF (1.0 M, 0.22 mL, 0.22 mmol) at 273 K, and the mixture was stirred at rt for 3 h. The mixture was added to an ether (3.3 mL) and pyridine (1.1 mL) solution of Cu(OAc) 2 (228 mg, 1.3 mmol), and the mixture was stirred at 323 K for 3 h. The mixture was poured into sat. NH 4 Cl aqueous solution and AcOEt, and the organic and aqueous layers were separated. The aqueous layer was extracted with AcOEt, and the combined organic layer was washed with water and brine. After drying over MgSO 4 , the solution was evaporated. The residue was subjected to column chromatography on silica gel (hexane/CH 2 Cl 2 , 9:1) to provide 1 (29.6 mg, 0.090 mmol, 90% yield).

Optical properties
To evaluate the electronic effects of the molecular structure of (E,E)-1 on its optical properties, UV-Vis absorption and photoluminescence spectra were recorded in CHCl 3 (Fig. 4). In the UV-Vis absorption spectrum, (E,E)-1 showed the longest and the maximum absorption bands at 377 nm (" 0.45 Â 10 4 L mol À1 cm) and 299 nm (" 7.4 Â 10 4 L mol À1 cm), respectively. The former absorption band was assignable to the HOMO-LUMO transition of (E,E)-1 by DFT calculations performed at the B3LYP/6-31G(d) level of theory; 419 nm and f = 0.0415 were obtained as the first excitation energy and oscillator strength after calibration by multiplying by 0.96. The DFT calculations also revealed that the HOMO and LUMO of (E,E)-1 expanded in the whole molecule (Fig. 5). When UV light was irradiated to the CHCl 3 solution of (E,E)-1 and in the powdered state, blue and greenish blue-colored emissions were recorded at 468 nm (È F 0.26) and 504 nm (È F 0.24), respectively (Fig. 4). UV-Vis absorption and photoluminescence spectra of (E,E)-1.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 1. All H atoms were refined using a riding model with d(C-H) = 0.93 Å , U iso (H) = 1.2U eq (C) for aromatic H, 1.00 Å , U iso (H) = 1.2U eq (C) for CH, 0.98 Å .  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.