*2.2. Structure and Packing Mode of 3*

The double aza-oxa[7]helicene structure of **3** was definitely confirmed by X-ray crystallography using a single crystal, grown from its racemic solution. We used the liquid/liquid diffusion technique between ethyl acetate and *n*-hexane to prepare this crystal slowly over three days in a dark environment at −20 ◦C. As expected, the two helicene moieties are connected via a phenylene linker (Figure 2a,b). The dihedral angles between the phenylene linker's plane and the pyrrole (ring B') are −41.86◦ for (C1-C6-N7-C8), and 54.38◦ for (C5-C6-N7-C9). Although the experimental values of (C5-C6-N7-C9) dihedral angle (54.38◦) is comparable to that of the optimized structure using DFT calculations at MN15/6-311G(d,p) level of theory (54.72◦), (C1-C6-N7-C8) dihedral angle was smaller than optimized structures at various levels (Table 1). This can be attributed to the intermolecular

interactions between the double helicene molecules **3** in the packed structure. Only meso isomer (*P*,*M*)-**3** was observed in the crystal packing with achiral molecules packed along the b-axis (Figure 2c,d). The packing of **3** shows a herringbone pattern with π-π distance of 4.458 A◦. This characteristic arrangement is optimum for many material-based applications, especially semiconductors [49–51]. In addition, it maximizes the optical and electronic properties of the obtained double helicene upon self-assembly [49,52–54]. Most of these larger or multiple helicenes showed significant variations during DFT calculations owing to the long-range conjugation and the effects of charge transfer [55,56]. Among the functions we screened, Minnesota 15 (MN15) function was found to be the most suitable parameters for our molecules (Table 1) [57].

**Figure 2.** Single crystal structure of the double helicene **3**: (**a**,**b**) ORTEP drawing structure of (*P*,*M*)-**3** obtained by X-ray crystal analysis with ellipsoids at 50% probability (H atoms were omitted for clarity); (**c**) crystal packing of (*P*,*M*)-**3** with ellipsoids at 30% probability; (**d**) packing structure of (*P*,*M*)-**3** is viewed along the c-axis to show the herringbone arrangement.


**Table 1.** The selected experimental and calculated structural parameters of double aza-oxa[7]helicene **3**.

<sup>1</sup> All calculations are carried out using 6-311G(d,P) basis set at three different functions (B3LYP, wB97XD, and MN15).

Nucleus-independent chemical shift (NICS) calculations revealed the low aromaticity of the central phenylene linker with a NICS (0) value of −5.8 ppm (Figure 3a), much lower than that of benzene −7.6 ppm calculated at the same level of theory. The largest NICS (0) values (between −7.3 ppm and −8.6 ppm) were found on the benzene of 6*H*-furo[3,2 *e*]indole (ring C ), pyrrole (ring B ) and naphthalene (rings F' and H'). While the lowest NICS (0) values (around −5.8 ppm) were found on the phenylene linker (ring A') and furan rings (D') which is consistent with the aromatic character of this ring. Generally, symmetric double hetero[*n*]helicenes (*n* ≥ 4) have three isomers, those being two chiral enantiomers (*P*,*P*) and (*M*,*M*), and one meso diasteromer (*P*,*M*) [30]. All three isomers of **3** were afforded under our reaction conditions which was confirmed by HPLC separation using DAICEL CHIRALPAK IA column (eluent: *n*-hexane/*i*-PrOH = 20/1) (Figure 3b). The experimental ratio among the three isomers was found to be around (1:2:1) with the meso isomer (*P*,*M*)-**3** as the major formed product (confirmed by the absence of optical rotation). After HPLC chiral resolution, the epimerization rate of **3** was studied at three different temperatures (See SI). Eyring plot (Figure 3c) indicated a low chiral stability of **3** (epimerization barrier ~24.2 kcal mol−1) with an estimated half-life of the epimerization <6.5 h at 25 ◦C. These observations were matching with our DFT calculations that showed similar epimerization barriers 25.32 kcal mol−<sup>1</sup> and 25.62 kcal mol−<sup>1</sup> (Figure 3d).
