*2.3. Photophysical Properties*

Our double aza-oxa[7]helicene **3** shows high luminescence upon photo-irradiation, which can be attributed to the rigid scaffold that hinders the thermal energy loss upon structural changes. The UV/vis absorption of **3** was recorded in different solvents to show its high solubility in most of the organic solvents which increases the potential for some applications that require good solubility such as solution-processed electronics [58–60]. In all measured solvents, compound **3** showed similar UV/vis absorption patterns (Figure 4a). The maximum absorbance exhibited in chloroform was shown at 407 nm (absorption coefficient: 7.5 × 104 <sup>M</sup>−<sup>1</sup> cm−1) corresponding to an optical energy gap of (2.18 eV). According to TD-DFT calculations at the MN15/6-311G(d,p) level of theory, this lowenergy absorption can be accountable to the HOMO→LUMO transition. The absorption band at 385 nm possibly attributed to the equal contribution of both HOMO−1→LUMO and HOMO→LUMO+1 transitions. The band at 368 nm is estimated to be due to the HOMO−1→LUMO+1 transition, while the higher energy absorption band at 328 nm would be attributed to HOMO−2→LUMO corresponding to an optical energy gap of (2.58 eV). Molecular orbital calculations indicated that the HOMO is spread mainly over the phenylene linker (ring A ) and pyrroles (rings B') and LUMO is spread over the whole scaffold rather than the phenylene linker (ring A ), accounting for the substantial stability. Photoluminescence PL spectrum of **3** was recorded in a pure chloroform solution exhibiting emission maxima shifted in a bathochromic way at 415 nm and 440 nm.

**Figure 3.** (**a**) NICS (0) values of (*P*,*M*)-**3** calculated at the MN15/6-311G+(2d,p) level; (**b**) HPLC chromatogram determined by (Daicel Chiralpak IA, *n*-hexane/*i*-PrOH = 20/1, flow rate 1.0 mL/min, T = 25 ºC, 240 nm): t1= 10.36 min, t2 = 14.30 min, and t3 = 18.32 min; (**c**) Eyring plot of compound **3** epimerization and thermodynamic parameters; (**d**) Epimerization process from (*P*,*M*)-**3** isomer to (*M*,*M*)-**3** and (*P*,*P*)-**3** isomers. The relative Gibbs free energies are calculated in (kcal mol−1) at the MN15/6-311G(d,p) level.

**Figure 4.** (**a**) UV/vis absorption and PL spectra of **3** in various solvents (20 μM); (**b**) Frontier Kohn-Sham molecular orbitals of **3** and TD-DFT calculated electronic transitions at MN15/6-311G (d,p) level of theory.
