New aza[6]helicenes: Synthesis, enantiomeric separation, (chir)optical properties and DFT calculations

Herein, we report the design, synthesis, and characterization of new aza[6]helicene derivatives, focusing on their photophysical and chiroptical properties. The target compounds were synthesized in good overall yields (43%– 71%) through a four-step sequence starting from quinoline–3–carbaldehyde. Their structures were confirmed by NMR and FT–IR spectroscopies, mass spectrometry, and single-crystal X–ray diffraction analysis. Photophysical investigations revealed a noticeable UV/Vis absorption and a fluorescence emission in the blue region of the electromagnetic spectrum. The racemic helicenes were successfully separated by chiral HPLC, resulting in P– and M–enantiomers in excellent optical purity (> 99.5% ee). These enantiomers exhibited large specific optical rotations (e.g., +2350–3200 for the P–enantiomer at λ = 589 nm) and remarkable electronic circular dichroism (ECD) signals. Density functional theory (DFT) was used to clarify the structure–property relationships and to reproduce the experimental observations with good accuracy. The comparative analysis between computational results and experimental data highlights the potential of these N–heteroaromatic scaffolds as organic small– molecules for applications as blue-emitting chiral dopants in OLED devices and as hole-transport units.