Enhancement of Dissymmetry Factors Through Axial Elongation of Hetero[ n ]helicenes

ABSTRACT

Helicenes are attractive emitters of circularly polarized light, yet strategies to enhance their luminescence dissymmetry factor ( g _lum ) remain underdeveloped. Herein, we report a scalable, facile synthetic route to hetero[9]helicenes utilizing a benzofuran scaffolding strategy. This efficient one‐pot protocol provides access to the hetero[9]helicenes at gram scale, enabling progressive axial elongation to hetero[12]‐ and hetero[15]helicenes. Structural characterization by NMR and single‐crystal X‐ray diffraction confirms increasing helical distortion and reduced helical pitch upon elongation. Chiroptical investigations reveal a pronounced enhancement of both absorption ( g _abs ) and emission ( g _lum ) dissymmetry factors with increasing ring count. Notably, annulation with a three‐ring extension via a naphtofuran unit yields a twofold increase in g _lum , reaching 1.95 × 10 ⁻² at 475 nm for the [15]helicene. TD‐DFT calculations reproduce these trends, revealing that the enhancement originates from a systematic decrease in the electric transition dipole moment while the magnetic transition dipole moment remains constant, thereby increasing the relative magnetic contribution to the S ₁ ^→ S ₀ transition as well as the angle between the dipole moments. These findings establish axial elongation as a robust design principle for amplifying chiroptical responses in helicenes.