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

Helicenes are attractive emitters of circularly polarized light, yet strategies to enhance their luminescence dissymmetry factor (glum) 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 xX-ray diffraction confirms increasing helical distortion and reduced helical pitch upon elongation. Chiroptical investigations reveal a pronounced enhancement of both absorption (gabs) and emission (glum) dissymmetry factors with increasing ring count. Notably, annulation with a three-ring extension via a naphtofuran unit yields a twofold increase in glum, 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 S1 →S0 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.