Spring-Like Behavior in [8]Helicene Diimides: How Helical Pitch Governs Optical Anisotropy and Electronic Conjugation.

Helicenes, with their corkscrew-shaped geometry, have emerged as prototypical molecular springs for engineering chiral functional materials through precise structural modulation. Here we introduce the design and synthesis of [8]helicene diimides ([8]HDIs) and demonstrate that the helical pitch of their backbone can be precisely tuned by bridging the imide nitrogen atoms with alkyl chains of varying lengths (C3-C6). This approach constrains the molecular geometry to systematically control optical anisotropy, chiroptical response, and electronic communication. Remarkably, modulation of the helical pitch leads to high optical dissymmetry factors (up to 6.0 × 10-2) and enhanced through-space conjugation. Furthermore, we investigate how variations in the helical pitch affect crystal packing in both enantiopure and racemic samples. Complementary quantum chemical calculations provide insights into the origins of these properties, highlighting the potential of this strategy for designing advanced chiral materials.