Synergistic Chirality in Spiro-Fused Chiral Conjugated Helices

This study presents the design and synthesis of novel spiro-fused chiral conjugated helices via a bilateral bent-π-extension strategy to elucidate chirality synergy across mixed chiral centers. Notably, six stereoisomers of SDPD6H, including three enantiomeric pairs, were successfully obtained and fully resolved. Single-crystal structural analysis reveals that helicene fusion onto the spirodiperylenetetraimide (SDP) skeleton induces significant bending of the originally planar perylene diimide subunits into bowl-shaped geometries, breaking molecular symmetry and thus raising the isomerization barrier. Furthermore, the stereoisomers exhibit distinct chiroptical responses, with P,S,P-6 showing the strongest chiroptical activity (Cotton effects: |Δε| reaching 310 M^-1 cm^-1). Through in-depth theoretical analysis of rotatory strength (R), we elucidate the origin of the circular dichroism (CD) behavior in the 425-600 nm region. In homochiral bishelicene configurations (P,P or M,M), the magnetic transition dipole moments (m) from the two molecular fragments differ significantly in magnitude, leading to non-canceling R₃ and R₄, and thus pronounced CD signals. In contrast, heterochiral configurations (P,M) produce magnetic dipoles of similar magnitude but opposite sign, resulting in near-complete cancellation and a suppressed CD response. This work provides a mechanistic understanding of stereochemical interplay in hybrid multi-chiral-centers systems and establishes a new design strategy for advanced chiroptical materials.