Modulation of photoswitching profiles by 10,11-dialkoxymethyl substituents in c(2)-symmetric dibenzosuberane-based helicenes

A series of C₂‐symmetric, 10,11‐disubstituted dibenzosuberane (DBS)‐based helicenes 6 a – c with a common 7‐bromo‐α‐tetralin‐based bottom fragment were synthesized. Their absolute stereochemistry was determined to be 10R,11R,P after reductive desulfurization of the corresponding (10R,11R,1′S)‐episulfides with complete stereospecificity. Photoisomerization of the diastereomerically pure (P)‐ 6 c in hexane led to virtually exclusive formation of the opposite M‐form diastereomer (P/M′, <1:>99) at 290 nm. The preferential return of (M′)‐ 6 c to (P)‐ 6 c was also achieved with high selectivity (P/M′, 90:10) at 330 nm. Molecular simulations of (P)‐ 6 c and (M′)‐ 6 c with both DBS conformations suggest that the selectivities of photoswitching are controlled by the conformation of the top DBS template as evidenced by their ¹H NMR spectra. Doping 6 c into a nematic liquid crystal (E7) led to a cholesteric mesophase with modulated pitches, reversible helical senses, and with a switch memory of ternary logic.