Modulation of Photoswitching Profiles by 10,11‐Dialkoxymethyl Substituents in C 2 ‐Symmetric Dibenzosuberane‐Based Helicenes

A series of C 2 ‐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 10 R ,11 R , P after reductive desulfurization of the corresponding (10 R ,11 R ,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 1 H 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.