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[摘要]：A series of C-2-symmetric, 10,11-disubstituted dibenzosuberane (DBS)-based helicenes 6a-c with a common 7-bromo-a-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)-6c 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')-6c to (P)-6c was also achieved with high selectivity (P/M', 90:10) at 330 nm. Molecular simulations of (P)-6c and (M')-6c with both DBS conformations suggest that the selectivities of photoswitching are controlled by the conformation of the top DBS template as evidenced by their H-1 NMR spectra. Doping 6c 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. |
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