An unprecedented dual-mode optical switch by combining helicene and bis-azobenzene moieties in proximity allows for cooperative, highly diastereoselective chirochromic (92/8 to 3/97) and photochromic (>99/<1 to 19/81) switchings both in solution and in nematic LC with tunable pitches and handedness by permutation of four different metastable states.
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 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 (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.
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