Three C2-symmetric (10R,11R)-diethyl substituted dibenzosuberane (DBS)-based helicenes with varying steric and conjugation demands of their bottom fragments were synthesized. Only the helicene-7a[with the bottom part derived from tetralone was found photo-switchable in reasonable time scale. Photoisomerization of the diastereomerically pure (10R,11R,P)-helicene (7a) at 280 nm led to virtually exclusive formation of the opposite M form-diastereomer 7a′ (7a′/7a = 99.6/0.4). The preferential return of 7a′ to 7a can be effected upon irradiation at 254 nm (7a′/7a = 3G/67) or thermally at 130 7a′/7a = 0/100). The photo-induced switching process amounts to a 133% difference in d.e. (from 99.2% to-34%). The concomitant change of helicene chirality between these two diastereomeric photostationary states augurs well for their potential application as an optical switch in LC materials. To our knowledge, our system serves as the best chirochromic optical switch as compared to the examples possessing similar photochromic properties.
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