C2-symmetric dibenzosuberane-based helicenes as potential chirochromic optical switches

Three C-2-symmetric (10R,11R)-diethyl substituted dibenzosuberane (DBS)-based helicenes with varying steric and conjugation demands of their bottom fragments were synthesized. The X-ray analyses of their precursors-episulfides reveal the preferred conformation of the DBS skeleton possessing equatorially oriented ethyl groups at C-8 and C-9 positions. In all cases, the absolute configuration in the episulfide moiety is S, which can lead stereospecifically to the corresponding P helicenes after the reductive desulfurization of the episulfides. Only the helicene 7a with the bottom part derived from alpha-tetralone was found photoswitchable 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 = 33/67) or thermally at 130 degrees C (7a'/7a = 0/100). The photoinduced switching process amounts to a 133% difference in de (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.