A search for chiral photochromic optical triggers for liquid crystals: Photoracemization of 1,1 '-binaphthylpyran through a transient biaryl quinone methide intermediate

The photochromic pair 1,1'-binaphthylpyran (1) and 2-hydroxy-2'-hydroxymeth yl-[1,1']-binaphthylene (2) was examined to assess suitability as optical t riggers in a liquid crystal switch. Both 1 and 2 are chiral and they were r esolved into enantiomers by HPLC. The absorption and circular dichroism spe ctra of optically active 1 and 2 were recorded and the Kuhn anisotropy fact ors (g(lambda)) were calculated. The photochemistry of 1 and 2 was studied. Irradiation of optically active 1 in an CH3CN/H2O solution leads to its ra cemization and the formation of optically active 2. Irradiation of opticall y active 2 under these conditions gives optically active 1 but no racemizat ion of 2. Time-resolved absorption spectroscopy shows that irradiation of 1 generates an intermediate assigned to a 1,1'-biaryl quinone methide (3). A nalysis of the mechanism for photoracemization and photochromism reveals th at 3 is the key intermediate in both processes. This system is not well-sui ted for development as an optical trigger for a liquid crystal switch becau se the magnitude of g(lambda) is too small in the useful spectral region.