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'-hydroxymethyl-[1,1']-binaphthylene (2) was examined to assess suitability as optical triggers in a liquid crystal switch. Both 1 and 2 are chiral and they were resolved into enantiomers by HPLC. The absorption and circular dichroism spectra of optically active 1 and 2 were recorded and the Kuhn anisotropy factors (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 racemization and the formation of optically active 2. Irradiation of optically active 2 under these conditions gives optically active 1 but no racemization of 2. Time-resolved absorption spectroscopy shows that irradiation of 1 generates an intermediate assigned to a 1,1'-biaryl quinone methide (3). Analysis of the mechanism for photoracemization and photochromism reveals that 3 is the key intermediate in both processes. This system is not well-suited for development as an optical trigger for a liquid crystal switch because the magnitude of g(lambda) is too small in the useful spectral region.