Photoracemization broadening of selective reflection and polarization bandof glassy chiral-nematic films

Glass-forming liquid crystals consisting of a cyclohexane central core with (S)-1-phenylethylamine and (4-cyanophenyl) naphthalene pendants were synth esized as hosts for racemizable (R)-dinaphtho[2,1-d : 1',2'-f][1,3]dioxepin , a chiral dopant. Chiral-nematic films 14, 22, and 35 mu m thick were prep ared for thermal and photoinduced racemization at temperatures from 95 to 1 30 degrees C, i.e. in the mesomorphic temperature range, over a period of h ours to days. Spatially modulated photoracemization was accomplished with a n insignificant contribution from the thermal process at temperatures aroun d 100 degrees C over a period of up to 3 h. With an absorbance per unit thi ckness of 6.2 mu m(-1) at 334 nm, the photochemical process was essentially confined to the irradiated surface, thereby setting up counter-diffusion o f the two enantiomers through the film, and hence the pitch gradient as vis ualized by atomic force microscopy. The significantly widened selective ref lection band was interpreted with the Good-Karali theory extended for a gra dient-pitch film. Furthermore, the bandwidth was found to increase with dec reasing racemization temperature or with increasing film thickness, further validating the presence of a pitch gradient as a result of controlled phot oracemization.