MODIFICATION OF THE PITCH OF CHIRAL NEMATIC LIQUID-CRYSTALS BY MEANS OF PHOTOISOMERIZATION OF CHIRAL DOPANTS

New photoisomerizable chiral dopants have been studied. The dopants us ed were menthone derivatives, a chiral stilbene derivative and a nemat ic copolymer of a menthone derivative and a benzoyloxybenzonitrile der ivative. NMR, HPLC and UV results showed that the E-Z-isomerization of all the compounds indicated proceeded rapidly upon UV exposure, witho ut the formation of undesired by-products. Isomerization of the mentho ne derivatives, including the copolymer, induced a substantial decreas e in the helical twisting power. The changes in helical twisting power induced by the isomerization of the chiral stilbene derivative were l imited to a factor of 2. In mixtures of the photoisomerizable dopants with commercial dopants of opposite twisting senses and a nematic host mixture, the sign of the twisting sense could be reversed by illumina ting the mixture with UV light. The viewing angle dependence of irradi ated regions of a 90 degrees-twisted nematic cell was rotated 90 degre es with respect to the viewing angle dependence of the non-irradiated regions. It is expected that this approach may be useful in the prepar ation of dual domain TN cells with a reduced viewing angle dependence.