UV Induced Photoalignment and Colour Change in Nematic Liquid Crystals with Provitamin D Dopant I. Gvozdovskyy T. Orlova I. Terenetskaya Institute of Physics, National Academy of Sciences of Ukraine, Kiev, Prospekt Nauki, Ukraine Adsorption of provitamin D 3 molecules at glass=quartz substrate surface of a LC cell provided uniform alignment of nematic LCs in homeotropic orientation that was changed to the planar one under linearly polarized UV irradiation. Reversible switching between the two orientations has been achieved by applied electric field. The effects revealed are usable in optical imaging and data storage, in LC display technology and other applications. Keywords: data storage; optical imaging; photoalignment, INTRODUCTION Provitamin D photoisomerization is the first step in the complex network of vitamin D synthesis. UV irradiation of initial provitamin D within its absorption band (240–315 nm) leads to formation of pre- vitamin D by photoinduced hexadiene ring opening which is further converted into vitamin D by thermoinduced intramolecular hydrogen shift. However, previtamin D is not stable to UV radiation and under- goes a number of side photoconversions, its cis-trans photoisomeriza- tion into tachysterol is the most efficient one (quantum yield u  0:42) (Fig. 1) [1]. We are thankful to Merck for supplying liquid crystal materials and chiral dopants. The work was supported by the Science and Technology Centre in Ukraine (STCU Project No Gr-50). Address correspondence to I. Terenetskaya, Institute of Physics, National Academy of Sciences of Ukraine, 403028 Kiev-285, Prospekt Nauki, 46 Ukraine. E-mail: teren@iop.kiev.ua Mol. Cryst. Liq. Cryst., Vol. 430, pp. 199–203, 2005 Copyright # Taylor & Francis Inc. ISSN: 1542-1406 print=1563-5287 online DOI: 10.1080/15421400590946389 199