Light-induced pitch transitions in photosensitive cholesteric liquid crystals: Effects of anchoring energy

Abstract

We experimentally study how the cholesteric pitch, P , depends on the equilibrium pitch P 0 in planar liquid crystal (LC) cells with both strong and semistrong anchoring conditions. The cholesteric phase was induced by dissolution in the nematic LC the right-handed chiral dopant 7-dehydrocholesterol (7-DHC, provitamin D 3 ) which transforms to left-handed tachysterol under the action of uv irradiation at the wavelength of 254 nm. By using the model of photoreaction kinetics we obtain the dependencies of isomer concentrations and, therefore, of the equilibrium pitch on the uv irradiation dose. The cholesteric pitch was measured as a function of irradiation time using the polarimetry method. In this method, the pitch is estimated from the experimental data on the irradiation time dependence of the ellipticity of light transmitted through the LC cells. It is found that the resulting dependence of the twist parameter 2D/P (D is the cell thickness) on the free twisting number parameter 2D/P 0 shows jump-like behavior and agrees well with the known theoretical results for the anchoring potential of Rapini-Papoular form.

Multiple minimum-energy paths and scenarios of unwinding transitions in chiral nematic liquid crystals

“…1). Such a situation may occur in the light-induced pitch transitions in CLCs doped with photosensitive chiral dopants [15] when under the action of irradiation the free twisting number q 0 changes from the initial value close to −π/L to the value close to zero.…”

“…An efficient method widely used to prepare CLCs is doping nematic LC mixtures with chiral additives that induce a helical structure [2,8]. For photosensitive chiral dopants (photoswitches), their helical twisting power and thus the CLC equilibrium helix pitch P 0 can be controlled by light through photoinduced changes in chiral molecular switch conformation that influence the LC's helical twisting power [11][12][13][14][15][16][17][18]. Phototunability of the helix pitch leads to a variety of technologically promising effects such as the phototunable selective reflection, i.e.…”

Effects of chiral dopant on electro-optical properties of nematic liquid crystal cells under in-plane switching and non-uniform vertical electric fields

“…The total rise time of the blue N*LC cell is about 2.1 ms. When the IPS electric field is released, a part of the LH N*LC molecular structure affected by the anchoring effect will start reorientating to return to the original SH N*LC molecular structure, and the parallel helical axis will be slightly tilted [53][54][55]. This causes the light transmittance at t > 0 to decrease to 0.98.…”

Hysteresis and Freedericksz thresholds for twisted states in chiral nematic liquid crystals: Minimum-energy path approach

“…The latter plays the role of the governing parameter for the pitch transitions theoretically studied in Refs. [13,[23][24][25][26][27][28][29]. These transitions occur between different branches of metastable states and, in particular, manifest themselves in a jump-like temperature dependence of selective light transmission spectra [30][31][32][33].…”

“…An efficient method widely used to prepare CLCs is doping nematic LC mixtures with chiral additives that induce a helical structure [2,8]. For photosensitive chiral dopants (photoswitches), their helical twisting power and thus the CLC equilibrium helix pitch P 0 can be controlled by light through photoinduced changes in chiral molecular switch conformation that influence the LC's helical twisting power [9][10][11][12][13][14][15][16]. Phototunability of the helix pitch leads to a variety of technologically promising effects such as the phototunable selective reflection, i.e.…”