Evolution of light-induced anchoring in dye-doped nematics: Experiment and model

Abstract

A series of experiments was carried out to describe the evolution of light-induced anchoring in dye-doped nematic liquid crystals (LCs) at irradiation with polarized light. The experiments included cells filled with a pure pentyl-cyanobiphenyl (5CB) and containing a layer of azo dye deposited on an aligning film, as well as cells filled with azo dye doped 5CB, which allowed us to distinguish the role of "surface" and "bulk" dye molecules in the evolution of light-induced anchoring. Modifications of the spectra of spontaneously adsorbed dye molecules under illumination enabled us to assert that light-induced desorption is a mechanism responsible for producing an easy orientation axis in a dark-adsorbed layer. We found that the evolution of light-induced anchoring involves a competition between light-induced desorption and adsorption of the dye molecules on the aligning surface, and the final anchoring is determined by the total light irradiation dose. These data allowed introducing a theoretical model of light-induced anchoring of dye-doped nematic LCs that quantitatively described the experimental results and portrayed the whole evolution of the dye-doped LC cell at irradiation.

Effect of surface condition on liquid crystal photoalignment by light-induced azo dye adsorption phenomena

“…The photoalignment technique ensures that LCs are typically aligned on a substrate by irradiating the surface that contains photoreactive molecules with a linearly polarised light (visible or ultraviolet light). Over the past decades, the experimental results of anchoring transition caused by several external factors, such as temperature, [11] electric field [12] and photoisomerisation, [7,[13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32] have been reported.…”

P-140: Holograms Recording in AZO Nematic Liquid Crystal and in its Solid Polymer Solutions

“…Result of these processes may vary photochromic isomeric composition of the mixture, and influences the temperature range of the nematic phase and the optical properties of the LC layer. However, the unusual behavior of dark relaxation makes us suggest that the role of molecules LC-440 adsorbed in the surfactant is high [8,9]. One can assume that these surfactants adsorbed molecules are sufficiently photochemically stable, and the phase transition "nematicisotropic liquid" occurs due to accidental refocusing of adsorbed LC molecules during absorption and non-radiative relaxation quanta activating radiation and as a consequence, the transition of the non-oriented surface structure the amount of the LC layer.…”

Reversible Photoinduced Anisotropy of Azoxybenzenes in Nematic Liquid Crystal Phase and in Solid Polymer Solutions

“…В результате подобных фотохромных процессов может изменяться изомерный (химический) состав смеси, влияющий на температурный интервал нематической фазы и оптические характеристики ЖК-слоя. Однако необычность поведения темновой релаксации (возможность перехода в НЖК-состояния не только нагревом, но и охлаждением, характерное микроскопическое поведение при прямом и обратном процессе) позволяет считать, что роль поверхностно адсорбированных молекул ЖК-440 высока [22,23]. При этом можно пред-положить, что данные поверхностно адсорбирован-ные молекулы достаточно фотохимически стабиль-ные, а фазовый переход нематик -изотропная жидкость происходит за счет случайной пере-ориентации адсорбированных ЖК-молекул при поглощении и безызлучательной релаксации кван-тов активирующего излучения и, как следствие, переходе данной неориентированной поверхност-ной структуры в объем ЖК-слоя.…”