Photochromic and nonlinear optical properties of azo-functionalized POSS nanoparticles dispersed in nematic liquid crystals

Abstract

It is well known that addition of a small amount of nanoparticles to liquid crystals may enhance their nonlinear optical properties. To date, in most reports, semiconductor- or metallic-core nanoparticles in LCs have been described. Here, we report on the synthesis of two types of novel nanoparticles based on a cubic siloxane cage (RSiO_3/2)₈, known as polyhedral oligosilsesquioxane (POSS) with the Si–O skeleton surrounded by covalently attached eight groups (R) of azo-benzene mesogenes differing in the length of their aliphatic chains. These dielectric-core nanoparticles substituted with mesogenic groups are well adapted to the calamitic liquid crystal matrix. Azo-benzene units add an important light-driven functionality to the system via their conformational photoisomerisation. We report on the photochromic properties of these new compounds in chloroform. By embedding the nanoparticles in a nematic liquid crystalline matrix, a functional hybrid nanocomposite material has been obtained. Our research was focused on the characterization of the third order nonlinear optical properties in this system linked to photochromism. The experimental results of all-optical switching in an optical Kerr effect configuration are discussed in the framework of light-induced modification of the aliphatic chains containing azo-benzene photoisomerizing moieties and following local director reorientation in nematic liquid crystals. The described properties are useful in the field of real-time holography.