Probing the pore structure of a chiral periodic mesoporous organosilica using liquid crystals

Abstract

Periodic mesoporous organosilicas (PMO) are prepared by the surfactant-templated condensation of bridged organosilsesquioxane monomers. By controlling the nature of the organic segment, the type of surfactant and the condensation conditions, one can control the physical and chemical properties of the resulting PMO and produce highly ordered porous structures with a periodicity on the nanometer scale. The development of chiral PMO materials has been of significant interest given their potential in heterogeneous asymmetric catalysis, chiral chromatography and non-linear optics. Characterization of the chirality of pore structures in these materials thus far has been achieved by indirect methods including polarimetry and solid-state circular dichroism. We report herein a general and convenient approach to probe directly the pore structure of chiral PMO materials based on their interactions with inexpensive liquid crystalline solvents, which result in the induction of measurable chiral properties in the nematic (N) and smectic A (SmA) phases of the liquid crystals. The templated co-condensation of a biphenylene organosilsesquioxane monomer and a chiral binaphthyl organosilsesquioxane monomer produced a new chiral PMO material that was investigated as dopant in two different liquid crystal hosts. Measurements of induced circular dichroism and helical pitch in the nematic phase of the cyanobiphenyl liquid crystal 5CB, and the measurement of an induced electroclinic effect in the SmA phase of the phenyl benzoate liquid crystal 9OO4 were carried out. The induced chiral properties measured in these experiments are consistent with chirality transfer taking place inside the pores, and suggest that the inner structure of the pores in the PMO material is indeed chiral.