Organic–inorganic hybrid liquid crystals derived from octameric silsesquioxanes. Effect of the peripheral groups in mesogens on the formation of liquid crystals

Abstract

A series of octameric silsesquioxane-based organic–inorganic hybrid compounds 7a–h was synthesised by reacting octakis(dimethylsiloxy)silsesquioxane with 4′-(undec-10-enyloxy)biphenyl-4-yl mono-, di-, or trialkoxybenzoateviaplatinum-catalysed hydrosilylation reaction. The chemical structures of 7a–h were characterised by ¹H, ¹³C, and ²⁹Si nuclear magnetic resonance spectroscopy, mass spectroscopy and elemental analysis. Liquid crystallinity of 7a–h was investigated by differential scanning calorimetry (DSC) and polarised optical microscopy (POM). Temperature-dependent X-ray diffraction was used to verify liquid crystal phases, revealing that 7a–d assembled to smectic C and smectic A phases. The effects of the number and length of peripheral flexible alkoxy groups in 7a–h on the formation of liquid crystals were studied using DSC and POM, which revealed that 7a–d bore one alkoxy terminal group in each mesogen exhibited a mesophase, 7e–f and 7g–h bore two and three flexible alkoxy terminal groups in each mesogen, respectively, and did not show mesomorphic behaviour. The concept of “flexible group density” (FGD), which was defined as the number of the CH₂ groups per unit volume in the periphery of the POSS molecule, was introduced to interpret the formation of liquid crystals. The results revealed that the POSS molecules with di- and tri-substituted mesogens exhibited much larger FGD than those with mono-substituted mesogens, hence the absence of mesophases for 7e–f and 7g–h. This study provides a useful archetype to predict the formation of liquid crystals when designing and synthesising liquid crystalline materials, particularly star-like or dendrimer-like molecular materials.