Shape and Confinement Effects of Various Terminal Siloxane Groups on Supramolecular Interactions of Hydrogen-Bonded Bent-Core Liquid Crystals

To investigate the shape and confinement effects of the terminal siloxane groups on the self-assembled behavior of molecular arrangements in hydrogen-bonded (H-bonded) bent-core complexes, four H-bonded bent-core complexes Si, P1, C4, and P8 with string-, ring-, and cagelike siloxane termini (i.e., linear siloxane unit -Si-O-Si-O-Si, cyclic siloxane unit (Si-O)(4), and silsesquioxane unit PUSS, respectively) were synthesized and investigated. By X-ray diffraction measurements, different types of SmCG (B8) phases and leaning angles were controlled by the shape effect of the string- and cage-like siloxane termini for Si and P1 (with only one arm of H-bonded bent-core), respectively. In addition, the confinement effect of Pi, C4, and P8 (accompanied by increasing the numbers of attached H-bonded bent-core arms) resulted in higher transition temperatures and the diminishing of mesophasic ranges (even the disappearance of mesophase in P8). Moreover, AFM images showed the bilayer smectic CG phases of Si and P1 were aligned to reveal highly ordered thread-like structures by a DC field. By spontaneous polarization measurements within the mesophasic ranges, Si and P1 showed ferroelectric transitions but C4 displayed antiferroelectricity. Finally, the electro-optical performance of B8 phases could be optimized through binary mixtures of Si and P1, and a well aligned modulated ribbon phase could be formed via specific molar ratios of the binary mixtures.