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 <b>S1</b>, <b>P1</b>, <b>C4</b>, and <b>P8</b> with string-, ring-, and cage-like siloxane termini (i.e., linear siloxane unit −Si–O–Si–O–Si–, cyclic siloxane unit (Si–O)₄, and silsesquioxane unit POSS, 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 <b>S1</b> and <b>P1</b> (with only one arm of H-bonded bent-core), respectively. In addition, the confinement effect of <b>P1</b>, <b>C4</b>, and <b>P8</b> (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 <b>P8</b>). Moreover, AFM images showed the bilayer smectic CG phases of <b>S1</b> and <b>P1</b> were aligned to reveal highly ordered thread-like structures by a DC field. By spontaneous polarization measurements within the mesophasic ranges, <b>S1</b> and <b>P1</b> showed ferroelectric transitions but <b>C4</b> displayed antiferroelectricity. Finally, the electro-optical performance of B8 phases could be optimized through binary mixtures of <b>S1</b> and <b>P1</b>, and a well aligned modulated ribbon phase could be formed via specific molar ratios of the binary mixtures