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

Abstract

To investigate the shape and confinement effects of the terminal siloxane groups on the self-assembled behavior of molecular arrangements in hydrogen-bonded (Hbonded) bent-core complexes, four H-bonded bent-core complexes S1, 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 POSS, respectively) were synthesized and investigated. By Xray 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 S1 and P1 (with only one arm of H-bonded bent-core), respectively. In addition, the confinement effect of P1, 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 S1 and P1 were aligned to reveal highly ordered thread-like structures by a DC field. By spontaneous polarization measurements within the mesophasic ranges, S1 and P1 showed ferroelectric transitions but C4 displayed antiferroelectricity. Finally, the electro-optical performance of B8 phases could be optimized through binary mixtures of S1 and P1, and a well aligned modulated ribbon phase could be formed via specific molar ratios of the binary mixtures. ■ INTRODUCTIONBent-core or banana-shaped liquid crystals (LC) were first synthesized and investigated by Vorlander et al. in 1932. 1,2 Since the discovery of their special electro-optical properties by Niori et al., 3 many scientists became interested in the complicated mesophasic behavior, molecular biaxiality, layer polarity, and supramolecular chirality of achiral bent-core molecules. 4−7 These excellent electro-optical properties and structural characteristics might enable many applications, e.g., fast responsive LC displays, 8,9 light-scattering LC displays, 10 storage devices, 11 nonlinear optical appliances, 12 biosensors, 13 piezoelectric objects, 14 charge-generation interfaces, 15 bluephase materials, 16,17 supramolecular complexes, 18−20 biaxial mesogens, 21,22 and tunable chromophores. 23 On the basis of texture features and the results of X-ray diffractions, eight mesophases with smectic or columnar or three-dimensional orders, namely B1, B2, ..., B8 in chronological order, have been found in bent-core LCs with the effects of their configurations, including the zenithal constituents, the shapes of the rigid cores, the lengths of the flexible chains, and the roles of substituents at selected positions of rigid cores, on the properties of LCs. 6,7,24−27 As a result of the splay of polarization in smectic layers, 28 the modulation induces the breaking layers or ribbons to undergo long-range ordering in two-dimensional (2D) lattices of oblique (Col ob ), rectangular (Col r ), or hexagonal (Col h ) packing, classified as the B1 phase. 29 Depending upon the directio...