Chirality and Macroscopic Polar Order in a Ferroelectric Smectic Liquid-Crystalline Phase Formed by Achiral Polyphilic Bent-Core Molecules

Abstract

Materials with a macroscopic polar order have a variety of useful properties, such as piezo-and pyroelectricity and second-order nonlinear optical activity [1, 2] Especially ferroelectric (FE) and antiferroelectric (AF) liquid crystalline (LC) materials are of great interest, because they can be rapidly switched between different states by means of external electrical fields. [3, 4] These properties makes them useful for numerous applications, such as electrooptic devices, information storage, switchable NLO (nonlinear optic) devices and light modulators, which may be of interest for optical computing and other future technologies. At first, smectic LC phases with tilted arrangements of nonracemic chiral rodlike and disclike molecules have been used for this purpose and for a long time molecular chirality appeared to be essential for obtaining such materials. [3] However, the discovery by Niori et al. that bent-core mesogenic compounds (banana-shaped molecules) without molecular chirality, can also organize in fluid smectic phases with a polar order opened a new area in the field of LC research. [5,6] The polar structure of the smectic layers of such molecules is provided by the dense directed packing of their bent aromatic cores. However, to escape from a macroscopic polar order the bent direction in adjacent layers is antiparallel, so that the layer polarization alternates from layer to layer, which leads to a macroscopic apolar AF structure. [7] In most cases of such mesophases the molecules are additionally tilted relative to the layer normal. [8] Therefore these phases (also known as ™B2∫-phases) can be described as tilted smectic phases (SmC) with a polar order of the molecules (P) within the layers, and an antiparallel polarization in adjacent layers (A), which leads to the notation SmCP A . Because the molecules in adjacent layers can have either a synclinic (molecules in adjacent layers are tilted in the same direction, C S ) or an anticlinic (molecules in adjacent layers are tilted in opposite directions, C A ) interlayer correlation, the four different phase structures shown in Figure 1 may result for such mesophases. [7] Usually, the AF phases represent the ground states, whereas the FE states (SmC S P F and SmC A P F ) can only be achieved after with response times of less than one minute. The average response ratio at 3 ppm NO 2 was 1.16.Individual SnO 2 nanoribbons are small, fast and sensitive devices for detecting ppm-level NO 2 at room temperature under UV light. These nanodevices can be operated under laboratory conditions over many cycles without loss of sensitivity. The advantages of low-temperature, potentially drift-free operation make SnO 2 nanoribbons good candidates for miniaturized, ultrasensitive gas sensors in many applications. Further sensitivity increases should be achievable by using thinner nanoribbons, developing ohmic SnO 2 ± metal contacts and decorating these structures with catalysts. With such innovations, the chemical detection of single molecules on nanowires may soon b...

Unusual electric-field-induced transformations in the dark conglomerate phase of a bent-core liquid crystal

“…One of them is that under polarizing microscope, we see no chiral domains of opposite handedness in the ground state of the DC phase. As we discussed, the DC phase consists of SmCP structure with short range interlayer correlation length [10][11][12][13][14][15][16][17][18][19]. The absence of opposite handed domains in our sample indicates two possibilities for the ground state arrangement of the layers: The adjacent layers are either homochiral (SmC s P S or SmC a P A type), making the overall structure chiral, though the opposite handed domains are smaller than the wavelength of the visible light.…”

“…During this process a simultaneous reversal in the polarization and the tilt direction takes place giving rise to a switching from SmC s P A to SmC a P S and SmC a P A to SmC s P S . Bent-core mesogens are well known for the optically isotropic phases they exhibit, which include the B4 phase [7][8][9], the dark conglomerate (DC) [10][11][12][13][14][15][16][17][18][19] and the dark enantiomeric (DE) phases [20]. The DC phase, which is the topic of interest of this paper, is usually found to occur below the isotropic phase.…”

Dark conglomerate phases of bent-core liquid crystals

“…The in-layer frustration caused by orthogonal tilting of the molecular segments can be relieved by the saddle-splay curvature: in three dimensions, the DC phases have significant negative Gaussian curvature. Depending on the local structure, the DC phases are classified as liquid crystalline sponge-like phases [28][29][30][31][32][33][34][35][36][37] and Helical Nano-Filament (HNF) phases or B4 phases [38][39][40]. In addition to these, a number of DC phases with intermediate or distinct nanostructures have been observed [41][42][43][44][45][46][47][48].…”

Electric‐ and Light‐Responsive Bent‐Core Liquid Crystals: From Molecular Architecture and Supramolecular Nanostructures to Applications

“…Incorporation of a perfluorinated chain segment or silicon-containing unit (e.g., siloxane and carbosilane) into the hydrocarbon chain as an end group is useful to modulate mesophase properties. It was reported that mesophase stabilization was observed for bent-core molecules including two partially perfluorinated tails [36,94], and bent-core molecules incorporating nanosegregated siloxane end groups into one or two tails display a transformation from AF to FE switching [95].…”

“…The Tschierske group reported that incorporation of terminal siloxane units at one tail of bent-core molecules leads to a transition from AF to FE switching [95]. A large number of such molecules including siloxane and carbosilane units were subsequently reported [134][135][136][137][138][139][140][141][142][143][144][145][146][147], and many of them exhibit DC phases with surface-stabilized bistable FE switching.…”