Chirality and Macroscopic Polar Order in a Ferroelectric Smectic Liquid-Crystalline Phase Formed by Achiral Polyphilic Bent-Core Molecules This work was supported by the Deutsche Forschungsgemeinschaft and the Fonds der Chemischen Industrie.

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...

Banana‐shaped side chain liquid crystalline siloxanes

“…The origin of the chiral domains is not yet clear. It seems that this is not due to formation of a helical superstructure, as initially proposed [20]. Presently, there are at least two models under discussion.…”

“…In contrast to most low molar mass bent-core molecules, with an antiferroelectric ground state [16,17], banana compounds with siloxane [15,[18][19][20] or carbosilane [21] groups attached, tend to show a ferroelectric organization due to a microsegregation of the three incompatible units [8]. The typical optically isotropic textures with domains of opposite handedness is often observed for bent-core molecules with siloxane segments.…”

Ferroelectric behavior of orthogonal smectic phase made of bent-core molecules

“…Walba et al attempted to suppress the antiferroelectric interlayer order by introducing branched terminal chains into bent-core molecules [5]. Several bent shaped materials with FE switching were reported, having chiral alkyl chains [6,7], oligosiloxane [8] or oligocarbosilane [9] terminal bulky groups. However, until recently ferroelectric properties were encountered only in tilted smectic phases, while the ferroelectric orthogonal smectic phase has been searched for as a promising candidate for new displays with the advantages of high response speed, high contrast, continuous gray level, and wide viewing angle [10].…”

Non‐symmetric bent‐core mesogens with one terminal vinyl group

“…When using the term 'asymmetry' this should not be confused with 'chirality'. Non-symmetry for banana-shaped compounds can and has been achieved in a number of ways: (1) by a non-symmetrically substituted central group [9,[13][14][15][16][17]; (2) by two different terminal end groups or chains [18][19][20][21][22][23][24][25][26][27][28]; (3) by different linking groups in each wing of the molecule [19,23,28]; (4) by different (polar) substituents in each wing of the molecule [20][21][22]26]. In most of these cases, however, the non-symmetric nature of the investigated compounds arises from a combination of two of these possibilities.…”