A state of matter in which molecules show a long-range orientational order and no positional order is called a nematic liquid crystal. The best known and most widely used (for example, in modern displays) is the uniaxial nematic, with the rod-like molecules aligned along a single axis, called the director. When the molecules are chiral, the director twists in space, drawing a right-angle helicoid and remaining perpendicular to the helix axis; the structure is called a chiral nematic. Here using transmission electron and optical microscopy, we experimentally demonstrate a new nematic order, formed by achiral molecules, in which the director follows an oblique helicoid, maintaining a constant oblique angle with the helix axis and experiencing twist and bend. The oblique helicoids have a nanoscale pitch. The new twist-bend nematic represents a structural link between the uniaxial nematic (no tilt) and a chiral nematic (helicoids with right-angle tilt).
Some hydrocarbon linked mesogenic dimers are known to exhibit an additional nematic phase (N x ) below a conventional uniaxial nematic (N u ) phase. Although composed of non-chiral molecules, the N x phase is found to exhibit linear (polar) switching under applied electric field. This switching has remarkably low response time of the order of a few microseconds. Two chiral domains with opposite handedness and consequently opposite responses are found in planar cells. Uniformly lying helix, electroclinic, and flexoelectric effects are given as possible causes for this intriguing phenomenon. A change in the direction of the optical axis by a moderate electric field is the basis of the use of liquid crystals (LCs) in contemporary display technologies. However, further exploitation of this property in optical telecommunication technologies is hindered by the limited speed of the optical axis switching. Amongst the fastest electro-optic effects in LCs are surface stabilized ferroelectric smectic LCs, 1 uniformly lying helix (ULH) geometry in flexoelectric cholesteric LCs, 2 electro-optic effect in blue phases, 3 and the electroclinic effect 4 found in both chiral smectic and cholesteric LCs. These switching modes are defined by the asymmetry of the chiral molecules forming the corresponding LC phases.Meanwhile, non-chiral dimers of mesogenic molecules linked with a flexible hydrocarbon chain with odd number of alkyl units have recently attracted attention due to the presence of an unusual liquid crystalline phase (currently designated as N x ) in the temperature range below the classical nematic phase (N u ).  Although identified as a nematic phase by x-ray diffraction studies, the phase exhibits clearly different patterns in polarised optical microscopy (POM) observations as well as a difference in the enthalpy from N u measured by the differential scanning calorimetry. 5 The ability of this class of materials to spontaneously form unusual stripe patterns with periodicity defined by the gap between containing surfaces is promising for applications in photonics. A theoretical explanation connecting the molecular properties to the macroscopic self-assembly properties is still to be developed.In this Letter, we report one more intriguing property of the N x phase: it exhibits polar switching with remarkably low switching time under electric field in a similar manner as the above mentioned materials involving chirality.The molecular structures of materials under investigation are shown in Fig. 1. We have investigated both pure dimers (M1-3) and mixtures of M4 with 4-4 0 pentyl-cyano-biphenyl (5CB) (70/30% w/w) and of M2 with its monomer (65/35% w/w). A number of cells with cell gaps varying from 2 to 25 lm and with different alignment layers have been used. These include anti-parallel planar commercial cells (EHC. Co., KSRP-XX-A2 jj P1NSS), homemade planar cells (planar aligning agent RN1175, Nissan Chemicals, Japan), and homemade hybrid aligned cells (homeotropic aligning agent AL60702 JSR, Korea). The experim...
Liquid crystals (LCs) represent one of the foundations of modern communication and photonic technologies. Present display technologies are based mainly on nematic LCs, which suffer from limited response time for use in active colour sequential displays and limited image grey scale. Herein we report the first observation of a spontaneously formed helix in a polar tilted smectic LC phase (SmC phase) of achiral bent-core (BC) molecules with the axis of helix lying parallel to the layer normal and a pitch much shorter than the optical wavelength. This new phase shows fast (∼30 μs) grey-scale switching due to the deformation of the helix by the electric field. Even more importantly, defect-free alignment is easily achieved for the first time for a BC mesogen, thus providing potential use in large-scale devices with fast linear and thresholdless electro-optical response.
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