Abstract
The two main classes of liquid-crystal (LC) phases of rodlike molecules are nematics, where the rods align in the same direction (the nematic director n), and smectics, where the rods not only are aligned but also form layers. The electro-optic effects in LC devices that are a backbone in today’s display industry mainly use the Freedericksz transition, which is the bulk reorientation of a surface-anchored nematic by an electric field. Conventional (uniaxial) smectics do not present a Freedericksz transition, because, due to their layered structure, the director reorientation would distort the layers, which would cost too much energy. In a worldwide ongoing effort to extend the variety of LC compounds suitable for applications in the display industry, bent- shaped molecules have recently raised much attention, since they present multiple new LC phases with unusual properties. In this paper, we report on a structural and electro-optic study of the LC phases of a bent-shaped dimer. On cooling from the isotropic liquid, this compound shows a usual nematic (N), a twist- bend nematic (NTB), and a biaxial smectic-A phase (Sm Ab). Quite surprisingly, contrary to usual smectics, Sm Ab presents a remarkable electro-optic response, with low ( < 4 V) voltage threshold, no reorganization of the smectic layers, and low ( < 1 ms) response time (i.e., 30 times faster than the N phase at higher temperature). We interpret this unexpected electro-optic effect as a Freedericksz transition affecting the secondary director m of the Sm Ab, and we model it by analogy with the usual Freedericksz transition of the n director of the uniaxial N phase. Indeed, a Freedericksz transition affecting only m in this biaxial fluid smectic does not alter its layered structure and costs little energy. From the point of view of applications, thanks to its low relaxation time, this “biaxial” Freedericksz transition could be exploited in electro-optic devices that require fast switching.
Highlights
IntroductionLiquid crystals (LCs) are intermediate phases (mesophases) observed, upon melting, between the crystalline state and the usual (isotropic) liquid state
Liquid crystals (LCs) are intermediate phases observed, upon melting, between the crystalline state and the usual liquid state
The most common nematic (N) phase used in electro-optic displays has uniaxial symmetry around n, but there are other kinds of nematics, such as chiral ones, the still elusive biaxial nematic, which has lower orientational symmetry, and the spontaneously modulated nematics where the orientational order is periodic in space
Summary
Liquid crystals (LCs) are intermediate phases (mesophases) observed, upon melting, between the crystalline state and the usual (isotropic) liquid state. Smectics have lower symmetry than usual nematics, because they have, in addition to orientational order, long-range positional order of the centers of mass of the LC molecules in at least one direction of space, resulting in a layered structure. The most common smectic, called smectic A (SmA), is a uniaxial phase where the director is parallel to the normal to the layers and the molecules have no long-range positional order within the layers, which are, fluid. The molecules may be tilted with respect to the normal to the layers (SmC), or they may have long-range positional order within the layers (SmB) or even both (SmG)
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