Field-induced switching of chirality in undulated ferroelectric and antiferroelectric SmCP phases formed by bent-core mesogens.
Field-induced switching of chirality in undulated ferroelectric and antiferroelectric SmCP phases formed by bent-core mesogens.
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DOI：
10.1002/anie.200461490
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发表时间：
2005-01
影响因子：
--
通讯作者：
R. A. Reddy;M. Schröder;M. Bodyagin;H. Kresse;S. Diele;G. Pelzl;W. Weissflog
中科院分区：
文献类型：
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作者：
R. A. Reddy;M. Schröder;M. Bodyagin;H. Kresse;S. Diele;G. Pelzl;W. Weissflog
手征性是物质的基本属性。在大多数情况下，手性是一种固有的分子性质，但它也可以发生在固体晶体中，即使组成部分是非手性的，例如，作为结晶过程中自发区分的结果。在由非手性分子组成的液体体系中，由于粒子的流动性，手性似乎是不可能的。然而，非手性双核分子形成的新型中间相也能够表现出相手性。分子可以被包装在一个极性的方式，因为它们的弯曲形状，并引起一个长程相关的横向偶极子，并因此在近晶（SmC）层的宏观极化。[1]在最广泛研究的SmCP相（P=极性），极性包装的分子相对于层的法线倾斜。如Link et al. [2]根据相邻近晶层中分子的堆叠，可以区分出四种结构（图1）。分子可以具有向斜或背斜层间相关性，这在相符号SmCP中由C后的下标S和A表示。极性顺序由下标A和F指定，其中A对应于反铁电结构，F表示铁电结构（因此可能的相位符号是SmCAPA，SmCSPA，SmCAPF和SmCSPF）。[2]SmCP相分子的极性堆积和倾斜是对称性破缺的两个步骤，这两个步骤产生了没有任何分子手性的层手性。如图1所示，对于给定的倾斜方向，存在两个具有反平行极轴的等效层结构，它们是彼此的镜像。相反的手性由实心（黑色）或空心（白色）分子符号表示。在SmCAPA和SmCSPF结构中的宏观结构域的所有层具有相同的层手性（纯手性状态），而在SmCSPA和SmCAPF结构中的层与层之间的手性交替（外消旋状态）。为了避免体极化，反铁电基态存在于大多数情况下，其中极化在相邻层中交替。[3]反铁电态可以切换到相应的铁电态（图1）。这种fieldinduced的重新取向发生通过旋转的董事周围的垂直倾斜锥上的层，如发生在铁和反铁电相的calamitic化合物。层的手性在此切换过程中被保留，并且对于两个铁电状态之间的切换也是如此。最近，人们发现，在“香蕉相”的极性转换也可以通过另一种机制，这是基于集体旋转的分子围绕他们的长轴（图2a）。在极性近晶A（SmAP）相[4，5]和由SmAP样层碎片形成的B1 rev相中[6]，这种机制是极性转换的唯一可能性。有几个例子表明，这种转换机制也可以发生在倾斜的SmCPA相[7-9]以及具有SmCP样层碎片的B1 rev相中[6]，主要是在特殊的实验条件下。Nakata等人[8]和Bedel et al. [9]能够在临界电场之上检测到这种开关机制。Schröder等人[7]观察到这种极性转换只有在非常缓慢的增加或减少的电场。应该强调的是，场致开关在倾斜
Chirality is a fundamental property of matter. In most cases chirality is an inherent molecular property, but it can also occur in solid crystals even though the constituent moieties are nonchiral, for example, as a consequence of a spontaneous discrimination during the crystallization process. Chirality in liquid systems formed by nonchiral molecules seems to be impossible because of the mobility of the particles. However, there are new kinds of mesophases formed by nonchiral bentcore molecules which are also able to exhibit phase chirality. The molecules can be packed in a polar fashion because of their bent shape and give rise to a long-range correlation of the lateral dipoles and therefore to a macroscopic polarization in the smectic (SmC) layers.[1] In the most widely studied SmCP phase (P= polar), the polar packed molecules are tilted with respect to the normal of the layer. As first shown by Link et al.[2] four structures can be distinguished depending on the stacking of the molecules in adjacent smectic layers (Figure 1). The molecules can have either a synclinic or an anticlinic interlayer correlation which is indicated in the phase symbol SmCP by the subscripts S and A after C. The polar order is specified by the suubscripts A and F, where A corresponds to an antiferroelectric structure and F designates a ferroelectric structure (the possible phase symbols are thus SmCAPA, SmCSPA, SmCAPF, and SmCSPF).[2] The polar packing as well as the tilt of the molecules in the SmCP phases are two steps of symmetry breaking which create a layer chirality without any molecular chirality. As seen in Figure 1, two equivalent layer structures with antiparallel polar axes exist for a given tilt direction which are mirror images of each other. The opposite handedness is depicted by filled (black) or open (white) molecule symbols. All layers of a macroscopic domain in the SmCAPA and SmCSPF structures have the same layer chirality (homochiral state) whereas the chirality alternates from layer to layer in the SmCSPA and SmCAPF structures (racemic state). To avoid bulk polarization an antiferroelectric ground state exists in most cases where the polarization alternates in adjacent layers.[3] The antiferroelectric states can be switched to the corresponding ferroelectric states (Figure 1). This fieldinduced reorientation takes place through rotation of the director around the normal to the layer on the tilt cone such as occurs in ferro-and antiferroelectric phases of calamitic compounds. The chirality of the layers is preserved during this switching process and is also the case for a switch between two ferroelectric states. Recently, it was found that the polar switching in “banana phases” can also take place by another mechanism, which is based on the collective rotation of the molecules around their long axes (Figure2a). In polar smectic A (SmAP) phases [4, 5] and in B1rev phases formed by SmAP-like layer fragments [6] this mechanism is the only possibility for polar switching. There are a few examples that this switching mechanism can also occur in tilted SmCPA phases [7–9] as well as in B1rev phases with SmCP-like layer fragments,[6] mostly under special experimental conditions. Nakata et al.[8] and Bedel et al.[9] were able to detect this switching mechanism above a critical electric field. Schröder et al.[7] observed this kind of polar switching only on very slow increase or decrease of the electric field. It should be emphasized that the field-induced switching in tilted