SmCP phases

Stereogenic Elements in SmCP Phases SmCP Supermolecular Diastereomers... 

One of the key experimental results leading to the elucidation of this overall structural puzzle involved depolarized reflected light microscopy (DRLM) studies on NOBOW freely suspended films in the high-temperature SmCP phase.48 In the freely suspended films it appears that only one phase is observed, which is assumed to be the phase forming the majority domains in the EO cells. The DRLM experiment provides two key results. First, thin films of any layer number have a uniformly tilted optic axis, suggesting all of the layer interfaces are synclinic. Second, films of even-layer number are nonpolar, while films of odd-layer number are polar, with the polar axis oriented normal to the plane of the director tilt (lateral polarization). 

Figure 8.26 Illustration of supermolecular structure and pairwise relationships for isomeric SmCP phases is given.

Hydrogen bonding has also offered a convenient and versatile approach to the design of bent-core mesomorphism. For example, the use of carboxylic acids and stilbazoles 9 and 10, which act as H-donor and H-acceptor, respectively, has afforded bent-shaped complexes. The SmCP phases (Fig. 5.3) represented by such... 

M.W. SchrSder, S. Diele, G. Pelzl and W. Weissflog, Field-induced switching of the layer chirality in SmCP phases of novel achiral bent-core hquid crystals and their unusual large increase in clearing temperature under electric field application, Ghem. Phys. Chem. 5(1), 99-103, (2004). 

FIGURE 6.9 (a) AF and FE switching in SmCP phases formed by bent-core molecules (b) two basic switching mechanisms switching around the molecular long axes (A) reverses layer chirality, and switching on a smectic cone (B) retains layer chirality. The polar direction is not illustrated in (b) since it can be either parallel or antiparallel to the bend direction depending on the molecular structures. 

Since most of bent-core mesophases (e.g., Bl, B2, B5, B7, B7-like, and B8) possess intrinsic polar order, they are responsive upon applying an external stimulus such as an E-field, exhibiting either AF or EE switching or both. In this section, the author will focus on polar SmCP phases including B2 and DC, SmAP, and polar Col phases and review their field-induced switching, chirality transformation, and structural transformation. 

Besides the normal B2 phases, DC phases forming by different types of bent-core moleculesx [122-130], also belong to SmCP phases. The most striking feature of DCs is to have little or no birefringence like isotropic liquids, and their textures are... 

SmCP phases exhibit high spontaneous polarization (Pg = 500-800 nC cm ) which indicates a high degree of polar order (>50%). In contrast, only a few percent of chromophores are aligned in poled polymers, or 20% in conventional FLCs. However, since the early bent-core molecules incorporated chromophores with small molecular hyperpolarizabilities, the measured NLO coefficients (1.9 < D (pmA ) < 8 1 < d (pmA ) <4, X. = 1.064 p,m) via SHG for bent-core compounds is insufficient for NLO applications [182-185]. 

Schroder, M. W Diele, S. Pelzl, G. Dunemann, U. Kresse, H. Weissflog, W. Different nematic phases and a switchable SmCP phase formed by homologues of a new class of asymmetric bent-core mesogens. J. Mater. Chem. 2003, 13, 1877-1882. 

Weissflog, W. Schrikler, M. W. Diele, S. Pelzl, G. Field-induced formation of the polar SmCP phase above the SmCP-isotropic transition. Adv. Mater. 2003, 15, 630-633. 

Note The phase was identified as polar tilted smectic (SMCP) phase. Originally only one By phase was proposed, but it turned out that they correspond to at least two different phases (Byj and By.n, where Byj is basically a columnar phase, and Byjj has modulated smectic layer structure. 

In the tilted polar smectic (SmCP) phase, the texture depends on the overall chirality, i.e., whether the director field is synclinic or anficlinic (see a few examples in Figure 6.14). 

Textures in the SmCP phase, (a) 4-Chloro-l,3-phenylene bis[4-4(4-octyloxy phenyliminomethyl) benzoates in the chiral state (b) the same material in the racemic state (both at 125°C) (c) a sulphur-containing material B-IO(S) at 120°C and at 130°C (d). The domains with red color comespond to anticlinic structure, whereas those with green color have syncHnic structure. Bar 100 im. 

The natures of the different subphases and their main characteristics are still the subject of active research. Some models state that in the subsequent layers the polarization is always either parallel or antiparallel, and the macroscopic behavior depends on the number of repeating units, which should be more than of two layers range. For example, in the so-called SmCp phase, the zero field polarization is 2/3 of the saturated vale, which in frame of the orthogonal model would mean interaction of six layers in five consecutive layers in one direction, and in the sixth the opposite ((5 - l)/(5 + 1) = 2/3). This long-range interaction seems to be very unlikely. For this reason, the so-called "clock model" was suggested. This assumes only interactions of two layers, but does not require that the polarization directions in the neighbor layers be either parallel or antiparallel. This seems to be supported by... 

In this case the smectic layers have monoclinic chiral symmetry C2 It is denoted as SmCP phase (P stands for "polar"). Most of the experimental results concern this phase, and will be summarized below. 

Sketch of the racemic and chiral structures of the SmCP phase of achiral banana-shape molecules in antiferroelectric (at E = 0) and ferroelectric (at E > Etj, 5 V/ om) states. Left column Racemic structure, in which the chirality alternates in the adjacent layers. The APE domains are synclinic with coexisting opposite tilt directions. Right column chiral structure, in which the adjacent layers have the same handedness, but domains of different chirality coexist. The shading illustrates the bent or tilted shape of the molecules (brighter parts are closer to the reader). R (L) is the chirality descriptor corresponding to right (left(-handed layer conformations. 

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