Conical mesogen

Mesogen composed of molecules containing a semi-rigid conical core. 

Very recently, chiral 1,4-diphenylbutadiene-based mesogens 16 have been found to show isothermal phase transition behavior from the SmA phase [56]. As shown in Figure 5.13, the thin film of 16 exhibited an SmA phase with a focal conic texture at 118 °C, and the photoirradiation with 360 nm light resulted in the disappearance of the focal conic texture accompanied with the appearance of an oily streak texture, indicating the phase transition from SmA to N (Fig. 5.13). Upon further irradiation, the complete conversion was observed. 

The stmcture of the SmA phase does not change if the mesogens are chiral, but some of its properties do, e.g. the response to an applied electric field [21], Therefore, the chiral SmA phase is denoted as SmA. Between crossed polarizers, the SmA, the SmA as well as the lamellar L phase initially form so-called batonnets under planar anchoring conditions if emerging directly from the isotropic phase. These batonnets then condense into a focal conic fan-shaped textiue. If the director is oriented parallel to the viewing direction, the texture appears black between crossed polarizers, which is referred to as homeotropic . It is also possible to find oily streaks within the homeotropic texture, if the alignment of the sample is imperfect. 

Fig. 6 Schematic representation of the conical layer distribution of polydomain Sc elastomers exposed to uniaxial mechanical deformation (a) and corresponding X-ray pattern (b). 0 (denoted < > in this figure) is the Sc tilt angle, d the smectic layer spacing, and / the length of the mesogenic units. The layer normal k is conically distributed around the stress axis z. Reprinted with permission from [87]. Copyright (2008) American Chemical Society...

Nierengarten et al. reported liquid crystalline pillar[5]- and pillar[6]ar-enes. They synthesized pillar[5]arene 6.32 and pillar[6]arene 6.33 with 10 and 12 mesogenic cyanobiphenyl units, respectively (Figure 6.20). Both compounds showed smectic A phase (focal-conic fan texture and homotropic areas) over a broader temperature range when compared with the model compound. This can be explained as the macrocyclic effect Inter-molecular 71-71 interactions between neighboring pillar[n]arene cores stabilized the smectic A phase. 

Pyramidal or bowlic mesogen - a mesogen composed of molecules derived from a semirigid conical core. 

Figure 1. A smectic A phase extended on the left-hand side, and showing defect lines in the form of conics a smectic rodlet and small germs are floating in the isotropic phase. The mesogenic product, 4-cyano-4 -n-octylbiphenyl (8CB), was added together with a small amount of Canada balsam, and the mixture was observed between crossed polarizers at room temperature. Canada balsam is an isotropic but optically active and fluid resin, extracted from the conifer Abies balsamea, which facilitates the production of regular smectic textures in several thermotropic liquids. Collophony, or rosin, is a stabilized pine tree resin, used to rub violin bows Friedel used this substance not only for his violin, but also to obtain remarkable smectic textures. Scale bar 20 pm.

We have already considered the polygonal networks of cholesteric liquids, which assemble saddle-shaped layers close to Dupin s cyclides, and the focal segments of which differ from those of focal conics (see Sec. 7.4.3). Friedel and Grandjean [21] described in smectic A phases a texture much closer to Dupin s cyclides, with genuine focal conics. As for cholesterics, this texture is obtained when a thin film of the isotropic phase of the mesogenic compound separates the mesophase from the two glasses. 

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