Clearing alkyl substituted mesogens

Recently, Hoffmann et al. [12] have described the molecular and crystalline solid structure of 2-n-nonyl-1,4-phenylene bis(4-n-octyloxybenzoate), as estimated using X-ray diffraction [12]. The observed structure deviates from the model for the nematic phase in the crystalline solid state, as the lateral chains exist in -trans conformation. The type of link between the lateral hydrocarbon chain and the mesogenic core influences the course of the clearing points. A comparison performed for the 2-alkyl, acyl and alkyloxycarbonyl substituted derivatives 2a-2c in Table 1 showed a break in the clearing-point curve at 2b and 2c for five single units (including the -CO- or -CO-0 groups) within the lateral chains. The lower the transition temperatures the more pronounced the break in the curve [13, 15]. 

Three-ring mesogens with two longer lateral alkyl chains (e.g. 12) do not show mesophases. However, cross-shaped compounds (30) have nematic phases (uniaxial) with clearing points above 100°C [46, 59]. That is surprising because 1,2,4-tris(4-n-alkyl-oxybenzoyloxy)benzenes (19f) are not liquid crystalline, but l,3,5-tris(4-n-alkyloxy-phenyl)benzoates exhibit nematic discotic phases [83]. Apparently, such substituted benzene derivatives are located at the boundary between calamitic and discotic compounds. However, there is no example of this type that shows both columnar and nematic or smectic phases, as occurs in poly-catenar and double-swallow tailed compounds (see Sec. 5 and Chap. XII of this volume). 

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