Hydrocarbon substituted mesogens

Keating theory, ehiral nematics 366 Kelvin chirality, ferroelectrics 543 Kerr effect, nematics 175, 181 Kerr ellipsometry, charge transfer systems 956 keto groups, hydrocarbon cores 714 ketoximesters, substituted mesogens 841 kink angles, ferroelectric devices 639 Knorr condensation, heterocyclic cores 727 Krafft temperature, chromonics 982... 

Any increase of polarity of the hydrocarbon, for example by substitution with chlorine or by replacing CH2 groups by ether oxygens, also increases incompatibility with perfluoroalkanes as polar interactions become more important [89, 96]. Polar groups are fundamental constituents of the mesogenic cores, especially as linking units and as substituents at aromatic moieties, and these groups increase the incompatibility between the aromatic cores and Rp-chains. 

Spurred on by discussions on whether the smectic layers of the fluorocarbon-substituted polymers XXIII-m-n are induced by the immiscibility between hydrocarbon and fluorocarbon segments or by the fact that fluorocarbon segments form rod-like mesogenic units by themselves, Pugh et al. synthesized a series of siloxane-terminated monomers XXIV-m-n, with m=l,2 and n=4-S [79]. In these monomers, the rigid fluorocarbon segment was replaced by short polydimethylsiloxane (PDMS) segments that are also immiscible with hydrocarbons but are very flexible (Fig. 15). 

Triblock partially fluorinated hydrocarbons F/nHiiFin(m = 12, n = 8 m = 10, n = 10 m = 12, n = 12) have also been examined and all are found to exhibit very narrow (< ) range of thermotropic smectic B phases between their crystalline and isotropic liquid phases [88]. When the polymethylene portions of such molecules are replaced by a much stiffer pam-substituted aromatic ring, the mesogenic character is lost [89]. 

To construct helical columns several studies have used a different strategy where the core structure is dissymmetric. Saturated mesogens that are derived from chiral saccharides [80,81] and inositols [82] have a core that is chiral and are substituted with hydrocarbon chains. These materials stack into columnar structures but do not show any macroscopic manifestation of the core chirality. This is likely due to poor chiral information transfer from members of the stack. 

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]. 

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