Alkyl lateral substituents

The decoration of the ribbon structure with various substituents has appeared as a crucial step for obtaining sufficient polymer solubility. On the other hand, the presence of the long alkyl or alkoxy substituents typically used may have unwanted consequences (1) the presence of lateral substituents drastically changes the mol ular dimensions of the macromolecules (length/ width ratio) the mechanical and rheological properties, as well as the chemical... 

FIGURE 14.1 A general schematic representation of (a) rod-like and (b) bent-core liquid crystal molecules with two terminal alkyl chains. L is the linking group, C is the terminal chain connector, Z is a lateral substituent, Z is a chain substituent, and H is the head ring. 

In laterally alkyl substituted mesogens, each branch in a terminal or lateral substituent further decreases the mesophase stability dramatically (e.g. 2g, 2h, 3b, 6c, 6d, 7 and lOh). This fact is important in the synthesis of optically active compounds. 

Besides normal alkyl chains, alkenyl chains (-C H2 CH CHCH3) are also important because they exhibit particularly beneficial elastic constants and low rotational viscosities desirable for use in supertwisted nematic (STN) displays. Branched alkyl chains Iowct transition temperatures and increase the viscosity. Thus, chiral liquid crystal compounds, which usually incorporate a branched chain alcohol, e.g., 2-methyl butanol or 2-octanol, are relatively viscous. Compounds with two long alkyl chains favor smectic C phase formation (3, K-S, 48°C S.-S 122°C S -N 128°C N-I 166°C), especially when the core system contains some kind of nonlinear stmcture (such as a lateral substituent and/or este group) which aids the formation of tilted phases. 

Thus such polar units in the torninal chains are best avoided in the genontion of nematic matoials, unless other structural features are included to disrupt tiie lamellar attractions (e.g. lateral substituents). The use of an all loxy tominal chain with liquid crystals (compounds 33) based on aromatic core units tends to gena ate higho phase stability than compounds with analogous alkyl chains (compounds 9). 

Ultraviolet photoelectron spectroscopy allows the determination of ionization potentials. For thiazole the first experimental measurement using this technique was preformed by Salmona et al. (189) who later studied various alkyl and functional derivatives in the 2-position (190,191). Substitution of an hydrogen atom by an alkyl group destabilizes the first ionization potential, the perturbation being constant for tso-propyl and heavier substituents. Introduction in the 2-position of an amino group strongly destabilizes the first band and only slightly the second. 

Later work examined substituent effects on kinetically controlled alkylations [68, 69] (Scheme 32). Substitution at the 5-position is well tolerated in these reactions. Reductive lithiation of a series of 4-phenylthio-l,3-dioxanes and quenching of the axial alkyllithium intermediate with dimethyl sulfate provided the flzzfz -l,3-diols in good yield, with essentially complete selectivity. 

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