Synthesis, characterisation and investigation of phase diagrams of the chiral liquid crystals

Liquid crystals which are the unique state of matter have a wide application area such as digital displays, sensors and LCD screens. Especially chiral liquid crystal molecules are a fascinating research subject and field of technological application due to their mesomorphic and electro-optic properties. The appearance of ferroelectricity and anti-ferroelectricity in tilted smectic phases of chiral liquid crystals are especially important due to their applications in display devices showing fast switching behavior. Most of ferroelectric materials consist of a rod-like molecule formed by a rigid core with two flexible terminal chains, at least one of which is chiral. The materials used frequently in liquid crystal displays are thermotropic liquid crystals which have a calamitic structure and exhibit chiral mesophase.Most chiralities of liquid crystal phases are due to the introduction of a group containing a chiral carbon to the molecular structure. The mesogen may have one or more stereogenic center and they are found either along the terminal chain of the mesogen or in the central core. It is known that a simple change in length of the aliphatic tail affects the liquid crystalline polymorphism strongly. Modification of the chiral group in the liquid crystal molecule significantly alters the electro-optical properties and polymorphism. Siloxane groups attached to the terminals of olefinic groups of mesogenic molecules having different molecular structure allow expanded temperature range of liquid crystal phases and provide mesophase stabilization.In this study, olefinic terminated and hydroxyl terminated new chiral calamitic molecules, where ester groups are included in the molecular structure as a connecting group, have been synthesized. Siloxane derivatives of olefinic terminated compounds have been obtained by hydrosilylation reaction. Moreover, the effects of changes in the direction of ester connecting group on mesogenity have been investigated. The structures of new chiral calamitic mesogens studying on the effect of the presence of a chiral moiety and siloxane units within the same molecule on the mesogenity have been characterized using classical spectroscopic methods (1H-NMR, 13C-NMR, 29Si-NMR and MS) and elemental analysis. The mesomorphic properties of the new compounds have been investigated using polarization microscope, differential scanning calorimetry (DSC) and electro-optic methods. It is observed that, the mesomorphic temperature range of smectic mesophase, which appears in olefine terminated new chiral calamitic molecules, becomes wider with the introduction of siloxane unit to structure but melting and clearing points significantly decrease.Keywords: Liquid crystals, thermotropic, chiral, calamitic, mesomorphic properties, electro-optic, (anti)ferroelectricity.