Investigation of a Design Strategy For Liquid Crystals with De Vries-Like Properties

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Song, Qingxiang
design strategy , De Vries-like , liquid crystals
Ferroelectric liquid crystals (FLCs) have been investigated as the basis for a new type of liquid crystal display because of their fast switching times. In applications the layer contraction at the smectic A-smectic C transition is a severe problem that leads to buckling of the smectic layers and results in zigzag defects that drastically degrade the optical quality of FLC films. To solve this problem, researchers are focusing on a new class of liquid crystals with minimal smectic layer contraction at the SmA-SmC transition which is referred to as ‘de Vries-like’. In the first part of this thesis, we have developed a rational design strategy based on a concept of frustration between two structural elements, one promoting a SmA phase and another promoting a SmC phase. In this chapter, we show that one can further improve ‘de Vries-like’ properties by substituting the 2-phenylpyrimidine core in our first-generation siloxane-terminated mesogens with one stronger SmC-promoting core. We also address the hydrolytic instability of siloxane-terminated mesogens, by substituting the siloxane with a carbosilane end-group. As a result of this study, we found QL6-6 that forms a SmC phase at room temperature with ‘de Vries-like’ properties that are comparable to those of bona fide ‘de Vries-like’ liquid crystals. Next, the dialkoxy 2-phenyl-1,3,4-thiadiazole core which is a very powerful SmC promoting element was investigated. We prepared the analogue of QL6-6, compound QL13-6, which only forms a SmC phase despite having a chloro-terminated alkyl chain. Making a ‘de Vries-like’ liquid crystal without carbosilane terminal side-chain was also attempted, although ultimately unsuccessful. The terminal group effect on the ‘de Vries-like’ properties was also investigated. We prepared a series of mesogens derived from 2-phenylpyridine core with different end-groups to investigate whether the X-group might interact with the core in a bilayer structure. Finally, we inves¬tigated the effect of mixing ’de Vries-like’ liquid crystals with conventional smectic liquid crystals. We show that binary mix¬tures of ‘de Vries-like’ and conventional SmC meso¬gens with a molecular length ratio of 1.34 undergo a SmA-SmC phase transition with a maximum layer contraction ranging from 1.0 to 1.9% depending on the mixture composition.
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