| 摘要: | 本論文主要是設計並合成由 Benzoxazole 所衍生之對稱彎曲型分子，探討連接基的改變對這些對稱的結構會有什麼影響。所有化合物皆由偏光顯微鏡(POM)來觀測液晶相種類，以及熱微差掃描分析儀(DSC)測量相變化時之溫度與熱焓值，最後以變溫粉末 X-ray 繞射儀(XRD)來確認液晶相的種類。 我們發現這些彎曲型分子皆為層列型液晶分子。然而，其液晶相的形成受中間的軟段數目所影響，若彎曲型分子含有奇數的連接基 ( m = 3, 5, 7, 9, 11) ，將為層列 C 型液晶分子；相反的，若其含有偶數連接基 ( m = 4, 6, 8, 10, 12)， 則是層列 A 型液晶分子。 Liquid crystalline dimers, known so-called twins or bimesogens have attracted great attentions. This type of mesogenic structures was generally constructed by connecting either two identical or non-identifcal mesogenic moieties through a flexible central spacer. The structural conformation and mesomorphic properties observed by this type of LC dimers are quite different from conventional molecular mesogens, and also significantly influenced by the structure and the spacer length and the linker group as well. The mesomorphic properties of such dimeric compounds were known to be critically dependent on the length and the parity of the central spacer. A majority of the dimeric compounds showed a significant odd-even effect on the mesomorphic behavior with spacer length or/and terminal length. The mesogenic dimers with an even spacer have generally more parallel orientations than dimers with an odd spacer. The difference observed in phase behavior was mostly attributed to the dependence of the average molecular shape and/or geometry adopted on the parity of the central spacer when considered in the all-trans- conformation. In this work a series of rod-like molecules derived from benzoxales was prepared and studied, and these molecules were found to exhibit layer smectic mesophases. The formation of phases was sensitive to the numbers of the carbon chains appended to the core. The results indicated that dimeric comounds with odd spacers (n = 3, 5, 7, 9, 11) exhibited smectic C mesophases, however, compounds with even spacers (m = 4, 6, 8, 10, 12) showed smectic A mesophase. |
