| Summary: | 碩士 === 國立中央大學 === 化學研究所 === 95 === 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.
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