探討具有二甲醚基為旋光中心側邊取代基之旋光性液晶材料的液晶相及其光電性質

Study on the Mesophases and Electro-optical Properties of Chiral Liquid Crystals Possessing Lateral Methoxymethyl Substituent at Chiral Center

何宗禹(Zong-Yu He)

指導教授 ： 吳勛隆

大同大學/工程學院/化學工程學系所/碩士(2009年)

摘要

本研究主要目的為進一步了解旋光中心側邊取代基為二甲醚基的分子結構與誘電性液晶相穩定度的關係。以一旋光中心化合物 (R)-glycidyl methyl ether為起始物進行Grignard reaction得到1-methoxy-2-pentanol、1-methoxy-2-heptanol、1-methoxy-2-octanol 和 1-methoxy-2-nonanol，並以上述醇類合成出四系列旋光液晶材料來分別探討(a)改變旋光(n)及非旋光末端烷鏈長度 (m)，觀察其液晶相的變化和光電性質的特性，(b)比較其液晶相在旋光中心側邊取代基為二甲醚基和丙烷基時的不同。四系列液晶化合物其結構如下:

藉由偏光紋理圖及DSC的鑑定可分別得知液晶相及相轉移溫度的變化，經
由電流轉換行為及介電性質的量測可進一步鑑定誘電性液晶相的存在。
實驗結果顯示:四個系列液晶化合物皆觀察到SmA*相及誘電性液晶SmC*相的生成，第二系列 II-6 (m=8, n=4)化合物更發現有SmCγ*相的產生，其中四個系列液晶化合物的澄清點溫度、SmA*相的溫度範圍及SmA*相轉SmC*相的溫度都隨著非旋光末端烷鏈(m)增長而降低。而四個系列液晶化合物的澄清點溫度及SmC*相的生成溫度，都隨著旋光末端烷鏈(n)增長而下降。
旋光側邊取代基為二甲醚基與側邊為丙烷基的液晶材料相比較，前者具有較低的熔點及穩定的SmC*相，但卻會抑制SmCA*相的生成。
藉由光電量測得知Ps最大值範圍介於32和115 (nC/cm2)之間，且Ps值會隨著旋光末端烷鏈(n)增長而增加，推測原因為增加旋光末端烷鏈的長度時，會讓烷鏈與旋光中心花費較長時間沿躺在分子長軸上，這時旋光中心的偶極容易被固定在同一個方向上，使得整體Ps值上升。然而，Ps值和非旋光末端烷鏈(m)長度變化卻無明顯規則性。另一方面，傾斜角最大值範圍介於20°和40°之間，其值對於旋光(n)及非旋光末端烷鏈(m)的長度改變亦無顯著規則性。
總而言之，旋光中心側邊取代基為二甲醚基的液晶材料已經被成功合成出來，並建立起分子結構(旋光與非旋光末端烷鏈長度)與誘電性液晶相之間的關係，此結果在學術上提供給後續誘電性液晶材料的設計與合成方面的參考。

關鍵字

誘電性液晶相 ； 旋光性材料 ； 光電性質 ； 光學傾斜角

並列摘要

The major research work of this study is an attempt to correlate the structure-property relationship of chiral liquid crystals in terms of variation of achiral and chiral alkyl chains in the chiral liquid crystal materials possessing methoxymethyl group attached to the chiral centre. Thus, the optically active alcohols, 1-methoxy-2-pentanol, 1-methoxy-2-heptanol, 1-methoxy-2-octanol and 1-methoxy-2-nonanol, for the use as the building blocks of the chiral liquid crystals were designed and synthesized by the Grignard reaction of (R)-glycidyl methyl ether with alkyl magnesium chlorides. In consequence, four novel homologous series of chiral materials, (S)-1-methoxy-2-pentyl 4-{4’-[(4”-alkoxyphenyl)phenylcarbonyloxy]}benzoate, I-6 (m=8-12, n=2), (S)-1-methoxy-2-heptyl 4-{4’-[(4”-alkoxyphenyl)phenylcarbonyloxy]}benzoate, II-6 (m=8-12, n=4), (S)-1-methoxy-2-octyl 4-{4’-[(4”-alkoxyphenyl)phenylcarbonyloxy]}benzoate, III-6 (m=8-12, n=5) and (S)-1-methoxy-2-nonyl 4-{4’-[(4”-alkoxyphenyl)phenylcarbonyloxy]}benzoate, IV-6 (m=8-12, n=6) derived from these alcohols were synthesized for the investigation of the mesophases and electro-optical properties. In addition, the differences in mesomorphic phases and electro-optical properties of the chiral materials with methoxymethyl group (-C*HCH2OCH3) and propyl group (-C*HCH2CH2CH3) were compared.
The mesomorphic phases and their corresponding transition temperatures were primarily characterized by the microscopic textures and DSC thermograms, and the ferroelectric phases were further identified by the measurements of electric switching behavior and dielectric constant ε'.
The results show that, all compounds display enantiotropic mesophases of the SmA* and SmC* phases. The compound II-6 (m=8, n=4) exhibit an additional SmCγ* phase. An increasing aliphatic alkyl chain lengths (m) decreases the clearing point, the thermal stability of the SmA* phase and transition temperature of SmA* phase to SmC* phase. In addition, the extension of terminal chiral alkyl chain lengths (n) in all series of compounds results in the decreasing cleaning point and transition temperature of SmA* phase to SmC* phase.
A comparison of structurally similar chiral materials possessing methoxymethyl substituent to that possessing propyl substituent at the asymmetric carbon of the chiral group, show that the formers have lower melting point, facilitate the formation of the SmC* phase and suppress the formation of the SmCA* phase.
The physical properties of the chiral materials in ferroelectric SmC* phases were measured. The maximum magnitudes of spontaneous polarization measured for the materials in the SmC* phase are in the range of 32 to 115 (nC/cm2). These results indicate that the maximum Ps values reveal no significant correlation to the change of achiral alkyl chains, m. However, the Ps values are increased by the extending of chiral alkyl chains, n. The results are due to, as the chiral alkyl chain is extended, the chiral center and a certain portion of the alkyl chain are supposed to spend more time lying along the long axis of the molecule, such that the chiral center has a fixed spatial arrangement with respect to its local environment, this in tern will fix the orient of the diple at the chiral center.
The maximum apparent title angles (θ) measured for materials in the SmC* phase are in the range of 20° to 40°. These results of maximum apparent title angle (θ) reveal no significant correlation to the change of achiral and chiral alkyl chains (m and n).
In conclusion, the chiral materials possessing lateral methoxymethyl substituent attached at chiral center were successfully synthesized, and the structure-property correlation in terms of achiral and chiral alkyl chain lengths for the formation of ferroelectric liquid crystals was established. The establishment of this correction will be beneficial for the future design and synthesis of more ferroelectric liquid crystals.

並列關鍵字

Chiral materials ； ferroelectric phase ； electro-optical property ； title angle

參考文獻

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