探討具氯化甲烷旋光基的旋光性液晶材料的液晶相及光電性質

Study on the Mesophases and Electro-optical Properties of Chiral Liquid Crystals Derived from Chloromethyl Substituent Positioned at Asymmetric Carbon

游逸群(Yi-Chun Yu)

指導教授 ： 吳勛隆

大同大學/工程學院/化學工程學系所/碩士(2008年)

摘要

影響誘電性SmC* 液晶相其性質的因素有很多，像是旋光中心、硬核構造、連接基、末端烷鏈的長度和光學純度等等。本研究的目的是為進一步了解分子結構與液晶相性質的關係。
因此，本實驗以含有氯化甲烷基的(S)-1-alkyloxy-3-chloro-2-propanols為起始物，合成六系列新型含氯化甲烷基旋光中心的液晶材料來分別探討(i)旋光中心的側邊取代導入氯化甲烷基，觀察是否有其他液晶相的生成(ii)改變液晶分子旋光及非旋光末端柔軟鏈長度(n, m)，觀察其液晶相變化(iii)改變硬核中心的結構，探討其對物理性質的影響。
實驗結果顯示: 除了IV(m=8)和VI(m=8) 僅有SmA*液晶相的生成，所有化合物都具有SmA*及SmC*液晶相。六系列化合物I(m=8-12, n=3), II(m=8-12, n=4), III(m=8-12, n=5), IV(m=9-12, n=3), V(m=8-12, n=4)和VI(m=9-12, n=5)的Ps最大值顯示化合物的旋光碳鏈為偶數時具有較大的自發極化值。而傾斜角其結果顯示和旋光碳鏈數沒有明顯的關係。在光電性質的量測上，化合物 I(m=10, n=3), II(m=10, n=4), III(m=10, n=5), IV(m=10, n=3), V(m=10, n=4) and VI(m=10, n=5)在特定的頻率及溫度下具有無閥的光電V型轉換行為。
當旋光中心的側邊取代用氯化甲烷基取代甲烷基可觀察到SmA* 和 SmC* 液晶相的溫度範圍降低，也抑制了SmX*液晶相的生成。且具有較低的澄清點溫度和較高的熔點。其自發極化值和傾斜角，由結果可知旋光中心側邊取代基由甲烷基變成氯化甲烷基時，由於氯化甲烷基具有較大的體積和電負度，因此會產生較大的自發極化值及傾斜角。(Size: H=1.25, Cl=1.75; Electronegativity: H=2.1, Cl=3.0)
當硬核結構的組成為PhPhCOOPh時具有較寬廣的液晶相，且SmC*液晶相有較佳的熱穩定性。而當硬核結構由PhPhCOOPh改變為PhCOOPhPh時在相同溫度下會產生較大的自發極化值，但傾斜角的大小沒有顯著的關聯性。

關鍵字

新型旋光性材料 ； 誘電性液晶 ； V型光電應答行為

並列摘要

There are many factors affect the properties of ferroelectric chiral Smectic C phase such as chiral group, core structure, linking group, terminal chain length and optical purity. The purpose of this research is looking for a better understanding the relationship between molecular structures and mesomorphic properties.
Thus, new chiral precursor, (S)-1-alkyloxy-3-chloro-2-propanols containing chlomethyl chiral group were designed and synthesized by the reaction of (S)-epichlorohydrin with alcohols under the acid condition. Consequently, six homologous series of chiral compounds, (R)-1-alkyloxy-3-chloro-2-propyl 4-{4’-[(4’’-alkyloxyphenyl)phenylcarbonyloxy]}benzoates, I(m=8-12, n=3), II(m=8-12, n=4) and III(m=8-12, n=5), and (R)-1-alkyloxy-3-chloro-2-propyl 4”-[(4’-alkyloxy)benzoyloxy]biphenyl-4-carboxylate, IV(m=8-12, n=3), V(m=8-12, n=4) and VI(m=8-12, n=5), were synthesized and the mesophases and their corresponding electo-optical properties were investigated. The effect of changing chiral groups from methyl to chloromethyl groups at asymetric carbon, aliphatic alkyl chain length (m), chiral chain length (n), and core structure of the chiral compounds on the mesomorphic and electro-optical properties were compared and discussed.
The results show that, with the exception of compounds IV(m=8, n=3) and VI(m=8, n=5) which possessed exclusively SmA* phase, the rest of compounds displayed SmA* and SmC* phases. The maximum magnitudes of the spontaneous polarization for the chiral compounds, I(m=8-12, n=3), II(m=8-12, n=4), III(m=8-12, n=5), IV(m=9-12, n=3), V(m=8-12, n=4) and VI(m=9-12, n=5), show that compound with even number (n=4) of chiral alkyl chain length has the lager Ps values than that with odd numbers (n=3, 5), but the optical tilt angles have no significant correlation change with chiral chain length. The electro-optical responses of chiral compounds in SmC* phase displayed typical ferroelectric switching behavior. Compounds of I(m=10, n=3), II(m=10, n=4), III(m=10, n=5), IV(m=10, n=3), V(m=10, n=4) and VI(m=10, n=5) in the SmC* phase displayed V-shaped switching property at appropriate frequency and temperature.
The comparison of structurally similar chiral compounds between compounds with chloromethyl substituent and that with methyl substituent at the asymetric carbon of the chiral groups, show that the formers have narrower temperature range of mesophase as well as SmC* phase, higher melting point and lower clearing point than the later ones. In addition, the SmX* phase was surpressed. The elctro-optical measurements show that the maximun Ps values and optical tilt angles are larger for compounds with chloromethyl group than that with methyl group. This may be presumarily due to the lager size and/or electronegativity of the chlorine atom than hydrogen atom (Size: H=1.25, Cl=1.75; Electronegativity: H=2.1, Cl=3.0).
The comparison of chiral compounds with difference core structures shows that compounds with PhPhCOOPh core structure have better thermal stability in the the mesophase and ferroelectric SmC* phase than that with PhCOOPhPh core structure. In contrast, the results of the maximun Ps values indicate that compounds with PhCOOPhPh core structure have larger Ps values at the temperature below Curie point than that with PhPhCOOPh core structure. The apparent tilt angles revealed no significant correlation between these two core structures of the chiral compounds.

並列關鍵字

ferroelectric liquid crystal ； V-shaped electro-optical response ； new chiral material

參考文獻

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