探討具有氯化甲烷基為旋光中心側邊取代基之旋光性液晶材料的合成及其液晶相和光電性質

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

莊敏慈(Min-tzu Chuang)

指導教授 ： 吳勛隆

大同大學/工程學院/化學工程學系所/碩士(2010年)

摘要

影響誘電性SmC*液晶相性質的因素有很多，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。液晶分子在旋光中心延伸或增加側邊取代基尺寸或導入鹵素元素可能會在其堆疊時造成立體上的不對稱，而當在旋光末端位置具有燕尾結構時，容易形成鋸齒狀的排列方式，產生反誘電性液晶相的結構。本研究為了進一步了解分子結構與液晶相性質的關係，在旋光中心的側邊取代導入氯化甲烷基和改變液晶分子非旋光末端柔軟鏈長度(m)及旋光末端柔軟鏈長度(n)來探討其對液晶相變化及物理性質的影響。此外將液晶分子旋光末端烷鏈改變為燕尾型結構，同樣來探討其對液晶相以及光電性質的影響。
因此本實驗以(S)-epichlorohydrin為起始物，合成五系列新型含氯化甲烷基的旋光性液晶材料I(m=8-12, n=3), II(m=8-12, n=4), III(m=8-12, n=5), IV(m=8-12, n=6), V(m=8-12, n=8) 以及一系列新型含氯化甲烷基的旋光性燕尾型液晶材料VI(m=8-12)。
實驗結果顯示: 六系列所有化合物皆具有SmA*和SmC*液晶相。而化合物VI(m=8)則另外有SmX*液晶相的生成。比較不同旋光末端烷鏈長度(n=3, 4, 5, 6, 8)的化合物時，可以觀察到澄清點溫度、熔點、SmA*- SmC*液晶相轉移溫度會隨著旋光末端烷鏈長度的增長而降低。五系列化合物I(m=8-12, n=3), II(m=8-12, n=4), III(m=8-12, n=5), IV(m=9-12, n=6) 和V(m=8-12, n=8)的Ps最大值具有奇偶效應，其結果顯示化合物的旋光末端碳鏈長度為偶數(n=4, 6, 8)時具有較大的自發性極化值，但傾斜角則和旋光及非旋光末端碳鏈長度沒有明顯的關係。
當旋光中心的側邊取代用氯化甲烷基取代甲烷基時，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。其自發性極化值和傾斜角，由結果可知旋光中心的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體積和電負度的氯原子(Size: H=1.25Å, Cl=1.75Å; Electronegativity: H=2.1, Cl=3.0)，因此會產生較大的自發性極化值及傾斜角。
旋光側邊具氯化甲烷基之燕尾型化合物(VI)具有較低的澄清點溫度和熔點，且有較寬廣的SmC*液晶相，但含甲烷基之燕尾型化合物則有較寬廣的SmCA*液晶相生成。此外前者化合物只在較短的非旋光末端烷鏈長度(m=8)時有SmX*液晶相的出現。因此旋光側邊具氯化甲烷基的化合物抑制了SmCA* 和SmX*液晶相的生成。而結果顯示具有較大體積和電負度的氯化甲烷基則對燕尾型化合物的自發性極化值及傾斜角沒有顯著的影響。

關鍵字

誘電性液晶 ； 旋光性材料

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. Liquid crystal molecule with extending or increasing size of lateral substituent or incorporation of a halogen lateral substituent at chiral center may cause some degree of steric asymmetry when packed together and it is easy to form the zigzag-shaped arrangement that facilitates the formation of antiferroelectric phase while it possesses swallow-tailed structure at chiral terminal group. For these reasons, 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 and (S)-1-(1-ethylpropoxy)-3-chloro-2-propanal 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) , III(m=8-12, n=5) , IV(m=8-12, n=6) and V(m=8-12, n=8), and (R)-1-(1-ethylpropoxy)-3-chloro-2-propyl4-{4’-[(4’’-
alkyloxyphenyl)phenylcarbonyloxy]}benzoates, VI(m=8-12), were preparared for the study. The effect of changing chiral groups from methyl to chloromethyl groups at asymmetric carbon, achiral alkyl chain length (m) and chiral alkyl chain length (n) of the chiral compounds on the mesomorphic and electro-optical properties were compared and discussed. In addition, the effect of changing chiral terminal alkyl chain to swallow-tailed structure on the mesophases and electro-optical properties was also investigated.
The results show that with the exception of compound VI(m=8) which displays the phase sequence of Iso.-SmA*-SmC*-SmX*-Cr., the rest of compounds display the phase sequence of Iso.-SmA*-SmC*-Cr.. The comparison of chiral compounds with different chiral chain length (n=3, 4, 5, 6, 8) shows the temperature of clearing point, the melting point and the transition temperature of SmA*- SmC* decrease with increasing terminal chiral chain. 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=6) and V(m=8-12, n=8), display an odd-even effect in which compounds with even numbers (n=4, 6, 8) of chiral alkyl chain length have the larger Ps values than that with odd numbers (n=3, 5), but the optical tilt angles exhibit no significant correlation change with chiral and achiral chain length.
The comparison of structurally similar chiral compounds between compounds with chloromethyl substituent and that with methyl substituent at the asymmetric carbon of the chiral groups shows that the formers have narrower temperature range of mesophases as well as SmC* phase, higher melting point and lower clearing point than the later ones. In addition, the SmX* phase was suppressed. The electro-optical measurements show that the maximum Ps values and the optical tilt angles are larger for compounds with chloromethyl group than that with methyl group. This may be presumably due to the larger size and/or electronegativity of the chlorine atom than hydrogen atom (Size: H=1.25Å, Cl=1.75Å; Electronegativity: H=2.1, Cl=3.0).
When comparing structurally similar chiral swallow-tailed compounds between compounds with chloromethyl group and that with methyl group at the asymmetric carbon, the results show that the formers have wider temperature range of the SmC* phase, lower melting point and clearing point than the later ones, but the chiral swallow-tailed compounds with methyl substituent have wider temperature range of the SmCA* phase. Furthermore, the SmX* phase of the compounds with chloromethyl group only appears at short achiral alkyl chain length (m=8). Therefore, compounds with chloromethyl substituent suppress the existence of SmCA* and SmX* phases. The electro-optical measurements show that the larger size and higher electronegativity of chlorine atom (Size: H=1.25Å, Cl=1.75Å; Electronegativity: H=2.1, Cl=3.0) have no significant effect to the spontaneous polarization and the apparent tilt angle in the homolog series of swallow-tailed compounds.

並列關鍵字

new chiral compounds ； Ferroelectric liquid crystal

參考文獻

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