- 學位論文
雙氟取代基對TGBA*相生成的影響
Effect of Difluoro Substituents on the Formation of TGBA* Phase
摘要
從近來的文獻顯示，影響TGB液晶相生成的因素有很多，如立體障礙、連結基和硬核結構。本研究的目的為進一步的了解分子結構與TGB液晶相生成的關係。以旋光性乳酸甲酯(methyl(S)-lactate)為起始物合成一系列旋光性液晶材料，並分別探討(i)非旋光末端烷鏈長度‘m’、(ii)在硬核中心導入雙氟取代基和(iii)延長靠近旋光末端的烷鏈長度“n”對TGB相的影響。 旋光性化合物的液晶相和相轉移的溫度是用偏光顯微鏡以及微差掃描熱量計來做鑑定。第一系列化合物(m, n, x; m=8-11, n=3, x=H) 顯示SmA*和SmC* 兩種液晶相。第二系列化合物(m=8-12, 3, F) 具有SmA*和SmC* 兩種液晶相，其中第二系列化合物(m=8-9, 3, F) 顯示增加了N*和TGBA*兩種液晶相。明顯顯示具較短非旋光烷鏈的液晶材料上導入雙氟取代基會出現N*和TGBA*兩種液晶相。第三系列化合物(m, n, x)，化合物(m=8, n=2-5, F)和化合物(m=9, n=2-3, F) 顯示了N*，TGBA*，SmA*和SmC*的液晶相順序，而當化合物(m=9, n=4-5, F)具有較長的旋光烷鏈會壓制了TGBA*相結果產生了N*，SmA*和SmC*的液晶相順序。這個結果顯示出此化合物在較短的旋光烷鏈上較易生成TGBA*相。 旋光性化合物的自發性極化值與光學傾斜角都是在誘電性液晶相下量測。結果顯示第一系列化合物(m=8-12) 的最大自發性極化值的範圍是70-110 nc/cm2，而第二系列化合物(m=8-12) 最大的自發性極化值的範圍是32-81 nc/cm2。這個結果顯示出雙氟側邊取代基的偶極與羧基之耦合方向呈現相反狀態造成此化合物會降低自發性極化值。第一系列化合物(m, n, x; m=8-12, n=3, x=H) 最大的傾斜角範圍是27-32°，而第二系列化合物(m, n, x; m=8-12, n=3, x=F) 最大的傾斜角範圍是17-24°，這個結果顯示出化合物具有雙氟側邊取代的話也會降低傾斜角的範圍。 綜合以上結果可以顯示出增加雙氟取代基於分子的苯環上，在較短的非旋光烷鏈及旋光烷鏈上會形成扭轉晶界相(TGB)的構型。扭轉晶界相是由穩定的層列型結構與分子的螺旋結構相互競爭所形成的。因此，實驗結果很明顯的顯示雙氟取代基可能會降低層列結構的穩定度；降低分子在層列中的相互作用力，因此由具螺旋的旋光向列型液晶轉移至層列型A相時造成扭轉晶界相(TGB)產生。在誘電性液晶相物理性質的量測結果顯示，增加雙氟取代基於分子的苯環上會降低自發性極化值與傾斜角。添加雙氟取代基的化合物會減少自發性極化值可能是歸因於雙氟取代基與旋光甲基的偶極相反之緣故。
並列摘要
From the results of recent papers, there are many factors affect the TGB phase formation such as steric, linking group and rigid core structure. The purpose of this research is apt for a better understanding the relationship between molecular structure and TGB phase formation. A homologous series of chiral materials derived from optically active methyl (S)-(-)-lactate has been successfully synthesized and the structures-property relationship investigated in the chiral liquid crystal system. The target compounds were modified independently by the effects of (i) the nonchiral peripheral alkyl chain length ‘m’, (ii) the lateral difluoro substituents in the phenyl ring of the core and (iii) the extending chiral peripheral alkyl chain length “n” on the TGB phase properties. The msomorphic phases and their corresponding transition temperatures for the chiral compounds were determined by polarizing microscopic textures and DSC thmograms. The results in compounds I(m, n, x; m=8-11, n=3, x=H) display SmA* and SmC* phases. Compounds II(m=8-12, 3, F) possess enantiotropic SmA* and SmC*phases. Among them, compounds II(m=8-9, 3, F) display additional the N* and TGBA*, suggesting that an introduction of difluoro-substituents could induce N* and TGBA* phases at the shorter achiral alkyl chain length of the molecules. In the series of compounds III(m, n, x), compounds III(m=8, n=2-5, F) and III(m=9, n=2-3, F) display a mesophase sequence of N*, TGBA*, SmA* and SmC* phases, while compounds III(m=9, n=4-5, F) suppress the TGBA* phase resulting in a phase sequence of N*, SmA* and SmC* phases. This result indicates that compounds with the shorter chiral alky chains are favorable of the formation of TGBA* phase. The magnitudes of spontaneous polarization and apparent title angle in the SmC* phase of the chiral compounds were also measured. The results show that compounds I(m=8-12) have the maximum PS values of about 70-110 nc/cm2, whereas compounds II(m=8-12) have the maximum PS values of about 32-81 nc/cm2. The results indicate that the compounds with lateral difluoro-substituents have lower PS values due to the fluoro-substituents have the transverse dipole opposite to the dipole associated with the carbonyl groups. Compounds I(m, n, x; m=8-12, n=3, x=H) have the maximum tilt angles of 27-32°, whereas compounds II(m, n, x; m=8-12, n=3, x=F) have the maximum tilt angles of 17-24°. These results indicate that the compounds with lateral difluoro-substituents have lower tilt angles values. In conclusion, the results show that addition of difluoro-substituents in the phenyl ring of the molecules induces the formation of TGBA* phase at shorter achiral and chiral alkyl chain length. The TGBA* phase is generated as a result of the competition between the stabilization of the smectic layered structure and the desire for the molecules to form a helix structure. Thus, it is strongly suggested that difluoro-substituents may reduce the stability of the layered structure; reduce the molecular interactions in the layer structure, thus, the TGBA* phase is generated. The results obtained from the measurements of physical properties indicate that the addition of difluoro-substituents on the phenyl ring of the molecules decreases the spontaneous polarization and apparent title angle. The decrease in spontaneous polarization in the difluoro-substituted compounds may be attributed to the difluoro-substituents point away from the direction of the chiral methyl group.