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    Title: 彎曲形液晶分子之合成與光電性質研究
    Other Titles: Synthesis and property investigation of bent core mesogens
    Authors: 俞方正;Yu, Fang-cheng
    Contributors: 淡江大學化學學系博士班
    余良杰;Yu, Liang-jye
    Keywords: 二苯乙烯;曲棍形液晶;發光彎曲形液晶質;Stilbene;Hockey stick liquid crystals;Fluorescent bent core mesogens
    Date: 2008
    Issue Date: 2010-01-11 02:40:37 (UTC+8)
    Abstract: 依分子之外形與所含官能基特性,彎曲形液晶分子可以構成數種性質特殊的液晶相,與典型條形以及盤形分子的液晶相有顯著的差異。以二苯乙烯為基本架構,隨著苯環數不同,合成三類不同構形之彎曲形液晶材料,以偏光顯微鏡、熱差分析儀、X 光繞射、電場效應以及吸收與放射光譜探討相關液晶相性質。
    第一類為曲棍形液晶分子‐彎曲形分子兩翼各為烷氧鏈及含二苯乙烯之三環(n‐m 系列)或四環(n‐mB 系列)。變化烷氧鏈長度及苯環數可使此類形分子呈現雙向的N、SmA、SmC及SmCa 等相。所觀察到之液晶相紋理圖與X 光繞射圖形皆相似於條形液晶相,故沿用條形液晶相之命名。受電場驅動時,向列相呈現類似於Williams Domain 之直條紋理,但直條之長軸平行原配向方向,為電動對流(electroconvection)所造成之現象。受配向而呈現均勻排列的紋理圖中,雙折射隨著電場強度變化,顯示因電場驅動而分子極化方向隨之改變。SmCa 相的Schlieren 紋理圖中觀察到2‐brush 的缺陷,為反傾結構之特徵。依據Kajitani 於非旋光分子的N 相中觀察到對掌性區塊紋理,推測在平行配向的ITO cell 中,SmC 與SmCa相紋理圖的不同傾斜方向之層相區塊應具有構形旋光性質。施加正反向電場時,觀察到相同的紋理圖,但關閉電場時則回復到零電場的紋理圖,推測SmC 與SmCa 相皆具有類似於反鐵電性質之三穩定態。
    第二類為香蕉形分子。(Ac)2‐A 系列以甲苯醚2, 4 位置連接兩丙烯酸為彎曲單元,合成一系列之五環架構分子。位於中心苯環之甲基氧為大分子取代基,具有顯著的立體阻障,導致高規律度液晶相消失,且液晶相熱穩定性大幅降低。 (XST)2‐B 與(XBST)2‐B 系列之分子彎曲單元為雙苯乙烯苯,分別為五環以及七環架構之香蕉形液晶質。(XST)2‐B 系列中,中心苯環加入推拉電子電子取代基時皆生成B2 相,而兩翼尾端苯環加入側取代氯基,僅中心苯
    環以胺基取代之香蕉形分子生成B1 相,其餘亦為B2 相液晶質。以光電實驗鑑定B2 相為SmCAPA 次相,並觀察到自發極化值隨著推拉電子基不同而有顯著的變化。由於雙苯乙烯苯為發光團,故此系統之香蕉形液晶分子為發光材料 。加入推電子基氟基僅致使發射波長產生少許藍位移(Emmax388nm),但強度卻增加,而加入推電子基胺基則使得吸收與放射波長皆產生紅位移(Emmax 473nm),同時加入推拉電子基之放射波長亦為顯著的紅位移( Emmax 500nm),顯示推拉電子與雙苯乙烯苯之共振結構改變電荷密度,致使吸收與發射波長產生變化。相較於(XST)2‐B 系列,(XBST)2‐B 系列於彎曲形分子之兩翼各增加一苯環,分子極化率隨之增強,進而生成高規律度之B1 相。側取代氯基之立體效應導致分子間作用力減弱,液晶相由B1 相轉變為B2 相,液晶相結構經X 光繞射鑑定之。
    第三類為彎曲形分子,以萘環與二苯乙烯為中心架構,並在兩翼接上不同長度之取代基,觀察不對稱彎曲構形對液晶相性質的影響。以萘環為彎曲單元的1, 6‐N 系列中,兩翼接上相同取代基較不同取代基時容易生成香蕉形液晶相。R’‐ST 與R’‐BST 系列以二苯乙烯為彎曲單元,在3 與4’位置接上不同取代基,當間位(3 位置)取代基共軛結構長度逐漸增加時,液晶相由曲棍形液晶相轉變為香蕉形液晶相,得知彎曲形分子兩翼極化率明顯不對等時為曲棍形液晶分子,而兩翼極化率接近時為香蕉形液晶分子。
    三類分子中皆具有以二苯乙烯為中心彎曲架構之液晶分子,比較分子構形與液晶相性質的關聯,發現不對稱之彎曲形液晶分子中,兩翼共軛長度差異大時生成曲棍形液晶相,而極化率比例接近時生成香蕉形液晶相,對稱彎曲分子則皆為香蕉形液晶質。非旋光彎曲形液晶分子皆具有鐵電或反鐵電性質之液晶相,其原因為曲棍形液晶相具有兩位能簡併之鏡相分子構形所生成之構形旋光性(conformational chirality),而香蕉形液晶相則為傾斜層相
    分子傾斜向量相反形成之超結構旋光性(superstructural chirality)所導致。
    Depending upon the shape of molecule and the polarizability of constituting moiety, a number of peculiar mesophases can be formed with molecules consisting of various bent cores. Three types of bent shape mesogens are synthesized: hockey stick molecule, banana molecule and bent core molecule. The properties of these mesophases are studied with optical polarizing microscopy, differential scanning calorimetry, X-ray diffraction, and electric field effect.

    Type 1: Hockey stick molecules—bent molecules have two wings differing greatly in length. Enantiotropic anticlinic smectic C (SmCa), smectic C (SmC), smectic A (SmA) and nematic (N) phases of hockey stick molecules are obtained by tuning the length of hydrocarbon chain located at the meta-position to the carbon-carbon double bond of stilbenyl moiety of calamitic mesogenic skeletons. The thermodynamic properties and some aspects of optical appearance of these phases are indiscernible from those of calamitic mesogens. Distinctions are noted such as appearance of 2-brush defects and two types of domains in the tilted smectic phases. X-ray results indicate existence of clusters in the nematic phase and simple layer structure in the smectic phases. These phases exhibit electric field driven color switching, and all the results imply that all three molecular axes are simultaneously aligned in the N and SmC phases—a behavior unprecedented and different from those of conventional calamitic mesogens. Doping with a chiral calamitic mesogen results in formation of corresponding chiral N and ferroelectric SmC phases, but effect for the SmCa phase is not clear. Small and comparable values of spontaneous polarization are obtained for the doped and non-doped phases. Molecular organization of SmC and SmCa phases are proposed and discussed.

    Type 2: Banana molecules—bent molecules with two wings equivalent or nearly so. Ani series employed 2,4-disubstituted anisole as the bending unit (BU) and consisted of five phenyl rings linked with ester functional groups. The bulky methoxy group at the bending unit weakens intermolecular force and affects the molecular packing so the higher ordered mesophases disappear and thermal stability of mesophase decreases. For the system with distyrylbenzene as the BU, two systems were synthesized—five and seven rings, (XST)2-B and (XBST)2-B, rspectively. Substitutents NH2 and/or F were also attached to the BU. B2 phases were observed for all the (XST)2-B derivatives but B1 for NH2-(ClST)2-B. The B2 phases are identified to be SmCAPA subphase according to the electrooptic study and the values of spontaneous polarization varies with the electron donating and withdrawing property of the substituents. Comparing to (XST)2-B, molecules (XBST)2-B have two more phenyl rings, one at each wing, and therefore higher polarizability, so the higher ordered phase B1 is observed. The lateral chloro substituent at the terminal phenyl ring of the wings increases the steric effect and lowers the packing order, the B1 phase is converted back to B2. Compounds (XST)2-B are fluorescent for distyrylbenzene being a chromophore. The emission wavelength is blue shifted (388nm) and red shifted (473nm) when F and NH2, respectively, is attached at the BU. This behavior is attributed to the resonance stuctures formed when the electron donating and withdrawing groups are covalent bonded to the distyrylbenzene core.

    Type 3: Bent core molecules—both wings consist of phenyl rings but are highly un- symmetrical. 1,6-N series consists of naphthalene as the BU and with substitutents located at 1 and 6 positions. Banana phases would appear preferrably with equivalent substituents. For the series R’-ST and R’-BST, 3,4’-disubstituted stilbene is the BU. Increasing the polarizability from aliphatic chain to aromatic ring system for the substituents located at the 3-position would convert the hockey stick mesophase to banana phase.

    This study shows that bent molecule is capable to form mesophases provided that bent conformation, a mesophase destroying factor, and polarizability of moieties, a mesophase constructing factor, are properly balanced.
    Appears in Collections:[化學學系暨研究所] 學位論文

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