紫外光可聚合三苯基雙乙炔液晶單體在寬頻譜膽固醇偏光板與偏極化電激發光上的應用

Abstract

本研究之主要目的為合成三個系列含側向取代基之三苯基雙乙炔衍生物液晶單體，探討其在光電方面的應用。本研究合成出三苯基雙乙炔衍生物之單體分別為monoacrylate單體、monooxirane單體、diacrylate單體，所有不對稱之三苯基雙乙炔衍生物皆具有互變型（enantiotropic）向列型液晶相，在單體設計方面，三苯基雙乙炔衍生物在苯環上導入側取代基可以降低液晶相溫度，在末端苯環上具有不對稱之末端基與側取代基能有效降低液晶之熔點，在中央苯環上之側取代基能干擾液晶之結晶，液晶在降溫過程有延遲結晶的現象，其液晶相溫度可延伸至低於0 ℃。所合成出三苯基雙乙炔側鏈液晶聚合物分別為polyacrylate及polyoxirane聚合物，皆具有互變型向列型液晶相，其液晶相溫度範圍涵蓋室溫。 本研究第二部份將探討三苯基雙乙炔液晶對於膽固醇偏光板的應用，三苯基雙乙炔液晶的折射異率方向性隨著波長減少而增大，由紅光區域0.30增逐漸增大至藍光區域的0.58，與膽固醇 chiral dopant 摻混，得到的膽固醇液晶兼具大的折射率異方向性與低熔點。膽固醇液晶迴火排列之後，以紫外光交聯造成 pitch gradient，可得到涵蓋全可見光波段之膽固醇反射式偏光板。 本研究第三部分將探討三苯基雙乙炔液晶在電激發光的應用。三苯基雙乙炔液晶的UV吸收波峰都在330 nm 左右，在螢光發射光譜方面，三苯基雙乙炔液晶在THF溶液中之螢光發射波長在380 nm左右，而其在薄膜之光激發光波峰在430 nm左右，三苯基雙乙炔液晶在薄膜之螢光發光波峰較在THF溶液中紅位移約50 nm，由於液晶分子間彼此有很強的aggregation傾向，分子之間的耦合較好，其共軛長度較長，螢光發射光譜大幅紅位移。三苯基雙乙炔液晶電激發光波長與其螢光發射光譜相近，顯示其發光機構相同，然而其電激發光效率不高，只有1.2 × 10-2 lm/W。本研究最後探討三苯基雙乙炔液晶在偏極化電激發光的應用，對於不同配向材料對液晶的配向效果，以polyimide（PI）最好，polyaniline（PA）次之，poly(3,4-diethylene dioxythiophene)（PEDOT）較差，其配向能力與分子的剛硬程度成正比。三苯基雙乙炔液晶偏極化之的UV-vis 吸收光譜、螢光發射光譜與電激發光光譜，其平行於定向摩擦方向光譜強度皆大於垂直於定向摩擦方向，顯示發光基團排列方向皆與定向摩擦方向相同。UV-vis 吸收光譜最佳之偏極化值為15.2 ，螢光發射光譜最佳之偏極化值為13.8，電激發光光譜最佳之偏極化值為7.1，其液晶排列秩序度為0.67。

The goal of this study is aimed to synthesize three series of highly conjugated bistolane liquid crystals (LC) with lateral substituents and their corresponding polymers. The corresponding monoacrylate, monooxirane, and diacrylate monomers based on bistolane mesogenes are synthesized. All of the obtained asymmetric monomers reveal an enantiotropic nematic phase. Bistolane liquid crystals with lateral substituents exhibit low melting point and wide nematic range. The synthesized polyacrylates and polyoxiranes containing bistolane side groups exhibit the glass transition temperatures at about 5℃ and the nematic to isotropic temperatures at about 90℃. In the second part of this study, the reflective cholesteric polarizers based on the bistolane monomers are investigated. The birefringences of the LC monomers are in the range from 0.35 - 0.6 depending on the measuring wavelength. Blending the bistolane LC with the chiral dopant, the corresponding cholesteric LC exhibits a low melting point and high birefringence. The cholesteric LC mixtures were aligned between two glass substrates and cured by UV radiation to form the broad-band cholesteric liquid crystal polarizer. In the third part of this study, the optical properties of obtained monomers and polymers are investigated. All of the bistolane liquid crystals present UV absorption peaks at about 330 nm, and both solution and film samples show very similar absorption spectra. The PL emissions of bistolane liquid crystals measured in THF and in solid state are at about 380 nm and 430 nm, respectively. The emission maxima of the films are red-shifted compared to the maxima in the solution. These phenomena indicate that the mesogenic groups have the tendency to aggregate in the solid-state and resulted in the decrease of band gap. The EL spectra of bistolane liquid crystals are similar to the PL spectra of bistolane LC. These results indicate that the PL and EL mechanisms of bistolane LCs are similar. Finally, the polarized EL based on the bistolane monomers was investigated. Polyimide (PI), polyaniline (PA), and poly(3,4-diethylene dioxythiophene) (PEDOT) are used as alignment materials for LCs. The dichroic ratios of the PI-, PA-, and PEDOT-aligned LCs are PI > PA > PEDOT. PI shows the highest alignment ability. The reason could be due to a more linear and rigid main chain structure of PI. The dichroic ratio is defined as the parallel to perpendicular emission intensity. The best dichroic ratios for UV, PL, and EL are 15.2, 13.8, and 7.1, respectively. The order parameter of LC that was coated on PEDOT in the EL device is 0.67.