本論文利用全原子分子動態模擬和量子化學計算來研究香蕉形液晶的 結構、交互作用力及其動態行為。研究的香蕉形液晶分子(圖1.16)具有B6 液晶相，不同於一般常見的香蕉形液晶結構，中間連接基為剛硬的核心， 此一分子中間連接基為柔軟的飽和碳鏈連接兩邊硬核結構。 研究結果顯示，Schiff’s base 和苯環上的OH 基形成了分子內氫鍵，這 個氫鍵增強了Schiff’s base 和苯環的π-電子共振，使得這個分子的硬核較原 本預期的苯環來得大；此外，由於連接烷氧基的碳-氧鍵結具有部分雙鍵的 特性，限制了烷氧基連接端的構形，使得硬核的部分擴大到烷氧基的連接 端。 模擬結果顯示，多分子系統中分子的彎曲角度約120o 和單晶的結果 (128o)類似，屬於典型的香蕉形彎曲角度(110o ~140o)。由於分子內的的π- π堆疊穩定能量，使得單分子系統的彎曲角度(~40o)較小，傾向形成髮夾型 (“hairpin”-like)的構形。單分子和多分子截然不同的彎曲角度顯示分子間交 互作用力不僅影響分子間的堆疊，也對香蕉形構形的形成，扮演著重要的 角色。此外，我們也觀察到分子間存在著重要的π-π 堆疊及分子間的氫鍵網 絡，顯示分子間排列的情況主要由這兩個作用力決定。 長時間的分子動態模擬顯示，烷氧基長碳鏈和硬核的夾角為一動態平 衡，分佈主要集中在0o 左右，擺幅大約 ±30o 左右。研究也顯示烷氧基長碳 鏈一方面可以增加硬段的大小; 另外一方面，它柔軟度也較飽和長碳鏈來得 大。 In this study, long time-scale all-atom molecular dynamics simulations and density functional theory calculations were performed to study the structures, interactions and dynamics of a banana-shaped liquid crystal mesogen. This molecule (see figure 1.16 for details) owns a flexible alkyl linkage connecting to two rigid cores which is differential from the common rigid one. The results show that the intra-molecular hydrogen bond between the Schiff’s base and benzene’s OH group enhances the π-e resonance between them and integrates these two moieties into one rigid core. In addition, the characteristics of carbon-oxygen partially double bond between the pendent chain and benzene limits its conformational space. Thus, the rigid core extends from the moiety of benzene to the Schiff’s base and to the O-CH2 group of the alkoxy chain. The bending angle from the molecular dynamics simulations of the bulk system is ~120o, in the reasonable range of 110o-140o for banana-shaped mesogens. Moreover, this calculated bending angle is in good agreement with the observed 128o from the single crystal data. Arising from the stabilizing energy of intra-molecular π-π stacking, the isolated molecule prefers to form the hairpin-like conformation with a small bending angle of ~40o. The bending angle of the isolated molecule is pretty different from the one of bulk system indicating that the inter-molecular interactions can not only affect the organizations between molecules, but also their bending angles. Additionally, the networks of inter-molecular hydrogen bonds and inter-molecular π-π stacking were also observed from the 200 ns long time-scale molecular dynamics simulations. The angles between the alkoxy chain and rigid core are dynamical in liquid crystal phase where the chains have the highest population to align with the same plane of the rigid core. However, they fluctuate within a magnitude of ±30o up and down of the rigid core plane. More interesting, our simulations show the alkoxy chain enlarges the rigid core, on the other hand, it is more flexible than the saturated alkyl chain.
