Department of Materials Science and Engineering, National Chiao Tung University, Hsinchu, 30049, Taiwan.
J Am Chem Soc. 2011 Oct 5;133(39):15674-85. doi: 10.1021/ja205550b. Epub 2011 Sep 12.
In this study, we synthesized three analogous bent-core molecules, a hydrogen-bonded complex and a covalent-bonded compound with branched siloxane units (H-SiO and C-SiO, respectively) and a hydrogen-bonded complex with an alkyl unit (H-Alk), and investigated the effects of the hydrogen bonding and branched siloxane terminal units on their mesomorphic properties. The covalent-bonded compound C-SiO and the hydrogen-bonded complex H-Alk exhibited typical SmCP phases; in contrast, the hydrogen-bonded complex H-SiO exhibited a series of general tilt smectic (SmCG) phases with highly ordered layer structures (i.e., SmC̃G(2)P(F)-USmCG(2)P(A)-SmCG(2)P(F)-SmCGP(F) upon cooling). During the SmCG-type phase transition process, a 2D-modulated ribbon structure transferred into highly ordered layers via undulated layers, as the hydrogen-bonding strength increased with reduced temperatures. As the SmCG domains were aligned under dc electric fields, a gradual decrease in the leaning angle from ca. 60° to 50° (while the tilt angle kept at ca. 31°) could be determined by in situ wide-angle X-ray scattering (WAXS). Combined with Fourier transform infrared and Raman spectroscopic data, our results suggest that the change in the leaning angle was governed by the competition of the hydrogen bonds and microsegregation of siloxane units within the bilayer structure of the hydrogen-bonded complex H-SiO. In addition, the ferroelectric-(antiferroelectric)-ferroelectric transitions proven by the switching current responses in the SmCG-type phases of H-SiO reveal that the polar switching occurred through collective rotations around the long axis of H-SiO. Therefore, novel SmCG phases with a series of highly ordered 2D-structures were induced by the effects of the hydrogen bonding and branched terminal siloxane unit in the bent-core hydrogen-bonded LC complex H-SiO.
在这项研究中，我们合成了三种类似的弯型分子，即氢键复合物和带有支化硅氧烷单元的共价键化合物（分别为 H-SiO 和 C-SiO）以及带有烷基单元的氢键复合物（H-Alk），并研究了氢键和支化硅氧烷末端单元对它们介晶性质的影响。共价键化合物 C-SiO 和氢键复合物 H-Alk 表现出典型的 SmCP 相；相比之下，氢键复合物 H-SiO 在冷却过程中表现出一系列具有高度有序层结构的一般倾斜向列相（即 SmC̃G(2)P(F)-USmCG(2)P(A)-SmCG(2)P(F)-SmCGP(F)）。在 SmCG 型相变过程中，随着温度的降低，二维调制带状结构通过起伏层转变为高度有序层，因为氢键强度增加。当 SmCG 畴在直流电场下排列时，通过原位广角 X 射线散射（WAXS）可以确定倾斜角从约 60°逐渐减小到 50°（而倾斜角保持在约 31°）。结合傅里叶变换红外和拉曼光谱数据，我们的结果表明，倾斜角的变化受氢键和氢键复合物 H-SiO 双层结构中硅氧烷单元的微观分离竞争的控制。此外，在 H-SiO 的 SmCG 型相中的开关电流响应证明了铁电-（反铁电）-铁电转变，表明通过围绕 H-SiO 的长轴的集体旋转发生了极性切换。因此，新型 SmCG 相具有一系列高度有序的 2D 结构，这是由弯型氢键 LC 复合物 H-SiO 中的氢键和支化末端硅氧烷单元的影响引起的。
