Department of Physics, Kent State University, Kent, OH 44242, USA.
Materials Science Graduate Program, Advanced Materials and Liquid Crystal Institute, Kent State University, Kent, OH 44242, USA and Advanced Light Source, Lawrence Berkeley National Lab, Berkeley, CA 94720, USA.
Phys Chem Chem Phys. 2021 Feb 25;23(7):4055-4063. doi: 10.1039/d0cp06633h.
The helical pitch formed by organic molecules, such as the α-helix of proteins, usually requires hydrogen bonding between chiral units and long-range positional order. It was recently found that certain liquid crystal oligomers can have a twist-bend nematic (NTB) phase with nanoscale heliconical structure without hydrogen bonding, molecular chirality or positional order. To understand the nature of this unique structure, here we present hard and resonant tender X-ray scattering studies of two novel sulfur containing dimer materials. We simultaneously measure the temperature dependences of the helical pitch and the correlation length of both the helical and positional order. In addition to an unexpected strong variation of the pitch with the length of the spacer connecting the monomer units, we find that at the transition to the NTB phase the positional correlation length drops. The helical structure was found not only in the NTB phase but observed even in the upper range of a smectic phase that forms just below the NTB state. The coexistence of smectic layering and the heliconical order indicates a layered (SmATB) phase wherein the rigid units of the dimers are tilted with respect to the smectic layer normal in order to accommodate the bent conformation of the dimers and the tilt direction rotates along the heliconical axis.
有机分子（如蛋白质的α螺旋）形成的螺旋桨距通常需要手性单元之间的氢键和长程位置有序。最近发现，某些液晶低聚物可以具有扭曲向列（NTB）相，具有纳米级螺旋结构，而没有氢键、分子手性或位置有序。为了理解这种独特结构的性质，我们在这里介绍了两种新型含硫二聚体材料的硬和共振软 X 射线散射研究。我们同时测量了螺旋桨距和螺旋和位置有序的相关长度随温度的变化。除了连接单体单元的间隔物长度对螺距的意外强烈变化外，我们发现，在向 NTB 相转变时，位置相关长度下降。螺旋结构不仅在 NTB 相中观察到，甚至在 NTB 状态下形成的近晶相的上限范围内也观察到。近晶层和螺旋有序的共存表明存在层状（SmATB）相，其中二聚体的刚性单元相对于近晶层法线倾斜，以适应二聚体的弯曲构象，并且倾斜方向沿螺旋轴旋转。
