Reddy M Guruprasad, Lobo Nitin P, Roy Arun, Ramanathan K V, Narasimhaswamy T
Polymer Science and Technology, CSIR-Central Leather Research Institute, Adyar, Chennai 600020, India.
Centre for Analysis, Testing, Evaluation & Reporting Services (CATERS), CSIR-Central Leather Research Institute, Adyar, Chennai 600020, India.
Phys Chem Chem Phys. 2020 Nov 7;22(41):23986-23997. doi: 10.1039/d0cp03933k. Epub 2020 Oct 19.
For a successful design of functional mesogens, it is paramount to understand factors that contribute to molecular organisation such as molecular shape, the non-covalent interactions of the constituent moieties as well as nanosegregation of incompatible molecular parts. In this study on four tetracatenar mesogens, we show that by a slight change in the length of the terminal chain, the molecular organization changes from lamellar to columnar phase and that the orientational order experiences profound change between the lamellar, the center rectangular columnar and the hexagonal columnar mesophases. We consider here, mesogens that exhibit lamellar and columnar mesophases with five phenyl rings in the central rod-like core which are subjected to XRD and high resolution solid state C NMR investigations in their mesophases. The XRD studies indicate that the lower homologs exhibit a lamellar mesophase while the higher homologs show either a centre rectangular columnar phase or a 2D hexagonal columnar mesophase. C NMR investigations reveal interesting and strikingly different molecular orientations in each of these phases. For example, values of order parameters of one of the phenyl rings in the core region of the mesogens vary from 0.75 and 0.77 for the lamellar mesogens to 0.45 and 0.17 for the centre rectangular columnar and the hexagonal columnar mesogens respectively. While these values indicate that the mesogenic molecules are oriented along the magnetic field as expected in the lamellar phases, the very low order parameter in the hexagonal columnar phase arises due to molecules distributed azimuthally in layers and undergoing motion about the columnar axis which itself is oriented orthogonal to the magnetic field. Such cutting edge information extracted from the combined use of XRD and C NMR studies on tetracatenar mesogens is expected to be of significant use for the study of π-conjugated polycatenar systems where functional properties depend on the molecular orientation and order.
对于功能性液晶基元的成功设计而言，了解有助于分子组织的因素至关重要，这些因素包括分子形状、组成部分的非共价相互作用以及不相容分子部分的纳米分离。在这项对四种四链液晶基元的研究中，我们表明，通过末端链长度的轻微变化，分子组织从层状相变为柱状相，并且在层状相、中心矩形柱状相和六方柱状中间相中，取向有序经历了深刻的变化。我们在此考虑的是，在中心棒状核中有五个苯环的呈现层状和柱状中间相的液晶基元，它们在中间相状态下接受X射线衍射（XRD）和高分辨率固态碳核磁共振（C NMR）研究。XRD研究表明，较低同系物呈现层状中间相，而较高同系物则显示中心矩形柱状相或二维六方柱状中间相。C NMR研究揭示了这些相中每一个相中有趣且显著不同的分子取向。例如，液晶基元核心区域中一个苯环的有序参数值，对于层状液晶基元而言，从0.75和0.77变化到对于中心矩形柱状和六方柱状液晶基元分别为0.45和0.17。虽然这些值表明液晶分子如在层状相中预期的那样沿磁场取向，但六方柱状相中的极低有序参数是由于分子在层中沿方位角分布并围绕与磁场正交取向的柱状轴运动所致。从对四链液晶基元联合使用XRD和C NMR研究中提取的此类前沿信息，预计对于研究功能性质取决于分子取向和有序性的π共轭多链体系具有重要用途。
