Solid-state 13C NMRspectroscopy— A powerful characterization tool for thermotropic liquid crystals

Abstract

Thermotropic liquid crystals are quintessential materials. The self organization of them at molecular level leads to orientational order, an important property both for academic and industrial interest. Additionally, their sensitivity to external stimuli like temperature, pressure, mechanical, electrical, magnetic field etc. make them as natural choice for smart advanced materials. The contemporary molecular designing of mesogens has undergone dramatic change and accordingly different molecular topologies are being explored for inducing the novel mesophases and morphologies. The understanding of their interactions at molecular level is very crucial in designing future advanced materials for challenging applications. To address these challenges, understanding of current mesogenic molecular structures and their organization at different levels is utmost important. This article focuses on the role of solid state 13C NMR as a characterization tool for molecular level understanding of mesogens. As the 13C NMR spectroscopy in mesophase is marred by anisotropic interactions, it differs from conventional solution technique. Hence, this article is aimed at explaining the role of anisotropic interactions that influence the 13C chemical shifts and 13C-1H dipolar couplings of a mesogen in liquid crystalline phase. A brief historical development of 13C NMR (1D) and separated local field spectroscopy (2D) is presented and their role for deducing the important information in the mesophase is highlighted. The relevant subtleties like alignment induced chemical shifts and 13C-1H dipolar couplings for range of calamitic mesogens is shown while the work on other molecular shapes are cited based on their significance. Finally, the future role of 13C NMR spectroscopy for understanding complex mesogens is briefly mentioned.