Understanding Structural and Molecular Properties of Liquid Crystal Dimers: A Density Functional Approach

Abstract

Liquid crystals are molecular systems that exhibit partial ordering of molecules similar to solids while maintaining the ability to flow like liquids. Depending upon the amount of ordering in the material, there are many types of liquid crystalline phases. The nematic phase is the most common and technologically most important one due to its use in display applications. In the nematic phase, the molecules tend to have the same alignment, but their positions are not correlated. In part due to fundamental scientific interest and driven by new technological motivations apart from displays, the existence of new stable nematics has been continuously searched. The continuing search led to a recent discovery of a new type of nematic phase, 1 , 2 known as the twist-bend nematic (Ntb), in certain bent-shaped liquid crystal dimers that have been supported by various independent experimental studies. Since the Ntb phase has been discovered recently, its properties have not been fully explored and a detailed description and understanding at the molecular level are still far from complete. 3 The Ntb phase’s formation is highly sensitive to any slight changes in the molecular shape arising from the chemical makeup of the linking spacer, terminal moieties and mesogenic units. 4 , Such structural features are not accessible directly through experiments. Thus, in this work, we present a set of DFT calculations on a series of liquid crystal dimers. This work aims to probe the role of certain structural features in driving the formation of the Ntb phase. This study also reveals why this phase occurs in certain bent molecules, but not in all. Since the constituent molecules are flexible and exist in a range of conformers, comparing the conformational landscapes of the dimers-exhibiting-the-Ntb-phase against those-do-not would identify the molecular conformations promoting the formation of the Ntb phase. Overall, this study evaluates ideal molecular structural features and conformational ensembles potentially responsible for the appearance of the Ntb phase.