Bent-Shaped Liquid Crystals

Structures and Physical Properties

Hideo Takezoe, Alexey Eremin

• Publisher: CRC Press
• Year: 2017
• ISBN:
  9781315355412
• Edition: 1
• Topic: Physical Sciences
• Subtopic: Chemistry
• Index: Chemistry

Bent-Shaped Liquid Crystals: Structures and Physical Properties provides insight into the latest developments in the research on liquid crystals formed by bent-shaped mesogens. After a historical introduction, the expert authors discuss different kinds of mesophase structures formed by bent-shaped molecules. This book devotes the majority of its pages to physical properties such as polar switching, optics and non-linear optics, and behavior in restricted geometries. However, as chemistry is often highly relevant to the emergence of new phases, particularly with reflection symmetry breaking, it also involves a broad spectrum of interesting chemistry viewpoints.

1
Introduction

Liquid crystals formed by bent-shaped molecules opened up a vast variety of morphologies and structures that became an exciting new research area in the last decade of the twentieth century. In this book, we first introduce liquid crystals particularly focusing on the shape of molecules and introduce a brief history of mesogens consisting of bent-shaped molecules. It is important to know how bent-shaped liquid crystals were discovered and what the scientific background was at that time.

1.1Brief Introduction to the Liquid Crystal State

Liquid crystal is an intermediate state of matter between liquid and crystalline states. Because of this, the liquid crystalline state is characterized by fluidity and anisotropy, which are the main features of fluids and crystals, respectively. Hence, to realize liquid crystal states in a condensed molecular system, intermolecular interactions should not be too strong, and molecules have to be at least anisometric. Otherwise, they form crystals or isotropic liquids, respectively. In soft matter, entropic contribution to the free energy of the system, that is, repulsive interaction based on excluded volume effect, is particularly important. In the late 1940s, Onsager showed that the isotropic (Iso)-nematic (N) transition can be driven by entropy without considering intermolecular attractive interaction (Onsager 1949). Actually, Onsager explained the Iso-N transition in an aqueous solution of Tabaco mosaic virus, which is of a cylinder shape, by considering only repulsive force (entropy) due to the excluded volume effect without considering attractive force. In this sense, molecular shapes are very important for mesogenic phase formation. As for the molecular shape, rods and discs are typical mesogenic designs in conventional liquid crystals. Actually, rod- and disc-shaped molecules form a variety of liquid crystalline phases such as nematic, smectic, discotic nematic, columnar nematic, and various columnar phases, which are summarized in Figure 1.1. Here, the long axes of rod-shaped molecules and the disc normals of disc-shaped molecules align along a certain direction, which is called the director. We also define an orientational order parameter, which gives a measure of how well molecules align; the order parameter is unity when molecules perfectly align and zero when the orientation is random. Smectic and columnar phases are subdivided on the basis of molecular tilt with respect to layer’s and column’s axes, intralayer and intercolumnar orders, respectively. To optimize intermolecular interactions, molecules usually bear flexible chains and intermolecular interactions through flat and large π orbitals. Sometimes the introduction of substituted groups is also effective to realize liquid crystallinity.

FIGURE 1.1 Typical liquid crystalline phases formed by rod- and disc-shaped molecules. (a) Nematic, (b) smectic A, (c) smectic C, (d) discotic nematic, (e) columnar nematic, and (f) hexogonal columnar.

In this way, molecular structure and shape are very important to form liquid crystalline phases. Actually, various mesophase structures can be c...