Dichroic Dyes for Liquid Crystal Displays

Aleksandr V. Ivashchenko

• Publisher: CRC Press
• Year: 2018
• Print ISBN: 9781315892269
• eBook ISBN: 9781351088268
• Topic: Physical Sciences
• Subtopic: Chemistry
• Index: Chemistry

This book provides a systematic presentation of issues pertaining to the development of dichroic dyes applied in electrooptical systems for displaying and processing data. It explains the theory of the guest-host effect and the methodology of engineering dichroic dye (DD) molecules with specified characteristics. The book then examines the properties of currently known DDs, including the most interesting examples of synthesis. Various aspects of designing LCM for guest-host devices and available designs of guest-host LCDs are considered as well. Characteristics of dichroic dyes are presented in the Appendix as an added benefit to readers.

Chapter 1

Types of Dichroic Dyes and Their Order Parameters

When a dye with geometric anisotropy is dissolved in a liquid crystal the dye molecules tend to arrange in such a way that their long molecular axes align along the LC director, n→ (the direction of the predominant orientation of the LC long molecular axes). One of the main criteria of the DD eff iciency is the degree of ordering (or, in other words, the order parameter), S, of the long-wave electron transition oscillator (LETO), responsible for the color of the dye, with respect to n→. S can be easily found from the electron polarization spectra of the DD solution in LC using the following formula:

S=(D||−D⊥)/(D||−2D⊥)⁢(1)

where D_|| and D_⊥ are optical densities of the aligned solution of DD in LC measured for light polarizations parallel and normal to n→. As a rule, D_N and D_x are measured at the maximum of the absorption band. Often, to characterize the DD efficiency, the dichroic ratio N = D_||/D_⊥ is used which is related to S as follows:

S=(N−1)/(N+2)⁢(2)

If LETO coincides with the long geometrical axis x of the DD molecule (Figure 1a), then the value of S is equal to that of the geometrical ordering (S^G) of the DD long molecular axes in LC and can be found in the same way as the LC order parameter^3,14

S=SG=0.5(3cos2θ−1)⁢(3)

where θ is the angle between LETO (long axis) of an individual DD molecule and n→ averaged over all the DD molecules in the sample.

In the overwhelming majority of cases the DD LETO is polarized at an angle β relative to the axis x (Figure lb), hence the relationship between S and S° can be given by¹¹

S=SG=(1−3/2sin2β)⁢(4)

Dependence of S on the angles β and θ, and relationships between S and S^G for such dyes are presented in Figure 2. As can be seen from Figure 2b, for θ = 0 the order parameter decreases with β, and S >0 for the angles in the range 0° ≤ β < 54°44′8″. The dyes with S >0 are called DD of positive dichroism, or L-dyes (L = longitudinal). The L-dyes can be exemplified by 4-dialkylamino-4’-cyanoazobenzenes (Structure 1). The dyes with S <0 (54°44′8″ (< β ≤ 90°) are called DD of negative dichroism, or T-dyes (T = transverse), for example, 3,6-disubstituted tetrazines (Structure 2).

Also known are dyes with S = 0 (β = 54°44′8″ for θ = 0, or β = 90° for θ = 54°44′8″). They are referred to as isotropic dyes, orl-dyes, because in the electron polarization spectra of their aligned LC solutions the values of D_|| and D_⊥ are equal. DD of Structure 3 is an example of such a dye. Further below, dyes with -0.05 ≤ S ≤ 0.1 will be called I-dyes.

Figure 1 Types of dichroic dyes: (a) LETO coincides with the long molecular axis x of the DD molecule; (b) LETO does not coincide with the axis x.

Figure 2 Dependence of the DD order parameter, S, on the angles β and θ (a to c) and on the DD geometric ordering in LC (d).

Finally, there are dichromatic DD having in the visible spectral range two or more absorption bands with opposite signs of S. Such dyes are named L,T- and T,L-dyes, respectively. It should be noted that, similarly, there can be other dichromatic dyes, namely, LI- and IL- or IL- and TI-dyes, the latter, however, being represented by only a few examples. This type can be exemplified by the bism...

Table of contents

• Cover
• Title Page
• Copyright Page
• Table of Contents
• Introduction
• Chapter 1 Types of Dichroic Dyes and Their Order Parameters
• Chapter 2 Design of a Dichroic Dye Molecule
• Chapter 3 Dichroic Dyes
• Chapter 4 Liquid Crystalline Materials
• Chapter 5 Electrooptical Effects and Color Displays Based on Them
• Chapter 6 Commercial Dichroic Dyes and Liquid Crystalline Materials
• References
• Appendix 1. Bis(Arylideneamino)Anthraquinones
• Index