US-12624212-B2 - Bis-azo dyes for thin film polarizers and synthesis method

US12624212B2US 12624212 B2US12624212 B2US 12624212B2US-12624212-B2

Abstract

A dichroic bis-azo biphenyl dye of the following formula: The substituents R 1 , R 2 , R 3 , R 4 are alkyl (C n H 2n+1 ) or alkenyl (C n H 2n−1 ) groups with n=1-18. At least one substituent among R 1 R 2 R 3 R 4 is different from the other substituents. A benzidine-free method of making a dichroic bis-azo biphenyl dye is provided in which the biphenyl portion of the dye is formed from different first and second mono-azo dyes by a cross-coupling reaction, which may be a Suzuki reaction. A polarizer is formed from a photoaligned layer of the bis-azo biphenyl dye on a substrate and exhibits a dichroic ratio of at least 40. Chemical modification of the photoaligned bis-azo biphenyl dye layer creates a broad-band thin film polarizer with low light scattering and high thermal and photo tolerances.

Inventors

Valerii Vladimirovich Vashchenko
Olena VASHCHENKO
Hoi Sing Kwok
Cuiling MENG
Yeuk Lung HO

Assignees

THE HONG KONG UNIVERSITY OF SCIENCE AND TECHNOLOGY

Dates

Publication Date: 20260512
Application Date: 20220315

Claims (10)

1 . An absorptive thin film polarizer, comprising: a substrate; an aligned dye layer comprising at least one dichroic bis-azo biphenyl dye of the following formula: wherein substituents R 1 , R 2 , R 3 , R 4 are individually selected from alkyl (C n H 2n+1 ) or alkenyl (C n H 2n−1 ) groups with n=1-18 and wherein at least one substituent among R 1 and R 2 is different from at least one substituent among R 3 and R 4 such that the dichroic bis-azo biphenyl dye is molecularly asymmetrical; and wherein the haziness of the thin film polarizer is in a range of 0.5-1%, and a dichroic ratio of the thin film polarizer is at least 40.
2 . The absorptive thin film polarizer of claim 1 , wherein the aligned dye layer is a photoaligned dye layer.
3 . The absorptive thin film polarizer of claim 1 , wherein the aligned dye layer is a protonated aligned dye layer.
4 . The absorptive thin film polarizer of claim 1 , further comprising a protective layer positioned over the aligned dye layer.
5 . The absorptive thin film polarizer of claim 1 , wherein the substrate is a flexible substrate.
6 . The absorptive thin film polarizer of claim 5 , wherein the flexible substrate is a polymeric substrate.
7 . A display including the thin film polarizer of claim 1 .
8 . The absorptive thin film polarizer of claim 1 , wherein the dichroic bis-azo biphenyl dye being formed from first and second benzidine-free mono-azo precursors by a cross-coupling reaction, the first and second benzidine-free mono-azo precursors being different from each other.
9 . The absorptive thin film polarizer of claim 8 , wherein the first benzidine-free mono-azo precursor is a bromoazobenzene dye synthesized by diazotization of 4-bromoaniline following by azo-coupling with an appropriate N,N-dialkylaniline.
10 . The absorptive thin film polarizer of claim 8 , wherein the second benzidine-free mono-azo precursor is a boronic ester derivative of azobenzene synthesized by borylation of bromoazobenzene.

Description

FIELD OF THE INVENTION The present invention relates to thin film polarizers and, more particularly, to thin film polarizers that use bis-azo dyes and methods for their production. BACKGROUND Display technologies are evolving towards thin, flexible and foldable displays; these displays require polarizers, typically thin film polarizers. Several approaches to film polarizer exist; for absorptive polarizes, one technique uses a dichroic dye coated on a substrate and aligned with a photoalignment technique. This type of polarizer may be fabricated with a thickness of less than 0.5 micron and a high dichroic ratio (DR). Among the numerous dichroic dyes, the bis-azo derivative of benzidine-4,4′-(biphenyl-4,4′-diylbis(diazene-2,1-diyl))bis(N,N-dibutylaniline), named AD1, is characterized by a unique high individual dichroism in photooriented films (DR up to 90). Chemically modified AD1 films, e.g., by protonation, show increased absorption in the visible and near infrared region as well as resistance to light exposure; these modified films may be used in the manufacture of broad band polarizers. However, film polarizers based on AD1 in either protonated or basic form, have several disadvantages. Aligned AD1 films have a rather high level of light scattering, which further increases after chemical modification thus degrading the optical quality of the polarizer. The addition of plasticizers during the film fabrication weakly impacts the light scattering but significantly reduces the dichroic ratio. Variation in solvents and/or temperature during film preparation does not lead to the desired improvement in film quality. Further, the dichroic ratio of AD1 films critically depends on the AD1 chemical purity. The highest dichroic ratio values are achieved only when more materials of more than 99% purity material are used; however various impurities (such as those intrinsically formed during AD1 synthesis via benzidine, which are difficult to remove) in amounts as small as 3% reduce the dichroic ratio of the resulting AD1 film to below 10. Therefore, purification of the dichroic dyes, or development of new method of their synthesis that avoid difficult-to-remove impurities, are critical for polarization film fabrication. A commonly-employed AD1 synthesis technique involves the diazotization of benzidine (1,1′-biphenyl-4,4′-diamine) followed by azo-coupling reactions with N,N-dibutyl aniline (see FIG. 3). The crude product isolated from the reaction mixture contains large amounts of various colored impurities, which have similar physical properties to the target product. Purification via multiple recrystallizations and/or flash-chromatography are commonly used, but these methods yield AD1 with a purity <97%, and a dichroic ratio <10. Only purification of AD1 by preparative column chromatography provides a material of the required purity and high dichroic ratio. However preparative column chromatography is costly both in terms materials and processing time. It is important to note that the starting material for the synthesis of AD1, benzidine, is a hazardous chemical, because it is a human carcinogen. Therefore, recent years have seen a significant reduction in the use of benzidine in commercial processes. Thus, there is a need for alternatives to the use of benzidine to fabricate dichroic dyes. Among dichroic dyes with fundamentally different structures to AD1, there are not dyes that provide the benefits of AD1 without its weaknesses. Further, it has been determined that even small modifications to the core structure of the AD1 molecule (e.g., the introduction of side substituents in benzene rings or replacement of benzene rings with naphthalene) significantly worsen the dichroism of the new dyes. Therefore, the search for new dye structures similar to the AD1 structure (AD1-like dyes), combining an ability to photo-align with lower crystallinity is a challenge. Thus, there is a need in the art for alternatives to AD1, that is, “AD1-like” new materials, which are free from the above-mentioned disadvantages. This invention provides such materials, along with benzidine-free manufacturing techniques. Summary of the Invention The present invention provides new high dichroic ratio bis-azo dyes, an ecologically friendly method of their synthesis with high purity reaction products, and the use of the dyes in thin film polarizers. The new high dichroic ratio azo dyes are derivatives of 4,4′-Bis(4-N,N-dialkylaminophenylazo)-biphenyl with different lengths of alkyl substituents in the amino groups. The different lengths of the alkyl substituents create an asymmetrical structure of the bis-azo dyes, resulting in high optical quality/low haziness of the polarizers. The ecologically friendly method of bis-azo dye synthesis includes formation of the central biphenyl fragment of the dye from two different mono-azo dyes by means of a cross-coupling reaction. The method provides a safe, benzidine-free, synthetic approach to forming bis