Hirokazu Takada

Synthesis and properties of azo dye aligning layers for liquid crystal cells

The synthesis and properties of azo dyes that can be used for photoaligning liquid crystals (LCs) have been investigated. The structures and the synthetic procedure for the azo dyes are presented. The photoaligning of azo dyes takes place purely due to the reorientation of the molecular absorption oscillators perpendicular to the UV light polarization. The qualitative model for the phenomenon in terms of the rotational diffusion of the azo dye molecules in the field of the polarized light is discussed. The order parameters S = -0.4 (80% of the maximum absolute value S m = -0.5) were measured from the polarized absorption spectra at the wavelength 372 nm. A temperature stable pretilt angle of 5.3° was obtained by a two-step exposure of the azo dye film using normally incident polarized light followed by oblique non-polarized light. The azimuthal anchoring energy of the photoaligned substrate was Aϕ , 10−4 J m−2, which is the same as the anchoring of the rubbed polyimide (PI) layer. The voltage holding ratio value of a photoaligned LC cell was found to be even higher than for a rubbed PI layer, which enables the applications of azo dyes as aligning layers in active matrix liquid crystal displays. The thermal stability of the photoaligned azo dye layers is sufficiently high, but UV stability has to be improved, e.g. by polymerization. A new LCD aligning technology based on polymerized azo dye layers is envisaged.

New developments in liquid crystal photo-aligning by azo-dyes

Liquid crystal (LC) photo-alignment using azo-dyes is reviewed. This alignment method is very different from previously reported ones, which are due to mechanisms such as photo-crosslinking, photo degradation, and photo-isomerization. We present the basic physical mechanisms of the photo-induced orientational order in various photo-aligning materials and in azo-dye layers in particular. This method is based on rotational diffusion in a potential created by the light field as well as intermolecular forces. It will be shown that this photo-aligning method can provide a controllable pretilt angle and strong anchoring energy of the LC cell, as well as having high thermal and ultraviolet (UV) stability. The application of this method to the alignment and fabrication of various types of LC displays is also discussed.

Photo‐aligning by azo‐dyes: Physics and applications

Liquid crystal photo‐alignment using azo‐dyes is reviewed. This alignment method is very different from previously reported ones, such as photo‐crosslinking, photo degradation and photo‐isomerization. It will be shown that this photo‐aligning method can provide a controllable pretilt angle and strong anchoring energy of the liquid crystal cell, as well as high thermal and UV stability. The application of this method to the alignment and fabrication of various types of liquid crystal displays is also discussed.

Photoaligned Vertical Aligned Nematic Mode in Liquid Crystals

The photoaligned vertical aligned nematic (VAN) mode in liquid crystals (LC) was developed using the photosensitive polyimide (PI) JALC 2021-R2, illuminated by obliquely incident nonpolarized light. It has been found that the combination of PI with water soluble sulfonic azo-dye (SD) yields a perfect electro-optical performance of a liquid crystal in the VAN mode. The polar and the azimuthal anchoring strengths of LC in VAN mode were measured both for rubbed and photoaligned surfaces. Increasing the illumination time results in the exponential growth of the contrast ratio in the VAN LC cell. Analysis of anchoring strength and electrooptic behaviour of the photo-aligned LC cells confirm that the mechanism of photo-alignment in our materials includes not only the photodegradation, but mostly the reorientation of both SD-1 and PI molecules to get the average absorption oscillator almost parallel to the direction of the obliquely incident UV light.

P‐104: Aligning Layers Using Azo Dye Derivatives for Liquid Crystal Devices

The photo-induced alignment of an azo dye film and liquid crystal (LC) alignment on it are discussed for the liquid crystal display applications. Considerable dichroism is induced in the azo dye SD1 film not only by polarized UV light but also by obliquely incident non-polarized UV light irradiation. It was found that sulfo groups in the azo dye molecule play an important role in the LC alignment on the film. Large pretilt of LC on the photo-aligned film was achieved simultaneously, because the azo dye molecules can be easily reoriented out of the film plane by obliquely incident non-polarized light irradiation. In order to improve stability of the photo-aligned film, a monomer derived from the azo dye was synthesized. After thermal polymerization, the film of it also exhibit good photo-alignment properties. Improvement of durability against light exposure and moisture was observed in the LC cell using it.

Liquid‐crystal photoaligning by azo dyes (Invited Paper)

— Liquid-crystal (LC) photoalignment using azo dyes is described. It will be shown that this photoaligning method can provide a highly uniform alignment with a controllable pretilt angle and strong anchoring energy of the LC cell, as well as a high thermal and UV stability. The application of LC photoalignment to the fabrication of various types of liquid-crystal displays, such as VAN-LCDs, FLCDs, TN-LCDs, and microdisplays, on glass and plastic substrates is also discussed. Azo-dye photoaligned super-thin polarizers and phase retarders are considered as new optical elements in LCD production, in particular for transflective displays.

Azo-dye aligning layers for liquid-crystal cells

The photo-induced alignment of liquid crystal onto a photochemical stable azo-dye film was studied for liquid-crystal display (LCD) applications. The photo-aligning of azo dye takes place due to the pure reorientation of the molecular absorption oscillators perpendicular to the UV-light polarization. The order parameters S = -0.4 (80% of the maximum absolute value S m = -0.5) was measured at a wavelength of 372 nm from the polarized absorption spectra. The temperature-stable pretilt angle of 5.3° was obtained by a two-step exposure of azo-dye film using normally incident polarized light followed by oblique non-polarized light. The azimuthal anchoring energy of the photo-aligned substrate was A φ > 10 -4 J/m 2 , which is of the same magnitude as the anchoring of the rubbed PI layer. The VHR value of the photo-aligned LC cell was also found to be very high (98-99%) at room temperature and more than 95% at T= 80°C. The thermal stability of the photo-aligned azo-dye layers is sufficiently high, while UV stability has to be improved, e.g., by polymerization. The new LCD aligning technology based on photochemical stable azo-dye layers is envisaged.

38.2: Azodye Aligning Layers for Liquid Crystal Cells

The photo-induced alignment of liquid crystal onto a photochemical stable azo dye film is studied for the liquid crystal display (LCD) applications. The photo-aligning of azo dye takes place due to the pure reorientation of the molecular absorption oscillators perpendicular to the UV-light polarization. The order parameters S= −0.4 (80% of the maximum absolute value Sm= −0.5) was measured at the wavelength of 372 nm from the polarized absorption spectra. The temperature stable pretilt angle of 5.3° was obtained by a two-step exposure of azo- dye film using normally incident polarized light followed by oblique non-polarized light. The azimuthal anchoring energy of the photo-aligned substrate was Aɛ> 10−4 J/m2, which is of the same magnitude as the anchoring of the rubbed PI layer. VHR value of the photo-aligned LC cell was also found to be very high (98–99%) at room temperature and more than 95% at T=80°C. The thermal stability of the photo-aligned azo-dye layers is sufficiently high, while UV-stability has to be improved e.g. by polymerization. The bright perspectives for the new LCD aligning technology are envisaged.

Novel photoaligned twisted nematic liquid crystal cell

A novel photoaligned twisted nematic liquid crystal display (TN-LCD) cell is fabricated by one-step illumination with oblique nonpolarized UV light of an empty cell with azo-dye layers coated on indium tin oxide (ITO) substrates. If the incident angle is sufficiently large (>75°), the p-polarization of the light is more pronounced for the bottom azo-dye layer in comparison with that for the upper one. Thus, the photo-alignment of the azo-dye layer on the bottom substrate is perpendicular to that of the upper one, and LC directors on the top and bottom substrates also become perpendicular. The method provides a new simple way of manufacturing photoaligned TN-LCDs.

45.3: Azo Dye Materials for the Alignment of Liquid Crystal

In this paper, the photo-induced alignment of liquid crystal on an azo dye film is discussed for the liquid crystal display (LCD) applications. This is a non-contact technique to align the liquid crystals so that the dust particles and static charges generated by the rubbing process can be eliminated. To prepare for the photo-alignment layer, the azo dye film is exposed to a linearly polarized or non-polarized light. As a result of the dipole absorption, the photo-induced molecular distribution is non-uniform and has an angular dependence. Consequently, since the dispersion forces between the dye and the liquid crystal molecules dominate, the induced orientation of the liquid crystal molecules is possible. The oblique incidence of the light exposure will also favour a non-zero pretilt angle. We find that the order parameter associated with the aligned liquid crystal medium is high, and very good contrast has been measured in the Twist Nematic (TN) LCD.