Photoalignment of liquid crystals: basics and current trends

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.

The ability of suitably derived prop-2-enoates from 3-(thiophen-2-yl)- and 3-(thiophen-3-yl)-prop-2-enoic acid and appropriately fluorinated and non-fluorinated 4-[(4-alkoxyphenyl)diazenyl]phenols to promote excellent photoalignment of a bulk commercial nematic liquid crystal is reported. The performed study showed that the photoalignment of the liquid crystal is influenced by the number and disposition of fluoro-substituents and the position of attachment of the terminal thiophene moiety, either 2- or 3-substituted, in the molecular structure of the alignment material. Inclusion of a lateral fluoro-substituent(s) induces an excellent photoalignment whereas the non-fluorinated counterpart tends to give an unsatisfactory alignment. 3-Substituted thiophenes give a better photoalignment than their 2-substituted isomeric counterparts. A prototype display, which contains an alignment layer made from one of the proposed materials, possessing good alignment ability, is presented in order to demonstrate the application potential of these materials in LCDs.

New class of highly efficient polymers for liquid crystal (LC) photoalignment is proposed. These polymers are based on methacryloylaminoarylmethacrylates having two methacryloyl groups of different reactivity. The more active O-methacryloyl group is subjected to polymerization, while the less active NH-methacryloyl group undergoes photocrosslinking reaction in combination with Fries rearrangement. The latter process is anisotropic in case of polarized light irradiation that results in LC photoalignment with readily controlled pretilt angle and anchoring energy. The induced alignment shows high thermal and photo-stability.

The photoalignment techniques and materials for liquid crystal display (LCD) have attracted considerable interests, and been studied extensively as an alternative method to the rubbing technique. The alignment of liquid crystals (LCs) is possible by using a rubbing-free process, photo-irradiation of linearly polarized UV light (LPUVL) on the photoreactive thin film surface. The photoinduced chemical anisotropy of the alignment surface is transferred to LC layers to cause homogeneous alignment of LC molecules. Photoalignment is also attractive because its photo-patterning capability can give it an extra potential value for the manufacture of next generation LCDs, such as large area, multidomain, vertically aligned, and/or in-plane switching (IPS) mode displays. Wide range of photoreactive materials have been investigated so far, which are mainly based on various photochemical reactions such as cis-trans conformational transition, photodimerization, and photo-degradation. During the last decade, there have been significant advances improving the performance of photoalignment materials by designing the molecular structure, and the alignment performance of the best photoalignment materials approaches that of the rubbed polyimide. However, as an organic thin film to be used in the LCD industry, more improvements in physical properties are required. These include solution processibility, thermal and photochemical stability, adhesion, and optical transparency in addition to a high photosensitivity and a superior aligning performance. Polymers containing photodimerizable chromophores, such as cinnamate, coumarin, and chalcone units, at the side groups or in the polymer main chain, have been investigated more extensively due to their advantageous properties than other photochemical systems. Unlike azobenzene-type cistrans isomerizable polymers, these are photochemically irreversible and chemically stable after UV irradiation. Also, these polymers are relatively more sensitive to UV light, requiring a much lower exposure dose to achieve a saturated alignment than the photodegradation-type polymers. Poly(vinyl cinnamate) derivatives are among the most studied photoalignment materials based on photodimerization. LPUVL irradiation on their film provides homogeneous alignment of LCs with significantly high photosensitivity. However, the main problem hindering the commercial application of these polymers is their poor thermal stability due to their high chain flexibility. The deterioration of aligning performance was observed when the LC cells were annealed at high temperature. Recently, there have been many attempts to improve the thermal stability by using various polymer backbones, such as polyesters, poly(arylene ether), and polyimides, with crosslinkable chromophores in the structure. Polyimide is the most attractive one owing to its verified properties as an alignment material in the LC industry. It has stable alignment ability, good adhesive strength, high moisture resistance, and dimensional stability. The LC alignment on a soluble PI film with cinnamate side units was reported to be thermally stable up to 200 C, as described by Ree et al. On the other hand, the photosensitivity of these polymers was usually compromised by their rigid main chain. A high exposure dose of 0.5 J/cm was required for the saturated alignment of LCs in these systems. Therefore, it is necessary to explore new photoalignment materials having higher photosensitivity and superior thermal stability. In this communication, we report a soluble fluorinated polyimide bearing methylene cinnamate side groups, PIMC (Figure 1), as a photoalignment material of excellent thermal

Hereinafter we discuss our last five years investigations in the field of new highly efficient polymers for liquid crystal (LC) photoalignment. New polymers finally proposed are based on methacryloylaminoarylmethacrylates with two methacryloyl groups of different reactivity. Fries rearrangement and photocrosslinking/photopolymeryzation are such photochemical reactions which take place in their films under the action of UV light. In a case of polarized light irradiation rearrangement results in LC photoalignment. The problem of pretilt angle and anchoring energy control as well as thermal stability of the induced alignment are discussed.

We report a liquid-crystal (LC) photoalignment material with a high photosensitivity based on the fluorinated poly(arylene ether) containing a chalcone unit in the main chain. The fluorinated poly(arylene ether) exhibited defect-free homogeneous alignment of LCs upon irradiation of linearly polarized UV light for 10s. Spectroscopic analyses revealed that [2+2] cycloaddition between the chalcone moieties generated the surface anisotropy to induce an efficient alignment of LCs.

Photoalignment possesses obvious advantages in comparison with the usually "rubbing" treatment of the substrates of liquid crystal display (LCD) cells. The liquid crystal photoalignment is nano-technology, as the thickness of the alignment layer is about 2-15 nm. The photoalignment materials can be very useful for a new generation of liquid crystals displays and Photonics Devices. The advantages of the photoalignment technology and its application to Photonics LC devices will be reviewed here based on research in HKUST. Full Text: PDF

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 photoisomerization. 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.

A novel liquid crystal (LC) photoalignment method, based on a super thin azo dye molecular layer is proposed. The basic idea of this method is to form a very neat textile knitwear and uniform alignment by azo dye layer without spin coating and rubbing processes. The thickness of the alignment layer is smaller than 3 nm, which is much thinner than traditional PI alignment film. In addition to the advantages of a conventional photoalignment method, the use of super thin layer simplifies the alignment procedure, making possible a high electrooptical performance, good photo-tolerance and thermal stability, better adhesion on indium tin oxide (ITO) surface and compatibility with roll-to-roll process.

Liquid crystals photoalignment by films of side-chain azobenzene-containing polymers with different molecular structure

In this original review we briefly consider the novel azo-dye photo-aligning technology: history and the perspectives for future applications in liquid crystal (LC) devices. The review describes the following items. The brief introduction to the history of photoalignment and the basic classes of the photoaligning materials: photosensitive polymers, azodyes and monolayers will follow with an introduction to the physical mechanisms of the photo-aligning and photo-patterning technology. The advantages and drawbacks of various photo-aligning materials are analyzed from the point of view of practical applications. The detail description of the diffusion photo-aligning in azo-dye materials is provided. The characteristics of azo-dye photoaligning LC layers are compared with those ones prepared by polyimide rubbing method. The characterization of LC-surface interaction, such as pretilt angle and azimuthal anchoring energy is discussed. The newly developed materials should have a controllable pretilt angle and anchoring energy, thus enabling to develop a new generation of the LC devices: with low voltage, fast response and wide viewing angles. The problem of image sticking can be considerably reduced due to the high anchoring energy of azo-dye materials. Promising results, obtained for voltage holding Ratio (VHR) and residual DC voltage (RDC) in azodye photo-aligning materials are also shown. This implies that the azo-dyes can be applied as aligning layers in active matrix liquid crystal displays (AM-LCDs). The possibility to use the photo-aligning layers for new types of liquid crystal displays such as FLCD, VAN-LCD, p-BTN LCD, optical rewritable memory, microdisplays, and TN-LCD on plastic substrates is demonstrated. The photoaligning of liquid crystal polymers (LCP) and the new classes of devices based on them (optical retarders and compensators) is discussed. Special types of 3D LC alignment, LC alignment inside thin micro tubes, and grating surface were concerned. New superthin photo-aligned polarizers based on azo-dye layers were demonstrated. The polarizers are based both on photo-aligned lyotropic LC as well as pure azo-dye layers. The polarizers can be patterned and put inside LC display cell to serve as internal polarizers. Both color and neutral internal polarizers can be fabricated with the thickness 0.3-0.7 mm. The electo-optic response of TN-LCD with internal polarizers is practically the same as in case of usual external polarizers. New applications in transflective and 3D displays are envisaged.

Brownian motors in the photoalignment of liquid crystals

Highly sensitive photoalignment of liquid crystals (LCs) can be realized by axis-selective triplet energy transfer. Addition of a triplet photosensitizer (phosphorescent donor) into a photocrosslinkable polymer tethering E-cinnamate side chains ensures dramatic enhancement of photosensitivity to generate the optical anisotropy of polymer film and surface-assisted LC photoalignment. Photoirradiation of triplet photosensitizer-doped polymer films with linearly polarized 365 nm light for the selective excitation of triplet sensitizer gives rise to optical anisotropy of cinnamates as a result of axis-selective triplet energy transfer. By analyzing phosphorescence spectra with theoretical Perrin's formula, we find that triplet energy transfer is efficient within a radius of ∼0.3 nm from the triplet photosensitizer. Such photoaligned polymer films can be used for the surface-assisted orientation photocontrol of not only calamitic LC, but also discotic LC, even for extremely low exposure energies. The present procedure would be greatly advantageous for high-throughput fabrication of optical devices by photoalignment techniques.

In this paper the photo‐induced alignment of liquid crystals by azo‐dyes was investigated in spin‐coated films of individual azo‐dyes and azo‐dyes incorporated in polymer. It was studied the photo‐induced optical anisotropy (POA) properties of different azodyes; the influence of azo‐dye molecular structure with different terminal and lateral substituents on the POA properties and on the aligning ability of LC. The inclined LC alignment was controlled by two‐steps irradiation: 1) with normal linearly polarized light and 2) with oblique non‐polarized light. It was shown that the type and the quality of the LC alignment as well as the value of pretilt angle depend on the exposure time to the nonpolarized light, on the nature of the solvent for spin‐coating process and on the treatment of the orienting layers. The correlation between POA of azo‐dyes and their aligning capability was explored. It was shown that it has nonlinear character: even small anisot‐ropy gives a good LC alignment. Moreover, photo‐aligning azo‐dye layers have a good thermostability and are stable under weak UV irradiation as well. In this case all the main parameters of these films, such as optical anisotropy, pretilt angle and degree of ordering are practically independent of temperature.

To improve the characteristics and quality of the photoalignment of liquid crystals has been a main goal of many relevant studies. We demonstrate in this work, following the general concept of composite materials, that the photoalignment of liquid crystal can substantially be improved when using alignment layers made from photoalignment composite materials. These composites contain at least one photoalignment material. The other components of the photoalignment composite material may or may be not photosensitive, as conventional polyimide materials, for instance. We show that by a proper choice of the components, the properties of the photoalignment materials, made by photoalignment composites, can be tailored in a broad range, resulting in higher sensitivity, and thus shorter exposure time, uniform and smooth photoalignment topography (without domains and sandy structures) resulting in high image contrast. High-quality photo-patterning of the liquid crystal alignment is demonstrated in this work with a photoalignment composite material containing one of our photoalignment materials and a conventional polyamide alignment material.