Photo‐isomerization, photodimerization, and photodegradation polyimides for a liquid crystal alignment layer

Abstract

We review the important features of LC orientation for the alignment layer, namely, production method, alignment mechanisms, and required properties. Photoalignment, which could eliminate the disadvantages of the rubbing method is explored. Applicable materials and techniques for photoalignment are also examined. Many materials for photoalignment layers have been proposed, but polyimide‐based materials are commonly used because of their transparency and insulation properties, which are needed for optical electronic devices. Furthermore, photoisomerization, photodimerization, and photodecomposition as photoalignment methods are reviewed, and specific photoreactive groups and alignment mechanisms are used to induce LC alignment. In summary, polyimides cause photodecomposition and have the following disadvantages; (1) requirement of relatively short‐wavelength ultraviolet (UV) irradiation, (2) difficulty in reorienting polymer chains owing to their rigid polymer matrix, and (3) low‐photo reactivities. The solutions for each disadvantage are already proposed, but the required properties should be determined during material design. Polyimide‐based materials introduced with the azobenzene and cinnamate groups are proposed and found to have the ability of fulfilling the required properties. The mechanism of polyimide photodecomposition is not fully understood. It likely has the advantage in terms of LC alignment stability because of the absence of E/Z isomerization. Therefore, the alignment mechanism and availabilities should be further explored.

Light-responsive bent-core liquid crystals as candidates for energy conversion and storage

“…Even though azobenzenes have been studied for several decades [5][6][7], they continue to attract interest due to their potential to promote long-range macroscopic changes induced by short-range molecular modifications. [8][9][10][11][12] Bent-core liquid crystals, BCLCs, exhibit anti-ferroelectric and ferroelectric behaviour with high figures of merit, which can be useful for energy storage applications. BCLCs can form columnar (Col, former B 1 ), polar smectic C (SmCP, former B 2 ), twist grain boundary dark conglomerate (DC) or helical nanofilament (HNF, former B 4 ) phases among others [13][14][15][16][17][18][19][20].…”

“…Even though azobenzenes have been studied for several decades, 5–7 they continue to attract interest due to their potential to promote long-range macroscopic changes induced by short-range molecular modifications. 8–12…”

Binary geometric-phase holograms

“…Photoalignment is a versatile method that can accommodate small feature sizes and spatially-varying patterning [61,62,63,64]. A photoalignment layer (PAL) is applied to the substrate and then locally orients relative to the linear polarization angle of a UV source during exposure.…”

Shape‐Morphing Liquid‐Crystal polymers

“…For example, the photoresponsive liquid crystal mesogens can be arranged by light. [38][39][40] and the mesogens can be aligned along the direction of magnetic induction intensity because strongly diamagnetic of the mesogens containing benzene ring structure. [41,42] It is also possible to arrange mesogens using surface alignment layers.…”

“…In addition, the alignment of liquid crystal mesogens can be realized by the induction of external fields. For example, the photoresponsive liquid crystal mesogens can be arranged by light [38–40] . and the mesogens can be aligned along the direction of magnetic induction intensity because strongly diamagnetic of the mesogens containing benzene ring structure [41,42] .…”