Photo‐Induced Orientation of a Film of Ladderlike Polysiloxane Bearing Dual Photoreactive Side Groups

Abstract

The control of the orientation of liquid crystalline molecules is essential in liquid crystal displays (LCDs), and is usually achieved by coating an alignment film on the conductive ITO (indium tin oxide) glass surface. The photo-alignment (PA) technique as one promising alternative to the widely used conventional rubbing process of a polyimide (PI) substrate has been studied extensively in the last decade, because it provides a simple and non-contact fabrication process suitable for advanced thin film transistor (TFT) LCDs. In principle, the PA materials can be primarily divided into two categories based on the type of functional groups employed. The first ones are capable of undergoing photo-induced isomerization from trans isomer (E-isomer) to cis-isomer (Z-isomer) and vice versa; this category includes molecules such as azobenzene-containing polymers.[1] The second category consists of polymers that can be anisotropically photo-crosslinked on illumination with linear polarized ultraviolet (LPUV) light; this includes examples such as polymers containing cinnamoyl or coumarin groups. [2,3] In particular, Schadt, [2b±d] and O'Neill and Kelly [4] recently demonstrated that the films based on coumarin exhibit high sensitivities and high pretilt angles (up to 7.5), although the alignment was ascribed to unstable photo-degradation rather than cross-linking. However, despite the promising future of the PA technique, the search for materials possessing fine comprehensive performance still remains a great challenge.As an inorganic±organic hybrid, the ladder-like polysilsesquioxanes (LPSs) possess many excellent properties, such as being colorless and transparent, glasslike film-forming ability, high adhesion to glass and silicon chips, and superior solubility in common organic solvents, and are potentially ideal skeleton materials for PA applications. [5,6] Accordingly, a novel LPS containing dual photo-reactive groups, cinnamoyl and p-nitroazobenzene, attached in a side-on fashion, was synthesized (Scheme 1, which also shows the structure of the final polymer). The molecular design was based on the following considerations:l the azobenzene-containing polymer films often have a higher degree of tilted orientation and photosensitivity than the photo-cross-linking ones; [7] l the threshold dose required to obtain a superior alignment for the side-on azo-grafted polymer is substantially smaller than that of the end-on ones; [8] l both the photo-orientation and photo-cross-linking of the neighboring dual moieties would take place collaboratively after proper irradiation and most likely result in better alignment stability. As expected, the LC cell assembled using the LPS-CA30 coated substrates exhibits a uniformly homogeneous alignment (almost mono-domain) with a long temporal stability (over one year at room temperature) and an adjustable tilt angle (h = 1~7) by variance in UV-light intensity (2~5 mW cm ±2 ). The macroscopic orientation behavior of the LC cell was observed by polarizing optical microscopy (POM). It wa...

Influence of Side‐Chain Structure and Irradiation Condition on Photoalignment of Ladder‐Like Polysiloxane Films

“…According to our previous description of the photoalignment mechanism, [15] it is suggested that the influence of exposure conditions is related to the rate of two photoreactions. Only when UV light with proper intensity and dose is applied, are the photo-induced isomerization and crosslinking triggered simultaneously and the reorientation both in and out of plane results.…”

“…[3a,4a,6,7,9±11] On the other hand, the polymers containing photocrosslinkable cinnamate or coumarin groups have been regarded as preferred candidates for PA films since Schadt and co-workers demonstrated their good performance in inducing stable, uniform alignment. [2±4,12±14] As recently stated in our previous report, [15] a novel ladder-like polysiloxane containing dual photoreactive groups, cinnamoyl and p-nitroazobenzene, attached in a side-on fashion, was designed and synthesized for generating high-quality photoalignment layers with controllable pretilt angle. The mechanism of photo-induced alignment and generation of pretilt angle was elucidated by some integrated spectroscopic methods as follows: the photo-induced isomerization and cycloaddition reactions take place cooperatively and give rise to consistent contributions to the homogeneous alignment; the direction-selective E±Z±E isomerization cycle of gradually enriched azobenzene groups leads mainly to an out-of-plane (spatial) reorientation and a reasonably large tilt angle.…”

Promising Functional Materials Based on Ladder Polysiloxanes

“…Additionally, moderate annealing before and after illumination can markedly improve the alignment uniformity by self-healing of defects. [12] 4. Pseudo-SPS-Based SHG/NLO Polymer Materials SHG/NLO organic polymers have attracted much attention owing to their high NLO activity and good processability.…”

A bridge-like polymer synthesized by tandem metathesis cyclopolymerization and acyclic diene metathesis polymerization

“…It is noteworthy that ladder-like polymers have greater resistance to irradiation as well as thermal and chemical degradation in comparison to their single-stranded counterparts. 16,17 Besides, ladder-type arrays should have planar and rigid π-π structures that facilitate electron delocalization and enhance conjugation. 17 A series of ladder polymers with various linkers, including the ferrocene moiety, the azobenzene group, multilayer planar oligoaryl chromophores, hexaperi-hexabenzocoronenes, cubane, cuneane, and cyclooctatetraene have been synthesized by the ROMP of bis(norbornene) derivatives.…”