Polyimide-Free Planar Alignment of Nematic Liquid Crystals

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Surfaces, Interfaces, and Applications

Polyimide-free Planar Alignment of Nematic Liquid Crystals: Sequential Interfacial Modifications through Dual-Wavelength In Situ Photoalignment Aboozar Nasrollahi, Vineet Kumar, Myong-Hoon Lee, Shin-Woong Kang, Heung-Shik Park, Ho Lim, Keun Chan Oh, and Jae Jin Lyu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b02601 • Publication Date (Web): 02 Apr 2019 Downloaded from http://pubs.acs.org on April 3, 2019

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ACS Applied Materials & Interfaces

Polyimide-free Planar Alignment of Nematic Liquid Crystals: Sequential Interfacial Modifications through Dual-Wavelength In Situ Photoalignment

Aboozar Nasrollahi,† Vineet Kumar,† Myong-Hoon Lee,‡ Shin-Woong Kang,†,* Heung-Shik Park,§ Ho Lim,§ Keun Chan Oh,§ and Jae Jin Lyu,§ †Department

of BIN Convergence Technology, Chonbuk National University, Jeonju, Korea 54896

‡The

Graduate School of Flexible and Printable Electronics, Chonbuk National University, Jeonju 54896, Korea §Samsung

Display Co., Ltd., Yongin 17084, Korea

KEYWORDS: liquid crystals, photoalignment, photochromic dyes, polymer stabilization, liquid crystal displays

ACS Paragon Plus Environment

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ABSTRACT High quality alignment control of liquid crystal (LC) for ultrahigh definition large-size display is a challenging task. A conventional rubbing method has obvious limitations for fabricating large size displays with a small pixel size and uneven inner surface. To comply with the current trend, we propose a simple and reliable PI-less in situ photoalignment. It was achieved using visible-light-sensitive azo-dye and mesogenic acrylate, both doped to host LCs. Without using pretreated alignment layer, mono- and multi-domain uniaxial alignments of LC molecules were induced by linearly polarized visible light (LPVL) and subsequently stabilized by unpolarized UV-light irradiation. The stepwise process was monitored by adopting fluorescent indicator. By loading the mixture into a confined cell, azo-dyes were spontaneously adsorbed at inner surfaces of the cell while reactive mesogens (RMs) were homogeneously dissolved in a LC host. The molecular orientational anisotropy of dyes at the surface, induced by LPVL, aligned LC director perpendicular to the polarization direction. Upon the second step UV-irradiation, the RMs in a LC host were photopolymerized into thin interfacial layers, stabilizing aligned LC director. The overlaid crosslinked RM-layers secured a thermal and radiative stability of LC alignment. The RM-layers completely screened the effect of azo-dyes, which can be easily randomized by heat and irradiation. The interfacial RM-layer functioned as a permanently stable alignment layer. It provided sufficient azimuthal anchoring strength together with heat and light stabilities, which are essential for practical applications. Such sequential interfacial modifications through dual-wavelength processes can completely avoid interference between forming alignment and stabilization layers, inevitable if the same wavelength light is used. The proposed method provides a simple fabrication process and reliable alignment characteristics by employing effective in situ photoalignment and without using traditional


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alignment layer. Therefore, it meets a current trend in display market toward ultrahigh resolution and large area displays.

1. INTRODUCTION Alignment control of liquid crystal (LC) molecules on a solid surface is a crucial prerequisite for the optical and electro-optical applications of liquid crystals. Conventionally for uniform planar alignment, mechanically rubbed polyimide (PI) films were predominantly employed for a mass production. Although the classical rubbing method yields high quality LC alignment, it possesses unavoidable drawbacks. In addition to the multistep process for coating, soft- and hard-baking prior to rubbing, the production of electrostatic charges and debris requires additional wet-cleaning and drying processes. Due to a very sensitive effect of rubbing, achieving a uniform LC alignment for large area and uneven surface is extremely challenging. The current trends toward ultrahigh definition, small pixel size, and large area display applications make the rubbing method laborious to be employed for commercial productions. Therefore, the photoalignment has been attracting a great attention due to its merits to resolve such problems and demands. Over the last two decades, the development of photo-alignment techniques, as a non-contact method, has been considerably progressed with various approaches. In general, the photoalignment can be classified into three different categories based on the alignment materials and relevant mechanisms, including azobenzene based materials,1-6 photochemical crosslinking of cinnamate or coumarin containing polymers,7-12 and photodegradation of polymeric materials.13-14 Photoinduced molecular orientational anisotropy of alignment materials at the surface promotes uniform planar alignment of LCs. The reorientation of azo-chromophores is facilitated by the reversible photochromic trans- to cis- and cis-to trans-


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isomerizations.15-16 For cinnamoyl and coumarin compounds, however, the surface anisotropy is induced by irreversible [2+2] cycloaddition reaction of vinyl groups, while in some cases it is accompanied by cis/trans-isomerization.17 For both cases, linearly polarized light induces selective response of the chromospheres that leads to the surface anisotropy and consequent LC alignment. In case of the azobenzene containing systems, LC director is aligned perpendicular to the polarization direction. For the cinnamoyl polymers, both parallel8-9 and perpendicular10-11 alignment to the polarization direction have been reported. In the third group, however, the surface anisotropy is induced through selective photodecomposition of polymer chains, caused by high energy deep UV-irradiation. The LC director is aligned perpendicular to the polarization direction and the process is irreversible.18-19 The photo-controlled surface alignment was first reported by Ichimura et al. in the late 1980’s. The self-assembled monolayer of azobenzene derivative on a glass surface initially aligned LCs homeotropic from the surface. The in situ anchoring transition to a random planar state was demonstrated by trans-to-cis isomerization with UV-light and the initial homeotropic state was restored by reversible cis-to-trans isomerization with visible light irradiation.20 Shortly after the first report, Gibbons et al. presented rewritable planar photoalignment by employing dye-doped polymer layers and polarized laser light.21 Various forms of dichroic azobenzene derivatives - doped to polymers, covalently attached to polymer, and solution cast monomeric films – were used for photo-controlled homogeneous and homeotropic alignment.1,

22-30

Photoalignment based on the side-chain cinnamate polymer was first reported by Schadt et al. and followed by numerous reports using cinnamate, coumarin and chalcone containing polymers.9,

12, 31-34

Homogeneous planar alignment of LC molecules was achieved by the

irradiation of linearly polarized UV-light. The surface anisotropy was instigated by the concerted


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[2+2] cycloaddition of vinyl groups, which is selective to the polarization direction of polymerizing light. Although many were successful in a laboratory level, just a few of them could reach to a display market. The crucial deficiencies were instability of LC alignment and insufficient anchoring strength of LC molecules at solid surfaces. In addition, one of the common features of aforementioned approaches is a pretreatment of alignment layers, such as coating, baking, and photo-irradiation. These processes were completed for each substrate prior to cell assembly. Photoalignment, achieved by reversible photochromic process, has an intrinsic drawback in stability against heat and radiation. It has a crucial limitation for permanent alignment, required for practical device applications. For the stability, it was shown that polymer stabilization using reactive mesogen (RM) noticeably improved heat and light stabilities.35-38 As an alternative approach to create a stable alignment layer, the dye-RM mixtures were solution processed on substrates and followed by irradiation of UV-light prior to cell assembly.39-40 Recently, we demonstrated a successful stabilization of uniform planar alignment, induced by photoresponsive polyimides (PIs), through the sequential interfacial modifications with visible and ultraviolet lights.41 Regarding the fabrication process, two major approaches of fabrication procedure were proposed to produce uniform uniaxial orientation of LC molecules on a solid surface. In the first conventional class, top and bottom substrates are pretreated for LC alignment and subsequently assembled to an electro-optical cell, accompanied by LC filling. In the other class, the LC aligning process, specifically can be achieved by photoalignmnet, is performed after assembling top and bottom substrates into an electro-optical cell. If the photoaligning process is performed with the LC-loaded cell, it is often called in situ photoalignment.


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Contrary to the conventional approach, several studies have been reported for LC alignment without using a pretreated alignment layer (so called, PI-less alignment) for both homeotropic and planar alignments.24,

42-45

However, intrinsic obstacles, such as strong aggregation of

nanoparticles and resulted non-uniformity,46 poor stability and weak azimuthal anchoring strength,47 can cause serious problems for practical applications. In the present study, we demonstrate the PI-less in situ photoalignment for stable planar alignment, accomplished by a simple one-bottle process. It was realized using a group of visiblelight-sensitive azobenzene compounds and reactive mesogens, both doped to a LC host. The LC cells were fabricated without using a pretreated alignment layer. The sequential in situ photoalignment and stabilization were carried out using visible and ultraviolet light, respectively. The sequential processes resulted in consecutive interfacial modifications for the photoalignment and subsequent stabilization. The stepwise mechanisms on sequential interfacial adjustments were investigated using a fluorescent model compound. The samples for each step were characterized by utilizing confocal fluorescence microscopy (CFM), polarized fluorescence microscopy (PFM), polarized fluorescence spectroscopy (PFS), polarized optical microscopy (POM), field emission scanning electron microscopy (FE-SEM), and atomic force microscopy (AFM). The alignment stability, anchoring strength, and electro-optical characteristics, required for practical display applications, were examined by using in-plane-switching (IPS) LC cells, prepared by the proposed method. 2. EXPERIMENTAL SECTION 2.1 Materials. A group of visible-light-sensitive azo-benzene compounds (chrysophenine, thiazole yellow G, congo red, acid yellow 9, methyl orange, metanil yellow) from Sigma Aldrich were used as received. Chrysophenine was modified to nonionic compound by acidifying sodium


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sulfonate group. Nematic LCs, MLC 15600-100 and MLC 6686 (both from Merck), with positive dielectric anisotropy were used as host LCs in the experiments. Photo-responsive azodyes and reactive mesogen RM257 were used as additives. Tiazole Yellow G (TYG) was used as fluorescence indicator. For the representative experiments, 0.2 wt.% of acidified chrysophenine (ACP), 0.3 wt.% of TYG and 0.5 wt.% of reactive mesogen RM257 were homogeneously mixed in a LC host by stirring in the isotropic temperature. No photoinitiator was added for the polymerization of RM 257. 2.2 Acidification of Chrysophenine. Chrysophenine sodium salt (0.5 g) was dissolved in chloroform (20 ml). 1 N HCl (5 ml) was added dropwise with continuous stirring at room temperature to the above solution. Acidified solution was further extracted in separating funnel using chloroform and brine solution (water + NaCl) in 1:1 weight ratio for 5 times. Separated organic layer of chloroform was dried over rotary evaporator. Further drying of sample was done over high vacuum, keeping it for a few days to remove any trace of solvent or water. Modification of the chrysophenine was confirmed by NMR. 2.3 Cell Fabrications. The bare ITO-coated glass and cover glass were used as substrates. The cleaning was performed in sonic bath with surfactant for one hour. Subsequently, substrates were rinsed with deionized water, acetone and isopropyl alcohol for three times and then dried in oven at 100 °C for one hour. The cell gap was maintained by the adhesive tapes with 10 µm or 60 µm thickness. The in-plane-switching (IPS) cells were prepared using interdigitated electrodes on one substrate. Monodomain striped pattern consisted of 4 µm wide electrode and 10 µm period. No electrode was formed on the opposing substrate. The cell gap was maintained at 6.4 m. The mixtures were injected into the empty cells by a capillary action at a few degrees


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above the nematic to isotropic transition temperature (TNI) and slowly cooled down to room temperature. 2.4 In situ Photoalignment and stabilization. In situ photoalignment and stabilization were performed by the two-step photoirradiation with separate wavelengths of light (i.e., dualwavelength photoalignment). The pristine random planar cells were irradiated by LPVL in the isotropic phase by using Instec HCS 402 hot stage with STC 200 temperature controller. The LPVL treatment was performed by two different light sources. The metal halide lamp with a bandpass filter for 400~700 nm was used to irradiate the LC cell with 3.0 mw cm-2 intensity for varied time. The LPVL treatment for LC alignment was also performed by Nikon Eclipse LV100 POL polarizing optical microscope (POM), equipped with LV-HL50W 12V 50W LONGLIFE halogen lamp, in the reflection mode whit no objective lens. A schematic diagram of the setup was shown in Figure S-1. The USAF photomask was served for the patterned LC alignment by two-step LPVL treatment. The second-step stabilization process was carried out at room temperature for the entire cell by employing LED-UV Curing System with 365 nm wavelength. No polarizer was used. The irradiation energy was 54 J cm-2 (30.0 mw cm-2 for 30 minutes). 2.5 Confocal fluorescence Microscopy (CFM). Confocal fluorescence images were taken by confocal laser scanning microscope (LSM 510 Meta, Carl Zeiss, Inc., Germany) equipped with VIS laser module. The LC cell consisted of a bare ITO-glass and 100 m-thick cover slip as substrates. The TYG was used not only as an aligning agent but also as a fluorescent marker. The LC mixture was prepared by doping 0.3 wt.% TYG and 0.5 wt.% RM257 and loaded into the cell with ~ 60 m gap. The LC cell was exposed to the LPVL using metal halide UV lamp at 3.0 mw cm-2 intensity for 20 minutes (3.6 J cm-2) in the isotropic temperature. The short wavelength UV was taken away by using a cut-off filter (400 nm) and near IR was removed by IR-mirror.


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Confocal images were taken with 0.98 m z-scan resolution in 750 ｘ750 m2 area before and after LPVL-treatment. The stacked images were visualized by the perpendicular and crosssectional perspectives from the substrates 2.6 Polarized fluorescence Microscopy (PFM) and polarized fluorescence spectroscopy (PFS). The PFM study was performed in a reflective mode using the fluorescence microscope (Nikon Eclipse TE2000-U), equipped with a Nikon LWD 0.52 CCD camera. The PFS properties were measured by the Photoluminescence spectrometer (RMS-1000, Olis, Inc.). To measure anisotropic photoluminescence, a linear polarizer was employed to examine either polarized excitation or polarized emission. The same LC cell, used for photoalignment, was examined before and after LPVL-treatment. In this case, the TYG was used for both LC alignment and fluorescence marking. The fluorescent spectroscopic properties were measured by the spectrofluorophotometer (Shimadzu) in 0.01 mM aqueous solution. 2.7 Characterizations. The UV-vis absorption spectra of the ~ 10-5 M dye solutions in ethanol or water were acquired by the UV-vis spectrophotometer (Jasco, ARSN-733). For UV-vis spectral changes, a metal halide lamp was used for an illumination source. The quality of LC alignment was examined by using the Nikon Eclipse LV 100 POL. POM images were taken by the Nikon DS-Ri1 CCD camera. To investigate the surface morphology, the LC cells were dipped in the hexane for 12 hours to selectively dissolve out the LC host, followed by disassembling the cells. The polymerized RM-layers, formed for stabilization at the solid-LC interfaces, were characterized by using AFM (Multimode-8, Bruker, Germany) and FE-SEM (Hitachi S-4800, Japan). Anchoring stabilities of the cells before and after stabilization were examined by continuously applying AC-field for prolonged time. AC-field (5.0 Vpp, 1 kHz) was produced by using an Agilent 30 MHz Function/Arbitrary Waveform Generator. The electro-


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optical characterizations of V-T curves were performed by LCMS-200 (Sesim Photonic Technology, Korea). 3. RESULTS AND DISCUSSION 3.1 Conceptual approach for PI-less in situ photoalignment. Figure 1 illustrates the schematic process how a uniform planar alignment can be achieved by employing photochromic dichroic dyes and subsequently stabilized by photopolymerization of reactive mesogens (RMs). For the first step, the LC host doped by photochromic azo-dye and RM is loaded into the cell, where the inner surfaces are untreated for LC alignment. A random planar alignment is supposed with uncontrolled manner. However, one can expect the azo-dye molecules to be adsorbed at the solid-LC interfaces while RMs are homogeneously dissolved in LCs, by properly choosing the molecular structure of the additives. In this case, the adsorbed dye molecules show no orientational order at the surface as illustrated in Figure 1a. The in-plane orientation is random on average. When the surface is subjected to the linearly polarized light with proper wavelength, the dye molecules can be reoriented through photochromic trans-cis isomerization with a dichroic property.15 As shown in Figure 1b, the dye molecules are uniaxially aligned in the irradiated area, marked by the red circle. As a result, anisotropic surfaces induce uniform LC alignment. However, the alignment is unstable against heat and radiation due to the thermal diffusion and reversible trans-cis isomerization. Upon circularly polarized irradiation, the alignment is completely randomized (Figure 1b to Figure 1a). If the uniformly aligned state in Figure 1b can be stabilized so that the LC alignment become irreversible, it can be beneficially adopted for industrial applications. This can be achieved by employing RMs as depicted in Figure 1b and 1c. The homogeneously dissolved RMs (blue ellipses) in a LC host (yellow ellipses in Figure 1b) can be localized at the interface by


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photopolymerization. Consequently, a thin polymer layer is formed at the interface and overlays dye-adsorbed surface as shown in Figure 1c. Because cross-linked RM-networks are stable against heat and radiation, and function as a new alignment layer, the LC director retains suitable orientation permanently.

Figure 1. Schematic illustration of the proposed sequential interfacial modifications, achieved by dual-wavelength in situ photoalignment. (a) Random planar alignment after the injection of dyeand RM-doped LC mixture and prior to the LPVL-treatment. Dye molecules are randomly adsorbed at the surface while RM is homogeneously dissolved in the LC host. (b) Photo-induced reorientation of adsorbed dye molecules at a surface and the corresponding LC alignment in the circled area after the LPVL-irradiation. The double-ended arrow represents polarization direction. (c) Stabilized LC alignment achieved by polymerization of RM with UV-light and resulting polymer overlay. The green, red, and blue layers at the inner surfaces represent randomly adsorbed dyes, uniaxially aligned dyes, and RM-stabilization layers, respectively.


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To accomplish such in situ controlled stable alignment, it is important to modify the surface in a proper sequence through surface adsorption of dye molecules, photo-induced reorientation, and RM-stabilization. Each process should avoid interference with others for the best results. The polymer stabilization process should be completed at the last step. In general, however, photochromic isomerization of azo-benzene and photopolymerization of RM are performed by using a similar wavelength of radiation, raging 300 nm to 400 nm UV-light. Therefore, some degree of interference is inevitable. To efficiently separate these processes, it will be optimal to employ lower energy radiation for the photoalignment and followed by the higher energy RMstabilization (i.e., dual wavelength approach).

3.2 Interfacial adsorption and photochemical responsiveness. In addition, appropriate chemical nature is required for each additive. Immediately after injection, dye-molecules are supposed to be spontaneously adsorbed at the surfaces while RMs are dissolved in a LC host (Figure 1a). In this respect, we selected additives by focusing on their compatibility to surface and LC host. We chose RM and azo-dye for high and limited miscibility to LCs, respectively. Figure 2 displays a group of azo-benzene compounds used for the study. It includes acidified chrysophenine (ACP), thiazole yellow G (TYG), congo red (CR), acid yellow 9 (AY9), methyl orange (MO), and metanil yellow (MY). The common features of the dyes are extended conjugation of azo-benzene chromophore and polar substituents in the periphery of the core. Extended -conjugation and electron donating/withdrawing substituents shift absorption to a visible range. The UV-vis spectra are shown in Figure 2 and Figure S-2. Polar groups limit solubility in LCs and enhance interfacial adsorption.


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Figure 2. Chemical structures of additive compounds and photochromic properties of representative azobenzene chromophores. (a) Spectral changes in UV-vis absorption of the 0.01 mM ACP solution in ethanol: black (0 min), red (1 min), blue (90 min), green (after heating at 90 °C), inset (isobestic point) and (b) spectral changes in UV-vis absorption and photoluminescence of the 0.01 mM TYG solution in water: black (0 min), red (5 min), pink (20 min), green (40 min). The blue curve above 500 nm corresponds to the photoluminescence excited at 407 nm. For both solutions, a metal halide lamp was used for illumination source. Based on the prerequisite conditions, we carried out sequential interfacial modifications with a dual-wavelength approach. All compounds shown in Figure 2 exhibited essentially the same


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effects although the quality of LC alignment was different to some degree. In this report, therefore, we present detailed experimental results for the representative model compounds (ACP and TYG). The ACP was obtained by acidifying chrysophenine for enhancing miscibility to LCs and reducing ionic groups. The solubility was increased in the limited range (< ~ 1 wt.%). Photochromic properties of the ACP in ethanol solution (0.01 mM) were characterized by UVvis spectroscopy. Its absorption changes upon visible light ( > 400 nm) irradiation are shown in Figure 2a. The pristine solution showed a strong peak at 402 nm which was attributed to the π-π* transition of the trans-isomer. Upon visible light irradiation, the intensity of the π-π* band rapidly decreased and then a little change in the peak intensity was observed for extended exposure. Simultaneously, a slight increase was taken place near 520 nm, corresponding to n-π* transition of the cis-isomer. By heating the sample, the pristine spectrum was recovered and superimposed to the original one (green and black in Figure 2a). It was clear that a relaxation from cis- to trans-isomer occurs both by heat and longer wavelength radiation. The existence of an isobestic point at 485 nm, marked by the arrow and enlarged inset, indicates that the process is reversible and both isomers are in equilibrium photostationary state under illumination. The photoluminescent Thiazole Yellow G (TYG) was used to prove our hypothesis on the sequential interfacial modifications. Its absorption and luminescence spectra, measured for 0.01 mM aqueous solution, are shown in Figure 2b. The fresh solution showed an intense peak and weak peak at 407 nm and 325 nm. Photochromic property was influenced by visible light illumination ( > 400 nm). After visible light illuminations, the absorption intensity exhibited a considerable decrease near 407 nm and increase near 325 nm as shown in Figure 2b. In addition, a slight increase was observed near 540 nm with the isobestic point at 500 nm. The existence of two isobestic points at 500 nm and 360 nm (marked by the arrow) indicates that more than two


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species are in equilibrium. Due to its dichroic absorption and photoluminescence, TYG benefited a stepwise mechanism study for proving our hypothesis.

Figure 3. Polarized optical characteristics of the planar aligned LC cells, fabricated by in situ LPVL treatments. (a-i)/(a-ii)/(a-iii) correspond to the macroscopic images of the cell under crossed polarizers before and after LPVL treatment; (b) angular dependence of transmitted light intensity of the exposed area; (c-i)/(c-ii)/(c-iii) show the POM images, corresponding to the unexposed and exposed areas; (d-i)/(e-i) macroscopic and (d-ii)/ (e-ii) microscopic images with different orientation under crossed polarizers. The numbers in (d-ii) correspond to a pitch of the resolution pattern. The crossed arrows denote crossed polarizers. The white and blue arrows represent the polarization directions of the first and second LPVL, respectively.


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3.3 Interfacial modification by in situ photoalignment. To demonstrate the proposed approach, the ACP- and RM-doped LC mixture was sandwiched by bare ITO-glass with no alignment layer and followed by LPVL-treatment in the isotropic temperature. Figure 3a shows polarized optical images of the cell. The image in 3(a-i) exhibits typical random planar state as expected. After LPVL illumination with 3.6 J cm-2 (3.0 mw cm-2 for 20 min), however, LC director was uniformly aligned in the exposed area (marked by the circle) as presented in 3(a-ii) and 3(a-iii). It was evident that the LPVL induced the anchoring transition from a random to uniform planar state at the surfaces. Under crossed polarizers, dark and light states sinusoidally varied by rotating a sample in the plane of substrates, indicating uniaxial planar alignment of the optic axis. As observed in Figure 3b, the contrast of dark and light states was increased with irradiation energy. The poor contrast of a random alignment (black curve) was abruptly improved with the increase of irradiation energy (0, 0.9, 1.8, 2.7, 3.6 J cm-2, respectively). Although the irradiation energy was rather large, the efficiency could be improved by properly optimizing the conditions such as chromophores, wavelength of light, and temperature. Figure 3c presents the POM images, corresponding to unexposed (3c-i) and exposed area (3c-ii, 3c-iii) with different orientations. In the exposed area, the LC director was uniformly aligned perpendicular to the polarization direction (denoted by a double-ended arrow), the same as previous reports.21, 41, 48 Photopatterning of surface anchoring was also accomplished by two-step LPVL exposure. As the first step, the circled area was uniaxially aligned by in situ photoalignment as in Figure 3a. The second step LPVL-treatment was performed by applying USAF photomask to the uniformly aligned area with the same irradiation energy in the isotropic temperature. In this case, polarization direction of the second LPVL was aligned by 45 ° with respect to the first one.


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During the second treatment, LC director was realigned perpendicular to the polarization direction in the exposed area while the unexposed area retained its orientation. As a result, the patterned photoalignment was observed in the circled area as shown in Figure 3d and 3e. Macroscopic and microscopic images show patterned LC-alignment with different orientations under crossed polarizers. It was evident that the pattern consisted of two different domains with 45° difference in their director orientation. The patterning of LC alignment was attributed by the reversibility of trans-cis isomerization and subsequent reorientation of azo-dye molecules at the surfaces.

Figure 4. Interfacial adsorption and polarized photoluminescence of the TYG in LC cells. Crosssectional and normal views of confocal fluorescence microscope images (a-i)/(a-ii) before and (b-i)/(b-ii) after LPVL-treatment. (c-i) unpolarized and (c-ii)/(c-iii) polarized fluorescence microscope images. (d) Polarized fluorescence spectra for the excitation polarization (red, P//) parallel and (blue, P┴) perpendicular to LPVL direction. The white and yellow arrows denote polarizer and LPVL directions, respectively. The ellipses represent LC molecules.


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To elucidate the sequential interfacial modifications and the origin of LC alignment, we monitored stepwise processes by adopting photoluminescent TYG as a photoaligning and photoprobing guest compound. The confocal fluorescence images were taken immediately after LC filling and shown in Figure 4(a-i) and 4(a-ii). The cross-sectional and normal views clearly demonstrate a localization of dye molecules at the solid-LC interfaces. Due to the polar nature and limited miscibility to LCs, most of the dye molecules were spontaneously adsorbed at the surfaces. No noticeable fluorescence was observed in a bulk of the LC host. After LPVL treatment, the spatial distribution of TYG was not changed much as shown in Figure 4(b-i) and 4(b-ii). However, significant anisotropy in fluorescence was induced by LPVL illumination. Figure 4c shows polarized fluorescence of the LPVL-treated cell. Figure 4(c-i) reveals green fluorescence with no use of polarizer. Figure 4(c-ii) and 4(c-iii) exhibit polarized emission, measured by placing a polarizer between the excitation filter and sample cell. As noticed in Figure 4(c-ii) and 4(c-iii), the minimum and maximum fluorescence were observed when the polarizer (white arrow) was aligned parallel and perpendicular to the LPVL (yellow arrow), respectively. Polarized photoluminescence was further confirmed by the polarized fluorescence spectroscopy (PFS) as shown in Figure 4d. When the polarized excitations were arranged parallel and perpendicular to the LPVL, the corresponding emission spectra, P// and P┴ respectively, showed significant difference in luminescence. Although it was not accurately measured, the observed anisotropy indicated rather limited order parameter of the uniaxially aligned chromophores. Prior to the LPVL-illumination, however, the adsorbed dyes at the substrate-LC interface are expected to be random in orientation. In fact, the PFM and PFS results showed no meaningful anisotropy (see Figure S-3). The polarized photoluminescence indicates the anisotropy in


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molecular orientation of chromophores. The induced anisotropy could be explained by the in situ interfacial modification of molecular order by the remote LPVL-treatment. Dichroic fluorescent azo-dyes were reoriented perpendicular to the polarization direction of LPVL presumably through reversible tran-cis and cis-trans isomerizations.

Therefore, uniaxially aligned

fluorophores emit linearly polarized light. According to the molecular orientational anisotropy at the surface, LC molecules anchor in uniaxial planar state at the surface, as illustrated by the ellipses in Figure 4c. It was corroborated by the polarized emission and optical axis of LCs, confirmed by PFM, PFS and POM.

Figure 5. Degradation of LC alignment against heat and unpolarized light treatments. (a-i)/(a-ii) macroscopic and (b-i)/(b-ii)/(b-iii) microscopic polarized photographs with different orientations. POM images showing appearance of defects after (b-i) 15 hours of heat at 110 °C and (b-ii)/(biii) additional radiation treatments at 110 °C. The single arrow denotes the polarization direction of LPVL and crossed ones correspond to the polarizer and analyzer, used for inspection. The scale bar corresponds to 10 m.


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3.4 Stabilities against heat and light irradiation. The LPVL-induced surface anisotropy and resulting LC alignment were reversible as reported earlier.16, 18 The polarized luminescence and corresponding LC alignment were completely randomized by irradiating circularly polarized light and rewritable by using LPVL with different polarization directions. Even in a dark state, the induced alignment was unstable against a heat treatment. Figure 5 shows degraded LC alignment after heat and unpolarized light treatments, observed from the uniformly aligned cell, shown in Figure S-4. The emergence of defects was evidenced in the macroscopic (Figure 5(ai)/(a-ii)) and microscopic polarized optical images with different orientations. Figures 5(b-i) and Figure 5(b-ii)/(b-iii) correspond to the POM images after 15 hours at 110 ° C and additional unpolarized radiation at 10.0 mw cm-2 for 30 minutes (18 J cm-2), respectively. 3.5 Interfacial stabilization of LC alignment. The rewritable LC alignment, achieved by reversible trans-cis isomerization of azobenzene compounds, attracted a great attention for a special applications.18,

21

In general, however, most of the LC devices in markets require

permanent LC alignment with a specific configuration. In this respect, the reversibility is an intrinsic drawback of the alignment process utilizing photo-responsive dyes. The LC alignment is not stable enough against heat and radiation for commercial applications. Therefore, we propose an additional process to permanently stabilize a specific alignment, induced by the in situ photoalignment. As illustrated in Figure 1b and 1c, small amount of RMs can be polymerized in a uniformly aligned LC host, resulting in the overlaid stabilization layers on top of the photo-induced alignment surfaces. To form a robust stabilization layer, RM257 (0.5 wt.%) was polymerized in the uniformly aligned monodomain LCs, formed by the first step in situ LPVL-treatment (see Figure S-5), using unpolarized 365 nm LED-UV (30.0 mw cm-2) at room temperature for 30 minutes. After the stabilization, the thermal and light stabilities were


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inspected under 24 hours of heat at 110 °C (see Figure S-6) and supplementary UV-vis light treatments (10.0 mw cm-2) at 110 °C for 90 minutes (Figure 6). The macroscopic (Figure 6a) and microscopic (Figure 6b) polarized optical images present uniform LC alignment in a circled area. The LC host retained its uniaxial alignment, induced by the first step LPVL. No indication of degraded LC alignment was observed. In addition, Figure 6c and 6d further confirmed the effect of RM-polymerization for the dual-domain patterned alignment. After the consecutive LPVL treatments as in Figure 3d, the unpolarized UV-light irradiation at room temperature resulted in the formation of RM-stabilization layer and thus secured the heat and light stabilities of the patterned alignment. As shown in Figure 6(c-i)/(c-ii) and Figure 6(d-i)/(d-ii), both macroscopic and microscopic photographs exhibited a good patterned alignment after the same stability test.


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Figure 6. Heat and light stabilities of the monodomain and dual-domain-patterned LC alignments after RM-stabilization. Polarized optical images with different orientations: (a-i)/(aii) macroscopic monodomain, (b-i)/(b-ii) microscopic monodomain, (c-i)/(c-ii) macroscopic dual-domain-pattern, and (d-i)/(d-ii) microscopic dual-domain-pattern. The ellipses represent aligned LC molecules. The white and blue arrows denote the polarization direction of 1st and 2nd LPVL, respectively. The crossed arrows correspond to the polarizer and analyzer, used for inspection. The scale bar in (b-i)/(b-ii) corresponds to 20 m. The second interfacial modification, executed by the RM-polymerization, was investigated by employing field emission scanning electron microscopy (FE-SEM) and atomic force microscopy (AFM). Figure 7a shows a bare ITO-surface with various grains. The azo-dye adsorbed surface also showed essentially the same morphology. No noticeable distinction was observed. After RM-polymerization and removal of LCs, the FE-SEM image in Figure 7b exhibited featureless monotonic surface from the normal view. However, it was very distinct from the bare ITOsurface, indicating the formation of continuous polymer layer at the surface. It was more prominent from highly tilted view as in Figure 7c. It was manifested that a thin polymer layer with a nanoscopic bumpy surface was formed at the ITO-LC interface. The thickness was approximately 70 ~ 100 nm. No in-plane anisotropy was notable depending on the direction of LC alignment. The AFM image also corroborated the formation of interfacial layers. Figure 7d and 7e present the AFM images for the same scan viewed from different perspective, rotated by 90°. Both AFM images consisted of numerous protrusions. In this case, a slight in-plane anisotropy was noticeable for multiple samples. By considering 90° in-plane rotation, the protrusions in Figure 7d were wider than those in Figure 7e, indicating a slight elongation of protrusions in the


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Figure 7. Surface inspections for the second interfacial modification, achieved by photopolymerization of RMs. FE-SEM images taken from the top view of the (a) bare ITOsurface and (b) polymerized RM-layer, and taken from (c) the tilted view of polymerized RMlayer. (d)/(e) AFM images of the RM-modified interface with different perspectives, rotated by


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90°. The arrows indicate the direction of LC director. All data were taken from the top (near UVside) substrate. plane of a substrate. The average roughness values along and transverse direction to the LC director were 9.8 nm and 11.5 nm, respectively. The corresponding 2D-AFM image was shown in Figure S-7. As marked by the arrow, the LC director aligned along more elongated direction of the protrusions on average. It means that LC molecules align along less roughened direction in the plane of the surface. Contrariwise, it is reasonable to say that such in-plane anisotropy in the RM-layer was induced by the LC director during photopolymerization in aligned LCs. Essentially the same results were observed for the opposing bottom substrate. The corresponding FE-SEM and 3D-AFM images were shown in Figure S-8 in the supporting information. The overlaying polymer was formed from diacrylic RMs and hence it is cross-linked system, which is stable against chemical and heat treatments. It is also stable for UV and visible light because the polymerized RM has no photoresponsive reconfigurable group. Therefore, the stable overlaying interfacial layers on both substrates accomplished a permanent LC alignment as discussed above in Figure 1 and Figure 6. In this respect, a complete coverage of the surface is important for the stability of uniform LC alignment. This can be optimized by tuning polymerization conditions such as monomer concentration, radiation intensity and energy, etc. 3.6 Electro-optical characteristics. To scrutinize a practical application of the proposed technique, we prepared in-plane-switching (IPS) LC cells without using pretreated alignment layers. Figure 8a and 8b show polarized optical images after the sequential interfacial modifications through in situ dual-wavelength photoalignment. The LC director was uniformly aligned at 15 ° off from the stripe-patterned electrodes throughout the active area. The same


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quality of alignment was maintained after the stability test with unpolarized UV-vis light irradiation (10.0 mw cm-2 for 90 minutes) at 110 °C. Upon applied voltage, the LC director was reoriented parallel to the in-plane electric field and the cell turned bright as shown in Figure 8c. The corresponding voltage-transmission (V-T) curve was presented by the blue line in Figure 8d. The inset in Figure 8d correspond to the microscopic dark and light states observed during switching.

Figure 8. Electro-optical switching and anchoring stability of the IPS LC-cell, fabricated by the proposed dual-wavelength in situ photoalignment. (a-i)/(b-i) Macroscopic and (a-ii)/(b-ii) microscopic polarized optical photographs after fabrication; (c) Light turn-on state of the cell with 5.0 Vpp AC field; (d) V-T characteristics before (blue) and after (red) the AC-stress test; (e) unrelaxed state of the cell with a weak anchoring strength. The inset shows POM images of dark and light switching states. The single arrow indicates the polarization direction of LPVL and crossed ones correspond to the polarizer and analyzer, used for inspection. The pitch of IPSelectrode corresponds to 10 m. Since a weak anchoring strength is a critical problem in the conventional photoalignment,40 the anchoring strength of proposed cells was examined by the AC-stress test, employed for


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industrial applications. The relaxation to a pristine anchoring state at zero-field was investigated after continuously applying 60 Hz AC-voltage with 8.0 Vpp amplitude for 24 hours. If anchoring strength is not strong enough, the relaxation is very slow or not occurring for a long period of time. For the aligned LC cell, obtained by the LPVL, with no RM-stabilization, the AC-stress test showed a weak anchoring of LCs at the surface. As shown in Figure 8e, the cell showed bright striped lines unrelaxed back to the dark state, indicating an insufficient anchoring strength in those areas. However, a fast and complete relaxation was accomplished in the entire active area for the RM-stabilized cells as shown in the inset of Figure 8d. In addition, no hysteresis on V-T responses was observed after the completion of RM-stabilization. The V-T curves were completely overlapped before and after the AC-stress test as presented in Figure 8d (blue and red curve, respectively). Based on the Ac-stress test, it was confirmed that the proposed technique can provide sufficiently strong anchoring of LC at the surface of RM-stabilization layers. In general, the approach adopting ionic additives has intrinsic drawbacks regarding voltage holding ratio (VHR), residual DC (RDC), and image sticking characteristics. Therefore, it is important to avoid ionic impurities in a LC mixture. In this respect, the nonionic ACP may have advantages over other ionic azobenzene chromophores although it wasn’t confirmed by experiments yet. Like we treated the ACP, additional process, producing nonionic additives, could be an essential for their practical device applications. The complete understanding on such impurity-sensitive parameters requires further detailed investigations. The other potential disadvantage is a colored nature of azobenzene chromophores, which may hamper a spectral performance. In this case, the previous report, proposing a photo-bleaching of a colored component by UV-light irradiation, could be a good option.35


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4. CONCLUSION We successfully demonstrated the “PI-less in situ photoalignment” of nematic liquid crystals with the one-bottle approach. With a homogeneous one-bottle mixture, instead of using pretreated alignment layer, the in situ planar photoalignment of LCs and its stabilization were accomplished by sequential interfacial modifications with the dual-wavelength method. We offered a group of visible-light-sensitive azo-dyes by suggesting the structural requirements, effective for interfacial adsorption and sequential photoalignment. The dual-wavelength approach was very beneficial to interpose interfacial LC-aligning and stabilizing layers in a proper sequence. The stepwise process was unambiguously elucidated by investigating the model compounds (ACP and TYG). The uniaxial orientational anisotropy of adsorbed azo-dyes at surfaces was efficiently induced by the first step LPVL-irradiation. The resulting LC alignment was reversible and unstable again heat and radiation treatments. The invariable stabilization of LC alignment was achieved by photopolymerizing RMs with the second step UV-light illumination. Such consecutive interfacial modifications produced an overlaid polymerized RMlayer, serving as a permanently stable alignment layer. In addition to stability, the optical and electro-optical properties, examined using IPS cells, showed reliable performances with a sufficient anchoring strength for practical device applications. In conclusion, the proposed “PIless in situ photoalignment” has a great potential for industrial applications. It is cost-effective due to a simple fabrication procedure. As a noncontact technique, it complies well with the current requirements for fabricating large area, ultrahigh definition, and small pixel-size LC displays with uneven inner surfaces.

ASSOCIATED CONTENT


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Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: xx.xxxx/xxxxxxx. Schematic representation of the setup, Chemical strucures and UV-vis absorption spectra of the in situ photoaligning guest compounds, Fluorescent microscopic images, Polarized photographs of the LC cells, 2D AFM images, FE-SEM and AFM images for the opposing substrate (PDF) Corresponding Author *E-mail: [email protected] Department of BIN Convergence Technology, Chonbuk National University, Jeonju 54896, Korea Author Contributions S.-W.K., M.-H.L. planed and supervised the project. A.N. and V.K. performed the experiments. H.-S.P., H.L., K.C.O., and J.J.L. advised the preparations of IPS cells, electro-optical characterizations and data analysis. All authors discussed the data and participated in writing the manuscript. Funding Sources This research was supported by the Samsung Display Company in Korea and the “BK21 Plus Project” through the National Research Foundation of Korea funded by the Korea Government (MSIT). Notes The authors declare no competing financial interest.


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ACKNOWLEDGMENT This research was supported by the Samsung Display Company in Korea and the “BK21 Plus Project” through the National Research Foundation of Korea funded by the Korea Government (MSIT).

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Polyimide-Free Planar Alignment of Nematic Liquid Crystals

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