Linearly Polarized Ultraviolet Research Articles

Synthesis and Study of Copolymers Bearing Two Coumarin Side Groups in Monomer Unit: Toward Photoalignment of Liquid Crystals

AbstractPhotodimerization of coumarin derivatives in photoactive polymers under linearly polarized UV (LPUV) light irradiation is a photochemical reaction often used to prepare the surface alignment layer that can orient liquid crystals (LCs) due to the anisotropic interfacial interactions. Herein, the synthesis of a series of novel coumarin‐containing copolymers and their use for the photoalignment surface layer of LCs is reported. These copolymers have the particular feature of bearing two coumarin side groups in each monomer unit, which is designed to have a high content of the chromophores. The alignment of LCs induced by the photoalignment layers of the coumarin‐containing copolymers is investigated. It is found that the photoalignment layers can effectively induce orientation of LC molecules along the direction of the UV light polarization used to prepare the surface layers, with an order parameter of around 0.5 achieved by adjusting the preparation conditions. Moreover, the electro‐optical behaviors of LC cells made with the photoalignment layers are investigated, confirming the alignment of LCs without the use of rubbed surfaces. Optically inscribing the photoalignment layer for nonuniform and organized LC alignment is also demonstrated.

Photoalignment in Polysiloxane Liquid‐Crystalline Elastomers with Rearrangeable Networks

AbstractMolecular alignment in liquid‐crystalline elastomers with polysiloxane backbone is controlled with the aid of photoalignment and exchangeable links to enhance performance as photoactuators. The photoalignment is triggered by trans‐cis‐trans isomerization of azobenzene moieties, which are introduced at crosslinking points, upon irradiation with linearly polarized UV (LPUV) light under heating. The induced alignment is memorized by rearrangement of network structures through exchange reactions of covalent bonds. When irradiated with LPUV light, freestanding films show bending along with the alignment change in surface regions. Macroscopic shapes are also rememorized by exchange reactions at crosslinks. Combination of photoalignment and network rearrangement allows continuous deformation of freestanding films upon exposure to LPUV light with varying polarization directions. This approach enhances the degree of freedom in alignment control, photoactuation, and reshaping of polysiloxane‐based liquid‐crystalline elastomers, which are feasible as soft actuators.

Alignment of liquid crystals generated by [2 + 2] cycloaddition in alignment layers

In this manuscript, photo-sensitive polysulfone thin layers, CMPSF200-HCx thin layers, are utilized to vertically align liquid crystals (LC) accompanied with linearly-polarized UV light (LPUV) exposure. CMPSF200-HCx thin layers are highly transparent and the optical performances of CMPSF200-HCx thin layers are little alternated after LPUV exposing. Vertical alignment of LC between CMPSF200-HC70 thin layers is due to the surface energy alternation generated from [2 + 2] cycloaddition between >C=C< unites, which occurs under LPUV exposing. LC vertically aligned between CMPSF200-HC70 thin layers could be electrically driven to switch at 3.45V, and the total response of LC has been evaluated as 27.941ms. Highly transparent and photosensitive CMPSF200-HC70 thin layers are a competitive candidate in photo-aligning LC.

Polarization axis-selective realignment of a photoreactive liquid crystalline composite with homogeneous alignment

We investigated the photoalignment properties of a liquid crystalline composite. The composite consisted of a low molecular weight nematic liquid crystal (LC) and a photoreactive liquid crystalline polymer containing 4-(4-methoxycinnamoyloxy)biphenyl side groups. Homogeneously aligned LC composite cells were fabricated using rubbed substrates and were then exposed to a linearly polarized (LP) ultraviolet (UV) laser beam. The alignment states of the LC composites were characterized using a LP visible laser beam. The LC composite in the cell was realigned by LP UV exposure and subsequent annealing. The direction of the realignment could be controlled by the polarization direction of the LP UV beam. The relationship between the realignment direction and the rubbing strength of the substrates was also investigated. A large realignment was induced in the LC composite cell when the azimuthal anchoring strength of the rubbed substrates was relatively weak.

Multi-Dimensional Switching of Ring Currents in Nonplanar Chiral Aromatic Molecules by Ultrashort Linearly Polarized UV Laser Pulses

Nonplanar chiral aromatic ring molecules are candidates for use as ultrafast multi-dimensional optical switching devices. Nonplanar (P)-2, 2’-biphenol exhibits four patterns of π-electron rotations along the two phenol rings. It can induce two-dimensional coherent ring currents. Each rotational pattern can be driven by a coherent excitation of two quasi-degenerate electronic states using linearly polarized ultrashort UV pulses. We present the quantum dynamical simulation results of a four-step switching of the two-dimensional ring currents within 300 fs.

Influence of End Groups in Photoinduced Reorientation of Liquid Crystalline Polymer Films Based on Axis-Selective Photo-Fries Rearrangement

Photoreactive liquid crystalline polymethacrylates with phenylbenzoate side groups with different molecular terminal were synthesized and their photoresponsive behavior was investigated. All polymer films underwent axis-selective photo-Fries rearrangement and generated photoinduced optical anisotropy upon exposure to linearly polarized ultraviolet (LPUV) light. In addition, annealing the photoreacted films enhanced molecular reorientation perpendicular to polarization direction of LPUV light, and the molecular terminal groups strongly affected the reorientation behavior.

Reorientation of photoreactive liquid crystalline polymer pattern fabricated by hybrid nanoimprinting

The molecular orientation of photoreactive liquid crystalline polymers can be controlled by linearly polarized ultraviolet (LPUV) irradiation and thermal treatment. Polymethacrylate with a hexamethylene spacer group terminated with 4-oxycinnamic acid on its side chain (P6CAM) is a photoreactive liquid crystalline polymer, and is reoriented parallel to the imprinted line upon thermal nanoimprinting. Thus, the molecular orientation of P6CAM could be controlled by LPUV irradiation or thermal nanoimprinting. Three dimensional molecular orientation may be induced in the P6CAM pattern by combining LPUV irradiation and thermal nanoimprinting (denoted hybrid nanoimprinting). Hybrid nanoimprinting of the photoreactive liquid crystalline polymer was carried out.

Photo-assisted molecular engineering in solution-processed organic thin-film transistors with a blended semiconductor for high mobility anisotropy

This paper reports a viable method for enhancing mobility anisotropy in solution-processed organic thin-film transistors (TFTs) by irradiating linearly polarized ultraviolet (LPUV) light onto a 6,13-bis(triisopropylsilylethynyl)-pentacene (TIPS-pentacene)/poly (vinyl cinnamate) (PVCi) blended semiconductor film. The needle-like crystalline domains in the blended semiconductor film were aligned along the polarization direction of the LPUV light, suggesting that TIPS-pentacene molecules are oriented anisotropically due to an intermolecular interaction and aggregation with the photo-aligned PVCi molecules. The mobility anisotropy reached up to about 50, which is the highest value achieved in solution-processed organic TFTs with a polymeric insulator.

Effect of Multi-Dimensional Ultraviolet Light Exposure on the Growth of Pentacene Film and Application to Organic Field-Effect Transistors

We report on the multi-dimensional alignment of pentacene molecules on a poly(methyl methacrylate)-based photosensitive polymer (PMMA-polymer) and its effect on the electrical performance of the pentacene-based field-effect transistor (FET). Pentacene molecules are shown to be preferentially aligned on the linearly polarized ultraviolet (LPUV)-exposed PMMA-polymer layer, which is contrast to an isotropic alignment on the bare PMMA-polymer layer. Multi-dimensional alignment of pentacene molecules in the film could be achieved by adjusting the direction of LPUV exposed to the PMMA-polymer. The control of pentacene molecular alignment is found to be promising for the field-effect mobility enhancement in the pentacene FET.

광분해성 고분자를 이용한 액정배향에서의 광조사 및 열처리 효과

광분해형 CBDA-ODA 폴리이미드의 배향막에 대하여 선편광(LPUV) 조사와 이후의 열처리가 배향막, 액정 배향 그리고 LCD 특성에 어떠한 영향을 미치는지에 대하여 조사하였다. 가장 우수한 액정 배향은 배향막의 이방성을 최대로 발생시키는 광조사 에너지보다 상당히(약 5~10배) 작은 양의 광조사에 의하여 유도되었다. 광조사에 이후에 이어지는 배향막에 대한 열처리는 광분해 반응에 의하여 배향막 표면에 형성된 저분자 부산물들을 효과적으로 제거함과 동시에 광조사된 배향막을 안정화시킴으로써 액정의 배향성과 LCD의 전기적 특성을 향상시키는 결과를 가져왔다. The effects of the linearly polarized UV (LPUV) irradiation and thermal treatment of a photo-degradable polyimide (CBDA-ODA) alignment layer (AL) on its AL properties, liquid crystal (LC) alignment, and LCD characteristics have been investigated. The best quality of LC photo-alignment have been induced by the LPUV-irradiation with much (about 5~10 times) less dosage than that generating the maximum anisotropy of the AL. A thermal treatment of the LPUV-irradiated AL has effectively removed the undesirable, low-M.W. fragments of the AL generated during the photo-decomposition and increased the stability of the AL, which has resulted in improvement of the LC alignment and the LCD property.

Photo-controlled molecular growth and electrical performances of a pentacene-based organic transistor with a photo-reactive insulator

We investigate the initial molecular growth of the pentacene and the corresponding transistor performance by varying the exposure angle of linearly polarized ultraviolet (LPUV) light on a poly(methylmethacrylate) (PMMA)-based photo-reactive insulator. Based on the optically verified alignment features of pentacene molecules on the LPUV illuminated PMMA-based insulator, it is found that the initial size and the anisotropy of pentacene grains are gradually increased and finally saturated with increasing the exposure angle of the LPUV light. In such situation, the resultant electrical performance of the pentacene-based organic transistor are found to be monotonically enhanced up to a certain maximum at an optimal exposure angle of 30° and finally saturated.

Photoinduced molecular reorientation in photoreactive liquid crystalline copolymers in a phase retarder application

Polymethacrylate copolymers, which have hexamethylene spacer groups terminated with 4-methoxyphenyl-4′-oxycinnamate (MPC) and 4-oxybenzoic acid (BA) in the side chain, were synthesized. Thin films underwent thermally enhanced photoinduced cooperative molecular reorientation using linearly polarized ultraviolet (LPUV) light and subsequent annealing. Moreover, low exposure energy (4–80 mJ cm −2) and annealing temperature (<140 °C) provided sufficient cooperative molecular reorientation of both side groups with a large reorientational order ( S > 0.5). Tuning the copolymer composition adjusted the birefringence (∆ n) of homogeneously reoriented films between 0.10 and 0.22 at 517 nm. Finally, a phase retarder (∆ nd = 130 nm) using an oriented copolymer on a triacetylcellulose (TAC) film substrate, which exhibited a thermal stability up to 140 °C, was fabricated.

Evaluation of imprinting characteristics of photoinduced liquid crystalline polymer

The orientation of molecules in photoinduced liquid crystalline polymer film was studied by means of a thermal nanoimprinting technique using a SiO2/Si mold with a line-and-space (L&S) pattern. If nanoimprinting reorients molecules in photoinduced liquid crystalline polymer, unique characteristics may result from carrying out nanoimprinting and linearly polarized UV (LPUV) irradiation at the same time. We found that the imprinted photoinduced polymer was oriented parallel to the line pattern on the mold. Furthermore, we found that photoinduced liquid crystalline polymer films that were pre-aligned by LPUV irradiation and a heat treatment could be reoriented by thermal nanoimprinting.

Substitute Effect of Fluorine-containing Polyimides with Hexafluoroisopropylidene Group on Photo Alignment of Liquid Crystal Molecule

The substituent effect of fluorine-containing polyimides with 4,4-(hexafluoro-isopropylidene) diphthalic anhydride (6FDA) group on the photo alignment of the liquid crystal (LC) molecule and the effect of 254 nm linearly polarized ultraviolet (LPUV) irradiation on the chemical structure were systematically investigated. 6FDA-2,3,5,6-tetramethyl-1,4-phenylene diamine (TeMPD) showed the photo alignment characteristic of the LC molecule. Based on the infrared and ultraviolet absorption spectra, the photo-scission reaction of LPUV irradiation was not observed, though photoreaction between polymer segments was observed. The photo alignment characteristic of LPUV-irradiated 6FDA-TeMPD strongly depended on the photoreaction between the C=O group of the imide ring and the CH3 group for phenyl-group π-π* transitions. The chemical structure changed with the excitation of C=O double bonds and C-H bonds and the formation of hydroperoxide O-H between the intermolecular polymer segment through the LPUV irradiation mechanism. The electron-donating benzene ring and the electron-accepting imide ring then formed charge transfer complexes (CTCs) between the two sides of polymer chains; the combination with the alternation sequence to both cross sides was clearly formed. In one direction of the photoreaction in the CTC structure, 6FDA-TeMPD showed the photo alignment of the LC molecule.

Thermal Nanoimprinting on Prealigned Photoreactive Polymer Liquid Crystal for Molecular Reorientation

Molecular orientation in a photo-crosslinkable liquid crystalline polymer (PLCP) film was explored by a thermal nanoinprinting technique using a SiO2/Si mold with a line-and-space (L&S) pattern. A molecular oriented structure was confirmed by polarization optical microscopy and diffraction efficiency measurements, at which the reoriented direction was parallel to the mold lines. We demonstrated thermal nanoimprinting on prealigned PLCP films by linearly polarized UV (LPUV) and evaluated the optical characteristics of imprinted PLCP films. As a result, the prealigned PLCP films were reoriented by thermal nanoimprinting.

Polarized Light Emission in a Liquid Crystalline Copolymer Film Comprised of Fluorenevinylene and Photo-Cross-Linkable Mesogenic Side Groups

A polymethacrylate copolymer comprised of light-emissive cyano-substituted dihexylfluorenevinylene (CN-DHFV) side groups and photo-cross-linkable 4-(4-methoxycinnamoyloxy)biphenyl (MCB) mesogenic side groups was synthesized and characterized. Irradiating a thin copolymer film with linearly polarized ultraviolet (LPUV) light and subsequent annealing generated a cooperative molecular reorientation of both side groups parallel to the polarization (E) of LPUV light. The resulting reoriented film showed a polarized light emission of the CN-DHFV groups with a polarization ratio of 5.6.

Effect of molecular weight of biscinnamated poly(ethylene glycol) oligomers on the photocycloaddition reaction

Poly(ethylene glycol) (PEG) is of great importance in the biomedical applications, owing to their hydrophilicity, nontoxicity, and absence of antigenicity and immunogenicity. Recently, much interest is focused on the utilization of light as a source of stimulus for the applications for biomedical field. In line with this photoreactive PEG has been utilized for photografting onto surface and fabrication of 3D structured hydrogels. In order to allow photoreaction for PEG, mostly conjugation of the photoreactive moiety at the hydroxyl groups in PEG is used. Cinnamoyl group is well known photoreactive group which undergoes [2+2] photocycloaddition reaction when UV is irradiated and photoalignment layer could be obtained by the irradiation of linearly polarized UV (LPUV) onto the coated films of polymer with cinnamoyl groups. Interestingly, it is reported that cinnamoyl group is spontaneously metabolized in the body and thus has been proven to be nontoxic. Additionally, nematic liquid crystal (LC) materials are emerging as promising candidates for a wide range of applications from optoelectronic devices to biosensing. Although there is lack of report on the biscinnamated PEG as a 2D in-plane orientation layer, it can be expected that biscinnamated PEG could provide biocompatible photoaligned substrate for biomedical applications. From the early studies from our group, molecular weight of PEG whether it is attached as a side chain or itself affect on the performance of application field such as ionic conduction behavior, interaction with DNA, and gelatinization of starch. Considering photoalignment layer, we also reported beneficial phenomenon of the utilization of oligomeric cinnamate rather than polymeric cinnamates in some aspects. In this note, we prepared photoalignable oligomers based on the different molecular weight of PEG oligomers and investigated the photoreactivity and the LC aligning property of the prepared oligomers triggered by the linealy polarized UV irradiation.

Photo-Sensitive Polyimide Containing Methoxy Cinnamate Derivatives on Photo-Alignment of Liquid Crystal

Photosensitive polyimide containing methoxy cinnamate derivatives as a photo-alignment layer is investigated. 4-Methoxy cinnamic acid, 2-methoxy cinnamic acid and 2,5-dimethoxy cinnamic acid were irradiated and the absorption spectra were obtained with irradiation time. 4-methoxy cinnamic acid and 2-methoxy cinnamic acid are observed isosbestic points at 258 nm and 250 nm, respectively and 2,5-dimethoxy cinnamic acid has no isosbestic pioint. 2-Methoxy cinnamic acid was most photosensitive with 30 sec irradiation time. Polyimide containing methoxy cinnamte derivatives was synthesized and the anisotropy of alignment layer was induced by irradiation linearly polarized UV (LPUV). UV irradiation of a film a of polyimide with 2-methoxy cinnamate proceeded predominantly by the photo-isomerizaiton.

Uniform Alignment of Liquid Crystals Induced by Perfluoropolyether Film Exposed to Linearly Polarized Ultraviolet Light

We have succeeded in inducing uniform alignment of liquid crystal (LC) molecules by using end-functionalized-perfluoropolyether (PFPE) (Fluorocur® made by Liquidia Technologies) films exposed to linearly polarized ultraviolet light (LPUVL). The average orientation direction of LC molecules was parallel to the polarization direction of LPUVL, which is opposite to most cases of other photo-alignment treatments. In the photo-alignment treatment, use of deep UV light and the existence of oxygen in the atmosphere was found to be important for inducing uniform LC alignment.

High in-plane alignment of liquid crystalline methacrylate copolymers bearing photoreactive 2-styrylpyridine and mesogenic 4-methoxybiphenyl groups

This paper describes the thermally enhanced in-plane alignment and other properties of liquid crystalline (LC) copolymers (1, 2, and 3) consisting of a methacrylate with a 2-styrylpyridine side group as a photocyclodimerizable group and a methacrylate with a methoxy biphenyl side group as a mesogenic group. 1H NMR spectroscopy indicated that the contents of the photoreactive monomer unit in copolymers 1, 2, and 3 were 0.65, 0.46 and 0.19 (mole fraction), respectively. The copolymers in the film showed a transmittance of 92% at wavelengths > 450 nm, even at 100 °C, as well as thermal stability up to approximately 360 °C. Copolymers 1 and 2 gave a nematic texture at 155 and 150 °C, respectively, while copolymer 3 showed a smectic texture at 130 °C. The copolymer films aligned parallel to the linearly polarized UV(LPUV) light after LPUV light irradiation and subsequent thermal annealing. The respective in-plane order parameters of copolymers 1, 2, and 3 in the film at 315 nm were 0.65 at 0.001 J/cm2, 0.59 at 0.01 J/cm2, and 0.48 at 0.025 J/cm2 by the same treatment in the LC temperature ranges. This suggests that the photoreactive LC copolymers not only have high optical transparency and good thermal stability but also have controllable, high in-plane alignment with the contents of the photoreactive monomer unit.