Homeotropic Alignment of Nematic Liquid Crystals by a Photocross

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2009, 113, 2961–2965 Published on Web 02/16/2009

Homeotropic Alignment of Nematic Liquid Crystals by a Photocross-Linkable Organic Monomer Containing Dual Photofunctional Groups Dongyu Zhao, Wei Huang, Hui Cao, Yudong Zheng, Guojie Wang, Zhou Yang, and Huai Yang* Department of Materials Physics and Chemistry, School of Materials Science and Engineering, UniVersity of Science and Technology Beijing, Beijing 100083, China ReceiVed: NoVember 17, 2008; ReVised Manuscript ReceiVed: January 1, 2009

In this paper, we report a novel organic monomer containing dual photocross-linkable groups and success in realizing photoinduced homeotropic alignment of nematic liquid crystals (LCs) with it. It was first revealed that direct irradiation of the photoalignment thin film with nonpolarized ultraviolet (UV) light at 365.0 nm brought out homeotropic orientation of the photopolymer as a result of the photocross-linking of the dual photoreactive groups. When the thin film was obliquely irradiated with nonpolarized UV light, the pretilt angles of nematic LC were generated. Interestingly, we find that the hydrophobicity of the photopolymer increases with increasing irradiation time. In discussing the mechanism of the homeotropic alignment, it was found that the incorporation of the dual photofunctional group of the photoalignment molecules as well as the extreme hydrophobicity of the photopolymer play the essential roles. This monomer cross-linked film is expected as a promising homeotropic alignment film with rubbing-free processing for the fabrication of advanced vertical alignment LC displays. Introduction The control of the orientation of liquid crystalline molecules is essential in liquid crystal displays (LCDs). The photoalignment technique, as one promising alternative to the conventional rubbing technique, has attracted great attention for its contactfree and photopatternable characteristics.1 It is considered that the essential driving force for photoinduced alignment of LC is the intermolecular interaction between LC molecules and photooriented molecules or residues localized at the outermost film surface. So far, three kinds of photoreactive materials have been employed for the orientation of LC molecules exhibiting photoisomerization,2-4 photocross-linking,5-7 and photodecomposition.8 Among them, the photocross-linkable materials are thought to be suitable for the production of LC-alignment films due to their high thermal stability arising from the threedimensional cross-linked structure. Their photofunctionalities arise from (2 + 2) cycloaddition of the neighboring cinnamate,9-16 coumarin,17-25 or diphenylacetylene7 residues by irradiation with UV light, resulting in the insolubilization in organic solvents and hence align LC homogeneously on the resultant film. Currently, vertically aligned nematic (VAN)-LCDs which require homeotropic (perpendicular to the surface) LC alignment are becoming the most popular LCD because of their excellent quality such as high contrast, wide viewing angle, and fast response time.26 In addition, the generation and control of pretilt angle is also vital to the fabrication of a defect-free homeotropic alignment LC cell by eliminating the appearance of reverse tilt disclinations.27 So far, only a few reports have been published on the fabrication of VAN LC cells utilizing photoalignment * Corresponding author. Phone: 86-10-62333969. Fax: 86-10-62333969. E-mail: [email protected].

10.1021/jp8101089 CCC: $40.75

layers,28-35 because the anchoring strength of homeotropic alignment (10-4 J m-2) is very much weaker than the planar anchoring (10-3 J m-2) of homogeneous alignment.36,37 Hence, developing promising photoalignment materials to obtain vertically aligned LCs continues. On the basis of photoisomerization of azobenzene moieties, several researchers reported obtaining homeotropic as well as slightly tilted homeotropic alignment by the use of slantwise nonpolarized UV light irradiation. However, a major problem in using the azobenzene-containing materials is the lack of thermal stability of the photoaligned state because no covalent cross-linking is performed. In this work, our object is to obtain a homeotropic alignment layer for nematic LC using a novel organic monomer, 4-propyldiphenylacetypenecarboxylic acid cinnamyl ester (PDACE), which has not been reported to our knowledge. We characterized the photocross-linking reaction of PDACE by nonpolarized UV light irradiation and successfully obtained a homeotropic alignment of the LC on the resultant film. In this alignment procedure, oblique irradiation of the photoalignment film is utilized to generate the pretilt angles of LC. Irradiation of the spin-coated film with nonpolarized UV light generated good-quality PDACE thin films with smooth surfaces and ensured homeotropic alignment of LC, as a consequence of the three-dimensional orientation of the cross-linked photopolymer through the incorporation of the dual photocross-linkable groups. In discussing the mechanism of LC homeotropic alignment, it is interesting to find that the hydrophobicity of the irradiated film also plays an important role. Since photoalignment has patterning capability, this photoaligned film is expected to be promising for multidomain vertical alignment mode LC displays.  2009 American Chemical Society

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Letters

Figure 1. (a) Chemical structure of PDACE. (b) UV-vis absorption spectral change of a spin-coated PDACE film upon UV irradiation. (c) FT-IR spectra of a PDACE film before and after UV irradiation for 60.0 s. (d) Scheme of the possible photoreaction of PDACE.

Experimental Section Material Synthesis. The reaction scheme as well as the synthesis and characterization results for PDACE are included in the Supporting Information. 4-Propyldiphenylacetypenecarboxylic Acid Cinnamyl Ester (PDACE). 1H NMR spectral data (400 MHz, CDCl3): δ, ppm 0.92 (t, J ) 7.2 Hz, 3H, sCH3), 1.621 (m, J ) 7.6 Hz, 2H, sCH3CH2CH2), 2.583 (t, J ) 7.6 Hz, 2H, sCH3CH2CH2), 4.98 (d, J ) 7.8 Hz, sCOOCH2s, 2H), 6.39 (m, J ) 15.6 Hz, ArsCHdCHs, 1H), 6.73 (d, J ) 16.0 Hz, ArsCHdCHs,1H), 7.16 (d, J ) 8.0 Hz, 2H, ArsH), 7.25 (t, J ) 5.6 Hz, 2H, ArsH), 7.31 (t, J ) 6.0 Hz, 1H, ArsH), 7.35 (t, J ) 6.8 Hz, 2H, ArsH), 7.41 (dd, J ) 6.8 Hz, 1.6 Hz, 4H, ArsH), 8.04 (d, J ) 8.4 Hz, 2H, ArsH). Preparation of Photoalignment Films and LC Test Cells. PDACE thin films were prepared from a 1.0 wt % toluene solution by spin-casting onto a quartz or an indium tin oxide (ITO) substrate (2000 rpm, 30 s). The films were then irradiated with nonpolarized UV light (365.0 nm, 6.5 mW/cm2) from a 500 W high-pressure mercury lamp, and the light intensity was monitored with an UV radiometer (UV-A). Test LC cells (15 µm thick LC layer) were assembled with the irradiated PDACE films in an antiparallel type. The nematic LC (SLC1717, Slichem Co. Ltd., TNI ) 91 °C) was filled into the vacant cells by capillary action. Physical Measurements. UV-vis absorption spectra were taken on a JASCO V-510 UV/vis/NIR spectrophotometer. Polarized UV-visible absorption spectra were recorded by setting a polarizer at a probe light path. FT-IR spectra measurements were performed with a Perkin-Elmer Spectrum One spectrometer, and a calcium fluoride plate was used as the substrate. The AFM studies were carried out using a Nanoscope-

IIIa scanning probe microscope in a tapping mode. The POM (polarizing optical microscopy) image was gained by an Olympus BX-51 polarizing optical microscope. The pretilt angles of the nematic LC (SLC1717) were measured by a crystal rotation method.38 Contact angles were measured using a DataPhysics OCA-20 measuring device. All measurements were performed at room temperature, and contact angles were measured at least three times for each sample and averaged. Results and Discussion A novel organic monomer, 4-propyldiphenylacetypenecarboxylic acid cinnamyl ester (PDACE), was synthesized, and its chemical structure is shown in Figure 1a. A thin film was prepared by means of a conventional solution spinning-casting and then exposed to nonpolarized 365.0 nm light. To investigate the photolability of PDACE, the absorption spectral changes of PDACE film upon UV irradiation were first examined by UV-vis spectroscopy. As shown in Figure 1b, prolonged irradiation results in the decrease of absorption bonds in a range from 280 to 330 nm due to coupling sCdCs and sCtCs groups, while absorbance at λmax ) 260 nm increases which is attributable to the generation of benzoid products of dimerized tolane moieties including 1,2,3-triphenylnaphthalene, 1,2,3triphenylazulene, and others.7,39,40 To obtain further information concerning the photocross-linking in PDACE films, the FT-IR spectra of PDACE film before and after UV irradiation were measured, as shown in Figure 1c. The characteristic absorption peaks at 967 and 2210 cm-1 are associated with out-of-plane bending vibrations of the dCsH bonds and the sCtCs stretching vibrations of PDACE, respectively. A great decrease in the intensity of the dCsH vibration at 967 cm-1 supports the fact that majority of the sCdCs groups do undergo

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Figure 2. AFM images of PDACE film before (a) and after (b) 60.0 s of UV light irradiation.

photocycloaddition reaction rather than photoisomerization. Meanwhile, after prolonged photoirradiation, the intensity of the sCtCs vibration at 2210 cm-1 decreases, which should be attributed to the photocycloaddition between sCtCs groups to form dimerized photoproducts of tolane moieties. These results are in line with the photodimerization of the double and triple bonds,7 confirming that the photocross-linking of both the sCdCs and the sCtCs groups jointly participate in the generation of the product. It is noteworthy that the absorption peak intensity at 967 cm-1 decreases drastically after UV irradiation compared to the slow decrease of the peak intensity at 2210 cm-1, revealing the faster photocycloaddition ratio of the sCdCs group. On the basis of the above results, the possible photocross-linking reaction is speculated in Figure 1d; however, further insight concerning the exact process is not available due to the complexity of the photoreaction owing to the dual photosensitive groups. Figure 2 shows the AFM images of PDACE thin film before and after irradiation with nonpolarized UV light. Before irradiation, the film surface appeals quite rough; the root-meansquare (rms) roughness is 11.29 nm over the area of 3.0 × 3.0 µm2 (Figure 2a), while it apparently becomes smooth and reveals an optimal rms surface roughness of 0.22 nm over the area of 3.0 × 3.0 µm2 (Figure 2b) after an irradiation time of 60.0 s. This change of surface topography in the film confirmed that the photocross-linking between PDACE molecules took place upon UV irradiation, giving birth to the photopolymer which exhibits a quite smooth surface. In order to gain more insight into the 3D orientation of the PDACE in nonpolarized UV-irradiated films, absorption spectral analysis as a function of incident angle (θm) of probing light was performed, as depicted in Figure 3a. Here, Ap and As

Figure 3. (a) Schematic illustration of polarized absorption spectra measurement of the PDACE film. θm denotes the incident angle of probing light with respect to the surface normal. (b and c) Ap/As values of PDACE film as a function of incident angle of the probe light at different incident angles (θa). θa ) (b) 0° and (c) 45°.

represent absorbances at λmax measured by p-polarized light with the electric vector parallel to the plane of incidence and s-polarized light with the electric vector perpendicular to the plane of incidence, respectively. Measurements of Ap and As can give significant information concerning the spatial orientation of chromophoric molecules as a function of incident angles of probing light, since Ap is sensitive to the out-of-plane orientation, whereas As is relatively inert.41,42 Parts b and c of Figure 3 show the ratios of Ap300/As300 for PDACE films which were irradiated with nonpolarized UV light at incident angles (θa) of 0 and 45°, respectively. When θa ) 0°, UV-irradiated PDACE film displayed obviously symmetric curves of Ap300/ As300 with a minimum value at θm ) 0° (Figure 3b), revealing that the irradiated PDACE film orients homeotropically to the substrate surface. When θa ) 45°, the asymmetric plots of Ap300/ As300 with a minimum value at θm ) 10° indicate the tilted alignment of PDACE irradiated film, as shown in Figure 3c. On the basis of the results mentioned above, the homeotropicially aligned LC cell was assembled and the alignment was identified in several different ways. As shown in Figure 4a, the uniform dark image when studied with crossed polarizers was observed without defects, regardless of the cell rotation angle. This observation shows that the axis of the homeotropicially

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Letters TABLE 1: Contact Angle of PDACE Irradiated Films with Different Exposure Times

a

Figure 4. (a) POM image of the homeotropic alignment in LC cells with substrates of irradiated PDACE films. (b) Experimental result of the pretilt angle measurement (crystal rotation method) for a direct nonpolarized light exposure. (c) Pretilt angles of a nematic LC (SLC1717) as a function of irradiation time, generated by PDACE thin films after being exposed by nonpolarized UV light with various irradiation angles.

aligned LC molecules is perpendicular to the view direction. Apart from that, the pretilt angle of the nematic LC was determined by means of the crystal rotation method. The crystal rotation signals of the pretilt angle measurement correspond to 3D birefringence of the nematic LC cell as a function of incident angles of probing light. Figure 4b shows the result of the pretilt angle measurement in a LC (SLC1717) cell, whose inside walls were covered with PDACE films, upon direct irradiation with nonpolarized 365.0 nm light. A symmetrical curve of the transmitted light intensity means that LC molecules align uniaxially at a pretilt angle of about 90°, which was at last determined to be 89.8 ( 0.02° by fitting the result in Figure 4b with a theoretical calculation.38 On the other hand, when the film is obliquely irradiated, the inclined homeotropic alignment of the LC cell can be induced. Figure 4c shows the photogeneration of pretilt angles as a function of exposure time of nonpolarized UV light at various incident angles (θa). The pretilt angle decreases with increasing exposure time and incident angle. From this result, the suitable pretilt angle for fabricating VA-LCD cells of 87-88° is obtainable by appropriate choices of the irradiation angle as well as the exposure time. One of the requirements to be fulfilled in the application to rubbing-free LC photoaligning films for LCDs is the prominent

exposure timea (s)

contact angle

0.0 ( 0.1 30.0 ( 0.1 60.0 ( 0.1 90.0 ( 0.1 120.0 ( 0.1

50.1 ( 0.5° 68.4 ( 0.5° 72.8 ( 0.5° 82.0 ( 0.5° 82.1 ( 0.5°

UV light wavelength, 365.0 nm; light intensity, 6.5 mW/cm2.

stability of photoaligned states. Hence, the thermal stability of the photoalignment of the present system was evaluated by monitoring the change in POM image. Under ambient conditions, the LC cells displaying homeotropic alignment were found quite to be stable for at least 6 months; when the cells were heated at 120 °C, the LC photoalignment remained even after heating for 5 h. At last, when heated at 130 °C for 1 h, the photoaligned states were slightly destroyed. The results are shown in Supporting Information Figure S1. The excellent thermal stability of photoinduced LC alignment is owing to the photocross-linking of the dual photocross-linkable groups, which gives birth to the photopolymer network. Finally, we would like to propose the following photoalignment mechanism determined by PDACE irradiated film. In this method, the homeotropically oriented alignment layer could be generated after UV irradiation. In the first step, PDACE thin film forms after being spin-coated and the molecules orient in a random fashion. Upon UV irradiation, the sCdCs group of the PDACE molecule undergoes the [2 + 2] photocycloaddition, giving rise to the cyclobutane ring product. This [2 + 2] photocycloaddition product tends to orient out-of-plane because the rod-shaped tolane units are so hydrophobic that the tolane units would be localized at the topmost surface of a film to minimize surface energy. At the same time, unreacted PDACE molecules near the generated cyclobutane ring products are induced to orient perpendicularly. After prolonged irradiation, these photoproducts would continue to react with unreacted groups to attain the homeotropically oriented polymer network. In order to confirm the strong hydrophobicity of the irradiated molecules, measurements of water contact angles were made on PDACE irradiated films, as given in Table 1. Before UV exposure, the contact angle of the film was about 50.1°. After prolonged irradiation, the water contact angle increased with increasing irradiation time and attained a saturation value of 82.1°. This result reveals increasing hydrophobicity of PDACE irradiated film. Thus, the homeotropically oriented PDACE photoalignment layer formed as a result of the difference of photoreaction ratio of the dual photocross-linkable groups as well as the hydrophobicity of the tolane units. Finally, as a result, the vertical alignment of the LC was induced by the homeotropicially oriented PDACE photocross-linked film. Conclusions In this report, we have synthesized a novel organic monomer containing dual photoreactive groups, the cinnamate and tolane moieties, and successfully utilized it to fabricate homeotropic alignment layers for LC molecules. The photocross-linking of the dual photoreactive groups was elucidated by UV-vis, FTIR, as well as AFM spectroscopy. The out-of-plane polarized absorption spectral analysis was used to indicate the homeotropic orientation of the photoalignment layer upon irradiation. It was found that the incorporation of the dual photofunctional group as well as the hydrophobicity of the photopolymer play essential

Letters roles in the homeotropic alignment. Further studies to elucidate the mechanism in detail are underway. Acknowledgment. This research was supported by National Natural Science foundation (Grant No. 20674005), Program of National High Technology 863 program of China (Grant No. 2006AA03Z108), and Science and Technology Program of Beijing, China (Grant No. Y0405004040121). Supporting Information Available: The detailed synthetic and characterization results of PDACE, the results of the stability of the LC homeotropic alignment when the cells were heated, and the POM image of the LC alignment with substrates of spin-coated PDACE film. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Ichimura, K. Chem. ReV. 2000, 100, 1847. (2) Hahm, S. G.; Lee, S. W.; Lee, T. J.; Cho, S. A.; Chae, B.; Jung, Y. M.; Kim, S. B.; Ree, M. J. Phys. Chem. B 2008, 112, 4900. (3) Gibbons, W. M.; Shannon, P. J.; Sun, S.-T.; Swetlin, B. J. Nature 1991, 351, 49. (4) Shannon, P. J.; Gibbons, W. M.; Sun, S.-T. Nature 1994, 368, 532. (5) Furumi, S.; Ichimura, K. J. Phys. Chem. B 2007, 111, 1277. (6) Kim, C.; Wallace, J. U.; Trajkovska, A.; Ou, J. J.; Chen, S. H. Macromolecules 2007, 40, 8924. (7) Obi, M.; Morino, S.; Ichimura, K. Chem. Mater. 1999, 11, 1293. (8) Wang, Y. H.; Xu, C. Y.; Kanazawa, A.; Shiono, T.; Ikeda, T.; Matsuki, Y.; Takeuchi, Y. J. Appl. Phys. 1998, 84, 181. (9) Schadt, M.; Schmitt, K.; Kozinkov, V.; Chirgrinov, V. Jpn. J. Appl.Phys 1992, 31, 2155–2164. (10) Kawatsuki, N.; Ono, H.; Takatsuka, H.; Yamamoto, T.; Sangen, O. Macromolecules 1997, 30, 6680–6682. (11) Ichimura, K.; Akita, Y.; Akiyama, H.; Kudo, K.; Hayashi, Y. Macromolecules 1997, 30, 903–911. (12) Obi, M.; Morino, S.; Ichimura, K. Jpn. J. Appl. Phys 1999, 38, L145-L147. (13) Cull, B.; Shi, Y.; Kumar, S.; Schadt, M. Phys. ReV. E 1996, 53, 3777–3781. (14) Kawatsuki, N.; Matsuyoshi, K.; Hayashi, M.; Takatsuka, H.; Yamamoto, T. Chem. Mater. 2000, 12, 1549–1555. (15) Kawatsuki, N.; Takatsuka, H.; Yamamoto, T.; Ono, H. Jpn. J. Appl. Phys 1997, 36, 6464–6469. (16) Perny, S.; Barny, P. L.; Delaire, J.; Buffeteau, T.; Soursseau, C.; Dozov, I.; Forget, S.; Martinot-Lagarde, P. Liq. Cryst. 2000, 27, 329–340.

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