Photocontrol of Liquid Crystal Alignment by Polymethacrylates with

Chem. Mater. , 1999, 11 (5), pp 1293–1301 ... Publication Date (Web): April 17, 1999 ... Irradiation of thin films of DPA polymers before cell assem...
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Chem. Mater. 1999, 11, 1293-1301

1293

Photocontrol of Liquid Crystal Alignment by Polymethacrylates with Diphenylacetylene Side Chains Masaki Obi, Shin’ya Morino, and Kunihiro Ichimura* Research Laboratory of Resources Utilization, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan Received November 17, 1998

Whereas diphenylacetylene (DPA) exhibits no photoisomerization because of the involvement of a triple bond in the chromophore, prolonged irradiation of the films with 313-nm light resulted in the reduction of absorption, leading to insolubilization as a result of photocross-linking. When the actinic light is linearly polarized, polarized UV absorbances perpendicular to the electric vector of the light decreased more preferentially than those in parallel with that to generate dichroism. Homogeneous alignment of LCs was induced, when cells were fabricated with substrate plates covered with thin films of DPA polymers, followed by linearly polarized light irradiation at temperatures above the transition temperature of LC. Irradiation of thin films of DPA polymers before cell assembly was more convenient because much less exposure doses were required for the LC photoalignment control. The photogenerated LC alignment was highly thermally stable and was not altered even after heating at 100 °C for 1 week. Discussion was made on the mechanism of the photocontrol of LC alignment on the basis of the photochemical behavior of the DPA polymers.

Introduction Current efforts have been focused on the photocontrol of liquid crystal (LC) alignment by photochemical treatment of molecular as well as polymer thin films deposited on substrate surfaces from fundamental as well as practical interest.1 In particular, the irradiation of thin films of photoactive polymers with linearly polarized light has attracted extensive interest from an industrial standpoint of view2 because this procedure is applicable to manufacturing LC aligning layers which are inevitable materials in the production of LC displays and have been so far fabricated conventionally by the socalled rubbing treatment. The LC alignment control by polarization photochemistry possesses the other extensive applications including the preparation of versatile optical elements, optical information storage, optically anisotropic dye films, and so on.3 There have been three classes of polymers which are capable of bringing about homogeneous LC alignment with linearly polarized light irradiation. The first involves polymers having photoisomerizable units tethered to polymer backbones. The photoisomerizable moieties employed so far for this purpose include azobenzenes4-7 and benzylidenephthalimidines.8,9 They * Author to whom correspondence should be addressed: Prof. Kunihiro Ichimura, Research Laboratory of Resources Utilization, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan. Phone: +81-45-924-5266. Fax: +81-45-924-5276. E-mail: [email protected]. (1) Ichimura, K. In Polymers as Electrooptical and Photooptical Active Media; Shibaev, V., Ed.; Springer: New York, 1996; pp 138172. (2) Schadt, M.; Seiberle, H.; Schuster, A. Nature 1996, 381, 212. (3) ) Gibbons, W. M.; Shannon, P. J.; Sun, S.-T. Mol. Cryst. Liq. Cryst. 1994, 251, 191. (4) Ichimura, K.; Akiyama, H.; Ishizuki, N.; Kawanishi, Y. Makromol. Chem., Rapid Commun. 1993, 14, 813.

display reversible photoisomerization. The second class of polymers incorporates photodimerizable chromophores including cinnamates10-12 and coumarins2 in polymer side chains. It should be noted that cinnamates10 and benzylidenephthalimidines8 display dual photochemical reactions of trans (E)/cis (Z) photoisomerization and [2+2] photodimerization while coumarin exhibits [2+2] photodimerization exclusively, owing to its cyclic structure.2 The third class of polymers is polyimides which have no well-characterized photoreactive chromophores though homogeneous LC alignment arises from linearly polarized light irradiation of this type of polymer films.13 The mechanism of surface-assisted photocontrol of LC alignment has been extensively discussed for azobenzenes as photoactive moieties,4-6,14-16 whereas other photochromic units such as spiropyranes,17 spirooxazines,18 and stilbenes19 have been also employed to (5) Akiyama, H.; Kudo, K.; Ichimura, K.; Yokoyama, S.; Kakimoto, M.; Imai, Y. Langmuir 1995, 11, 1033. (6) Akiyama, H.; Kudo, K.; Ichimura, K. Macromol. Chem., Rapid Commun. 1995, 16, 35. (7) Akiyama, H.; Kudo, K.; Hayashi, Y.; Ichimura, K. J. Photopolym. Sci. Technol. 1996, 9, 49. (8) Suh, D.-H.; Hayashi, Y.; Ichimura, K.; Kudo, K. Macromol. Chem. Phys. 1998, 199, 363. (9) Suh, D.-H.; Ichimura, K.; Kudo, K. Macromol. Chem. Phys. 1998, 199, 375. (10) (a) Schadt, M.; Schmitt, K.; Kozinkov, V.; Chigrinov, V. Jpn. J. Appl. Phys. 1992, 7, 2155. (b) Schadt, M.; Seiberle, H.; Schuster, A.; Kelly, S. M. Jpn. J. Appl. Phys. 1995, 34, 3240. (11) Marusii, T. Y.; Reznikov, Y. A. Mol. Mater. 1993, 3, 161. (12) Ichimura, K.; Akita, Y.; Akiyama, H.; Kudo, K.; Hayashi, Y. Macromolecules 1997, 30, 903. (13) Hasegawa, M.; Taira, T. J. Photopolym. Sci. Technol. 1995, 2, 241. (14) Ichimura, K.; Hayashi, Y.; Akiyama, H.; Ishizuki, N. Langmuir 1993, 9, 3298. (15) Ichimura, K.; Hayashi, Y.; Kawanishi, Y.; Seki, T.; Tamaki, T.; Ishizuki, N. Langmuir 1993, 9, 857. (16) Ichimura, K.; Akiyama, H.; Kudo, K.; Ishizuki, N.; Yamamura, S. Liq. Cryst. 1996, 20, 423.

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generate homogeneous LC alignment by linearly polarized light irradiation. It has been accepted that linearly polarized light irradiation of thin layers incorporating azobenzenes brings about dichroism as a result of the repetition of E/Z photoisomerization of the chromophores which absorb polarized light preferentially when the electric vector of the light is in line with a transition moment of azobenzene chromophores.20 Homogeneous LC alignment is obtained by this kind of optically anisotropic properties of the topmost surface layers. It has been assumed that the other photochromic moieties such as stilbenes undergo photoinduced polarizationphotoisomerization in a manner similar to the azobenzene cases, leading to the emergence of optical anisotropy.19 This sort of optical anisotropy caused by polarization photochemistry triggers the reorientation of LC molecules when they are in contact with photooriented chromophores located at a topmost surface of substrate plates. Quite a similar process has been proposed for cinnamate polymers which display photodichroism induced by E/Z photoisomerization around the CdC bond,12 whereas there has been the alternative interpretation that the orientational direction of LC molecules is determined specifically by the nature of [2+2] cyclobutane ring(s) as photoproduct(s).10 In contrast, the generation of homogeneous LC alignment by polyimide thin films has not been fully understood because of the complexity of photochemical transformation of the polymer skeletons. From a photochemical point of view, these key photoreactions capable of the generation of homogeneous LC alignment can be divided into two categories: reversible and irreversible photoreactions that are initiated by linearly polarized light irradiation. The former involves photochromic reactions such as E/Z photoisomerization and ring opening/closure of [4+2] π-electronic systems including spiropyrans. Photoinduced anisotropy can arise from the reorientation of molecular axis of this kind of photochromic units in this case. On the other hand, [2+2] photocycloaddition of coumarins and the photodegradation of polyimide ring systems belong to the latter. This type of anisotropic phenomena has been proposed to refer to as axial-selective photochemistry. In this context, we took notice of unique structure of diphenylacetylene (DPA) moieties on the basis of the following consideration. DPA does not exhibit any E/Z photoisomerization, owing to the triple bond. However, it was shown that rod-shaped acetylene moieties are converted into a bent structure with a nonbonding orbital in excited states, which is quite similar to ground-state stilbenes.20 This fact led us to evaluate the molecular reorientation of DPA in polymer matrixes under assumption that the bent DPA in excited states formed by the absorption of linearly polarized light suffers statistically from the alteration of molecular orientation more or less during the relaxation to the ground-state rod-shaped structure. (17) Ichimura, K.; Hayashi, Y.; Goto, K.; Ishizuki, N. Thin Solid Films 1993, 235, 101. (18) Goto, K.; Ichimura, K. Unpublished results. (19) Ichimura, K.; Tomita, H.; Kudo, K. Liq. Cryst. 1996, 20, 161. (20) (a) Dumont, M.; Sekkat, Z. SPIE 1992, 1774, 188. (b)Fischer, T.; La¨sker, L.; Stumpe, J.; Kostromin, S. G. J. Photochem. Photobiol. A: Chem. 1994, 80, 453. (c) Scho¨nhoff, M.; Mertesdorf, M.; Lo¨sche, M. J. Phys. Chem. 1996, 100, 7558.

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Figure 1. Chemical structures of DPA polymers.

Figure 2. Chemical structures of liquid crystals and a dichroic dye molecule used in this study.

This paper looks at the photochemical behavior of DPA moieties in polymer films (Figure 1) under polarized light irradiation to achieve LC photoalignment. In contrast to our expectation, it failed to detect the molecular reorientation of DPA as a result of photophysical processes initiated by irradiation with linearly polarized light. It was revealed that DPA demonstrates irreversible photoreaction(s) giving rise to photo-crosslinking between polymer chains while thin films of DPA polymers are able to control LC alignment upon linearly polarized light irradiation. This phenomenon may be of practical significance since photoaligned states are quite stable toward heat treatment. Experimental Section Materials. Starting materials for DPA polymers were purchased from Tokyo Kasei Co. Ltd. and used without further purification. Nematic LCs, NPC-02 of TNI ) 35.0 °C and 5CB of TNI ) 35.4 °C, were gifted by Lodic. Co. Ltd. The chemical structures of LCs are shown in Figure 2. 4-(2-Phenylethynyl)phenol (1). Triphenylphosphine (60 mg), bis(triphenylphosphine)palladium(II) chloride (30 mg) and copper(I) iodide (10 mg) were dissolved in 20 mL of triethylamine. To this solution was added dropwise a solution of 5.7 g of 4-bromophenylacetate and 5.0 g of phenylacetylene dissolved in 10 mL of triethylamine under nitrogen stream. The mixture was stirred at 80 °C for 7 h, cooled at room temperature, and filtered through Celite, followed by the removal of triethylamine under a reduced pressure. A brown oily residue was mixed with 500 mL of hexane, and the mixture was stirred vigorously. Subsequently, a yellow insoluble solid was purified by recrystallization from hexane to give pale yellow crystals of 4-(2-phenyl)ethynylphenyl acetate

Photocontrol of Liquid Crystal Alignment of mp 102-103 °C in a 46% yield. 1H NMR (CDCl3) δ (ppm): 2.3 (s, 3H, CH3), 7.1-7.6 (m, 9H, aromatic). 13C NMR (CDCl3) δ (ppm): 21 (CH3), 88, 89 (acetylene), 120-169 (aromatic), 210 (carbonyl). A solution of 4-(2-phenyl)ethynylphenyl acetate (1.6 g) thus prepared in 50 mL of methanol was stirred at 50 °C for 1 h in the presence of 1.0 g of potassium carbonate. Slight acidification (pH ) 6) of the solution at room temperature with dilute hydrochloric acid gave a precipitate which was collected by filtration and recrystallized from hexane to afford a crystalline mass of 4-(2-phenyl)ethynylphenol (2) of mp 124125 °C in a 79% yield. Anal. Found: C; 86.34, H; 5.10. Calcd. for C14H10O: C 86.60 H 5.15. 1H NMR (CDCl3) δ (ppm): 5.0 (s, 1H, OH), 6.5-7.5 (m, 9H, aromatic). 13C NMR (CDCl3) δ (ppm): 88, 89 (acetylene), 115-155 (aromatic). 4-[4-(2-Phenyl)ethynylphenoxy]butanol (2). To a solution of 1.8 g of 2 in 50 mL of N,N-dimethylformamide (DMF) were added 2.0 g of potassium carbonate and a catalytic amount of potassium iodide. 2-(4-Chlorobutyloxy)tetrahydropyrane (3.0 g) was added to the mixture and stirred at 65 °C for 10 h. After cooling, the reaction mixture was treated with water and diethyl ether, and the organic layer was washed with water, followed by drying over magnesium sulfate and the removal of the solvent. A brown oily residue was dissolved in 50 mL of ethanol containing 2.1 g of p-toluenesulfonic acid and a few drops of trifluoroacetic acid and stirred overnight at room temperature. Diethyl ether and water were poured into the solution, and the organic layer was washed with water thoroughly and dried over magnesium sulfate. The evaporation of the solvent gave a yellowish residue, which was purified by column chromatography on silica gel using a 3:2 (v/v) mixture of hexane and ethyl acetate as an eluent and recrystallized from ethyl acetate to afford colorless crystals of mp 95 °C in a 73% yield. 1H NMR (CDCl3) δ (ppm): 1.6-2.9 (m, 4H, CH2), 3.7 (t, 2H, -CH2CH2O), 4.0 (t, 2H, ArOCH2), 6.8-7.5 (m, 9H, aromatic). 13C NMR (CDCl3) δ (ppm): 62 (-CH2CH2O), 68 (ArOCH2), 88, 89 (acetylene). Anal. Found: C; 80.00, H; 6.64. Calcd. for C18H18O2: C; 81.20, H; 6.77. 4-(4-(2-Phenylethynyl)phenoxy)butyl Methacrylate (3). Compound 2 (3.2 g) and 1.4 g of triethylamine were dissolved in 50 mL of benzene. Benzene containing 1.4 g of methacryloyl chloride was added dropwise into the solution under stirring. After 3 h of stirring and the removal of a white precipitate, the solvent was evaporated to give a yellowish oil. The oily residue was dissolved in diethyl ether and washed with water, followed by drying over magnesium sulfate and by evaporation to give a solid material which was purified with column chromatography on silica gel using a 4:1 mixture of hexane and ethyl acetate as an eluent. Recrystallization from hexane gave 3 as white needles of mp 40 °C in a 78% yield. 1H NMR (CDCl3) δ (ppm): 1.6 (s 1H OH) 1.8-2.0 (t, 4H, CH2), 4.0 (t, 2H, ROCH2), 4.2 (t, 2H, ArOCH2), 5.6 (s, 1H, dCH), 6.1 s, 1H, dCH), 6.8-7.5 (m, 9H, aromatic). 13C NMR (CDCl3) δ (ppm): 18 (CH3), 88 (ethyne), 89 (ethyne), 125 (ethylene), 136 (ethylene), 167 (carbonyl) Anal. Found: C; 78.91, H; 6.53. Calcd. for C22H22O3: C; 79.04, H; 6.59. 4-(2-Phenylethynyl)phenyl Methacrylate (4). To a solution of 3.0 g of 1 and 3.0 mL of triethylamine in benzene was added dropwise a solution of 3.0 g of methacryloyl chloride in benzene. After the conventional workup colorless prisms of mp 103-104 °C was obtained in a 68% yield. 1H NMR (CDCl3) δ (ppm): 2.1 (s, 3H, CH3), 5.8 (s, 1H, dCH), 6.4 (s, 1H, dCH), 7.1-7.6 (m, 9H, aromatic). Anal. Found: C; 82.34, H; 5.62. Calcd. for C18H14O2: C; 82.44, H; 5.34. Poly{4-[4-(2-Phenylethynyl)phenoxy]butyl methacrylate} (PDPA4). A degassed solution of 940 mg of 3 and 10 mg of AIBN in 10 mL of benzene placed in an ampule was kept at 65 °C for 10 h, followed by pouring into hexane to isolate 700 mg of the polymer ((PDPA4). Mw ) 53 000, Mw/Mn ) 2.3, and Tg ) 128 °C. Poly[4-(2-Phenylethynyl)phenyl methacrylate] (PDPA0). Polymerzation was carried out in the same way, and the polymer was isolated by pouring a polymerization solution into methanol. Mw ) 143 000, Mw/Mn ) 3.2, and Tg ) 142 °C.

Chem. Mater., Vol. 11, No. 5, 1999 1295 Physical Measurements. 1H and 13C NMR spectra were recorded on an AC 200 Bruker. Molecular weight of polymers was determined by Gel Permeation Chromatography with a polystyrene standard, using K-800P and KF-805L for analytical columns, a 875-UV (JASCO) as a detector and a 807-IT for integrater. Glass transition temperatures (Tg) were measured by a DSC200 (Seiko Electronic Co.) in a temperature range from 20 °C to 300 °C. Film thickness was measured by a Dektak 3ST (ULVAC Japan). UV-vis absorption spectra were taken on a diode array HP8452A spectrometer. FT-IR spectral measurement was performed with a Bio-Rad FTS 6000 spectrometer, and a calcium fluoride plate was used as a substrate. Polarized absorption spectra were recorded by setting a polarizer at a probe light path. LC Cell Fabrication. Fused silica plates were washed with acetone, 12 N nitric acid, a saturated aqueous solution of sodium hydrogen carbonate, and finally with pure water. DPA films were prepared by spin-coating (2000 rpm, 20 s) of a 1 wt % PDPA toluene solution on a clean, fused silica plate and annealed at 100 °C. Thickness was ∼40 nm. A hybrid LC cell was fabricated by placing a nematic LC between a substrate plate coated with a PDPA film and the other one, which was treated with lecithin in advance, at 40 °C with capillary action. A cell gap was adjusted by colloidal silica spacers of a 5-µm diameter. Photoirradiation. Light from a 500-W high-pressure mercury lamp (Ushio Electric Co.) was passed through a UVD35 glass filter (Toshiba) and a solution filter consisting of an aqueous solution of potassium chromate and sodium hydroxide of a 10-mm path, to select 313-nm line. A Gran-Taylor prism as a polarizer was combined to obtain linearly polarized 313nm light. Light intensity was monitored with an Advantest optical powermeter, TQ8210. A thermocontroller, FP800 thermosystem (Mettler), was used to control temperatures of samples. Determination of Photogelation. A thin film of 40 nmthicknes was spin-cast on a fused silica plate from a toluene solution of a PDPA polymer and dried at 120 °C for 30 min and illuminated with 313-nm light, followed by development with toluene and by measurement of thickness of a remaining film. Determination of LC Photoalignment. An LC cell was subjected to linearly polarized 313-nm light at various exposure doses, and photoinduced LC alignment was evaluated by monitoring the angular dependence of a He-Ne laser beam intensity through an LC cell. The direction of LC alignment was determined by evaluating the angular dependence of absorbances of a dichroic dye, LCD118, which was dissolved in LC before cell fabrication.

Results and Discussion Photoreaction of DPA Tethered to Polymethacrylate Chains. The key compound in this sudy, 4-(2phenylethynyl)phenol (1), was prepared according to the procedure reported by S. Jayaraman et al.21 Two types of polymethacrylates with DPA side chains were synthesized by radical homopolymerization to examine the spacer effect on the photochemical behavior of DPA moiety. PDPA4 has a C4 spacer, whereas DPA is directly attached to polymer backbones of PDPA0. Thermal analysis of PDPA0 and PDPA4 showed endothermic peaks due to Tg in a heating process at 142 and 128 °C, respectively, and neither an exo- nor an endothermic peak attributable to phase transition was detected, indicating that they are amorphous. As mentioned in the Introduction, our motivation to employ DPA moiety as a photoactive chromophore was to verify whether the reorientation of molecules or (21) Ingold, C. K.; King, G. W. J. Chem. Soc. 1953, 2702.

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Figure 4. Photoproducts of diphenylacetylene in solution.

Figure 3. Spectral changes of films of (a) PDPA0 and (b) PDPA4 during irradiation with 313-nm light of 0, 1, 2, 5, 10, 15, and 20 J cm-2 doses.

residues absorbing linearly polarized light would be caused through a photophysical process without any photochemical transformation. In this context, the photolability of DPA polymers was examined first. Figure 3 shows UV-vis spectral changes of films of both DPA polymers upon 313-nm light irradiation. Before irradiation, a film of PDPA0 shows absorption bands centered at 288 and 308 nm while absorption bands of a PDPA4 film are at 296 and 316 nm. The red shift of the PDPA4 film is obviously due to the conjugation of the ether oxygen with DPA moiety. These spectra in films state were not far from those of the corresponding DPA monomers in acetonitrile solution and of poly(methyl methacrylate) films doped with 1 wt % of the monomers, indicating that no intimate interaction exists between the chromophores. This makes a contrast to the observation made by Farahat et al. who reported that DPA forms H-aggregation in LB films which display a marked blue shift of the absorption peak to λmax ) 260 nm.22 No spectral shift in PDPA films reflects unequivocally the amorphous nature of the polymers, where DPA chromophores orient in a random fashion, even though DPA chromophores are closely packed in the homopolymers. This situation is very similar to that of polymethacrylates with azobenzene side chains, as described in our previous papers.4,6 Prolonged irradiation of films with 313-nm light resulted in the disappearance of absorption bands in a range from 260 to 330 nm for both polymers while (22) Jayaraman, S.; Srinivasan, R.; Mcgrath, J. E. J. Polym. Sci. A 1995, 33, 1551.

absorbances increased at λmax ) ∼250 nm, as shown in Figure 3. These results imply that the conjugated π-electronic system of DPA moiety is cleaved to give birth to the generation of benzenoid band(s) which is centered at around ∼250 nm. The photoreaction of DPA moiety was studied by Bu¨chi et al. in solution.23 They isolated dimerization products. K. Ota et al. suggested also that spectral changes of DPA moieties is attributable to the formation of dimerized photoproducts including 1,2,3-triphenylnaphthalene, 1,2,3-triphenylazulene, and others (Figure 4).24 Furthermore, they reported that the dimerization processes of DPA are accelerated in viscous solutions. Taking into account of the solution photochemisty reported so far, it is very likely that the spectral alteration of the PDPA polymers, shown in Figure 3, arises from the photodimerizations of DPA side chains. Photoreaction of the DPA moiety in polymer side chains was investigated from UV-vis and FT-IR spectra. It is noteworthy here that the extent of the increase of absorbances at 250 nm in the DPA films are smaller than that observed in the solution photochemisty of DPA.24 This suggests that the photochemical transformation of DPA in polymer solid is more or less different from that in solution. But further insight concerning the difference in the photoproduct distribution between solution and polymer solid was not available, owing to the difficulty of the identification of photoproduct(s) in polymer solids. The results given in Figure 3 were subjected to spectral analysis by means of ED (extinction difference) diagrams which are conveniently used to confirm that a reaction concerned progresses in such a way that it involves a single process solely involving starting compound(s) and product(s).25 As shown in Figure 5, both films of PDPA0 and PDPA4 gave linear plots in their ED diagrams, supporting that the DPA units are converted into photoproducts which are not subjected to any secondary reaction. Considering the solution photochemistry of DPA chromophores, it is reasonable to assume that the decrease of absorbances is predominantly due to the dimerization of DPA. The photo-cross-linking was confirmed by determining the behavior of photoinsolubilization of thin films of the PDPA polymers. The results are shown in Figure (23) Farahat, C. W.; Penner, T. L.; Ulman, A.; Whitten, D. G. J. Phys. Chem. 1996, 100, 12616. (24) Bu¨chi, G.; Perry, C. W.; Robb, E. W. J. Org. Chem. 1962, 27, 4106. (25) Ota, K.; Murofushi, K.; Hoshi, T. Tetrahedron Lett. 1974, 15, 1431.

Photocontrol of Liquid Crystal Alignment

Figure 5. Absorbance difference (ED) diagrams for films of (a) PDPA0 and (b) PDPA4 upon irradiation with 313-nm light. Absorption differences before and after irradiation (∆A) at 244 (O), 266 (b), 308 (0), and 314 (9) nm were plotted against ∆A at 296 nm.

Figure 6. Photogelation behavior of PDPA0 films (open circle) and PDPA4 films (closed circle) upon irradiation with 313nm light. Toluene was used as a developing solvent.

6. Minimum exposure doses for photoinsolubilization of PDPA0 and PDPA4 are approximately 4.7 and 65 mJ cm-2, respectively. A PDPA4 film displays a photosensitivity that is 13.8 times lower, despite the presence of a tetramethylene spacer which may ensure the molecular mobility of DPA residues. A minimum exposure dose required for photogelation (Eg) is determined by the following expression:

Eg ) rd/(MwcΦ) where Mw, r, d, , and c denote a weight-average

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Figure 7. Changes of polarized absorbances (a) of a PDPA0 film at 290 nm and (b) of a PDPA4 film at 294 nm, respectively, upon irradiation with linearly polarized 313-nm light Open circles and closed circles denote absorbances measured by a linearly polarized probe light with an electric vector in parallel with (A|) and perpendicularly to (A⊥) that of the actinic light, whereas open squares are corresponding to A⊥ - A|.

molecular weight, film thickness, specific gravity, absorption coefficient, and concentration of a photosensitive chromophore, respectively, while Φ stands for quantum efficiency for photo-cross-linking.27 Among these parameters, Mw should be taken into consideration since Mw (5.3 × 104) of PDPA4 is 2.7 times lower than that of PDPA0 (Mw )14.3 × 104). Even after the correction of Eg values based on the difference in Mw, the photosensitivity of PDPA0 is 5.1 time higher than PDPA4. This suggests that the higher sensitivity of PDPA0 arises from a higher quantum efficiency for photo-cross-linking. Polarization Photochemistry. Before the determination of LC photoalignment control by the DPA polymers, the behavior of thin films of the polymer upon irradiation with linearly polarized 313-nm light was studied. Photoinduced optical anisotropy of polymer thin films was followed by measuring absorbances at 290 nm in parallel with (A|) and perpendicular to (A⊥) the electric vector of the light. The results are shown in Figure 7. The optical anisotropy is expressed here as dichroism defined as difference between A⊥ and A|. Figure 7 shows A⊥ - A| values as a function of exposure doses. A| values decrease slightly faster than those of (26) Gauglitz, G. In Photochromism; Molecules and Systems; Du¨rr, H., Bouas-Laurent, H., Eds. Elsevier: Amsterdam, 1990; p 15. (27) (a) Reiser, A.; Egerton, P. L. Macromolecules 1979, 12, 670. (b) Reiser, A. J. Chem. Phys. 1980, 77, 469.

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A⊥ to generate dichroism although A⊥ - A| values are relatively small. This fact means that the axis-selective disappearance of DPA moieties takes place as a result of preferential light absorption of DPA chromophores with a transition moment in parallel with the electric vector of the polarized light. The aspect of the photodichroism generation for both of PDPA0 and PDPA4 films was somewhat different from each other. When a film of PDPA0 was exposed to the linearly polarized light, A⊥ - A| increased with the increase in exposure doses of less than ∼5 J cm-2, followed by a gradual decline upon further irradiation. On the other hand, the reduction of A⊥ - A| for a PDPA4 film took place not so markedly upon further irradiation of exposure doses of even more than ∼5 J cm-2. The generation of dichroism induced by irradiation with linearly polarized light has been predominantly discussed with the use of polymer films incorporating azobenzenes.4-7 It has been accepted that the photodichroism of azobenzenes arises from two processes: the axial-selective photoisomerization involving no molecular reorientation and the photoinduced molecular reorientation. Although the dichroism of a PDPA0 film at a maximum is ∼0.01 and larger than that of a PDPA4, this value is far smaller than that of a thin film of polymethacrylates with azobenzene side chains. Larger photodichroism of azobenzene chromophores in polymer films is related to the molecular reorientation as a result of repetition of E/Z photoisomerization under linearly polarized light irradiation. As stated above, our initial interest in this DPA system was to understand the possibility of molecular reorietation of the nonisomerizable chromophores through photophysical processes. However, it is hard at the present to discuss the molecular reorientation through this mechanism because of the small A⊥ - A| values and of the involvement of the irreversible photochemistry. FT-IR measurements were performed before and after linearly polarized light irradiation of polymer films to obtain further information concerning the photochemistry in polymer films. The results are shown in Figure 8. The absorption band due to the triple bond of a PDPA4 film of 40 nm-thickness at 2200 cm-1 disappeared completely after prolonged photoirradiation. Because the absorption of the triple bond of PDPA0 was so weak, FT-IR measurements before and after 313-nm light irradiation were performed by using a film of 400 nm-thickness. Prolonged photoirradiation resulted again in the disappearance of the band at 2220 cm-1. These results are in line with the photodimerization of the triple bond. The wavenumber of a band due to ester Cd O is not greatly altered after prolonged photoirradiation, while the band was broadened as a result of the partial transformation into the other CdO group(s). Photocontrol of LC Alignment. It has been occasionally observed that exposure energy for LC alignment photocontrol by photoreactive layers is markedly influenced by whether linearly polarized light irradiation is performed before or after LC cell assembly.28 Usually, more than one order magnitude of greater exposure energies are required for the generation of homogeneous alignment upon linearly polarized light (28) Kawanishi, Y.; Tamaki, T.; Sakuragi, M.; Seki, T.; Ichimura, K. Langmuir 1992, 8, 2601.

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Figure 8. FT-IR spectra of a PDPA0 film (a) before and (b) after irradiation with 313-nm light of an exposure dose of 10 J cm-2 and a PDPA4 film (c) before and (d) after the photoirradiation.

irradiation after LC cell assembly, when compared with the irradiation of photoreactive layers before cell assembly. Nematic LCs used here are NPC-02 (TNI ) 35.0 °C), EXP-CIL (TNI ) 31.8 °C), and 5CB (TNI ) 35.4 °C) (Figure 2). It was revealed by polarized microscopy observation that no homogeneous alignment of NPC02 or EXP-CIL emerges when polarized light irradiation of a cell, the walls of which are covered with a spincast PDPA film, is achieved at 25 °C even though exposure energies exceeded 20 J cm-2. In a contrast to this, homogeneous alignment was generated when the irradiation of cells was performed at temperatures, for instance, at 40 °C, higher than TNI of the nematics. Both PDPA0 and PDPA4 were active for the LC alignment photocontrol. But 5CB displayed different behavior. Whereas homogeneous alignment of this LC was observed when a film of PDPA0 was employed to assemble a cell that was to be subjected to polarized light irradiation, a PDPA4 film gave rise to homeotropic (perpendicular) alignment which was not altered even prolonged irradiation with the light. It is very likely that polar 5CB molecules interact with DPA moieties in such a way that DPA moieties reorient perpendicularly, owing to the presence of a flexible spacer, resulting in the homeotropic alignment of the nematics. The LC alignment was determined by our conventional method to monitor transmitted light intensity of linearly polarized He-Ne laser beam as a probe light as a function of rotational angle of a cell. Figure 9 shows typical results. Irradiation with linearly polarized 313nm light resulted in the control of azimuthal orientation of LC for both types of PDPA polymers, as revealed by the regular appearance of peaks and valleys in transmittance at 90° separation. The extent of the LC alignment was leveled off at exposure energies of around 1 J cm-2. The LC alignment was not achieved by heating

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Figure 10. Circular diagrams of absorbances of the 1 wt % dichroic dye (LCD118) dissolved in NPC-02 after polarized light irradiation of a cell modified with a PDPA0 film (open circles) and with a PDPA4 film (closed circles), respectively. The electric vector of the actinic 313-nm light is shown by the arrow.

Figure 9. Angular dependence of transmitted intensity of a linearly polarized He-Ne laser beam passed through (a) an LC (NPC-02) cell modified with a PDPA0 film and (b) an LC cell modified with a PDPA4 film before (open circle) and after (closed circle) irradiation with linearly polarized 313-nm light of an exposure dose of 1 J cm-2 of irradiation at 40 °C. Rotation angles are defined as angles contained by the electric vector of 313-nm light and that of a monitoring He-Ne laser beam.

the LC cell at T > TNI (40 °C) after linearly polarized light irradiation at room temperature, implying that no optical anisotropy is generated in a polymer film in the presence of LC under these irradiation conditions. The irradiation of cells at T > TNI is a necessary condition to attain the photocontrol of azimuthal LC orientation. Next, LC alignment photocontrol was achived by the irradiation of films of the DPA polymers at room temperature, followed by cell assembly. Exposure doses required for the generation of homogeneous alignment were only 50 mJ cm-2 and 200 mJ cm-2 for PDPA0 and PDPA4 films, respectively. Alhough photoinduced anisotropy of the films is quite minute for both cases, the optical quality of photoaligned LC cells was pretty good, owing to the self-assembling nature of LC molecules. Note that exposure doses are much smaller than those in the cases after cell assembly, as mentioned just above, and that the photoirradiation was performed at an ambient temperature. This contrastive behavior has been also observed for polymer thin films embedding azobenzenes5 and may arise from the fact that DPA moieties located at a topmost surface of a polymer film can intimately contact with molecules in a photorespon(29) Ichimura, K. Mol. Cryst. Liq. Cryst. 1997, 298, 221. (30) Obi, M.; Morino, S.; Ichimura, K. Macromol. Rapid Commun., in press. (31) Kobayashi, S.; Iimura, Y. SPIE 1997, 3015, 40.

sive cell to form a supramolecular assemblage so that the molecular motion of the photoactive residues is highly suppressed. Although the results shown in Figure 9 exhibit uniaxial aligment of LC molecules, no information concerning the orientational direction is available from them. According to our conventional method, the orientational direction of LC molecules was determined by measuring dichroism of a dichroic guest dye, LCD118 (Figure 1), since the transition moment of the dye is in parallel with the director of host LCs.28 Films of the DPA polymers were illuminated with linearly polarized 313-nm light, followed by the cell assembly to measure the dichroism of the dye. The results shown in Figure 10 are amazing. The orientational direction was quite different from each other although the same chromophore was used. Whereas the orientational direction of LC was perpendicular to the electric vector of the light in the case of PDPA0, a photoirradiated PDPA4 film resulted in homogeneous LC alignment with the direction in parallel with the electric vector of the light, except for 5CB on PDPA4. This kind of the photoalignment mode influenced by the presence of a spacer was not affected by the nature of LCs. Since an order parameter (S) of the guest dye is a measure of optical quality of photoaligned LC layers, S values of the present systems were compared with those of guest-host cells based on the other photoreactive polymer films. S is defined here as S ) (A| - A⊥)/(A| + 2A⊥), where A| and A⊥ denote absorbances of the guest dye perpendicular to and in parallel with the electric vector of polarized light. S of the dichroic dye for cells modified with the PDA polymers was 0.28, being irrespective of the chemical structure of the polymer. This value is comparable to those of cells based on films of polymers bearing side-chain azobenzenes (S ) -0.28 to -0.35),29 benzylidenephthalimidines (S ) -0.28 to -0.35),9 and cinnamates (S ) -0.20 to -0.25),12 respectively The description of the mechanism of LC photoalignment determined by the DPA homopolymers remains speculative because of the following. First, it is hard to obtain clear-cut evidence supporting the photoinduced

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molecular reorientation of DPA units because of low levels of photodichroism and the concurrent occurrence of photobleaching of the chromophores. Second, no information concerning the structure of photoproducts in films is as yet available. Although there are such limitations in the present system, we would like to propose the following photoalignment mechanism. As discussed previously, the generation of optical anisotropy induced by polarization photochemistry consists of two processes.6 They include the axis-selective photochemical transformation of molecules with a transition moment in parallel with the electric vector of actinic light and the reorientation of the molecular axis of the photoisomerizable moieties, leading to the preferential formation of molecules or residues oriented perpendicularly to the electric vector of the actinic light. If the molecular photoreorientation of DPA may determine the LC alignment, the alignment direction should be perpendicular to the electric vector of polarized light, taking notice of the rod-shaped DPA moiety. This is not the case at least for a PDPA4 film which gives rise to a parallel photoalignment. In other words, it is very likely that no molecular reorientation of DPA induced by linearly polarized light takes place. The possilibity of the photoreorientation of DPA units of PDPA0 is consequently eliminated since DPA is tethered directly to polymer backbones so that molecular mobility should be much more suppressed. It follows that the LC photoalignment is determined by optical anisotropy generated by axis-selective photochemistry which results in the consumption of DPA and the formation of photoproduct(s). We have quite recently observed that the reversion of photoalignment from parallel to perpendicular direction is induced during photoirradiation of a film of a polymethacrylate with coumarin side chains directely tethered to the polymer backbone with linearly polarized light.29 This was reasonably interpreted in terms of axis-selective photochemistry of the nonphotoisomerizable coumarin residues. Namely, the parallel alignment at the early stage of photoirradiation is caused by photodimer(s) as a result of the axisselective photochemistry. Prolonged irradiation results in fragments of coumarin rings with a transition moment perpendicular to the electric vector of the light, leading to perpenducular orientation of LC molecules. Quite the same interpretation can be applied to the present system. As shown in Figure 7, the fraction of DPA with the transition moment in parallel with the polarization plane of the light disappears slightly faster. This observation suggests that the photoalignment triggered by a PDPA0 film is determined rather by unconsumed DPA residues, the molecular axis of which lies perpendicularly to the electric vector of the actinic polarized light. On the other hand, the parallel LC orientation caused by a PDPA4 film may thus arise from a different mechanism. Although no spectral evidence has yet been available due to the small dichroic nature of photoirradiated films, it is assumed that the parallel orientation is governed by the anisotropically formed photoproduct(s) of DPA moieties. It is assumed that the flexible spacer plays an essential role in determining the mode of LC photoalignment, although further studies are necessary.

Figure 11. Angular dependence of transmitted intensity of a linearly polarized He-Ne laser beam passed through (a) an LC cell modified with a PDPA0 film before (open circle) and after (closed circle) heat treatment at 100 °C for 20 h and (b) an LC cell modified with a PDPA4 film before (open circle) and after (closed circle) heat treatment at 100 °C for 1 h, respectively. LC cells based on PDPA0 and PDPA4 were exposed to linearly polarized 313-nm light of exposure doses of 50 and 200 mJ cm-2, respectively.

Thermal Stability of the LC Alignment. Because the LC photoalignment has been attracting practical interest in its application to the preparation of LCaligning films without conventional rubbing treatment of polymer films,13 the thermal stability of the photoalignment of the presenst system was evaluated according to the following procedures. A PDPA film on a silica plate was irradiated first with linearly polarized 313nm light of various exposure doses and used for a cell assembly. LC used here was NPC-02. The cells displaying homogeneous alignment were heated at 100 °C, and changes in the LC alignment were monitored in the same way as described in Figure 9. Typical results are shown in Figures 11. Minimum exposure doses of 200 mJ cm-2 for PDPA4 to generate homogeneous alignment were not sufficient at all to attain the thermal stability of the photoalignment, which disappered totally after heating for 1 h at 100 °C. When exposure doses for a PDPA4 film exceeded 500 mJ cm-2, the LC photoalignment remained even after heating for 20 h. Interestingly, PDPA0 film exhibited higher thermal stability of photogenerated LC alignment; an exposure energy of only 50 mJ cm-2 was found to be sufficient to attain the thermally stable homogeneous alignment which remained even after 20 h at 100 °C. The excellent stability of homogeneous alignment toward heat treatment is owing to the cross-linking of

Photocontrol of Liquid Crystal Alignment

DPA units. Difference in the alignment stability between PDPA0 and PDPA4 films may be ascribable to the quantum efficiency for photo-cross-linking, as indicated by the photogelation experiments shown in Figure 6; the gelation of PDPA0 occurs at smaller exposure energies when compare with PDPA4. The presence of the spacer of PDPA4 also plays a crucial role in the deterioration of photoaligned state by heating because of the flexibility of the side chains which determine the photoalignment. Conclusions Photoirradation of thin films of polymers with DPA side chains resulted in insolubilization because of photodimerization. Quantum efficiencies for the insolubilization of PDPA0 without a spacer were larger than that of PDPA4 having a C4 spacer. Linearly polarized light irradiation generated photodichroism although relatively small, as a result of the preferential consumption of DPA moieties with the transition moment in parallel with the electric vector of the actinic light. Thin films of both DPA polymer were capable of achieving the azimuthal alignment photocontrol by irradiation with linearly polarized 313-nm light. Exposure doses of the actinic light for the generation of homogeneous alignment were markedly influenced by whether photoirradiation was performed before or after cell assembly. The emergence of homogeneous alignment was not observed until a cell was heated at temperatures above TNI of LC used. Furthermore, much

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greater exposure doses were required for the LC alignment photocontrol. This situation may arise from the fact that the mobility of photoactive DPA groups at an uppermost surface of a polymer film is highly suppressed because the DPA moieties are incorporated in a liquid crystalline assemblage. Homogeneous alignment was obtained by polarized light irradiation of much smaller exposure doses when cell assembly was made after the photoirradiation. A profound spacer effect was observed on the orientational direction of photogenerated homogeneous alignment. While the LC director controlled by irradiation of a PDPA0 film with linearly polarized light was perpendicular to the electric vector of the actinic polarized light, parallel alignment was generated when a film of PDPA4 was employed. These contrastive results correspond to the difference in the photoalignment mechanism. It is very likely that the orientational direction of LC is determined by the molecular direction of remaining DPA units of PDPA0 after polarized light irradiation, whereas photoproducts of DPA units of PDPA4 should participate in the determination of the photoalignment direction. The homogeneous alignment induced by polarization photochemistry of DPA polymer films was so significantly thermally stable that no deterioration of the photoalignment was observed even after heating at 100 °C for 1 week. This fact is of great improtance for practical applications, including the preparation of LCaligning films for LC display devices. CM981075T