Surface-Assisted Photoalignment of Discotic Liquid Crystals by

Discotic Liquid Crystals for Opto-Electronic Applications ... Aggregate-induced emission in light-emitting liquid crystal display technology. Dongyu Z...
0 downloads 0 Views 473KB Size
Download PDF
J. Phys. Chem. B 2007, 111, 1277-1287

1277

Surface-Assisted Photoalignment of Discotic Liquid Crystals by Nonpolarized Light Irradiation of Photo-Cross-Linkable Polymer Thin Films Seiichi Furumi*,‡ and Kunihiro Ichimura*,† National Institute for Materials Science (NIMS), 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan, and Faculty of Science, Toho UniVersity, 2-2-1 Miyama, Funabashi, Chiba 274-8510, Japan ReceiVed: September 1, 2006; In Final Form: NoVember 28, 2006

In this article, we describe the surface-assisted photoalignment of discotic liquid crystals (DLCs) on thin films of photo-cross-linkable polymers with cinnamoyl moieties as the side chains. Oblique irradiation of the polymer thin films with nonpolarized UV light at 313 nm brought about inclined orientation of the cinnamoyl residues as a result of their direction-selective photoisomerization and photodimerization. The DLC molecules on the photoirradiated polymer films were aligned in a tilted hybrid manner. This means that the DLC directors are continuously altered from the substrate to the DLC film surface so as to minimize the elastic free energy. Interestingly, we found that the tilted direction of aligned DLC molecules is clearly influenced by the chemical structures of the cinnamate-containing polymers. When a poly(vinyl cinnamate) thin film was obliquely exposed to nonpolarized UV light, the DLCs were inclined to the direction opposite to the UV light propagation. In a keen contrast, the thin film of poly(methacrylate)s tethering cinnamoyl groups, which was obliquely exposed to nonpolarized UV light in advance, provided the tilting DLC direction in parallel with the light propagation. The results were supported by tilted orientation of calamitic (rod-shaped) liquid crystal on the obliquely irradiated polymer films. Such photoalignment behavior of the DLCs can be rationalized by anchoring balance between intermolecular interaction of the DLC molecules with the photodimers of polymer films and those with the remaining E-isomers of cinnamoyl side chains at the film interface. The present technique of DLC photoalignment opens promising ways not only to understand anisotropic physical properties of DLCs, but also to design and fabricate novel nanodevices for photonics and electronics applications.

1. Introduction Photoinitiated polymerization has attracted the long-standing interest of polymer chemists not only to understand the basic mechanism and kinetics of the polymerization, but also to fabricate the photolithographic images for electronic and photonic applications.1 In conventional photopolymerizations, photoirradiation promotes the initiating step to liberate either free radicals or ions, which then propagate the chain reaction of the monomers to yield high molecular weight products. After this discovery, novel photopolymerization has been proposed by utilizing photocycloaddition.2 The photocycloaddition of the organic molecules with olefin bonds is one of the most common photoinduced reactions. According to the Woodward-Hoffmann rule, the photoinduced cycloaddition of two olefins to give their cyclobutane formation is allowed photochemically as a (2 + 2) reaction in the solid states when topological requirements such as molecular arrangements are fulfilled to minimize the molecular motion due to the bimolecular reaction. For instance, cinnamic acid and its derivatives show the topochemical reactions of both reversible E-to-Z photoisomerization and irreversible photodimerization (photocycloaddition) in the crystalline states, as shown in Scheme 1.3 The E-isomer crystallizes into three principal types of crystal forms: the R-form, in which the double bonds of neighboring molecules contact at a distance * To whom correspondence should be addressed. E-mail: FURUMI. [email protected] (S.F.), [email protected] (K.I.). † Toho University. ‡ NIMS.

SCHEME 1: The Photochemical Reactions, E-to-Z Photoisomerization, and (2 + 2) Photodimerization of Cinnamoyl Derivatives

of ∼3.7 Å across a center of symmetry; the β-form, characterized by a lattice having one axial length of 4.0 ( 0.1 Å between each pair of neighboring molecules; and the γ-form, in which the double bonds of neighboring molecules separate over 4.7 Å. By irradiation of the cinnamate crystals with ultraviolet (UV) light, the R-type crystal leads to a centrosymmetric dimer related to R-truxillates, the β-type crystal gives a mirror-symmetric dimer of β-truxinates, and the γ-form exhibits no photodimerization. Introduction of diolefinic linkages into the molecular structures, for example, 2,5-distyrylpyrazine,4 gives rise to their insoluble linear polymers as a result of their stepwise topochemical (2 + 2) photodimerization.2

10.1021/jp065686h CCC: $37.00 © 2007 American Chemical Society Published on Web 01/23/2007

1278 J. Phys. Chem. B, Vol. 111, No. 6, 2007 A similar photocycloaddition takes place even in polymer solid films, including the cinnamoyl moieties, leading to versatile applications to optical devices, such as optical switching, channel waveguide, waveguide lithography, and so forth.5 The poly(vinyl cinnamate) (pVCi) derivatives, as conventional negativetone photoresists, have attracted considerable interest from both fundamental and technological viewpoints of the photopolymers since the invention made by Eastman Kodak Company five decades ago.5a Their photofunctionalities happen from cyclobutane formation of the neighboring cinnamoyl residues through (2 + 2) photodimerization by irradiation with UV light, resulting in not only the large changes in optical refractive index but also the insolubilization in organic solvents. Two decades after the invention, it was reported that the photochemical transformation shows a peculiar optical property: linearly polarized photoirradiation of the pVCi films gives rise to the emergence of optical birefringence for holographic recording.6 The advance in linear polarization photochemistry of molecular and polymeric thin films leading to the surface-assisted orientational photocontrol of calamitic (rod-shaped) liquid crystals (CLCs) has made the potential use of this type of conventional photoresists significantly more attractive.7 The exposure of a thin film of the pVCi and the related polymers to linearly polarized UV light enables uniaxially planar alignment of the CLCs with thermal stability as a result of angular selective photoisomerization and photodimerization of the cinnamoyl residues with respect to the polarization plane.8 Much attention has been devoted to the orientational manipulation of CLCs by photochemical reactions as well as orientations of photoreactive moieties such as azobenzenes, cinnamates, coumarins, and so forth settled on the outmost surface of substrates.9 Therefore, the surface-assisted photoalignment of CLCs is a widely investigated research topic from an industrial viewpoint because of its versatile practical applicabilities, including LC display (LCD) devices, optical memories and anisotropic optical elements. In particular, current attention is paid to the fabrication of LC-aligning films embedded in the LCDs, aiming at the optimization for their driven modes in the devices, such as multidomain twisted nematic10 and vertical alignment.11 However, this kind of LC photoalignment technique has been hitherto applied to only the CLC molecules. We reported previously a novel and comprehensive approach of the surface-assisted photoalignment of discotic liquid crystals (DLCs) on a thin film of an azobenzene-containing polymer.12 It is necessary for the generation of DLC photoalignment to treat the azobenzene-containing polymer film by oblique exposure to nonpolarized light, followed by annealing at 240 °C in order to enhance and stabilize the photoaligned state of azobenzene moieties by dipole-dipole interaction. As mentioned above, irradiation of the photo-cross-linkable polymers with linearly polarized light suppresses the thermodynamically segmental motion of the photoaligned chains through the covalent photocrosslinking reaction. As a result, homogeneous alignment of CLC molecules shows excellent thermal stability.8 Taking account of this advantage, the pVCi and its related polymers seem the suitable photoalignment films for surfaceassisted orientation of the DLC molecules.13 In this paper, we elaborate on the photoalignment of DLC molecules by using thin films of a poly(vinyl cinnamate) and poly(methacrylate)s with cinnamoyl side chains, as shown in Figure 1a, exposed solely to nonpolarized UV light. In this alignment procedure, the annealing treatment of photoaligning films is not necessary to generate the DLC orientation. Oblique illumination of the polymer film with nonpolarized UV light

Furumi and Ichimura

Figure 1. (a) Chemical structures of photo-cross-linkable polymers tethering cinnamoyl groups (top, pM0CinR; middle, pM2CinMe; bottom, pVCi) used as photoaligning films. (b) Chemical structures of a discotic liquid crystal (top, DLC) and a calamitic liquid crystal (bottom, DON-103) used in this study. The DLC consists of a mixture of triphenylene derivatives with the polymerizable R′ units at the terminal positions, corresponding typically to an acrylate group and its derivatives.18,20 The calamitic liquid crystal of DON-103 is a mixture of cyclohexanoic acid 4-alkoxypheny ester derivatives.

ensured inclined alignment of DLC molecules, exhibiting discotic nematic (ND) phase, as a consequence of the threedimensional orientation of the cinnamoyl side chains through their direction-selective photochemical reactions. Interestingly, we found out that the tilted direction of DLCs is strongly dependent on the chemical structures of the cinnamate-containing polymers. Such photoalignment behavior of DLCs can be rationalized by anchoring balance between intermolecular interaction of the DLC molecules with the photodimers of polymer films and those with the remaining E-isomers of cinnamoyl side chains at the interface of the photoaligning thin films. On the basis of the spectral and optical measurement results, we discuss the mechanism of surface-mediated DLC photoalignment on the cinnamate polymer thin films at the molecular level of the film surface. 2. Experimental Section Materials. Figure 1a shows the chemical structures of the photo-cross-linkable polymers used in this study. A poly(vinyl cinnamate) was purchased from Aldrich Chemical Co., Inc. and used after reprecipitation from methanol two times. Four kinds of poly(methacrylate)s with cinnamoyl side chains, pM0CinR (R ) Me, Et, and n-Pr) and pM2CinMe, were prepared by radical polymerization of the corresponding methacrylate monomers of methyl 2-(4-methacryloyloxyphenyl)ethenylcarboxylate (M0CinMe), ethyl 2-(4-methacryloyloxyphenyl)ethenylcarboxylate (M0CinEt), n-propyl 2-(4-methacryloyloxyphenyl)ethenylcarboxylate (M0CinPr), and methyl 2-[4-(2-methacryloyloxyethyl)phenyl]ethenylcarboxylate (M2CinMe).8d Before polymerization, the monomers were characterized by nuclear magnetic resonance (NMR) spectral measurement, recorded on a NMR spectrometer (Bruker, AC-200) with TMS as an internal standard, and elemental analysis. The results are summarized in the following lists.

Photoalignment of Discotic Liquid Crystals Methyl 2-(4-Methacryloyloxyphenyl)ethenylcarboxylate (M0CinMe). mp: 84-85 °C. 1H NMR (200 MHz, CDCl3, TMS) δ (ppm): 2.00 (3H, m, CH2dC-CH3), 3.70 (3H, s, COOCH3), 5.78 (1H, br s, Ha-CdC), 6.36 (1H, br s, Hb-CdC), 6.40/7.67 (2H, d, J ) 16.0, Ar-CHdCH-COO), 7.15 (2H, d, J ) 8.6, Ar-H), 7.55 (2H, d, J ) 8.6, Ar-H). Anal. Calcd. (%): C 68.30, H 5.69. Found: C 68.33, H 5.78. Ethyl 2-(4-Methacryloyloxyphenyl)ethenylcarboxylate (M0CinEt). mp: 61-62 °C. 1H NMR (200 MHz, CDCl3, TMS) δ (ppm): 1.34 (3H, t, J ) 7.1, -CH2-CH3), 2.06 (3H, m, CH2dC-CH3), 4.27 (2H, m, COO-CH2-CH3), 5.78 (1H, br s, Ha-CdC), 6.36 (1H, br s, Hb-CdC), 6.40/7.67 (2H, d, J ) 16.0, Ar-CHdCH-COO), 7.15 (2H, d, J ) 8.6, Ar-H), 7.55 (2H, d, J ) 8.6, Ar-H). Anal. Calcd. (%): C 69.15, H 6.15, O 24.59. Found: C 69.32, H 6.36, O 24.64. n-Propyl 2-(4-Methacryloyloxyphenyl)ethenylcarboxylate (M0CinPr). mp: 51-52 °C. 1H-NMR (200 MHz, CDCl3, TMS) δ (ppm): 1.00 (3H, t, J ) 7.1, -CH2-CH3), 1.72 (2H, m, -CH2-), 2.06 (3H, m, CH2dC-CH3), 4.17 (2H, m, COOCH2-CH2-), 5.78 (1H, br s, Ha-CdC), 6.36 (1H, br s, HbCdC), 6.40/7.67 (2H, d, J ) 16.0, Ar-CHdCH-COO), 7.15 (2H, d, J ) 8.6, Ar-H), 7.55 (2H, d, J ) 8.6, Ar-H). Anal. Calcd. (%): C 70.00, H 6.57, O 23.33. Found: C 69.80, H 6.52, O 23.75. Methyl 2-[4-(2-Methacryloyloxyethyl)phenyl]ethenylcarboxylate (M2CinMe). mp: 85-86 °C. 1H-NMR (200 MHz, CDCl3, TMS) δ (ppm): 1.94 (3H, s, -CH3), 3.72 (3H, s, -OCH3), 3.99 (2H, q, CH2), 4.16 (2H, t, COO-CH2-CH2-), 5.54 (1H, br s, Ha-CdC), 6.09 (1H, br s, Hb-CdC), 6.30/7.64 (2H, d, J ) 16.0, Ar-CHdCH-COO), 6.88 (2H, d, J ) 8.8, Ar-H), 7.46 (2H, d, J ) 8.6, Ar-H). Anal. Calcd. (%): C 66.20, H 6.25. Found: C 65.93, H 6.23. Polymerization. Radical polymerization of the monomers was carried out in 10 wt % solution in the following procedure. The monomer (1.0 g) and azobisisobutyronitrile (AIBN; 10 mg) as a polymerization initiator, which was beforehand recrystalized from methanol, were dissolved in 10 mL of dried benzene. The solution was placed in an ampoule and degassed through repeated freeze and thaw cycles. The sealed ampoule was heated at 60 °C for 10 h. The resultant solution was poured into methanol to separate the crude polymer, which was purified by reprecipitation from methanol for several times. Finally, the polymer was dried in vacuo at room temperature for 6 h or more. The thermal properties of the polymers were analyzed at a heating rate of (10 °C min-1 with a differential scanning calorimeter (DSC; Seiko Electronics, DSC 200). The molecular masses and their distributions were determined by gel permeation chromatography (GPC; Jasco, 880-PU intelligent highpressure liquid chromatography pump, 860-CO column oven, and 807-IT integrator) equipped with a UV detector (Jasco, 875UV) at a flow rate of 1.0 mL min-1 using tetrahydorofuran (THF) as an eluent on the basis of calibration with polystyrene standards. Liquid Crystals. Figure 1b shows the discotic liquid crystal (DLC) and calamitic liquid crystal (CLC) used in this study. The photopolymerizable DLC material, supplied from Fuji Photo Film Co., Ltd, consists of a mixture of triphenylene derivatives possessing cross-linkable moieties at their peripheral positions, abbreviated as the R′ in Figure 1b, with a small amount of photoinitiator. The R′ substituents correspond to typically acrylate group and its derivatives.18,20 By polarized optical microscopic observation of the DLC, discotic nematic phase emerged in the range of temperatures between 115 and 150 °C. The CLC material of DON-103, donated from Rodic Co., LTD,

J. Phys. Chem. B, Vol. 111, No. 6, 2007 1279 consists of mixtures of cyclohexanoic acid 4-alkoxypheny esters in order to compare photoorientational behavior of the DLC. The DON-103 showed the nematic-isotropic phase transition at 74 °C. Sample Preparation. A thin film of the photo-cross-linkable polymers with a thickness of ∼60 nm was obtained on a fusedsilica substrate by spin-coating of their 1.5 wt % solutions in an equivolume mixture of monochlorobenzene and methylene chloride. The film was obliquely exposed to nonpolarized light at 313 nm. The oblique photoirradiation was performed by a 150-W Hg-Xe lamp (San-ei Electric Mfg. Co., UV Supercure230S) through a solution filter of K2CrO4 in 0.1 N aq NaOH and a band-pass filter (Toshiba, UV-D35) to isolate 313-nm light. The light intensity was measured with an optical power meter (Advantest, TQ8210). Subsequently, a 20 wt % solution of the polymerizable DLCs in 4-methyl-2-pentanone, including a photoinitiator in ∼0.5 wt %,18 was spin-coated onto the photoirradiated polymer thin film. After the DLC orientation was carried out by heating at the ND phase, we exposed them to UV light at the ND phase to generate photopolymerization of the DLC molecules. To measure the film thickness, the spincast film of polymer or DLC was scratched, and then, the trench was measured by a mechanical profilometry (Japan Vacuum Technology Co., Dektak 3ST). Physical Measurements. Absorption spectra were taken on a photodiode array spectrometer (Hewlett-Packard, 8452 A) or a scanning spectrophotometer (Hitachi, 320). The polarized absorption spectra of the polymer thin films were taken on the spectrometer by the attachment of a Glan-Thomson prism. Optical textures of the DLCs were observed by a polarized optical microscope (Olympus, BH-2) combined with a high gain color CCD camera (Flovel Co., HCC-600) and a hot stage (Mettler, FP800 thermosystem) upon a heating and cooling process. Optical phase retardation of the films was measured by means of a polarization-modulating transmission ellipsometer (Jasco, BFA-150) equipped with a probing He-Ne laser beam of 633 nm and a photoelastic modulator (PEM) operating at the modulation frequency operating of 50 kHz.12 Spectral Analysis for Photoproduct Distribution. Thin films of pM0CinR, pM2CinMe, and pVCi were obliquely illuminated with nonpolarized 313-nm light and subjected to the polarized absorption spectral measurements at the intervals. The distributions of photoproducts, which correspond to the Eand Z-isomers and photodimer, of cinnamoyl groups in the polymer films during the course of the photoirradiation were calculated by Reiser’s method with a slight modification, as reported in the previous report.8d,e The following equations give us the photoproduct distributions in the polymer films.

fE ) E/(E - Z)[A/A0 - (Z/E)(Aiso/A0iso)]

(1)

fZ ) E/(E - Z)[Aiso/A0iso - (A/A0)]

(2)

fDimer ) 1 - Aiso/A0iso

(3)

A: absorbance at 274 nm for pM0CinR and pVCi and at 306 nm for pM2CinMe; A0: initial absorbance at 274 nm for pM0CinR and pVCi and at 306 nm for pM2CinMe; Aiso: absorbances at the isosbestic points; A0iso: initial absorbances at the isosbestic points; E; molar extinction coefficients of the E-isomer; and Z; molar extinction coefficients of the Z-isomer.

1280 J. Phys. Chem. B, Vol. 111, No. 6, 2007

Furumi and Ichimura

TABLE 1: Spectral Parameters for Photoproduct Distribution Estimation

TABLE 2: Physical Properties of Polymers with Cinnamoyl Side Chains

 at λmax c polymer

λmaxa (nm)

λisob (nm)

E × 10-4

Z × 10-4

pM0CinR pM2CinMe pVCi

274 294 274

252 268 304

2.29 2.29 2.12

0.67 2.90 1.20

a Wavelength at the absorption maximum of E-isomers. b Wavelength at the isosbestic point during the photoisomerization in hexane. c Molar extinction coefficients (dm3 mol-3 cm-1) at λmax of the corresponding E-isomer and Z-isomer, respectively.

Figure 2. (a) Illustrative representation of oblique irradiation of the polymer film with nonpolarized 313-nm light and spectral measurement for evaluation of the azimuthal and spatial orientation of the film. (bd) Polarized absorption spectral changes of pM0CinMe (b), pM2CinMe (c), and pVCi films (d) as a function of exposure energies of oblique irradiation with nonpolarized 313-nm light. The UV exposure energies were 0, 10, 50, 100, 500, 1000, and 5000 mJ cm-2 according to the arrow direction. Solid and dashed curves correspond to spectra monitored by probing the polarized light with the electric vector in parallel with and perpendicular to the incident light plane, respectively. Notice that the spectra are closely overlapped with each other.

The molar extinction coefficients at the maximum absorption wavelengths of pM0CinR, pM2CinMe and pVCi are summarized in Table 1. Polarized Spectral Evaluation of Polymer Films. Polarized absorption spectra of the obliquely exposed polymer films were conducted to obtain the azimuthal (in-plane) orientation, as illustrated in Figure 2a. A1 and A2 are absorbances at the maximum wavelength monitored by polarized probing light with the electric vector in parallel with and perpendicular to the incident plane of the irradiation light, respectively. Here, the differential value of the absorbances (∆A) is defined as the following equation.

∆A ) A1 - A2

(4)

3. Results and Discussion 3.1. Photodichroism of Polymer Films by Oblique Exposure to Nonpolarized UV Light. Four kinds of photocross-linkable homopolymers incorporating with cinnamoyl side chains (pM0CinMe, pM0CinEt, pM0CinPr, and pM2CinMe), as depicted in Figure 1a, were prepared by radical polymerization of the corresponding monomers: methyl 2-(4-methacryloyloxyphenyl)ethenylcarboxylate, ethyl 2-(4methacryloyloxyphenyl)ethenylcarboxylate, n-propyl 2-(4-methacryloyloxy-phenyl)ethenylcarboxylate, and methyl 2-[4-(2-

polymer

MW (×10-4)a)

MW/MNb

Tg (°C)c

yield (%)

pM0CinMe pM0CinEt pM0CinPr pM2CinMe pVCi

5.1 12.8 10.8 5.9 7.2

3.2 3.3 3.2 3.2 2.5

125 96 80 63 95

81 77 70 72 NA

a MW: weight-average molecular weight. b MW/MN: molecular weight distribution. c Tg: glass transition temperature.

methacryloyloxyethyl)phenyl]ethenylcarboxylate, as mentioned in the Experimental Section. The resultant polymers exhibited no liquid crystallinity, meaning that they possessed an amorphous nature. The physical properties are compiled in Table 2. The glass transition temperatures (Tg) of the poly(methacrylate)s decreased as a result of not only the introduction of an ethylene spacer but also the attachment of longer alkyl chains at the terminus of the cinnamoyl ester connected directly with the polymer backbone of the pM0CinR. A thin film of the photopolymers of ∼60 nm in thickness on a fused silica plate, obtained by spin-coating of their 1.5 wt % solutions, was obliquely exposed to nonpolarized 313-nm light at an incidence angle of 45° from the surface normal, as depicted in Figure 2a. Absorption spectral measurements of the polymers revealed that the wavelengths at absorption maxima (λmax) and spectral shape between the solutions and the spin-coated films are similar to each other for all polymers. This result suggests that no aggregation of the cinnamoyl moieties takes place in the polymer matrixes. The λmax of pM2CinMe appeared at 294 nm, whereas the pM0CinMe thin films exhibited an absorption band with λmax at 274 nm. The red-shifted absorption band of pM2CinMe results from the conjugation of the electron-donating alkoxy ether connected with the π-electron delocalized system of the cinnamoyl residue. In the case of pM0CinMe, direct attachment of the cinnamoyl residues to the polymer backbones leads to electron-drawing from the aromatic system of the cinnamoyl group. A thin film of pVCi displayed a similar spectral appearance of pM0CinMe. Figure 2b-d shows polarized absorption spectral changes of the spin-coated films of pM0CinMe (b), pM2CinMe (c), and pVCi (d) upon the oblique UV irradiation. Before irradiation, the polymer films exhibited no optical anisotropy. Oblique irradiation of the polymer films with nonpolarized 313-nm light gave rise to a monotonic decrease in absorbances around the λmax by both E-to-Z photoisomerization and (2 + 2) photodimerization of cinnamoyl groups, as shown in Figure 2b-d. Prolonged UV irradiation of the energies with more than several joules per square centimeter promoted to the photostationary states of the all polymer films with a reduction of nearly 85% with respect to the initial absorbances. As stated above, there were no differences between the solutions and films in the peak positions and the half-widths of absorption band ascribable to the cinnamoyl moieties. This experimental fact enables us to analyze distribution of the photoproducts of both geometrical E- and Z-isomers and photodimers of the cinnamoyl moieties in the polymer films by the slightly modified Reiser’s method,8d,e as stated in the Experimental Section. The results are shown in Figure 3. Distributions of the E-isomer in the all polymer films were immediately reduced at the early stages of photoirradiation, and even after further irradiation with 5.0 J cm-2 doses. As is evident from this figure, the consumption rates of each component were considerably affected by molecular structures of the cinnamate-containing polymers. In the pM0CinMe films, E-to-Z photoisomerization of the cinnamoyl side chains took

Photoalignment of Discotic Liquid Crystals

Figure 3. Photoproduct distribution of E-isomer (open circles), Z-isomers (open triangles), and photodimers (closed circles) of the cinnamoyl groups in pM0CinMe (a), pM2CinMe (b), and pVCi films (c) as a function of exposure energy of UV light at 313 nm.

place preferably, even at the early stage of the exposure of a 0.5 J cm-2 dose (triangles in Figure 3a). Prolonged irradiation of a 5.0 J cm-2 dose gave 60% photodimer and 20% E-isomer, whereas the E-to-Z photoisomerization leveled off. To the contrary, the pM2CinMe and pVCi films preferably produced the dimerized formation of cinnamoyl moieties, even at the early stage, so that their abilities of photoisomerization were highly suppressed, as compared with those of pM0CinMe. The photochemical behavior arises probably from enhancement of the mobility of cinnamoyl side chains introduced by the flexible chains of cinnamic acid esters, such as ethylene spacer for pM2CinMe and polymer backbone for pVCi.8e It was found that absorption spectra taken with polarized light of the electric vector in parallel with the incident plane of the nonpolarized actinic UV light are slightly larger than those perpendicular to the incident plane, even at the exposure energy of a 5.0 J cm-2 dose for all polymer films. The results mean that the azimuthal orientation of the cinnamoyl groups is determined even by oblique exposure to nonpolarized UV light. We measured the changes in difference between A1 and A2 (∆A ) A1 - A2), as expressed in eq 4, as a function of exposure energy. The A1 and A2 stand for the absorbances at λmax monitored by the polarized probe light with the electric vector

J. Phys. Chem. B, Vol. 111, No. 6, 2007 1281 in parallel with (solid curves in Figure 2b-d) and perpendicular to (dashed curves in Figure 2b-d) the incident plane of the irradiated light. The positive value of ∆A value means azimuthal orientation of the cinnamate polymer films is preferentially parallel to the incident plane of the nonpolarized actinic light. The results are shown in the Supporting Information (Figure S1). Although the azimuthal dichroic ratios of ∆A values were quite minute, the emergence of their maxima was observed at an exposure energy of a 0.1 J cm-2 dose, giving ∼50% of the E-isomer for all polymers. This implies that the directionselective photochemical consumption, involving E-to-Z photoisomerization and photodimerization, of the (E)-cinnamoyl moieties occurs predominantly at the early stage of the oblique exposure to nonpolarized 313-nm light, and the successive exposure gave rise to gradual decline of the ∆A values due to the depletion in component ration of (E)-cinnamates. The small ∆A values of pM2CinMe and pVCi may be reflected by the enhancement of segmental motion of the chromophores due to the present of the flexible chains. As discussed in the previous reports,15 the azimuthal optical anisotropy of azobenzene-containing polymer thin films was generated by oblique irradiation with nonpolarized light to minimize the light absorption as a consequence of two photochemical processes. The processes consist of the directionselective photochemical reactions and photoinduced molecular reorientation, similar to the case of photoinduced dichroism by exposure to linearly polarized light.9a,15 Although the former process of (E)-azobenzenes is direction-selective photoisomerization, the (E)-cinnamoyl compounds exhibit two kinds of direction-selective photoreactions of photoisomerization as well as photodimerization. To the contrary, the latter process of molecular reorientation is based on the photoinduced repetition of structural changes of the chromophores due to the reversibility of photochemical transformation, such as photoisomerization. We have so far determined the dichroic ratio (DRNPL) of azobenzene-containing polymer thin films by oblique irradiation with nonpolarized light, defined in the following equation.15

DRNPL ) (A1 - A2)/(A1 + A2)

(5)

In this study, the DRNPL values of the pM0CinR, pM2CinMe, and pVCi films are quite small, when compared with those of the azobenzene-containing polymers. In this way, the low level in the photoinduced dichroic ratio at prolonged irradiation may stem from the contribution of irreversible photochemical process, considering the fact that the photodimerization dose not bring about molecular reorientation. 3.2. Evaluation of Three-Dimensional Orientation of Polymer Films. To monitor the photoinduced polar (out-ofplane) orientation of the cinnamoyl group in the polymer films, absorption spectral measurements were performed at variant incident angles (θm) of the probing light with respect to the surface normal. As depicted in Figure 2a, the θm is defined as an angle between the propagation direction of probing light and the surface normal. Here, we take account of both the refractive index (n) and the optical path length of the polymer thin film to correct the observed absorbances (Absob) at θm on the basis of Snell’s law, that is, nair sin θm ) nfilm sin θr, where nair and nfilm stand for the refractive indexes of air and films, respectively. The incident and refractive angles of the probing light are denoted as θm and θr. Accordingly, the Abs is described in the following equation,

Abs ) Absob × (nfilm2 - sin2 θm)1/2/nfilm

(6)

1282 J. Phys. Chem. B, Vol. 111, No. 6, 2007

Furumi and Ichimura

Figure 5. (Top) Structures of dimerized model compounds of pM0CinMe, pM2CinMe, and pVCi. (Bottom) Calculated absorption spectra of dimerized model compounds of pM0CinMe (open bars), pM2CinMe (shaded bars) and pVCi (solid bars) in the wavelength range between 270 and 300 nm.

Figure 4. Changes in Abs of pM0CinMe (a), pM2CinMe (b), and pVCi film (c) as a function of incident angle (θm) of monitoring light. Oblique irradiation with nonpolarized UV light was performed at an incident angle of θm ) +45° from the surface normal, as illustrated in Figure 2a. The Abs profiles were measured before (i) and after the oblique photoirradiation of 0.5 (ii) and 10 J cm-2 doses (iii).

where Abs stands for the corrected absorbance. The out-of-plane orientation of the polymer films was assessed by angular dependence of the Abs value as a function of the incident angle of the probing light.15c Figure 4 shows the changes in the corrected absorbance at 274 nm, that is, λmax, for pM0CinMe and pVCi and at 280 nm for pM2CinMe as a function of incident angles (θm) of probing light. Before UV irradiation, the all polymer films exhibited symmetric curves of the Abs with respect to θm ) 0°, as shown in Figure 4a-i, 4b-i, and 4c-i. The minimum θm angle of Abs was shifted by the oblique irradiation with exposure doses of 0.5 J cm-2, providing 40% or less of the E-isomer. Importantly, the shifted direction of Abs was essentially dependent on the intrinsic chemical structures of the photopolymers. The Abs minima of photoirradiated pM0CinMe and pM2CinMe films were slightly shifted to θm < 0° (Figure 4a-ii and 4b-ii). In contrast, the pVCi film displayed the shift direction of Abs toward θm > 0° by oblique irradiation with nonpolarized UV light of a 0.5 J cm-2 dose (Figure 4c-ii). The photoorientation behavior of pVCi is similar to that of azobenzene-containing polymer films upon oblique exposure,15 anticipating that the remaining (E)-cinnamoyl groups of pVCi are inclined to the nonpolarized light propagation. After prolonged irradiation of a 10 J cm-2 dose, the absorption band was remarkably reduced to ∼0.06. Note that asymmetric profiles with minimum Abs

values can be observed for pM0CinMe, pM2CinMe, and pVCi films, even though the Abs values are so tiny. The results indicate decisively that the orientational director of polymer films tilts toward the opposite direction with respect to the nonpolarized light propagation for pM0CinMe and pM2CinMe film and the parallel direction for pVCi film. Electronic absorption spectra of the dimerized model compounds of pM0CinMe, pM2CinMe, and pVCi were simulated by the semiempirical molecular orbital calculation (MOPAC) to elucidate the difference of the tilting direction between the pVCi and the methacrylates with cinnamoyl side chains, that is, pM0CinR and pM2CinMe by oblique UV irradiation. Figure 5 shows simulated absorption spectra of their dimerized model compounds in a wavelength range from 270 to 300 nm. It is obvious that both model compounds of pM0CinMe and pM2CinMe have strong absorption bands around 280 nm, when compared with the model compound of pVCi. In particular, this result is supported by an experimental absorption spectrum of the pM2CinMe film irradiated with 313-nm light of a huge exposure dose, as can been seen in Figure 2c. From the experimental spectrum, we can observe a slight absorption band of the UV-irradiated pM2CinMe film around 280 nm. It is plausible that the photodimers of the cinnamate-containing polymers, especially pM2CinMe, contribute to the inclined Abs profile. Therefore, oblique irradiation of pM0CinR and pM2CinMe films with nonpolarized UV light gives rise to their tilted orientation opposite that of the nonpolarized light propagation, as presented in Figure 4a and b. In contrast, the photodimers of the pVCi dissociate from contribution to the optical anisotropy around λmax so that the remaining E-isomers might be inclined toward the nonpolarized actinic light propagation, as shown in Figure 4c. The three-dimensional ordering of the cinnamoyl moieties in the pM0CinMe, pM2CinMe, and pVCi films is induced by oblique nonpolarized photoirradiation according to the following

Photoalignment of Discotic Liquid Crystals

Figure 6. Illustrative representation of direction-selective photochemical reactions of E-isomer of cinnamoyl side chains attached to polymers. Black and red ellipses represent the photoreacted and remaining (E)cinnamoyl groups of polymers.

mechanism. The (E)-cinnamoyl moieties harvest preferably all photons coming from a direction perpendicular to the molecular axis (black ellipses in Figure 6), whereas the cinnamates cannot absorb at all when the light propagation comes in parallel with the molecular axis (red ellipse in Figure 6). This situation results in the direction-selective photochemical reactions in the film. The E-to-Z photoisomerization and (2 + 2) photodimerization of the light-harvested cinnamates take place so as to consume thoroughly the E-isomers. On the other hand, the (E)-cinnamoyl moieties located along the light propagation remain without the photochemical reactions. As a result, the tilted Abs profile of pM0CinMe and pM2CinMe films stems mostly from their photodimers induced by prolonged exposure to obliquely nonpolarized light, whereas the remaining (E)-cinnamoyl moieties of pVCi generate the photoinduced Abs tilted profile of the film.16 3.3. Orientation of Discotic Liquid Crystals on UVIrradiated Polymer Films. After the photo-cross-linkable polymer film was obliquely irradiated with nonpolarized UV light of a 10 J cm-2 dose in advance, a 3.5 wt % solution of discotic liquid crystal material in 4-methyl-2-pentanone was spin-coated onto the photoirradiated polymer thin films to form a DLC thin film of ∼100 nm in thickness. The exposure energy of the polymer film at 10 J cm-2 resulted in a sufficient reduction of absorbance around 270 nm attributed cinnamoyl moieties, as shown in Figure 2b-d, to avoid overlapping of the absorbances of the polymers with that of the DLC molecules. The DLC used in this study is a mixture of triphenylene derivatives possessing cross-linkable moieties at their peripheral positions, as shown in the top of Figure 1b. The material exhibited a discotic nematic (ND) phase between 115 and 150 °C, as determined by polarized microscope observation.

J. Phys. Chem. B, Vol. 111, No. 6, 2007 1283

Figure 7. (a) UV absorption spectra of a DLC film aligned on a pVCi film before (solid) and after annealing at 130 °C of ND phase for 10 min and subsequent photopolymerization of the aligned DLC molecules by UV-irradiation (dash). The pVCi film was irradiated beforehand with nonpolarized UV light of a 10 J cm-2 dose at an incident angle of 45°. The spectra of DLC films before and after alignment were taken at room temperature. (b-d) Changes in Abs at 268 nm of the 100-nm DLC films on photoirradiated pM0CinMe (b), pM2CinMe (c) and pVCi films (d) as a function of incident angle (θm) of the probing light. The Abs profiles were measured before (open circles) and after annealing (solid circles).

Figure 7a shows typical changes of absorption spectra of the DLC thin film before and after annealing at 130 °C, corresponding to the ND phase temperature, for 10 min. The annealing treatment of the DLC film gave rise to the considerable increment of π-π* absorbances assigned to the triphenylene core of the DLCs as a result of orientational changes of the DLC molecules induced by the annealing treatment at the ND phase. To obtain further insight into the three-dimensional orientation of DLC molecules, absorption spectral analysis was performed by rotating the probing angle, taking into account the fact that absorbances of the cinnamate are negligibly small by the photoirradiation of a large exposure dose. Figure 7b-d presents the changes in corrected absorbance [Abs ) Absob × (nfilm2 sin2 θm)1/2/nfilm], as described in the eq 6, at 268 nm as a function of incident angle (θm) of probing light. This procedure is the same as in Figure 4. The Abs profile showed symmetry with respect to θm ) 0° before annealing for all DLC films, indicating that no orientational ordering of the DLCs occurs in the polar angle. Subsequently, annealing treatment of the DLC film at the ND phase gave rise to the asymmetrical Abs profiles with respect to θm ) 0°. As shown in the closed circles of Figure 7b and c, the photoirradiated thin films of the pM0CinMe and pM2CinMe brought about the inclined orientation of the DLC molecules toward the light propagation owing to the orientational director of the DLCs lying perpendicular to the plane of the triphenylene core. The tilted DLC alignment along the light propagation contributes to the direction-selective formation of photodimers in the pM0CinR and pM2CinMe films, as sup-

1284 J. Phys. Chem. B, Vol. 111, No. 6, 2007 ported by the results in Figure 4a-iii and b-iii. As shown in the Supporting Information (Figure S2), the pM0CinMe film showed the steepest slope in Abs profiles of pM0CinR (R ) Me, Et, and n-Pr), suggesting that the high pretilt angle17 of the DLC is generated by the pM0CinMe film. The pM0CinMe and pM2CinMe displayed maximum Abs values at θm ) 30° and 40° so that the average pretilt angles of the DLC induced were calculated to be ∼70° and 65°, respectively. The average pretilt angles of the DLCs on the pM0CinEt and pM0CinPr films could not be estimated, because no maximum Abs value appeared. In the case of pVCi film, the tilted DLC orientation showed the direction opposite to that of the nonpolarized UV light propagation, with an average tilt angle of 65°, as shown in the closed circles of Figure 7d. This inclined DLC direction is similar to that on obliquely exposed azobenzene polymer film.12 It is plausible that the DLC molecules are tilted according to the orientational direction of the remaining (E)-cinnamoyl moieties induced by the oblique photoirradiation of a pVCi film with nonpolarized UV light. This is because the photoaligned (E)-azobenzene moieties generate the tilted orientation of DLC molecules near the surface of azobenzene-containing polymer film. 3.4. Optical Evaluation of Photoinduced Orientation of Discotic Liquid Crystals. The orientation behavior of DLC molecules was studied in more detail by means of polarized microscope observation and optical birefringence measurements of the thicker DLC film. The DLC films with ∼1.0-µm thickness were placed on pM0CinR, pM2CinMe, and pVCi films, which were obliquely exposed to nonpolarized UV light of a 10 J cm-2 dose in advance. The DLC films were prepared by spin-coating of a 20 wt % DLC solution in 4-methyl-2-pentanone. Polarized microscope observation of the DLC spin-coated film under the crossed-Nicols confirmed an initial polycrystalline texture at room-temperature owing to its crystalline phase. Upon heating, the optical texture of the DLC film changed at 110 °C from the polycrystalline structure to Schlieren texture corresponding to the ND phase, followed by the gradual disappearance of many brushing defects within 10 min to give eventually an oriented texture at 130 °C. Subsequently, UV-exposure of the aligned DLC films for 3 min at ND phase provided stable DLC films at room temperature. We found they had excellent thermostability under ambient conditions over 6 months as a result of hte photopolymerization of peripheral polymerizable units of the DLCs, as shown in Figure 1b.18 In addition, the aligned DLC films would show excellent photostability due to no intrinsic light absorption band of the polymers as well as the DLCs above 330 nm, as shown in Figure 7. Interestingly, we confirmed that the optical quality of the oriented DLC films under the crossed-Nicol is significantly influenced by the molecular structures of the cinnamatecontaining polymers. Figure 8 shows the polarized optical microphotographs of the DLC films at 130 °C. A typical monodomain structure with an excellent optical quality could be obtained by the use of photoirradiated pM0CinR and pM2CinMe films (Figure 8a). In contrast, several brushing defects of the DLC film still remained by using obliquely exposed pVCi film (Figure 8b), probably because of a weak anchoring strength of the tilted (E)-cinnamoyl groups with respect to the DLC molecules. Such DLC alignment behavior exhibited reproducibility. It is worth noting that the anchoring strength to the DLC is entirely different from the poly(methacrylate)s with a cinnamoyl side chain (pM0CinR and pM2CinMe) and poly(vinyl cinnamate).

Furumi and Ichimura

Figure 8. Polarized optical microphotographs of the DLC films with 1.0 µm on photoaligned pM0CinMe (a) and pVCi films (b) under crossed Nicols, observed at 130 °C. Obliquely irradiated films of pM0CinEt, pM0CinPr, and pM2CinMe films provided monodomain optical textures similar to that of pM0CinMe, as shown in part a.

To obtain further insight into the three-dimensional alignment of DLC molecules on the cinnamate-containing polymer films, birefringence measurement of the DLC film as a function of incident angles (φ) of a probing He-Ne laser beam was carried out by a polarization-modulating transmission ellipsometer. This system gives a convenient optical phase difference, meaning a difference between the phase velocities of vertically and horizontally polarized light (s- and p-polarized light).12 Figure 9 presents the optical phase differences of photooriented DLC films on pM0CinMe (a), pM2CinMe (b), and pVCi thin films (c) irradiated obliquely with variant exposure doses. Homeotropic orientation with several defects of the DLC was generated on the photoirradiated thin films of the cinnamate-containing polymers when exposure energies were 0.01 J cm-2 or less for pM0CinR, 0.05 J cm-2 for pM2CinMe, and 0.1 J cm-2 for the pVCi film. Taking account of the fact that the orientational director of the DLC molecule is perpendicular to the π-extended plane of the discotic core, the birefringence measurements revealed that the oblique irradiation with nonpolarized UV light using a 0.05 J cm-2 dose or more for pM0CinMe and 0.25 J cm-2 for pM2CinMe resulted in the tilted orientation of the DLCs in parallel with the actinic light propagation, as illustrated in Figure 10a. Interestingly, this is in sharp contrast to the results obtained for pVCi films irradiated with nonpolarized light of a 0.5 J cm-2 dose or more, which gave rise to the inclined DLC alignment opposite to the light propagation, as shown in Figure 10b. This behavior is supported by the spectral measurements shown in Figure 7b-d, as mentioned above. Prolonged illumination with nonpolarized UV light did not change the inclined birefringence of the photoaligned DLC film for all polymers. These results imply that the inclined alignment of the DLC molecules is generated on the photoirradiated thin films of cinnamate-containing polymers, even though the photoinduced optical anisotropy of the polymer films is negligibly small, as given from Figure 2. 3.5. Pretilt Angles of Discotic Liquid Crystals. The orientational directors of DLCs are continuously altered from the polymer/DLC film interface to the uppermost free surface of the DLC film so as to minimize the elastic free energy of the outermost surface of the DLC spin-coated film, as represented in Figure 10c.12,14,18,19 The DLC molecules are anchored with a pretilt angle of R on the pVCi film surface, whereas the pretilt angle β at the interface between air and the DLC film is changed. The angles can be estimated by means of the theoretical optical simulation.19 As shown in Figure 10c, the angles of R and β stand for the angles between the substrate surface and the DLC plane at the interfaces of the aligning layer/DLC and the DLC/ air, respectively. The pretilt angles on various polymer films

Photoalignment of Discotic Liquid Crystals

J. Phys. Chem. B, Vol. 111, No. 6, 2007 1285

Figure 10. (a, b) Illustrative representation of the inclined alignment of DLC molecules, denoted as disks, induced by photo-cross-linkable polymer films obliquely irradiated with nonpolarized UV light. The incident light propagation is denoted as a broad arrow. (c) Schematic illustration of the tilted hybrid orientation of the DLCs on the films. The angle of R indicates a pretilt angle of the aligned DLCs near the alignment layer from substrate plane, whereas β is a pretilt angle of the aligned DLCs near the air interface.

TABLE 3: Discotic Liquid Crystal Photoalignment by Cinnamate Polymer Thin Films tilt angle polymer pM0CinMe pM0CinEt pM0CinPr pM2CinMe pVCi

exposure energies for inclined DLC alignment cm-2

0.05 J 0.05 J cm-2 0.05 J cm-2 0.25 J cm-2 0.5 J cm-2

optical quality of DLC films

Ra

βb

uniform uniform uniform uniform several defects

5° 15° 40° 1° -2°

50° 75° 80° 50° -50°

a R indicates a pretilt angle of the aligned DLCs near the alignment layer from substrate plane, as depicted in Figure 10c. b β indicates a pretilt angle of the aligned DLCs near the air interface from substrate plane, as depicted in Figure 10c.

Figure 9. Optical phase differences of DLC films on photoaligned pM0CinMe (a), pM2CinMe (b), and pVCi films (c) as a function of incident angle of a He-Ne laser beam. Oblique exposure of the cinnamate-containing polymer films was performed at an incident angle of 45°. The exposure energies were 0.01 (open circles), 0.05 (solid triangles), 0.1 (solid circles), and 1.0 J cm-2 doses (solid squares) for hte pM0CinMe film; 0.05 (open circles), 0.25 (solid triangles), 0.5 (solid circles), and 1.0 J cm-2 doses (solid squares) for the pM2CinMe film; 0.025 (open circles), 0.05 (open triangles), 0.1 (open squares), 0.5 (solid circles), 1.0 (solid triangles), 5.0 (solid squares), and 10 J cm-2 doses (solid diamonds) for the pVCi film.

are compiled in Table 3. Both R and β angles by pM0CinMe, pM2CinMe, and pVCi films did not make much difference. Note that pM0CinPr films yields extremely high pretilt angles of DLC molecules at both interfaces, that is, R and β, as compared with those induced by the pM0CinMe, pM2CinMe, and pVCi films. The optical birefringence results of the DLC/pM0CinPr film are shown in the Supporting Information (Figure S3). No optical axis appeared in the optical birefringence profiles of the aligned DLC/pM0CinPr film, indicating that the pM0CinPr film induces a high pretilt angle of the DLC molecules by obliquely nonpolarized UV exposure. The pretilt angles of DLC were not affected by the incident angles of oblique UV exposure. Accordingly, the surface-assisted DLC photoalignment is strongly attributable to the chemical structures of photoaligning polymer films. To inspect the three-dimensional orientation reversion of DLC molecules, we measured the pretilt angles of a nematic, calamitic liquid crystal. A vacant cell was composed of two polymer thin films, which were obliquely irradiated with nonpolarized light

of a 3.0 J cm-2 dose before the CLC cell assemblage in an antiparallel manner, then a nematic CLC of DON-103, as shown in Figure 1b, was filled in the vacant cell via the capillary force at its isotropic state. Polarized optical microscopic observation revealed that the CLCs show homogeneous alignment, with several disclination lines of reverse tilting for all LC cells, suggesting that the pretilt angles of DON-103 are of a low order of several degrees or less. The disclination lines of the CLCs emerged probably due to a weak azimuthal anchoring strength of the nonpolarized irradiated polymer films with respect to the DON-103. Subsequently, the pretilt angles of nematic CLCs were evaluated by the crystal rotation method.15a,d,e Although a pretilt angle of +0.5° was observed for the LC cell fabricated with pVCi films, the UV-irradiated films of pM0CinMe, pM0CinEt, pM0CinPr, and pM2CinMe generated -1.8°, -2.3°, -3.0°, and -0.4°. The CLCs by the pVCi films were inclined toward the light propagation of the nonpolarized UV light, indicating decisively that the remaining (E)-cinnamoyl groups control three-dimensional alignment of not only the CLC but also the DLC molecules. This result shows a contrast to the observation using pM0CinR and pM2CinMe that the CLC molecules are tilted opposite to the nonpolarized UV light propagation as a result of direction-selective formation of the photodimers. Taking into account the overall results, the difference of the inclined direction of the DLC molecules can be interpreted in the following mechanism. When the pVCi film was obliquely exposed to nonpolarized UV light, the remaining (E)-cinnamoyl moieties were inclined toward the light propagation through their direction-selective photochemical reactions. As a result, the tilted direction of DLC on the pVCi film was opposite to the propagation of actinic nonpolarized light at oblique incidence, as depicted in Figure 10b, and was consistent with that on the

1286 J. Phys. Chem. B, Vol. 111, No. 6, 2007 azobenzene-containing polymer film.12 Considering the previous results of surface-assisted photoalignment that there is no contribution of the (Z)-cinnamate to the LC orientation,9 the inclined DLC orientation was generated not by tilted orientation of the photodimers but by that of the remaining (E)-cinnamoyl moieties orienting along with the light propagation. Photodimerization of the cinnamoyl side chains plays significant roles in enhancing both the thermostability of orientation of remaining (E)-cinnamoyl residues required for the annealing of a DLC film with a high mesophase temperature and thorough insolubility of the polymer film to 4-methyl-2-pentanone as the DLC spin-coating solvent. These situations led to the inclined alignment of the DLC molecules with several defects, as seen in Figure 8b, owing to quite a low content of the E-isomer in the pVCi film obliquely irradiated with nonpolarized UV light of a 0.5 J cm-2 dose generating DLC alignment. On the other hand, it is plausible that the inclined DLC alignment showing clear birefringence without any defects is caused by photodimers of the cinnamoyl groups of the pM0CinR and pM2CinMe films, because the tilted direction orients preferentially toward the nonpolarized light propagation, as illustrated in Figure 10a. The inclined orientation of the photooriented dimers opposite to the light propagation has been reported by N. Kawatsuki and co-workers, who used thin films of liquid crystalline poly(methacrylate)s with both cinnamoyl moieties and calamitic biphenyl mesogens in the side chains.16a According to the literature, annealing treatment at smectic phase of the polymer thin film, which was beforehand irradiated with nonpolarized UV light at an oblique incidence, brought about the inclined birefringence as a consequence of coplanar alignment of the calamitic mesogens, so it should be noticed that three-dimensional orientation reversion of DLC molecules is attributable to the anchoring balance between intermolecular interaction of the DLC molecules with the photodimers of polymer films and those with the remaining E-isomers of cinnamoyl side chains at the interface molecular level. 4. Conclusions In this report, we have developed the surface-assisted photoalignment of discotic liquid crystals (DLCs) on thin films of photo-cross-linkable polymers tethering cinnamoyl moieties in the side chains. Oblique illumination of thin films of the poly(methacrylate)s incorporated with the cinnamoyl side chains (pM0CinR and pM2CinMe) with nonpolarized UV light gave rise to inclined orientation of the photodimers of cinnamoyl moieties opposite to the light propagation through their directionselective photochemical reactions. On the other hand, a thin film of poly(vinyl cinnamate) brought about tilting optical anisotropy of the remaining (E)-cinnamoyl side chains in parallel with the actinic nonpolarized light propagation. The DLCs on the photoirradiated polymer film were aligned in a tilted hybrid manner. Unexpectedly, we found out that the inclined direction of the photooriented DLC molecules is distinctly influenced by the molecular structure of the cinnamate-containing polymers because of the participation of both remaining E-isomers and photodimers of the cinnamoyl side chains in the DLC photoalignment. The three-dimensional orientation reversion is comprehensible in terms of the anchoring balance between intermolecular interaction of the DLC molecules with the photoinduced dimers of the polymers and those with the remaining E-isomer of the cinnamoyl side chains. Furthermore, we investigated that the pretilt angles of the DLCs at both aligning film/DLC and DLC/air interfaces are strongly governed by the alkyl chains at the terminus of the cinnamoyl ester directly attached to the poly(methacrylate)s (pM0CinR).

Furumi and Ichimura In this way, the present DLC photoalignment strategy provides a novel guideline to comprehend the orientation mechanism of DLCs at a molecular level, leading not only to the understanding anisotropic physical properties of DLCs but also to the fabrication of unique molecular devices for nextgeneration photonics and electronics.20,21 Our DLC-aligned films by the photo-cross-linkable polymer films possess intrinsic physical properties, such as optical transparency in the visible wavelength range (Figure 7), tilted hybrid orientation of negative birefringence molecules (Figure 9), and thermally stable orientation of photopolymerized DLC molecules. Therefore, it is suitable for optical compensatory sheets for twisted nematic LC displays, which widen remarkably our viewing angles.18,19,22 Acknowledgment. This study was carried out entirely at the Chemical Resources Laboratory in Tokyo Institute of Technology (T.I.T.). S.F. thanks the support of Research Fellowships of Japan Society for the Promotion of Science (JSPS) for Young Scientists during his Ph.D studies at the T.I.T. Supporting Information Available: Supporting Information includes the changes in the azimuthal dichroic ratio of polymer thin film by oblique exposure to nonpolarized UV ligh; the results of polar angle orientation of the pM0CinMe, pM0CinEt, and pM0CinPr films by oblique exposure to nonpolarized UV light; and the optical birefringence of the DLC film on the photoaligned pM0CinPr film as a function of the probing incident angle. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Oster, G.; Yang, N.-L. Chem. ReV. 1968, 68, 125. (2) Hasegawa, M. Chem. ReV. 1983, 83, 507. (3) (a) Cohen, M. D.; Schmidt, G. M. J. J. Chem. Soc. 1964, 1969. (b) Schmidt, G. M. J. Pure Appl. Chem. 1971, 27, 647. (4) (a) Koelsch, C. F.; Gumprecht, W. H. J. Org. Chem. 1958, 23, 1603. (b) Hasegawa, M.; Suzuki, Y. J. Polym. Sci., Part B 1967, 5, 813. (5) (a) Minsk, L. M. U.S. Patent 2 725 377, 1955. (b) Minsk, L. M.; Smith, J. G.; Van Deusen, W. P.; Wright, J. F. J. Appl. Polym. Sci. 1959, 2, 302. (c) Reiser, A. In PhotoreactiVe Polymers: The Science and Technology of Resists; Wily-Interscience: New York, 1989; p 1. (6) (a) Barachensky, V. A. Proc. SPIE 1991, 1559, 184. (b) Barachensky, V. A. J. Photopolym. Sci. Technol. 1991, 4, 177. (7) (a) Gibbons, W. M.; Shannon, P. J.; Sun, S.-T.; Swetlin, B. J. Nature 1991, 351, 49. (b) Kawanishi, Y.; Tamaki, T.; Sakuragi, M.; Seki, T.; Suzuki, Y.; Ichimura, K. Langmuir 1992, 8, 2601. (c) Aoki, K.; Seki, T.; Suzuki, Y.; Tamaki, T.; Hosoki, A.; Ichimura, K. Langmuir 1992, 8, 1007. (d) Seki, T.; Sakuragi, M.; Kawanishi, Y.; Suzuki, Y.; Tamaki, T.; Fukuda, R.; Ichimura, K. Langmuir 1993, 9, 211. (e) Ichimura, K.; Hayashi, Y.; Akiyama, H.; Ikeda, T.; Ishizuki, N. Appl. Phys. Lett. 1993, 63, 449. (f) Akiyama, H.; Kudo, K.; Ichimura, K.; Yokoyama, S.; Kakimoto, M.; Imai, Y. Langmuir 1995, 11, 1033. (g) Furumi, S.; Akiyama, H.; Morino, S.; Ichimura, K. J. Mater. Chem. 1998, 8, 65. (8) (a) Schadt, M.; Schmitt, K.; Kozenkov, V.; Chigrinov, V. Jpn. J. Appl. Phys., Part 1 1992, 31, 2155. (b) Marusii, T. Y.; Reznikov, Y. A. Mol. Mater. 1993, 3, 161. (c) Kawatsuki, N.; Takatsuka, H.; Yamamoto, T.; Sangen, O. Macromol. Rapid Commun. 1996, 17, 703. (d) Ichimura, K.; Akita, Y.; Akiyama, H.; Kudo, K.; Hayashi, Y. Macromolecules 1997, 30, 903. (e) Obi, M.; Morino, S.; Ichimura, K. Jpn. J. Appl. Phys., Part 2 1999, 38, L145. (f) Obi, M.; Morino, S.; Ichimura, K. Chem. Mater. 1999, 11, 1293. (g) Furumi, S.; Nakagawa, M.; Morino, S.; Ichimura, K. Polym. AdV. Technol. 2000, 11, 427. (h) Yamaki, S.; Nakagawa, M.; Morino, S.; Ichimura, K. Macromol. Chem. Phys. 2001, 202, 325. (i) Kawatsuki, N.; Takatsuka, H.; Yamamoto, T. Jpn. J. Appl. Phys. 2001, 40, L209. (j) Furumi, S.; Ichimura, K. Appl. Phys. Lett. 2005, 85, 224. (k) Kawatsuki, N.; Tachibana, T.; An, M.-X.; Kato, K. Macromolecules 2005, 38, 3903. (9) (a) Ichimura, K. Chem. ReV. 2000, 100, 1847. (b) O’Neil, M.; Kelly, S. M. J. Phys. D 2000, 33, R67. (c) Ichimura, K. In Reflexible Polymers and Hydrogels: Understanding and Designing Fast-responsiVe Polymeric Systems; Yui, N., Mrsny, R. J., Park, K., Eds.; CRC Press: London, 2004; p 283. (10) (a) Iimura, Y.; Saitoh, T.; Kobayashi, S.; Hashimoto, T. J. Photopolym. Sci. Technol. 1995, 8, 257. (b) Schadt, M.; Seiberle, H.; Schuster, A. Nature 1996, 381, 212.

Photoalignment of Discotic Liquid Crystals (11) (a) Yoshida, H.; Koike, Y. Jpn. J. Appl. Phys., Part 2 1997, 36, L428. (b) Seo, D.-S.; Hwang, L. Y.; Kobayashi, S. Liq. Cryst. 1997, 23, 923. (c) Furumi, S.; Nakagawa, M.; Morino, S.; Ichimura, K.; Ogasawara, H. Appl. Phys. Lett. 1999, 74, 2438. (d) Park, B.; Han, K.-J.; Jung, Y.; Choi, H.-H.; Hwang, H.-K.; Lee, S.; Jang, S.-H.; Takezoe, H. J. Appl. Phys. 1999, 86, 1854. (e) Kawatsuki, N.; Takatsuka, H.; Yamamoto, T. Jpn. J. Appl. Phys. 2000, 39, L230. (f) Jain, S. C.; Tanwar, V. K.; Dixit, V. Jpn. J. Appl. Phys. 2002, 41, L1106. (g) Furumi, S.; Ichimura, K. AdV. Funct. Mater. 2004, 14, 247. (12) (a) Ichimura, K.; Furumi, S.; Morino, S.; Kidowaki, M.; Nakagawa, M.; Ogawa, M.; Nishiura, Y. AdV. Mater. 2000, 12, 950. (b) Furumi, S.; Kidowaki, M.; Ogawa, M.; Nishiura, Y.; Ichimura, K. J. Phys. Chem. B 2005, 109, 9245. (13) (a) Chandrasekhar, S. In Handbook of Liquid Crystals; Demus, D., Goodby, J., Gray, G. W., Spiess, H.-W., Vill, V., Eds.; Wiley-VCH: Weinheim, 1988; Vol. 2B, p 749. (b) Kumar, S. Chem. Soc. ReV. 2006, 35, 83. (c) Kumar, S. Liq. Cryst. 2005, 32, 1089. (d) Kumar S. Liq. Cryst. 2004, 31, 1037. (14) The preliminary results concerning DLC photoalignment by a pVCi film have been previously reported. See: Furumi, S.; Ichimura, K.; Sata, H.; Nishiura, Y. Appl. Phys. Lett. 2000, 77, 2689. (15) (a) Ichimura, K.; Morino, S.; Akiyama, H. Appl. Phys. Lett. 1998, 73, 921. (b) Wu, Y.; Ikeda, T.; Zhang, Q. AdV. Mater. 1999, 11, 300. (c) Ichimura, K.; Han, M.; Morino, S. Chem. Lett. 1999, 85. (d) Furumi, S.; Nakagawa, M.; Morino, S.; Ichimura, K. J. Mater. Chem. 2000, 10, 833. (e) Furumi, S.; Ichimura, K. Thin Solid Films 2006, 499, 322. (16) (a) Kawatsuki, N.; Takatsuka, H.; Yamamoto, T. Appl. Phys. Lett. 1999, 75, 1386. (b) Kawatsuki, N.; Yamamoto, T.; Ono, H. Polym. J. 1999,

J. Phys. Chem. B, Vol. 111, No. 6, 2007 1287 31, 630. (c) Kawatsuki, N.; Kawakami, T.; Yamamoto, T. AdV. Mater. 2001, 13, 1337. (17) The pretilt angle of DLCs is defined as an angle between the orientational director, lying perpendicular to the plane of DLC core, and the substrate plane. (18) Okazaki, M.; Kawata, K.; Nishikawa, H.; Negoro, M. Polym. AdV. Technol. 2000, 11, 398. (19) Mori, H.; Ito, Y.; Nishiura, Y.; Shinagawa, Y. Jpn. J. Appl. Phys., Part 1 1997, 36, 143. (20) (a) Adam, D.; Schuhmacher, P.; Simmerer, J.; Ha¨ussling, L.; Siemensmeyer, K.; Etzbach, K.; Ringsdorf, H.; Haarer, D. Nature 1994, 371, 141. (b) Liu, C.-Y.; Pan, H.-L.; Fox, M. A.; Bard, A. J. Science 1993, 261, 897. (c) Van de Craats, A. M.; Warman, J. M.; Fechtenko¨tter, A.; Brand, J. D.; Harbison, M. A.; Mu¨llen, K. AdV. Mater. 1999, 11, 1117. (d) Monobe, H.; Awazu, K.; Shimizu, Y. AdV. Mater. 2000, 12, 1495. (e) Schmidt-Mende, L.; Fechtenko¨tter, A.; Mu¨llen, K.; Moons, E.; Friend, R. H.; Mackenzie, J. D. Science 2001, 293, 1119. (f) Monobe, H.; Kiyohara, K.; Terasawa, N.; Heya, M.; Awazu, K.; Shimizu, Y. AdV. Funct. Mater. 2003, 13, 919. (g) Tracz, A.; Jeszka, J. K.; Watson, M. D.; Pisula, W.; Mu¨llen, K.; Pakula, T. J. Am. Chem. Soc. 2003, 125, 1682. (h) Pisula, W.; Menon, A.; Stepputat, M.; Lieberwirth, I.; Kolb, U.; Tracz, A.; Sirringhaus, H.; Pakula, T.; Mu¨llen, K. AdV. Mater. 2005, 17, 684. (21) (a) Furumi, S.; Janietz, D.; Kidowaki, M.; Nakagawa, M.; Morino, S.; Stumpe, J.; Ichimura, K. Chem. Mater. 2001, 13, 1434. (b) Furumi, S.; Janietz, D.; Kidowaki, M.; Nakagawa, M.; Morino, S.; Stumpe, J.; Ichimura, K. Mol. Cryst. Liq. Cryst. 2001, 368, 4285. (22) Kawata, K. Chem. Rec. 2002, 2, 59.