Langmuir 1995,11, 2341-2343
2341
Surface-AssistedPhotolithography To Form Anisotropic Dye Layers as a New Horizon of Command Surfaces’ Kunihiro Ichimura,” Masayuki Momose, Kazuaki Kudo, and Haruhisa Akiyama Research Laboratory of Resources Utilization, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226, Japan
Norio Ishizuki Chemical Research Laboratory, Nippon Kayaku Co., Ltd., Shimo-cho, Kita-ku, Tokyo 115, Yokohama 115, Japan Received February 13, 1995. I n Final Form: May 1, 1995@ A novel method to produce multiaxial polarizers has been developed. A thin film of poly(4methacryloyloxyazobenzene) was coated on a substrate plate and irradiated with linearly polarized light to induce the reorientation of the azo chromophore. An aqueous solution of a dye was spread on the film by the spin-coating technique to form a dichroic dye layer. The direction of the electric vector of the polarized light for the photoisomerization determined the orientation direction of dye molecules.
Introduction Regulation of uniaxially aligned orientation of organic molecules is a fundamental technique to offer various elements and devices with optical anisotropy and has been carried out by physical procedures including electric field poling, shear alignment, mechanical stretching, and surface treatment. An alternative way to regulate molecular orientation is based on photochemical reactions. Exposure offilms of polymers substituted with azobenzene units to linearly polarized light (LPL) gives rise to a directed reorientation of the azobenzene chromophores as a result of the repeated photoisomerization.l A novel way to amplify the photoinduced arrangement of photochromic moieties has been achieved by covering substrate surfaces with a polymeric film dissolving an azo dye2 or with a monolayer of photoisomerizable molecule^^,^ to trigger the reorientation of nematic liquid crystal molecules upon LPL irradiation to lead to a drastic alteration of birefringence. The insight into this type of the photocontrol of liquid crystals by the command surface effect has led us to attempt to anisotropic orientation of organic dye molecules on an LPL-irradiated polymer film, assuming that the orientational order is governed by a photoinduced alignment of photoactive units localized on the surface to give rise to anisotropically “epitaxial growth” which has been reported to occur on a stretched polymer thin film.5 The function of polarizers is to divide natural unpolarized light into two polarized components with an electric vector perpendicular to each other and to convert into polarized light by absorption or dispersion of one of the components. Conventional polarizers invented by Land about 70 years ago have been manufactured by mechanical
* To whom correspondence should be addressed. ‘Command surfaces. 11. Part 10: Akiyama, H.; Kudo, K.; Ichimura, K. Macromol. Chem., Rapid Commun. 1995,16, 35. Abstract published in Advance A C S Abstracts, J u n e 1, 1995. (1)Fischer, T.; Lasker, L.; Stumpe, J.;Kostromin, S. G. J.Photochem. Photobiol. A: Chem. 1994,80,453, and references cited therein. (2) Gibbons, W. M.; Shannon, P. J.; Sun, S.-T.;Swetlin, B. J. Nature 1991,351, 49. (3)Kawanishi, Y.;Tamaki, T.; Sakuragi, M.; Seki, T.; Suzuki, Y.; Ichimura, K. Langmuir 1992,8, 2601. (4)Ichimura, K.;Hayashi, Y.; Akiyama, H.; Ishizuki, N. Langmuir 1993,9,3298. ( 5 ) Wittman, J. C.; Smith, P. Nature 1991,352,414.
stretching of a Polvmeric sheet doDed with dichroic moleculeswhich dispiay axially se1ecti;e light absorption.6 Here we report a novel method to produce dichroic polarizers by the action of a command surface. A thin film of an azobenzene pendent polymer is exposed to linearly polarized light to result in molecular reorientation of azobenzene groups, followed by coating a dye solution to cause the surface-assisted emergence of optical anisotropy of a dye layer. This is of a practical significance to fabricate optical elements with polarized multiaxes by means of a photolithographic technique.
Results and Discussion Our previous work revealed that a thin film of poly(4-methacryloyloxyazobenzene) gives rise to a uniaxial alignment of nematic liquid crystals when the film is exposed to LPL for the photoisomerization prior to a cell assembly to result in the photoinduced dichroism of the azobenzene chromophore^.^ It followed that a homopolymer as well as copolymers with methyl methacrylate were employed to attempt the anisotropic crystallization on a polymeric film. A toluene solution of a polymer with azobenzene side groups was spin-coated on a glass substrate to yield a thin film which was subsequently illuminated with linearly polarized light to wavelengths longer than ca. 430 nm to bring about the photodichroism of the azobenzene moieties in the film. Since the photoinduced optical anisotropy ofthe azo chromophore faded away in contact with organic solvents, an aqueous solution ofa water-soluble azo dye (C.I. Direct Blue 67)was selected and spin-coated on an LPL-irradiated film. A surfactant was added in the solution to enhance the solubility of the dye and the wettability ofthe polymeric film to a n aqueous solution. Figure 1 shows the surface-assisted emergence of photodichroism of a dye layer placed on LPL-irradiated films of polymers having different copolymerizationratios. The dichroism of the dye layer was checked by measuring the transmittance of polarized light of the absorption maximum wavelength of the dye a t 574 nm as a function ~
( 6 ) Shurcliff, W. In Polarized Light: Production and Use; Harvard University Press: Cambridge, MA, 1962. ( 7 )Ichimura, K.;Akiyama, H.; Ishizuki, N.; Kawanishi, Y. Macromol. Chem., Rapid Commun. 1993,14,813.
0743-746319512411-2341$09.0010 0 1995 American Chemical Society
Letters
2342 Langmuir, Vol. 11, No. 7, 1995
270" Figure 1. Angular dependence of the absorbance at 574 nm of a dye layer placed on LPL-irradiatedfilms of polymers having different copolymerizationratios. The copolymerizationratio of 4-methacroyloxyazobenzene to methyl methacrylate was 1:0 (0),1:1.2(A),and 1:8.1(0).The arrow indicates the direction of the electric vector of the actinic light.
Relative humidity / 96 Figure2. Dependenceof the contrastratio on relativehumidity under which a thin film of an azobenzene-pendenthomopolymer coated on a glass plate was exposed to linearly polarized light, followed by spin-coating of a dye solution. of the rotational angle of the plate. An optical anisotropy is evidently brought about. The results show clearly that the preferential orientation of the azo-dye molecules is parallel to the electric vector of polarized actinic light. The homopolymer gave the best result. Microscopic observation has revealed that a photoinduced dichroic layer is composed of many dye domains of a few micrometers in size with different optical axis. The distribution of the orientational direction of each domain was analyzed by means of a polarized microscope. Preliminary results indicated that the macroscopic dichroism arises from the averaged anisotropy of a number of domains with variant orientational direction. The photodichroism of a dye layer was found to be influenced decisively by various factors including the coating solvent, environmentalconditions, heat treatment of a film of an azobenzene-pendentpolymer before LPL irradiation, and so forth. For further discussion, the photodichroism of a dye layer was evaluated by measuring polarized absorbances of the dye at A,, = 574 nm to calculate a contrast ratio (CR) which is defined by
where AI and All stand for the polarized absorbances perpendicular and parallel to a polarization plane of the actinic light, and A1 =AI and A2 =All when All > A l while A1 = All and A2 = A1 when All < Al, respectively. Figure 2 showsrepresentative results carried out under a variety of relative humidity. Optimal CR values were obtained in a humidity not far from 60%.
Figure 3. Microphotographs ( x 40) displaying the photodichroic inversion of a dye layer. A 3 w t % aqueous solution of the dye was spin-coated on a film of the azo homopolymer (a) immediately after the polymer film formation and (b)after the heat treatment of the film at 100 "C for 10 min, followed by spin-coatingof a dye solution to form dye layers. Pictures on the right side and the left side were taken through a polarizer with the polarization plane perpendicular and parallel to the electric vector of actinic light, respectively.
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visible light (1) sheet ----azo-polymer glass plate
----
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-
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n
dye layer
Figure 4. Principle of photolithographic image formation of a multiaxial dichroic dye layer. The whole area of a thin film of poly(4-methacryloyloxyazobenzene)spin-coated on a glass plate was exposed (a) to >430 nm light through a polarizer sheet and (b) subsequently with the same light through a photomask after rotating the polarization plane 90". (c) A n aqueous solution of the dye was spin-coated on the photoirradiated film. Photographs show a polarized pattern of a dye layer visualized by a polarized sheet with a polarizationplane angle (1)parallel and (2)perpendicular to the electric vector of actinic light for the second irradiation.
The coating solvent plays also a crucial role. A thin film of the homopolymer coated from a tetrahydrofuran solution resulted in no surface-assisted photodichroism of the dye whereas xylene with a relatively higher boiling point gave a slightly better result than toluene did. These facts led us to the assumption that the residual solvent in polymeric films may enhance the photoinduced azimuthal reorientation of the azobenzene chromophores in a polymeric matrix. In order to confirm this assumption, a coated film was heated a t 100 "C slightly higher than Tg= 92 "C of the polymer to remove a residual solvent. The subsequent irradiation of the annealed film with LPL has led us to a queer observation. As shown in Figure 3, the orientational direction of the dye layer became unexpectedly perpendicular to the electric vector of the
Langmuir, Vol. 11, No. 7, 1995 2343
Letters actinic light. This photodichroic “inversion” was reproducible and independent of the nature of the solvent. We have no reasonable explanation for this surface phenomenon a t present. The dye with a n ionic structure is highly hydrophilic so that there is essentially no affinity of dye molecules to polymer chains. It is very likely that the formation of a photodichroic dye layer is triggered by anisotropic molecular interactions of dye molecules with photoaligned azobenzene residues on a topmost polymeric layer. The present phenomenon is closely related with the previous observation that the dichroism of a dye layer is induced by coating a dye solution on a substrate plate which is rubbed in one direction in a d v a n ~ e . ~It, ~has been suggested that a n anisotropic dye layer on a uniaxially rubbed substrate plate is formed through a lyotropic liquid crystalline phase.1° However, there has been no distinct confirmation of the existence of a lyotropic mesophase on rubbed substrate surfaces. Although the alignment mechanism of the dye is still speculative a t the present stage, the optical anisotropy of dye layers is governed exclusively by the orientational nature of a n uppermost surface. This is in line with the fact that the extent of the dichroism is affected crucially by environmental conditions ( 8 ) Zocher, H.; Coper, K. 2.Phys. Chem. 1928,132,295. (9)Anderson, S.J . Opt. SOC.Am. 1949,39,49. (10)Dreyer, J. F. J . Phys. Colloid Chem. 1948,52,808.
since a surface layer of polymeric films is dynamically responsive to environment to result in the inversion of residual groups linked to a polymeric backbone. The photodichroic “inversion” mentioned above is evidently a n extreme case of this dynamic surface phenomenon. Under optimized conditions, latent photoimages, which are visualized by a polarized sheet, are readily formed by LPL irradiation. Figure 4 shows a photolithographic formation of anisotropic images. Because the latent images can be overwritten readily by changing the electric vector of polarized actinic light, complicated polarized patterns are readily available by means of a photolithographic technique. Such polarized images themselves can be used as photomasks to duplicate them. The surface-assisted anisotropic alignment of dye molecules disclosed in this work provides a promising way to produce unique types of optically anisotropic elements which are applicable not only to conventional polarizer sheets but also to multiaxial polarizers and even threedimensional display elements.
Acknowledgment. This work was supported partly by a Grand-in-Aid for Priority-Area-Research on “Photoreaction Dynamics” from the Ministry of Education, Science and Culture. LA950107D
