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Langmuir 1992,8,2601-2604
Photochemical Induction and Modulation of Nematic Homogeneous Alignment by the Polarization Photochromism of Surface Azobenzenest Yuji Kawanishi,' Takashi Tamaki, Masako Sakuragi, Takahiro Seki, and Yasuzo Suzuki Research Institute for Polymers and Textiles, 1-1-4 Higashi, Tsukuba, Ibaraki 305, Japan
Kunihiro Ichimura Research Laboratory of Resources Utilization, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 227, Japan Received October 15,1991. In Final Form: September 15, 1992 The alignment of nematicliquid crystals (LC)can be regulated by controlling the photochromicreaction of azobenzene (Az)chromophoresbeing attached to the LC substrate surface. Respondingto the trans-cis photohomerhation of Az, the LC alignment switches between the perpendicular state and the parallel state with respect to the substrate surface. As demonstrated, the system exhibits another significant feature, novel in-plane rearrangement of the parallel state induced by exposure to the linearly polarized W light, to attain formation of a homogeneous domain with a specific orientation axis. The induced orientation axis was found to be perpendicular to the electric polarization of the incident UV light. Thus, the orientation axis was rotated intentionallyby changing the incident polarization plane. The induced anisotropy of the homogeneous domain was quite high, providing a dichroic ratio of more than 6. Present in-plane regulation of the nematic orientation was explained in terms of the polarization-selective photochrommm of surface Az.
Introduction Controlling structures of organized molecular systems by the effect of light is a substantial scientific matter of which many photochemists have been dreaming. Liquid crystals (LC) are attractive media from this point of view, since they possess fluid, flexible, and highly-ordered structures.l-12 It has been known that excitation by the linearlypolarized light often lea& a photoreactivematerial to an organized structure with characteristic optical properties1+l7 mostly due to anisotropy in the photoabsorption event. The most recent example was presented by Gibbons et al. reporting modulation of the nematic homogeneous alignment by polarized laser excitation to the LC cell consisting of two rubbed polyimide films, one of which
* To whom correspondence should be addressed.
+ Reversible Alignment Change of Liquid Crystals Induced by Photochromic Molecular Filme. 8. Part 7: see ref 27. (1)Otruba,J. P.,V.; Weisa, R. G. J. Org. Chem. 1983,48,3448. (2)Otruba, J. P.,V.; Web, R. G. Mol. Cryst. Liq. Cryst. 1982,80,165. (3)Ganapathy,S.;Zimmermann, R.; Weisa, R. J. Org. Chem. 1986,51, 2629. (4) Seki,T.; Ichimura, K. J. Colloid Interface Sci. 1989,129,353. (6)Umaneki, P.;Kryweweki, M. Acta Polym. 1988,39,613. (6)Sackmann, E. J. Am. Chem. SOC.1971,93,7088. (7)Haae, W. E.;Nelson, K. F.; Adams,J. E.;Dir,G. A. J. Electrochem. SOC.1974,121,1667. (8) h a , K.; Hirabayashi,H.;Uejima,A.;Nakamura,K.Jpn. J . Appl. Phys. 1982,21,969. (9)Tazuke, S.;K u r k a , S.;Ikeda, T. Chem. Lett. 1987,911. (10)Ikeda, T.; Horiuchi, S.;Karanjit, D.B.; Kurihara, S.;Tazuke, S. Chem. Lett. 1988,1679. (11) Ikeda,T.; Horiuchi, S.;Karanjit, D. B.; Kurihara, S.; Tazuke, S. Macromolecules 1990,23,42. (12)Suzuki,Y.;Ozawa, K.; Hosoki, A.;Ichimura, K. Polym. Bull. 1987, 17,285. (13)Jones,P.;Jon-, W. J.; Williams,G. J . Chem. Soc.,Faraday Trans. 1990.86.1013. - - - -,- -,- - - -. (14)Jonen, P.;Darcy, P.; Attard, G.; Jones, W. J.; Williams, G. Mol. Phys. 1989,67,1053. (15)Tredgold, R. H.;Allen, R. A.; Hodge, P.; Khoehdel, E. J. Phys. D Appl. Phys. 1987,20,1386. (16)Tcdorov,T.; Nikolova, L.; Tomova, N. Appl. Opt. 1984,23,4309. (17)&ami, C.;Nakaawa, K.; Fujiwara, H. Jpn. J. Appl. Phys. 1990, 29,LlW-.
was doped with a d i d a m i n e dye.18 Although the system offers an enormous change of optical anisotropy,there are still questions about how the photochemical processes of a doped dye overcome or change the qualified aligning force of the rubbed polyimide surface, and whether there is any thermal assistance from nonradiative decay of the dye or from high-energy laser light in the rearrangement of the polyimide bulk structure. Anderle et al. also reported laser-induced realignment of a LC polymer bearing azobenzene(Az)moieties and possibleapplication to holographic re~ordings.~g~20 The phenomenon might involvethermally assisted processes by local heating. They explained the anisotropy induction in terms of thermal trans isomerization accompanied by 90' reverse cis rotation of the molecular axis to the original trans-Az. As we reported previously, totally photochemical regulation of nematic LC alignment has been achieved by the photochromic trans-cis isomerization of surface-attached Az chromophores.21-n LC orientation is switched reversibly between two states, the homeotropic state (H, optical axis of the LC phase is normal to the surface) and parallel state (P, the axis lies down on the surface) by altemate exposure to ultraviolet (UV)and visible light. The system utilizes an inherent property of the LC phase, reflecting
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(18)Gibbons, W. M.;Shannon, P. J.; Sun,S.-T.;Swetlin, B. J. Nature 1991,351,49. (19)Anderle, K.; Birenheide, R.; Eich, M.; Wendorff, J. Makromol. Chem., Rapid Commun. 1989,10,477. (20)Anderle, K.; Birenheide, R.; Werner, M. J. A.; Wendorff, J. H. Liq. Cryst. 1991,9,691. (21)Ichimura.. K.: . Suzuki,. Y.:. Seki, T.;Hoeoki, A.; Aoki. K. Langmuir 1988,4, 1214. (22)Ichimura, K.;Suzuki, Y.; Seki, T.; Kawanishi, Y.; Tamaki, T.; Aoki, K. Makromol. Chem., Rapid Commun. 1989,10,5. (23)Seki, T.; Tamaki, T.; Suzuki, Y.; Kawanishi, Y.; Ichimura, K.; Aoki, K. Macromolecules 1989,22,3606. (24)Ichimura, K.; Suzuki, Y.; Seki, T.; Kawanishi, Y.; Tamaki, T.; Aoki, K. Jpn. J. Appl. Phys., Suppl. 1989,28,289. (25)Aoki, K.; Ichimura, K.; Tamaki, T.; Seki, T.; Kawaniehi; Y. Kobunshi Ronbunshu 1990,47,771. (26)Kawanishi,Y.;Seki,T.;Tamaki,T.;Ichimura,K.;Ikeda,M.;Aoki, K. Polym. Adv. Technol. 1991,1, 311. (27)Kawaniehi, Y.;Tamaki, T.; Seki, T.; Sakuragi, M.; Suzuki, Y.; Ichimura, K. Langmuir 1991,7,1314.
0743-7463/92/2408-2601$03.00/0 0 1992 American Chemical Society
2602 Langmuir, Vol. 8, No.11, 1992
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Figure 1. Introduction of the azobenzene chromophore onto a quartz surface by the Michael reaction. 0
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LCD118 Figure2. Chemicalstructures of nematic DON103 and dichroic LCDllS used in this study.
surfaceinformationon the mesoscopiclevel. Polarization excitationis not an exception to change ourface information and gives an excellent homogeneous LC alignment as mentioned in our preliminary paper.28 This is our first report for this fascinating feature of the LC/Az surface system providingabsolutelyphotochemicalinduction and modulation of the LC homogeneous alignment axis with preciseness.
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Experimental Section Clean glaas plates (30 X 30 mm) were treated with gaseous (aminopropy1)triethoxyailane (1 mmHg) a t 450 K for 30 min. The glasa plates were dipped into the 1.0 w t 96 solution of p-(acryloy1oxy)-p’-methoxyazobenzenein chloroform, air-dried, and heated for 1 h at 373 K to bring about Michael addition (Figure 1). Two kinds of photoresponsiveLC systems were then fabricated by placing a nematicmixture of p,p’-substituted phenyl cyclohexanecarboxylate derivatives (Rodic DON103, K-290-N344-1, A€< 0) with 8-pm spacers between (i) two Az glass plates (Az/LC/Az normal cell) or (ii) Az glass and octadecylsilylated glasa (Az/LC/ODS hybrid cell). In order to find the right alignment axis, guest-host (GH) cells were prepared by doping the LC phase with 1.0 w t 96 dichroic dye LCDllS (Nippon Kayaku). Chemical structures of DON103 and LCD118 are shown in Figure 2. As a reaction light source, a conventional 500-Whigh-pressureHg lamp was used with a Glan-ThompsonTaylor prism polarizer, and two seta of optical filters to obtain linearly polarized UV (365 nm) and visible (440 nm) light, respectively.
Results and Discussion Figure 3 shom the polarizing micrographs of photoinduced texture changes in the Az/LC/Az normal cell. On preparation of the cell, the LC alignmentwas H, exhibiting a dark image (a). Ordinary (not polarized) UV exposure resulted in an inhomogeneous multidomain texture, suggesting low orientationalorder of the P alignment (b), and subsequentvisible exposure reproduced the H alignment. When we applied linearly polarized UV light instead, a (28)Kawaniehi, Y.; Suzuki, Y.; Tamaki, T.; Ichimura, K. Abetract of the aymptmium on Photochemical Processes in Organized Molecular Syeteme, Yokohama, 1990;p 65.
Figure 3. Polarizing micrographs of the photoresponsive LC cell: (a) on preparation or after visible light exposure; (b) after nonpolarized UV exposure; (c) after linearly polarized UV exposure. The bar represents 100 pm.
drastic effectappear&, i.e., a highly homogeneous P phase was developed within a few minutes (c). The homogeneous P alignment was stable even after terminating the UV exposure. Visible light exposure reproduced the H alignment again. Anisotropyof the photoinduced homogeneousphase was studied for the dichroic dye doped Az/LC/Az GH cell. Configurationsof the polarized exposure experiments are illustrated in Figure 4. We define L as the tentative axis of the LC cell, 8 as the polarization angle between L and the electric polarization of the reaction light PR,and 6 as the rotation angle between L and the electric polarization of the monitor light. Parts a-d of Figure 5 show transmittance surfaces of the LC cell at 633 nm, each one of
Langgmuir, Vol. 8, No. 11,1992 2603
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Figure 4. Configurations of the polarized UV exposure (a)and the cell anisotropy measurement (b): A, the induced orientation axis; L,the tentative axis of the cell; PR,electric polarization of the reection UV light; PM,electric polarization of the monitoring light; 4, rotation angle of the cell; 8, polarization angle of the reaction UV liiht.
a)
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Figure 6. Polarizing transmittance surfaces of the homogeneously aligned Az/LC/Az normal cell at 633nm after the linearly polarized UV exposure with various polarization angles 0. Outer and inner circles represent 75% and 50% transmittance, respectively. A' is the hypothetical orientationaxis expected from the cross-relation to 8. 8 = ' 0 (a),-15' (b), -30' (c),and 40' (d). some complication arose (Figure 6). The cross-relation was not retained anymore; Le., the induced axis A was not exactly perpendicular to the polarization of the UV light but was rotated irregularly. The differences between two systems,the normal cell and the GH cell, may be explicable by considering the difference in the photoisomerization yield between two Az surfaces, the front Az surface at the intake of the reaction light and the rear surface at the outlet of that. It has been found that the H P alignment change takes place more efficiently above a critical yield of trans cis isomerization of surface Az.29 In the GH cell, the reaction yield of the rear Az surface is not high enough to give the alignment change due to the inner filter effect of the doped dichroic dye. Thus, the front Az surface can principally control the H P alignment change and the macroscopic orientation axis as well. In the normal cell,both the Az surfacescan absorbalmost the same amount of light enerp, enough to induce the H P alignment change. However, each surface provides an individualorientationaxis because the liiht polarization is not the same on two Az surfaces. Deviation of the macroscopic orientation axis from the expected position implies a twisted structure of the nematic phase, but no simple relations between A and PRhave been found in the normal cell. By replacing the second Az surface with a photoinert ODS glass, the cross-relation between A and PR was again obtained as shown in Figure 7. The present homogeneous axis modulation is supposed to originate from two individual effects of the Az surface acquired by a polarization-selectivephotochromicreaction (polarization photochromi~m).~3?~~ One is an inherent ability of the cis-Azsurfacewhich tends to givethe parallel LC alignment,and the other is the surfaceanisotropywhich establishes the homogeneous axis into the parallel alignment. According to semiempirical LCAO-SCF-CI calculations,3O either the lowest T-U* transition vector of transAz or that of cis-Az is almost parallel to their -N=N-
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.,. I
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Figure 5. Transmittancesurfaces of the homogeneously aligned Az/LC/Az GH cell at 633 nm after the linearly polarized UV exposure with various polarization angles 8. Angles along the outer circle indicate the cell rotation angle 4. Outer and inner circles represent 75% and 50% transmittance, respectively. 8 = -30' (a),-60' (b), -90' (c), and -120O (d).
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which is obtained after repeating the alignment change; Le., H P H P, by exposure to the nonpolarized visible light and the polarized UV light with a specific 8. Two important results were readily obtained. First, the transmittance of the cell is a function of the rotation angle 4 with ?r of periodicity, corresponding to a uniaxially oriented structure of the P alignment. Second, the transmittance is also the function of the W polarization angle 8, and takes minimum and maximal values at 4 = B f [(2n + l)r/23 and 4 = 8 f n?r ( n = integral numbers), respectively. These clearly indicate that the orientation axis (A) of the homogeneous P domain is induced perpendicularlyto the electric polarizationof the incident W light, which can be rotated intentionally in the way to hold the cross-relationto 8. Such intentional rotation of the orientation axis should be understood only by a reversible nature of the surface photochromic reaction. When the LC phase did not contain the dichroic dye,
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(29) Kawanishi, Y.; et al. Unpublished results. (30) Beveridge, D. L.; Jaffe, H. H. J. Am. Chem. SOC.1966,88,1948.
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2604 Lagmuir, Vol. 8, No. 11,1992
of Az molecules in terms of the -N=N- axis should become perpendicular to the incident polarization plane due to the polarizationphotochromism. It is not clear at present which isomer is the dominant in homogeneousorientation induction. However, as alreadypointed out previously,26n LC molecules tend to align their long axes parallel to those of surface trans-Az units, but cis-Az is rather known to destroy an ordered structure of the LC phase.*11 It should be noted that induced anisotropyof the present photochromicLC system is significantlyhigh (the dichroic ratio is more than 6) compared with that of polymer-based systems.15-17Jg~20 The advantage of the LC phase is its self-organizing structure which can amplify the surface anisotropy.
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1
180'
Figure 7. Polarizing transmittance surfaces of the homogeneously aligned Az/LC/ODS hybrid cell at 633 nm after the linearly polarized UV exposure with various polarization angles 8. Outer and inner circles represent 75% and 50% transmittance, respectively. 8 = -3OO (a), doo (b), -90" (c), and -120O (d).
bond axes. On the polarized UV exposure corresponding to the lowest r-x* transitions of both isomers, the surface becomes cis-Az rich, and besides, the average alignment
Concluding Remarks The present system provides absolute photochemical control of the LC alignment. The cell is essentiallyhybrid and apparently exhibits a high anisotropy with the photorotatable alignment axis. The application of the system will be wide and not only limited to an aligning technique in the existent LC device manufactured, but future optical devices like erasable direct read-and-write photomemories, displays, spatial light modulators in opticalcomputing systems, image processing,etc. Further experiments are in progress to elucidate the dominant molecular interaction in homogeneous orientation induction.
