Polymers for Microelectronics - American Chemical Society

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Chapter 31

Photoregulation of Liquid-Crystalline Orientation by Anisotropic Photochromism of Surface Azobenzenes 1

1

2

Yuji Kawanishi , Takashi Tamaki , and Kunihiro Ichimura

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1

Agency of Industrial Science and Technology, National Institute of Materials and Chemical Research, 1-1 Higashi, Tsukuba, Ibaraki 305, Japan Research Laboratory of Resources Utilization, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 227, Japan 2

Liquid crystals (LC) are fluid with highly ordered molecular orientation. Because of their responsiveness in orientation as well as in optical properties to an applied electric field, LCs have been materialized in production of thin displays driven by small batteries. The L C orientation is also influenced by bringing other molecules into the system, i.e., dopants and substrate surfaces. This makes special orientation in marketed L C displays possible such as twisted nematic, super twisted nematic, surface stabilized ferroelectric, and dye doped guest-host systems, etc. Any mechanisms modifying the physicochemical nature of molecules on the surface will be available to control the L C orientation. Introduction of photochemistry is particularly interesting since it enables us to acquire high density and fast accessible optical memories as well as new sights on molecular interactions in the L C phase. Here, photochemical approaches to regulate the L C orientation are briefly reviewed. Afterwards, our new findings on precise 3D control of the L C orientation by anisotropic surface photochromism will be introduced. INDUCED PHASE TRANSITION BY PHOTOCHROMIC REACTIONS OF DOPED MOLECULES Photochemical transformation of molecules with a reversible nature is termed a photochromic reaction (1, 2) or photochromism as represented by photoisomerization of azobenzene (Az) derivatives (1). Direct application to optical memories based on a spectral change in photochromism seems relatively difficult, since reading light always induces the reverse photoreaction with an efficiency as long as the light is absorbed by the photoisomer. 0097-6156/94/0537-0453$06.00/0 © 1994 American Chemical Society

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N=N N=N

UV

(i)

visible

cw-Az

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trans-Az Combining photochromism with L C makes photochemical switching of molecular orientation possible, and the event can be monitored by the light not causing either any photochemistry of photochromic systems or any damage of records. Since Sackmann (3) demonstrated elongation of helical pitch in a cholesteric mixture induced by photochromic trans—* cis isomerization of Az dopant, analogous systems consisting of L C and photochromic dopants have been proposed with a view to applications in optical memories (4-8). Most of those are based on the photoinduced phase transition (PPT) from a mesophase to the isotropic phase. PPT is believed to be caused by a tremendous structural change in molecular shape of the photochromic dopant (Figure 1). Although one could write images in a L C bulk by this fashion, the records easily disappear due to diffusion of dopant and flow of the L C bulk. PPT of polymer LCs (9, 10) affords long-term storage more than one month if kept below the glass transition temperature, while heating up may be required to write as fast as low-mass LCs. Long-term storage without sacrificing response properties has been demonstrated by applying the polymer dispersed L C film (PDLC) (11). PDLC is a polyvinylalcohol film in which microcapsules of a mixture of cyanobiphenyl L C analogues and 4-butyl-4'-methoxy-Az are embedded. Optical resolving power of the PDLC corresponds to the diameter of the L C capsule, that is about 1-2 /im. INDUCED ALIGNMENT CHANGE BY PHOTOCHROMIC REACTIONS OF SUBSTRATE SURFACE More sophisticated control of the L C orientation has been provided by photochromic transformation of surface attached molecules (12-15). When a nematic L C is placed on the substrate surface modified with a thin layer of Az molecules, the alignment (orientation-direction) of the L C can be changed reversibly between two states, homeotropic state (H, the optical axis of the L C phase is normal to the surface) and parallel state (P, the axis lies down on the surface), responding to the photochromism of the Az skeleton as shown in Figure 2. The phenomenon is not involved in PPT, but should be classified as a new kind of orientation change, photoinduced alignment change (PAC). Photochemical efficiency of PAC, in which a couple of surface Az molecules is supposed to command approximately 10 L C molecules, is much higher than that of PPT systems requiring high concentration (> 10wt%, typically) of a 4

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UV

visible Figure 1. Photoinduced phase transition of a nematic L C by the photochromic isomerization of the doped Az. Homeotropic Alignment

Parallel Alignment

WooVo

LC Layer

oooooooo

0

0

Azobenzene Layer Glass Substrate

trans-Az

cis-Az

y or P when < y . In fact, cis-Az surface tends to give higher y than trans-Az surface so that the H -> P alignment change seems to be explicable. However, quantitative calculation by the F C K rule did not explain all the experimental results. Deviation may be caused by the use of macroscopic values of y. Spectroscopic investigation suggests that more specific molecular interaction must be concerned to interpret the phenomena. Spectral changes of surface trans-Az moieties can be useful indices of molecular interaction at the surface. Their absorption peaks locating around ca. ~ 350nm and ca. ~ 240nm are IT - TT* transitions, whose transition dipoles are parallel and perpendicular to the Az long axis, respectively (18). The absorption spectrum of Az on the quartz plate is quite similar to that in solution suggesting no specific alignment of Az on the substrate in average. It has been noticed that the H alignment of L C is never obtained right after cell preparation but requires some period to be formed from the initial P alignment. There may be a reorientation process after the L C attaches to the Az surface. As expected, the spectrum changed drastically after the Az surface was coated with a thin nematic L C layer (Figure 3). The absorption of the transition perpendicular to the molecular axis became more emphasized, which means the perpendicular alignment of the surface trans-Az was induced after interacting with a nematic phase. The effective interaction will probably be brought about by the rod-like shape of trans-Az skeleton. If so, cis-Az moiety with a bend structure and a higher y has handicap to induce the H alignment in the L C phase. Importance of the interaction between the Az moiety and the L C molecule has been pointed out in PAC by Az pendent methacrylic polymers (19-21). When the pendent moieties are close together and separated far from the main chain, a considerable amount of aggregates of trans-Az is formed in the polymer film even at the room temperature. The L C phase does not take the H alignment but the P on such a polymer surface, probably due to difficulty of insertion into paired trans-Az moieties. Photoirradiation to the polymer film causes isomerization of Az and affects the equilibrium on aggregation. Three interchangeable states, A , B and C, are obtained, separately. s

L C

s

s

s

s

aggregated trans-Az * (A) m

M

e

* monomelic trans-Az * ^ * cis-Az (B) () t

v m b l e

c

Thompson et al.; Polymers for Microelectronics ACS Symposium Series; American Chemical Society: Washington, DC, 1993.

(2)

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As responding to the reaction (2), the L C cell exhibits three discrete textures of the nematic L C phase (marbled parallel (PI) on A, H for B, and then schlieren parallel (P2) for C, Figure 4). The result clearly indicates that the H alignment is highly sensitive to the amount of the monomelic trans-Pa. on the surface. To promote the H alignment, highly interactive monomeric trans-Pa. should be required.

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ORIENTATION C O N T R O L OF L C M O L E C U L E S B Y SURFACE ANISOTROPIC PHOTOCHROMISM The natural texture of the induced P alignment in PAC is marbled or schlieren, suggesting lack of macroscopic orientation axis in the plane. Very recently, we have found that a highly homogeneous P mono-domain can be induced by applying linearly polarized U V reaction light (22). We have also found that slantwise U V exposure is effective for formation of a large P monodomain even the light is not polarized (Figure 5) (25). Induction, rotation, and erasure of the uniaxially oriented P alignment are possible by changing light characteristics. The effects are explained in terms of the surface anisotropy brought about by the photoselective isomerization of Az moieties. Experimental Photoresponsive L C cells were fabricated by sandwiching a nematic mixture (DON103, K-290-N-344-I, Rodic) between a pair of Az modified glass plates ( A z / L C / A z normal cell), or between a Az glass plate and a octadecylsilylated glass plate ( A z / L C / O D S hybrid cell) with 8 /xm spacers. For investigation of the orientation axis, guest-host (GH) cells containing 1.0 wt% of a dichroic dye (LCD118, Nippon-Kayaku) were used. Structures of chemicals and surface Az are shown in figure 6. A 500W high-pressure Hg arc lamp was used for stationary irradiation experiments (Figure 7a) with optical filters to obtain U V (365nm) and visible (440nm) light, and with a Glan-Thompson-Taylor prism polarizer for linearly polarized exposure. A pulse laser system shown in Figure 7b was used to analyse L C relaxation profiles. Induction and Rotation of Homogeneous Orientation by Linearly Polarized UV Light On preparation of the cell, the L C alignment was homeotropic. Ordinary U V exposure resulted parallel multi-domains with no unique macroscopic orientation axis, and subsequent visible exposure reproduced the homeotropic alignment. When the reaction U V light was linearly polarized, a large domain of homogeneous P alignment was obtained. The orientation was stable unless the visible light was applied. Results of angular dependent transmittance for the G H cell clearly indicate that the induced orientation axis is perpendicular to the electric polarization of the reaction U V light, and rotatable correlating with the light polarization (Figure 8). Alignment relaxation was analyzed by means of laser pulse experiments. The time constants are 110 msec for the H -> P (random) change by the unpolarized U V excitation, 90msec for the H -* P (homogeneous) change by Thompson et al.; Polymers for Microelectronics ACS Symposium Series; American Chemical Society: Washington, DC, 1993.

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0.02

0.00 200

300

500

400

Wavelength(nm)

Figure 3. Absorption specta of the surface A z ( C H A z O C H CONHC H Si(CH ) 0-glass) before ( ) and after (•••) contact to a nematic LC. Transition vectors corresponding to two absorption peaks are perpendicular (~ 240 nm) and parallel (~ 350 nm) to the molecular long axis of Az as indicated. 6

3

6

3

1 3

1 0

2 0

2

Figure 4. Textures of a nematic L C (DON103) and corresponding absorption spectra of the Az polymer — [ C C H ( C O O C H O A z C H ) — C H ] — film. 3

n

22

6

13

2

Thompson et al.; Polymers for Microelectronics ACS Symposium Series; American Chemical Society: Washington, DC, 1993.

n

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Figure 5. Photochemical induction of the uniaxially oriented parallel L C phase by the linearly polarized U V exposure (i) and the slantwise U V exposure (ii). L: tentative axis of the L C cell, 0: polarization angle between the incident electric polarization (P ) and L in (i), or incident angle between the projection of the light path (I) and L in (ii). A: induced orientation axis. R

Figure 6. Chemical structures and abbreviations.

Thompson et al.; Polymers for Microelectronics ACS Symposium Series; American Chemical Society: Washington, DC, 1993.

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a) He-Ne

laser Polaroid

500W Hg lamp

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Q P LC c e l l Filters

h

o u

t T

o diode

Prismpolarizer Data

Amplifier

recorder

b) Exci t a t ion XeCI

Excimer

DMQ Dye

Pulse

fck Ml

A ~~ 360nm He-Ne

LI

Trigger Generator

L2 PI

|$|

/Monitor Beam

Sample MicroComputer

6PIB

Digital Storage Osci I lo.

Figure 7. Experimental setup for the photoinduced alignment change in nemati< L C cells. a) Steady exposure experiments. Polarizer is placed in front of the Hg lamp for PAC by polarization pho tochromism. b) Laser pulse experiments. L 1 , L 2 : lenses, P1,P2: polarizers (PI is used for PAC by polarization pho tochromism), PP: PIN photodiode.

Thompson et al.; Polymers for Microelectronics ACS Symposium Series; American Chemical Society: Washington, DC, 1993.

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linearly polarized U V excitation, and 240msec for rotation of the P orientation axis 45 ° to the original by changing U V light polarization, respectively. A l l the relaxation curves were well explained by the monoexponential function. The results show that the macroscopic orientation axis is immediately formed after the polarized U V exposure without experiencing the random P state, since no contribution from the time constant corresponding to the in-plane rearrangement was involved in the relaxation profile.

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Homogeneous Orientation Induced by Slantwise UV Exposure The homogeneous P domain can be also obtained by the alternative method, slantwise exposure to unpolarized U V light, in which the reaction light path is tilted about 10 to the normal to the cell surface. The results on the angular dependent transmittance of the G H cell show that the orientation axis is induced parallel to the projection of the light path on the cell surface (Figure 9). Again, the orientation axis is rotatable by changing the incident direction of the reaction light. Dynamic conoscopic observation affords us to investigate how the orientation changes upon exposure (Figure 10). The crosspoint in the conoscopic image, which represents the optical axis of the L C phase, moves to the right when the reaction U V light comes from the left side of the microscope. By changing the amount of U V or visible irradiation, the cross point can be moved back and forth intentionally. This means that the slantwise exposure provide the uniaxial orientation axis with a tunable tilt angle. 0

0°

180°

Figure 8. Absorption surfaces (angular dependence of optical density) of the MeOAzO/DON103-LCD118/MeOAzO G H cell at 633nm induced by the linearly polarized exposure at 0 = - 3 0 ° (1), - 6 0 ° (2), - 9 0 ° (3) and -120 ° (4). Along the circle is plotted the cell rotation angle with respect to the polarization of the monitor He-Ne. Concentric circles represent 0.5, 1.0 and 1.5 of optical density, respectively.

Thompson et al.; Polymers for Microelectronics ACS Symposium Series; American Chemical Society: Washington, DC, 1993.

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180°

Figure 9. Absorption surfaces of PMeOAzO/DON103-LCD118/ODS G H cell at 633nm obtained by the slantwise exposure at 0 = 0 ° (1), 45 ° (2) and 90 ° (3). The circle represents 0.6 of optical density.

Supposed Mechanism for In-plane Orientation By Anisotropic Photochromism Present photoinduced homogeneous orientation is supposed to originate from two individual effects of the Az surface. One is an inherent ability of the os-Az rich surface giving the P alignment. The other is the surface anisotropy acquired through the photoselective isomerization of Az, which may provide the inplane orientation axis. According to semiempirical LCAO-SCF-CI calculations (18), either the lowest m - IT* transition vector of trans-Az or that of cw-Az is almost parallel to their — N = N — bond axes. Along with the polarized U V exposure (at 365m) corresponding to the lowest TT - TT* transitions of both isomers, the surface becomes as-Az rich, and the average alignment of Az molecules in terms of — N = N — axis should become perpendicular to the incident polarization plane due to the polarization-selective photochromism (24). It is not clear at present which isomer is the dominant in homogeneous orientation induction. Our preliminary results show L C molecules tend to align their long axes parallel to those of surface trans-Az units. On the other hand, ds-Az is rather known to destroy an ordered structure of L C phase as in the case of PPT. Similar anisotropy should be produced in the Az surface by the slantwise exposure. Using FresnePs formulas, we can expect that polarization due to reflection at interfaces of the system is negligible. To explain, we suppose that trans-Az lying its long axis parallel to the reaction light path hardly photoisomerize since the incident electric vector is perpendicular to the transition vector of the Az, and would give the specific orientation axis.

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31.

Figure 10. Conoscopic observation of induced alignment change of PMeOAzO/DON103/ODS cell by slantwise exposure to U V (a) and visible (b) lights. Exposure was made from the left of the microscope. Pictures were taken every 2s for (a) and 0.5s for (b) as the order shown above the pictures.

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OTHER SYSTEMS BASED ON POLARIZATION PHOTOCHROMISM Induction of organized structure in L C media by polarized light has recently attracted much attention. Induction of orientation axis perpendicular to the light electric field has been found in analogues of L C polymers bearing Az and mesogenic units as their side chain (25-29). Photoinduced reorientation depends strongly upon the driving temperature and often exhibits much slower rate below polymer's Tg. One proposed mechanism is that: thermal reverse isomerization of os-Az may produce 90 ° rotated trans-Pa with respect to the original one, which will be accumulated during photoirradiation due to photoselection. Surface induced orientation has been demonstrated in the system consisting of two rubbed polyimide films, one of which is doped with a diazodiamine dye (30). Again the induced axis is found to be perpendicular to the light polarization. The system offers an enormous change in optical anisotropy, regardless of understandings how the photochemical process of the dye overcome the qualified aligning force of polyimide surface. SUMMARY Photochemical control of organized structures in materials is one of substantial scientific objects. Precise control of orientation may be difficult in PPT systems, because the system is rather based on destruction of organized structure induced by a large topological change of photochromic dopant and self-organizing character of the L C phase. When PAC systems are combined with anisotropic photochromism, precise control of the L C orientation becomes possible. Tilt angle including H-P alignment change is controlled by changing the fraction of surface photochromic molecules. In-plane orientation of the P alignment is regulated by controlling the light polarization or the incident direction. Thus, anisotropic surface photochromism will afford precise 3D control of the L C orientation by changing the light characteristics. ACKNOWLEDGMENT This work has been carried out under the project "Photoreactive Materials" conducted by Agency of Industrial Science and Technology, Ministry of International Trade and Industry. REFERENCES 1. Techniques of Chemistry, Vol. 13, Photochromism; Brown, G. H., Ed.: Wiley Interscience: New York, 1971. 2. Photochromism: Molecules and Systems: Dürr, H.:Bouas-Laurent,H.,Eds.: Elsevier: Amsterdam, 1990. 3. Sackmann, E. J. Am. Chem. Soc. 1971, 93, 7088 4. Haas, W. E.; Nelson, K. F.; Adams, J. E.; Dir, G. A., J. Electrochem. Soc. 1974, 121, 1667

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5. Ogura, K.; Hirabayashi, H.; Uejima, A.; Nakamura, K.; Jpn. J. Appl. Phys. 1982, 21, 969 6. Tazuke, S.; Kurihara, S.; Ikeda, T.; Chem. Lett. 1987, 911 7. Ikeda, T.; Horiuchi, S.; Karanjit, D. B.; Kurihara, S.; Tazuke, S.; Chem. Lett. 1988, 1679 8. Suzuki, Y.; Ozawa, K.; Hosoki, A.; Ichimura, K., Polym. Bull., 1987, 17, 285 9. Ikeda,T.: Horiuchi,S.: Karanjit,D.B.: Kurihara,S.; Tazuke,S., Macromolecules, 1990, 23, 42 10. Ikeda,T.: Kurihara,S.: Karanjit, D.B.: Tazuke,S., Macromolecules, 1990, 23, 3938 11. Kawanishi,Y.:Tamaki,T.:Ichimura,K., J.Phys.D: Appl. Phys, 1991, 24, 782 12. Ichimura, K.: Suzuki, Y.: Seki, T.: Hosoki, A.: Aoki, K. Langmuir 1988, 4, 1214 13. Ichimura, K. In Photochemical Processes in Organized Molecular Systems: Honda,K.,Ed.: North-Holland: Amsterdam, 1991: p343. 14. Ichimura, K.: Suzuki, Y.: Seki, T.: Kawanishi, Y.: Tamaki, T.: Aoki, K. Makromol. Chem., Rapid Commun. 1989, 10, 5 15. Seki, T.: Tamaki, T.: Suzuki, Y.: Kawanishi, Y.: Ichimura, K.: Aoki, K., Macromolecules 1989, 22, 3505 16. Ichimura, K.: Suzuki, Y.: Seki, T.: Kawanishi, Y.: Tamaki, T.: Aoki, K., Jpn. J. Appl. Phys., Suppl. 1989, 28, 289 17. Alignment of Nematic Liquid Crystals and Their Mixtures, Mol. Cryst. Liq. Cryst. Suppl. Ser. 1: Cognard,J.,Ed.: Gordon and Breach: New York, 1982 18. Beveridge, D. L.: Jaffe, H . H . J. Am. Chem. Soc. 1966, 88, 1948 19. Kawanishi,Y.: Seki,T.: Tamaki,T.: Ichimura,K.: Ikeda,M.: Aoki, K., Polym. Adv. Tech. 1991, 1, 311 20. Kawanishi,Y.: Tamaki,T.: Seki,T.: Sakuragi,M.: Suzuki,Y.: Ichimura, K., Langmuir 1991, 7, 1314 21. Kawanishi, Y . : Tamaki, T.: Seki, T.: Sakuragi, M . : Ichimura, K., J. Photopolym. Sci. Tech. 1991, 4, 271 22. Kawanishi, Y.: Tamaki, T.: Seki, T.: Sakuragi, M.: Ichimura, K., Mol. Cryst. Liq. Cryst. 1992, 218, 153; Kawanishi, Y.: Tamaki, T.: Sakuragi, M.: Seki, T.: Suzuki, T.: Ichimura, K., Langmuir 1992, 8, 2601 23. Kawanishi, Y.: Tamaki, T.: Ichimura, K. Polym. Mat. Sci. Eng. 1992, 66, 263 24. Jones, P.: Jones, W. J.: Williams, G., J. Chem. Soc. Faraday Trans. 1990, 86, 1013 25. Tredgold, R.H.: Allen, R.A.: Hodge, P.: Khoshdel,E. J. Phys. D: Appl. Phys. 1987, 20, 1385 26. Todorov, T.: Nikolova, L.: Tomova, N . Applied Optics, 1984, 23, 4309 27. Anderle, K.: Birenheide, R.: Eich, M.: Wendorff, J. Makromol. Cem., Rapid Commun. 1989, 10, 477 28. Anderle, K.: Birenheide, R.: Werner, M . J. A.: Wendorff, J. H . Liquid Crystals 1991, 9, 691 29. Ivanov, S.: Yakovlev, I.: Kostromin, S.: Shibaev, V.: Lasker, L.: Stumpe,J.: Kresig, D. Makromol.Chem., Rapid Commun. 1991, 12, 709 30. Gibbons, W. M.: Shannon, P. J.: Sun, S.-T.: Swetlin, B. J. Nature 1991, 351, 49 Received May 25, 1993 Thompson et al.; Polymers for Microelectronics ACS Symposium Series; American Chemical Society: Washington, DC, 1993.