Langmuir 1993,9,867-860
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pcyanoazobenzene as a Command Molecule for Azimuthal Anisotropy Regulation of a Nematic Liquid Crystal upon Exposure to Linearly Polarized Visible Light+ Kunihiro Ichimura’ and Yuko Hayashi Photochemical Process Division, Research Laboratory of Resources Utilization, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 227, Japan
Yuji Kawanishi, Takahiro Seki, and Takashi Tamaki Research Institute for Polymers and Textiles, 1-1 -4 Higashi, Tsukuba, Ibaraki 305, Japan
Norio Ishizuki Chemicals Research Laboratories] Nippon Kayaku Company Ltd., 3-26-8 Shimo-cho, Kita-ku, Tokyo 115, Japan Received June 1,1992. In Final Form: November 18,1992 UV exposure of a nematic liquid crystalline cell fabricated with a quartz plate surface-modified with p-cyanoazobenzene units resulted in no reversible alignment change. On the contrary, irradiation with linearly polarized visible light p430 nm)brought about in-plane anisotropy of the mesophase although the photostationarystate under the visible light illumination consisted predominantly of the tram isomer with no apparent geometricalphotoisomerization. The direction of the photoinduced azimuthal anisotropy was approximatelyperpendicular to the polarizationplane of the actinic light and controllableby changing the polarization plane. This technique was applicable to form photoimages on the basis of the difference in the direction of anisotropy. The photoinduced birefringenceof the cell was stable at room temperature even upon exposure to light unless the light was not polarized.
Introduction The photoinduced alignment regulation of a nematic liquid crystal has been operated by putting a liquid crystal cell between substrate plates1which are surface-modified with molecular layers incorporating photochromic units such as azobenzenes? stilbenes? and a-hydrazono-8ketoesters.4 Rodlike trans isomers of the geometrically photoisomerizable units sprouted from the surfaces bring about homeotropic alignment. In the case of azobenzene, exposure of the cell to UV light for the ~ - r *excitation converts the units into the correspondingcis isomer having a V-shaped form which gives rise to the reorientation of liquid crystalline molecules in a planar mode through a “domino effect”. Alternate exposure to UV and visible light leads consequently to the reversible alignment alteration between the homeotropic and planar modes. It has been proposed to call such a photoactive surface for the alignment regulation the command surface.2a The alignmentregulation is markedlyinfluenced by the molecular structure of photochromicunitsa2Particularly, a substituent on an azobenzene moiety as a head group plays a crucial role in the photoregulation. Whereas hydrophobic substituents like hexyl and cyclohexyl give rise to the reversible transformation of liquid crystalline
alignment between the homeotropic and planar modes, hydrophilic ones including chloro and methoxy groups result in the planar orientation even thoughthe azobenzene unit is in the trans form and causes no remarkable alignmentmodification upon photoisomerization into the cis isomer. Such a hydrophilic head group on azobenzene has been thus thought to be inactive for the alignment regulation between homeotropic and planar modes. On the one hand, a new mode of photoregulation of liquid crystalline alignment by the command surface has been recently explored by employing linearly polarized light.s When a nematic liquid crystalline cell is exposed to linearly polarized UV light for the r-r* transition to isomerize the trans isomer to the cis counterpart, the mesophase has been found to be converted from the homeotropic into the uniaxially planar (homogeneous) alignmentes The direction of the homogeneous alignment is in the crossed position to the polarized light plane. A similar phenomenon has been observed when a substrate plate is covered with a photochromic spiropyran monolayer.6 Another type of surface-mediated alignment photocontrol of a nematicliquid crystal with use of linearly polarized light has been reported.7 In this w e , a polyimide thin film doped with dichroic dye molecules is employed as a photoactive layer. On the other hand, exposure of * To whom correspondence should be addressed. side chain liquid crystallimepolymers substituted with an t CommandSurfaces. 3. Part2 Sakuragi,M.;Seki,T.;Kawanishi, Y.; Tamaki, T.; Ichimura, K. Chem. Lett. 1992, 1763. azobenzene moiety to visible light for the n-r* transition (1) Ichimura, K. In Photochemical A.ocesses in Organized Molecular leadsto the reorientation of the mesogenic groups although Systems; Honda, K., Ed.;Elsevier: Amsterdam, 1991; p 343. (2) a)Ichimura,K.;Suzuki,Y.;Seki,T.;Hosoki,A.;Aoki,K.Langmuir the photostationary state under these expoeweconditions 1988,4,1214. b) Aoki, K.; Seki, T.; Suzuki, Y.; Tamaki, T.; Hosoki, A.; Ichimura, K. Langmuir 1992,8,1007. c) Aoki, K.; Tamaki, T.; Seki, T.; Kawaniehi, Y.; Ichimura, K. Langmuir 1992,8,1014. (3) A0ki.K.; Ichimura, K.;Tamaki,T.:Seki,T.:Kawanishi,Y.Kobumhi Ronbunshu 1990,47,771. (4) Yamamura, S.; Tamaki, T.; Seki, T.; Sakuragi, M.; Kawaniehi, Y.; Ichimura, K. Chem. Lett. 1992,543.
(5) Kawanishi, Y.; Tamaki, T.;, Sakuragi,M.; Seki, T.; Suzuki, Y.; Ichimura, K. Langmuir 1992,8, 2601. (6) Ichimura, K.; Hayashi, Y.; Iehizuki, N. Chem. Lett. 1992, 1063. (7) Gibbons, W. M.; Shannon,P. J.; Sun,S.-T.; Swetlin, B. J. Nature 1991, 351, 49.
Q743-7463/9312409-0867$04.oO/Q0 1993 American Chemical Society
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contains a major part of the trans-azobenzene.8 These facts have led us to an idea that a command surface covered with a hydrophilic trans-azobenzene monolayer causing planar alignment could indub the reorientationof nematic liquid crystalline molecules by linearly polarized "visible" light, We report here on the reorientation behavior of a nematic liquid crystal inducedby linearlypolarized visible light irradiation.
7
Experimental Section Materials. Preparation of 5-[4-((4-~yanophenyl)azo)phenoxylcaproic acid (1) is described in our previous paper.2 The condensation of 1 with freshly distilled (3-aminopropy1)triethoxysilane was carried out in dichloromethane with use of dicyclohexylcarbodiimideto give the silylatingreagent 2, and the product was used without further purification. Lecithin from egg yolk was purchased from Wako Chemicals Co., Ltd. A nematic liquid crystal (EXP-CIL, T N I= 30.6 "C), a mixture of a cyclohexanecarboxylate and two cyclohexenylcyclohexanes,was a gift from Rodic Co., Ltd.
Surface Modification andcell Fabrication. Quartz plates (1X 3 cm2)were cleaned in the reported way.2 The plates were immersed in a 1 w t 7% ethanol solution of crude 2 for 10 min, air-dried, and heated at 120"C for 30 min, followed by sonication in chloroform for 15min, in NJV-dimethylformamide for 15 min, and finally in chloroform for 15 min. Another plate was dipped in a 0.1 w t 7% hexane solution of lecithin for 5 min, followed by rinsing with chloroform and drying at 120 "C. The nematic liquid crystal EXP-CIL containing spherical glass spacers of 5-pm diameter (Tokuyama Soda Co., Ltd., SP-5OF) was sandwiched between an azo-modified quartz plate and a quartz plate modified with lecithin to afford a hybrid cell. Exposure Experiment and Birefringence Observation. The photoisomerization of the azobenzene on a surface-modified plate was undertaken by means of a JASCO CRM-FA irradiator. Visible light from a 500-Whigh-pressure mercury arc passed through a glass filter (Toshiba V-43, >430 nm) and a polarizer (Nippon Kayaku, KN-18242T) was incident vertically with the aid of a mirror upon a liquid crystal cell placed on a hot plate at 58 OC. After cooling, the exposed cell set in a holder was subjected to the vertical incidence of a linearly polarized He-Ne laser (NEC; GC-3500) beam. Light intensity of the monitoring laser beam passed through the cell and a crossed polarizer was measured with a light power meter. The birefringence of the cell was measured by followingthe light intensity at various rotation angles of the cell.
Results and Discussion The p-cyanoazobenzene unit was employed here as a surface modifier because the trans isomer does not induce the homeotropic mode. Quartz plates were treated with (8)a) Anderle, K.;Birenheide,R.;Eich,M.;Wendorff, J. H.Makromol. Chem., Rapid Commun.1989,10,477. b) Anderle, K.; Birenheide, R.; Werner, M. J. A.; Wendorff, J. H. Liq. Cryst. 1991, 9,691. c) Wiesner, U.;Antonietti,M.; Boeffel, C.; Spiess, H. W. Makromol. Chem. 1990,191, 2133. d) Ivanov, S.;Yakovlev, I.; Kostromin, S.;Shibaev,V.; Laesker, L.; Stump, J.; Kreyaig, D. Makromol. Chem., Rapid Commun. 1991, 12, 709. e) Eich, M.; J. H. Wendorff, Reck, B.; Ringdorf, H . Makromol. Chem., Rapid Commun. 1987,8, 59.
3 70 490 Wavelengthlnm
610
Figure 1. Electronic absorption spectra of p-cyanoazobenzene residues attached on a quartz surface before (-) and after (- - -) UV irradiation.
an ethanol solution of the silylating reagent 2 having a p-cyanoazobenzene moiety.2 It is well known that the photoisomerized trans to cis ratio of azobenzene depends strongly on the excitation wavelength; while illumination with UV light for the r ? r * excitation results in the predominant formation of the cis isomer, a major component is the trans isomer upon exposure to visible light for the n l r * excitation. Irradiation of the azobenzenecarboxylic acid 1 in ethanol with visible light at a wavelength longer than about 440 nm gave a photostationary state containing 94% of the trans isomer. The photoisomerizability of the azobenzene sprouted from a quartz surface from the trans to the cis isomer was confirmed by irradiation with UV light at 365 nm, as shown in Figure 1. When the UV-exposed plate was illuminated with visible light at 445 nm, the absorption spectrum was almost identical with that of the trans form, confirming that the photostationary state of the p-cyanoazobenzene on the surface consists of the predominant trans isomer under visible light illumination. The figure also indicates that the average area occupied by each azobenzene unit is estimated spectroscopically to be ca. 100 A2 under the assumption that the absorption coefficient (e = 18 OOO at ,A, = 361 nm) of the chromophore in a solution is not affected by binding onto the surface. A hybrid cell was made by sandwiching the nematic liquid crystal EXP-CILbetween two quartz plates. One of the plates was modified with 2 whereas the other was modified with lecithin for the homeotropic alignment. Therefore, the cell has a single photoreactive molecular layer. The actinic light was incident first upon the azomodified plate of the cell to avoid the distortion of polarizabilitycaused by the birefringence of the mesophase layer. Irradiation of the cell with nonpolarized UV light caused no marked alteration of the transmittance of a linearly polarized He-Ne laser beam as a monitoring light, confirming our previous observation that the azobenzene substituted with a hydraphilic group brings about no alignment change between homeotropicand planar modes2 because the trans isomer itself gives rise to a planar alignment. The cell was placed on a hot plate and exposed to linearly polarized visible light (>430 nm). The linearly polarized light was incident vertically first upon the quartz plate modified with 2 in order to avoid the distortion of the polarization plane during the course of passing through the anisotropic mesophasic layer. The photoinduced azimuthal anisotropy was checked by measuring the intensity of a He-Ne laser beam through the cell and a crossed polarizer placed behind it as a function of the rotation angle of the cell. The definition of the polarized
Azimuthal Anistropy Regulation of a Liquid Crystal
Langmuir, Vol. 9, No. 3, 1993 869
er .!
20 0
20 0
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Figure 4. Angular dependence of the light intensity of a monitoring linearly polarized He-Ne laser beam through the liquid crystalline cell exposed to the polarized visible light (a) at 0 = 90" for 5 min, subsequently at B = -45" (b)for 20 8, (c) for 40 s, and (d) for 7 min, and finally at 8 = 90" (e) for 10 s and (f) for 6 min. Figure 2. Definition of rotation angles 0 and 3. contained by the cell axis and the light plane axes.
:;pvavy 0
20 10 0, L I- 2 o t
b)
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90
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(y)
Figure 5. Angular dependence of the light intensity of a
0
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Rotation angle(Y)
Figure 3. Angular dependence of the light intensity of a monitoring linearly polarized He-Ne laser through the liquid crystalline cell (a) before and after exposure to the polarized visible (>430 nm) light with the plane axis of B = 0" (b) at 38 "C for 10 s, (c) at 38 "C for 2 min, and (d) at 58 "C for 3 min.
light plane angles is illustrated in Figure 2. The cell axis is tentatively equivalent to the direction of the longer sides of the rectangular cell while is the angle between the cell axis and the polarization plane of the actinic visible light, and $ is the angle between the cell axis and the polarized light plane of the monitoring He-Ne laser beam. Because no clear reorientation of the liquid crystalline molecules was observed when the cell was irradiated at an ambient temperature, the exposure was made under heating the cell above TNI. Whereas the intensity of the He-Ne laser beam through an unexposed cell displayed no distinct dependence on the cell angle and hence no birefringence, photoirradiation with the linearly polarized visible (>430 nm) light at B = 0" induced unequivocally the optical anisotropy showing minimum transmittance roughly at $ = 0" 90° X n (Figure 3). This evidently supporta the observation that the direction of liquid crystalline orientation is parallel or perpendicular to the polarization axis. The direction of in-plane alignmentof the liquid crystal was altered at will by rotating the axis of polarization of the visible light. Figure 4 presents the angular dependence of the transmittance of the cell exposed to the linearly polarized visible light successively at B = -goo, -45", and Oo, as typical examples. The transmittance maxima appeared roughly at $ = 45" + 90° X n for e = -90° or 0" and at $ = 0" + 90" X n for B = - 4 5 O .
+
monitoring linearly polarized He-Ne laser beam through the liquid crystalline cell initially exposed to the linearly polarized visible light at 0 = 0"as a function of the time of exposure to the linearly polarized visible light at 0 = -60". Exposure times at 58 "Cwere 0 s (-), 3 s (. .),12 s (- - -),32 s (- * -), and 212 s (- -).
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-
Figure 5 shows the change in the angular dependence during the course of exposing the cell to the light at $ = +60° after the initialization with irradiation at $ = Oo. At the early stage, the optical anisotropy was distorted considerably, accompanied by the reduction of the transmittance. Further photoirradiation resulted in the increase in the transmittance at different angles and subsequentlyin a slight shift of the angles at the maximum (or minimum) transmittance. The surface-mediated orientation of the liquid crystal was found to be relatively stable to heat treatment and to exposure to nonpolarized visible light. It should be stressed here that the azimuthalalignment resulting from the transcis photoisomerization of the surface azobenzene under irradiation with linearly polarized UV light disappears completely by heat treatment because of the thermal reversion into the trans isomer which induces the homeotropic alignmentwith no birefringence. No change in the azimuthal anisotropy was observed in the present cell upon prolonged storage at an ambient temperature and even after heating the aligned cell at 60 "C for 10 min. However, the anisotropy was considerably destroyed after heating at 120 "C for 10 min (Figure 6). The formation of azimuthal anisotropy may be triggered by the photoselection of the surface azobenzene units. It has been reported that irradiation with linearly polarized visible light causes the reorientation of azobenzene units attached on side chains of liquid crystalline polymers.8 Similar photoinducedreorientation of an azobenzeneunit
860 Langmuir, Vol. 9, No. 3, 1993
Ichimura et al.
J
0
90
180 Rot at ion angle Cy)
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Figure 6. Angular dependence of the light intensity of a monitoring linearly polarized He-Ne laser beam through the liquid crystalline cell exposed to the polarized visible light a t 0 = 4 0 " for 1 min (-), subsequently exposed to nonpolarized visible (>430nm) light for 10 min at an ambient temperature (- - -), and finally exposed to the nonpolarized visible light for 12 min at 60 "C (- -).
was recently observed even in an amorphous p01ymer.~In the case of azo-modified liquid crystalline polymers, the rearrangementof the chromophoresinduced by the linearly polarized visible light brings about the reorientation of the mesogensto afford a significant birefringent alteration of the polymer matrices. It may be assumed that quite a similar phenomenon takes place at a boundary region of our present cell. The linearly polarized light forces the azochromophoreson the surface to reorient perpendicular to the polarization plane after the cumulative cis-trans photoisomerization. The uniaxial reorientation of the azobenzeneunits induces the rearrangement of the liquid crystalline molecules surrounding the azo groups to give the homogeneous alignment. The emergence of optical anisotropy of the liquid crystalline (LC)cell induced by the linearly polarized visible light is thus assumed to be the molecular amplification of uniaxially oriented monolayeredchromophores and enables us to apply it to photomemory. The whole area of the cell is first irradiated with linearly polarized visible light to make the liquid crystal alignment homogeneous for the initialization. Optical information storage is subsequently performed by exposingthe cell imagewise to the linearly polarized light, the polarization plane of which is set here at 4 5 O with respect to the initial one. The photoimages can be visualized by means of polarizers as a negative as well as a positive tone, depending on the angle of the polarizer axes. Figure 7 shows a polarizing micrograph of the cell exposed to the visible light through a test chart. The cell was initially irradiated with the visiblelight with the polarization plane parallel to the cell axis to attain homogeneous alignment and subsequently exposed through the test chart to the visible light after rotating the polarization plane to 45'. Although there are some defects in the picture, clear images are observed. Optimization of surface modification would improve the quality of images. It has been suggested that the uniaxial alignment of molecules displaying anisotropic absorption with the aid of linearly polarized light may be applicable to photorecording on the basis of "photochromism in orientation," as has been discussed by Michle.lo One of the crucial (9) Rochon, P.; Gosselin,J.; Natansohn, A.; Xie, S. Appl. Phys. Lett. 1992,60, 4. (10)Michle, J. In Photochromism: Molecules and Systems; Diirr, H., Bouas-Laurent,H., Eds.; Elsevier: Amsterdam, 1990; p 919.
Figure 7. A polarizing micrograph of the liquid crystalline cell irradiated with the linearly polarized visible light through a test chart. The whole area of the cell was exposed to the light a t 0 = 0" a t 38 "C before the imagewise irradiation with the same light a t 0 = 45 O at 38 "C.
requirements for such an optical recording is the prevention of the uniaxial molecular alignment from thermal randomization during memory storage. Our present liquid crystalline cell fulfillsthis requirement. Furthermore, the followingshould be stressed from a practical point of view for the application to rewritable photomemory. First, the fatigue resistance of the LC cell should be markedly improved because visible light for the n-7r*excitation leads scarcely to the degradation of the chromophore while the photofatigue of azobenzenesis brought about specifically by irradiation with UV light for the r 7 r * excitation. Indeed, although the quantum yield for the disappearance of azobenzenes amounts to about 104 upon exposure to 308-nm light, no observable loss of the chromophores was found even after exposure with an Ar+ laser emitting 488nm light of 4 kJ/cm2, indicating that the quantum yield for the degradation of azobenzenes with visible light for the n-rr* excitation is assumed to be far less than 10-6.'' Second, only a single wavelength region is required here whereastwodifferent wavelengthsare essentiallynecessary for erasable photomemory using photochromism. Third, the recording energy for the cell is about a few hundred mJ/cm2and much smallerthan (ca. 1or more than 1J/cm2) that for polymeric liquid crystalline systems containing azobenzeneunits which exhibit the molecular axis change upon exposure to linearly polarized visible The maximum transmittance of the cell is still not so high that the cell shows relatively poor contrast, as seen in Figures 3 and 4. This reflects obviously insufficient planar alignment; an angle contained by the substrate surface and the LC molecular axis is relatively large. The improvement of the contrast by attaching azobenzenes laterally onto glass surfaces will be reported elsewhere. In summary, p-cyanoazobenzene residues attached on a silica substrate surface induce reversible alteration of in-plane alignment of a nematic liquid crystal with use of linearly polarized visible light which causes no apparent photoisomerization of the chromophore. The photoinduced azimuthal anisotropy of the liquid crystalline layer is of potential value for erasable photomemory. (11)Suzuki,Y.;Sakuragi,M.;Asano,N.;Abe,Y.;Tamaki,T.;Ichimura, K. Unpublished results.
