Langmuir 1993,9, 3298-3304
3298
Photoregulation of In-Plane Reorientation of Liquid Crystals by Azobenzenes Laterally Attached to Substrate Surfaces? Kunihiro Ichimura,’ Yuko Hayashi, and Haruhisa Akiyama Research Laboratory of Resources Utilization, Tokyo Institute of Technology, 4259 Nagatsuta Midori-ku, Yokohama 227, Japan
Norio Ishizuki Chemical Research Laboratory, Nippon Kayaku Co., Ltd., Shimo-cho, Kita-ku, Tokyo 115, Japan Received February 17,1993. I n Final Form: August 12,1999 An effective photochemicalway to regulate the in-planealignment of a nematic liquidcrystalis presented. A quartz plate surface was modified with 4-hexyl-4’-hexyloxyazobenzene substituted with a triethoxysilyl group through a spacer at the 2‘-position to introduce the side-on type azobenzene unit on the surface. A liquid crystal hybrid cell was fabricated with the azo-modified plate and a quartz plate modified with lecithin. Irradiation of the cell with visible light linearly polarized for the n--R* transition resulted in the in-planereorientation of the liquid crystal molecules to afford a homogeneousalignment. The rate of the photoinduced reorientation was considerably enhanced by heating the cell above the nematiclisotropic transition temperature and was markedly stable toward heat and unpolarized light. The mechanism of the in-plane alignment is discussed, and the concept of a molecular rotor is proposed.
Introduction Current knowledge indicates that the modes of nematic liquid crystal (LC) alignment are determined by the nature of substrate surfaces.l It has been recently showed that the reversible regulation of alignment between homeotropic and planar modes can be achieved by geometrical photoisomerization of monolayered photochromic units2 including azobenzenes? a stilbene: and a-hydrazono-j3ketoesters5 which are bound covalently onto substrate surfaces called command surfaces. For example, the E(trans)-isomer azobenzene monolayer causes homeotropic alignment whereas the planar alignment is induced by the corresponding Z(cis)-isomer. Such photochromic molecules which control the LC alignment are referred to as command molecules because of their ability to trigger the collective reorientation of more than a thousand LC molecules. This photoinduced alignment regulation can be regarded as an amplificationof the reversible molecular shape change between the rodlike E-form and bent 2-form of the command molecules. The molecular shape alteration causesthe reorientation of LC molecules surrounding the command molecules to result in the alignment change owing to the long range order of the mesophase. The photochemical alteration of the alignment of nematic LCs mediated by a monolayer of command molecules has been veryrecently extended to the regulation
* To whom correspondence should be addressed.
+ Command Surfaces. Part 5. Part 4 Ichimura, K.; Hayashi, Y.; Ikeda, T.; Ishizuki, N. Appl. Phys. Lett., in press. Abstractpubliihedin Advance ACSAbatracts, October 15,1993. (1)Cognard, J. Mol. Cryst. Liq. Cryst. 1982, Supplement 1, 1. (2) Ichimura, K. In Photochemical Processes in Organized Molecular Systems; Honda, K., Ed.; Elsevier: Amsterdam, 1991; p 343. (3) (a) Ichimura, K.;Suzuki,Y.;Seki,T.; Hosoki,A.;Aoki,K.Langmuir 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.; Kawanishi, Y.; Ichimura, K. Langmuir 1992,8, 1014. (4) Aoki,K.;Ichimura,K.;Tamaki,T.;Seki,T.;Kawanishi,Y. Kobunshi Robunshu 1990,47, 771. (5) Yamamura, S.; Tamaki, T.; Seki, T.; Sakuragi, M.; Kawanishi, Y.; Ichimura, K. Chem. Lett. 1992, 543. @
0743-746319312409-3298$04.0010
of in-plane reorientation of LC molecules with use of linearly polarized UV lighte6When a nematic LC is placed on a quartz plate whose surface is modified with an azobenzene molecular layer, UV irradiation results in an alignment alteration from the homeotropic mode to the homogeneous one, in which the direction of the uniarially arranged molecular axis of LC is roughly 90° to the actinic polarized light plane. This photoinduced homogeneous alignment decays away due to the photochemical as well as thermal reversion from the 2 to the E isomer of the azobenzene moiety to bring about the homeotropic alignment. From a practical point of view, this situation has a disadvantage with regard to optical information storage. According to our previous work, exposure to linearly polarized UV light of a quartz substrate modified with a photochromic spiropyran molecular layer enables one in the same way to rotate the alignment direction at will without such decay of the homogeneousalignment because the spiropyran monolayer itself inducesthe planar mode.7 This implies that regulation of the persistent in-plane alignment with use of linearly polarized light should be available if azobenzene command molecules attached on the substrate surface lead to the planar mode even in their E form. Azobenzenes are selectively excited to give a Z-formrich photostationary state with UV light for the m* transition whereas the E-form is a major component upon exposure to visible light for the n i * transition. This means that no decay of the photoalignment can be realized when the surface azobenzenes are subjectedto visible light irradiation for the n-* transition. In fact, our recent work has revealed that p-cyanoazobenzene units attached on a quartz surface command the azimuthal reorientation of nematic liquid crystals upon exposure to linearly polarized visiblelight because this azochromophore yields (6) (a) Kawanishi, Y.;Tamaki, T.; Seki, T.; Sakuragi, M.; Ichimura, K. Mol. Cryst. Liq. Cryst. 1992,218, 153. (b) Kawmiahi, Y.;Tamaki,T.; Sakuragi, M.; Seki, T.; Suzuki, Y.; Ichimura, K. Langmuir 1992,8,2601. (7) Ichimura, K.; Hayashi, Y.; Ishizuki, N. Chem. Lett. 1992, 1063.
0 1993 American Chemical Society
Langmuir, Vol. 9,No.11,1993 3299
Photoregulation of Liquid Crystals
a planar mode.8 However, further molecular design has been required t o enhance the photoinduced birefringence. This work aims at investigating the binding of azobenzene moieties laterally onto substrate surfaces instead of the conventional surface modification using head-on type azobenzenes, as a means of enabling even the E form to induce planar alignment of the nematic LCs effectively. It was confirmed that the n-u* excitation with linearly polarized visible light causes azimuthal reorientation of the LCs, even though the photostationary state of the azobenzene on the surface consists predominantly of the E isomer. Experimental Section Materials. Quartz plates (1X 3 cm2)were cleaned according to our previous papers? Hexylaniline and resorcinol monobenzoate were used as received. (3-Aminopropy1)triethoxysilane was distilled before use. The following nematic LCs were a kind gift of Rodic Co., Ltd.: DON-103, a mixture of cyclohexanecarboxylates, of TNI= 74 OC and EXP-CIL, a ternary mixture of a cyclohexanecarboxylateand two cyclohexylcyclohexenes,of TNI = 32.6 "C. 2-(Benzoyloxy)-4-hydroxy-4'-hexylazobenzene(2).To a mixture of 33.2 g of concentrated hydrochloric acid and 48 g of water was added 17.7 g of hexylaniline dropwise under ice-cooling.An aqueous solution of 7.2 g of sodium nitrite in 20 mL of water was added dropwise to the mixture below 10 OC to afford a diazotized solution. The cooled solution was neutralized with a sodium carbonate aqueous solution and added dropwise to a solution of 21.4 g of resorcinol monobenzoate in 200 mL of methanol below 10 OC. The mixture was neutralized with a saturated aqueous solution of sodium acetate and stirred for 30 min at ca. 10 "C followed by 2 h at room temperature. The precipitate was collectedby fiitration and washed with water and hexane, followed by drying in vacuo. The crude product (33.0 g) was recrystallized from ethyl acetate to yield the azobenzene derivative of mp 137 OC as orange needles. NMR 6 (CDCb): 0.89 (3H, t, CHa-C), (8H, m, - C H r ) , 2.58 (2H, t, CHz-arom), 6.71 (lH, d, H-arom), 6.73 (lH, d,H-arom), 7.11 (2H,d, H-arom), 7.55 (3H,d, H-arom), 7.67 (2H,d, H-arom), 7.80 (lH, d, H-arom), 8.26 (2H, d, H-arom). Anal. Calcd for C=H&J203: C; 74.60, H; 6.51, N 6.96. Found C; 74.71, H; 6.60, N,6.95. 2-Hydroxy-4-(hexyloxy)-4'-hexylazo benzene (4). 2-(Benzyloxy)-4-hydroxy-4'-hexylazobenzene(5.0 g) was dissolved in 20 mL of DMF and stirred magnetically under reflux for 2 h after adding 2.4 g of hexyl bromide and 2.7 g of anhydrous potassium carbonate. To the stirred reaction mixture were added water and hexane, and the organic layer was separated; the aqueous layer was then extracted with hexane. The combined hexane solution was dried over anhydrous sodium sulfate, followed by removal of the solvent. The residual crystalline mass (3) was dissolved in methanol, and the solution was stirred for 3 h at room temperature after addition of 1.0 g of potassium hydroxide and further stirred for 1h at 50 OC with an additional 0.5 g of potassium hydroxide. The solution was diluted with water and acidified with hydrochloric acid, followed by extraction with a 1:l mixture of hexane and ethyl acetate. The removal of the solvent from the dried solution gave a crystalline mass which was recryatallizsd from hexane to yield 33g of the hydroxyazobenzene. The product shows TKN= 36 OC and TNI= 76 "C. Anal. Calcd for C=HuNzOz: C, 75.35; H, 8.96; N, 7.32. Found C, 76.32; H, 8.97; N, 7.32. MS (m/e)= 382 (MW = 382). IR (KBr) Y(cm-1): 3440,1625. 'H-NMR (CDCb) (ppm): 0.89 (3H, t, CH3C), 0.91 (3H, t, CHa-C), 1.2-1.8 (16H, m, C - C H d ) , 2.68 (2H, t, CHrarom), 4.05 (2H, t, O-CHz), 6.46 (lH, d, H-arom), 6.58 (lH, d, H-arom), 7.26 (lH, 8, H-arom), 7.30 (2H, d, H-arom), 7.72 (2H, d, H-arom), 7.76 (lH, d, H-arom). 2-(10-Carboxydecyloxy)#-(hexyloxy)-4'-hexylazobenzene(1). To a solution of 2-hydroxy-4-(hexyloxy)-4'-hexylabenzene(1.0 g) in 10 mL of DMF was added 1.05 g of tetrahydropyranylllbromoundecanoate and 0.50 g of potassium carbonate, and the (8) Ichimura, K.; Hayashi, Y.; Kawanishi, Y.; Seki, T.; Tamaki,T.; Ishizuki, N. Langmuir 1993, 9, 867.
u F i iter
Hg-lamp
t Polarizer
LC cell
Hot
plate
(b)
LC cell
Photo detector
ser Polarizer
Figure 1. Experimental setup (a) for irradiation of an LC cell with linearly polarized visible light and (b) for measurement of the photoinduced birefringence of the cell. mixture was stirred for 4 h at 60 OC. After the precipitate was removed, the fiitrate was mixed with hexane and water. The hexane layer was separated and washed thoroughly with water, followed by drying over anhydrous magnesium sulfate. The solution was evaporated to give a residual oil, which was hydrolyzed in dioxane containing concentrated hydrochloric acid toyield the carboxylicacid; the crystallinemaas was reclystallized from hexane. Mp 67 OC. Anal. Calcd for C&uNzO,: C, 74.16; H, 9.60; N, 4.94. Found C, 73.96; H, 9.69; N, 4.78. 'H-NMR 6 (CDCls): 0.89 (2H, t, CH3-C), 0.91 (3H, t, CHs-C), 1.2-1.9 (32H, m, -C-CHA-), 2.31 (2H, t, CHrarom), 2.68 (2H, t, C H d O ) , 4.02 (2H, t, CHfl), 4.13 (2H, t, CHz-O), 6.48 (lH, d, arom-H), 6.58 (lH, d, arom-H), 7.27 (2H, d, arom-H), 7.70 (lH, d, aromH), 7.80 (2H, d, mom-H). Surface Modification and Cell Fabrication. A solution of 50 mg of 2-(l0-carboxydecyloxy)-4-(hexyloxy)-4'-hexylazobenzene, 20 mg of (3-aminopropyl)triethoxysilane, and 19 mg of dicyclohexylcarbodiimide in 5 mL of dichloromethanewas stirred for 2 h under ice-cooling. Precipitated dicyclohexylurea was removed by fitration, and the filtrate was evaporated under reduced pressure, followed by dilution of the crude residue (5) with ethanol to prepare a ca. 1wt % silylating solution. Quartz plates (1X 3 cm9 cleaned according to our previous papersawere immersed in the ethanol solution of 1 for 20 min, air-dried and heated at 120 OC for 20 min. The plates were purified by ultrasonication in dichloromethanefor 20 min, followed by drying at 120 OC. Another quartz plate was immersed in a 0.1 wt % hexane solution of lecithin for 5 min and rinsed with chloroform to give plates modified with lecithin. An LC cell was fabricated by sandwiching a nematic LC(D0N-103) suspending 5-pm spherical glass spacers (SP-5OF Tokuyama Soda Co., Ltd.), between the azo-modified quartz plate and the quartz plate absorbing lecithin for homeotropic alignment. Photoinduced Birefringence Observation. The experimental setup is illustrated in Figure 1. An LC cell placed on a hot stage was illuminated with a 500-W high-pressure Hg arc lamp through a glass filter (Y-43, Toshiba, >430 nm) and a polarizer (Figurela). The birefringence alteration was monitored by following the intensity of transmitted linearly polarized HeNe laser beam through the cell and a crossed polarizer as a function of the rotation angle of the cell (Figure lb).
Results Surface Modification. Our previous work on the photoinduced alignment alteration between homeotropic and planar modes with the aid of the command surfaces suggests that the side by side interaction between command molecules linked upon substrate surfaces and liquid
Ichimura et al.
3300 Langmuir, Vol. 9,No. 11, 1993 Scheme I
*30*
a025t 0
t
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I\
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crystalline molecules plays an essential role. The working principle of the alignmentregulation has been interpreted as follows. The command-molecular shapes, including azobenzenes? stilbene: and a-hydrazono-&ketoesterst exhibit reversible changes in the geometrical molecular structure of the rodlike E forms and are quite similar to LC molecules, so that they are assimilated into the LC mesophase. It follows that the command surfaces, monomolecularly covered with the E-forms in a head-on type manner,are favorablefor the homeotropic (perpendicular) alignment. The molecular shape conversion into the V-shaped Z-forms induces distortion of the molecular arrangement of the nearest neighbor LC molecules and leads to the alignment alteration into the planar mode. The molecular design of the azobenzene moiety for modification of silicasurfaces has been undertakento fulfiu the following conditions; the chromophore is substituted with alkyl residues to mimic the size and shape of nematic LC, and the linkage of the chromophore on substrate surface is made lateral so that the long axis of the photoisomerizable units is oriented preferentially in parallel with the surface. We have thus designed an azobenzene (1) which bears two long chains, hexyl and hexyloxy, at the opposite positions and a carboxylic acid group at a lateral position of the chromophore through decamethylene spacer. The preparation of the compoundsis shown in Scheme I. In order to avoid further diazotization of the resorcinol moiety, resorcinol monobenzoate was employed to react with diazotized hexylaniline in a good yield.g The monobenzoated azobenzene (2) was alkylated and subsequently hydrolyzed to give an o-hydroxyazobenzene(4) which displays a nematic mesophase of TNI= 76 OC. The condensation of 4 with the w-bromocarboxylate and the subsequent hydrolysis gave 1. Surface modification of quartz plates was achieved in a conventional way with use of the azobenzene silylating reagent (5) which was obtained by the condensation of 1 with (3-aminopropy1)triethoxysilanewith use of dicylohexylcarbodiimide. Figure 2 shows theabsorption spectra of a quartz plate treated with the azobenzene derivative (5). The average density of the azobenzene was estimated spectroscopically under the assumption that the molar absorption coefficient of the chromophore in ethanol (e = 13 400 dm3mol-l cm-' at 360 nm) is not markedly affected by the chemical adsorption on the surface. The calculated average density was 4.5 azobenzene molecules per 100A2, suggesting that the silylated azobenzenes possess no monomolecularly layered structure since a slightly larger (9) Barton, D.H.R.;Linnel, W.H.;Senior, N.J. Chem. Soc. 1946,4, 436.
I
'
"
~
"
"
'
'
'
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'
250 300 350 400 450 500 550 600
/ nm Figure2. Electronicabsorptionspectra of a quartz plate surfacemodified with the azobenzene (1)before (-) and after (- - -) W irradiation and subsequently after visible light irradiation
Wavelength
(- -)*
occupied area of 25 A2was observed for the corresponding head-on type amphiphilic azobenzene,1° although the occupied area of the side-on type azobenzene in amonomolecular fh is expected to be much larger than that of the head-on counterpart. Photochemistry. Reversible photoisomerization of the azobenzene was confirmed by alternate irradiation of 6 with W (365 nm) and visible light (>430 nm) in ethanol. The E:Z isomer ratio was approximately 1:9 in a photostationary state after W irradiation while the ratio was 91after prolonged irradiation with visible light according to the electronic absorption spectra. The reversible EIZ photoisomerization was observed for the azobenzene moleculesattached on quartz surfaces. Figure 2 illustrates the spectral change of the azobenzene adsorbed chemically on a quartz surface upon exposureto UV light. Subsequent illumination of the UV-exposed plate with the visible light resulted in ca. 70% recovery of the E isomer. There appears no isoebesticpoint in the absorption spectra shown in Figure 2 probably because of insufficient setting of the plate in a sample holder. Because the azobenzene units are in contact with LC molecules in the present cell, further confirmation of the photoisomerizability was carried out on an azo-modified quartz plate which was covered with a thin layer of a UVtransparent nematic LC (EXP-CIL),a ternary mixture of cyclohexanecarboxylateand two cyclohexylcyclohexenes. Polarized photoisomerization was achieved by irradiating an azo-modified quartz plate with polarized visible light P 4 3 0 nm). Figure 3 shows that there exists photoinduced dichroism with a relatively small dichroic ratio (AIIAII = 1.18).Sincethe photostationary state under this illumination condition consists of major part of the E-isomer, as stated above, the dichroism is ascribed to the E-isomeric chromophores on the surface. Figure 3 shows that the absorbance (AJ of the chromophore with the molecular axis perpendicular to the polarization plane of the actinic light is larger than when it is parallel to the polarization plane (All). To the authors' knowledge, this is the first such example of the photodichroism of dye molecules adsorbed on a solid surface. Photoresponsiveness. A cell was fabricated by sandwiching a nematic LC mixture of alkoxylphenyl cyclo(10) Seki, T.;Ichimura, K.Polym. Commun. 1989,30,109.
Langmuir, Vol. 9, No. 11,1993 3301
Photoregulation of Liquid Crystals
.,,I .
0
3
~
itoring
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1
/ nm Figure 3. Polarized absorptionspectraof a quartz plate surfacemodified with the azobenzene(1)after irradiationwith polarized visible light (>430nm). The polarization plane of the probing light is (a) parallel (-) and (b) perpendicular (- - -) to that of the actinic light. Wavelength
I
>430nm
0
I 90 Rotatlon 180 angle (
4 ) /270 degree
360
Figure 5. Angular (4) dependenceof transmittanceof monitoring He-Ne laser beam through the cell upon exposure at 20 'C to linearlypolarized visible light with 8 = Oo as a function of exposure time: 0 min, 0; 1 min, 0; 3 min, A;5 min, 0 ; 10 min, 0 ;20 min, A.
Figure 4. Cell structure and actinic light exposure conditions. hexanecarboxylates of TNI= 74 "C, DON-103, between a quartz plate modified with 5 and a quartz plate surfacemodified with lecithin for homeotropic alignment (Figure 4). The hybrid cell thus prepared was bright between a couple of crossed polarizers, indicating that a surface modified with 5 causes a planar mode. Exposure of the cell to UV light induced no transmittance alteration of linearly polarized light. When the cell was irradiated with linearly polarized visible light of wavelengths longer than ca. 430 nm for the n-u* excitation, the emergence of birefringence was observed. The polarized light was incidentperpendicularly f i t upon the azo-modified plate in order to prevent the distortion of the linear polarization of the actinic light. The cell axis is defined here as the direction of the longer sides of the rectangular cell, and the polarization plane angle (8) contained by the cell axis and the polarization plane of actinic light is illustrated in Figure 4. The birefringence was evaluated by measuringthe intensity of a linearly polarized He-Ne laser beam as a monitoring light through the cell and a crossed polarizer placed behind it as a function of the rotation angle (4) of the cell. 4 is defined as an angle between the cell axis and the polarization plane of monitoring H e N e laser beam, indicated in Figure Ib. Figure 5 shows the dependence of the transmittance of the monitoring light through the cell upon the rotation angle (4) during the course of the exposure to light with the polarization plane at B = 0". Figure 5 shows that although the initially prepared cell displayed an irregular dependence of the transmittance upon 4 became of the
0
90
180 Rotation angle
270
360
a / degree
Figure6. Angular (4) dependenceof transmittanceof monitoring He-Ne laser beam through the cell upon exposure at 100 ' C to linearly polarized visible light with polarization plane of (a)8 = ' 0 (O), (b) -30' (VI,(c) -45' (A),and (d) -60' (01,respectively.
lack of in-plane anisotropy, prolonged irradiation with the linearly polarized visible light at 20 "C resulted in the appearance of a regular pattern due to the optical anisotropy. The longer the exposure period was,the larger became the transmittance at the maxima occurring roughly at 45" + 90" X n, suggesting that the direction of the LC molecules exists in parallel with or perpendicular to the polarization plane. The unevenness of the maximum transmittances may reflect the imperfection of the homgeneous alignment under these irradiation conditions. Temperature Effect on the Emergence of Birefringence. It was found that irradiation of the cell at an elevated temperature improves considerablythe regularity of the transmittance as a function of 4 and boosts the birefringence. Figure 6 shows the birefringence of the cell exposed to the linearly polarized visible light with various polarization planes (0) at 100 "C, as a typical example. It
Ichimura et al.
3302 Langmuir, Vol. 9,No. 11,1993 loo S
i I I
"0
90
180
270
360
0
90
180
Rotation angle
270
0 /degree
360
Figure 8. Angular (4) dependenceof transmittanceof monitoring He-Ne laser beam through the cell exposed to linearly polarized light with B = Oo before (0) and after heating at 100 "C for 1 h (A)and after subsequent exposure to the unpolarized visible
light at 100 OC ( 0 ) .
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0' 0
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I80
270
--I
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Rotation
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Figure 7. Shift of angular (4) dependence of transmittance of monitoring He-Ne laser beam through the cell upon exposure (a) at 20 "C for 0 min (O),3 min (A),and 20 min (O), (b) at 50 O C for 0 min ( O ) , 3 min (A),and 10 min ( O ) , and (c) at 100 OC for 0 min (O),0.5 min (A),and 3 min ( O ) , respectively,to linearly polarized visible light with polarization plane of B = 4 5 O . The cell is illuminated with linearly polarized light where 0 = OD in advance for generation of the homogeneous alignment. should be noticed that the mesophase is converted into the isotropic phase at the exposure temperature and that the birefringence appears by cooling the sample down at ambienttemperature. It is shownclearlythat the direction of in-plane alignmentcan be controlled at will by selecting a suitable value of 8. The dynamic alteration of birefringence was followed by exposing the cell, which was irradiated in advance with the light of 8 = Oo at 100OC for the homogeneousalignment, to the same light after rotating the polarization plane at 0 = -45O at various temperatures. The shift of the birefringence pattern took place reluctantly at 20 OC. The patterns displayed a parallel shiftwithout any modification of their shapeat higher temperaturea. Figure 7s u m " the angular shift of the birefringence patterns as a function of the exposure energy. The figure indicates that the temperature affects not only the reorientation rate but also the extent of the shifted angle; at room temperature the transmittance at 4 = Oo was hardly modified while a marked change in the transmittance occurred at 100 OC. The azimuthal reorientation is complete at an exposure energy of ca.200 mJ/cm2 at higher temperatures. Stability. A characteristic feature of the present photoresponsive cell driven by linearly polarized light is the anomalous stability to heat treatment and also exposure to nonpolarized visible light. As shown in Figure 8, the birefringence of the cell induced by irradiation with
actinic light at 100 "Cwas not altered at all after heating at 100 OC for 1 h. Interestingly, even more severe storage conditionsinvolvingillumination with nonpoiarized visible light at 100 OC for 1 h did not affect the birefringence. The birefringence is not affected at even more higher temperature of 120OC for 1 h while an LC cell surface-mudified with p-cyanoazobenzene units loses ita photoinduced birefringence completely after heating at the same temperature only for 5 min.13 In relation to the applicability for storage as well as display devices of optical informations, preliminary evaluation was made on the image formation. The image exposure of the cell through a photomask reduces the resolution power owing to interference of the actinic light which passes through an ammodified quartz plate of 1mm thickness before absorption by the azobenzene molecular layer. Consequently, the azo-modified plate was first wetted with the LC and covered with a glass plate, followed by irradiating the cell uniformly to result in the homogeneous alignment. The glass plate was subsequently removed and replaced by a photomask made from a quartz plate to put the LC between the azo-modified plate and the photomask plate. The cell was exposed to the actinic light after rotating the polarization plane 45O. Figure 9 shows polarized micrographs of the illuminated azomodified plate covered with the LC layer which is exposed to air directly; clear images are obtained. Positive as well as negative tones are observed by rotating the plate 45O or 135O.
Discussion The emergence of the azimuthal anisotropy induced by laterally attached azobenzeneunits may be interpreted as follows. The linearly polarized light is preferentially absorbed by the E-isomer azobenzeneunits which possess the transition moment for the n-r* transition in parallel with the electric vector of the visible light yielding isomerization to the 2-isomer. With a slightly larger absorption coefficient and a higher quantum yield for the photoisomerization of the 2-isomer in the visible region, the 2-isomer units thus formed are reversed readily again into their E-counterparts to result in a photostationary state containing a major amount of the E-isomer. Continuous excitation with the polarized light leadsto selective excitation of the E-isomer with the transition moment parallel with respect to the electric vector of the incident light to bring about the photodichroism,asshown in Figure 3. As a result, the longer molecular axis of the azobenzenes bound on the surface rotates roughly perpendicularly to the polarization plane. In this context, the azobenzene unita bearing two long chain substituents acta as ''molecular rotors" which rotate around the spacer as a "molecular
Photoregulation of Liquid Crystals
Langmuir, Vol. 9, No. 11, 1993 3303
V
uartz date
Figure 10. Illustrativerepresentation of in-plane reorientation of LC molecules induced by azobenzene units as “molecular rotors”attached laterally onto substrate surface upon excitation with the linearly polarized light.
Figure 9. Polarized micrographs of the photoimages recorded by the azo-modified plate covered with the LC layer. The homogeneouslyaligned cell was illuminated through a photomask with visible light of a polarization plane which was tilted 45’ toward the direction of the homogeneous alignment. Positive as well as negative tones are observed by rotating the cell 45’ or 135’ under a polarized microscope.
axle”. Rotation of the molecular rotor induces subsequent rearrangement of the LC molecules surrounding the azobenzene units to cause the bulk homogeneous alignment. Such a photoselection of azobenzenes upon excitation with linearly polarized visible light has been reported in a series of polymeric LCs substituted with azobenzenes in their side chains.ll The photoselection mechanism for this system has been proposed, as follows. The excitation of the azo-chromophores with linearly polarized light brings about the photoisomerizationfrom the E- into the 2-isomer which reverses into the E-isomer. The repeated isomerization cycles cause the alteration of the original orientational distribution to result in the rotation of azochromophores. It has been recently reported that a rubbed polyimide thin film doped with dye molecules triggers the reorientation of a nematic LC by the action of a linearly polarized Ar+ laser beam to afford a novel photoresponsiveLC ce11.12 A mechanism based on a combinationof the photochemical and thermal isomerizations has been mentioned for this (11) (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.;Yokovlev,I.; Kostromin,S.;Shibaev, V.; Laesker,L.; Stumpe,J.; Leysig, D. Makromol.Chem.,Rapid Commun. 1991, 12, 709. (e) Eich, M.; Wendorff, J. H.; Reck, B.; Ringsdorf, H. Makromol. Chem., Rapid Commun. 1987,8,59.
surface-assisted photoalignment contr01.l~The involvement of the thermal reversion process is disputable; no direct evidence has been presented for the photoisomerizability and the existence of the short-lived 2-isomer of the dye dopant. Although the working principle seems to be closely related to that of our systems, the reorientation of the dye moleculesmay involve rather a thermal process of local heating since much larger exposure energies (540 J/cm2)13*are required, whereas smaller exposureenergies of about 100 mJ/cm2 yield photoimages in our cells. The photostationary state contains about 30% of the Z-isomer under the present irradiation condition, as stated above (Figure 2). Because the 2-isomer possesses a strongly bent form like a V-shape, it is assumed that this isomer may play a certain role in the alignment control. This is not the case, however, since there was no modification of the pattern shapes due to the birefringence even though the thermal reversion to the E-isomer was complete after heat treatment. This means that the rotation of the director of homogeneously aligned LC molecules is induced specifically by the rotation of the azobenzene units in the rodlike E-form (Figure 10). This mechanism is in line with the observationthat no distinct alignment control was caused by irradiation with linearly polarized UV light which produces a major part (ca. 90 % ) of the 2-isomer. The present phenomenon is obviously closely related with the LC alignment induced by various substrates modified by various methods using rubbed polymericthin films,14transferred Langmuir-Blodgett molecular films,15 oblique evaporation of inorganic materials,16and photolithographic gratings1’ for the fabrication of conventional LC displays. Recent studies on the LC molecular alignment at the interface between LC layer and modified surfaces based on the SHG (second harmonic generation) technique have revealed that the homogeneity of the bulk LC alignment is caused by the in-plane order parameter of the surface LC molecules when rubbed polyimide thin filmsare employed for the alignment regu1ation.l8,l9 This ordered structure of the surface LC molecules is induced by the molecular interaction with the polyimide polymer chains which are oriented by the rubbing treatment. In analogy to the cases, the alignment of the surface LC (12) (a) Gibbons, W. M.; Shannon, P. J.; Sun, S.-T.; Swetlii, B. J. Nature 1991,351,49. (b) Gibbons, W. M.; Shannon, P. J.; Sun, S.-T.; Swetlin,B. J. Proc. SPIE 1992,1665,184. (c) Gibbons, W. M.; Shannon, P. J. Liq. Cryst. 1992,12,869. (d) Gibbons, W. M.; Shannon, P. J. Proc. SPIE 1992,1815,59. (13) Jones, C.; Day, S. Nature 1991,351, 15. (14) Greary, J. M.; Goodby, J. W.; Kmetz, A. R.; Pate, J. S. J. Appl. Phys. 1987,62,4100. (15) Kakimoto, M.; Suzuki, M.; Tonishi, T.; Imai, Y.; Iwamoto, M.; Hino, T. Chem. Lett. 1986, 823. (16) Urbach, W.; Boix, M.; Guyon, E. Appl. Phys. Lett. 1974,25,479. (17) Wolf, U.; Greubel, W.; Kruger, H. Mol. Cryst. Liq. Cryst. 1977, 23, 187. (18) (a) Chen, W.; Feller, M. B.; Shen, T. R. Phys. Rev. Lett. 1989,63, 2665. (b) Feller, M. B.; Chen, W.; Shen, Y. R. Phys. Rev. 1991,A43,6778. (19) Barmentlo, M.; van Aerle, N. A. J. M.; Hollering, R. W. J.; Damen, J. P. M. J. Appl. Phys. 1992, 71,4799.
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3304 Langmuir, Vol.9,No.11,1993
molecules of the present system is governed by the molecular interaction between the LC molecules and the azobenzeneunits which are more or less uniaxially oriented by the linearly polarized light irradiation. One of the characteristic features of the cell is the extreme stabilitytoward heat and unpolarized light (Figure 8). This means that little randomization of azimuthal reorientation of the surface azobenzene units takes place during the irradiation with linearly polarized light even above TNI.According to our previous works, although the photoalignment induced by a surface spiropyran or by p-cyanoazobenzene is stable at room temperature, it disappeared completely after heating at 120 OC only for 5 min.8 The anomalous thermal stability of the photoinduced birefringence reflects obviously the fact that the photoselectiontakes place even at temperature higher than TNIwhere the bulk LC is converted into the isotropicphase. These imply that the boundary region between the substrate surface and the LC layer differs markedly from the bulk LC in the molecular aggregation properties and is termed often as a "smectic-like 1ayeP.l The presence of wall-induced,pretransitional birefringence of a nematic LC above ita TNIhas been confirmed experimentally.20It has also been reported that flow-induced homogeneous alignmentof a nematic mesophase induced by anisotropic adsorption of the LC molecules onto an evaporated Si0 film in not affected even by heating above TNIfor a
prolonged period.21 Thus, the thermal stability of the photoinduced anisotropy of the present cell most likely arises from the pretransitional ordered structure of a ultrathin mesophase layer, in which the thermally induced randomizationof the azobenzenes tethered to the surface is highly suppressed because the collective molecular rotations of azo chromophore around the molecular axle provided by the spacer group is restricted in the LC molecular layer closely packed at the surface. The present LC cells promise the applicability to a novel type of photon-mode erasable photomemory with excellent storage stability and display devices stable toward ambient light, as discussed by Gibbons et al.'2 In marked contrast to the conventional LC devices using a couple of transparent electrodes, image formation can be achieved by a single photoactive molecular layer, as shown in Figure 9. Furthermore, it should be emphasized that the photoimages are formed by light absorption at the n-* band of an azo-modified plate possessing an absorbanceof only ca. 0.001. The pesent photoactive LC cells are interesting from a practical standpoint because they can be overwritten repeatedly by changing solely the polarization plane angle of a single light source. By contrast other types of photon-mode memory media based on photuchromism requires at least two light sources for writing-in and erasure,so that overwritingis impossible until a special device for light source is undertaken.
(20) (a) Miyano, K. Chem. Phys. Lett. 1979,43,61. (b) Myano, K. J. Chem, Phys. 1979, 71,4108.
2845.
(21) Yokoyama, H.; Kobayashi, S.;Kamei, H. J. Appl. Phys. 1984,66,
