Langmuir 1992,8, 1007-1013
1007
Factors Affecting Photoinduced Alignment Regulation of Cyclohexanecarboxylate-TypeNematic Liquid Crystals by Azobenzene Molecular Films? Koso Aoki Toda Kogyo Corp., 4-1 -2 Funairi-Minami, Naka-ku, Hiroshima 730, Japan
Takahiro Seki, Yasuzo Suzuki, and Takashi Tamaki Research Institute for Polymers and Textiles, 1 - 1 -4 Higashi, Tsukuba, Zbaraki 305, Japan
Akira Hosoki ZCI Japan Ltd., Japan Technical Center, 47 Wadai, Tsukuba, Zbaraki 305, Japan
Kunihiro Ichimura’ Research Laboratory of Resources Utilization, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 227, Japan Received September 17,1991 Azobenzene units attached on quartz plates through Si-0 bonding exhibit reversible photoisomerization and regulate the alignment of a nematic liquid crystal (LC) between homeotropic and planar modes when LC cells are constructed by putting the LC between two plates covered with the azo-modified surfaces which have been called “command surfaces”. The efficiency of the command surfaces is affected crucially by the molecular structure and the occupied area of azobenzene unit. Concerning the molecular structural effect, the hydrophobic nature of a substituent at the azobenzene moiety holds an important role in the alignment regulation. The mode of silylation and the length of a spacer unit between the azobenzene moiety and the silylating group are key factors in commanding the alignment regulation as well. There is a critical area occupied by an azobenzene unit for the photoregulation of LC alignment. An occupied area larger than ca. 120 A2 results in no photoregulation. The occupied area also affects the photoresponse pattern, and discussion is given on the existence of a threshold of the command surface property for the photoregulation. Effects of cell thickness and the exposure cycles on the photoresponse are also presented.
Introduction The nature of substrate surfaces plays an essential role in the orientation of liquid crystalline molecules, and various methods for the surface modification have been developed to control the alignment of liquid crystals (LCs) as a key technology in the production of LC display devices.’ Typical examples of the alignment control are the surface modification with long alkyl silylating reagents for homeotropic alignment and the coating of substrate plates with a thin film of polymers like a polyimide for parallel alignment. These facta indicate that the alignment of LC molecules throughout the cell is determined by the molecular interaction between the substrate surface and nearest neighbor LC molecules. During the course of our investigation on photochromic LC systems, the cholesteric mesophase of nematic liquid crystals doped with chiral azobenzene (Az) was found to be converted into the isotropic phase after the elongation of cholesteric pitch upon UV exposure for the photoisomerization of the Azs from the trans to the cis isomer. Similar systems demonstrating photoinduced mesophase alteration between nematic and isotropic phases have been
* To whom correspondence should be addressed. + Reversible Alignment Change of Liquid Crystals Induced by Photochromic Molecular Films: Part 13. Part 12: Kawanishi, Y.; Tamaki, T.; Seki, T.; Sakuragi, Y.; Ichimura, K. Proceedings of the 5th International Conference on Unconventional Photoactive Solids (Okazaki), 1991;pp 565-570. (1) Cognard, J. Mol. Cryst. Liq. Cryst., Suppl. Ser. 1982, 1, 1.
0743-7463/92/2408-1007$03.00/0
studied in more detail by Tazuke and his co-workers who have employed nematic LCs and achiral Azs as photoactive dopants.2 The systems have been extended to polymeric LCS.~These photoinduced mesophase changes can be interpreted as the result of drastic alteration of the molecular shape of Azs between the rodlike trans isomer, which mimicks the size and shape of nematic LC molecules, and the V-shaped cis isomer, which disturbs the unidirectional orientation of LC molecules to bring about the destruction of the mesophase. Such an interpretation for the photoinduced mesophase change and the essential role of molecular interaction between substrate surfaces and LC molecules led us to an idea that alignment regulation is possible when Az units are attached to a substrate surface since the photoisomerization of Az units results in the reorientation of LC molecules surrounding the photoactive units. Indeed, the photoinduced regulation of the alignment took place when a nematic LC was sandwiched between two glass plates, the surface of which was modified with an Az monolayer by silylation, as reported preliminarily (Figure l).4Since a couple of the monolayered Az units on both substrates regulate the alignment of about lo4LC molecules between them, we have referred to these photochromic surfaces as “command surfaces”. Our subsequent papers have re(2) Tazuke,
s.;Kurihara, s.;Ikeda, T. Chem. Lett. 1987, 911.
(3) Ikeda, T.; Kurihara, S.;Karanjit, D. B.; Tazuke, S. Macromolecules 1990,23, 3938. (4) Ichimura, K.; Suzuki, Y.; Seki, T.; Hosoki, A.; Aoki, K. Langmuir 1988, 4 , 1214.
0 1992 American Chemical Society
1008 Langmuir, Vol. 8, No. 3, 1992
Aoki et al. liquid cry stollim molecules
M/f
mobenzene monolayer
M
1
1P
He Ne laser
C
,
A
F
Figure 1. Illustrative representation of liquid crystallinealignment change induced by azobenzene monolayer. Table I. Azobenzene Carboxylic Acids
Ri Azc-1 Azc-2
C6H13 ca17
R2 O(CH2)6 O(CH2)6
c1 H CH30 C6H130 C6H13 C6H13 C6H13 C6H13
O(CH2)5 O(CH2)5 O(CH2)3 O(CH2)2 OCH2 O(CH2)lO
Recorder
Figure 2. Experimental setup for measuring the LC alignment alteration: C, LC cell; P, polarizer; F, filter; A, analyzer.
mp, "C
144-146 142-143
Azc-3 Azc-4 Azc-5 Azc-6 Azc-7 AZC-8 Azc-9 Azc-10 AZC-13
I
165169 143-145 >300 278-282 162-162 15Ck159 189-191 106-108
vealed that the methods of surface modification other than the silylation involve the spin-coating or adsorption of Az pendent polymer^^^ and the deposition of a LangmuirBlodgett film of the Az polymers on substrate plates.8 This paper will deal with the detailed studies on the photoresponsive LC cells driven with the aid of the command surfaces, focusing on the factors affecting reversible alignment change of a cyclohexanecarboxylatetype nematic LC.
Experimental Section Materials. Nematic LC (DON-103), a gift from Dainippon Ink and Chemicals,Inc., is a mixture of 4-alkoxyphenyl4-alkylcyclohexanecarboxylates(1) of K-17-N-73-1, showing An = 0.09 and E = -1.3. Commercially available silane compounds, ethyltriethoxysilane (ETS), octyltriethoxysilane(OTS), (&aminopropyl)triethoxysilane (ATS),(3-aminopropy1)methyldiethoxysilane, and (3-aminopropyl)dimethylethoxysilane,were all purified by distillation before use. Azobenzenes. Para-substituted benzenediazonium hydrochlorides were coupled with phenol to afford 4-substituted 4'hydroxyazobenzeneswhich were purified by column chromatography on silica gel and subsequent recrystallization. The hydroxyazobenzeneswere reacted with tetrahydropyranyl esters of w-halocarboxylic acid in the presence of potassium carbonate in DMF under efficient stirring to give the corresponding Az esters which were hydrolyzed to give Az carboxylic acids. 4(Methoxy)- and (hexy1oxy)azobenzenecarboxylic acids were prepared by the alkylationand the subsequenthydrolysisof ethyl 4-(4-hydroxyphenyl)azobenzoate. The structural elucidationof the azobenzenecarboxylicacids listed in Table I was made by means of NMR spectroscopy. General Procedure for the Preparation of Azobenzene Silylating Reagents. To an ice-cooled solution of an equimolar mixture of Az carboxylicacid and aminoalkylsilanein dichloromethane was added a slight excess amount of dicyclohexylcarbodiimide, and the solution was stirred for 3 h at room (5) Ichimura, K.; Suzuki, Y.; Seki, T.; Kawanishi, Y.; Aoki, K. Makromol. Chem., Rapid Commun. 1989,10, 5. (6) Kawanishi, Y.; Seki, T.; Tamaki, T.; Ichimura, K.; Ikeda, M.; Aoki, K. Polym. Adv. Technol. 1990, 1, 311. (7) Kawanishi, Y.; Tamaki, T.; Seki, T.; Sakuragi, M.; Suzuki, Y.; Ichimura, K. Langmuir 1991, 7,1314. (8)Seki, T.; Tamaki, T.; Suzuki, Y.; Kawanishi, Y.; Ichimura, K.; Aoki, K. Macromolecules 1989,22,3505.
temperature. After removal of the urea, the solvent was evaporated under reduced pressure to give a residual oil which was dissolved in absolute ethanol. The concentration was adjusted to 0.5 w t %. Surface Modification. Quartz plates were purified ultrasonically in acetone,in concentrated nitric acid and in saturated sodium bicarbonate aqueous solution in this sequence. Each ultrasonication was carried out for 10 min, followed by washing in deionized water. After drying at 120 "C for 30 min, the plates were dipped in an ethanolic solution containing an Az silylating reagent along with or without ETS, OTS, or ATS at room temperature for 10 min. The modified plates were dried at 120 "C for 30min, washed ultrasonicallyin dichloromethanefor 10 min, and dried at 120 "C for 30 min. Alternatively, the cleaned quartz plates were dipped in a toluene solution of 1w t % ATS and heated at 90 "C for 24 h. The plates were supersonicated in benzene for 10 min, followed by dryingat 120"C for 10min to afford aminated plates. Azobenzene carboxylicacid was treated with thionyl chloride to give the corresponding acyl chloride which was dissolved in benzene to ! solution. The aminated plates were dipped prepare a 0.7 w t % in the solution for 2 h, ultrasonicated in pure benzene for 5 min, and dried at 120 "C for 10 min to afford Az-modified plates. The occupied area per Az ( S in A2/molecule)was estimated according to eq 1,where E and A are the absorption coefficient respectively. in ethanol and an absorbance at, ,A
S = lO"'J(6.023A)
(1)
Physical Measurements. NMR spectra were taken on a Nicole NT-360 NMR spectrometer, and electronic absorption spectra were recorded on a Shimadzu UV-220 spectrometer, respectively. Measurement of Photoresponse. The experimental setup is shown in Figure 2. A 500-Whigh-pressure mercury arc was used as a light source,and glass filterswere employed for selecting UV (365 nm) and visible (ca. 440 nm) light. An LC cell was exposed to UV and subsequentlyto visible light. The alignment change was monitored by following the intensity of a linearly polarized H e N e laser beam passed through the celland a crossed polarizer.
Results and Discussion Surface Modification. Surface modification of silica has been achieved readily with use of various kinds of silane coupling reagent^.^ It has been reported that monolayer deposition on Si02 substrates takes place readily through Si-0 bond formation with use of long-chain alkyltrichlorosilanes to give closely packed monolayered films quite similar to the Langmuir-Blodgett films.1° We designed the molecular structure of silylating reagents bearing Az units (2) and prepared a family of the surface modifiers. Alkoxysilyl residues instead of trichlorosilyl were employed here because of the ease in the synthesis. The surface modification of quartz plates with these reagents was carried out in an ethanolic solution, followed by heat treatment (method A). An alternative method (9) Plueddemann, E. P. Silane Coupling Agents; Plenum Press: New York and London, 1982. (10) Sagiv, J. J. Am. Chem. SOC.1980,112, 92.
Langmuir, Vol. 8, No. 3, 1992 1009
Photoinduced Alignment Regulation of Liquid Crystals
Scheme I
1
I
for attaching Az units onto substrate consists of the acylation of aminopropylated silica surface with Az acyl chlorides (method B) (Scheme I). The amount of Az unit attached onto the plates was estimated with use of the absorption coefficient of Az in ethanol under the assumption that t on the surface does not much differ from t in a solution. For example, the occupied area per Az-1 unit ( E = 3.19 X lo4) substituted with hexyl as the head group prepared accordingto method A was estimated to be 36 A2according to eq 1. The validity of the estimation was examined by treating the modified quartz plate with a strongly alkaline methanolic solution to detach the Az-1 unit thoroughly from the surface and by measuring the absorbance in the solution. The occupied area obtained by this technique was calculated to be 32 A2 and fairly in line with the aforementioned value. The occupied areas for other Az units calculated according to eq 1are compiled in Table I1 for further qualitative discussions. An occupied area of the closely packed Az unit was obtained through the *-A curve measurement of the amphiphilic Az carboxylic acid (3) at a water-air interface and was 25 A2 per Az molecule." The fact that the occupied areas of Az units listed in Table I1 are larger than that in the LB film may arise from the flexibility of the molecular chain of the Az silylating reagents due to the ether linkage and the hydrophilic character of the amide bond of Az-1. n
(Nematic L C )
( Amphiphilic Az derivative)
Photoisomerization. The relatively loose packing of Az units attached to the quartz surface through Si-0 bond(s) is rather favorable for the photoisomerization of the chromophore. Indeed, no photoisomerization takes place from the trans to the cis isomer when the transferred LB membrane of the amphiphilic Az derivative (3) showing the H-aggregation absorption maximum at 298 nm is exposed to UV light," possibly because the isomerization is accompanied by a considerable expansion of the occupied area which is suppressed by the rigidity of the LB membrane. The reversible photoisomerization of Az unit on the quartz surface was confirmed, as shown in Figure 3 for Az-1 unit as a typical example. The figure shows the presence of two absorption maxima a t 336 and 352 nm for the T-T* transition. I t has been reported that the electronic absorption spectra of azobenzene units are strongly influenced by their aggregation. The absorption maximum at 336 nm is assignable to the face to face dimer (11)Seki, T.; Ichimura, K. Polym. Commun. 1989, 30, 109.
a 1
I
300 LOO 500 Wavelength ( n m )
0.02 I
I b
I
Wavelength ( nm )
Figure 3. Spectral changes for azobenzene-modified quartz plates on exposure to 365 nm at intervals: (a) a plate modified with Az-1; (b) a plate modified with a 1:9 mixture of Az-1 and
ATS.
of Az units whereas that at 352 nm is of the monomeric unit according to Shimomura and his co-workers.12 The deviation from the isosbestic points in the spectral change of Az unit upon photoirradiation reflects the existence of the two kinds of molecular environment of the Az unit. When the surface modification was made with use of a mixture of Az-1 and ATS in a 1:9 molar ratio, the Az unit showed a single maximum at 349 nm, and the spectral change during the course of the photoisomerization exhibited an isosbestic point. ATS acts as a twodimensional diluent for the Az unit to result in the monomerization. It was confirmed that other Az units listed in Table I1 were all photoisomerizable on the silica surface. Alignment Alteration. LC cells were constructed by sandwiching a nematic LC of a mixture of cyclohexanecarboxylates (1) between two plates modified with Az-1 and those modified with a 1:9 mixture of Az-1 and ATS. The cell thickness was adjusted by suspending tiny glass rod spacers of 8 Fm diameter in the LC. I t was observed that it took few minutes for the cells to become homeotropic throughout the cell. The cell, for instance, appeared bright between crossed polarizers just after the cell construction. Soon after, small dark spots came out in the bright zone, and the areas of the spots became larger and spread finally throughout the whole of the cell. When the cells were exposed to UV light, they became bright between two crossed polarizers, indicating the appearance of bifringence of the LC layer. This is reasonably interpreted as the LC alignment alteration into a planar alignment where the long axis of the LC molecules is reoriented in parallel with the substrate surface. The alignment alteration was followed by monitoring the light intensity of a transmitted linearly polarized He-Ne laser beam through each cell and a crossed polarizer placed behind the cell. The photoresponse upon alternate exposure to UV and visible light is illustrated in Figure 4. Intermittent exposure to UV and visible light caused the discontinuous increase and decrease in the transmittance, respectively, as given in Figure 5. The gradual decrease or increase of the transmittance after the interruption of the photoirradiation reflects the thermal reversion of the cis isomer. These findings led us to carry out systematical examination of factors affecting the photoinduced alignment alteration. Molecular Structure of Azobenzenes. A variety of Az derivatives (2) having different head groups (RI), (12)Shimomura, M.; Ando, R.; Kunitake, T. Ber. Bunsen-Ges.Phys. Chem. 1983,87, 1134.
1010 Langmuir, Vol. 8, No. 3, 1992
Aoki et al.
Table 11. Surface Modification of Quartz Plates with Azobenzene Silylating Reagents
Table 111. Effect of the Molecular Structure of Azobenzene Silylating Reagents (2) on the Alignment Regulation of the Cyclohexanecarboxylate-TypeNematic Liquid Crystal alignmenta
too
0
Exposure time
200 (sec)
Figure 4. Photoresponse of an LC cell fabricated with a couple of plates modified with a 1:9 mixture of Az-1 and ATS upon alternate exposure to UV and visible light.
OTF
22
:+a c
2I-
cell no.
plate no.
initial
c-1
c-2 c-3 c-4 c-5 C-6 c-7
Az-1 Az-2 Az-3 Az-4 AZ-5 Az-6 Az-7
h h h P
C-8 c-9 c-10
Az-8 AZ-9 Az-10
c-11
Az-11 Az-12 Az-13
C-12 C-13
OFF
a
O
50
100 150 Time ( s e c )
200
Figure 5. Photoresponse of an LC cell fabricated with a couple of plates modified with a 1:9 mixture of Az-1 and A T S upon intermittent exposure to UV and visible light.
spacers (Rzand R3),and silylating units (SiXYZ) were prepared by the condensation of Az carboxylic acid (Table I) with (aminoalky1)silane and used for the surface modification (method A) of quartz plates. The acylation of aminated quartz plates (method B)was also employed as an alternative route to the Az surface modification. The construction of LC cells was done by using Az-modified quartz plates which are compiled in Table 11. The cells
exposure to UV vis
+ + + -
P ~h P P P
h P P
P
P
P
P
-
h h h
P P P
h h h
+ + +
P P h
P P P
P P h
P P P
h
photoregulation
-
-
+
Key: h, homeotropic alignment; p, parallel alignment.
were irradiated alternately with UV and visible light. The results are compiled in Table 111. LC cells for Az-1,Az-2, and Az-3 were all quite dark between a couple of crossed polarizers, indicating that the LC molecules are in a homeotropic alignment. In contrast to these observations, other cells for Az-4, Az-5,Az-6,and Az-7 were inhomogeneously bright between polarizers, suggesting that the trans form of these Az units does not bring about homeotropic alignment, but random parallel.. As summarized in Table 111,the photoregulation of LC alignment is dependent on the nature of the head group (R1). When the cells constructed with plates modified with Az reagents bearing alkyl aubstituents (Az-1, Az-2, and Az-3) were exposed to UV, they became quite bright. They were recovered into the initial dark states upon subsequent exposure to visible light, and the alternate exposure to UV and visible light caused the reversible dark/bright change of the cell. On the contrary, no photoresponse was observed for the Az derivatives without substituents (Az-5)and with chloro (Az-41, methoxy (Az-
Langmuir, Vol. 8, No. 3, 1992 1011
Photoinduced Alignment Regulation of Liquid Crystals Table IV. Surface Modification of Quartz Plates with 0.6% (wt/vol) Ethanol Solutions of Az-1 and Ethyltriethoxysilane (ETS) or Octyltriethoxysilane (OTS)
71
7,
LC
weight ratio ,A,, occupied align- photoreg plateno. Az-1 ETS OTS nm area,& menta ulation 36 H 340 Azl 1 0 346 43 H Azl-E1 1 1 Azl-E3 1 3 346 49 H + H 350 71 Azl-E6 1 6 H 350 120 Azl-E9 1 9 R 350 190 Azl-E50 1 50 R 352 470 Azl-E200 1 200
+ + + +
Azl-00 Azl-09 Azl-050 Azl-0200
1 1 1 1
0 9 50 200
342 350 356 356
34 66 156 192
H H R R
Y h
n U
1
0
20 40 60 Irradiation time of UV light ( sec )
2
0
20 40 60 Irradiation time of VIS light ( s e c )
0
20 40 60 Irradiation time of VIS light ( s e c )
+ +
-
a Alignment mode before exposure to U V H, homeotropic; R, random parallel.
6), and hexyloxy (Az-7),although the photoisomerization took place in their monolayers. Therefore, the homeotropic alignment due to the action of the trans Az isomer seems to be one of the necessary conditions for the photoregulation. The spacer effect is not clear when Az has hexyl as the head group and triethoxysilyl unit since the second family in Table 111(Az-8, Az-9, and Az-10) are all active for the photoregulation. The situation becomes quite different when the Az group is attached to a glass surface through a monofunctional Si-0 bond. While Az-12 does not result in the photoregulation, Az-13having a longer spacer makes the cell photoresponsive. The photoregulaton is also influenced by the mode of silylation, as mentioned above. Mono- and difunctional silylation using Az-12 and Az-11, respectively, failed to afford a photoresponsive LC cell. It may be assumed that the mono- or difunctionalsilylation results in the reduction of the angle contained by the Az molecular axis and the glass surface while the trifunctional silylation using Az-1 is favorable for introducing Az units perpendicularly to the surface. Occupied Areas of Azobenzene Units. The area occupied by one Az unit was controlled by treating plates with a mixture of CsH13AzO(CH2)5CONH(CH2)3Si(OC2H5)3 and ETS or OTS. The results are summarized in Table IV. LC cells were constructed with these Az modified plates and exposed alternately to UV and visible light. No photoresponse was observed when the occupied area per Az exceeds 120A2in the case of ETS. Similar results were observed for OTS. This value of the critical occupied area is quite in line with that observed in one of the other techniques for the preparation of command surfaces. Poly(viny1 alcohol) substituted with Az units has an amphiphilic character and affords a polymeric LB membrane incorporating Az units." The area per Az is readily controlled in this case by regulating the surface pressure of the Az pendent polymer a t a water-air interface and by the subsequent deposition of the membrane onto quartz plates. This novel technique has revealed that occupied areas less than 100 A 2 are active for the photoregulation of the LC alignment and that no photoresponse was observed when the area is larger than 150 A2.8 The occupied area of Az was found to affect the photoresponse pattern. The relationship between the extent of the photoisomerization and the induced alignment alteration was examined. The results for the cell (cell-1) constructed with a couple of plates (plate Az-1) modified solely with Az-1 and the cell (cell-2) with plates (plate
*I,, ,
0
,
,
, /
20 40 60 Irradiation time of UV light ( sec)
Figure 6. Relationshipbetween the extent of the photoisomer-
ization and the induced liquid crystalline alignment alteration: (a) cell-1 upon UV irradiation; (b) cell-1 upon visible light irradiation;(c) cell-2 upon UV irradiation;(d)cell-2 upon visible light irradiation.
Azl-E9) modified with a 1:Qmixture of Az-1 and ETS, respectively, are shown in Figure 6, as typical examples. Upon UV exposure, the transmittance change of cell-1 took place after an induction period whereas cell-2 exhibited an immediate increase in the transmittance although the disappearance of the trans form occurred in the same rate on both plates. These implies that the alignment change from homeotropic to planar mode starts at a critical point determined not by the isomer ratio but rather by the absolute number of the remaining trans-Az unit during the trans-cis photoisomerization. The immediate photoresponse of cell-2 reflects the fact that the occupied area (120 A2)of the Az unit on the Azl-E9 plate is close to the threshold value for the homeotropic alignment, as mentioned above, and the reorientation of LC molecules to form planar alignment is induced quite readily by the isomerization of the trans form. This situation is confirmed by illustrating the relationship between the transmittance of monitoring light and the absorbance at ,A, of the trans isomer (Figure 7). The values of the absorbance were employed here instead of the absolute number of trans isomer because of inaccuracy in the estimation of the latter. The figure reveals the following. First, the abrupt increase in the transmittance of cell-1 upon exposure to UV emerges after more than half of the absorbance of the trans isomer diminishes while the alignment alteration occurs immediately upon UV exposure for cell-2. Second, the reverse process of cell-1 caused by the visible light irradiation is not superimposed on the forward process, showing a hysteresis curve. The implication that the change in the number of trans Az controls the alignment alteration was further supported by the observation that the decrease in the occupied area causes the increase in photoresponse time which is defined as the UV exposure time giving a final transmittance, as presented in Figure 8. Under the present photoirradiation condition using a mercury arc, it is reasonable to assume that the alignment alteration takes place essentially simultaneously with the photoisomerization since the
Aoki et al.
1012 Langmuir, Vol. 8,No. 3, 1992
IM;,
uv
I
n
0.01
0 0.01 5
0.005 0.010 Absorbance
t
30
VIS
60
Az ETS = 1 9
05 \
t
0005 0.010 Absorbance
0015
Figure 7. Dependence of the liquid crystalline alignment alteration upon the trans Az fraction.
011,
'
30 '
60 '
211L222
O0
30
60
Exposure time ( sec
1
Figure 9. Dependence of photoresponseof cells fabricated with the plates modified with Az-1 on the cell thickness.
0
50
100
150
occupied a r m of AZ unr t (
A'
Figure 8. Dependence of the photoresponse time of the cell on the occupied area of the Az unit.
alignment alteration completes within about 0.1 s as revealed by a pulsed laser technique.13 It is noteworthy that there is a break at ca. 50 A2 per Az unit in the figure. It is also suggested that the value is double the minimum occupied area of trans-Az of 25 A2 obtained by tkie LB technique. It has been reported that a loosely packed LB membrane of lecithin molecules transferred onto substrate plates causes homeotropic alignment in contrast to a densely packed LB membrane which affords rather only a distorted alignment.I4 The considerable dependence of LC alignment on the packing of the mphiphilic molecules has been interpreted in terms of specific interaction of lecithin molecules with LC molecules which may stick on the loosely packed LB membrane to result in the homeotropic alignment. There might be a similar situation in the appearance of the break point in the photoresponse timeoccupied area relationship. When the occupied area of Az unit is and exceeds 50 A2,trans& units may interact readily with LC molecules which can stick to the Az molecular layer because of the reduction of space filling of Az on the surface. Under these circumstances, the molecular deformation of the Az units due to the photoisomerization induces more rapid reorientation of the nearest neighbor LC molecules. (13) Ichimura, K.; Suzuki, Y.; Seki, T.; Kawanishi, Y.; Tamaki, T.; Aoki, K.Jpn. J . Appl. Phys., Suppl. 1989,28, 289. (14)Hiltrop, K.; Stegemeyer, H. Ber. Bunsen-Ges. Phys. Chem. 1978, 82, 884.
In the reverse process upon exposure to visible light, the recovery of the homeotropic alignment proceeds more quickly even in the case of cell-1, while the rate of the recovery of the trans form is not influenced by the occupied area. This is agreeable with the above-mentioned interpretation that the criticalpoint for the alignment alteration is determined by the number of trans Az units. Cell Thickness. The effect of cell thickness on the photoresponse was studied using Az-1 plates. The thickness was adjusted by using tiny glass rods of diameters 8, 12, and 20 pm, respectively, or a polyester film of 100 pm thickness. All cells exhibited the homeotropicalignment a t the initial state. Figure 9 represents the photoresponee pattern of the cells upon alternate exposure to UV and visible light. In contrast to the response pattern shown in Figure 4, patterns illustrated in Figure 9 exhibit more than one peak. Although the cells of 8- and 12-pm thickness both showed a similar behavior in the photoresponse, the thicker cells exhibited more complicated response patterns. In particular, the photoresponse of a cell of 100 pm thickness was not only quite poor but also so unstable that the intensity of monitoring He-Ne laser beam was fructuated after UV exposure. Similar resulta were obtained when the cells were made from Azl-Es plates which were prepared by the surface modification with a mixture of Az-1 and ETS in 1:9 ratio. These results allow us to concluded that the critical cell thickness is about 100 pm for constructing effective cells. Such complicated response patterns can be explained quantitativelyaccording to eq 215 where polarized light of A, wavelength with intensity IOis passed through a layer of thickness dof an LC having bifringence An and a crossed polarizer, giving transmitted light intensity I. This equation was originally given to elucidate the relationship of transmittance with applied voltage of electrode-driven LC cells and is modified to eq 2 by replacing An a t applied voltage V with An at tilted angle a. Here, B stands for an angle contained by the polarization axis and a homogeneous axis.
I = I, sin2 (28) sin2 ( T C ~ A ~ ( ~ ) / X )
(2)
Since rotation of ouz LC cells after UV exposure affected no remarkable change in the transmittance of the mon115) Schiekel,M. F.; Fahrenschon, K. Appl. Phys. Lett. 1971,19,391.
Langmuir, Vol. 8, No. 3, 1992 1013
Photoinduced Alignment Regulation of Liquid Crystals
1
12 um
'i
U
,c 0.05E
5
e
b [
0'
1000
2000
3000
Alternate exposure cycle ( times)
1.0
100 urn
0
30 Tilt angle
60
90
(degree)
Figure 10. Relationship between the transmittance of polarized light and a tilted angle according to eq 1 where X = 633 nm, An = 0.09, and d = 8 pm.
itoring polarized light, the alignment is not in homogeneous mode, but the LC layer in the planar alignment forms a microdomain structure so that the direction of the LC molecular axis is highly dispersed. This means that /3 is indefinite in this case, and it is reasonable to assume that sin228 is not influenced by the extent of the photoisomerization and to be constant although it is smaller than unity. The light intensity during the alignment alteration is dependent solely on the change in the tilted angle (a)because other parameters of eq 1are constant. Assuming X = 633 nm and An = 0.09, the relationship between the relative transmitted linearly polarized light intensity defined as I / ( l o sin2 2 /3) and the tilted angle (a)for each cell having 8,12,20, or 100 pm thickness is depicted as in Figure 10, where peaks are shown. The comparison of the response curves given in Figure 9 with the corresponding theoretical curves indicates that the change in the tilted angle induced by the photoisomerization of both Az-1 alone and Az-1 diluted with ETS takes place between 0" and approximately 80". The photoresponse pattern showing no peak as shown in Figure 4 suggests that the tilted angle change induced by the command surface made from a mixture of Az-1 and ATS does not exceed the angle giving a maximal light intensity, 40" for the 8 pm thick cell. Such a smaller tilted angle change results possibly from the effect of the polar comodifier, ATS. Reversibility. A cell fabricated with plates modified with Az-1 was alternately irradiated with UV (365 nm) and visible (ca. 440 nm) light to evaluate the fatigue property. As given in Figure 11,the intensity of polarized monitoring He-Ne laser light transmitted through the UVexposed cell and a crossed polarizer began to decrease after lo00 cycles and completely disappeared at about 2600 cycles. The inactivated cell was decomposed, and
Figure 11. Change in the transmittance of a UV-exposed cell fabricated with Az-1 modified plates after repeating alternative exposure to UV and visible light.
the surface-modified glass plates were washed thoroughly with dichloromethane to reconstruct an LC cell. The cell demonstrated no photoresponse any longer. This means that the degradation of Az units is responsibleto the fatigue of the cell. The repeated exposure resulted in the modification of the photoresponse patterns. This may be caused by the degradation of Az units which reduces the number of effective command molecular units.
Conclusion The alignment regulation by Az molecular films is affected by various factors including the molecular structure of Az moiety, the occupied area per Az unit on substrate surface, cell thickness, and repeating cycles. In particular, the ability of Az moiety to regulate the LC alignment photochemically depends definitely upon a hydrophobic substitutent as a head group like hexyl, octyl, or cyclohexyl. This implies that the homeotropic alignment a t the initial state induced by the trans isomer is one of the requirements for photoregulation of LC. A spacer group and a mode of silylation act also in crucial roles to bring about the photoregulation. Considering the relationship between the alignment alteration and the extent of photoisomerization, the alignment change from the homeotropic to the planar mode takes place at the time when the decrease in trans Az unit exceeds a critical amount of the unit upon UV exposure. This results in faster photoresponse when the occupied area per Az unit attached on substrate plates increases, unless the area does not exceed about 120 A2 where no photoregulation was observed at all. Registry No. DON-l03,115288-48-7;ETS, 78-07-9;ATS,91930-2; OTS, 2943-75-1; Az-l,l15271-04-0; Az-2, 138724-07-9;Az3,1387240&0;Az-4,138724-09-1~Az-5,138724-10-4;A~-6,13872411-5; Az-7, 138724-12-6;Az-8, 138724-13-7;Az-9, 138724-14-8; Az-lO,138724-15-9;Az-ll,138724-16-0; Az-12, 138724-17-1;Az13,138724-18-2;(3-aminopropyl)methyldiethoxysilane, 3179-768; ( 3 - a m i n o p r o p y l ) ~ e ~ y l e ~ o x18306-79-1; ~ i l ~ e , 4-(methoxy)azobenzene carbonyl chloride, 138724-05-7; 4-(hexyloxy)azobenzenecarbonyl chloride, 138724-06-8;quartz, 14808-60-7.
