13002
J. Phys. Chem. 1995,99, 13002-13007
Photochemical Control of Properties of Ferroelectric Liquid Crystals. 1. Effect of Structure of Host Ferroelectric Liquid Crystals on the Photochemical Switching of Polarization Takeo Sasakit and Tomiki Ikeda*” PRESTO, JRDC, Research Laboratory of Resources Utilization, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226, Japan Received: June 1, 1995@
The photochemical switching of polarization was examined for several structures of ferroelectric liquid crystals, and the effects of the structures of the core mesogen and the chiral unit and the length of the flexible tail unit were discussed. Introduction of polar groups into the mesogen resulted in a high efficiency of the photochemical switching of polarization.
Introduction
Ferroelectric LlquM Crystals F
Ferroelectric liquid crystals (FLCs) have been explored for new types of electrooptic materials.’-* FLCs are characterized by the existence of spontaneous polarization (Ps) and specific phase structure, particularly when thinly sandwiched between two glass plates.2 This phase is called the surface-stabilized state, in which FLC molecules are orderly packed in C2 symmetry. When alternating voltage is applied across the two plates, the aligning direction of the long axis of each FLC molecule changes in accordance with the applied voltage. This phenomenon is called an electrooptic effect, with which the light transmittance can be switched through a pair of crossed polarizers placed in front and behind the FLC. The response of FLCs against the applied voltage is very fast, so that FLCs are expected to be applied to a wide range of electrooptical
n I 8,9,10,11 C,Hm+lO
0g-
0 0 0 - C 0 -{H CI -{H-C2H5 CH3
n 9,10,l1,13
device^.^ FLCs possess bistable states with opposite orientation of Ps and distinct critical values of electric field (coercive force, Ec) on switching between the two states in the surface-stabilized states. We have reported a new application of FLC based on a novel working prin~iple.’,~FLC doped with a few mol % of an azobenzene derivative showed photochemically induced switching of polarization. This works in the following sequence of events. (1) The polarization of FLC is aligned in one direction with the aid of the extemal electric field. (2) An electric field ( E ) is applied in the opposite direction to the polarization, which should be small enough to retain the initial polarization unchanged (E < Ec). (3) The cell is irradiated with light which causes a photoisomerization of the photoresponsive guest molecule (azobenzene derivatives). (4) The photochemical reaction of the guest molecule simultaneously lowers the critical value (Ec) of the guestlFLC mixture, and when the Ec value is lowered below E, the polarization will flip to the opposite direction. This process was very fast, and the optical property of the switched area showed a sharp contrast against the unirradiated area. Factors considered to affect the photochemical switching of polarization are magnitude of coercive force, spontaneous polarization, and uniformity of the surfacestabilized state. The FLC used in the previous report was a three-ring FLC with a rather special structure.’.* Thus, to
T To whom correspondence should be addressed. ’ Present address: Institute for Chemical Reaction Science, Tohoku University, Katahira 2, Aoba-ku, Sendai 980, Japan. A fellow of PRESTO, JRDC (1992-1995). Abstract published in Advance ACS Absrracts, August 1, 1995. @
0022-365419512099- 13002$09.00/0
PhotoresponsiveMolecule
Figure 1. Structures of FLCs and a photoresponsive molecule used in this study.
explore the structure-efficiency relationship for the photochemical switching of polarization, studies on FLCs with different structure are necessary. The relationship between the molecular structure of FLCs and Ps has been extensively investigated: a variety of FLCs have been synthesized which contain biphenyl, phenyl benzoate, and pyrimidine moieties for the core mesogen, and a large number of chiral groups were examined. Especially toward the application for display devices, much effort has been directed to obtain FLCs with a high spontaneous polarization and low viscosity. However, for the purpose of the photochemical switching of polarization, those FLCs are not always useful. In 0 1995 American Chemical Society
J. Phys. Chem., Vol. 99, No. 34, 1995 13003
Effect of Structure of Host Ferroelectric Liquid Crystals
TABLE 1: Physical Properties of F'LCs Used in This Study"' phase transition temperature (°C)b structure n 1 2-n 3-n
4-n
5 6
7 8-n
9-n
7 8 9 8 9 10 11 9 10 11 13 8 9 8 8 9 10 11 9
10 11
C C C C C C C C C C C C C C C C C C C C C C
38 32 33 41 41 45 51 59 64 66 69 62 44 33 48 50 65 89 55 53 62 62
SmC* SmC* SmC* SmC* SmC* SmC* SmC* SmC* SmC* SmC* SmC* SmC* SmC* SmC* SmC* SmC* SmC* SmC* SmC* SmC* SmC* SmC*
64 62 52 54
SmA SmA SmA SmA
73 77
SmA SmA
(34) (36) (38) (38) (45) 48 50 57 82 98 73 (50) (49) (52)
SmA SmA SmA SmA SmA SmA SmA SmA SmA SmA SmA SmA SmA SmA
110 70 66 67 77 75 78 98 (50) (53) (55) (57) 70 63 85 120 113 115 112
64 68 72
N
112
I I I I I I I I I I I I I I I I I I I I I
I
Ps (nClcm2)
tilt angle (deg)
82 95 180 120 72 80 75 42 110 124 116 108 2 10 0.6 12 15 77 50 40 48 42
20.1 14.5 16.2 15.3 16.2 21.0 23.0 19.1 12.1 13.6 13.1 14.6 13.3 11.8 4.7 20.0 20.0 20.0 21.0 18.2 15.5 16.2
C, crystal; SmC*, chiral smectic C phase; SmA, smectic A phase; I, isotropic. Values in parentheses were observed only in the cooling process.
this paper, a series of FLCs which possess a simple structure were examined to clarify what factors affect the photochemical switching of polarization. Samples. Structures of FLCs and an azobenzene derivative are shown in Figure 1. A series of FLCs were prepared from a-chloro acid or chiral alcohols, and phase transition temperatures and Ps of these FLCs are listed in Table 1. FLCs with a-chloro acid derived from L-isoleucine are known to exhibit a large spontaneous p o l a r i z a t i ~ n . ~The ~ ' ~a-chloro acid, (2S,3S)2-chloro-3-methylpentanoicacid, was synthesized by means of a nucleophilic substitution of the amino group of L-isoleucine by the chlorine atom via the diazonium salt." FLCs with other chiral were also prepared in order to investigate the effect of the structure of the chiral unit. Commercially available FLC mixtures were also used as references; the ZLI series from MERCK and the NM series and DOF from Dainippon Ink and Chemicals, Inc. An azobenzene derivative with a flexible tail and a chiral group (10) was used as a photoresponsive guest molecule. Introduction of a chiral moiety into azobenzene was aimed at improving the miscibility with FLCs. FLCs were dissolved in chloroform solutions of the azobenzene derivative, and the resulting solutions were evaporated and heated under vacuum for several hours to remove chloroform. The concentration of the azobenzene derivative was 3 mol %. The mixture was then injected into a 2 pm glass cell (with 1 cm2 IT0 electrode, polyimide coated). The samples were heated until they showed a complete isotropic phase and then cooled gradually at 1 "C/min to obtain a fine alignment of FLCs. The sample was annealed at 50 OC overnight before measurement. No aggregation of the photoresponsive molecule was observed in the UV absorption spectrum and also under microscopic observation. Measurements. Phase transition temperatures of FLCs were determined by DSC (Seiko I&E SSC-5000) and polarizing microscopy, and Ps values were measured by the triangular voltage method (50 Hz, f 1 0 Vp-p). The photochemical switching of polarization was measured with the experimental setup as described in the previous paper.8 A Spectron HL-21 Nd:YAG laser (the third harmonic, 355 nm; pulse width, 12 ns fwhm) was used as an excitation light source, and a He-Ne laser (633 nm, 5 mW) was used as an analyzing light source.
The sample was thermostated, placed between two crossed polarizers, and connected to a function generator and an amplifier. The He-Ne laser passed through the sample was focused onto the entrance slit of a Jabin-Ybon HR-320 monochromator. The change in transmittance of the He-Ne laser was measured with a Hamamatsu R-928 photomultiplier and recorded with an Iwatsu TS-8123 storage scope. Measurement of the photochemical switching was performed as follows. Under the static electric field of +10 V, the molecular axis of FLC was aligned in one direction. At this stage, the direction of the incident polarizer was adjusted to coincide with the alignment direction, so that the transmitted light intensity was minimal. The electric field was then turned to the opposite polarity (- 10 V). The molecular axis of the FLC changed its direction by twist angle shift from the original orientation, and the transmittance became large (electrooptic effect'). Thus, the switching of polarization can be monitored by the intensity of the transmitted light through the sample. Then, the electric field was increased gradually (1 V/min) to opposite polarity (+1-4 V). After these procedures, the sample was irradiated with a laser pulse, and the induced switching of the FLC was monitored by the transmitted light intensity.
Results and Discussion Photochemical Switching of Polarization in FLCs with Different Structures. Figure 2 shows the time-resolved observation of the photochemical switching of polarization in two types of FLC. Both FLCs were doped with 3 mol % of azobenzene derivative 10. As seen in this figure, the photochemical switching of polarization is not always induced effectively in any types of FLC. FLC 1 exhibited a significant transmittance change after the pulse irradiation, while FLC 2-8 showed only a small change. The transmittance through the crossed polarizers is plotted as a function of the applied voltage (T-V profile) in Figure 3. In this experiment, the voltage of +10 V was first applied and then reduced gradually at a rate of < 1 V/min to - 10 V. Thus, in the range of 0 to -4 V, the FLC was exposed to the electric field of opposite polarity. There is a distinct difference between (a) and (b). FLC 1 showed a sharp rising-up profile around
Sasaki and Ikeda
13004 J. Phys. Chem., Vol. 99, No. 34, 1995
Transmittance
(a)
0
8
4
12
16
20
Time (ms)
t 'E =cne
Transmittance
U
40-
20-
Figure 4. Schematic illustration of the effect of the T-V profile on the transmittance change in the photochemical switching of the polarization: (-) trans form of azobenzene; (- - -) cis form of azobenzene.
100
1001
-10
-5
0 5 Voltage (V)
I
I
10
-
0 -10
-5
0
5
10
Voltage (V) Figure 5. Optical hysteresis (T-Vprofile) of FLC 3-9. Measurement was performed at T = Tc - 3 "C.
-10
-5
0 5 Voltage (V)
10
Figure 3. Optical hysteresis (T-Vprofile): (a) FLC 1; (b) FLC 2-8; (0)before irradiation; (0)after irradiation at 355 nm. Both FLCs were doped with 3 mol % of 10. Measurements were performed at T = Tc - 3 "C where Tc indicates the SmC*-SmA phase transition temperature.
the coercive force (Ec), while FLC 2-8 showed a gentle slope at this voltage. After irradiation, the azobenzenes were isomerized and the transmittance profile changed as indicated in Figure 3. The switching process can be represented as illustrated in Figure 4, in which the solid lines indicate T-V profiles for the trans-1OIFLC mixture and the dotted lines denote those for the cis-lO/ELC mixture. FLCs with a profile like (a) can induce a large transmittance change only on a small decrease of Ec, while FLCs with a profile like (b) cannot induce such a large change. Thus, for the photochemical switching of polarization to take place effectively, FLCs with the profile shown in (a) in Figure 4 are favorable. The factors favorable for the photochemical switching are (1) a large Ec with which large bias voltage can
be applied without switching of polarization and (2) a steep curve around Ec in the 7'-V profile. Effect of Molecular Structure on the T-V Profile. ( 1 ) Effect of Structures of Mesogens. Properties like T- V profiles are usually determined by the interaction between FLC molecules and the surface in the surface-stabilizedstates. Switching is closely related to phase properties such as domain creation, growth of domain, and polarization reversal in the surface layer. Control of the phase properties at the molecular level is difficult; however, studies on structures of FLCs would give us a concrete guide to obtain adequate FLC materials for the photochemical switching. The Ec value and the steepness of the T-V profile are expected to be affected by interactions between FLC molecules. Thus, the strategy we adopted at first was to introduce a polar group into the core unit. The T- V profile of FLC 3-9 is shown in Figure 5 . FLC 3-9 possesses carbonyl groups at both sides of the central biphenyl moiety. As seen in this figure, transmittance rose steeply around Ec. The photochemical switching of polarization was measured with a pulsed laser on FLC 3-9 doped with azobenzene 10, and the results are shown in Figure 6. Significant transmittance change was brought about on a single-pulse irradiation. The difference in structure between FLC 2-n and 3-n lies in the existence of
J. Phys. Chem., Vol. 99, No. 34, 1995 13005
Effect of Structure of Host Ferroelectric Liquid Crystals I
-1
-L IUV
8
loor--r--l
80
0
8
4
12
16
20
Time (ms) Figure 6. Time-resolved observation of photochemical switching of the polarization of FLC 3-9 doped with azobenzene derivative 10 monitored by the electrooptic effect; laser power, 120 mJ/cm2. 100
". -6
.
,
-4
-2
.
0
. 2
I
I
4
6
Voltage (V) Figure 7. Optical hysteresis (T-Vprofile) of FLC 4-9. Measurement was performed at T = Tc - 3 "C. an additional carbonyl group at the end of the core unit. Although the effect of molecular structure on the architecture of the molecular assembly seems to be complicated, it is at least clear that the steepness of the T-V profile was caused by the introduction of a carbonyl group into the mesogen. The FLC with phenyl benzoate as mesogen (4-n) was also examined. FLC 4-n possesses a structure in which an additional carbonyl group is introduced into the center of the core unit of FLC 2 4 . FLC 4-9 showed a large Ps; however, the SmC* phase of this FLC appeared only in a supercooled phase and was unstable. As shown in Figure 7, the T-V profile of this FLC showed a steep curve around Ec. Thus, the introduction of polar groups into the mesogen is evidently favorable to obtain steep T-V profiles, although it is not clear at the present stage in which part of the molecules the polar groups work most effectively. We also examined the T- V profiles of commercially available FLCs which are composed of phenylpyrimidine-type host LCs and chiral dopants of cyanocyclopropane and lactone derivatives. These FLCs were prepared for use as active media in display devices, so they possess low viscosity. The T-Vprofile of these FLCs showed small slopes and small Ec values, which are not adequate for the photochemical switching. The required properties of FLC materials for the photochemical switching of polarization turned out to be different from those needed in the display which is driven only by electric field. (2) EfSecr of Tail Length. The length of the flexible tail also influenced the T-V profile. The effect of tail length on properties of LCs has been reported to be ~ignificant.'.~ For instance, many smectic LCs possess a critical tail length which determines appearance or nonappearance of the SmC* phase. Also, many studies on the effect of flexible tail length on Ps have been reported; however, the prediction of Ps as a function of tail length is very difficult. Thus, on analysis of the T-V
0-10-8 -6 -4 -2 0 2 4 6 8 10
Voltage (V) Figure 8. Optical hysteresis (T-V profile) of FLC 3 4 : (0)n = 8; (0)n = 9;(0)n = 10; (W) n = 11. Measurement was performed at T = TC - 3 "C.
-10-8 -6 -4 -2 0 2 4 6 8 10
Voltage (V) Figure 9. Optical hysteresis (T-V profile) of FLC 4-n: (0)n = 9; (0)n = 10; (0)n = 11; (W) n = 13. Measurement was performed at T = TC - 3 "C. 50
I
40{
0
0
30 v
U
20
0
1
0
lo 0
7
1 8
9
10
11
12
n Figure 10. Slope a of the T-V profile of FLC 3-n as a function of the tail length. T = Tc - 3 "C.
profiles, the effect of the tail length is significant. The T-V profiles of FLC 3-n and 4-n are shown in Figure 8 and 9. As seen in these figures, the steepness of the T-V profile was strongly affected by the tail length. To discuss the steepness of the T-V profiles, we define the slope a as the gradient of the T-V profile at Ec. Figure 10 shows the slope a of FLC 3-n as a function of the number of methylene units in the flexible tail. When n = 8 or 9, the slope of the T-V profile was steep enough to induce significant transmittance change on the photochemical switching of polarization. However, as the value of n increased, the slope became small, making it difficult to bring about a large transmittance change. This result may be attributed to the difference in the FLC-surface interaction caused by the different tail length. The packing structure of FLCs near the surface may be strongly affected by the tail length. This difference in packing structure may lead to different interactions between FLC molecules and between the FLC and the surface. As the tail length increases, it seems that
Sasaki and Ikeda
13006 J. Phys. Chem., Vol. 99, No. 34, 1995
40.
5
E
0
1.-
A
30.
0
8
Y
0
20
U
E
E
1°i I
,
,
,
,
,
,
8
9
10
11
12
13
14
Figure 11. Slope a of the T-V profile of FLC 4-n as a function of the tail length. T = Tc - 3 "C.
TABLE 2: Slope a (% V-l) of T-V Profile 9
2-n 3-n 4-n
21 33
20 41
21
5 6 7
10
11
14 31
28
18 38 18
7
>
21
8
Y
U
10
:1!20
10
20
the anchoring of FLC molecules to the surface is weakened and simultaneously interlayer spacing increases. As a result, the interlayer interaction between mesogens is weakened, and hence the rotational viscosity is reduced. Furthermore, when the interlayer interaction is weak, the switching of polarization of each layer occurs individually, so that on increasing the applied voltage with the opposite polarity, the switched domains are created gradually. From the data of slope a, it seems that the shorter tail length ggve the preferable property. However, the phase diagram changed abruptly when the tail length became shorter than n = 9 in FLC 3-n. FLC 3-8 did not exhibit the SmA phase in the higher temperature region of the SmC* phase, so that it was very difficult to align the FLC molecules in one direction by the usual alignment technique and heat treatment. The slope a of FLC 4-n is plotted as a function of tail length in Figure 11. The same tendency was observed as for FLC 3-n. The long tail length reduced the steepness of the T-V profiles. (3) Effect of Structures of Chiral Units. There have been many studies on the relationship between the chiral structure and the magnitude or the direction of Ps.' Quite a number of chiral moieties have been employed as chiral parts in FLCs.It has been reported that the structure of the core mesogen is responsible for thermal stability of the LC phase, while the structure of the chiral unit dominates the ferroelectric properties. Then, the effect of the structure of the chiral unit on the T-V profile was examined. The chiral parts employed in this study were those derived from (2S,3S)-3-methyl-2-chloropentanoic acid, (R)-( -)-2-methylbutanol, and (R)-( -)-2-octanol. The values of the slope a of these FLCs are listed in Table 2. It was observed that the steepness of the T-V profiles was mainly determined by the structure of the core mesogen, not by the structure of the chiral unit. The response time of the FLC to the extemal electric field is determined by the magnitude of the Ps, and the Ps is mainly determined by the structure of the chiral part. Thus, to obtain a fast response in the photochemical switching, introduction of the chiral units into the core mesogen which possesses a steep T-V profile seems to be effective. FLC 3-9 tumed out to be an example we obtained on the basis of this rule. ( 4 ) Effect of Size of Core Unit. To explore the size of the
0
Voltage (V) Figure 12. Optical hysteresis (T-Vprofile) of FLC 8-9 (0) and 8-10 (0). Measurement was performed at T = Tc - 3 "C.
number of carbons in flexible tails, n 8
30-
10 -10
n
I
50-
t:
0
structure
-
70
0
h 7
8
9
10
11
12
n Figure 13. Slope a of the T-V profile of FLC 8-n as a function of the tail length. T = Tc - 3 "C.
8 60 h
70
Q)
0
5 5
'E 2 +
5040-
30-
20-10
0
10
Voltage (V) Figure 14. Optical hysteresis (T-Vprofile) of FLC 9-9 (0)and 9-10 (0). Measurement was performed at T = Tc - 3 "C.
core unit on the T-V profiles, the T-V profiles were also evaluated on three-ring FLCs. Figure 12 shows the T-V profiles of FLC 8-9 and 8-10. As seen in this figure, these FLCs with three-ring structure exhibited a steep slope as well as large values of Ec. The slope a of FLC 8-n is plotted as a function of the number of methylene units in the flexible tail in Figure 13. Different from the results obtained for the two-ring FLCs (Figures 10 and ll), the slope a was not affected significantly by the tail length. The T-V profiles and the slope a of FLC 9-n are shown in Figures 14 and 15. The slope a of FLC 9-n was again not influenced by the length of the tail unit. The large core unit of three-ring FLCs may produce large interactions between FLC molecules and between the FLC and the surface. These interactions might be large enough to ignore the effect of the tail length. The photochemical switching of polarization of 10/8-n and 10/9-n mixtures was explored by the time-resolved measurements. The response times of -1 ms were obtained in these FLCs,and it tumed out that three-ring FLCs are favorable for
Effect of Structure of Host Ferroelectric Liquid Crystals
-
. ;
40
were affected effectively by factors such as the structure of the core unit and the length of the flexible tail (Figure 16). However, the structures of the chiral groups rarely influenced the T-V profiles, while they dominate the ferroelectric properties.
0
30-
>
ae
Y
20-
U
References and Notes
10 -
-
0 8
9
11
10
12
n Figure 15. Slope a of the T-V profile of FLC 9-n as a function of the tail length. T = Tc - 3 "C. Flexlblr tall
J. Phys. Chem., Vol. 99, No. 34, 1995 13007
Chlnl group
Core unlt
t
Polar group
Figure 16. Schematic illustration of the structure of the FLC.
the photochemical switching of polarization because of the large Ec and the large a.
Conclusion The relationship between molecular structure of FLCs and ferroelectric property (T-V profile) was investigated with respect to molecular design of efficient FLC materials for the photochemical switching of polarization of FLCs. Comparison among several structures of FLCs revealed that T-V profiles
(1) Skarp, K.; Handschy, M. A. Mol. Cryst. Liq. Cryst. 1988, 165,439. (2) Clark, N. A.; Lagerwall, S. T. Appl. Phys. Lett. 1980, 36, 899. (3) (a) Ozaki, M.; Sadohara, Y.; Yoshino, K. Jpn. J . Appl. Phys. 1990, 29, 843. (b) Fukushima, S.; Kurosawa, T.; Matsuo, S.; Kozawaguchi, H. Opt. Lett. 1990, 15, 285. (c) Gomes, C. M.; Sekine, H.; Yamazaki, T.; Nakagawa, T.; Kobavashi, S. Neural Nehvorks 1992, 5 , 169. (4)Kondo, K.; Takezoe, H.; Fukuda, A,; Kuze, E. Jpn. J . Appl. phys. 1983, 22, L85. (5) Ishikawa, K.; Ouchi, Y.; Uemura, T.; Tsuchiya, T.; Takezoe, H.; Fukuda, A. Mol. Cryst. Liq. Cryst. 1985, 122, 175. (6) Hatano, T.; Yamamoto, K.; Takezoe, H.; Fukuda, A. Jpn. J . Appl. Phys. 1986, 25, 1762. (7) Ikeda, T.; Sasaki, T.; Ichimura, K. Nature 1993, 361, 428. (8) Sasaki, T.; Ikeda, T.; Ichimura, K. J . Am. Chem. SOC.1994, 116, 625. (9) Sakurai, T.; Mikami, N.; Higuchi, R.; Honma, M.; Ozaki, M.; Yoshino, K. J . Chem. Soc., Chem. Commun. 1986, 978. (10) Tinh, N. H.; Salleneuve, C.; Babeau, A.; Galvan, J. M.; Destrade, C. Mol. Cryst. Liq. Cryst. 1987, 151, 147. (11) Fu; S. J.;-Bimbaum, S. M.; Greenstein, J. P. J . Am. Chem. Soc. 1954, 76, 6054. (12) Mikami, N.; Higuchi, R.; Sakurai, T.; Ozaki, M.; Yoshino, K. Jpn. J . Appl. Phys. 1986, 25, L833. (13) Keller, P. Ferroelectrics 1984, 58, 3 . (14) Furukawa, K.; Terashima, K.; Ichihashi, M.; Saitoh, S.; Miyazawa, K.; Inukai, T. Ferroelectrics 1988, 85, 451. (15) Kitazume, T.; Ohnogi, T.; Ito, K. J . Am. Chem. Soc. 1990, 112, 6608.
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