Langmuir 1993,9,3045-3049
3045
Highly Oriented Polysilane Films Prepared by the Langmuir-Blodgett Technique Rikako Kani,* Hiroshi Yoshida, Yoshihiko Nakano, Shinji Murai, Yasushi Mori, Yasushi Kawata, and Shuzi Hayase Materials and Devices Research Laboratories, Research and Development Center, Toshiba Corporation, 1, Komukai Toshiba-cho, Saiwai-ku, Kawasaki 210, Japan Received December 21, 1992. In Final Form: July 19, 199P Highly orientedpolysilane thin filmswere provided by the Langmuir-Blodgett technique. All polysilanes bearing phenol moieties provided monolayers on the aidwater interfaceand were transferred to a substrate step by step. Among these polysilanes, only poly((3-hydroxypheny1)n-buty1)silaneprovided thin f i i s oriented parallel to the dipping direction of the substrate. The orientation was evaluated by polarized UV absorption and polarized fluorescence spectroscopy. The order parameter reached 0.5, which corresponded to those of conventional nematic liquid crystals. Order parameters were independent of the excitation wavelengths, whose feature was distinct from that in a solution. This suggests that fast and efficient energy migrations take place along the Si-Si main chains.
Introduction Polysilanes have various interesting optical and semiconductive properties,14 which are attributed to u-u* conjugations.gJ0 It is necessary to obtain thin films in which the Si-Si chains orient in one direction in order to evaluate the polysilane chracteristics, because the u-u* conjugations develop along the Si-Si main chains. Stretching and rubbing treatments have been known to provide oriented f i l m ~ . ~The ~ J ~Langmuir-Blodgett (LB) technique is an excellent method for preparation of ultrathin films with a well-defined molecular order.13 The LB technique has recently been shown to provide stable oriented polymer thin films.1P18 One method to prepare thin films is to use rod-like polymers with conformationally flexible side groups bearing minimal By applying this idea, Embs et al. have e Abstract Dublished in Aduance ACS Abstracts. SeDtember 15, 1993. (1) Wreber, P.;Guillon, D.; Skoulioe, A.;Miller, R. D. J. Phys. (Paris) 1989,50,793. (2) Wreber, P.; Guillon, D.; Skoulios, A. Liq. Cryst. 1990,8,825. (3) Asuke, T.; West, R. Macromolecules 1991,24, 343. (4) Trefonaa, P., III.;Damewood, J. R., Jr.; West, R. Organometallics 1986,4, 1318. (5) Harrah,L. A.; Zeigler, J. M. J. Polym. Sci., Polym. Lett. Ed. 1986, 23,209. ( 6 ) Miller, R. D.; Hofer, D.; Rabolt, J . J. Am. Chem. SOC.1986,107, 2172. (7) Schilling,F.C.;Bovey,F.A.;Davie,D.D.;Lovinger,A.J.;Macgregor, R. B.; Walsh, C. A.; Zeigler, J. M. Macromolecules 1989,22, 4645. (8)Song,K.;Miller,R.D.;W&aff,G. M.;Rabolt, J.F.Macromolecuks 1991,24,4084. (9) West, R. The Chemistry of Organic Silicon Compounds;Patai, S., Rappoport, Z.,E&.; John Wdey & Sone La.: New York, 1989; p 1207. (10) Miller, R. D. Chem. Rev. 1989,89,1359. (11) McCrary, V. R.; Sette, F.;Chen, C. T.; Lovinger, A. J.; Robin, M. B.; Stohr, J.; Oiler, J. M. J. Chem. Phys. 1988,88, 5925. (12) Tachibana, H.; Kawabata, Y.;Yamaguchi, A.; Moritomo, Y.; Koehihara, S.; Tokura, Y. Phys. Reu. B 1992,45,8752. (13) Good, R. D.; Stromberg, R. R.; Patrick,R. L. Technique of surface and colloid chemistry and physics; Marcel Dekker: New York, 1972; VOl. 11, (14)Niehikata, Y.; Morikawa, Y.; Takiguchi, Y.; Kanemoto, M.; Kakimoto, M.; Imai,Y. Jpn. J . Appl. Phys. 1988,27, L1163. (lS)Nbhikata, Y.; Morikawa, Y.; Takiguchi, Y.; Kanemoto, M.; Kakimoto, M.; Imai, Y. Nippon Kagaku Kaishi 1987,2174. (16) Duda, G.; Schouten, A. J.; Amdt, T.; Lieser, G.; Schmidt, G. F.; Bubeck, C.; Wegner, G. Thin Solid F i l m 1988,159,221. (17) Duda, G.; Wegner, G. Makromol. Chem. Rapid Commun. 1988, 9, 496. (18) Orthmann, E.; Wegner, G. Angew. Chem. 1986,98, 114. I
_
reported that poly(dipheny1)silane derivatives provided LB films with Si-Si chains oriented parallel to the dipping direction." An alternative method is to use polymers bearing both hydrophilic and hydrophobic substituents, affording a much wider selection of application than that mentioned above.14Js However, all polymers which provide stable Langmuir films on the aidwater interface do not necessarily yield oriented polymer films when the monolayers are transferred. Amphiphilic polysilaneswith cationicsites have been reported to provide LB films by Seki et al.20 However, highly oriented polysilane has not been obtained yet. The goal of the authors' work is to prepare highly oriented polysilane LB films by applying polysilanes bearing phenol groups which have been newly prepared by the authors.21 This paper describes the preparation of ordered thin films by the LB technique and the degrees of orientations determined by the polarized UV absorption and the polarized emission. The mechanism of how the orientations take place is also presented.
Experimental Section Synthesis. Polysilanes were synthesized by methods described in the previous paper.21 Film Preparation. LB fiis were prepared by the vertical deposition method on a Kyowa Kaimen Kagaku Model HBM Langmuir trough. The surface pressurearea isotherms were monitoredwitha Wilhelmy plate. A0.4 g/L solution of polyailane was spreadon the airlwater interface. Cyclohexanone and toluene solutions were used for polysilanes with and without the phenol group, respectively. The monolayers were compressed at a compression rate of 20 mm/min to 15 mN/m and transferred step by step onto a quartz or GaAs plate at 15 O C . The dipping speed waa 5 mmlmin. The quartz plate and GaAs plate were treated with a mixture of sulfuric acid and a hydrogen peroxide aqueous solution and then rinsedwithdeionizedwater. The plates were further treated with a dilute hydrofluoric acid aqueous solutionand rinsed again with deionizedwater. The procedure was repeated 3 times. After (19) Embe, F. W.; Wegner, G.; Neher, D.; Albouy, P.; Miller R. D.; Willson, C. G., Schrepp, W. Macromolecules 1991,24,6068. (20) Seki, T.; Tamaki,.T.;Ueno, K. Macromolecules 1992,25, 3825. (21) Nakano, Y.; Murru, S.; Kani, R.; Hayase, S. J. Polym. Sci.,Polym. Chem. Ed., in press.
0743-7463/93/2409-3045$04.00/0 0 1993 American Chemical Society
Kani et al.
3046 Langmuir, Vol. 9,No. 11,1993
Table I. List of Polymer Structures for LB Films
I
I
baking, the plate was treated with hexamethyldisilazane for 12 hat room temperaturein order to render the surfacehydrophobic. Spectroscopy. The absorption spectra of LB f i i were recorded with a Shimadzu Model W-260 visible recording spectrometer. All measurements were made in transmission geometry wing quartz as the substrate. The incident beam was polarized with a Mellos Griot Glan-Taylor polarizing prism to measure the anisotropy of UV absorption. Polarized fluorescence spectra were recorded with a Hitachi M-850fluorescence-visible recording spectrometer. The depolarization ratios, R1 and R2,were determined in the right angle arrangement (Figure 1)with the polarization of the incident and exit beams parallel or perpendicularto the symmetry axis along 2. The sensitivity ratio between the parallel beam and perpendicular beam on the spectrometerwas corrected with a standard RodaminB solution. The depolarization ratios were determined at 260, 285, 310, and 340 nm. The samples were annealed at various temperatures from 90to 180 O C in order to measure the orientational change of the polysilane chains. Orientation of the Liquid Crystals on Aligned Polysilanes. Liquid crystal was injected between a glass plate coated with a rubbed polyimide f i b and that coated with polysilanes oriented by the LB technique. The geometry of the polyimide orientationdirection was perpendicularto that of the polysilane orientationdirection as shownin Figure 2a. The spacingwas set at 6 Mm with glass fiber spacers. The texture formation was observed by a polarized light microscope after the liquid crystal waa heated to the isotropic state and cooled. The orientation of the liquid crystal on the LB films was observed with 8-carotene. The mixture of @-carotene(1w t % to the liquid crystal) and the liquid crystal (4-heptyEl'-cyanobiphenyl, 5CB) was injected between two glass plates covered with polysilanes oriented by the LB technique. The geometry of the polysilane orientations on the two plates was parallel, as shown in Figure 2b. The orientationof &carotenewaa evaluated by the polarized UV absorption.
Results and Discussion Figure 3 shows the surface pressure-area isotherm for various polysilanes. The surface pressure for polysilanes bearing phenol moieties increased and leveled off when the molecules on the water were pressurized. It has been believed that molecules provide monolayers on the air/ water interface at the region between the point where the surface pressure begins to appear and the point where the surface pressure levels 0ff.13 It seems likely that polysilanes bearing phenol moieties provide monolayers on the air/ water interface. On the contrary, the surface pressurearea isotherms for polysilanes without phenol moieties or with phenol groups protected with silyl ethers showed different features from those for polysilanes bearing phenol groups. The areas where the surface pressure appeared were very small and the level-off points were not observed at all. These results suggest that these polysilanes did not form monolayers at the aidwater interface. Transfer ratios defined as (areaof transferred polysilanes onto plate)/(decreasedarea on the aidwater interface)were close to 1.0 for all polysilanes bearing phenol groups, as
shown in Table 11. This implies that all polysilanes bearing phenol groups were transferred to the substrate step by step. Among these polysilanes, only polysilane VI1 provided thin films in which the polysilane main chains were oriented to be each other along the dipping direction. Figure 4 shows the polarized UV absorption spectra for polysilane VI1 and polysilane VI. The polarized UV absorption assigned to the u-u* transition of polysilane VI1was larger in the dipping direction than in the direction perpendicular to it. On the contrary, the polarized UV absorption assigned to the U-T* transition was smaller in the dipping direction than in the direction perpendicular to it. This implies that the Si-Si main chains align along the dipping direction and the phenol moieties are located in the direction perpendicular to the main chain aa shown in Figure 4. The anisotropic nature in the UV absorptions was not seen for the polysilane VI and other polysilanes bearing phenol groups. The dichroic ratio R defined as A(parallel)/A(perpendicular),whereA(paralle1)and A(perpendicular) represent the absorbance parallel or perpendicular to the dipping directions,respectively, reached 4.0. The order parameter defined as ( R - l)/(R 2) was 0.5, which was comparable to those of conventional liquid crystals such as p-(methoxybenzy1idene)-p'-n-butylaniline.22 It is well-known that polysilanes emit fluorescence.lO Fluorescence anisotropy increases in a solution as the excitation wavelength becomes longer.23 The anisotropy decreases within the picosecond 0rder.~**~6 The following model has been provided. The energy once absorbed migrates along the Si-Si main chains and the emission takes place from the site of Si-Si chains whose excited levels are lower than those where absorption took place. However, emission spectra for oriented polysilanes have not been reported except for those for poly(methy1 methacrylate) films in which polysilanes were orientedaZ6 Figure 5 shows the emission anisotropies afor oriented polysilane VII. Polysilane VI1 emitted fluorescence parallel to the main chains more strongly than that perpendicular to the main chains, independently of whether the polysilane was excited by light parallel to or perpendicular to the Si-Si main chains. Emission peaked at 400-500 nm reported in other aromatic polysilanes was not detected for this LB film at 6 K. The depolarization ratios RI and RZ are defied as follows: Ri = (Izz- Izx)/(Izz+ ~ z x R2 ) ; = UYZ-IYX)/(IYZ
+
(22) Chapoy, L. L.; DuPre, D. B. J. Chem. Phys. 1979, 70, 2560. (23) Klingensmith, K. A.; Downing,J. W.; Miller, R. D.;Michl, J. J. Am. Chem. SOC.1986,108,7438. (24) Kim,Y.R.;Lee,M.;Thorne,J.R.G.;Hochstrasser,R.M.;Zeigler, J. M. Chem. Phys. Lett. 1988,146,75. (25) Thome, J. R. G.; Hochstraaser,R. M.; Zeiiler, J. M. J. Phys. Chem. 1988,92,4275. (26) Harrah, L. A.; Zeigler, J. M. Macromolecules 1987,20,601.
Highly Oriented Polysilane F i l m
f
Langmuir, Vol. 9, No. 11,1993 3047
,,a'
Emittd.,,Hght
45 40
f
35
1s
30 25
R 2 = Iyz ' I Y X
Incident light
In+2hx
Polarizer
X
T
2
a
20
4;
15 10
5
Figure 1. Right angle arrangementfor measuringdepolariztaion
ratios R1 and Rs. Arrows indicate the polarization directionsof incident and scattered light. (a)
0 10
30
50
40
Area ( A*/ monomer unit)
(b) Polarizer
20
Figure 3. Surfacepressure-areaisotherma of varioue polysilanee at 15 O C .
olyrilane LB film Liquld crystal
0.3
-
Polysilane LB fil bbingfilm
I1 \
Polarizer
Figure 2. Cell structures to determine orientation of liquid crystale: (a) angle between direction of polysilane orientation and rubbing direction, No;(b) angle between two oriented polysilanes, oo. + 21~x1.R1 and Rz were independent of the excitation wavelength at a range between 250 and 340 nm, as shown in Figure 6. In comparison, R1and Rz for polysilane VI1 in ethanol are also shown in the figure. These values largely depended on the excitation wavelength, which was consistent with Michl's resulta.23 These results imply that most fluorescence was emitted from expanded Si-Si main chains whose LUMO are the lowest, even when ?r electrons of the phenyl groups and short Si-Si main chains whose delocalization was interrupted by kinks are excited. This result was consistent with reports that efficient and fast energy migrations take place from LUMO having the high energy to that having the lower energy.s2s It is very curious that the long Si-Si chains aligned parallel to the dipping direction. In order to clarify the alignment, an attempt was made to determine the orientations of polysilane main chains by using polarized UV absorption. As mentioned above, polysilane VI1has long Si-Si chains oriented parallel to the dipping direction in the LB film. Thisprompted us to make liquid crystal cells using oriented polysilane films as the aligning layers. It has been reported that liquid crystals are oriented along the oriented polymer backbones of rubbed polymer films.n Liquid crystal 5CB was injected into gaps between two glass substrates. The cell was heated up to 50 OC to give (27) Geary, J. M.;Goodby, J. M.;Kmetz, A. R.;Patel, J. S.,J. Appl. Phys. Chapoy, L.L.;W r e , D.B. J. Chem. Phys. 1987,62,4100.
P
0
Wavelength (nm)
Figure 4. Polarized UV absorption spectra for LB f i i s of polysilanes VI1 and VI deposited as 20 layers. Angles between polarizer and dipping direction are Oo (-) and No(- -). Table 11. Transfer Ratios for Various Polysilanes
polymer I I1
I11
Iv
transfer ratio 0.1 0.1 1.0 0.95
polymer
v
VI VI1
transfer ratio 1.05 1.0 1.0
the isotropic state of the liquid crystal and then cooled slowly. One of the two substrates was coated with a polyimide film which was rubbed with a cloth, and the other was coated with oriented polysilane VII films made by the LB technique. The geometries of the two plates are shown in FIgure 2a. Figure 7 which was photographed through crossed-polarizers, shows the texture of the liquid crystal cell. In the region where light passed through, oriented polysilanes make the liquid crystal aligned so that the liquid crystal should be parallel to the polarizer. In the dark region, the liquid crystal must be aligned with each other by the rubbed polyimide films, where the
3048 Langmuir, Vol. 9,No. 11, 1993 I
I
I
1
9
5
W
.-fn>I c
C c C
a
v
350
~~
400
450
500
550
Wavelength (nm)
Figure 7. Textures of the liquid crystal by polarized light. For
cell structures, see Figure 2a and Figure 8a.
Polarizer 350
400
450
500
550
Wavelength (nm)
Figure 5. Emission anisotropy for the oriented polysilane VI1 observed with excitation at 340 nm at room temperature.
Liquid crystal \
Rubbing film Polarizer
Figure 8. Orientations of liquid crystals in cells.
rr" a'
I
300
Wavelength (nm)
Figure 9. Polarized UV absorption spectra for @-carotene(0.1 w t % ) in the liquid crystal (a, b) before heating and (c, d) cooled
Figure 6. Temperaturedependenceof depolarization ratios R1 and R2 for polysilane VII: R1 in LB film(0); R2 in LB film (A);
R1 in solution ( 0 ) ;R2 in solution (A).
polarizer is perpendicular to the liquid crystal. The geometry of the liquid crystal cell is shown in Figure 8a. The orientation of the liquid crystal in the cell described above was evaluated with @-caroteneincorporated into the liquid crystal phase.28 The direction of the @-carotene was consistent with that of the liquid crystal. The geometry of the liquid crystal cell is shown in Figure 2b. When the mixture of 5CB and 8-carotene was injected, the direction of @-carotenewas perpendicular to that of the Si-Si chain of polysilane, as shown in Figure 9a. (28) Yoshida, H.;Nakajima, Y.;Kobinata, S.; Maeda, S. J. Phys. SOC. Jpn. 1980,49,1140.
after heating at 50 OC: (a) perpendicular to Si-Si chains; (b) parallel to Si-Si chains; (c) parallel to Si-Si chains; (d) perpendicular to Si-Si chains. However, after the cell was heated to 50 "C followed by slow cooling to room temperature, it was found that the direction of @-carotenebecame consistent with that of the polysilane chains, as shown in Figure 9b. This result suggests that at first 5CB was oriented perpendicular to the Si-Si chain and then changed its orientation to the direction parallel to the Si-Si chains. The orientation of the liquid crystal did not change at all after repeat cycles of heating and cooling. The orientation perpendicular to the Si-Si chain takes place only before heating. This reveals that the orientation of the polysilane films does not change during the heating cycles. The same phenomenon has been reported in the case of po1ystyrene.B The liquid crystals would be aligned by the polarity of the
Highly Oriented Polysilane Film phenyl-Si likage which lies perpendicularto the Si-Si main chains. The former orientation of 5CB, orientation perpendicular to the Si-Si chain, may be governed by phenyl groups which lie perpendicular to the Si-Si main chains.
Conclusion Highly oriented polysilane thin films were fabricated for the firsttime with polysilanes bearing both hydrophobic (29) Nakajima, K.;Wakemoto, H.; Sato, S.;Yokotani, F.;Iehihara, S.; Matsuo, Y.Mol. Cryat. Liq. Cryat. 1990, 180B,223.
Langmuir, Vol. 9, No. 11,1993 3049 and hydrophilic substituents by the LB technique. Only poly((3-hydroxyphenyl)-n-butylsilane)provided oriented thin films. The orientation was as high as those of conventional liquid crystals, which was ascertained from the polarized UV absorption and the polarized emission. The liquid crystal aligned along the Si-Si chains and in some cases along the long alkyl side chains. This provided a way to control the direction of the Si-Si chains.
Acknowledgment. We thank Professor K. Horie and Dr. T. Torii of the University of Tokyo for helpful discussion about polarized fluorescence spectra.
