Chem. Mater. 2004, 16, 3919-3923
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Helical Shape Memory of Screw-Sense Switchable Polysilanes in Cast Films Akihiro Ohira,† Masashi Kunitake,*,‡,§ Michiya Fujiki,*,†,§ Masanobu Naito,† and Anubhav Saxena†,§ Graduate School of Materials Science, Nara Institute of Science and Technology, 8916-5, Takayama, Ikoma, Nara 630-0101, Japan, Department of Applied Chemistry and Biochemistry, Kumamoto University, 2-39-1 Kurokami, Kumamoto 860-8555, Japan, and CREST-JST, 4-1-8 Hon-cho, Kawaguchi, Saitama 332-0012, Japan Received March 16, 2004. Revised Manuscript Received June 18, 2004
The optically active poly[{(R)-3,7-dimethyloctyl-(S)-3-methylpentylsilane}-co-{decylisobutylsilane}](PS-1) underwent a thermodriven helix-helix transition at 13 °C in isooctane. The casting of polymer solution from the pre- and post-transition temperatures led to P (righthanded) or M (left-handed) helical films, which gave positive and negative Cotton circular dichroism (CD) signals, respectively. This result suggests that the helical sense of PS-1 below and above transition temperature in the homogeneous solution was memorized in the cast films. Furthermore, the CD spectra of P- and M-helical films were mirror images to each other at 319 nm, indicating the 73 helical pitch of the polymers with opposite helical sense.
Introduction Recently, a precise control over the optical activity of chiral polymers and supramolecules has received much attention because of its widespread applications in material science.1 Among these, synthetic helical polymers exhibit many unique phenomena, for example, photoisomerization and chiroptical amplification,2 formation of thermotropic cholesteric liquid crystals,3 molecular chirality recognition,4 and helix-helix (P-M) transition.4-13 Especially, the P-M transition phenomenon, * To whom correspondence should be addressed. For M.K.: phone, +81-96-342-3675; fax, +81-96-342-3679; e-mail: kunitake@chem. kumamoto-u.ac.jp. For M.F.: phone, +81-743-72-6040; fax, +81-74372-6049; e-mail:
[email protected]. † Nara Institute of Science and Technology. ‡ Kumamoto University. § CREST-JST. (1) Materials Chirality: Topics in Stereochemistry; Green, M. M., Nolte, R. J. M., Meijer, E. W., Eds.; Wiley: New York, 2003; Vol. 24. (2) (a)Mayer, S.; Maxein, G.; Zentel, R. Macromolecules 1998, 31, 8522. (b) Langeveld-Voss, B. M. W.; Janssen, R. A. J.; Meijer, E. W. J. Mol. Struct. 2000, 521, 285. (3) (a)Watanabe, J.; Fukuda, Y.; Gehani, R.; Uematsu, I. Macromolecules 1984, 17, 1004. (b) Watanabe, J.; Ono, H.; Uematsu, I.; Abe, A. Macromolecules 1985, 18, 2141. (c) Watanabe, J.; Ono, H. Macromolecules 1986, 19, 1079. (d) Watanabe, J.; Goto, M.; Nagase, T. Macromolecules 1987, 20, 298. (e) Watanabe, J.; Nagase, T. Macromolecules 1987, 21, 171. (f) Okoshi, K.; Kamee, H.; Suzaki, G.; Tokita, M.; Fujiki, M.; Watanabe, J. Macromolecules 2002, 35, 4556. (4) (a) Yashima, E.; Matsushima, T.; Okamoto, Y. J. Am. Chem. Soc. 1995, 117, 11596. (b) Yashima, E.; Nimura, T.; Matsushima, T.; Okamoto, Y. J. Am. Chem. Soc. 1996, 118, 9800. (c) Yashima, E.; Matsushima, T.; Okamoto, Y. J. Am. Chem. Soc. 1997, 119, 6345. (d) Yashima, E.; Maeda, K.; Okamoto, Y. J. Am. Chem. Soc. 1998, 120, 8895. (e) Yashima, E.; Maeda, K.; Okamoto, Y. Nature 1999, 399, 449. (f) Nakako, H.; Mayahara, Y.; Nomura, R.; Tabata, M.; Masuda, T. Macromolecules 2000, 33, 3978. (g) Nakako, H.; Nomura, R.; Masuda, T. Macromolecules 2001, 34, 1496. (h) Maeda, K.; Goto, H.; Yashima, E. Macromolecules 2001, 34, 1160. (i) Onouchi, H.; Maeda, K.; Yashima, E. J. Am. Chem. Soc. 2001, 123, 7441. (j) Yashima, E.; Maeda, K.; Sato, O. J. Am. Chem. Soc. 2001, 123, 8159. (5) (a) Pohl, F. M.; Jovin, T. M. J. Mol. Biol. 1972, 67, 375. (b) Pohl, F. M. Nature 1976, 260, 365.(c) Pohl, F. M.; Thomae, R.; DiCapua, E. Nature 1982, 300, 545. (6) Toriumi, H.; Saso, N.; Yasumoto, Y.; Sasaki, S.; Uematsu, I. Polym. J. 1979, 11, 977. (7) Maeda, K.; Okamoto, Y. Macromolecules 1998, 31, 5164.
which involves reversible switching between the P (plus, right-handed) and M (minus, left-handed) screw-sense segments along the helical backbone, has attracted much interest in chiroptical materials.4-13 The moleculebased chiroptical properties, such as memorizing and switching of helicity using the P-M transition phenomenon in helical polymers, may have potential applications in data storage, optical devices, chromatographic chiral separation, and liquid crystals for display.14 The P-M transition phenomenon in solution was virtually unknown three decades ago, except in synthetic DNA5 by changing in salt concentration (B-Z transition), and in synthetic polypeptide6 by varying the temperature. Several examples of P-M transitions driven by external stimuli, such as temperature,9,12 light,2a,10 and additives,4j have already been observed in a homogeneous solution of specially designed synthetic helical polymers, such as polyisocyanates,2a,8,10 polyphenylacetylenes,4d,f,g,j,9 polythiophenes,15a,c,d,16,17 poly(triarylmethyl methacrylate)s,18 poly(N-propargylamide)s,19 and polysilanes.12 The chiroptical inversion of certain helical polymers dispersed in solution have also been demon(8) (a) Maeda, K.; Okamoto, Y. Macromolecules 1998, 31, 5164. (b) Maeda, K.; Okamoto, Y. Macromolecules 1999, 32, 974. (c) Cheon, K. S.; Selinger, J. V.; Green, M. M. Angew. Chem., Int. Ed. 2000, 39, 1482. (d) Tang, K.; Green, M. M.; Cheon, K. S.; Selinger, J. V.; Garetz, B. A. J. Am. Chem. Soc. 2003, 125, 7313. (9) Schenning, A. P. H. J.; Fransen, M.; Meijer, E. W. Macromol. Rapid Commun. 2002, 23, 265. (10) Maxein, G.; Zentel, R. Macromolecules 1995, 28, 8438. (11) Nakashima, H.; Fujiki, M.; Koe, J. R.; Motonaga, M. J. Am. Chem. Soc. 2001, 123, 1963. (12) (a) Koe, J. R.; Fujiki, M.; Nakashima, H.; Motonaga, M. Chem. Commun. 2000, 389. (b) Fujiki, M. Macromol. Rapid Commun. 2001, 22, 539. (c) Fujiki, M. J. Am. Chem. Soc. 2000, 122, 3336. (d) Fujiki, M.; Koe, J. R.; Motonaga, M.; Nakashima, H.; Terao, K.; Teramoto, A. J. Am. Chem. Soc. 2001, 123, 6253. (e) Fujiki, M.; Tang, H.-Z.; Motonaga, M.; Torimitsu, K.; Koe, J. R.; Watanabe, J.; Sato, T.; Teramoto, A. Silicon Chem. 2002, 1, 67. (13) Watanabe, J.; Okamoto, S.; Satoh, K.; Sakajiri, K.; Furuya, H.; Abe, A. Macromolecules 1996, 29, 7084. (14) Feringa, B. L.; van Delden, R. A.; Koumura, N.; Geertsema, E. M. Chem. Rev. 2000, 100, 1789.
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strated in the aggregated state prepared by the addition of poor solvents11,15 or metal ions.16 In addition, the P-M transition of helical polymers in the solid state has been recently reported.13,16 For example, poly(L-aspartic acid ester) exhibits a sharp P-M transition not only in solution but also in the solid state.13 Meijer et al. reported that the optical activity of regioregular chiral polythiophene in the cast film shows chiroptical inversion switching.17 In this case, the helicity of the polymer in the film was controlled by the proper choice of cooling rate from the melted state, leading to the potential application as a circular polarization recording/erasing system. In the present study, we successfully demonstrate a facile control of preferential screw sense and chiroptical switching of polysilane copolymer (PS-1) in the thin films by manipulating its casting temperature only. Previously, we reported that the P-M transition temperature is easily controllable by the ratio of chiralchiral or chiral-achiral comonomer systems during copolymerization.12b,c These unique characteristics of polysilane encouraged us to design optically active thin films with one of the two possible optical activities (P or M) from the same polysilane, which can undergo the P-M transition by the control of the casting temperature. Circular dichrosim (CD) studies suggest that the polymer adopts the P-screw-sense in the thin film cast from the polymer solution below the P-M transition temperature, while adopting the M-screw-sense above the transition temperature. Furthermore, the resulting solid films exhibit thermodriven chiroptical switching based on the change of optical activity by CD intensities. Materials and Methods Materials. The new thermodriven screw-sense switchable copolymer, poly[{(R)-3,7-dimethyloctyl-(S)-3-methylpentylsilane}-co-{decylisobutylsilane}] (PS-1, Scheme 1), was synthesized via Wurtz coupling from its corresponding dichlorosilanes with sodium, as described in the literature.12b,20,21 Monomer Preparation. The synthetic procedure of (R)(+)-3,7-dimethyloctyl-(S)-3-methylpentyldichlorosilane (1) is described. (S)-3-methylpentyltrichlorosilane (2) was obtained by coupling 68 g (0.40 mol) of tetrachlorosilane (Shin-Etsu) with the Grignard reagent formed from 29.5 g (0.25 mol) of (S)-(+)-3-methylpentyl chloride (3) (Chemical Soft): colorless liquid; yield 33.3 g (76%); bp 70-73 °C/6.5 mmHg; [R]24D +2.25° (neat); 29Si NMR (CDCl3, 30 °C, ppm) 13.66; 13C NMR (CDCl3, 30 °C, ppm) 19.00, 21.66, 22.61, 22.70, 24.67, 27.95, 28.93, 34.38, 36.39, 39.24. The dichlorosilane (1) was obtained by slowly adding Grignard reagent (5) obtained from 33.0 g (0.15 mol) of (R)-(-)-3,7-dimethyloctyl bromide [4; [R]24D -5.96° (neat), 96.6% ee] to 17.4 g (79 mmol) of 2 in dry diethyl ether at room temperature. The bromide 4 was prepared at Chemical Soft by bromination of (R)-(+)-3,7-dimethyloctanol [6; [R]24D +3.90° (neat), 95.7% ee] with PPh3 and Br2 in CCl4, followed (15) (a) Bidan, G.; Guillerez, S.; Sorokin, V. Adv. Mater. 1996, 8, 157. (b) Fujiki, M.; Toyoda, S.; Yuan, C.-H.; Takigawa, H. Chirality 1998, 10, 667. (c) Lermo, E. R.; Langeveld-Voss, B. M. W.; Janssen, R. A. J.; Meijer, E. W. Chem. Commun. 1999, 791. (d) Goto, H.; Okamoto, Y.; Yashima, E. Macromolecules 2002, 35, 4590. (16) (a) Goto, H.; Yashima, E. J. Am. Chem. Soc. 2002, 124, 7943. (b) Zhang, Z.-B.; Fujiki, M.; Motonaga, M.; Nakashima, H.; Torimitsu, K.; Tang, H.-Z. Macromolecules 2002, 35, 941. (17) Bouman, M. M.; Meijer, E. W. Adv. Mater. 1995, 7, 385. (18) Okamoto, Y.; Nakano, T.; Ono, E.; Hatada, K. Chem. Lett. 1991, 525. (19) Tabei, J.; Nomura, R.; Sanda, F.; Masuda, T. Macromolecule 2004, 37, 1175. (20) West, R. J. Organomet. Chem. 1986, 300, 327. (21) Miller, R. D.; Michl, J. Chem. Rev. 1989, 89, 1359.
Ohira et al. Scheme 1. Chemical Structures of Screw-Sense Switchable Polysilane Copolymers, PS-1, and Homopolymer, PS-2
by hydrogenation of (R)-(+)-β-citronellol [7; Fluka, [R]24D +4.46° (neat), 97.4% ee]. Filtration of the reaction mixture and vacuum distillation of the filtrate afforded pure 1: colorless liquid; yield 9.8 g (42%); bp 108-113 °C/0.20 mmHg; [R]24D +3.18° (neat); 29Si NMR (CDCl3, 30 °C, ppm) 34.73; 13C NMR (CDCl3, 30 °C, ppm) 11.33, 17.38, 18.64, 22.63, 22.72, 24.76, 27.99, 28.70, 28.73, 29.14, 34.81, 36.44, 36.48, 39.32. The synthetic procedure for the achiral monomer n-decylisobutyldichlorosilane (8) is described as follows. Dichlorosilane (8) was obtained by slowly adding Grignard reagent (9) obtained from 18.6 g (0.084 mol) of 1-bromodecane (10) to 19.3 g (0.1 mol) of isobutyltrichlorosilane (Shin-Etsu) in dry diethyl ether at room temperature. Filtration of the reaction mixture and vacuum distillation of the filtrate afforded pure 8: colorless liquid; yield 13.8 g (55%); bp 50 °C/0.35 mmHg; 29Si NMR (CDCl3, 30 °C, ppm) 32.64; 13C NMR (CDCl3, 30 °C, ppm) 14.12, 21.28, 22.41, 22.69, 24.18, 25.59, 29.12, 29.31, 29.44, 29.59, 30.38, 31.90, 32.49. Polymer Preparation. A typical synthetic procedure for PS-1 is described as follows: To a mixture of 0.39 g (17 mmol) of sodium (Wako) and 0.06 g (0.09 mmol) of 18-crown-6 (Wako) in 15 mL of dry toluene (Kanto) were added dropwise 1.15 g (3.5 mmol) of 1 and 0.116 g (0.39 mmol) of 8 in dry nitrogen atmosphere. The mixture was then slowly stirred at 120 °C. The hot reaction mixture slurry was passed immediately through a 5-µm PTFE filter under argon gas pressure. To the colorless clear filtrate were carefully added 2-propanol, ethanol, and methanol as poor solvents. The white precipitate was collected by centrifugation and dried overnight at 120 °C in a vacuum. PS-2 was obtained by the same procedure as described for PS-1. To a mixture of 12 mL of dry toluene (Kanto), 0.50 g (22 mmol) of sodium (Wako), and 0.06 g (0.23 mmol) of 18crown-6 (Wako) was added dropwise 1.6 g (4.9 mmol) of 1 in an argon atmosphere. After the reaction mixture was slowly stirred at 120 °C for 2 h, the reaction was worked up by following the earlier mentioned procedure for PS-1. Methods. The polymer was dissolved in isooctane solution (500 µg/mL). The solution was then deposited onto a quartz substrate and dried in air for 1 h. All UV, CD, and LD signals were recorded simultaneously on a JASCO J-725 spectropolarimeter. The weight-average molecular weight of polymer (Mw) and number-average molecular weight of polymer (Mn) were determined using gel permeation chromatography (Shimadzu A10 instruments, Plgel 10 µm MIXED-B as a column and HPLC-grade tetrahydrofuran as an eluent) at 40 °C, based on a calibration with polystyrene standards. 13C and 29Si NMR spectra were measured in CDCl3 with a JEOL EX-400 spectrometer using tetramethylsilane as an internal standard. Optical rotation at the Na-D line was measured with a JASCO
Screw-Sense Switchable Polysilanes in Cast Films
Chem. Mater., Vol. 16, No. 20, 2004 3921 Table 1. Characterization Data for Polysilane PS-1 and PS-2a polymer
achiral monomer contents (x, molar fraction)
10-3Mw
Mw/Mn
Tc, °C
PS-1 PS-2
0.1 0.0
88 79
1.60 1.76
13 3
a Molecular weights were determined by gel permeation chromatography (GPC) in tetrahydrofuran at 40 °C with polystyrene standards. The P-M transition temperature, Tc, was evaluated by the temperature dependence of gabs ()∆/) in homogeneous isooctane solution.
Figure 1. CD and UV absorption spectra for homogeneous solutions of PS-1: (A) in isooctane at 0 and 80 °C and (B) in hexane at 0 and 50 °C. Either the or ∆ value was normalized per the Si repeating unit. P-1020 polarimeter using a quartz cell with a path length of 10 mm at room temperature (24 °C).
Results and Discussion In the Homogeneous Solution System of ScrewSense Switchable Polysilanes. We have previously reported that the rodlike polysilanes, bearing enantiopure γ-branched chiral side chains, underwent thermodriven helix-helix transition in solution.12b,12c The occurrence of the P-M transition was detected spectroscopically as an inversion of the CD band. The transition temperatures (Tc) of the copolymer (PS-1), which can undergo a P-M transition, were controllable between 3 and 40 °C based on the composition ratio between the chiral alkyl comonomer and the achiral alkyl comonomer in the copolymer system. The homopolymer (PS-2) undergoes a helix-helix transition at 3 °C in isooctane (see the Supporting Information, Figure S1A).22 Figure 1A shows the CD and UV absorption spectra of PS-1 at 0 °C (below Tc) and at 80 °C (above Tc) in isooctane. The positive-sign CD spectrum with an extremum of 320 nm observed below Tc was almost the inverse of the negative-sign CD spectrum with an extremum of 325 nm observed at temperatures above Tc. An increase in content of the achiral comonomer gave rise to Tc of the copolymer systems. The Tc for PS-1 (achiral monomer contents, x, 10 mol %) was 13 °C as estimated by the temperature dependence of Kuhn’s dissymmetry ratio gabs ()∆/) in homogeneous isooctane solution.23 These data are given in Table 1. The change in Tc with the comonomer content is consistent with that of the well-studied semirigid poly(alkylisocyanate)s.8c,8d (22) Teramoto, A.; Terao, K.; Terao, Y.; Nakamura, N.; Sato, T.; Fujiki, M. J. Am. Chem. Soc. 2001, 123, 12303.
Green and co-workers previously reported the nature of the P-M transition by theoretical and experimental methods based on the “majority rule” and “sergeants and soldier principle” in polyisocyanate copolymers. They have suggested that the different chiral bias energies of chiral (S)-chiral (R) or chiral (S or R)-achiral comonomers in the copolymer system can change the helical properties, such as helicity, and the P-M transition with temperature.8d Similar behavior has also been reported for polysilane copolymers.12b,12c The P-M transition is very sensitive not only to the temperature but also to the type of solvents.12d In hexane solution, the Tcs of the polymers were drastically decreased. At temperatures above -10 °C, PS-1 in hexane showed a negative CD signal, indicating the M state, as shown in Figure 1B. This indicates that PS-1 can recognize the shape of molecules, such as the degree of branching of solvents, as in the case of homopolymer system (PS2).12d In Thin Solid Films. The cast films of PS-1 on a quartz substrate were prepared by several different conditions in terms of the casting temperature (below or above Tc) and the solvent used. Parts A and B of Figure 2 show UV and CD spectra of PS-1 films prepared below and above the Tc in homogeneous isooctane solution, respectively. The film prepared by casting at approximately 5 °C below Tc revealed a positive signal (designated as P film, CD λext ) 319 nm) in the CD spectrum at 20 °C, as shown in Figure 2A. Conversely, a negative signal in the CD spectrum (designated as M film, CD λext ) 319 nm) was observed for the film prepared by casting at approximately 30 °C above Tc (Figure 2B). The sign of the CD signal in the solid film cast below and above Tc was always consistent with that in the solution. Moreover, the sign of the CD signal was not affected by the evaporation rate. Certainly, when the film was prepared by casting from hexane solution below Tc (typically 5 °C), the copolymer always revealed a negative CD signal, indicating the post-transition states, as shown in Figure 2C. The film was essentially the same as that prepared from the isooctane solution at temperatures above the Tc with respect to the extremum wavelength and the polarity of the CD spectra.24 These results clearly indicate that the helical senses of cast films were similar to that of the polymer in the solution during casting. In other words, the helical sense in a homogeneous solution was frozen into the cast film. It should be noted that the CD signals of all P and M films were almost mirror images to each other, although the signal intensities (23) Temperature dependence of the dissymmetry ratio of PS-1 and PS-2 in isooctane is shown in the Supporting Information section (Figure S1).
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Figure 3. Temperature dependence of λmax in UV and λext in CD spectra of PS-1 with P-state film prepared from isooctane solution at 5 °C (filled circles for UV and open circles for CD), the M-state film prepared from isooctane solution at 30 °C (filled squares for UV and open squares for CD), and the M-state film prepared from hexane solution at 5 °C (filled triangles for UV and open triangles for CD).
Figure 2. UV and CD absorption spectra of PS-1 thin films prepared by casting from isooctane solution at 5 °C (A) and at 30 °C (B) and casting from hexane solution at 5 °C (C). The film thickness (approximately 30 nm) was roughly estimated from the density of PS-1 (0.878 g/cm3), the film area on the quartz substrate, and the film weight. The film weight was obtained from the concentration of solution and the amount of solution used for the cast film.
were not regulated. This observation indicates that all P and M films possessed the same 73 helical pitch of the polymer, although the helical sense differed.25 All P and M films showed temperature-dependent UV and CD spectra, as shown in Figure 2. The red shift and decrease in the intensities of UV and CD spectra of the films were observed at high temperature. Figure 3 shows the temperature dependence of λmax for the UV spectra and λext for the CD spectra of cast films. The red shift in polysilanes at higher temperatures is (24) Many researchers pointed out the difficulty of measuring and interpreting the CD spectra of cast films. The linear dichroism (LD) signal may contribute to the observed apparent CD spectra of the films and makes it difficult to estimate the real CD enhancement of the films. Concerning these problems, we provided Supporting Information for the rotation angle dependence of the CD and LD spectrum of PS-1 cast films as shown in Figure S2. For further references, see: Gillgren, H.; Stenstam, A.; Ardhammar, M.; Norden, B.; Sparr, E.; Ulvenlund, S. Langmuir 2002, 18, 462. Davidsson A.; Norden, B. Chem. Phys. Lett. 1980, 70, 313. Norden, B. J. Phys. Chem., 1977, 151, 81.
Figure 4. Thermal cycle responses of extremum of CD intensities for the P film prepared from isooctane solution at 5 °C (open circles) and the M film prepared from isooctane solution at 30 °C (filled circles). The thermal cycles were conducted between heating to 50 °C and cooling to -10 °C. The heating/cooling rates were around 1-5 °C/min.
generally attributed to the elongation of the Si-Si bonds and a loosening in the screw-pitch, deviating from the ideal 73-helical structure. In the solid film state, a thermodriven P-M transition was not observed at all, until 110 °C, although the UV and CD intensities of all films were decreased with the increase of temperature. Indeed, concerning the thermal property in the solid state, glass transition and any other transitions were not observed at all by differential scanning calorimetry (DSC) measurement in the heating and cooling cycles, presumably due to the rigid rodlike polymer chain. The UV and CD spectra of cast films showed relatively high thermal reversibility against the thermal cycles of heating (+50 °C) and cooling (-10 °C), as shown in Figure 4. It reveals that the intensity of optical activities (25) We can clearly observe the 73-helical structure of polysilanes by wide-angle X-ray diffraction pattern (WAXD). For the first report, see: Miller, R. D.; Farmer, B. L.; Fleming, W.; Sooriyakumaran, R.; Rabolt, J. J. Am. Chem. Soc. 1987, 109, 2509. For another optically active polysilane, we have demonstrated that poly[n-decyl-(S)-2methylbuthylsilane] has the 73-helical structure in solid state by WAXD pattern. See: Okoshi, K.; Kamee, H.; Suzaki, G.; Tokita, M.; Fujiki, M.; Watanabe, J. Macromolecules 2002, 35, 4556. It has also been reported that the λmax in UV absorbance of this polymer in the solid state was observed at 320 nm. Therefore, in this case, PS-1 is thought to possess the 73-helical structure in the solid film.
Screw-Sense Switchable Polysilanes in Cast Films
in the cast films can be easily controlled by the film temperature, indicating the chiroptical switching. However, a several percent decrease in CD intensity was observed at each cycle. When the temperature changed from +20 to +50 °C, for instance, the decrease in UV and CD intensities was approximately 2.2% and 6.7% for the P film prepared from isooctane (T < Tc) and 6.5% and 8.0% for the M film prepared from isooctane (T > Tc), respectively. The thermally induced change in the CD signals was consistently greater than the change in the UV absorbance in all films, as observed for the polymer in solution at temperatures below the Tc. This observation indicates that the changes in the CD spectra might be not only due to the thermally induced changes of helical pitch but also to the decrease in helical uniformity with partial racemization or P-M transition. Conclusions In summary, both P- and M-state cast films with positive and negative Cotton CD signals were prepared
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from the same polysilane copolymer by controlling its casting temperature only. The intensities of optical activities in the cast films can be switched by manipulating the film temperature. These results show promise for the development of chiroptical devices such as switching and memory using thermodriven screw-sense switchable polysilanes. Acknowledgment. This work was supported by CREST-JST, Japan. A.O. thanks the JSPS Research Fellowships (No. 01682) for Young Scientists. The authors thank Dr. Sun-Young Kim for the fruitful discussion. Supporting Information Available: CD and UV data of PS-2 in isooctane and the temperature dependence of gabs ()∆/) of PS-1 and PS-2, the anisotropic dependence of apparent CD signals for the films, and the dependency of rotation angle of PS-1 cast film, which is perpendicular against the light axis in CD and LD spectra (PDF). This material is available free of charge via the Internet at http://pubs.acs.org. CM0495616
