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Highly Oriented Langmuir-Blodgett Films of Helical Polysilanes and Their Optical Properties Hiroaki Tachibana,*,†,‡ Hideo Kishida,§ and Yoshinori Tokura†,§ Joint Research Center for Atom Technology (JRCAT), Tsukuba 305-8562, Japan, National Institute of Materials and Chemical Research, Tsukuba 305-8565, Japan, and Department of Applied Physics, University of Tokyo, Tokyo 113-8656, Japan Received July 28, 2000. In Final Form: November 2, 2000
Thin films of a helical polysilane with chiral substituents were fabricated using a Langmuir-Blodgett (LB) method. The polarized absorption and photoluminescence spectra show the high anisotropy for the LB films with the polymer backbone lying in-plane along the dipping direction. The anisotropy of the lowest exciton absorption band can be enhanced by annealing the LB films. With use of the oriented LB film, we have revealed the one-dimensional exciton structures characteristic of polysilanes with helical conformations by measurements of electroabsorption spectra, compared with those of polysilanes of other conformations such as trans-planar and alternating trans-gauche backbone structures. The large binding energies of the lowest exciton are generic in the one-dimensional regular polysilanes, irrespective of the conformations.
Introduction Langmuir-Blodgett (LB) technique has been considered as a canonical method of the fabrication of thin films with the structures defined at the molecular level. During the past few decades there have been numerous studies on the fabrication, structural characterization, and functions in the LB films.1-4 As an advantage for the LB method, anisotropic LB films have been obtained for various π-conjugated polymers such as poly(diacetylene),5 polythiophene,6,7 poly(p-phenylenevinylene),8 and poly(pphenylene),9 and σ-conjugated polysilanes,10-15 in which polymer backbone lies along the dipping direction. The family of σ-conjugated polysilanes is of particular interest as these polymers exhibit unique linear and * Corresponding author. Tel: +81-298-61-4668. Fax: +81-29861-4669. E-mail:
[email protected]. † JRCAT. ‡ National Institute of Materials and Chemical Research. § University of Tokyo. (1) Robert, G. Langmuir-Blodgett Films; Plenum: New York, 1990. (2) Ulman, A. An Introduction to Ultrathin Organic Films: From Langmuir-Blodgett to Self-Assembly; Academic Press: Boston, MA, 1991. (3) Petty, M. C. Langmuir-Blodgett Films: an introduction; Cambridge University Press: New York, 1996. (4) Kajiyama, T.; Aizawa, M. New Developments in Construction and Functions of Organic Thin Films; Elsevier: Amsterdam, 1996. (5) Tomioka, Y.; Imazeki, S.; Tanaka, N. Chem. Phys. Lett. 1990, 174, 433. (6) Rikukawa, M.; Nakagawa, M.; Abe, H.; Sanui, K.; Ogata, N. Thin Solid Films 1996, 284-285, 636. (7) Bjornholm, T.; Greve, D. R.; Reitzel, N.; Hassenkam, T.; Kjaer, K.; Howes, P. B.; Larsen, N. B.; Bogelund, J.; Jayaraman, M.; Ewbank, P. C.; McCullough, R. D. J. Am. Chem. Soc. 1998, 120, 7643. (8) Hagting, J. G.; Vorenkamp, E. J.; Schouten, A. J. Macromolecules 1999, 32, 6619. (9) Cimrova, V.; Remmers, M.; Neher, D.; Wegner, G. Adv. Mater. 1996, 8, 146. (10) Embs, F. W.; Wegner, G.; Neher, D.; Albouy, P.; Miller, R. D.; Willson, C. G.; Schrepp, W. Macromolecules 1991, 24, 5068. (11) Kani, R.; Yoshida, H.; Nakano, Y.; Murai, S.; Mori, Y.; Kawata, Y.; Hayase, S. Langmuir 1993, 9, 3045. (12) Kani, R.; Nakano, Y.; Majima, Y.; Hayase, S.; Yuan, C.-H.; West, R. Macromolecules 1994, 27, 1911. (13) Seki, T.; Tanigaki, N.; Yase, K.; Kaito, A.; Tamaki, T.; Ueno, K.; Tanaka, Y. Macromolecules 1995, 28, 5609. (14) Yoshida, M.; Mori, M.; Yokokawa, S.; Nakanishi, F.; Sakurai, H. Mol. Cryst. Liq. Cryst. 1998, 322, 135. (15) Yoshida, M. Mol. Cryst. Liq. Cryst. 1999, 327, 71.
nonlinear optical properties inherent to the delocalized nature of σ-electrons on the Si-backbone chains.16 Polysilane possesses two side groups, R1 and R2, attached to a repeated unit of the backbone. Depending on the alkyl length of R1 and R2, the backbones of polysilanes take a variety of conformations, such as trans-planar, alternating trans-gauche, 7/3 helical, and disordered backbone structures.16 The optical properties depend significantly on the conformation structures of the backbone. Features of onedimensional exciton states for polysilanes have been revealed for a variety of conformations by the measurements of various linear and nonlinear optical spectroscopies.17,18 The Si-backbone of polysilane19-23 and oligosilane24 can take helical conformations with one screw sense by introducing chiral substituents such as the (S)2-methylbutyl group into R1. The helical screw sense can be controlled by changing solvent polarity25 and temperature26 of solution of helical polysilanes. Recently, it has been also reported that poly(methylphenylsilane) without the chiral substituents exhibits helical conformations in solution at room temperature.27 In this paper, we report the fabrication of the LB film of chiral polysilanes bearing hydrophilic groups, poly((5ethoxypentyl)-(S)-(2-methylbutyl)silane) (Figure 1). The (16) Miller, R. D.; Michl, J. Chem. Rev. 1989, 89, 1359. (17) Tachibana, H.; Matsumoto, M.; Tokura, Y.; Moritomo, Y.; Yamaguchi, A.; Koshihara, S.; Miller, R. D.; Abe, S. Phys. Rev. B 1993, 47, 4363. (18) Hasegawa, T.; Iwasa, Y.; Koda, T.; Kishida, H.; Tokura, Y.; Wada, S.; Tashiro, H.; Tachibana, H.; Matsumoto, M. Phys. Rev. B 1996, 50, 11365. (19) Frey, H.; Moller, M.; Matyjaszewski, K. Macromolecules 1994, 27, 1814. (20) Fujiki, M. J. Am. Chem. Soc. 1994, 116, 6017. (21) Ebihara, K.; Koshihara, S.; Yoshimoto, M.; Maeda, T.; Ohnishi, T.; Koinuma, H.; Fujiki, M. Jpn. J. Appl. Phys. 1997, 36, L1211. (22) Shinohara, K.; Aoki, T.; Kaneko, T.; Oikawa, E. Chem. Lett. 1997, 361. (23) Koe, J. R.; Fujiki, M.; Nakashima, H. J. Am. Chem. Soc. 1999, 121, 9734. (24) Obata, K.; Kabuto, C.; Kira, M. J. Am. Chem. Soc. 1997, 119, 11345. (25) Fujiki, M.; Toyoda, S.; Yuan, C.-H.; Takigawa, H. Chirality 1998, 10, 667. (26) Koe, J. R.; Fujiki, M.; Motonaga, M.; Nakashima, H. Chem. Commun. 2000, 389. (27) Toyoda, S.; Fujiki, M. Chem. Lett. 1999, 699.
10.1021/la0010774 CCC: $20.00 © 2001 American Chemical Society Published on Web 12/23/2000
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Figure 1. Polysilane used in this study.
electronic structure of the LB film has been investigated by measurements of polarized absorption, photoluminescence, and electroabsorption spectra. In the LB film the helical backbone was proved to lie along the dipping direction. In addition, we have revealed one-dimensional exciton states of polysilanes with helical conformation, compared with that of polysilanes with other conformations. The thermochromic properties and the effect of annealing treatment on the orientation were also studied. Experimental Section Materials. The poly((5-ethoxypentyl)-(S)-(2-methylbutyl)silane) (chiral PS) was synthesized by Wurtz coupling reaction as described previously.25 The molecular weight for the chiral PS was determined to be Mw ) 4.71 × 105 by use of polystyrene calibration. Langmuir Monolayer Formation. The surface pressurearea (π-A) isotherms were measured using a Lauda film balance. A toluene solution of chiral PS was spread at 17 °C onto a pure water, which was purified by distilling deionized water. The chiral PS formed stable monolayers up to 20 mN m-1. Monolayers were transferred at a surface pressure of 15 mN m-1 onto quartz plates hydrophobized with hexamethyldisilazane by the vertical dipping method. The layer number of the LB films used in this study is 20. For electroabsorption (EA) measurements, the chiral PS monolayers were transferred onto the ITO-coated quartz plates and a semitransparent aluminum electrode was vacuum deposited as a counter electrode on top of the LB films. Characterization. The helicity of the chiral PS backbone was investigated by circular dichroism (CD) spectrum in isooctane solution. Sharp and strong absorption band is observed at 3.86 eV in the absorption spectra of the solution. The CD spectra for the chiral PS shows the positive Cotton effect at the same energy as the absorption peak, indicating a right-handed helical conformation of the polysilane backbones. The introduction of ether group into the alkyl chain attached to the chiral PS does not influence the helical sense. Photoluminescence (PL) spectra were measured using a monochromatized (4.0-eV) light from a Xe lamp at 77 K. For measurements of the polarized absorption, photoluminescence, and EL spectra, the light E vector was set to be either parallel or perpendicular to the dipping direction of the LB films. The EA signal was detected at a frequency of 2f, where f (1 kHz) is the applied sinusoidal electric field perpendicular to the LB film. Intensities of the EA signals varied quadratically with the applied field strength. Temperature-dependent absorption spectra were measured by controlling the temperature of the LB films within (1 K. For the measurements of annealing effect, the LB films were heated at 373 K for a constant time. After cooling of the sample to room temperature, the polarized absorption spectra were measured at room temperature.
Results and Discussion Figure 2 shows polarized absorption spectra of the LB films and the spin-coated films of the chiral PS at 77 K. In both film an intense absorption peak is observed around 3.8 eV, which is assigned to the lowest exciton as in the case of polysilanes with other conformations.17 There is, however, a clear difference in spectral features of absorption between the spin-coated thin film and the LB film. The absorption bands of the LB films are sharper than
Figure 2. Polarized absorption spectra of (a) the spin-coated film and (b) the LB film of the chiral polysilane. Solid and dashed lines represent the spectra polarized parallel and perpendicular to the dipping direction, respectively.
Figure 3. Polarized (a) absorption and (b) photoluminescence spectra in the LB films of the chiral polysilane. Solid and dashed lines represent the spectra polarized parallel and perpendicular to the dipping direction, respectively.
those of the spin-coated thin films. Large in-plane spectral anisotropy was observed in the LB films, suggesting that the Si backbones with right-handed helical conformations are highly oriented along the dipping direction. It is wellknown that the dipping-induced flow orientation is observed in the case of rigid rodlike polymers or rodlike molecules.28-30 Since the chiral PS can be considered as rigid rodlike polymers, the anisotropy is perhaps due to a flow orientation effects. From the polarized absorption spectra in Figure 2, we can estimate the dichroic ratio for the lowest exciton band, D|/D⊥, where D| and D⊥ are absorption for the polarization of the incident light parallel and perpendicular to the dipping direction, respectively. The dichroic ratio was 4.5, which was observed to be independent of the dipping speed and the layer number. Similar anisotropy has been also observed at the airwater interface of helical polysilanes with substituents of ether and branched achiral alkyl group.14,15 Polarized photoluminescence spectra for LB films of the chiral PS at 77 K are shown in Figure 3, compared with the polarized absorption spectra. The photoluminescence peak for the chiral PS with the helical backbone appears at nearly the same energy as the absorption peak with minimal Stokes shift and no vibrational structure, indicating weak exciton-lattice interaction. Such a resonance emission has been observed for the regular polysilanes with the ordered trans-planar and alternating trans-gauche conformations.17 Furthermore, the photo(28) Minari, N.; Ikegami, K.; Kuroda, S.; Saito, K.; Saito, M.; Sugi, M. Solid State Commun. 1988, 65, 1259. (29) Minari, N.; Ikegami, K.; Kuroda, S.; Saito, K.; Saito, M.; Sugi, M. J. Phys. Soc. Jpn. 1989, 58, 222. (30) Wegner, G. Thin Solid Films 1992, 262, 105.
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Figure 5. Temperature dependence of absorption spectra in the LB films of the chiral polysilane.
Figure 4. Polarized spectra of (a) absorption and (c) electroabsorption in the LB films of chiral polysilane. Solid and dashed lines represent the spectra polarized parallel and perpendicular to the dipping direction, respectively. Lines in (b) represent the first energy derivative curve of the absorption spectra.
luminescence spectra also show very similar anisotropy to the absorption spectra. The ratio of luminescence intensity polarized parallel and perpendicular to the dipping direction was nearly the same as the dichroic ratio observed in the polarized absorption spectra. This result indicates that the photogenerated free excitons migrate to undergo radiative recombination on the rigid rodlike polysilane backbones with helical conformation. Electroabsorption (EA) spectra were measured to clarify the electronic structure of the chiral PS. Figure 4 shows polarized EA spectra of the LB film at 77 K. For comparison, the polarized absorption spectra and the calculated curves of the first energy derivative of the original absorption spectra are also shown in Figure 4a,b, respectively. The EA spectra of the chiral PS with helical backbone also show very similar features to those of the polysilanes with the other conformations as reported previously, such as trans-planar and alternating transgauche backbone structures apart from the energy positions.17 The Stark shift signals are observed in the same energy region of the absorption peak due to the lowest singlet exciton (1B1u), which are well reproduced by the calculated curves for the first energy derivative of the original absorption spectra. In addition, the characteristic EA signals are observed at 0.9 eV higher energy above the lowest exciton, which have no counterpart in the absorption spectrum. Like the corresponding ones for other polysilanes,17,18 this higher-lying EA signals can be assigned to the first excited states (1Ag) of the onedimensional excitons. These results have revealed features of one-dimensional exciton states characteristic of the helical conformation with one helical sense. The large binding energies (ca. 1.0 eV) of the lowest singlet exciton are almost the same as that of polysilanes with other conformation as reported previously,17,18 which is inherent in the one-dimensional nature of σ-conjugated polysilanes. We have investigated thermal effects on spectral change in the LB film of the chiral PS. Figure 5 shows temperature dependence of absorption spectra in the LB films of the chiral PS. The absorption peak of polysilanes with conformations such as trans-planar and alternating transgauche backbone structures have been known to shift to higher energy drastically, accompanied with orderdisorder conformational change of the polymer backbones.17 Such a drastical spectral change is, however, not
Figure 6. Dichroic ratio as a function of annealing time in the LB films of the chiral polysilane. The annealing temperature was 373 K. The inset shows polarized absorption spectra of the chiral PS LB films (A) before and (B) after annealing treatment. Solid and dashed lines represent the spectra polarized parallel and perpendicular to the dipping direction, respectively.
observed in the LB film of the chiral PS with helical conformation, probably due to the rigidity of the backbone structures. With the increase of temperature, the exciton peak shifts to lower energy, becomes broader, and decreases in the intensity. This is probably due to the thermal expansion of helical pitch. Similar phenomena have been also observed in solution of helical polysilane with chiral substituents.23,25 The effect of the annealing treatment on the orientation of the chiral PS in the LB films was also studied. The inset to Figure 6 shows the polarized absorption spectra before and after annealing. The anisotropy of the lowest exciton absorption is enhanced by annealing the LB films. The orientational degree of polymer backbones can be estimated from the dichroic ratio of the lowest exciton absorption. The change in the dichroic ratio as a function of the annealing time at 373 K is plotted in Figure 6. The dichroic ratio increases up to 9, twice that for the asdeposited LB film, with the annealing time and tends to saturate over 12 h. Conclusions The electronic structure in a Langmuir-Blodgett (LB) film of helical polysilane with chiral substituents have been investigated by measurements of polarized UVvisible absorption, photoluminescence, and electroabsorption spectra. The polarized UV-visible absorption and photoluminescence spectra prove the high structural anisotropy of the LB film, in which the polymer backbone lies along the dipping direction. The anisotropy can be enhanced by annealing the LB films at 373 K. The electroabsorption spectra have revealed the one-dimensional exciton structure characteristic of the helical polysilane with chiral substituents, which exhibits a
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common feature for σ-conjugated polysilanes with regular backbone conformations. Acknowledgment. This work was supported by a Grant-In-Aid for Scientific Research from the Ministry
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of Education, Science, and Culture of Japan and by the New Energy and Industrial Technology Development Organization (NEDO). LA0010774
