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Langmuir 1996,11, 4639-4641
Incorporation of Pyrene into an Oriented Transparent Film of Layered Silica-Hexadecyltrimethylammonium Bromide Nanocomposite Makoto Ogawa? The Institute of Physical and Chemical Research (RIKEN), Hirosawa 2-1, Wako-shi, Saitama 351-01,Japan Received November 22, 1994. In Final Form: October 25, 1995@ A layered silica-hexadecyltrimethylammonium halide nanocomposite was used as a novel transparent support to immobilize pyrene. For the incorporation of pyrene into the layered composites, pyrene was first solubilized into an aqueous solution of hexadecyltrimethylammonium halide and the solution was allowed to react with partially hydrolyzed tetramethoxysilane. By spin coating the solution on a quartz substrate, thin films were obtained. The films are transparent in a wavelength region from the visible to the near-infrared regions and possessed a layered structure. Fluorescence spectra of the incorporated pyrene suggest that the pyrene molecules were incorporatedin the hydrophobic part ofthe layered composites without aggregation. The present system is a novel state of surfactant aggregates. Applications such as the immobilization of guest species (especially photoactive ones) are promising because of the simplicity of preparation, the micro- and macroscopic anisotropy, and transparency of these layered composites.
The self-organization of molecules into highly ordered architectures has attracted increasing attention from a wide range of both scientific and practical interests. For constructing ordered nanomaterials, inorganic-organic nanocomposites with ordered structures have advantages such as well-defined structure and The processing of them into oriented films, in which microscopic anisotropy can be converted directly into macroscopic anisotropy, is of importance for' both their practical applications and their detailed characterization. Along this line, oriented films of intercalation comp~unds,~" fixations of molecular sieves into thin films,8 and selfassembled multilayers of zirconium phosphonatesg have been reported so far. The encapsulation of organic molecules into alkoxysilane-derived inorganic solids through sol-gel processeslO has been reported to probe sol-gel processes as well as to prepare materials with specific optical properties.11J2 However, it is difficult to obtain well-defined nanostructures for the organic guest species. Incorporation of pyrene into ordered inorganic+ Present address: Institute of Earth Science, Waseda University, Nishiwaseda 1-6-1,Shinjuku-ku, Tokyo 169-50,Japan. Abstract published in Advance ACS Abstracts, December 1, 1995. (1)Ozin, J. A. Adv. Mater. 1992,4,612. (2)Stucky, G. D. Prog. Inorg. Chem. 1992,40,99. (3)Fendler, J. H. Membrane-Mimetic Approach to Advanced Muterials; Springer-Verlag: Berlin, 1994. (4)Intercalation Chemistry; Whittingham, M. S., Jacobson, A. J., Eds.; Academic Press: New York, 1982. (5)Mann, S.,Webb, J., Williams, R. J. P., Eds. Biomineralization; VCH Publishers, Inc.: Weinheim, 1988. (6)Ogawa, M.; Handa, T.; Kuroda, K.; Kato, C.; Tani, T. J. Phys. Chem. 1992,96,8116. Ogawa, M.; Takahashi, M.; Kato, C.; Kuroda, K. J. Mater. Chem. 1994,4,519.Ogawa, M.; Takahashi, M.; Kuroda, K. Chem. Mater. 1994,6,715. (7)Kim, R.M.; Pillion, J . E.; Burwell, D. A,; Groves, J. T.; Thompson, M. E. Inorg. Chem. 1993,32,4509. (8)Feng, S.;Bein, T. Nature 1994,368,834. (9)Cao, G.; Hong, H.-G.; Mallouk, T. E. Acc. Chem. Res. 1992,25, 420.Lee, H.; Kepley, L. J.;Hong, H.-G.; Mallouk, T. E. J. Am. Chem. SOC.1988,110,618. Vermeulen, L.A.; Thompson, M. E. Nature 1992, 358,656.Katz, H. E.; Scheller, G.; Putvinski, T. M.; Schilling, M. L.; Wilson, W. L.; Chidsey, C. E. D. Science 1991,254, 1485. (10)Brinker, C. J.; Scherer, G. W. Sol-Gel Science The Physics and Chemistry of Sol-Gel Processing; Academic Press, Inc.: San Diego, CA, 1990. (11)Avnir, D.; Braun, S.;Lev, 0.;Levy, D.; Ottolenghi, M. InSol-Gel Optics: ProcessingandApplications;Klein, L. C., ed.; Kluwer Academic Publishers: Boston, MA, 1994;Chapter 23. (12)Dunn, B.; Zink, J. I. J . Muter. Chem. 1991,1, 903.
surfactant nanocomposites, organoammonium-exchanged layered clay minerals (montmorillonite),has been reported so far.13 Although unique environments for pyrene have been obtained, it is difficult to process them into thin films with high optical quality. On the other hand, the formation of nanostructured silicate~l~ and -~~ other metal oxides17 using surfactant aggregates as structure directing agents has attracted increasing interest. Recently, I have successfullyprepared novel oriented thin films of layered silica-alkyltrimethylammonium salt nanocomposites.22 Their transparency in a wavelength region from ultraviolet to near-infrared opens a way to apply these novel inorganic-organic composites for supports of photoactive species. In this communication, the incorporation of pyrene into the layered silica-hexadecyltrimethylammonium bromide composite film is reported.
molecular structure of pyrene
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The thin films of silica-surfactant nanocomposites were prepared by the method described in a previous com(13)Ogawa, M.; Aono, T.; Kuroda, K.; Kato, C. Lungmuir 1993,9, 1529. (14)Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.;Vartuli, J. C.; Beck, J. S. Nature 1992,359,710. (15)Beck, J. S.;Vartuli, J . C.; Roth, W. L.; Leonowicz,M. E.; Kresge, C. T.; Schmidt, K. D.; Chu, C. T.-W.; Olson, D. H.; Sheppard, E. W.; McCullen. S. B.:. Hiapins. 1992, -- J. B.: Schlenker, J. L. J.Am. Chem. SOC. 114,10834. (16)Monnier, A,; Schuth, F.; Huo, Q.; Kumar, D.; Margolese, D.; Maxwell, R. S.; Stucky, G. D.; Krishnamurty, M.; Petroff, P.; Firouzi, A.; Janicke, M.; Chmelka, B. F. Science 1993,261,1299. (17)Huo, Q.; Margolese, D. I.; Ciesla, U.; Feng, P.; Gier, T. E.; Sieger, P.; Leon, R.; Petroff, P. M.; Schiith, F.; Stucky, G. D. Nature 1994,368, 317. (18)Tanev, P. T.;Chibwe, M.; Pinnavaia, T. J. Nature 1994,368, 321. (19)Huo, Q.;Margolese, D. I.; Ciesla, U.; Demuth, D. G.; Feng, P.; Gier, T. E.; Sieger, P.; Firouzi, A.; Chmelka, B. F.; Schiith, F.; Stucky, G. D. Chem. Muter. 1994,6, 1176. (20)Sakata, K.; Kunitake, T. J. Chem. SOC.,Chem. Commun. 1990, 504. Sakata, K.; Kunitake, T. Chem. Lett. 1989,2159. (21)Dubois, M.; Guik-Krzywicki, Th.; Cabane, B. Lungmuir 1993, 9, 673.Dubois, M.; Cabane, B. Langmuir 1994,10, 1615. (22)Ogawa, M. J. Am. Chem. SOC.1994,116,7941.
0 1995 American Chemical Society
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4640 Langmuir, Vol. 11, No. 12, 1995
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Figure 1. Absorption spectra of the silica-CTAB-pyrene composite films (the molar ratio of pyrene t o CTAB is W100 (a), 1/10 (b), and W5 (c)) .
munication.22 Tetramethoxysilane (abbreviated as TMOS) was partially hydrolyzed by a substoichiometric amount of water (the molar ratio of TM0S:HzO was 1:2) under acidic conditions (pH = 3) for 2 h a t room temperature. Then an aqueous solution of hexadecyltrimethylammonium bromide (abbreviated as CTAB, 0.25 M) was added. After the mixture was stirred for a few minutes a t room temperature, the homogeneous solution was spin coated on a quartz substrate and dried in air. The molar ratio of TM0S:CTAB was 4:l in this study. For the incorporation of pyrene into the layered composites, pyrene was first solubilized into a n aqueous solution of CTAB. The molar ratios of pyrene to CTAB varied from M O O to l/5. The absorption spectrum ofthe thin film (the thickness of the film was ca. 1 pm) of the silica-CTAB-pyrene composite (the molar ratio of pyrene to CTAB is 1/10) is shown in Figure lb. The films were highly transparent in the wavelength region from the visible to the nearinfrared region (to 2500 nm). The absorption bands due to n-nm transition of pyrene were observed a t 340,320, 310, 275, 265, 243, and 235 nm and the spectrum is consistent with that of a pyrene solution. Transparent thin films were obtained for the composites containing different amounts of pyrene (1/100 or 115 mol of pyrene to CTAB) and the absorption spectra of the films are also shown in Figure 1. There is a trend toward a n increase in the absorbance with increased amount of pyrene in a n almost linear relationship, indicating that the starting mixtures have been converted to final products. Due to the solubility of pyrene in CTAB solution, the amount of pyrene incorporated into the layered composite was as much as 115 mol t o CTAB under the experimental conditions used in this study. When alkyltrimethylammonium salts with shorter alkyl chains were used, the amount of pyrene embedded in the layered composites was smaller due to lower solubility of pyrene in the surfactant solutions. Figure 2 shows the X-ray diffraction patterns of the silica-CTAB and the silica-CTAB-pyrene (the molar ratio of pyrene to CTAB are 1/10 and 1/5) composite films. As reported in the previous communication, a very sharp diffraction peak which shows the d spacing of ca. 3.9 nm was observed in the XRD pattern of the silica-CTAB composite film (Figure 2a). In the XRD pattern of the silica-CTAB-pyrene composite film (Figure 2b,c), a very sharp diffraction peak with the d value of ca. 4.1 nm appeared and no diffraction attributable to pure pyrene or pure CTAB crystals was detected. These observations indicate that the pyrene-containing composite films retain the layered structure characteristic of the silica-CTAB films.
3 20 / O (CuKa) Figure 2. X-ray diffraction pattern of (a) the silica-CTAJ3 and (b, c) silica-CTAB-pyrene (the molar ratio of pyrene t o CTAB is 1/10 (b) and 1/5 (c)) composite films. There is a general trend toward larger d spacings with increasing concentration of pyrene. As mentioned in the previous communication,22the d values of the layered silica-surfactant composites were affected by the relative ratios of organic to inorganic components. The d value of the silica-CTAB composite (TM0S:CTAB= 4:l) is slightly larger than that of the composite (TM0S:CTAB = 2.51). Further increase in the d value was observed by the addition of pyrene. Therefore, I attribute the larger d values observed for the silica-CTAB-pyrene composites to the change in the relative ratio of silica to organic components. The mechanism responsible for the increasing d values may involve an increase in the thickness of the silica layer andor a change in the packing of the organic components (possibly including reorientation of the alkyl chains). In the study on the preparation ofmesoporous silicates, 1,3,5-trimethylbenzene (TMB) was used as a n “expander”.15 The pore size of MCM-41 can be varied as a function of the concentration of TMB. The molar ratios of TMB to CTAB ranged from 1to 2.5,15which were much higher than those used in this study. In the present system, the added amount of pyrene to be incorporated is lower. Therefore, it was difficult to elucidate the location of pyrene molecules in the layered composite from the relationships between the d values and the amount of pyrene. In contrast to conventional alkoxysilane-derived materials,1° the present composites contained only a trace amount of water, which was revealed by thermogravimetric analysis. This observation supports the structure ofthe layered silica-surfactant nanocomposites proposed in the previous communication.22 The surface of the silica layers interacts with hydrophilic head groups of the cationic surfactants and there is no available silica surface for water to adsorb. Figures 3 and 4 show the emission and excitation spectra of the silica-CTAB-pyrene composite film. (The excitation wavelength for the emission spectrum is 340 nm and the excitation spectrum was monitored for the emission a t 390 nm.) When pyrene is forced into close proximity or in high concentration solution, a n excited state dimer (excimer) forms and the emission from the excimer is observed a t around 475 nm in the fluorescence spectrum.23 Although the amount of pyrene was high (the composition of the composite [the molar ratio of pyrene to CTAB was 1/10] was determined to be 38, 58, and 3 wt % for Si02, CTAB, and pyrene), the contribution of the excimer
Langmuir, Vol. 11, No. 12, 1995 4641
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Figure 3. Emission spectra of (a, b) the silica-CTAB-pyrene composite films (the molar ratios of pyrene to CTAB are 1/10 (a)and 1/5 (b))and (c) pyrene in CTAB solution (the molar ratio of pyrene to CTAB is 1/100).Excitation wavelength was 340 nm.
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Figure 4. Excitation spectrum of the silica-CTAB-pyrene (the molar ratio of pyrene to CTAB is 1/10) composite film. The excitation spectrum was monitored for the emission at 390 nm. emission (at around 475 nm) was very small for the present composites. The excitation spectrum of the excimer emission (475nm) was consistent with that ofthe monomer emission (monitored at 390 nm), suggesting that there were no significant ground state interactions between the incorporated pyrene molecules. If compared with the fluorescence spectra of the CTAB solutions containing pyrene, the contribution of excimer fluorescence was significantly suppressed in the spectra of the silicasurfactant composite films. These observations indicate that the added pyrene molecules are solubilized molecularly in the silica-CTAB nanocomposite and that the (23)(a)Kalyanasundaram,K.;Thomas, J. K. J.Am. Chem. SOC.1977, 99, 2039. (b) Nakajima, A. Bull. Chem. SOC.Jpn. 1971,44, 3272. (c) Dong, D. C.; Winnik, M. A. Photochem. Photobiol. 1982,35, 17.(d) Kalyanasundaram, K.In Photochemistry in Organized & Constrained
Media; Ramamurthy, V., Ed.; VCH Publishers, Inc.: New York, 1991; Chapter 2.
mobility of the pyrene molecules is restricted. It is not likely that the high loading of pyrene (1/5 for pyrene to CTAB) can be achieved without there being any nearest neighbor pyrene molecules. The location and the distribution of the pyrene molecules especially a t high loading are not clear a t present. The vibronic structure of the fluorescence spectrum of pyrene is strongly dependent on the polarity of microenv i r o n m e n t ~ . The ~ ~ change in the relative intensity of fluorescence peaks I (at 373 nm) and I11 (at 385 nm) is commonly used to monitor the polarity. (Peaks I and I11 are marked in Figure 3.) For the present composite films, MI1 intensity ratios are ca. 0.8 irrespective of the amounts ofpyrene, and these values are similar to that ofthe pyrene in CTAB solution (pyrene:CTAB = 1:lOO). Takingthe Py scale of solvent polarity which has been proposed by Dong and Winnik23cinto consideration, the incorporated pyrene molecules are thought to be surrounded by the alkyl chain of CTAB with some interactions of the hydrophilic head group of CTAB which is similar to the location proposed for the TMB embedded in hexagonal silica-CTAB comp o s i t e ~ Since . ~ ~ the surfactant aggregates are immobilized by the inorganic parts, the surfactants are thought to form ordered structures even when additives such as 1,3,5trimethylbenzene and pyrene are included. It was shown that the layered silica-CTAB composites could incorporate pyrene molecules a t the concentrations of as high as a few weight percent while retaining their highly ordered layered structures. Surfactant aggregates in solution^^^-^^ and their transformation into solids such as LB f i l m P and immobilized synthetic bilayer memb r a n e have ~ ~ ~widely been investigated for constructing molecularly ordered architectures. The present system is a novel state of surfactant aggregates stabilized by the thin silica layer. Applications such as the immobilization of guest species (especiallyphotoactive ones) are promising because of the simplicity of preparation, the micro- and macroscopic anisotropy, and the transparency of these layered composites.
Acknowledgment. This work was supported by a grant for “Special Researcher’s Basic Science Program” from the Science and Technology Agency of the Japanese Government. The author thanks reviewers for their helpful comments. LA940927A (24)Turro,N. J.; Gratzel, M.; Braun, A. M. Angew. Chem., Int. Ed. Engl. 1980,19, 675. (25)Clint, J. H.Surfactant Aggregation; Blackie: New York, 1992. (26)Ringsdorf, H.; Schlarb, B.; Venzmer, J. Angew. Chem., Znt. Ed. Engl. 1988,27,113. (27)Goddard, E. D.; hathapadmanabhan, K. P., Eds. Interactions ofSurfactants with Polymers and Proteins; CRC Press, Inc.: Boca Raton, 1993. (28)Ulman, A. An Introduction to Ultrathin Organic Films from Langmuir Blodgett to Self-Assembly;Academic Press, Inc.: San Diego, CA, 1991. (29)Kunitake, T.Angew. Chem. Int. Ed. Engl. 1992,39, 709.
