Nanoscale Aggregation of Fullerene in Nafion Membrane - Langmuir

The well-defined microstructures, together with their exceptional optical, chemical, thermal, and mechanical properties, have made Nafion films an ide...
0 downloads 0 Views 125KB Size
Download PDF
Langmuir 2002, 18, 9017-9021

9017

Nanoscale Aggregation of Fullerene in Nafion Membrane Zhi-Xin Guo,*,† Na Sun,† Junxin Li,† Liming Dai,*,†,‡ and Daoben Zhu† Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, China, and Department of Polymer Engineering, College of Polymer Science and Polymer Engineering, The University of Akron, Akron, Ohio 44325-2909 Received May 14, 2002. In Final Form: August 11, 2002 Nanoscale aggregation of a new methanofullerene derivative with peripheral hydrophilic moieties was achieved within the confined space of a Nafion membrane. Transmission electron microscopic examination revealed an average size of 5.0 nm for the aggregates, while optical, fluorescence, and X-ray photoelectron spectroscopic measurements indicated excimer formation within the well-defined aggregated structure. These nanocomposites based on fullerene derivatives exhibit a significant optical-limiting capability attractive for many potential applications.

Introduction Owing to their unique chemical and physical properties, fullerenes have been continuously attracting strong interest from many areas of science and technology.1-5 Recent research has extended to the fabrication of various selfassembled structures of fullerenes and their derivatives,6-22 for example, through chemical-modification-induced hydrophobic and/or amphiphilic interactions.6-16 In particular, Tour and co-workers6 reported self-assembling of rodlike and vesicular aggregated structures of C60-N,Ndimethylpyrrolidinium iodide from appropriate solutions of the C60 derivative, while Chu et al.15 demonstrated the * Corresponding authors. E-mail: [email protected] (L.D.), [email protected] (Z.-X.G.). † Institute of Chemistry. ‡ The University of Akron. (1) Hirsch, A., Ed. Top. Curr. Chem. 1999, 199. (2) Fullerenes: Chemistry, Physics, and Technology; Kadish, K. M., Ruoff, R. S., Eds.; Wiley-Interscience: New York, 2000. (3) Prato, M. J. Mater. Chem. 1997, 7, 1097. (4) Makarova, T. L.; Sundqvist, B.; Ho¨hne, R.; Esquinazi, P.; Kopelevich, Y.; Scharff, P.; Davydov, V. A.; Kashevarova, L. S.; Rakhmanina, A. V. Nature 2001, 413, 716. (5) Scho¨n, J. H.; Kloc, C. H.; Batlogg, B. Science 2001, 293, 2432. (6) Cassell, A. M.; Asplund, C. L.; Tour, J. M. Angew. Chem., Int. Ed. 1999, 38, 2403. (7) Murakami, H.; Shirakusa, M.; Sagara, T.; Nakashima, N. Chem. Lett. 1999, 815. (8) Oishi, K.; Ishi-i, T.; Sano, M.; Shinkai, S. Chem. Lett. 1999, 1089. (9) Sano, M.; Oishi, K.; Ishi-i, T.; Shinkai, S. Langmuir 2000, 16, 3773. (10) Brettreich, M.; Burghart, S.; Bottocher, C.; Bayerl, T.; Bayerl, S.; Hirsch, A. Angew Chem. Int. Ed. 2000, 39, 1845. (11) Shigeru, D.; Rossiza, G. A.; Kaoru, T. Langmuir 2001, 17, 6013. (12) Fujihara, H.; Nakai, H. Langmuir 2001, 17, 6393. (13) Angelini, G.; Maria, P. D.; Fontana, A.; Pierini, M. Langmuir 2001, 17, 6404. (14) Rossiza, G. A.; Shigeru, D.; Kaoru, T. J. Am. Chem. Soc. 2001, 123, 10460. (15) Zhou S.; Burger, C.; Chu B.; Sawamura, M.; Nagahama, N.; Toganoh, M.; Hackler, U. E.; Isobe, H.; Nakamura, E. Science 2001, 291, 1944. (16) Nakashima, N.; Ishii, T.; Shirakusa, M.; Hiroto, T.; Sagara, M. Chem. Eur. J. 2001, 7, 1766. (17) Brust, M.; Kiely, C. J.; Bethell, D.; Schiffrin, D. J. J. Am. Chem. Soc. 1998, 120, 12367. (18) Fu, K.; Kitaygorodskiy, A.; Sun, Y.-P. Chem. Mater. 2000, 12, 2073. (19) Sun, N.; Wang, Y.; Song, Y.; Guo, Z.; Dai, L.; Zhu, D. Chem. Phys. Lett. 2001, 344, 277. (20) Sun, N.; Guo, Z.; Dai, L.; Wang, Y.; Song, Y.; Zhu, D. Chem. Phys. Lett. 2002, 356, 175. (21) Sudeep, P. K.; Ipe, B. I.; Thomas, G.; George, M. V.; Barazzouk, S.; Hotchandani, S.; Kamat, P. V. Nano Lett. 2002, 2, 29. (22) Jenekhe, S. A.; Chen, X. L. Science 1998, 279, 1903.

formation of spherical bilayer vesicles from potassium salt of pentaphenyl fullerene. Likewise, Nakashima16 prepared both fibrous and disklike aggregated nanostructures from C60 ammonium amphiphile derivatives. On the other hand, a few self-assembled structures consisting of functionalized fullerene derivatives (e.g. with terminated thiol groups) and nanometals (e.g. Au and Ag) have recently been demonstrated to show certain unusual optoelectronic properties.17-21 In this context, we19,20 have recently reported interesting optical-limiting properties for methanofullerene-Ag nanocomposites, while Sudeep et al.21 constructed a self-assembled photoactive antenna based on nanostructured gold nanoparticles as a central nanocore and thiol-derivatized fullerene molecules as the photoreceptive hydrophobic shell. Furthermore, certain hollow structures have been utilized to facilitate the formation of fullerene aggregates with well-defined geometries, as exemplified by the encapsulation of C60 in self-assembled hollow cavities formed by amphiphilic poly(phenylquinoline)-block-polystyrene rod-coil diblock copolymers22,23 and hollow cores of carbon nanotubes.24,25 Although the aggregation of C60 and/or its derivatives within a confined space is interesting and of practical significance, this field is still in its infancy. Nafion membranes constructed from perfluorosulfonate ionomers possess network structures with interconnected reverse micellelike ion clusters,26,27 as schematically shown in Figure 1. The well-defined microstructures, together with their exceptional optical, chemical, thermal, and mechanical properties, have made Nafion films an ideal template for investigating various chemical and physical processes within a confined geometry.28-35 In this paper, (23) Chen, X. L.; Jenekhe, S. A. Langmuir 1999, 15, 8007. (24) Smith, B. W.; Monthioux, M.; Luzzi, D. E. Chem. Phys. Lett. 1999, 315, 31. (25) Hornbaker, D. J.; Kahng, S. J.; Misra, S.; Smith, B. W.; Johnson, A. T.; Mele, E. J.; Luzzi, D. E.; Yazdani, A. Science, 2002, 295, 828. (26) Hsu, W. Y.; Gierker, T. D. J. Membr. Sci. 1983, 13, 307. (27) Perfluorinated Ionomer Membranes; Eisenberg, A., Yeager, H. L., Eds.; American Chemical Society, Washington, DC, 1982. (28) Krishnan, M. White, J. R.; Fox, M. A.; Bard, A. J. J. Am. Chem. Soc. 1983, 105, 7002. (29) Wang, Y.; Mahler, W. Opt. Commun. 1987, 61, 233. (30) Hilinski, E. F.; Lucas, P. A.; Wang, Y. J. Chem. Phys. 1988, 89, 3435. (31) Wang, Y.; Suna, A.; McHugh, J.; Hilinski, E. F.; Lucas, P. A.; Johnson, R. D. J. Phys. Chem. 1990, 92, 6927. (32) Smotkin, E. S.; Brown, R. M.; Radenburg, L. K.; Salomon, K.; Bard, A. J.; Campion, A.; Fox, M. A.; Mallouk, T. E.; Webber, S. E.; White, J. M. J. Fluorine Chem. 1990, 94, 7543.

10.1021/la0259454 CCC: $22.00 © 2002 American Chemical Society Published on Web 10/17/2002

9018

Langmuir, Vol. 18, No. 23, 2002

Guo et al.

fluorescence emission due to excimer formation. A strong optical-limiting effect was also observed on a nanosecond Nd:YAG pulse laser system at a wavelength of 532 nm. Experimental Section

Figure 1. The ion cluster model proposed for the Nafion membrane in the literature.27 Scheme 1

we report a novel approach toward C60 nanoaggregates within a Nafion membrane. By so doing, we synthesized a new methanofullerene derivative with peripheral hydrophilic chains (Scheme 1). As we shall see later, the tailor-made hydrophilic terminal groups can effectively “drive” the modified fullerene molecules into the hollow cavities of a Nafion membrane through the hydrophilic interaction with those surrounding polar groups (e.g. -SO3Na, Figure 1). The fullerene aggregates thus formed have a comparable size (∼5.0 nm in diameter) to the cavities of the Nafion membrane and show a red-shift in (33) Zen, J.-M.; Chen, G.; Fan, F.-R. F.; Bard, A. J. Chem. Phys. Lett. 1990, 169, 23. (34) Albu-Yaron, A.; Arcan, I. Thin Solid Films 1999, 185, 181. (35) Rollions, H. W.; Whiteside, T.; Shafer, G. J.; Ma, J.-J.; Tu, M.H.; Liu, J.-T.; DesMarteau, D. D.; Sun, Y.-P. J. Mater. Chem. 2000, 10, 2081.

Materials. Scheme 1 shows the route to methanofullerene (TDC60). 2-(10-Bromodecyloxy)tetrahydropyran (compound 1) was synthesized from 1,10-didecanediol according to a published method,36,37 whereas 2-4 were prepared via reported procedures with slight modifications to be detailed below.38,39 Synthesis of 2-(10-{2-[2-(2-Ethoxyethoxy)ethoxy]ethoxy}decyloxy)tetrahydropyran (Compound 2). A 1.5-g portion of sodium hydride (52% in oil, 30 mmol) was added in portions to 20 mL of tri(ethylene glycol) monoethyl ether in an ice-cooled flask. After 2 h, the ice bath was removed and the mixture was stirred for another 2 h at 100 °C. Then 6.5 g of 1 (20 mmol) was added and the mixture was kept at 100 °C for 20 h. The resulting opaque mixture was cooled, diluted with 50 mL of water, and extracted with dichloromethane (4 × 10 mL). The combined organic layers were washed with water (2 × 10 mL) and dried with MgSO4. After removing the solvent, compound 2 was obtained and used without further purification. Synthesis of 10-{2-[2-(2-Ethoxyethoxy)ethoxy]ethoxy}decan-1-ol (Compound 3). A 4.07-g portion of 2 (10 mmol) and 3.32 g of CBr4 (0.5 mmol) in 50 mL of anhydrous methanol was refluxed for 3 h. After cooling, the solution was poured into 100 mL of 5% aqueous NaHCO3 solution and then extracted with diethyl ether (3 × 100 mL). The combined organic layer was washed twice with brine (100 mL) and dried with MgSO4. Then 3.0 g of compound 3 was obtained as clear oil through vacuum distillation (160-165 °C/0.05 mmHg). Yield: 45%. 1H NMR (300 MHz, CDCl3): δ (ppm) 3.68-3.60 (m, 14H), 3.58 (q, 2H), 3.46 (t, 2H), 1.57 (m, 4H), 1.30 (m, 12H), 1.22 (t, 3H). 13C NMR (75 MHz, CDCl3): δ (ppm) 71.46, 70.52, 69.97, 69.74, 66.58, 62.70, 32.68, 29.54, 29.47, 29.38, 26.00, 25.72, 15.08. FAB-MS: 335. Synthesis of Bis(10-{2-[2-(2-ethoxyethoxy)ethoxy]ethoxy}decyl) Propanedioate (Compound 4). A mixture of 1.3 mL of pyridine (15.6 mmol) and 5.5 g of compound 3 (16.4 mmol) in 100 mL of dry dichloromethane was cooled to 0-5 °C. Then 0.80 mL of propanediol dichloride (8.16 mmol) was added dropwise over 15 min. After 1 h, the solution was brought to room temperature and stirred overnight. The solution was washed twice with brine and dried with MgSO4. After removing the solvent, the mixture was separated with column chromatography (SiO2, CH2Cl2/MeOH ) 50/1, v/v). Finally, 3.0 g of compound 4 was obtained as a light yellow liquid. Yield: 50%. 1H NMR (300 MHz, CDCl3): δ (ppm) 4.13 (t, 4H), 3.67-3.56 (m, 24H), 3.54 (q, 4H), 3.45 (t, 4H), 3.36 (s, 2H), 1.66-1.55 (m, 8H), 1.28 (m, 24H), 1.21 (t, 6H). 13C NMR (75 MHz, CDCl3): δ (ppm) 166.60, 71.4, 70.5, 70.0, 69.8, 66.6, 65.6, 41.6, 29.6, 29.4, 29.1, 28.4, 26.0, 25.7, 15.1. FAB-MS: 759 (M + Na+). Synthesis of Bis(10-{2-[2-(2-ethoxyethoxy)ethoxy]ethoxy}decyl) 1,2-Methano[60]fullerene-61,61-dicarboxylate (TDC60). A mixture of 47.6 mg of C60 (0.066 mmol), 32.8 mg of CBr4 (0.100 mmol), 73.6 mg of compound 4 (0.100 mmol), and 15.2 µL of 1,8-diazabicyclo[5,4,0]undec-7-ene (DBU, 0.198 mmol) in 30 mL of dry toluene was stirred at room temperature for 6 h. The solvent was evaporated under reduced pressure, and the residue was purified by flash chromatography on silica gel (toluene, then dichloromethane); TDC60 was obtained as dark brown slurry. Yield: 25.3 mg (26.4%). 1H NMR (400 MHz, CDCl3): δ (ppm) 4.48 (t, 4H), 3.64-3.56 (m, 24H), 3.51 (q, 4H), 3.43 (t, 4H), 1.58-1.40 (m, 8H), 1.28-1.25 (m, 24H), 1.20 (t, 6H). 13C NMR (100 MHz, CDCl ): δ (ppm) 163.66, 145.42 (4C), 145.26 3 (4C), 145.19 (8C), 144.88 (2C), 144.69 (4C), 144.67 (2C), 144.61 (4C), 143.89 (4C), 143.09 (2C), 143.02 (4C), 142.99 (4C), 142.21 (4C), 141.92 (4C), 140.95 (4C), 139.00 (4C), 71.72, 71.53, 70.67 (36) Camps, M.; Casamor, J. M.; Coll, J.; Guerrero, A.; Riba, M. Org. Prep. Proc. Int. 1983, 19(1-2), 63. (37) Schwarz, M.; Graminski, G. F.; Waters, R. M. J. Org. Chem. 1986, 51, 260. (38) Johnson, H.; Degraw, J.; Engstrom, J.; Skinner, W. A.; Brown, V. H.; Skidmore, D.; Maibach, H. I. J. Pharm Sci. 1975, 64, 693. (39) Lee, A. S.-Y.; Su, F.-Y.; Liao, Y.-C. Tetrahedron Lett. 1999, 40, 1323.

Nanoscale Aggregation of Fullerene (2C), 70.61, 70.07, 69.83, 67.47, 66.63, 29.67, 29.56, 29.54, 29.52, 29.24, 28.61,26.12, 26.00, 15.17. MOLDI-TOF-MS: 1477.3 (M + Na+). Activation of Nafion Membrane: Nafion 117 (1100 EW, 117 µm thick) was purchased from Aldrich. The film was sonicated for 1 h in a mixture of distilled water and methanol (1:1, v/v). Subsequently, it was washed repeatedly in distilled water. Then, the Nafion-H+ membrane was prepared by immersing the clean Nafion film in 20% nitric acid under stirring at 60 °C for 24 h, followed by washing thoroughly with distilled water. Thereafter, the Nafion-H+ membrane was converted into the sodium form by soaking it in a 0.1 M aqueous solution of sodium hydroxide under stirring for 24 h, followed by washing thoroughly with distilled water until the washing liquid becomes neutral. The membrane in the sodium form is clear and optically transparent down to 200 nm. Preparation of TDC60-Trapped Nafion Film. The sodium form of the Nafion film was soaked in a saturated methanol solution of TDC60 at room temperature for several days. After washing thoroughly with chloroform to remove free TDC60 residuals, if any, the film was dried in a vacuum at 50 °C overnight. Characterization. 1H and 13C NMR spectra were recorded on Bruker DMX-300 and Bruker ARX 400 spectrometers with tetramethylsilane as internal reference. MALDI-TOF-MS spectra were obtained on a Bruker BIFLEX III machine. Optical absorption was measured on a computer-controlled Shimadzu UV2501-PC spectrophotometer. Fluorescence spectra were recorded in front-face geometry on a Spex Fluorolog-3 photon-counting emission spectrometer equipped with a 450 W xenon lamp, using a Spex 600 grooves/mm dual-grating (blazed at 1000 nm) as emission monochromator and 1200 grooves/mm grating (blazed at 600 nm) as excitation monochromator. The thermoelectronically cooled detector consists of a near-infraredsensitive Hamamatsu R2658P photomutiplier tube operated at -1500 V. For fluorescence measurements, an Schott 500 nm (KV 500) color glass sharp-cut filter was placed before the emission monochromator to eliminate the excitation scattering. Fluorescence spectra were corrected for nonlinear response by using a predetermined correction factor provided by the manufacture. Transmission electron microscopy (TEM) images were taken from ultrathin cross-sectional slices of the Nafion films on an Hitachi H-800 transmission electron microscope to ensure a clear view of the films. Briefly, the Nafion film was placed in an epoxy resin for hardening, followed by cutting on an ultramicrotome with a glass knife. TEM images taken from such a sliced sample reveal to a cross-sectional view of the Nafion membrane. X-ray photoelectron spectroscopy (XPS) measurements were performed using an ESCALab 220I-XL photoelectron spectrometer with Al KR radiation at 1486.6 eV. The optical-limiting measurements were performed with linearly polarized 8-ns pulses at 532 nm generated from a Q-switched frequency-doubled Nd:YAG laser. The spatial profiles of the optical pulses were of nearly Gaussian transverse mode. The pulsed laser was focused onto the sample with a focusing mirror of 30 cm focal length. The input and output pulse energy was measured simultaneously by Rjp-735 energy probes, while the incident pulse energy was attenuated by a Newport Com. Attenuator.

Results and Discussion The incorporation of colored TDC60 into Nafion membrane was accompanied by a color change of the membrane from colorless to light brown upon soaking the polymer film in a pale yellow, saturated methanol solution of TDC60 at room temperature for 2 h. Further soaking up to 2 days led to a homogeneously colored dark brown film. The observed deep dark color for the TDC60-incorporated Nafion film indicates the large amount of TDC60 in the Nafion membrane. The driving force for the effective introduction of TDC60 in the Nafion membrane arises, most probably, from the hydrophilic-hydrophilic interaction between those polar groups surrounding the hollow

Langmuir, Vol. 18, No. 23, 2002 9019 Scheme 2

Figure 2. Absorption spectra of TDC60 aggregation in Nafion membrane (s) and a saturated solution of TDC60 in methanol (- - -).

cavities in the Nafion membrane (Figure 1)26,27 and the hydrophilic tri(ethylene glycol) monoethyl ether chains of TDC60. The above expectation is consistent with the observation made for pristine C60 and DTC60 of hydrophobic peripheral decyl chains (Scheme 2),19 both of them shown no incorporation with the Nafion membrane under the same conditions. The above observation, together with the subtle difference in the side-chain structure between TDC60 and DTC60, prompted us to suggest that the hydrophilic peripheral tri(ethylene glycol) monoethyl ether chains of TDC60 play an important role in facilitating the inward-diffusion of TDC60 molecules into the hollow cores of the Nafion membrane through their interaction with the hydrophilic groups in a Nafion membrane. As more and more TDC60 molecules accumulate within the confined space of the Nafion membrane, the hydrophilic-hydrophilic interaction could be counterbalanced by the hydrophobic-hydrophobic interaction between C60 skeletons of the TDC60 molecules. It is this rather delicate force balance between the hydrophilic and hydrophobic interactions that leads to the aggregate formation of TDC60 molecules within the Nafion membrane. The aggregation of TDC60 is further evidenced by the optical absorption spectra shown in Figure 2. As can be seen, TDC60 in methanol shows typical absorption features characteristic of a monomethanofullerene derivative with a maximum absorption peak at ∼470 nm and the very weak red onset absorption at ∼700 nm,40 whereas the Nafion membrane soaked with TDC60 gives a featureless spectrum in the wavelength range covered by this study due to aggregation of TDC60 molecules and/or an effect of more inhomogeneous environmental interactions within the Nafion membrane. More direct evidence for the presence of TDC60 aggregates in the Nafion membrane comes from transmission electron microscopic (TEM) images. The TDC60 aggregates are clearly evident in Figure 3 as the dark spots dispersed (40) Sun, Y.-P.; Guduru, R.; Lawson, G. E.; Mullins, J. E.; Guo, Z.; Quinlan, J.; Bunker, C. E.; Gord, J. R. J. Phys. Chem. B 2000, 104, 4625.

9020

Langmuir, Vol. 18, No. 23, 2002

Figure 3. TEM image of TDC60-incorporated Nafion membrane.

Figure 4. XPS C 1s spectrum of TDC60-incorporated Nafion membrane.

in the more bright networklike structures associated with the perfluorinated polymer chains of the Nafion membrane.35 A statistical analysis of more than 80 dark spots yields an average spot size of 5.0 nm with a size distribution standard deviation of 0.8 nm. This size is slightly bigger than that of the pore size (which is 4 nm) of the Nafion film, probably due to a possible aggregation-induced expansion of the membrane pores.35 Therefore, the size of the fullerene aggregates formed within the Nafion membrane appears to be controlled by the size of the reverse micellelike ion clusters in the Nafion membrane. The chemical nature of the TDC60-incorporated Nafion membrane was investigated by X-ray photoelectron spectroscopic measurements. While the XPS survey spectrum (0-1200 eV) of the Nafion film before and after the TDC60 incorporation shows no new peak, the increased atomic ratios of C/F and O/F from 1.130 to 1.316 and 0.264 to 0.349, respectively, by soaking the Nafion membrane in the TDC60 solution in methanol indicate the occurrence of TDC60 incorporation into the Nafion film. More detailed information on the Nafion-supported TDC60 can be obtained from the C 1s spectrum shown in Figure 4. Proper curve-fitting of the C 1s spectrum reveals the presence of F-C-O (292.84 eV) and -C-F (291.41 eV) groups characteristic of the Nafion membrane. The peaks at 286.34 and 288.73 eV can be attributed to -C-O and -CdO groups, respectively, associated with the tri(ethylene glycol) monoethyl ether moieties in TDC60. While the blank Nafion film shows a very weak peak at 284.59 eV due to possible carbon or hydrocarbon contamination, its peak intensity increases significantly upon the TDC60 incorporation, arising from TDC60. It was anticipated that aggregation could lead to fluorescence quenching via intermolecular energy migration or excimer formation between adjacent molecules in

Guo et al.

Figure 5. Emission spectra of TDC60-incorporated Nafion membrane (s) and TDC60 in a toluene solution (- - -).

the aggregate.41 Possible effects of aggregation on fluorescence properties of TDC60 molecules were investigated by carrying out the fluorescence spectroscopic measurements for TDC60-incorporated Nafion membranes using a toluene solution of TDC60 as reference. As seen in Figure 5, the TDC60 solution shows a maximum emission at ∼710 nm with a shoulder peak over ∼780 nm, resembling those of other methanofullerenes reported earlier.40 In contrast, the corresponding maximum emission for aggregated TDC60 in Nafion film was found to red-shift by about 40 nm to ∼750 nm and the associated shoulder band shifted to ∼ 810 nm. In view of similar red-shifts reported for well-studied pure C60 or C70 aggregates41 and C60 single crystals,42-44 we attributed the newly observed changes in the emission spectrum of TDC60 aggregates to the excimer formation via an aggregation-enhanced coupling between the adjacent excited TDC60 molecules within the confined geometry of the Nafion membrane, though it could also be associated with effects of the localized environment (e.g. local field effects, Stokes shift). The larger red-shift in excimer emission for TDC60 aggregates (∼750 nm) than that of pristine fullerene aggregates (∼725 nm)41 indicates a weaker dipolar-dipolar interaction in the Nafionsupported TDC60 aggregates than that of the pristine C60 aggregates due, most probably, to the presence of the grafted chains in TDC60 molecules and/or their hydrophilic interaction with the Nafion membrane. Solutions of C60 and its derivatives have been reported to possess excellent optical-limiting properties.45,46 Since many practical applications require solid-film-type optical limiters, C60 and its derivatives blended in sol-gel glasses,47-51 polymethyl methacrylate matrixes,52,53 and (41) Rudalevige, T.; Francis, A. H.; Zand, R. J. Phys. Chem. A 1998, 102, 9797. (42) Pippenger, P. M.; Averit, R. D.; Papanyan, V. O.; Nordlander, P.; Halas, N. J. J. Phys. Chem. 1996, 100, 2854. (43) Ahn, J. S.; Suzuki, K.; Iwasa, Y.; Otsuka, N.; Mitani, T. J. Lumin. 1998, 76&77, 201. (44) van den Henvel, D. J.; Chan, I. Y.; Groenen, E. J. J.; Matsushita, M.; Schmidt, J.; Meijer, G. Chem. Phys. Lett. 1995, 233, 284. (45) Tutt, L. W.; Krost, A. Nature 1992, 356, 225. (46) Sun, Y.-P.; Riggs, J. E. Int. Rev. Phys. Chem. 1999, 18, 43. (47) Schell, J.; Brinkmann, D.; Ohlmann, D.; Ho¨nerlage, B.; Le´vy, R.; Joucla, M.; Rehspringer, J.-L.; Serughetti, J.; Bovier, C. J. Chem. Phys. 1998, 108, 8599. (48) Schell, J.; Ohlmann, D.; Brinkmann, D.; Le´vy, R.; Joucla, M.; Rehspringer, J.-L.; Ho¨nerlage, B.; J. Chem. Phys. 1999, 111, 5929. (49) Bentivegna, F.; Canva, M.; Georges, P.; Brun, A.; Chaput, F.; Malier, L.; Boilot, J.-P. Appl. Phys. Lett. 1993, 62, 1721. (50) Felder, D.; Guillon, D.; Le´vy, R.; Mathis, A.; Nicoud, J.-F.; Nierengarten, J.-F.; Rehspringer, J.-L.; Schell, J. J. Mater. Chem. 2000, 10, 887. (51) Signorini, R.; Meneghetti, M.; Bozio, R.; Maggini, M.; Scorrano, G.; Prato, M.; Brusatin, G.; Innocenzi, P.; Guglielmi, M. Carbon, 2000, 38, 1653. (52) Riggs, J. E.; Sun, Y.-P. J. Chem. Phys. 2000, 112, 4221.

Nanoscale Aggregation of Fullerene

Figure 6. Optical-limiting response to 8 ns, 532 nm optical pulses of Nafion film incorporated with TDC60.

glass-polymer composites54 have been investigated. However, the relatively low solubilities of C60 and its derivatives limited the amount of C60 entities to be incorporated into the polymer matrixes without phase separation, and hence the optical-limiting capability. The well-defined porous structure and the ease with which TDC60 can be introduced, coupled with its excellent processibility, chemical, thermal, optical, and mechanical properties, make Nafion membrane an excellent matrix for constructing solid optical-limiting devices. Therefore, the optical-limiting property of TDC60-trapped Nafion film was investigated with 8 ns laser pulses from a Q-switched frequencydoubled Nd:YAG laser at 532 nm with an optical path length of 117 µm (the same as the thickness of the Nafion film). The linear transmittance is T ) 20%. As shown in Figure 6, the fluence transmitted is ∼20 mJ/cm2 when the intensity of the laser pulses increases to ∼200 mJ/ cm2. The optical limiting threshold, which is defined as the input light fluence at which the output light fluence is 50% of what is predicted by the linear transmittance, is 0.57 mJ/cm2. These results indicate that the TDC60(53) Kost, A.; Tutt, L.; Klein, M. B.; Dougherty, T. K.; Elias, W. E. Opt. Lett. 1993, 18, 334. (54) Gvishi, G.; Bhawalkar, J. D.; Kumar, N. D.; Ruland, G.; Narang, U.; Prasad, P. N.; Reinhardt, B. A. Chem. Mater. 1995, 7, 2199.

Langmuir, Vol. 18, No. 23, 2002 9021

trapped Nafion film possess strong optical-limiting capability at moderate input light fluences, implying potential applications for the Nafion-TDC60 nanocomposite as optical limiters. In summary, we have successfully incorporated certain fullerene derivatives into Nafion membranes by chemical modification of fullerene molecules with tailor-made structures and hence specific interactions with the Nafion film. It was demonstrated that fullerene derivatives with hydrophilic peripheral groups (e.g. TDC60) could be introduced into the hydrophilic hollow cores of the Nafion membrane, whereas no fullerene incorporation was observed for pure C60 with or without hydrophobic peripheral groups (e.g. DTC60). While the hydrophilic-hydrophilic interaction between the guest molecules and host cores was shown to be responsible for the successful incorporation of TDC60 into the Nafion membrane, the counterbalance of the hydrophilic interaction with the hydrophobichydrophobic interaction among the hosted TDC60 molecules is the driving force for the formation of TDC60 aggregates within the confined space of the Nafion membrane. The aggregated structure of TDC60 facilitates the excimer formation, as evidenced by a red-shift in fluorescence emission. The resulting Nafion-TDC60 nanocomposites exhibit significant optical-limiting effects of practical importance. The use of Nafion membrane as a template for assembling fullerene derivatives into nanostructured aggregates of interesting optoelectronic properties could open up avenues not only for constructing various novel fullerene nanoentities for optoelectronic devices but also for developing new functional polymeric membranes useful in many practical applications, for example, as a protective layer for preventing damage to optical devices by brightlight sources (e.g. laser beams). Acknowledgment. We thank Prof. Yinglin Song of Harbin Institute of Technology for the optical-limiting measurements. This work was supported by the Chinese Academy of Sciences and the Major State Basic Research Development Program (Grand No. G2000077500). LA0259454