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Magnetic Vesicles of Amphiphilic Spiropyran Containing Iron Oxide Particles on a Solid State Substrate Minori Taguchi,† Guangming Li,†,§ Zhongze Gu,‡ Osamu Sato,‡ and Yasuaki Einaga*,† Department of Chemistry, Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi, Yokohama 223-8522, Japan, Kanagawa Academy of Science and Technology, KSP, 3-2-1 Sakado, Kawasaki 213-0012, Japan, and Faculty of Chemistry and Chemical Engineering, Heilongjiang University, Harbin 150080, P. R. China Received June 8, 2003. Revised Manuscript Received September 29, 2003
Incorporation of photochromic compounds into magnetic systems is one of the new strategies being implemented to realize the control of magnetic properties by photoillumination. We have designed composite materials comprising iron oxide particles and photochromic spiropyran vesicles as cast films on substrates. Photofunctional magnetic vesicles of amphiphilic spiropyran (SP1822) containing iron oxide particles exhibit superparamagnetic properties at room temperature. Both photoisomerization and photoinduced aggregation of the photoresponsive magnetic vesicles were observed, even without any supporting media such as solvents or polymer matrixes. As a result of the assembly of the vesicles, magnetic dipolar interactions among adjacent iron oxide particles were induced by photoillumination at room temperature, even in the solid state.
Introduction The design of photocontrollable materials has been an attractive topic in recent years.1-11 In particular, there has been great interest in studies to develop novel compounds whose magnetic properties can be controlled by photoillumination, especially at room temperature.4-11 These compounds are important in the development of electronic devices such as molecular switches. On the other hand, research into artificial membrane-forming lipids, such as liposomes, has also attracted much attention. There have been many opportunities for the development of new functional materials with superior physicochemical properties by using synthetic bilayer membranes and by providing ordered molecular architectures.5,9-15 Herein, we have focused on the * To whom correspondence should be addressed. Phone: 81-45-5661704. Fax: 81-45-566-1697. E-mail:
[email protected]. † Keio University. ‡ Kanagawa Academy of Science and Technology. § Heilongjiang University. (1) Kimura, K.; Sakamoto, H.; Kado, S.; Arakawa, R.; Yokoyama, M. Analyst 2000, 125, 1091. (2) Bahr, J. L.; Kodis, G.; de la Garza, L.; Lin, S.; Moore, A. L.; Moore, T. A.; Gust, D. J. Am. Chem. Soc. 2001, 123, 7124. (3) Bobrovsky, A. Y.; Boiko, N. I.; Shibaev, V. P. Adv. Mater. 1999, 11, 1025. (4) Pejakovic´, D. A.; Manson, J. L.; Miller, J. S.; Epstein, A. J. Phys. Rev. Lett. 2000, 85, 1994. (5) Einaga, Y.; Sato, O.; Iyoda, T.; Fujishima, A.; Hashimoto, K. J. Am. Chem. Soc. 1999, 121, 3745. (6) Sato, O.; Iyoda, T.; Fujishima, A.; Hashimoto, K. Science 1996, 272, 704. (7) Nakatani, K.; Yu, P. Adv. Mater. 2001, 13, 1411. (8) Benard, S.; Leaustic, A.; Riviere, E.; Yu, P.; Clement, R. Chem. Mater. 2001, 13, 3709. (9) Einaga, Y.; Gu, Z.-Z.; Hayami, S.; Fujishima, A.; Sato, O. Thin Solid Films 2000, 374, 109. (10) Einaga, Y.; Yamamoto, T.; Sugai, T.; Sato, O. Chem. Mater. 2002, 14, 4846. (11) Einaga, Y.; Taguchi, M.; Li, G.; Akitsu, T.; Gu, Z.-Z.; Sugai, T.; Sato, O. Chem. Mater. 2003, 15, 8.
intercalation of inorganic magnetic materials into organized photoresponsive organic assemblies.5, 9-11 Recently, to realize photocontrol of magnetic properties at room temperature, we have focused on the combination between iron oxide particles as magnetic materials at room temperature and the photoinduced aggregation of amphiphilic spiropyran (SP1822) vesicles.11 As a result, we have succeeded in increasing the magnetization values of magnetic vesicles of spiropyran in poly(vinyl alcohol) (PVA) films. A polymer matrix was employed to engender photochromism (photoisomerization and accompanying thermal aggregation of chromophores) for spiropyran, even in the solid state, because the photoisomerization was accompanied by a large volume change. However, the system containing the polymer was relatively complicated because the properties of the polymer itself, such as phase-transition, turbidity, and stability, need to be taken into account. In general, spiropyran with certain structural features undergoes reversible changes between colorless and colored forms with the application of either heat or light.16,17 The colorless “closed” form has the typical structure of a spiropyran (SP), and the colored “open” form has the structural features of a photomerocyanine (PMC) dye. The equilibrium is dependent upon both the thermal and the light exposure conditions. Furthermore, (12) Fendler, J. H. Membrane Mimetic Chemistry; Wiley: New York, 1982. (13) Kunitake, T. Angew. Chem., Int. Ed. Engl. 1992, 31, 709. (14) Ichinose, I.; Kimizuka, N.; Kunitake, T. J. Phys. Chem. 1995, 99, 3736. (15) Archibald, D. D.; Mann, S. Nature 1993, 364, 430. (16) Brown, G. H. Photochromism; Wiley: New York, 1971. (17) Tork, A.; Boudreault, F.; Roberge, M.; Ritcey, A. M.; Lessard, R. A.; Galstian, T. V. Applied Optics 2001, 40, 1180.
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it is known that amphiphilic PMC compounds that have two long alkyl chains undergo J-aggregation (J-PMC)18-22 into ammonium-type bilayer membranes21 and Langmuir-Blodgett films,19,20 etc., by UV light illumination.
Figure 1. Absorption spectra of a dispersed aqueous solution of SP1822 (0.1 mM) on illumination with UV light. The spectra were recorded during illumination from t ) 0 min to t ) 60 min at room temperature.
In the present work, a novel system comprising iron oxide particles and SP1822 vesicles without any media such as liquid solvents or polymer matrixes has been designed and fabricated. The new type of films could be formed very easily by casting onto a substrate without any polymer and they showed photo effects similar to those of the previous polymer system, i.e., a photoinduced magnetization increase.11 Experimental Section Preparation of Composite Materials. Photochromic spiropyran, 1′,3′-dihydro-3′,3′-dimethyl-6-nitro-1′-octadecyl-8docosanoyloxy-methylspiro[2H]-1-benzopyran-2,2′-[2H]indol (SP1822) was purchased from Hayashibara Biochem Lab, Inc. (Japan). The SP1822 was used without further purification, and was suspended in deionized water. An ultrasonicator (model VCX-750, Sonic and Materials) was used for 0.5 h at a power of 200 W to produce a dispersed solution of SP1822 vesicles. Commercially available magnetic fluids were used as the source of the iron oxide particles. The diameters of the particles were almost homogeneous (10 nm) due to the use of a coating surfactant, which made the surface of each iron oxide particle hydrophilic. The composite materials were obtained by mixing the dispersed solution of SP1822 (0.2 mL, 2 × 10-7 mol) and the solution of magnetic fluids (0.1 mL, 1.16 × 10-11 mol) in aqueous media. Hereafter the composite material is designated as 1. Furthermore, films of 1 were prepared by casting the above solution onto substrates. (18) Bohn, P. W. Annu. Rev. Phys. Chem. 1993, 44, 37. (19) Tachibana, H.; Yamanaka, Y.; Matsumoto, M. J. Mater. Chem. 2002, 12, 938. (20) Tachibana, H.; Yamanaka, Y.; Sakai, H.; Abe, M.; Matsumoto, M. J. Lumin. 2000, 87, 800. (21) Seki, T.; Ichimura, K.; Ando, E. Langmuir 1988, 4, 1068. (22) Uznanski, P. Synth. Met. 2000, 109, 281.
Physical Methods. UV-Visible absorption spectra were recorded on a V-560 spectrophotometer (JASCO). An environmental scanning electron microscope (E-SEM, model XL30, NIKON Intec.) was used to image the SP1822 dispersed solution and the composite materials. The local chemical compositions of the composite materials were obtained using an energy-dispersive X-ray spectrometer (EDX). To enhance their visibility, the amphiphiles were investigated with a 0.5% aqueous solution of RuO4. The magnetic properties were investigated with a superconducting quantum interference device magnetometer (SQUID, model MPMS-5S, Quantum Design). UV illumination (filtered light, λmax ) 360 nm, 10 mW/ cm2) was carried out using a xenon light source (model XFL300, Yamashita Denso).
Results and Discussion Photoisomerization (SP f PMC) of a Dispersed Solution of SP1822 at Room Temperature. Figure 1 shows the UV-visible absorption spectra for a dispersed aqueous solution of SP1822 vesicles at room temperature. It was almost transparent before illumination with UV light (absorption bands for SP1822 are observed at ca. 240, 270, and 340 nm (SP state)).20 These absorption bands can be interpreted as originating from the indoline ring and the chromene ring on spiropyran. In the initial regime using UV light illumination, Cspiro-O bond cleavage takes place, leading to the formation of the open zwitterrionic colored form (the photo merocyanine isomer (PMC state)), which has strong absorption in the visible region. The open form of SP has a fully delocalized π-electron system, and a red shift in the absorption spectrum accompanies the ring opening. Two absorption bands appear at ca. 380 and 580 nm on UV light illumination. With prolonged illumination, only the intensities of the two bands gradually increase, without any noticeable band-shift-
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Figure 2. Absorption spectra of a cast of the SP1822 cast on a quartz plate on illumination with UV light. The spectra were recorded during the illumination from t ) 0 min to t ) 180 min at room temperature (a) before illumination, and after UV illumination for (b) 1 min, (c) 10 min, (d) 30 min, (e) 60 min, and (f) 180 min.
ing. This explains the decrease in intensity of the absorption bands at ca. 240 and 270 nm and the concomitant increases in the intensities of the bands at ca. 340, 380, and 580 nm. These absorption bands were assigned on π-π*, n-π*, and intramolecular chargetransfer absorption.16 The color of the SP1822 dispersed solution was changed to purple by the UV light illumination. The results indicate the typical photoisomerization of spiropyran to the open-colored PMC in the monomeric form. Moreover, the open form (PMC state) reverted to the closed colorless form (SP state) on visible light illumination, and the change was reversible. Photoinduced Aggregation of Cast Films of SP1822. The dispersed solution of SP1822 was cast onto a quartz plate at room temperature and the photochromic reaction of the film was investigated (Figure 2). Before UV light illumination, the cast film of the dispersed solution of SP1822 was almost transparent (SP state, Figure 2a). In the initial stages of UV light illumination (after 1 min) the main photoreaction was the isomerization of SP1822 to the PMC isomer (Figure 2b). After prolonged UV light illumination for 10 min it exhibited a sharp and intense band at a longer wavelength, ca. 620 nm, which was assigned to the Jaggregate (Figure 2). That is, we could observe the photoinduced aggregation of SP1822 just by casting onto a substrate, whereas we could not observe it in the dispersed solution. Figure 3 shows SEM images of a cast film of SP1822 on a silicon substrate before and after illumination with UV light. The SEM images before UV light illumination indicate the presence of spherical domains of heterogeneous diameter (diameter 500-1000 nm) (Figure 3A). An estimation of the size of the spherical domains indicates that the vesicles consisted of multi-bilayers. Interestingly, several assembled vesicles of SP1822 were observed after UV illumination (Figure 3B), whereas they could not be observed before illumination. We consider that the UV light illumination induced the PMC isomer, leading to J-aggregate formation, thereby resulting in the assembly of vesicles by migration. As we have reported previously, spherical vesicles of SP1822 can aggregate to form a cylindrical structure by UV light illumination in PVA (poly(vinyl alcohol)) matrix films.
Figure 3. SEM images of SP1822 vesicles cast onto a silicon wafer, (A) before illumination, and (B) after 60 min UV illumination.
Figure 4. Absorption spectra of 1 on illumination with UV light. The spectra were recorded during the illumination from t ) 0 min to t ) 60 min at room temperature.
That example implies that molecules can migrate relatively freely in the polymer films. In the present case, however, it was reasonable to assume that the migration of molecules in the cast film on the solid substrate was restricted to some extent. Composite Material 1. Figure 4 shows the UVvisible absorption spectra of 1 in a deionized water solution at room temperature. The results are consistent with the results in Figure 1. The photochromic reaction of the cast film of 1 on a quartz plate at room temperature was investigated by UV-visible spectra. The spectral changes are shown in Figure 5. These results are also similar to the cast film of the dispersed solution of SP1822 in Figure 2. Figure 6 shows SEM images of a cast film of 1 on a silicon wafer before and after illumination with UV light. The SEM images before illumination with UV
Photofunctional Magnetic Vesicles of Amphiphilic Spiropyran
Figure 5. Absorption spectra of 1 cast onto a quartz plate on illumination with UV light. The spectra were recorded during the illumination from t ) 0 min to t ) 180 min. at room temperature (a) before illumination, after UV illumination, (b) 1 min, (c) 10 min, (d) 30 min, (e) 60 min, (f) 120 min, (g) 180 min.
Figure 6. SEM images of 1 cast onto a silicon wafer (A) before illumination and (B) after 60 min UV illumination. The inset in (A) shows EDX analysis along the line indicated in part (A), showing the relative intensity for Fe along the line (arrow indicates increasing intensity).
light show spherical domains of heterogeneous diameters (diameter 500-1000 nm) (Figure 6A). The relative amount of Fe (iron oxide) in the SP1822 vesicles was analyzed using EDX along the line indicated in Figure 6A. This analysis suggests that the intercalated iron oxide exists inside the SP1822 vesicles (inset of Figure 6A), not outside them. The formation mechanism of SP1822 vesicles containing iron oxide can be explained by geometric, electrostatic, and dipole-dipole interactions within the SP1822 vesicles, i.e., the affinity of the headgroup of the amphiphilic spiropyran (SP1822) to the iron oxide surface (hydrophilic groups of SP1822 and
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Figure 7. Magnetization vs applied magnetic field for 1 cast onto a glass substrate at 300 K.
the hydrophilic surface of iron oxide particles). The SEM images changed markedly after the isomerization using UV light illumination. Several assembled SP1822 vesicles were observed after UV illumination (Figure 6B), whereas they could not be observed before illumination. We consider that the assembled SP1822 resulting from the UV light illumination induced the PMC isomer, leading to J-aggregate formation. Increase in Magnetization of Magnetic Vesicles by UV Light Illumination. The magnetic properties of the cast film of 1 on the glass substrate were analyzed by SQUID. The hysteresis loop, measured at room temperature, is shown in Figure 7. The cast of 1 exhibited superparamagnetic behavior as indicated by the zero coercivity and remanence on the magnetization curve.23-28 The influence of UV light illumination on a cast of 1 at room temperature with an external magnetic field of 10 G is shown in Figure 8. After UV light illumination for 180 min the magnetization value increased from 2.53 to 2.58 × 10-1 emu/g. Even after the illumination was stopped, this increased magnetization was maintained for at least 1 h. The photoinduced magnetization increase could not be observed in the case of only SP1822, only iron oxide particles, and only glass substrate. That is, the combination between SP1822 and iron oxide particles plays an important role for the photoinduced magnetization increase. Figure 9 shows the zero-field-cooled magnetization (ZFC), which plots the measured susceptibility versus the temperature before and after UV light illumination. ZFC was measured to confirm the interparticle interaction effects in the cast of 1. ZFC, measured in a 5 G field, was plotted versus temperature from 5 to 300 K, before and after UV light illumination. The curves exhibited features typical of an assembly of magnetic nanoparticles presenting a volume (V) distribution, implying a distribu(23) El-Hilo, M.; O’Grady, K.; Chantrell, R. W. J. Magn. Magn. Mater. 1992, 114, 295. (24) Kim, D. K.; Zhang, Y.; Voit, W.; Rao, K. V.; Muhammed, M. J. Magn. Magn. Mater. 2001, 225, 30. (25) Johansson, C.; Hanson, M.; Pedersen, M. S.; Mørup, S. J. Magn. Magn. Mater. 1997, 173, 5. (26) Bui, Q. T.; Pankhust, Q. A.; Zulqarnain, K. IEEE Trans. Magn. 1998, 34, 2117. (27) Testa, A. M.; Foglia, S.; Suber, L.; Fiorani, D.; Casas, LI.; Roig, A.; Molins, E.; Grene`che, J. M.; Tejada, J. J. Appl. Phys. 2001, 90, 1534. (28) Dormann, J. L.; Fiorani, D.; Tronc, E. Adv. Chem. Phys. 1997, 98, 283.
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Figure 8. Magnetization for 1 cast onto a glass substrate induced by UV light illumination at 300 K with an external magnetic field of 10 G. The magnetizations were recorded during the illumination from t ) 0 min to t ) 180 min.
Figure 9. Normalized zero-field-cooled magnetization vs temperature for 1 cast onto a glass substrate (O) before illumination and (b) after 180 min UV light illumination.
tion of anisotropy energy barriers EB (EB ) KaV for uniaxial anisotropy) and consequently of relaxation times τ. For an assembly of noninteracting particles with a certain size distribution, the temperature of the ZFC susceptibility maximum Tm is found to increase with particle size, and, in general, it is related to the average blocking temperature (TB) according to relationships that depend on the volume distribution func-
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tion.27,28 Here, for the magnetic vesicles before and after UV light illumination, the ZFC curve showed a blocking temperature (TB) at 215 K and 220 K, respectively. Such curves are typical of the blocking transition in superparamagnetic particles. Minor differences were apparent before and after UV light illumination of the magnetic vesicles, with the latter having the higher TB. Given that strong interparticle interaction effects, such as assembly, are known to increase TB, we inferred that the particle size of the iron oxide after UV light illumination was larger than that before illumination because of some assembly among adjacent iron oxide particles. The assembly was induced by the PMC isomer, leading to the J-aggregate form of SP1822. Although the magnetization of 1 was increased by UV light illumination, it could not be decreased by any stimuli such as visible light illumination and thermal treatment. That is, the photoinduced-change in magnetization of 1 was not reversible. In general, spiropyran compounds undergo reversible changes between SP and PMC form by photoillumination or thermal treatment. Actually, reverse reaction (i.e., PMC to SP) of SP1822 in 1 by visible light illumination was observed by UVvisible absorption spectra. However, as for the magnetic properties of 1, magnetization value did not decrease even after visible light illumination. This is because the iron oxide particles assembled due to the aggregation of SP1822 never separate by any stimuli. Therefore, studies are now in progress to design an appropriate reversible photocontrollable system. In summary, we have shown new strategies for designing photocontrollable magnetic vesicles by applying the technique of incorporating organic photochromic compounds into magnetic systems. The magnetization value of the composite magnetic vesicles was increased by UV light illumination, even in the solid state at room temperature. Acknowledgment. This work was supported by a Grant-in-Aid for Scientific Research on Priority Areas (417) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of the Japanese Government. CM0344515
