Temperature-Controlled Photooxygenation with Polymer

Jul 26, 2008 - Yasuhiro Shiraishi*, Yumi Kimata, Hisao Koizumi and Takayuki Hirai. Research Center for Solar Energy Chemistry, and Division of Chemica...
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Langmuir 2008, 24, 9832-9836

Temperature-Controlled Photooxygenation with Polymer Nanocapsules Encapsulating an Organic Photosensitizer Yasuhiro Shiraishi,* Yumi Kimata, Hisao Koizumi, and Takayuki Hirai Research Center for Solar Energy Chemistry, and DiVision of Chemical Engineering, Graduate School of Engineering Science, Osaka UniVersity, Toyonaka 560-8531, Japan ReceiVed May 17, 2008. ReVised Manuscript ReceiVed June 16, 2008 A cross-linked poly-N-isopropylacrylamide (polyNIPAM) nanocapsule, TH@PC, containing thionine (TH), an organic photosensitizer, has been synthesized. This capsulated polymeric photosensitizer promotes a singlet oxygen oxygenation (1O2) accurately controlled by temperature: it shows high oxygenation activity at low temperature, but shows activity decrease with a rise in temperature, resulting in almost zero activity at >40 °C. The clear on-off activity control is driven by a heat-induced structure change of the capsule from the swollen single capsule to contracted state, and then to aggregate, behaving as an intelligent 1O2 filter. At low temperature, the capsule exists as the swollen single capsule, which allows 1O2 diffusion to bulk water, resulting in high oxygenation activity. A rise in temperature leads to contraction of the capsule, reducing the mesh size of the capsule wall. This suppresses 1O2 diffusion to bulk water and shows decreased activity. Intercapsule aggregation at >30 °C further suppresses 1O2 diffusion and shows almost no activity. The capsule promotes reversible activity control regardless of the heating/cooling process and can be reused with a simple recovery process.

1. Introduction Fabrication of uniform and well-defined organic or inorganic nanocapsules has attracted a great deal of attention.1 Among them, polymer nanocapsules2 have been studied extensively for application to various kinds of materials such as drug delivery media,3 heterogeneous catalysts,4 and containers for guest molecules.5 Recently, intelligent polymer nanocapsules that show stimuli-responsive reversible swelling/contraction have been proposed. In these, the mesh size of the capsule wall is controlled by external stimuli such as pH,6 ions,7 and salts8 and, hence, allows controlled permeation of the guest molecules. Temperature can also be used as the stimulus. This is based on the temperaturedependent hydration/dehydration behavior of poly-N-isopropylacrylamide (polyNIPAM) in water.9 PolyNIPAM nanocapsules that are usually synthesized with a cross-linker demonstrate clear * To whom correspondence should be addressed. E-mail: shiraish@ cheng.es.osaka-u.ac.jp. Fax: +81-6-6850-6273. Tel: +81-6-6850-6271. (1) (a) Caruso, F. Chem.;Eur. J. 2000, 6, 413–419. (b) Trewyn, B. G.; Giri, S.; Slowing, I. I.; Lin, V. S.-Y. Chem. Commun. 2007, 3236–3245. (c) Tazaki, K. Clays Clay Miner. 1997, 45, 203–212. (d) Yoon, S. B.; Sohn, K.; Kim, J. Y.; Shin, C.-H.; Yu, J.-S.; Hyeon, T. AdV. Mater. 2002, 14, 19–21. (2) (a) Meier, W. Chem. Soc. ReV. 2000, 29, 295–303. (b) Chen, D.; Jiang, M. Acc. Chem. Res. 2005, 38, 494–502. (c) Sukhorukov, G.; Fery, A.; Mo¨hwald, H. Prog. Polym. Sci. 2005, 30, 885–897. (3) (a) Zelikin, A. N.; Becker, A. L.; Johnston, A. P. R.; Wark, K. L.; Turatti, F.; Caruso, F. ACS Nano 2007, 1, 63–69. (b) Son, Y.-H.; Park, M.; Choy, Y. B.; Choi, H. R.; Kim, D. S.; Park, K. C.; Choy, J.-H. Chem. Commun. 2007, 2799– 2801. (4) (a) Wu, H.; Liu, Z.; Wang, X.; Zhao, B.; Zhang, J.; Li, C. J. Colloid Interface Sci. 2006, 302, 142–148. (b) Price, K. E.; Mason, B. P.; Bogdan, A. R.; Broadwater, S. J.; Steinbacher, J. L.; McQuade, D. T. J. Am. Chem. Soc. 2006, 128, 10376–10377. (c) Supsakulchai, A.; Ma, G. H.; Nagai, M.; Omi, S. J. Microencapsulation 2003, 20, 19–33. (5) (a) Shchukin, D. G.; Ko¨hler, K.; Mo¨hwald, H. J. Am. Chem. Soc. 2006, 128, 4560–4561. (b) Andreeva, D. V.; Gorin, D. A.; Mo¨hwald, H.; Sukhorukov, G. B. Langmuir 2007, 23, 9031–9036. (6) (a) Zhang, Y.; Jiang, M.; Zhao, J.; Wang, Z.; Dou, H.; Chen, D. Langmuir 2005, 21, 1531–1538. (b) Sauer, M.; Streich, D.; Meier, W. AdV. Mater. 2001, 13, 1649–1651. (7) (a) Okahata, Y.; Lim, H.-J. J. Am. Chem. Soc. 1984, 106, 4696–4700. (b) Chu, L.-Y.; Yamaguchi, T.; Nakao, S.-i. AdV. Mater. 2002, 14, 386–389. (8) Ibarz, G.; Da¨hne, L.; Donath, E.; Mo¨hwald, H. AdV. Mater. 2001, 13, 1324–1327. (9) (a) Cheng, H.; Shen, L.; Wu, C. Macromolecules 2006, 39, 2325–2329. (b) Rusu, M.; Wohlrab, S.; Kuckling, D.; Mo¨hwald, H.; Scho¨nhoff, M. Macromolecules 2006, 39, 7358–7363.

temperature-dependent swelling/contraction:10 at low temperature, the capsule exists as a swollen state, but a rise in temperature leads to dehydration of the polymer chain, resulting in the formation of a contracted capsule. Recently, the design of polymers containing photosensitizing units has attracted much attention,11 because the polymeric architecture provides functional microenvironment to alter the photochemical behavior affecting photoreaction activity and selectivity. Earlier, we reported a linear polyNIPAM copolymerized with an organic photosensitizer that behaves as the first sensitizer showing temperature-driven photosensitization activity.12 In that, the photoexcited sensitizer transfers the energy to molecular oxygen (O2) and produces singlet oxygen (1O2), an oxidizing agent. During reaction with substrate at low temperature, the polymer exists as an extended structure and allows 1O2 diffusion, resulting in high oxygenation activity. A rise in temperature leads to aggregation of the polymer, and the sensitizer units are confined within the aggregate. This suppresses diffusion of 1O2 formed inside the polymer to bulk solution and shows decreased oxygenation activity. This is the first photoreaction system enabling reversible activity control without contaminating the reaction mixture. However, in this linear polymer system, the activity response against temperature is insufficient. The sensitizer units are randomly arranged along the polymer chain; therefore, even at high temperature, parts of the sensitizer units are exposed on the surface of the aggregate. This allows 1O2 (10) (a) Zha, L.; Zhang, Y.; Yang, W.; Fu, S. AdV. Mater. 2002, 14, 1090– 1092. (b) Nayak, S.; Gan, D.; Serpe, M. J.; Lyon, L. A. Small 2005, 1, 416–421. (c) Gao, C.; Chen, B.; Mo¨hwald, H. Colloid. Surf., A 2006, 272, 203–210. (d) Gao, H.; Yang, W.; Min, K.; Zha, L.; Wang, C.; Fu, S. Polymer 2005, 46, 1087– 1093. (11) (a) Hecht, S.; Fre´chet, J. M. J. J. Am. Chem. Soc. 2001, 123, 6959–6960. (b) Jensen, A. W.; Maru, B. S.; Zhang, X.; Mohanty, D. K.; Fahlman, B. D.; Swanson, D. R.; Tomalia, D. A. Nano Lett. 2005, 5, 1171–1173. (c) Bhyrappa, P.; Young, J. K.; Moore, J. S.; Suslick, K. S. J. Am. Chem. Soc. 1996, 118, 5708–5711. (d) Oar, M. A.; Dichtel, W. R.; Serin, J. M.; Fre´chet, J. M. J.; Rogers, J. E.; Slagle, J. E.; Fleitz, P. A.; Tan, L-S.; Ohulchanskyy, T. Y.; Prasad, P. N. Chem. Mater. 2006, 18, 3682–3692. (e) Nishikubo, T.; Uchida, J.; Matsui, K.; Iizawa, T. Macromolecules 1988, 21, 1583–1589. (f) Nishimura, I.; Kameyama, A.; Nishikubo, T. Macromolecules 1998, 31, 2789–2796. (g) Shiraishi, Y.; Koizumi, H.; Hirai, T. J. Phys. Chem. B 2005, 109, 8580–8586. (12) Koizumi, H.; Shiraishi, Y.; Tojo, S.; Fujitsuka, M.; Majima, T.; Hirai, T. J. Am. Chem. Soc. 2006, 128, 8751–8753.

10.1021/la8015194 CCC: $40.75  2008 American Chemical Society Published on Web 07/26/2008

Polymer Nanocapsules Encapsulating Photosensitizer

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Scheme 1. (a) Synthesis of TH@PC, (b) Structure of a Noncapsulated Random Polymer, Poly(NIPAM-co-TH-co-MBA), and (c) Schematic Representation of Temperature-Dependent Change in Structure of TH@PC and 1O2 Diffusion

diffusion to bulk solution and promotes substrate oxygenation even at high temperature. In the present work, we synthesized a cross-linked polyNIPAM nanocapsule (TH@PC) encapsulating thionine (TH), a typical photosensitizer13 (Scheme 1a). We found that this capsulated photosensitizer accurately controls the oxygenation activity by temperature, with a heat-induced activity decrease at 10-40 °C and complete activity termination at >40 °C. This is driven by temperature-induced swelling/contraction of the capsule wall behaving as an intelligent 1O2 filter, which accurately controls 1O diffusion to bulk water. To the best of our knowledge, this 2 is the first application of polymer nanocapsule to photosensitized reactions.

2. Results and Discussion The capsulated photosensitizer, TH@PC, was synthesized in four steps, as summarized in Scheme 1a, by a silica-templated polymerization with methylene blue (MB) as a precursor (see Experimental Section). Scanning electron microscopy (SEM) images of the materials obtained in each step are shown in Figure 1. The silica template was made by hydrolysis of tetraethyl orthosilicate (TEOS) in the presence of MB (Step 1). The template (average diameter, 171 nm; Figure 1a) was treated with 3-(trimethoxysilyl)propylmethacrylate (Step 2). NIPAM and N,N′-methylenebisacrylamide (MBA), a cross-linker, were polymerized in the presence of the methacrylate-modified silica (183 nm; Figure 1b) to form silica/cross-linked polyNIPAM core-shell particles (272 nm; Figure 1c) (Step 3). The particles were then washed with hydrofluoric acid (Step 4) to form monodispersed spheres of TH@PC (292 nm; Figure 1d), where the silica core is completely removed while leaving the sensitizer. The complete removal of the silica core is confirmed by IR and thermogravimetrical (TG) analysis: IR analysis reveals that SiO2 absorption at 1100 cm-1 disappears completely (Figure S2, Supporting Information). TG analysis scarcely detects any mass remaining at >700 °C (Figure S3). In the above procedures, MB is used as a precursor, but the resulting capsule encapsulates TH. This is because MB is (13) (a) Usui, Y. Chem. Lett. 1973, 743–744. (b) Kaanumalle, L. S.; Shailaja, J.; Robbins, R. J.; Ramamurthy, V. J. Photochem. Photobiol. A: Chem. 2002, 153, 55–65. (c) van Laar, F. M. P. R.; Holsteyns, F.; Vankelecom, I. F. J.; Smeets, S.; Dehaen, W.; Jacobs, P. A. J. Photochem. Photobiol. A: Chem. 2001, 144, 141–151.

converted to TH by base-catalyzed demethylation14 during hydrolysis of TEOS (Step 1). This is confirmed by the fluorescence spectrum of TH@PC (Figure S4): it shows distinctive TH fluorescence at 550-650 nm, while MB shows red-shifted fluorescence at 650-700 nm. MB is necessary as a precursor for production of TH-encapsulated capsule. Use of TH is unsuccessful because TH is not charged strongly15 and is scarcely doped in silica. The TH amount within TH@PC is determined by mass balance to be 18.3 µmol/g (Table 1), where the encapsulated TH molecules do not leak out at any temperature and in any solvent. The width of TH molecule is estimated to be 0.57 nm by semiempirical (PM3) molecular orbital calculation (Figure S5),16 suggesting that the mesh size of the capsule wall is less than this. Sensitized oxygenation activity of TH@PC was estimated with the transformation of phenol (1) to p-benzoquinone (2), a typical 1O oxygenation reaction17 (Figure 2a). The reaction was 2 performed by photoirradiation (λ > 530 nm) of an O2-saturated aqueous solution (pH 10) containing 1 (10 µmol) and TH@PC (2.35 mg containing 0.04 µmol TH). Figure 2b shows the temperature-dependent change in turnover number (TON) for 2 formation [) (2 yield)/(TH amount)]. With bulk TH (i), activity is always high for the entire temperature range. In contrast, TH@PC (ii) shows high oxygenation activity at low temperature, but the activity decreases with a rise in temperature. The activity at >40 °C is almost zero, showing a clear “on-off” activity change against the temperature window. As shown by (iii), use of TH together with TH-free PC (see Experimental Section) is unsuccessful for activity control. As shown by (iv), a noncapsulated TH-appended polymer, poly(NIPAM-co-TH-co-MBA) (Scheme 1b; see Experimental Section), shows a heat-induced activity decrease, but the decrease at high temperature is very small. These data clearly indicate that the encapsulation of TH within the capsule is crucial for accurate oxygenation activity control. (14) (a) Schaefer, F. C.; Zimmermann, W. D. Nature 1968, 220, 66–67. (b) Hoppe, R.; Schulz-Ekloff, G.; Wo¨hrle, D.; Kirschhock, C.; Fuess, H.; Uytterhoeven, L.; Schoonheydt, R. AdV. Mater. 1995, 7, 61–64. (15) Zanjanchi, M. A.; Ebrahimian, A.; Alimohammadi, Z. Opt. Mater. 2007, 29, 794–800. (16) Shiraishi, Y.; Saito, N.; Hirai, T. J. Am. Chem. Soc. 2005, 127, 8304– 8306. (17) (a) Li, C.; Hoffman, M. Z. J. Phys. Chem. A 2000, 104, 5998–6002. (b) Koizumi, H.; Kimata, Y.; Shiraishi, Y.; Hirai, T. Chem. Commun. 2007, 1846– 1848.

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Figure 2. (a) Sensitized 1O2 oxygenation. (b) Temperature-dependent change in TON for compound 2 formation during reaction (λ > 530 nm, 3 h) in O2-saturated aqueous solution (pH 10) with respective sensitizers. The systems are: (i) TH (0.04 µmol), (ii) TH@PC (2.4 mg), (iii) TH (0.04 µmol) with TH-free PC (2.4 mg), (iv) poly(NIPAM-co-TH-coMBA) (2.4 mg), (v) capsule B (2.4 mg), (vi) capsule C (2.4 mg), and (vii) capsule D (2.4 mg).

Figure 1. SEM images of (a) sensitizer-doped silica, (b) methacrylatemodified silica, (c) core-shell silica-polymer particles, and (d) TH@PC. Photographs of the respective materials are shown in Figure S1. Table 1. Feed Composition of NIPAM and MBA (Cross-linker) and the TH Amount in the Capsules capsule

NIPAM/SiO2 (weight/weight)

MBA/NIPAM (mol/mol)

TH content (mol/g)

A TH@PC B C D

1/1 2/1 3/1 4/1 2/1

0.055 0.055 0.055 0.055 0.147

NDa 1.83 × 10-5 1.18 × 10-5 6.97 × 10-6 1.52 × 10-5

a

Not detected.

The size of substrate 1 is estimated to be 0.53 nm (Figure S5), which is similar to TH (0.57 nm). This suggests that 1 scarcely permeates through the capsule wall and oxygenation of 1 occurs in bulk water with 1O2 out of the capsule interior. As shown in Scheme 1c, the clear on-off activity control by TH@PC is driven by a heat-induced structure change of the capsule from the swollen single capsule to the contracted state, and then to aggregate,

Figure 3. Temperature-dependent change in turbidity (A400nm) of an aqueous solution (pH 10) containing TH@PC. (Inset) Change in absorption spectra.

suppressing 1O2 diffusion to bulk water. Figure 3 shows temperature-dependent change in turbidity (A400nm) of an aqueous solution containing TH@PC. The solution turbidity increases with a rise in temperature at 10-30 °C, indicating that the polymer contracts via heat-induced dehydration of the polymer chain18 (formation of contracted single capsules; Scheme 1c). This is confirmed by 1H NMR analysis of the capsule in D2O (Figure S6): integrated intensity of CH resonance for the polymer chain and the NIPAM units decreases with a rise in temperature,19 indicative of the polymer contraction. Change in the capsule size further confirms this. As shown in Figure 4a, at 20 °C, TH@PC has a narrow unimodal distribution with an average diameter 550 nm. As shown in Figure 4b (black), the capsule size actually (18) (a) Zhang, Q.-S.; Zha, L.-S.; Ma, J.-H.; Liang, B.-R. J. Appl. Polym. Sci. 2007, 103, 2962–2967. (b) Chen, X.; Ding, X.; Zheng, Z.; Peng, Y. Macromol. Rapid Commun. 2004, 25, 1575–1578. (19) (a) Cao, Z.; Liu, W.; Gao, P.; Yao, K.; Li, H.; Wang, G. Polymer 2005, 46, 5268–5277. (b) Shiraishi, Y.; Miyamoto, R.; Hirai, T. Langmuir 2008, 24, 4273–4289.

Polymer Nanocapsules Encapsulating Photosensitizer

Figure 4. Temperature-dependent change in (a) distribution of hydrodynamic radius (Rh) of TH@PC and (b) average Rh for (black) single capsules and (white) aggregated capsules measured in aqueous solution (pH 10). The data for TH-free PC are shown in Figure S7.

Figure 5. (a) ESR spectra of TEMP-1O2 spin adduct obtained by photoirradiation (at 40 °C) of an aerated aqueous solution (pH 10) containing TEMP with (i) TH and (ii) TH@PC. (b) 1O2 quantum yield (Φ1O2) for the respective systems with (white) TH and (black) TH@PC, obtained by double integration of the lowest magnetic field signal of the adduct, where Rose Bengal was used as a reference (Φ1O2 ) 0.75 at 25 °C; ref 20a).

decreases with a rise in temperature; the size becomes 365 nm (34% decrease) at 31 °C. As a result of this, the mesh size of the capsule wall becomes smaller. This suppresses 1O2 diffusion to bulk water, resulting in heat-induced oxygenation suppression (Scheme 1c). This mechanism is fully confirmed by 1O2-trapping electron spin resonance (ESR) analysis with 2,2,6,6-tetramethylpiperidine (TEMP) as a probe molecule (Figure 5a). Photoirradiation of an aqueous solution containing TH or TH@PC with TEMP gives TEMP-1O2 spin adduct signals (aN ) 17.3 G, g ) 2.0053).20 Figure 5b shows 1O2 quantum yield (Φ1O2) determined by double (20) (a) He, Y.-Y.; An, J.-Y.; Jiang, L.-J. J. Photochem. Photobiol. B: Biol. 1999, 50, 166–173. (b) Lion, Y.; Delmelle, M.; van de Vorst, A. Nature 1976, 263, 442–443. (c) Li, H.-R.; Wu, L.-Z.; Tung, C.-H. J. Am. Chem. Soc. 2000, 122, 2446–2451.

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integration of the signals.20a Φ1O2 obtained with bulk TH (white) monotonously increases with a rise in temperature, which agrees well with the oxygenation activity profile (Figure 2b, i). With TH@PC (black), Φ1O2 value decreases as the temperature rises, which is reasonably consistent with the activity profile (Figure 2b, ii) and the change in capsule size (Figure 4b, black). TEMP has a size (0.72 nm; Figure S5) much larger than the mesh size of the polymer wall; therefore, the reaction with 1O2 takes place in bulk solution, as is the case for the substrate 1. These findings clearly indicate that, in the TH@PC system, 1O2 diffusion to bulk solution is actually suppressed with the heat-induced shrinkage of the polymer wall. As shown in Figure 2b (ii), at >30 °C, the oxygenation activity of TH@PC becomes almost zero. This is due to the change in capsule structure from the contracted single capsule to aggregate (Scheme 1c), leading to complete suppression of 1O2 diffusion to bulk solution. As shown in Figure 4a, at >30 °C, large size polymer aggregates appear. The size of the aggregates increases as the temperature rises (Figure 4b, white). This indicates that strong dehydration of the polymer chain at high temperature leads to intercapsule aggregation. At this temperature range, the solution turbidity decreases obviously (Figure 3) because the solution transparency increases by the intercapsule aggregation.21 In this condition, 1O2 diffusion to bulk water is much more suppressed. As shown in Figure 5b (black), Φ1O2 obtained with TH@PC becomes almost zero at >30 °C, which agrees with the activity profile (Figure 2b, ii). The mechanism of the accurate temperature-driven oxygenation activity control by TH@PC can therefore be summarized in Scheme 1c. At low temperature, TH@PC exists as the swollen single capsule, which allows 1O2 diffusion to bulk water, resulting in high oxygenation activity. A rise in temperature leads to reduction of the mesh size of the polymer wall. This suppresses 1O diffusion, resulting in activity decrease. Intercapsule ag2 gregation at >30 °C further suppresses the 1O2 diffusion and, hence, the system shows almost no activity. As shown in Figure S8, the capsule can control the oxygenation activity reversibly regardless of the heating/cooling sequence. In addition, the capsule can be reused with a simple recovery process: heating the reaction mixture to 40 °C followed by centrifugation (5 min, 2 × 104 rpm) affords >98% polymer recovery, and the recovered polymer shows the same activity as does the virgin polymer (Figure S8). It must be noted that the amounts of NIPAM and cross-linker (MBA) in the capsule synthesis are quite important for the TH encapsulation and the oxygenation activity control. As summarized in Table 1, with low NIPAM amount (capsule A), TH is scarcely encapsulated, probably due to the incomplete capsule morphology. With high NIPAM amount (capsules B and C), oxygenation activity at low temperature decreases (Figure 2b, v and vi). This may be because the thick polymer wall strongly suppresses the 1O2 diffusion to bulk water even at low temperature. With high cross-linker (MBA) amount (capsule D), the activity decrease at high temperature is very small (Figure 2b, vii). As described,10a aggregation of polyNIPAM is suppressed with an increase in the cross-linker amount due to the decrease in polymer flexibility. This leads to insufficient contraction of the capsule wall even at high temperature. This allows 1O2 diffusion and shows high oxygenation activity even at high temperature.

3. Conclusion We found that a polymer nanocapsule containing an organic photosensitizer, TH@PC, enables the accurate control of 1O2 (21) (a) Shiraishi, Y.; Miyamoto, R.; Zhang, X.; Hirai, T. Org. Lett. 2007, 9, 3921–3924. (b) Shiraishi, Y.; Miyamoto, R.; Hirai, T. Tetrahedron Lett. 2007, 48, 6660–6664.

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oxygenation activity by temperature. This is driven by a heatinduced contraction of the capsule wall, behaving as an intelligent 1O filter. The capsule can reversibly control the oxygenation 2 activity. In addition, the capsule can be reused with a simple recovery process. The concept of the capsulated sensitizer has other significant advantages: (i) a homogeneous sensitizer can be handled like a heterogeneous one, and (ii) the sensitizer can be physically isolated from the substrate by the polymer wall, which suppresses the direct reaction of the sensitizer with substrate. The concept presented here may contribute to the design of more efficient photosensitizing materials.

4. Experimental Section 4.1. Materials. All of the reagents used were of the highest commercial quality, which were supplied from Wako, Aldrich, and Tokyo Kasei, and used without further purification. NIPAM monomer was recrystallized from n-hexane prior to use. Water was purified by the Milli-Q system. TH@PC. The synthesis route is summarized in Scheme 1a. The silica template was prepared based on Sto¨ber’s method:22 MB (15 mg, 47 µmol) and NH3 (28% aqueous solution, 7.3 g) were dissolved in EtOH (120 mL). TEOS (5.0 g, 24 mmol) was added to the solution and stirred at room temperature for 48 h. The formed particles were washed with EtOH (Step 1). 3-(Trimethoxysilyl)propylmethacrylate (0.63 g, 2.5 mmol) and the obtained particles were stirred in EtOH at room temperature for 48 h, and the resultant was washed with EtOH (Step 2). The obtained particles were dispersed in water (75 mL) containing NIPAM (0.60 g, 5.3 mmol) and MBA (45 mg, 29 µmol). Potassium persulfate (15 mg, 56 µmol) was added to the mixture and stirred at 70 °C for 4 h under N2. The resultant was washed with water, affording core/shell silica-polymer particles (Step 3). The obtained particles were stirred in an aqueous HF solution (5%; 60 mL) at room temperature for 4 h. The resultant was washed with water (Step 4), affording blue solid of TH@PC (0.53 g, 83%). 1H NMR (400 MHz, DMSO-d , tetramethylsilane (TMS)): δ (ppm) 6 ) 1.07 (s, br, 6H, -C(CH3)2), 1.2-2.2 (m, 3H; -CHCH2), 3.86 (s, br, 1H; -CH-), 6.73 (br, 1H, -NH-). PC (TH-Free). This was synthesized with a sensitizer-free silica template prepared by hydrolysis of TEOS (2.5 g, 12 mmol) with NH3 (28% aqueous solution, 3.7 g) in EtOH (60 mL). The subsequent procedures similar to TH@PC afford TH-free PC as a white solid (0.17 g, 80%). 1H NMR (400 MHz, DMSO-d6, TMS): δ (ppm) ) 1.08 (s, br, 6H, -C(CH3)2), 1.2-2.2 (m, 3H; -CHCH2), 3.86 (s, br, 1H; -CH-), 6.73 (br, 1H, -NH-). Poly(NIPAM-co-TH-co-MBA). NIPAM (0.60 g, 5.3 mmol), MBA (45 mg, 0.29 mmol), and vinylbenzyl chloride (VBC, 1.8 mg, 1.2 µmol) were dissolved in water (25 mL). Potassium persulfate (18 mg, 65 µmol) was added to the mixture and stirred at 70 °C for 24 h under N2. The resultant was washed with water affording poly(NIPAM-co-VBC-co-MBA) as a fluffy white solid (0.62 g, 96%). The polymer (0.10 g), MB (18 mg, 59 µmol), and triethylamine (TEA, 0.61 g, 6.0 mmol) were stirred in dimethylformamide (DMF) (10 mL) at 80 °C for 48 h under N2.23 The resultant was purified by reprecipitation with MeOH (1 mL) and diethyl ether (100 mL) affording poly(NIPAM-co-MB-co-MBA) as a fluffy blue solid (92 mg, 77.5%). The polymer (61 mg) and NH3 (28% aqueous solution, 0.608 g) were stirred in EtOH (10 mL) at room temperature for 48 h. The product was washed with water, affording poly(NIPAM-coTH-co-MBA) as a fluffy blue solid (60 mg, 98%). 1H NMR (400 (22) Sto¨ber, W.; Fink, A.; Bohn, E. J. Colloid Interface Sci. 1968, 26, 62–69. (23) Amat-Guerri, F.; Botija, J. M.; Sastre, R. J. Polym. Sci. A: Polym. Chem. 1993, 31, 2609–2615. (24) Shiraishi, Y.; Saito, N.; Hirai, T. J. Am. Chem. Soc. 2005, 127, 12820– 12822.

Shiraishi et al. MHz, DMSO-d6, TMS): δ (ppm) ) 1.09 (s, br, 6H, -C(CH3)2), 1.3-2.2 (m, 3H; -CHCH2), 3.86 (s, br, 1H; -CH-), 6.74 (br, 1H, -NH-). 4.2. Analysis Photooxygenation12,17b,24 Each sensitizer material was added to a buffered aqueous solution (5 mL; pH 10; consisting of 0.025 M NaHCO3 and 0.011 M NaOH) containing 1 within a Pyrex glass tube (capacity: 20 mL). Each tube was sealed using a rubber septum cap, and O2 was bubbled through the solution for 5 min. The sample was photoirradiated with magnetic stirring by a high-pressure Hg lamp (100 W; Eikohsha Co. Ltd., Osaka, Japan), filtered through a Corning color filter (CS3-67) to give light wavelengths of λ > 530 nm, The light intensity at 530-630 nm (through the filter) is 317 W/m2. Substrate and product concentrations were measured by GC-FID (Shimadzu GC-14B). ESR Measurement12,16,24 The spectra were recorded at the X-band using a Bruker EMX-10/12 spectrometer (scan conditions: microwave frequency, 9.7 GHz; microwave power, 10 mW; modulation amplitude, 1.0 G; modulation frequency, 100 kHz; time constant, 0.66 s; scan time, 335 s; receiver gain, 4.0 × 105; center field setting at 3438 G), where microwave power saturation of the signals does not occur. 1,1-Diphenyl-2-picrylhydrazyl (DPPH) was used for magnetic field calibration. TEMP (250 µmol) and sensitizer were added to a buffered aqueous solution (5 mL; pH 10) within a Pyrex glass tube, and O2 was bubbled through the solution for 5 min. The tube was photoirradiated for 1 h by a high-pressure Hg lamp in a manner similar to the photooxygenation experiments. The resulting solution was introduced to a conventional quartz ESR tube, and the measurement was carried out at room temperature. Other Analysis. All spectroscopic measurements were carried out with a 10 mm path length quartz cell. Absorption spectra were measured on a UV-visible photodiode-array spectrophotometer (Shimadzu; Multispec-1500) with a temperature controller (Shimadzu; S-1700).25 Fluorescence spectra were measured on Hitachi F-4500 spectrometer (excitation and emission slit width: 5.0 nm).26 SEM measurement was performed on a Hitachi S-5000. 1H NMR spectra were obtained by JEOL JNM-AL400 (400 MHz) with TMS as standard. Light scattering measurements were performed on static laser scattering spectrometer (LA-910, HORIBA, detection range 10 nm to 700 µm).19b IR spectra were measured on an FT-IR-610 infrared spectrophotometer (Jasco Corp.) with KBr disk.27 TG analysis was performed on a DTG-50 (Shimadzu) under N2 (25-1000 °C) with a heating rate of 10 °C/min.27

Acknowledgment. This work was supported by the Grantin-Aid for Scientific Research (Nos. 19760536 and 20360359) from the Ministry of Education, Culture, Sports, Science and Technology, Japan (MEXT). We thank Mr. M. Kawashima of the “Gas Hydrate Analyzing System (GHAS)”, Osaka University, for SEM measurement. H.K. thanks the Japan Society for Promotion of Science (JSPS) for Young Scientist. Supporting Information Available: Supplementary data (Figures S1-S8). This material is available free of charge via the Internet at http://pubs.acs.org. LA8015194 (25) (a) Shiraishi, Y.; Maehara, H.; Ishizumi, K.; Hirai, T. Org. Lett. 2007, 9, 3125–3128. (b) Shiraishi, Y.; Tokitoh, Y.; Hirai, T. Chem. Commun. 2005, 5316–5318. (c) Shiraishi, Y.; Tokitoh, Y.; Hirai, T. Org. Lett. 2006, 8, 3841– 3844. (26) (a) Shiraishi, Y.; Tokitoh, Y.; Nishimura, G.; Hirai, T. Org. Lett. 2005, 7, 2611–2614. (b) Nishimura, G.; Shiraishi, Y.; Hirai, T. Chem. Commun. 2005, 5313–5315. (c) Nishimura, G.; Maehara, H.; Shiraishi, Y.; Hirai, T. Chem.;Eur. J. 2008, 14, 259–271. (27) (a) Shiraishi, Y.; Teshima, Y.; Hirai, T. J. Phys. Chem. B 2006, 110, 6257–6263. (b) Shiraishi, Y.; Morishita, M.; Teshima, Y.; Hirai, T. J. Phys. Chem. B 2006, 110, 6587–6594.