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Temperature-Controlled Photosensitization Properties of Benzophenone-Conjugated Thermoresponsive Copolymers Hisao Koizumi, Yasuhiro Shiraishi,* 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 27, 2008; ReVised Manuscript ReceiVed: August 6, 2008
We previously found that a copolymer, poly(NIPAM-co-BP), consisting of N-isopropylacrylamide (NIPAM) and benzophenone (BP) units, behaves as a photosensitizer showing temperature-controlled oxygenation activity by singlet oxygen (1O2) in water (J. Am. Chem. Soc. 2006, 128, 8751-8753). This polymer shows a heatinduced oxygenation enhancement at 20 °C. This is driven by a heat-induced phase transition of the polymer from coil to micelle and then to globule state, controlling the stability and diffusion of 1O2 and the location of substrate. In the present work, effects of polymer concentration and BP content of the polymer on the oxygenation activity were studied at 5-35 °C. Increase in the polymer concentration leads to activity decrease at >20 °C due to strong polymer aggregation, suppressing incident light absorption of the BP units. With a decrease in BP content of the polymer, heat-induced oxygenation enhancement at 20 °C; high polymer concentration leads to a formation of huge globule state polymer particles and shows very low activity. In contrast, the BP content of the polymer affects the activity at 320 nm, 3 h) to an O2-saturated aqueous solution (pH 10) containing 1 (10 µmol) and different amount of P6. At all tested temperatures and P6 concentrations, 2 is produced as a sole product. Figure 1 shows temperature-dependent change in 2 yield. With 0.004 g/L P6, 2 yield is quite low at the entire temperature range. With >0.02 g/L P6, 2 yield increases with a rise in temperature at 20 OC, showing an off-on-off activity profile. With 0.02-0.04 g/L P6, 2 yield increases with an increase in the polymer concentration at the entire temperature range. In contrast, with higher polymer amount, 2 yield at 20 OC becomes lower. As described previously,6 the heat-induced oxygenation enhancement at 0.02 g/L (log[P6] > -1.7), indicating that hydrophobic domain forms. However, at 29 °C.7 However, 2 yield decreases at >20 °C (Figure 1). This is because globule formation occurs partially at >20 °C.6 As shown in Figure 5B, dynamic light scattering (DLS) analysis detects the scattered light at >20 OC, implying that globule formation occurs partially at >20 OC (Scheme 2).13 The removal of 1 from the globule polymer is confirmed by 1H NMR analysis of 1 with polymer (Figure 3). The intensity of 1 increases at >20 °C, which is consistent with the globule formation, resulting in heat-induced oxygenation suppression at >20 °C (Figure 1).6 As shown in Figure 1, the sensitization activity at >20 OC decreases entirely with >0.04 g/L P6, although the BP amount in solution is higher. This is because strong interpolymer aggregation leads to formation of huge polymer particles, suppressing incident light absorption by the BP units (Scheme 3). As shown in Figure 5A and 5B, at higher temperature, the turbidity and polymer particle size becomes much larger with the polymer concentration increase. This is because higher
Figure 5. (A) Temperature-dependent change in turbidity (A500 nm) of P6 polymer solution. (B) Change in hydrodynamic radius (Rh) of P6 polymer particles. (C) Change in transmittance of the incident light through the polymer solution (%T320-400 nm ) 100 × I/I0). I0 is the intensity measured without polymer, and I is the intensity measured with polymer, where the intensity was determined by integration of the spectral irradiance at 320-400 nm. The numeric numbers in the figure denote P6 concentration (g/L).
polymer concentration enhances interpolymer aggregation.10a As shown in Figure 6, with 0.04 g/L P6, the BP unit still absorbs light (ca. 320-400 nm) at >30 °C. However, with >0.12 g/L, the aggregated polymer shows large absorption at 20 °C, with 0.004-0.04 g/L P6, %T scarcely decreases, suggesting that BP units absorb the incident light sufficiently. However, with >0.12 g/L, %T decreases drastically, meaning that incident light absorption of the BP units is much suppressed by huge polymer particles. As shown in Figure 4B, at >20 °C, Φ1O2 decreases with >0.04 g/L P6; this agrees with the 2 yield profile (Figure 1). These suggest that, at high polymer concentration, formation of huge polymer particles suppresses the incident light absorption of the BP units, leading to 2 yield decrease at >20 °C. 2.2. Effect of BP Content of Polymer. Effect of the BP content of poly(NIPAMx-co-BPy) was studied with two more types of polymers, P0.6 (x/y ) 99.4/0.6) and P0.3 (x/y ) 99.7/ 0.3) (Table 1). The singlet energy (ES) and triplet energy (ET) of these polymers are similar to that of P6. Figure 7A shows temperature-dependent change in 2 yield obtained with the respective polymers at constant concentration (0.04 g/L). These
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SCHEME 3: Schematic Representation of Globule State P6 Polymer at Different Concentrations
Figure 7. Temperature-dependent change in (A) 2 yield and (B) turnover number (TON) for 2 formation during reaction (λ >320 nm, 3 h) in O2-saturated aqueous solution (pH 10) with respective polymers at constant polymer concentration (0.04 g/L). Changes in turbidity and particle size of the respective polymer solutions are shown in Figure S1 (Supporting Information).
two polymers also show off-on-off activity profile. The highest 2 yield is obtained with P6 because of the highest BP amount in solution. Figure 7B shows change in turnover number (TON) for 2 formation [) (2 yields)/(BP amount in solution)].14 The highest TON is obtained with P0.3, meaning that the micelle state of P0.3 has the highest activity. As reported,15 the size of the hydrophobic domain of polyNIPAM containing hydrophobic fragments increases with an increase in the fragment amount. As shown in Figure 8, CH resonance intensity of 1 measured with P6 (black circle) at ca. 20 °C is lower than that with P0.3 (black square) and P0.6 (black triangle). This indicates that P6 with large amount of hydrophobic BP moiety indeed produces larger-size micelle, in which a large number of 1 exists. However, TON of P0.3 and P0.6 is much higher than that of P6 (Figure 7B). The higher activity of these polymers may be due to the smaller-size micelle. Within the hydrophobic domain
Figure 6. Temperature-dependent change in absorption spectra of P6 polymer solution (pH 10) at concentration of (A) 0.16 g/L, (B) 0.12 g/L, and (C) 0.04 g/L.
Figure 8. Signal intensity of 1H NMR spectrum of 1 (2 mM) measured with the respective polymers. The intensity was determined by integration of all CH resonances of 1, where the intensity measured at 35 °C without polymers is set as 1.
of the micelle, 1O2 lifetime is lengthened; however, at the same time, diffusion of 1O2 and substrate is suppressed by weak polymer aggregation.6 The access of 1O2 and 1 may occur more easily within the smaller micelle. Although the direct measurement of 1O2 and substrate diffusion within the micelle was not performed, the enhanced access of 1O2 and 1 within the small micelle may be the reason for high oxygenation activity of P0.3 at middle temperatures (Scheme 4).16 A notable feature of the polymer containing low BP content appears when used at high concentration. Figure 9 (square) shows 2 yield obtained by photoirradiation to a solution containing 1 and high concentration of P0.3 polymer (0.88 g/L containing 0.1 µmol BP unit). The activity at 5 °C is quite low (20% yield). However, at >20 °C, the activity decreases significantly and becomes almost zero at 25 °C, demonstrating sharp off-on-off activity. The white triangle and circle symbols in Figure 9 show the results obtained with P0.6 (0.38 g/L) and P6 (0.04 g/L) polymers, where the BP amount in solution is adjusted to be same as that of P0.3 (0.1 µmol). Both P0.6 and P6 polymers show less sharp activity response. Higher concentration (0.88 g/L) of P0.3 polymer produces a large number of small-size micelles. A large number of 1 molecule is therefore contained in these micelles, in which the reaction of 1O2 and 1 is enhanced, thus showing pronounced
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SCHEME 4: Schematic Representation of the Micelle State of P6 and P0.3 Polymers at Constant Polymer Concentration
heat-induced oxygenation enhancement at 20 °C (Figure 9). This is due to the formation of huge polymer particles. As shown in Figure 10C, %T of the solution decreases significantly at >20 °C. This leads to significant decrease in the incident light absorption of the BP unit, resulting in pronounced 2 yield decrease (Figure 9). With P0.6 (0.38 g/L) and P6 (0.04 g/L) polymers (Figure 10B), smaller particles form due to lower polymer concentration; therefore, %T decrease is smaller than the case with P0.3 (Figure 10C), resulting in gradual activity decrease at >20 °C (Figure 9). The overall reaction mechanism of P0.3 polymer at high concentration can therefore be summarized as Scheme 5. The above findings clearly suggest that poly(NIPAMx-co-BPy) with low BP content, when used at high concentration, demonstrates sharp off-on-off sensitization activity response against the temperature window. 3. Conclusion Photosensitization properties of polymeric photosensitizer, poly(NIPAMx-co-BPy), have been studied in water. These polymers promote a heat-induced oxygenation enhancement at 20 °C. Effects of the polymer concentration and the BP content of the polymer on the photosensitization activity were studied, with the following results:
Figure 10. (A) Temperature-dependent change in turbidity (A500 nm) of the respective polymer solutions containing constant BP amount in solution (0.1 µmol). (B) Change in hydrodynamic radius (Rh) of polymer particles. (C) Change in transmittance (%T320-400 nm) of the respective polymer solutions. Temperature-dependent changes in turbidity and Rh of P0.6 and P0.3 polymers as a function of the polymer concentration are summarized in Figures S2 and S3 (Supporting Information).
(1) With an increase in the polymer concentration, photosensitization activity at 20 °C decreases. The activity enhancement at 20 °C is due to the formation of huge polymer particles, which suppresses incident light absorption of the BP units within the polymer. (2) With a decrease in the BP content of the polymer, the heat-induced activity enhancement at 5 °C and >25 °C and very high activity at middle temperature range. This is due to the formation of small-size micelles at middle temperatures and huge polymer particles at high temperatures. 4. Experimental Section
Figure 9. Temperature-dependent change in 2 yield (3 h) in O2saturated aqueous solution (pH 10, 5 mL) with the respective polymers at constant BP amount in solution (0.1 µmol).
General. All of the reagents used are the highest commercial quality and was used without further purification. Water was purified by the Milli Q system. Synthesis of Poly(NIPAMx-co-BPy). Required amounts of NIPAM, 4-allyloxyBP,17 and AIBN (Table 1) were dissolved in toluene (5 mL) and degassed by two freeze-pump-thaw
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SCHEME 5: Schematic Representation of Temperature-Dependent Change in Structure of P0.3 Polymer at High Polymer Concentration (0.88 g/L)
cycles. The solution was kept at 60 OC for 18 h under dry N2. The resultant solution was added to diethyl ether (100 mL). The precipitate formed was collected by centrifugation and purified by reprecipitation with MeOH (1 mL) and diethyl ether (100 mL), affording poly(NIPAM-co-BP) as a fluffy white solid: 1H NMR (400 MHz; CDCl ; TMS): δ (ppm) ) 1.07 (s, br, 3 C(CH3)2), 1.2-2.2 (m, CHCH2), 3.88 (s, br, CH), 6.72 (br, NH), 7.65 (br, Ar-H). The amount of BP units on the polymers was determined by comparison of absorbance (A290 nm) with 4-methoxyBP ( ) 7.98 × 103 L/mol cm) in methanol (298 K). Photooxygenation. Each sensitizer was dissolved in 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 at 5 OC to avoid evaporation of the materials. The sample was photoirradiated with magnetic stirring by a highpressure Hg lamp (100 W; Eikohsha Co. Ltd., Osaka, Japan), filtered through an aqueous CuSO4 (10 wt %) solution to give light wavelengths of λ >320 nm. The light intensity at 320-400 nm (through the filter) is 905 mW/m2. Substrate and product concentrations were measured by GC-FID (Shimazu GC-14B). ESR Measurement. Spectra were recorded at the X-band using a Bruker EMX-10/12 spectrometer18 (scan condition: 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 3455 G), where microwave power saturation of the signals does not occur. 1,1′-Diphenyl-2picrylhydrazyl (DPPH) was used for calibration. Each sensitizer and TEMP (250 µmol) were dissolved in an 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 10 min by a high-pressure Hg lamp in a similar manner to the photooxidation experiment. After photoirradiation, the solution was introduced to a conventional quartz ESR tube, and measurement was then started at room temperature.19 Analysis. Absorption spectra were recorded on an UV-visible photodiode-array spectrophotometer (Shimadzu; Multispec1500)20 with a temperature controller (Shimadzu; S-1700) with a 10 mm path length quartz cell. 1H NMR spectra were obtained by JEOL JNM-AL400 (400 MHz). Fluorescence and phosphorescence spectra (77 K) were measured on a Hitachi F-4500 fluorescence spectrophotometer, using an EtOH/diethyl ether glass (2/1 v/v) within a 4 mm cylindrical quartz tube. The singlet energy (ES) and triplet energy (ET) of the sensitizers were determined by literature procedure.21 Dynamic light scattering
(DLS) measurement was carried out with a laser scattering spectrometer (LB-500, HORIBA),22 where the light source was a 5 mW semiconductor laser (λ ) 650 nm) and the scattering angle was 90O. Spectral irradiance was measured with a spectroradiometer (USR-40, Ushio Inc.). Molecular weight of the polymers was determined by gel permeation chromatography (GPC)23 using a JASCO HPLC system equipped with a PU980 pump (JASCO) and refractive index detector RI-930 (JASCO), with KF-806 L column (Shodex). The oven temperature was 40 OC, and DMF containing LiBr (0.01 M) was used as the carrier solvent (flow rate: 0.6 mL/min). Acknowledgment. This work was supported by the Grantin-Aid for Scientific Research (No. 19760536) from the Ministry of Education, Culture, Sports, Science and Technology, Japan (MEXT). We are grateful to the Division of Chemical Engineering for the Lend-Lease Laboratory System. We thank Mr. C. Yamashita for his experimental help. H.K. thanks the Japan Society for the Promotion of Science (JSPS) Research Fellowships for Young Scientists. Supporting Information Available: Supplementary data (Figures S1-S3). This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Wahlen, J.; De Vos, D. E.; Jacobs, P. A.; Alsters, P. L. AdV. Synth. Catal. 2004, 346, 152–164. (b) Chavan, S. A.; Maes, W.; Gevers, L. E. M.; Wahlen, J.; Vankelecom, I. F. J.; Jacobs, P. A.; Dehaen, W.; De Vos, D. E. Chem. Eur. J. 2005, 11, 6754–6762. (2) (a) Kitamura, N.; Yamada, K.; Ueno, K.; Iwata, S. J. Photochem. Photobiol. A: Chem. 2006, 184, 170–176. (b) Hino, T.; Anzai, T.; Kuramoto, N. Tetrahedron Lett. 2006, 47, 1429–1432. (c) Fuchter, M. J.; Hoffman, B. M.; Barrett, A. G. M. J. Org. Chem. 2006, 71, 724–729. (d) Griesbeck, A. G.; Bartoschek, A.; El-Idreesy, T. T.; Ho¨inck, O.; Miara, C. J. Mol. Catal. A: Chem. 2006, 251, 41–48. (e) Griesbeck, A. G.; El-Idreesy, T. T. Photochem. Photobiol. Sci. 2005, 4, 205–209. (3) (a) Dichtel, W. R.; Baek, K.-Y.; Fre´chet, J. M. J.; Rietveld, I. B.; Vinogradov, S. A. J. Polym. Sci. Part A: Polym. Chem. 2006, 44, 4939–4951. (b) Hecht, S.; Fre´chet, J. M. J. J. Am. Chem. Soc. 2001, 123, 6959–6960. (c) Shiraishi, Y.; Koizumi, H.; Hirai, T. J. Phys. Chem. B 2005, 109, 8580–8586. (d) 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. (4) (a) Irie, S.; Irie, M.; Yamamoto, Y.; Hayashi, K. Macromolecules 1975, 8, 424–427. (b) Bhyrappa, P.; Young, J. K.; Moore, J. S.; Suslick, K. S. J. Am. Chem. Soc. 1996, 118, 5708–5711. (c) Bhyrappa, P.; Young, J. K.; Moore, J. S.; Suslick, K. S. J. Mol. Catal. A: Chem. 1996, 113, 109–116. (5) (a) 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. (b) Nishikubo, T.; Uchida, J.; Matsui, K.; Iizawa, T. Macromolecules 1988, 21, 1583–1589. (c) Nishimura, I.; Kameyama, A.; Nishikubo, T. Macromolecules 1998, 31, 2789–2796.
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