Photoinduced Hydrogen Production with Artificial Photosynthesis

To develop a photoinduced hydrogen production system based on an artificial photosynthesis model, the anionic water-soluble carotenoid dye crocetin, w...
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J. Phys. Chem. C 2009, 113, 16811–16815

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Photoinduced Hydrogen Production with Artificial Photosynthesis System Based on Carotenoid-Chlorophyll Conjugated Micelles Yutaka Amao,* Yuko Maki, and Yoshiko Fuchino Department of Applied Chemistry, Oita UniVersity, Dannoharu 700, Oita 870-1192, Japan ReceiVed: January 4, 2009; ReVised Manuscript ReceiVed: July 27, 2009

To develop a photoinduced hydrogen production system based on an artificial photosynthesis model, the anionic water-soluble carotenoid dye crocetin, which has two carboxylate groups (a π electron conjugated polyene material with a carbon number of 20), electrostatically immobilized onto the surface of cationic surfactant cetyltrimethylammonium bromide (CTAB) micelles that included Mg chlorophyll-a and b (MgChl-a and b) (Cro/MgChl), was prepared, and its photochemical properties were studied using UV-vis absorption and fluorescence spectroscopy. The fluorescence of MgChl-a and b was induced, with the excitation wavelength attributed to the absorption band of crocetin, indicating that a photoinduced energy transfer from the photoexcited state of crocetin to MgChl-a and b occurs via the CTAB micelle interface. The photostability of MgChl-a and b in Cro/MgChl was investigated under continuous visible irradiation. After 60 min of irradiation, the absorbance at 660 nm due to MgChl-a and b in Cro/MgChl and MgChl-a/b, which did not contain crocetin, decreased by 3.0% and 17%, respectively. Little photoinduced decomposition of the crocetin moiety of Cro/MgChl was observed. These results indicate that the photoinduced decomposition rate of MgChla/b in Cro/MgChl against irradiation was suppressed by the crocetin molecule on the surface of the micelles. An effective hydrogen production system in the presence of NADH as an electron donor, methylviologen (MV2+) as an electron carrier, and platinum colloid as a hydrogen producing catalyst under visible light irradiation was developed using Cro/MgChl as a photosensitizer (3.6 µmol in 3 h), which was found to be better than using MgChl-a/b without crocetin (2.1 µmol in 3 h). Introduction A photosynthesis protein consists of two reaction centers. The photosynthesis system consists of two photosystems. One is photosystem I (PSI) with an absorption band of 700 nm, which involves the photoreduction of NADP+, and the other is photosystem II (PSII) with an absorption band of 680 nm, which involves water oxidation at an oxygen evolving site with a manganese-calcium cluster catalyst. In principle, the photolysis of water into hydrogen and oxygen could be developed using PSI and PSII in the presence of a suitable hydrogen-producing catalyst. However, the effective photolysis of water into hydrogen and oxygen using photosystems has not yet been accomplished. From the viewpoint of the need to develop new energy sources, the photoinduced production of hydrogen from water molecules based on water oxidation using artificial photosynthesis in imitation of higher green plants has been studied extensively with the goal of finding a means of converting solar energy into chemical energy. Many studies on the photoinduced hydrogen production systems containing an electron donor, a photosensitizer, an electron relay, and a hydrogen production catalyst have been reported.1-7 In this system, hydrogenase from DesulfoVibrio Vulgaris (Miyazaki)1-4 and platinum colloid5-7 are widely used as catalysts. Magnesium chlorophylls act as effective photosensitizers in the photosynthesis of green plants8 and have maximum absorption at 670 nm. Thus, chlorophylls are attractive compounds for a visible photosensitizer. Photoinduced hydrogen production systems with chemically modified chlorophyll and hydrogenase,9,10 along with chlorophyll derivatives,11-13 have been previously reported. We * Corresponding author. Phone: +81-97-554-7972. Fax: +81-97-5547972. E-mail: [email protected].

have already reported some photoinduced hydrogen production with chlorophyll-a and platinum colloid.14-17 However, purified chlorophylls are unstable against irradiation. Thus, photosensitizer molecules with high stability against irradiation and highly photosensitized activity are desired for the development of photoinduced hydrogen production. On the other hand, the light-harvesting sites in natural photosynthesis proteins consist of Mg chlorophyll-a,b (MgChl-a,b) and carotenoid dyes, such as a β-carotene.8 These dye molecules play important roles in photosynthesis, such as in light harvesting, photoinduced energy, and electron transfer, and so on. The carotenoid dyes in the protein absorb light in the ultraviolet and blue-green regions, within which chlorophyll has a low extinction coefficient, and transfer excitation energy to the chlorophylls. Many studies have been performed on the excitation energy transfer from carotenoids to chlorophylls in the natural photosynthesis proteins of green plants, cyanobacteria, or photosynthetic bacteria.18-21 In contrast, carotenoid dyes also have important functions in the absorption of UV light, photoprotection of MgChl-a and b, photosynthesis proteins, and so on.22,23 Artificial photosynthesis systems based on carotenoid and chlorophyll conjugation via covalent bonds have been reported.24,25 In a photosynthesis protein, MgChl-a,b and carotenoid dyes are assembled via the hydrogen bond, hydrophobic interaction, and coordination bond, not covalently to amino acid residues of the R-helix structure of the photosynthesis protein.26-28 Moreover, to develop an artificial photosynthesis model, minimizing the separation between the energy donors (carotenoids) and acceptors (chlorophylls) is a requirement dictated by the extremely short excited state lifetimes of the carotenoids. Carotenoid dye immobilized onto the surface of a surfactant micelle that includes MgChl-a

10.1021/jp900063r CCC: $40.75  2009 American Chemical Society Published on Web 09/01/2009

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and b in the hydrophobic site as an artificial photosynthesis protein will be an attractive photosensitized system, because the carotenoid dye molecule on the surface of the micelle will act as a UV light cutoff filter, suppressing the degradation of the MgChl-a/b. We have already reported, through a communication letter, the optical properties of the anionic watersoluble carotenoid dye crocetin (λmax ) 440 and 460 nm) electrostatically immobilized onto the surface of cationic surfactant cetyltrimethylammonium bromide (CTAB) micelles that included MgChl-a and b (Cro/MgChl).29 In a previous report,29 however, detailed investigation about photochemical and physical properties, such as photoinduced energy transfer mechanism in Cro/MgChl, improvement of photostability of MgChl induced crocetin immobilization, and so on, have not been clarified. Moreover, no application for photoinduced hydrogen production using Cro/MgChl also has yet been developed. In this work, an artificial photosynthesis system, the anionic water-soluble carotenoid dye crocetin electrostatically immobilized onto the surface of CTAB micelles that included MgChl-a and b (Cro/MgChl), was prepared, and its photophysical properties, such as the photoprotection effect of the crocetin and photoinduced energy transfer mechanism between crocetin moiety and MgChl, were investigated. Moreover, the photoinduced hydrogen production was examined with the Cro/MgChl and platinum colloid in the presence of methylviologen (MV2+) and NADH as an electron acceptor and an electron donor, respectively. Materials and Methods General. MgChl-a and b were purchased from the Juntech Corporation. Crocetin sodium salt was purified from gardenia yellow dyes obtained from Kiriya Chemical Co. Ltd. (Osaka, Japan), with the purification performed by recrystallization from a water-methanol (5:1) solution. NADH was obtained from Oriental Yeast Co. Ltd. (Tokyo, Japan). Methylviologen dichloride (MV2+) and cetyltrimethylammonium bromide (CTAB) were purchased from Tokyo Kasei Co. Lid (Tokyo, Japan). Hydrogen hexachloplatinate hexahydrate and sodium citrate dihydrate were obtained from Kanto Chemical Co. Ltd. (Tokyo, Japan). All the other reagents were the highest grade available. Preparation of Cro/MgChl. Cro/MgChl was prepared according to the method in a previous report.29 First, MgChl-a and b were solubilized in a 20 mM cetyltrimethylammonium bromide (CTAB) micellar medium containing potassium phosphate buffer (pH 7). Then, the crocetin solution containing potassium phosphate buffer (pH 7) was added to the CTAB micellar solution containing MgChl-a and b. The concentrations of crocetin and the MgChl-a and b in the Cro/MgChl were 0.43, 0.40, and 0.03 mM, respectively. Spectroscopy of Cro/MgChl. The UV-vis absorption spectra of the MgChl-a/b, crocetin, and Cro/MgChl solution were recorded using a spectrophotometer (Multispec-1500 Shimadzu). The fluorescence and excitation properties of the MgChl-a/b, crocetin, and Cro/MgChl were measured using a spectrofluorophotometer with a 150 W xenon lamp as a visible excitation light source (RF-5300PC Shimadzu). The excitation and emission bandpasses were both 5.0 nm. Preparation of Platinum Colloid. A suspension of platinum colloid was prepared by refluxing hydrogen hexachloplatinate(IV) hexahydrate and sodium citrate.6 A solution of 400 mL of water containing 30 mg of hydrogen hexachloplatinate hexahydrate was heated until it boiled using a heater mantle and a magnetic stirrer for 1.5 h, and then a solution of 30 mL

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Figure 1. UV-vis absorption spectra of the Cro/MgChl (solid line), MgChl-a/b (dotted line), and crocetin (dashed line) solutions in the presence of CTAB micelles.

of water containing 600 mg of sodium citrate dihydrate was added and it was refluxed at 100 °C for 4 h. The particle size was estimated to be 1.5 nm. The prepared platinum colloid had the ability to release 0.7 µmol of hydrogen in the reaction system of 10 µL platinum colloid, 1.2 × 10-5 mmol MV2+, and 7.7 × 10-5 mmol sodium dithionite in 4 mL of 10 mM Tris/HCl buffer (pH 7.4) at 30 °C for 10 min. One unit of platinum colloid activity was defined as the release of 1.0 µmol of hydrogen per min. Photoreduction of MV2+. The photoreduction of the MV2+ was tested in a reaction mixture containing NADH (2.0 mM), Cro/MgChl (Cro: 9.0 µM/MgChl: 9.0 µM), and MV2+ (2.0 mM) in 3.0 mL of 50 mM potassium phosphate buffer (pH 7.0). The solution was deaerated by repeated freeze-pump-thaw cycles and irradiated with a 200 W tungsten lamp at a distance of 3.0 cm. Wavelengths shorter than 390 nm were removed using a Toshiba L-39 cutoff filter. The reduction of the MV2+ was monitored using a UV-vis spectrophotometer set at 605 nm and assuming a molar extinction coefficient of 1.3 × 104 M-1 cm-1.30 The photoreduction of the MV2+ was tested in a reaction mixture containing NADH (2.0 mM), photosensitizer (MgChl-a and -b: 18 µM or crocetin: 18 µM), and MV2+ (2.0 mM) in 3.0 mL of 50 mM potassium phosphate buffer (pH 7.0), as a reference experiment. Photoinduced Hydrogen Production. The photoinduced hydrogen production was tested in a reaction mixture containing NADH (2.0 mM), Cro/MgChl (Cro: 9.0 µM/MgChl: 9.0 µM), MV2+ (2.0 mM), and platinum colloid (0.5 unit) in 3.0 mL of 50 mM potassium phosphate buffer (pH 7.0). The solution was deaerated by six freeze-pump-thaw cycles and flushed with argon gas. The amount of hydrogen produced and the other gases produced were measured using a Shimadzu GC-14B gas chromatograph (detector, TCD; column temperature, 40 °C; column, active charcoal with a particle size of 60-80 mesh; carrier gas, nitrogen gas; carrier gas flow rate, 24 mL min-1). The photoinduced hydrogen production was tested in a reaction mixture containing NADH (2.0 mM), photosensitizer (MgChl-a and -b: 18 µM or crocetin: 18 µM), MV2+ (2.0 mM), and platinum colloid (0.5 unit) in 3.0 mL of 50 mM potassium phosphate buffer (pH 7.0), as a reference experiment. Results and Discussion Spectroscopy of Cro/MgChl. The UV-vis absorption spectra of the Cro/MgChl and MgChl-a/b solutions in the presence of CTAB micelles were measured. Figure 1 shows the UV-vis absorption spectra of the Cro/MgChl, MgChl-a/b (dotted line), and crocetin (dashed line) solutions in the presence

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J. Phys. Chem. C, Vol. 113, No. 38, 2009 16813 SCHEME 1: Mechanism of Photinduced Energy Transfer between Crocetin and Chlorophyll Moieties in Cro/MgChl

Figure 2. Fluorescence emission spectra of Cro/MgChl with excitation 460 nm (a), MgChl with excitation 460 nm (b), crocetion (dashed line) with excitation 460 nm, and Cro/MgChl with excitation 660 nm (dotted).

of CTAB micelles. From the UV-vis absorption spectrum measurements, the absorption maxima of the Cro/MgChl were 407, 430, 460, and 670 nm. The sholder absorption of the Cro/ MgChl also was observed at 500 nm. The absorption bands at 407, 430, and 670 nm were attributed to MgChl-a and b. As shown in UV-vis absorption spectrum, a small absorption band at 740 nm due to the aggregate of MgChl-a and b molecules was observed. From absorbance at 740 nm, the concentration of the aggregate of MgChl-a and b molecules was calculated to be less than 1.0%. Thus, the formation of the aggregate of MgChl-a and b molecules can almost be disregarded. The absorption bands at 460 nm and shoulder absorption at 500 nm due to crocetin were observed in the Cro/MgChl. The shape of the absorption spectrum of Cro/MgChl was similar to that of the summation of the spectra of MgChl-a/b (dotted line) and crocetin (dashed line). This result shows that there was no electrostatic interaction between crocetin and MgChl-a and b in Cro/MgChl at the ground state. Figure 2 shows the fluorescence emission spectra of the Cro/ MgChl, MgChl-a/b, and crocetin solutions. When crocetin was excited with 460 nm due to the absorption band, weak fluorescence was observed at 522 nm (dotted line).21 The fluorescence was observed when there was more than 1.0 mM of crocetin. The fluorescence emission spectrum of the Cro/ MgChl, with excitation attributed to the absorption band of crocetin (460 nm), is shown in Figure 2a. Fluorescence at 680 nm due to MgChl-a and b was observed. However, the fluorescence at 522 nm due to crocetin disappeared. In contrast, fluorescence in Cro/MgChl was also observed at 680 nm with excitation to the absorption band of MgChl-a and b (660 nm), as shown in Figure 2 (dotted line). However, no fluorescence at 680 nm was observed with 460 nm excitation in the CATB micelles that included MgChl-a and b without crocetin (MgChla/b), as shown in Figure 2b. Thus, the fluorescence of crocetin was quenched by the MgChl-a/b. Next, let us focus on the photoinduced energy transfer from the photoexcited state of the crocetin moiety to the MgChl in the Cro/MgChl. The energies of the first excited singlet state of the crocetin and MgChl were calculated from the average value of the frequencies of the

longer wavelengths of the absorption maxima and the shortest wavelengths of the fluorescence maxima. No correction for Coulomb effects was attempted, because any interaction among the chromophores, crocetin, and MgChl, at the ground state results from the UV-vis absorption spectrum of Cro/MgChl. The first photoexcited singlet state of crocetin lies at 2.72 eV above the ground state. On the other hand, the first photoexcited singlet state of MgChl lies at 1.83 eV above the ground state. From UV-vis absorption and fluorescence spectra measurement, the possible photoenergy transfer from the photoexcited singlet state of crocetin (1*Cro) onto the surface of the micelles to the MgChl moiety is shown in Scheme 1. When light of wavelength 460 nm was irradiated (process 1), the crocetin moiety in the Cro/MgChl absorbed the light and transmitted to the photoexcited singlet state of the crocetin moiety (1*Cro/MgChl). In the absence of MgChl, the fluorescence of crocetin was observed with the excitation of 460 nm light (process 2). Next, a photoenergy transfer occurs from 1*Cro to the photoexcited singlet state of MgChl (1*MgChl) moiety via the CTAB micelle interface (process 3), and the fluorescence emission from the 1 *MgChl moiety occurred (process 4). The fluorescence emission from the 1*MgChl moiety also was observed with direct excitation (660 nm) of MgChl moiety (process 1′). There was also a possibility in the process from 1*Cro to the photoexcited triplet state of MgChl (3*MgChl) moiety, as phosphorescence from the 3*MgChl moiety was not observed and radiationless deactivation occurred at room temperature. If the process from 1 *Cro to 3*MgChl proceeded, the fluorescence from the 1 *MgChl moiety with excitation of 460 nm would disappear. However, fluorescence from the 1*MgChl moiety was observed with excitation of 460 nm. These results suggest that photoenergy transfer from 1*Cro onto the surface of the micelles to the MgChl moiety proceeded as shown in Scheme 1. Photostability of MgChl in the Cro/MgChl. Figure 3 shows the absorbance changes at 660 nm attributed to the absorption band of the MgChl-a/b in the Cro/MgChl (closed circle) and the MgChl-a/b (closed square) with irradiation time. After 60 min of irradiation, the absorbance decreases at 660 nm in Cro/ MgChl and MgChl-a/b were 3.0% and 17%, respectively. The

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Figure 3. Time dependence of absorbance changes at 660 nm attributed to MgChl-a and b in Cro/MgChl (circle) and MgChl (square) with irradiation. The absorbance change at 460 nm due to the absorption band of crocetin in Cro/MgChl (open squre) and only crocetin (diamond) with irradiation. The absorbance changes at 660 nm in MgChl-a/b with irradiation through an optical short wave cutoff filter with Sigma-Koki SCF-50S-48Y, transmitted above 480 nm light (X), and SCF-50S-39 L, transmitted above 390 nm light (open circle).

absorbance decrease rate for the MgChl-a/b in Cro/MgChl was slower than that of MgChl. As a reference experiment, the absorbance changes at 660 nm in MgChl-a/b with irradiation through an optical short wave cutoff filter (Sigma-Koki SCF50S-48Y: transmitted above 480 nm light and SCF-50S-39 L: transmitted above 390 nm light) are also shown in Figure 3(X) and (open circle), respectively. After 60 min of irradiation, the absorbance decrease at 660 nm for MgChl-a/b was less than 1.0% in both cases. These results indicate that the photobreaching rate of the MgChl-a/b in Cro/MgChl against irradiation was suppressed by the crocetin molecule on the surface of the micelles. Figure 3 also shows the absorbance change at 460 nm due to the absorption band of the crocetin in the Cro/MgChl (open square) and for crocetin alone (closed diamond) with irradiation. As shown in Figure 3, little change in the absorbance at 460 nm was observed in the two systems, indicating that no crocetin degradation occurs under steady-state irradiation. As shown in Figure 1, moreover, crocetin absorbed light with wavelengths shorter than 400 and 460 nm on the surface of the CTAB micelles that included MgChl-a/b. From the fluorescence of Cro/MgChl, the photoinduced energy transfers from crocetin to the MgChl-a/b via the interface of the CTAB micelles, as shown in Scheme 1. Moreover, the degradation of MgChl-a/b was suppressed with irradiation through an optical short wave cutoff filter. Since the crocetin molecule on the surface of the micelles acted as a cutoff filter, the degradation of MgChl-a/b was suppressed. Photoreduction of MV2+. The photoreduction of MV2+ is the most important step in a photoinduced hydrogen production system. When a sample solution containing NADH (2.0 mM), Cro/MgChl (Cro: 9.0 µM/MgChl: 9.0 µM), and MV2+ (2.0 mM) in 3.0 mL of 50 mM potassium phosphate buffer (pH 7.0) was irradiated at 30 °C, a reduced accumulation of methylviologen was observed, as shown in Figure 4. After 60 min irradiation,

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Figure 4. Time dependence of reduced MV2+ (MV.+) concentration with the system containing NADH, photosensitizer, and MV2+ under irradiation. Photosensitizer: Cro/MgChl (closed circle), MgChl-a/b (closed square), and crocetin (closed diamond). Open square: without irradiation.

the reduced MV2+ concentrations using Cro/MgChl and MgChla/b were 0.61 and 0.41 mM, respectively. In contrast, Figure 4 also shows the time dependence of the MV2+ photoreduction using crocetin instead of Cro/MgChl or MgChl-a/b as a photosensitizer. After 60 min of irradiation, the reduced MV2+ concentration was 0.03 mM. This result shows that the lower photoreduction activity of MV2+ was due to the shorter lifetime of the photoexcited state of the crocetin. Moreover, no photoreduction of MV2+ was observed without irradiation (open square). In the case where Cro/MgChl was used, although the concentration of MgChl in Cro/MgChl was half that of MgChla/b, the reduction in the MV2+ concentration was increased. These results indicate that an effective photoreduction system could be developed by increasing the photosensitized activity due to the energy transfer from the crocetin site to the MgChl in Cro/MgChl. Photoinduced Hydrogen Production. Photoinduced hydrogen production with colloidal platinum was attempted using the conditions described above. When a sample solution containing NADH (2.0 mM), Cro/MgChl (Cro: 9.0 µM/MgChl: 9.0 µM), MV2+ (2.0 mM), and platinum colloid (0.5 unit) in 3.0 mL of 50 mM potassium phosphate buffer (pH 7.0) was irradiated at 30 °C, hydrogen production was observed, as shown in Figure 5. The amount of hydrogen produced increased with the irradiation time. After 3 h of irradiation, the amount of hydrogen produced with the system using Cro/MgChl and MgChl-a/b was ca. 3.6 and 2.1 µmol, respectively. In contrast, Figure 5 also shows the time dependence of the hydrogen production using crocetin instead of Cro/MgChl or MgChl-a/b as a photosensitizer. After 3 h of irradiation, no hydrogen production was observed due to the shorter lifitime of the photoexicited state of the crocetin. Moreover, no hydrogen production was observed without irradiation (open square). In the case where Cro/MgChl was used, although the concentration of MgChl in the Cro/ MgChl was half that of MgChl-a/b, the hydrogen production rate also increased. Consequently, an effective hydrogen production system was developed using Cro/MgChl.

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J. Phys. Chem. C, Vol. 113, No. 38, 2009 16815 Acknowledgment. This work was partially supported by The Asahi Glass Foundation, The Yazaki Science and Technology Foundation, Nippon Sheet Glass Foundation for Materials Science and Engineering, and The Sumitomo Foundation. References and Notes (1) Okura, I. Coord. Chem. ReV. 1985, 68, 53–99. (2) Okura, I. Biochimie 1986, 68, 189–199. (3) Okura, I.; Aono, S.; Kusunoki, S. Inorg. Chim. Acta 1983, 71, 77– 80.

Figure 5. Time dependence of hydrogen produced with the system containing NADH, photosensitizer, MV2+, and platinum colloid under irradiation. Photosensitizer: Cro/MgChl (closed circle), MgChl-a/b (closed square), and crocetin (closed diamond). Open square: without irradiation.

Conclusion To develop an artificial photosynthesis model, the anionic water-soluble carotenoid dye crocetin electrostatically immobilized onto the surface of cationic surfactant CTAB micelles that included Mg chlorophyll (MgChl) (Cro/MgChl) was prepared, its photophysical properties were studied using UV-vis absorption and fluorescence spectroscopy, and it was applied to photoinduced hydrogen production in the presence of NADH, MV2+, and platinum colloid. The fluorescence of the MgChl moiety was observed with the excitation wavelength attributed to the absorption band of crocetin, indicating that photoinduced energy transfer occurs from the photoexcited state of crocetin to the MgChl in Cro/MgChl. The photostability of the MgChl in Cro/MgChl was investigated under continuous irradiation. After 60 min of irradiation, the absorbance at 660 nm due to MgChl in Cro/MgChl and MgChl without crocetin decreased by 3.0% and 17%, respectively. These results indicate that the photobleaching rate of MgChl in Cro/MgChl against irradiation is suppressed by the crocetin molecule on the surface of micelles. An effective photoinduced hydrogen production system using Cro/MgChl was developed.

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