Energy Transfer between Chlorophyll Derivatives in Silica

Mar 22, 2005 - Figure 2a depicts a visible absorption spectrum of Cu-. APTES-Chl a ..... Culture, Sports, Science and Technology (MEXT), Japan. The 21...
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Langmuir 2005, 21, 3992-3997

Energy Transfer between Chlorophyll Derivatives in Silica Mesostructured Films and Photocurrent Generation Hiroyasu Furukawa,*,† Natsuka Inoue,‡ Tadashi Watanabe,§ and Kazuyuki Kuroda*,‡,| Energy Electronics Institute, National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japan, Department of Applied Chemistry, Waseda University, 3-4-1 Okubo, Shinjuku-ku, Tokyo 169-8555, Japan, Institute of Industrial Science, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8505, Japan, and CREST, Japan Science and Technology Agency (JST) Received August 30, 2004. In Final Form: December 12, 2004 Layered silica/surfactant mesostructured thin films containing chlorophyllous pigments [C132demethoxycarbonyl-pheophytin b (pyroPheo b) or zinc C132-demethoxycarbonyl-chlorophyll b (Zn-pyroChl b)] have been prepared on an indium tin oxide (ITO) electrode grafted with a chlorophyll derivative possessing a triethoxysilyl group (copper C132-demethoxycarbonyl-chlorophyllide a 3-triethoxysilyl propylamide, CuAPTES-Chl a) to achieve effective light harvesting and successive photocurrent generation by the mesostructured films. The incorporation of pyroPheo b and Zn-pyroChl b in the mesostructured film resulted in 1.2- and 1.6-fold increases of the photocurrent density, respectively, as compared to the case of an antenna pigment-free film also grafted to a surface-modified ITO electrode. The difference action spectra, between the electrodes with and without the antenna pigments, coincided well with the absorption spectra of the immobilized pigments. Because direct electron injection from the antenna pigments in the mesostructured films to the ITO electrode was scarcely observed, the energy transfer from the antenna pigments to Cu-APTES-Chl a plays an important role for the increase in photocurrent density. The usefulness of the mesostructured films as model systems is discussed in relation to the photosynthetic primary processes of higher plants.

Introduction It is of general interest to construct artificial photoredox systems for conversion and storage of solar energy as an alternative to using fossil fuels.1,2 Several researchers have demonstrated photocurrent generation at electrodes modified with chlorophyllous pigments.3-5 However, in the case of a chlorophyll (Chl) monolayer deposited on a SnO2 electrode,4 a large photocurrent density cannot be expected due to the small absorbance. To overcome this problem, two approaches have emerged. One is to introduce any specific electron acceptor or donor groups at appropriate moieties of porphyrin derivatives.6,7 In this case, back electron transfer is suppressed by the introduced functional groups, and an increase in the charge separation lifetime may be expected, leading to an increase of the * To whom correspondence should be addressed. E-mail: [email protected] (H.F.); [email protected] (K.K.). † AIST. Present address: Department of Chemistry, University of Michigan, 930 North University Avenue, Ann Arbor, MI 481091055. ‡ Waseda University. § The University of Tokyo. | Japan Science and Technology Agency (JST). (1) Kobuke, Y.; Ogawa, K. Bull. Chem. Soc. Jpn. 2003, 76, 689. (2) (a) Gust, D.; Moore, T. A.; Moore, A. L. Acc. Chem. Res. 2001, 34, 40. (b) Imahori, H.; Fukuzumi, S. Adv. Mater. 2001, 13, 1197. (c) Imahori, H.; Fukuzumi, S. Adv. Funct. Mater. 2004, 14, 525. (3) (a) Tributsch, H.; Calvin, M. Photochem. Photobiol. 1971, 14, 95. (b) Fong, F. K.; Polles, J. S.; Galloway, L.; Fruge, D. R. J. Am. Chem. Soc. 1977, 99, 5802. (4) (a) Miyasaka, T.; Watanabe, T.; Fujishima, A.; Honda, K. Nature 1979, 277, 638. (b) Miyasaka, T.; Watanabe, T.; Fujishima, A.; Honda, K. J. Am. Chem. Soc. 1978, 100, 6657. (c) Watanabe, T.; Machida, K.; Suzuki, H.; Kobayashi, M.; Honda, K. Coord. Chem. Rev. 1985, 64, 207. (5) (a) Yang, Y.-G.; Zhou, R.-L.; Han, Y.-Y.; Jiang, Y.-S. J. Photochem. Photobiol., A 1993, 76, 111. (b) Bedja, I.; Hotchandani, S.; Carpentier, R.; Fessenden, R. W.; Kamat, P. V. J. Appl. Phys. 1994, 75, 5444. (c) Bedja, I.; Kamat, P. V.; Hotchandani, S. J. Appl. Phys. 1996, 80, 4637.

photocurrent density at the onset of illumination. However, it is difficult to absorb a large number of photons per area, because these systems have no antenna moieties for light harvesting. Another approach is to attach antenna layers to porphyrin self-assembled monolayers (SAMs) on the electrode through hydrogen8 or covalent bonds.9 Though this is effective for the increase of the photocurrent density, pigments must possess a linker to the function of antenna pigments. Moreover, these two strategies may suffer from the following drawbacks: (i) the design and synthesis of appropriate antenna pigments are generally not easy, and (ii) neither Langmuir-Blodgett (LB) films nor SAMs may be suitable in view of the strength of the substrates. Thus, there is a continuous demand for developing energy conversion systems prepared with a simple synthetic procedure while possessing mechanical strength. We have recently proposed the use of inorganic media (i.e., mesoporous silica powders and mesostructured thin (6) (a) Imahori, H.; Norieda, H.; Ozawa, S.; Ushida, K.; Yamada, H.; Azuma, T.; Tamaki, K.; Sakata, Y. Langmuir 1998, 14, 5335. (b) Imahori, H.; Norieda, H.; Nishimura, Y.; Yamazaki, I.; Higuchi, K.; Kato, N.; Motohiro, T.; Yamada, H.; Tamaki, K.; Arimura, M.; Sakata, Y. J. Phys. Chem. B 2000, 104, 1253. (c) Imahori, H.; Yamada, H.; Nishimura, Y.; Yamazaki, I.; Sakata, Y. J. Phys. Chem. B 2000, 104, 2099. (d) Yamada, H.; Imahori, H.; Fukuzumi, S. J. Mater. Chem. 2002, 12, 2034. (e) Yamada, H.; Imahori, H.; Nishimura, Y.; Yamazaki, I.; Fukuzumi, S. Adv. Mater. 2002, 14, 892. (f) Yamada, H.; Imahori, H.; Nishimura, Y.; Yamazaki, I.; Ahn, T. K.; Kim, S. K.; Kim, D.; Fukuzumi, S. J. Am. Chem. Soc. 2003, 125, 9129. (7) (a) Uosaki, K.; Kondo, T.; Zhang, X.-Q.; Yanagida, M. J. Am. Chem. Soc. 1997, 119, 8367. (b) Lahav, M.; Heleg-Shabtai, V.; Wasserman, J.; Katz, E.; Willner, I.; Durr, H.; Hu, Y.-Z.; Bossmann, S. H. J. Am. Chem. Soc. 2000, 122, 11480. (c) Lahav, M.; Gabriel, T.; Shipway, A. N.; Willner, I. J. Am. Chem. Soc. 1999, 121, 258. (d) Nomoto, A.; Mitsuoka, H.; Ozeki, H.; Kobuke, Y. Chem. Commun. 2003, 1074. (8) Nomoto, A.; Kobuke, Y. Chem. Commun. 2002, 1104. (9) Hasobe, T.; Imahori, H.; Yamada, H.; Sato, T.; Ohkubo, K.; Fukuzumi, S. Nano Lett. 2003, 3, 409.

10.1021/la047845z CCC: $30.25 © 2005 American Chemical Society Published on Web 03/22/2005

Energy Transfer & Photocurrent Generation of Chls

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current generation (i.e., electron transfer from pigments to electrode) was evaluated by action spectra measurements. Materials and Methods

Figure 1. Molecular structure of chlorophyllous pigments (left) and schematic representation of the energy and electron transfer in mesostructured thin films (right).

films) as adsorbents for chlorophyllous pigments10,11 and reported energy transfer between incorporated pigments in the mesopores.10b,d Light harvesting and the following energy and electron transfers in the natural photosynthesis are performed not by proteins but by pigments themselves, despite these pigments being immobilized into protein matrixes in living organisms.12 Therefore, the immobilization of organic pigments on inorganic media instead of proteins may be useful for the design of energy conversion devices. In this study, for mimicking energy transfer between chlorophyllous pigments and successive electron transfer to an indium tin oxide (ITO) electrode, novel Chl-modified ITO electrodes were fabricated. The advantages of the Chl-modified ITO electrodes over the conventional ones are as follows. (i) A conceptual approach and structure of the Chl modified electrodes are very simple, because the electrodes have only two kinds of layers (i.e., acceptor and antenna layers) which can be fabricated by grafting and spin-coating. (ii) Various antenna pigments can be introduced into the mesostructured thin films through solubilization of pigments by surfactant molecules.11,13 (iii) Because Chls in the mesostructured thin films probably absorb solar energy and the energy can be transferred to the copper C132-demethoxycarbonyl-chlorophyllide a 3-triethoxysilyl propylamide (Cu-APTES-Chl a) layer on ITO electrodes,8,9 effective solar energy collection may be expected. (iv) The energy transfer quenching of the excited state of Chls by the electrode surfaces could be suppressed, because Cu-APTES-Chl a is grafted on the ITO electrode.9,14 Here, we report the fabrication of layered silica/ surfactant mesostructured thin films containing chlorophyllous pigments on the Cu-APTES-Chl a grafted ITO electrode (Figure 1, inset). The efficiency of the photo(10) (a) Murata, S.; Hata, H.; Kimura, T.; Sugahara, Y.; Kuroda, K. Langmuir 2000, 18, 7106. (b) Murata, S.; Furukawa, H.; Kuroda, K. Chem. Mater. 2001, 13, 2722. (c) Furukawa, H.; Kuroda, K.; Watanabe, T. Chem. Lett. 2000, 1256. (d) Furukawa, H.; Watanabe, T.; Kuroda, K. Chem. Commun. 2001, 2002. (11) Hata, H.; Ogawa, M.; Sugahara, Y.; Kuroda, K. J. Sol.-Gel Sci. Technol. 2000, 19, 543. (12) (a) Jordan, P.; Fromme, P.; Witt, H. T.; Klukas, O.; Saenger, W.; Krauss, N. Nature 2001, 411, 909. (b) Liu, Z.; Yan, H.; Wang, K.; Kuang, T.; Zhang, J.; Gui, L.; An, X.; Chang, W. Nature 2004, 428, 287. (13) (a) Ogawa, M. Langmuir 1995, 11, 4639. (b) Ferrer, M.; Lianos, P. Langmuir 1996, 12, 5620. (c) Bekiari, V.; Ferrer, M.-L.; Lianos, P. J. Phys. Chem. B 1999, 103, 9085. (14) Yamada, H.; Imahori, H.; Nishimura, Y.; Yamazaki, I.; Fukuzumi, S. Chem. Commun. 2000, 1921.

Materials. The preparation of Chl derivatives was described elsewhere.10d,15 Briefly, chlorophyllous pigments were extracted from lyophilized spinach leaf tissues. To suppress pigment denaturation, all chlorophyllous pigments were pyrolyzed (Figure 1). Copper C132-demethoxycarbonyl-chlorophyllide a was condensed with 3-aminopropyltriethoxysilane (APTES) in dry dichloromethane to obtain Cu-APTES-Chl a.10d Zinc C132demethoxycarbonyl-chlorophyll b (Zn-pyroChl b) was obtained as follows: Chl b was demetalated and refluxed in 2,4,6trimethylpyridine (Wako) to remove the C132-methoxycarbonyl group. Obtained pyroPheo b was metalated with Zn(OAc)2‚2H2O (Wako) in dichloromethane/ethanol (4:1, v/v).15 All crude compounds were purified by flash column chromatography over silica gel and by preparative high-performance liquid chromatography. Preparation of Chl-Immobilized ITO Electrodes. ITO electrodes were washed successively with hot petroleum ether, hot acetone, and hot methanol and then treated with concentrated nitric acid and rinsed with pure water.16 A washed ITO substrate was soaked in dichloromethane containing 1 mM (1 M ) 1 mol dm-3) Cu-APTES-Chl a for 12 h in the dark at room temperature. The recovered substrate was washed with acetone to remove unreacted Cu-APTES-Chl a molecules and dried under a N2 atmosphere. For the formation of mesostructured thin films,17 tetramethoxysilane (TMOS) was acid-hydrolyzed in water with continuous stirring for 2 h (TMOS/H2O ) 1:2, pH ≈ 2). Then, an aqueous solution of hexadecyltrimethylammonium bromide (C16TAB) was added to the mixture [the final molar ratio: TMOS/H2O/C16TAB/HCl ) 4:120:1:(4 × 10-7)] and stirred for 5 min. The obtained sol was spin-coated onto a Cu-APTES-Chl a grafted ITO substrate (3000 rpm, 10 s), and the films were dried in air at room temperature. The mesostructured films containing Chls were prepared in a similar manner. Prescribed amounts of pyroPheo b (or Zn-pyroChl b) dissolved in acetone were mixed in surfactant solutions [pyroPheo b (or Zn-pyroChl b)/C16TAB ) 1:70, i.e., the pigment concentration in the precursor solution was 4 mM]. A schematic structure of the mesostructured thin film is illustrated in Figure 1 (inset). Instrumentation. X-ray powder diffraction (XRD) patterns were obtained with a Mac Science M03XHF22 diffractometer (Mn filtered Fe KR radiation). Diffuse reflectance visible absorption and fluorescence spectra were recorded on a Shimadzu spectrophotometer (UV-3101PC) and a Hitachi spectrofluorophotometer (F-4500), respectively. The thickness of the mesostructured films were measured using a surface roughness measuring instrument SE1700 (Kosaka Laboratory, Ltd.). The photocurrent measurement setup was essentially the same as that reported previously.4b,16 A 500-W xenon lamp of Ushio Electric, Inc. (UXL-500D-O), was used as the light source. Infrared radiation was removed with an 18-cm path length water cell. For the action spectra measurements, interference filters (Koshin Kogaku, 10-nm steps) were used, and the photon flux was measured with a power meter (Anritsu, MA9411A). A Chl-immobilized ITO electrode was mounted as a window (1 cm in diameter) of a photoelectrochemical cell. The solution containing 0.1 M sodium sulfate (Wako) and 0.03 M hydroquinone (Aldrich) as the supporting electrolyte and reductant, respectively, was used. The pH value of the electrolyte was adjusted to 6.9 with a phosphate buffer (Junsei Kagaku). The potential of the ITO electrode was controlled at 0.1 V versus Ag/AgCl by use of a Toho Technical Research potentiostat model 2000. Photocurrents were recorded with a Sony Tecktronix oscilloscope (15) Furukawa, H.; Oba, T.; Tamiaki, H.; Watanabe, T. Bull. Chem. Soc. Jpn. 2000, 73, 1341. (16) Saga, Y.; Watanabe, T.; Koyama, K.; Miyasaka, T. J. Phys. Chem. B 1999, 103, 234. (17) (a) Ogawa, M.; Igarashi, T.; Kuroda, K. Bull. Chem. Soc. Jpn. 1997, 70, 2833. (b) Ogawa, M.; Igarashi, T.; Kuroda, K. Chem. Mater. 1998, 10, 1382.

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Figure 3. Absorption spectra of (a) ITO/Cu-Chl(g)/Film(-), (b) ITO/Cu-Chl(g)/Film(Zn-Chl), and (c) ITO/Film(Zn-Chl) electrodes. Figure 2. Absorption spectra of (a) ITO/Cu-Chl(g), (b) ITO/ Cu-Chl(g)/Film(-), (c) ITO/Cu-Chl(g)/Film(Pheo), and (d) ITO/ Film(Pheo) electrodes. (TDS-340). A low-pass filter (NF Electric Instruments, E-3201B) was also used in the measurements of photocurrent action spectra.

Results Visible Absorption Spectra of Substrates. The ITO substrate grafted with Cu-APTES-Chl a was green-colored after washing with acetone [hereafter, ITO/Cu-Chl(g), where “/” represents the interface and (g) means grafted]. The contact angle between the substrate and water on the substrate changed from 15 to 60° after the modification with Cu-APTES-Chl a. This suggests that the grafted pigments are covalently bonded to the ITO substrate, because the chlorin macrocycle should be more hydrophobic than the pristine ITO surface. Figure 2a depicts a visible absorption spectrum of CuAPTES-Chl a grafted on the ITO substrate. The spectrum exhibits the broadened Soret and Qy bands at 420 and 668 nm, respectively.18 The Qy peak showed a 20-nm bathochromic shift from that of Cu-Chl a in acetone.4c,19 A similar spectral red shift was observed for porphyrin monolayers on a gold substrate6b and Chl LB films.4b,20 In general, the red shift of the absorption band from the corresponding monomeric peak indicates the aggregation of pigments. Therefore, it is reasonable to consider that chlorin macrocycles on the ITO/Cu-Chl(g) took a highly packed structure such as Chls on LB films. From the absorbance value at the Qy band, the number of chromophores on ITO/Cu-Chl(g) per unit area was estimated to be 0.8 molecules nm-2. This is near the value deduced from the area occupied by a Chl a molecule in the LB films (ca. 1 molecule nm-2),4b,c,20 though the surface of ITO is not flat. The absorption spectrum of ITO/Cu-Chl(g)/Film(-), carrying mesostructured thin films on ITO/Cu-Chl(g), showed a Qy band at 664 nm and a Soret peak at 420 nm (Figure 2b). The spectral feature was similar to that of ITO/Cu-Chl(g) because both samples contain no pigment molecule in the mesostructured film. On the other hand, (18) The elevation of the baseline in the wavelength shorter than 500 nm is probably attributable to the ITO electrode rather than CuAPTES-Chl a. (19) Jones, I. D.; White, R. C.; Gibbs, E.; Denard, C. D. J. Agric. Food Chem. 1968, 16, 80. (20) Tweet, A. G.; Bellamy, W. D.; Gaines, G. L., Jr. J. Chem. Phys. 1964, 41, 2068.

the ITO/Cu-Chl(g)/Film(Pheo) substrate, containing pyroPheo b in the mesostructured film, showed the absorption bands at 420 and 664 nm with a shoulder at around 443 nm (Figure 2c). This shoulder is attributable to the presence of pyroPheo b, indicating the formation of antenna layers on the ITO/Cu-Chl(g) substrate. As a reference, a mesostructured film containing pyroPheo b was fabricated on the ITO substrate [ITO/Film(Pheo), Figure 2d]. The ITO/Film(Pheo) substrate showed the Soret peak at around 440 nm assigned to pyroPheo b.21 It should be noted that the mesostructured thin films on ITO/Cu-Chl(g) gave distorted absorption spectra due to an optical interference effect of the thin films (Figure 2b-d). The mesostructured thin films containing Zn-pyroChl b as the antenna pigments were also fabricated (denoted as ITO/Cu-Chl(g)/Film(Zn-Chl), Figure 3b). A new absorption band at around 470 nm, as compared to the spectrum of ITO/Cu-Chl(g)/Film(-) (Figure 3a), is attributed to the Soret peak of Zn-Chl b,19 suggesting the formation of antenna layers of Zn-Chl b. This is in line with the absorption spectrum of ITO/Film(Zn-Chl) possessing the Soret and Qy peaks at 472 and 656 nm, respectively (Figure 3c). Characterization of Mesostructured Thin Films. For the pristine ITO substrate, no XRD peaks originating from the mesostructure were observed (Figure 4a). In contrast, ITO/Film(Pheo) showed a sharp diffraction peak at 3.0° (d ) 3.7 nm) with a second-order reflection (d ) 1.9 nm), which is characteristic of an ordered lamellar structure of thin films (Figure 4b).17 A similar XRD profile was obtained for the substrates of ITO/Cu-Chl(g)/Film(Pheo) (Figure 4c). This indicates that the mesostructured thin films were successfully constructed even though CuAPTES-Chls were grafted on the ITO substrates. It is worth noting that these mesostructured films also preserved their structures after electrochemical measurements. The thicknesses of the obtained thin films were roughly 0.36 µm, and the value is similar to that observed by scanning electron microscopy (data not shown). On the basis of the relationship between the XRD results and the thickness of thin films, the number of silica layers in the mesostructured films was calculated to be about 100. In the case of ITO/Cu-Chl(g)/Film(-) and ITO/Cu-Chl(g)/ (21) The broadened absorption band around 400 nm may arise from an optical interference effect of thin films, as is supported by the fact that the ITO/Film(Pheo) substrate showed an excitation peak at 443 nm.

Energy Transfer & Photocurrent Generation of Chls

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Figure 6. Photocurrent spectra of ITO/Cu-Chl(g)/Film(Pheo) (circles), ITO/Cu-Chl(g)/Film(-) (squares), and ITO/Film(Pheo) (triangles) electrodes. The experimental conditions are the same as in Figure 5. Figure 4. XRD patterns of (a) pristine ITO, (b) ITO/Film(Pheo), and (c) ITO/Cu-Chl(g)/Film(Pheo) electrodes.

Figure 5. Photocurrent spectrum of the ITO/Cu-Chl(g) electrode (circles); incident monochromatic photon flux, 2.5 × 1016 cm-2 s-1. The solid curve represents its absorption spectrum (same as in Figure 2a). (Inset) Temporal evolution of the photocurrent intensity for the ITO/Cu-Chl(g) electrode (white light, 12 mW cm-2).

Film(Zn-Chl) substrates, the XRD patterns, indicating the formation of a lamellar structure, were also observed. Photoelectrochemical Measurements. Figure 5 (inset) illustrates a typical photocurrent response pattern from the ITO/Cu-Chl(g) electrode. An anodic photocurrent was observed by the onset of illumination and the photocurrent density that slightly decreased with time. When the light was turned off, the current rapidly decreased down to the zero level. Such a behavior is essentially the same as that of the Chl monolayer deposited on a SnO2 electrode.4b,5a Similar photocurrent responses were observed for all the substrates with grafted CuAPTES-Chl a on the ITO surface. The photocurrent density under the white light radiation was larger in the order of ITO/Cu-Chl(g)/Film(Zn-Chl) > ITO/Cu-Chl(g)/Film(Pheo) > ITO/Cu-Chl(g) ≈ ITO/Cu-Chl(g)/Film(-) electrode. This trend indicates that the generation of photocurrent is not hampered by the presence of the mesostructured films on the ITO electrode, because O2 molecules possibly act as an electron carrier in the photoelectrochemical cell.22 In contrast, when we fabricated mesostructured films on the pristine ITO electrodes, their photocurrent densities were suppressed at a low level (less than 2% of that for the Cu-APTES-Chl a grafted electrode). The result implies that direct electron injection does not takes place from Chls in the mesostructured films to the ITO electrode. For the pristine ITO electrode and the mesostructured films deposited on a glass substrate, no photocurrent response was observed with turning on and off of illumination. (22) Imahori, H.; Norieda, H.; Yamada, H.; Nishimura, Y.; Yamazaki, I.; Sakata, Y.; Fukuzumi, S. J. Am. Chem. Soc. 2001, 123, 100.

Figure 7. Photocurrent spectra of ITO/Cu-Chl(g)/Film(Zn-Chl) (circles), ITO/Cu-Chl(g)/Film(-) (squares), and ITO/Film(ZnChl) (triangles) electrodes. The experimental conditions are the same as in Figure 5.

Figure 5 depicts the action spectrum of Cu-APTES-Chl a grafted on the ITO electrode. The action spectrum gave a peak at 670 nm, and the shape of the action spectrum resembled the absorption spectrum of the ITO/Cu-Chl(g) electrode (Figure 5, solid curve). A similar tendency has been reported for a Chl layer deposited on a SnO2 electrode.4b The quantum yield for photocurrent generation at the ITO/Cu-Chl(g) electrode was estimated to be 4.1% (i ) 3.7 µA cm-2, A ) 0.01, I ) 2.5 × 1016 cm-2 s-1), based on the absorbance at the Qy band (i.e., 670 nm) of the electrode, incident photon number, and observed photocurrent density. This value is similar to that of the CuChl a monolayer on a SnO2 electrode.4c When the ITO electrode carried the mesostructured films, the shape of the action spectra was also reminiscent of the shape of the corresponding absorption spectra. The action spectra of the ITO/Cu-Chl(g)/Film(-) and ITO/CuChl(g)/Film(Pheo) electrodes showed a peak at 670 nm (Figure 6) originating from the Qy band of Cu-APTES-Chl a. However, the photocurrent density of ITO/Cu-Chl(g)/ Film(Pheo) was slightly larger than that of ITO/Cu-Chl(g)/Film(-). This indicates that incorporated Chls in the mesostructured films can enlarge the photocurrent density. As a reference, the photocurrent generation of the ITO/Film(Pheo) electrode was investigated (Figure 6, triangles). Though an anodic photocurrent was obtained, the current density was drastically small as compared to that of Cu-APTES-Chl grafted on the ITO electrodes. This finding strongly suggests that almost all the anodic photocurrent is attributable to electron injection from grafted Chls to the ITO electrodes and not from Chls in the mesostructured films. The photocurrent response of the ITO/Cu-Chl(g)/Film(Zn-Chl) electrodes was basically similar to that of ITO/ Cu-Chl(g)/Film(Pheo) (Figure 7). The spectral features generally coincided with those of the absorption spectra,

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Figure 9. Mechanism of photocurrent generation from the Chl-containing mesostructured electrode. ECB and EVB denote the conduction and valence band edges, respectively.

Figure 8. (a) Difference photocurrent spectra between ITO/ Cu-Chl(g)/Film(Pheo) and ITO/Cu-Chl(g)/Film(-) electrodes. (b) Difference photocurrent spectra between ITO/Cu-Chl(g)/ Film(Zn-Chl) and ITO/Cu-Chl(g)/Film(-) electrodes. (c) Absorption spectrum of pyroPheo b containing a mesostructured film on a quartz substrate. (d) Absorption spectrum of the ZnChl b containing a mesostructured film (same as in Figure 3c).

suggesting the generation of photocurrent due to CuAPTES-Chl a grafted on the ITO electrodes. Because the photocurrent intensity of ITO/Cu-Chl(g)/Film(Zn-Chl) was larger than that of ITO/Cu-Chl(g)/Film(-), the immobilized Zn-pyroChls contribute to the generation of photocurrent to some extent. On the other hand, a photocurrent response was barely detected for the ITO/Film(Zn-Chl) electrode, pointing to a difficulty in directly injecting electrons to the ITO electrode from Zn-pyroChl b in the antenna layers. Discussion As seen in Figures 6 and 7, incorporation of chlorophyllous pigments in the mesostructured films [i.e., ITO/CuChl(g)/Film(Pheo) and ITO/Cu-Chl(g)/Film(Zn-Chl)] enlarges the photocurrent density in comparison with the ITO/Cu-Chl(g)/Film(-) electrode. Figure 8a shows the difference action spectrum obtained by subtracting the spectrum of the ITO/Cu-Chl(g)/Film(-) electrode from that of ITO/Cu-Chl(g)/Film(Pheo), and the spectrum yields the bands at 450 and 670 nm. The spectral shape of the action spectrum was similar to the absorption spectrum of pyroPheo b in the mesostructured thin film (Figure 8c). A similar trend was observed in the thin film containing Zn-pyroChl b. The difference spectrum showed the peaks at 600 and 650 nm and a shoulder around 470 nm (Figure 8b), and these peaks can be assigned to the Qx, Qy, and Soret bands of Zn-pyroChl b, respectively (Figure 8d). These findings indicate that the enhancement of photocurrent density should originate in the incorporated pigments in the antenna layers. As Figures 6 and 7 (triangles) show, direct electron transfer from the pigments in the mesostructured thin films to the ITO electrodes is negligible. Consequently, it should be reasonable to suppose that (i) absorbed light energy by the antenna pigments was transferred to Cu-APTES-Chl a grafted on the ITO electrode and that (ii) the photocurrent was generated by subsequent electron injection from excited Cu-APTES-Chl a. Figure 9 illustrates a schematic energy diagram of photoinduced energy transfer and the follow-up electron transfer. When chlorophyllous pigments immobilized in the mesostructured films (antenna layers) are excited, the energy transfers from these pigments to Cu-APTES-

Chl a grafted on the ITO substrate. Because Cu-Chls are generally nonfluorescent,4c,20 the energy level of CuAPTES-Chl a can reach the triplet excited level quickly. Namely, electron injection from the singlet excited level of Cu-APTES-Chl should be negligible. Because the level of the triplet excited level of Cu-APTES-Chl [ca. -0.24 V vs standard hydrogen electronde (SHE)4c] is higher than that of the conduction band edge of ITO (ca. 0.0 V vs SHE23),24 the energetic requirements are satisfied for electron injection from the triplet state of Cu-APTESChl. The photocurrent densities of the ITO/Cu-Chl(g)/Film(Pheo) and ITO/Cu-Chl(g)/Film(Zn-Chl) electrodes were 1.2- and 1.6-fold larger than that of the ITO/Cu-Chl(g)/ Film(-) electrode, respectively.25 This trend demonstrates that the pigments in the antenna layer improve both the photocurrent density and the photon numbers absorbed by the Chl-modified electrodes. However, the increment of the photocurrent density is not so large in comparison with the case that antenna pigments were linked with porphyrin SAMs on electrodes through hydrogen bonds or covalent bonds (ca. 5 times).8,9 The low light-harvesting properties of the antenna layers are probably attributable to the large donor/acceptor (antenna pigment/Cu-APTESChl a) ratio. Because the critical distance of the Fo¨rster type energy transfer between chlorophyllous pigments is estimated to be 10 nm (ca. three layers in the mesostructured films) or below, only a small fraction of pigments near the electrode interface can transfer excitation energy to the grafted Cu-APTES-Chl a. Indeed, the ratio of donor/ acceptor was roughly calculated to be 3:100 in the present system. This consideration is supported by the result that the emission from pyroPheo b in the mesostructured film was not quenched completely by the acceptor Cu-APTESChl a molecules. Therefore, if the concentration of donor pigments near the interface between the ITO electrode (23) Sereno, L.; Silber, J. J.; Otero, L.; del Valle Bohorquez, M.; Moore, A. L.; Moore, T. A.; Gust, D. J. Phys. Chem. 1996, 100, 814. (24) In general, the absorption and emission properties of chlorophyllous pigments are affected by the difference in their chlorin macrocycle structures.15 Because the APTES group of Cu-APTES-Chl is out of the π-electronic system, the energy level of Cu-APTES-Chl should be almost identical to that of Cu-Chl. (25) The extent of photocurrent density increase was estimated from the peak area originating from the action spectra. The difference in the enhancement ratio between the ITO/Cu-Chl(g)/Film(Pheo) and ITO/ Cu-Chl(g)/Film(Zn-Chl) electrodes could be attributed to the energy transfer efficiency from the antenna pigments to Cu-APTES-Chl a. Thus, Zn-pyroChl b possesses (i) a fluorescence quantum yield comparable to that of pyroPheo b and (ii) larger spectral overlap between the emission spectrum of Zn-pyroChl b and the absorption one of Cu-Chl a in the Qy band than in the case of pyroPheo b.19,26 (26) Watanabe, T.; Hongu, A.; Honda, K.; Nakazato, M.; Konno, M.; Saitoh, S. Anal. Chem. 1984, 56, 251.

Energy Transfer & Photocurrent Generation of Chls

and mesostructured film is increased, we can expect a much more improved efficiency for photocurrent generation. The present electrode system collects light energy by the antenna moieties and drives successive energy and electron transfers. This is essentially equivalent to the natural photosynthesis where sunlight is absorbed by the light-harvesting complexes and then transferred to the reaction center to cause charge separation.12a Though the energy transfer efficiency of the present electrodes is not so high due to the small donor/acceptor (antenna pigments/ Cu-APTES-Chl a) ratio, it is of interest that inorganicorganic thin films can act as the immobilization media, like protein matrixes, for photosynthetic pigments in higher plants. Another advantage of using such thin films can be pointed out. When the mesostructured film was formed on the Chl-modified ITO electrode, the inactivation of Chls by the electrolyte solution during electrochemical measurements was suppressed to some extent as compared with the case of a “bare” ITO/Cu-Chl(g) electrode (data not shown). Namely, the stability of acceptor Chls may be improved by coating them with the mesostructured film. Such a protection is reminiscent of the core part of the reaction center complex which is surrounded by not only a light-harvesting apparatus but also some lipids. Therefore, the present mesostructured films would exhibit a significant potential as an immobilization medium for various organic molecules. Conclusions Silica mesostructured films containing pyroPheo b or Zn-pyroChl b were fabricated on ITO/Cu-Chl(g) electrodes

Langmuir, Vol. 21, No. 9, 2005 3997

as light-harvesting antenna layers for the first time. From the increase in the photocurrent density by the presence of the antenna pigments in the thin films, energy transfer from the antenna layers to Cu-APTES-Chl a grafted on the ITO electrode and subsequent electron transfer to the ITO electrode took place. Though the photocurrent generation efficiency is not as high because the present electrode design is still at the initial stages, it is worth noting that the chlorophyllous pigments immobilized into inorganic media work as both antenna layers and a denaturation suppressor for charge-separating pigments (i.e., Cu-APTES-Chl a). This finding potentially paves the way for developing photosynthetic model systems. A study on the arrangement of Chls in the inorganic media for improving photocurrent generation efficiency is now in progress.

Acknowledgment. The authors are grateful to Prof. M. Ogawa (Waseda University) for measurements of thickness of mesostructured films and Dr. S. Yoshida and Mr. T. Atake (The University of Tokyo) for photocurrent measurements. This work was partially supported by a Grant-in-Aid for COE Research, Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. The 21st century COE program (Practical nanochemistry at Waseda University) is also acknowledged. H.F. is grateful for his JSPS Research Fellowships for Young Scientists and to Dr. A. P. Coˆte´ (University of Michigan) for his patience with the correction of English. LA047845Z