NANO LETTERS
Enhancement of Light Harvesting and Photocurrent Generation by ITO Electrodes Modified with meso,meso-Linked Porphyrin Oligomers
2003 Vol. 3, No. 3 409-412
Taku Hasobe,† Hiroshi Imahori,*,‡ HirokoYamada,† Tomoo Sato,‡ Kei Ohkubo,† and Shunichi Fukuzumi*,† Department of Material and Life Science, Graduate School of Engineering, Osaka UniVersity, CREST, Japan Science and Technology Corporation, Suita, Osaka 565-0871, Japan, and Department of Molecular Engineering, Graduate School of Engineering, Kyoto UniVersity, PRESTO, Japan Science and Technology Corporation (JST), Sakyo-ku, Kyoto 606-8501, Japan, and Fukui Institute for Fundamental Chemistry, Kyoto UniVersity, 34-4, Takano-Nishihiraki-cho, Sakyo-ku, Kyoto 606-8103, Japan Received December 27, 2002; Revised Manuscript Received January 23, 2003
ABSTRACT ITO electrodes modified with a meso,meso-linked porphyrin oligomer have been prepared for the first time to achieve improved light-harvesting efficiency and a larger quantum yield for photocurrent generation as compared to the corresponding porphyrin monomer system. An increase in the number of the porphyrins in meso,meso-linked porphyrins results in an improvement of the photoenergy conversion efficiency per mol cm-2 in the visible light region as well as a larger quantum yield of photocurrent generation. The relative integrated values of the photocurrents for all of the frequencies in the action spectrum per mol cm-2 in the porphyrin dimer and tetramer systems are determined to be 2.5 and 6.5, respectively, as compared with the values of the porphyrin monomer system.
Self-assembled monolayers (SAMs) of porphyrins have been frequently employed in artificial photosynthetic systems1-10 because they are highly promising in constructing welldefined molecular assemblies on metal electrodes. In these systems, however, there remain two difficult problems to be solved: one is a strong energy-transfer (EN) quenching of the excited states of chromophores by the metal surface, which has precluded the achievement of a high quantum yield for charge separation (CS) on the metal surface as attained in photosynthesis, and the other is the poor light-harvesting efficiency of monolayers, which results in low values of the incident photon-to-photocurrent efficiency (IPCE). These problems may be surmounted by preparing indium-tin oxide (ITO) electrodes modified with meso,meso-linked porphyrin arrays11-14 since ITO can suppress the EN quenching on the surface15,16 and meso,meso-linked porphyrin arrays11-14 can absorb visible light more widely than a linear combination of the corresponding porphyrin monomer because of the exciton coupling of the porphyrins. However, there has so * Corresponding authors. E-mail:
[email protected];
[email protected]. † Osaka University. ‡ Kyoto University. 10.1021/nl025974z CCC: $25.00 Published on Web 02/20/2003
© 2003 American Chemical Society
far been no example in which meso,meso-porphyrin arrays are applied to photoenergy conversion systems on electrodes. We report herein the first preparation of ITO electrodes modified with meso,meso-linked porphyrin oligomers (denoted as (H2P)2/ITO and (H2P)4/ITO) and the much improved photoelectrochemical properties in comparison with those of the ITO electrode modified with porphyrin monomer (denoted as H2P/ITO). (See Figure 1.) The general strategy employed for the preparation of porphyrin references and SAMs is described in Schemes 1 and 2. Starting materials, H2P-ref and 2 were synthesized previously.14 Meso,mesolinked porphyrin dimer ((H2P)2-ref) was synthesized by a cross-coupling reaction.17 To improve the solubility, the isoamyloxy (i-AmO) groups were introduced at meta positions of meso phenyl groups in the porphyrin tetramer ((H2P)4-ref). Preparations of H2P/ITO, (H2P)2/ITO, and (H2P)4/ITO were carried out according to our previously reported method.15,16 Figure 2a displays absorption spectra of H2P/ITO and H2P-ref in THF. The Soret band of H2P/ITO becomes broader than that of H2P-ref in THF, whereas no significant change is seen in the Q bands. The λmax value of the Soret
Scheme 1.
Synthesis of Reference Compounds in This Study
Figure 1. Porphyrin compounds employed in this study.
band of H2P/ITO (418 nm) is red shifted by 6 nm as compared with that of H2P-ref in THF (412 nm). Similar red shifts are also seen for the λmax values of (H2P)2/ITO (424, 453 nm) relative to those of (H2P)2-ref (413, 448 nm) in THF and for the λmax values of (H2P)4/ITO (418, 486 nm) relative to those of (H2P)4-ref (411, 478 nm) in THF, respectively (Figure 2b,c). This indicates that the porphyrin environment of H2P/ITO, (H2P)2/ITO, and (H2P)4/ITO is perturbed significantly because of the aggregation.1The sharp splitting of the Soret bands (∼420 nm and ∼450 or ∼480 nm) is characteristic of the meso,meso-linked porphyrins.11-14 More importantly, the porphyrin tetramer exhibits a larger absorptivity in the Q band region relative to that of the porphyrin monomer and dimer. This enables us to harvest the light widely across the visible region, as compared with the harvesting region for porphyrin monomers. Integration of the area of the absorption spectra provides an estimate of the surface coverage of the porphyrin (Γ),18,19 after correcting for the surface roughness (roughness factor ) 1.3).16 The Γ value (mol cm-2) decreases in the order of H2P/ITO (8.5 × 10-11), (H2P)2/ITO (6.1 × 10-11), and (H2P)4/ITO (1.7 × 10-11).20 This implies that the surface coverage of the porphyrins is largely dependent on the steric hindrance around the porphyrins.21 Photoelectrochemical measurements were performed in an argon-saturated 0.1 M Na2SO4 aqueous solution containing 50 mM triethanolamine (TEA) acting as an electron sacrificer, using H2P/ITO, (H2P)2/ITO, and (H2P)4/ITO as the working electrode, a platinum counter electrode, and an Ag/ AgCl (sat. KCl) reference electrode, respectively (hereafter represented by ITO/H2P/TEA/Pt, ITO/(H2P)2/TEA/Pt, and ITO/(H2P)4/TEA/Pt, respectively, where / denotes an interface). An increase in the anodic photocurrent was observed with an increase in the positive bias (from -200 to 400 mV) to the gold electrode. The action spectra for photocurrent generation largely agree with the absorption spectra of H2P/ ITO, (H2P)2/ITO, and (H2P)4/ITO within the range of 380410
Scheme 2.
Preparation of Porphyrin SAMs in This Study
Nano Lett., Vol. 3, No. 3, 2003
Figure 2. Absorption spectra of (a) H2P/ITO and H2P-ref in THF and (b) (H2P)2/ITO and (H2P)2-ref in THF and (c) (H2P)4/ITO and (H2P)4-ref in THF. The spectra are normalized at the Soret band for comparison. Action spectra of (a) ITO/H2P/TEA/Pt system and (b) ITO/(H2P)2/TEA/Pt system and (c) ITO/(H2P)4/TEA/Pt system; input power: 500 µW cm-2; applied potential: +0.40 V vs Ag/ AgCl (sat.KCl); an argon-saturated 0.1 M Na2SO4 aqueous solution containing 50 mM TEA. Excitation spectra of (a) H2P/ITO and (b) (H2P)2/ITO and (c) (H2P)4/ITO.
700 nm (Figure 2).22 Such an agreement shows clearly that the porphyrin is the photoactive species responsible for the photocurrent generation. The action spectra are in good agreement with the excitation spectra on ITO. These results reveal that the anodic photocurrent flows from the electrolyte to the ITO electrode via the excited state of the porphyrin SAM. The quantum yields (φ) of photocurrent generation were determined for the ITO/H2P/TEA/Pt, ITO/(H2P)2/TEA/Pt, and ITO/(H2P)4/TEA/Pt systems at an applied potential of Nano Lett., Vol. 3, No. 3, 2003
+0.40 V versus Ag/AgCl (sat. KCl) using the input power (λ ) 419.5 ( 5.3 nm light of 500 µW cm-2), the photocurrent density, and the absorbance on the electrodes (ITO/H2P/TEA/Pt system: i ) 150 nA cm-2, A ) 0.022; ITO/(H2P)2/TEA/Pt system: i ) 140 nA cm-2, A ) 0.014; ITO/(H2P)4/TEA/Pt system: i ) 87 nA cm-2, A ) 0.007).23 The φ values of ITO/H2P/TEA/Pt, ITO/(H2P)2/TEA/Pt, and ITO/(H2P)4/TEA/Pt systems are 1.7 ( 0.3%, 2.6 ( 0.4%, and 3.2 ( 0.4%, respectively. This trend demonstrates that an increase in the number of porphyrins in the porphyrin oligomers improves the photocurrent generation efficiency. To evaluate the total incident light-to-current generation efficiency, the integrated values of the photocurrents for all of the frequencies in the action spectra per mol cm-2 of the ITO/(H2P)2/TEA/Pt and ITO/(H2P)4/TEA/Pt systems are also determined to be 2.5 and 6.5 relative to that of the ITO/ H2P/TEA/Pt system under the same experimental conditions.24 This reveals that the photoenergy conversion efficiency and the light-harvesting properties of ITO/(H2P)2/ TEA/Pt and ITO/(H2P)4/TEA/Pt systems are significantly improved as compared with those of the ITO/H2P/TEA/Pt system. Taking into account the above results together with the well-established photodynamics of porphyrin-linked systems on electrodes,25 the mechanism of photocurrent generation in ITO/(H2P)n/TEA/Pt system (n ) 1, 2, and 4) is summarized as follows. An electron transfer (ET) takes place from TEA (+0.61 V vs Ag/AgCl (sat. KCl))15 to the singlet excited state 1H2P* (+0.77 V vs Ag/AgCl (sat. KCl)) or 1 (H2P)2* (+0.79 V vs Ag/AgCl (sat. KCl) or 1(H2P)4* (+0.84 V vs Ag/AgCl (sat. KCl)), yielding the porphyrin radical anion (H2P•- or (H2P)2•- or (H2P)4•-)26 and the TEA radical cation (TEA•+). The occurrence of photoinduced electron transfer from TEA to 1H2P* was confirmed by the laserflash photolysis experiments. (See Supporting Information S2.27) The resulting TEA•+ is rearranged to its reducing form by H-atom abstraction from another TEA.28 Such an irreversible transformation of TEA•+ prevents the back electron transfer from H2P•- to TEA•+, resulting in efficient anodic photocurrent generation. In conclusion, we have successfully constructed novel photoelectrochemical systems including ITO electrodes modified with meso,meso-linked porphyrin oligomers in which the relative integrated values of photocurrents for all of the frequencies in the action spectra per mol cm-2 as well as the quantum yields of photocurrent generation are improved as compared with those of the corresponding porphyrin monomer. Further improvements in the photoelectrochemistry may be made possible by increasing the number of porphyrins in the present systems. Acknowledgment. This work was partially supported by a Grant-in-Aid for Scientific Research and the Development of Innovative Technology (no. 12310) and a Grant-in-Aid for the Scientific Research Priority Area (no. 11228205) from the Ministry of Education, Culture, Sports, Science and Technology, Japan. H.I. thanks the Nagase Science and Technology Foundation for financial support. 411
Supporting Information Available: AFM image of (H2P)4/ITO, transient absorption spectra observed in photoinduced electron transfer from TEA to 1H2P*, and UVvis spectrum of H2P•-. This material is available free of charge via the Internet at http://pubs.acs.org. References (1) 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. (2) Imahori, H.; Norieda, H.; Yamada, H.; Nishimura, Y.; Yamazaki, I.; Sakata, Y.; Fukuzumi, S. J. Am. Chem. Soc. 2001, 123, 100. (3) Imahori, H.; Hasobe, T.; Yamada, H.; Kamat, P. V.; Barazzouk, S.; Fujitsuka, M.; Ito, O.; Fukuzumi, S. Chem. Lett. 2001, 784. (4) Guldi, D. M.; Pellarini, F.; Prato, M.; Granito, C.; Troisi, L. Nano Lett. 2002, 2, 965. (5) Nomoto, A.; Kobuke, Y. Chem. Commun. 2002, 1104. (6) Uosaki, K.; Kondo, T.; Zhang, X.-Q.; Yanagida, M. J. Am. Chem. Soc. 1997, 119, 8367. (7) Morita, T.; Kimura, S.; Imanishi, Y. J. Am. Chem. Soc. 1999, 121, 581. (8) 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. (9) Gryko, D. T.; Clausen, C.; Roth, K. M.; Dontha, N.; Bocian, D. F.; Kuhr, W. G.; Lindsey, J. S. J. Org. Chem. 2000, 65, 7345. (10) Yamada, T.; Hashimoto, T.; Kikushima, S.; Ohtsuka, T.; Nango, M. Langmuir 2001, 17, 4634. (11) Osuka, A.; Shimidzu, H. Angew. Chem., Int. Ed. Engl. 1997, 36, 135. (12) Bonifazi, D.; Diederich, F. Chem. Commun. 2002, 2178. (13) Susumu, K.; Shimidzu, T.; Tanaka, K.; Segawa, H. Tetrahedron Lett. 1996, 37, 8399. (14) Imahori, H.; Tamaki, K.; Araki, Y.; Sekiguchi, Y.; Ito, O.; Sakata Y.; Fukuzumi, S. J. Am. Chem. Soc. 2002, 124, 5165. (15) Yamada, H.; Imahori, H.; Nishimura, Y.; Yamazaki, I.; Fukuzumi, S. Chem. Commun. 2000, 1921. (16) Yamada, H.; Imahori, H.; Nishimura, Y.; Yamazaki, I.; Fukuzumi, S. AdV. Mater. 2002, 14, 892. (17) Aratani, N.; Osuka, A. Org. Lett. 2001, 3, 4213. (18) Mccallien, D. W. J.; Burn, P. L.; Anderson, H. L. J. Chem. Soc., Perkin Trans. 1 1997, 2581. (19) The extinction coefficients of (H2P)n in THF are 530 000 M-1 cm-1 (412 nm), 280 000 M-1 cm-1 (413 nm), and 450 000 M-1 cm-1 (411 nm) in (H2P)n-ref for n ) 1, 2, and 4, respectively.
412
(20) A somewhat smaller Γ value (3.5 × 10-11 mol cm-2) was obtained from the cyclic voltammogram of (H2P)2/ITO by dividing the first anodic peak currents of the porphyrin by the Faraday constant and roughness factor ) 1.3. Because of the larger experimental error involved in the CV measurements, the Γ values were determined from the area of the absorption spectra. (21) Our tentative AFM observation seems to imply that (H2P)4 molecules lie on the surface and make the striped structure. (See Supporting Information S1.) This may account for the low Γ value of (H2P)4/ ITO. However, the insufficiently smooth ITO surface has precluded the definitive characterization of the structure of (H2P)4 on ITO. (22) It was confirmed that there was no photocurrent generation in the blank experiment with TEA alone. The photocurrent generation is stable under the present experimental conditions, although the current decreases by about 30% at prolonged irradiation times (several tens of minutes). The difference between the action spectrum and the absorption spectrum at 470 nm may be attributed to the difference in the electron-transfer rate of the splitting S1 states judging from the difference between the absorption and excitation spectra. (23) The lamp intensity at each wavelength was determined by an optical power meter (Anritsu ML 9002A) and corrected. (24) The area of the action spectra was calculated using the wavenumber unit. The values were divided by the surface coverage to give the integrated value of the action spectrum per mol cm-2. The IPCE value of the ITO/(H2P)4/TEA/Pt system is determined to be 0.051% (illuminated at 420 nm), which is not improved as compared with the value of the corresponding monomer system (0.089%, illuminated at 420 nm) due to the smaller surface coverage. Although the lightharvesting efficiency per mol cm-2 has been improved significantly by introducing the porphyrin tetramer, the surface coverage remains to be improved. (25) Imahori, H.; Fukuzumi, S. AdV. Mater. 2001, 13, 1197. (26) The first reduction potentials of H2P, (H2P)2, and (H2P)4, which are equivalent to the one-electron oxidation potentials of the corresponding radical anions, were determined by differential pulse voltammetry to be -1.16 V, -1.11 V and -1.03 V vs Ag/AgCl (sat. KCl), respectively. (27) After the triplet-triplet absorption decay at 430 nm, the residual absorption band due to H2P•- was clearly observed at 1.5 ms. (See Supporting Information S2.) The absorption spectrum of H2P•- was measured independently by the reduction of H2P by the benzophenone radical anion. (See Supporting Information S3.) (28) (a) Chan, S.-F.; Chou, M.; Creutz, C.; Matsubara, T.; Sutin, N. J. Am. Chem. Soc. 1981, 103, 369. (b) Matsuoka, S.; Yamamoto, K.; Ogata, T.; Kusaba, M.; Nakashima, N.; Fujita, E.; Yanagida, S. J. Am. Chem. Soc. 1993, 115, 601.
NL025974Z
Nano Lett., Vol. 3, No. 3, 2003
