1406
Langmuir 2006, 22, 1406-1408
Light-Harvesting Energy Transfer and Subsequent Electron Transfer of Cationic Porphyrin Complexes on Clay Surfaces Shinsuke Takagi,*,† Miharu Eguchi,† Donald A. Tryk,† and Haruo Inoue*,‡ Department of Applied Chemistry, Faculty of Urban EnVironmental Sciences, Tokyo Metropolitan UniVersity, Minami-ohsawa 1-1, Hachiohji, Tokyo 192-0397, Japan, and SORST, JST (Japan Science and Technology), Tokyo 192-0397, Japan ReceiVed October 29, 2005. In Final Form: December 15, 2005 A novel energy-transfer system involving nonaggregated cationic porphyrins adsorbed on an anionic-type clay surface and the electron-transfer reaction that occurs after light harvesting are described. In the clay-porphyrin complexes, photochemical energy transfer from excited singlet zinc porphyrins to free-base porphyrins proceeds. The photochemical electron-transfer reaction from an electron donor in solution (hydroquinone) to the adsorbed porphyrin in the excited singlet state was also examined. Because the electron-transfer rate from the hydroquinone to the excited singlet free-base porphyrin is larger than that to the excited singlet zinc porphyrin, we conclude that the energy transfer accelerates the overall electron-transfer reaction.
In photosynthetic bacteria, aggregates of bacteriochlorophylls form the light-harvesting system, which plays an important role in collecting sunlight efficiently.1 A fair amount of research has been carried out in the quest for efficient artificial light-harvesting systems. Supramolecular assemblies,2-4 covalently linked systems,5-7 and dendrimer systems8-10 have been examined in this quest. These systems have demonstrated that they can play at least partial roles that are analogous to those in natural photosynthetic systems. However, challenges remain in developing simple preparative procedures and in developing ways to couple the light-harvesting event to subsequent synthetic reactions. In this letter, a novel light-harvesting system consisting of nonaggregated porphyrins on a clay surface and the electrontransfer reaction that occurs after light harvesting are described. Recently, we have developed a new technique for the conformational control of dye adsorption on clay surfaces.11-13 We have been able to prepare unique complexes in which the porphyrin molecules adsorb on clay surfaces without aggregation, even though organic molecules tends to aggregate easily on the surfaces of inorganic materials. The crucial factor for high-density adsorption of porphyrin molecules with controlled intermolecular distances is the matching of intercharge distances on the clay and porphyrin (i.e., the distance between negatively charged * Corresponding author. E-mail:
[email protected]. † Tokyo Metropolitan University. ‡ SORST. (1) Tamiaki, H. Coord. Chem. ReV. 1996, 148, 183 and references therein. (2) Prokhorenko, V. I.; Holzwarth, A. R.; Muller, M. G.; Schaffner, K.; Miyatake. T.; Tamiaki, H. J. Phys. Chem. B 2002, 106, 5761. (3) Miyatake. T.; Tamiaki, Holzwarth, A. R.; Schaffner, K. Photochem. Photobiol. 1999, 69, 448. (4) Miyatake. T.; Tamiaki, Holzwarth, A. R.; Schaffner, K. HelV. Chim. Acta 1999, 82, 797. (5) Imahori, H.; Norieda, H.; Yamada, H.; Nishimura, Y.; Yamazaki, I.; Sakata, Y.; Fukuzumi, S. J. Am. Chem. Soc. 2001, 123, 100. (6) Miller, M. A.; Lammi, R. K.; Prathapan, S.; Holten, D.; Lindsey, J. S. J. Org. Chem. 2000, 65, 6634. (7) Cho, H. S.; Song, N. W.; Kim, Y. H.; Jeoung, S. C.; Hahn, S.; Kim, D.; Kim, S. K.; Yoshida, N.; Osuka, A. J. Phys. Chem. A 2000, 104, 3287. (8) Choi, M.-S.; Aida, T.; Yamazaki, T.; Yamazaki, I. Chem.sEur. J. 2002, 8, 2667. (9) Gilat, S. L.; Adronov, A.; Frechet, Jean M. J. Angew. Chem., Int. Ed. 1999, 38, 1422. (10) Bar-Haim, A.; Klafter, J.; Kopelman, R. J. Am. Chem. Soc. 1997, 119, 6197. (11) Takagi, S.; Shimada, T.; Yui, T.; Inoue, H. Chem. Lett. 2001, 128. (12) Takagi, S.; Shimada, T.; Eguchi, M.; Yui, T.; Yoshida, H.; Tryk, D. A.; Inoue, H. Langmuir 2002, 18, 2265. (13) Takagi, S.; Tryk, D. A.; Inoue, H. J. Phys. Chem. B 2002, 106, 5455.
sites on the clay sheet and that between positively charged sites in the porphyrin moleculesthe size-matching effect11-13). In these complexes, the electron density distribution on the clay surface is the deciding factor in determining the distance between porphyrins to be 2.4 nm. In this arrangement, there is a discernible interaction between porphyrins in the ground state, and the excited-state lifetimes for the porphyrins adsorbed on clay are sufficient to participate in photochemical reactions. Therefore, these porphyrin-clay complexes should be suitable prototypes for the construction of artificial light-harvesting systems. Smecton SA (SSA, Kunimine Industries), a synthetic cationexchangeable clay that provides a unique chemical reaction environment, was used.14-18 The stoichiometric formula for SSA is [(Si7.20Al0.80) (Mg5.97Al0.03) O20 (OH)4 ]-0.77 (Na0.49Mg0.14)+0.77, the surface area is 750 m2 g-1, and the cation-exchange capacity (CEC) is 0.997 meq g-1. The average area per anionic site was calculated to be 1.25 nm2, and the average distance between anionic sites was estimated to be 1.2 nm on the basis of a hexagonal array. The free-base porphyrin tetrakis(1-methyl-4-pyridiniumyl)porphyrin (H2TMPyP) and the zinc porphyrin tetrakis(N, N, N-trimethyl-4-aniliniumyl)porphyrin (ZnTMAP) (Figure 1) were used as functional dyes.19,20 Their absorption and fluorescence spectra are easily distinguishable from each other. The clay-porphyrin complexes were prepared by mixing an aqueous SSA solution with the respective aqueous porphyrin solutions. The SSA concentration was 8.3 mg L-1 unless otherwise noted. The concentration of anionic sites in solution was calculated to be 8.3 × 10-6 eq L-1 from the cation-exchange capacity. The UV-visible spectra of the porphyrin samples show that, for each of the porphyrins, the absorption spectra for each sample type exhibited clear differences (14) Shiragami, T.; Nabeshima, K.; Matsumoto, J.; Yasuda, M.; Inoue, H. Chem. Lett. 2003, 32, 484. (15) Ogawa, M.; Kuroda, K. Chem. ReV. 1995, 95, 399. (16) Takagi, K.; Shichi, T. In Solid State and Surface Photochemistry; Ramamurthy, V., Schanze, K. S., Eds.; Marcel Dekker: New York, 2000; Vol. 5, p 31. (17) Takagi, K.; Shichi, T. J. Photochem. Photobiol., C: Photochem. ReV. 2000, 1, 113. (18) Lucia, L. A.; Yui, T.; Sasai, R.; Takagi, S.; Takagi, K.; Yoshida, H.; Whitten, D. G.; Inoue, H. J. Phys. Chem. B 2003, 107, 3789. (19) The Porphyrin Handbook; Kadish, K. M., Smith, K. M., Guilard, R., Eds.; Academic Press: New York, 1999. (20) Takagi, S.; Inoue, H. In Multimetallic and Macromolecular Inorganic Photochemistry; Ramamurthy, V., Schanze, K. S., Eds.; Marcel Decker: New York, 1999; Vol. 6, p 214.
10.1021/la052911y CCC: $33.50 © 2006 American Chemical Society Published on Web 01/14/2006
Letters
Langmuir, Vol. 22, No. 4, 2006 1407 Table 1. Parameters of the Energy Transfer and Electron Transfer to Hydroquinone of Porphyrins in Individual System (H2TMPyP-SSA and ZnTMAP-SSA) and in the Light-Harvesting System (H2TMPyP-ZnTMAP-SSA)a individual systemb H2TMPyP ZnTMAP contribution ratio for fluorescence/% KSV/M-1 kq/109 M-1 s-1
Figure 1. Tetrakis(1-methyl-4-pyridiniumyl)porphyrin (H2TMPyP) and zinc tetrakis(N,N,N-trimethyl-4-aniliniumyl)porphyrin (ZnTMAP).
Figure 2. Fluorescence spectra of SSA-H2TMPyP-ZnTMAP complexes excited at 428 nm ([SSA] ) 8.3 mg L-1, [H2TMPyP] + [ZnTMAP] ) 15.4%, [H2TMPyP] ) [ZnTMAP] vs the CEC of the clay). The concentration of hydroquinone was varied over the range of 0-0.41 M. max from each other (λHmax ) 450 nm and λZnTMAP ) 428 nm). 2TMPyP As reported before,11-13 the complexes were identified as those in which SSA exists as either single, completely nonassociated sheets or very loosely associated sheets and the porphyrins coadsorb on the clay surfaces. It was found that aggregation is completely inhibited and that the porphyrin exists as a single molecular unit for every loading level (0-100% vs CEC of the clay). The excitation wavelength was set at 428 nm, which is the λmax of ZnTMAP, for all experiments. The contribution ratio for the fluorescence of the respective porphyrins was obtained via fitting of the overall spectral shape because the individual fluorescence spectra are clearly distinguishable from each other max (λHmax ) 689 and 752 nm and λZnTMAP ) 624 and 666 nm). 2TMPyP When the total loading level of [porphyrin] () [ZnTMAP] + [H2TMPyP], [ZnTMAP] ) [H2TMPyP]) was 15.4% versus the CEC of the clay, the fluorescence contribution of H2TMPyP was 83%, despite mostly selective excitation of ZnTMAP. Under such conditions, the photochemical electron transfer from excited singlet porphyrin molecules adsorbed on the clay surfaces to electron donors in the aqueous solution was examined by the means of fluorescence quenching analysis. First, the characteristics of the electron transfer between each of the porphyrins and the electron donor in the solution were examined as reference experiments, without light-harvesting. H2TMPyP-SSA and ZnTMAP-SSA complexes, in which the loading level of porphyrin was 7.7%, were used for the fluorescence quenching experiments. Hydroquinone (HQ, Eox ) 0.234 V vs Ag/AgCl)21 was used as the electron donor. The fluorescence of both excited singlets, H2TMPyP (Ered ) -0.07 V vs Ag/AgCl)22 and ZnTMAP (Ered ) -1.02 V vs NHE)23 was quenched by the addition of HQ,
(21) Encyclopedia of Electrochemistry of the Elements; Bard, A. J., Lund, H., Eds.; Marcel Dekker: New York, 1973; Organic Section, Vol. XI.
100 68.5 18
100 4.0 5.7
light-harvesting systemc H2TMPyP 83 62.2 16
ZnTMAP 17 3.0 d
[SSA] ) 8.3 mg L-1. b Loading level of porphyrin is 7.7% vs CEC of the clay. c Loading level of porphyrin (H2TMPyP + ZnTMAP) is 15.4% vs CEC of the clay ([H2TMPyP] ) [ZnTMAP]). d The kq value of excited singlet ZnTMAP cannot be obtained because the excited singlet lifetime of ZnTMAP in the H2TMPyP-ZnTMAP-SSA system is unknown. a
Figure 3. Stern-Volmer plots for electron transfer from an excited singlet porphyrin to hydroquinone. Plots at 624 and 689 nm correspond to ZnTMAP and H2TMPyP, respectively.
and linear Stern-Volmer plots were obtained with the SternVolmer constants KSV ) 68.5 M-1 for H2TMPyP and 4.0 M-1 for ZnTMAP. From the KSV values and the singlet lifetimes of the metal-free and zinc porphyrins,12 the rate constants of electron transfer were estimated to be 1.8 × 1010 and 5.7 × 109 M-1 s-1, respectively. The HQ molecule was considered to diffuse and collide with porphyrin molecules adsorbed on the clay surface without restraint. These results indicate that the clay sheets are not stacked in the clay-porphyrin complexes. These fluorescence quenching experiments demonstrate that photochemical reactions that involve the diffusion process can be carried out efficiently in the clay-porphyrin complexes. Post-light-harvesting electron transfer between the porphyrins and HQ in the solution was examined. In this reaction system, TMAP is the light absorber and energy donor, and H2TMPyP is the energy acceptor. The total loading level of the porphyrin was set at 15.4% in the fluorescence quenching experiment. The fluorescence spectra of the samples with 428 nm excitation are shown in Figure 2. From the curve fitting of the fluorescence spectra without HQ, the contribution ratio of each porphyrin to the fluorescence can be obtained. The contribution ratio would be 50% in the case without energy transfer between porphyrins; however, the contribution of H2TMPyP was calculated to be 83% at the 15.4% porphyrin loading level, as shown in Table 1. The fluorescence of both excited singlets in the light-harvesting system (i.e., H2(22) Oliver Su, Y.; Kuwata, T.; Chen, S.-M. J. Electroanal. Chem. 1990, 288, 177. (23) Kalyanasundaram, K. J. Chem. Soc., Faraday Trans. 2 1983, 79, 1365.
1408 Langmuir, Vol. 22, No. 4, 2006
Figure 4. Schematic illustration of light harvesting and the subsequent electron-transfer reaction in clay-porphyrin complexes.
TMPyP and ZnTMAP) was quenched by HQ. When the HQ concentration was 0.41 M, the spectral shape was almost the same as that of ZnTMAP. Linear Stern-Volmer plots at 689 nm for H2TMPyP and 624 nm for ZnTMAP were obtained, as shown in Figure 3. The KSV values and electron-transfer rate constants obtained are summarized in Table 1. The quenching rate constant of H2TMPyP in the light-harvesting system was in good agreement with that of H2TMPyP alone. The series of experiments described here demonstrates this novel and unique light-harvesting process and the subsequent electron-transfer reaction. In the present light-
Letters
harvesting system, the energy transfer from the excited singlet ZnTMAP to H2TMPyP, which is a more effective sensitizer for electron transfer with HQ, and the efficient fluorescence quenching, by electron transfer from HQ in the solution, of the energy-accepted H2TMPyP were observed by the irradiation of ZnTMAP, as shown in Figure 4. It should be noted here that the total electron-transfer efficiency in the light-harvesting system was larger than that for ZnTMAP alone. Thus, these clayporphyrin complexes should be promising as architectural components in light-harvesting systems. In the future, it may be fascinating to increase the dye component in the clay-dye complex for the construction of even more effective artificial light-harvesting systems. Acknowledgment. This work has been partially supported by a Grant-in-Aid for Exploratory Research and Scientific Research on Priority Areas (417) and a Grant-in-Aid for Young Scientists (B) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of the Japanese Government. Supporting Information Available: Absorption and fluorescence spectra of cationic porphyrins with clay. This material is available free of charge via the Internet at http://pubs.acs.org. LA052911Y
