Structure and Photophysical Properties of Porphyrin-Modified Metal

Langmuir , 2004, 20 (1), pp 73–81 .... Dynamic and Static Quenching of Fluorescence by 1−4 nm Diameter Gold ..... Gold(0) Porphyrins on Gold Nanop...
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Langmuir 2004, 20, 73-81

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Structure and Photophysical Properties of Porphyrin-Modified Metal Nanoclusters with Different Chain Lengths Hiroshi Imahori,*,† Yukiyasu Kashiwagi,‡ Yoshiyuki Endo,‡ Takeshi Hanada,§ Yoshinobu Nishimura,| Iwao Yamazaki,*,| Yasuyuki Araki,⊥ Osamu Ito,*,⊥ and Shunichi Fukuzumi*,‡ Department of Molecular Engineering, Graduate School of Engineering, Kyoto University, PRESTO, Japan Science and Technology Agency (JST), Katsura, Nishikyo-ku, Kyoto 615-8510, Japan, Fukui Institute for Fundamental Chemistry, Kyoto University, 34-4, Takano-Nishihiraki-cho, Sakyo-ku, Kyoto 606-8103, Japan, Department of Material and Life Science, Graduate School of Engineering, Osaka University, CREST, JST, Suita, Osaka 565-0871, Japan, The Institute of Scientific and Industrial Research, Osaka University, 8-1 Mihoga-oka, Ibaraki, Osaka 567-0047, Japan, Department of Molecular Chemistry, Graduate School of Engineering, Hokkaido University, Sapporo 060-8628, Japan, and Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, CREST, JST, Katahira, Aoba-ku, Sendai 980-8577, Japan Received August 6, 2003. In Final Form: September 23, 2003 Three-dimensional porphyrin-monolayer-protected gold clusters with different chain lengths (MPCs) have been prepared to examine the structure and photophysical properties, in comparison with selfassembled monolayers (SAMs) of the porphyrins on a flat gold surface. The three-dimensional porphyrin MPCs exhibit electrochemical and photophysical properties that are much closer to those of a porphyrin reference compound in solution than those of two-dimensional porphyrin SAMs on the flat gold surface. The three-dimensional architectures of porphyrin MPCs with large surface area have improved the lightharvesting efficiency relative to the corresponding porphyrin SAM on the two-dimensional flat gold surface. Time-resolved single photon counting fluorescence and transient absorption spectroscopic studies have demonstrated that undesirable quenching of the porphyrin excited singlet state via energy transfer to the gold surface of the three-dimensional MPCs is much suppressed, as compared to the quenching of the porphyrin SAMs on the two-dimensional flat gold surface. Both the quenching rate constants of the porphyrin excited singlet state by the surfaces of bulk gold and gold nanoclusters reveal weak chain length dependence of the energy transfer quenching.

Artificial multi-porphyrin systems have attracted much attention in relation to natural photosystems such as lightharvesting complexes and photosynthetic reaction center and molecular electronics including photonic and electronic molecular devices.1,2 In this context, a number of covalently linked multi-porphyrin systems have been prepared to understand the factors controlling photosynthetic energy transfer (EN) and electron transfer (ET) * To whom correspondence should be addressed. E-mail: [email protected]; [email protected]; ito@tagen. tohoku.ac.jp; [email protected] † Department of Molecular Engineering, Graduate School of Engineering, Kyoto University, PRESTO, Japan Science and Technology Agency (JST), and Fukui Institute for Fundamental Chemistry, Kyoto University. ‡ Department of Material and Life Science, Graduate School of Engineering, Osaka University, CREST, JST. § The Institute of Scientific and Industrial Research, Osaka University. | Department of Molecular Chemistry, Graduate School of Engineering, Hokkaido University. ⊥ Institute for Multidisciplinary Research for Advanced Materials, Tohoku University, CREST, JST. (1) (a) Anoxygenic Photosynthetic Bacteria; Blankenship, R. E., Madigan, M. T., Bauer, C. E., Eds.; Kluwer Academic: Dordrecht, 1995. (b) The Photosynthetic Reaction Center; Deisenhofer, J., Norris, J. R., Eds.; Academic Press: San Diego, 1993. (c) McDermott, G.; Prince, S. M.; Freer, A. A.; Hawthornthwaite-Lawless, A. M.; Papiz, M. Z.; Cogdell, R. J.; Isaacs, N. W. Nature 1995, 374, 517. (2) Molecular Electronics; Jortner, J., Ratner, M., Eds.; Blackwell: London, 1997.

processes and also to obtain valuable information about the realization of molecular electronics.3-10 These include porphyrins linked by saturated or unsaturated linkages, as well as planar fully conjugated systems, and directly meso-meso linked oligomers.3-10 These porphyrin oligo(3) (a) Vincente, M. G. H.; Jaquinod, L.; Smith, K. M. J. Chem. Soc., Chem. Commun. 1999, 1771. (b) Anderson, H. L. Chem. Commun. 1999, 2323. (c) Gust, D.; Moore, T. A. In The Porphyrin Handbook, Vol. 8; Kadish, K. M., Smith, K., Guilard, R., Eds.; Academic Press: San Diego, CA, 2000; pp 153-190. (d) Aratani, N.; Osuka, A.; Cho, H. S.; Kim, D. J. Photochem. Photobiol. C 2002, 3, 25. (e) Wasielewski, M. R. Chem. Rev. 1992, 92, 435. (4) (a) Crossley, M. J.; Burn, P. L. J. Chem. Soc., Chem. Commun. 1991, 1569. (b) Crossley, M. J.; Govenlock, L. J.; Prashar, J. K. J. Chem. Soc., Chem. Commun. 1995, 2379. (c) Officer, D. L.; Burrell, A. K.; Reid, D. C. W. J. Chem. Soc., Chem. Commun. 1996, 1657. (5) (a) Maruyama, K.; Osuka, A. Pure Appl. Chem. 1990, 62, 1511. (b) Osuka, A.; Tanabe, N.; Nakajima, S.; Maruyama, K. J. Chem. Soc., Perkin Trans. 2 1996, 199. (c) Kim, Y. H.; Jeong, D. H.; Kim, D.; Jeoung, S. C.; Cho, H. S.; Kim, S. K.; Aratani, N.; Osuka, A. J. Am. Chem. Soc. 2001, 123, 76. (d) Tsuda, A.; Osuka, A. Science 2001, 293, 79. (e) Cho, H. S.; Jeong, D. H.; Cho, S.; Kim, D.; Matsuzaki, Y.; Tanaka, K.; Tsuda, A.; Osuka, A. J. Am. Chem. Soc. 2002, 124, 14642. (6) (a) Lin, V. S.-Y.; DiMagno, S. G.; Therien, M. J. Science 1994, 264, 1105. (b) Lin, V. S.-Y.; Therien, M. J. Chem.sEur. J. 1995, 1, 645. (c) Fletcher, J. T.; Therien, M. J. J. Am. Chem. Soc. 2000, 122, 12393. (d) Susumu, K.; Therien, M. J. J. Am. Chem. Soc. 2002, 124, 8550. (7) (a) Anderson, H. L.; Martin, S. J.; Bradley, D. D. C. Angew. Chem., Int. Ed. Engl. 1994, 33, 655. (b) Anderson, H. L. Inorg. Chem. 1994, 33, 972. (c) Anderson, H. L.; Anderson, S.; Sanders, J. K. M. J. Chem. Soc., Perkin Trans. 1 1995, 2231. (d) Screen, T. E. O.; Thorne, J. R. G.; Denning, R. G.; Bucknall, D. G.; Anderson, H. L. J. Am. Chem. Soc. 2002, 124, 9712. (e) Mak, C. C.; Bampos, N.; Sanders, J. K. M. Angew. Chem., Int. Ed. 1998, 37, 3020.

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mers seem to be superior to the corresponding porphyrin monomers with respect to the structural control and photophysical properties, but the synthetic difficulty has precluded development of such porphyrin oligomers as artificial photosynthetic and molecular electronic materials. Another promising approach for achieving these goals is the self-assembly of porphyrin-bearing molecular recognition units.11,12 These porphyrin self-assemblies are prepared easily but often afford incomplete structural control and stability. We13,14 and others15,16 have focused on self-assembled monolayers (SAMs) of porphyrins on flat gold substrates or equivalents, because they can provide densely packed, highly ordered structures of porphyrins on two-dimensional gold electrodes suitable for developing artificial photosynthetic materials. However, the light-harvesting efficiency in the two-dimensional (2D) systems has so far been limited due to the porphyrin monolayer exhibiting poor light absorptivity. On the other hand, metal nanoclusters have attracted widespread interest, since their nanosize physical properties are quite different from those of the bulk materials depending upon their size, shape, and packing density.17,18 The potential (8) (a) Wagner, R. W.; Johnson, T. E.; Lindsey, J. S. J. Am. Chem. Soc. 1996, 118, 11166. (b) Li, J.; Ambroise, A.; Yang, S. I.; Diers, J. R.; Seth, J.; Wack, C. R.; Bocian, D. F.; Holten, D.; Lindsey, J. S. J. Am. Chem. Soc. 1999, 121, 8927. (c) Lammi, R. K.; Ambroise, A.; Balasubramanian, T.; Wagner, R. W.; Bocian, D. F.; Holten, D.; Lindsey, J. S. J. Am. Chem. Soc. 2000, 122, 7579. (d) Kuciauskas, D.; Liddell, P. A.; Lin, S.; Johnson, T. E.; Weghorn, S. J.; Lindsey, J. S.; Moore, A. L.; Moore, T. A.; Gust, D. J. Am. Chem. Soc. 1999, 121, 8604. (9) (a) Arnold, D. P.; Heath, G. A. J. Am. Chem. Soc. 1993, 115, 12197. (b) Wytko, J.; Berl, V.; McLaughlin, M.; Tykwinski, R. R.; Schreiber, M.; Diederich, F.; Boudon, C.; Gisselbrecht, J.-P.; Gross, M. Helv. Chim. Acta 1998, 81, 1964. (c) Sugiura, K.; Tanaka, H.; Matsumoto, T.; Kawai, T.; Sakata, Y. Chem. Lett. 1999, 1193. (d) Ruhlmann, L.; Schulz, A.; Giraudeau, A.; Messerschmidt, C.; Fuhrhop, J.-H. J. Am. Chem. Soc. 1999, 121, 6664. (10) (a) Kato, T.; Maruo, N.; Akisada, H.; Arai, T.; Nishino, N. Chem. Lett. 2000, 890. (b) Yeow, E. K. L.; Ghiggino, K. P.; Reek, J. N. H.; Crossley, M. J.; Bosman, A. W.; Schenning, A. P. H. J.; Meijer, E. W. J. Phys. Chem. B 2000, 104, 2596. (c) Choi, M.-S.; Aida, T.; Yamazaki, T.; Yamazaki, I. Chem.sEur. J. 2002, 8, 2667. (11) (a) Haycock, R. A.; Yartsev, A.; Michelsen, U.; Sundstro¨m, V.; Hunter, C. A. Angew. Chem., Int. Ed. 2000, 39, 3616. (b) Haycock, R. A.; Hunter, C. A.; James, D. A.; Michelsen, U.; Sutton, L. R. Org. Lett. 2000, 2, 2435. (c) Ogawa, K.; Kobuke, Y. Angew. Chem., Int. Ed. 2000, 39, 4070. (d) Tamiaki, H.; Miyatake, T.; Tanikaga, R.; Holzwarth, A. R.; Schaffner, K. Angew. Chem., Int. Ed. Engl. 1996, 35, 772. (12) (a) Drain, C. M.; Lehn, J.-M. J. Chem. Soc., Chem. Commun. 1994, 2313. (b) Fan, J.; Whiteford, J. A.; Olenyuk, B.; Levin, M. D.; Stang, P. J.; Fleischer, E. B. J. Am. Chem. Soc. 1999, 121, 2741. (c) Kumar, R. K.; Balasubramanian, S.; Goldberg, I. Inorg. Chem. 1998, 37, 541. (d) Imamura, T.; Fukushima, K. Coord. Chem. Rev. 2000, 198, 133. (13) (a) Fukuzumi, S.; Imahori, H. In Electron Transfer in Chemistry; Balzani, V., Ed.; Wiley-VCH: Weinheim, 2001; Vol. 2, pp 927-975. (b) Imahori, H.; Sakata, Y. Eur. J. Org. Chem. 1999, 2445. (c) Imahori, H.; Fukuzumi, S. Adv. Mater. 2001, 13, 1197. (14) (a) 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. (b) Imahori, H.; Yamada, H.; Nishimura, Y.; Yamazaki, I.; Sakata, Y. J. Phys. Chem. B 2000, 104, 2099. (c) Imahori, H.; Norieda, H.; Yamada, H.; Nishimura Y.; Yamazaki, I.; Sakata, Y.; Fukuzumi, S. J. Am. Chem. Soc. 2001, 123, 100. (d) Yamada, H.; Imahori, H.; Nishimura, Y.; Yamazaki, I.; Fukuzumi, S. Chem. Commun. 2000, 1921. (e) Yamada, H.; Imahori, H.; Nishimura, Y.; Yamazaki, I.; Fukuzumi, S. Adv. Mater. 2002, 14, 892. (15) (a) Uosaki, K.; Kondo, T.; Zhang, X.-Q.; Yanagida, M. J. Am. Chem. Soc. 1997, 119, 8367. (b) Kondo, T.; Kanai, T.; Iso-o, K.; Uosaki, K. Z. Phys. Chem. 1999, 212, 23. (16) (a) Lahav, M.; Gabriel, T.; Shipway, A. N.; Willner, I. J. Am. Chem. Soc. 1999, 121, 258. (b) Shipway, A. N.; Katz, E.; Willner, I. ChemPhysChem 2000, 1, 18. (17) (a) Schmid, G. Clusters and Colloids. From Theory to Applications; VCH: New York, 1994. (b) Alivisatos, A. P. Science 1996, 271, 933. (c) Bawendi, M. G.; Steigerwald, M. L.; Brus, L. E. Annu. Rev. Phys. Chem. 1990, 41, 477. (d) Wang, Y. In Advances in Photochemistry; Neckers, D. C., Volman, D. H., von Bu¨nau, G., Eds.; Wiley: New York 1995; pp 179-234.

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applications of metal nanoclusters involve chemical and biochemical sensors, catalysts, quantum dot, and optoelectronic devices, nanostructure fabrication, and scaffolds.17,18 In particular, alkanethiolate-monolayer-protected metal clusters (MPCs), which can provide threedimensional (3D) architectures, have drawn much attention, because they are stable in air and soluble in both nonpolar and polar organic solvents, thereby being capable of facile modification with other functional thiols through exchange reactions or by couplings and nucleophilic substitutions.19 Thus, MPCs have been variously modified with functional molecules such as ferrocenes, quinones, cyclodextrins, azobenzenes, nucleic acids, and so on.20-24 Construction of the 3D architectures of porphyrin MPCs which have large surface area would improve the lightharvesting efficiency as compared to the 2D porphyrin SAMs. Furthermore, the interaction of the porphyrin excited state with the gold nanocluster would be reduced significantly, relative to the bulk gold surface, due to the “quantum effect”.17,18 However, there has so far been no report on the preparation of porphyrin-modified gold nanoclusters which can be applied to such photofunctional materials. We report herein the first comprehensive studies on porphyrin-modified metal nanoclusters H2PCnAuC (n ) 3, 5, 7, 11) with different spacers, as shown in Figure 1.25 The effects of chain length of the spacer (n ) 3, 5, 7, 11) on the structure and photophysical properties were investigated by 1H NMR, UV-visible, steady-state fluorescence, time-resolved fluorescence and transient absorption spectroscopies, electrochemistry, elemental analysis, and transmission electron microscopy (TEM). The structure and photophysical properties of H2PCnAuC (18) (a) Kamat, P. V. In Semiconductor Nanoclusters: Physical, Chemical and Catalytic Aspects; Kamat, P. V., Meiser, D., Eds.; Elsevier Science: Amsterdam, 1997; pp 237-259. (b) Kamat, P. V. J. Phys. Chem. B 2002, 106, 7729. (c) Henglein, A. Ber. Bunsen-Ges. Phys. Chem. 1995, 99, 903. (d) Pileni, M. P. New J. Chem. 1998, 693. (e) Link, S.; El-Sayed, M. A. J. Phys. Chem. B 1999, 103, 4212. (19) (a) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 801. (b) Templeton, A. C.; Wuelfing, W. P.; Murray, R. W. Acc. Chem. Res. 2000, 33, 27. (20) (a) Mirkin, C. A.; Letsinger, R. L.; Mucic, R. C.; Storhoff, J. J. Nature 1996, 382, 607. (b) Alivisatos, A. P.; Johnsson, K. P.; Peng, X.; Wilson, T. E.; Loweth, C. J.; Bruchez, M. P.; Schultz, P. G. Nature 1996, 382, 609. (c) Elghanian, R.; Storhoff, J. J.; Mucic, R. C.; Letsinger, R. L.; Mirkin, C. A. Science 1997, 277, 1078. (d) Mucic, R. C.; Storhoff, J. J.; Mirkin, C. A.; Letsinger, R. L. J. Am. Chem. Soc. 1998, 120, 12674. (e) Storhoff, J. J.; Mirkin, C. A. Chem. Rev. 1999, 99, 1849. (21) (a) Ingram, R. S.; Hostetler, M. J.; Murray, R. W. J. Am. Chem. Soc. 1997, 119, 9175. (b) Liu, J.; Mendoza, S.; Roma´n, E.; Lynn, M. J.; Xu, R.; Kaifer, A. E. J. Am. Chem. Soc. 1999, 121, 4304. (c) Templeton, A. C.; Cliffel, D. E.; Murray, R. W. J. Am. Chem. Soc. 1999, 121, 7081. (d) Hicks, J. F.; Zamborini, F. P.; Osisek, A. J.; Murray, R. W. J. Am. Chem. Soc. 2001, 123, 7048. (22) (a) Boal, A. K.; Ilhan, F.; DeRouchey, J. E.; Thurn-Albrecht, T.; Russell, T. P.; Rotello, V. M. Nature 2000, 404, 746. (b) McIntosh, C. M.; Esposito, E. A.; Boal, A. K.; Simard, J. M.; Martin, C. T.; Rotello, V. M. J. Am. Chem. Soc. 2001, 123, 7626. (c) Evans, S. D.; Johnson, S. R.; Ringsdorf, H.; Williams, L. M.; Wolf, H. Langmuir 1998, 14, 6436. (d) Frankamp, B. L.; Boal, A. K.; Rotello, V. M. J. Am. Chem. Soc. 2002, 124, 15146. (23) (a) Fujiwara, H.; Yanagida, S.; Kamat, P. V. J. Phys. Chem. B 1999, 103, 2589. (b) Thomas, K. G.; Kamat, P. V. J. Am. Chem. Soc. 2000, 122, 2655. (c) Sudeep, P. K.; Ipe, B. I.; Thomas, K. G.; Gerorge, M. V.; Barazzouk, S.; Hotchandani, S.; Kamat, P. V. Nano Lett. 2002, 2, 29. (d) Ipe, B. I.; Thomas, K. G.; Barazzouk, S.; Hotchandani, S.; Kamat, P. V. J. Phys. Chem. B 2002, 106, 18. (e) Kamat, P. V.; Barazzouk, S.; Hotchandani, S. Angew. Chem., Int. Ed. 2002, 41, 2764. (24) (a) Zhang, J.; Whitesell, J. K.; Fox, M. A. Chem. Mater. 2001, 13, 2323. (b) Hu, J.; Zhang, J.; Liu, F.; Kittredge, K.; Whitesell, J. K.; Fox, M. A. J. Am. Chem. Soc. 2001, 123, 1464. (c) Stellacci, F.; Bauer, C. A.; Meyer-Friedrichsen, T.; Wenseleers, W.; Marder, S. R.; Perry, J. W. J. Am. Chem. Soc. 2003, 125, 328. (25) Preliminary results on the preparation and photophysical properties of H2PC11AuC and H2PC11-C11AuC were reported. Imahori, H.; Arimura, M.; Hanada, T.; Nishimura, Y.; Yamazaki, I.; Sakata, Y.; Fukuzumi, S. J. Am. Chem. Soc. 2001, 123, 335.

Porphyrin-Modified Metal Nanoclusters

Figure 1. Schematic structures of porphyrin-modified metal nanoclusters H2PCnMC and the porphyrin SAM on Au(111) (H2PCnAu(111)) (ref 14a).

(n ) 3, 5, 7, 11) are also compared with those of the corresponding two-dimensional system H2PCnAu(111) (n ) 3, 5, 7, 11) using the same compound on flat Au(111).14a The present study provides a new type of artificial photosynthetic materials as photocatalysts and lightharvesting materials. Results and Discussion Preparation and Characterization. The synthetic route to the starting material, porphyrin alkanethiol 1 (a, n ) 3; b, n ) 5; c, n ) 7; d, n ) 11), is shown in Scheme 1. To increase the solubility and reduce self-quenching of the porphyrin excited state, bulky tert-butyl substituents were introduced into meta positions of the meso-phenyl ring on the porphyrin core. Condensation of aminoporphyrin26 with the corresponding ω-bromopolymethylenecarboxylic acid in the presence of 2-chloro-4,6-dimethoxy1,3,5-triazine (CDMT) and N-methylmorpholine14a or 1-[3(dimethylamino)propyl]-3-ethylcarbodiimide (EDC) and 4-(dimethylamino)pyridine (DMAP) afforded 2. Bromide 2 was converted to porphyrin alkanethiol 1 via thioesterification with potassium thioacetate and subsequent base deprotection of 3.14a Acetoamidoporphyrin (Por-ref) was also prepared as a reference (Scheme 1).26 Place-exchange reactions of the MPCs with ω-functionalized alkanethiols or amide and ester coupling reactions19-24,27-29 have been generally used for function(26) Imahori, H.; Hagiwara, K.; Aoki, M.; Akiyama, T.; Taniguchi, S.; Okada, T.; Shirakawa, M.; Sakata, Y. J. Am. Chem. Soc. 1996, 118, 11771. (27) Hostetler, M. J.; Green, S. J.; Stokes, J. J.; Murray, R. W. J. Am. Chem. Soc. 1996, 118, 4212.

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alization of MPCs. However, the extent of functionalization was incomplete and generally unsatisfactory. In the case of place-exchange reactions, all the functionalized alkanethiolates are unlikely to penetrate into the preformed MPCs and in turn bind covalently to the gold surface because of the severe steric hindrance near the gold nanoclusters. In this study, the porphyrin-modified gold nanoclusters [H2PCnAuC (n ) 3, 5, 7, 11)] were directly prepared by reduction of HAuCl4 with NaBH4 in toluene/ water containing porphyrin alkanethiol 1 (1/HAuCl4 ) 1:1) to increase the extent of functionalization, as shown in Scheme 2 (vide infra). A porphyrin-alkanethiol mixed system (H2PC11-C11AuC) was then obtained by placeexchange reactions of H2PC11AuC with 1-dodecanethiol in toluene for 48 h. Alkanethiolate-MPC (C8AuC) as a reference was also synthesized from toluene containing a 1:1 ratio of 1-octanethiol and HAuCl4. The porphyrin MPCs (H2PCnAuC and H2PC11-C11AuC) and a reference MPC (C8AuC) were purified by repeated gel permeation chromatography and reprecipitation and characterized by 1H NMR, UV-visible, fluorescence spectroscopies, electrochemistry, elemental analysis, and TEM. The mean diameter of the gold core determined by TEM was 2RCORE ) 2.1 nm (with a standard deviation σ ) 0.3 nm) for H2PC11AuC (Figure 2a) and H2PC11-C11AuC (Table 1). Similar values were obtained for C8AuC [2RCORE ) 2.5 nm (σ )0.8 nm)], H2PC3AuC [2RCORE ) 2.0 nm (σ ) 0.5 nm)], H2PC5AuC [2RCORE ) 2.1 nm (σ ) 0.6 nm)], and H2PC7AuC [2RCORE ) 2.5 nm (σ ) 0.4 nm)]. These results reveal that the size of gold nanoclusters is not susceptible to the ω-functionalization even in the case of a large porphyrin. The TEM image of H2PC3AuC exhibits hexagonal packing of H2PC3AuC (Figure 2b). The separation distance between the gold core is estimated as 3.8 nm, which is 2 times larger than the thickness of the porphyrin monolayer determined by ellipsometric thickness (1.8 nm).14a Such hexagonal packing of H2PC3AuC can be ascribed to the densely packed, rigid structure of the porphyrin moieties near the gold cluster due to the short methylene spacer (n ) 3).30 In accordance with this observation, no similar hexagonal packing was seen for the other porphyrin MPCs with a longer spacer (n ) 5, 7, 11). Taking the gold core as a sphere with density FAu (58.01 atoms/nm3)29 covered with an outermost layer of hexagonally close-packed gold atoms (13.89 atoms/nm2)29 with a radius of RCORE - RAu (RAu ) 0.145 nm),29 the model predicts that the core of H2PC11AuC contains 280 Au atoms, of which 143 lie on the Au surface (Table 1). Given the values for elemental analysis of H2PC11AuC (see the Experimental Section), the number of porphyrin alkanethiolate chains on the gold surface of H2PC11AuC is determined as 57. The coverage ratio of porphyrin alkanethiolate chains of H2PC11AuC to surface Au atoms (γ) is thereby determined as 40% (porphyrin alkanethiolate/Au atom ) 57:143), which is remarkably increased relative to the coverage ratio (γ ) 6.5%) of the 2D porphyrin SAM H2PC11Au(111).14a Such enhanced packing of the large porphyrins may be achieved due to the highly curved outermost surface of Au clusters, where the spacer is splayed outward from the gold core to relieve steric (28) Templeton, A. C.; Hostetler, M. J.; Warmoth, E. K.; Chen, S.; Hartshorn, C. M.; Krishnamurthy, V. M.; Forbes, M. D. E.; Murray, R. W. J. Am. Chem. Soc. 1998, 120, 4845. (29) Terrill, R. H.; Postlethwaite, T. A.; Chen, C.-h.; Poon, C.-D.; Terzis, A.; Chen, A.; Hutchison, J. E.; Clark, M. R.; Wignall, G.; Londono, J. D.; Superfine, R.; Falvo, M.; Johnson, C. S., Jr.; Samulski, E. T.; Murray, R. W. J. Am. Chem. Soc. 1995, 117, 12537. (30) Similar packing was reported for fluorinated gold nanoparticles. See: Yonezawa, T.; Onoue, S.; Kimizuka, N. Langmuir 2001, 17, 2291.

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Imahori et al. Scheme 1

Scheme 2

crowding significantly. These results clearly indicate that light-harvesting efficiency of 3D systems (H2PCnAuC) is much improved as compared to that of 2D systems (H2PCnAu(111)). 1 H NMR spectra of porphyrin MPCs were measured in CDCl3. The 1H NMR spectra of H2PC11AuC and porphyrin alkanethiol 1d in CDCl3 are shown in Figure 3. The complete disappearance of -S-CH2- and -CH2-CONHsignals in H2PC11AuC due to the broadening of the sharp peaks observed in 1d indicates that all the porphyrin alkanethiolates are covalently linked to the gold surface to leave no parent molecules 1d. The end-group NMR resonances (i.e., methylene spacer) in H2PC11AuC are also much broader than those of the terminal porphyrin moiety. Such broadening of the NMR signal is a typical characteristic of MPCs.19-24,27-29 Similar 1H NMR behavior was seen for H2PC11-C11AuC, H2PC7AuC, H2PC5AuC, and H2PC3AuC. The complete disappearance of -S-CH2- and -CH2-CONH- signals was also seen in H2PC11-C11AuC, indicating that there is no intercalation of the porphyrin alkanethiols between the alkane chains. Electrochemical and Photophysical Properties. Cyclic voltammograms of H2PCnAuC are also compared with that of Por-ref (1.0 × 10-4 M based on the number of the porphyrins) in CH2Cl2 containing 0.1 M n-Bu4NPF6 with a sweep rate of 0.01 V s-1. The first oxidation potential of H2PC11AuC [1.00 V vs Ag/AgCl (saturated KCl)] is the same as that of Por-ref, as shown in Figure 4.31 The first oxidation potentials of H2PCnAuC [n ) 3, 5, 7: 1.00 V vs Ag/AgCl (saturated KCl)] are also the same as that of Por-ref [1.00 V vs Ag/AgCl (saturated KCl)].

Figure 2. TEM image and the particle size distribution of (a) H2PC11AuC and (b) H2PC3AuC. The sample was formed by solvent evaporation from a toluene solution containing compound. The dark spots correspond to the metal core.

UV-visible spectra of MPCs were taken in benzene, THF, and CHCl3. The absorption due to the surface plasmon resonance of H2PC11AuC (Figure 5) is much weaker than that of the porphyrin moiety (see the Supporting Information, Figure S1, for the surface plasmon absorption of C8AuC). The λmax values of the Soret band of H2PC11AuC in benzene, THF, and CHCl3 are also nearly identical to that of Por-ref in the three solvents, whereas the λmax value of H2PC11Au(111) is red-shifted (6 nm) relative to that of Por-ref in benzene and CHCl3 (Table 1).14a This also indicates that the porphyrin environment of H2PC11AuC is less perturbed than that of the 2D SAM H2PC11Au(111). Steady-state fluorescence spectra of porphyrin MPCs and Por-ref were measured in various solvents with the excitation of the Soret band where the absorbance at the (31) When H2PC11AuC interacts with the electrode, only a part of the porphyrins in H2PC11AuC adsorbed on the electrode surface can be redox active. In addition, the diffusion coefficient of a very bulky H2PC11AuC as compared with that of Por-ref in solution is likely to be much smaller. This may be the reason the current intensity of H2PC11AuC is reduced as compared to that of Por-ref. The possible desorption of the porphyrin SAM at the examined potentials may be negligible because of the absence of the redox wave due to the desorbed porphyrin.

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Langmuir, Vol. 20, No. 1, 2004 77

Table 1. Diameter, Composition of Au Atom and Porphyrin, UV-Visible Spectral and Electrochemical Data of Porphyrin MPCs and Reference Porphyrin diameter, 2RCORE (standard deviation)/nm

number of metal atoms (number of porphyrin)

λmax of Soret band/nma

2.1 (0.3) 2.5 (0.4) 2.1 (0.6) 2.0 (0.5) 2.1 (0.3)

280 (57) 470 (51) 280 (55) 240 (44) 280 (45)

422(422)b 421 420 420 422 428c 427c 427c 425c 422(422)b

H2PC11AuC H2PC7AuC H2PC5AuC H2PC3AuC H2PC11-C11AuC H2PC11Au(111) H2PC7Au(111) H2PC5Au(111) H2PC3Au(111) Por-ref a

b

c

Eox0/+1/Vd

Ered0/-1/Ve

1.00 1.00 1.00 1.00

-1.51

1.10c 1.10c 1.08c 1.05c 1.00c

-1.51

d

In benzene. In chloroform. From ref 14a. First oxidation potentials of the porphyrin moiety were determined by cyclic voltammetry in CH2Cl2 containing 0.1 M n-Bu4NPF6 with a sweep rate of 10 mV s-1, a glassy carbon working electrode, a platinum wire counter electrode, and a Ag/AgCl (saturated KCl) reference electrode. e First reduction potentials of the porphyrin moiety were determined by cyclic voltammetry in benzonitrile containing 0.1 M n-Bu4NPF6 with a sweep rate of 10 mV s-1, a platinum working electrode, a platinum wire counter electrode, and a Ag/AgNO3 reference electrode.

Figure 3. 1H NMR spectra of H2PC11AuC (bottom) and porphyrin alkanethiol 1d (top) in CDCl3.

Soret peak was adjusted to be identical (1.0). The peak position (655 nm) and shape of the emission are similar, but moderate quenching of the steady-state fluorescence of porphyrin MPCs is seen relative to Por-ref. This agrees with the previous results that the excited singlet state of the dye on the surface of metal nanoclusters is quenched by EN or ET.18b,23c,23d The relative fluorescence intensity of H2PCnAuC versus Por-ref in benzene decreases slightly with decreasing spacer length in the order H2PC11AuC (0.10), H2PC7AuC (0.10), H2PC5AuC (0.09), and H2PC3AuC (0.08); see the Supporting Information, Figure S2. Time-Resolved Laser Spectroscopies. We have previously shown that the fluorescence decay of the 2D porphyrin SAM H2PCnAu(111) (n ) 1-11) obeys firstorder kinetics and the lifetime (τ) becomes extremely short (0.01-0.04 ns) as compared to that of Por-ref in THF (9.8 ns) due to fast EN or ET from the porphyrin excited singlet state to the gold surface.14a To examine the effects of chain length on the fluorescence of porphyrin MPCs, fluorescence lifetime measurements were carried out in various solvents. The fluorescence decays of the porphyrin MPCs (λex ) 420 nm and λobs ) 650 nm) could be analyzed by two components (Figure 6). For example, the lifetime of the minor component of H2PC11AuC in benzene [8.9 ns (4%)] is close to that of Por-ref in benzene (9.7 ns) as shown in Table 2. It is unlikely that the long-lived, minor component

Figure 4. Cyclic voltammograms of H2PC11AuC (1.0 × 10-4 M based on the number of the porphyrins) and Por-ref (1.0 × 10-4 M) in CH2Cl2 containing 0.1 M n-Bu4NPF6 with a sweep rate of 0.01 V s-1 using a glassy carbon working electrode.

is due to the parent compound 1 which should have been completely removed by the separation procedure. The 1H NMR spectra in Figure 3 also indicate that no parent compound 1 remains in porphyrin MPCs. Since the bulky tert-butyl groups are introduced at the meta positions of the meso-phenyl groups on the porphyrin ring, selfquenching due to the porphyrin aggregation is significantly reduced, thereby resulting in no apparent effect on the porphyrin fluorescence lifetimes under the present conditions.14a,32 Thus, the double exponential fluorescence decay in porphyrin MPCs suggests that there are at least two types of porphyrin monolayer structures that are different from those in the corresponding 2D porphyrin SAMs.33 This may result from different ligation sites (vertex, edge, terrace, and defect) on the truncated octahedral Au core surface. The lifetimes of the short-lived, major component [0.10 ns (96%) for H2PC11AuC] are significantly longer than (32) Anikin, M.; Tkachenko, N. V.; Lemmetyinen, H. Langmuir 1997, 13, 3002. (33) Aguila, A.; Murray, R. W. Langmuir 2000, 16, 5949.

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Imahori et al. Table 2. Fluorescence Lifetimes (τ) of Porphyrin MPCs and the Reference in Benzene, CHCl3, THF, and Benzonitrilea fluorescence lifetime (τ)/nsb system H2PC11AuC

Figure 5. UV-visible absorption spectra of H2PC11AuC (solid line) and Por-ref (dotted line, 1.0 × 10-6 M) in CHCl3. The spectra are normalized at the Soret band for comparison.

Figure 6. Fluorescence decay curve of H2PC11AuC in benzene monitored at 650 nm (λex ) 420 nm). The decay curve was fitted by two components (major component: τ ) 0.10 ns, amplitude (A) ) 0.018 (relative amplitude ) 96%); minor component: τ ) 8.9 ns, amplitude (A) ) 0.00084 (relative amplitude ) 4%); χ2 ) 1.5).

that of H2PC11Au(111) (0.040 ns).14a A similar trend was also observed at different emission wavelengths (λobs ) 720 nm) and in different solvents (i.e., CHCl3 and THF). The average lifetime of H2PC11AuC (0.45 ns) in benzene is 11 times as long as that of H2PC11Au(111) (0.040 ns), which is consistent with the moderate quenching of the steady-state fluorescence of H2PC11AuC in benzene (relative intensity, 0.10) (vide supra). Longer lifetimes are also obtained for H2PCnAuC (n ) 3, 5) and a porphyrinalkanethiol mixed SAM (H2PC11-C11AuC) as compared to the lifetimes of the corresponding 2D porphyrin SAM (Table 2). The fluorescence lifetimes of the porphyrin moiety in the mixed SAMs have been reported to remain almost constant irrespective of the relative ratio of the alkanethiol to the porphyrin.34 These results clearly demonstrate that the quenching of the porphyrin excited singlet state by the gold clusters via an EN or ET is much suppressed relative to the EN or ET quenching by the Au(111) surface. To establish the excited-state deactivation pathways, nanosecond transient absorption spectra were recorded (34) Imahori, H.; Hasobe, T.; Yamada, H.; Nishimura, Y.; Yamazaki, I.; Fukuzumi, S. Langmuir 2001, 17, 4925.

benzene

0.10 (96%) 8.9 (4%) H2PC7AuC d H2PC5AuC 0.053 (89%) 9.0 (11%) H2PC3AuC 0.046 (95%) 8.0 (5%) H2PC110.11 (71%) C11AuC 9.2 (29%) H2PC11Au(111) 0.040c H2PC7Au(111) 0.024c H2PC5Au(111) 0.021c H2PC3Au(111) 0.016c Por-ref 9.7 a

CHCl3

THF

benzonitrile

0.074 (91%) 7.4 (9%) d 0.050 (90%) 7.2 (10%) 0.044 (94%) 7.1 (6%) d

0.11 (92%) 9.2 (8%) d 0.060 (89%) 9.0 (11%) 0.057 (94%) 8.7 (6%) 0.17 (97%) 8.2 (3%)

0.13 (93%) 9.6 (7%) d 0.057 (90%) 9.4 (10%) d

d

9.8

d

d

b

Excited at 420 nm and monitored at 650 nm. Numbers in parentheses are the relative amplitudes of the pre-exponential factors in the exponential functions. c From ref 14a. d Not measured.

Figure 7. Nanosecond time-resolved transient absorption spectra of H2PC11AuC (solid line with white circles), H2PC3AuC (solid line with black circles), and Por-ref (solid line with black triangles) in argon-saturated benzonitrile at a time delay of 1000 ns using 550 nm excitation light.

following the 550 nm laser pulse as the excitation of porphyrin MPCs and the reference. Por-ref exhibits an absorption at 700-800 nm, which is characteristic of the porphyrin excited triplet state (Figure 7).35 In contrast, the transient absorption spectra of H2PC11AuC and H2PC3AuC reveal similar but weak absorption throughout the UV-visible region, which can be assigned as the triplet excited state.36 No transient absorption due to the porphyrin radical cation is seen for H2PC11AuC and H2PC3AuC. The intensity of transient absorption for H2PC3AuC is smaller than that of H2PC11AuC. These results are in good agreement with the fluorescence lifetime measurements. Namely, most of the porphyrin excited singlet state on the metal nanoparticles is quenched by the metal surface via ET or EN processes, whereas residual porphyrin excited triplet state is generated via the intersystem crossing from the unquenched porphyrin (35) Luo, C.; Guldi, D. M.; Imahori, H.; Tamaki, K.; Sakata, Y. J. Am. Chem. Soc. 2000, 122, 6535. (36) The transient absorption of H2PC11AuC and H2PC3AuC in benzonitrile is quenched by addition of hexyl viologen (HV2+) to yield hexyl viologen radical cation (HV•+), which is detected by using nanosecond transient absorption spectroscopy.

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Figure 8. Picosecond transient absorption spectra of H2PC11AuC in benzonitrile as a function of the time delay between the pump and probe laser beams. The insets show the time profiles at 460 nm (dotted curve with white squares) and 600 nm (solid curve with black circles) using 540 nm excitation light.

excited singlet state. Picosecond transient absorption spectra were also taken following the 540 nm laser pulse as the excitation of porphyrin MPCs (Figure 8). For instance, immediately after the excitation (20 ps delay) of H2PC11AuC in benzonitrile, transient absorption due to the porphyrin excited singlet state at around 450 nm is seen. The rate constant for the decay of the band (7.7 × 109 s-1) at 460 nm agrees well with that for the rise of the excited plasmon band at around 600 nm37 (7.7 × 109 s-1). The rate constant is in good agreement with the value obtained from the fluorescence lifetime measurements (7.7 × 109 s-1). There is no evidence for the formation of the porphyrin radical cation. These results indicate that the porphyrin excited singlet state in the present systems is quenched by the metal surface via energy transfer rather than electron transfer.38 As seen in the steady-state fluorescence, the fluorescence lifetime slightly decreases with decreasing spacer length, which is in accordance with the quenching trend in the 2D porphyrin SAMs. In general, there are two mechanisms for EN: Fo¨rster dipole-dipole coupling39 and Dexter exchange EN.40 The dipole-dipole interaction is a longrange EN process in which the EN rate decreases with the inverse sixth power of r (the separation distance between donor and acceptor). On the other hand, the EN rate constant (kq) in a short-range EN process decreases exponentially with increasing r as given by eq 1,

kq ) k0 exp[-β(r - r0)]

(1)

where r0 is the van der Waals contact distance separating the centers of the donor and acceptor, and β is a decay coefficient (damping factor) that determines the rate of falloff with distance. The exchange interaction is normally expected to become negligibly small as r increases beyond 10 Å.41 However, long-range EN is made possible by a type of electron exchange mechanism involving throughbond coupling with the spacer orbitals.42-45 (37) Ahmadi, T. S.; Logunov, S. L.; El-Sayed, M. A. J. Phys. Chem. 1996, 100, 8053. (38) However, the electron transfer followed by fast charge recombination to give the plasmon absorption cannot be ruled out completely. (39) Fo¨rster, T. Fluorezenz Organische Verbindungen; Vadenhoech and Rprech: Gottingen, Germany, 1951. (40) Dexter, D. L. J. Chem. Phys. 1953, 21, 836.

Figure 9. Distance dependence of the quenching rate constant for 3D porphyrin MPCs (dashed line with squares) and 2D porphyrin SAMs (solid line with circles). The plots of ln(kq) vs d ) (r - r0) gave straight lines with the slope (-0.1 ( 0.01) according to eq 1.

The slope of a double logarithmic plot of ln kq versus ln d ()r - r0) (see the Supporting Information, Figure S3) is -1.5 ( 0.1 for H2PCnAuC and H2PC11Au(111), respectively, which is much smaller than the value of -6 predicted by the Fo¨rster dipole-dipole coupling EN. The edge-to-edge distance (d ) r - r0) between the gold surface and porphyrin moiety is determined from Corey-PaulingKoltun (CPK) models,14a assuming that the alkyl chain is fully extended. Plots of ln kq versus d yield the β values in eq 1 for H2PCnAuC and H2PCnAu(111) as the same value, 0.1 ( 0.01 Å-1, as shown in Figure 9. The slower EN rate of the 3D surface as compared to that of the 2D surface may result from fewer gold atoms on the 3D surface of the nanoparticles involved in EN as compared to those on the 2D surface of the flat electrode. The β value in this study is remarkably small as compared to those for intramolecular triplet energy transfer through DNA (1.7 Å-1),46a saturated hydrocarbons (1.33 Å-1),46b polyphenyl (0.32-0.64 Å-1),46c,d and steroids (1.7 Å-1)46e and is similar to those for polyalkynes (0.05-0.17 Å-1).47 The small β value in Figure 9 suggests that the alkyl chain is not fully extended as the chain length increases. In conclusion, we have successfully developed a variety of gold nanoclusters covered with porphyrin monolayers including different lengths of the spacer and compared (41) Turro, N. J. Modern Molecular Photochemistry; Benjamin/ Cummings: Menlo Park, CA, 1978; Chapter 9. (42) Speiser, S. Chem. Rev. 1996, 96, 1953. (43) (a) Lokan, N.; Paddon-Row, M. N.; Smith, T. A.; Rosa, M. L.; Ghiggino, K. P.; Speicer, S. J. Am. Chem. Soc. 1999, 121, 2917. (b) Kroon, J.; Oliver, A. M.; Paddon-Row, M. N.; Verhoeven, J. W. J. Am. Chem. Soc. 1990, 112, 4868. (44) (a) Tung, C.-H.; Zhang, L.-P.; Li, Y.; Cao, H.; Tanimoto, Y. J. Am. Chem. Soc. 1997, 119, 5348. (b) Agyin, J. K.; Timberlake, L. D.; Morrison, H. J. Am. Chem. Soc. 1997, 119, 7945. (45) Closs, G. L.; Johnson, M. D.; Miller, J. R.; Piotrowiak, P. J. Am. Chem. Soc. 1989, 111, 3751. (46) (a) Brun, A. M.; Harriman, A. J. Am. Chem. Soc. 1994, 116, 10383. (b) Oevering, H.; Verhoeven, J. W.; Paddon-Row, M. N.; Cotsaris, E.; Hush, N. S. Chem. Phys. Lett. 1988, 143, 488. (c) Schlicke, B.; Belser, P.; De Cola, L.; Sabbioni, E.; Balzani, V. J. Am. Chem. Soc. 1999, 121, 4207. (d) Barigelletti, F.; Flamigni, L.; Guardigli, M.; Juris, A.; Beley, M.; Chadorowsky-Kimmes, S.; Collin, J.-P.; Sauvage, J.-P. Inorg. Chem. 1996, 35, 136. (e) Closs, G. L.; Piotrowiak, P.; MacInnis, J. M.; Fleming, G. R. J. Am. Chem. Soc. 1988, 110, 2652. (47) (a) Grosshenny, V.; Harriman, A.; Ziessel, R. Angew. Chem., Int. Ed. 1995, 34, 1100. (b) Harriman, A.; Khatyr, A.; Ziessel, R.; Benniston, A. C. Angew. Chem., Int. Ed. 2000, 39, 4287.

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Langmuir, Vol. 20, No. 1, 2004

the structure and photophysical properties with those of a 2D porphyrin SAM on the flat bulk surface for the first time. The 3D porphyrin MPCs prepared in this study contain a high porphyrin coverage ratio to surface Au atoms in the nanoclusters, which suppress an undesirable quenching relative to the corresponding 2D SAM systems. We have also demonstrated that energy transfer quenching occurs from the porphyrin excited singlet state to the metal surface using transient absorption spectroscopy and fluorescence lifetime measurements. The porphyrinmodified metal nanoclusters developed in this study may be highly promising as a new type of light-harvesting materials, photocatalysts, chemical and biochemical sensors, and molecular devices. Experimental Section General. Melting points were recorded on a Yanagimoto micromelting-point apparatus and were not corrected. 1H NMR spectra were measured on a JEOL EX-270 (270 MHz) or a JEOL JMNAL300 (300 MHz). Matrix-assisted laser desorption/ionization (MALDI) time-of-flight (TOF) mass spectra were measured on a Kratos Compact MALDI I (Shimadzu). Elemental analyses were performed on a Perkin-Elmer model 240C elemental analyzer. Materials. All solvents and chemicals were of reagent grade quality, obtained commercially and used without further purification unless otherwise noted (vide infra). HAuCl4 (99.999%) and tetraoctylammonium bromide (98%) were purchased from Aldrich. Tetrabutylammonium hexafluorophosphate (n-Bu4NPF6) used as a supporting electrolyte for the electrochemical measurements was obtained from Tokyo Kasei Organic Chemicals. THF was purchased from Wako Pure Chemical Industries, Ltd., and purified by successive distillation over calcium hydride. Thin-layer chromatography (TLC) and flash column chromatography were performed with Art. 5554 DC-Alufolien Kieselgel 60 F254 (Merck) and Fujisilicia BW300, respectively. 5-(4Aminophenyl)-10,15,20-tris(3,5-di-tert-butylphenyl)porphyrin26 and 2d14a were prepared by the same procedures as described in the previous paper. Transmission Electron Microscopy. The diameters of gold nanoclusters and the distance of clusters were obtained as bright field images using a JEOL 3000F operating at 300 kV. The samples were prepared by placing CHCl3 containing MPCs on water and allowing the solvent to evaporate and then scooped up with amorphous carbon supporting film. Synthesis and Characterization. 1a-d. To a stirred solution of 3a-d (300 mg, 0.270 mmol) in deaerated THF (5 mL), a deaerated MeOH (10 mL) solution of potassium hydroxide (200 mg, 3.56 mmol) was added and then refluxed under nitrogen for 15 min. The resulting reaction mixture was evaporated, poured into water, extracted with CHCl3, and dried over Na2SO4. After evaporation, the residue was purified by column chromatography (SiO2, CHCl3), and subsequent reprecipitation from CHCl3methanol gave 1a (70.0 mg, 0.0655 mmol, 24%), 1b (70%), 1c (43%), and 1d (35%). 1a. mp > 300 °C; 1H NMR (300 MHz, CDCl3) δ 8.88 (s, 4H), 8.86 (d, J ) 5 Hz, 2H), 8.84 (d, J ) 5 Hz, 2H), 8.19 (d, J ) 8 Hz, 2H), 8.06 (d, J ) 2 Hz, 2H), 8.05 (d, J ) 2 Hz, 4H), 7.94 (d, J ) 8 Hz, 2H), 7.87 (br s, 1H), 7.78 (m, 3H), 2.98 (t, J ) 7 Hz, 2H), 2.71 (t, J ) 7 Hz, 2H), 1.4-1.8 (m, 57H), -2.71 (s, 2H); MALDITOF-MS 1069 (M + H+). 1b. mp > 300 °C; 1H NMR (300 MHz, CDCl3) δ 8.90 (s, 4H), 8.88 (d, J ) 5 Hz, 2H), 8.86 (d, J ) 5 Hz, 2H), 8.16 (d, J ) 8 Hz, 2H), 8.09 (d, J ) 2 Hz, 4H), 8.07 (d, J ) 2 Hz, 2H), 7.85 (d, J ) 8 Hz, 2H), 7.79 (t, J ) 2 Hz, 2H), 7.78 (t, J ) 2 Hz, 1H), 7.57 (br s, 1H), 2.77 (m, 4H), 1.1-1.6 (m, 61H), -2.69 (s, 2H); MALDITOF-MS 1097 (M + H+). 1c. mp > 300 °C; 1H NMR (300 MHz, CDCl3) δ 8.89 (s, 4H), 8.87 (d, J ) 5 Hz, 2H), 8.85 (d, J ) 5 Hz, 2H), 8.18 (d, J ) 8 Hz, 2H), 8.08 (d, J ) 2 Hz, 4H), 8.07 (d, J ) 2 Hz, 2H), 7.89 (d, J ) 8 Hz, 2H), 7.79 (t, J ) 2 Hz, 2H), 7.78 (t, J ) 2 Hz, 1H), 7.44 (br s, 1H), 2.61 (m, 4H), 1.1-1.9 (m, 65H), -2.71 (s, 2H); MALDITOF-MS 1125 (M + H+).

Imahori et al. 1d. mp > 300 °C; 1H NMR (300 MHz, CDCl3) δ 8.90 (s, 4H), 8.87 (d, J ) 5 Hz, 2H), 8.86 (d, J ) 5 Hz, 2H), 8.18 (d, J ) 8 Hz, 2H), 8.08 (d, J ) 2 Hz, 4H), 8.07 (d, J ) 2 Hz, 2H), 7.91 (d, J ) 8 Hz, 2H), 7.78 (t, J ) 2 Hz, 2H), 7.77 (t, J ) 2 Hz, 1H), 7.49 (br s, 1H), 2.55 (t, J ) 8 Hz, 2H), 2.54 (q, J ) 8 Hz, 2H), 1.2-1.9 (m, 73H), -2.72 (s, 2H); MALDI-TOF-MS 1181 (M + H+). 2a. To a stirring solution of 4-bromobutyric acid (1.57 g, 9.38 mmol) and 2-chloro-4,6-dimethyoxy-1,3,5-triazine (CDMT) (1.63 g, 9.28 mmol) in THF (50 mL) was added N-methylmorpholine (1.10 mL, 9.96 mmol) at 0 °C, and stirring was continued at 0 °C for 5 h. To the crude solution, 5-(4-aminophenyl)-10,15,20tris(3,5-di-tert-butylphenyl)porphyrin (580 mg, 0.600 mmol) was added at 0 °C. Stirring continued for 20 h at room temperature, and the reaction mixture was evaporated to dryness. Flash column chromatography on silica gel with 6% ethyl acetate/ toluene as an eluent and subsequent reprecipitation from CHCl3methanol gave 2a (1.12 g, 1.00 mmol, 41% yield). mp > 300 °C; 1H NMR (300 MHz, CDCl ) δ 8.90 (s, 4H), 8.88 (s, 2H), 8.84 (d, 3 J ) 5 Hz, 2H), 8.16 (d, J ) 8 Hz, 2H), 8.09 (d, J ) 2 Hz, 4H), 8.08 (d, J ) 2 Hz, 2H), 7.81 (d, J ) 8 Hz, 2H), 7.79 (d, J ) 2 Hz, 2H), 7.78 (d, J ) 2 Hz, 1H), 7.44 (br s, 1H), 3.61 (t, J ) 7 Hz, 2H), 2.63 (t, J ) 7 Hz, 2H), 1.4-1.8 (s, 56H), -2.70 (s, 2H); MALDI-TOFMS 1116 (M + 2H+). 2b and 2c. A solution of THF containing 4-(dimethylamino)pyridine (DMAP, 244 mg, 2.00 mmol) and 4-bromobutanoic acid or 6-bromohexanoic acid (4.00 mmol) was treated with 1-ethyl3-(3-dimethylaminopropyl) carbodiimide hydrochloride (EDC, 230 mg, 1.20 mmol) under nitrogen at 0 °C. The mixture was stirred for 1 h at 0 °C. 5-(4-Aminophenyl)-10,15,20-tris(3,5-ditert-butylphenyl)porphyrin (400 mg, 0.414 mmol) was added to the mixture and stirred at 0 °C for 30 min. The mixture was stirred at room temperature overnight. The mixture was concentrated in vacuo and poured onto a 5% HCl aqueous solution and extracted with chloroform (50 mL × 3). The organic layer was washed with saturated NaHCO3 aqueous solution, dried over anhydrous Na2SO4, and evaporated. Flash column chromatography (SiO2, CHCl3) gave 2b (464 mg, 0.406 mmol, 98%) or 2c (97%) as a purple solid. The compound data were described in the previous paper.14a 3a-3d. A solution of 2a-2d (0.879 mmol) in THF was added to a solution of potassium thioacetate (456 mg, 4.00 mmol) in EtOH (25 mL) and refluxed under nitrogen for 3 h. The resulting reaction mixture was poured into water and extracted with CHCl3 and dried over Na2SO4. After evaporation, the residue was purified by column chromatography on silica gel eluted with CHCl3, and subsequent reprecipitation from CHCl3-methanol gave 3a (690 mg, 0.621 mmol, 71% yield), 3b (80%), 3c (88%), and 3d (93%), respectively. 3a. mp > 300 °C; 1H NMR (300 MHz, CDCl3) δ 8.89 (s, 4H), 8.87 (d, J ) 5 Hz, 2H), 8.86 (d, J ) 5 Hz, 2H), 8.19 (d, J ) 8 Hz, 2H), 8.08 (d, J ) 2 Hz, 4H), 8.07 (d, J ) 2 Hz, 2H), 8.03 (s, 1H), 7.97 (d, J ) 8 Hz, 2H), 7.79 (t, J ) 2 Hz, 2H), 7.78 (t, J ) 2 Hz, 1H), 3.11 (t, J ) 7 Hz, 2H), 2.60 (t, J ) 7 Hz, 2H), 2.44 (s, 3H), 1.4-1.8 (m, 56H), -2.71 (s, 2H); MALDI-TOF-MS 1111 (M + H+). 3b. mp > 300 °C; 1H NMR (300 MHz, CDCl3) δ 8.90 (s, 4H), 8.87 (d, J ) 5 Hz, 2H), 8.86 (d, J ) 5 Hz, 2H), 8.17 (d, J ) 8 Hz, 2H), 8.09 (d, J ) 2 Hz, 2H), 8.08 (d, J ) 2 Hz, 4H), 7.87 (d, J ) 8 Hz, 2H), 7.79 (t, J ) 2 Hz, 2H), 7.78 (t, J ) 2 Hz, 1H), 7.55 (s, 1H), 2.94 (t, J ) 7 Hz, 2H), 2.49 (t, J ) 7 Hz, 2H), 2.36 (s, 3H), 1.4-1.7 (m, 60H), -2.70 (s, 2H); MALDI-TOF-MS 1139 (M + H+). 3c. mp > 300 °C; 1H NMR (300 MHz, CDCl3) δ 8.90 (s, 4H), 8.87 (d, J ) 5 Hz, 2H), 8.85 (d, J ) 5 Hz, 2H), 8.17 (d, J ) 8 Hz, 2H), 8.08 (d, J ) 2 Hz, 2H), 8.07 (d, J ) 2 Hz, 4H), 7.88 (d, J ) 8 Hz, 2H), 7.79 (t, J ) 2 Hz, 1H), 7.78 (t, J ) 2 Hz, 2H), 7.49 (s, 1H), 2.91 (t, J ) 7 Hz, 2H), 2.49 (t, J ) 7 Hz, 2H), 2.34 (s, 3H), 1.1-1.7 (m, 64H), -2.71 (s, 2H); MALDI-TOF-MS 1167 (M + H+). 3d. mp > 300 °C; 1H NMR (300 MHz, CDCl3) δ 8.90 (s, 4H), 8.87 (d, J ) 5 Hz, 2H), 8.86 (d, J ) 5 Hz, 2H), 8.18 (d, J ) 8 Hz, 2H), 8.08 (d, J ) 2 Hz, 4H), 8.07 (d, J ) 2 Hz, 2H), 7.89 (d, J ) 8 Hz, 2H), 7.79 (m, J ) 2 Hz, 3H), 7.47 (s, 1H), 2.88 (t, J ) 7 Hz, 2H), 2.52 (t, J ) 7 Hz, 2H), 2.33 (s, 3H), 1.7-1.2 (m, 72H), -2.71 (s, 2H); MALDI-TOF-MS 1223 (M + H+).

Porphyrin-Modified Metal Nanoclusters H2PCnAuC (n ) 3, 5, 7, 11). HAuCl4 (115 mg, 0.205 mmol) was dissolved in 5 mL of deionized water. This solution was shaken in a separatory funnel with 18 mL of a toluene solution of tetraoctylammonium bromide (320 mg, 0.345 mmol). Once the chloroauric anion was completely transferred into the toluene phase, 3.3 mL of this solution was isolated and combined with 1a-1d (0.065 mmol). To the vigorously stirring resulting solution was added an aqueous solution (3.3 mL) of sodium borohydride (67 mg, 1.8 mmol). After further stirring for 3 h, the organic phase was separated and washed with water. After evaporation, the residue was purified by gel permeation chromatography (BioRad Bio-Beads SX-1 (4.6 × 82 cm), toluene) and repeated gel permeation chromatography (JAIGEL-4H, CHCl3) to give H2PC3AuC as a purple solid (41 mg), H2PC5AuC (73 mg), H2PC7AuC (118 mg), and H2PC11AuC (20 mg), respectively. Anal. Calcd. for H2PC11AuC, C4560H5700N285O57S57Au280: H, 4.69; C, 44.74; N, 3.26. Found: H, 4.88; C, 44.77; N, 3.10. Anal. Calcd. for H2PC7AuC, C3876H4692N255O51S51Au470: H, 3.15; C, 31.02; N, 2.38. Found: H, 3.23; C, 30.96; N, 2.24. Anal. Calcd. for H2PC5AuC, C4070H4840N275O55S55Au280: H, 4.23; C, 42.36; N, 3.34. Found: H, 4.59; C, 42.40; N, 3.01. Anal. Calcd. for H2PC3AuC, C3168H3696N220O44S44Au240: H, 3.96; C, 40.44; N, 3.28; Found: H, 4.49; C, 40.41; N, 2.90. H2PC11-C11AuC. To a solution of H2PC11AuC (20 mg) in toluene (10 mL) was added 1-dodecanethiol (3.5 µL). After stirring for 48 h at room temperature under N2 in the dark, the reaction mixture was evaporated to dryness. The residue was purified by reprecipitation (methanol/toluene) and washed with methanol and acetone. Then, further reprecipitation (n-hexane/2-propanol) yielded H2PC11-C11AuC as a purple solid (7.1 mg). C8AuC. HAuCl4 (310 mg, 0.91 mmol) was dissolved in 25 mL of deionized water. This solution was shaken in a separatory funnel with 80 mL of a toluene solution of tetraoctylammonium bromide (1.50 g, 2.74 mmol). Once the chloroauric anion was completely transferred into the toluene phase, this solution was isolated and combined with 1-octanethiol (0.28 mL, 1.6 mmol). To the vigorously stirring resulting solution was added sodium borohydride (420 mg, 11.1 mmol) aqueous solution (25 mL). After further stirring for 3 h, the organic phase was separated and washed with water. After evaporation, the residue was purified by reprecipitated (ethanol/toluene) to give C8AuC as a black solid (118 mg). H2PC3Au(111), H2PC5Au(111), H2PC7Au(111), H2PC11Au(111) were prepared by the same procedures as described in the previous paper.14a Spectral Measurements. UV-visible spectra were obtained on a Shimadzu UV-3100PC spectrometer or a Hewlett-Packard 8452A diode array spectrophotometer at 298 K. Corrected fluorescence spectra were taken using a SPEX Fluorolog 2 spectrometer or a Perkin-Elmer LS50B fluorescence spectrophotometer or a Shimadzu spectrofluorophotometer (RF5000PC). The solutions were deaerated by argon purging for 7 min prior to the measurements. Fluorescence decay curves on a gold surface and in solutions were measured by means of a time-correlated single photon counting method using the second harmonic (435 nm) of a Ti:sapphire laser (Coherent MIRA 900). Nanosecond Transient Absorption. Nanosecond transient absorption measurements were carried out using an OPO laser (Continuum, Surelite OPO, full width at half-maximum (fwhm) 4-6 ns, 560 nm) pumped by a Nd:YAG laser (Continuum, SLII10, 4 ns fwhm) as an excitation source. Photoinduced events in

Langmuir, Vol. 20, No. 1, 2004 81 nanosecond time regions were monitored by using a continuous Xe lamp (150 W) and a Si-APD photodiode (Hamamatsu 2949) as a probe light and a detector. The output from the photodiode was recorded with a digitizing oscilloscope (HP, 54810A, 500 MHz). All the samples in a quartz cell (1 × 1 cm) were deaerated by bubbling argon through the solution for 15 min. Fluorescence Lifetime Measurements. Fluorescence decays were measured by using a femtosecond pulse laser excitation and a single photon counting system for fluorescence decay measurement.48 The laser system was a mode-locked Ti:sapphire laser (Coherent, Mira 900) pumped by an argon ion laser (Coherent, Innova 300). The repetition rate of a laser pulse was 2.9 MHz with a pulse picker (Coherent, model 9200). The second harmonic generated by an ultrafast harmonic system (Inrad, model 5-050) was used as an excitation source. The excitation wavelength was set at 420 nm, and temporal profiles of fluorescence decay and rise were recorded by using a microchannel plate photomultiplier (Hamamatsu R3809U). Full-width at halfmaximum of the instrument response function was 36 ps where the time interval of the multichannel analyzer (Canberra, model 3501) was 2.6 ps in the channel number. The fluorescence decays were measured at 650 nm for the porphyrin moiety. Criteria for the best fit were the values of χ2 and the Durbin-Watson parameters, obtained by nonlinear regression. Electrochemical Measurements. Cyclic voltammetry measurements were performed at 298 K on a BAS 100W electrochemical analyzer or a BAS CV-50W voltammetric analyzer in deaerated CH2Cl2 containing 0.1 M n-Bu4NPF6 as the supporting electrolyte at 298 K. A conventional three-electrode cell was used with a platinum (i.d. ) 1.6 mm) or platinum button or glassy carbon working electrode (i.d. ) 3.0 mm) and a platinum wire as the counter electrode. The measured potentials were recorded with respect to the Ag/AgNO3 or Ag/AgCl (saturated KCl) reference electrode. The E1/2 value of ferrocene used as a standard is 0.37 V versus saturated calomel electrode in CH2Cl2 under the present experimental conditions. All electrochemical measurements were carried out under an atmospheric pressure of argon.

Acknowledgment. This work was supported by a Grant-in-Aid (No. 13440216 to S.F.) and the Development of Innovative Technology (No. 12310) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. S.F. thanks MEXT, Japan (21st Century COE on Osaka University Creation of Integrated EcoChemistry), for financial support through a Grant-in-Aid. H.I. also thanks MEXT, Japan (21st Century COE on Kyoto University Alliance for Chemistry), for financial support through a Grant-in-Aid. Supporting Information Available: UV-vis absorption spectra of C8AuC in benzene (Figure S1); steady-state fluorescence spectra of H2PC11AuC, H2PC7AuC, H2PC5AuC, and H2PC3AuC in benzene (Figure S2), and plots of ln kq vs ln(r - r0) (Figure S3). This material is available free of charge via the Internet at http://pubs.acs.org. LA035435P (48) (a) Boens, N.; Tamai, N.; Yamazaki, I.; Yamazaki, T. Photochem. Photobiol. 1990, 52, 911. (b) Nishimura, Y.; Yasuda, A.; Speiser, S.; Yamazaki, I.; Chem. Phys. Lett. 2000, 323, 117.