Synthesis of Dendron-Protected Porphyrin Wires and Preparation of a

The ca. 2.3 nm difference between the height (5.4 ± 0.7 nm, Figure 5a) of the polymer after .... Brust, M.; Bethel, D.; Schifrin, D. J.; Kiel, C. J. ...
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Langmuir 2007, 23, 6365-6371

6365

Synthesis of Dendron-Protected Porphyrin Wires and Preparation of a One-Dimensional Assembly of Gold Nanoparticles Chemically Linked to the π-Conjugated Wires Hiroaki Ozawa, Masahiro Kawao, Hirofumi Tanaka, and Takuji Ogawa* Research Center for Molecular-Scale Nanoscience, Institute for Molecular Science, 5-1 Higashiyama, Myodaiji, Okazaki, 444-8787, Japan, Graduate UniVersity for AdVanced Studies, Hayama, Miura, 240-0193, Japan, and Core Research for EVolutional Science and Technology (CREST) of Japan Science and Technology Agency (JST), Honcho 4-1-8, Kawaguchi, Saitama, 332-0012, Japan ReceiVed NoVember 27, 2006. In Final Form: March 12, 2007 A one-dimensional assembly of gold nanoparticles chemically bonded to π-conjugated porphyrin polymers was prepared on a chemically modified glass surface and on an undoped naturally oxidized silicon surface by the following methods: π-conjugated porphyrin polymers were prepared by oxidative coupling of 5,15-diethynyl-10,20-bis-((4dendron)phenyl) porphyrin (6), and its homologues (larger than 40-mer) were collected by analytical gel permeation chromatography (GPC). The porphyrin polymers (>40-mer) were deposited using the Langmuir-Blodgett (LB) method on substrate surfaces, which were then soaked in a solution of gold nanoparticles (2.7 ( 0.8 nm) protected with t-dodecanethiol and 4-pyridineethanethiol. The topographical images of the surface observed by tapping mode atomic force microscopy (AFM) showed that the polymers could be dispersed on both substrates, with a height of 2.8 ( 0.5 nm on the modified glass and 3.1 ( 0.5 nm on silicon. The height clearly increased after soaking in the gold nanoparticle solution, to 5.3 ( 0.5 nm on glass and 5.4 ( 0.7 nm on silicon. The differences in height (2.5 nm on glass and 2.3 nm on silicon) corresponded to the diameter of the gold nanoparticles bonded to the porphyrin polymers. The distance between gold nanoparticles observed in scanning electron microscopic images was ca. 5 nm, indicating that they were bonded at every four or five porphyrin units.

Introduction Metal nanoparticles have attracted considerable recent interest as materials due to their characteristic optical and electronic properties.1-8 The electrical properties of assembled gold nanoparticles (AuNPs) are strongly influenced by the dimensionality of the structures.9 In particular, one-dimensional structures have gathered a great deal of attention due to their potential for applications in areas such as sensors, catalysis, medical diagnostics, and electric and optical devices.10-18 These * To whom correspondence should be addressed. E-mail: ogawat@ ims.ac.jp. (1) Barton, M. I. Synthesis, Functionalization and Surface Treatment of Nanoparticles; American Scientific Publishers: Los Angeles, 2003. (2) Schmidt, G. Nanoparticles; WILEY-VCH: Weinham, 2004. (3) Chi, L. F.; Hurting, M.; Dresser, T.; Schwas, T.; Seidel, C.; Fuchs, H.; Schmidt, G. Appl. Phys. A 1998, 66, S187-S190. (4) Brust, M.; Bethel, D.; Schifrin, D. J.; Kiel, C. J. AdV. Mater. 1995, 7, 795-797. (5) Ogawa, T.; Kobayashi, K.; Masuda, G.; Takes, T.; Maeda, S. Thin Solid Films 2001, 393, 374-378. (6) Huang, W.; Masuda, G.; Maeda, S.; Tanaka, H.; Ogawa, T. Chem. Eur. J. 2006, 12, 607-619. (7) Sato, T.; Ahmed, H.; Brown, D.; Johnson, B. F. G. J. Appl. Phys. 1997, 82, 696-701. (8) The Lander, C.; Magnusson, M. H.; Depart, K.; Samuelson, L.; Paulsen, P. R.; Niggard, J.; Berggren, J. Appl. Phys. Lett. 2001, 79, 2106-2108. (9) Grabert, H.; Devoret, M. H. Single charge tunneling: Coulomb blockade phenomena in nanostructures; Plenum Press: New York, 1992. (10) Remacle, F.; Levine, R. D. Nano Lett. 2002, 2, 697-701. (11) Remacle, F.; Beverly, K. C.; Heath, J. R.; Levine, R. D. J. Phys. Chem. B 2002, 106, 4116-4126. (12) Sample, J. L.; Beverly, K. C.; Chaudhari, P. R.; Remacle, F.; Heath, J. R.; Levine, R. D. AdV. Mater. 2002, 14, 124-128. (13) Beverly, K. C.; Sampaio, J. F.; Heath, J. R. J. Phys. Chem. B 2002, 106, 2131-2135. (14) Haiss, W.; Nichols, R. J.; Higgins, S. J.; Bethell, D.; Hobenreich, H.; Schiffrin, D. J. Faraday Discuss. 2004, 125, 179-194. (15) Hassenkam, T.; Moth-Poulsen, K.; Stuhr-Hansen, N.; Norgaard, K.; Kabir, M. S.; Bjornholm, T. Nano Lett. 2004, 4, 19-22. (16) Fan, H. Y.; Yang, K.; Boye, D. M.; Sigmon, T.; Malloy, K. J.; Xu, H. F.; Lopez, G. P.; Brinker, C. J. Science 2004, 304, 567-571.

assemblies have been prepared in various ways, the most straightforward method being the use of dimensional templates. These templates include copolymers, modified carbon nanotubes, and DNA.19-27 However, AuNPs were physically adsorbed on these templates. We are interested in the physical properties of assemblies in which AuNPs are chemically bonded to their templates. Here we report the preparation of a one-dimensional array of AuNPs chemically linked to π-conjugated systems (Figure 1). Porphyrin polymers with dendron groups were used as templates to increase the solubility of the long polymer molecules and to enlarge the diameter of the molecular chains for easy observation by AFM. AuNPs were capped with 4-pyridineethanethiol (pyAuNPs), whose pyridinyl moiety could bind chemically to zinc atoms of the porphyrin units. After porphyrin polymers were deposited by the Langmuir-Blodgett (LB) method on substrates, they were soaked in a solution of the py-AuNPs. This procedure was expected to form one-dimensional arrays of AuNPs on the conjugated porphyrin polymers. These assemblies were observed (17) Snow, a. W.; Ancona, M. G.; Kruppa, W.; Jernigan, G. G.; Foos, E. E.; Park, D. J. Mater. Chem. 2002, 12, 1222-1230. (18) Imura, K.; Nagahara, T.; Okamoto, H. J. Am. Chem. Soc. 2004, 126, 12730-12731. (19) Reuter, T.; Vidoni, O.; Torma, V.; Schmid, G. Nano Lett. 2002, 2, 709711. (20) Correa-Duarte, M. A.; Sobal, N.; Liz-Marzan, L. M.; Giersig, M. AdV. Mater. 2004, 16, 2179-2184. (21) Quinn, B. M.; Dekker, C.; Lemay, S. G. J. Am. Chem. Soc. 2005, 127, 6146-6147. (22) Fullam, S.; Cottell, D.; Rensmo, H.; Fitzmaurice, D. AdV. Mater. 2000, 12, 1430-1432. (23) Kimura, M.; Kobayashi, S.; Kuroda, T.; Hanabusa, K.; Shirai, H. AdV. Mater. 2004, 16, 335-338. (24) Berry, V.; Rangaswamy, S.; Saraf, R. F. Nano Lett. 2004, 4, 939-942. (25) Patolsky, F.; Weizmann, Y.; Willner, I. Nature Mater. 2004, 3, 692-695. (26) Fu, X. Y.; Wang, Y.; Huang, L. X.; Sha, Y. L.; Gui, L. L.; Lai, L. H.; Tang, Y. Q. AdV. Mater. 2003, 15, 902-906. (27) Harnack, O.; Ford, W. E.; Yasuda, A.; Wessels, J. M. Nano Lett. 2002, 2, 919-923.

10.1021/la0634544 CCC: $37.00 © 2007 American Chemical Society Published on Web 04/25/2007

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Figure 1. Procedure for assembly of AuNPs on porphyrin polymer. Porphyrin polymer was dispersed on modified glass or silicon wafers by the Langmuir-Blodgett method. These substrates were then soaked in a solution of py-AuNPs (10 mg/mL in methanol) to connect the pyridine moiety of the nanoparticles to the zinc of the porphyrin units.

by atomic force microscopy (AFM) and scanning electron microscopy (SEM). Spectroscopic studies of the assemblies were performed to study the interactions between the AuNPs and the π-conjugated porphyrin system. Experimental Section Instrumentation. UV-vis absorption spectra were recorded with a Shimadzu UV-3150 double-beam spectrophotometer and fluorescence spectra with a JASCO FP-6600 spectrometer. Analytical gel permeation chromatography (GPC) data were recorded on a JASCO MD-2015 Plus with a Shodex GPC KF-805L column, using THF as the mobile phase at a flow rate of 1 mL/min. NMR spectra were recorded on a JEOL JNM-LA400 spectrometer relative to the TMS internal standard (δ ) 0.00). Matrix assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF-MS) was performed on an Applied Biosystems Voyager DE-STR spectrometer with 2-aminopyridine and dithranol as the matrices. Porphyrin wires were deposited on substrates using a Nihon Laser NL-LB 400 LB film system. AFM observation was performed with a JEOL JSPM4210. All images were collected in tapping mode in air with a silicon cantilever (Mikromasch, silicon cantilevers NSC35/AIBS/50). A JEOL JSM-6700F scanning electron microscope was used in this work. Transmission electron microscopy (TEM) images were obtained with a JEOL JEM-3200FS. Materials. The porphyrin 1 and dendron molecule 8 were synthesized according to the literature.28-30 5,15-Dibromo-10,20-bis(4-methoxyphenyl)porphyrin (2). 5,15-Bis(4-methoxyphenyl)porphyrin (100 mg, 0.19 mmol) and N-bromosuccinimide (65 mg, 0.38 mmol) in CHCl3 were stirred for 30 min. The solvent was evaporated to obtain a precipitate, which was filtered, washed by methanol, and dried. The crude product was purified by passing through a silica gel chromatography column (CHCl3) to give dark purple solid (102 mg, 78.5%). 1H NMR (400 MHz, CDCl3): 9.61 (d, J ) 5 Hz, 4H, β-position of the porphyrin (28) Wiehe, A.; Shaker, Y. M.; Brandt, J. C.; Mebs, S.; Senge, M. O. Tetrahedron 2005, 61, 5535-5564. (29) Hawker, C. J.; Frechet, J. M. J. J. Am. Chem. Soc. 1990, 112, 7638-7647. (30) Hawker, C.; Frechet, J. M. J. J. Chem. Soc., Chem. Commun. 1990, 10101013.

Ozawa et al. ring), 8.87 (d, J ) 5 Hz, 4H, β-position of the porphyrin ring), 7.96 (d, J ) 9 Hz, 4H, Ph), 7.28 (d, J ) 9 Hz, 4H, Ph), 4.12 (s, 6H, OCH3), -2.71 ppm (s, 2H, HN). MALDI-TOF-HRMS (m/z): M+ calcd for C34H24Br2N4O2, 678.02656; found, 678.02605. UV-vis (CHCl3): λmax ) 424, 523, 560, 603, 661 nm. IR (KBr): 2995, 2956, 2926, 2851, 1243, 794 cm-1. 5,15-Dibromo-10,20-bis(hydroxyphenyl)porphyrin (3). 5,15Dibromo-10,20-bis(4-methoxphenyl)porphyrin (41 mg, 0.06 mmol) was dissolved in 5 mL of CH2Cl2, and the solution was cooled at -78 °C under an argon atmosphere. Borane tribromide/CH2Cl2 1 M solution (BBr3) (5 mL) was added dropwise and stirred for 20 h at room temperature. The reaction mixture was washed with water and aqueous NaHCO3 to give a solid product, which was separated by filtering. The solid was dissolved with THF, and the excess solvent was evaporated off. By adding small amount of methanol to the solution, the product was crystallized as purple crystals (31 mg, 78.9%). 1H NMR (400 MHz, THF-d8): 9.62 (d, J ) 5 Hz, 4H, β-position of the porphyrin ring), 8.92 (d, J ) 5 Hz, 4H, β-position of the porphyrin ring), 7.98 (d, J ) 9 Hz, 4H, Ph), 7.19 (d, J ) 9 Hz, 4H, Ph), -2.65 ppm (s, 2H, HN). MALDI-TOF-HRMS (m/z): M+ calcd for C32H20Br2N4O2, 649.99504; found, 649.99475. UVvis (THF): λmax ) 422, 522, 559, 604, 662 nm. IR (KBr): 3397, 2925, 1608, 794 cm-1. [5,15-Dibromo-10,20-bis((4-dendron)phenyl)porphyrinato]zinc(II) (4). 5,15-Dibromo-10,20-bis(4-hydroxyphenyl)porphyrin (3) (900 mg, 1.38 mmol), G2-Br (8) (4.62 g, 3.40 mmol), K2CO3 (3 g, 21.7 mmol), and 18-crown-6-ether (800 mg, 3.03 mmol) were dissolved in 150 mL of THF. The solution was refluxed for 2 days. Chloroform (400 mL) was added to the reaction solution, which was washed several times with water (300 mL). The organic layer was filtered through a column of alumina to remove the crown ether. After the solvent was evaporated off, the residue was dissolved in 100 mL of CHCl3 to which Zn(CH3COO)2‚2H2O (3 g, 13.7 mmol) was added and stirred for 12 h. The solution was washed several times with water (300 mL), and the organic layer was roughly purified by passing through a column of alumina using chloroform as the eluent. The product was isolated as purple solid by passing through an open GPC column (2.67 g, 73.2%). 1H NMR (400 MHz, CDCl3): 9.61 (d, J ) 5 Hz, 4H, β-position of the porphyrin ring), 8.87 (d, J ) 5 Hz, 4H, β-position of the porphyrin ring), 7.96 (d, J ) 8 Hz, 4H, Ph), 7.28 (d, J ) 8 Hz, 4H, Ph), 6.80∼6.00 (m, 42H, dendrimerArH), 5.27 (s, 4H, CH2), 5.01 (s, 8H, CH2), 4.89 (s, 16H, CH2), 3.60 ppm (s, 48H, OCH3). MALDI-TOF-HRMS (m/z): M+ calcd for C146H134Br2N4O30Zn, 2644.6735; found, 2644.6793. UV-vis (CHCl3): λmax ) 430, 564, 606 nm. IR (KBr): 2928, 2837, 1597, 1155 cm-1. [5,15-Bis-((4-dendron)phenyl)-10,20-bis-(trimethylsilylethynyl)porphyrinato]zinc(II) (5). Compound 4 (460 mg, 174 µmol), CuI (4 mg, 21.1 µmol), Pd(PPh3)4 (21 mg, 18.2 µmol), 1 mL of Et3N, and 0.3 mL of trimethylsilylacetylene were dissolved in 10 mL of THF and refluxed for 24 h. The reaction solution was washed several times with water (200 mL), after which the organic layer was dried and evaporated. The product was purified by passing through a silica gel chromatography column (CHCl3/hexane ) 5:5) to give a dark green solid (321 mg, 68.8%). 1H NMR (400 MHz, CDCl3): 9.65 (d, J ) 5 Hz, 4H, β-position of the porphyrin ring), 8.89 (d, J ) 5 Hz, 4H, β-position of the porphyrin ring), 8.02 (d, J ) 8 Hz, 4H, Ph), 7.23 (d, J ) 8 Hz, 4H, Ph), 6.85-6.15 (m, 42H, dendrimer-ArH), 5.30 (s, 4H, CH2), 5.08 (s, 8H, CH2), 4.92 (s, 16H, CH2), 3.62 (s, 48H, OCH3), 0.59 ppm (s, 18H, (CH3)3Si). MALDITOF-HRMS (m/z): M+ calcd for C156H152N4O30Si2Zn, 2680.9316; found, 2680.9349. UV-vis (CHCl3): λmax ) 439, 577, 636 nm. IR (KBr): 2930, 2837, 2138, 1597, 1156 cm-1. [5,15-Diethynyl-10,20-bis-((4-dendron)phenyl)porphyrinato]zinc(II) (6). Compound 5 (100 mg, 37 µmol) and a THF solution of tetrabutylammonium fluoride (1 M, 0.1 mmol, 0.1 mL) were dissolved in 50 mL of CHCl3, and the solution of this mixture was stirred for 5 min. The solvent was evaporated, and the residue was purified by passing through a silica gel chromatography column (CHCl3/AcOEt ) 9:1) to give a green solid (90 mg, 94.8%). 1H NMR (CDCl3): 9.67 (d, J ) 5 Hz, 4H, β-position of the porphyrin

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Figure 3. 1H NMR spectra of (a) porphyrin polymer 7, (b) a mixture of 7 with py-AuNPs, and (c) a mixture of 7 and t-dodeAuNPs in CDCl3. β-Protons of the porphyrin unit (δ ≈ 9.6) clearly shift to upfield only in the case of (b).

Figure 2. (a) UV-vis absorption spectra of porphyrin polymer 7 (n ) 40-200) (solid line: Soret band, λmax ) 495 nm; Q-band, λmax ) 805 nm) and monomer porphyrin 5 (dotted line: Soret band, λmax ) 439 nm; Q-band, λmax ) 590, 642 nm), and (b) fluorescence spectra of 7 (λmax ) 841 nm, excitation at 465 nm). Measurements were performed in DMF solution. The large peak at around 930 nm was attributed to stray excitation light. ring), 8.90 (d, J ) 5 Hz, 4H, β-position of the porphyrin ring), 8.01 (d, J ) 8 Hz, 4H, Ph), 7.29 (d, 4H, Ph), 6.84∼6.12 (m, 42H, dendrimer-ArH), 5.31(s, 4H, CH2), 5.08 (s, 8H, CH2), 4.91 (s, 16H, CH2), 4.15 (s, 2H, CCH), 3.61 ppm (s, 48H, OCH3). MALDI-TOFHRMS (m/z): M+ calcd for C150H136N4O30Zn, 2536.8525; found, 2536.8455. UV-vis (CHCl3): λmax ) 433, 575, 625 nm. IR (KBr): 3276, 2927, 2838, 2095, 1596, 1155 cm-1. Porphyrin Polymer 7. To a pyridine solution (1 mL) of compound 6 (12 mg, 4.7 µmol), Cu(OAc)2 (10 mg, 55 µmol) was added, followed by stirring for 12 h. Water (50 mL) was added to this solution, and the resulting precipitate was filtered then washed with water and methanol. The precipitate was redissolved in DMF and then filtered to remove insoluble solids. The solvent was evaporated to dryness, and the solid was washed with methanol, yielding a blackgreen solid (9 mg). Analytical GPC (Figure 1S) showed that the molecular weight ranged from 4 × 103 to 4 × 105, centered at 4 × 104 Da. 1H NMR (400 MHz, DMF-d7): 9.98 (m, β-position of the porphyrin ring), 9.05 (m, β-position of the porphyrin ring), 8.30 (m, Ph), 7.30 (m, Ph), 6.80-6.20, (m, dendron), 5.45 (m, CH2), 5.20 (m, CH2), 5.09 (m, CH2), 3.78 ppm (m, OCH3). UV-vis (DMF): λmax ) 467, 587, 805 nm. Fluorescence (DMF, λex ) 460 nm), λem ) 841 nm. Synthesis of Pyridineethanethiol-Capped Gold Nanoparticles. t-Dodecanethiol-modified AuNPs (t-dodeAuNPs) were prepared by modification of a reported method.31 In the original report, n-dodecane thiol was used to protect the particles; however, we found that t-dodecanethiol is a better protecting molecule because the substitution reaction with another thiol is much accelerated by using the sterically hindered thiol. The pyridineethanethiol-capped AuNPs (py-AuNPs) were synthesized by ligand-exchange methods.32 To a t-dodeAuNP (10 mg) solution in 10 mL of toluene, 4-pyridineethanethiol (10 mg, 0.72 µmol) was added with stirring for 1 day. The reaction solution was centrifuged (1450g, 30 min), and the supernatant was removed. The residue was dissolved in MeOH and centrifuged (1450g, 30 min) again. The precipitate was removed, and the solvent was (31) Brust, M.; Walker, M.; Bethel, D.; Schifrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 801-802. (32) Hostetler, M. J.; Green, S. J.; Stokes, J. J.; Murray, R. W. J. Am. Chem. Soc. 1996, 118, 4212-4213.

evaporated. The solid was washed three times with toluene and dried to give the product as a black powder (8 mg). Anal. Found: C, 7.22; H, 0.95; N, 0.94; S, 2.84%. IR (KBr) ν: 2956, 2927, 1637, 1601, 1116, 803, 620 cm-1. The TEM image (Figure S2) showed the average diameter of py-AuNPs to be 2.7 ( 0.8 nm. Surface Treatment of Glass and Silicon Substrates. Cover glasses (MATSUNAMI, micro cover glass, 18 × 18 mm, thickness No. 1, 0.12-0.17 mm) were treated with (N-phenyl-3-aminopropyltrimethoxysilane (98%, Shin-Etsu Silicone Co.) to prepare a hydrophobic glass surface as follows. N-Phenyl-3-aminopropyltrimethoxysilane (0.5 mL) was dissolved in 40 mL of 5% aqueous acetic acid solution and stirred for 5 min. Cover glasses were soaked in this solution for 15 min, followed by washing with water, acetone, and ethanol, and were dried with nitrogen gas. Undoped naturally oxidized silicon substrates were washed with acetone and isopropanol and dried with nitrogen gas. Deposition of Porphyrin Polymer. Porphyrin polymers were deposited on the substrates by the LB method. Porphyrin polymers with molecular weight higher than 100 000 Da (about 40-mer) were collected by analytical GPC. They were dissolved in DMF/chloroform (1:1) and diluted with the same solvent to adjust the absorbance to about 0.1 at 461 nm. Droplets of this solution were spread on the water surface of the LB trough. After being left undisturbed for 10 min, the barrier was moved at 3 mm/min to compress the surface film, while the dipper with the substrates was moved vertically upward at 3 mm/min, keeping the interface pressure at 1 mN/m. Substrates with porphyrin wires deposited on them were dried in air. Assembly of py-AuNPs on Porphyrin Polymer. The substrates deposited with porphyrin polymer were soaked in a methanol solution of py-AuNPs (0.5 mg/mL) for 5 min, then washed with methanol and dried with nitrogen gas.

Results and Discussion Preparation and Characterization of Porphyrin Polymer 7. Porphyrin polymer was prepared by oxidative coupling of 6 with Cu(OAc)2 in pyridine at room temperature for 12 h (Scheme 1).33 By adding water, the product was precipitated, then filtered and washed with methanol. The molecular weight was determined by GPC (mobile phase, THF; reference, polystyrene standards; Figure 1S), which indicated the presence of porphyrin polymers with molecular weights greater than 500 000 Da. This corresponded to about 200 porphyrin units, based on the molecular weight of a single porphyrin unit (2536 Da). For the present experiments, porphyrin polymers with a degree of polymerization of 40-200 were used after purification by GPC. Absorption spectra of the porphyrin polymer 7 showed large red-shifts in both the Soret band and Q-band compared with the monomer 6, indicating a high degree of conjugation in 7 (Figure 2). Preparation and Characterization of Pyridine-Capped Gold Nanoparticles. Py-AuNPs were prepared by ligand exchange of t-dodeAuNPs with 4-pyridineethanethiol in toluene.32 (33) Ozawa, H.; Kawao, M.; Tanaka, H.; Ogawa, T. Tetrahedron 2006, 62, 4749-4755.

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Scheme 1. Synthesis of Dendron-Protected Porphyrin Polymer 7

As the ligand exchange progressed, the solubility of AuNPs in toluene gradually decreased and precipitates appeared. Py-AuNPs were characterized by 1H NMR, UV-vis, and IR spectra, elemental analysis, and TEM. The IR spectra of py-AuNPs exhibited a peak at 1601 cm-1, assigned to the stretching of the pyridine ring. The atomic ratio of sulfur to nitrogen was estimated to be 1.32 from the elemental analysis. Assuming only t-dodecanethiol and pyridineethanethiol were present as organic molecules in the AuNPs, the pyridineethanethiol/tdodecanethiol molar ratio was ca. 3.1. The core diameter of AuNPs was determined to be 2.7 ( 0.8 nm from the TEM images (Figure S2). Substrates for Assembly of the Molecular Wires and AuNPs. Glass is a convenient substrate for both electronic and optical measurements because it is electrically insulating and transparent to visible light. However, untreated glass surfaces are hydrophilic, and hydrophobic porphyrin polymer cannot be dispersed homogeneously on such surfaces. In order to make the glass surfaces hydrophobic, they were treated with N-phenyl3-aminopropyltrimethoxysilane. Undoped naturally oxidized silicon substrates were used without this treatment. Assembly of the py-AuNPs on the Porphyrin Polymer. In the first method, porphyrin polymers were dispersed on the

substrate surfaces using an LB method, which resulted in the network structures observed by AFM (Figure 3a). These substrates were immersed in the solution of py-AuNPs (10 mg/mL in methanol) for 2 min and then rinsed with methanol (Figure 1). In the second method, the porphyrin polymer solution (absorbance about 0.72 at 463 nm, CHCl3) was mixed with the solution of py-AuNPs (0.1 mg/mL, CHCl3). The resulting solution was used for further spectroscopic analysis. 1H NMR Spectra of the py-AuNPs Assemblies Connected to Porphyrin Monomer 5. In order to confirm that py-AuNPs could coordinate to the Zn-porphyrin, the 1H NMR spectrum of a mixture of porphyrin monomer 5 (5 mg) and py-AuNPs (10 mg) in CDCl3 was analyzed. Before addition of py-AuNPs, the protons of β-position of the porphyrin ring exhibited peaks at 9.63 and 8.87 ppm (Figure 3a), which shifted to 9.55 and 8.82 ppm, respectively, when py-AuNPs were added (Figure 3b). New broad peaks that appeared at around 2-3 and 6-7 ppm were assigned to the coordinated pyridine moiety of the pyAuNPs after comparison with reported data.34 As the controlexperiment, t-dodeAuNPs were added to the solution of 5 to show no peak shift, as shown in Figure 3c. AFM Observations of py-AuNPs Assembled with Porphyrin Polymer on a Modified Glass Surface. Figure 4 shows

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Figure 4. AFM images of (a) porphyrin polymers 7 and (b) assemblies of py-AuNPs connected to 7, both on modified glass surfaces. Histograms (c) of the height of 7 (2.8 ( 0.5 nm) and (d) of the height of py-AuNPs assembled on 7 (5.3 ( 0.5 nm).

Figure 5. (a) AFM and (b) SEM images of py-AuNP assemblies connected to 7, on undoped naturally oxidized silicon substrates. Histograms (c) of height of py-AuNPs assembled on 7 (5.4 ( 0.7 nm) and (d) of distance between AuNPs on 7 (4.7 ( 0.6 nm).

AFM images of the porphyrin polymer on a modified glass surface before (Figure 4a) and after (Figure 4b) treatment with py-AuNPs. The AFM observations showed similar topographic patterns in both samples. However, the height (5.3 ( 0.5 nm, Figure 4d) of the treated porphyrin polymer was ca. 2.5 nm greater than that of the as-prepared porphyrin polymer (2.8 ( 0.5 nm, Figure 4c). This height difference was almost equal to the diameter of the AuNP cores (2.7 ( 0.8 nm), strongly indicating that the py-AuNPs were connected to the porphyrin polymer molecules. As a control experiment, porphyrin polymers 7 on modified glass surface were treated with t-dodeAuNPs instead of pyAuNPs. (Figure 3Sa) The histogram shows that the average height of structures was 3.0 ( 0.5 nm, which is almost the same as the (34) Anderson, H. L.; Hunter, C. A.; Meah, M. N.; Sanders, J. K. M. J. Am. Chem. Soc. 1990, 112, 5780-5789.

height of the porphyrin polymer themselves. This result strongly supports that alkylthiol protected Au nanoparticles cannot attach to the porphyrin polymers. AFM and SEM Observations of Porphyrin Polymer and py-AuNPs Assembled on a Silicon Surface. Figure 5a shows an AFM image of a porphyrin polymer on a silicon surface after treatment with py-AuNPs. The structures and heights of the treated polymer were similar to that of the analogous polymer examined on the modified glass surface. The ca. 2.3 nm difference between the height (5.4 ( 0.7 nm, Figure 5a) of the polymer after pyAuNP treatment and that before the treatment (3.1 ( 0.5 nm, Figure 4S) was also in good agreement with the diameter of the AuNPs on the silicon surface. Shown in Figure 5b are representative SEM images of the assembly on a silicon surface. Since individual AuNPs could be resolved by SEM, the average center-to-center distance between

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Figure 6. (a) UV-vis absorption spectra in CHCl3 of 7 (dotted line: Soret band, λmax ) 463 nm; Q-band, λmax ) 769 nm), a mixture of 7 and py-AuNPs (solid line: Soret band, λmax ) 450 nm; Q-band, λmax ) 790 nm) and a mixture of 7 and tdodeAuNPs (dashed dotted line: Soret band, λmax ) 463 nm; Q-band, λmax ) 770 nm). (b) UV-vis absorption spectra of pyAuNPs assembled on 7 (solid line: Soret band, λmax ) 498 nm; Q-band, λmax ) ∼840 nm) and 7 (dotted line: Soret band, λmax ) 495 nm; Q-band, λmax ) ∼840 nm) on the modified glass substrates.

adjacent AuNPs was determined to be about 5 nm (Figure 5d). It has been reported that for close-packed AuNPs protected by n-dodecanethiol, the average gap (1.3 nm) between AuNPs was substantially less than twice the thickness of the alkylthiol (2.4 nm). Alkylthiol molecules attached to AuNPs have been known to interpenetrate in the region between adjacent AuNPs.35 In the present assemblies the average gap was ca. 2.3 nm (5-2.7 nm), significantly more than twice the thickness of t-dodecanethiol (2 × 1.0 ) 2.0 nm). This observation showed that the AuNPs in these assemblies were not closed-packed. The pyridine nitrogen of py-AuNPs was likely coordinated to the zinc atom of every fourth porphyrin unit of the polymer, since the distance between zinc atoms of neighboring porphyrin units was ca. 1.3 nm, and 4 × 1.3 nm ) 5.2 nm closely matched the observed center-tocenter distance between adjacent AuNPs. UV-Vis Absorption Spectra of py-AuNPs Assembled on Porphyrin Polymers. UV-vis absorption spectra of the assemblies in solution and on the glass substrate are shown in Figure 6. When t-dodeAuNPs were added to the porphyrin polymer solution, the Soret and Q-bands in the resulting assembly did not shift. When py-AuNPs were added to the solution, the Soret and Q-bands were broad ended and λmax of the Q-band shifted from 769 to 790 nm. On the glass surface the Soret band of the porphyrin polymer appeared at a longer wavelength (Figure 6b, λmax ) 495 nm) than in solution (Figure 6a, λmax ) 463 nm). The red-shift of this band in the polymer on the solid surface was probably due to the adoption of more ordered planar conformations, with little torsional disorder.36,37 Additionally, the Soret band of the polymer/py-AuNP assemblies was redshifted by about 5 nm in comparison with that of the polymers (35) Andres, R. P.; Bielefeld, J. D.; Henderson, J. I.; Janes, D. B.; Kolagunta, V. R.; Kubiak, C. P.; Mahoney, W. J.; Osifchin, R. G. Science 1996, 273, 16901693. (36) Taylor, P. N.; Huuskonen, J.; Rumbles, G.; Aplin, R. T.; Williams, E.; Anderson, H. L. Chem. Commun. 1998, 909-910. (37) Anderson, H. L. Inorg. Chem. 1994, 33, 972-981.

Figure 7. Fluorescence and excitation spectra of (a) 7 (λmax ) 780 nm), (b) assemblies of py-AuNPs bonded to 7 (λmax ) 767 nm) and (c) py-AuNPs in CHCl3 (excitation at 460 nm and observation at 760 nm).

alone. This red-shift could be attributed to the extension of resonance between the pyridine molecules and the porphyrin units.37,38 However, this effect was not significant because the coordination interaction between porphyrin and py-AuNPs was weak. Fluorescence Spectra of Assemblies of Porphyrin Polymer 7 with py-AuNPs. Emission and excitation spectra of the porphyrin polymer, py-AuNPs and their assemblies are shown in panels a, b, and c of Figure 7, respectively. The fluorescence peak of porphyrin polymer 7 in CHCl3 was 780 nm (Figure 7a). By adding the py-AuNPs to the solution of 7, the intensity of this peak was significantly reduced and a new weak band appeared at around 885 nm (Figure 7b). The new band could be attributed to the plasmon band of the py-AuNPs by comparison with the fluorescence spectrum of pure py-AuNPs (Figure 7c). The excitation spectrum of py-AuNP/7 mixtures observed at 760 nm showed peaks at 451, 465, and 558 nm. The former two peaks corresponded to the Soret band of 7 and the latter peak probably corresponded to the plasmon peak of the py-AuNPs. The foregoing results implied that in the py-AuNP/7 composites, (38) Screen, T. E. O.; Thorne, J. R. G.; Denning, R. G.; Bucknall, D. G.; Anderson, H. L. J. Mater. Chem. 2003, 13, 2796-2808.

Synthesis of Dendron-Protected Porphyrin Wires

energy transfer occurred from 7 to py-AuNPs and also from py-AuNPs to 7.

Conclusion One-dimensional assemblies were fabricated by the binding of py-AuNPs onto conjugated porphyrin polymers. The assembled structures were examined by AFM and SEM on glass or silicon surfaces. The average distance between AuNPs was about 5 nm, and they were connected to every fourth porphyrin unit in the polymer chain. Spectroscopic studies of the assemblies of pyAuNPs with porphyrin polymers showed that there were electronic interactions between these two moieties. Nanoscale conductance measurements on the assemblies are in progress.

Langmuir, Vol. 23, No. 11, 2007 6371

Acknowledgment. This work was supported by a Grantin-Aid for Scientific Research (No. 15201028 and No. 14654135) from the Ministry of Culture, Education, Science Sports, and Technology of Japan. Supporting Information Available: Details of synthesis of porphyrin derivatives; UV-vis and fluorescence spectra; GPC data for porphyrin polymer; UV-vis spectra and TEM images of t-dodecanethioland pyridine-capped AuNPs; and AFM image of porphyrin polymer on silicon substrate. This material is available free of charge via the Internet at http://pubs.acs.org. LA0634544