Controllable Growth of Well-Defined Regular Multiporphyrin Array

Apr 21, 2005 - A dipper from KSV Instrument Co. (Finland) was used to transfer multiporphyrin nanocrystal layers with the dipping rate of 1 mm/min. To...
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Langmuir 2005, 21, 5079-5084

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Controllable Growth of Well-Defined Regular Multiporphyrin Array Nanocrystals at the Water-Chloroform Interface Bing Liu,† Dong-Jin Qian,*,† Hong-Xiang Huang,†,‡ Tatsuki Wakayama,§ Shigeki Hara,‡ Wei Huang,† Chikashi Nakamura,§ and Jun Miyake§ Institute of Advanced Materials, Fudan University, Handan Road 200433, Shanghai, China, Research Institute for Innovation in Sustainable Chemistry, National Institute of Advanced Industrial Science and Technology, Tsukuba 305-8565, Japan, and Research Institute of Cell Engineering, National Institute of Advanced Industrial Science and Technology, Amagazaki 661-0974, Hyogo, Japan Received January 8, 2005. In Final Form: March 27, 2005 On the basis of the coordination geometry of metal ions, regular cubic, clubbed, and wirelike nanocrystals of Cd2+-/PtCl62--mediated, and Hg2+-/Ag+-/PtCl42--mediated multiporphyrin arrays have been grown at the water-chloroform interface. The nanocrystal growth process was monitored by the transmission electron microscopy (TEM), which revealed (1) an intrinsic rule for coordination polymers, that is, the geometries of metal ions (as connects for the coordination polymers) dominate the frameworks of the related polymeric nanocrystals, and (2) one kind of intuitive nanocrystal growth processes at the interfaces. Both electron diffraction and X-ray diffraction patterns indicated the formation of well-defined nanocrystals. It was found that single-/microcrystals were formed at first, and then they grew into polycrystals. The nanocrystal layer was transferred onto Si and quartz substrate surfaces by the Langmuir-Blodgett method, with its composition analyzed by X-ray photoelectron spectroscopy as well as the arrangement of porphyrin macrocycles in the nanocrystals by UV-vis absorption spectroscopy.

Introduction Predetermined building blocks based on tetrasubstituted porphyrins and metalloporphyrins have recently attracted much attention in the crystal engineering of coordination frameworks.1,2 The nature of such building blocks can lead to synthetic supramolecular arrays with potential applications in material chemistry as photonic devices, molecular switches, conductive polymers, chemical sensors, and receptors for selective catalysis, and with potential interest as zeolite-like materials.3,4 Moreover, * To whom correspondence may be addressed: e-mail, [email protected]; tel, +86-21-65643666; fax, +86-21-55664192. † Institute of Advanced Materials, Fudan University. ‡ Research Institute for Innovation in Sustainable Chemistry, National Institute of Advanced Industrial Science and Technology. § Research Institute of Cell Engineering, National Institute of Advanced Industrial Science and Technology. (1) (a) Milic, T.; Garno, J. C.; Batteas, J. D.; Smeureanu, G.; Drain, C. M. Langmuir 2004, 20, 3974-3983. (b) Shmilovits, M.; Vinodu, M.; Goldberg, I. Cryst. Growth Des. 2004, 4, 633-638. (c) Redman, J. E.; Feeder, N.; Teat, S. J.; Sanders, J. K. M. Inorg. Chem. 2001, 40, 24862499. (d) Cheng, K. F.; Drain, C. M.; Grohmann, K. Inorg. Chem. 2003, 42, 2075-2083. (e) Stulz, E.; Scott, S. M.; Ng, Y. F.; Bond, A. D.; Teat, S. J.; Darling, S. L.; Feeder, N.; Sanders, J. K. M. Inorg. Chem. 2003, 42, 6564-6574. (f) Alessio, E.; Ciani, E.; Iengo, E.; Kukushkin, V. Y.; Marzilli, L. G. Inorg. Chem. 2000, 39, 1434-1443. (2) (a) Iengo, E.; Zangrando, E.; Alessio, E. Eur. J. Inorg. Chem. 2003, 2371-2384. (b) Hagrman, D.; Hagrman, P. J.; Zubieta, J. Angew. Chem., Int. Ed. 1999, 38, 3165-3168. (c) Iengo, E.; Milani, B.; Zangrando, E.; Geremia, S.; Alessio, E. Angew. Chem., Int. Ed. 2000, 39, 10961099. (d) Suslick, K. S.; Rakow, N. A.; Kosal, M. E.; Chou, J.-H. J. Porphyrins Phthalocyanines 2000, 4, 407-413. (3) (a) Burrell, A.; Officer, D.; Plieger, P.; Reid, D. C. W. Chem. Rev. 2001, 101, 2751-2796. (b) Milic, T. N.; Chi, N.; Yablon, D. G.; Flynn, G. W.; Batteas, J. D.; Drain, C. M. Angew. Chem., Int. Ed. 2002, 41, 2117-2119. (c) Drain, C. M. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 5178-5182. (d) Drain, C. M.; Batteas, J. D.; Flynn, G. W.; Milic, T.; Chi, N.; Yablon, D. G.; Sommers, H. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 6498-6502. (4) Kitagawa, S.; Kitaura, R.; Noro, S. Angew. Chem., Int. Ed. 2004, 43, 2334-2375.

porphyrin-based building blocks (usually called multiporphyrin arrays) can be use to mimic natural synthetic process, where multiple chromophoric assemblies (lightharvesting complex) capture sunlight and transfer the energy/electrons into the photosynthetic reaction centers.5 These porphyrin building blocks or multiporphyrin arrays are usually synthesized or assembled in organic solutions, and some of their crystal structures have been analyzed by X-ray crystallography measurements.6 On the basis of the large amount of interesting work,1a,d,3b-d and recently on the self-organization of tetrameric porphyrin arrays on the surfaces, Drain and co-workers have demonstrated that features, such as the number, position, and nature of the peripheral groups of porphyrins, are of key importance to the two-dimensional (2D) and 3D selforganization of assemblies.7 We are currently investigating the metal-mediated multiporphyrin arrays at the interfaces of air-water or water-chloroform or directly on the solid surfaces,8-10 (5) (a) Choi, M. S.; Yamazaki, T.; Yamazaki, I.; Aida, T. Angew. Chem., Int. Ed. 2004, 43, 150-158. (b) Holten, D.; Bocian, D. F.; Lindsey, J. S. Acc. Chem. Res. 2002, 35, 57-69. (c) Imamura, T.; Fukushima, K. Coord. Chem. Rev. 2000, 198, 133-156. (6) (a) Sharma, C. V. K.; Broker, G. A.; Huddleston, J. G.; Baldwin, J. W.; Metzger, R. M.; Roger, R. D. J. Am. Chem. Soc. 1999, 121, 11371144. (b) Gupta, I.; Agarwal, N.; Ravikanth, M. Eur. J. Org. Chem. 2004, 1693-1697. (c) Iengo, E.; Zangrando, E.; Geremia, S.; Graff, R.; Kieffer, B.; Alessio, E. Chem. Eur. J. 2002, 8, 4670-4674. (7) Milic, T.; Garno, J. C.; Batteas, J. D.; Smeureanu, G.; Drain, C. M. Langmuir 2004, 20, 3974-3983. (8) (a) Qian, D. J.; Nakamura, C.; Miyake, J. Langmuir 2000, 16, 9615-9619. (b) Qian, D. J.; Nakamura, C.; Miyake, J. Thin Solid Films 2001, 397, 266-275. (9) (a) Qian, D. J.; Nakamura, C.; Miyake, J. Chem. Commun. 2001, 2312-2313. (b) Qian, D. J.; Nakamura, C.; Wakayama, T.; Miyake, J. J. Porphyrins Phthalocyanines 2003, 7, 414-418. (10) (a) Qian, D. J.; Nakamura, C.; Ishida, T.; Wenk, S.-O.; Wakayama, T.; Takeda, S.; Miyake, J. Langmuir 2002, 18, 10237-10242. (b) Qian, D. J.; Wakayama, T.; Nakamura, C.; Miyake, J. J. Phys. Chem. B 2003, 107, 3333-3335.

10.1021/la050064t CCC: $30.25 © 2005 American Chemical Society Published on Web 04/21/2005

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Scheme 1. Schematic Frameworks for the Multiporphyrin Arrays with Cd2+/PtCl62- and Ag+/Hg2+/PtCl42- as Connectors

where structures of the building blocks can be tailored by selecting various metal ions and/or tetrasubstituted porphyrins. Although there are some advantages for the assembly of multiporphyrin arrays at the air-water interface and on the solid surface, the liquid-liquid interface shows unique features for the design and growth of regular multiporphyrin array nanoparticles and nanocrystals, since the particles at the fluid interface are highly mobile and can rapidly achieve an equilibrium assembly.11,12 The rapid diffusion of nanoparticles and reagents in either fluid also lead to very efficient interfacial chemistry, including interface reaction and molecular assembly. Moreover, the interfaces between two immiscible solutions provide a defect-free junction, which is very important for the products with high purity. Some of metal (e.g., Au, Ag), metal oxide (e.g., ZnO, TiO2), and metal sulfide (e.g., CdS, ZnS) nanoparticles have been prepared at liquid-liquid interfaces in the past decade.13,14 We report here structure-controllable (predictable or predetermined) nanocrystal growth of well-defined multiporphyrin arrays at the liquid-liquid interfaces via metal ions as connects and porphyrin derivative as linkers. The used porphyrin contains four pyridyl (Py) groups, which are able to coordinate with metal ions (Cd2+, PtCl62-/ PtCl42-, Ag+, and Hg2+) to form “coordination bond M-Py” (M refers to the metal ions). Thus, depending on the different geometries of the metal ions, coordination multiporphyrin arrays with different frameworks can be predicted. Some research groups have revealed such features through synthesis of the relative multiporphyrin arrays and analyzed their crystal structure with the use of X-ray crystallography. For example, Rogers and co(11) (a) Lin, Y.; Skaff, H.; Emrick, T.; Dinsmore, A. D.; Russell, T. P. Science 2003, 299, 226-229. (b) Liljeroth, P.; Ma¨lkia¨, A.; Cunnane, V. J.; Kontturi, A. K.; Kontturi, K. Langmuir 2000, 16, 6667-6673. (c) Bowden, N.; Arias, F.; Deng, T.; Whitesides, G. M. Langmuir 2001, 17, 1757-1765. (12) (a) Rao, C. N. R.; Kulkarni, G. U.; Thomas, P. J.; Agrawal, V. V.; Saravanan, P. J. Phys. Chem. B 2003, 107, 7391-7395. (b) Jensen, H.; Kakkassery, J. J.; Nagatani, H.; Fermı´n, D. J.; Girault, H. H. J. Am. Chem. Soc. 2000, 122, 10943-10948. (c) Eugster, N.; Fermı´n, D. J.; Girault, H. H. J. Am. Chem. Soc. 2003, 125, 4862-4869. (13) (a) Su, B.; Abid, J.-P.; Fermı´n, D. J.; Girault, H. H.; Hoffmannova´, H.; Krtil, P.; Samec, Z. J. Am. Chem. Soc. 2004, 126, 915-919. (b) Kumar, A.; Mandal, S.; Mathew, S. P.; Selvakannan, P. R.; Mandale, A. B.; Chaudhari, R. V.; Sastry, M. Langmuir 2002, 18, 6478-6483. (14) (a) Sathsye, S. D.; Patil, K. R.; Paranjape, D. V.; Mitra, A.; Mandale, A. B. Langmuir 2000, 16, 3487-3490. (b) Wu, P. W.; Gao, L.; Guo, J. K. Thin Solid Films 2002, 408, 132-135.

workers have pointed out that several multiporphyrin arrays can be formed by coordination reaction of MX2 (M ) Cd, Hg, Pb; X ) Br, I) with tetrapyridylporphyrin (TPyP) in a mixed solution of 1,1,2,2-tetrachloroethane and methanol.6a On the basis of the different geometries, either 1D or 2D coordination networks can be achieved.15 As shown in Scheme 1, the Cd-TPyP and PtCl62--TPyP multiporphyrin arrays adopt octahedral (Oh) geometry with trans-halides and coordination of four pyridyl moieties to form isostructural nanoporous coordination polymers. According to this geometric feature and our previous work on the assembly of multiporphyrin arrays, it can be predicted that a regular three-dimensional (3D) nanocrystal of the Cd-/PtCl62--TPyP multiporphyrin arrays might be grown at the water-chloroform interface. On the other hand, Ag+, Hg2+, and PtCl42- are tetrahedrally (Td) coordinated with TPyP,6a,15 and beltlike (rodlike) 2D nanocrystals are predicted (Scheme 1). In this paper, the water-chloroform interface was used as a mediator for coordination assembly of multiporphyrin nanocrystals, and the Langmuir-Blodgett (LB) method was used to transfer the nanocrystal layer to solid surfaces. The nanocrystal formation process and its frameworks were characterized by using transmission electron microscopy (TEM), electron diffraction (ED) and X-ray diffraction (XRD). Moreover, X-ray photoelectron spectra (XPS), and UV-vis absorption spectra were used to analyze the composition and porphyrin arrangement of the nanocrystals. Experimental Section Materials. 5,10,15,20-Tetra(4-pyridyl)-21H,23H-porphyrin (TPyP) was purchased from Aldrich Chemical Co.; CdCl2, HgCl2, and AgNO3 were from Shanghai Chemical Reagent Co.; and K2PtCl4 and K2PtCl6 were from Wako Pure Chemicals, Ltd. Chloroform was from Fisher Chemicals Co. All chemicals were used as received without further purification. Double distilled water (first deionized) was used to prepare aqueous solutions. Growth of Multiporphyrin Array Nanocrystals at the Water-Chloroform Interface. The reaction process was (15) (a) Carlucci, L.; Ciani, G.; Proserpio, D. M.; Porta, F. Angew. Chem., Int. Ed. 2003, 42, 317-322. (b) Gibson, V. C.; Long, N. J.; White, A. J. P.; Williams, C. K.; Williams, D. J. Organometallics 2000, 19, 4425-4428. (c) Fornie´s, J.; Martı´n, A.; Sicilia, V.; Villarroya, P. Organometallics 2000, 19, 1107-1114. (d) Albano, V. G.; Monari, M.; Orabona, I.; Panunzi, A.; Roviello, G.; Ruffo, F. Organometallics 2003, 22, 1223-1230. (e) Han, L.-B.; Shimada, S.; Tanaka, M. J. Am. Chem. Soc. 1997, 119, 8133-8134.

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Figure 1. TEM images of the Cd-TPyP multiporphyrin array nanocrystals grown at the water-chloroform interface after (a) 1 h, (b) 12 h, (c) 24 h, (d) 2 days, (e) 5 days, and (f) 14 days. Concentrations of CdCl2 and TPyP were 0.04 M and 0.3 mM, respectively. similar to our previous work on the preparation of multiporphyrin array monolayers.9b Briefly as an example, a 0.04 M CdCl2 solution was used as the water phase and 0.3 mM TPyP chloroform solution as the organic phase. During experiments, the aqueous solution of metal ions was slowly added on the chloroform phase surface at which interfacial reaction occurred. Experimental conditions for other metal ions with TPyP are described in the Supporting Information (Table S1). Transfer of Multiporphyrin Array Nanocrystals onto Solid Surfaces. Layers of nanocrystals grown at the interface were transferred on to substrate surfaces with the use of LB lifting method. A dipper from KSV Instrument Co. (Finland) was used to transfer multiporphyrin nanocrystal layers with the dipping rate of 1 mm/min. To avoid the influence from water phase, hydrophobic substrates were used to deposit the nanocrystal layer with a vertical dipping technique. The hydrophobic substrates were prepared by immersing a hydrophilic substrate in 10% (CH3)3SiCl chloroform solution for about 1 h. Characterization of Multiporphyrin Array Nanocrystals. TEM images were taken on a Hitachi H-600 electron microscope operated at 75 kV. The nanocrystals were deposited on 230-mesh copper grid covered with Formvar by the LB method. X-ray diffraction for the nanocrystal LB films on the Si substrate surface was carried out with a Rigaku D/max-γB diffractometry in transition mode and Cu KR radiation. The scan range of 2θ was 10-67° with a step interval of 0.02°. UV-vis spectra for porphyrins in chloroform solution and the nanocrystal LB films were measured with the use of Shimadzu UV-2550 UV-vis spectrophotometer. X-ray photoelectron spectroscopy was measured using a PHI5600ci system with a monochromatic Al KR X-ray source (PerkinElmer). The samples (nanocrystals) were immobilized on hydrophobic quartz substrate surface by the LB method. Photoelectrons from quartz substrate surfaces on the XPS measurements were detected at an angle of 75°. All peaks were resolved using the spectral processing program in the XPS system.

Results and Discussion Formation of Metal-Mediated Multiporphyrin Array Nanocrystals. A very important condition for the successful growth of molecular crystals at the waterchloroform interface is that the products (nanocrystals) are unable to dissolve into either water or chloroform solution; thus the crystals of many inorganic salts cannot

be prepared at the interface. However, we have confirmed that the Cd2+-mediated multiporphyrin arrays do not dissolve in either water or chloroform solution,8-10 which makes it possible for their crystals to be able to grow at the water-chloroform interface. Similar dissolubility was found for the Pt-, Ag-, and Hg-TPyP multiporphyrin nanocrystals. During the experiments, we further found that the formation rate of multiporphyrin nanocrystals was strongly dependent on the kinds of metal ions; the fastest one was Hg-TPyP in the present work, some fine particles could be found in the chloroform phase after 1 h of interfacial reaction when the concentration of HgCl2 used was about 0.01 M. Conversely, the lowest one was Cd-TPyP; to get its regular nanocrystals, 2 days were needed when 0.04 M CdCl2 and 0.3 mM TPyP were used. TEM Images of Metal-Mediated Multiporphyrin Array Nanocrystals. To clarify the growth process for the multiporphyrin nanocrystals, we observed their TEM images as a function of reaction time as well as concentrations of either metal ions or TPyP. Their formation process, sizes, and frameworks are individually described in the following sections. 1. Cd-TPyP Nanocrystals. Figure 1 shows a set of TEM images of the Cd-TPyP multiporphyrin arrays in 1 h, 12 h, 1 day, 2 days, 5 days, and 14 days after the interfacial reaction started. As shown in the figures, many fine nanocrystals with sizes of 10-30 nm were formed within 1 h. With the increase of reaction time, these TEM images revealed the following two features: (1) The size of the nanocrystals became larger and larger, for example, about 30-80 nm after 12 h, and about 200-300 nm after 48 h. (2) The nanocrystals were round or irregular at first (within 1 day), then became mixtures of round crystals and cubic crystals (1-2 days, Figure 1c,d), and finally grew into well-defined regular cubic crystals (after 2 days). The TEM image with both round and cubic nanocrystals in Figure 1c indicates that the cubic crystals are formed not only by coordination reaction with Cd2+ and TPyP in solutions but also by fusing the neighboring fine crystals. To our knowledge, although crystals of some metal-

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Figure 2. ED images of the Cd-TPyP multiporphyrin array nanocrystals prepared (a) within 1 day and (b) after 2 days.

Figure 4. TEM images of the Hg-TPyP multiporphyrin array nanocrystals. Concentrations of HgCl2 and TPyP were 0.01 M and 0.3 mM, respectively.

mediated multiporphyrin arrays have been analyzed by X-ray crystallography,6 there is no report on this kind of “direct” observation of the nanocrystal growth process up to now. Electron diffraction for the nanocrystals formed within 1 day and after 2 days was measured. As shown in Figure 2, regular hexagonal spots were recorded when the multiporphyrin nanocrystal layer was transferred within 1 day, and more spots were recorded when the nanocrystal layer was transferred after 2 days. These ED features may be attributed to the Cd-TPyP multiporphyrin nanocrystals being single-/microcrystals when the first monomolecular layer or regular ultrathin layer is formed and then growing into polycrystals with more and more Cd2+ ions and TPyP molecules coordinated. With the use of different concentrations of either CdCl2 or TPyP, we did not find significant difference for the frameworks of the nanocrystals; however, the formation rate, nanocrystal size, and number clearly increased with the increase of CdCl2 or TPyP concentration. (Figures are shown in the Supporting Information.) 2. Ag-TPyP Nanocrystals. Figure 3 shows a set of TEM images of the Ag-TPyP multiporphyrin array nanocrystals 3, 30, and 60 min after the interfacial reaction started. It can be seen that clubbed nanocrystals were formed quickly with their width about 200-400 nm and that no obvious change was observed for the width of AgTPyP nanocrystals at different reaction times. Conversely, the length of the clubbed nanocrystals was largely increased with the reaction time (from part a to part c of Figure 3). These phenomena suggest that the Ag+ ions and pyridyl groups at the end of the nanorods are not completely coordinated even when the nanorods were formed. This is in agreement with Scheme 1b, where it is predicted that Ag-TPyP multiporphyrins can grow to be a linear (or zig-type) framework. The crystal engineering studies on the multiporphyrin arrays have shown that, different from Cd2+ (Oh geometry), when the connector is Ag+, it is tetrahedrally (Td geometry) coordinated with two pyridyl moieties from different TPyP

molecules (Scheme 1).15 On the basis of the Ag+ geometry feature, it is expected that the Ag-TPyP multiporphyrin arrays might form “2D” crystal structure (nanorod or nanowire). The TEM images in Figure 3 confirm this expectation and strongly support the crystal analysis for the Ag-TPyP supramolecular arrays.15 The difference is that crystal analysis is a theoretical calculation from X-ray crystallography, here we revealed “visible” TEM photos for the formation of the regular nanocrystals and clarified the intrinsic relationship between the geometric features of metal ions and the crystal frameworks of the relative coordination polymers. 3. Hg-TPyP Nanocrystals. Time dependence for the TEM images of Hg2+-mediated multiporphyrin nanocrystals was also investigated with the features similar to that of Ag-TPyP nanocrystals. Thus we ignored their formation processes, just showing the final TEM image here. As shown in Figure 4, wirelike nanocrystals were formed for the Hg-TPyP multiporphyrin arrays with the width (diameter) up to about 200 nm and length over 10 µm. 4. Pt-TPyP Nanocrystals. It has been reported that PtCl42- and PtCl62- are of Td and Oh geometries,15 respectively; thus a comparison of their multiporphyrin array nanocrystals is interesting. Figure 5 shows a set of TEM images of the PtCl42--TPyP and PtCl62--TPyP multiporphyrin array nanocrystals in 1 h and 5 days after the interfacial reaction started. Although it is not so clear as that in the cases of Cd-, Ag-, and Hg-TPyP nanocrystals, the photos did indicate that, at the beginning of the reaction (1 h), irregular nanorods and round nanoparticles were formed for the PtCl42-- and PtCl62--TPyP multiporphyrin array nanocrystals, respectively (Figure 5A(a) and 5B(a)). After 5 days, PtCl42--TPyP nanocrystals grew into longer nanorods and PtCl62--TPyP nanocrystals became irregular squares, respectively. Figure 5A(a) indicated that the length and width of the PtCl42--TPyP nanorods were about 200 and 20 nm after 1 h of interfacial reaction, respectively. Similar to the Hg-TPyP and AgTPyP nanorods/nanowires, the width did not change a lot

Figure 3. TEM images of the Ag-TPyP multiporphyrin array nanocrystals grown at the water-chloroform interface after (a) 3, (b) 30, and (c) 60 min. Concentrations of AgNO3 and TPyP were 0.1 M and 0.3 mM, respectively.

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Table 1. Deconvolution of the XPS Peaks for Cd-TPyP and Ag-TPyP Multiporphyrin Array Nanocrystals on Quartz Surfaces multiporphyrins

Si(2P)

Cl(2P)

C(1s)

CdTPyP AgTPyP

103.2 103.6

198.4

284.1 284.6

after 5 days while the length of the crystals increased to over 1 µm (Figure 5A(b)). These different shapes of nanocrystals, together with those obtained by the connector ions of Cd2+, Hg2+, and Ag+, suggest that geometry of metal or metal complex ions (here, Oh and Td) dominates the frameworks of the produced coordination polymers and strongly supports our assumptions in the Scheme 1.

Ag(3d3/2, 3d5/2) 373.5, 367.3

Cd(3d3/2, 3d5/2)

N(1s)

O(1s)

411.6, 404.8

399.1 399.8

531.8 532.5

groups of patterns, there are also some small peaks. Hence, the XRD measurements support our conclusions obtained from TEM and ED observation, that is, the multiporphyrin array nanocrystals are in the form of micro-/polycrystals. The lattice spacing corresponds to the interlayer stacking distances of the multiporphyrin nanocrystals. The obtained three distances may relate to the different arrangement of Cd-TPyP microcrystals as indicated by its ED photo (Figure 2b). Another explanation may be due to intercalation of other small molecules (solvents) in the multiporphyrin sheet, as stated in the literature.6a However, since the formed nanocrystals were so small that we were unable to analyze them by the X-ray crystallography, it is difficult to clarify the relevant nanostructure.

Figure 5. TEM images of the (A) PtCl42--TPyP and (B) PtCl62--TPyP multiporphyrin array nanocrystals after (a) 1 h and (b) 5 days. Concentrations of K2PtCl4, K2PtCl6, and TPyP were 5, 5, and 0.3 mM, respectively.

XPS Analysis of Multiporphyrin Array Nanocrystals. A chemical analysis of the transferred Cd-TPyP and Ag-TPyP multiporphyrin nanocrystals on the quartz surfaces was carried out by XPS measurements. The spectra exhibit several peaks in the binding energy between 90 and 540 eV, which are summarized in Table 1.16 The detected elements agree with the polymer composition. Si is from the substrate surface. No Cl element was detected in the Ag-TPyP multiporphyrin arrays because AgNO3 solution was used as water phase during assembly of Ag-TPyP. X-ray Diffraction. A diffractogram is capable of providing structural information about the crystals and powders;17 thus layers of the multiporphyrin array nanocrystals were transferred onto Si substrate surface for the XRD measurement. As an example, Figure 6 shows the XRD patterns for the LB films of the Cd-TPyP multiporphyrin nanocrystals transferred after one-day of interfacial reaction. The diffraction peaks recorded could be divided into three groups, which were marked as stars, triangles, and circles, respectively (Figure 6). According to Bragg’s equation (2d sin θ ) λ/n), these three groups of patterns correspond to lattice spacing of about 7.24, 5.97, and 4.92 Å, respectively. Except for the above three (16) (a) Brun, M.; Berthet, A.; Bertolini, J. C. J. Electron Spectrosc. Relat. Phenom. 1999, 104, 55-60. (b) Liu, M.; Xu, G.; Liu, Y.; Chen, Q. Langmuir 2001, 17, 427-431. (c) Pavageau, M.-P.; Morin, A.; Seby, F.; Guimon, C.; Krupp, E.; Pecheyran, C.; Poulleau, J.; Donard, O. F. X. Environ. Sci. Technol. 2004, 38, 2252-2263. (17) (a) Waddon, A. J.; Coughlin, E. B. Chem. Mater. 2003, 15, 45554561. (b) El-Kouedi, M.; Foss, C. A., Jr.; Bodolosky-Bettis, S. A.; Bachman, R. E. J. Phys. Chem. B 2002, 106, 7205-7209.

Figure 6. XRD spectrum for the Cd-TPyP multiporphyrin array nanocrystals.

UV-vis Absorbance of the Multiporphyrin Array Nanocrystals. Layers of the nanocrystals after the different reaction time were transferred from the waterchloroform interface to the hydrophobic quartz substrate surface by the LB method. The dipping rate was kept at 1 mm/min in order to retain the structure of the multiporphyrin arrays. As an example, we discussed here UVvis absorption properties of the Cd-TPyP multiporphyrin nanocrystals. As shown in Figure 7, the Soret band

Figure 7. Absorption spectra for the films of Cd-TPyP multiporphyrin array nanocrystals assembled at different reaction times: 1 h (s), 3 h (- - -), and 1 day (- ‚ -).

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Figure 8. Absorption spectra for the film of the Cd-TPyP multiporphyrin array nanocrystals (s), the TPyP LB film transferred from the pure water surface (- - -), casting film (- ‚ ), and dilute TPyP chloroform solution (‚‚‚).

appeared at about 427, 428, and 429.5 nm for the LB film of the Cd-TPyP nanocrystals transferred in 1 h, 3 h, and 1 day after the reaction started, respectively. This small red shift is probably not from the size effect of the nanocrystals since the size of the crystals falls in the range of 10-30 nm even when the films were transferred immediately after the reaction started, but from the interaction between porphyrin rings, the larger the size of the nanocrystals, the stronger the π-conjugated interaction,18 which leads to red shift of the Soret band. The absorption spectrum for the film of the Cd2+-TPyP nanocrystals was further compared with the spectra of the dilute TPyP chloroform solution, the TPyP casting film, and the TPyP LB film prepared from the pure water surface. As shown in Figure 8, the Soret band is at about 417 nm in the dilute TPyP chloroform solution where the TPyP molecules are in monomer form,19 at about 431 nm in the casting film, and at about 442 nm in the TPyP LB film where the TPyP molecules are in aggregates.8 Figure 6 indicates the Soret band at about 427-429.5 nm for the Cd-TPyP nanocrystals; hence compared to the monomer state, there is a 10-12.5 nm red shift for the present nanocrystals, a 14 nm red shift for the casting film, and a 25 nm red shift for the TPyP LB film. As has been pointed (18) (a) Choudhury, B.; Weedon, A. C.; Bolton, J. R. Langmuir 1998, 14, 6192-6198. (b) Khairutdinov, R. F.; Serpone, N. J. Phys. Chem. B 1999, 103, 761-769. (19) Qian, D. J.; Planner, A.; Miyake, J.; Fra¸ ckowiak, D. J. Photochem. Photobiol., A 2001, 144, 93-99.

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out that the red shift of the Soret band absorption is due to the π-π* interaction between two porphyrin rings, we can conclude that porphyrin interaction in the present nanocrystals is stronger than that in solution but weaker than that in the TPyP LB films, slightly weaker than that in the casting film. This is in agreement with the previous observations for the Cd-TPyP multiporphyrin arrays prepared from the air-water interface and the crystals of Cd-TPyP multiporphyrin arrays synthesized in organic solutions.8 The different interaction is related to the distance between two porphyrin rings. Since the porphyrins exist as monomers in the solution, aggregates via π-π* interaction in the LB films, and multiporphyrin arrays via coordination bond in the nanocrystals, the distance of two porphyrin rings increases in the order LB films, multiporphyrin array, solution, thus leading to the decrease of the porphyrin-porphyrin interaction in the same order. On the basis of the above UV-vis, XPS, XRD, and ED analysis, we suggest that the M-TPyP multiporphyrin nanocrystals are of similar crystal structure to those synthesized from organic solutions.6,15 Conclusions We have demonstrated that rodlike or wirelike nanocrystal frameworks can be formed by the interfacial coordination reaction of HgCl2, AgNO3, or K2PtCl4 with TPyP, and that a square nanocrystal be formed by CdCl2 or K2PtCl6 with TPyP at the water-chloroform interface. On the basis of the consideration of Hg2+, Ag+, and PtCl42(tetrahedral) and of Cd2+ and PtCl62- (octahedral) geometric features, it can be concluded that the frameworks of coordination polymers produced are dominant by the geometric features of the metal ions. Thus, from the features of metal ions and ligands, it is possible to predict and design the coordination polymer nanocrystals with desired structure. Acknowledgment. This work was financially supported by NEDO’s International Joint Research Grant Program (Japan) and by the Shanghai Commission of Science and Technology under Grants 022261042 and 0216nm040 (China). Supporting Information Available: Table of the experimental conditions for the M-TPyP multiporphyrin nanocrystals and TEM photos for the Cd-TPyP nanocrystals at different concentrations of CdCl2 or TPyP. This material is available free of charge via the Internet at http://pubs.acs.org. LA050064T