Porphyrin Gels Reinforced by Sol−Gel Reaction via the Organogel

Publication Date (Web): September 8, 2005. Copyright © 2005 ..... Synthesis chemistry and application development of periodic mesoporous organosilica...
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Porphyrin Gels Reinforced by Sol-Gel Reaction via the Organogel Phase Takanori Kishida, Norifumi Fujita, Kazuki Sada, and Seiji Shinkai* Department of Chemistry and Biochemistry, Graduate School of Engineering, Kyushu University, 6-10-1 Hakozaki, Higashi-ku, Fukuoka, Fukuoka 812-8581 Received June 13, 2005. In Final Form: August 1, 2005 Porphyrins bearing four urea-linked dodecyl groups (3a) or four urea-linked triethoxysilylpropyl groups (3TEOS) at their peripheral positions were synthesized. 3a tends to assemble into a sheetlike two-dimensional structure due to the predominant hydrogen-bonding interaction among the urea groups and acts as a moderate gelator of organic solvents. On the other hand, its Cu(II) compelx (3a‚Cu) tends to assemble into a fibrous one-dimensional structure due to the predominant porphyrin-porphyrin π-π stacking interaction and acts as an excellent gelator of many organic solvents. 3TEOS and 3TEOS‚Cu, which also act as gelators, afforded similar superstructures as those of 3a and 3a‚Cu, respectively, and as evidenced by SEM and TEM observations and XRD measurements, the original superstructures could be precisely immobilized by in situ sol-gel polycondensation of the triethoxysilyl groups. The TEM images of 3a gels and 3TEOS gels after sol-gel polycondensation showed a fine striped structure, the periodical distance of which was either 2 or 4 nm. X-ray crystallographic analysis of a single crystal obtained from a reference porphyrin bearing four urea-linked butyl groups revealed that there are two different porphyrinstacked columns in the crystal and both the 2 nm distance and the 4 nm distance can appear, depending on the observation tilting angle. The hybrid gel prepared from 3TEOS‚Cu by sol-gel polycondensation showed unique physicochemical properties such as a high sol-gel phase-transition temperature (>160 °C), sufficient elasticity, high mechanical strength, etc. Thus, the present study has established new concepts for molecular design of porphyrin-based gelators on the basis of cooperative and/or competitive actions of hydrogen-bonding and π-π stacking interactions and for immobilization of their superstructures leading to development of new functional organic/inorganic hybrid materials.

Introduction It is well-known that porphyrins and phthalocyanines tend to align into one-dimensional aggregates and therefore are of much concern in relation to creation of novel supramolecular architectures such as nanowires, discotic liquid crystals, helical ribbon structures, etc.1-6 The major driving forces operating in these architectures are considered to be π-π stacking and/or van der Waals interactions. It is possible to fabricate these one-dimensional aggregates by inorganic materials, utilizing electrostatic and/or hydrogen-bonding interactions, to produce novel fibrous organic-inorganic hybrid materials.4,7-9 More * To whom correspondence should be addressed. Phone: +81 (0)92 642 3583. Fax: +81 (0)92 642 3611. E-mail: seijitcm@ mbox.nc.kyushu-u.ac.jp. (1) Engelkamp, H.; Middelbeek, S.; Nolte, R. J. M. Science 1999, 284, 785-788. (2) (a) Fuhrhop, J.-H.; Bindig, U.; Siggel, U. J. Am. Chem. Soc. 1993, 115, 11036-11037. (b) Patel, B. R.; Suslick, K. S. J. Am. Chem. Soc. 1998, 120, 11802-11803. (3) Drager, S. A.; Zangmeister, R. A. P.; Armstrong, N. R.; O’Brien, D. F. J. Am. Chem. Soc. 2001, 123, 3595-3596. (4) (a) Kimura, M.; Kitamura, T.; Muto, T.; Hanabusa, K.; Shirai, H.; Kobayashi, N. Chem. Lett. 2000, 1088-1089. (b) Kimura, M.; Wada, K.; Ohta, K.; Hanabusa, K.; Shirai, H.; Kobayashi, N. J. Am. Chem. Soc. 2001, 123, 2438-2439. (5) (a) Imada, T.; Murakami, H.; Shinkai, S. Chem. Commun. 1994, 1557-1558. (b) Arimori, S.; Takeuchi, M.; Shinkai, S. J. Am. Chem. Soc. 1996, 118, 245-246. (6) For comprehensive reviews for organogels, see: (a) Terech, P.; Weiss, R. G. Chem. Rev. 1997, 97, 3133-3159. (b) Melendez, R. E.; Carr, A. J.; Linton, B. R.; Hamilton, A. D. Struct. Bond. 2000, 31-61. (c) van Bommel, K. J. C.; Friggeri, A.; Shinkai, S. Angew. Chem. Int. Ed. 2003, 42, 980-999. (d) Shimizu, T.; Masuda, M.; Minamikawa, H. Chem. Rev. 2005, 105, 1401-1443. (7) Tanamura, Y.; Uchida, T.; Teramae, N.; Kikuchi, M.; Kusaba, K.; Onodera, Y. Nano Lett. 2001, 1, 387-390. (8) (a) Tamaru, S.-i.; Takeuchi, M.; Sano, M.; Shinkai, S. Angew. Chem. Int. Ed. 2002, 41, 853-856. (b) Kawano, S.-i.; Tamaru, S.-i.; Takeuchi, M.; Fujita, N.; Shinkai, S. Chem. Eur. J. 2004, 10, 343-351.

recently, we and others found that the hydrogen-bondforming groups introduced into the peripheral positions also play an important role in the determination of the final aggregation mode.10,11 Through these studies, we learned that the aggregation mode of porphyrin rings can be tuned not only in a one-dimensional fashion but also in a two-dimensional fashion by programming the structure of these peripheral groups: for example, 1 tends to assemble into a one-dimensional (1-D) aggregate, whereas 2 tends to assemble into a two-dimensional (2-D) aggregate.11,12 It thus occurred to us that the in situ fabrication utilizing sol-gel polycondensation of covalently linked, peripheral triethoxysilyl groups would be useful to immobilize these different porphyrin-based superstructures and eventually provide reinforced “porphyrin wire” and “porphyrin sheet” architectures from the 1-D and the 2-D aggregate, respectively. We thus designed 3a and 3b bearing 1-D-hydrogenbonding urea groups through trimethylene spacers from meso-phenyl groups and their polymerizable derivative 3TEOS bearing triethoxysilyl groups.13 In addition, to

seek for a variety of new stacking modes and gelation properties, we introduced a few metal ions into the (9) Jung, J. H.; Shinkai, S.; Shimizu, T. Chem. Rec. 2003, 3, 212224.

10.1021/la0515569 CCC: $30.25 © 2005 American Chemical Society Published on Web 09/08/2005

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Table 1. Gelation Properties of 3a, 3a‚M, 3TEOS, and 3TEOS‚M (M ) Cu, Zn)a state solvent

3a

3a‚Cu

3a‚Zn

3TEOS

3TEOS‚Cu

benzene toluene p-xylene anisole pyridine diphenyl ether methanol ethanol hexane cyclohexane chloroform tetrachloromethane 1,1,2,2-tetrachloroethane THF 1,4-dioxane acetone DMSO ethyl acetate DMF acetonitlile

G (5.0) G (15.0) G (15.0) G (10.0) S G (2.5) I I I I G (15.0) P G (30.0) G (30.0) G (10.0) G (30.0) G (25.0) P S P

I G (1.0) G (0.71) G (2.0) P G (0.71) P P I I G (10.0) G (7.5) G (30.0) G (30.0) G (15.0) G (2.5) P G (4.0) S I

P P P G (50.0) P P P P I I S I S P G (15.0) P P P S I

G (100)

G (2.0)

S S

G (1.0) G (1.0)

a G, P, I, and S denote gelation, precipitation, insoluble, and soluble, respectively. The critical gelation concentration (g dm-3) is showed in a parenthesis. P, I, and S are at [gelator] ) 30 g dm-3.

porphyrin nuclei. We have found that in the Cu(II) complex the morphology is transformed to a one-dimensional fibrous structure, which can be immobilized by sol-gel polycondensation.14 We here report our very interesting findings that the resultant porphyrin-based hybrid gel gains a very high thermal stability and behaves as a novel “elastic” gel.15 Results and Discussion Gelation Test. The gelation properties of 3a and its derivatives have been tested for 20 different solvents (Table 1). The gelation test was carried out as follows: the gelator was mixed in a capped test tube with the appropriate amount of solvent, and the mixture was heated until the solid was dissolved. The sample vial was cooled in air to 25 °C, left for 1 h at this temperature, and then turned upside down. When the gelator formed a clear or slightly opaque gel by immobilizing the solvent at this stage, it was denoted by a “G” mark in Table 1. It is seen from Table 1 that 3a and its Cu(II) complex (3a‚Cu) act as good gelators which can gelatinize 11 solvents, whereas its Zn(II) complex (3a‚Zn) gelatinizes only 2 solvents. Introduction of triethoxysilyl groups generally enhances (10) (a) Luboradzki, R.; Gronwald, O.; Ikeda, M.; Shinkai, S.; Reinhoudt, D. N. Tetrahedron 2000, 56, 9595-9599. (b) Hishikawa, Y.; Sada, K.; Watanabe, R.; Miyata, M. Chem. Lett. 1998, 795-796. (11) (a) Tamaru, S.-i.; Nakamura, M.; Takeuchi, M.; Shinkai, S. Org. Lett. 2001, 3, 3631-3634. (b) Tamaru, S.-i.; Uchino, S.-y.; Takeuchi, M.; Ikeda, M.; Hatano, T.; Shinkai, S. Tetrahedron 2002, 43, 3751-3755. (12) Shirakawa, M.; Kawano, S.-i.; Fujita, N.; Sada, K.; Shinkai, S. J. Org. Chem. 2003, 68, 5037-5044. (13) Kishida, T.; Fujita, N.; Sada, K.; Shinkai, S. Chem. Lett. 2004, 33, 1002-1003. (14) The gel formation followed by polymerization of gelators has been reported by several groups: (a) de Loos, B.; van Esch, J.; Stokroos, L.; Kellogg, R. M.; Feringa, B. L. J. Am. Chem. Soc. 1997, 119, 1267512676. (b) Matsuda, M.; Hanada, T.; Yase, K.; Shimizu, T. Macromolecules 1998, 31, 9403-9405. (c) Inoue, K.; Ono, Y.; Kanekiyo, K.; Kiyonaka, S.; Hamachi, I.; Shinkai, S. Chem. Lett. 1999, 225-226. (d) Inoue, K.; Ono, Y.; Kanekiyo, K.; Hanabusa, K.; Shinkai, S. Chem. Lett. 1999, 429-430. (e) Tamaoki, N.; Shimada, S.; Okada, Y.; Belaissaoui, A.; Kruk, G.; Yase, K.; Matsuda, H. Langmuir 2000, 16, 7545-7547. (f) Moreau, J. J. E.; Vellutini, L.; Man, M. W. C.; Bied, C.; Bantignies, J.-L.; Dieudonne, P.; Sauvajol, J.-L. J. Am. Chem. Soc. 2001, 123, 79587959. (g) Moreau, J. J. E.; Vellutini, L.; Man, M. W. C.; Bied, C. Chem. Eur. J. 2003, 9, 1594-1599. (h) Barboiu, M.; Cerneaux, S.: van der Lee, A.; Vaughan. G. J. Am. Chem. Soc. 2004, 126, 3545-3550. (15) Preliminary communication: Kishida, T.; Fujita, N.; Sada, K.; Shinkai, S. J. Am. Chem. Soc. 2005, 127, 7298-7299.

the solubility of gelators and frequently takes away the gelation ability. We confirmed, however, that 3TEOS‚Cu retains the gelation ability for benzene, p-xylene, and anisole, whereas 3TEOS can gelatinize only benzene. Furthermore, the critical gelation concentration for 3TEOS‚Cu is lower by about 2 orders of magnitude than that for 3TEOS, reflecting the strong stacking nature of the porphyrin‚Cu(II) complex. To further characterize these gels we mainly used benzene for 3a and 3TEOS and anisole for 3a‚Cu and 3TEOS‚Cu. Sol-Gel Polycondensation. Sol-gel polycondensation of these gels was carried out according to the following method: 3TEOS (4.0 mg) was dissolved in benzene (200 µL) by heating. After cooling, TFA (0.2 equiv with respect to 3TEOS) and water (1.0 µL) were added, and the resultant mixture was left at room temperature for 3 days. The purple precipitates thus obtained were isolated by centrifugation and washed with benzene. This operation was repeated with chloroform/ethanol (1:1 v/v) and with ethanol to remove TFA and unreacted 3TEOS. After drying in vacuo, the sample was subjected to spectral and microscopic analyses. On the other hand, sol-gel polycondensation of 3TEOS‚Cu was also performed according to the above-mentioned method: the concentrations were 3TEOS‚Cu (2.0 mg), anisole (400 µL), and HCl (0.04 equiv with respect to 3TEOS‚Cu). The progress of sol-gel polycondensation was followed by a FT-IR spectral method (Figure 1 and Figure S5 in the Supporting Information). The FT-IR spectrum (KBr) of 3TEOS showed that 1101 and 1076 cm-1 peaks assignable to the Si-OEt groups disappear, while a broad 1044 cm-1 peak assignable to the Si-O-Si group newly appears, indicating that sol-gel polycondensation proceeds successfully. The peaks assignable to the urea groups are scarcely changed before and after the reaction (1627 f 1633 cm-1, 1572 f 1564 cm-1, and 3319 f 3318 cm-1). This result suggests that the hydrogen-bonding network is scarcely damaged by the reaction. The FT-IR spectra (KBr) of 3TEOS‚Cu before and after sol-gel polycondensation also give the similar results as those of 3TEOS. UV-vis Absorption and Fluorescence Spectra. The UV-vis absorption spectrum of a homogeneous benzene solution of 3a (1.4 × 10-6 M) gave the Soret band at 422.5 nm. In the benzene gel phase ([3a] ) 11.4 mM) it shifted

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Figure 3. SEM images of the xerogel of (a) 3a + benzene gel and self-assembled 3TEOS in benzene (b) before and (c) after sol-gel polycondensation; [3a] ) 20 g dm-3 (11.4 mM), [3TEOS] ) 20 g dm-3 (10.6 mM), and [TFA] ) 0.2 equiv for 3TEOS.

Figure 1. FT-IR spectra for self-assembled 3TEOS in benzene (a) before and (b) after sol-gel polycondensation; [3TEOS] ) 10.6 mM, [TFA] ) 0.2 equiv with respect to 3TEOS.

Figure 2. UV-vis absorption spectra of (a) the aggregate in benzene before (solid line, 10.6 mM) and after (dashed and dotted line, [3TEOS] ) 10.6 mM, [TFA] ) 0.2 equiv with respect to 3TEOS) sol-gel polycondensation and the benzene solution (dotted line, 2.0 × 10-6 M) of 3TEOS and (b) the anisole gel before (solid line, 2.6 mM) and after (dashed and dotted line, [3TEOS‚Cu] ) 2.6 mM, [HCl] ) 0.2 equiv with respect to 3TEOS‚Cu) sol-gel polycondensation and the anisole solution (dotted line, 2.0 × 10-6 M) of 3TEOS‚Cu.

to longer wavelength (428.0 nm), indicating that it assembles into a J-aggregate in the gel phase (Figure S7 in the Supporting Information). The similar spectral change supporting J-aggregate formation was also observed for 3TEOS (422.5 nm in the homogeneous benzene solution to 427.5 nm in the benzene gel phase) (Figure 2a).16 This implied that even in 3TEOS the aggregation

Figure 4. SEM images of xerogels prepared from the anisole gel of (a) 3a‚Cu and 3TEOS‚Cu (b) before and (c) after sol-gel polycondensation; [3a‚Cu] ) 5.0 g dm-3 (2.8 mM), [3TEOS‚Cu] ) 5.0 g dm-3 (2.6 mM), and [HCl] ) 0.2 equiv for 3TEOS‚Cu.

mode is primarily governed by the porphyrin and hydrogen-bonding groups, whereas the triethoxysilyl groups scarcely affect it. Interestingly, sol-gel polycondensation of 3TEOS gave the gel sample which showed the UV-vis absorption spectrum (Soret band 426.0 nm) similar to that of 3TEOS + benzene gel. In addition, the fluorescence spectra of 3a and 3TEOS also support these results.17 On the other hand, a homogeneous anisole solution of 3a‚Cu (2.0 × 10-6 M) gave the Soret band at 421.0 nm. In the anisole gel phase (2.8 mM) it shifted to shorter wavelength (402.5 nm), indicating that it assembles into an Haggregate in the gel phase (Figure S8 in the Supporting Information). As expected, the Soret band for 3TEOS‚Cu + anisole gel also appeared at shorter wavelength region (402.5 nm) than that in the homogeneous anisole solution (421.0 nm) (Figure 2b). Interestingly again, the Soret band after sol-gel polycondensation of 3TEOS‚Cu was observed at 406.5 nm, which is comparable with that of the gel phase. These results consistently support the view that introduction of Cu(II) facilitates the H-aggregation of porphyrin rings, and both of the resultant J- and H-aggregate structures can be immobilized by in situ solgel polycondensation of the peripheral triethoxysilyl groups. (16) The concentration prepared herein was lower than the CGC of 3TEOS, but we confirmed by SEM and TEM that the aggregate structure constructed in the sol phase is almost the same as that constructed in the gel phase. (17) The fluorescence spectra of 3a gel and 3TEOS aggregates after sol-gel polycondensation in benzene was carried out (Figure S11 in the Supporting Information). Upon excitation of the porphyrin unit at 518 nm, the fluorescence spectra of a homogeneous benzene solution of 3a gave a peak at 655.0 nm. In the 3a gel it shifts to longer wavelength (661.5 nm), indicating that this shifted peak stems from the porphyrin J-aggregate. Interestingly, the 3TEOS aggregate after sol-gel polycondensation also gave a peak shifted to longer wavelength (659.0 nm), supporting that the porphyrin J-aggregate is still maintained in the organic-inorganic hybrid.

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Figure 5. TEM images of xerogels prepared from (a), (b) 3a + benzene gel in the different focus, and (c) the microcrystal grown up from 3b. The insets show cross-sectional analysis in each image. The white lines in the images represent the place scanned.

Figure 6. (a) TEM image of xerogel prepared from selfassembled 3TEOS after sol-gel polycondensation and (b) its magnified image. The inset shows cross-sectional analysis in the image. The white line in the image represents the place scanned.

SEM Observation. The foregoing view was further confirmed by SEM observation. As shown in Figure 3a-c, both 3a and 3TEOS construct a two-dimensional sheetlike structure, and this structure is well maintained even after sol-gel polycondensation. On the other hand, Figures 4a-c show that both 3a‚Cu and 3TEOS‚Cu construct a one-dimensional fibrous structure, and it is maintained even after sol-gel polycondensation. It is clear from these results that we can successfully gain the different porphyrin hybrids composed of one- and two-dimensional molecular assemblies created by J- and H-aggregated porphyrin stacks, respectively. When sol-gel polycondensation of 3TEOS and 3TEOS‚Cu was carried out in the solution phase (e.g., at the concentration below the critical gelation concentration), these characteristic structures could not be obtained at all. This difference clearly indicates that the preorganization in the gel phase is an indispensable process for immobilization of the organic superstructures by the sol-gel reaction. TEM Observation of 3a, 3b, and 3TEOS. To obtain more resolved visual images of the sheetlike gel structures obtained from 3a and 3TEOS, we took TEM pictures of the xerogels. As shown in parts a and b of Figure 5, the TEM pictures of 3a gels show a fine striped structure, the periodical distance of which is either 2 nm (Figure 5a) or 4 nm (Figure 5b). The microcrystal obtained from 3b (reference compound) was also subjected to TEM observation. The result (Figure 5c) shows that it also possesses the striped structure with the 4 nm periodical distance. Very interestingly, we found that the TEM picture after sol-gel polycondensation still keeps the fine striped structure in a very wide area (Figure 6a), and the periodical distance that can be determined with an enlarged picture (Figure 6b) is 4 nm in accord with that before sol-gel

Figure 7. Two-dimensional sheet structure composed of alternate columns A and B, the hydrogen bond pattern, and side view of the one columnar structure obtained from the crystal structure of 3b.

polycondensation. These findings can be raised as another evidence that the sol-gel polymerized 3TEOS hybrid firmly maintains the original 2-D sheet structure. X-ray Crystallographic Analysis of the Single Crystal of 3b and X-ray Diffraction Analysis of 3a and 3TEOS. To obtain a reasonable rationale for the periodical distance of either 2 or 4 nm, we attempted X-ray analysis of the single crystal grown up from 3b (Table S1 in the Supporting Information). Although the resolution accuracy is not satisfactorily high at present (R1 ) 0.24), it is still useful for the present purpose because one can visualize how the porphyrin rings are arranged in the crystal (see Figure 7): that is, (1) as expected, they adopt a columnar J-aggregate, (2) these columns are assembled into a two-dimensional sheet structure, and (3) looking at the plane angle of the porphyrins in the column, they are tilted by ca. 90° alternately from one column (column A) to the neighboring column (column B). Based on these lines of X-ray information, one can reasonably explain the TEM image and the XRD data. In the TEM image of the xerogel prepared from 3a + benzene gel, a stripe structure with 4 nm space was clearly observed (Figure 5b). This corresponds to the distance between column A and the next column A. Interestingly, when the focus depth was modified, another stripe with 2 nm space appeared (Figure 5a). One can regard this to be a distance between column A and column B. When the crystalline powder of 3b (used for the growth of the single crystal) was observed

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Table 2. X-ray Diffraction Date of the Xerogel Prepared from Organogels of 3a, 3TEOS, 3a‚Cu, and 3TEOS‚Cu entry

2θ (°) (d (nm))

3aa 3TEOS (before)b 3TEOS (after)c 3a‚Cud 3TEOS‚Cu (before)e 3TEOS‚Cu (after)f

6.6 (1.34), 10.5 (0.85), 20.4 (0.44) 20.6 (0.43) 20.6 (0.43) 3.5 (2.5), 20.6 (0.43) 3.7 (2.4), 20.8 (0.43) ca. 20.6 (0.43)

a 3a + benzene gel ([3a] ) 57.0 mM). b,c 3TEOS + benzene gel before and after sol-gel polycondensation ([3TEOS] ) 53.0 mM, [TFA] ) 0.2 equiv with respect to 3TEOS‚Cu). d 3a‚Cu + anisole ([3a‚Cu] ) 16.8 mM). e,f 3TEOS‚Cu + benzene gel before and after sol-gel polycondensation ([3TEOS‚Cu] ) 15.6 mM, [HCl] ) 0.2 equiv with respect to 3TEOS‚Cu).

Figure 8. TEM images of xerogels prepared from the anisole gel of (a) 3a‚Cu and 3TEOS‚Cu (b) before and (c) after sol-gel polycondensation and (d) the magnified image of (c); [3a‚Cu] ) 5.0 g dm-3 (2.8 mM), [3TEOS‚Cu] ) 5.0 g dm-3 (2.6 mM), and [HCl] ) 0.2 equiv for 3TEOS‚Cu.

by TEM, the presence of the stripe with 4 nm space was confirmed (Figure 5c). The results imply that as suggested from the results of X-ray crystallographic analysis, the distance between the porphyrin columns is determined by the hydrogen-bonding interaction among the urea groups but not by the length of the alkyl chains. The complementary structural data were obtained from XRD measurements of the xerogels prepared from 3a and 3TEOS. The XRD peaks at 2θ ) 20.4° (d ) 0.44 nm), 10.5° (d ) 0.85 nm), and 6.6° (d ) 1.34 nm) obtained from the xerogel of 3a can be assigned to the distance between the hydrogen-bonding urea groups, that between the porphyrin planes, and that between the two-dimensional sheets, respectively. In the xerogel of 3TEOS, the latter two peaks become broad but the main peak still appears clearly at 2θ ) 20.6° (d ) 0.43 nm) (Table 2). TEM Observation of 3a‚Cu and 3TEOS‚Cu. The more resolved visual images of 3a‚Cu and 3TEOS‚Cu before and after sol-gel polycondensation are obtained from TEM observation. As shown in Figure 8, parts a and b, the fibers constructed from these gelators feature a very uniform diameter of 20 ( 2 nm and a very long length, reaching 15 µm. These images are complementary to those obtained from SEM observations (Figure 4). As seen from Figure 8c, this fibrous structure is scarcely varied even after sol-gel polycondensation. This implies that the H-aggregated columns are immobilized in situ by sol-gel polycondensation of the peripheral triethoxysilyl groups. Further magnification of one fiber shows the presence of a stripe structure (Figure 8d), indicating that it is a bundle of one-dimensional porphyrin columnar aggregates. X-ray Diffraction Analysis of 3a‚Cu and 3TEOS‚ Cu. The results of the above TEM observations are also supported by XRD data of the xerogels (Table 2). The main XRD peaks for 3a‚Cu + anisole gel appeared at 2θ ) 20.6° (d ) 0.43 nm) and 3.5° (d ) 2.5 nm). As the

Figure 9. Plots of Tgel vs concentration of (a) 3a‚Cu and 3TEOS‚Cu (b) before and (c) after sol-gel polycondensation in anisole.

intramolecular urea-to-urea distance in 3a‚Cu is estimated to be 2.7 nm, the d ) 2.5 should stem from the H-aggregated porphyrin column. The d ) 0.43 is assigned to the intermolecular urea-to-urea distance. The similar peaks at 2θ ) 20.8° (d ) 0.43 nm) and 3.7° (d ) 2.4 nm) are also observed for the xerogel prepared from 3TEOS‚ Cu + benzene gel before sol-gel polycondensation, indicating that 3TEOS‚Cu also assembles into a onedimensional column similar to 3a‚Cu. After sol-gel polycondensation, these peaks became so broad that the d values could not be determined precisely. The TEM observation (Figure 8) supports, however, that the basic morphology of 3TEOS‚Cu is maintained even after solgel polycondensation. Sol-Gel Phase-Transition Temperature and Rheological Properties. It is known that the gel stability is sometimes enhanced by post-modification, such as polymerization, cross-linking, etc.14 To assess the influence of in situ sol-gel polycondensation on the gel stability, we measured Tgel values for the anisole gels of 3a‚Cu and 3TEOS‚Cu (Figure 9). The Tgel values for 3a‚Cu and 3TEOS‚Cu increased with increasing gelator concentration, which is a general trend observed for low molecular weight gelators.18 The Tgel values for 3a‚Cu are somewhat higher than those for 3TEOS‚Cu, which is attributed to the enhanced solubility of 3TEOS‚Cu by introduction of the triethoxysilyl groups. Very interestingly, the Tgel values for 3TEOS‚Cu after sol-gel polycondensation are enhanced up to 160 °C and independent upon the concentration. At 160 °C, the solvent either oozed out or volatilized. The findings clearly support the view that to improve the gel stability by postmodification, the maintenance of the original molecular network constructed in the gel phase is very essential. Surprisingly, the gelatinous mass obtained from 3TEOS‚Cu after sol-gel polycondensation showed sufficient “elasticity”. To assess the viscoelasticity of 3TEOS‚ Cu + anisole gel before and after sol-gel polycondensation, the ocillatory shear measurements were carried out using a stress-controlled rheometer with a parallel platetype geometry as a function of angular frequency from 100 to 0.1 rad s-1 at 25 °C. As shown in Figure 10, both of the storage modulus G′ values for 3TEOS‚Cu + anisole gel before and after sol-gel poly condensation are scarcely dependent on the oscillatory frequency. Very interestingly, we found that the G′ values for the gel after sol-gel (18) (a) Murata, K.; Aoki, M.; Suzuki, T.; Harada, T.; Kawabata, H.; Komori, T.; Ohseto, F.; Ueda, K.; Shinkai, S. J. Am. Chem. Soc. 1994, 116, 6664-6676. (b) Yoza, K.; Amanokura, N.; Ono, Y.; Akao, T.; Shinmori, H.; Takeuchi, M.; Shinkai, S.; Reinhoudt, D. N. Chem. Eur. J. 1999, 5, 2722-2729.

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Figure 10. Angular frequency ω dependence of the storage modulus G′ and the loss modulus G′′ of the 3TEOS‚Cu + anisole gel before and after sol-gel polycondensation; b (G′ after reaction), 9 (G′′ after reaction), O (G′ before reaction), 0 (G′′ before reaction); [3TEOS‚Cu] ) 2.6 mM, [TFA] ) 0.04 equiv with respect to 3TEOS‚Cu.

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the solvent and eventually became a thin membrane with 24 pieces (Figure 11c). We immersed this membrane in anisole but it did not recover its original figure by swelling (even with treatment by sonication or heating). On the other hand, the 3TEOS‚Cu + anisole gel (before sol-gel polycondensation) readily collapsed even by putting the glass plate on it (Figure 11a). Conclusion. We have demonstrated that one and twodimensional molecular assemblies created by J- and H-aggregated porphyrin stacks can be immobilized, without a morphological change, by in situ sol-gel polycondensation of the peripheral triethoxysilyl groups. The resultant gel prepared according to this flowchart has gained a very high thermal stability as well as a unique mechanical stability. One may regard, therefore, that this is a unique organic/inorganic hybrid gel obtainable by using the low molecular weight gel as a template. Supramolecular chemistry has been playing a historical role in providing a variety of unique and attractive architectures, and the superstructure created in the gel system are classified as one of those architectures. However, there is a fatal drawback in that architectures constructed by noncovalent bonds are unstable, like a house of cards. It is clear from the present results that in situ polycondensation is useful not only to immobilize the architectures by covalent bonds but also to reinforce them stably. We believe that the thermodynamically metastable architectures thus immobilized should show new chemical and physical properties that cannot be attained with the thermodynamically stable architectures. Experimental Section

Figure 11. Photographs of the 3TEOS‚Cu + anisole gel (a) before sol-gel polycondensation, which collapses by putting on only a glass plate, (b) after sol-gel polycondensation, which a glass plate and 10 10-yen coins are put on, and (c) after sol-gel polycondensation, in which the solvent is squeezed out by putting on a glass plate and 24 10-yen coins; (i) a plate and/or 10 yen coins are put on the 3TEOS‚Cu + anisole gel and (ii) they are taken off of the 3TEOS‚Cu + anisole gel.

polycondensation exceed the loss modulus G′′ by more than 1 order of magnitude. However, that of the gel before sol-gel polycondensation is greater only by about 6-fold. Thus, one can regard the gel before sol-gel polycondensation to be a “weak” gel, whereas that after sol-gel polycondensation can be considered to be a “true” gel.6a,19,20 Furthermore, the G′ for the gel before sol-gel polycondensation (25 Pa) became larger by 14 times after sol-gel polycondensation (350 Pa). These results indicate that the sol-gel process has remarkably improved the viscoelastic properties of the 3TEOS‚Cu + anisole gel. To demonstrate the improved elasticity of the 3TEOS‚Cu + anisole gel after sol-gel polycondensation (Figure 11), we put a glass plate (4.5 g) on it and piled up 10 yen coins (4.5 g/piece). Up to 13 pieces, it still kept its original figure (total weight ) 4.5 g (glass) + 4.5 g × 13 ) 63.0 g) (Figure 11b). It began to collapse from 14 pieces, squeezing out (19) (a) Yao, S.; Beginn, U.; Gress, T.; Lysetaka, M.; Wu¨rthner, F. J. Am. Chem. Soc. 2004, 126, 8336-8348. (b) Sawant, P. D.; Liu, X.-Y. Chem. Mater. 2002, 14, 3793-3798. (20) The “elastic” gel as reported herein was obtained only when we used TFA as a catalyst. When HCl was used, we obtained the more fragile gel.

General. All starting materials and solvents were purchased from Tokyo Kasei Organic Chemicals, Wako Organic Chemicals, or Aldrich and used as received. The 1H NMR spectra were recorded on a Bruker DRX 600 (600 MHz) spectrometer. Chemical shifts are reported in ppm downfield from tetramethylsilane as the internal standard. Mass spectral data were obtained using a Perseptive Voyager RP matrix assisted laser desorption/ ionization-time-of-flight (MALDI-TOF) mass spectrometer. UVvis spectra were recorded with a Shimadzu UV-2500 PC spectrophotometer. FT-IR Measurement of Xerogels and Solids. All FT-IR measurements of xerogels and solids were performed in the attenuated total reflection (ATR). The KBr pellets mixed with samples after sol-gel polycondensation were measured on the transparent mode. SEM Observation. The gel prepared in a sample tube was frozen by liquid nitrogen or placed in a plate without being frozen by liquid nitrogen. The sample was evaporated by a vacuum pump under reduced pressure for 1 day at room temperature. The obtained sample was shielded with platin. The accelerating voltage of the transmission electron microscope was 120 kV, and the beam current was 65 µA. TEM Observation. A piece of the gel was placed in a carboncoated copper grid. The sample was dried by a vacuum pump under reduced pressure for 1 day at room temperature. The accelerating voltage of the transmission electron microscope was 120 or 200 kV, and the beam current was 65 or 105 µA. Powder X-ray Diffraction. The gel prepared in a sample tube was frozen by liquid nitrogen or placed in a glass plate without being frozen by liquid nitrogen. The sample was evaporated by a vacuum pump under reduced pressure for 1 day at room temperature. An X-ray diffraction was recorded on an imaging plate using Cu radiation (λ ) 1.54178 at a distance of 15 cm). Crystal Structure Determination. X-ray diffraction data were collected on a Rigaku R-AXIS RAPID diffractometer with a 2D area detector using graphite-monochromatized Cu K radiation (λ ) 1.5418). Lattice parameters were obtained by leastsquares analysis from reflections for three oscillation images. Direct methods (SIR92) were used for the structure solution. All

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Kishida et al. Scheme 1

calculations were performed using on the TEXSAN21 or Crystal Structure22 crystallographic software packages. For 3b, the low single crystallinity and small size made it difficult to find out the electron density for the four n-butyl groups of the porphyrin and some solvated molecules. As a result, only the limited number of the atoms around the porphyrin ring could be found and refined with anisotropic displacement parameters by the full matrix leastsquares procedure using observed reflections (>2.0σ(I)) based on F2. The relative high R-value is due to disorders of the butyl (21) TEXSAN, X-ray Structure Analysis Package; Molecular Structure Corporation: The Woodlands, TX, 1985. (22) Crystal Structure, X-ray Structure Analysis Package; Molecular Structure Corporation.

group and the solvated molecules. Crystallographic parameters are summarized in Table S1 in the Supporting Information. Measurement of Sol-Gel Phase-Transition Temperatures. The sealed tube containing the gel was immersed inversely in a thermostated oil bath. The temperature was raised at a rate of 1 °C min-1. Here, the Tgel was defined as the temperature at which the gel was broken. Synthesis. The following operations were conducted according to a procedure described by Dick et al.23 5,10,15,20-Tetrakis[4(3-aminopropoxy)phenyl]porphyrin 6 was obtained from 5,10, (23) Dick, D. L.; Venkata, T.; Rao, S.; Sukumaran, D.; Lawrence, D. S. J. Am. Chem. Soc. 1992, 114, 2664-2669.

Porphyrin Gels Reinforced by Sol-Gel Reaction 15,20-tetraphthalimideporphyrin 5, which was synthesized from 4-[3-(1,3-dioxo-1,3-dihydro-2H-isoindol-2-yl)propoxy]benzaldehyde 4 and pyrrole by the Adler method. The deprotection reaction was carried out with hydrazine to give 5,10,15,20-tetrakis[4(3-aminopropoxy)phenyl]porphyrin 6. 4-[3-(1,3-Dioxo-1,3-dihydro-2H-isoindol-2-yl)propoxy]benzaldehyde 4 was prepared by Willamson reaction from 4-hydroxybenzaldehyde and N-(3bromopropyl)phthalimide. The synthesis of the following compounds is shown in Scheme 1. 5,10,15,20-Tetrakis[4-[3-(3-N-dodecylureido)propoxy]phenyl]porphyrin (3a). A mixture of 6 (150 mg, 0.165 mmol), dodecylisocyanate (697 mg, 3.3 mmol), and 1,8-diazabicyclo[5.4.0]7-undecene (DBU) (250 mg, 1.65 mmol) in 100 mL of dry DMF was warmed at 60 °C for 2 days under a nitrogen atmosphere. The solvent was evaporated under reduced pressure, and the solid residue was purified through chromatography [silica gel, CHCl3/MeOH ) 30:1 (v/v)] to give 3a in 73% yield (211 mg): mp 132-133 °C. 1H NMR (600 MHz, CDCl3/MeOH-d4 ) 1:1 (v/v), TMS, rt) δ: 0.82 (t, J ) 6.9, 12 H), 1.20-1.36 (m, 72 H), 1.511.55 (m, 8 H), 2.17-2.19 (m, 8H), 3.18 (t, J ) 7.1, 8 H), 3.51 (t, J ) 6.6, 8 H), 4.35 (t, J ) 5.8, 8H), 7.33 (d, J ) 8.2, 8 H), 8.12 (d, J ) 8.2, 8 H), 8.87 (br, 8H). MALDI-TOF-MS [dithranol]: m/z calcd 1753.5, found 1753.9. IR (ATR) 3318, 2920, 1627, 1574 cm-1. Anal. Calcd for C108H158N12O8‚1.10H2O: C, 73.19; H, 9.11; N, 9.48. Found: C, 73.43; H, 9.01; N, 9.22. 5,10,15,20-Tetrakis[4-[3-(3-N-butylureido)propoxy]phenyl]porphyrin (3b). A mixture of 6 (50 mg, 0.055 mmol), n-butylisocyanate (109 mg, 1.1 mmol), and 1,8-diazabicyclo[5.4.0]7-undecene (DBU) (84 mg, 0.55 mmol) in 100 mL of dry DMF was warmed at 60 °C for 2 days under a nitrogen atmosphere. The solvent was evaporated under reduced pressure, and the solid residue was washed with chloroform to give 3b in 95% yield (68 mg): mp 327 °C dec. 1H NMR (600 MHz, CDCl3/MeOH-d4 ) 1:1 (v/v), TMS, rt) δ: 0.96 (t, J ) 7.3, 12H), 1.35-1.44 (m, 8H), 1.49-1.52 (m, 8H), 2.17 (t, J ) 6.3, 8H), 3.19 (t, J ) 7.1, 8H), 3.50 (t, J ) 6.7, 8H), 4.34 (t, J ) 5.9, 8H), 7.32 (d, J ) 8.3, 8H), 8.12 (d, J ) 8.2, 8H), 8.88 (s, 8H). MALDI-TOF-MS [dithranol]: m/z calcd 1304.6, found 1304.4. IR (ATR) 3315, 2927, 1621, 1571 cm-1. Anal. Calcd for C76H94N12O8‚0.10CHCl3: C, 69.48; H, 7.21; N, 12.78. Found: C, 69.39; H, 7.23; N, 12.78. 5,10,15,20-Tetrakis[4-[3-[3-(3-triethoxysilyl-N-propyl)ureido]propoxy]phen yl]porphyrin (3TEOS). A mixture of 6 (100 mg, 0.110 mmol) and 3-(triethoxysilyl)propylisocyanate (327 mg, 1.32 mmol) in 100 mL of dry CH2Cl2 was refluxed for 2 days under a nitrogen atmosphere. The solvent was evaporated under reduced pressure, and the solid residue was purified by reprecipitation from chloroform to hexane to give 3TEOS in 86% yield (180 mg): mp 103-104 °C. 1H NMR(600 MHz, CDCl3/ DMSO-d6 ) 1:1 (v/v), TMS, rt) δ: -2.86 (s, 2 H), 0.59 (t, J ) 8.4, 8 H), 0.82 (t, J ) 6.9, 36 H), 1.51-1.54 (m, 8 H), 2.08-2.10 (m, 8 H), 3.07 (t, J ) 6.5, 8 H), 3.40 (t, J ) 6.2, 8 H), 3.78 (t, J ) 7.0, 24 H), 4.31 (t, J ) 5.8, 8 H), 5.84(s, 4 H), 5.97 (s, 4 H), 7.34 (d, J ) 8.2, 8 H), 8.09 (d, J ) 8.2, 8 H), 8.86 (s, 8 H). MALDI-TOF-MS [dithranol]: m/z calcd 1897.6, found 1898.1. IR (ATR) 3318, 2926, 1627, 1527 cm-1. Anal. Calcd for C96H142N12O20Si4‚0.20CHCl3: C, 60.16; H, 7.46; N, 8.75. Found: C, 60.23; H, 7.24; N, 8.91. 5,10,15,20-Tetrakis[4-[3-[3-(3-triethoxysilyl-N-propyl)ureido]propoxy]phen yl]porphine zinc (3a‚Zn). To a solution of 3a (30 mg, 0.012 mmol) in 20 mL of chloroform/methanol ) 1:1 (v/v) was added zinc acetate dihydrate (26 mg, 0.12 mmol), and the mixture was warmed at 60 °C for 6 h. The solution was evaporated under reduced pressure. The solid residue was purified through chromatography [silica gel, CHCl3/MeOH ) 40:1 (v/v)] to give 3a‚Zn in 78% yield (24 mg): mp 110-111 °C. 1H NMR (600 MHz, CDCl3/MeOH-d4 ) 1:1 (v/v), TMS, rt) δ: 0.82 (t, J ) 6.9, 12 H), 1.20-1.36 (m, 72 H), 1.50-1.52 (m, 8 H), 2.11-2.15 (m, 8H), 3.16 (t, J ) 7.1, 8 H), 3.47 (t, J ) 6.6, 8 H), 4.29 (t, J ) 5.8, 8H), 7.25 (d, J ) 8.2, 8 H), 8.08 (d, J ) 8.2, 8 H), 8.86 (s, 8H). MALDI-TOF-MS [dithranol]: m/z calcd 1815.9, found 1815.2. IR (ATR) 3322, 2921, 1629, 1572 cm-1. Anal. Calcd for C108H156N12O8Zn‚1.25H2O: C, 70.56; H, 8.69; N, 9.14. Found: C, 70.37; H, 8.73; N, 8.99.

Langmuir, Vol. 21, No. 21, 2005 9439 5,10,15,20-Tetraphthalimideporphine Copper (7). To a solution of 5 (200 mg, 0.140 mmol) in 50 mL of chloroform/ methanol ) 3:1 (v/v) was added copper acetate anhydride (254 mg, 1.40 mmol), and the mixture was stirred at room temperature for 5 h. The solution was evaporated under reduced pressure. To the solid residue was added chloroform, and the resulting precipitate was filtered off to remove the copper acetate. The obtained filtrate was purified through chromatography [silica, chloroform] to give 7 in 82% yield (170 mg): mp >300 °C. MALDITOF-MS [dithranol]: m/z calcd 1487.4, found 1487.3. IR(ATR) 2927, 1770, 1705, 1242 cm-1. Anal. Calcd for C88H64N8O12Cu‚ 0.60H2O: C, 70.47; H, 4.38; N, 7.47. Found: C, 70.44; H, 4.35; N, 7.52. 5,10,15,20-Tetrakis[4-(3-aminopropoxy)phenyl]porphine Copper (8). The mixture of 7 (170 mg, 0.114 mmol) and hydrazine (20 mL) in 90 mL of THF/chloroform ) 2:1 (v/v) was refluxed for 5 h. The solution was evaporated under reduced pressure, and subsequently to the residue was added dichloromethane. The hydrazine layer of the mixed solution was discarded, and the remaining dichloromethane layer was extracted with aqueous K2CO3 and water, and then purified by reprecipitation from chloroform and hexane to give 8 in 90% yield (100 mg): mp 262-263 °C. MALDI-TOF-MS [dithranol]: m/z calcd 968.4, found 968.5. Anal. Calcd for C56H56N8O4Cu‚ 0.75CHCl3: C, 64.41; H, 5.41; N, 10.59. Found: C, 64.46; H, 5.63; N, 10.39. 5,10,15,20-Tetrakis[4-[3-[3-(3-triethoxysilyl-N-propyl)ureido]propoxy]phen yl]porphine Copper (3a‚Cu). The mixture of 8 (70 mg, 0.072 mmol), dodecylisocyanate (184 mg, 1.45 mmol), and 1,8-diazabicyclo[5.4.0]-7-undecene (DBU) (66 mg, 0.43 mmol) in 100 mL of dry DMF was refluxed for 2 days under a nitrogen atmosphere. The solution was evaporated under reduced pressure, and the solid residue was purified by reprecipitation from chloroform and hexane to give 3a‚Cu in 75% yield: mp 202-203 °C. MALDI-TOF-MS [dithranol]: m/z calcd 1815.0, found 1815.3. IR (ATR) 3319, 2921, 1624, 1571 cm-1. Anal. Calcd for C108H156N12O8Cu‚0.9H2O: C, 70.87; H, 8.69; N, 9.18. Found: C, 70.82; H, 8.58; N, 9.08. 5,10,15,20-Tetrakis[4-[3-[3-(3-triethoxysilyl-N-propyl)ureido]propoxy]phen yl]porphine copper (3TEOS‚Cu). The mixture of 8 (80 mg, 0.093 mmol) and 3-(triethoxysilyl)propylisocyanate (511 mg, 1.86 mmol) in 100 mL of dry dichloromethane was refluxed for 2 days under a nitrogen atmosphere. The solution was evaporated under reduced pressure, and the solid residue was purified by reprecipitation from chloroform and hexane to give 3TEOS‚Cu in 87% yield (140 mg): mp 120-121 °C. MALDI-TOF-MS [dithranol]: m/z calcd 1958.1, found 1958.0. IR (ATR) 3321, 2926, 1633, 1568 cm-1. Anal. Calcd for C96H140N12O20Cu‚0.40CHCl3: C, 57.72; H, 7.06; N, 8.38. Found: C, 57.61; H, 7.04; N, 8.56.

Acknowledgment. This work was partially supported by a Grant-in-Aid for Scientific Research (S) (15105004) and the 21st Century COE Program “Functional Innovation of Molecular Informatics” from the MEXT of Japan. We would like to thank Professor Y. Takahashi, Dr. A. Takada, and Mr. D. Tabata of Kyushu University for oscillatory shear measurements. Supporting Information Available: 1H NMR spectra of 3a, 3b, 3TEOS, and 3a‚Zn; FT-IR spectra of 3TEOS‚Cu + anisole gel before and after sol-gel polycondensation; UV-vis spectra of 3a, 3a‚Cu, and 3TEOS‚Cu before and after sol-gel polycondensation; SEM image of 3TEOS‚Cu + anisole gel after sol-gel polycondensation with TFA as a catalyst; crystallographic data of 3b; fluorescence spectra of 3a and 3TEOS after sol-gel polycondensation. This material is available free of charge via the Internet at http://pubs.acs.org. LA0515569