Construction of Microbelts through the Coassembly of a Disclike

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Construction of Microbelts through the Coassembly of a Disclike Molecule and Primary Alkyl Ammoniums: A Noncovalent Strategy to Mimic Covalently Bonded π-Core Alkyl Chain Structure Ze-Yun Xiao, Xin Zhao,* Xi-Kui Jiang, and Zhan-Ting Li* State Key Laboratory of Bioorganic and Natural Products Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 345 Lingling Lu, Shanghai 200032, China Received June 1, 2010. Revised Manuscript Received July 9, 2010 In this letter, we report the fabrication of microbelts through the coassembly of hexa-2-pyridyl-hexaazatriphenylen (HPHAT), a disklike π-conjugated molecule, with primary alkyl ammonium triflate. The strategy is first to construct hydrogen-bonded complexes between HPHAT and primary alkyl ammoniums to mimic covalently bonded π-core alkyl chain structures, and then the complexes self-assemble into microbelts driven by π-π stacking in the π core and van der Waals interactions between the peripheral alkyl chains. The morphology of as-prepared microbelts has been characterized with scanning electron microscopy (SEM), optical microscopy, polarizing microscopy, and transmission electron microscopy (TEM). Spectroscopic and crystallographic investigations were also carried out to reveal the formation mechanism of the microbelts, through which a sequential self-assembly process has been proposed.

Introduction One dimensional (1D) micro/nanostructures fabricated from π-conjugated molecules have gained a great deal of attention in recent years because of their applications in the field of organic and supramolecular electronics and photonics.1 In this context, the supramolecular self-assembly of small organic molecules into well-defined micro/nanowires or micro/nanobelts represents a simple, efficient approach to constructing micro/nanostructures with high aspect ratios.2 The self-assembly is generally believed to be driven by the π-π stacking interaction between the π cores, and the attachment of peripheral substituents (usually flexible alkyl chains) onto the π core is necessary not only to enhance the *Corresponding authors. E-mail: [email protected], [email protected]. Fax: þ86-21-64166128. Tel: þ86-21-54925023.

(1) (a) Schenning, A. P. H. J.; Meijer, E. W. Chem. Commun. 2005, 3245–3258. (b) Zhao, Y.; Fu, H.; Peng, A.; Ma, Y.; Liao, Q.; Yao, J. Acc. Chem. Res. 2010, 43, 409–418. (c) Zan, L.; Che, Y.; Moore, J. S. Acc. Chem. Res. 2008, 41, 1596–1608. (d) Zhou, Y.; Liu, W.-J.; Ma, Y.; Wang, H.; Qi, L.; Cao, Y.; Wang, J.; Pei, J. J. Am. Chem. Soc. 2007, 129, 12386–12387. (e) Che, Y.; Datar, A.; Balakrishnan, K.; Zan, L. J. Am. Chem. Soc. 2007, 129, 7234–7235. (f) Xie, H.; Luo, S.-C.; Yu, H.-h. Small 2009, 5, 2611–2617. (g) Xiao, K.; Rondinone, A. J.; Puretzky, A. A.; Ivanov, I. N.; Retterer, S. T.; Geohegan, D. B. Chem. Mater. 2009, 21, 4275–4281. (h) Puigmartí-Luis, J.; Laukhin,
. P.; Vidal-Gancedo, J.; Rovira, C.; Laukhina, E.; Amabilino, D. B. V.; del Pino, A Angew. Chem., Int. Ed. 2007, 46, 238–241. (2) (a) Hoeben, F. J. M.; Jonkheijm, P.; Meijer, E. W.; Schenning, A. P. H. J. Chem. Rev. 2005, 105, 1491–1546. (b) Lee, S. J.; Hupp, J. T.; Nguyen, S. T. J. Am. Chem. Soc. 2008, 130, 9632–9633. (c) Elemans, J. A. A. W.; van Hameren, R.; Nolte, R. J. M.; Rowan, A. E. Adv. Mater. 2006, 18, 1251–1266. (d) Xiao, Z.-Y.; Zhao, X.; Jiang, X.-K.; Li, Z.-T. Org. Biomol. Chem. 2009, 7, 2540–2547. (e) Hameren, R. Van; Sch€on, P.; Buul, A. M. van; Hoogboom, J.; Lazarenko, S. V.; Gerritsen, J. W.; Engelkamp, H.; Christianen, P. C. M.; Heus, H. A.; Maan, J. C.; Rasing, T.; Speller, S.; Rowan, A. E.; Elmans, J. A. A. W.; Nolte, R. J. M. Science 2006, 314, 1433–1436. (f) Shklyarevskiy, I. O.; Jonkheijm, P.; Christianen, P. C. M.; Schenning, A. P. H. J.; Guerzo, A. D.; Desvergne, J.-P.; Meijer, E. W.; Maan, J. C. Langmuir 2005, 21, 2108–2112. (3) (a) Houmadi, S.; Coquiere, D.; Legrand, L.; Faur
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emita, S. Langnuir 2007, 23, 4849–4855. (b) Hill, J. P.; Jin, W.; Kosaka, A.; Fukushima, T.; Ichihara, H.; Shimomura, T.; Ito, K.; Hashizume, T.; Ishii, N.; Aida, T. Science 2004, 304, 1481–1483. (c) McGrath, K. K.; Jang, K.; Robins, K. A.; Lee, D.-C. Chem.;Eur. J. 2009, 15, 4070–4077. (d) Seo, S. H.; Jones, T. V.; Seyler, H.; Peters, J. O.; Kim, T. H.; Chang, J. Y.; Tew, G. N. J. Am. Chem. Soc. 2006, 128, 9264– 9265. (e) Yoo, Y.-S.; Choi, J.-H.; Song, J.-H.; Oh, N.-K.; Zin, W.-C.; Park., S.; Chang, T.; Lee, M. J. Am. Chem. Soc. 2004, 126, 6294–6230. (f) Moon, K.-S.; Kim, H.-J.; Lee, E.; Lee, M. Angew. Chem., Int. Ed. 2007, 46, 6807–6810.

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solubility of the whole molecule but also to manipulate the morphologies of the aggregates. To this end, a popular design principle for supramolecular building blocks is to incorporate peripheral substitutents into a large π core. Although the manipulation of side groups with respect to the self-assembly properties of π-conjugated molecules has been extensively studied to date,3 the synthesis of such structures is sometimes tedious and timeconsuming. This obstacle prompted us to investigate whether a noncovalently bonded complex can be used to mimic the covalently bonded π-core peripheral substituents structure, and thus the noncovalently bonded peripheral substituents could also provide a modulation of the morphology of the aggregates (Supporting Information Scheme 1). In addition, dynamic libraries might also be constructed that profit from the reversible nature of the noncovalent bond,4 through which the fabrication of highly ordered micro/nanostructures might be screened in a high-throughput way. To secure a stable complex formed between the π core and suitable peripheral substitutents, the intermolecular interaction connecting them must be strong enough. In this context, the hydrogen-bonding interaction was chosen because of its strong bonding strength and high directionality. On the basis of the above consideration, we started our design by choosing a suitable π core that could provide an excellent π-π stacking ability as well as an attractive electro-optical property. For this reason, hexaazatriphenylene (HAT) was chosen as the basic scaffold. HAT is a conjugated heterocycle that has been used to develop n-type semiconductors,5 magnetic materials,6 (4) (a) Corbett, P. T.; Leclaire, J.; Vial, L.; West, K. R.; Wietor, J.-L.; Sanders, J. K. M.; Otto, S. Chem. Rev. 2006, 106, 3652–3711. (b) Bruin, B. de; Hauwert, P.; Reek, J. N. H. Angew. Chem., Int. Ed. 2006, 45, 2660–2663. (c) Lehn, J.-M. Chem.; Eur. J. 1999, 5, 2455–2463. (5) (a) Gearba, R. I.; Lehmann, M.; Levin, J.; Ivanov, D. A.; Koch, M. H. J.; Barber
a, J.; Debije, M. G.; Piris, J.; Geerts, Y. H. Adv. Mater. 2003, 15, 1614–1618. (b) Kaafarani, B. R.; Kondo, T.; Yu, J.; Zhang, Q.; Dattilo, D.; Risko, C.; Jones, S. C.; Barlow, S.; Domercq, B.; Amy, F.; Kahn, A.; Br
edas, J.-L.; Kippelen, B.; Marder, S. R. J. Am. Chem. Soc. 2005, 127, 16358–16359. (c) Ishi-i, T.; Yaguma, K.; Kuwahara, R.; Taguri, Y.; Mataka, S. Org. Lett. 2006, 8, 585–588. (6) Marshall, S. R.; Rheingold, A. L.; Dawe, L. N.; Shum., W. W.; Kitamura, C.; Miller, J. S. Inorg. Chem. 2002, 41, 3599–3601.

Published on Web 07/15/2010

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Figure 1. (a) SEM, (b) optical microscopy, (c) polarizing microscopy, and (d) TEM images of microbelts fabricated from the HPHAT/DAT mixture. The samples were prepared by mixing HPHAT (10 mM) and DAT (30 mM) in chloroform.

nonlinear optical chromophores,7 liquid crystals,8 and ligands for coordination chemistry.9 Its large, planar disklike structure facilitates highly ordered self-assembly by π-π stacking. Introducing hydrogen-bonding sites into the HAT core brought about the design of the target molecule, hexa-2-pyridyl-hexaazatriphenylene (HPHAT), which was first developed here. In its chemical structure, the HAT core was extended by merging six pyridine units that could provide three hydrogen-bond acceptor pockets. The peripheral substituents, which should provide corresponding hydrogen bond donors to facilitate hydrogen bonding between HPHAT and the side groups, were also believed to be crucial to constructing the complex. Therefore, primary alkyl ammoniums with different chain lengths were selected for the ammonium group, which could provide three NH protons to bind to the nitrogen atoms of HPHAT. In this letter, we demonstrate a realization of the noncovalent mimic concept through the fabrication of well-defined microbelts by coassembling HPHAT and primary alkyl ammoniums with chain lengths of no fewer than eight carbon atoms.10 The formation mechanism of the microbelts was also discussed on the basis of spectroscopic and crystallographic investigations.

Results and Discussion HPHAT was prepared via the condensation of hexaaminobenzene and 1,2-di(pyridin-2-yl)ethane-1,2-dione in refluxing acetic acid (details in Supporting Information). Initially, the self-assembly behavior of HPHAT itself was investigated. No well-defined aggregates could be observed from the sample obtained from its chloroform solution (Supporting Information Figure S2), a solvent could provide acceptable solubility for HPHAT. Such a (7) Cho, B. R.; Lee, S. K.; Kim, K. A.; Son, K. N.; Kang, T. I.; Jeon, S. J. Tetrahedron Lett. 1998, 39, 9205–9208. (8) (a) Chang, T.-H.; Wu, B.-R.; Chiang, M. Y.; Liao, S.-C.; Ong, C. W.; Hsu, H.-F.; Lin, S.-Y. Org. Lett. 2005, 7, 4075–4078. (b) Ishi-i, T.; Hirayama, T.; Murakami, K.-i.; Tashiro, H.; Thiemann, T.; Kubo, K.; Mori, A; Yamasaki, S.; Akao, T.; Tsuboyama, A.; Mukaide, T.; Ueno, K.; Mataka, S. Langmuir 2005, 21, 1261–1268. (c) Pieterse, K.; Hal, P. A. van; Kleppinger, R.; Vekemans, J. A. J. M.; Janssen, R. A. J.; Meijer, E. W. Chem. Mater. 2001, 13, 2675–2679. (9) Kitagawa, S.; Masaoka, S. Coord. Chem. Rev. 2003, 246, 73–88. (10) For examples on the formation of discotic liquid crystals by coassembly, see (a) Kleppinger, R.; Lillya, C. P.; Yang, C. J. Am. Chem. Soc. 1997, 119, 4097–4102. (b) Schmidt-Mende, L.; Fechtenk€otter, A.; M€ullen, K.; Moons, E.; Friend, R. H.; MacKenzie, J. D. Science 2001, 293, 1119–1122. (c) Percec, V.; Glodde, M.; Bera, T. K.; Miura, Y.; Shiyanovskaya, I.; Singer, K. D.; Balagurusamy, V. S. K.; Heiney, P. A.; Schnell, I.; Rapp, A.; Spiess, H.-W.; Hudson, S. D.; Duan, H. Nature 2002, 419, 384– 387. (d) Kadam, J.; Faul, C. F. J.; Scherf, U. Chem. Mater. 2004, 16, 3867–3871. (e) Reczek, J. J.; Villazor, K. R.; Lynch, V.; Swager, T. M.; Iverson, B. L. J. Am. Chem. Soc. 2006, 128, 7995–8002.

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result indicated that the π core itself was not a good building block for self-assembly, suggesting that a side chain might be necessary to promote ordered aggregation. However, when n-dodecyl ammonium triflate (DAT) was introduced into a solution of HPHAT in chloroform, flocculent material precipitated out after standing for several minutes. An SEM image revealed that it consisted of welldefined microbelts of 1 to 2 μm in width and tens of micrometers in length (Figure 1a). It is noteworthy that these microbelts are large enough that they could even be observed directly by optical microscopy (Figure 1b). Furthermore, the hierarchical selfassembly was also reflected by the crystalline feature of the microbelts, which was revealed by polarizing microscope observation (Figure 1c). TEM further confirms the solid feature of the asprepared microbelts by displaying almost no contrast throughout the belts (Figure 1d). Differential scanning calorimetry (DSC) was also performed to obtain more information about the microbelts. Although a DSC investigation of HPHAT indicated the existence of a liquid-crystalline (LC) phase, a DSC investigation of the microbelt from HPHAT and DAT revealed no LC phase but the decomposition of the complex (Supporting Information Figure S3). In addition, it was found that microbelts could also be fabricated by adding HPHAT to a chloroform solution of DAT, suggesting that the sequence of addition has no influence on the fabrication of microbelts. It should be noted that DAT itself also could not generate ordered aggregates in chloroform (Supporting Information Figure S2). Therefore, the microbelts should come from the coassembly of HPHAT and DAT. Further investigation of the influence of the length of the alkyl chains on the self-assembly property of HPHAT was conducted. It was found that adding n-butyl ammonium triflate (BAT) to a solution of HPHAT in chloroform did not result in any welldefined microstructures (Supporting Information Figure S4). However, wirelike aggregates were obtained again when an ammonium with a longer alkyl chain, such as n-octyl or n-hexadecanyl ammonium triflate, was mixed with HPHAT in chloroform (Supporting Information Figure S4). These results suggested that the alkyl chain of the ammonium salt also played a crucial role in dictating the 1D self-assembly process. This result prompted us to question whether a hydrogen-bonding donor other than the ammonium group could also be utilized for this purpose. Hence, neutral aliphatic amides were also tested as potential inducers to mediate the self-assembly of HPHAT because the protons of amides are also good hydrogen-bonding donors. However, no well-defined aggregates were generated when dodecanamide was mixed with HPHAT, although it has the same chain length as its ammonium counterpart. This result may be attributed to the weaker bonding strength between protons of the amides and HPHAT than that of protons of the ammonium and HPHAT (vide infra). To understand the formation mechanism of the above microstructures, spectroscopic and crystallographic investigations were carried out. The bonding behavior of HPHAT with alkyl ammonium was investigated by the 1H NMR titration experiment. n-Butyl ammonium triflate was used for this purpose because DOI: 10.1021/la1022104

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Figure 2. Packing patterns of (a) HPHAT and (b) HPHAT- BAT in their crystals. The hydrogen atoms that are not involved in hydrogen bonding, the solvent molecules, and CF3SO3- in image b have been omitted for clarity.

flocculent material always precipitated out when ammonium salts with longer chain lengths were employed. It was found that the solubility of n-butyl ammonium triflate in chloroform is extremely poor. However, the presence of HPHAT could cause its solubilization in chloroform, which indicated that a soluble complex formed from them. Upon addition of the ammonium salt, the signals of H-5 and H-6 of the pyridine unit of HPHAT and the n-butyl unit of the salt were all shifted downfield, suggesting that intermolecular hydrogen bonds were formed between the pyridine N of HPHAT and the three hydrogen atoms of the ammonium salt (Supporting Information Figure S5). No further shifting was exhibited after ca. 3 equiv of the salt was added, suggesting that the bonding was saturated by forming a complex with 1:3 stoichiometry, which was consistent with the fact that HPHAT possess three hydrogen-bonding accepting pockets. Furthermore, the 1H NMR of the as-prepared microbelts from HPHAT and DAT further confirmed the 1:3 stoichiometry of HPHAT and DAT based on the integration of the protons (Supproting Information Figure S6). Microbelts were still formed even when a 1:2 ratio of HPHAT and ammonium salt was used. However, a 1H NMR study of the resulting microbelts revealed that the stoichiometry of HPHAT and the ammonium salt in the microbelts was still 1:3. We think that once the complexes of HPHAT and the ammonium salt with a 1:3 ratio formed, they self-assembled into microbelts and then the microbelts precipitated out, whereas excess HPHAT remained in the solution. Also from the 1H NMR titration experiment, the apparent association constant of the single HPHAT with three BATs in chloroform was estimated to be 4.3  103,11 a value reflecting a very high stability for the complex. The formation of hydrogen bonds was further confirmed by the disappearance of the absorption of the stretching vibration of pyridyls at 3050 cm-1 and the considerably decreasing absorption of NH3þ of the ammonium at 1606 and 1531 cm-1 in the IR spectrum (Supporting Information Figure S7). In addition, the photophysical property of HPHAT before and after the addition of ammonium salt was examined via UVvis and fluorescence spectroscopy. The UV-vis absorption decreased, and red shifting was observed upon the addition of n-butyl ammonium triflate, whereas the fluorescence intensity was increased after the addition of the ammonium salt (Supporting Information Figure S8). Both results could also be attributed to the binding between HPHAT and the ammonium salt. In contrast, the titration of HPHAT with dodecanamide showed almost no change in the chemical shifting for the H-5 and H-6 pyridine protons, except a downfield movement of the signal of the amide protons after it was mixed with HPHAT, which suggested weak bonding between HPHAT and dodecanamide. These results (11) Wu, Z.-Q.; Shao, X.-B.; Li, C.; Hou, J.-L.; Wang, K.; Jiang, X.-K.; Li, Z.-T. J. Am. Chem. Soc. 2005, 127, 17460–17468.

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demonstrated that a stable complex of HPHAT and peripheral groups was a condition for the creation of the microbelts. Although the discotic HAT scaffold itself is rigid and planar and a good aromatic stacking building block, face-to-face stacking was not observed for HPHAT, as revealed by its crystal structure (Figure 2a). This could be attributed to the steric hindrance of the six peripheral pyridyl groups, which twisted significantly out of the plane of the HAT scaffold. Such a conformation might account for the ill-defined aggregate generated from HPHAT itself. Fortunately, a single crystal of complexed HPHAT and butyl ammonium triflate (BAT) suitable for X-ray crystallography was also obtained by the slow evaporation of a 1:3 mixture of HAHAT and BAT in tertrahydrofuran. Although the crystal was just bonded to two BAT molecules, the crystallographic study of the cocrystal of HPHAT and BAT clearly showed that HPHAT was hydrogen bonded to the NH protons of the ammonium groups of BAT, which further facilitated the face-to-face packing of its HAT core with an average distance of 3.62 A˚ (Figure 2b). The packing structure also showed a perpendicular arrangement of the pairs in the HPHAT-BAT complex. Therefore, the formation of the microbelts might be realized through the 1D growing process of such a packing motif. From the crystal structure of HPHAT-BAT, it also could be found that the butyl chains were too short to cause an interaction between them. The lack of such an interaction might be used to explain why no welldefined microstructures formed from HPHAT and BAT. However, when alkyl ammoniums with longer chain lengths were used, the van der Waals interaction between the alkyl chains should be expected, which further stabilized the 1D packing of the complexes between HPHAT and the alkyl ammoniums and led to the formation of the microbelts. A powder X-ray diffraction (XRD) study was also performed to obtain deep insight into the microstructure of the microbelts. The powder XRD revealed that the microbelts prepared from HPHAT and DAT displayed strong, sharp diffraction peaks, suggesting highly crystalline character that is consistent with the polarizing microscope observation. A diffraction peak (d spacing of 3.80 A˚) corresponding to π-π stacking was observed,12 and the value is very close to the distance between the two face-to-face-stacked HAT cores in the cocrystal of HPHAT-BAT. In addition, the XRD pattern was comparable to that calculated from the crystallographic data of the HPHATBAT complex in the low-2θ region, although HPHAT was just bonded to two BATs in the crystals and BAT was different from DAT in the structure (Supporting Information Figure S9). Therefore, it is reasonable to assume a similar arrangement of the HAT core in the microbelts to that in the crystal. On the basis of the above results, a sequential self-assembly process was proposed. As shown in Figure 3, a complex of (12) Sarma, B.; Reddy, L. S.; Nangia, A. Cryst. Growth Des. 2008, 8, 4546–4552.

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Figure 3. Tentative model for the formation of microbelts from HPHAT and primary alkyl ammoniums through a sequential self-assembly process.

HPHAT-primary alkyl ammoniums was first formed that was driven by the hydrogen bonding between HPHAT and the alkyl ammoniums. The hydrogen-bonded complex then self-assembled into microbelts, in which an arrangement of the complex similar to the crystal structure of HPHAT-BAT was adopted under the cooperative interactions of the π-π stacking of the HAT cores and the van der Waals interaction between the alkyl chains. An one-step coassembly mechanism, that is, the hydrogen bonding, π-π stacking, and van der Waals forces work simultaneously to generate the microbelts, may be argued. We propose that the aforementioned two-step process is more reasonable. As revealed by the crystallographic study, the π-π stacking of HAT cores is facilitated by the formation of a hydrogen bond. Therefore, hydrogen bonding should come into play first, followed by π-π stacking and van der Waals interactions.

into microbelts driven by the cooperation of π-π stacking of the HAT cores and the van der Waals interaction between the alkyl chains. Both the structure of peripheral groups and the type of hydrogen-bond donors play crucial roles in the construction of wellordered entities. This approach might offer a promising way to construct novel supramolecular architectures via the coassembly of simple components without the need to synthesize supramolecular building blocks with complicated chemical structures and should be applicable to other noncovalent forces such as transition-metal coordination interaction. Furthermore, the effective construction of micro/nanostructures might be screened from dynamic libraries generated from the coassembly of the π core and multifarious sidechain components. Considering the versatile applications of HATs in materials science, the 1D microbelt might be further explored to be used as semiconducting molecular wires, sensors, or other functional nanomaterials. We are currently investigating these potentials.

Conclusions The one dimensional self-assembly of π-conjugated systems into well-ordered aggregates provides a powerful tool for fabricating functional micro/nanostructures. Whereas the synthesis of the π cores that bear peripheral substitutents represents the major design principle for the construction of the supramolecular tectons, in this letter we demonstrate that the hydrogen-bonded π-core alkyl chain complexes can also serve as their noncovalently bonded analogues to fulfill the same mission. The formation of a stable hydrogen-bonded HPHAT-alkyl ammonium complex is believed to be the condition for this sequential self-assembly process, through which the complex could further self-assemble

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Acknowledgment. We thank the NSFC (20972180, 20921091, 20974118, and 20732007), the NBRP (2007CB808001), and the Science and Technology Commission of Shanghai Municipality (09XD1405300) for financial support. Supporting Information Available: Experimental details and characterizations, additional microscope images, DSC thermograms, 1H NMR, IR, UV-vis, and fluorescence spectra, an XRD profile, and X-ray crystallographic data (cif). This material is available free of charge via the Internet at http://pubs.acs.org.

DOI: 10.1021/la1022104

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