NANO LETTERS
A Linear Chain of Water Molecules Accommodated in a Macrocyclic Nanotube Channel
2009 Vol. 9, No. 1 122-125
Katsuhiko Ono,*,† Kenichi Tsukamoto,† Ryohei Hosokawa,† Masaki Kato,‡ Motohiro Suganuma,‡ Masaaki Tomura,§ Katsuya Sako,† Keijiro Taga,† and Katsuhiro Saito† Department of Materials Science and Engineering, Nagoya Institute of Technology, Gokiso, Showa-ku, Nagoya 466-8555, Japan, Aichi Industrial Technology Institute, Hitotsugi, Kariya 448-0003, Japan, and Institute for Molecular Science, Myodaiji, Okazaki 444-8585, Japan Received September 3, 2008; Revised Manuscript Received December 6, 2008
ABSTRACT A macrocyclic tetramer of 2-phenyl-1,3,4-oxadiazole was synthesized, and its self-assembly was investigated. The macrocycle was stacked to form a one-dimensional (1D) columnar structure containing water molecules. The nanotube self-assembled into a bundle, which grew into a molecular wire. The association of the water molecules in the tubular cavity resulted in shielding of the 1D chain of water molecules by the nanotube; these macrocyclic nanotube channels are promising candidates for nanotechnological applications.
Supramolecular chemistry has been a subject of great research interest in various fields such as materials science, biochemistry, crystal engineering, and chemical synthesis since molecular aggregations possess novel properties and unique functionalities.1 Aromatic macrocycles have attracted significant interest as building components in the study of supramolecular chemistry.2-6 A variety of aromatic macrocycles have been synthesized and investigated.3 These compounds are capable of arranging themselves into onedimensional (1D) columnar structures; this self-assembly is governed by strong π-π stacking, in addition to van der Waals, dipole, and hydrophobic interactions. These columnar structures are useful in electron and hole conduction.4 In addition, the cavities of some of the macrocycles can accommodate various molecules and metals as guests.5 Therefore, these columnar structures also serve as molecular capsules and tubes,6 which control the arrangement of guest molecules into 1D structures. Thus, macrocycles have attracted significant attention as building components for hybrid composite materials for nanotechnological applications. Therefore, we have designed macrocyclic oligomers of 2-phenyl-1,3,4-oxadiazole and synthesized tetramer 1, together with a small amount of trimer 2 (Figure 1). In the crystal of 1, a 1D chain of water molecules is formed in the channel of the self-assembled macrocycles. In this paper, * Corresponding author. E-mail:
[email protected]. † Nagoya Institute of Technology. ‡ Aichi Industrial Technology Institute. § Institute for Molecular Science. 10.1021/nl802672u CCC: $40.75 Published on Web 12/23/2008
2009 American Chemical Society
we report the synthesis, properties, and self-assembly of 1 from the viewpoint of supramolecular chemistry. A mixture of 5-tert-butylisophthalic acid and thiosemicarbazide in polyphosphoric acid (PPA) was stirred at 180 °C for 4 h. Subsequently, a white solid was obtained as a mixture of 1 (86%) and 2 (14%). The solid was recrystallized from CHCl3/AcOEt (2:1) to afford a pure sample of 1 in 2.1% yield. 1H NMR analysis revealed that the internal and external protons of the benzene rings in 1 underwent greater chemical shifts than the protons of the benzene rings in trimer 2 and acyclic compound 3. The external protons of 1 showed a greater downfield shift than those of 2 and 3 (1, 8.67; 2, 8.43; 3, 8.38 ppm). On the other hand the internal protons of 1 and 2 showed greater upfield and downfield shifts, respectively, than those of 3 (1, 8.40; 2, 9.04; 3, 8.65 ppm). The signal due to the internal protons of 1 shifted to a higher field than that due to the external protons. These results are attributed to the molecular structures of 1, 2, and 3 and/or their behavior in solution. From B3LYP/6-31G(d) calculations,7 the molecular structure of 1 was found to be saddleshaped, while that of 2 was planar and rigid. Tetramer 1 could interact with water molecules in solution. The elemental analysis of recrystallized 1 revealed that the macrocycle was hydrated by one water molecule. Sublimation of this recrystallized compound at 400 °C under 10-3 Torr afforded an anhydrous compound, indicating the existence of cavities for accommodating water molecules in the solid. Furthermore, recrystallization of the anhydrous compound from
Figure 1. Structure of macrocyclic and acyclic compounds.
Figure 2. Crystal structure of 1: (a) molecular stacking (spacefill), (b) columnar structure containing water molecules (ellipsoid), and (c) bundle structure (spacefill).
Figure 3. PL spectra of 1: (a) spectra in concentrations less than 1.3 × 10-6 M; (b) spectra in concentrations greater than 6.2 × 10-5 M; (c) time-dependent spectra after the addition of ethyl acetate (2.4 mL) to a solution of 1 in chloroform (3.2 × 10-4 M, 0.8 mL); (d) spectra of the self-assembled structures in solution and the solid.
CHCl3/AcOEt or CHCl3/MeOH reproduced the hydrated sample. The molecules in the crystal were planar and exhibited Ci symmetry (Figure 2a). The macrocyclic molecules were uniformly stacked along the c axis with an intermolecular distance of 3.389 Å; this value is close to the sum of the van der Waals radii of carbon atoms (3.40 Å). The molecules Nano Lett., Vol. 9, No. 1, 2009
overlap each other with a rotation angle of 33°. Strong faceto-face stacking is induced by electrostatic intermolecular interactions between the oxadiazole and benzene rings. In addition, four tert-butyl groups lock the neighboring macrocycles, and this contributes to form the tubular assembly. The water molecules are located at the center of the cavities in the macrocycles, where they vibrate thermally along the 123
Figure 4. (a) SEM image and (b) XRD profiles.
stacking direction of the macrocycles. Figure 2b shows the linear chain of the oxygen atoms of water in the channel of the self-assembled macrocycles. This self-assembling property facilitates the arrangement of water molecules into 1D chain structures, and these water molecules are expected to possess unique properties and applications.8 In the crystal structure of 1, the nanotubes self-assemble to form a bundle (Figure 2c). The tert-butyl groups also play a crucial role in the formation of the bundle. The nanotubes come in contact with each other in the vicinity of the tert-butyl groups. These groups alternately interpose between the neighboring tertbutyl groups to result in a tight bundle. In order to investigate the self-assembly of 1 in solution, we conducted spectral analysis using its photoluminescence (PL) spectra. Emission maxima in chloroform were observed between 330 and 370 nm. The intensity of these bands increased when the concentration of the solution was increased from 1.3 × 10-7 to 1.3 × 10-6 M (Figure 3a) and decreased due to the self-quenching of 1 when the concentration was increased from 6.2 × 10-5 to 3.2 × 10-4 M (Figure 3b). When ethyl acetate was added to a solution whose concentration was greater than 2.5 × 10-4 M, a white solid gradually precipitated, and its PL spectrum changed with time, as shown in Figure 3c. The bands between 330 and 370 nm gradually diminished, and new bands between 370 and 420 nm emerged. After the mixture was allowed to stand for 24 h, the shape of its PL spectrum dramatically changed. The resulting mixture consisted of the tubular assembly and others including the monomer, because the emission bands at 395 and 412 nm in the solid indicate the self-assembly of 1 (Figure 3d). To a solution of tetramer 1 in chloroform (3.2 × 10-4 M, 10 mL) ethyl acetate (10 mL) was added, and this resulted in the gradual precipitation of a white cotton-like solid (see Supporting Information). After the sample was dried, fieldemission scanning electron microscopy (FE-SEM) was performed in order to gain insight into the aggregation morphology of 1. Figure 4a shows the SEM image of a selfassembled molecular wire, whose diameter and length are 380 nm and almost 100 µm, respectively. The molecular wires are assumed to be composed of self-assembled nanotube bundles because the wires revealed an X-ray diffraction (XRD) profile similar to the powder pattern of the single crystal (Figure 4b). The diffraction peaks at around 2θ ) 26° indicate π-π stacking distances of 3.411 and 3.453 124
Figure 5. DRIFTS of hydrated 1: OH and CH stretching.
Å at -100 °C and at room temperature, respectively, indicating that the molecular wires undergo stretching with an increase in the temperature. The π-π stacking distance at -100 °C is longer by 0.022 Å than that in the single crystal. Differential scanning calorimetry (DSC) of the hydrated sample showed no significant peaks for endothermic and exothermic phase transitions for temperatures up to 470 °C. Tetramer 1 underwent thermal decomposition above 470 °C. Thermogravimetric analysis (TGA) of the hydrated sample also showed no significant weight loss up to 470 °C. These results indicate that water molecules are retained in the sample under the measurement conditions due to the dense molecular packing in the crystal structure. The hydrated sample showed a broad band due to OH stretching at around 3457 cm-1, indicating that the water molecules exist in an associated form. Therefore, the water molecules interact with one another in the tubular cavity formed by 1. Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) of hydrated 1 was measured, and Figure 5 shows the OH and CH stretching bands. The OH-stretching band was Nano Lett., Vol. 9, No. 1, 2009
observed even after heating at 300 °C for 1 h under He flow, although its band disappeared after heating at 400 °C for 1 h. The 1D chain of water molecules is shielded by the macrocyclic nanotube channel; hence, these macrocyclic channels are promising candidates for nanotechnological applications such as optical nanofibers. Further investigation pertaining to these studies is underway. Acknowledgment. This work was supported by a Grantin-Aid for Scientific Research (No. 19550034) from the Ministry of Education, Culture, Sports, Science and Technology, Japan and a grant from the Ichihara International Scholarship Foundation. We thank the Instrument Center of the Institute for Molecular Science for the X-ray structure analyses. We thank Dr. M. Fujiwara (Institute for Molecular Science) for the XRD measurements and M. Suto and K. Eguchi (Dow Corning Toray Co., Ltd.) for the DSC and TGA.
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