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Guest-Assisted Self-assembly of Organostannoxane Nanotubules on a Mica Surface V. Chandrasekhar,*,† P. Thilagar,*,‡ and A. Steiner§ †
Department of Chemistry, Indian Institute of Technology, Kanpur, Kanpur-208016, India Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore-560012, India § Department of Chemistry, University of Liverpool, Liverpool L69 7ZD, U.K. ‡
bS Supporting Information ABSTRACT: The reaction of [n-BuSn(O)OH]n and 9-hydroxy-9-fluorenecarboxylic acid in the presence of p-X-C6H4-OH (X = F, Br) afforded hydroxyl-rich hexameric organostannoxane prismanes. The crystal structures of these prismanes reveal guest-assisted supramolecular structures. Self-assembly of these compounds on a mica surface affords organooxotin nanotubules.
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omplex supramolecular architectures are ubiquitous in biological systems and biomaterials. These are built through the cumulative effect of various noncovalent interactions, including hydrogen bonding and electrostatic interactions.1,2 Inspired by such natural systems, there is currently widespread interest in designing systems that can self-assemble to form nano- or micrometer-scale morphologies. These include systems such as micelles, vesicles, ribbons, tubes, and tubules.3 Another challenge is to be able to form such organized structures on solid surfaces in view of their potential applications in molecular sensing, in catalysis, as well as in molecular devices such as optoelectronic and logic circuits. However, most of the surface-supported assemblies reported to date are predominantly generated from organic molecules.35 Recently, Yaghi and co-workers reported the assembly of metalorganic frameworks on a graphite surface.5 No examples of surface-assembled organometallic supramolecular architectures are known. Herein, we demonstrate the generation of organostannoxane nanotubule structures on a mica surface. This phenomenon is unprecedented among this family of compounds.6 Hexanuclear organostannoxane prismanes [{n-BuSn(OH)L}6 3 6H2O 3 6ROH 3 CHCl3] (1) (LH2 = 9-hydroxy-9-fluorenecarboxylic acid; ROH = p-fluorophenol) and [{n-BuSn(OH)L}6 3 6H2O 3 6ROH 3 CHCl3] (2) (LH2 = 9-hydroxy-9- fluorenecarboxylic acid; ROH = p-bromophenol) were synthesized by the reaction of [n-BuSn(O)(OH)]n with 9-hydroxy-9-fluorenecarboxylic acid in the presence of the corresponding phenol.7 The molecular structures of 1 and 2 are similar and contain C3 symmetric hexanuclear tin assemblies where every tin occupies the vertex of a perfect trigonal prism. Each pair of tin atoms that eclipse each other in the trigonal prismatic structure is bridged by two μ-OH groups. Each of the triangular faces of the trigonal prism is capped by three dianionic r 2011 American Chemical Society
tridentate 9-hydroxy-9-fluorene-carboxylate ligands. The binding action of these ligands generates a spherical cagelike structure8 (see the Supporting Information for the details on the molecular structures of 1 and 2). A notable feature of the crystal structures of 1 and 2 is the guest-assisted columnar supramolecular formation. It is interesting to note that the molecular symmetry of 1 and 2 is carried over to the crystal, leading to the formation of hexagonal two-dimensional layers. Intermolecular OH---O interactions between the geminal hydroxyl oxygen of L and the OH groups of the guest phenol and water molecules generate hexagonal two-dimensional layers in the crystallographic ab plane (see the Supporting Information, Figure S8). The arrangement of the two-dimensional layers in 1 and 2 is slightly different. In 1, layer B is disposed at an angle of 60° about the crystallographic c axis with respect to layer A and undergoes a translational movement along the ab plane (Figure 1a and b). In contrast, in 2, layer B, while being still disposed at 60°, does not undergo any translational movement in the ab plane, resulting in a direct stacking of the layers on top of each other (Figure 2a and b). The influence of the guest phenol molecules on the supramolecular structures of 1 and 2 is significant. In 1, the p-fluorophenol and water guest molecules glue successive layers though CH---F, O---HO, and ππ interactions, leading to columnar structures in the crystallographic c axis (see the Supporting Information for a detailed description of the supramolecular structures). The internal space of the repeating unit of the column has the dimensions 11.015 Å 11.015 Å 11.015 Å. The fluorenyl moiety and the butyl substituents on tin protrude Received: February 9, 2011 Published: April 12, 2011 1446
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Figure 1. (a) Space filling model of the basic unit of the columnar packing of 1. The guest phenol molecules are shown in light brown, and chloroform molecules are shown in pink. Alternate organostannoxane hexamer layers are shown in green and blue. (b) Cartoon representation of the crystal packing of 1. Individual stannoxane prismanes are averaged to spheres. Green spheres represent one layer while blue spheres represent the immediate lower layer. (c) AFM image of 1. (d) Enlarged image of the highlighted area shown in image c.
into the internal space of the column. At regular intervals, these hydrophobic columns are occupied by chloroform molecules. In 2, columnar structures are formed as a result of interlayer CH--π interactions (Figure 3). The voids present in these columns are occupied alternately by p-bromophenol and chloroform molecules (Figure S9 of the Supporting Information). The former is involved in OH---O, CH---π, and CH---O interactions while the latter is involved in Cl---HC interactions. The major difference in the supramolecular structures of 1 and 2 is that the former contains completely hydrophobic channels in its crystal structure whereas in the latter the channel structure contains alternate hydrophobic and hydrophilic voids (Figure S9). The surface self-assembly of 1 and 2 was systematically investigated by noncontact mode atomic force microscopy (AFM). Crystals of 1 or 2 were dissolved in dichloromethane, and the resulting solution (10 μL, 10 μM) was spread on a freshly cleaved mica surface with the help of a micropipet. After solvent evaporation at STP, the resultant thin films were dried in vacuum. AFM images of 1 and 2 are shown in Figures 1 and 2, respectively. The columnar packing of 1 is clearly seen in its AFM image (Figure 1c). The dimensions of the individual columnar tubules in 1 are 300 (width) and 350 nm (height), respectively. This represents about 60 tubular layers (each with a width of 5 nm) found in the crystal structure of 1. The AFM image of 2 confirms the retention of the columnar tubule structures for this compound on a mica surface (Figure 2c and d). The formation of a columnar structure can be rationalized in the following way; first, they form two-dimensional layer structures as described in Figures 1 and 2. These then aggregate to form small disks which
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Figure 2. (a) Space filling model of the basic unit of the columnar packing of 3. The guest phenol molecules are shown in light pink color. (b) Cartoon representation of the crystal packing of 3. Notice the direct stacking of the adjacent layers shown in green and blue. (c) AFM image of 3. (d) Enlarged part of the selected area in image c.
Figure 3. Hexagonal packing of 2. The guest molecules (six 4-bromophenol molecules and six water molecules) trapped in the trigonal voids are shown as a space filling model.
are stacked together to form tubules. These experiments reveal that the crystal structure features of 1 and 2 are replicated on deposition of their solutions on a solid surface. Thus, the morphology of the surface assembly of 1 and 2 closely matches the columnar packing present in their crystal structures. The 1447
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13, crystal and hierarchical supramolecular structural description of 13, SEM images of 13, optical spectroscopy images of 13, and solution and solid state absorption spectra and solution emission spectra of compounds 13. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*V.C.: fax, (þ91)-512-2597436; e-mail,
[email protected]. P.T.: fax, 0091-80-23601552; e-mail,
[email protected].
crucial role of the phenol guest molecules in the supramolecular formation (in crystals and surface) of 1 and 2 becomes evident from the studies on the phenol-free organostannoxane prismane [{n-BuSn(OH)L}6 3 2H2O 3 2CHCl3] (3), which reveals the absence of surface self-assembly (see Supporting Information). Compounds 13 have also been examined by scanning electron microscopy (SEM) (Figure S11 of the Supporting Information). Recrystallized samples of 13 were placed on a copper disk (1 cm 1 cm), and a drop of isopropanol was added to the sample. The samples were then dried in vacuum and examined by SEM. The SEM images of 1 (Figure S11a of the Supporting Information) and 2 (Figure S11b of the Supporting Information) show regions where columnar arrangement is seen while 3 shows a complete absence of such structures (Figure S11c of the Supporting Information). UVvis absorption and fluorescence spectroscopy studies of 1, 2, and 3 (Figures S12S14 of the Supporting Information) also confirmed their solid-state structural behavior. In solution, 1 and 2 emit strongly at 340 nm (Figure S13 of the Supporting Information). In the case of thin films, the emission maxima of 1 and 2 are red-shifted by 50 nm (Figure 4). An additional emission peak at 480 nm is also noticed (Figure 4). The shift in emission maxima and the appearance of new peaks strongly support aggregation in the solid-state, as determined in X-ray crystal structure and AFM studies. The thin films used for photochemical studies were investigated by optical microscopy (Figures S15S17 of the Supporting Information). These images also strongly support the columnar packing of 1 and 2 in the solidstate self-assembly as well as the absence of such packing in 3 (Supporting Information). In conclusion, we demonstrate that hexanuclear organostannoxane prismanes 1 and 2 show guest-assisted three-dimensional columnar architectures in their crystal structures. The nanotubular features of the crystal structures are retained upon deposition of the solutions of 1 or 2 on a mica surface, owing to surface self-assembly. Photophysical studies of these fluorophore-rich surface assemblies should be rewarding. These studies are in progress.
’ ASSOCIATED CONTENT
bS
Supporting Information. Crystallographic information file for 1 and 2, experimental technique describing synthesis of
DEDICATION Dedicated to Prof. S. S. Krishnamurthy on the occasion of his 70th birthday.
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Figure 4. Solution (solid line) and solid state (broken line) photoluminescence spectra of compounds 1 (black) and 2 (red).
’ ACKNOWLEDGMENT The authors acknowledge the Council of Scientific and Industrial Research, New Delhi, India, and Department of Science and Technology, New Delhi, India, for funding.
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