CRYSTAL GROWTH & DESIGN 2006 VOL. 6, NO. 7 1559-1562
Communications Self-Assembled, Discrete Organic Tubular Crystals with Controllable Sizes by Simple Sublimation Hongding Tang, Jian Ping Gao, Ying Xiong, and Zhi Yuan Wang* Department of Chemistry, Carleton UniVersity, 1125 Colonel By DriVe, Ottawa, Ontario, Canada K1S 5B6 ReceiVed April 19, 2006; ReVised Manuscript ReceiVed May 14, 2006
ABSTRACT: Large, free-standing, and fluorescent crystalline tubules with controllable sizes were obtained by simply sublimation of 1,4-bis(5-pentafluorophenyl-1,3,4-oxadiazol-2-yl)benzene under ambient conditions. Besides the intrinsic layered structure, which may be essential to the formation of tubular architecture, the substrate and source temperature and sublimation time also play important roles in this self-assembling tube growth process. Following the discovery of carbon nanotubes,1 nano- or microsized inorganic tubular crystals consisting of atoms other than carbon, such as BN, MxQy (M ) W, Mo, Cu, Cd, Ag, Sb; Q ) S, Se),2 metal oxides,3 and platinum complex,4 have been prepared and characterized. Most inorganic tubular crystals are derived from compounds possessing layered structures.5 At the same time, various preparative methods have been explored and typically include vapor deposition, laser ablation, arc-discharge, self-assembly and solvothermal, electrochemical, and template-directed syntheses. Organic cylindrical nanotubes reported to date are mainly constructed from lipids6 and cyclic compounds7 in solution through the intermediate bilayers and molecular stack. However, less attention has been paid to the formation of discrete tubules from small organic compounds,8 despite the availability of a vast amount of organic compounds with unique electrical and optical properties and the anticipated applications of organic functional materials with various tubular structures in microelectronics and optoelectronics. We report herein the formation of free-standing crystalline tubules with controllable sizes from a small fluorescent organic compound by simple sublimation under ambient conditions. In connection with our work on developing new n-type organic semiconductors and light-emitting materials, a series of fluorescent fluorinated bis-oxadiazole compounds were synthesized. These bisoxadiazole compounds are crystalline and can be sublimed. Among them, 1,4-bis(5-pentafluorophenyl-1,3,4-oxadiazol-2-yl)benzene (BPFOB) is able to form discrete tubules on a glass substrate by sublimation at 250-300 °C (Figure 1). Examination by scanning electron microscopy (SEM) indicates that the tube sizes range from less than one hundred nanometers to over hundred micrometers in diameter and a few hundred nanometers to a few centimeters in length, depending on the sublimation conditions. The largest tube obtained was more than 1 mm wide and more than 1 cm long. The cross section of tubes is polygonal with sharp corners. The corner angles may vary, and some tubes possess a square cross-section, with two neighboring basal planes almost located at 90 degrees to each other. Most tubes have open-ended structures on one side and * To whom correspondence should be addressed. E-mail: wayne_wang@ carleton.ca.
are sealed on the other end. The tubes are thermally stable up to 200 °C without deformation and are resistant to most organic solvents. However, the structural analogues, 2,6-bis(5-pentafluorophenyl-1,3,4-oxadiazol-2-yl)pyridine (BPFOP) and 2,6-bis(5pentafluorophenyl-1,3,4-oxadiazol-2-yl)naphthalene (BPFON) (Figure 1), failed to yield crystals with tubular structures under similar sublimation conditions. To gain insight into the molecular organization of the tubular architecture, crystal structures were characterized by X-ray crystallography. Single-crystal X-ray analysis reveals that the BPFOB molecule exists in nearly planar conformation in the triclinic space group (P1h) (Figure 1A) with the cell parameters of a ) 7.3223 Å, b ) 7.5889 Å, c ) 8.7140 Å, R ) 83.383°, β ) 77.723°, γ ) 89.083° (see Supporting Information). Along the b-axis, the molecules are arranged side-by-side into one-dimensional arrays, which further extended along the (201h) plane to form twodimensional sheets (Figure 1B). Each BPFOB molecule forms four hydrogen bonding with two adjacent molecules, as evidenced by a shorter intermolecular H‚‚‚N bond (2.43 Å) relative to the sum of their van der Waals radii (2.75 Å).9 The sheets may also be stabilized by weak van der Waals interactions between fluorine atoms from two head-to-head neighboring pentafluorophenyl rings that feature a shorter F‚‚‚F distance (2.79 Å) than their van der Waals radii sum (2.94 Å) (Figure 1B).9b,10 The distance between two adjacent layers is calculated to be ca. 3.07 Å (Figure 1C), which is significantly shorter than that observed in graphite (3.34 Å). However, each sheet is shifted along the c-axis by one cell parameter c with respect to its neighbors. Instead of a face-to-face alignment, the adjacent sheets are held together by offset π-π stacking between the bridging benzene ring and two electronegative pentafluorophenyls from the adjacent layers, with hydrogen and fluorine atoms almost centered on the top of each other’s aryl rings (Figure 1D). The planar conformation of BPFOB with a layered crystalline structure satisfies the critical requisite for the formation of tubular architecture concluded from inorganic materials.5 For BPFOP, X-ray crystallography indicates that the molecule adopts a twisted conformation in its monoclinic crystal system (a ) 34.287 Å, b ) 6.4516 Å, c ) 9.079 Å, and β ) 104.595°), in which a displacement angle (ca. 16°) exists between the oxadiazole and the
10.1021/cg0602313 CCC: $33.50 © 2006 American Chemical Society Published on Web 06/01/2006
1560 Crystal Growth & Design, Vol. 6, No. 7, 2006
Communications
Figure 1. Chemical structures of BPFOB, BPFOP, and BPFON. Crystal structure of BPFOB: (A) The triclinic unit cell; (B) layered structure viewed along the b-axis; (C) interaction of hydrogen bonding (C-H‚‚‚N) and halogen bonding (F‚‚‚F) between adjacent molecules (unit Å) within one sheet; (D) Illustration of the packing of two adjacent layers (the layer marked in red is on top). Carbon, black; fluorine, green; oxygen, red; nitrogen, light blue; hydrogen, dark blue.
pentafluorophenyl rings, and no layerlike structure is observed. Sublimation or crystallization of BPFON did not afford diffractionquality crystals. X-ray diffraction (XRD) rotating around the long axis of the BPFOB tubular crystal was taken to determine the molecular orientation relative to the tube axis. The XRD pattern analyses show that the b-axis of the single crystal almost parallels the tube shaft (see Supporting Information). Therefore, BPFOB molecules lay parallel to each other side-by-side along the shaft of the tubular crystal, with their molecular axis about 70° to and the molecular plane parallel to the long axis of the tubular crystal. Moreover, the orientation of molecular packing within the crystalline tube is further evident by its anisotropic fluorescent characteristics.11 BPFOB crystals emit strong blue light around 380 nm, when being excited by 300-nm light (Figure 2). This emission corresponds to the HOMO-LUMO transition moment along the axis of molecules.
Thus, when the fluorescence of tubular crystals was measured through a polarizer, the intensity was much stronger if the polarizer was placed perpendicularly to the shaft of the tubular crystal. The formation of self-assembled tubular crystals is attributed to uniqueness of the molecular structure of BPFOB and also very much depends on the sublimation conditions, such as substrate temperature, source temperature, and sublimation time or crystal growth time. The substrate temperature was found to have a profound influence on crystal morphology and growth. A detailed study of the tubular crystal growth was carried out using a long glass tube placed in a cylindrical oven with different heating zones. The temperatures at different zones along the glass tube were monitored and recorded during the experiments. Figure 3 shows representative SEM images of crystals grown on the glass tube at different temperature zones when the BPFOB sample was heated at 250 °C for 200 min. Large tubules and crystals were found on the substrate
Communications
Figure 2. (A) UV-Vis and fluorescence spectra of BPFOB in DMF solution, vapor deposited as thin film on quartz substrate and as discrete tubular crystals. (B) Fluorescence spectra of BPFOB tubular crystals excited at 300 nm with a polarizer placed perpendicular and parallel to the crystal shaft, respectively.
Figure 3. The representative SEM images of BPFOB crystals grown on the glass tube at different temperature zones (source temperature 250 °C sublimation time 200 min.). The substrate temperatures are (A) 210 ( 5 °C, (B) 180 ( 5 °C, (C) 140 ( 5 °C and its high magnification image (inset), and (D) 75 ( 5°C, respectively.
where the temperature was controlled around 210 ( 5 °C (Figure 3A), while at relatively lower substrate temperature (180 ( 5 °C) smaller tubular crystals were formed (Figure 3B). Interestingly, in
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Figure 4. The representative SEM images of BPFOB crystals taken as the function of sublimation time (glass substrate temperature 180-210 °C, source temperature 280 °C). (A) 50-100 min, (B) 300 min, (C) 600 min, and (D) 1000 min (1-mm scale bar, gold grains shown in background), respectively.
the next adjacent zone where temperature was below 150 °C (140 ( 5 °C), long needlelike rods formed together with some platelike crystals (Figure 3C). A high magnification SEM image (Figure 3C, inset) reveals the presence of open-channel, curled crystal sheets, suggesting a roll-up process from crystal sheets to the needles. When the substrate temperature fell below 90 °C (80 ( 5 °C), only opaque featureless crystals were deposited on the substrate (Figure 3D). Crystals with different shapes were found to exist in the area between two adjacent zones. Furthermore, the sublimation or source temperature can affect the sublimation rate or molecular concentration in the vapor phase, which in turn influences the crystal growth. BPFOB begins to sublime over 230 °C under ambient atmosphere conditions and melts at 320 °C. The sublimation temperature suitable for growing highquality tubules was found to be between 250 and 280 °C. Below 240 °C, BPFOB sublimes too slowly to form any sizable crystals within a reasonable time frame. Above 300 °C, large crystals are formed but often have rough, irregular inner and outer surfaces. The crystal growth was further monitored over time at given temperatures. Figure 4 shows SEM images of tubular crystals grown on glass substrate (180-210 °C) with a source temperature of 280 °C for (A) less than 100 min, (B) 300 min, (C) 600 min, and (D) 1000 min. In the initial 100 min (Figure 4A), small, ill-defined hollow-structured crystals were formed, along with some irregular crystalline grains, with the length and width ranging from hundreds of nanometers to a few micrometers. As sublimation proceeds, tubular structures gradually form into shapes as shown in Figure 4B. The tube diameter increased to tens of micrometers, and the length reached hundreds of micrometers in 600 min (Figure 4C).
1562 Crystal Growth & Design, Vol. 6, No. 7, 2006 Large well-defined tubules of ca. 4.2 mm in length and ca. 100 µm in width were readily formed on the substrate within the hightemperature region when the sublimation time was extended over 600 min (Figure 4D). Some interesting phenomena were observed during the preparation of tubular crystals. (1) Small tubular crystals, which had been removed from the oven, continued to grow faster along the crystal shaft than other directions if the sublimation process resumed. This could be explained by the fact that the shaft direction is also the direction where the hydrogen bonding is strongest. (2) Some of the irregular hollow structures formed in the early stage appeared to have a circular interior. This implies a process of tube growth similar to the one for carbon nanotubes and tubules made of lipids, involving the thermodynamically favored sheet curling-up process. (3) The tube diameters vary significantly in the early stage and then remain almost constantly after about 200-min of sublimation, while its length continues to increase. (4) Although the outer wall of the tube appears to be smooth and continuous, a bifurcation point of two planes exists in the inner wall of a large tube. This observation strongly suggests a process of linking of crystal sheets to form a polygonal tube, similar to the cases for some inorganic polygonal tubes.2f (5) Many small irregular crystals formed in the high-temperature region during the initial stage disappeared over time while the tubular structures were growing into shapes. Thus, the evaporation-deposition cycle taking place during the crystal growth is essential for the formation of large and well-defined tubular crystals. On the basis of the observed phenomena, the growth process for tubular crystals of BPFOB is believed to involve at least three steps. First, the irregular holelike nuclei were formed on the substrate (180-210 °C) during the first stage of sublimation (before 100 min, Figure 3A). As sublimation continued, the crystal nuclei induced the crystal growth, leading to the formation of larger, better-defined hollow crystals. Finally, epitaxal growth along the b-axis of the growing tubular crystals led to well-defined hallow tubules. The driving force for this preferred expitaxal growth arises from the strong hydrogen bonding between two adjacent molecules (Figure 1B). In conclusion, the intrinsic planar conformation of the BPFOB molecule and the sheetlike crystalline structure held by intermolecular hydrogen bonding and π-π stacking are essential for the formation of tubular crystals. Substrate temperature, source temperature, and sublimation time affect the self-assembling tube-growth process. The strong polarized blue light emission of BPFOB discrete tubular crystals demonstrates the anticipated novel properties arising from its unique structure. Given the diversity of functional organic compounds, the ability to make discrete tubules by simple sublimation and fabricate two-dimensional microstructures on various substrates opens a new avenue to exploration of organic functional materials for innovation in optoelectronics and photonics. Acknowledgment. We thank the Natural Sciences and Engineering Research Council of Canada for financial support. We also thank X. Ou’Yang of the University of Ottawa, K. A. Udachin of
Communications the Steacie Institute for Molecular Sciences of the National Research Council of Canada, and Chun Ye and Er-chang Chen of Peking University for X-ray diffraction measurements. Supporting Information Available: Experimental procedures; crystal data for BPFOB; X-ray diffraction pattern of BPFOB tubular crystals; crystallographic information file (cif). This material is available free of charge via the Internet at http://pubs.acs.org.
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