Crystal Hierarchical Supramolecular Architectures from 1-D Precursor

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J. Phys. Chem. C 2010, 114, 2925–2931

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Crystal Hierarchical Supramolecular Architectures from 1-D Precursor Single-Crystal Seeds Jialiang Xu,†,‡ Haiyan Zheng,†,‡ Huibiao Liu,*,† Chunjie Zhou,†,‡ Yingjie Zhao,†,‡ Yongjun Li,†,‡ and Yuliang Li*,† Beijing National Laboratory for Molecular Sciences (BNLMS), CAS Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China and Graduate UniVersity of Chinese Academy of Sciences, Beijing 100190, P. R. China ReceiVed: December 7, 2009; ReVised Manuscript ReceiVed: January 12, 2010

Hierarchical architectures with highly ordered 3-D micro/nanosized hierarchical structures with promising applications in electronics and optoelectronics are highly desirable in current materials synthesis. In this paper, a fluorene-bridged intramolecular charge-transfer (ICT) compound (bis(4-(dimethylamino)phenyl)-fluoren9-ylidene)malononitrile (DMPFM) has been synthesized and fabricated into 1-D microrods and 3-D microflowers in dioxane and THF, respectively. A stepwise growth method has been developed for constructing crystal micro/nanohierarchical superarchitectures by using the single-crystal 1-D microrods as precursor crystal seeds. The morphological controllability in both growth stages offers opportunities of rational tuning of the size, shapes, and dimensions of hierarchical self-assembled products. This stepwise growing method of hierarchical architectures and the exhibited semiconducting character of the material endows the superstructures with potential applications in the field of electronic devices. Introduction Organic supramolecular architectures with well-defined morphologies and novel functions have attracted increasing attention in recent years because of their potential applications in diverse fields such as molecular electronics, light-energy conversion, catalysis, sensors, etc.1,2 Scientists have successfully constructed low-dimensional (0-D, 1-D, and 2-D) supramolecular microor nanostructures from low-weight organic molecules with the help of hydrogen bonding,3 π-π stacking,4 electrostatic interactions,5 and other nonconvalent interactions.6 Driving forces for constructing such supramolecular self-assemblies with desirable sizes, shapes, and therefore functions derive mainly from tailormade organic dye molecules with distinct chemical structures afforded by organic synthesis.7 Among them, the selfassembly of low-weight intramolecular charge-transfer (ICT) organic molecules is very important8,9 because these D-π-A molecular systems have fascinating application in material fields such as nonlinear optical (NLO) materials,10 electrogenerated chemiluminescence materials,11 fluorescence probes,12 and dyesensitized solar cells.13 Fabrication of complex architectures with highly ordered 3-D micro/nanosized hierarchical structures with promising advanced applications in electronics and optoelectronics is highly desirable in current materials synthesis.14 Considerable efforts have been devoted to the fabrication of hierarchical superstructure with inorganic materials including metal, metal oxide, hydrate, sulfide, etc.15 However, most of these fabrication processes involve high temperatures or introduction of hard or soft templates.16 Furthermore, the examples of organic 3-D hierarchical self-assemblies with intrinsic merits of molecular tailorability and optoelectronic actitivity have been rarely reported,17 and the controllability over the orientations, sizes, and * To whom correspondence should be addressed. E-mail: [email protected]. † Institute of Chemistry, Chinese Academy of Sciences. ‡ Graduate University of Chinese Academy of Sciences.

structures of the superstructures still remains a challenge. In this work, a fluorene-bridged donor-acceptor substituted ICT compound DMPFM has been synthesized and assembled to form crystal superstructures with controllable morphologies and sizes in tetrahydrofuran (THF) and dioxane. A stepwise growing method has been developed for the first time for constructing crystal micro/nanohierarchical superarchitectures by fabricating 1-D microrods in dioxane first and using these single-crystal microrods as precursor crystal seeds in the subsequent solution process in THF. The conductive examination of the microrods demonstrates the semiconductive and Ohmic behavior of the material, which endows the superstructures with potential applications in the field of electronic devices. Results and Discussion The ICT compound bis(4-(dimethylamino)phenyl)-fluoren-9ylidene)malononitrile (DMPFM) was synthesized by a four-step procedure, as shown in Scheme 1 (see Experimental Section for detailed stepwise synthesis procedures). A Suzuki coupling reaction between the as-synthesized starting materials of 2,7-diiodofluorenone (1) and N,N-dimethyl(pinacolatoboron)aniline (2) afforded bis(4-(dimethylamino)phenyl)-fluorenone (DMPFO), which underwent a condensation reaction with malononitrile in the presence of piperidine to get the final olive-drab ICT compound DMPFM. The saturated solution of DMPFM was heated to reflux and cooled to room temperature in dioxane or THF, and the compound precipitated from the solutions in nearly quantitative yield. The observations of scanning electron microscopy (SEM) revealed that DMPFM grew into well-formed supramolecular architectures when precipitated from the solutions in dioxane or THF, and the morphologies of the formed architectures from the two solvents were obviously different (see Experimental Section for detailed preparation). As shown in Figure 1a, largescale 1-D microrods were observed after the refluxing solution of DMPFM in dioxane was cooled and kept undisturbed at 10

10.1021/jp911595m  2010 American Chemical Society Published on Web 01/29/2010

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SCHEME 1: Synthesis Strategy of DMPFM

°C for more than 30 min. The microrods were about 4-5 µm in width and 50 µm in length (Figure 1b) with smooth surfaces and rectangular cross sections (inset of Figure 1a). The typical length-to-width ratio of the microrods was about 10. However, when the saturated refluxing solution of DMPFM in THF was cooled and aged at 10 °C, the precipitates were observed to be “flower-like” 3-D superstructured assemblies (Figure 1c and 1d). These flower-like supramolecular architectures were 50-60 µm in diameter and constructed from nanobelts with a thickness of 300-500 nm and width of 3-5 µm (Figure 1d and Figure S1 (Supporting Information)). The morphologies of the selfassemblies from both dioxane and THF were affected by solution concentration and operation temperatures (Figure S2

Figure 1. Typical morphologies of DMPFM self-assemblies. (a) SEM images of large-scale DMPFM microrods precipitated from its hot solution in dioxane. (Inset) Rectangular cross sections of the microrods. (b) Magnified SEM image of the microrods. (c) SEM images of largescale DMPFM microflowers precipitated from its hot solution in THF. (d) Magnified image of a typical microflower. (e) SEM image of largescale hierarchical microstructures when refluxing a suspension of asprepared DMPFM microrods in THF which were cooled and stabilized at 0 °C. (f) Magnified image of a typical hierarchical microstructure with flowered ends.

(Supporting Information)). The lower concentration and higher environment temperature caused the precipitation process of the solute to be much slower and therefore decreased the growth rate of the microstructures. For the case of 1-D microrods grown from dioxane, the slowed growth process afforded microstructures with larger sizes and multidimensional orientations (Figure S2 (Supporting Information)). For example, while the hot solution was placed and left to cool naturally at 25 °C, the precipitates collected after 5 h were observed to be microrods with diameters of 10-20 µm and lengths of about 100 µm. When the hot solution was diluted, growing was further slowed and precipitates from the cooled solution consisted of microrods with diameters of tens of micrometers and lengths of hundreds. The results in THF were similar with the observation in dioxane, and the observed 3-D microflowers grew much bigger in size when growth was slowed down (Figure S2 (Supporting Information)). A stepwise method was successfully developed for constructing crystal hierarchical structures concerning the previous 1-D microrods and 3-D microflowers. The typical manipulation might be described as follows. First, microrods were fabricated by rapidly cooling the refluxing solution of DMPFM in dioxane as described and harvested by centrifuge. Then a refluxing suspension of the as-prepared microrods in THF was cooled and precipitated completely at 0 °C (Figure 2 and Experimental Section for detailed manupulation). The SEM observations of the collected precipitates indicated a view of large-scale hierarchically structured microsized suprastructures (Figure 1e and 1f). A typical hierarchical self-assembly consisted of a rigid rod with rectangular cross section and two fractal splitting grown ends, as shown in Figure 1f. The width of the rigid rod was almost the same as that of the previously mentioned microrods, suggesting the hierarchical assemblies were grown from the precursor crystal seeds of 1-D microrods. The investigation of SEM images at various stages of the morphogenesis led to a fractal splitting crystal growth model18 for the formation of the flowered microrods (Figure 2). As described, the operation parameters, such as operation temperature and solution concentration, obviously affected the morphologies of both the microrods formed in dioxane and the microflowers fabricated in THF, so the stepwise growth mode

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Figure 2. Schematic representation of the stepwise growth procedure and morphology transition process of the fractal growth at different growing stages.

Figure 3. Hierarchical architectures with different morphologies fabricated at different operation conditions. (a) Step I: 30 min, 8 °C. Step II: 10 min, 10 °C. (b) Step I: >3 h, 25 °C. Step II: 30 min, 10 °C. (c) Step I: 30 min, 8 °C. Step II: 1 h, 10 °C. (d) Step I: 30 min, 8 °C. Step II: >3 h, 0 °C. (e) Step I: 30 min, 25 °C. Step II: 1 h, 25 °C. (f) Step I: 30 min, 25 °C. Step II: > 3 h, 25 °C. (g) Single dumbbell-liked superstructure. (h) Intersected architecture with two dumbbell-like structures. (i) Intersected architecture with three dumbbell-like structures.

involing the two solvents offered opportunities for multiplexly tunable fabrication of superstructures with diverse morphologies by changing operation parameters (Figure 3). Therefore, hierarchical architectures with different axis length and fractal generations were constructed at different operation conditions, as shown in Figure 3. The morphologies of the final formed architectures were also affected by the shape of their precursor crystal seeds. Interestingly, some of these microrods were found to intersect each other at the center and led to the final formation of intersected dumbbell-like hierarchical structures (Figure 3g-i). The intersected precursor microrods and intermediates were also observed as shown in Figure S3 (Supporting Information). Single-crystal X-ray analysis of a DMPFM microrod grown from dioxane demonstrated that the rigid π-backbone of the DMPFM molecule was partially distorted with a dihedral angle of about 24° (Scheme 2a and 2b; see Supporting Information for a cif file). DMPFM molecules form a monoclinic unit cell, and the microrod crystals belong to the P2(1)/n space group with unit cell parameters a ) 18.382 (4) Å, b ) 5.7303 (11) Å, and c ) 25.034 (5) Å (Scheme 2c). In the crystal packing, a DMPFM molecule (molecule A, shown in “ball and stick style” in Figure S4a (Supporting Information)) tends to form dimers via a C-H · · · N hydrogen bond (2.55 Å, Figure S4a,b (Sup-

porting Information)) with a neighboring molecule B. At the same time, molecule A interacts with the other five DMPFM molecules (molecule C-G) via C-H · · · π interactions (∼2.80 Å). These interactions draw near the molecular layers (3.67 Å) and facilitated the partially packed π-π interactions between the packed molecules to form columns along the b axis (Scheme 2d). The columns along the b axis contacted by C-H · · · π interactions and C-H · · · N hydrogen bondings offer a cavum suitable for chain linking of solvent molecules via hydrogen bonding (C-H · · · O, 2.66 Å) along the b axis (Scheme 2d and Figure S4c (Supporting Information)). Such chain linking of dioxane molecules is very important for the formation of the 1-D single crystal in dioxane, although no interactions such as hydrogen bondings between DMPFM and dioxane molecules have been observed in the crystal packing. The self-assembled suprastructures with different growing stages were further characterized by transmission electron microscope (TEM) and corresponding selected area diffraction (SAD) patterns (Figure 4), which revealed definitely that the microrods were single crystal in nature. The SAED pattern was indexed with lattice constants, and the microrod was demonstrated to grow along the direction of [010] (Figure 4a and 4b), agreeing with the fact that DMPFM molecules tends to stack up along the b axis to form columns. The SAED pattern of the hierarchical self-assembly at a fractal end also confirmed the crystalline nature of the hierarchical structures, although at least two series of diffraction spots were found in the pattern which made the index difficult (Figure 4c and 4d). Powder X-ray diffraction (XRD) patterns were also taken to characterize the as-prepared superstructures (Figure 5). For the microrods prepared from the solution in dioxane, a distinct strong peak corresponding to the (010) plane is clearly observed in the diffraction profile, revealing that perfect crystal packing was formed and the preferred growth direction of the microrods was along the b axis, which agrees well with the TEM examination. The XRD patterns of the microflowers and flowered microrods grown from the solvent of THF also had sharp peaks and smooth baseline, and the main peaks were at the same positions as those of the pattern of the microrods. This demonstrated that the hierarchically structured assemblies grown in THF were also crystal in nature, and the crystals were isomorphous with that of the microrods grown in dioxane. The conductive property of a single microrod was investigated tentatively with bottom-contact devices. A typical bottomcontact device (see inset of Figure 6a) was fabricated by drop casting the as-prepared microrods across the patterned electrodes (Figure S5 (Supporting Information)). The patterned Au electrodes were fabricated on a SiO2 (300 nm thick)/Si wafer by using UV photolithography. The measurement was performed

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SCHEME 2: (a) Labeled ORTEP Drawings of DMPFM with Front View, (b) Top View of DMPFM Molecule, (c) Molecular Packing of DMPFM in a Unit Cell, and (d) Molecular Packing of DMPFM in the Crystal View Along the b Axisa

a

Solvent molecules were omitted in a and b for clarity.

using a two-probe method under ambient conditions, and the collected current-voltage (I-V) curves were nearly linear with a small contact barrier (Figure 6a). The electrical conductivity of DMPFM was estimated by collecting the average conductivity of five bottom-contact devices with microrods with different lengths and widths, and the value of the electrical conductivity was estimated to be 8.3 × 10-8 S/m,19 showing the semiconducting character of the material. The conductive property of a single microrod was further investigated by atomic force microscopy (AFM) using a conductive cantilever contacting (Figure 6b) with the DMPFM microrod under a bias voltage at a number of sites. The I-V curve collected at a bias voltage from -0.3 to 0.3 V as shown was linear through the zero point,

demonstrating the semiconducting property of DMPFM and the Ohmic behavior of the DMPFM microrods (Figure 6c and 6d).

Figure 4. (a) TEM image of a typical DMPFM microrod. (b) Corresponding SAED pattern of the DMPFM microrod. (c) TEM image of a typical hierarchical self-assembly. (d) SAED pattern of the hierarchical self-assembly at the fractal ends.

Figure 5. Powder XRD patterns of the as-prepared microrods (bottom), microflowers (middle), and hierarchically structured supraarchitectures (top). The peaks marked with asterisks are diffraction peaks of the Si (100) substrate.

Experimental Section General Methods. 1H and 13C NMR spectra were obtained on a Bruker ARX400 spectrometer using tetramethylsilane (TMS) as the internal standard, and chemical shifts (δ) are given

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Figure 6. (a) Typical I-V curve for a single DMPFM microrod. (Inset) SEM image of the measured device. (b) Schematic representation of the experimental setup of the AFM for conductive investigation. (c) AFM images of the microrods. (d) I-V curves of DMPFM microrods at the arrow pointed position.

in ppm relative to TMS. Elemental analyses were carried out on a Carlo Erba 1106 elemental analyzer. Melting points were obtained from TGA spectra recorded by using a DTG-60 SHIMADZU apparatus. SEM images were taken from a Hitachi S-4300 FESEM microscope at an accelerating voltage of 15 kV. TEM images and SAED patterns were taken from a JEOL JEM-2011 microscope at an accelerating voltage of 200 kV. The assembles were also characterized by X-ray diffraction (XRD, Japan Rigaku D/max-2500) with Cu KR radiation. All single-crystal X-ray diffraction data were collected on a Rigaku Saturn X-ray diffractometer with graphite monochromator Mo KR radiation (λ ) 0.71073 Å) at 173 K. Intensities were collected for absorption effects using the multiscan technique SADABS. The structures were solved by direction methods and refined by a full-matrix least-squares technique based on F2 using the SHELXL 97 program (Sheldrick, 1997). The extended packing plots and data from crystal packing were obtained using the software Mercury 1.4.1 and ORTEP 3. Materials. Synthesis used chemicals and solvents that were reagent grade purchased from Aldrich, ACROS Chemical Co., and used without further purification. The silica gel for column chromatography was purchased from JIYIDA Silica Gel Corp. in Qing Dao. Preparation. The self-assemblies were prepared by a simple solution method without any addition of surfactant, catalyst, or template. Three milligrams of DMPFM was totally dissolved in 6 mL of dioxane when refluxed (110 °C) for 30 min, and microrods were afforded by cooling this hot solution at an invariant temperature of 10 °C. The microrods can be collected by a centrifugal machine. Microflowers were prepared by using a similar method with THF (3 mg of DMPFM in 6 mL of THF). Before the preparation of hierarchical structures, it should be mentioned that 3 mg of DMPFM would be totally dissolved in

6 mL of THF to form a transparent solution while only partially dissolved in 5 mL of THF to form a suspension when refluxed. Therefore, the hierarchical structures were constructed by a twostep procedure. In the second step, 3 mg of as-prepared microrods was mixed with 5 mL of THF and refluxed for 30 min at 80 °C, forming a suspension. SEM observations of the collected samples were partially dissolved microrods (Figure S6 (Supporting Information)). The resulting suspension was cooled and aged at 0 °C for more than 30 min, and the precipitates were the final hierarchical flowered microrods. Synthesis of N,N-Dimethyl(pinacolatoboron)aniline (1). Under an argon atmosphere, to a stirred mixture of 4-bromoN,N-dimethylaniline (1.0 g, 5 mmol), bis(pinacolato)diboron (1.5 g, 6.0 mmol), and potassium acetate (1.5 g, 15 mmol) in 20 mL of THF was added PdCl2(dppf)CH2Cl2 (123 mg, 0.15 mmol). The mixture was heated to 80 °C and refluxed overnight. After cooling and removing most of the solvent in vacuum, the mixture was dissolved in 250 mL of CH2Cl2, the organic phase was washed with water (100 mL × 3) and dried with Na2SO4, and the product was purified by column chromatography (silica gel, petrol ether:dichloromethane ) 2:1, Rf ) 0.3) to give 1.17 g of white solids with a yield of 95%. Mp: 113.0 °C. 1H NMR (400 MHz, CDCl3, δ): 1.32 (s, 12H; CH3), 2.97 (s, 6H; N-CH3), 6.68 (d, J ) 8.2 Hz, 2H; Ar H), 7.69 (d, J ) 8.2 Hz, 2H; Ar H). 13C NMR (100 MHz, CDCl3, δ): 25.34, 40.60, 83.61, 84.04, 112.01, 135.49, 153.00. EI-MS m/z calcd for C14H22BNO2, 247.2; found, 247.0. Anal. Calcd (%) for C14H22BNO2: C, 68.04; H, 8.97; N, 5.67. Found: C, 68.08; H, 8.91; N, 5.59. Synthesis of 2,7-Diiodo-9-fluorenone (2). 9-Fluorenone (5.40 g, 30 mmol) was dissolved in 50 mL of boiling solvent (CH3COOH: H2O:H2SO4 ) 100:20:3) with stirring, followed by cooling to 70 °C, adding periodic acid dihydrate (3.45 g, 11 mmol) and iodine (7.68 g, 22 mmol). After reacting at 85 °C

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for more than 5 h, the elementary iodine almost disappeared and the precipitate was formed. Upon cooling, the yellow solid collected by filtration and the crude product was washed with water. Recrystallization from acetic acid afforded 6.75 g of needle-like product with a yield of 75%. Rf ) 0.6 (petrol ether: dichloromethane ) 5:1). Mp: 210.7 °C. 1H NMR (400 MHz, CDCl3, δ): 7.27 (d, J ) 7.8 Hz, 2H; Ar H), 7.83 (d, J ) 7.8 Hz, 2H; Ar H), 7.96 (s, 2H; Ar H). 13C NMR (CDCl3, 100 MHz): δ (ppm) 94.96, 122.61, 134.09, 135.28, 143.42, 143.99, 191.44. MS (EI, m/z) [M]+ calcd for calcd for C13H6I2O, 431.9; found, 431.7. Anal. Calcd (%) for C13H6I2O: C, 36.14; H, 1.40. Found: C, 36.09; H, 1.56. Synthesis of DMPFO. Under an argon atmosphere, to a stirred mixture of 4-pinacolato-N,N-dimethylaniline (247 mg, 1.0 mmol) and 2,7-diiodo-9-fluorenone (216 mg, 0.5 mmol) in 10 mL of THF and 3 mL of 2 M K2CO3 in water was added PdCl2(dppf)CH2Cl2 (25 mg, 0.03 mmol). The reaction mixture was refluxed overnight. After cooling and removing most of the solvent in vacuum, the mixture was dissolved in 100 mL of CH2Cl2, the organic phase was washed with water (50 mL × 3) and dried with Na2SO4, and the product was purified by column chromatography (silica gel, dichloromethane) to give 347 mg of red solids with a yield of 83%. Rf ) 0.4 (petrol ether: dichloromethane ) 1:3). Mp: 320.8 °C. 1H NMR (400 MHz, CDCl3, δ): 3.05 (s, 12H; CH3), 6.86 (m, 4H; Ar H), 7.53-7.56 (m, 4H; Ar H), 7.60 (s, 2H; Ar H), 7.69 (d, J ) 7.8 Hz, 2H; Ar H), 7.90 (s, 2H; Ar H). HRMS (EI m/z) [M]+ calcd for C29H26N2O, 418.2045; found, 418.2050. Anal. Calcd (%) for C29H26N2O: C, 83.22; H, 6.26; N, 6.69. Found: C, 83.15; H, 6.33; N, 6.75. Synthesis of DMPFM. To a solution of DMPFO (420 mg, 1 mmol) and malononitrile (300 mg, 4 mmol) in 15 mL of DMF, three drops of piperidine was added and the reaction mixture was stirred at room temperature for 3 h. The color changed from deep red into glaucous. The reaction mixture was poured into 100 mL of CH2Cl2, the organic phase was washed with water (30 mL × 3) and dried with Na2SO4, and the product was purified by column chromatography (silica gel, dichloromethane, Rf ) 0.4) and recrystallized from toluene to give 387 mg of olive-drab solids with a yield of 83%. Mp: 324.0 °C. 1H NMR (300 MHz, C6D4Cl2, 100 °C, δ): 2.65 (s, 12H; CH3), 6.45 (d, J ) 8.0 Hz, 4H; Ar H), 7.16 (d, J ) 7.9 Hz, 2H; Ar H), 7.32 (d, J ) 8.0 Hz, 4H; Ar H), 7.39 (d, J ) 7.9 Hz, 2H; Ar H), 8.43 (s, 2H; Ar H). HRMS (EI m/z) [M]+ calcd for C32H26N4, 466.2157; found, 466.2159. Anal. Calcd (%) for C32H26N4: C, 82.38; H, 5.62; N, 12.01. Found: C, 82.45; H, 5.61; N, 11.94. CCDC-751074 contains the supplementary crystallographic data for DMPFM. Conclusion We demonstrated the approach for the controlled fabrication of 1-D and 3-D crystal architectures in dioxane and THF. The single-crystal 1-D microrods could act as precursor crystal seeds for driving generation of the 3-D hierarchically structured superarchitectures in the subsequent solution process. The morphological controllability in both growth stages offers opportunities of rational tuning of the size, shapes, and dimensions of hierarchical self-assembled products. TEM and SAED analyses agree well with the results of single-crystal X-ray analysis that DMPFM molecules tend to stack up along the b axis to form columns in the 1-D microrods. The bottomcontact devices and conductive AFM examination of the microrods demonstrate the semiconductive behavior of the material. The stepwise growing method of hierarchical archi-

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