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1D Nano- and Microbelts Self-Assembled from the Organic-Inorganic Hybrid Molecules: Oxadiazole-Containing Cyclotriphosphazene Shu-Zheng Liu,† Xiong Wu,† Ai-Qing Zhang,‡ Jin-Jun Qiu,† and Cheng-Mei Liu*,† † ‡
School of Chemistry and Chemical Engineering, Huazhong University of Science & Technology, Wuhan 430074, P. R. China Hubei Key Laboratory for Catalysis and Material Science, College of Chemistry and Material Science, South-Central University for Nationalities, Wuhan 430074, P. R. China
bS Supporting Information ABSTRACT: A new oxadiazole-containing cyclotriphosphazene, namely, hexakis-(4-(5-phenyl-1,3,4-oxazodiazol-2-yl)phenoxy)-cyclotriphosphazene (HPCP) was synthesized. Single-crystal nano- and microbelts of HPCP were self-assembly via two simple solution methods. The shapes of the as-prepared nano- and microstructures can be readily controlled by varying the solvent and aging time in the self-assembly process. A growth mechanism was proposed for the formation of the 1D morphological structures. Crystal structure analysis demonstrated that the overlap between the aryl units attached to the cyclotriphosphazene backbone forms effective intermolecular π-π linking for crystal growth. Electronic and optical properties of the as-prepared nano- and microstructures are investigated.
’ INTRODUCTION Since the first report of the preparation of semiconducting oxides nanobelt by Wang’s group,1 one-dimensional (1D) nanostructures have been studied extensively.2-8 Such materials possess unique physical and chemical properties and potential applications in electronic and optoelectronic nanodevices,9-11 due to their high surface-to-volume ratios and rationally designed surfaces.12,13 In recent years, great progress has also been made in fabricating nanowires or nanobelts from small organic semiconductor molecules, for which the molecular π-π stacking is mostly along the long axis of the nanowire (or nanobelt).14,15 Examples of self-assembled nanostructures include (but are not limited to) nanofibers, nanowires, and nanoribbons synthesized from hexabenzocoronene derivatives,16,17 perylene tetra-carboxylic diimide derivatives,18-25 macrocyclic aromatic molecules,26 C60,27 triazine derivatives,28 and metal phthalocyanines.29-33 Recently, progress using polymercoded nanoparticle organization to induce the self-assembly of small aromatic molecules into 1D superstructures also has been reported.34 Clearly, it is of practical interest to obtain organic nanostructures with controlled morphology, including shape, size, and surface. In this work, we study the nano- and microstructures of hexakis(4-(5-phenyl-1,3,4-oxazodiazol-2-yl)-phenoxy)-cyclotriphosphazene (HPCP), which has six groups of 2,5-diphenyl1,3,4-oxazodiazol attached to the cyclotriphosphazene backbone as shown in Scheme 1-(3). Phosphazenes comprise a broad class of molecules based on the repeating unit [NPR2] and include cyclic or linear oligomers and polymers. The most striking characteristic of this type of compound is its associated synthetic versatility, which enables the introduction of almost any substitute group R at phosphorus and allows properties to be tailored by the choice of appropriate functional groups.35,36 r 2011 American Chemical Society
Phosphazenes arouse interest in materials such as flame-retardant and thermally stable macromolecules, low-temperature elastomers, biomaterials, photosensitive and/or photo inert substrates, solid-state electrolytes, variable types of hybrid membranes, solid-state fuel cells, and optical and electro-optical polymers.37,38 Recently, poly(cyclotriphosphazene-co-4,40 sulfonyldiphenol) nanotubes were reported via an in situ template approach.39-41 In other hand, compounds containing the 1,3,4-oxadiazole ring as a basic building block are known as scintillator materials or as biologically active agents. Modifications of their chemical structures reveal possibilities for new technical applications, for instance, as potential electroluminescent materials or as active sensoric substances.42 Small molecule oxadiazole derivatives have been commonly used as electron transporting and hole-blocking materials in OLEDs.43-45 Herein, the 1,3,4-oxadiazole groups were incorporated in the molecules in view of their ability to undergo efficient π-π stacking. In this paper, we report that single-crystal one-dimensional nanoand microbelts of HPCP with different thickness of several tens to several hundred nanometers could be controllably obtained by changing the solvent or the aging time. A possible growth mechanism is proposed. The X-ray diffraction (XRD) analysis was performed, and showed that the overlap between the aryl units forms effective intermolecular π-π linking in the crystals. Current-voltage (I-V) measurement was performed to explore the electronic properties, and optical study showed nano- and microbelts having similar photoluminescence spectra. Received: November 17, 2010 Revised: January 31, 2011 Published: March 04, 2011 3982
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Langmuir Scheme 1. Synthesis of Hexakis(4-(5-phenyl-1,3,4-oxazodiazol-2-yl)-phenyloxy)-cyclotriphosphazene (3)
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performed with a Bruker Daltonics Inc. MALDI-TOF. Thermo gravimetric (TG) analysis was performed using a Netzsch STA 409 PC/PG. The measurement was carried at a heating rate of 10 K min-1 under nitrogen atmosphere. The sizes and shapes of the nano- and microstructures were observed with a Sirion 200 field-emission scanning electron microscope (FESEM). The samples for SEM observations were prepared by casting several drops of the suspension onto a clean glass coverslip, followed by drying in air and then annealing overnight in an oven at 40 °C. To minimize sample charging, the dried samples were coated with gold prior to the SEM imaging. The transmission electron microscopy (TEM) was performed with a FEI Tecnai G20 microscope operating at an accelerating voltage of 200 kV. The samples for TEM studies were prepared by dropping the ethanol suspension containing the uniformly dispersed products onto a holey copper TEM grid (i.e., coated with carbon film) and then dried in air for a few hours. The X-ray diffraction (XRD) patterns were recorded with a X’ Pert PRO diffractometer equipped with Cu KR radiation (λ = 1.540 598 Å), employing a scanning rate of 4°/min in the 2θ range from 3°to 50°. The current-voltage measurement was performed with a CHI830C Electrochemical Analyzer. Computational Method. The configurations of HPCP was optimized from the corresponding minimum energy process using the DFT (B3LYP/6-31G*) method in Gaussian 03. The XRD analysis was performed by using the PowderX.47
Synthesis of Hexakis(4-tetrazolylphenyloxy)-cyclotriphosphazene 2. 2 was synthesized from hexakis(4-cyanophenyl)-cyclotriphosphazene (1) (which was synthesized according to literature48) following the literature method.49 Hexakis(4-cyanophenyl)-cyclotriphosphazene (10.0 g, 11.8 mmol) was added to a dried DMF solution (125 mL) containing NaN3 (7.0 g, 107.7 mmol) and ammonium chloride (5.8 g, 107.7 mmol). The mixture was slowly heated to 100 °C for 24 h under nitrogen atmosphere. Excess salts remained as a suspension even at high temperature. After the reaction mixture cooled, it was then acidified with 2 N HCl solutions (attention: hydrazoic acid is formed!) until acidic conditions were reached, after which a white powder slowly appeared. The product was isolated by filtration and washed thoroughly with water to eliminate excess salts. The product was dried under reduced pressure in the presence of P2O5. After recrystallization from ethanol and DMF (solvent ratio 3:1), the product was washed by ether and dried again. Yield: 89% (17.4 g). There is some solvent adsorption by the product due to the property of tetrazole ring; however, this does not affect the next reaction. 1H NMR (d6-DMSO): δ [ppm] 7.23-7.25 (d, 2H), 7.90-7.92 (d, 2H). 13C NMR (d6-DMSO): 162.24, 154.77, 151.36, 128.64, 121.86, 121.32. 31P NMR (d6-DMSO): 9.37. MS: calcd 1101, anal m/e = 1124 (Naþ), 1140 (Kþ).
’ EXPERIMENTAL SECTION
Synthesis of Hexakis(4-(5-phenyl-1,3,4-oxazodiazol-2-yl)phenyloxy)-cyclotriphosphazene 3. A suspension of hexakis
Materials. Hexachlorocyclotriphosphazene (HCCP) (synthesized as described in the literature46) was recrystallized from dry hexane followed by sublimation (60 °C, 0.05 mmHg) twice before use (mp 112.5-113.0 °C). 4-Cyanophenol, benzoyl chloride, and sodium azide were used as received from Aldrich. The K2CO3 was dried at 140 °C for 2 h, and NH4Cl was dried at 100 °C for 4 h prior to use. The acetone used as the solvent was predistilled from KMnO4, and further distilled from anhydrous CaSO4. The N,N-dimethylformamide (DMF) was dried with anhydrous magnesium sulfate, and distilled under vacuum. The pyridine was refluxed with CaH2 for 6 h and distilled before use. The other solvents such as chloroform, ethanol, and ethyl acetate were used without further purification. Instrumentation and Characterization. The Fourier transform infrared (FTIR) spectra were recorded in the range 4000-400 cm-1 with a Bruker FT Equinox 55 spectrometer, and the UV-vis-NIR spectra were recorded in the range 200-500 nm with a Vmini UV-vis 2550 spectrometer. NMR spectra were recorded on Bruker AV-400 M instruments. 1H and 13C {1H}-NMR are given in δ relative to TMS. 31P {1H}-NMR are given in δ relative to external 85% aqueous H3PO4. Mass spectrometry was
(4-tetrazolylphenyloxy)-cyclotriphosphazene (2.2 g, 2.0 mmol) in 100 mL pyridine (partly dissolved) was added benzoyl chloride (3.4 g, 24.3 mmol), and the mixture was stirred for 12 h at 110 °C under nitrogen. The suspension was dissolved in the heated process. After reaction, the solution was concentrated and the residue was dropped into a mixture solvent of water and ethanol (1:1). The off-white powder was collected by vacuum filtration. The solid was then washed twice with 10% KOH (100 2 mL) and brine (100 2 mL), respectively, and then dried in vacuum at 40 °C. After recrystallization from chloroform and ethyl acetate (1:3), a white solid was obtained. Yield: 61% (1.9 g). 1H NMR (CDCl3): δ [ppm] 7.23-7.26 (d, 2H), 7.45-7.54 (m, 3H), 8.01-8.04 (d, 4H). 13C NMR (CDCl3): 164.76, 163.51, 152.67, 131.76, 129.05, 128.57, 126.97, 123.65, 121.48, 121.43. 31P NMR (CDCl3): 7.84. MS: calcd 1557, anal m/e = 1580 (Naþ), 1596 (Kþ). Anal. found (calcd) for C84H54N15O12P3: C, 64.50 (64.74); H, 3.59 (3.49); N, 13.45 (13.48). Preparation of Nano- and Microbelts. Self-assembly of the nano- and microbelts of HPCP was performed through two simple solution methods.15 Method A (solution volatile processing): the nano3983
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Figure 1. SEM images of the nano- and microbelt structures obtained by different volatizing times of HPCP/DMF solution: (a,b,c) nanobelts obtained at 6 days; (d,e,f) nanobelts obtained at 12 days; (g,h,i) microbelts obtained at 1 month. and microbelts were fabricated in the solution of chloroform, DMF, and the mixture of chloroform/ethyl acetate and DMF/ethyl acetate, respectively, with the volatilization of solvent. Method B (a solvent exchange processing): the nanobelt were produced by transferring the molecule from a “good” solvent (such as chloroform, DMF) into a “poor” solvent (ethanol), where the molecules have limited solubility, and thus self-assembly occurs via molecular stacking.
’ RESULTS AND DISCUSSION Synthesis and Chemical Structure Characterization. The cyclotriphosphazene appended with six 1,3,4-oxadiazole-containing derivative, 3 (HPCP), investigated in the present study was synthesized by a multistep process (Scheme 1), in which the final step involved the conversion of tetrazole to 1,3,4-oxadiazole by the Huisgen mechanism.50-52 In order to ensure complete reaction of the multitetrazole, the benzoyl chloride was freshly distilled, and the benzoyl chloride amount was 2 equiv to the tetrazole group. The tetrazole derivative (a multitetrazole, which was synthesized in our laboratory for the first time) was obtained by reaction between sodium azide and hexakis(4-cyanophenyl)cyclotriphosphazene. All intermediates and final product were characterized by FTIR, NMR, and FAB-MS. MALDI-TOF and elemental analysis were also conducted for the final product. In the 31P NMR spectrum, compounds 2 and 3 showed only one signal each, at d = 9.37 and 7.84 ppm relative to external 85% aqueous H3PO4, respectively (see the Supporting Information), which indicates that these compounds are symmetrical in the sense that all three phosphorus atoms displayed the same chemical shift; this result supports hexa-substitution on the cyclotriphosphazene ring. Synthetic procedures and characterization details are provided in the Experimental Section and Supporting Information. Microscopy Characterization. Figure 1 shows the typical scanning electron microscopy (SEM) images of the resulting nanoand microstructures with different morphologies obtained by different volatizing time of HPCP/DMF solution. Briefly, 20.0 mg of HPCP was dissolved in 10 mL of DMF at 60 °C (the solubility of HPCP in DMF was about 6 mg/mL at 60 °C), producing a solution
Figure 2. (a,b,c) Enlarged images showing the morphology of the layers of the microbelts. (d,e,f) Structure of a single microbelt.
of HPCP at a concentration of 1.3 mM. After about 6 days, with the slow volatility of DMF (8 mL was remained), the nanobelts were formed through slow self-assembly (Figure 1a,b,c). The nanobelts had a width of about 1-3 μm, a thickness of 100-150 nm, and a length about 50 μm. When the volatizing time was increased to 12 days, dendritic nanobelts were obtained (Figure 1d,e,f) with a width of 2-3 μm and a thickness of 100-200 nm, and the length of the nanobelts was substantially increased up to several millimeters long. It can be seen that the surface of the nanobelts was not as flat as before. When the volatizing time was further increased to 1 month (2 mL solvent was remained, yield 90%), the length and the thickness of the belts continued to increase (Figure 1g,h,i). In addition, it's very interesting that most of the microbelts are composed of several layers (two, three, or more) as shown in Figure 2a,b,c and Supporting Information Figure S14. The width of the microbelts did not obviously increase, and the thicknesses of most three-layer belts are 450-500 nm with each layer about 150 nm, which is close to the thickness of the single nanobelts obtained at 12 days. Figure 2(d,e,f) described the layered structure in a single fiber, and it was obvious that three layers were presented in the middle part of the fiber and each layer consists of many sublayers from the top of the fiber. The belt-like morphology is also evidenced the transmission electron microscopy (TEM) imaging over a single nanobelt (Figure S16a) and microbelts (Figure 3 and Figure S16b,c,d) deposited on holey carbon film. Careful study of the SEM and TEM images of the microbelts showed that the layered structures of the microbelts are likely formed by the integration of the nanobelt bundle. The aggregation properties of HPCP in different solvents, such as chloroform and the mixture of chloroform and ethyl acetate, via solution volatizing, have also been investigated. HPCP dissolved easily in chloroform (10 mg/mL); this high solubility makes the self-assembly process very difficult to 3984
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Figure 3. TEM images of HPCP microstructures.
control. So, the resulting structure of HPCP is flake-like (see Figure S15a in the Supporting Information). Also, the microbelts fabricated in mixed solvent of CHCl3/ethyl acetate had a length of more than 50 μm and a width 5-10 μm (see Figure S15 b-f). Self-assembly of the nanobelts of HPCP was also performed, in which the molecules were transferred from a “good” solvent (such as chloroform, DMF) into a “poor” solvent (such as ethanol) where the molecules have limited solubility and thus self-assemble into 1D nanobelts via molecular stacking. Such a self-assembly approach takes advantage of the strong intermolecular π-π interaction, which is enhanced in a solvent where the solvophobic interaction is maximized. Similar methods have previously been used for self-assembly of a one-dimensional nanostructure of symmetric PTCDIs and other planar aromatic macromolecules.18-24 Briefly, 5.5 mg of HPCP was dissolved in chloroform, followed by addition of ethanol, producing a solution of HPCP (10 mL) at a concentration of 0.5 mM, with the volume ratio of chloroform to ethanol of 2:1, 1:1, 1:2, 1:4, and 1:6, respectively. The volume ratio of chloroform to ethanol of the binary solvent was found to be critical for controlling the morphology and size of the self-assembled materials. As shown in Figure 4a and Figure S17a, intertwining nanobelts were obtained after about 4 days through the slow self-assembly when the volume ratio was 2:1. These nanobelts have a width of about 5 μm, and most have thickness of about 100 nm and a length of more than 100 μm. However, these nanobelts were seriously intertwined, and there are also many twisted nanobelts as described in Figure 4a. There are also some very thin nanobelts (Figure 4b) which attached to other belts to make a layer-like structure similar to the that of the microbelts obtained by the first method (Figure 1j). When the volume ratio was 1:1, the fabrication time was substantially decreased, and the nanobelts were obtained after 1 day through the self-assembly process. The resulting nanobelts have a width of 2-4 μm and a thickness of 70-80 nm, and a length of up to 50 μm as demonstrated in Figure 4c,d and Figure S17b. Not similar to the nanobelts obtained at volume ratio of 2:1, the nanobelts have smooth surfaces and do not have very thin nanobelts. When decreasing
Figure 4. SEM images of HPCP nanostructures prepared by solvent exchange processing (chloroform to ethanol), with the volume ratio of chloroform to ethanol (a,b) 2:1; (c,d) 1:1; (e) 1:4, and (f) 1:6, respectively.
Figure 5. SEM images of HPCP nanostructures prepared by solvent exchange processing (DMF to ethanol), with the volume ratio of DMF to ethanol (a,b) 1:1; (c) 1:2; and (d) 1:6, respectively.
the volume ratio of chloroform to ethanol to 1:2, the nanobelts fabricated immediately when ethanol was added to the chloroform solution, and the nanobelts formed became significantly shortened, about 5-6 μm (Figure S17c,d). When the volume ratio was decreased to 1:4, many particulate aggregates were formed along with the short nanobelts (Figure 4e). If the volume ratio was further decreased to 1:6, only particulate aggregates were formed from the self-assembly process as depicted in Figure 4f. 3985
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Figure 6. XRD patterns of HPCP bulk powder materials (a), nanobelts (b), and microbelts (c).
Further, self-assembly of the nanobelts has also been investigated when DMF was used instead of chloroform to dissolve HPCP. The HPCP DMF solution was added to ethanol, producing a solution of HPCP at the same concentration of 0.5 mM, with the volume ratio of DMF to ethanol of 1:1, 1:2, 1:4, and 1:6, respectively. The volume ratio of DMF to ethanol of the binary solvent was also found to be critical in controlling the morphology and size of the self-assembled materials. When the volume ratio was 1:1, the nanobelts were obtained after two days with a width of 1-3 μm, a thickness of 40-60 nm, and a length of 30-50 μm as demonstrated in Figure 5a, b and Figure S18a,b. Different from chloroform, when DMF was used as a good solvent, the nanobelts formed were obviously induced by seeding, and the nanobelts did not twining together, and the seeding particles were obvious as shown in Figure 3b. When the volume ratio was 1:2, the nanobelts were fabricated immediately when ethanol was added to the DMF solution; after stabilization for 1 day, the resulting nanobelts have a width of 2-3 μm, a thickness of 100 nm, and a length of about 20 μm as shown in Figure 5c and Figure S18c. Apparently, the nanobelts were also induced by seeding; however, the seeding particles were not as obvious as the ratio of 1:1. By further decreasing the volume ratio to 1:6, only particulate aggregates were formed from the self-assembly process as depicted in Figure 5d. Change of solvent composition (solubility) may have the following possible effect in the crystal growth process. The change of solubility of HPCP in the preparation system would have a great impact on the supersaturation profile during the nucleation process and then subsequent growth kinetics in different directions, which
may possibly change the shape of the crystals.53 As shown in the SEM results, when DMF was used as solvent (in both methods) the nanobelts formed were obviously induced by seeding, and the seeding particles are dendrite-like. XRD Spectrum. Structures of the powder and nano- and microbelts were also investigated by XRD analysis (Figure 6) and the d-spacings were given in Table S1, S2, and S3. The sharp peaks in the XRD spectra further confirm the nano- and microstructures to be highly crystalline. The XRD patterns of nano- and microbelts are essentially the same and they are similar to that of starting powder materials, but the characteristic peaks of nanobelts are obviously smaller than those of powder and microbelts. Analysis shows that the crystal structure is monoclinic with lattice parameters of a = 25.944 Å, b = 20.303 Å, c = 14.419 Å, R = 90°, β = 98.895°, γ = 90°, and lattice type P. It should be noted that the relative intensity of the characteristic (-110), (110), (020), (-220), (-301), (-320), (420), (231), and (-602) peaks of microbelts remarkably increased as compared to that of the HPCP powder and nanobelts. The strong π-π interaction was indicated by the X-ray diffraction, the typical π-π stacking peak (with d-spacing value around 3.57 Å) was observed, and the relative intensity of microbelts also increased markedly as compared to the that of HPCP bulk powder and nanobelts. Molecular Packing Model Proposed for the HPCP Nanobelts. On the basis of the above SEM and XRD results, the HPCP should have a highly oriented nature of assembly. This property makes it difficult for us to obtain the ideal crystal 3986
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Figure 7. Energy-minimized configuration of HPCP obtained by DFT calculation (B3LYP/6-31 g*) using Gaussian 03.
Figure 9. Schematic illustration of the possible self-assembly process of HPCP.
Figure 8. Two possible packing models of HPCP molecules: side and top views of the first packing model (a,c); side and top views of the second packing model (b,d).
suitable for single-crystal XRD identification, and more importantly, the structure of the HPCP molecular is nonplanar (as shown in Figure 7), which makes it difficult to understand the molecular packing model. Taking into account of HPCP molecular sizes and the stacking characteristic of 2,5-diphenyl-1,3,4oxadiazole (DPO) molecules, we proposed a molecular packing model for the HPCP super molecules (see Figure 8). 2,5-Diphenyl-1,3,4-oxadiazole (DPO) molecules are characterized by the formation of stacks leading to intense π-π acceptordonor interactions between oxadiazole and phenyl rings.54 Figure S19 shows the molecular pairs in the crystal structure of DPO (copyright J. Mol. Struct. 2003, 649, 219-230). The strong electron acceptor character of the oxadiazole moiety induces the formation of π-complexes resulting from the interaction between one π-donor (phenyl ring) and a π-acceptor (oxadiazole ring).55 Figure 7 gave HPCPs two possible structures obtained by DFT calculation (B3LYP/6-31 g*) using Gaussian 03. Obviously, the six arms of HPCP have similar structure to diphenyl-1,3,4-oxadiazole (DPO); therefore, it can self-assemble through the intense π-π acceptor-donor interactions between oxadiazole and phenyl rings like DPO. This was also confirmed by the typical π-π stacking peak (with d-spacing value around 3.57 Å) observed in powder X-ray diffraction. So, we speculate that the driving force to self-organize nanobelts is attributed to the π-π interaction of the aryl unit in HPCP. For simplicity, we just chose the first possible structure of HPCP and one packing model of the aryl unit (as in Figure S20 C and A) to propose the molecular packing model for the HPCP nanobelts. As shown in Figure 8, the HPCP molecules still have two possible packing models for their aryl unit. However, the first model has smaller steric hindrance than the second, apparently. Herein, we chose the first to propose a packing model, and the second was also given in the Supporting Information (Figure S20, which has been proven incorrect). First, with lower solubility, some HPCP
molecules assembled together through the π-π acceptor-donor interactions to form a seed, as shown in Figure 9. Before stacking with another molecule, the six arms of HPCP are equal; however, after stacking the arms along the stacking axis (as shown in Figure 8 arrow A and B) have reduced steric hindrance, which gives the selfassembled structure a better orientation. Second, the other HPCP molecules continue to be arranged with the seeds via aromatic π-π stacking (Figure 9). As pointed out, the molecules preferably arranged along the stacking axis (as shown in Figure 8 arrow A and B), and thus obtained a one-dimensional structure. Note that each HPCP molecular can arrange with six other HPCP molecules, in theory, but with the increase of the combined molecules, the remaining steric hindrance increases. This accentuates the different growth rate among different crystallographic directions, resulting in the final belt-like morphology. Also, the ability to assemble with a number of other molecules can explain the dendritic structure obtained by volatizing HPCP/DMF solution (Figure 1d). In this packing model, the face-to-face distance between the two arrangements of aryl units is determined to be 0.35 nm by the XRD studies above. Figure 10 demonstrated the proposed crystal structure obtained by this packing model (it should be noted that the proposed crystal is just an ideal condition), and the calculated unit cell with a = 26.08 Å, b = 20.30 Å, c = 14.57 Å, R = 90°, β = 98.895°, and γ = 90° is consistent with the monoclinic unit cell obtained from the XRD studies above. As shown in Figure 10, in the surface of the nanobelts covered with oxadiazole functional groups, this gives the nanobelts the ability to interact through the π-π acceptor-donor interactions resulting in layered microbelts (Figure 2). Electronic Properties. We investigated the electronic properties of the nano- and microbelts by fabricating and characterizing the current-voltage (I-V) measurement from a single HPCP nano- or microbelt on gold/glass substrate. The electrodes were fabricated by sputter-coating of gold (∼30 nm) onto glass substrate, followed by scratching the gold film with a razor blade to create a pair of film electrodes with a gap ranging from 80 to 100 μm (Figure 11 inset). Typical I-V curves of the blank electrode device (without single belt) and nano- and microbelts are shown in Figure 11. The inset in Figure 11 shows a representative photo image of a Au/glass topcontact device constructed from a single HPCP microbelt. Although the π-π interactions of the oxadiazole are separated by the phosphazene ring, which prevents long-range π-stacking in the cofacially stacked molecules such as PTCDIs,18-24 and the nonconjugated phosphazene ring is considered nonconductive,37 the microbelt of the HPCP shows small electrical conductivity (Figure 11). However, the current of the HPCP nanobelt is too small in this experimental condition and too close to the background, which leads to difficulty determining its conductivity. So, we just 3987
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Figure 12. (a) UV-vis absorption spectra of HPCP (2.6 10-6 mol/ L) dissolved in DMF/ethyl acetate (1:5). (b) Photoluminescence spectra of HPCP solution, nanobelts, and microbelts.
Figure 10. Proposed crystal structure of the nanobelts. (The H atoms are omitted for clarity.) Figure 13. Fluorescence microscopy image of the nano- (a) and microbelts (b) of HPCP cast on glass slide. Excitation: 330-385 nm.
Figure 11. I-V curve measured on a HPCP nanobelt. The inset shows a representative microscopy image of a Au/glass top-contact device (separated by 80 μm) constructed from a single HPCP microbelt.
calculated the conductivity of the microbelt (the cross-sectional area of the microbelt was obtained by AFM; Figure S22). For the three devices measured, the conductivity extracted from the quasi linear
region at low bias (up to 6 V) is ca. 6.0 10-5 S cm-1, a value belonging to the scope of the semiconductor, which higher than that measured from PTCDI nanobelts56 and lower than that of PANI nanostructures.57 Two factors that may have caused this result: the good electron transporting oxadiazole groups58,59 and the ordered π-π stacking give the HPCP microbelts conductivity, while the nonconjugated phosphazene ring limits any further increases in conductivity. UV/vis and Luminescence Spectra. The self-assembly process of HPCP was also manifested by the results of UV-vis measurement. Figure 12a shows the absorption spectra of HPCP at different temperatures and different aging times (HPCP was dissolved in a mixture of 1:5 DMF/ethyl acetate with concentration of 2.6 10-6 mol/L, and a 10 mm quartz cell was used). By lowering the temperature, the wavelength of the maximum absorption peak indicated a red-shift tendency and the half-width of the absorption peak became a little wider with aging. However, 3988
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Langmuir compared to the flat molecules such as PTCDIs, the red-shifted absorption led by the strong π-π stacking is not significant, and also, no new peak appeared. The reason for this difference may be that the HPCP molecular is not flat, and the core (cyclotriphosphazene) prevents long-range π-π stacking (the π-π stacking is limited to just between two arms). The photoluminescence (PL) properties of the as-prepared nanoand microbelts were also investigated. Figure 12b shows the fluorescence spectra of the HPCP/CHCl3 solution and as-prepared nanoand microbelts, respectively. For HPCP solution and nanobelts, the PL spectra have a bifurcated peak structure. As explained below, one peak is from the monomers and the other is from the aggregates (the red-shifted peaks). Of course, if the aggregate has poor overlap between the molecular subunits, the excitation will be more localized and therefore at a wavelength closer to the monomer emission. The solution exhibits strong luminescence with two peaks centered at 342 and 356 nm, respectively, and the peaks of the nanobelts have slight red-shifts to 346 and 361 nm compared to the solution. In comparison to the former, the microbelts only exhibit a strong luminescence with peak centered at 362 nm. Figure 13 shows that the nano- and microbelts reported herein demonstrate emission, which can easily be imaged with a fluorescence microscope. The observable fluorescence of these new nano- and microbelts is apparently due to the π-π stacking mode and the special structure of HPCP. In addition, the thermal stability of the as-prepared nanobelts was investigated by thermogravimetric analysis (TGA). As shown in Figure S21, the decomposition temperature of nanobelts was over 400 °C and the char yield is about 55% at 600 °C, which shows that the nanobelts are highly thermal stable materials.
’ CONCLUSIONS In summary, a new oxadiazole-containing cyclotriphosphazene, namely, hexakis(4-(5-phenyl-1,3,4-oxazodiazol-2-yl)-phenoxy)-cyclotriphosphazene (HPCP), was synthesized. Also, via two simple solution processes, single-crystal nano- and microbelts of HPCP have been prepared by exploiting the strong π-π interactions between neighboring HPCP molecules as the driving force. By tuning the solubility of HPCP by varying the solvent and the volume ratio of solvents, the thickness and the length of belts gradually changed from tens to 500 nm and tens of micrometers to several millimeters, respectively. A growth mechanism for the 1-D self-assembly has been proposed. Crystal structure analysis supports that the contact between the oxadiazole groups of the phosphazene backbone forms effective π-π intermolecular linking for 1-D crystal growth. Current-voltage (I-V) measurement of single nano- and microbelts show that these new nano- and microbelts have little conductivity. These new nanoand microbelts also demonstrate dramatic emission. The ability to form fluorescent nano- and microstructures though these n-type side groups via π-π stacking and little conductivity as semiconductors make them potentially useful as candidates for investigation as molecular wires and emitters in supramolecular electronic devices. In addition, the associated synthetic versatility of phosphazene35,36 derivatives provides a useful platform for molecular design and engineering in the fabrication of nanomaterials.60 ’ ASSOCIATED CONTENT
bS
Supporting Information. FTIR, NMR, and MS spectroscopy of compounds 1-3, and the additional SEM, TEM, AFM images, DPOs packing model, TGA of the nanobelt, the
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d-spacing match. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT We thank the support from National Science Foundation of China (50703013). We are grateful to Prof. Cheng Dong (Physical institute of Chinese academy of Science) for the XRD analysis and Prof. Zhi-Xiang Wang (Graduate School of Chinese academy of Science) for Gaussian calculation. We also thank Analytical and Testing Center of HUST for NMR, SEM, and XRD measurements. ’ REFERENCES (1) Pan, Z. W.; Dai, Z. R.; Wang, Z. L. Science 2001, 291, 1947– 1949. (2) Wu, N. Q.; Wang, J.; Tafen, D. N.; Wang, H.; Zheng, J. G.; Lewis, J. P.; Liu, X. G.; Leonard, S. S.; Manivannan, A. J. Am. Chem. Soc. 2010, 132, 6679–6685. (3) Chen, J. Y.; Lim, B. K.; Lee, E. P.; Xia, Y. N. Nano Today 2009, 4, 81–95. (4) Tian, N.; Zhou, Z. Y.; Sun, S. G.; Ding, Y.; Wang, Z. L. Science 2007, 316, 732–735. (5) Tsung, C. K.; Kuhn, J. N.; Huang, W. Y.; Aliaga, C.; Hung, L. I.; Somorjai, G. A.; Yang, P. D. J. Am. Chem. Soc. 2009, 131, 5816–5822. (6) Li, L. S.; Wang, Z. J.; Huang, T.; Xie, J. L.; Qi, L. M. Langmuir 2010, 26, 12330–12335. (7) Lieber, C. M.; Wang, Z. L. MRS Bull. 2007, 32, 99–104. (8) Hasegawa, M.; Iyoda, M. Chem. Soc. Rev. 2010, 39, 2420–2427. (9) Xia, Y. N.; Yang, P. D.; Sun, Y. G.; Wu, Y. Y.; Mayers, B.; Gates, B.; Yin, Y. D.; Kim, F.; Yan, H. Q. Adv. Mater. 2003, 15, 353–389. (10) Novo, C.; Mulvaney, P. Nano Lett. 2007, 7, 520–524. (11) Novo, C.; Funston, A. M.; Mulvaney, P. Nat. Nanotechnol. 2008, 3, 598–602. (12) Fang, X. S.; Bando, Y.; Shen, G. Z.; Ye, C. H.; Gautam, U. K.; Costa, P. M. F. J.; Zhi, C. Y.; Tang, C. C.; Golberg, D. Adv. Mater. 2007, 19, 2593–2596. (13) Li, L.; Yang, Y. W.; Li, G. H.; Zhang, L. D. Small 2006, 2, 548–553. (14) Hoeben, F. J. M.; Jonkheijm, P.; Meijer, E. W.; Schenning, A. P. H. J. Chem. Rev. 2005, 105, 1491–1546. (15) Zang, L.; Che, Y.; Moore, J. S. Acc. Chem. Res. 2008, 41, 1596–1608. (16) Xiao, S.; Tang, J.; Beetz, T.; Guo, X.; Tremblay, N.; Siegrist, T.; Zhu, Y.; Steigerwald, M.; Nuckolls, C. J. Am. Chem. Soc. 2006, 128, 10700–10701. (17) 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. (18) Yan, P.; Chowdhury, A.; Holman, M. W.; Adams, D. M. J. Phys. Chem. B 2005, 109, 724–730. (19) Zang, L.; Che, Y.; Moore, J. S. Acc. Chem. Res. 2008, 41, 1596–1608. (20) Wurthner, F. Chem. Commun. 2004, 1564–1579. (21) Newman, C. R.; Frisbie, C. D.; da Silva Filho, D. A.; Bredas, J.-L.; Ewbank, P. C.; Mann, K. R. Chem. Mater. 2004, 16, 4436–4451. (22) Che, Y.; Datar, A.; Balakrishnan, K.; Zang, L. J. Am. Chem. Soc. 2007, 129, 7234–7235. (23) Balakrishnan, K.; Datar, A.; Naddo, T.; Huang, J.; Oitker, R.; Yen, M.; Zhao, J.; Zang, L. J. Am. Chem. Soc. 2006, 128, 7390–7398. (24) Balakrishnan, K.; Datar, A.; Oitker, R.; Chen, H.; Zuo, J.; Zang, L. J. Am. Chem. Soc. 2005, 127, 10496–10497. 3989
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