One- or Semi-Two-Dimensional Organic Nanocrystals Induced by

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One- or Semi-Two-Dimensional Organic Nanocrystals Induced by Directional Supramolecular Interactions Xiujuan Zhang,§,†,‡ Xiaohong Zhang,*,† Bo Wang,† Chengyi Zhang,† Jack C. Chang,‡ Chun Sing Lee,‡ and Shuit-Tong Lee*,‡ Functional Nano & Soft Materials Laboratory (FUNSOM), Soochow UniVersity, Suzhou, Jiangsu 215123, China, Nano-organic Photoelectronic Laboratory and Laboratory of Organic Optoelectronic Functional Materials and Molecular Engineering, Technical Institute of Physics and Chemistry, Chinese Academy of Science, Beijing 100101, People’s Republic of China, and Center of Super-Diamond and AdVanced Film (COSDAF) and Department of Physics and Materials Science, City UniVersity of Hong Kong, Hong Kong SAR, People’s Republic of China ReceiVed: April 24, 2008; ReVised Manuscript ReceiVed: July 9, 2008

One- or semi-two-dimensional (semi-2-D) nanostructures of a variety of intramolecular charge transfer organic materials have been obtained by using directional dipole-dipole interaction between neighboring molecules as the driving force. It was found that (1) increasing the dipole strength could increase the tendency of directional assembly; (2) introducing an additional dipole component into the molecule, growth in an additional direction could be promoted and lead to the formation of semi-2-D nanostructures; and (3) molecular geometry also affects the growth process of nanostructures, as molecular planarity decreases, rearrangement and stacking between molecules become increasingly difficult, which would hinder the formation of directional nanostructures. In addition, we showed that besides dipole-dipole interaction, strong directional supramolecular interactions such as π-π stacking and hydrogen-bonding interactions are also effective in directing 1-D and semi-2-D nanostructure growth. Introduction Crystalline one-dimensional (1-D) and semi-two-dimensional (semi-2-D) organic nanostructures are of fundamental and technological importance due to their unique properties for use in electronic, optoelectronic, and photonic nanodevices.1-8 Increasing interest has been focused on the synthesis of crystalline 1-D or semi-2-D organic nanostructures.9-13 A rational and simple approach to synthesize a wide variety of single crystalline organic nanomaterials is still highly desirable and is a very important element for the future advancement of n-D organic nanoscience and nanotechnology. Intramolecular charge-transfer (ICT) compounds are an important class of organic molecular materials possessing unique properties that have found wide applications in electroluminescent devices, field-effect transistors, nonlinear optical devices, and solid-state lasers.14-16 An ICT molecule has an electron-donating (D) group and an electron-accepting (A) group connected through a π-conjugated system. Charge transfer or charge separation between the D and A groups renders this class of compounds a highly anisotropic structure. Consequently, crystallization tends to occur preferentially along the direction of the D-A dipole-dipole interaction, in a way similar to II-VI compound semiconductors that show fast growth normal to the polar lattice planes of cations and anions.17-19 Thus, ICT materials are expected to become 1-D or semi-2-D nanostructures when they are properly crystallized. Herein, we describe a facile route to produce this kind of relative common but most useful ICT * Corresponding author. X.Z.: e-mail [email protected]; fax +8610-62554670;phone+86-10-82543510.S.-T.L.:[email protected]; fax +852-27844696. § SooChow University. † Chinese Academy of Science. ‡ City University of Hong Kong.

materials into single crystalline nanostructures such as nanowires, nanorods, and nanoribbons by exploring the strong supramolecular dipole-dipole interactions as the main driving force. Experimental Section Materials. Organic compounds for the preparation of different nanostructures were all synthesized in-house and their structures (see the Supporting Information, Part 1 Table 1) confirmed with nuclear magnetic resonance (NMR) and mass spectrometry (MS). High-purity water (resistivity ) 18.2 MΩ cm) was produced with a Milli-Q apparatus (Millipore) and filtered by using an inorganic membrane with a pore of 0.02 µm (Whatman International, Ltd.) just before use. Tetrahydrofuran (THF) was obtained from Beijing Chemical Agent and used without further treatment. Preparations. The ICT nanomaterials in this study were prepared by first dissolving the source organic compounds in THF to a concentration of 1 × 10-3 M. Next 400 µL of the solution was injected into 5 mL of water with vigorous stirring at 298 K. After being stirred for 3 min, the samples were left undisturbed for about 4 h to stabilize the crystal growth; For the compounds having intermolecular π-π stacking interactions, perylene was first dissolved in THF to a concentration of 2 × 10-3 M. Next 400 µL of the solution was injected into 5 mL of a mixed solution of water and methanol (v:v ) 2:3) with vigorous stirring at 298 K. After being stirred for 3 min, the sample was left undisturbed for about 4 h to stabilize the crystal growth; 9,10-diphenylanthracene was dissolved in THF to a concentration of 2 × 10-3 M, and then 100 µL of the solutions was injected into 5 mL of water with vigorous stirring at 298 K. After being stirred for 3 min, the sample was left undisturbed for about 4 h to stabilize the crystal growth.

10.1021/jp803572f CCC: $40.75  2008 American Chemical Society Published on Web 09/27/2008

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SCHEME 1: Proposed 1-D Organic Nanostructure Growth Modela

a Key: (I) Organic chemical is first dissolved in a good solvent. (II) A small amount of good solvent containing organic material is micromixed to a poor solubility environment to gain supersaturation. (III) The nucleation process to produce nuclei together with some tiny meta-stable aggregates. (IV1) Directional interactions will act as the driving force to direct the crystal growth process. Under vigorous stirring, the meta-stable aggregates would dissolve and transform to grow onto the crystallites along the orientation of the directional interactions. (IV2) Growth termination: as the size of the crystal grows larger, it becomes more stable and, eventually, the solubility of the crystal in the solvent limits the rate of growth and dissolution.

For the compounds having intermolecular hydrogen bonding, they were first dissolved in N,N-dimethylformamide (DMF) to a concentration of 1 × 10-3 M, then some of the solution was injected into 5 mL of water with vigorous stirring at 298 K. After being stirred for 3 min, the sample was left undisturbed for about 12 h to stabilize the crystal growth. Characterization. For scanning electron microscopic (SEM) studies, a few drops of the sample were placed onto silicon substrates, and the solvent was left to evaporate. The samples were then examined with a field emission SEM (Hitachi, S-4300) operated at an accelerating voltage of 5 kV. To minimize sample charging, an ultrathin layer of Au was deposited onto the samples before SEM examination. Samples for transmission electron microscopy (TEM) were prepared by placing a drop of suspension on a copper grid coated with carbon film, and then dried in air for a few hours before observation. The TEM study was performed in a Philips CM200FEG operating at an accelerating voltage of 200 kV. Computational Method. Dipole moments of all ICT compounds were obtained from the corresponding minimum energy configuration optimization process by energy minimization, using the B3LYP/6-31G * method in Gaussian 98. Results and Discussion In a typical synthesis, the ICT compound is first dissolved in a good solvent to a certain concentration, and then a portion of the solution is introduced to a poor solvent under vigorous stirring. The process is very simple with no addition of any surfactant, template, or catalysis. Typical SEM (Figure 1a,c) and TEM images (Figure 1b,d) show the 1-D nanostructures produced from ICT compounds, whose molecular structures are displayed in the insets. The SEM images reveal the nanostructures have a high degree of monodispersity, with no nanoparticles or other impurity phases (see the Supporting Information, part S2). The high phase purity is attributed to the absence of surfactant, template, or catalysis in the solutions. To confirm their composition and purity, the nanowires were redissolved in THF, and the measured absorption and emission spectra of the resulting solution were identical with those of the original THF solution. NMR and MS measurements further confirmed that the nanowires only consist of original molecules with no detectable impurities. The TEM images in Figure 1b,d also show the wire morphology of the nanostructure, and the sharp spots in the corresponding electron diffraction (ED) patterns in the insets clearly demonstrate the nanowires are single crystalline (see the Supporting Information, part S3, for details). To better understand the formation progress, we proposed a growth model as shown in Scheme 1. When organic molecules

Figure 1. (a, c) SEM images of the as-prepared 1-D organic nanostructures; the inset in each SEM image shows the molecular structure. (b, d) The corresponding TEM images of compound 1 and 2, respectively; the inset in each TEM image is the electron diffraction (ED) pattern of a single nanoscaled object.

dissolved in a good solvent are injected into a poor solvent, the solvophobic tendency between the organic molecules and the poor solvent yields only a small amount of the organic molecules solubilized, resulting in supersaturation of the organic molecules in the two-solvent solution.20 Supersaturation leads to precipitation of the organic molecules to form nuclei and some metastable aggregates. Under vigorous agitation the less stable aggregates would dissolve and transform onto crystalline nuclei, which would grow preferentially along the direction of the dipole-dipole interaction. Once crystal growth begins, continual addition of molecules to the growing site along the preferred direction would form 1-D or semi-2-D organic nanostructures. The growth would terminate when all the molecules, except for those remaining solubilized, have participated to from 1-D nanostructures. In this process, the linear morphology of the final product is determined by the intrinsic anisotropy of the building blocks. The proposed growth process is very consistent with the experimental observations. Figure 2 shows the growth process of nanowires of compound 2 by temporal characterization of TEM. According to the model, the morphology of ICT nanostructures should be a function of (1) the strength of the dipole-dipole interactions which determine the driving force for anisotropic molecular stacking, (2) the direction(s) of the dipole moment that determines the favorable direction(s) of molecular stacking,

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Figure 2. TEM images of the temporal evolution process of growth of nanowires from compound 2.

Figure 3. SEM image of zero-dimensional organic nanoparticles synthesized by compound 3 whose structure is shown in the inset.

Figure 4. (a,c) SEM image of nanoribbons prepared by compounds 6 and 7 with dipole moment in two directions; the inset is the molecular structure. (b) TEM image of the nanoribbon; the inset is the corresponding electron diffraction (ED) pattern of a single nanoribbon.

and (3) the molecular geometry or planarity that affect molecular stacking in the crystal. The influence of dipole moment strength was studied by using compound 3 (inset of Figure 3), which has a similar structure to compound 1 except that the N,N-dimethyl donor group is replaced by a methoxy (CH3O-) group. While these two compounds have similar structures, the calculated dipole moment of compound 3 (8.98 D) is significantly smaller than that of compound 1 (11.05 D). It is thus expected that formation of 1-D nanocrystals would be less favorable in compound 3. Indeed, no nanowires were obtained from compound 3 when the same experiment was performed; instead only nanoparticles were observed (Figure 3). Similar trends were also observed for another pair of molecules (compounds 4 vs 5) in which only the acceptor group was changed (see the Supporting Information, part S4 for details). This shows the tendency of forming 1-D nanowires in the D-A type of organic molecules can be adjusted by tuning molecular dipole strength via substituent groups in either the donor or the acceptor part of the molecule. The influence of directions of dipole-dipole interactions was also studied. ICT compounds having a dipole moment in two directions in the D-A-D type of structure such as compounds 6 and 7 (Figure 4) were synthesized. The nanostructure obtained from compounds 6 and 7 was semi-2-D nanoribbon instead of 1-D nanowire, as shown in Figure 4a,c. This can be understood because the second direction of dipole moment in the D-A-D type of molecules enhances the stacking in another direction (i.e., the width-wise direction of the ribbon). To corroborate this supposition, we show that nanoribbons again were obtained

Figure 5. (a, c, d) SEM images of the respectively by compounds 8, 9, and 10; corresponding molecular structures. (b) TEM formed by compound 8; the inset is the diffraction pattern of a single nanowire.

nanostructures formed the insets show their image of the nanowires corresponding electron

from another D-A-D molecule (part S6b in the Supporting Information). These observations support that differences in dipole moment direction play a key role in determining the stacking geometry and thus the resulting crystal morphologies.21,22 (see the Supporting Information, parts S5 and S6, for more details). Molecular geometry also affects the growth process of nanostructures. For example, compound 8 with a large dipole moment of 13.42 D, shown in Figure 5a,b, also forms singlecrystal 1-D nanowires as expected. Compound 9 has almost the same structure as compound 8 except for the four methyl groups in the donor segment of the molecule, and the calculated dipole moment of compound 9 is 13.13 D, close to that of compound 8. However, the four additional methyl groups substantially reduce the planarity of compound 9, which leads to a decreased tendency of molecular stacking. As a result, while some 1-D nanostructures were obtained, the major product from compound 9 was nanoparticles, as shown in Figure 5c. When molecular planarity of compound 9 is further decreased by adding a tertbutyl group to the acceptor segment of compound 10 (dipole moment ) 14.09 D), only nanoparticles were obtained (Figure 5d). These results indicate that as molecular planarity decreases, rearrangement and stacking between molecules becomes increasingly difficult, which would hinder the formation of 1-D nanostructures in favor of nanoparticles. The above results show that single-crystal nanostructures of small molecular organic materials can be readily obtained by manipulating the directional dipole-dipole attraction between molecules. Further, nanoparticles, 1-D nanowires and nanorods, or semi-2-D nanoribbons can be obtained by adjusting the strength and direction of molecular dipole moments, and steric

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J. Phys. Chem. C, Vol. 112, No. 42, 2008 16267 composition-melting point dependence of the metal-inorganic alloy. While the anisotropic growth of inorganic nanostructures is frequently dictated by such confining factors such as metal tips, surfactants, or templates,27-30 notably the directional growth of the organic nanostructure is navigated by intermolecular forces such as dipole-dipole interactions, π-π stacking, and hydrogen bonding interactions. As elaborated above, we expected that a myriad of nanostructures can be synthesized in a designed fashion based on the rational approach. Conclusion

Figure 6. SEM images of nanostructures formed by π-π interaction (a, b) and hydrogen-bonding interaction (c, d). Each inset is the corresponding molecular structure.

hindrance to molecular stacking. While all the molecules employed have little solubility in water, we can anticipate that for mildly soluble molecules, the degree of solubility would affect crystallization kinetics and serve as an additional controlling parameter.23 The good agreement between the predications and the experimental data suggests that this method would be possible for the synthesis of many other 1-D organic nanostructures. Because, in contrast to inorganic materials, various interactions such as π-π stacking and hydrogen bonding interaction etc. can also offer solids with anisotropic structures and act as the driving force for the aggregation of 1-D nanostructures. Here we demonstrate this by using perylene and 9,10-diphenylanthracene, which are important organic molecules with strong π-π bonding between molecules. We show that uniform sheetlike nanocrystals (Figure 6a) and nanoribbons (Figure 6b) indeed can be readily formed directed by the strong π-π bonding between molecules. The same method can also generate 1-D single-crystal nanostructures from organic molecules having hydrogen bonding interactions (Figure 6c,d).24-26 In addition to bonding types, other parameters can also be used to control the nanocrystal formation process in the method. For example, besides solvent exchange, supersaturation and nucleation can also be induced by lowering solution temperature or slowing solvent evaporation (see the Supporting Information, part S7). Supersaturation and nucleation can also be achieved by jetting solutions, mixing with stirring, or ultrasonication. Experimental studies with different concentrations of the initial solutions further provide us with a way to control the size of the nanostructure. For example, as the initial concentration of compound 2 increased from 5 × 10-4 to 1 × 10-3 and 2 × 10-3 mol/L, the diameter of the nanowires synthesized changed from 120 to 250 and 380 nm, respectively (see the Supporting Information, part S8). This indicates that by adjusting the concentration of the initial organic compound, it is possible to obtain nanostructures of custom defined sizes. Moreover, much of the understandings can be compared with the formation of inorganic nanomaterials by the use of intrinsically anisotropic crystallographic structure of a solid to accomplish 1-D nanostructures. The method also shares some similarity with the typical vapor-liquid-solid (VLS) or solution-liquid-solid (SLS) process for inorganic nanostructure growth. In the VLS growth, the metal catalyst is the solvent, and supersaturation and nucleation are governed by the

In summary, by exploiting the directional supramolecular interaction induced crystal growth strategy, 1-D and semi-2-D nanostructures of a variety of low molecular weight organic materials have been synthesized. In the present work, the ICT molecule system is investigated in particular. It was found that morphologies of the nanostructure obtained can be controlled by adjusting the molecular geometry, dipole-dipole strength and direction via fine-tuning of the molecular structure. All the nanostructures synthesized were found to have high purity and narrow size distribution and are single crystalline. In addition, we have confirmed that the process can be extended straightforwardly to other directional supramolecular interaction systems such as π-π stacking and hydrogen bonding interactions. We expect that these functional organic nanostructures obtained through this approach could be used as the building blocks for the next generation of organic semiconductor nanodevices. Acknowledgment. This work was supported by CityU Strategic Research Grants (No. 7002075 and 7002275) of the City University of Hong Kong. X.H.Z. thanks the National Basic Research Program of China (973 Program) (Grant Nos. 2006CB933000 and 2007CB936000) and the National Hightech R&D Program of China (863 Program) (Grant No. 2006AA03Z302). Supporting Information Available: Supporting results mentioned in the text. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Chui, J. J.; Kei, C. C.; Perng, T. P.; Wang, W. S. AdV. Mater. 2003, 15, 1361. (2) Veinot, J. G. C.; Yan, H.; Smith, S. M.; Cui, J.; Huang, Q.; Marks, T. J. Nano Lett. 2002, 2, 333. (3) Takazawa, K.; Kitahama, Y.; Kimura, Y.; Kido, G. Nano Lett. 2005, 5, 1293. (4) Yamamoto, Y.; Fukushima, T.; Suna, Y.; Ishii, N.; Saeki, A.; Seki, S.; Tagawa, S.; Taniguchi, M.; Aida, T. Science 2006, 314, 1761. (5) Che, Y.; Datar, A.; Balakrishnan, K.; Zang, L. J. Am. Chem. Soc. 2007, 129, 7234. (6) Nguyen, T. Q.; Martel, R.; Avouris, P.; Bushey, M.; Nuckolls, C.; Brus, L. E. J. Am. Chem. Soc. 2004, 126, 5234. (7) Wang, Z. C.; Medforth, C. J.; Shelnutt, J. A. J. Am. Chem. Soc. 2004, 126, 15954. (8) de la Escosura, A.; Martinez-Diaz, M. V.; Thordarson, P.; Rowan, A. E.; Nolte, R. J. M.; Torres, T. J. Am. Chem. Soc. 2003, 125, 12300. (9) Pantos, G. D.; Pengo, P.; Sanders, J. K. M. Angew. Chem., Int. Ed. 2007, 46, 194. (10) Al-Kaysi, R. O.; Bardeen, C. J. Chem. Commun. 2006, 1224. (11) Liu, H. B.; Li, Y. L.; Xiao, S. Q.; Gan, H. Y.; Jiu, T. G.; Li, H. M.; Jiang, L.; Zhu, D. B.; Yu, D. P.; Xiang, B.; Chen, Y. F. J. Am. Chem. Soc. 2003, 125, 10794. (12) Fu, H. B.; Xiao, D. B.; Yao, J. N.; Yang, G. Q. Angew. Chem., Int. Ed. 2003, 42, 2883. (13) Hu, J. S.; Guo, Y. G.; Liang, H. P.; Wan, L. J.; Jiang, L. J. Am. Chem. Soc. 2005, 127, 17090. (14) Albota, M.; Beljonne, D.; Bredas, J. L.; Ehrlich, J. E.; Fu, J. Y.; Heikal, A. A.; Hess, S. E.; Kogej, T.; Levin, M. D.; Marder, S. R.; McCordMaughon, D.; Perry, J. W.; Rockel, H.; Rumi, M.; Subramaniam, C.; Webb, W. W.; Wu, X. L.; Xu, C. Science 1998, 281, 1653.

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