STM Study on Quinacridone Derivative Assemblies: Modulation of the

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Langmuir 2005, 21, 7225-7229

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STM Study on Quinacridone Derivative Assemblies: Modulation of the Two-dimensional Structure by Coadsorption with Dicarboxylic Acids Xunyu Yang, Zhongcheng Mu, Zhiqiang Wang, Xi Zhang,* Jia Wang, and Yue Wang* Key Lab of Organic Optoelectronics and Molecular Engineering, Department of Chemistry, Tsinghua University, Beijing 100084, and Key Lab of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130021, People’s Republic of China Received April 23, 2005. In Final Form: June 10, 2005 We describe the two-dimensional (2D) assemblies of N,N’-dialkyl-substituted quinacridone derivatives on highly orientated pyrolytic graphite observed by scanning tunneling microscopy, and focus our discussion on whether the supramolecular organization can be modulated by the coadsorption of dicarboxylic acids. Our experiments have demonstrated that the quinacridone derivatives can form different 2D nanostructures when coadsorbed with dicarboxylic acids of different length at the liquid/graphite interface. Interestingly, N,N’-dihexadecyl-substituted quinacridone derivative alternately takes two different conformations in two columns for its coadsorption with pentadecanedioic acid and form a gridlike structure. It is shown that a cooperative effect of different interactions can be modulated by introducing guest molecule, leading to formation of different self-assembled nanostructures.

* To whom correspondence should be addressed. E-mail: xi@ mail.tsinghua.edu.cn (X.Z.).

systems because their supramolecular structure can be modulated not only by the molecular structure of the component itself but also by the interplay among the mutlicomponent. Rabe et al. studied early on the selfassembly of two components of 5-alkoxyisophthalic acid and pyrazine on the surface.12 Since then, Qian et al. used 4,4′-bipyridine as a marker for identifying the carboxylic group of a fatty acid.13 Bai et al. reported the formation of an array structure by coadsorption of phthalocyanines and functionalized alkanes.14 De Feyter et al. fabricated 2D suprastructure based on the complementary interaction of hydrogen bonding donors and acceptors including a diaminotriazine derivative, merocyanine barbituric acid dye, and perylene bisimide derivatives.15 In most of the multicomponent systems, the coadsorption often leads to phase separation in nanometer scale16 or a randomly mixed monolayer on the surface.17 There are a few cases of multicomponent systems that form a uniform monolayer structure. Quinacridone and its derivatives are well known as chemically stable pigments and can be used as photo-

(1) (a) Frommer, J. Angew. Chem., Int. Ed. Engl. 1992, 31, 1298. (b) Cyr, D. M.; Venkataraman, B.; Flynn, G. W. Chem. Mater. 1996, 8, 1600. (c) De Feyter, S.; De Schryer, F. C. J. Phys. Chem. B 2005, 109, 4290. (2) Foster, J.; Frommer, J. Nature 1988, 333, 542. (3) McGonigal, G. C.; Bernhadt, R. H.; Thomson, D. J. Appl. Phys. Lett. 1990, 57, 28. (4) (a) Rabe, J. P.; Buchholz, S. Science 1991, 253, 424. (b) Venkataraman, B.; Breen, J. J.; Flynn, G. W. J. Phys. Chem. 1995, 99, 6608. (5) (a) Hibino, M.; Sumi, A.; Hatta, I. Jpn. J. Appl. Phys. 1995, 34, 3354. (b) Zou, B.; Dreger, K.; Muck-Lichtenfeld, C.; Grimme, S.; Schafer, H. J.; Fuchs, H.; Chi, L. Langmuir 2005, 21, 1364. (6) (a) Venkataraman, B.; Flynn, G. W.; Wilbur, J.; Folkers, J. P.; Whitesides, G. M. J. Phys. Chem. 1995, 99, 8684. (b) Rabe, J. P.; Buchholz, S.; Askadskaya, L. Synth. Met. 1993, 54, 339. (7) (a) Wawkuschewski, A.; Cantow, H.-J.; Magonov, S. N.; Mo¨ller, M.; Liang, W.; Whangbo, M.-H. Adv. Mater. 1993, 5, 821. (b) Wawkuschewski, A.; Cantow, H.-J.; Magonov, S. N. Langmuir 1993, 9, 2778. (c) Magonov, S. N.; Wawkuschewski, A.; Cantow, H.-J.; Liang, W.; Whangbo, M.-H. Appl. Phys. 1994, 59, 119. (8) Askadskaya, L.; Boeffel, C.; Rabe, J. P. Ber. Bunsen-Ges. Phys. Chem. 1993, 97, 517. (9) Qiu, X. H.; Wang, C.; Zeng, Q. D.; Xu, B.; Yin, S. X.; Wang, H. N.; Xu, S. D.; Bai C. L. J. Am. Chem. Soc. 2000, 122, 5550.

(10) (a) Samori, P.; Francke, V.; Enkelmann, V.; Mu¨llen, K.; Rabe, J. P. Chem. Mater. 2003, 15, 1032. (b) Mu, Z.; Yang, X.; Wang, Z.; Zhang, X.; Zhao, J.; Bo, Z. Langmuir 2004, 20, 8892. (c) Gong, J.-R.; Zhao, J.-L.; Lei, S.-B.; Wan, L.-J.; Bo, Z.-S.; Fan, X.-L.; Bai, C.-L. Langmuir 2003, 19, 10128. (11) (a) Qiu, D.; Ye, K.; Wang, Y.; Zou, B.; Zhang, X.; Lei, S.; Wan, L. Langmuir 2003, 19, 678. (b) Mu, Z.; Wang, Z.; Zhang, X.; Ye, K.; Wang, Y. J. Phys. Chem. B 2004, 108, 19955. (12) Eichhorst-Gerner, K.; Stabel, A.; Moessner, G.; Declercq, D.; Valiyaveettil, S.; Enkelmann, V.; Mu¨llen, K.; Rabe, J. P. Angew. Chem. Int. Ed. Engl. 1996, 35, 1492. (13) (a) Qian, P.; Nanjo, H.; Yokoyama, T.; Suzuki, T. M. Chem. Commun. 1999, 1197. (b) Qian, P.; Nanjo, H.; Yokoyama, T.; Suzuki, T. M.; Akasaka, K.; Orhui, H. Chem. Commun. 2000, 2021. (14) Lei, S. B.; Wang, C.; Yin, S. X.; Bai, C. L. J. Phys. Chem. B 2001, 105, 12272. (15) De Feyter, S.; Miura, A.; Yao, S.; Chen, Z.; Wurthner, F.; Jonkheijm, P.; Schenning, A. P. H. J.; Meijer, E. W.; De Schryver, F. C. Nano Lett. 2005, 5, 77. (16) (a) Venkataraman, B.; Breen, J. J.; Flynn, G. W. J. Phys. Chem. 1995, 99, 6608. (b) Hibino, M.; Sumi, A.; Hatta, I. Thin Solid Films 1996, 281-282, 594. (c) Baker, R. T.; Mougous, J. D.; Brackley, A.; Patrick, D. L. Langmuir 1999, 15, 4884.

Introduction Two-dimensional (2D) self-assembled nanostructures of organic molecules self-assembled on a substrate surface have been extensively studied with scanning tunneling microscope (STM) due to their potential application in the field of nanoscience and nanotechnology.1 The related research has ever focused on monocomponent systems and gradually switched to multicomponent systems. For the investigation on monocomponent systems, researchers have acquired valuable information on the 2D ordering of various types of organic molecules, including a liquid crystal,2 alkanes,3 alcohols,4 fatty acids,4a,5 thiols, sulfides, and disufides,6 cycloalkanes,7 alkylbenzene derivatives,7b,8 phthalocyanines and porphyrins,9 p-phenylene-ethynylene derivatives,10 quinacridone derivatives,11 and other complicated molecules.1 In addition to the investigation on monocomponent systems, there is an increasing interest in the self-assembled nanostructures of multicomponent

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Scheme 1. Chemical Structure of Quinacridone Derivatives

voltaic, photoconductive, and light-emitting materials.18 Many investigations on quinacridone derivatives have been performed to explore the effects of different structural parameters on their physical properties. Our group has investigated the 2D ordering of N,N′-dialkyl-substituted quinacridone derivatives and observed the formation of chiral racemates or domains by coadsorbing with monofunctional acid.11 In the self-assembly process, the hydrogen-bond between the molecules is a dominant factor in the formation of 2D nanostructure because it is much stronger than the van der Waals interaction between the substrate and molecules. However, one monofunctional fatty acid molecule can only form one hydrogen bond with one quinacridone derivative molecule. We are curious if it is possible to modulate the 2D assemblies of quinacridone derivatives more efficiently and more controllable by coadsorbing quinacridone derivatives with bifunctional dicarboxylic acids. In the present article, we attempted to investigate the two-component assemblies of N,N’-dialkyl-substituted quinacridone derivatives with dicarboxylic acids by STM and focus our discussion on whether the supramolecular organization can be modulated by coadsorption of dicarboxylic acids bearing different lengths of alkyl chains. We hope our results can provide valuable information for further design and control of 2D supramolecular assemblies. Experimental Section Quinacridone derivatives (Scheme 1) were prepared according to the published method.19 Hexadecanedioic acid (HOOC(CH2)14COOH) was purchased from Acros Organics; pentadecanedioic acid (HOOC(CH2)13COOH) and 1,18-octadecanedicarboxylic acid (HOOC(CH2)18COOH) were purchased from Sigma-Aldrich and used without further purification. Saturated solutions of quinacridone derivatives and dicarboxylic acids in 1-phenyloctane were prepared, respectively. Quinacridone derivative solution and dicarboxylic acid solution were subsequently mixed in a 1:1 molar ratio. STM investigation was performed by using a commercial Multimode Nanoscope IV scanning tunneling microscope (Digital Instrument Co., Santa Barbara, CA) with mechanically cut Pt/Ir (90:10) tips at ambient temperature. All images shown were recorded in the constant-current mode. For measurements at the solution-substrate interface, a saturated solution of a quinacridone derivative or a mixed solution was applied to a freshly cleaved surface of highly orientated pyrolytic graphite (HOPG; Digital Instruments Co.), and then the tip was immersed into the solution. Measurement conditions are given in the corresponding figure captions. Different tips and samples were used to check for reproducibility and to ensure that there are no image artifacts caused by the tips or samples. Flattening of the images was carried out to compensate for tilting of the substrate (17) (a) Hipps, K. W.; Lu, X.; Wang, X. D.; Mazur, U. J. Phys. Chem. 1996, 100, 11207. (b) Padowitz, D. F.; Messmore, B. W. J. Phys. Chem. B 2000, 104, 9943. (18) Hiramoto, M.; Kawase, S.; Yokoyama, M. Jpn. J. Appl. Phys., Part 2 1996, 35, L349. (19) Keller, U.; Mu¨llen, K.; De Feyter, S.; De Schryver, F. C. Adv. Mater. 1996, 8, 490.

Figure 1. STM image of an adlayer of the quinacridone derivative TmQA-C22 adsorbed on HOPG in 1-phenyloctane (22.79 nm × 22.79 nm, U ) 800 mV, I ) 50.0 pA). and scan line artifacts, and a low-pass filtered transform was employed to remove scanning noise in the STM images.

Result and Discussion A quinacridone derivative bearing long alkyl chains, TmQA-C22, forms a stable monolayer after adsorption on HOPG, as shown in Figure 1. The orientation of the molecular cores and substituted alkyl chains are well resolved: the short, bright bands are quinacridone cores, the dim stripes correspond to the alkyl chains. The distance (∆L) between two adjacent quinacridione cores along the direction of the alkyl chains is 2.2 ( 0.1 nm. The angle (R) between the direction of the long axis of an alkyl chain and the boundary of a lamella is 138.8 ( 2°. To control the 2D assembly structure of TmQA-C22, we mixed TmQAC22 and 1,18-octadecanedicarboxylic acid and observed structure change induced by coadsorption. As seen from the large-scale STM image, Figure 2a, the two kinds of molecules in a 1:1 ratio form many small domains that contain different oriented stripes, which is different from the assemblies of pure TmQA-C22. From the high-resolution STM image, Figure 2b, we can see that three alkyl chains between two adjacent bright rows form a group, as shown by the yellow model. The two side alkyl chains belong to the quinacridone derivatives, and the middle one is the coadsorbed 1,18-octadecanedicarboxylic acid. The angle (R) between the direction of the long axis of an alkyl chain and the boundary of a lamella is 117.36 ( 2°, about 20° less than that in the 2D assemblies of the pure TmQA-C22. The distance (∆L) between two adjacent quinacridione cores along the direction of the alkyl chains has changed to 2.8 ( 0.1 nm, larger than that of the pure TmQA-C22. These structure changes must be caused by the introduction of 1,18-octadecanedicarboxylic acid: one bifunctional acid forms two hydrogen bonds with two adjacent TmQA-C22, and then the two kinds of molecules form a one-dimensional hydrogen-bonding network, therefore enhancing the molecule-substrate interaction. The cooperative effect makes TmQA-C22 change its conformation to form such a structural array in the coadsorption in order to minimize its free energy level. We wondered if it would be possible to induce a structural change by coadsorbing with a bifunctional acid bearing a short alkyl chain. For this purpose, TmQA-C22

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Figure 3. STM image of adlayer of the mixture of TmQA-C22 and pentadecanedioic acid adsorbed on HOPG in 1-phenyloctane (23.40 nm × 23.40 nm, U ) 700 mV, I ) 50.0 pA).

Figure 2. STM images of an adlayer of the mixture of TmQAC22 and 1,18-octadecanedicarboxylic acid adsorbed on HOPG in 1-phenyloctane. (a) Large-scale image (153.3 nm × 153.3 nm, U ) 800 mV, I ) 50.0 pA). (b) High-resolution STM image (23.30 nm × 23.30 nm, U ) 800 mV, I ) 50.0 pA).

is used as a host molecule and coadsorbed with pentadecanedioic acid. In contrast to the fatty acid bearing very long alkyl chains, seemingly, pentadecanedioic acid is not matching TmQA-C22 as well as 1,18-octadecanedicarboxylic acid. However, the two components, when mixed, change conformations to adapt each other, forming stable 2D assemblies. As shown in Figure 3, the angle (R) between the directions of alkyl chains and quinacridone cores is 65.6 ( 2°, the angle (β) between the quinacridone core’s long axis and the row is 57.7 ( 2°. The distance (∆L) between two quinacridione in adjacent rows is 2.2 ( 0.1 nm, which is consistent with the length of pentadecanedioic acid, 2.15 nm. Because the length of the bifunctional acid has shortened, the quinacridone core has to adjust its conformation in order to form hydrogen bonds with such a bifunctional acid. As a result, the angel (R) changed from 117.36 ( 2° to 65.6 ( 2°, the distance (∆L) changed from 2.8 ( 0.1 to 2.2 ( 0.1 nm, indicating that quinacridone derivatives TmQA-C22 take a contractive conformation.

Figure 4. STM image of an adlayer of the mixture of TmQAC16 and 1,18-octadecanedicarboxylic acid (29.90 nm × 29.90 nm, U ) 800 mV, I ) 80.0 pA).

If the coadsorption as indicated above can lead to formation of delicate 2D assemblies, it should be also true for other two-component structures of the bifunctional acids with other quinacridone derivatives. Figure 4 shows the STM observation on a coadsorbate of TmQA-C16 bearing a short alkyl chain with 1,18-octadecanedicarboxylic acid. The angle (R) between the alkyl chains of the quinacridone derivatives and quinacridone is 132.6 ( 2°, the angle (β) between the quinacridone core’s long axis and the row is 57.7 ( 2°, and the distance (∆L) between two quinacridiones in adjacent rows is 2.19 ( 0.1 nm. There are small holes existing at the end of the alkyl chains of the quinacridone derivatives. Obviously, to accommodate the longer dicarboxylic acid, quinacridione derivatives have taken such an extended conformation that the angle (R) is much larger than that of the mixture of TmQA-C22 and 1,18-octadecanedicarboxylic acid. We have also studied the coadsorption of TmQA-C16 with bifunctional acids hexadecanedioic acid and penta-

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Figure 5. STM image of an adlayer of the mixture of TmQAC16 and hexadecanedioic acid (28.30 nm × 28.30 nm, U ) 800 mV, I ) 80.0 pA).

decanedioic acid, respectively. As shown in Figure 5, the STM observation on a coadsorbate of TmQA-C16 with hexadecanedioic acid, it can be observed that the angle (R) between the alkyl chains of quinacridone derivatives and the quinacridone cores is 65.0 ( 2° and the distance (∆L) between two quinacridione cores in adjacent rows is 1.8 ( 0.1 nm. Moreover, there are excessive hexadecanedioic acid molecules adsorbed among quinacridone derivatives. Due to the shorter alkyl chain of the hexadecanedioic acid, the TmQA-C16 adopts a slightly contractive conformation to accommodate the hexadecanedioic acid, similar to the case of the mixture of TmQA-C22 and pentadecanedioic acid, but different from the case of the mixture of TmQA-C16 and 1,18-octadecanedicarboxylic acid. These results further support our assumption that the guest molecules with different length can modulate the conformation of the host molecules through the formation of hydrogen bonds between the guest molecules and host molecules. The most interesting structural feature has occurred to the coadsorbate of TmQA-C16 and pentadecanedioic acid. As shown in Figure 6, we have observed that TmQA-C16 takes two different conformations in two different columns (column A, column B) and these columns arrange alternately on the HOPG surface forming a gridlike structure. In column A, the angle (R) between the quinacridone cores and the alkyl chain is 120.7 ( 2° and the distance (∆L1) between the two adjacent quinacridone cores is 1.2 ( 0.1 nm. In column B, the angle (β) between the quinacridone cores and the alkyl chains is 96.0 ( 2° and the distance (∆L2) between the two adjacent quinacridone cores is 1.6 ( 0.1 nm. The angle (γ) between the quinacridone cores in the column A and column B is 24.0 ( 2° The distance (∆L3) between the adjacent quinacridone cores in column A and column B is longer than the distance between the adjacent quinacridone cores in the same column. Therefore, bifunctional pentadecanedioic acid fit well into the space between the adjacent TmQA-C16 cores in the adjacent columns A and B, instead of coadsorption between the adjacent TmQA-C16 cores in the same column, as shown in the tentative molecular model of Figure 6b. It should be noted that pentadecanedioic acid and hexadecanedioic acid are very similar, except that the hexadecanedioic acid has an even number

Figure 6. (a) STM image of an adlayer of the mixture of TmQAC16 and pentadecanedioic acid (28.30 nm × 28.30 nm, U ) 800 mV, I ) 80.0 pA). (b) Tentative molecular model of the conformation of TmQA-C16 in the different columns.

of carbon atoms, 16, and the pentadecanedioic acid has an odd number of carbon atoms, 15. This slight difference alone has induced a very different self-assembled nanostructure for their coadsorption with same TmQA-C16. One plausible explanation for such an odd-even effect20 could result from the zigzag conformation of the alkyl chains. The odd and even number of alkyl chains can modulate the direction of headgroups of the dicarboxylic acids that lead to formation of different 2D structures in multicomponent assemblies. Conclusions In summary, we have demonstrated that the mixture of the quinacridone derivatives and bifunctional acid can form uniform adlayers and the introduction of bifucntional acid can control the 2D structures formed by quinacridone (20) Yablon, D. G.; Wintgens, D.; Flynn, G. W. J. Phys. Chem. B 2002, 106, 5470.

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Table 1. Parameters of Quinacridone Derivatives in the Multicomponent samples

TmQA-C22 and HOOC(CH2)18COOH

TmQA-C22 and HOOC(CH2)13COOH

TmQA-C16 and HOOC(CH2)18COOH

TmQA-C16 and HOOC(CH2)14COOH

angle (R) distance (∆L)

117.36 ( 2° 2.8 ( 0.1 nm

65.6 ( 2° 2.2( 0.1 nm

132.6 ( 2° 2.19( 0.1 nm

65.0 ( 2° 1.8( 0.1 nm

derivatives. No matter how bifunctional acids of relatively long or short alkyl chains are used, the host and guest molecules can adjust their conformations to adapt to each other simultaneously, which are indicated by the parameters in Table 1, forming energy-favored 2D assemblies. In all the cases investigated, we only observed that the coadsorption of TmQA-C16 and pentadecanedioic acid regularly arrange on the surface with two different conformations. This study provides a new approach for

modulating the patterned structure of quinacridone derivatives with strong light-emitting properties. Acknowledgment. This research is supported by the Major State Basic Research Development Program (G2000078102,2002CB613401),NSFC(20334010,50225313, 20473045), “863” project (2003AA302140), and Ministry of Education, P.R. China. LA051087A