STM Study on 2D Molecular Assemblies of Luminescent Quinacridone

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Langmuir 2007, 23, 1287-1291

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STM Study on 2D Molecular Assemblies of Luminescent Quinacridone Derivatives: Structure Fine-tuned by Introducing Bulky Substitutes and Co-adsorption with Monofunctional/Bifunctional Acid Xunyu Yang, Jia Wang, Xi Zhang,* Zhiqiang Wang, and Yue Wang* Key Lab of Optoelectronics and Molecular Engineering, Department of Chemistry, Tsinghua UniVersity, Beijing 100084, People’s Republic of China, and Key Lab of Supramolecular Structure and Materials, Jilin UniVersity, Changchun 130012, People’s Republic of China ReceiVed August 15, 2006. In Final Form: October 29, 2006 We describe the 2D assemblies of a series of N,N′-dialkyl-substituted quinacridone derivatives on highly oriented pyrolytic graphite observed by scanning tunneling microscopy. Our experiments have demonstrated that pure quinacridone derivatives take contractive conformations, but quinacridone derivatives take extended conformations when coadsorbed with dicarboxylic acid. Interestingly, by co-adsorption with monofunctional acid stearic acid, quinacridone derivative bearing two smaller substituted groups of trifluoromethyl takes an extended conformation, while quinacridone derivative bearing two larger substituted groups of tert-butyl still takes a contractive conformation. Therefore, the 2D structure of the quinacridone derivatives can be fine-tuned by co-adsorbing with monofunctional/bifunctional acid through hydrogen bonds.

Introduction Since Frommer and co-worker introduced scanning tunneling microscopy (STM) in studying two-dimensional (2D) molecular assemblies in 1988,1 it has been extended from one-component, such as alkanes, alcohols, carboxylic acids,2 and so on,3 to twocomponents systems.4 In two-components systems, the interplay between host-guest pairs allows for fine-tuning their 2D structures. For example, Rabe and co-workers studied early on the self-assembly of two components of 5-alkoxyisophthalic acid and pyrazine on the surface.5 Qian et al. used 4,4′-bipyridine as a marker for identifying the carboxylic group of a fatty acid.6 Bai and co-workers reported the formation of an array structure by co-adsorption of phthalocyanines and functionalized alkanes.7 Wan investigated the molecular self-organization of calixarene and its complex with C60.8 De Feyter et al. fabricated a 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.9 Quinacridone and its derivatives (QA) are well-known as organic pigments10 and dopant emitters11 that show excellent chemical stability and fastness properties as well as pronounced * Authors to whom correspondence should be addressed. E-mail: [email protected]; [email protected]. (1) Foster, J.; Frommer, J. Nature 1988, 333, 542. (2) Rabe, J. P.; Buchholz, S. Science 1991, 253, 424. (3) Cry, D. M.; Venkataraman, V.; Flynn, G. W. Chem. Mater. 1996, 8, 1660. (4) Hoeppener, S.; Chi, L.; Fuchs, H. Chem. Phys. Chem. 2003, 4, 494. (5) Eichhorst-Gerner, K.; Stabel, A.; Moessner, G.; Declercq, D.; Valiyaveettil, S.; Enkelmann, V.; Muellen, K.; Rabe, J. P. Angew. Chem., Int. Ed. 1996, 35, 1492. (6) (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. (7) Lei, S. B.; Wang, C.; Yin, S. X.; Bai, C. L. J. Phys. Chem. B 2001, 105, 12272. (8) Wan, L. Acc. Chem. Res. 2006, 39, 334. (9) 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. (10) Hiramoto, M.; Kawase, S.; Yokoyama, M. Jpn. J. Appl. Phys., Part 2 1996, 35, L349. (11) Jabbour, G. E.; Kawabe, Y.; Shaheen, S. E.; Wang, J. F.; Morrell, M. M.; Kippelen, B.; Peyghambarian, N. Appl. Phys. Lett. 1997, 71, 1762.

photovoltaic and photoconductive activities.12-15 Because they have good electrochemical stability in the solid state and high photoluminescent efficiency in dilute solution, quinacridone and its derivatives are promising materials for fabricating highperformance organic light-emitting devices (OLEDs). Many investigations on quinacridone derivatives have been devoted to the exploration of the effects of different structural parameters on their physical properties. For example, Nakahara et al.13 have synthesized quinacridone derivatives with four alkyl chains and used them in Langmuir-Blodgett films to control the orientation and packing of the chromophores. De Feyter and co-workers16,17 have studied the aggregation behavior and two-dimensional ordering of 2,3,9,10-tetra(dodecyloxy)quinacridone, 2,9-di(2undecyltridecyl-1-oxy)quinacridone, and N,N′-dimethyl-substituted analogues. We have reported the 2D assemblies of N,N′dialkyl-substituted quinacridone derivatives,18-20 observed the formation of chiral racemates or domains by co-adsorbing with monofunctional acid,19 and then tried to modulate the 2D assemblies of quinacridone derivatives by co-adsorbing quinacridone derivatives with bifunctional dicarboxylic acids.20 To enhance the stability of quinacridone derivatives, we have synthesized quinacridone derivatives substituted with trifluoromethyl groups, D(TFM)QA. In the mean time, for reducing the self-quenching effect in high-concentration solution of quinacridone derivatives, we have synthesized quinacridone derivatives substituted with tert-butyl groups, DTBuQA. In this article, we study the 2D assemblies of the double-substituted (12) Shichiri, T.; Suezaki, M.; Inoue, T. Chem. Lett. 1992, 1717. (13) Nakahara, H.; Kitahara, K.; Nishi, H.; Fukuda, K. Chem. Lett. 1992, 711. (14) Nakahara, H.; Fukuda, K.; Ikeda, M.; Kitahara, K.; Nishi, H. Thin Solid Films 1992, 210/211, 555. (15) Shi, J.; Tang, C. W. Appl. Phys. Lett. 1997, 70, 1665. (16) Keller, U.; Mu¨llen, K.; De Feyter, S.; De Schryver, F. C. AdV. Mater. 1996, 8, 490. (17) De Feyter, S.; Gesquiere, A.; De Schryver, F. C. Chem. Mater. 2002, 14, 989. (18) Qiu, D.; Ye, K.; Wang, Y.; Zou, B.; Zhang, X.; Lei, S.; Wan, L. Langmuir 2003, 19, 678. (19) Mu, Z.; Wang, Z.; Zhang, X.; Ye, K.; Wang, Y. J. Phys. Chem. B 2004, 108, 19955. (20) Yang, X.; Mu, Z.; Wang, Z.; Zhang, X.; Wang, J.; Wang, Y. Langmuir 2005, 21, 7225.

10.1021/la0624034 CCC: $37.00 © 2007 American Chemical Society Published on Web 12/02/2006

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Scheme 1. Chemical Structure of Quinacridone Derivatives

series of quinacridone derivatives by STM and attempt to modulate their 2D assemblies by co-adsorbing monofunctional acid and dicarboxylic acid, respectively. We have found interestingly that the double-substituted series of quinacridone derivatives takes a different conformation when co-adsorbed with different kinds of carboxylic acid. We hope our result can be helpful in understanding the 2D assemblies of quinacridone derivatives and provide valuable information for the design and control of 2D supramolecular assemblies of quinacridone derivatives. Experimental Section STM investigation was performed by using a commercial Multimode Nanoscope IV scanning tunneling microscope (Digital Instrument Co., Santa Barbara, CA). Highly oriented pyrolytic graphite (HOPG) was also purchased from Digital Instrument Co. The tips are mechanically cut from Pt/Ir (90/10) wires purchased from MaTeck. 1-Phenyloctane was purchased from Aldrich. All the samples were dissolved in 1-phenyloctane at ∼1 mg/mL concentration. The ratio of quinacridone derivatives and fatty acids is around 1:2. When dicarboxylic acid is involved, this ratio becomes 1:1. All images shown were recorded in the constant-current mode. For measurements at the solution-substrate interface, a solution of a quinacridone derivative or a mixed solution was applied to a freshly cleaved surface of HOPG, and then the tip was immersed into the solution. Measurement conditions were given in the corresponding figure captions. Different tips and samples were used to check for reproducibility and to ensure no image artifacts caused by the tips or samples. Flattening of the images was carried out to compensate for tilting of the substrate and scan line artifacts, and a low-pass filtered transform was employed to remove scanning noise in the STM images. The N,N′-dialkyl-substituted quinacridone derivatives studied here were 2,9-trifluoromethyl-N,N′-dialkyl-substituted-quinacridone derivatives (D(TFM)-C22, D(TFM)-C16) and 2,9-tert-butyl-N,N′dialkyl-substituted-quinacridone derivatives (DTBuQA-C22, DTBuQA-C16), as shown in Scheme 1, which were synthesized according to the procedure reported previously.21 D(TFM)-Cn (n ) 16 and 22) and DTBuQA-Cn (n ) 16 and 22) have been characterized by 1H NMR, mass, and element analysis and the synthesis details will be published elsewhere.

Results and Discussion 2D Assembly of Pure Quinacridone Derivatives. Before observing co-adsorption of quinacridone derivatives and carboxylic acids, we expected to understand clearly how the pure quinacridone derivatives arrange at the liquid/solid interface and how the steric interaction of trifluoromethyl groups affects their 2D assemblies. Figure 1 shows the STM images of D(TFM)QA-C22 bearing two trifluoromethyl groups at the 2, 9 positions of the quinacridone core physically adsorbed at the liquid/HOPG (21) Ye, K. Q.; Wang, J.; Sun, H.; Liu, Y.; Mu. Z. C.; Li, F.; Jiang, S. M.; Zhang, J. Y.; Zhang, H. X.; Wang, Y.; Che, C. M. J. Phys. Chem. B 2005, 109, 8008.

Figure 1. STM images of 2D assemblies of D(TFM)QA-C22 adsorbed on HOPG in 1-phenyloctane. (a) Large-scale image (82.0 nm × 82.0 nm, U ) 800 mV, I ) 100.0 pA). (b) High-resolution STM image (26.3 nm × 26.3 nm, U ) 700 mV, I ) 70.0 pA).

interface. As seen from the large-scale STM image (Figure 1a), D(TFM)QA-C22 spontaneously forms a uniform and stable stripe pattern on HOPG. Figure 1b presents a high-resolution STM image in which the direction of the long axis of a quinacridone core is a little tilted (angle β ) 20 ( 2°) in the direction of the stripes that make up the quinacridone cores. The orientation of the molecular cores and substituted alkyl chains are wellresolved: the short, bright bands are quinacridone cores; the dim stripes correspond to the alkyl chains. The angle R between the direction of the long axis of an alkyl chain and the direction of the long axis of a quinacridone core is 57 ( 2°. The distance (∆L) between the two adjacent quinacridone cores at adjacent stripes is 1.7 ( 0.1 nm. Because the trifluoromethyl group is not big enough to be visualized and it has a low tunneling efficiency, the quinacridone cores in the STM image are still displayed as rectangles. In addition, the formation of monolayer structure was clearly indicated by the defect marked by a circle in Figure 1a. In an attempt to study the influence of bulky groups on 2D assemblies, we have replaced the trifluoromethyl groups by tertbutyl group in DTBuQA-C22. As shown in Figure 2, DTBuQAC22 molecules also form a uniform and stable strip structure after physical adsorption on the HOPG. The angle β between the long axis of a quinacridone core and the stripe formed by the

2D Molecular Assemblies of Quinacridone DeriVatiVes

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Figure 3. STM image of 2D assemblies of D(TFM)QA-C16 adsorbed on HOPG in 1-phenyloctane (35.30 nm × 35.30 nm, U ) 700 mV, I ) 70.0 pA).

Figure 2. (a) STM image of 2D assemblies of DTBuQA-C22 adsorbed on HOPG in 1-phenyloctane. (b) Tentative molecular model of the conformation of DTBuQA-C22 (36.2 nm × 36.2 nm, U ) 800 mV, I ) 100.0 pA).

quinacridone cores is 21 ( 2° and the angle R between the direction of the long axis of an alkyl chain and the direction of the long axis of a quinacridone core is 50 ( 2°. The distance (∆L) between the two adjacent quinacridone cores at adjacent stripes is 1.3 ( 0.1 nm. The quinacridone cores are shown as a tilt parallelogram because the tert-butyl group is big enough to influence the shape of quinacridone cores visualized by STM. Thereby, the quality of STM images for DTBuQA-C22 cannot be as good as that of D(TFM)QA-C22. If the angle R between the direction of the long axis of an alkyl chain and the direction of the long axis of a quinacridone core is bigger than 90°, quinacridone is regarded as an extended conformation; otherwise, it is named as a contractive conformation. Comparing the data obtained from Figures 1 and 2, D(TFM)QA-C22 and DTBuQA-C22 both take a contractive conformation. As we have found that the quinacridone derivatives with substitutes at 1, 3, 8, 10 positions take an extended conformation,19 the quinacridone with substitutes at 2, 9 positions, asymmetric substitutes, should be the main reason for its contractive conformation. This contractive conformation is helpful for either trifluoromethyl or tert-butyl groups to occupy the bare surface of HOPG between the adjacent quinacridone cores, as shown in the tentative molecular model of Figure 2b, minimizing the free energy level of the 2D assemblies.

Figure 4. STM images of 2D assemblies of the mixture of D(TFM)QA-C22 and stearic acid. (a) Large-scale image (110.1 nm × 110.1 nm, U ) 800 mV, I ) 100.0 pA). (b) High-resolution STM image (26.1 nm × 26.1 nm, U ) 700 mV, I ) 100.0 pA). Unit cell: a ) 4.4 ( 0.1 nm, b ) 2.2 ( 0.1 nm, γ ) 57 ( 2°.

Similarly, we also investigated 2D assemblies of D(TFM)QA-C16 and DTBuQA-C16 by STM. For the shorter alkyl chain, only the cores of D(TFM)QA-C16 can be visualized, as shown

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Figure 5. STM image of 2D assemblies of the mixture of DTBuQAC22 and stearic acid (31.0 nm × 31.0 nm, U ) 700 mV, I ) 80.0 pA). Unit cell: a ) 3.5 ( 0.1 nm, b ) 2.4 ( 0.1 nm, γ ) 51 ( 2°.

Figure 7. (a) STM image of 2D assemblies of the mixture of DTBuQA-C22 and 1,18-octadecanedicarboxylic acid (42.8 nm × 42.8 nm, U ) 700 mV, I ) 100.0 pA). Unit cell: a ) 3.6 ( 0.1 nm, b ) 1.9 ( 0.1 nm, γ ) 75 ( 2°. (b) Tentative molecular model of the conformation of the mixture of DTBuQA-C22 and 1,18octadecanedicarboxylic acid. Figure 6. STM image of 2D assemblies of the mixture of D(TFM)QA-C22 and 1,18-octadecanedicarboxylic acid (25.9 nm × 25.9 nm, U ) 800 mV, I ) 100.0 pA). Unit cell: a ) 3.4 ( 0.1 nm, b ) 2.0 ( 0.1 nm, γ ) 68 ( 2°.

in Figure 3, which indicates the unstable adsorption on the HOPG surface. For DTBuQA-C16 with shorter alkyl chains and more bulky tert-butyl groups, we are unable to observe stable 2D assemblies on a HOPG surface by STM. The shorter alkyl chain derivatives of D(TFM)QA-C16 and DTBuQA-C16 showed poor adsorption behaviors on HOPG because of their weaker interaction with HOPG than that of the longer alkyl chain derivatives. For short alkyl chain derivatives, it may form a stable adlayer by co-adsorption by fatty acid sometimes. However, for D(TFM)QA-C16 and DTBuQA-C16, the introduction of bulky substitutes can weaken further their interaction with the substrate; therefore, they cannot form a stable adlayer structure even by co-adsorption. 2D Assembly of Quinacridone Derivatives with Monofunctional Acid. We wondered if it would be possible to modulate the 2D assemblies of quinacridone derivatives as host molecules by co-adsorption with guest molecules of monofunctional acid bearing an alkyl chain of suitable length. For this purpose, we mixed D(TFM)QA-C22 and stearic acid and observed the

structure change induced by co-adsorption. As seen from the large-scale STM image Figure 4a, the mixed solution forms many small domains that contain different oriented stripes, which is different from the assemblies of pure D(TFM)QA-C22. All the angles between the different oriented domains are about either 60° or 120°, indicating that there is a template effect of the hexagonal structures of the graphite substrate on the molecular orientation of the 2D assemblies. Figure 4b is the high-resolution STM image of the co-adsorption of D(TFM)QA-C22 and stearic acid. The angle β between the long axis of a quinacridone core and the stripe formed by the quinacridone cores is 34 ( 2°. The angle R between the direction of the long axis of an alkyl chain and the direction of the long axis of a quinacridone core is 148 ( 2°. The distance (∆L) between the two adjacent quinacridione cores at adjacent stripes is 2.1 ( 0.1 nm. D(TFM)QA-C22 molecules take an extended conformation in the co-adsorption with stearic acid, which is different from the 2D assemblies of pure D(TFM)QA-C22. The change of quinacridone conformation is induced by the fatty acids, which may modulate intermolecular interactions by interacting with the quinacridone host molecules through hydrogen bonding and enhancing interaction with a substrate, thus probably controlling the structure of 2D su-

2D Molecular Assemblies of Quinacridone DeriVatiVes

pramolecular assemblies on HOPG. It is observed that every quinacridone molecule has co-adsorbed with two stearic acid molecules, which are arrayed on the two sides of quinacridone cores, respectively. The arrangements of the alkyl chains from the host and the guest molecules are alternated and interdigitated. The end of an alkyl chain of D(TFM)QA-C22 also extends into the space between two adjacent cores in a row for occupying the bare HOPG surface to minimize the whole system’s energy level. Interestingly, DTBuQA-C22 has displayed a different patterned structure from D(TFM)QA-C22 when co-adsorbed with stearic acid. Figure 5 is the STM image of co-adsorption of DTBuQAC22 and stearic acid. The high-resolution image reveals that DTBuQA-C22 takes a different conformation when co-adsorbed with stearic acid. The angle β between the long axis of a quinacridone core and the stripe formed by the quinacridone cores is almost zero. The angle R between the direction of the long axis of an alkyl chain and the direction of the long axis of a quinacridone core is 60 ( 2°. The distance (∆L) between the two adjacent quinacridione cores at adjacent stripes is 2.0 ( 0.1 nm. The main difference between DTBuQA-C22 and D(TFM)QA-C22 lies only in the bulky substitutes: DTBuQA-C22 substituted by tert-butyl groups and D(TFM)QA-C22 substituted by trifluoromethyl groups. The different bulky substitutes result in DTBuQA-C22 taking a contractive conformation with D(TFM)QA-C22 taking an extended conformation during coadsorption with stearic acid, thereby leading to their different structures of 2D assemblies. 2D Assembly of Quinacridone Derivatives with Dicarboxylic Acids. As discussed above, the 2D assemblies of quinacridone derivatives can be modulated by introduction of a monofunctional acid, stearic acid. However, one monofunctional fatty acid molecule can form only one hydrogen bond with one quinacridone derivative molecule. We are curious if it is possible to modulate the 2D assemblies of quinacridone derivatives in a different way by co-adsorbing the quinacridone derivatives with bifunctional dicarboxylic acids. Figure 6 presents the 2D assemblies of the co-adsorption of D(TFM)-C22 and 1,18octadecanedicarboxylic acid. The high-resolution STM image, Figure 6, indicates that every two quinacridone molecules coadsorbed with one 1,18-octadecanedicarboxylic acid molecule. 1,18-Octadecanedicarboxylic acid is contained in the middle of the two alkyl chains which belong to the two adjacent quinaridone molecules, respectively. Although the end of the alkyl chain of D(TFM)QA-C22 extends into the space between two adjacent cores in a row, there are still some free volumes shown as small, black holes in the STM image. The angle β between the long axis of a quinacridone core and the stripe formed by the quinacridone cores is 24 ( 2°. The angle R between the direction of the long axis of an alkyl chain and the direction of the long axis of a quinacridone core is 118 ( 2°, indicating that the quinacridone molecule takes a slight extended conformation. The distance (∆L) between the two adjacent quinacridione cores at adjacent stripes is 2.4 ( 0.1 nm.

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Likewise, DTBuQA-C22 also takes an extended conformation on HOPG co-adsorbed with 1,18-octadecanedicarboxylic acid indicated by Figure 7. There are two DTBuQA-C22 molecules co-adsorbed with one 1,18-octadecanedicarboxylic acid molecule forming two hydrogen bonds, respectively. The distance (∆L) between the two adjacent quinacridione cores at adjacent stripes is 2.4 ( 0.1 nm. Also there are some small holes in the space between the end of the alkyl chain and the quinacridone cores as shown in Figure 7, the high-resolution STM image of the co-adsorption of DTBuQA-C22 and 1,18-octadecanedicarboxylic acid. The angle R between the direction of the long axis of an alkyl chain and the direction of the long axis of a quinacridone core is 123 ( 2°, indicating that the DTBuQA-C22 takes an extended conformation under the modulation of the dicarboxylic acid 1,18-octadecanedicarboxylic acid, different from the situation under the modulation of the monofunctional acid stearic acid. So it suggests that the conformation change of quinacrindone from contractive to extended conformations is not energyfavorable; however, the formation of two hydrogen bonds can compensate for the energy change and form energy-favorable 2D assemblies. The fact that DTBuQA-C22 takes a contractive conformation when co-adsorbed with monofunctional stearic acid but takes an extended conformation when co-adsorbed with 1,18-octadecanedicarboxylic acid clearly indicates that the formation of two hydrogen bonds can compensate for the energy change induced by conformation change of quinacridone, whereas formation of one hydrogen bond is not enough to compensate the energy change.

Conclusion In summary, different from the quinacridone derivatives reported, we have demonstrated that a series of double-substituted quinacridone derivatives take contractive conformation for its 2D assemblies on HOPG. Co-adsorption with the monofunctional acid stearic acid by hydrogen bond, D(TFM)QA-C22, bearing two smaller substituted groups of trifluoromethyl takes an extended conformation, while DTBuQA-C22, bearing two larger substituted groups of tert-butyl, still takes a contractive conformation. In the case of co-adsorption with bifunctional dicarboxylic acid, both D(TFM)QA-C22 and DTBUQA-C22 take extended conformations. The larger the double-substituted group, the more quinacridone derivatives tend to take contractive conformation. Clearly, bifunctional dicarboxylic acid can modulate the 2D assemblies of quinacridone derivatives in a more efficient and controlled manner than monofunctional fatty acid. Acknowledgment. This research was funded by NSFC (20334010, 20174013, and 50225313) and National Basic Research Program (2005CB724400, 2007CB808000). LA0624034