Alkyl Chain Length Dependence of the Self-Organized Structure of

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4708

Langmuir 2008, 24, 4708-4714

Alkyl Chain Length Dependence of the Self-Organized Structure of Alkyl-Substituted Phthalocyanines Koji Miyake,*,† Yukari Hori,‡ Taichi Ikeda,‡,§ Masumi Asakawa,‡ Toshimi Shimizu,‡ and Shinya Sasaki†,| AdVanced Manufacturing Research Institute (AMRI), National Institute of AdVanced Industrial Science and Technology (AIST), 1-2-1 Namiki, Tsukuba, Ibaraki 305-8564, Japan, and Nanoarchitectonics Research Center (NARC), AIST, Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan ReceiVed August 20, 2007. In Final Form: February 14, 2008 The alkyl chain length on alkyl-substituted phthalocyanines (CnOPc) dependence of their self-organized structures was examined in this study. STM results indicated that the symmetry of ordered structures decreased as the alkyl chain became longer, with the exception of C6OPc, which preferentially formed a quasi-3-fold symmetrical structure. This could be explained by the fact that the CnOPc molecules are most likely to form densely packed structures. With CnOPc, when n ) 4 to 10, the self-organized structures were dependent on the competition between how densely the molecules were arranged and how loose the intermolecular interaction energy was, caused by the formation of the densely packed structure. However, with CnOPc, when n ) 10-18, the molecules tended to form densely packed structures by reducing the symmetry, even though the CnOPc molecules were distorted. When C12OPc and cobalt phthalocyanine were coadsorbed, the mixed system exhibited a four-fold symmetrical structure, which is rarely observed in C12OPc.

Introduction One of the important points to achieve future molecular scale devices is to create samples that demonstrate the suitable properties of the devices even in a solid state.1-3 Effective techniques are required to immobilize functionalized molecules and to form their ordered structures on substrates. The selfassembly of organic thiols or disulfides on a gold (Au) surface is a promising candidate to realize the formation of well-ordered stable monolayers of functionalized molecules.4-6 There have been many studies on self-assembled monolayers (SAMs) of alkanethiols or their substituted analogues, such as aromatic thiols,7-12 azobenzene thiols,13-15 cyclohexanthiol,16 hydrocarbon * Corresponding author. E-mail: [email protected]. † AMRI, AIST. ‡ NARC, AIST. § Present address: Functional Modules Group, Organic Nanomaterials Center, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki, 305-0044 Japan. | Present address: Department of Mechanical Engineering, Tokyo University of Science, 1-14-6 Kudankita, Chiyoda-ku, Tokyo 102-0073 Japan. (1) Shipway, A. N.; Willner, I. Acc. Chem. Res. 2001, 34, 421. (2) Pease, A. R.; Jeppesen, J. O.; Stoddart, J. F.; Luo, Y.; Collier, C. P.; Heath, J. R. Acc. Chem Res. 2001, 34, 433. (3) Ishida, T. Chemistry of Nanomolecular Systems; Springer Publishing: Berlin, 2003; Chapter 6. (4) Ulman, A. Chem. ReV. 1996, 96, 1533. (5) Poirier, G. E. Chem. ReV. 1997, 97, 1117. (6) Nuzzo, R. G.; Zegarski, B. R.; Dubois, L. H. J. Am. Chem. Soc. 1987, 109, 733. (7) Sabatani, E.; Cohen-Boulakia, J.; Bruening, M.; Rubinstein, I. Langmuir 1993, 9, 2974. (8) Tao, Y.-T.; Wu, C.-C.; Eu, J.-Y.; Lin, W.-L.; Wu, K.-C.; Chen, C.-H. Langmuir 1997, 13, 4018. (9) Wan, L.-J.; Terashima, M.; Noda, H.; Osawa, M. J. Phys. Chem. 2000, 104, 3563. (10) Yang, G.; Qian, Y.; Engtrakul, C.; Sita, L. R.; Liu, G.-Y. J. Phys. Chem. B 2000, 104, 9059. (11) Liao, S.; Shnidman, Y.; Ulman, A. J. Am. Chem. Soc. 2000, 122, 3688. (12) Ishida, T.; Mizutani, W.; Azehara, H.; Sato, F.; Choi, N.; Akiba, U.; Fujihira, M.; Tokumoto, H. Langmuir 2001, 17, 7459. (13) Wolf, H.; Ringsdorf, H.; Delamarche, E.; Takami, T.; Kang, H.; Michel, B.; Gerber, Ch.; Jaschke, M.; But, H.-J.; Bamberg, E. J. Phys. Chem. 1995, 99, 7102. (14) Wang, R.; Iyoda, T.; Jiang, L.; Tryk, D. A.; Hashimoto, K.; Fujishima, A. J. Electroanal. Chem. 1997, 438, 213.

cages with methane thiolates,17 organosulfur compounds with C60,18 and DNA bases.19 These studies indicate that the packing arrangement and the ordering of the molecules are strongly influenced by the competition of various interactions, such as the Au substrate-sulfur interaction, or the intermolecular van der Waals interaction of alkyl chains or end groups. Thus, it is difficult to control the symmetry and periodicity of the molecular arrangement on the substrate in organothiol SAMs. In addition, the complicated molecular structure itself often prevents the formation of molecular ordering on the surface. To overcome the obstacles of SAMs of alkylthiol-substituted analogues, one can use mixtures of alkanethiols and organosulfur compounds with functionalized molecules.20,21 However, even in this mixed system, the control of the symmetry and periodicity of the arrangement of the functionalized molecules is difficult. The other approach to form well-ordered stable monolayers of functionalized molecules is the utilization of alkyl chainassisted SAMs represented by alkyl chain-substituted compounds on a graphite surface.22-28 Unsubstituted and substituted alkanes (15) Tamada, K.; Nagasawa, J.; Nakanishi, F.; Abe, K.; Ishida, T.; Hara, M.; Knoll, W. Langmuir 1998, 14, 3264. (16) Noh, J.; Hara, M. Langmuir 2002, 18, 9111. (17) Fujii, S.; Akiba, U.; Fujihira, M. J. Am. Chem. Soc. 2002, 124, 13629. (18) Shi, X.; Caldwell, W. B.; Chen, K.; Mirkin, C. A. J. Am. Chem. Soc. 1994, 116, 11598. (19) (a) Katsumata, S.; Ide, A. Jpn. J. Appl. Phys. 1994, 33, 3723. (b) Katsumata, S.; Ide, A. Jpn. J. Appl. Phys. 1995, 34, 3360. (20) For example, Cygan, M. T.; Dunbar, T. D.; Arnold, J. J.; Bumm, L. A.; Shedlock, N. F.; Burgin, T. P.; Jones, L.; Allara, D. L.; Tour, J. M.; Weiss, P. S. J. Am. Chem. Soc. 1998, 120, 2721. (21) Yasuda, S.; Nakamura, T.; Matsumoto, M.; Shigekawa, H. J. Am. Chem. Soc. 2003, 125, 16430. (22) (a) Plass, K. E.; Kim, K.; Matzger, A. J. J. Am. Chem. Soc. 2004, 126, 9042. (b) Scherer, L. J.; Merz, L.; Constable, E. C.; Housecroft, C. E.; Neuburger, M.; Hermann, B. A. J. Am. Chem. Soc. 2005, 127, 4033. (c) Yang, X.; Mu, Z.; Wang, Z.; Zhang, X.; Wang, J.; Wang, Y. Langmuir 2005, 21, 7225. (d) Wei, Y.; Kannappan, K.; Flynn, G. W.; Zimmt, M. B. J. Am. Chem. Soc. 2004, 126, 5318. (23) (a) Mamdouh, W.; Ujii, H.; Ladislaw, J. S.; Dulcey, E.; Percec, V.; De Schryver, F. C.; De Feyter, S. J. Am. Chem. Soc. 2005, 128, 317. (b) Lackinger, M.; Griessl, S.; Heckl, W. M.; Hietschold, M.; Flynn, G. W. Langmuir 2005, 21, 4984. (24) Florio, G. M.; Klare, J. E.; Pasamba, M. O.; Werblowsky, T. L.; Hyers, M.; Berne, B. J.; Hybertsen, M. S.; Nuckolls, C.; Flynn, G. W. Langmuir 2006, 22, 10003.

10.1021/la702564m CCC: $40.75 © 2008 American Chemical Society Published on Web 04/02/2008

Self-Organized Alkyl-Substituted Phthalocyanines

have been used as a model system to study the interactions that control molecular adsorption and self-assembly on graphite.29-33 It is well-known that the alkyl-substituted porphyrins and phthalocyanines rearranges in lines or discrete states with regular spacing on highly oriented pyrolytic graphite (HOPG) surfaces under ambient conditions.25a,25b,26 Metalloporphyrins and metallophthalocyanines have the ability to immobilize the desired organic molecules through axial coordination to the central metal atom. We succeeded in synthesizing porphyrin-stoppered rotaxane through the coordinate linkage of rhodium(III) (Rh(III)) porphyrin derivative.34 In addition, we demonstrated that the coordination bond between a pyridine unit and the Rh(III) ion of the porphyrin rhodium chloride is extremely stable. On the basis of these findings, we have proposed a technique to immobilize functionalized molecules and to form their ordered structures on substrates, which we call the molecular template method. Alkylsubstituted metalloporphyrins26a and hydroquinone-strapped alkyl-substituted porphyrin26b are used as templates to arrange functionalized molecules. The alkyl-substituted porphyrin molecules form a well-ordered array with a characteristic lamellar structure of 2-fold symmetry, which is almost the same as the structure of alkyl-substituted free base porphyrins, even though those molecules have bulky ligands. Furthermore, the intermolecular distance increased with longer substituted alkyl chain lengths. This is a significant step toward creating a molecular template method. However, the alkyl-substituted porphyrins only form a well-ordered array with a characteristic lamellar structure of 2-fold symmetry. The symmetry also does not change with changes in the alkyl chain length. Therefore, one has to control self-organized structures in order to create a sophisticated molecular template method. To control the structures of molecular templates, we examine the possibility of using phthalocyanine derivatives as molecular templates. Octaalkoxyl-substituted phthalocyanines (CnOPc; n is the number of carbon atoms) have 4-fold symmetrical structures as shown Figure 1. According to previous results, octyloxy(25) (a) Qiu, X.; Wang, C.; Zeng, Q.; Xu, B.; Yin, S.; Wang, H.; Xu, S.; Bai, C. J. Am. Chem. Soc. 2000, 122, 5550. (b) Wang, H.; Wang, C.; Zeng, Q.; Xu, S.; Yin, S.; Xu, B.; Bai, C. Surf. Interface Anal. 2001, 32, 266. (c) Gong, J.; Lei, S.; Wan, L.; Deng, G.; Fan, Q.; Bai, C. Chem. Mater. 2003, 15, 3098. (d) Pan, L.; Zeng, Q.; Lu, J.; Wu, D.; Xu, S.; Tan, Z.; Wan, L.; Wang, C.; Bai, C. Surf. Sci. 2004, 559, 70. (26) (a) Ikeda, T.; Asakawa, M.; Goto, M.; Miyake, K.; Ishida, T.; Shimizu, T. Langmuir 2004, 20, 5454. (b) Ikeda, T.; Asakawa, M.; Miyake, K.; Shimizu, T. Chem. Lett. 2004, 33, 1418. (c) Otsuki, J.; Nagamine, E.; Kondo, T.; Iwasaki, K.; Asakawa, M.; Miyake, K. J. Am. Chem. Soc. 2005, 127, 10400. (d) Otsuki, J.; Seki, E.; Taguchi, T.; Asakawa, M.; Miyake, K. Chem. Lett. 2007, 36, 740. (27) (a) Kikkawa, Y.; Koyama, E.; Tsuzuki, S.; Fujiwara, K.; Miyake, K.; Tokuhisa, H.; Kanesato, M. Langmuir 2006, 22, 6910. (b) Kikkawa, Y.; Koyama, E.; Tsuzuki, S.; Fujiwara, K.; Miyake, K.; Tokuhisa, H.; Kanesato, M. Chem. Commun. 2007, 1343. (c) Kikkawa, Y.; Koyama, E.; Tsuzuki, S.; Fujiwara, K.; Miyake, K.; Tokuhisa, H.; Kanesato, M. Surf. Sci. 2007, 601, 2520. (28) (a) Binnemans, K.; Sleven, J.; De Feyter, S.; De Schryver, F. C.; Donnio, B.; Guillon, D. Chem. Mater. 2003, 15, 3930. (b) Yang, Z. Y.; Gan, L. H.; Lei, S. B.; Wan, L. J.; Wang, C.; Jiang, J. Z. J. Phys. Chem. B 2005, 109, 19859. (c) Takami, T.; Arnold, D. P.; Fuchs, A. V.; Will, G. D.; Goh, R.; Waclawik, E. R.; Bell, J. M.; Weiss, P. S.; Sugiura, K.-i.; Liu, W.; Jiang, J. J. Phys. Chem. B 2006, 110, 1661. (d) Klymchenko, A. S.; Sleven, J.; Binnemans, K.; De Feyter, S. Langmuir 2006, 22, 723. (e) Otsuki, J.; Kawaguchi, S.; Yamakawa, T.; Asakawa, M.; Miyake, K. Langmuir 2006, 22, 5708. (29) Rabe, J. P.; Buchholts, S. Science 1991, 253, 424. (30) Okawa, Y.; Aono, M. Nature 2001, 409, 683. (31) Walba, D. M.; Stevens, F.; Clark, N. A.; Parks, D. C. Acc. Chem. Res. 1996, 29, 591. (32) (a) Cyr, D. M.; Venkataraman, B.; Flynn, G. W. Chem. Mater. 1996, 8, 1600. (b) Giancarlo, L. C.; Flynn, G. W. Annu. ReV. Phys. Chem. 1998, 49, 297. (c) Giancarlo, L. C.; Flynn, G. Acc. Chem. Res. 2000, 33, 491. (33) (a) De Feyter, S.; Gesquiere, A.; Abdel-Mottaleb, M. M.; Grim, P. C. M.; De Schryver, F. C.; Meiners, C.; Sieffert, M.; Valiyaveetttil, S.; Mu¨llen, K. Acc. Chem. Res. 2000, 33, 520. (b) De Feyter, S.; De Schryver, F. C. Chem. Soc. ReV. 2003, 32, 139. (34) (a) Asakawa, M.; Ikeda, T.; Yui, N.; Shimizu, T. Chem. Lett. 2003, 174. (b) Ikeda, T.; Asakawa, M.; Goto, M.; Nagawa, Y.; Shimizu, T. Eur. J. Org. Chem. 2003, 3744.

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Figure 1. Molecular structure of alkyl-substituted phthalocyanine derivative (CnOPc).

substituted phthalocyanine (C8OPc) formed 3-fold and 4-fold symmetrical structures on graphite surfaces.25a Therefore, we considered that the CnOPc molecules would have the possibility to form various two-dimensional ordered structures on graphite surfaces by changing the length of the substituted alkyl chain. Experimental Section Materials. The CnOPc molecules (n ) 4, 6, 10, 12, 14, 16, 18) were prepared according to the means described in previous paper.35 Column chromatography was performed on a Wakogel C-400HG (Wako Pure Chemical Industries). 1H and 13C NMR spectra were recorded on a Bruker AVANCE 400 spectrometer (400 and 100 MHz for 1H and 13C NMR, respectively) using residual solvent as an internal standard. These materials produced satisfactory 1H NMR and MALDI-TOFMS data. Only, octaoctylocy-substituted phthalocyanine (C8OPc) was purchased from Aldrich and used without further purification. In addition, cobalt phthalocyanine (CoPc) was used to change and control the self-organized structure of CnOPc by mixing it with CnOPc. The molar ratio of CoPc and CnOPc was fixed at 1:1. STM Observations. Samples were prepared by dissolving the CnOPc molecules in phenyloctane at a concentration of less than 1 mM. Phenyloctane was purchased from Aldrich and used without further purification. A droplet of the solution was deposited onto a freshly cleaved highly oriented pyrolytic graphite (HOPG) surface. Then, the STM tip was immersed into the solution to image the molecular structure formed at the liquid-solid interface. All STM observations were performed at room temperature (Nanoscope IIIa multimode SPM) using commercially available Pt/Ir tips (80:20). The tunneling current (It) and the sample bias voltage (Vs) were set between 1 and 200 pA and between -0.3 and - 2.0 V, respectively. In our experiment, the unit cell data was estimated from wide-area STM images that we obtained using a fast scanning speed to minimize the effect of thermal drift on the STM images. We compared two consecutive STM images and estimated the drift rate. Then, we calibrated the STM images using that estimated drift rate.

Results and Discussion Figure 2 shows examples of the STM images of the CnOPc molecules. For C4OPc, only a 4-fold symmetrical structure was observed as shown in Figure 2a. This structure is considered to reflect the geometric structure of the molecules. On the other hand, we observed only a quasi-3-fold symmetrical structure in the C6OPc (Figure 2b). Only 0.2 nm longer alkyl chains induced a drastic change in the self-organized structure. Interestingly, a 4-fold symmetrical structure was observed again in addition to a quasi-3-fold symmetrical structure in the C8OPc (Figure 2c). Both the quasi-3-fold and 4-fold symmetrical structures were also observed in the C10OPc (Figure 2d). When the alkyl chain length exceeded C12, a 2-fold symmetrical structure appeared. A 3-fold symmetrical structure was dominant in the C12OPc (35) Foley, S.; Jones, G.; Liuzzi, R.; McGarvey, D. J.; Perry, M. H.; Truscott, T. G. J. Chem. Soc., Perkin Trans. 1997, 2, 1725.

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Figure 2. STM images of (a) C4OPc (sample bias (Vs) ) - 1.1 V, tunneling current (It) ) 113 pA), (b) C6OPc (Vs) - 1.2 V, It ) 60 pA), (c) C8OPc (Vs) - 1.2 V, It ) 25 pA), (d) C10OPc (Vs ) - 1.1 V, It ) 60 pA), (e) C12OPc (Vs ) - 0.8 V, It ) 2.5 pA), (f) C14OPc (Vs ) - 1.3 V, It ) 58 pA), (g) C16OPc (Vs ) - 1.2 V, It ) 65 pA), and (h) C18OPc (Vs ) - 1.3 V, It ) 50 pA). The scanning size of all STM images is 50 nm × 50 nm. Table 1. The Unit Cell Data for Self-Organized Structures of CnOPc 4-fold b

n 4

lattice (nm) [plane group]

internal angle (deg) b

1.9(0.1) × 1.9(0.1) [P4 mm]

92(2)

6 8 10

2.6(0.1) × 2.6(0.2) [P4mm] 2.9(0.2) × 2.8(0.2) [P4mm]

92(4) 93(3)

12 14 16 18

3.4(0.3) × 3.4(0.3) [P4mm]

2-folda

quasi-3-fold

93(5)

(nm)b

lattice [plane group]

internal angle (deg) b

2.5(0.3) × 2.4(0.2) [C2mm] 2.6(0.1) × 2.6(0.2) [C2mm] 3.3(0.2) × 2.9(0.2) [P2] 3.4(0.3) × 3.4(0.3) [P2] 3.5(0.2) × 2.4(0.2)c [P2]

117(5)

d

lattice [plane group]

internal angle (deg)b

3.4(0.3) × 2.0(0.1) [P2mm] 3.7(0.7) × 2.0(0.1) [P2mm] 3.9(0.5) × 2.1(0.3) [P2mm]

94(3)

106(2) 106(4) 117(8) 109(8)c d

4.2(0.4) × 2.2(0.3)c [P2]

(nm)b

108(8)c

94(6) 93(3)

a Rectangular lattice. b Standard deviation is in parentheses. c Oblique lattice. d The observed area of oblique lattice is too small to estimate the unit cell data.

(Figure 2e). The C12OPc also formed 4-fold and 2-fold symmetrical structures, but they were in the minority. For the C14OPc, both 2-fold and 4-fold symmetrical structures were concurrently observed (Figure 2f). However, the C16OPc (Figure 2g) and C18OPc (Figure 2h) dominantly formed 2-fold symmetrical structures. In addition, two different 2-fold symmetrical structures were observed for C14OPC, C16OPc, and C18OPc. Rectangular lattice was dominant and oblique lattice was in the minority. Some of the structures, such as C10OPc, C14OPc, and C16OPc, have several defects. In our experience, it was difficult to obtain fine STM images of C10OPc, C14OPc, and C16OPc. We feel that the difference in the stability of the self-organized structures of CnOPc was attributable to the difference in the interaction between the molecules and substrate. We will discuss about the details of the interaction between the substrate and alkyl chains later. C10OPc was just in transition from 4-fold symmetry to 3-fold

symmetry. On the other hand, C14OPc and C16OPc were just in transition from 3-fold to 2-fold symmetry. However, we have confirmed the repeatability of the STM images. Our results indicated that C10OPc formed 3-fold and 4-fold symmetric structures, C14OPc formed 4-fold and 2-fold symmetric structures, and C16OPc preferentially formed a 2-fold symmetric structure. For this reason, we presented our discussion about the symmetries of self-organized structures, and we showed typical STM images of self-organized structures of CnOPcs. Consequently, we concluded that when the alkyl chain became longer the symmetry was reduced and the intermolecular distance became larger. The unit cell data for each structure are summarized in Table 1. Standard deviation of the mean is in parentheses in Table 1. We next discuss the origin of a variety of self-organized structures. Panels a and b of Figure 3show the two possible arrangements of alkyl-substituted phthalocyanines. In this model, we assumed that the alkyl chains adopted a fully extended all-trans

Self-Organized Alkyl-Substituted Phthalocyanines

Figure 3. CPK models of C8OPc for (a) 4-fold symmetrical and (b) quasi-3-fold symmetrical structures. The CPK models were optimized by molecular mechanics calculation (MM2 method). Schematic models of CnOPc for (a) 4-fold symmetrical and (b) quasi3-fold symmetrical structures.

conformation and that the CnOPc molecules formed the densely packed structures. The CPK models shown in panels a and b of Figure 3 were optimized by molecular mechanics calculation (MM2 method). In this calculation, the optimized structures strongly depend on the initial structures. Therefore, the structure we showed may not be a global minimum energy structure but a local minimum energy structure. However, in an actual system, the surface exists under the molecules, and we considered that the effect of the surface on the stability of the molecular structure was large in our system. The initial structures were decided based upon our STM results. Therefore, we proposed the simplified arrangement model as shown in Figure 3c,d. On the basis of Figure 3c, the size of a molecule was roughly estimated as

∼1.3 (size of phthalocyanine) + 0.125n (length of alkyl chain) × 2 We estimated the intermolecular distance (d), internal angle (β), and the area of unit cell from the molecular arrangement model shown in Figure 3c,d. In this model, we assumed that the separation between the edge of the alkyl chain and the neighboring phthalocyanine to be 0.3 nm. When the molecules were arranged as shown in Figure 3a,c, the molecules formed a 4-fold symmetrical structure, i.e., β ) 90°. On the other hand, the molecules formed a quasi-3-fold symmetrical structure when they were arranged as shown in Figure 3b,d. In this case, β becomes larger than 90°. Figure 4 shows the alkyl chain length dependence of (a) intermolecular distance (d), (b) internal angle (β), and (c) area of unit cell. The areas of the unit cells were defined as “|a| |b| sin β”, where a and b are the unit vectors. The gray squares, solid triangles, and diamonds correspond to the experimental results of the 4-fold, 3-fold, and 2-fold symmetrical structures, respectively. In 2-fold symmetric structures, the intermolecular distance along the molecular lines is about 2 nm. However, the distance

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Figure 4. Alkyl chain length dependence of (a) the intermolecular distance (d), (b) internal angle (β), and (c) area of the unit cell of the self-organized structures. Gray squares and solid triangles correspond to the experimental results of 4-fold and 3-fold symmetrical structures, respectively. Solid diamonds and gray circles correspond to rectangular and oblique lattice of 2-fold symmetrical structures, respectively. Solid and dashed lines correspond to the estimated values from the arrangement models of 4-fold (Figure 3a-c) and quasi-3-fold (Figure 3b,c) symmetries, respectively.

between the molecular lines varied with the alkyl chain length. Therefore, the unit cell sizes for 2-fold symmetric structures of C14-C18 are much smaller than those for 3-fold and 4-fold symmetric structures. We will discuss about the details of the 2-fold symmetric structure later. The solid and dashed lines correspond to the estimated values from the arrangement models of 4-fold (Figure 3c) and quasi-3-fold (Figure 3d) symmetries, respectively. At first, we consider CnOPc with n ) 4-10. The intermolecular distance and internal angle measured by the STM images are well compatible with our roughly estimated values. This indicated that the alkyl chains were interdigitated. In addition, the CnOPc formed the densely packed structure shown in Figure 3a,c for 4-fold and Figure 3b,d for quasi-3-fold symmetrical structures. The error values of the unit cell area for all arrangements were not very large compared to those of the intermolecular distance and the internal angles. This indicates that the occupied area per molecule did not change even when the distance between the neighboring molecules had fluctuated. Next, we consider the alkyl chain length on the CnOPc with n ) 4-10 dependence of the self-organized structures. The area of the unit cell of the quasi-3-fold symmetrical structure (A3-fold) was smaller than that of the 4-fold symmetrical structure (A4-fold). This result indicates that the alkyl-substituted phthalocyanines tend to form the quasi-3-fold symmetrical structure because the molecular density of the quasi-3-fold symmetrical structure is higher than that of a 4-fold structure. However, when the molecules form a quasi-3-fold symmetrical structure, the intermolecular distance of the alkyl chains surrounding the cavity, which is indicated by the bold line in Figure 3d, is so short that the molecules may lose the intermolecular interaction between neighboring alkyl chains. Therefore, the self-organized structures of the CnOPcs are dependent on the competition between the energy gain by the formation of the densely packed structure and the loss of the intermolecular interaction by steric hindrance between the neighboring alkyl chains. In the C4OPc, the loss of the intermolecular interaction by forming the densely packed structure

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model.36 In this model, each molecule contains n spherical monomers, which correspond to a CH2 unit. To simplify the model, we assumed that a molecule was described as a rigid and periodic arrangement of n spherical monomers including the ends (CH3). To estimate the interaction, we used 12-6 L-J potential.

U(r) )  Figure 5. Small scale STM image of (a) C4OPc (Vs ) - 1.1 V, It ) 113 pA, 10 nm × 10 nm) and (b) C6OPc monolayer (Vs ) - 1.2 V, It ) 58 pA, 10 nm × 10 nm). The CPK models of each structure were superimposed on each STM image.

may be large compared to the energy gain. As a result, the C4OPc formed a 4-fold symmetrical structure. On the other hand, we considered the energy gain to overcome the loss of the intermolecular interaction in the case of the C6OPc. Therefore, the C6OPc formed a 3-fold symmetrical structure. Figure 5 shows small scale STM images of (a) C4OPc and (b) C6OPc. The arrangement models of 4-fold and quasi-3-fold symmetrical structures were superimposed on STM images of C4OPc and C6OPc, respectively. The arrangement models matched well with the STM images. In the C4OPc and C6OPc, the intermolecular distance between Pcs was too short to observe the arrangement of the alkyl chain. In these images, however, we confirmed the intramolecular structure of the phthalocyanine molecules. Therefore, the arrangement models could easily fit the STM images. The CPK models optimized by molecular mechanics calculation of 4-fold symmetrical structure of C4OPc and quasi3-fold symmetrical structure of C6OPc were superimposed on the STM images of C4OPc and C6OPc, respectively. The arrangement models matched well with the STM images. These results indicate that the arrangement of CnOPc with n ) 4-10 can be explained by the proposed model in Figure 3. In the C8OPc and C10OPc, the loss of the intermolecular interaction by forming the densely packed structure may be comparable to the energy gain, and the C8OPc and C10OPc formed both quasi-3fold and 4-fold symmetrical structures. In the C12OPc, the internal angle measured by the STM images disagreed with our estimated values as shown in Figure 4b. Furthermore, when the alkyl chain length exceeded C12, a 2-fold symmetrical structure appeared. These results indicate that the arrangement structure will be different from the model shown in Figure 3 for the CnOPc with n ) 12-18. Furthermore, when the CnOPcs formed a 2-fold symmetrical structure, the molecular density was drastically increased. The cavities enclosed by the unit cells became larger with the increment of the length of the substituted alkyl chains, if the CnOPcs had the 4-fold symmetrical structure shown in Figure 3a,c. Even when the quasi-3-fold symmetrical structure shown in Figure 3b,d was formed, the decrement of the area of the unit cells was not so large. STM results regarding unsubstituted and substituted alkane molecules adsorbed on graphite surfaces have shown that they form a wellordered array with a characteristic lamellar structure of 2-fold symmetry.22-25 These results led us to conclude that to reduce the size of the cavities and to gain the intermolecular interaction energy of the alkane molecules as much as possible the molecules tend to form densely packed structures by reducing the symmetry. To reinforce these arguments, we roughly estimated the intermolecular interaction of the alkane molecules. We assumed that the alkane-alkane interactions were well-approximated by Lennard-Jones (L-J) interactions. Previous studies have shown that the alkane molecule was described with a bead-spring

{(σr ) - (σr ) } 12

6

(1)

where r is the distance between molecules and/or that between molecules and the substrate atom. The parameters  and σ are characteristic energy and length, respectively. Values that would be representative of hydrocarbons are  ∼ 9.7 × 10-22 J and σ ∼ 0.39 nm.37 Summing up the potential for all CH2 units in a molecule, we obtained the potential energy of a molecule. The intermolecular interaction energy of decane (C10) molecules (about 4.2 × 10-20 J) became comparable with the energy barrier for the torsional rotation of dimethyl ether of about 2100 cm-1 (∼4.2 × 10-20 J).38 This result indicates that the molecules had the possibility to form densely packed structures even though the CnOPc molecules were distorted in the CnOPc with n ) 12-18. Consequently, phthalocyanines with longer alkyl chains can form 3-fold and 2-fold symmetrical structures. Figure 6 shows (a) a possible schematic model, (b) the CPK model, and (c) small scale STM image of the 3-fold symmetrical structure of C12OPc. The CPK model was optimized by molecular mechanics calculation. The schematic model matched well with the CPK model optimized by molecular mechanics calculation. Furthermore, the arrangement of the alkyl chains was observed in the small scale STM image, as shown in Figure 6c. The CPK model of 3-fold symmetrical structure was superimposed on STM image. The arrangement models matched well with the STM image. In addition, as indicated by the arrows in Figure 6c, the arrangement of the alkyl chain agreed with the model shown in Figure 6a,b. Figure 7 shows (a) monoclinic and (b) orthorhombic cells of 2-fold symmetrical structures formed by CnOPc with n > 12. In addition, the CPK model of monoclinic cells of 2-fold symmetric structure and a small scale STM image of C14OPc were shown in panels c and d of Figure 7, respectively. The models shown in panels a and b of Figure 7 are not consistent with the CPK model in Figure 7c but instead is consistent with the STM image shown in Figure 7d. We believe that alkyl chains are arranged parallel to one another and are interdigitated with each other similar to the models shown in Figure 7a,b. The CPK model of the 2-fold symmetrical structure does not agree with our STM results. The yellow ellipses in Figure 7a indicate C-O-C bonds outside the Pc plane. Therefore, some alkyl chains may have been desorbed from the surface, while others may have been interdigitated with each other on the surface. We would not be surprised if the alkyl chains have been desorbed from the surface because the molecules had the possibility to form densely packed structures even though the CnOPc molecules were distorted in the CnOPc with n ) 12-18 on the basis of our interaction energy estimation mentioned above. The CPK models were optimized by the MM2 method. The structure we showed may not have been a global minimum energy structure but instead a local minimum energy structure. It was not possible to reproduce the model structures shown in Figure 7a,b by this calculation. As a result, the total energy gain of a system is likely to become large when alkyl chains tended to form densely packed structures, (36) Kremer, K.; Grest, G. S. J. Chem. Phys. 1990, 92, 5057. (37) Hentschke, R.; Schu¨rmann, B. L.; Rabe, J. P. J. Chem. Phys. 1992, 96, 6213. (38) Goodman, L.; Pophristic, V. Chem. Phys. Lett. 1996, 259, 287.

Self-Organized Alkyl-Substituted Phthalocyanines

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Figure 6. (a) Possible schematic model, (b) CPK model optimized by molecular mechanics calculation, (c) small scale STM image (Vs ) - 0.4 V, It ) 3 pA, 22 nm × 22 nm) of 3-fold symmetrical structure formed by C12OPc. CPK model was superimposed on the STM image.

Figure 8. Small scale STM image of insertion of CoPc into selforganized C12OPc template (Vs ) - 0.4 V, It ) 3 pA, 25 nm × 25 nm). The unit cell with CoPc is indicated by a square in the figure.

Figure 7. (a) CPK model of one possible arrangement of 2-fold symmetric structure optimized by molecular mechanics calculation. (b) Small scale STM image of C14OPc monolayer (Vs ) - 1.3 V, It ) 58 pA, 20 nm × 20 nm).

even though the C-O-C bonds were stressed and some alkyl chains were partly desorbed from the surface. STM could not detect the partly desorbed alkyl chains. Those alkyl chains might have been swept away by the STM tip during the scan. As a result, it is likely that 2-fold symmetrical structures are destabilized by desorbed alkyl chains and STM scans. The partly observed alkyl chains are possible evidence that some alkyl chains are partly desorbed from the surface in 2-fold symmetrical structures (arrows in Figure 7b). The relatively large errors in the

intermolecular distances of the 2-fold symmetrical structures are also possible evidence that the C-O-C bonds were stressed and some alkyl chains were partly desorbed from the surface. Therefore, we proposed the models of the 2-fold symmetrical structures that we have shown in Figure 7a,b, which were consistent with the STM images. However, further studies are required to clarify the molecular arrangements of these 2-fold symmetrical structures. Next, we attempted to control the self-organized structure of CnOPc. If small molecules, such as porphyrins or phthalocyanines, coexist, they may adsorb on the cavities and stabilize the 4-fold symmetrical structure even when the alkyl chain is long. Therefore, to control the symmetry and the intermolecular distance of the self-organized structure, we attempted to adsorb metallophthalocyanines on the cavities. We chose C12OPc as a matrix, since C12OPc did not form a 4-fold symmetrical structure but rather a 3-fold symmetrical structure. CoPc was used as an insert molecule, since it is well-known that the CoPc is observed brighter than the free-base phthalocyanine.39 Figure 8 shows a STM image of the insertion of CoPc into a self-organized C12OPc template. The unit cell with the CoPc is indicated by the square in the Figure 8. The C12OPc formed a 3-fold symmetrical structure as shown in Figure 6. However, the area of the 4-fold symmetrical structure was expanded by the adsorption of the CoPc, as shown in Figure 8. Bright spots were observed in the center of the unit cell of the 4-fold symmetrical structure. Figure 9 shows the schematic models of CoPc adsorbed on the cavity of (a) 4-fold and (b) 3-fold symmetrical structures of C12OPc. Dark gray square corresponds to a CoPc molecule. Panels c and d of Figure 9 show the CPK models of CoPc adsorbed on the cavity of 4-fold and 3-fold symmetrical structures of C12OPc, respectively. Panels b and d of Figure 9 obviously indicate that the size of the cavity (39) Scudiero, L.; Barlow, D. E.; Hipps, K. W. J. Phys. Chem. 2000, 104, 11899.

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symmetrical structure just fits the size of a CoPc molecule as shown in Figure 9a,c. This suggests that the CoPc adsorbed on the cavity and stabilized the 4-fold symmetrical structure. Therefore, the symmetry and intermolecular distance could be controlled by changing the length of the alkyl chain and by using a mixed system of alkyl-substituted phthalocyanine and metallophthalocyanine.

Conclusions

Figure 9. Schematic models of CoPc adsorbed on the cavity of (a) 4-fold and (b) 3-fold symmetrical structures of C12OPc. Dark gray square corresponds to a CoPc molecule. The CPK models optimized by molecular mechanics calculation of (c) 4-fold and (d) 3-fold symmetrical structures of C12OPc.

of 3-fold symmetrical structure is too small to adsorb a CoPc molecule. On the other hand, the size of the cavity of 4-fold

We examined the alkyl chain length on CnOPcs dependence of their self-organized structures. The arrangement of the CnOPcs changed depending on the alkyl chain length. The molecules preferred to form densely packed structures. When C12OPc and CoPc were coadsorbed, the mixed system exhibited a 4-fold symmetrical structure, which was not observed with C12OPc. These results led us to conclude that the symmetry and intermolecular distance could be controlled by changing the alkyl chain length and by using a mixed system of alkyl-substituted phthalocyanines and mettalophthalocyanines. This technique pursues the sophistication of the self-assembling method and will advance the development of molecular devices that use functionalized molecules as scaffolds. Acknowledgment. The authors are grateful to Ms. M. Sasou for help with STM measurements. LA702564M