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Langmuir 2006, 22, 6910-6914
Two-Dimensional Structure Control by Molecular Width Variation with Metal Coordination Yoshihiro Kikkawa,*,† Emiko Koyama,†,‡ Seiji Tsuzuki,‡,§ Kyoko Fujiwara,† Koji Miyake,| Hideo Tokuhisa,†,‡ and Masatoshi Kanesato†,‡ Nanoarchitectonics Research Center, Synthetic Nano-Function Materials Project (SYNAF), Research Institute for Computational Sciences, and AdVanced Manufacturing Research Institute, National Institute of AdVanced Industrial Science and Technology (AIST), 1-1-1 Higashi, Tsukuba, Ibaraki 305-8562, Japan ReceiVed March 7, 2006. In Final Form: May 11, 2006 The self-assembled monolayer of bipyridine derivative 1, which has two alkyl chains on each end, at the HOPG/ 1-phenyloctane interface was studied by in situ scanning tunneling microscopy (STM). The detailed mechanism of a spontaneous change in the monolayer packing pattern by Pd coordination was studied. Uncomplexed 1 existed in a bent form in the monolayer, and the alkyl chains were interdigitated, whereas Pd-complexed 1 was in a straight form and the alkyl chains were not interdigitated. An intermediate state of 1 was successfully observed during metal coordination. The structure was the bent form with noninterdigitated alkyl chains. Equilibrium intermolecular distances reported from ab initio calculations indicate that the molecular width of the central aromatic part of uncomplexed 1 (7.5 Å) is substantially smaller than that of the peripheral alkyl chain part (9.2 Å). The bent form was suitable for covering up the surface to maximize the packing density. However, the molecular width of the aromatic unit of Pd-complexed 1 (9.1 Å) was almost identical to that of the alkyl chain unit (9.2 Å). Therefore, Pd-complexed 1 took the straight form in the monolayer. The observation of surface coverage by STM suggests that the bent form increases the packing density by as much as 16% compared with that of the straight form. These results indicate that the control of molecular width can be used to design molecular templates for nanostructure formation.
Introduction Controlling the lateral assembly and spatial arrangement of nanomaterials at interfaces is one of the fundamental issues in realizing the efficient production of future molecule-based devices such as sensors and circuits.1 As a bottom-up approach for nanopatterning on a surface, the spontaneous organization of molecules into a stable state, so-called self-assembly, has been utilized, and nanoscale templates can be generated under ambient conditions. Scanning tunneling microscopy (STM) is widely accepted as a powerful tool for studying surface features and surface states. STM has been used to visualize the 2D structures of monolayers, which are physisorbed on highly oriented pyrolytic graphite (HOPG). STM has revealed the details of intermolecular interactions and their role in the formation of 2D structures.2 Dynamic and in situ STM observations have recently attracted much attention to the study of the structural changes in monolayers at the HOPG/solvent interface in real time. There are some examples, such as the STM-tip-induced polymerization of diacetylene derivatives, the surface molecular diffusion process, and the coadsorption of solvent and molecules.3 These studies have provided important information for understanding the intermediate states and formation process of 2D monolayers in addition to the static STM studies. * Corresponding author. E-mail:
[email protected]. Phone: +8129-861-2955. Fax: +81-29-861-3029. † Nanoarchitectonics Research Center. ‡ SYNAF. § Research Institute for Computational Sciences. | Advanced Manufacturing Research Institute. (1) (a) Giancarlo, L. C.; Flynn, G. W. Acc. Chem. Res. 2000, 33, 491-501. (b) De Feyter, S.; De Schryver, F. C. Chem. Soc. ReV. 2003, 32, 139-150. (2) (a) Cyr, D. M.; Venkataraman, B.; Flynn, G. W. Chem. Mater. 1996, 8, 1600-1615. (b) Kim, K.; Matzger, A. J. J. Am. Chem. Soc. 2002, 124, 87728773. (c) Ikeda, T.; Asakawa, M.; Goto, M.; Miyake, K.; Ishida, T.; Shimizu, T. Langmuir 2004, 20, 5454-5459. (d) Scherer, L. J.; Merz, L.; Constable, E. C.; Housecroft, C. E.; Neuburger, M.; Hermann, B. A. J. Am. Chem. Soc. 2005, 127, 4033-4041. (e) De Feyter, S.; De Schryver, F. C. J. Phys. Chem. B 2005, 109, 4290-4302 and references therein.
Metal complexes of pyridine derivatives have been used for the formation of ordered nanostructures on a solid/liquid interface.4 A variety of 2D structures have been demonstrated depending on the structure of pyridine derivatives and coordinated metals. Recently, De Feyter et al. reported a spontaneous change in the monolayer packing pattern of a bipyridine derivative (4,4′dialkyl-2,2′-bipyridine) by Pd coordination on a graphite/liquid interface using STM in situ.5 They found that the alkyl chains of uncomplexed bipyridine derivatives has no interdigitated structure, whereas those of the Pd complex are interdigitated. The molecular orientation of the bipyridine unit did not largely change after Pd coordination. They concluded that the increased distance between the neighboring molecules due to Pd coordination causes the alkyl chains to interdigitate. These results imply that the molecular width is the key factor in determining the 2D nanostructures on the surface and that metal coordination is a powerful method for controlling the nanostructures. Several types of bipyridine derivatives have been used for the formation of self-assembled monolayers. The bipyridine deriva(3) (a) Okawa, Y.; Aono, M. Nature 2001, 409, 683-684. (b) Miura, A.; De Feyter, S.; Abdel-Mottaleb, M. M.; Gesquie`re, A.; Grim, P. C. M.; Moessner, G.; Sieffert, M.; Klapper, M.; Mu¨llen, K.; De Schryver, F. C. Langmuir 2003, 19, 6474-6482. (c) Sullivan, S. P.; Schnieders, A.; Mbugua, S. K.; Beebe, T. P., Jr. Langmuir, 2005, 21, 1322-1327. (d) Gong, J. R.; Lei, S. B.; Pan, G. B.; Wan, L. J.; Fan, Q. H.; Bai, C. L. Colloids Surf., A 2005, 257-258, 9-13. (e) Gesquie`re, A.; Abdel-Mottaleb, M. M.; De Feyter, S.; De Schryver, F. C.; Sieffert, M.; Mu¨llen, K.; Calderone, A.; Lazzaroni, R.; Bre´das, J, -L. Chem.sEur. J. 2000, 6, 3739-3746. (4) (a) Semenov, A.; Spatz, J. P.; Mo¨ller, M.; Lehn, J.-M.; Sell, B.; Schubert, C. H.; Weidl, C. H.; Schubert, U. S. Angew. Chem., Int. Ed. 1999, 38, 2547-2550. (b) Ziener, U.; Lehn, J.-M.; Mourran, A.; Mo¨ller, M. Chem.sEur. J. 2002, 8, 951-957. (c) Kurth, D. K.; Severin, N.; Rabe, J. P. Angew. Chem., Int. Ed. 2002, 41, 3681-3683. (d) Mourran, A.; Ziener, U.; Mo¨ller, M.; Breuning, E.; Ohkita, M.; Lehn, J.-M. Eur. J. Inorg. Chem. 2005, 2641-2647. (e) Zell, P.; Mo¨gele, F.; Ziener, U.; Rieger, B. Chem. Commun. 2005, 1294-1296. (5) (a) Abdel-Mottaleb, M. M. S.; Schuurmans, N.; De Feyter, S.; Van Esch, J.; Feringa, B. L.; De Schryver, F. C. Chem. Commun. 2002, 1894-1895. (b) De Feyter, S.; Abdel-Mottaleb, M. M. S.; Schuurmans, N.; Verkuijl, B. J. V.; Van Esch, J. H.; Feringa, B. L.; De Scryver, F. C. Chem.sEur. J. 2004, 10, 11241132.
10.1021/la0606244 CCC: $33.50 © 2006 American Chemical Society Published on Web 06/27/2006
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Scheme 1. Chemical Structure of Bipyridine Derivative 1
tive by De Feyter et al.5 has a single alkyl chain on each end of an aromatic moiety. They have explained the packing pattern change for their pyridine derivative by metal coordination. However, the relationship between the molecular structures and monolayer packing patterns of other bipyridine derivatives was not clearly understood. Different bipyridine derivatives may have different mechanisms of packing pattern changes induced by metal coordination. A detailed understanding of the relationship and roles of metal coordination is essential for designing selfassembled nanostructures. In this study, we report the 2D nanostructure formation based on bipyridine derivative 1 (Scheme 1), which has two alkyl chains at each end of an aromatic moiety. The additional alkyl chains would increase the stability of the physisorbed bipyridine derivative. The self-assembled monolayer packing patterns of 1 and the metal-bipyridine complex are studied by STM at a solid/liquid interface. A substantial geometrical change in 1 (bent to straight form) was observed and was associated with the 2D nanostructure change due to Pd coordination. In addition, the packing pattern of an intermediate state during Pd coordination was directly observed. We have discussed the relationship between the molecular structures (uncomplexed and complexed 1) and their packing patterns on the basis of the equilibrium intermolecular distances from ab initio calculations and a quantitative evaluation of the surface coverage. Experimental Section Syntheses. General Procedure. 5,5′-Dibromo-2,2′-bipyridine was synthesized according to the reported procedure.6 Benzyltriethylammonium dichloroiodate as the iodation reagent was obtained by the reported procedure.7 Catechol (first grade, Aldrich Co., Ltd.), 1-bromododecane (first grade, Aldrich Co., Ltd.), trimethylsilylacetylene (first grade, Tokyo Chemical Industry Co., Ltd.), PdCl2 (first grade, Aldrich Co., Ltd.), Pd(PPh3)4 (first grade, Tokyo Chemical Industry Co., Ltd.), PPh3 (first grade, Aldrich Co., Ltd.), CuI (first grade, Kanto Co., Ltd.), triethylamine (first grade, Kanto Co., Ltd.), and anhydrous tetrahydrofuran (THF) (first grade, Kanto Co., Ltd.) were used as received. NMR spectra were recorded on a 500 MHz Bruker Avance 500 spectrometer using tetramethylsilane (TMS) as an internal standard. FTIR spectra were obtained with a Jasco FT/IR-420. 5,5′-Diethynyl-2,2′-bipyridine. To a mixture of 5,5′-dibromo-2,2′bipyridine (12.0 g, 38.2 mmol), PdCl2 (0.41 g, 2.3 mmol), PPh3 (1.20 g, 4.6 mmol), and CuI (0.44 g, 2.3 mmol) in three-necked round-bottomed flask were introduced into dry THF (600 mL) and triethylamine (300 mL). The flask was cooled, degassed, charged with N2 gas, and subsequently heated at 70 °C. Trimethylsilylacetylene (9.0 g, 91.6 mmol) was added to the solution, which was kept at 70 °C for 3 days. After the reaction, the solution was filtered through silica gel to remove insoluble inorganic salts, and the solvent was evaporated from the reaction mixture. The residue was dissolved in chloroform and washed with water. The organic phase was concentrated and dried in vacuo. The obtained crude mixture was dissolved in THF (300 mL) and methanol (200 mL). KF (5.2 g, 89.7 mmol) was added to the solution, which was stirred under N2 at ambient temperature overnight. After the deprotection, the solution was filtered through silica gel, and the solvent was evaporated from the reaction mixture. The residue was dissolved in chloroform and (6) Oae, S.; Kawai, T.; Furukawa, N. Phosphorus Sulfur Relat. Elem. 1987, 34, 123-132. (7) Kosynkin, D. V.; Tour, J. M. Org. Lett. 2001, 3, 991-992.
Figure 1. STM images of bipyridine derivative 1 physisorbed at the solid/liquid interface. (a) I ) 10 pA, V ) -200 mV; (b) I ) 6.7 pA, V ) -242 mV; (c) I ) 80 pA, V ) -261 mV, L1 ) 2.28 ( 0.11 nm, L2 ) 1.42 ( 0.09 nm; (d) tentative molecular model of monolayer estimated from the STM images. washed with water. The organic phase was concentrated and dried in vacuo. The obtained residue was purified by gel permeation chromatography (GPC) (eluent: chloroform) to afford a yellow solid in 69% yield. 1H NMR (CDCl3): δ 3.31 (s, 2H), 7.90 (dd, J1 ) 8.2 Hz, J2 ) 2.2 Hz, 2H), 8.40 (dd, J1 ) 8.2 Hz, J2 ) 0.8 Hz, 2H), 8.77 (dd, J1 ) 2.2 Hz, J2 ) 0.8 Hz, 2H). IR (KBr): 3268, 2104, 1586, 1532, 1465, 1364, 1234, 1026 cm-1. 1,2-Didodecyloxy-4-iodobenzene. To a mixture of catechol (0.92 g, 8.36 mmol) and KOH (1.13 g, 20.1 mmol) in dry EtOH solution was added 1-bromododecane (5.0 g, 20.1 mmol), and the solution was heated under N2 at 70 °C overnight. The obtained mixture was evaporated and subsequently washed with water, and then the organic phase was removed. The crude product was purified by silica gel column chromatography (eluent: hexane to 9:1 chloroform/hexane) to afford a colorless solid in 50% yield. The obtained 1,2didodecyloxy benzene (1.85 g, 4.4 mmol) was dissolved in acetic acid (10 mL). To the solution was added benzyltriethylammonium dichloroiodate as an iodation reagent (1.62 g, 4.4 mmol) and ZnCl2 (0.85 g, 6.6 mmol), and it was stirred at ambient temperature overnight under air. After the reaction, a 1 N NaOH solution and then ether were introduced into the solution. After washing, the organic phase was removed. Further purification was not carried out. The target iodated compound was obtained in 91% yield as a colorless solid.1H NMR (CDCl3): δ 0.88 (t, J ) 6.8 Hz, 6H), 1.23-1.31 (br s, 32H), 1.40-1.48 (m, 4H), 1.75-1.83 (m, 4H), 3.94 (t, J ) 6.6 Hz, 4H), 6.61 (d, J ) 8.5 Hz, 1H), 7.12 (d, J ) 2.1 Hz, 1H), 7.17 (dd, J1 ) 8.4 Hz, J2 ) 2.0 Hz, 1H). IR (KBr): 2920, 2850, 1582, 1504, 1466, 1394, 1254, 1216, 1135 cm-1. Compound 1. To a mixture of 5,5′-diethynyl-2,2′-bipyridine (0.24 g, 1.19 mmol), 1,2-didodecyloxy-4-iodobenzene (1.50 g, 2.62 mmol), Pd(PPh3)4 (82.5 mg, 0.07 mmol), and CuI (13.6 mg, 0.07 mmol) in three-necked round-bottomed flask were introduced dry THF (100 mL) and triethylamine (50 mL). The flask was cooled, degassed, charged with N2 gas, and subsequently heated at 60 °C for 4 days. After the reaction, the solution was filtered through silica gel to remove insoluble inorganic salts, and the solvent was evaporated from the reaction mixture. The residue was dissolved in chloroform and washed with water. The organic phase was concentrated and dried in vacuo. The obtained residue was purified by GPC (eluent: chloroform) to afford a yellow solid in 45% yield. ESI-MS: m/z 1094.1 [M + H+]. 1H NMR (CDCl3): δ 0.88 (t, J ) 7.0 Hz, 12H),
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Table 1. Unit Cell Parameters of Bipyridine Derivative 1 before and after Metal Coordination as Well as the Intermediate State and Intermolecular Distance (M) Measured from STM Imagesa
1 intermediate state complexed 1 a
a (nm)
b (nm)
γ (deg)
M (nm)
occupancy (nm2/molecule)
1.50 ( 0.07 1.34 ( 0.17 0.99 ( 0.04
2.96 ( 0.09 4.52 ( 0.13 5.18 ( 0.11
95.5 ( 1.7 87.6 ( 1.6 92.8 ( 1.0
0.80 ( 0.05 0.87 ( 0.08 0.99 ( 0.04
4.42 ( 0.12 6.05 ( 0.15 5.12 ( 0.09
The occupancy of the molecule was calculated from unit cell parameters.
Figure 2. STM images of bipyridine derivative 1 physisorbed at the solid/liquid interface after the addition of Pd(CH3CN)2Cl2 solution. Domain A is the uncomplexed region, whereas domain B shows complexed structure. (a) I ) 7.4 pA, V) -621 mV; (b) I ) 7.4 pA, V ) -621 mV; (d) I ) 7.8 pA, V ) -617 mV, L3 ) 4.19 ( 0.10 nm, L4 ) 2.30 ( 0.12 nm, L5 ) 5.12 ( 0.21 nm. (c and e) Tentative molecular models of the monolayer estimated from the STM images. (c) Intermediate state (domain B). (e) After Pd complexation. 1.22-1.40 (br s, 64H), 1.42-1.50 (m, 8H), 1.80-1.88 (m, 8H), 4.03 (t, J ) 6.6 Hz, 8H), 6.85 (d, J ) 8.4 Hz, 2H), 7.07 (d, J ) 1.7 Hz, 2H), 7.14 (dd, J1 ) 8.3 Hz, J2 ) 1.7 Hz, 2H), 7.92 (d, J ) 8.1 Hz, 2H), 8.41 (d, J ) 6.6 Hz, 2H), 8.79 (s, 2H). IR (KBr): 2920, 2851, 2208, 1513, 1467, 1254, 1220, 1125 cm-1. Anal. Calcd for C74H112N2O4: C, 81.27; H, 10.32; N, 2.56. Found: C, 80.95; H, 10.31; N, 2.53. STM Observation. A self-assembled monolayer of bipyridine derivative 1 was observed by low-current STM (SII Nanotechnology Inc., Japan: SPI4000/SPA400 and Veeco Instruments, Santa Barbara, CA: Nanoscope IIIa multimode SPM). The STM tip was prepared by mechanically cutting Pt/Ir wire (80:20). Bipyridine derivative 1 was first dissolved in CH2Cl2, followed by dispersion in 1-phenyloctane (Kanto) with a final concentration below 0.1 mM. The solution was dropped on freshly cleaved HOPG of ZYB grade (NT-MDT, Russia), and the CH2Cl2 was allowed to evaporate before STM imaging. In the case of the complexation experiment, a 1-phenyloctane solution of Pd(CH3CN)2Cl2 (below 0.1 mM) was prepared, and excess solution was added to the initial drop of bipyridine derivative 1. Then, STM observation was performed at the HOPG/1-phenyloctane interface. Samples were negatively biased. All STM images were obtained in constant current mode and are shown without any filtering. Experimental conditions are given in each Figure caption. To check the reproducibility of the STM image, different tips, samples, and two pieces of STM equipment were used. The STM images in Figures 1a and b and 2 were obtained by SII STM, whereas that of Figure 1c was obtained using a Veeco STM.
Results and Discussion Self-assembled nanostructures of bipyridine derivative 1 formed at the HOPG/1-phenyloctane interface were observed by STM. Parts a and b of Figure 1 show the STM images of 1
physisorbed onto HOPG. Two-dimensional patterns of 1 were spontaneously formed at the interface throughout the substrate. To reveal the molecular arrangement, high-resolution STM imaging was performed. Figure 1c shows the high-resolution STM image of 1, which allowed us to identify the alkyl chains as well as the π-conjugated aromatic unit (bipyridine moieties with two additional aromatic rings). The unit cell parameters are a ) 1.50 ( 0.07 nm, b ) 2.96 ( 0.09 nm, and γ ) 95.5 ( 1.7°. All of the unit cell parameters observed in our system are listed in Table 1. L1 (2.28 ( 0.11 nm) in the STM image corresponds to the length of an aromatic unit, whereas L2 (1.42 ( 0.09 nm) corresponds to the alkyl chain lengths (Figure 3). The STM images in Figure 1b and c show that the alkyl chains are interdigitated and form an angle of 118.4 ( 3.1° with respect to the aromatic unit. Figure 1d shows the plausible packing pattern model of 1 in the self-assembled monolayer observed in Figure 1b and c. After the formation of the self-assembled monolayer of 1, a 1-phenyloctane solution of Pd(CH3CN)2Cl2 was added to the initial drop of HOPG. Then, the complexation process was observed by STM in situ and in real time. An intermediate-state packing pattern (Figure 2a and b) was observed during the conversion from uncomplexed to Pd-complexed 1. Domain A is the monolayer of uncomplexed 1, whereas domain B is the newly formed packing pattern due to Pd complexation. Lowcurrent STM observation enabled us to analyze the molecular arrangement in domain B, as shown in the upper right corner of Figure 2b. The parameters of the unit cell (domain B) are a ) 1.34 ( 0.17 nm, b ) 4.52 ( 0.13 nm, and γ ) 87.6 ( 1.6°. The tentative molecular model of the intermediate state is shown in
2D Structure Control by Molecular Width Variation
Figure 2c. Newly formed column length L3 (4.19 ( 0.10 nm) is shorter than that of the calculated full length of 1 (5.13 nm). In addition, dark troughs, which are characteristic of terminal methyl groups, are recognized between the neighboring columns. These results suggest that 1 in domain B is in the bent form with noninterdigitated alkyl chains. After several minutes, another new packing pattern was observed throughout the monolayer (Figure 2d). Again, the packing pattern was submolecularly resolved. The dimensions of the unit cell are a ) 0.99 ( 0.04 nm, b ) 5.18 ( 0.11 nm, and γ ) 92.8 ( 1.0°. L4 (2.30 ( 0.12 nm) is identical to the length of the aromatic unit. L5 (5.12 ( 0.21 nm) is equivalent to the calculated full length of 1, indicating that Pd-complexed 1 has a straight form in the monolayer. In this case, parallel lamellar structures were defined by dark troughs, suggesting that there is no interdigitated structure of alkyl chains. A tentative 2D structural model of Pd-complexed 1 is shown in Figure 2e. Thus, we have successfully observed the conversion process of 1 due to metal coordination by using in situ STM. Here will discuss the origin of 2D structures before and after metal coordination as well as the intermediate state. Before metal coordination, 1 had a bent form with interdigitated alkyl chains (Figure 1). Basically, molecules physisorbed on the surface tend to cover the whole surface region to maximize the molecular packing density. Uncomplexed 1 had a bent form in the self-assembled monolayer, as shown in Figures 1c and 3a, suggesting that the bent form is advantageous to increasing the surface coverage. The occupancy of physisorbed uncomplexed 1 (bent form) was 4.42 ( 0.12 nm2/molecule (Figure 1d), whereas that of the Pd complex (straight form) was 5.12 ( 0.09 nm2/ molecule (Figure 2c). This result indicates that the bent form increases the molecular packing density by as much as 16% compared with the straight form if we assume that the surface coverage of straight uncomplexed 1 is the same as that of straight Pd-complexed 1. In addition to the surface coverage, the molecular geometry seems to affect the packing pattern of 1 in the self-assembled monolayer. Figure 3a shows the plausible geometry of 1 estimated from reported ab initio calculations.8,9 Recent high-level ab initio calculations show that the equilibrium distance between two n-alkane chains is 4.6 Å,8 which indicates that the molecular width of the alkyl chain unit of 1 is about 9.2 Å () 4.6 Å × 2). The calculated equilibrium intermolecular distance of the naphthalene dimer9 implies that the molecular width occupied by the aromatic unit of 1 is about 7.5 Å. These facts indicate that the molecular width of the aromatic unit is substantially smaller than that of the alkyl chain unit (Figure 3a). If uncomplexed 1 with a straight form covers up the surface, then a substantial void remains because of the difference in the molecular widths in compound 1. Therefore, the bent form was taken in the selfassembled monolayer of uncomplexed 1 to increase the surface coverage. Our in situ STM observation has demonstrated that the complexation of 1 with Pd drastically changed the self-assembled nanostructures formed at the HOPG/1-phenyloctane interface, which was different from the packing pattern change in the bipyridine derivative by metal coordination reported by De Feyter et al.5 Moreover, we have successfully captured the intermediate state (i.e., the interdigitated structure (domain A in Figure 2a) changed into the noninterdigitated structure (domain B in Figure (8) Tsuzuki, S.; Honda, K.; Uchimaru, T.; Mikami, M. J. Phys. Chem. A 2004, 108, 10311-10316. (9) Tsuzuki, S.; Honda, K.; Uchimaru, T.; Mikami, M. J. Chem. Phys. 2004, 120, 647-659.
Langmuir, Vol. 22, No. 16, 2006 6913
Figure 3. Intermolecular distance and molecular size of bare (a) and complexed (b) bipyridine derivative 1. The data are based on the MM2 and ab initio calculations reported previously8,9 as well as crystallographic parameters.10
2b)), whereas the bent form of 1 was maintained. The intermolecular distance of the aromatic unit for uncomplexed 1 was measured to be M1 ) 8.0 ( 0.5 Å, whereas that for the intermediate state of 1 was M2 ) 8.7 ( 0.8 Å, implying that Pd-complexed 1 was formed. Then, the bent form of intermediatestate 1 changed its conformation to be in a more thermodynamically stable state (straight form) because increased packing density can be achieved by the final state of Pd-complexed 1 (5.12 ( 0.09 nm2/molecule) in comparison with the intermediate state (6.05 ( 0.15 nm2/ molecule); that is, the final structure of Pdcomplexed 1 in Figure 2d has an 18% higher surface coverage than that of the intermediate state observed in Figure 2b in the right corner. After metal coordination, the straight form with noninterdigitated alkyl chains was observed in the monolayer (Figure 2d), and the plausible geometry of Pd-complexed 18-10 is shown in Figure 3b. The crystal structure of 2,2′-bipyridine dichloropalladium(II)10 suggests that the molecular width of the aromatic unit of Pd-complexed 1 is about 9.1 Å, which is substantially larger than that of uncomplexed 1 (7.5 Å). Actually, the STM images in Figures 1 and 2 revealed that the distance of the aromatic unit of Pd-complexed 1 from the neighboring one (M3 ) 9.9 ( 0.4 Å) was considerably larger than that of the uncomplexed one (M1 ) 8.0 ( 0.5 Å). Taking these crystallographic and STM data into account, the width of the aromatic part of the Pd complex is almost identical to that of the alkyl chain unit (Figure 3b), suggesting that it is more favorable for the straight form of Pdcomplexed 1 to cover the surface. These results clearly indicate that the change in the molecular width of the aromatic unit of 1 is the cause of the spontaneous change in the monolayer packing pattern by Pd coordination.
Conclusions A two-dimensional self-assembled monolayer of bipyridine derivative 1 was observed by STM at the solid/liquid interface. (10) Maekawa, M.; Munakata, M.; Kitakawa, S.; Nakamura, M. Anal. Sci. 1991, 7, 521-522.
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Uncomplexed 1 was in a bent form in the monolayer and alkyl chains were interdigitated, whereas Pd-complexed 1 was in a straight form in the monolayer and alkyl chains were not interdigitated. During the Pd coordination, our STM observation revealed the intermediate state, which had a straight form with noninterdigiated alkyl chains. Equilibrium intermolecular distances reported from ab initio calculations showed that the molecular width of the central aromatic unit of 1 was substantially smaller than that of the peripheral alkyl chain unit. The surface coverage observed by STM indicates that the bent form is suitable
Kikkawa et al.
to cover the surface because of the different molecular widths of the two units. However, the molecular width of the aromatic unit of Pd-complexed 1 was almost identical to that of the alkyl chain unit; therefore, Pd-complexed 1 had a straight form in the monolayer. Acknowledgment. This work was partly supported by NEDO under the Nanotechnology Materials Program. LA0606244
