pubs.acs.org/Langmuir © 2009 American Chemical Society
Bipyridine Derivatives at a Solid/Liquid Interface: Effects of the Number and Length of Peripheral Alkyl Chains Yoshihiro Kikkawa,*,† Emiko Koyama,*,† Seiji Tsuzuki,‡ Kyoko Fujiwara,† and Masatoshi Kanesato† †
Photonics Research Institute and ‡Research Institute for Computational Sciences, National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Higashi, Tsukuba, Ibaraki 305-8562, Japan Received August 24, 2009
Bipyridine derivatives (bpys) with various number and length of peripheral alkyl chains (with carbon numbers of n = 11-17) were synthesized, and their self-assembled monolayers were observed by scanning tunneling microscopy (STM) at a 1-phenyloctane/highly oriented pyrolytic graphite (HOPG) interface. The effects of the number, the substitution position, and the length of alkyl chains on the two-dimensional structures were systematically studied. Bpys substituted by a single alkyl chain in the p-position on each side adopted an almost linear form with zigzag-type alignment of the π-conjugated unit, whereas, in the case of m-substitutution, the bpys showed Z-shaped morphology with interdigitated alkyl chains. In both cases, no odd-even alkyl chain length effects were observed. The bpys with double alkyl chains at m- and p-positions displayed odd-even alkyl chain effects, suggesting that the formation of twodimensional structure is dominated by the interactions between alkyl chains. Bpys with triple alkyl chains at o-, m-, and p-positions also showed odd-even alkyl chain effects, but only for the higher number of carbon atoms in the alkyl chain unit (n = 14-17). These results indicate that concerted intermolecular interactions of the alkyl chain unit introduce the odd-even chain length effect on the self-assembled two-dimensional structure. After coordination of PdCl2, odd-even effects were quenched, and bpys were converged into the same lamellar structure, in which the molecules are almost linear. All the structural differences due to the odd-even alkyl chain length effect were explained in terms of intermolecular and molecule-substrate interactions.
1. Introduction Understanding of the intermolecular and molecule-substrate interaction is one of the prerequisite issues to realize the molecular scale devices on substrates.1 Self-assembly has been recognized as a powerful method to fabricate functional materials by using well-designed molecules as building blocks.2 Molecular assemblies on substrates to precisely integrate their functionalities are challenging toward the development of molecular nanosystems. Self-assembled monolayers have been obtained via either chemisorption3 or physisorption,4 and have been studied by scanning tunneling microscopy (STM). Especially, in the solid/ liquid system, molecular scale observation has been achieved. Self-assembled monolayers were formed via relatively weak *Corresponding author. (Y.K.) Phone: þ81-29-861-2955. Fax: þ81-29861-3029. E-mail:
[email protected]. (E.K.) Phone: þ81-29-861-2443. Fax: þ81-29-861-3029. E-mail:
[email protected].
(1) (a) Whitesides, G. M.; Grzybowski, B. Science 2002, 295, 2418–2421. (b) Theobald, J. A. (c) Barth, J. V.; Costantini, G.; Kern, K. Nature 2005, 437, 671– 679. (d) Nadal, E. G-.; Luis, J. P-.; Amabilino, D. B. Chem. Soc. Rev. 2008, 37, 490– 504. (2) (a) Giancarlo, L. C.; Flynn, G. W. Acc. Chem. Res. 2000, 33, 491–501. (b) Lehn, J.-M. Science 2002, 295, 2400–2403. (c) De Feyter, S.; De Schryver, F. C. Chem. Soc. Rev. 2003, 32, 139–150. (3) (a) Ulman, A. Chem. Rev. 1996, 96, 1533–1554. (b) Smith, R. K.; Lewis, P. A.; Weiss, P. S. Prog. Surf. Sci. 2004, 75, 1–68. (c) Madueno, R.; R€ais€anen, M. T.; Silien, C.; Buck, M. Nature 2008, 454, 618–621. (4) (a) Cyr, D. M.; Venkataraman, B.; Flynn, G. W. Chem. Mater. 1996, 8, 1600–1615. (b) De Feyter, S.; De Schryver, F. C. J. Phys. Chem. B. 2005, 109, 4290– 4302. (c) Plass, K. E.; Grzesiak, A. L.; Matzger, A. J. Acc. Chem. Res. 2007, 40, 287– 293. (5) For example, see (a) Otsuki, J.; Nagamine, E.; Kondo, T.; Iwasaki, K.; Asakawa, M.; Miyake, K. J. Am. Chem. Soc. 2005, 127, 10400–10405. (b) Tahara, K.; Furukawa, S.; Uji-I, H.; Uchino, T.; Ichikawa, T.; Zhang, J.; Mamodouh, W.; Sonoda, M.; De Schryver, F. C.; De Feyter, S.; Tobe, Y. J. Am. Chem. Soc. 2006, 128, 16613– 16625. (c) Pkrifchak, M.; Turner, T.; Pilgrim, I.; Johnston, M.; Hipps, K. W. J. Phys. Chem. C 2007, 111, 7735–7740. (d) Klymchenko, A. S.; Furukawa, S.; M€ullen, K.; Van der Auweraer, M.; De Feyter, S. Nano Lett. 2007, 7, 791–795.
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intermolecular interactions, such as van der Waals interactions,5 hydrogen bonds,6 metal coordination,7 and so on. Odd-even effects offer the alternate change of physical and chemical properties as well as crystalline structures of a molecule, depending on the odd/even nature of its chain.8 Such effects can be observed in many systems. There are many examples of twodimensional crystal system showing odd-even effect on the packing structure and chirality.9 For instance, fatty acids with alkyl chains of even carbon numbers in the alkyl chain revealed two chiral domain structures, whereas those with odd carbon atoms showed nonchiral structures.6 The other example of odd-even effect could be found in anthracene derivatives, which form a row structure.10 In the monolayer of anthracene derivatives with odd carbon atoms in the alkyl chain, the anthracene group points in the same direction between adjacent rows, (6) For example, see (a) Ziener, U.; Lehn, J.-M.; Mourran, A.; M€oller, M. Chem.;Eur. J. 2002, 8, 951–957. (b) 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. (c) 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.;Eur. J. 2004, 10, 1124–1132. (d) Mourran, A.; Ziener, U.; M€oller, M.; Breuning, E.; Ohkita, M.; Lehn, J. -M. Eur. J. Inorg. Chem. 2005, 2641–2647. (e) Zell, P.; M€ogele, F.; Ziener, U.; Rieger, B. Chem. Commun. 2005, 1294–1296. (7) For example, see (a) De Feyter, S.; Miura, A.; Yao, S.; Chen, Z.; W€urthner, F.; Jonkheijm, P.; Schenning, A. P. H. J.; Meijer, E. W.; De Schryver, F. C. Nano Lett. 2005, 5, 77–81. (b) Blunt, M.; Lin, X.; Gimenez-Lopez, M. C.; Schr€oder, M.; Champness, N. R.; Beton, P. H. Chem. Commun 2008, 2304–2306. (c) Gutzler, R.; Lappe, S.; Mahata, K.; Schmittel, M.; Heckel, W. M.; Lackinger, M. Chem. Commun. 2009, 680–682. (d) Ciesielski, A.; Schaeffer, G.; Petitjean, A.; Lehn, J. -M.; Samori, P. Angew. Chem., Int. Ed. 2009, 48, 2039–2043. (8) Tao, F.; Bernasek, S. L. Chem. Rev. 2007, 107, 1408–1453 and references cited therein. (9) Tao, F.; Bernasek, S. L. J. Phys. Chem. B 2005, 109, 6233–6238. (10) (a) Wei, Y.; Kannappan, K.; Flynn, G. W.; Zimmt, M. B. J. Am. Chem. Soc. 2004, 126, 5318–5322. (b) Wei, Y.; Tong, W.; Wise, C.; Wei, X.; Armbrust, K.; Zimmt, M. J. Am. Chem. Soc. 2006, 128, 13362–13361.
Published on Web 09/18/2009
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Article Chart 1. Chemical Structures of Bpys Used in This Study
whereas that with even carbon atoms showed opposite direction of anthracene groups. In both cases, the structural differences due to odd-even effect have been explained in terms of minimization of the steric repulsion between the end methyl groups. In our previous paper,11 we observed the odd-even alkyl chain length effects on the two-dimensional structures of bipyridine derivatives (bpys) containing two alkyl chains on each side. The nanostructures of bpys with appropriate alkyl chain length (n = 11-16) were alternately changed depending on the odd-even alkyl chain length. However, this structural regulation was disturbed by longer alkyl chains (n = 17, 18), possibly due to the increased van der Waals interactions. Metal coordination induced quenching of the odd-even effect, and the structural convergence was introduced to be the same lamellar structure, apart from the interlamellar distances. However, in this previous report, only bpys with double alkyl chains were investigated, and therefore a question has arisen: Does the number of alkyl chains determine the odd-even effects? There is no report on the effect of the number of alkyl chains on the odd-even effect. This prompted us to carry out a systematic study on bpys with various numbers and lengths of alkyl chain units at a solid/liquid interface. Herein, we investigate the two-dimensional structure formation of bpys with one, two, and three alkyl chain units on each side (Chart 1), which were observed by STM at a highly oriented pyrolytic graphite (HOPG)/1-phenyloctane interface. Possible odd-even chain length effects are examined for all the bpys. The bpys with a single alkyl chain on each side showed no odd-even effect on the two-dimensional structures. The double and some triple alkyl chain-substituted bpys displayed an alternative alteration of nanostructures, which is attributed to odd-even effects. It was proposed that intermolecular interactions of alkyl chain units have a dominant role for the emergence of odd-even effects on the surface, whereas the combination of the intermolecular interaction between bipyridine units and the molecule-substrate interaction impedes the alternative changes in the nanostructure and induces structural unification. Metal coordination of bpys changes the intermolecular interaction and annihilates odd-even effects in the system.
2. Experimental Section 2.1. Synthesis and Characterization of Bpys. The bpys
(Chart 1) were synthesized according to reported procedures.12 Typically, synthesized 5,50 -dibromo-2,20 -bipyridine was subsequently reacted with trimethylsilylacetylene to give 5,50 -diethynyl(11) Kikkawa, Y.; Koyama, E.; Tsuzuki, S.; Fujiwara, K.; Miyake, K.; Tokuhisa, H.; Kanesato, M. Chem. Commun. 2007, 1343–1345. (12) Kikkawa, Y.; Koyama, E.; Tsuzuki, S.; Fujiwara, K.; Miyake, K.; Tokuhisa, H.; Kanesato, M. Langmuir 2006, 22, 6910–6914.
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2,20 -bipyridine, and substituted by the appropriate iodo compound to give the desired bpys. Thus, reacting 1-alkyloxy4-iodobenzene, 1-alkyloxy-3-iodobenzene, 1,2-dialkyloxy-4-iodobenzene, or 1,2,3-trialkyloxy-4-iodobenzene with 5,50 -diethynyl2,20 -bipyridine generates the corresponding bpys with p-substituted single- (1Cnp), m-substituted single- (1Cnm), double- (2Cn), or triple alkyl chain units (3Cn) on the both ends, respectively. The length of the alkyl chain was varied from n = 11 to n = 16. In addition, only in the case of 3Cn was a molecule with n = 17 added to the series of experiments. The bpys were characterized by using 1 H and 13C nuclear magnetic resonance (NMR), Fourier transformed infrared (FT-IR), and matrix-assisted laser desorption ionization time-of-flight mass spectroscopy (MALDI-TOF-MS) (see Supporting Information). 2.2. STM Observation. Two-dimensional structures of bpys were observed by STM (Digital Instruments, Santa Barbara, CA: Nanoscope IIIa multimode SPM). The STM tip was mechanically cut and sharpened from Pt/Ir wire (90:10). The bpys were dissolved in 1-phenyloctane (Kanto) with a concentration below 0.1 mM. The 1-phenyloctane solution was deposited on a freshly cleaved HOPG of ZYB grade (NT-MDT, Russia). For the metal coordination experiment, an excess molar amount of Pd(CH3CN)2Cl2 in CH2Cl2 was added to bpys in CH2Cl2, and the resulting solution was diluted in 1-phenyloctane (c < 0.1 mM). Then, STM observation was performed at the HOPG/1-phenyloctane interface. Experimental conditions are given in each figure caption. To verify the reproducibility of the STM image, different tips and samples were used. All the STM images were recorded in the constant current mode, and analyzed by the SPIP software (Image Metrology, Denmark). The HOPG lattice was used as an internal standard to correct each STM image as well as to determine the lattice constants of the bpys.
3. Results 3.1. Mono-Alkyl-Chain-Substituted Bpys (1Cn). Figure 1a shows the STM image of p-substituted monoalkyl bpys with 16 carbon atoms in each alkyl chain unit (1C16p). In the STM images, the bright part is composed of π-conjugated units (bipyridine moieties with two additional aromatic rings), whereas alkyl chains appear as a dark contrast.11-13 In the case of 1C16p, the π-conjugated unit and alkyl chain unit are alternately aligned in a zigzag mode. Other 1Cnp’s showed similar two-dimensional structures except for 1C12p (see Figure S1 in the Supporting Information). In the case of 1C12p, the insertion of solvent molecules (1-phenyloctane) in the two-dimensional crystalline (13) For example, see (a) Jonkheijm, P.; Miura, A.; Zdanowska, M.; Hoeben, F. J. M.; De Feyter, S.; Schenning, A. P. H. J.; De Schryver, F. C.; Meijer, E. W. Angew. Chem., Int. Ed. 2004, 43, 74–78. (b) Miura, A.; Jonkheijm, P.; De Feyter, S.; Schenning, A. P. H. J.; Meijer, E. W.; De Schryver, F. C. Small 2005, 1, 131–137. (c) Nakanishi, T.; Miyashita, N.; Michinobu, T.; Wakayama, Y.; Tsuruoka, T.; Ariga, K.; Kurth, D. G. J. Am. Chem. Soc. 2006, 128, 6328–6329.
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Kikkawa et al. Table 2. Lattice Constants of 2Cn
2C11 2C12 2C13 2C14 2C15 2C16
a (nm)
b (nm)
γ (deg)
2.91 ( 0.11 1.50 ( 0.07 3.01 ( 0.04 1.44 ( 0.02 3.30 ( 0.17 1.48 ( 0.02
3.18 ( 0.09 2.96 ( 0.09 3.75 ( 0.18 3.25 ( 0.04 3.89 ( 0.29 3.61 ( 0.02
94.0 ( 1.4 95.5 ( 1.7 92.4 ( 1.7 94.3 ( 5.5 93.9 ( 2.7 93.3 ( 1.9
Figure 1. STM images of 1C16p (a) and 1C16m (b) physisorbed at the solid/liquid interface. Imaging conditions: (a) I = 75 pA, V = -1000 mV; (b) I = 69 pA, V = -1000 mV. Molecular models are superimposed on the STM images for clarification. Table 1. Lattice Constants of 1Cnp and 1Cnm
1C11p 1C12p 1C13p 1C14p 1C15p 1C16p 1C11m 1C12m 1C13m 1C14m 1C15m 1C16m
a (nm)
b (nm)
γ (deg)
1.36 ( 0.06 1.37 ( 0.07 1.65 ( 0.08 1.78 ( 0.13 1.70 ( 0.06 1.58 ( 0.01 1.00 ( 0.05 1.07 ( 0.01 1.06 ( 0.01 1.08 ( 0.08 1.09 ( 0.07 1.06 ( 0.01
2.08 ( 0.20 5.64 ( 0.22 1.89 ( 0.11 1.82 ( 0.16 2.07 ( 0.14 2.25 ( 0.07 2.73 ( 0.13 2.84 ( 0.03 2.90 ( 0.04 3.01 ( 0.06 3.13 ( 0.07 3.22 ( 0.11
102.4 ( 4.1 93.6 ( 1.8 95.3 ( 2.4 104.0 ( 2.3 100.6 ( 1.1 104.9 ( 0.7 93.7 ( 2.6 92.8 ( 0.6 93.3 ( 1.8 94.2 ( 3.5 92.2 ( 0.8 92.0 ( 1.2
lattice lead to a linear structure, as previously reported for the same system.14 Figure 1b shows the STM image of 1C16m. The Z-shape of the molecule was reflected to the two-dimensional structure, and the alkyl chains were interdigitated to form a close-packed structure. The morphologies of 1C11-15m were similar to those of 1C16m with intermolecular distances that are proportional to the alkyl chain length. All the STM images of 1Cnm are shown in Figure S2 (Supporting Information). Lattice constants of p- and m-substituted bpys were measured and are listed in Table 1. 3.2. Double-Alkyl-Chain-Substituted Bpys (2Cn). Odd-even alkyl chain length effects have been found in the double-alkylchain-substituted bpys (2Cn; see Figure S3 in the Supporting Information), as reported previously.11 Briefly, in the case of odd carbon numbers in the alkyl chain unit (2Codd), a pair of molecules were arranged in a L-form, and are symmetrically oriented, where the alkyl chains were aligned to allow van der Waals interactions. In contrast, 2Ceven adopted a bent form, and the alkyl chains were interdigitated. Thus, the two-dimensional structure was alternately changed as a result of the odd-even carbon numbers in the alkyl chain unit. For these molecules, lattice constants are listed in Table 2. 3.3. Triple-Alkyl-Chain-Substituted Bpys (3Cn). These molecules showed different types of two-dimensional structures, which were classified into two groups on the basis of the alkyl chain length. The molecules of 3C11-13 are linear and aligned in a way that there is little contact between bipyridine units, as shown in Figure 2a,b. In the case of 3C13 (Figure 2b), solvent molecules coadsorbed onto the HOPG surface, which is evidenced by the singular “dot” within the molecular lattice as well as increased lattice constants of the b-axis (Table 3). This result suggests that (14) Kikkawa, Y.; Koyama, E.; Tsuzuki, S.; Fujiwara, K.; Miyake, K.; Tokuhisa, H.; Kanesato, M. Surf. Sci. 2007, 601, 2520–2524.
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Figure 2. STM images of 3Cn at a solid/liquid interface. Frames a-e are the STM images of 3C12-17, respectively. Molecular models are superimposed on each image. In frame b, coadsorbed 1-phenyloctane molecules are expressed as yellow. Imaging conditions: (a) I = 1.1 pA, V = -263 mV; (b) I = 1.1 pA, V = -639 mV; (c) I = 1.6 pA, V = -625 mV; (d) I = 1.2 pA, V = -275 mV; (e) I = 1.1 pA, V = -660 mV; (f) I = 1.4 pA, V = -242 mV. Table 3. Lattice Constants of 3Cn
3C11 3C12 3C13 3C14 3C15 3C16 3C17
a (nm)
b (nm)
γ (deg)
1.59 ( 0.13 1.62 ( 0.21 2.12 ( 0.17 3.09 ( 0.12 1.24 ( 0.15 3.18 ( 0.28 1.31 ( 0.16
3.24 ( 0.14 2.78 ( 0.12 2.95 ( 0.18 4.25 ( 0.17 5.04 ( 0.22 4.38 ( 0.31 6.02 ( 0.22
101.4 ( 6.1 96.2 ( 4.2 102.3 ( 3.9 97.2 ( 4.3 91.7 ( 2.0 95.5 ( 4.3 93.5 ( 1.6
the two-dimensional structures of 3C11-13 are basically the same, and that there is no odd-even alkyl chain length effect within the alkyl chain length for n = 11-13. Langmuir 2010, 26(5), 3376–3381
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Figure 3. STM images of 1C16 (a) and 3C16 (b) after metal coordination. Molecular models are superimposed on each image. In frame b, one alkyl chain on each side is thought to dangle into the solvent from the STM image on the basis of lattice constants in Table 4. Imaging conditions: (a) I = 1.6 pA, V = -650 mV; (b) I = 1.3 pA, V = -654 mV. Table 4. Lattice Constants of Bpys after Metal Coordination
1C11p þ PdCl2 1C12p þ PdCl2 1C13p þ PdCl2 1C14p þ PdCl2 1C15p þ PdCl2 1C16p þ PdCl2 2C11 þ PdCl2 2C12 þ PdCl2 2C13 þ PdCl2 2C14 þ PdCl2 2C15 þ PdCl2 2C16 þ PdCl2 3C11 þ PdCl2 3C12 þ PdCl2 3C13 þ PdCl2 3C14 þ PdCl2 3C15 þ PdCl2 3C16 þ PdCl2 3C17 þ PdCl2
a (nm)
b (nm)
γ (deg)
0.91 ( 0.03 0.97 ( 0.02 0.93 ( 0.02 0.92 ( 0.10 0.99 ( 0.10 0.97 ( 0.06 0.93 ( 0.07 0.99 ( 0.04 0.95 ( 0.09 1.14 ( 0.07 1.04 ( 0.02 1.02 ( 0.07 0.97 ( 0.05 0.97 ( 0.05 0.90 ( 0.08 0.91 ( 0.09 0.87 ( 0.06 1.03 ( 0.02 0.91 ( 0.08
3.82 ( 0.29 4.06 ( 0.17 4.04 ( 0.11 4.18 ( 0.30 4.43 ( 0.19 4.52 ( 0.11 4.92 ( 0.05 5.18 ( 0.11 5.62 ( 0.06 5.69 ( 0.09 6.09 ( 0.09 6.14 ( 0.03 5.10 ( 0.20 5.22 ( 0.10 5.56 ( 0.29 5.75 ( 0.29 5.98 ( 0.31 6.19 ( 0.04 6.54 ( 0.24
93.8 ( 1.3 92.6 ( 3.0 95.5 ( 0.7 92.7 ( 2.9 94.4 ( 2.8 93.5 ( 2.1 93.8 ( 4.3 92.8 ( 1.0 93.9 ( 2.0 93.2 ( 2.1 91.4 ( 0.8 93.1 ( 0.8 94.7 ( 2.6 93.8 ( 2.9 93.1 ( 0.8 91.5 ( 1.5 94.0 ( 1.8 94.3 ( 2.0 91.6 ( 1.4
Surprisingly, odd-even effects were found in the molecules of 3C14-3C17 (Figures 2c-f). In the case of 3C14 and 3C16, all the alkyl chains are attached on the HOPG surface, and a pair of molecules exists parallel in the crystalline lattice. On the contrary, 3C15 and 3C17 exhibited a Z-shaped morphology, and all the alkyl chains were arranged in a tail-to-tail fashion. 3.4. Metal Coordination. For the metal coordination experiments, an excess molar amount of Pd(CH3CN)2Cl2 was added into the bpy solution. The coordination of palladium to the bpys lead to a characteristic change in the color of the mixed solution. The metalated bpy solution was then diluted in 1-phenyloctane, and STM observations of metal complexed bpys were performed at the solid/liquid interface. Typical STM images of 1C16p and 3C16 after metal coordination are shown in Figure 3. All the lattice constants of bpys after metal coordination are listed in Table 4. In the case of 1Cnp, the coordination to the metal center lead to different two-dimensional structures, as shown in Figure 3a and Figure S5 in Supporting Information. Indeed, the metalated 1Cnp formed a lamellar structure, in which the alkyl chain unit was interdigitated. However, structural change was not observed in the monolayer of 1Cnm, despite the confirmation of the characteristic color change of the 1Cnm solution and downfield shift in the R proton of nitrogen by 1H NMR after the addition of Pd(CH3CN)2Cl2 solution (see Supporting Information). Figure S6 shows the 1C16m after metal coordination treatment. Even when varying the ratio of 1Cnm versus Pd(CH3CN)2Cl2, the Langmuir 2010, 26(5), 3376–3381
two-dimensional structure remained unchanged after the metal coordination treatment. The metal complexed 2Cn molecules completely changed their nanostructure, and all the molecules formed a lamellar structure without interdigitation of their alkyl chains (Figure S7 in the Supporting Information). In other words, the structures were converged into the same lamellar structure with interlamellar distances related to the alkyl chain length.11 Two-dimensional structures of metalated 3Cn were almost same as those of 2Cn. This can be confirmed from the morphologies and comparison of the lattice constants in Table 4. The lattice constants are almost identical between 3Cn and 2Cn, suggesting that one of the alkyl chains on the side in the 3Cn molecule is dangling into the solvent rather than attaching onto the HOPG.
4. Discussion 4.1. Monolayers of 1Cnp and 1Cnm. Various two-dimensional structures were observed in the bpy assemblies depending on the substitution position, the number of alkyl chain units, and odd-even alkyl chain length. The molecules of 1Cnp and 1Cnm did not show any odd-even alkyl chain effects on the twodimensional structures. Two-dimensional structures of 1Cnp have relatively less intermolecular interactions, which is evidenced by the zigzag alignment of alkyl chains and bipyridine units (Figure 1a). In other words, alkyl chain and bipyridine units are separated from their own respective units, resulting in weaker intermolecular interactions. Therefore, interaction between the molecule and the HOPG substrate dominates the two-dimensional structure formation of 1Cnp. Coordination to a metal induced a spontaneous structural change. Indeed, metalated 1Cnp adopted a linear form, and the coordination cores stick together. The alkyl chain units have enough space to accommodate their neighboring alkyl chain between them and interdigitate.14 Therefore, the structures of metal-complexed 1Cnp are mainly governed by the intermolecular interactions of metalated bipyridine and alkyl chain units. In the case of 1Cnm, the crystalline packing was very dense, and the alkyl chain and bipyridine units are close together enough to have intermolecular interactions (Figure 1b). Metal coordination could be visually judged from the typical color change of the solution. In addition, downfield shift in the R proton of nitrogen, which is characteristic for metalation of the bipyridine unit, was confirmed by 1H NMR. These results indicate that metal coordination is certainly conducted and that metal complexed 1Cnm molecules exist in the solution. Nevertheless, the two-dimensional structure remained unchanged after the metal coordination treatment. Accordingly, there are two plausible explanations for the preservation of the 1Cnm structure before and after metal coordination. One is that the coordinated PdCl2 is dangling into the solvent rather than lying on the HOPG surface. In this case, the contrast of STM image should vary after metal coordination. Four “dots” derived from aromatic rings in the π-conjugated units are visible before metal coordination, whereas only three “dots” could be found in the STM images after metal coordination (see Figures S10 and S11). Although PdCl2 coordinated to the bipyridine unit is lying on the HOPG surface in the case of 2Cn, the appearance of the bipyridine unit of 1Cnm should also be altered more or less. However, such contrast change could not be recognized in the metal-treated 1Cnm (Figure S6 in the Supporting Information), suggesting that this first explanation cannot account for the conservation of the structures. The other possible explanation is that the crystalline packing of 1Cnm is too close and stable to change its molecular alignment after metal coordination. DOI: 10.1021/la903156m
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In the present solid/liquid interface system, there are two equilibriums such as metal coordination/decoordination and the attachment/detachment of the complexed molecules onto HOPG surface. Therefore, it can be supposed that PdCl2 is released when the molecules align on the HOPG surface in order to form more stable crystalline packing structure, i.e., original two-dimensional structure of 1Cnm. Exchange of the coordination metal from palladium to platinum may clarify the above-mentioned possibility, and this will be reported elsewhere. 4.2. Monolayers of 2Cn. The 2Cn molecules showed clear odd-even effect, and the nanostructure on the surface was alternately changed depending on the carbon numbers in the alkyl chain unit (Figure S3). After the metal coordination, odd-even effect was quenched as a result of the increased molecular width at the bipyridine unit.11,12 Two-dimensional structures of 2Cn were converged into the same lamellar structure with no interdigitation of the alkyl chains (Figure S7). Indeed, the molecular width of the bipyridine unit became almost identical to that of two peripheral alkyl chains. At the same time, the metalated bipyridine units aligned in a column, resulting in the strong interaction. This result suggests that the structure formation is dominated by the interactions of alkyl chains as well as metalated bipyridine units. 4.3. Monolayers of 3Cn. The 3C11-13 molecules have similar structures (Figures 2a,b and S4). Since the o-alkyl chains were parallel to the m- and p-ones, there is certain sterical strain at the stem of the o-alkyl chain. Both the alkyl chain and bipyridine units were almost parallel to the HOPG lattice (Table S1). From these results, it is reasonable to assume that those structures are stabilized by both van der Waals interaction between alkyl chains and molecule-substrate interactions. The 3C14-17 molecules exhibited odd-even alkyl chain effect (Figures 2c-f), and the structure alternately changed as in the case of 2Cn, suggesting that the two-dimensional structure is determined by the intermolecular interactions of alkyl chain unit. All the metal-coordinated 3Cn molecules exhibited the same lamellar structure, regardless of the interlamellar distances (Figures 3b and S8). The metal coordination induces a variation of the molecular width of the bipyridine unit, resulting in the molecular width of the bipyridine and two of the three peripheral alkyl chains becoming identical. Such molecular width variation changed the intermolecular interaction that controls the two-dimensional structure formation. In addition to the alkyl chain-HOPG interactions, the lamellar structure was formed not only by the intermolecular interactions of alkyl chains but also by the metal-complexed bipyridine units. 4.4. Emergence of the Odd-Even Effect. On the basis of the STM analysis, the molecular arrangements of bpy assemblies could be classified into three groups depending on their intermolecular and molecule-substrate interactions: the twodimensional structure formation is dominated by (i) moleculesubstrate interactions, (ii) intermolecular interactions of the alkyl chain unit in addition to the bipyridine units, or (iii) intermolecular interactions of the alkyl chain unit. As the special case of (ii), (ii0 ) is defined as follows: the intermolecular interactions of the alkyl chain unit and metal-complexed bipyridine units dominate the two-dimensional structure formation. The classification of the two-dimensional structure formation is shown in Figure 4. The two-dimensional structures of 1Cnp and 3C11-13 belong to group (i). Intermolecular interactions are weak because the molecular units are far from each other, indicating that the nanostructure is mainly controlled by the HOPG surface. The 1Cnm formed such a stable molecular packing structure that it remains even after metal coordination, suggesting that neighboring alkyl chain and bipyridine units have strong intermolecular interaction. 3380 DOI: 10.1021/la903156m
Kikkawa et al.
Figure 4. Classification of dominant interactions determining the molecular arrangements in bpy monolayers: (i) molecule-substrate interactions; (ii) intermolecular interactions of the alkyl chain unit in addition to the bipyridine units; (iii) intermolecular interactions of the alkyl chain unit. (ii0 ) is defined as follows: the intermolecular interactions of the alkyl chain unit and metalcomplexed bipyridine units.
Since odd-even effects were found in the 2Cn and 3C14-17, they are categorized into group (iii). In most cases, it has been proposed that the odd-even effect emerges as a result of the end methyl group in the alkyl chain unit in order to minimize the steric repulsion.8 Unless the functions of the end methyl group are disturbed by the other molecular interactions with the substrate and bipyridine units, odd-even effects of the alkyl chain could be observed. Therefore, only if the major driving force of the twodimensional structural formation is intermolecular interaction of alkyl chains, odd-even effects are anticipated to appear to the system. Such molecular interactions are strongly influenced by the coordination to a metal center. All the metal-complexed bpys exhibited similar two-dimensional structures with or without the interdigitation. This result suggests that the main interactions for the nanostructures are between both alkyl chain and metalcomplexed bipyridine units, respectively. As a result, odd-even effects in 2Cn and 3C14-17 are canceled. Thus, emergence of odd-even effects of bpys is determined by the molecular interaction, which dominates the two-dimensional structure formation.
5. Conclusions STM observations of bpys were performed at a solid/liquid interface. The bpys contained various numbers and lengths of alkyl chains. The two-dimensional structures were classified into three groups in terms of the molecule-molecule and moleculesubstrate interaction. The nanostructures of 1Cnp and 3C11-13 are HOPG surface controlled, whereas those of the others are formed not only by alkyl chain-HOPG interaction but also the intermolecular interactions. The 1Cnm formed a quite stable twodimensional nanostructure, which remained unchanged even after the metal coordination. In this case, the intermolecular interactions are composed of alkyl chain and bipyridine units, respectively. If the intermolecular interactions of the alkyl chain units are the main factor for the formation of the nanostructure, odd-even effects are observed in the system, namely in the cases of 2Cn and 3C14-17. These molecules showed alternately different structures dependent on the carbon numbers in the alkyl chain unit. After the metal coordination, odd-even effects were quenched as a result the alteration of intermolecular interaction of bipyridine unit with PdCl2. Thus, differences in molecular interaction controlling the two-dimensional ordering account for Langmuir 2010, 26(5), 3376–3381
Kikkawa et al.
the nanostructure formation of bpys in addition to the odd-even effect. Acknowledgment. We would like to thank Prof. Markus Albrecht of the Institut f€ur Organische Chemie, RWTH Aachen and Dr. Nathana€elle Schneider of the Photonics Research Institute, National Institute of Advanced Industrial Science and Technology (AIST) for their helpful discussions and suggestions.
Langmuir 2010, 26(5), 3376–3381
Article
We also thank Dr. Yasuo Norikane and Dr. Masaru Yoshida of the Nanotechnology Research Institute, AIST, for MALDI-TOF MS measurements. Supporting Information Available: Characterization of bpys; additional STM images; azimuthal setting against HOPG lattice; STM image contrast. This material is available free of charge via the Internet at http://pubs.acs.org.
DOI: 10.1021/la903156m
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