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Two-Dimensional Supramolecular Arrangements of Enantiomers and Racemic Modification of 1,1′-Binaphthyl-2,2′-Dicarboxylic Acid Masashi Kunitake,*,† Tetsutaro Hattori,‡ Sotaro Miyano,‡ and Kingo Itaya§ Department of Applied Chemistry and Biochemistry, Faculty of Engineering, Kumamoto University, Kurokami, Kumamoto 860-8555, Japan, Department of Biomolecular Engineering, Graduate School of Engineering, Tohoku University, Aoba-yama07, Sendai 980-8579, Japan, and Department of Applied Chemistry, Graduate School of Engineering, Tohoku University, Aoba-yama04, Sendai 980-8579, Japan Received April 19, 2005. In Final Form: July 21, 2005 Formation of adlayers of the optically active compound 1,1′-binaphthyl-2, 2′-dicarboxylic acid (BINAC) on iodine-modified Au (111) surfaces in perchloric acid was investigated by in situ scanning tunneling microscopy (STM). Highly ordered arrays formed on the surfaces via simple spontaneous adsorption from a solution of enantiomers or the racemic BINAC, in spite of the fact that BINAC has a three-dimensionally complex stereochemical structure. Adlayers of both enantiomers essentially shared the same enantiomorphous structure. Observed parameters of the rectangular unit cell lattice for arrays of both enantiomers of BINAC were a ) 2.3 ( 0.2 nm and b ) 0.7 ( 0.2 nm. On the other hand, racemic modification formed an entirely different adlayer, which consisted of an alternate alignment of the two enantiomers, with an oblique unit cell lattice with parameters of a ) 1.2 ( 0.2 nm, b ) 0.8 ( 0.1 nm, and 74 ( 3°. No domain composed of a single enantiomer was observed. The stronger hetero-intermolecular interactions of enantiomer couples led to the formation of an alternate arrangement in the array prepared by racemic modification.
Introduction Scanning tunneling microscopy (STM) has been used to visualize organic molecules and to determine their orientation, packing arrangement, and internal structure.1-3 Recently, chiral arrangement of molecules on surfaces has become a hot issue in the field of STM.3-25,30,33-36 STM now permits not only direct discrimination between chiral molecules in the isolated * Corresponding author. Tel: +81 96 342 3675. Fax: +81 96 342 3673. E-mail:
[email protected]. † Kumamoto University. ‡ Department of Biomolecular Engineering, Tohuku University. § Department of Applied Chemistry, Tohuku University. (1) Poggi, M. A.; Bottomley, L. A.; Lillehei, P. T. Anal. Chem. 2002, 74, 2851. (2) Wieckowski, A., Ed. ACS Monography in Electron Spectroscopy and STM/AFM analysis of the solid-liquid interface; 1997; Vol. 656, p 171. (3) Feyter, S. D.; De Schryver, F. C. J. Phys. Chem. B 2005, 109, 4290. (4) Lopinski, G. P.; Moffatt, D. J.; Wayner, D. D. M.; Wolkow, R. A. Nature 1998, 392, 909. (5) Messina, P.; Dmitriev, A.; Lin, N., Spillmann, H.; Abel, M.; Barth, J. V.; Kern, K. J. Am. Chem. Soc. 2002, 124, 14000. (6) Taniguchi, M.; Nakagawa, H.; Yamagishi, A.; Yamada, K. Surf. Sci. 2000, 454/456, 1005. (7) Weckesser, J.; De Vita, A.; Barth, J. V.; Cai, C.; Kern, K. Phys. Rev. Lett. 2001, 87, 096101. (8) Barth, J. V.; Weckesser, J.; Trimarchi, G.; Vladimirova, M.; Vita, A. D.; Cai, C.; Brune, H.; Gu¨nter, P.; Kern, K. J. Am. Chem. Soc. 2002, 124, 7991. (9) Bo¨hringer, M.; Morgenstern, K.; Schneider, W.-D.; Berndt, R. Angew. Chem., Int. Ed. 1999, 38, 821. (10) Bo¨hringer, M.; Schneider, W.-D.; Berndt, R. Angew. Chem., Int. Ed. 2000, 39, 792. (11) Li, C.-J.; Zeng, Q.-D.; Wu, P.; Xu, S.-L.; Wang, C.; Qiao, Y.-H.; Wan, L.-J.; Bai, C.-L. J. Phys. Chem. B 2002, 106, 13262. (12) Li, C.-J.; Zeng, Q.-D.; Wang, C.; Wan, L.-J.; Xu, S.-L.; Wang, C.-R.; Bai, C.-L. J. Phys. Chem. B 2003, 107, 747. (13) Schunack, M.; Laegsgaard, E.; Stensgaard, I.; Johannsen, I.; Besenbacher, F. Angew. Chem., Int. Ed. 2001, 40, 2623. (14) Stevens, F.; Dyer, D. J.; Walba, D. M. Angew. Chem., Int. Ed. Engl. 1996, 35, 900. (15) Ohtani, B.; Shitani, A.; Uosaki, K. J. Am. Chem. Soc. 1999, 121, 1, 6515.
state4,5 and in the ordered adlayer6 but also molecular scale visualization of one-dimensional (1-D)7 or twodimensional (2-D) chiral arrangements of supramolecular arrays consisting of achiral,8-12,25 racemic,14,33-36 chiral,15-22 and mixed molecules.23,24 (16) Lorenzo, M. O.; Baddeley, C. J.; Muryn, C.; Raval, R. Nature 2000, 404, 376. (17) Widmer, I.; Hubler, U.; Sto¨hr, M.; Merz, L.; Gu¨ntherodt, H. J.; Hermann, B. A.; Samori, P.; Rabe, J. P.; Rheiner, P. B.; Greiveldinger, G.; Murer, P. Helv. Chem. Acta 2002, 85, 4255. (18) Bernhard, S.; Takada, K.; Diaz, D. J.; Abrun˜a, H. D.; Mu¨rner, H. J. Am. Chem. Soc. 2001, 123, 10265. (19) De Feyter, S.; Grim, P. C. M.; Ru¨cker, M.; Vanoppen, P.; Meiners, C.; Sieffert, M.; Valiyaveettil, S.; Mu¨llen, K.; Schryver, F. C. D. Angew. Chem., Int. Ed. 1998, 37, 1223. (20) Han, M.-J.; Wang, D.; Hao, J.-M.; Wan, L.-J.; Zeng, Q.-D.; Fan, Q.-H.; Bai, C.-L. Anal. Chem. 2004, 76, 627. (21) France, C. B.; Parkinson, B. A. J. Am. Chem. Soc. 2003, 125, 12712. (22) Barlow, S. M.; Delphine, S. L.; Roux, L.; Williams, J.; Muryn, C.; Haq, S.; Raval, R. Langmuir 2004, 20, 7171. (23) Li, C.; Zeng, Q.; Wu, P.; Xu, S.; Wang, C.; Qiao, Y.; Wan, L.; Bai, C. J. Phys. Chem. B 2002, 106, 13262. (24) Yablon, D. G.; Wintgens, D.; Flynn, G. W. J. Phys. Chem. B 2002, 106, 5470. (25) Qian, P.; Nanjo, H.; Yokoyama, T.; Suzuki, T. M.; Akasaka, K.; Orhui, H. J. Chem. Soc., Chem. Commun. 2000, 2021. (26) Cunha, F.; Tao, N. J. Phys. Rev. Lett. 1995, 75, 2376. (27) Ohira, A.; Sakata, M.; Taniguchi, I.; Hirayama, C.; Kunitake, M. J. Am. Chem. Soc. 2003, 125, 5057. (28) Ishikawa, Y.; Ohira, A.; Skata, M.; Hirayama, C.; Kunitake, M.; Chem. Commun. 2002, 22, 2652. (29) Kunitake, M.; Batina, N.; Akiba, U.; Itaya, K. Langmuir 1997, 13, 1607. (30) Kunitake, M.; Miyano, S.; Itaya, K. International Conference on Electrochemistry of Ordered Interfaces, Sapporo, Japan, 1998, PB14. (31) Oi, S.; Matsuzaka, Y.; Yamashita, J.; Miyano, S. Bull. Chem. Soc. Jpn. 1989, 62, 956. (32) Tamai, Y.; Matsuzaki, Y.; Hattori, T.; Oi, S.; Miyano, S. Bull. Chem. Soc. Jpn. 1991, 64, 2260. (33) Oi, S.; Matsunaga, K.; Hattori, T.; Miyano, S. Synthesis 1993, 893. (34) Weber, E.; Csoregh, I.; Stensland, B.; Czugler, M. J. Am. Chem. Soc. 1984, 106, 3297. (35) Cai, Y.; Bernasek, S. L. J. Am. Chem. Soc. 2003, 125, 1655. (36) Cai, Y.; Bernasek, S. L. J. Phys. Chem. B 2005, 109, 4514.
10.1021/la051040p CCC: $30.25 © 2005 American Chemical Society Published on Web 08/25/2005
1,1′-Binaphthyl-2,2′-Dicarboxylic Acid
In their pioneering work, Walba and co-workers reported STM observations of chirality at tetrahedral stereocenters of chiral ferroelectric liquid crystals.14 Formation of two coexistent symmetric chiral domains from racemic molecules is due to 2-D crystallization of uni-chiral molecules on a surface, which is of interest as a molecular scale analogue of Pasteur’s classic experiment. As an example of covalently immobilized 2-D chirality, Ohtani and coworkers reported on the highly ordered molecular layers of enantiomers of a chiral binaphthyl derivative, (R)- and (S)-1,1′-binaphthalene-2,2′-dithiol (BNSH) prepared by self-assembly techniques utilizing chemical adsorption of thiol compounds on Au surfaces.15 These 2-D chiral arrangements of molecular adlayers on well-defined surfaces are essentially self-organized by intermolecular interactions rather than by epitaxial adsorption. Adsorption-induced self-organization (AISO), which is based on spontaneous mild adsorption from a solution onto a substrate followed by 2-D self-organization, is a potentially valuable wet process for the formation of highly ordered molecular arrays of various organic molecules.2,3,26-30 Here, we report results of in situ STM imaging of the 2-D supramolecular arrangement produced by means of AISO of the optically active organic molecule 1,1′-binaphthyl-2,2′-dicarboxylic acid (BINAC)30-33 on the iodine-modified Au (111) surface in 0.1 M perchloric acid. Experimental Procedures Synthesis and Purification of BINAC. BINAC compounds were synthesized using a convenient synthetic route via the Ullmann coupling of 1-bromo-2-naphthoic ester, and the optically pure enantiomers were separated by HPLC, as described previously.31-33 Purification was done by recrystallization. Before performing STM experiments, recrystallized BINAC samples were heat-treated (120 °C, 10 h) in a vacuum, to eliminate toluene used as the solvent during recrystallization. Construction of the BINAC Model. The molecular model for BINAC was calculated using the MOPAC computer program in the CAChe software package (CAChe Scientific Co. Ltd.). Numerical values for BINAC molecules including interatomic distances and size were estimated from this molecular model. Sample Preparation and in Situ STM Imaging. As a substrate, an Au (111) facet on an Au single-crystal bead was used. Iodine modified Au (111) surfaces were prepared by immersion into 1 mM KI aqueous solution after ordinary annealing-quenching treatment. Methods for in situ STM observations used in this research, including preparation of iodine-modified Au (111), were essentially the same as those reported in a previous article.29 A diluted aqueous solution of BINAC was injected into the STM cell filled with 0.1 M perchloric acid. Concentration of BINAC in the cell was typically ca. 10-6 M. Throughout the experiments, the electrode potential was kept at ca. 0.8 V versus RHE to fix the iodine lattice structure, a centered rectangular c(px3 - 30°) phase.2,29 A centered rectangular c(px3 - 30°) phase was found to appear in the applied potential range with a small change in the p value at about 2.5. In situ STM measurements were carried out with a Nanoscope III instrument (Digital Instruments, Santa Barbara, CA). The tunneling tip was prepared from an electrochemically etched W wire, and its sidewalls were sealed with transparent nail polish to reduce faradic current. All images were collected in the constant current mode. Two Pt wires were used as reference and counter electrodes in the STM cell, respectively.
Results and Discussion In BINAC, the structure consists of two connecting naphthalene rings whose planes are immobilized by steric hindrance of carboxylic groups and naphthalene rings, as shown in Figure 1. The resulting cross-shaped structure
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Figure 1. Chemical structure and top view of the space-filling model of R-BINAC in the upward orientation.
induces an axis of chirality. Axially dissymmetric 1,1′binaphthalene derivatives such as BINAC serve as highly efficient chiral inducers for a wide range of asymmetric reactions, and BINAC has many uses in the preparation of optically active derivatives.31-33 The present study investigated how molecular chirality was reflected in the molecular packing arrangement within the 2-D molecular architecture. Three sample solutions of the two enantiomers (R- and S-BINAC) and a racemic modification (RS-BINAC) were prepared in order to investigate the influence of chirality on structure of self-organized adlayers. Despite their asymmetric shapes, highly ordered arrays were formed on iodine-modified Au (111) surfaces by spontaneous adsorption from the various enantiomers, and even from RS-BINAC. In a typical experiment, a diluted aqueous solution of BINAC was injected in the STM cell filled with 0.1 M perchloric acid. Several minutes after addition of the BINAC solution, islands of BINAC arrays began to appear on large terraces, and a highly ordered adlayer extended to cover the surfaces after approximately 30 min. Relatively slow ordering confirmed weak adsorption and selforganization on surfaces. Adsorption and self-organization behaviors of the isomers (R- and S-BINAC) were essentially similar to those of RS-BINAC. It is noteworthy that spontaneous adsorption of BINAC on a bare Au (111) surface by a similar procedure gave only a disordered adlayer, in which individual molecular features were indistinct. The BINAC adlayer on Au (111) with quite low coverage was also prepared for the observation of an isolated state; nevertheless, we could not obtain a high-resolution image of isolated molecules so we could not discuss individual chirality. Resolution of isolated molecules is generally poor as compared to that in the arrays at ambient conditions. Poor resolution of isolated molecules could be due to fragmental perturbation of adsorbates. In a 2-D array, such perturbations might be strongly suppressed.29,37 An I/Au (111) surface is one of the most suitable substrates for the formation of highly ordered molecular layers of various organic molecules such as porphyrins, by means of AISO, because the chemically inert I/Au (111) surface shows relatively weak adsorption of organic adsorbates as compared to bare Au (111) surfaces.2,29,30 Figure 2A,B shows typical in situ STM images of highly ordered molecular arrays of R-BINAC. The lattice and STM features were entirely different from those of the iodine adlayer. Furthermore, the opposite enantiomer, S-BINAC, revealed similar STM images (Figure 2D), and the observed unit cell dimensions of S-BINAC were essentially the same as those of R-BINAC within experi(37) Uemura, S.; Sakata, M.; Hirayama, C.; Kunitake, M. Langmuir 2004, 20, 9198.
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Figure 2. In situ STM images (A, B, and D) and corresponding models (C and E) of chiral R-BINAC adlayer on I/Au (111) in 0.1 M perchloric acid containing ca. 10-6 M R-BINAC. Images (A and B) were obtained at Es ) 0.79 V, Et ) 0.22 V, and It ) 1.0 nA. This image (D) was obtained at Es ) 0.76 V, Et ) 0.35 V, and It ) 1.0 nA.
mental error. A rectangular unit cell lattice for adlayers of R-and S-BINAC could be seen with lattice parameters of a ) 2.3 ( 0.2 nm and b ) 0.7 ( 0.2 nm, as shown in Figure 2B,D. Symmetry and arrangements of each BINAC adlayer were investigated by STM at a submolecular resolution and by using a chemical structural model. The projected size of a BINAC molecule in the upward orientation, with parallel carboxylic groups facing the solution and the two ends of the symmetrically aligned naphthalene groups
touching the substrate, was approximately 1.2 × 1.0 nm (Figure 1). However, size could slightly vary if the angle was adjusted between the two planes of the naphthalene rings. This upward orientation is conceivable because the hydrophobic naphthalene moieties and hydrophilic carboxylic groups face the hydrophobic iodine adlayers and solution, respectively. The four bright spots in the STM images (Figure 2B,D), which were located at the edge of the unit cell, might be attributed to the upper carboxylic groups of BINAC with an upward orientation.
1,1′-Binaphthyl-2,2′-Dicarboxylic Acid
However, one side of the lattice for both enantiomers (R-and S-BINAC) and racemic BINAC was too short for a single BINAC molecule in the upward orientation. In arrays, BINAC molecules can overlap or combine with adjacent molecules to form a closely packed structure. This probably explains some of the difficulties encountered in interpreting the enantiomer images. A typical STM image of a highly ordered adlayer prepared from the enantiomer solution (R-and S-BINAC) acquired in an area of 10 × 10 nm2 is presented in Figure 2B,D. As we mentioned before, both enantiomers essentially revealed the same adlayer with a rectangular unit cell. The four bright ridges running lengthwise consisted of a complex regular pattern of protrusions in both images. In addition, in Figure 2D, a long-range strip pattern that overlapped the submolecular features could be seen. Such a long-range strip pattern, which was also observed in Figure 2A, was frequently observed for both enantiomers. Unexpectedly, a mirror image of the molecular arrangement of the enantiomers was not found, even at a higher resolution. After considering numerous molecular arrangements, we finally settled on a model with a sideby-side stacking structure and a sideways molecular orientation. Only an interdigitated molecular arrangement would give a closely packed structure that fits within the observed unit cell. Figure 2C,E illustrates the molecular arrangement for arrays of R- and S-BINAC enantiomers, modeled by using a framework that corresponded to that in Figure 2B,D in terms of scale and direction, respectively. Top and side views obtained using space filling models are shown in Figure 4A,B, respectively. It was expected that in the array, the bright ridges would consist of two molecules facing each other, and one of the naphthalene rings in the molecule would be stacked up alternately along the bright lines. One of the naphthalene rings of BINAC would therefore possess a vertical orientation for stacking with both sides of the adjacent molecules, and the other would possess an oblique orientation. The racemic modification, RS-BINAC, also formed a highly ordered adlayer with a completely different unit cell, yielding a unique feature in STM images. As shown in Figure 3A, the highly ordered molecular layer prepared from the solution of RS-BINAC was spread uniformly across the terrace. We mentioned previously that the monolayer of racemic molecules contained a mixture of two enantiomorphous domains in chiral liquid crystal compounds. In contrast, no domain originating from a single enantiomer was observed in BINAC. Moreover, STM features of the adlayer and unit cell lattice for the racemic BINAC were totally different from those for the adlayer prepared from the enantiomer solution. The observed array for RS-BINAC had an oblique unit cell lattice with a ) 1.2 ( 0.2 nm, b ) 0.8 ( 0.1 nm, and 74 ( 3°. This indicated that both enantiomers were uniformly mixed in the array, which consisted of an alternate alignment of R-BINAC/S-BINAC or a hetero-enantiomer coupling of R-BINAC/S-BINAC. Recently, Bernasek reported a chiral pair adlayer of iodine-substituted octadecanol molecules on HOPG prepared from a racemic mixture.35-36 Figure 3B is a typical high-resolution STM image of the RS-BINAC adlayer. This array resembles a twisted ribbon. As for the enantiomers, packing arrangement of naphthalene units based on π-π interactions would be the key to the formation of the alternate alignment. A smaller unit cell is expected in the overlapped packing arrangement of RS-BINAC. We propose a molecular arrangement consisting of a specific overlapped structure with alternate
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Figure 3. High-resolution STM images (A and B) and the corresponding model (C) of racemic RS-BINAC adlayer on I/Au (111) in 0.1 M perchloric acid containing ca. 10-6 M RS-BINAC. These images (A and B) were obtained at Es ) 0.75 V, Et ) 0.45 V, and It ) 1.0 nA.
molecular alignment of the RS-BINAC adlayer. The molecular framework in Figure 3C corresponds to that in Figure 2B, in terms of scale and direction. In addition, Figure 4C,D shows schematic representations of the top and side views of molecular arrangements in racemic BINAC. In our model, all adsorbed BINAC molecules have an upward orientation. R-and S-BINAC molecules are alternately aligned along row II. Here, the tilted naphthalene groups of a given BINAC molecule overlap those of adjacent molecules with the opposite chirality. This is more easily seen in the side view model shown in Figure
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Figure 4. Illustrative images of the top (A and C) and side views (B and D) of adlayers of enantiomer R-BINAC in a sideways orientation (A and B) and adlayers of RS-BINAC with an alternate alignment and upward orientation (C and D).
4D. The shape of a BINAC molecule resembles the character X in a side view seen from the direction of row III. It should be noted that this molecular arrangement is available only for the alternately aligned racemic couple. The BINAC enantiomer cannot be folded up into the lattice because of the crossing at the angle between adjacent naphthalene rings. As mentioned previously, a single enantiomer, R- or S-BINAC, seemed to possess not an upward but a sideways orientation in arrays. To test the array model for enantiomers with an upward orientation, it is possible to align molecules along the vertical toward the connecting C-C bond between naphthalene units (X direction in Figure 1) to avoid obstruction from tilted naphthalene groups of adjacent molecules. According to the space-filling model, aromatic rings of adjacent heterocouples of BINAC cannot attract each other until a sufficient distance is reached in the arrangement of the molecules because of repulsion and steric hindrance of carboxylic groups. This may explain the differences in conformation and arrangement between the arrays prepared from enantiomers and racemic modification.
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Among the expected intermolecular interactions, π-π interactions between naphthalene rings obviously play a dominant role in the formation and structure of the ordered adlayer in all BINAC arrays. Formation of an alternate arrangement from racemic BINAC indicates that heterointermolecular interactions (R-BINAC vs S-BINAC) are stronger than homo-intermolecular interactions (R-BINAC vs R-BINAC and S-BINAC vs S-BINAC). If the reverse were true, then a mixture of two ordered domains, each consisting of alternative enantiomers, would appear, as was observed for chiral liquid crystal compounds.14 These results are clearly consistent with the melting points of the enantiomers and racemic BINAC, which are 166 and 271 °C, respectively.31-33 The higher melting point of the racemic modification and the fact that it can be easily purified by recrystallization strongly support the hypothesis that hetero-intermolecular interactions between isomers are stronger than homo-intermolecular interactions. Hetero-enantiomer coupling would occur even in the bulk crystal phase. In general, structures of molecular adlayers are controlled by a balance between molecule-substrate interactions and intermolecular interactions in a thermodynamic point of view. The substrate also plays an important role in molecular adsorption. However, in the case of AISO, especially for the adsorption system of BINAC on I/Au (111), intermolecular interactions are dominant over the interactions from the substrate. Therefore, tiny structural differences between enantiomers are obviously reflected into BINAC adlayer structures. In conclusion, the stereochemical molecular arrangement in BINAC arrays obviously reflected the chirality of R-, S-, and racemic BINAC. The stronger heterointermolecular interaction of enantiomer couples led to the formation of an alternate arrangement in the array prepared by racemic modification. These results suggest that it is possible to visualize optically active reactions at the chirality center to elucidate mechanisms of the catalytic reaction. Visualization of 2-D supramolecular structures prepared by adsorption-induced self-organization provides fundamental knowledge on intermolecular interactions between chiral molecules. Acknowledgment. This work was supported in part by CREST-JST and ERATO-JST. The authors acknowledge Dr. Y. Okinaka for his assistance in writing this manuscript. LA051040P
