NANO LETTERS
Toward Two-Dimensional Supramolecular Control of Hydrogen-Bonded Arrays: The Case of Isophthalic Acids
2003 Vol. 3, No. 11 1485-1488
Steven De Feyter,*,† Andre´ Gesquie`re,† Markus Klapper,‡ Klaus Mu1 llen,‡ and F. C. De Schryver† Katholieke UniVersiteit LeuVen (K.U. LeuVen), Department of Chemistry, Celestijnenlaan 200 F, B-3001 LeuVen, Belgium, and Max-Planck-Institut fu¨r Polymerforschung, Ackermannweg 10, D-55021, Mainz, Germany Received June 23, 2003; Revised Manuscript Received September 25, 2003
ABSTRACT The two-dimensional pattern formation of hydrogen bonding isophthalic acid derivatives at the liquid/solid interface has been investigated using scanning tunneling microscopy. By varying the location and nature of alkyl substituents on the aromatic core in combination with the intrinsic hydrogen bonding properties of the isophthalic acid units, the two-dimensional supramolecular ordering has been controlled, leading to several different motifs.
Structuring surfaces is an important topic in the nanosciences. Quite some applications will need highly structured surfaces. Indeed, the control of the lateral assembly and spatial arrangement of micro- and nano-objects at interfaces is crucial when it comes to potential applications in the field of nanoscience. To create two-dimensional patterns, one can take advantage of ‘active’ manipulation techniques, such as photolithography or electron beam lithography,1 and ‘soft lithography’ techniques.2 Scanning probe microscopy (SPM) techniques, such as scanning tunneling microscopy (STM) and atomic force microscopy (AFM), are another class of techniques, which can be implemented for the controlled manipulation of matter.3-9 Self-assembly methods provide an alternative approach to making well-defined structures with dimensions on the nanometer scale. Self-assembly is a natural phenomenon that can be observed in many biological, chemical, and physical processes.10 Self-assembled monolayers (SAMs) are ordered molecular assemblies formed by the adsorption of an active surfactant on a solid surface.11 Many SAMs have been investigated, but monolayers of alkanethiolates on gold are the most popular ones. Several groups have taken advantage of the spontaneous formation of SAMs and the formation of nanometer-sized domains by the coadsorption of two or more adsorbates.12,13 * Corresponding author. E-mail:
[email protected] † Katholieke Universiteit Leuven. ‡ Max-Planck-Institut fu ¨ r Polymerforschung. 10.1021/nl034436z CCC: $25.00 Published on Web 10/18/2003
© 2003 American Chemical Society
Less attention has been given to the self-assembly of physisorbed layers at surfaces. In contrast to chemisorbed structures, physisorption is not very suitable for making ‘permanent’ architectures. Nevertheless, these physisorbed adlayers are model systems to investigate the interplay between molecular structure and the formation of ordered assemblies in two dimensions and can be studied in great detail with STM.14-18 A very convenient method for the formation of extended 2D structures is physisorption at the liquid/solid interface.19,20 The preparation is relatively simple, and STM allows a detailed investigation of the twodimensional patterns.21-24 Hydrogen bonding has been amply explored for selfassembly purposes in solution and in three-dimensional (3D) crystals.25,26 Although hydrogen bonding has been exploited for self-assembly at surfaces too,27-33 the control of the supramolecular patterns of a given hydrogen bonding synthon to form a variety of different 2D supramolecular structures has not received as much attention. Benzene carboxylic acid derivatives form an interesting class of hydrogen bonding compounds, of which a few members are shown in Scheme 1. Recently, it has been demonstrated that 1,3,5-benzenetricarboxylic acid (trimesic acid) forms various two-dimensional patterns, including the typical honeycomb pattern, both under UHV conditions and at the liquid/solid interface.34 1,4Benzenedicarboxylic acid (terephthalic acid) and 1,3-benzenedicarboxylic acid (isophthalic acid) are two other important members of this family. Terephthalic acid deriva-
Scheme 1. Chemical Structure of Trimesic Acid 1, Terephthalic Acid 2, and Isophthalic Acid 3
tives form typically one-dimensional rows when physisorbed at a surface.35 Crystal engineering deals with the study of the selfassembly in 3D, but those rules do not necessarily apply in two dimensions (2D) due to the reduced dimensionality and the presence of a substrate. It is our aim to gain insight into and control the two-dimensional pattern formation of hydrogen bonding compounds. Such control not only allows one to create ‘nice patterns’ but might eventually lead to highly functional surfaces. For instance the controlled positioning of certain reactive groups in a predefined pattern can give rise to successful topochemical reactions in two dimensions.36,37 In this letter, we demonstrate how completely different 2D motifs can be established by structural changes in the case of isophthalic acid 3 derivatives (Scheme 1), studied by scanning tunneling microscopy (STM) at the liquid/solid interface.20 Let’s consider, for example, the kind of motifs that could be formed when isophthalic acid assembles on a surface. The main feature of an isophthalic acid is the presence of two carboxylic acid groups in the 1,3-position. It is known that these carboxylic acid groups tend to dimerize, so the main interaction is expected to be hydrogen bonding. The two carboxylic acid groups are in the meta position, so the angle between both is 120°. The presence and relative position of the carboxylic acid groups leads to a number of different patterns, the more straightforward of which are shown in Scheme 2: a zig/zag tape (A); a socalled ‘crinkled tape’ (B),38 both types giving rise to rows; and ‘rosette’ structure (C), whereby a cyclic hexamer is formed. This study explores how to direct the supramolecular ordering of the isophthalic acid molecules to form specific supramolecular patterns on demand on a substrate, in casu graphite, and to investigate the balance between adsorbateadsorbate and adsorbate-substrate interactions. The approach followed is based on the substitution of the isophthalic acid groups with alkyl chains. Isophthalic acid derivatives with a linear alkyl chain in the 5-position (ISA-A) form a pattern as revealed by scanning tunneling microscopy at the liquid/solid interface (Figure 1A). The aromatic isophthalic acid (ISA) groups show up bright while the alkyl chains are darker. The ISA groups are arranged in a double row, adopting a distorted ‘zig/zag’ pattern. The distance between equivalent isophthalic acid rows measured along the alkyl chains (a) is 36 ( 2 Å. The alkyl chains are fully extended, interdigitated, closely packed, and almost perpendicular to the lamella axis. Both rows of adjacent ISA-A molecules are hardly shifted with respect to 1486
Scheme 2. Possible Optimal Hydrogen Bond Directed Patterns of Isophthalic Acid Derivatives ((A) zig/zag motif; (B) crinkled tape; (C) rosette)
each other, and the intermolecular distance between ISA moieties along a row measures only 9.4 Å ( 0.2 Å (b). This is in contrast with the situation in 3D crystals of the same type of molecule, where the distance between ISA groups is 15.7 Å.35b As a result, optimal hydrogen bonding between ISA groups based upon the classical carboxylic acid dimer is not formed at the liquid/solid interface. If one compares the ordering of the molecules with respect to the substrate underneath, it’s clear that the graphite surface has a pronounced influence on the ordering of the molecules, as the alkyl chains are running parallel to one of the main graphite symmetry axes. This has been observed for the adsorption of numerous alkyl-containing molecules at the liquid/graphite interface. The reason is the similarity between the periodicity of the carbon atoms (hydrogen atoms) along an alkyl chain and the distance between the hexagons of the graphite substrate.22 The interaction of the alkyl chains with the graphite support is also expressed by the orientation of adjacent domains, which reflects the 3-fold symmetry of the graphite lattice (not shown). In a second stage, an isophthalic acid derivative with two alkyl chains in the 4- and 6-positions was investigated. This substitution pattern should lead to pronounced differences in the packing, as doubling the number of alkyl chains with respect to the isophthalic acid groups should have an effect on the close packing of the molecules (ISA-B). In the STM image in Figure 1B, alkyl chains are fully extended, interdigitated, and are oriented perpendicularly with respect to the lamella axis. In contrast to ISA-A, the ISA groups form no continuous rows. Along a row, two bright spots and a gray spot appear in an alternating way: the ISA groups are bright; the gray spot suggests the absence of an ISA moiety: the molecules form a ‘quasi-crinkled’ tape pattern. The ‘empty’ space might be occupied by mobile solvent molecules or weakly adsorbed alkyl chains. The distance between adjacent ISA groups along a row measures 9.4 ( 0.2 Å (b). The distance between the bright dimers is 28 Å Nano Lett., Vol. 3, No. 11, 2003
Figure 1. Chemical structures, representative STM image and schematic motif of isophthalic acid derivatives physisorbed at the liquid/graphite interface: (A) ISA-A (Iset ) 1.0 nA; Vbias ≈ - 0.7 V (sample negative)). (B,C) ISA-B (A: Iset ) 1.0 nA; Vbias ) -0.51 V. B: Iset ) 1.0 nA; Vbias ) -0.29 V). (D) ISA-C (Iset ) 1.0 nA; Vbias ) -0.82 V). (E) ISA-D (Iset ) 1.0 nA; Vbias ) -0.8 V). The scale bar in the STM images (lower left) represents 2 nm. Some simple stick models are indicated in the STM images: the circle and the sticks represent the aromatic ISA groups and alkyl chains, respectively. The schematic motifs indicate the ordering of the ISA cores and the direction of the carboxylic acid groups. In all cases, the solvent is 1-heptanol or 1-octanol, except for ISA-A, where the solvent is 1-phenyloctane. In D, an arrow indicates coadsorbed solvent molecules (1-octanol).
(c). The distance between equivalent rows of isophthalic acid groups measured along the alkyl chains is 34.3 ( 1.3 Å (a). To achieve this ordering, not all of the molecules can have both alkyl chains adsorbed on the graphite substrate, as shown by the stick model. In addition to this pattern, other polymorphous 2D motifs are observed, such as given in Figure 1C. Two important differences in comparison with Nano Lett., Vol. 3, No. 11, 2003
the first type of lamellar ordering can be observed. First of all, the angle formed by the alkyl chains and the lamella axis is about 60°. For the packing discussed above this was 90°. Second, the hydrogen-bonding pattern is distinctly different. A zig/zag pattern of the ISA groups can be observed. The distance between the ISA groups (b) is 16.7 ( 1.6 Å. This value is in good agreement with the repeat distance of the ISA groups in a hydrogen bonding tape in 3D crystals of 4,6-dialkoxy substituted isophthalic acid derivatives (∼15.8 Å) and allows for almost optimal hydrogen bonding. Note that there is a one-to-one ratio between the isophthalic acid groups and the alkyl chains adsorbed on the substrate. Thus, only half of the alkyl chains are adsorbed. The distance measured along the direction of the alkyl chains (a) is 35.7 ( 1.1 Å, comparable with the value found for the first packing. How to induce the formation of hexameric ring-type patterns? One possibility could be the use of “branched” alkyl chains (see Figure 1D). The steric hindrance of the alkyl chains could lead to the formation of cyclic structures. However, when adsorbed from a 1-octanol solution the branched molecule (ISA-C) shows a pattern at the lamella level similar to that of ISA-A. The ratio between the ISA groups and the alkyl chains is 1:1, so it can be concluded that one alkyl chain is adsorbed on the surface, while the second chain is pointing out of the plane of the monolayer into the liquid phase. Note that in this case, 1-octanol molecules are co-adsorbed, separating adjacent isophthalic acid lamellae. Co-deposition of solvent molecules has been observed for a number of isophthalic acid derivatives but will not be discussed here.24 The intermolecular distance has been found to be (b) 9.6 ( 0.1 Å, and the lamella width (a) is 25.3 ( 0.6 Å. The width including a solvent lamella is 37.9 ( 0.5 Å. The adsorption of only one ‘arm’ is, in retrospect, not that surprising given that ISA-B also revealed that not all of the alkyl chains need to be adsorbed on the substrate. In contrast, a new motif is formed when bulky benzhydryl groups are introduced at the end of a long alkyl chain (ISA-D). Figure 1E is an STM image of this molecular arrangement at the liquid/graphite interface. ISA-D exclusively shows the formation of rosette structures containing six molecules. The rosettes are by themselves organized in a hexagonal arrangement. The bright features are attributed to the location of the aromatic ISA groups, and close inspection shows that indeed every ring is composed of six bright spots. The interior of these rosettes might be filled with mobile solvent molecules. The benzhydryl and the alkyl groups are not clearly visible in the STM image, which we attribute to the mobility of the chains on the graphite surface caused by the lack of interchain van der Waals interactions and free volume. In conclusion, we have demonstrated in 2D how, by the combination of hydrogen bonding units, their relative position, and the nature, number, and location of alkyl chains, completely different motifs can be formed. The competition between the intermolecular van der Waals contact and hydrogen bonding, combined with adsorbate-substrate interactions, determines the observed molecular patterning. 1487
Careful design of the molecules allows some control of the supramolecular ordering. The application of these twodimensional patterns as templates is in progress. Acknowledgment. The authors thank the DWTC, through IUAP-V-03, the Institute for the Promotion of Innovation by Sciences and Technology in Flanders (IWT). S.D.F. is a postdoctoral fellow of the Fund for Scientific ResearchFlanders. The collaboration was made possible thanks to the TMR project SISITOMAS and a Max-Planck Research Award. Supporting Information Available: Synthesis details. This material is available free of charge via the Internet at http://pubs.acs.org. References (1) Handbook of Microlithography, Micromachining, and Microfabrication; Rai-Choudhury, P., Ed.; SPIE Optical Engineering Press: London, 1997. (2) Xia, Y.; Whitesides, G. M. Angew. Chem., Int. Ed. Engl. 1998, 37, 550. (3) Binnig, G.; Rohrer, H.; Gerber, C.; Weibel, E. Phys. ReV. Lett. 1982, 49, 57. (4) Binnig, G.; Quate, C. F.; Gerber C. Phys. ReV. Lett. 1986, 56, 930. (5) Kim, Y.-T.; Bard, A. J. Langmuir 1992, 8, 1096. (6) Ross, C. B.; Sun, L.; Crooks, R. M. Langmuir 1993, 9, 632. (7) Liu, G.-Y.; Xu, S.; Qian, Y. Acc. Chem. Res. 2000, 33, 457. (8) Mu¨ller, W. T.; Klein, D. L.; Lee, T.; Clarke, J.; McEuen, P. L.; Schultz, P. G. Science 1995, 268, 272. (9) Maoz, R.; Cohen, S.R.; Sagiv, J. AdV. Mat. 1999, 11, 55. (10) ComprehensiVe supramolecular chemistry; Atwood, J. L., Davies, J. E. D., Macnicol, D. D., Vogtle, F., Eds.; Pergamon Press: New York, 1996. (11) Ulman, A. Chem. ReV. 1996, 96, 1533, and references therein. (12) Bain, C. D.; Whitesides, G. M. J. Am. Chem. Soc. 1988, 110, 6560. (13) Lewis, P. A.; Donhauser, Z. J.; Mantooth, B. A.; Smith, R. K.; Bumm, L. A.; Kelly, K. F.; Weiss, P. S. Nanotechnology 2001, 12, 231, and references therein. (14) Yokoyama, T.; Yokoyama, S.; Kamikado, T.; Okuno, Y.; Mashiko, S. Nature 2001, 413, 619. (15) Bo¨hringer, M.; Morgenstern, K.; Schneider, W.-D.; Berndt, R.; Mauri, F.; De Vita, A.; Car, R. Phys. ReV. Lett. 1999, 83, 324. (16) Weckesser, J.; De Vita, A.; Barth, J. V.; Cai, C.; Kern, K. Phys. ReV. Lett. 2001, 87, 6101. (17) Dmitriev, A.; Lin, N.; Weckesser, J.; Barth, J. V.; Kern, K. J. Phys. Chem. B 2002, 106, 6907. (18) Lorenzo, M. O.; Baddeley, C. J.; Muryn, C.; Raval, R. Nature 2000, 404, 376. (19) McGonical, G. C.; Bernhardt, R. H.; Thomson, D. J. Appl. Phys. Lett. 1990, 57, 28. (20) Rabe, J. P.; Buchholz, S. Science 1991, 253, 424.
1488
(21) Frommer, J. Angew. Chem., Int. Ed. Engl. 1992, 31, 1298. (22) Cyr, D. M.; Venkataraman, B.; Flynn, G. W. Chem. Mater. 1996, 8, 1600. (23) Giancarlo, L. C.; Flynn, G. W. Annu. ReV. Phys. Chem. 1998, 49, 297. (24) De Feyter, S.; Gesquie`re, A.; Abdel-Mottaleb, M. M.; Grim, P. C. M.; De Schryver, F. C.; Meiners, C.; Sieffert, M.; Valiyaveettil, S.; Mu¨llen, K. Acc. Chem. Res. 2000, 33, 8, 520. (25) Subramanian, S.; Zaworotko, M. J. Coord. Chem. ReV. 1994, 137, 357. (b) Fan, E.; Vicent, C.; Geib, S. J.; Hamilton, A. D. Chem. Mater. 1994, 6, 1113. (c) MacDonald, J. C.; Whitesides, G. M. Chem. ReV. 1994, 94, 2383. (26) Zaworotko, M. J. Chem. Commun. 2001, 1, 1. (b) Alcala, R.; Martinez-Carrera, S. Acta Crystallogr. 1972, B28, 1671. (c) Etter, M. C.; Urbanczyk-Lipkowska, Z.; Jahn, D. A.; Frye, J. S. J. Am. Chem. Soc. 1986, 108, 5871. (d) Valiyaveettil, S.; Enkelmann, V.; Mu¨llen, K. J. Chem. Soc., Chem. Commun. 1994, 2097. (e) Enkelmann, V.; Valiyaveettil, S.; Moessner, G.; Mu¨llen, K. Supramol. Sci. 1995, 2, 3. (d) Yang, J.; Marendaz, J.-L.; Geib, S. J.; Hamilton, A. D. Tetrahedron Lett. 1994, 35, 22, 3665. (27) Weckesser, J.; De Vita, A.; Barth, J. V.; Cai, C.; Kern, K. Phys. ReV. Lett. 2001, 87, 9, 6101. (28) Keeling, D. L.; Oxtoby, N. S.; Wilson, C.; Humphry, M. J.; Champness, N. R.; Beton, P. H. Nano Lett. 2003, 3, 9. (29) De Feyter, S.; De Schryver, F. C. Chem. Soc. ReV. 2003, 32, 3, 139, and references therein. (30) Gottarelli, G.; Masiero, S.; Mezzina, E.; Pieraccini, S.; Rabe, J. P.; Samori, P.; Spada, G. P. Chem. Eur. J. 2000, 6, 17, 3242. (31) Buchholz, S.; Rabe, J. P. Angew. Chem., Int. Ed. Engl. 1992, 31, 189. (32) Lei, S. B.; Wang, C.; Yin, S. X.; Wang, H. N.; Wi, F.; Liu, H. W.; Xu, B.; Wan, L. J.; Bai, C. L. J. Phys. Chem. B 2001, 105, 10838. (33) Wintgens, D.; Yablon, D. G.; Flynn, G. W. J. Phys. Chem. B 2003, 107, 1, 173. (34) Ishikawa, U.; Ohira, A.; Sakata, M.; Hirayama, C.; Kunitake, M. Chem. Commun. 2002, 2652. (b) Griessl, S.; Lackinger, M.; Edelwirth, M.; Hietschold, M.; Heckl, W. M. Single Mol. 2002, 1, 25. (c) Dmitriev, A.; Lin, N.; Weckesser, J.; Barth, J. V.; Kern, K. J. Phys. Chem. B 2002, 106, 6907. (d) Messina, P.; Dmitriev, A.; Lin, N.; Spillmann, H.; Abel, M.; Barth, J. V.; Kern, K. J. Am. Chem. Soc. 2002, 124, 27, 14000. (35) De Feyter, S.; Gesquie`re, A.; Abdel-Mottaleb, M. M.; Grim, P. C. M.; De Schryver, F. C.; Meiners, C.; Sieffert, M.; Valiyaveettil, S.; Mu¨llen, K. Acc. Chem. Res. 2000 33, 520. (b) Eichhorst-Gerner, K.; Stabel, A.; Moessner, G.; Declercq, D.; Valiyaveettil, S.; Enkelmann, V.; Mu¨llen, K.; Rabe, J. P. Angew. Chem., Int. Ed. Engl. 1996, 35, 1492. (36) Grim, P. C. M.; De Feyter, S.; Gesquie`re, A.; Vanoppen, P.; Ru¨cker, M.; Valiyaveettil, S.; Moessner, G.; Mu¨llen, K.; De Schryver, F. C. Angew. Chem., Int. Ed. Engl. 1997, 36, 2601. (b) Okawa, Y.; Aono, M. Nature 2001, 409, 683. (37) Abdel-Mottaleb, M. M. S.; De Feyter, S.; Gesquie`re, A.; Sieffert, M.; Mu¨llen, K.; De Schryver, F. C. Nano Lett. 2001, 1, 353. (38) Zerkowski, J. A.; Seto, C. T.; Whitesides, G. M. J. Am. Chem. Soc. 1992, 114, 5473.
NL034436Z
Nano Lett., Vol. 3, No. 11, 2003
