Two-Dimensional Self-Assembly into Multicomponent Hydrogen

NANO LETTERS

Two-Dimensional Self-Assembly into Multicomponent Hydrogen-Bonded Nanostructures

2005 Vol. 5, No. 1 77-81

Steven De Feyter,*,† Atsushi Miura,†,| Sheng Yao,‡ Zhijian Chen,‡ Frank Wu1 rthner,*,‡ Pascal Jonkheijm,§ Albertus P. H. J. Schenning,*,§ E. W. Meijer,§ and Frans C. De Schryver† Laboratory of Photochemistry and Spectroscopy, Department of Chemistry, Katholieke UniVersiteit LeuVen, Celestijnenlaan 200-F, 3001, LeuVen, Belgium, Institut fu¨r Organische Chemie, UniVersita¨t Wu¨rzburg, Am Hubland, D-97074 Wu¨rzburg, Germany, and Laboratory of Macromolecular and Organic Chemistry, EindhoVen UniVersity of Technology, P. O. Box 513, 5600 MB, The Netherlands Received October 5, 2004; Revised Manuscript Received November 30, 2004

ABSTRACT By means of scanning tunneling microscopy, we have explored the two-dimensional self-assembly of functional bicomponent hydrogenbonding dye systems, leading to well-defined patterns, different from those of the individual components, and providing design rules to immobilize multicomponent systems at the liquid−solid interface.

Gaining control of two-dimensional (2D) pattern formation might turn out to be crucial to the design and properties of functional surfaces.1 Noncovalent interactions are attractive tools to control the ordering of molecules on a surface. Hydrogen bonds are highly selective and directional, though moderately strong, and they have been extensively applied to direct the molecular ordering on surfaces, both under UHV conditions and at the air-solid or liquid-solid interface, as revealed by scanning tunneling microscopy (STM).2,3 Given that the fabrication of highly ordered monocomponent supramolecular structures by design at surfaces is not always trivial, the controlled formation of multicomponent assemblies with a well-defined ordering creates an even bigger challenge. Many binary mixtures investigated so far show phase separation on the nanometer scale or the formation of randomly mixed monolayers.4 A few reports deal with the successful formation of well-defined heterocomplexes based upon hydrogen bonding.5 In these cases, typically, only a small number of hydrogen bonding sites per molecule are involved, and/or the final outcome of the heterocomplexation process leads to patterns dominated by one of the components. An exception is the network formation between * Corresponding authors. E-mail: [email protected]; [email protected]; [email protected] † Katholieke Universiteit Leuven. ‡ Universita ¨ t Wu¨rzburg. § Eindhoven University of Technology. | Current address: Division of Microelectronic Device Science, Nara Institute of Science and Technology, Takayama 8916-5, 630-0192, Ikoma, Japan. 10.1021/nl048360y CCC: $30.25 Published on Web 12/21/2004

© 2005 American Chemical Society

perylene bisimide and 1,3,5-triazine-2,4,6-triamine,5d as observed under UHV conditions. Perylene bisimides and merocyanines with hydrogenbonding receptor sites are little soluble pigment dyestuffs which are the subject of intensive research due to their interesting optoelectronic6 and supramolecular organization properties.7 However, studies on heterocomplex formation of such little soluble pigment dyestuffs on surfaces is a difficult task, one which is possible only under UHV conditions and requires proper control of evaporation rate, substrate temperature, etc. Therefore, we have synthesized derivatives PBI and MBA (Figure 1) bearing tert-butylphenoxy (PBI) and alkyl (MBA) substituents which afford high solubility of these dyes in most organic solvents. Both dyes have two sets of hydrogen bonding sites, i.e., -CO-NHCO- (imide), where NH is a hydrogen bond donor (D) and CO a hydrogen bond acceptor (A), giving rise to an A-D-A sequence. Most importantly, the relative orientation of the hydrogen bonding units differs in both molecules: in PBI both A-D-A sets are parallel but facing opposite directions, while in MBA these sets are at an angle of ∼120°. In this contribution we aim at the formation of well-defined heterocomplexes based upon multitopic hydrogen bonding at the liquid-solid interface and to correlate the composition and structure of the observed heterocomplexes and the extent and quality of their long-range ordering with the orientation of the hydrogen bonding sites in the molecules. For the formation of well-defined heterocomplexes, an alkylated

Table 1. Unit Cell Parameters compound

a (nm)

b (nm)

γ (°)

DAT 2.27 ( 0.03 2.85 ( 0.06 66 ( 4 DAT-MBA-DAT 4.3 ( 0.1 4.72 ( 0.1 71 ( 4 DAT-PBI-DAT 2.24 ( 0.03 4.6 ( 0.1b 72 ( 4

space no. group moleculesa p2 p2 p2

2 6 3

a Number of molecules in unit cell. b Extended domains of DAT-PBIDAT are not formed. The unit cell only describes the parameters between two adjacent rows.

Figure 1. Chemical structures of the diamino triazine derivative (DAT), merocyanine barbituric acid dye (MBA), and perylene bisimide derivative (PBI), and the anticipated formation of DATMBA-DAT and DAT-PBI-DAT heterocomplexes.

diamino triazine derivative (DAT) having a D-A-D hydrogen bonding set, i.e., NH-N-NH, is selected to form complementary hydrogen bonding with the A-D-A sequence. This type of interaction should favor heterocomplex formation as recently found in solution.8 It is anticipated that the interaction between DAT and MBA will give rise to a termolecular complex having a triangle configuration. Also, DAT and PBI are expected to form a termolecular complex, though now in a linear fashion (see Figure 1). This difference may also strongly affect their long range ordering.

Before targeting heterocomplex formation, the self-assembly of the pure compounds was addressed. A drop of a solution of the compound in 1-phenyloctane was applied to a freshly cleaved surface of highly oriented pyrolytic graphite (HOPG). Upon spontaneous monolayer formation, STM images were acquired in the variable current mode by scanning with the STM tip immersed in solution. The measured tunneling currents are converted into a gray scale: black (white) refers to a low (high) measured tunneling current. Typical STM images of the 2D ordering of DAT are shown in Figure 2A-B, respectively. The large scale image in Figure 2A shows the formation of different domains. The bright spots correspond to the aromatic part of the molecule. Based on the unit cell parameters (Table 1), DAT is selfassembled into dimers (Figure 2B). All alkyl chains are adsorbed on the substrate. The molecules are expected to form two hydrogen bonds according to the tentative model of the unit cell indicated in Figure 2C. MBA self-assembles in rows (not shown), though the exact nature of the intermolecular interaction could not be established. It turned out not to be possible to visualize PBI molecules that we attribute to their high mobility, which results from the highly distorted π-conjugated system of tetraphenoxy-substituted perylene bisimides (torsion angle of ∼30° between the two naphthalene imide half units) leading to weak molecule-substrate interactions.9,10 In contrast, the parent flat perylene bisimide has been successfully imaged under UHV conditions11 and in the presence of melamine; a 2D honeycomb network is formed through triple hydrogen bonding.5d The mixtures give rise to heterocomplex formation and their structures are in line with the expectations based upon

Figure 2. STM images of monolayers of DAT at the liquid-solid interface. (A) Image size is 100 × 100 nm2. (B) Image size is 13.5 × 13.5 nm2. Unit cell is indicated in yellow. (C) Tentative model. 78

Nano Lett., Vol. 5, No. 1, 2005

Figure 3. STM image of a monolayer of DAT-MBA-DAT heterocomplexes at the liquid-solid interface self-assembled from a 1-phenyloctane solution of DAT/MBA. (A) Image size is 11.5 × 11.5 nm2. Unit cell is indicated in yellow. (B) Tentative molecular model with unit cell. Alkyl chains are colored in green to highlight the location and orientation of the hydrogen bonded structures.

the orientation of the hydrogen bonding units. Figure 3A shows the 2D self-assembled pattern formed after deposition of a drop from a 1-phenyloctane solution of DAT/MBA. Clusters of three bright spots indicate the formation of termolecular complexes. Such arrangement was never observed for the pure components. The orientation of the alkyl chains is clearly visible. All alkyl chains are adsorbed on the surface and the orientation of the alkyl chains runs close to parallel (within 10°) to a main graphite symmetry axis. There is a clear indication that two different species are involved in the termolecular complex formation. Two spots appear identical while the third one is longer and has a peculiar contrast. In line with our hypothesis (Figure 1), we identify the central one as being MBA flanked by two DAT Nano Lett., Vol. 5, No. 1, 2005

molecules, forming a DAT-MBA-DAT complex. The shape of the heterocomplexes (triangular) reflects the constraints imposed by the hydrogen bonding moieties on the MBA unit. A tentative model is shown in Figure 3B. In analogy, also mixtures of DAT and PBI give rise to mixed monolayers. Figure 4A is a large scale image of a physisorbed monolayer after applying a drop of a 1-phenyloctane solution of DAT/PBI on HOPG. In addition to the typical features of DAT monolayers (compare with Figure 2B), other structures appear, which are the PBI molecules, only locally leading to heterocomplexes. DAT-PBI-DAT complexes are indicated by the sequence of a yellow, red, and yellow arrow, indicating the two DAT molecules (yellow) and PBI moiety (red), respectively. DAT dimers are indicated by dashed yellow arrows. The high resolution image in Figure 4B clearly reveals that PBIs are stacked in rows flanked at both sides with a row of DAT molecules, in line with the interaction model shown in Figure 1. In this image, the PBIs appear with submolecular resolution and the substituents in the bay area can be identified. The location of and distance between PBIs is determined by the pattern formed by the DAT molecules (see Table 1). The PBIs seemingly fit within the DAT matrix (PBI rows run parallel with DAT dimer rows) and the intermolecular distance between adjacent PBI molecules is identical to the unit cell vector a of the DAT matrix. As a result, PBI is trapped and immobilized within the monolayer. The formation of termolecular complexes and their composition (2 DAT molecules and one MBA or PBI molecule) and geometry (at an angle of ∼120° for DATMBA-DAT or linear for DAT-PBI-DAT), which is in line with predictions (Figure 1), is a clear indication that hydrogen bonding is indeed involved. Heterocomplex formation is most easily achieved by hydrogen bonding if the intermolecular interaction strength of the different partners exceeds the one at play for the pure components, as observed in the present study. Though in both cases the hydrogen bonding interactions are similar (identical hydrogen bonding sets are involved) and termolecular complexes are formed, there are important differences at the monolayer level. A DAT/MBA mixture leads to the formation of true 2D crystals: heterocomplexes are exclusively formed and they organize in large domains. In contrast, DAT-PBI-DAT complexes are formed only locally, they assemble in rows, they do not cover complete domains, and they coexist with noncomplexed DAT dimers within the same domain. Moreover, also the conditions to achieve heterocomplex formation and 2D ordering are different. For the DAT/MBA system, the solution contains the compounds in a 2:1 ratio, anticipating and leading to termolecular (2:1) complex formation. Adding the compounds in sequence did not give rise to formation of monolayers of the complex: only domains with the pure compounds were observed. To successfully form DAT-PBIDAT complexes at the liquid-solid interface, it was necessary to add a large excess of PBI, typically in a 1:9 ratio. Experiments with DAT and PBI in a 2:1 ratio, as suggested by the anticipated termolecular (2:1) complex formation and 79

Figure 4. STM images of monolayers at the liquid-solid interface self-assembled from a DAT/PBI solution. (A) Image size is 23.5 × 23.5 nm2. Unit cell is indicated in yellow. (B) Image size is 12.3 × 8.8 nm2. The heterocomplexes are indicated by arrows: yellow (DAT), red (PBI), yellow (DAT). Dashed yellow arrows refer to DAT molecules which are not complexed with PBI. (C) Tentative model. A unit cell is indicated in yellow. Some alkyl chains are indicated in green to highlight the hydrogen bonding interaction between the DAT and PBI moieties.

bulk studies on related compounds which indicate the formation of termolecular complexes, did not turn out to be successful. Even at a large excess of PBI, the images obtained in the beginning of a measuring session reveal only domains composed of DAT dimers. In time, mixed domains start to appear. This time dependence was also observed for the coadsorption of 1-octanol molecules in a monolayer formed by alkylated isophthalic acid derivatives.5e What causes both systems to differ in the approach to achieve 2D ordering and the extent of 2D ordering? The differences in behavior between both systems do not imply that hydrogen bond interactions in the DAT-PBI-DAT complexes are weaker than in the DAT-MBA-DAT complexes. Differences in interaction of the pure components and the heterocomplexes with the substrate are more likely to play a key role. Both DAT and MBA show a higher affinity for graphite than PBI as expressed by the fact that both DAT and MBA self-assemble on the surface into ordered monolayers with laterally immobilized molecules, while it was not possible to visualize PBI under analogous experimental conditions, which can be understood qualitatively in the following way. In the case of DAT and MBA, both compounds have an equal number of alkyl chains of similar length. These alkyl chains lead to a favorable interaction with HOPG and give rise to a similar adsorption behavior of both compounds. In contrast, the lack of extended alkyl chains and the significant twisting of the π-system caused by the phenoxy substituents in the bay area of PBI do not favor adsorption to the same extent as observed for the other compounds. The very high excess of PBI required to induce heterocomplex adsorption, in combination with the fact that PBI 80

coadsorbs with DAT only gradually in time, leads to the conclusion that the formation of a DAT layer is at least kinetically favored compared to PBI and heterocomplex adsorption. The fact that successful DAT-MBA-DAT heterocomplex formation is observed at the anticipated ratio and that heterocomplexes are formed exclusively when a mixture of the compounds is applied stress the strength of the hydrogen bonding between both partners and suggest that both compounds and their complex have a similar affinity for the substrate and that their adsorption kinetics are similar. This is not in contradiction with the lack of heterocomplex formation upon adding both components in sequence. Given that a DAT layer is formed, the formation of heterocomplexes after addition of MBA requires drastic changes in the already formed monolayer packing of DAT, most likely involving desorption of the DAT molecules and adsorption of the complex. Note that the measurements are performed at the liquid-solid interface, allowing continuous exchange of adsorbed molecules with those in solution.12 However, it can take several tens of minutes before a given molecule desorbs.12 In contrast, coadsorption of PBI molecules does not require major changes in the DAT monolayer structure formed. DAT molecules do not need to desorb. Due to the symmetric location of the hydrogen bonding sites on PBI (at 180° for PBI versus at 120° for MBA), PBI molecules fit rather well within the DAT matrix without destroying the latter, leading to rows of heterocomplexes. The formation of 1D rows of heterocomplexes minimizes the effect on the DAT matrix. As a result, the formation of a pure 2D crystalline phase of these heterocomplexes is not favored. An option to increase the DAT-PBI-DAT content is to increase the PBI concentration. Nano Lett., Vol. 5, No. 1, 2005

The data presented suggest some guidelines for the successful preparation of heterocomplexes based upon hydrogen bonding at the liquid/solid interface and the creation of two-dimensional networks. Heterocomplex formation on surfaces will be favored if, in addition to noncovalent interactions between the different partners which exceed the intermolecular interactions between molecules of the same kind, the tendency of adsorption of the heterocomplexes is higher or comparable to the pure components. If so, heterocomplexes formed in solution may successfully adsorb on the substrate and grow to 2D crystals. In the case that heterocomplexes have a lower affinity for adsorption on the substrate than (one of) the pure components, the expected higher affinity of (one of) the pure compounds for the substrate will favor the adsorption of the pure compound(s) and destabilize the heterocomplexes formed in solution. Therefore, heterocomplex formation on the substrate should be disfavored. However, in case the heterocomplex formation does not disturb significantly the 2D ordering of the dominant species (i.e., DAT for the DAT/PBI system), heterocomplex formation might still be observed. Upon successful heterocomplex formation on the substrate, the formation of 2D crystalline domains will be favored if the structure of the heterocomplexes is not compatible with the ordering of (one of) the pure components. For a high degree of compatibility, heterocomplexes might form mixed domains with one of the pure components, though increasing the concentration of the weakly adsorbing compound should lead to an increase of the heterocomplex content. In conclusion, multitopic hydrogen bonding interactions may lead to the formation of well-defined heterocomplexes at the liquid-solid interface. These interactions can lead to immobilization of otherwise too mobile species. The structure of the heterocomplexes depends on the orientation of the hydrogen bonding sites. 2D networks may be formed if, upon successful heterocomplex formation on the surface, the supramolecular ordering differs sufficiently from that of the individual components, which in this case depends on the orientation of the hydrogen bonding sites. Furthermore, symmetry effects play an important role in the monolayer structure affecting 2D crystallization. The case study presented here provides additional insight in controlling selfassembly at surfaces, in particular of multicomponent systems, which is of importance to the construction of functional surfaces. Acknowledgment. The authors thank the Federal Science Policy through IUAP-V-03, the Institute for the promotion of innovation by Science and Technology in Flanders (IWT), the Fund for Scientific Research-Flanders (FWO), and the Deutsche Forschungsgemeinschaft (DFG). S.D.F. is a postdoctoral fellow of FWO. A.M. thanks FWO and KULeuven

Nano Lett., Vol. 5, No. 1, 2005

for a postdoctoral fellowship. The work in Eindhoven is supported by The Netherlands Organization for Scientific Research (NWO, CW). References (1) (a) Bumm, L. A.; Arnold, J. J.; Cygan, M. T.; Dunbar, T. D.; Burgin, T. P.; Jones, L. D., II; Allara, L.; Tour, J. M.; Weiss, P. S. Science 1996, 271, 1705. (b) 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. (c) Maoz, R.; Frydman, E.; Cohen, S. R.; Sagiv, J. AdV. Mater. 2000, 12, 725. (d) Ja¨ckel, F.; Watson, M. D.; Mu¨llen, K.; Rabe, J. P. Phys. ReV. Lett. 2004, 98, 188303. (2) De Feyter, S.; De Schryver, F. C. Chem. Soc. ReV. 2003, 32, 139, and references therein. (3) (a) Barth, J. V.; Weckesser, J.; Cai, C.; Gu¨nter, P.; Bu¨rgi, L.; Jeandupeux, O.; Kern, K. Angew. Chem., Int. Ed. Engl. 2000, 39, 1230. (b) 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. (c) Griessl, S.; Lackinger, M.; Edelwirth, M.; Hietschold, M.; Heckl, W. M. Single Mol. 2002, 3, 25. (d) Keeling, D. L.; Oxtoby, N. S.; Wilson, C.; Humphry, M. J.; Champness, N. R.; Beton, P. H. Nano Lett. 2003, 3, 9. (4) (a) Venkataraman, B.; Breen, J. J.; Flynn, G. W. J. Phys. Chem. 1995, 99, 6608. (b) Stevens, F.; Dyer, D. J.; Walba, D. M. Langmuir 1996, 12, 436. (c) Hipps, K. W.; Lu, X.; Wang, X. D.; Mazur, U. J. Phys. Chem. 1996, 100, 11207. (d) Baker, R. T.; Mougous, J. D.; Brackley, A.; Patrick, D. L. Langmuir 1999, 15, 4884. (e) Padowitz, D. F.; Sada, D. M.; Kemer, E. L.; Dougan, M. L.; Xue, W. A. J. Phys. Chem. B 2002, 106, 593. (f) De Feyter, S.; Larsson, M.; Schuurmans, N.; Verkuijl, B.; Zoriniants, G.; Gesquie`re, A.; AbdelMottaleb, M. M.; van Esch, J.; Feringa, B. L.; van Stam, J.; De Schryver, F. C. Chem. Eur. J. 2003, 9, 198. (5) (a) Eichhorst-Gerner, K.; Stabel, A.; Moessner, G.; Declercq, D.; Valiyaveettil, S.; Enkelmann, V.; Mu¨llen, K.; Rabe, J. P. Angew. Chem., Int. Ed. 1996, 35, 1492. (b) Qian, P.; Nanjo, H.; Yokoyama, T.; Suzuki, T. M.; Akasaka, K.; Orhui, H. Chem. Commun. 2000, 2021. (c) Wintgens, D.; Yablon, D. G.; Flynn, G. W. J. Phys. Chem. B 2003, 107, 173. (d) Theobald, A.; Oxtoby, N. S.; Phillips, M. A.; Champness, N. R.; Beton, P. H. Nature 2003, 424, 1029. (e) De Feyter, S.; Gequie`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. (6) For perylene bisimides see, for example: (a) Schmidt-Mende, L.; Fechtenko¨tter, A.; Mu¨llen, K.; Moons, E.; Friend, R. H.; MacKenzie, J. D. Science 2001, 293, 1119. (b) Law, K.-Y. Chem. ReV. 1993, 93, 449. For merocyanine dyes see, for example: (c) Wu¨rthner, F.; Wortmann, R.; Meerholz, K. ChemPhysChem 2002, 3, 17. (7) For a review on perylene bisimide dyes: Wu¨rthner, F. Chem. Commun. 2004, 1564. (8) Wu¨rthner, F.; Chen, Z.; Hoeben, F. J. M.; Osswald, P.; You, C.-C.; Jonkheijm, P.; van Herrikhuyzen, J.; Schenning, A. P. H. J.; van der Schoot, P. P. A. M.; Meijer, E. W.; Beckers, E. H. A.; Meskers, S. C. J.; Janssen, R. A. J. J. Am. Chem. Soc. 2004, 126, 10611. (9) Wu¨rthner, F.; Sautter, A.; Thalacker, C. Angew. Chem., Int. Ed. Engl. 2000, 39, 1243. (10) Hofkens, J.; Vosch, T.; Maus, M.; Kohn, F.; Cotlet, M.; Weil, T.; Herrman, A.; Mu¨llen, K.; De Schryver, F. C. Chem. Phys. Lett. 2001, 333, 255. (11) (a) Ludwig, C.; Gompf, B.; Petersen, J.; Strohmaier, R.; Eisenmenger, W. Z. Phys. B 1994, 93, 365. (b) Uder, B.; Ludwig, C.; Petersen, J.; Gompf, B.; Eisenmenger, W. Z. Phys. B 1995, 97, 389. (12) 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. Eur. J. 2000, 6, 20, 3739.

NL048360Y

81