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J. Phys. Chem. C 2008, 112, 15236–15240
Topological Selectivity in a Supramolecular Self-Assembled Host-Guest Network at the Solid-Liquid Interface Christoph Meier, Katharina Landfester, and Ulrich Ziener* Institute of Organic Chemistry III (Macromolecular Chemistry and Organic Materials), Albert-Einstein-Allee 11, 89081 Um, Germany ReceiVed: May 13, 2008; ReVised Manuscript ReceiVed: June 28, 2008
An oligopyridine forms a well-ordered two-dimensional network at the graphite-liquid interface stabilized by weak hydrogen bonds. This network offers two types of geometrically different voids of similar size, oval and circular in shape, respectively. Hierarchical coadsorption of C60 fullerene onto the oligopyridine monolayer leads to a host-guest network in which the fullerene molecules adsorb exclusively in the circularly shaped voids. This is attributed to the fit of the spherical molecules to the round void according to a geometric lock-and-key principle. 1. Introduction Two dimensional (2D) hydrogen-bonded networks (HBN) have attracted growing interest in the past several years as the self-assembly behavior of the appropriate building blocks can be controlled via slight variations of their chemical structures.1-4 The feasibility of precise control of the resulting network structures makes them ideal candidates for functionalizing inorganic surfaces for applications in molecular electronics5-10 and surface-supported sensors.11-13 From both scientific and technological points of view, the fabrication of highly ordered arrays of fullerenes on solid substrates under ambient conditions is of great importance. Fullerenes are well-known for their outstanding electronic properties,10,14,15 making them viable building blocks for the realization of molecular electronic devices working under ambient conditions. To investigate the electronic properties of individual fullerene molecules under ambient conditions, they have to be immobilized in a spatially well controlled manner.16 Within this context, the study of supramolecular binding phenomena at the solid-liquid interface is very important.4,16-24 On metal surfaces in ultrahigh vacuum, various organic template networks accommodate C60 through weak van der Waals interactions.25-29 At the solid-liquid interface, the solubility of fullerene in the supernatant solvent phase plays an important role in adsorption. It was shown that C60 is incorporated into trimesic acid (TMA) networks from saturated solutions in fatty acids, which do not physically dissolve fullerene molecules.16,30 As a result of the low solubility, their residence time at the interface is high and their incorporation into the host structure is a rare event. In typical organic solvents such as 1,2,4-trichlorobenzene (TCB), the formation of stable C60 monolayers on HOPG has not been documented, as a result of the low adsorption energy of C60 on the bare graphite surface.31 However, it was shown that stabilizing charge-transfer interactions between C60 and template building blocks, e.g., thiophenes, are a suitable method for immobilizing C60 at the solid-liquid interface.19,20 In this contribution, we report on the lock-and-key principlelike immobilization of C60 in a 2D supramolecular HBN, composed of the oligopyridine building block 4,3′-BTP (4,3′bisterpyridine) under ambient conditions at the solid-liquid interface (Scheme 1). We show that fullerene adsorption in the HBN is topologically selective even in the absence of charge-
SCHEME 1: Sketch of the Investigated Oligopyridine 4,3′-BTP and the Fullerene C60
transfer interactions. Therefore, the properties of the selfassembled HBN differ fundamentally from those of the individual 4,3′-BTP building blocks and are a result of the selfassembly process. 2. Experimental Procedure A drop of a 1,2,4-trichlorobenzene (Aldrich, >99%) solution, containing 4,3′-BTP (0.2 mg/mL), was deposited on a freshly cleaved surface of highly ordered pyrolytic graphite (HOPG) with the tip (80/20 Pt/Ir) in tunnel contact. After successful imaging of the oligopyridine network with the STM (SPM1000, RHK), a drop of a concentrated solution of C60 in TCB (∼15 mg/mL) was applied and imaged. The MM+ force field calculations were performed with Hyperchem 7.32 3. Results and Discussion In a previous publication, we reported on the C2V symmetric oligopyridine 4,3′-BTP which forms a 2D chiral HBN at the solid-liquid interface of highly ordered pyrolytic graphite (HOPG) and TCB, which was investigated with scanning
10.1021/jp804235a CCC: $40.75 2008 American Chemical Society Published on Web 09/10/2008
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Figure 1. (a) Reexamined STM image of the 4,3′-BTP HBN on HOPG from TCB. (b) Molecular model of the corresponding enantiomorphous phase. The monolayer exhibits oval (red) and circular (green) types of nanoscale holes.3
tunneling microscopy (STM) (Figure 1).3 A more precise investigation of the 2D pattern reveals the following parameters of the Cs-symmetric and nearly hexagonal unit cell: a ) 3.14 ( 0.06 nm, b ) 2.99 ( 0.05 nm, and ∠a,b ) 60 ( 1°. The homochiral phases are present in an equal amount on the surface, stabilized through weak C-H · · · N hydrogen bonds between the terminal pyridyl moieties.33,34 The chirality is solely a result of the asymmetric hydrogen bonding pattern interconnecting the achiral adsorbed oligopyridines and can be described as supramolecular chirality.3 As a result of the self-assembly process, the monolayer network exhibits two types of nanoscale holes, oval and circular cavities (Figure 1). The oval cavities are separated by the phenyl moieties of the oligopyridine molecules, whereas the circular cavities are confined by the terpyridine subunits of the molecular building blocks. Both types of cavities are bordered solely by hydrogen atoms and are chemically equivalent. They expose a well-defined area of the graphite substrate, imaged with dark contrast in the case of the oval voids. As the substrate is not imaged in the circular cavities with the same contrast as in the oval cavities, we assume weak coadsorption of TCB solvent molecules into the circular cavities. Solvent codeposition is proposed in several self-assembled monolayers at the solid-liquid interface.35-40 Taking the van der Waals radii into account, the oval cavities exhibit a length of 0.78 nm and a width of 0.58 nm, the circular cavities exhibit an inner diameter of 0.86 nm, and the diameter of the ball-shaped C60 is 0.88 nm. Simplified, its adsorption base can be considered as a disk with a diameter of 0.48 nm. Therefore, both types of cavities shall be suitable in size for fullerene adsorption. After addition of a drop of a concentrated solution of C60 in TCB to the preorganized oligopyridine network and partial evaporation of the solvent, bright disk-shaped spots appear in the STM images of the modified graphite surface (Figure 2a). Directly after application of the concentrated solution, a small number of bright spots appear on the HBN. The evaporation of the solvent leads to a further increase in the fullerene concentration at the interface. As a result, a nearly complete occupation of the binding sites (see below) is observed. The bright disks with a diameter of 1.2 ( 0.1 nm form a long-range ordered array with nearly hexagonal symmetry. The apparent height of the bright disks above the graphite surface was determined to be 1.1 ( 0.1 nm. On the bare graphite surface, the height of a C60 monolayer was reported to be 1.05 ( 0.05 nm.41 The
difference of approximately 0.05 nm is smaller than the thickness of the 4,3′-BTP monolayer of approximately 0.3 nm, which might arise from the weaker electron coupling of the fullerene molecules with the substrate at the larger fullerenesubstrate distance. Thus, the fullerene height is smaller than for fullerenes adsorbed on an unmodified graphite surface. From these results, we conclude that the bright disks can be associated with individual C60 molecules immobilized in the oligopyridine network. Measurements on independent STM images and on STM images during the same experiment after removal of the fullerene molecules from the host-guest network through dilution revealed that the deviation of the fullerene superstructure periodicities from the bare host network periodicities is smaller than 1%, which is on the order of the measurement error. Therefore, the host network structure remains basically unaffected by C60 adsorption. Furthermore, the similar unit cells of the host-guest network and the neat 4,3′-BTP HBN point to the presence of a preferential C60 adsorption site in the host network. In Figure 2a, an appropriate setpoint current and bias voltage, according to a tunneling resistance of 55.3 GΩ, were set to image the fullerene superstructure. Under these tunneling conditions, the outshining C60 image contrast does not allow for a sufficient imaging of the host network. Most of the C60 molecules are imaged as bright disks; a small number are imaged in parts or stripelike. In Figure 2b, a higher setpoint current and lower bias voltage were chosen compared to Figure 2a, according to a tunneling resistance of 19.5 GΩ. Under these conditions, the host network is imaged with nearly submolecular resolution, whereas the C60 molecules adsorbed on the HBN are partially imaged as stripes in individual scan lines. These stripes are oriented exclusively in the fast scan direction (Figure 2b). Finally, when they are imaged with a tunneling resistance of 12.1 GΩ (Figure 2c), only a few stripes can be observed on the HBN. In the STM images with medium tunneling resistance (Figure 2b,d), the stripes appear with a similar length and are located on preferential positions on the 4,3′-BTP HBN. In subsequent scan lines after a stripe, the host network void is clearly imaged (Figure 2d), indicating that the stripe-wise imaging is not a result of feedback lost or a scan artifact. The weak bonding of C60 to a macrocyclic bithiophene resulted in a similar stripelike imaging,20 whereas the strong binding of C60 to a cyclothiophene resulted in a complete disklike imaging of the fullerene molecules.19 On the bare graphite substrate, no
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Figure 2. STM image sequence of the C60@4,3′-BTP host-guest network recorded with different tip-sample separations. (a) Large-scale STM image, recorded with IT ) 8 pA, UV ) -442 mV, 55.3 GΩ, and νTip ) 59 µs/point, highlighting the C60 superstructure. The inset (23 nm × 23 nm) shows a magnified surface region. (b) STM image recorded with IT ) 11.6 pA, UV ) -227 mV, 19.5 GΩ, and νTip ) 101 µs/point. The white circle indicates an unoccupied circular void. (c) STM image showing the enantiomorphous phase recorded with IT ) 27.7 pA, UV ) -336 mV, 12.1 GΩ, and νTip ) 101 µs/point, shortly after the image shown in panel b. (d) STM image recorded with IT ) 12.6 pA, UV ) -227 mV, 18.0 GΩ, and νTip ) 101 µs/point, highlighting the host network structure. (e) Plotted height profiles across the C60@4,3′-BTP architecture in the direction of a lattice vector (1) and in the fast scan direction (2) as indicated in panel d. (f) Plot of the number of bright scan lines (N) per circular void vs the tunneling resistance (RT).
fullerene adsorption can be observed due to the weak graphite fullerene interactions and the high solubility of fullerenes in TCB. Without any graphite surface modifications, the C60 molecules are favored to remain dissolved in the supernatant solution.42 Therefore, we conclude that the observed stripes are a result of the weak bonding between C60 and the monolayer voids, allowing the tip to disturb the host-guest assembly. The stripelike image contrast of fullerene adsorbed on the HBN can be observed at a scan speed of 101 µs/point (slow), whereas the disklike fullerene contrast is imaged at a scan speed of 59 µs/point (fast). If the appearance of the C60 molecules were mainly governed by the adsorption and desorption kinetics of C60 on the HBN and not tip-induced, the opposite tendency would be expected, i.e., disklike contrast for slow imaging and stripelike contrast for fast scanning. Hence, the adsorption and desorption of C60 are much slower than the slowest applied scan speed. Therefore, we conclude that the stripe-wise C60 image contrast is not a result of fast adsorption and desorption of the fullerene molecules on the HBN but purely based on the weak bonding of fullerene to the oligopyridine monolayer. The stabilization of C60 in the monolayer strongly depends on the tunneling parameters and therefore on the tip sample distances. The number of stripes per void increases with increasing tunneling resistance, which is equivalent to an increase in the tip-sample separation (Figure 2f). It was shown that weakly bound fullerene molecules can be swept out of their binding sites with the STM tip by reducing the tunnel gap.16 In the C60@4,3′-BTP host-guest network, almost all cavities are filled with C60 in the absence of any disturbance by means of a significant fullerene-tip interaction, as approximately established in the case of a large tip-sample separation (Figure 2a). The ability of the tip to push the fullerenes out of their binding sites is a result of the weak bonding of fullerene to the host
network and the fact that a larger significant part of the C60 molecule points into the supernatant solution, resulting in a strong corrugation of the host-guest network surface (Figure 3b). With increasing disturbance, still most of the cavities are filled except those beneath the STM tip, which are emptied by the fullerene-tip interaction. The more stripe-wise imaging of C60 at smaller tip-sample separations arises from the fact that the probability of a successful manipulation event strongly depends on the tip-sample distance. At large tip-sample separations, only a few manipulation events occur during scanning. The occupation of an individual cavity with a fullerene molecule can be observed in almost every subsequent scan line. As a result, mostly disk-shaped features, corresponding to individually immobilized C60 molecules, are visible in the STM images. With a decreasing tip-sample separation, the number of manipulation events increases, lowering the probability of a C60 occupying the cavity beneath the STM tip. As a result, consecutive scan lines reproduce the cavity less occupied (bright line part) and more unoccupied (HBN line part), resulting in the stripe-wise C60 image contrast. The unoccupied cavity is a momentary situation. As the tip proceeds across the surface, the cavities are refilled with C60 immediately or shortly after the tip has passed the cavity. This tunneling parameter-dependent behavior could not be observed for C60@thiophene chargetransfer complexes,20 indicating a weaker van der Waals bonding of the fullerene molecules to the oligopyridine monolayer. The Cs-symmetric unit cell of the host network and the tunneling parameter-dependent experiments allow for an exact determination of the C60 binding position (Figure 2b). The centers of the bright stripes are located asymmetrically in the rhombic unit cell of the host network. In the images of the bare 4,3′-BTP network and the tentative molecular model (Figure 1), the circular void is located in a small angle corner
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Figure 3. Proposed model of the C60@4,3′-BTP host-guest interaction. The fullerene is colored red. (a) Top view. The arrow indicates an unoccupied circular void. (b) Perspective view of the C60@4,3′-BTP architecture. A larger portion of the fullerene molecule points into the supernatant solution. (c) Energy diagram showing the relative positions of the interaction energies of C60 with the circular cavity (C60@Circ), with an isolated 4,3′-BTP molecule (C60@BTP, placed on top of the pyrimidine moiety), and with the oval cavity (C60@Oval).
of the host network unit cell. In the images of the C60@4,3′BTP network, the bright stripes are located in a small angle corner of the host network unit cell (Figure 2b) as well. A closer look at the STM images with small tip-sample separation reveals that both the immobilized C60 molecules (bright lines) and the network voids (dark disks) correlate laterally with each other. The bright lines are exactly located on top of the circular cavities, whereas bright lines on top of the 4,3′-BTP molecules or the oval voids could not be observed. Therefore, the fullerene molecules are preferably adsorbed in the circular network cavities (Figure 3). It was shown that C60 forms charge-transfer complexes on surfaces with electron rich compounds such as thiophenes and pentacene.19,20,43,44 Between the electron poor oligopyridines and the electron poor fullerene molecules, no stabilizing chargetransfer interactions are expected, resulting in weak intermolecular interactions as confirmed by the experimental observations. Theoretically, the adsorption energy of C60 on graphite was calculated to -0.96 eV, with the six-membered ring parallel to the surface and a separation of 0.31 nm.31 In addition, we estimated the stabilization energy of C60 above the oval and circular voids of the monolayer with the MM+ force field to be -0.81 and -1.07 eV, respectively (Figure 3c). The stabilization energy above the pyrimidine moiety of an isolated 4,3′BTP molecule was estimated to be -0.44 eV. The influence of the solvent on the stabilization energy was neglected. Note that the C60 HOPG interaction is underestimated compared to the literature (-0.96 eV and 0.31 nm31 vs -0.68 eV and 0.32 nm, respectively). As both cavities are chemically equivalent, the energetic difference is mainly attributed to the different geometric shapes of the cavities, which adjust the noncovalent interactions of the C60 molecules with the cavity perimeters and the graphite substrate. Therefore, the adsorption of C60 into the circular cavity is preferred as it represents the specific complementary geometric shape to the spherical fullerene molecule. 4. Summary In summary, the successful immobilization of C60 at the solid-liquid interface into a highly ordered architecture has been realized by hierarchical self-assembly with a porous oligopyridine network. The oligopyridine network offers two distinguishable types of cavities, oval and circular. The fullerene molecules are trapped selectively in the circular voids. The
highly selective and specific interaction between the circular oligopyridine network voids and the spherical C60 molecules can be described as a simple lock-and-key interaction. The “lock” (void) is formed by the controlled self-assembly of the oligopyridine molecules into a highly ordered porous hydrogenbonded network. The results presented here highlight the fact that fullerene molecules can be immobilized in oligopyridine monolayers with topological selectivity, mimicking the wellknown enzymatic lock-and-key principle. These results contribute to the fundamental understanding of surface-supported host-guest architectures in the context of single-molecule chemical sensors. Acknowledgment. This work was financially supported by the Deutsche Forschungsgemeinschaft within the SFB 569. Supporting Information Available: Calculation results. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Lackinger, M.; Griessl, S. J. H.; Markert, T.; Jamitzky, F.; Heckl, W. M. J. Phys. Chem. B 2004, 108, 13652–13655. (2) Lackinger, M.; Griessl, S. J. H.; Kampschulte, L.; Jamitzky, F.; Heckl, W. M. Small 2005, 1, 532–539. (3) Meier, C.; Ziener, U.; Landfester, K.; Weihrich, P. J. Phys. Chem. B 2005, 109, 21015–21027. (4) Meier, C.; Landfester, K.; Ku¨nzel, D.; Markert, T.; Gross, A.; Ziener, U. Angew. Chem. 2008, 120, 3881–3885; Angew. Chem., Int. Ed. 2008, 47, 3821-3825. (5) Carroll, R. L.; Gorman, C. B. Angew. Chem. 2002, 114, 4556– 4579; Angew. Chem., Int. Ed. 2002, 41, 4378-4400. (6) Nitzan, A.; Ratner, M. A. Science 2003, 300, 1384–1389. (7) Selzer, Y.; Allara, D. L. Annu. ReV. Phys. Chem. 2006, 57, 593– 623. (8) Joachim, C.; Gimzewski, J. K.; Aviram, A. Nature 2000, 408, 541– 548. (9) Joachim, C.; Ratner, M. A. Proc. Natl. Acad. Sci. U.S.A. 2005, 102, 8801–8808. (10) Joachim, C.; Gimzewski, J. K. Chem. Phys. Lett. 1997, 265, 353– 357. (11) Rosi, N. L.; Mirkin, C. A. Chem. ReV. 2005, 105, 1547–1562. (12) Davis, J. J. Chem. Commun. 2005, 3509–3513. (13) Park, T. J.; Lee, S. Y.; Lee, S. J.; Park, J. P.; Yang, K. S.; Lee, K. B.; Ko, S.; Park, J. B.; Kim, T.; Kim, S. K.; Shin, Y. B.; Chung, B. H.; Ku, S. J.; Kim, D. H.; Choi, I. S. Anal. Chem. 2006, 78, 7197–7205. (14) Diederich, F.; Go´mez-Lo´pez, M. Chem. Soc. ReV. 1999, 28, 263– 277.
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