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2007, 111, 11342-11345 Published on Web 09/13/2007
Influence of Molecular Geometry on the Adsorption Orientation for Oligophenylene-Ethynylenes on Au(111) Sigrid Weigelt,† Carsten Busse,†,§ Morten Nielsen,‡ Kurt V. Gothelf,‡ Erik Lægsgaard,† Flemming Besenbacher,† and Trolle R. Linderoth*,† Interdisciplinary Nanoscience Center (iNANO) and Department of Physics and Astronomy, and Department of Chemistry, UniVersity of Aarhus, DK-8000 Aarhus C, Denmark ReceiVed: July 2, 2007; In Final Form: July 25, 2007
The adsorption structures formed from a class of oligophenylene-ethynylenes on Au(111) under ultrahigh vacuum conditions is compared based on high-resolution scanning tunneling microscopy (STM) measurements. The molecules consist of three or four benzene rings connected by ethynylene spokes and are all functionalized identically with an aldehyde, a hydroxyl, and a bulky tert-butyl group. Compounds with the conjugated spokes placed in the para, meta, and threefold configurations were previously found to exclusively form molecular layers with flat-lying adsorption geometries. In contrast, the associated compound with spokes in the ortho configuration surprisingly differs in its adsorption by forming only structures with an upright adsorption orientation. The packing density for the structures formed by the compound with the ortho configuration is less dense than that in conventional self-assembled monolayers while still keeping the conducting backbone in an upright orientation. These structures are thus interesting from the perspective of performing singlemolecule conduction measurements on the oligophenylene-ethynylene backbones.
Introduction The adsorption of organic molecules on metallic substrates has been studied extensively in recent years, motivated partly by applications in areas such as surface chemical functionalization and molecular electronics and partly by a fundamental interest in understanding how competing molecule-molecule and molecule-surface interactions control molecular selforganization on surfaces.1 The orientation of adsorbed molecules depends, in general, on the specific molecule-metal system. Flat-lying adsorption geometries, which allow for large molecular footprints and strong interactions between the surface and the electronic system of the molecular backbone, have been observed both for aliphatic hydrocarbons2 and for a large range of planar aromatic molecules such as pentacene,3 porphyrins,4 DNA bases,5 4-trans-2-(pyrid-4yl-vinyl) benzoic acid (PVBA)6 or oligophenylene-ethynylenes.7 In contrast, upright-standing adsorption geometries typically occur for layers of molecules that contain functional groups with a high surface affinity, such as thiols,8-10 pyridines,11 or carboxylic acids.12 In those latter cases, the closer packing allows for a higher density of interacting affinity groups, which compensates for the loss in backbone-surface interaction. The balance between the two situations may depend both on surface coverage8,11 and molecular chain length.9 * To whom correspondence should be addressed. E-mail:
[email protected]. Tel: +45 8942 5536. Fax: +45 8612 0740. † Department of Physics and Astronomy. ‡ Department of Chemistry. § Present address: II. Physikalisches Institut, Universita ¨ t zu Ko¨ln, D-50937 Ko¨ln, Germany.
10.1021/jp075123l CCC: $37.00
In this Letter, we demonstrate that it is possible to induce a change in molecular adsorption orientation, from flat-lying to upright-standing, by modifying the molecular geometry alone, without any alterations to the molecular size or functional groups. The presented work expands on our previous scanning tunneling microscopy (STM) studies of the adsorption of a class of oligophenylene-ethynylene molecules on Au(111).7 Compounds with conjugated spokes placed in the para, meta, or threefold position on a central benzene ring (see Figure 1) were all found to exclusively form molecular layers with flat-lying adsorption geometries, in agreement with the general expectations for conjugated organic adsorbates without any chemical groups with a strong affinity for the substrate. In Figure 1, examples of such adsorption structures are shown. Here, we investigate an associated compound which has the same chemical groups but a different geometry with the two spokes placed closer to each other in the ortho configuration (Figure 2) and surprisingly find that it differs qualitatively in its adsorption behavior by forming only structures with an upright adsorption orientation. The resulting low-density molecular overlayer with conjugated backbones placed upright on the substrate may facilitate single-molecule conductivity measurements by STM. Experimental Methods The experiments were performed in an UHV chamber equipped with standard facilities for sample preparation and characterization as well as a home-built Aarhus STM.13 An atomically clean Au(111) single-crystal surface was prepared by repeated cycles of Ar ion sputtering and annealing to 840© 2007 American Chemical Society
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Figure 1. STM images (50 × 50 Å2) and schematic drawings of three chemically similar oligophenylene-ethynylenes on Au(111). The molecules consist of a central benzene ring connected to two or three ethynylene spokes functionalized with a t-butyl-substituted salicylaldehyde moiety at the ends. The molecular spokes are attached in a (a) para, (b) meta, and (c) three-spoke configuration.
Figure 2. The ortho compound. Schematic models of possible planar conformations with indicated distance, d, between the t-butyl groups.
900 K, resulting in a well-ordered herringbone reconstruction. The ortho compound (1,2-bis[(5-t-butyl-3-formyl-4-hydroxyphenyl)ethynyl]benzene) was thoroughly outgassed at the deposition temperature and vapor deposited from a heated glass crucible onto the Au(111) sample under ultrahigh vacuum conditions. STM measurements were performed in situ at a sample temperature of 140-170 K. Results and Discussion The ortho compound forms two coexisting adsorption phases, termed hexagonal (Figure 3) and rhombic (Figure 4), both observed to form large self-assembled islands (see Figures 3c and 4c), with the Au(111) herringbone reconstruction being visible through the molecular layer. Two different imaging modes occur.7 In one mode (Figures 3a and 4a), the contrast primarily arises from the conjugated π-system of the molecules, while in the other mode (Figures 3b and 4b), the topography is dominated by bright protrusions arising from the bulky t-butyl groups. The occurrence of the two qualitatively different imaging modes does not correlate with the applied scanning parameters and is instead attributed to different states of the tip apex, possibly caused by adsorbed molecules. Images of the hexagonal structure obtained in the π-system imaging mode (Figure 3a) reveal the individual molecules as
crescent-moon-like shapes with six molecules forming a chiral rosette motif (see also Supporting Information Figure S1). A nearly hexagonal unit cell (|a| ) (19.6 ( 1.0) Å, |b| ) (22.3 ( 1.1) Å, φ ) 121.1 ( 5.0°) containing three molecules is marked on the STM images (Figure 3a,b). The hexagonal structure has a definite orientational relationship to the underlying substrate, with an angle of -5.9 ( 5.0° between the unit cell vector, b, and the [11h0] direction of the Au(111) surface. In the rhombic structure, four molecules join in a windmill-type arrangement, as marked on Figure 4a,b. In the π-system imaging mode, the molecules appear with a characteristic s shape. The unit cell (see Figure 4a,b,d) contains two molecules and is truly rhombic (|a| ) (15.4 ( 0.8) Å, |b| ) (14.9 ( 0.7) Å, φ ) 83.5 ( 5.0°). In contrast to the hexagonal structure, the rhombic structure assumes a range of orientations compared to the underlying substrate, suggesting a weaker molecule-substrate interaction in this case. These adsorption structures differ in two respects from those of the previously studied compounds; (i) the characteristic v shape anticipated from Figure 2 for a flat-lying ortho backbone is not observed, and (ii) the molecular packing density is considerably higher. The ratio between the estimated van der Waals footprint of a flat-lying molecule (AvdW) and the area occupied by a molecule in the experimentally observed struc-
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Figure 3. The hexagonal structure formed by the ortho compound. (a) and (b) High-resolution STM images (70 × 70 Å2, Vt ∼ 1.5 V, It ∼ 1 nA) obtained in (a) the π-system imaging mode and (b) the t-butyl imaging mode. (c) Large-scale STM image (500 × 500 Å2, It ) 0.86 nA, Vt ) 1.43 V). The Au(111) herringbone reconstruction is visible through the molecular layer. (d) Tentative adsorption model. The central benzene ring of each molecule is only visible as a line due to the projection.
tures (Amol) is AvdW/Amol ) 1.5 (1.6) for the hexagonal (rhombic) structures, respectively, while the similar ratios for the para, meta, and three-spoke compounds were found in ref 7a to lie between 0.58 and 0.96. Packing densities substantially larger than 1 do not conform with a flat-lying orientation of the molecular backbone. We therefore conclude that the ortho compounds adsorb in an upright-standing geometry. We assume that the molecules orient the spokes toward the surface to optimize the interaction between the oxygen lone pairs and the metal surface. In Figures 3b and 4b, the bright protrusions arising from the t-butyl groups can unambiguously be assigned to individual molecules by comparison to single-molecule vacancies in the structures. The distance, d, between the maxima for two protrusions belonging to the same molecule is d ) (7.3 ( 0.5) Å [(7.9 ( 0.5) Å] for the hexagonal (rhombic) structure. This may be compared to theoretical distances between t-butyl groups for molecules with planar backbones, as sketched in Figure 2 (d1 ) 5.9, d2 ) 14.2, d3 ) 10.9 Å). The poor agreement suggests that the molecular spokes are rotated such that the two outermost benzene rings are not parallel to the center-most one. In this way, any distance between d1 and d2 may be realized, allowing for agreement with the experimental observations. Tentative adsorption models for the two upright structures are shown in Figures 3d and 4d, assuming the molecular backbones to be perpendicular to the substrate and the two outermost benzene rings to be parallel to each other.
Compared to the para, meta, and three-spoke molecules, the two spokes of the ortho compound are in much closer proximity, implying larger steric effects between the t-butyl groups and stronger attractive interactions between the aromatic systems of the two spokes.14 The nonplanar conformation with rotated outermost benzene rings may result from these interactions.15 A nonplanar conformation is expected to weaken the backbonesubstrate interaction in the case of flat-lying adsorption geometries. We speculate that the upright orientation of the ortho molecules is induced by this effect and further stabilized by attractive π-π interactions with the aromatic systems of the neighboring molecules.16 In addition, the acute angle between the spokes allows both endgroups to be placed in close proximity on the substrate for an upright adsorption orientation which would not be possible for the other compounds studied. Conclusion In summary, we have demonstrated that the ortho compounds surprisingly form adsorption structures in which the molecular backbones are upright-standing. The molecular packing in the observed phases is less dense11 than that in conventional selfassembled monolayers while still keeping the conducting backbone in an upright orientation. This makes the structures highly interesting from the perspective of performing single-molecule conduction measurements on the oligophenylene-ethynylene backbones, which has otherwise only been possible by dispers-
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Figure 4. The rhombic structure formed by the ortho compound. (a) and (b) High-resolution STM images (70 × 70 Å2, Vt ∼ 1.5 V, It ∼ 1 nA) obtained in (a) the π-system imaging mode and (b) the t-butyl imaging mode. (c) Large-scale STM image (500 × 500 Å2, It ) 0.87 nA, Vt ) 1.25 V). The Au(111) herringbone reconstruction is visible through the molecular layer. (d) Tentative adsorption model. The central benzene ring of each molecule is only visible as a line due to the projection.
ing individual molecules in a nonconducting monolayer10a to avoid electron transport through neighboring molecules.10b Acknowledgment. We acknowledge funding from the EU Programs FUN-SMART, PICO-INSIDE and AMMIST, the Carlsberg Foundation, the Danish Natural Science Research Council through funding for the iNANO Center, the Danish Natural and Technical Research Council, the Danish National Research Foundation, and the Aarhus University Research Foundation. Supporting Information Available: Details on the synthesis of the ortho compound and an additional STM image. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Barlow, S. M.; Raval, R. Surf. Sci. Rep. 2003, 50, 201-341. (2) Marchenko, A.; Xie, Z. X.; Cousty, J.; Pham Van, L. Surf. Interface Anal. 2000, 30, 167-169. (3) Lukas, S.; Witte, G.; Wo¨ll, Ch. Phys. ReV. Lett. 2002, 88, 028301/ 1-028301/4. (4) Yokoyama, T.; Yokoyama, S.; Kamikado, T.; Okuno, Y.; Mashiko, S. Nature 2001, 413, 619-621. (5) (a) Otero, R.; Scho¨ck, M.; Molina, L. M.; Lægsgaard, E.; Stensgaard, I.; Hammer, B.; Besenbacher, F. Angew. Chem., Int. Ed. 2005, 44,
2270-2275. (b) Chen, Q.; Richardson, N. V. Nat. Mater. 2003, 2, 324328. (6) Barth, J. V.; Weckesser, J.; Cai, C. Z.; Gu¨nter, P.; Bu¨rgi, L.; Jeandupeux, O.; Kern, K. Angew. Chem., Int. Ed. 2000, 39, 1230-1234. (7) (a) Busse, C.; Weigelt, S.; Petersen, L.; Thomsen, A. H.; Nielsen, M.; Gothelf, K. V.; Lægsgaard, E.; Besenbacher, F.; Linderoth, T. R. J. Phys. Chem. B 2007, 111, 5850-5860. (b) Weigelt, S.; Busse, C.; Petersen, L.; Rauls, E.; Hammer, B.; Gothelf, K. V.; Besenbacher, F.; Linderoth, T. R. Nat. Mater. 2006, 5, 112-117. (8) Poirier, G. E.; Pylant, E. D. Science 1996, 272, 1145-1148. (9) Dhirani, A.-A.; Zehner, R. W.; Hsung, R. P.; Guyot-Sionnest, P.; Sita, L. R. J. Am. Chem. Soc. 1996, 118, 3319-3320. (10) (a) Bumm, L. A.; Arnold, J. J.; Cygan, M. T.; Dunbar, T. D.; Burgin, T. P.; Jones, L., II; Allara, D. L.; Trou, J. M.; Weiss, P. S. Science 1996, 271, 1705-1707. (b) Ishida, T.; Mizutani, W.; Akiba, U.; Umemura, K.; Inoue, A.; Choi, N.; Fujihira, M.; Tukumoto, H. J. Phys. Chem. B 1999, 103, 1686-1690. (11) Dougherty, D. B.; Lee, J.; Yates, J. T, Jr. J. Phys. Chem. B 2006, 110, 11991-11996. (12) Dmitriev, A.; Lin, N.; Weckesser, J.; Barth, J. V.; Kern, K. J. Phys. Chem. B 2002, 106, 6907-6912. (13) Lægsgaard, E.; Besenbacher, F.; Mortensen, K.; Stensgaard, I. J. Microsc. 1988, 152, 663-669. See also www.specs.de. (14) Pickholz, M.; Stafstro¨m, S. Chem. Phys. 2001, 270, 245-251. (15) Hunter, Ch. A.; Lawson, K. R.; Perkins, J.; Urch, Ch. J. J. Chem. Soc., Perkin Trans. 2 2001, 651-669. (16) Nelson, J. C.; Saven, J. G.; Moore, J. S.; Wolynes, P. G. Science 1997, 277, 1793-1796.
