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Langmuir 2006, 22, 10874-10876
Self-Assembled Monolayers Based on Phenanthroline-Gold(111) Bonding Franc¸ ois Lux,† Gilles Lemercier,*,† Chantal Andraud,† Guillaume Schull,‡ and Fabrice Charra*,‡ Laboratoire de Chimie, UMR n°5182 CNRS/EÄ cole Normale Supe´ rieure de Lyon, 46 Alle´ e d’Italie, 69364 Lyon Cedex 07, France, and SerVice de Physique et Chimie des Surfaces et Interfaces, Commissariat a` l’Energie Atomique, Centre de Saclay, F-91191 Gif-sur-YVette Cedex, France ReceiVed June 6, 2006. In Final Form: October 27, 2006 The self-assembly of long-alkyl-chain substituted phenanthroline derivatives on highly oriented pyrolitic graphite (HOPG) and gold(111) is compared. Whereas the adsorption on HOPG is controlled by the affinity of alkyl chains for the substrate, which leads to flat-lying adsorbed molecules, alignments of upright-oriented molecules are formed on gold(111). This situation is explained by the bonding of chelating species with gold(111) surfaces and by the π-stacking interaction between conjugated moieties. This intermediate situation between strong thiol-like chemical bonding and the weak n-alkane-like physical adsorption opens the route toward laterally organized functional molecular assemblies.
1. Introduction Molecular self-assembly onto atomically flat surfaces attracts considerable interest as a bottom-up route for the realization of nanoscale patterns.1 This technique exploits the interplay between molecule-substrate and molecule-molecule interactions. A prototype of self-assembly dominated by molecule-substrate interactions is given by thiol derivatives on Au(111). For example, n-alkyl-thiols form a dense brush-like monolayer with lateral organization driven by the sites favorable for sulfur-gold bonding.2 However, increasing the size of thiol-functionalized molecules leads to poor or no lateral order.3 On the other side, lateral organization of physically adsorbed monolayers results from an optimization of the packing governed by a subtle interplay between molecule-substrate attraction and steric hindrance. This may lead to well-ordered surface networks for large planar molecules lying flat on the surface, thus maximizing the weak bonding with the substrate.4 This geometry is not favorable to the generation of functional molecular systems, which requires 3D architectures. The observation that 2,2′-bipyridine,5 1,10-phenanthroline,6 or oxadiazole derivatives7 form edge-on π-stacked assemblies on Au(111) provides evidences for the strong interaction of the chelate species with gold(111) surfaces. This raises the question of the possible exploitation of this intermediate type of bonding for the formation of brushlike assemblies with controlled lateral organization. As a first step in this direction, we demonstrate the ability of long-chain substituted phenanthroline derivatives (we have * To whom correspondence should be adressed. (F.C.) E-mail:
[email protected]; Phone: +33/169089722. (G.L.) E-mail:
[email protected]; Phone: +33/472728733. † UMR n°5182 CNRS / E Ä cole Normale Supe´rieure de Lyon. ‡ Commissariat a ` l’Energie Atomique. (1) Barth, J. V.; Costantini, G.; Kern, K. Nature 2005, 437, 671-679. (2) Poirier, G. E. Chem. ReV. 1997, 97, 1117. Kumar, A.; Whitesides, G. M. Science 1994, 263, 60-62. Schoer, J. K.; Crooks, R. M. Langmuir 1997, 13, 2323-2332. Zhao, J. W.; Uosaki, K. Nano Lett. 2002, 2, 137-140. (3) Liang, T. T.; Azehara, H.; Ishida, T.; Mizutani, W.; Tokumoto, H. Synth. Met. 2004, 140, 139-149. (4) De Feyter, S.; De Schryver, F. C. J. Phys. Chem. B 2005, 109, 4290-4302. (5) Dretschkow, Th.; Wandlowski, Th. J. Electroanal. Chem. 1999, 467, 207216. (6) Cunha, F.; Jin, Q.; Tao, N. J.; Li, C. Z. Surf. Sci. 1997, 389, 19-28. (7) Kwon, K.-Y.; Lin, X.; Pawin, C.; Wong, K.; Bartels, L. Langmuir 2006, 22, 857-859.
Figure 1. (a) STM image of DYP on highly-oriented pyrolytic graphite (HOPG). The image size is 15 nm × 15 nm. The tunneling current was IT ) 38 pA, and the sample bias was VS ) 1200 mV. The full vertical color scale represents 1.0 Å (from black to white). (b) Auto-correlation-filtered image of part a showing the details of the unit cell: a ) 40 ( 2 Å, b ) 18 ( 1 Å. There are four molecules per unit cell. (c) Molecular structure of the DYP ligand.
focused our work on 5-(dec-1-ynyl)-1,10-phenanthroline, DYP, represented in Figure 1c and obtained according to a modified Sonogashira8 reaction) to form aligned self-assemblies of uprightoriented molecules on gold(111). This intermediate case between strong chemical bonding, weak physical adsorption, and π-stacking type interactions opens the route toward laterally organized functional molecular assemblies. The comparison with graphite, on which the adsorption is governed by the affinity of alkyl chains for the substrate, confirms this point of view. 2. Experimental Section 1H
13C
and NMR spectra were recorded on a Bru¨cker AC200. UV/vis spectra were recorded in the 200-800 nm range on a UV/vis Jasco V-550; λmax are given in nm, and molar extinction coefficients are given in L mol-1 cm-1. Melting points (mp) were recorded on a Perkin-Elmer DSC7. Microanalysis was performed by the microanalytical unit of the CNRS in Solaize, France. Emission spectra were obtained in dilute dichloromethane solutions with a PTI spectrofluorimeter. Synthesis of 5-(Dec-1-ynyl)-1,10-phenanthroline (DYP). A solution of 5-bromo-1,10-phenanthroline9 (1 g, 3.86 mmol), dec(8) Crisp, G. T.; Gore, J. Tetrahedron 1997, 53, 1505-1522.
10.1021/la061621u CCC: $33.50 © 2006 American Chemical Society Published on Web 11/10/2006
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Langmuir, Vol. 22, No. 26, 2006 10875
1-yne (640 mg, 4.63 mmol), and tetrakis-(triphenylphosphine)palladium(0) (223 mg, 0.193 mmol) in pyrrolidine (25 mL) was stirred at 70 °C overnight. Then, the reaction mixture was poured into a saturated NH4Cl solution (50 mL) and extracted with dichloromethane (3 × 50 mL). The combined organic phases were washed with an NH4Cl solution (3 × 25 mL) and a NaCl solution and dried with MgSO4, and then the solvent was removed. The residue was purified with chromatography (SiO2, ethyl acetate) to give 0.57 g of a pale-orange solid (47% yield), mp 59 °C. 1H NMR (200.13 MHz; CDCl3): δ 9.19 (1H, dd, H2 or H9, J ) 4.3, 1.7 Hz), 9.14 (1H, dd, H9 or H2, J ) 4.3, 1.7 Hz), 8.72 (1H, dd, H4, J ) 8.2, 1.7 Hz), 8.16 (1H, dd, H7, J ) 8.2, 1.7 Hz), 7.93 (1H, s, H6), 7.68 (1H, dd, H3 or H8, J ) 8.2, 4.3 Hz), 7.60 (1H, dd, H8 or H3, J ) 8.2, 4.3 Hz), 2.59 (2H, t, H12, J ) 6.9 Hz), 1.15-1.80 (12H, m, H13-H18), 0.88 (3H, t, H19, J ) 6.7 Hz). 13C NMR (50.32 MHz, CDCl3): δ 14.73, 20.341, 23.294, 29.390, 29.695, 29.756, 29.849, 32.475, 77.851, 97.693, 121.450, 123.827, 123.952, 128.772, 129.295, 130.655, 135.469, 136.161, 146.537, 151.078, 151.163. Anal. Calcd for C22H24N2, 0.33 H2O: C, 81.94; H, 7.71; N, 8.68. Found: C, 82.05; H, 7.50; N, 8.49. UV/vis (CHCl3): 217 (12 100), 242 (24 700), 274 (28 600), 304 (11 400), 315 (10700). Eu(DYP)2Cl3 Synthesis. A solution of EuCl3 (43.5 mg, 0.17 mmol) and DYP (160 mg, 5.05 mmol) was stirred in a small amount of dry ethanol overnight at reflux under an inert atmosphere to allow, after filtration, the obtainment of the desired white compound in 41% yield. Anal. Calcd for Eu(DYP)2Cl3, EtOH1.5 (C47H57N4Cl3O1.5): C, 58.78; H, 5.98; Eu, 15.8. Found: C, 58.52; H, 5.68; Eu, 15.76. UV/vis (CH2Cl2): 219 (27 100), 245 (72 500), 275 (82 600), 303 (35 300), 315 (32 800). Upon excitation at the ligand π-π* transition wavelength (340 nm), only the emission lines of Eu3+, 5D0-7FJ (J ) 1, 2, 3, 4 at 601, 622, 656, and 708 nm, respectively), were observed with the hypersensitive transition 5D -7F as the most prominent group. 0 2 Scanning Tunneling Microscopy (STM). The STM was operated at the liquid-solid interface. Nearly saturated solutions of either DYP or the europium complex in n-tetradecane (>99%, Aldrich) have been used. This solvent is well adapted for in-situ STM because it is hydrophobic, highly insulating, and has low vapor pressure. The STM junction was immersed in a droplet of solution immediately before scanning. The substrates were either HOPG (Goodfellow) or 100-nm-thick layers of gold epitaxially grown on mica following a known procedure so as to expose a (111) face.10 The substrates were prepared immediately before their use either by cleavage (HOPG) or flame annealing (Au). In the case of Au, the observation of the 22 × x3 reconstruction in the pure solvent was a prerequisite for further use of each substrate. The tips were mechanically formed from a 250-µm Pt-Ir wire (Pt80/Ir20, Goodfellow). STM imaging was achieved through a homemade digital system specially designed for low-current operation. The images reported here were obtained in height mode, also called constant current mode.
3. Results and Discussion The results obtained on HOPG are reported in Figure 1. The unit cell corresponds to an area of ∼7.1 nm2. Conjugated moieties, which provide lower-barrier tunneling paths through LUMO and other low-energy orbitals, appear as bright patterns in STM images. The correlation-averaged image strongly suggests the presence of 4 molecules per unit cell, thus corresponding to an area per molecule of ∼1.8 nm2 and a full vertical-contrast amplitude of 0.6 Å. The networks form pairs of lamellae, in which each lamella is deduced from the other upon rotation by 180° (C2 symmetry). The structure observed on Au(111) is completely different (Figure 2). Immediately after application of the droplet, islands of parallel strips are observed (Figure 2a,b). The strip-to-strip (9) Hissler, M.; Connick, W. B.; Geiger, D. K.; McGarrah, J. E.; Lipa, D.; Lachicotte, R. J.; Eisenberg, R. Inorg. Chem. 2000, 39, 447-457. (10) Golan, Y.; Margulis, L.; Rubinstein, I. Surf. Sci. 1992, 264, 312-326.
Figure 2. STM images obtained with solutions of DYP (a, b, and c) and of the europium complex (d) on gold(111). The image sizes are 42 nm × 42 nm (a), 73 nm × 73 nm (b), 47 nm × 47 nm (c), and 81 nm × 81 nm (c). The tunneling current was IT ) 15 pA, the sample bias was VS ) 800 mV (a, b, and d), IT ) 19 pA, and VS ) 600 mV (c). Image c has been acquired after ∼18 h of exposure to the solution. The distance between rows is 9 ( 0.8 Å. The inset in image b shows the height cross-section along the line displayed in the image. The tip height increase over rows is larger than 3 Å.
lateral period is ∼0.9 nm. The apparent height of these patterns relative to the naked Au surface is larger than 3 Å, as shown on the section view in Figure 2b. Nearly full coverage is observed after several hours of exposure to the solution (Figure 2c). The strip structure is very similar to those observed with phenanthroline.6 However, the overall organization is more disordered, and the kinetics of monolayer formation is much slower. The layer is also less stable upon STM imaging and cannot be scanned with a tunneling current larger than ∼20 pA. This indicates an increased mechanical interaction of the aliphatic alkyl chains on DYP with the tip, compared with that for conjugated phenanthroline. The layer is also removed by STM biases larger than ∼1.1 V. Intrastrip periodicity is not clear, but one discerns structures with typical dimensions of ∼0.4 nm (inset in Figure 2c), compatible with the period reported for phenanthroline. This leads to an area per molecule not larger than 0.4 nm2. In several attempts, we have not been able to observe selfassembly on HOPG when the europium complex was used in place of neat DYP. In contrast, the same patterns are obtained on Au(111) with solutions of both DYP and the complex. The deposition rate and domain size are similar. However, the complex systematically seems to produce better-ordered less-kinked lines. The area per molecule obtained on HOPG is ∼35% larger than the van der Waals area of the planar configuration of DYP (∼1.3 nm2, as estimated using van der Waals radii of 0.12 nm for hydrogen atoms11). For comparison, in the case of long n-alkanes adsorbed on HOPG in a highly compact linear structure, the area per CH2 period is 0.055 nm2,12 which is ∼30% larger than its van der Waals area, ∼0.042 nm2. Present STM observations are fully consistent with self-assembly following (11) Pauling, L. Nature of the Chemical Bond, 3rd ed.; Cornell University Press: Ithaca, NY, 1960; Chapter 7.12, pp 257-263. (12) Watel, G.; Thibaudau, F.; Cousty, J. Surf. Sci. Lett. 1993, 281, L297302.
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Figure 3. Suggested model for the molecular arrangement of DYP over Au(111).
the standard paradigm of molecules lying flat on the surface, bonded to HOPG through the adsorption of their alkyl chains in registry with HOPG13,14 and with a lateral organization maximizing the coverage density.15,16 The less compact packing of DYP compared with that of linear alkanes is a consequence of geometrical constraints. The situation on Au(111) is quite different. Variations between HOPG and Au(111) in the supramolecular organization of adsorbed molecules comprising aliphatic17 or conjugated16,18 parts is often observed. Schematically, it corresponds to a decreased adsorption energy of the alkyl chains and an increased interaction of the conjugated core with the surface when Au(111) is used as the substrate.16 Here, the small area per molecule precludes an organization with molecules lying flat on the Au(111) surface. The similarity with 1,10-phenanthroline points to an organization with conjugated nitrogen heteroatoms facing the gold surface, with the vertical conjugated planes being associated through π stacking (Figure 3). Also, similar observations made with cytosine,19 a N-rich DNA base, and deprotonated uracil20 give support to the chemisorption of such heteroconjugated moieties on gold(111) substrates. This “side-on” structure is further confirmed by the larger apparent height (∼3 Å) compared with that obtained for molecules lying “face-on” on HOPG (∼0.6 Å). This apparent height is less than the full height of the conjugated moiety, which is on the order of 6 Å including the triple bond, (13) Groszek, A. J. Proc. Roy. Soc. London, Ser. A 1970, 314, 473-498. (14) Foster, J. S.; Frommer, J. E. Nature 1988, 333, 542-545. (15) Qiu, X.; Wang, C.; Zeng, Q.; Xu, B.; Yin, S.; Wang, H.; Xu, S.; Bai, C. J. Am. Chem. Soc. 2000, 122, 5550-5556. (16) Perronet, K.; Charra, F. Surf. Sci. 2004, 551, 213-218. (17) Xie, Z. X.; Xu, X.; Mao, B. W.; Tanaka, K. Langmuir 2002, 18, 31133116. (18) Walzer, K.; Hietschold, M. Surf. Sci. 2001, 471, 1-10. (19) Wandlowski, Th., Lampner, D., Lindsay, S. M. J. Electroanal. Chem. 1996, 404, 215-226. (20) Dretschkow, Th.; Dakkouri, A. S.; Wandlowski, Th. Langmuir 1997, 13, 2843-2856.
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but at the applied bias of 0.8 V the tunneling is nonresonant and the apparent height is then usually significantly smaller than the actual one.21 The mechanism of contrast involves the reduction of the tunnel barrier by the presence of low-energy states such as LUMO. It has been shown that for monolayers of alkanethiol with similar lengths and comparable current setpoints the tip is in contact with the molecule.22 Hence, the strong mechanical interaction with the tip is consistent with an upright orientation of the low-transconductance alkyl chains over the conjugated moieties. There remains one uncertainty as concerns the orientation of the side chain, which can be uniformly oriented on one side, alternated on both sides of the columns, or randomly distributed on both sides. Because the conjugated system extends over the beginning of this chain, better π-stacking stabilization would be obtained with uniformly oriented molecules (Figure 3). An in-depth modeling study is currently in progress to describe the involved interactions better. This geometry should result in an asymmetric imaging of molecular rows, which is not observed experimentally. However, partial solvation of the alkyl chain16 may result in dynamic disorder with faster fluctuations than STM acquisition, thus preventing its imaging. The existence of the mobile alkyl-chain free end offers an opportunity for further functionalization. In particular, planar conjugated substituents at such places could form a second π-stacked column electronically isolated from the conducting substrate. The better organization observed when the complex is used for the deposition may be explained either by kinetics considerations or by the formation of a better-organized phase incorporating the ion. The presence of the Eu-containing solution prevents any chemical elemental analysis of the interface. The very close similarity between the patterns observed with the neat ligand and with the complex suggests that the composition of the layers is the same, thus supporting the kinetic hypothesis. The additional decomplexation step needed for the complex prior to adsorption of the chelate could slow down the deposition kinetics and favor a better organization.
4. Conclusions We have shown that long-chain substituted 1,10-phenanthroline derivatives form self-assembled monolayers on both HOPG and gold(111). In the latter case, they form π-stacked islands of aligned, upright-oriented molecules that are strongly anchored to the surface through the interaction of the chelate species with gold. The possibility of functionalizing the free end of the side chain opens a new route toward laterally organized functional molecular assemblies. Acknowledgment. This work has been supported by the French Ministry of Scientific Research through ACI “NanoDELO” NR161. LA061621U (21) Li, Y. W.; Zhao, J. W.; Yin, X.; Yin, G. P. J. Phys. Chem. A 2006, 110, 11130-11135. (22) Bumm, L. A.; Arnold, J. J.; Dunbar, T. D.; Allara, D. L.; Weiss, P. S. J. Phys. Chem. B 1999, 103, 8122-8127.
