Lactam Recognition Patterns

Albert-Einstein-Allee 11 D-89069 Ulm, Germany, Laboratoire de Chimie Supramoléculaire, ISIS, Université Louis Pasteur, Strasbourg, France, and D...
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Langmuir 2006, 22, 7579-7586

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Homo- and Heteroassemblies of Lactim/Lactam Recognition Patterns on Highly Ordered Pyrolytic Graphite: An STM Investigation Ahmed Mourran,†,‡ Ulrich Ziener,*,† Martin Mo¨ller,*,†,‡ Maurizio Suarez,§ and Jean-Marie Lehn§ Organische Chemie III/Makromolekulare Chemie der UniVersita¨t, Albert-Einstein-Allee 11 D-89069 Ulm, Germany, Laboratoire de Chimie Supramole´ culaire, ISIS, UniVersite´ Louis Pasteur, Strasbourg, France, and Deutsches Wollforschungsinstitut an der RWTH Aachen e.V., Pauwelsstr. 8, D-52074 Aachen, Germany ReceiVed April 6, 2006. In Final Form: April 27, 2006 The 2D assembly of phthalhydrazide 1 and aminopyrimidine 2 derivatives equipped with C16 and C8 alkyl chains, respectively, on highly ordered pyrolytic graphite (HOPG) was studied by scanning tunneling microscopy. Welldefined, rather complex surface layer patterns emerge resulting from a delicate balance of (self-) complementary (strong) hydrogen bonds and van der Waals force-driven ordering of the alkyl substituents on the HOPG surface. The four different compounds and their 1:1 mixtures yield seven different 2D structures. Phthalhydrazide offers in principle three tautomeric forms, with the lactim/lactam being the most stable. Depending on the solvent, different morphologies can be obtained. In one case, the special self-assembly of achiral 1a leads to a 2D chiral packing with the left- and right-hand motifs present in different domains. We assume that pure 1a is expressed in its lactim/lactam form, whereas in a 1:1 mixture with 2a it switches to the bislactam form. These features display a process of dynamic diversity generation through tautomerism resulting in different nanostructures in response to environmental parameters.

I. Introduction Self-processes such as self-assembly and self-organization at the molecular level have become in recent years a subject of strongly increasing activity in the fields of molecular design and engineering.1-6 Such processes allow by a bottom-up approach the fabrication of nanoarchitectures based on the specific interaction pattern of the programmed components. An ultimate goal of (solid-state) supramolecular chemistry is the prediction of nanostructures from the mere knowledge of the structure of the basic components that self-assemble into sophisticated nanostructures.7 As 3D crystal engineering still remains a big challenge, the introduction of directional forces8 and the reduction to the 2D problem on surfaces facilitate establishing a priori how the molecular units come together. Hydrogen bonding is predestinated for this purpose due to its directionality and selectivity. An excellent method to visualize these nanostructures with submolecular resolution on, for example, highly ordered pyrolytic graphite (HOPG) is represented by scanning tunneling microscopy (STM).9-15 In the past few years, an increasing number of references on self-assembly processes was found where hydrogen bonding was used as a directional tool for the design * Corresponding authors. E-mail: [email protected]; Moeller@ dwi.rwth-aachen.de. † Organische Chemie III/Makromolekulare Chemie. ‡ Present address: Deutsches Wollforschungsinstitut. § Laboratoire de Chimie Supramole ´ culaire, ISIS. (1) Lehn, J.-M. Science 2002, 295, 2400-2403. (2) Reinhoudt, D. N.; Crego-Calama, M. Science 2002, 295, 2403-2407. (3) Ikkala, O.; ten Brinke, G. Science 2002, 295, 2407-2409. (4) Hollingsworth, M. D. Science 2002, 295, 2410-2413. (5) Kato, T. Science 2002, 295, 2414-2418. (6) Whitesides, G. M.; Grzybowski, B. Science 2002, 295, 2418-2421. (7) Desiraju, G. R. Nat. Mater. 2002, 1, 77-79. (8) Kobayashi, K.; Sato, A.; Sakamoto, S.; Yamaguchi, K. J. Am. Chem. Soc. 2003, 125, 3035-3045. (9) De Feyter, S.; De Schryver, F. C. J. Phys. Chem. B 2005, 109, 4290-4302. (10) De Feyter, S.; Abdel-Mottaleb, M. M. S.; Schuurmans, N.; Verkuijl, B. J. V.; van Esch, J. H.; Feringa, B. L.; De Schryver, F. C. Chem.-Eur. J. 2004, 10, 1124-1132. (11) Samori, P. J. Mater. Chem. 2004, 14, 1353-1366. (12) De Feyter, S.; De Schryver, F. C. Chem. Soc. ReV. 2003, 32, 139-150.

of superstructures on surfaces from tailored subunits. In addition to a few examples where weak hydrogen bonds C-H‚‚‚X (X ) N, O)16-22 were employed as structurally directing motif, strong hydrogen bonds X-H‚‚‚Y (X, Y ) N, O, S) are dominating self-organization processes based on directional intermolecular forces. Apart from biological molecules such as, for example, nucleobases,23,24 synthetic molecules are utilized for self-assembly on surfaces investigated by STM. Among these compounds are urea derivatives,25 alcohols,26 aromatic thiols,27,28 ureido-functionalized molecules,29 2-pyrrolidone,30 imines,31 imides,32,33 (13) De Feyter, S.; Gesquiere, 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, 520-531. (14) Giancarlo, L. C.; Flynn, G. W. Annu. ReV. Phys. Chem. 1998, 49, 297336. (15) Cyr, D. M.; Venkataraman, B.; Flynn, G. W. Chem. Mater. 1996, 8, 1600-1615. (16) Yokoyama, T.; Yokoyama, S.; Kamikado, T.; Okuno, Y.; Mashiko, S. Nature 2001, 413, 619-621. (17) Ziener, U.; Lehn, J.-M.; Mourran, A.; Mo¨ller, M. Chem.-Eur. J. 2002, 8, 951-957. (18) Bo¨hringer, M.; Schneider, W. D.; Berndt, R. Surf. ReV. Lett. 2000, 7, 661-666. (19) Xing, L.; Ziener, U.; Sutherland, T. C.; Cuccia, L. A. Chem. Commun. 2005, 5751. (20) Meier, C.; Ziener, U.; Landfester, K.; Weihrich, P. J. Phys. Chem. B 2005, 109, 21015-21027. (21) Barth, J. V.; Weckesser, J.; Cai, C.; Gu¨nter, P.; Bu¨rgi, L.; Jeandupeux, O.; Kern, K. Angew. Chem. 2000, 112, 1285-1288; Angew. Chem., Int. Ed. 2000, 39, 1842-1845. (22) Stepanow, S.; Lin, N.; Vidal, F.; Landa, A.; Ruben, M.; Barth, J. V.; Kern, K. Nano Lett. 2005, 5, 901-904. (23) Nakagawa, T.; Tanaka, H.; Kawai, T. Surf. Sci. 1997, 370, L144-L148. (24) Gottarelli, G.; Masiero, S.; Mezzina, E.; Pieraccini, S.; Rabe, J. P.; Samori, P.; Spada, G. P. Chem.-Eur. J. 2000, 6, 3242-3248. (25) Gesquiere, A.; Abdel-Mottaleb, M. M. S.; De Feyter, S.; De Schryver, F. C.; Schoonbek, F.; van Esch, J.; Kellogg, R. M.; Feringa, B. L.; Calderone, A.; Lazzaroni, R.; Bredas, J. L. Langmuir 2000, 16, 10385-10391. (26) Le Poulennec, C.; Cousty, J.; Xie, Z. X.; Mioskowski, C. Surf. Sci. 2000, 448, 93-100. (27) Pinheiro, L. S.; Temperini, M. L. A. Surf. Sci. 1999, 441, 45-52. (28) Pinheiro, L. S.; Temperini, M. L. A. Surf. Sci. 1999, 441, 53-64. (29) Gesquiere, A.; Jonkheijm, P.; Hoeben, F. J. M.; Schenning, A. P. H. J.; De Feyter, S.; De Schryver, F. C.; Meijer, E. W. Nano Lett. 2004, 4, 1175-1179. (30) Xie, Z. X.; Charlier, J.; Cousty, J. Surf. Sci. 2000, 448, 201-211.

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carbamates,34,35 and carboxylic acids.21,22,36-42 The adsorption of terephthalic acid on Au(111) under UHV conditions leads to hydrogen-bonded linear chains,36 whereas on Cu(100) temperature-dependent changes of the morphology take place parallel to deprotonation.39 If a third carboxylic group is introduced to the aromatic core (trimesic acid, TMA), a two-dimensional extended flower and chickenwire-like structures are formed on graphite which can be controlled by the solvent and act additionally as hosts for further TMA molecules.37,41 Even five different morphologies are built if the same compound is selfassembled on Au(111) under potential control.38 In addition to these O-H‚‚‚O hydrogen bonds hetero bonds O-H‚‚‚N are described in self-assemblies of pyridylvinyl benzoic acid on Ag(111)21 and coadsorbates of bisterpyridine derivatives with azobenzene dicarboxylic acid on HOPG.40 Hydrogen bonding between lactam moieties is proposed in hexameric aggregates of 2-pyrrolidone on Au(111).30 Here, we report on a hydrogen bonding lactam (lactim) system based on alkoxy- and alkyl-substituted phthalhydrazide 1a,b in combination with pyrimidine derivatives 2a,b (Figure 1). Contrary to the above-mentioned examples from literature, phthalhydrazide is capable of undergoing tautomerism between three forms, which offers a variable hydrogen-bonding motif (Figure 1). In ethanol solution the lactim/lactam tautomer of a methyl derivative is the most stable isomer.43 In the solid state, too, phthalhydrazide44 and the nonaromatic compound maleic hydrazide with the corresponding hydrogen bonding capabilities are (exclusively) present in their lactam/lactim tautomeric form.45 It was recently shown that 1a,b form thermotropic, columnar, discotic mesophases built up from trimeric cyclic units based on the lactim/ lactam isomer.46 Even the 1:1 mixture of 1a and 2a exhibiting complementary hydrogen-bonding functionalities forms a mesophase that presumably is built by tetrameric units due to a transformation of the lactim/lactam isomer to the lactam/lactam isomer of 1a (Figure 1).46 This shows the strong dependency of the tautomerism on the environment of the hydrogen-bonding groups. In the following sections, we will present the self-assembly of 1 and 2 as well as mixtures of both on HOPG and discuss the influence of the solvent on the pattern formation which demonstrates a successful strategy for nanopatterning of graphite (31) Sto¨hr, M.; Wahl, M.; Galka, C. H.; Riehm, T.; Jung, T. A.; Gade, L. H. Angew. Chem. 2005, 117, 7560-7564. (32) Swarbrick, J. C.; Ma, J.; Theobald, J. A.; Oxtoby, N. S.; O’Shea, J. N.; Champness, N. R.; Beton, P. H. J. Phys. Chem. B 2005, 109, 12167-12174. (33) Thalacker, C.; Miura, A.; De Feyter, S.; De Schryver, F. C.; Wu¨rthner, F. Org. Biol. Chem. 2005, 3, 414-422. (34) Kim, K.; Plass, K. E.; Matzger, A. J. J. Am. Chem. Soc. 2005, 127, 4879-4887. (35) Kim, K.; Plass, K. E.; Matzger, A. J. Langmuir 2005, 21, 647-655. (36) Clair, S.; Pons, S.; Seitsonen, A. P.; Brune, H.; Kern, K.; Barth, J. V. J. Phys. Chem. B 2004, 108, 14585-14590. (37) Lackinger, M.; Griessl, S.; Markert, T.; Jamitzky, F.; Heckl, W. M. J. Phys. Chem. B 2004, 108, 13652-13655. (38) Li, Z.; Han, B.; Wan, L. J.; Wandlowski, T. Langmuir 2005, 21, 69156928. (39) Stepanow, S.; Strunskus, T.; Lingenfelder, M.; Dmitriev, A.; Spillmann, H.; Lin, N.; Barth, J. V.; Wo¨ll, C.; Kern, K. J. Phys. Chem. B 2004, 108, 1939219397. (40) Wu, D.; Deng, K.; Zeng, Q.; Wang, C. J. Phys. Chem. B 2005, 109, 22296-22300. (41) Griessl, S.; Lackinger, M.; Edelwirth, M.; Hietschold, M.; Heckl, W. M. Single Mol. 2002, 3, 25-31. (42) Otsuki, J.; Nagamine, E.; Kondo, T.; Iwasaki, K.; Asakawa, M.; Miyake, K. J. Am. Chem. Soc. 2005, 127, 10400-10405. (43) Elvidge, J. A.; Redman, A. P. J. Chem. Soc. 1960, 1710-1714. (44) Howell, R. C.; Edwards, S. H.; Gajadhar-Plummer, A. S.; Kahwa, I. A.; McPherson, G. L.; Mague, J. T.; White, A. J. P.; Williams, D. J. Molecules 2003, 8, 565-592. (45) Katrusiak, A. Acta Crystallogr., Sect. B 2001, 57, 697-704. (46) Suarez, M.; Lehn, J.-M.; Zimmermann, S. C.; Skoulios, A.; Heinrich, B. J. Am. Chem. Soc. 1998, 120, 9526-9532.

Figure 1. Phthalhydrazides (1) and pyrimidines (2), tautomerism of 1, trimeric cyclic units (1Tri) of 1, and tetrameric units (1,2Tetra) of a 1:1 mixture of 1 and 2 in the mesophases.

by self-assembly of a homo- and heterocomplementary hydrogenbonding system. II. Experimental Section STM measurements were carried out at ambient conditions with a low-current RHK 1000 control system. In situ STM imaging was performed at the internal interface between HOPG and concentrated solution of the compounds dissolved in either 1,2,4-trichlorobenzene or 1-chloronaphthalene. According to the procedure of Rabe and Buchholz,47 a drop of solution was placed on a freshly cleaved HOPG surface while the surface had already been scanned by STM under conditions that allowed atomic resolution of the graphite surface structure. The scan rate was 0.5 µm/s. All images presented were obtained at constant current mode using a Pt/Ir (90/10) tip, which was mechanically sharpened. The specific tunneling conditions are given in the figure captions. The images were analyzed by an external calibration with respect to the basal plane of HOPG. The overall error was then determined to be below (5%. All compounds have been obtained as described before.46 (47) Rabe, J. P.; Buchholz, S. Science 1991, 253, 424-427.

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III. Results and Discussion The presented molecular assemblies on HOPG are formed from solution and are visualized by in situ STM under ambient conditions. This simple procedure gives, in contrast to more sophisticated investigations such as those under UHV conditions and at low temperatures,16 access to a large variety of different assemblies. Our investigations were guided by the following questions: (i) can the 2D structures of 1 and 2 be formed on HOPG as predicted from the hydrogen-bonding motifs found in the mesophase structures (see Figure 1)? (ii) How does the surface interfere with the structures leading eventually to new features compared to the structures in the mesophases? (iii) How is the solvent interplay expressed in the 2D structures? From the features of the hydrogen-bonding motif of 1, either discrete cyclic (preferably trimeric) units or infinite polymeric bandlike structures are expected if the hydrogen bonds are fully saturated regardless of which tautomeric form is expressed. The cyclic units are formed if neighboring molecules show double hydrogen bonds between the left and right sides of the two N-C-O hydrogen-bonding moieties of the neighboring molecule (Figure 1). In the case of the infinite bands, the double hydrogen bonds are formed between the two left and the two right sides, respectively, of neighboring molecules. Figure 2 depicts representative STM images of 1a adsorbed at the interface between HOPG and a saturated solution of 1a in 1,2,4-trichlorobenzene. The large scan area shows multidomain adlayers consisting of molecular rows. While the bright contrast indicates a large tunneling current through the aromatic moieties of the adsorbed molecules, the hydrocarbon parts are dim. The relative orientation of the rows at the domain boundaries indicates that they are weakly aligned with the underlying HOPG. However, consideration of the orientation of the alkyl chains at the same boundaries indicates epitaxial alignment with the crystalline lattice of HOPG. The distance between the rows was measured to be 31 Å, which is larger than the extended length of a C16 alkyl chain but in the expected range of interdigitating chains. There is a substructure of a trapezoidal arrangement of spots within the bright (aromatic) part of the rows showing a repeating length of 20 Å along the rows and a translation of 4-5 Å in the orthogonal direction. The molecular rows separating aromatic and alkyl moieties as seen in Figure 2 exclude any discrete trimeric cyclic units as found in the mesophases46 and for the parent compound in the solid state44 but indicate (as suggested above) a 1D chainlike structure as proposed in Figure 3a. The model in Figure 3a elucidates a lamellar assembly of chainlike structure assuming the all-trans planar packing of the alkyl chains and the hydrogen bonding of the aromatic lactam and lactim units between two right and left sides of neighboring molecules (Figure 1), respectively. The lamellar width (31 Å) fits nicely the expected length of the interdigitating C16 alkyl chains and the aromatic units in accordance with the model. However, the scheme in Figure 3b suggests that (i) a periodicity of 20 Å is too small if six chains have to be adsorbed with 4.1-4.5 Å per single alkyl chain48 and (ii) on the other side the 20 Å is too large for three and too small for four aromatic units in the fully saturated (double) hydrogen-bonding state as it requires 5-5.5 Å per molecule regardless of which tautomeric form is expressed. Packing constraints suggest only fiVe chains out of six are adsorbed on the graphite as known from other examples.33,49,50 While the plane formed by the hydrogen bonds is (48) Yin, S.; Wang, C.; Qiu, X.; Xu, B.; Bai, C. Surf. Interface Anal. 2001, 32, 248-252. (49) Zell, P.; Mo¨gele, F.; Ziener, U.; Rieger, B. Chem. Commun. 2005, 12941296.

Figure 2. STM image of 1a from a 1,2,4-trichlorobenzene solution. Large scan area (a) and magnified view with trapezoidal arrangement of bright spots (b). Ub ) 690 mV, It ) 18 pA.

parallel to the surface, the orientations of the aromatic moieties in adjacent domains manifest weak surface affinity as compared to the aliphatic tail. We suggest that the energetic competition between the adsorption of the alkyl chains on the graphite and the hydrogen bonding34,35,42 leads to a compromise between both features as there is a mismatch of the spatial requirement of two alkyl chains (8-9 Å per molecule) and the aromatic moieties (5-5.5 Å). We assume that the system escapes from this energetically unfavorable situation by shifting every three molecules per lamella by ca. 4-5 Å perpendicular and ca. 3 Å parallel to the main axis of the lamella on the expense of a double hydrogen bond but gaining space for an additional (in total five) alkyl chain. This shift results in the formation of a periodic substructure (Figure 2) offering additional O-H‚‚‚O bonds between three neighboring molecules (see Figure 3b). As the observed pattern strongly depends on the solvent (see below), a direct involvement of trichlorobenzene solvent molecules in the 2D structure by additional hydrogen bonds cannot be ruled out. Still, there is no clear evidence in the STM image for the incorporation of solvent molecules. Other pos(50) Mourran, A.; Beginn, U.; Zipp, G.; Mo¨ller, M. Langmuir 2004, 20, 673679.

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Figure 3. (a) Linear trimeric unit of 1a in the lactim/lactam form from 1,2,4-trichlorobenzene solution on HOPG with the principal hydrogen-bonding motif as part of the infinite chainlike structure. Image (b) shows their surface assembly reproducing the periodic substructure of the aromatic part and interdigitations of the alkyl chains. R*: one arbitrary chain out of six alkyl chains pointing into the solution.

sibilities for breaking the hydrogen bond may be a mismatch between the crystalline structure of the graphite and the molecular arrangement of 1a or a switch from one tautomer to another. For the first case, the model does not require a defined tautomeric form. All three tautomers would fulfill the requirement with respect to the hydrogen-bonding features though in solution46 and in single crystals of phthalhydrazide44 and related compounds45 the lactim/lactam tautomer is preferred, whereas secondary effects such as packing or interactions with the surface might induce formation of different tautomers. For the second case, the switch from, for example, the bislactim to the bislactam tautomer (Figure 1) could give rise to the break of the hydrogen bonding after each third double hydrogen bond. It seems that in the arrangement of 1a on graphite (Figures 2 and 3) the surface coverage is optimized on the expense of full hydrogen bonding and maximal van der Waals interactions between the alkyl chains and the substrate (see above). Weakening the adsorption energy by shortening the length of the aliphatic tail from C16 down to C8 (compound 1b), we were not able to observe any pattern under similar conditions as for 1a. A reason might be that the high polarity of the aromatic moiety owing to the four oxygen atoms cannot be compensated by the relatively short alkyl chains; hence, no adsorption takes place. This demonstrates the delicate balance between the adsorption/lateral interaction of the alkyl chains and the hydrogen bonds, but also sets a lower limit for the change of free energy associated with adsorption from the TCB environment. The following paragraph shows the system’s lability as it reacts on subtle changes of the solvent molecule geometry and its tendency to form π-π interactions with the graphite surface.

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To provide more insight on the influence of the solvent on the self-assembly of 1a on graphite, we repeated the experiment with 1-chloronaphthalene instead of 1,2,4-trichlorobenzene. The naphthalene derivative exhibits properties, such as polarity, that are comparable to the benzene compound but require more space than the latter and should subsequently lead to a different arrangement of 1a on HOPG. The STM images in Figure 4 demonstrate remarkable effect of the solvent on the 2D arrangement of 1a on HOPG. The large scan area exhibits a closed packing of hexagons, and the edge has a length of 31 Å. The center-to-center distance of neighboring hexagons is 55 Å (Figure 4a). In some cases, defects are generated by breaking up the hexagons forming zigzag lines while keeping the 120° angle of the hexagons. Figure 4b shows that the edge of each hexagon bears asymmetric teeth that define the rotation angle of the hexagons. The arrangement of such hexagons on a surface implies that chirality of a single hexagon emerges from the self-assembly of achiral (prochiral) molecules. This can be seen on a closer look to the rotation direction of the hexagons in Figure 4b exhibiting a “left” and a “right” rotation angle. As we did not use a chiral environment it is expected that both 2D chiral forms are expressed, and indeed, different domains separated by the aforementioned zigzag lines show partially different 2D chirality. Symmetry breaking of prochiral molecules on a substrate or substrate-induced chirality22,51,52 or the formation of prochiral supramolecular aggregates53 and complexes54 composed of prochiral monomers are already known from the literature. We present a more sophisticated system created by hierarchical self-assembly of the lactim/lactam tautomer 1a to trimers and the subsequent arrangement to hexagons on HOPG. Figure 4c reveals that the hexagons are built up from symmetrical three-arm stars, which are hexagonally arranged (Figure 4d). The bright contrast at the center of each star is due to a high tunneling current through the aromatic assembly of 1a relative to the alkyl chains. The arms of the stars have an equal length of 18 Å corresponding to the length of C16 alkyl chains. Consequently, each star consists of three molecules of 1a, forming a trimer with six hydrogen bonds in the lactim/lactam tautomeric form similar to what was proposed in the mesophase46 and found in the solid state for the nonalkylated compound.44 It is worth noticing that only one out of two star cores shows up bright in Figure 4c. Thus far we cannot clearly explain this effect. Though different contrast might arise from different adsorption sites on HOPG, it is relevant to exclude here this possibility because of the observed periodic length scale. Each trimeric star shows chirality upon adsorption due to the nonsymmetric hydrogenbonding motif of the lactim/lactam tautomeric form within the trimers. We suggest that the chirality of the hexagons favors pairing of stars with complementary chirality of the single stars which are energetically equivalent but give rise to different tunneling current and hence contrast. In contrast to the infinite polymeric arrangement of 1a from TCB (Figures 2 and 3), the discrete trimers deposited from 1-chloronaphthalene are fully saturated with respect to the hydrogen bonds and the spatial requirement of the stars in the hexagons fits the measured dimensions (Figure 5). This arrangement allows the regular packing of the alkyl chains but also (51) Bo¨hringer, M.; Morgenstern, K.; Schneider, W.-D.; Berndt, R.; Mauri, F.; De Vita, A.; Car, R. Phys. ReV. Lett. 1999, 83, 324-327. (52) France, C. B.; Parkinson, B. A. J. Am. Chem. Soc. 2003, 125, 1271212713. (53) Vidal, F.; Delvigne, E.; Stepanow, S.; Lin, N.; Barth, J. V.; Kern, K. J. Am. Chem. Soc 2005, 127, 10101-10106. (54) Dmitriev, A.; Spillmann, H.; Lingenfelder, M.; Lin, N.; Barth, J. V.; Kern, K. Langmuir 2004, 20, 4799-4801.

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Figure 5. Schematic model of the 2D assembly of 1a from 1-chloronaphthalene at the HOPG interface. The lactim/lactam tautomer is expressed by building the trimeric units.

Figure 4. STM image of 1a adsorbed from 1-chloronaphthalene solution on HOPG. Images (a) and (b) show the honeycomb arrangement of 1a (Ub ) 540 mV, tunneling current of It ) 20 pA). Image (c) is taken with bias voltage of 250 mV and tunneling current of 13 pA, and at these scanning conditions a three-arm star-shaped substructure is observed. Image (d) depicts a scheme of the 2D packing constructed with two stars as repeating units. The unit cell is hexagonal with a ) b ) 55 Å and γ ) 120°. The scheme illustrates a 2D chiral structure that emerges through the self-assembly of achiral (prochiral) molecules.

generates voids in the centers of the hexagons. For steric reasons the size of the voids is too small to incorporate an additional trimer with the alkyl tails. We assume that the solvent molecules stabilize the assembly by filling the voids due to the more extended conjugated system of 1-chloronaphthalene compared to TCB and compensate for the lower surface coverage compared to the arrangement from TCB. We further argue that compound 1b with the shorter aliphatic tail was observed forming monolayers at the HOPG/1-chloronaphthalene interface. This is in contrast to the 1,2,4-trichlorobenzene environment since 1b does not form adlayers on HOPG which can be detected by STM. Thus, the 1-chloronaphthalene favors the trimeric arrangement and provides enough stability for shorter alkyl chains to self-assemble on HOPG. This is illustrated in Figure 6a, which shows a hexagonal arrangement of three-arm stars which are assigned to trimeric units as for 1a. Contrary to 1a, each hexagon contains seven triangular bright spots. A model is proposed in Figure 6b. Compared to that of 1a, for 1b all triangles are equivalent. As the resolution is not sufficient to reveal the arrangement of the alkyl chains, we suggest that the chains of one molecule 1b are parallel to each other and experience van der Waals interactions between the alkyl chains of neighboring molecules. This causes large voids in the model (Figure 6b) which are not seen on the STM image. The solvent might stabilize the voids as assumed in the case of 1a (see above). The distance between the centers of the stars in such an arrangement is 25 Å, which fits well to the measured one. These findings show the strong interplay of the solvent with the hydrogen-bonding motif and hence impact on the resulting 2D pattern.9,37,55 As for the pyrimidine derivatives 2a and 2b, in contrast to the phthalhydrazide derivatives 1a and 1b, no tautomerism is possible. There is effectively only one 2D packing motif expected with fully saturated hydrogen bonds. Indeed, Figure 7 shows the results of the STM investigations of 2a and 2b adsorbed from 1,2,4trichlorobenzene. Both compounds exhibit a lamellar structure, and widths of 26 Å for 2a and 19 Å for 2b, respectively, were measured. For both compounds, the lamellae consist of rows of equidistant bright spots (10-11 Å) that we attribute to the pyrimidine units, whereas the low contrast parts are assigned to the alkyl chains. The difference in the periodicities of the two compounds is congruent with the different lengths of the C16 and C8 alkyl chains, respectively. A high-resolution micrograph of 2a (Figure 7b) reveals the herringbone structure of the alkyl chains with a tilt angle of 60° relative to the aromatic moieties. On the basis of the self-complementary hydrogen-bonding motif, we modeled the 2D assembly of 2a or 2b in Figure 8 as (55) Mamdouh, W.; Uji-i, H.; Ladislaw, J. S.; Dulcey, A. E.; Percec, V.; De Schryver, F. C.; De Feyter, S. J. Am. Chem. Soc. 2006, 128, 317-325.

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Figure 6. STM image of 1b on HOPG from a 1-chloronaphthalene solution (a) and the model (b). Scanning conditions: Ub ) 251 mV and It ) 27 pA.

a one-dimensional chainlike structure of the heteroaromatic units and the alkyl chains, respectively. The periodicity along the aromatic backbone is measured to be 12 and 18 Å, respectively, and the interchain distance is consistent with interdigitated alkyl tails with a tilt angle of 60° relative to the pyrimidine units. The use of 1-chloronaphthalene as solvent does not alter the 2D assembly of 2a,b. The same morphologies were found as those used for 1,2,4-trichlorobenzene corresponding to fully saturated hydrogen bonds. Thus far, compounds 1a,b and 2a,b exhibited different 2D morphologies due to different hydrogen-bonding features in the self-assembled structures. These two classes of compounds were selected because they show (hetero-)complementary hydrogen bonding as it is presumably expressed in the tetrameric units in the mesophase (cf. Figure 1).46 Hence, in a third series of experiments we investigated the 1:1 mixtures of 1 and 2 to address the effect of heterocomplementary hydrogen bonding on the 2D self-assembly. In the case of the mixture of 1a with 2a in 1,2,4trichlorobenzene, a completely different surface pattern was obtained compared to that of the pure components (Figure 9). The homogeneous pattern confirms the complete miscibility of the two compounds excluding phase separation.56 The repeating

Figure 7. STM image of 2a (Ub ) 388 mV, It ) 20 pA (a), Ub ) 300 mV, It )15 pA (b)) and 2b from 1,2,4-trichlorobenzene (Ub ) 800 mV, It ) 22.5 pA (c)).

length of the alternating dark and bright regions is 29 Å. The fine substructure of the bright areas consists of rows of crosslike (56) Klymchenko, A. S.; Sleven, J.; Binnemans, K.; De Feyter, S. Langmuir 2006, 22, 723-728.

Assemblies of Lactim/Lactam Recognition Patterns

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Figure 8. Model for the arrangement of 2b adsorbed on HOPG irrespective of the solvent environment.

Figure 10. Proposed model for the arrangement of a 1:1 mixture of 1a and 2a from 1,2,4-trichlorobenzene. R*: three arbitrary chains out of eight alkyl chains pointing into the solution.

Figure 9. STM image of a 1:1 mixture of 1a and 2a from 1,2,4trichlorobenzene. Scanning conditions: Ub ) 870 mV, It ) 17 pA.

assemblies (periodicity 20 Å) that we assign to the aromatic moieties of the pyrimidine and phthalhydrazide molecules, respectively. The dark regions are attributed to the alkyl chains. From the crosslike feature and the lateral spacing, we assume the same discrete structure of 1a and 2a as proposed for the mesophase (Figure 1).46 Figure 10 depicts the model for such an arrangement with the estimated dimensions. This model implies that the less favored bislactam tautomer was preferred over the more stable lactim/lactam or the bislactim form (least stable). Expression of the alternative tautomeric forms (lactim/lactim or bislactam) would lead to more sterically hindred packing (not shown here) which could not be fitted to the STM data (Figure 9). Here we suggest a tetrameric arrangement of the 1:1 mixture of 1a and 2a (Figure 10). As already noticed, the requirement on 2D packing of 1a leads to partial overlap of the alkyl chains. Only five out of eight chains per tetramer adsorb on the HOPG; the others are pointing into the solution as found earlier (see above). We believe that formation of the large cyclic core increases the adsorption strength of the aromatic part relative to the aliphatic tail at the HOPG/liquid interface. This is consistent with our previous observation that shorter alkyl substituents form stable adlayers in the environment favoring the trimeric form from chloronaphthalene solution. However, in the 1:1 mixture of 1a and 2a the hydrogen-bonding motif is already predetermined in the presence of TCB as solvent favoring the other tautomeric

forms of the phthalhydrazide molecules. We strongly assume that preformed aggregates of 1a are already present in solution.20 This guided us to investigate the adsorption of the 1:1 mixture 1a/2a from 1-chloronaphthalene, a solvent that favors formation of trimers of 1a (see above). On the first glimpse, the large scan area image in Figure 11 (top) resembles the features found for pure 1a (cf. Figure 4) as it shows the hexagonal pattern built from three-arm stars. The characteristic dimensions of the pattern with the edge length of the hexagons 33 Å (Figure 11) are similar to those for pure 1a from 1-chloronaphthalene with 32 Å (cf. Figure 4). In addition to these similarities, there are two different arrangements of the hexagons in Figure 11 contrary to pure 1a (Figure 4). They either share edges or stand alone. Consequently, there are two different distances between two neighboring hexagons with 56 and 78 Å, respectively. Nevertheless, the image at the bottom in Figure 11 shows additional bright spots along the edges of the hexagons. Although the solution consists of a 1:1 mixture of 1a and 2a, we could not clearly assign 2a from the STM images. Homogeneity of the pattern excludes any phase segregation between the two compounds. We also exclude “conformational polymorphism” since we did not observe different patterns of the monolayer from the pure compounds within the same solvent. However, we do not exclude that the presence of 2a promotes polymorphism. Furthermore, from the characteristic dimension of the observed structure the trimeric assembly in 1-chloronaphthalene seems to be preserved. Whether coadsorption of 2a or conformational polymorphism induced by the presence of 2a in solution leads to this intriguing pattern is an open question that deserves further investigation. To get more proof for the proposed model, we examined also the adsorption of the mixed short-chain compounds 1b/2b to obtain the same pattern as for 1a/2a but with different periodicities.

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Figure 11. STM image of a 1:1 mixture of 1a and 2a from 1-chloronaphthalene as large scan area (top) and slightly magnified view with higher resolution (bottom). Scanning conditions: Ub ) -361 mV, It ) 28 pA (top), Ub ) -181 mV, It ) 18 pA (bottom).

bonding. Additionally, phthalhydrazide 1 exhibits in principle three different tautomeric forms (bislactam, lactim/lactam, and bislactim) that might be expressed under different conditions, with the lactim/lactam tautomer being the most stable in solution43 and in single crystals.44 We found that there is a strong influence of the solvent on the 2D pattern formation. The long-chain derivative 1a self-assembles from 1,2,4-trichlorobenzene in an infinite bandlike structure that can be formed in principle by all three forms. Yet, we suggest that the most stable form (lactim/ lactam) is expressed in that arrangement which is determined by a subtle interplay between the adsorption/crystallization of the alkyl chains and hydrogen-bond interactions. From 1-chloronaphthalene, 1a exhibits trimeric starlike units that arrange on HOPG in a hexagonal pattern. The same three-arm stars are expressed in the mesophase46 and solid state,44 too. The C8 compound 1b could be visualized by STM only from 1-chloronaphthalene showing a similar arrangement on HOPG as the long-chain derivative 1a under the same conditions. No effect of the solvent on the assemblies of 2a and 2b on graphite could be found showing in both cases bandlike structures that present the expected features for the fully saturated self-complementary hydrogen bonds. A switch from the lactim/lactam to the bislactam tautomer of 1a is found in a 1:1 mixture with 2a from 1,2,4trichlorobenzene forming cyclic tetrameric units as are found in the mesophase, too.46 Both the complementary hydrogen bonding between the pyrimidine and the phthalhydrazide units and the influence of the 2D graphite surface may cause the switch to the less stable tautomer. The presented rich 2D structures give an example of molecular recognition-directed nanostructuring of surfaces. They show that by a careful choice of the conditions (solvent) and by tailoring the self-assembling units the structures can be tuned toward quite different nanopatterns. These nanostructures will be further exploited as template structures41,57,58 in combination with metal complexation to achieve addressable functional devices for, for example, molecular or optical electronics. Finally, one may point out that the switching of the phthalhydrazide group between different tautomeric forms represents, as noted earlier,59 a process of dynamic combinatorial chemistry, which generates dynamic diversity through tautomerism between different recognition patterns. The resulting expression of a given recognition pattern amounts to adaptation of the system in response to external factors (such as interaction with the surface, packing, solvent binding) and results in the generation of different, adaptive nanostructures.

The STM investigations of the mixture of 1b with 2b from 1-chloronaphthalene or from 1,2,4-trichlorobenzene either did not lead to defined images or the interpretation of the images was too difficult to result in a conclusive model, respectively.

Acknowledgment. We thank the German Science Foundation (“Deutsche Forschungsgemeinschaft”) for financial support within the framework of the Research Center 569 (“Sonderforschungsbereich”) at the University of Ulm.

IV. Conclusion STM was used to investigate stable self-assemblies of aminopyrimidine 2 or phthalhydrazide 1 molecules equipped with C8 or C16 alkyl chains on graphite. Both types of compounds are capable of complementary and self-complementary hydrogen

LA0609266 (57) Theobald, J. A.; Oxtoby, N. S.; Phillips, M. A.; Champness, N. R.; Beton, P. H. Nature 2003, 424, 1029-1031. (58) Theobald, J. A.; Oxtoby, N. S.; Champness, N. R.; Beton, P. H.; Dennis, T. J. S. Langmuir 2005, 21, 2038-2041. (59) Lehn, J.-M. Chem.-Eur. J. 1999, 5, 2455-2463; see in particular p 2457 and Figure 2 therein.