Expression of Molecular Chirality and Two-Dimensional

AFM Visualization of Individual and Periodic Assemblies of a Helical Dendronized Polyphenylacetylene on Graphite. Virgil Percec, Jonathan G. Rudick, M...
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Langmuir 2004, 20, 7678-7685

Expression of Molecular Chirality and Two-Dimensional Supramolecular Self-Assembly of Chiral, Racemic, and Achiral Monodendrons at the Liquid-Solid Interface Wael Mamdouh,† Hiroshi Uji-i,† Andre´s E. Dulcey,‡ Virgil Percec,*,‡ Steven De Feyter,*,† and Frans C. De Schryver*,† Department of Chemistry, Laboratory of Photochemistry and Spectroscopy, Katholieke Universiteit Leuven, Celestijnenlaan 200-F, 3001 Leuven, Belgium, and Roy & Diana Vagelos Laboratories, Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19104-6323 Received March 15, 2004. In Final Form: May 19, 2004 We have investigated the two-dimensional ordering of chiral and achiral monodendrons at the liquidsolid interface. The chiral molecules self-assemble into extended arrays of dimers. As expected, the R enantiomer forms the mirror image type pattern of the chiral two-dimensional structure formed by the S enantiomer. A racemic mixture applied from solution onto the substrate undergoes spontaneous segregation: the enantiomers separate on the surface and appear in different domains. In contrast to the chiral molecules, the achiral analogue self-assembles into cyclic tetramers. Moreover, the pattern formed by the achiral molecule strongly depends on the solvent used. In the case of 1-phenyloctane, solvent molecules are coadsorbed in a 2:1 (dendron:solvent) ratio whereas in 1-octanol, no solvent molecules are coadsorbed. By the appropriate solvent choice, the distance between the potential “supramolecular containers” can be influenced.

1. Introduction Stereochemistry plays a major role in the material and life sciences. The nature of the atoms is of importance not only for the properties of chemical and biological substances but also for their exact positioning in space. For instance, amino acids, the building blocks of proteins and enzymes, are chiralsthe object and its mirror image are nonsuperimposable by any translation or rotationsand there is an evolutionary preference for one of both possible mirror forms (enantiomers). The sensitivity of living organisms to chirality is also expressed sometimes by the dramatic differences in the response of an organism to both enantiomers of a chemical substance. Chirality as a control tool for conformations at the molecular scale and at the supramolecular level is of great interest.1 Surface chirality is gaining increasing attention and is restricted not only to solid surfaces such as the ones described in this contribution but also to systems at air-liquid interfaces.2 The interest in surface chirality is a fundamental onesparticularly how to make a surface chiralsand is also very relevant in the area of catalysis.3 Some metal surfaces are intrinsically chiral4 while achiral surfaces can become chiral by decoration with chiral5 or * To whom correspondence may be addressed: [email protected], Steven.DeFeyter@ chem.kuleuven.ac.be, [email protected]. † Katholieke Universiteit Leuven. ‡ University of Pennsylvania. (1) (a) Solladie, G.; Zimmermann, R. G. Angew. Chem., Int. Ed. Engl. 1984, 23, 348-362. (b) Eliel, E. L.; Wilen, S. H. Stereochemistry of Organic Compounds; J. Wiley & Sons: New York, 1994. (c) Supramolecular Stereochemistry; Siegel, J., Ed.; NATO ASI Series C 473, Kluwer: Dordrecht, 1995. (d) Marks, T. J.; Ratner, M. A. Angew. Chem., Int. Ed. Engl. 1995, 34, 155-173. (e) Rowan, A. E.; Nolte, R. J. M. Angew. Chem., Int. Ed. 1998, 37, 63-68. (f) Green, M. M.; Park, J.-W.; Sato, T.; Teramoto, A.; Lifson, S.; Selinger, R. L. B.; Selinger, J. V. Angew. Chem., Int. Ed. 1999, 38, 3138-3154. (g) Feringa, B. L.; Van Delden, R. A. Angew. Chem., Int. Ed. 1999, 38, 3418-3438. (h) Mikami, K.; Terada, M.; Korenaga, T.; Matsumoto, Y.; Ueki, M.; Angelaud, R. Angew. Chem., Int. Ed. 2000, 39, 3532-3556.

achiral6 molecules, as revealed for example by scanning probe microscopes. Recent progress in the understanding of supramolecular chemistry in two dimensions and more in particular in chirality on solid surfaces is to a large extent realized thanks to scanning tunneling microscopy studies revealing molecular to submolecular resolution, initially at the liquid-solid interface and later under ultrahigh vacuum (UHV) conditions.5,6 As the studies at the solid-liquid interface are limited to extended twodimensional (2D) patterns, under temperature-controlled UHV conditions, submonolayer coverage and clusters can be probed too.7 The molecular decoration approach is a (2) (a) Kuzmenko, I.; Kjaer, K.; Als-Nielsen, J.; Lahav, M.; Leiserowitz, L. J. Am. Chem. Soc. 1999, 121, 2657-2661. (b) Zhong, O. Y.; Xu, X. B.; Wu, C. X., Iwamoto, M. Phys. Rev. E 1999, 59, 2105-2108. (c) Prus, P.; Pietraskiewicz, M.; Bilewicz, R. Supramol. Chem. 1998, 10, 17-25. (d) Scalas, E.; Brezesinski, G.; Kaganer, V. M.; Mo¨hwald, H. Phys. Rev. E 1998, 58, 2172-2178. (e) Yamagishi, A.; Sasa, N.; Taniguchi, M. Langmuir 1997, 13, 1689-1694. (f) Scalas, E.; Brezesinski, G.; Mo¨hwald, H.; Kaganer, V. M.; Bouwman, W. G.; Kjaer, K. Thin Solid Films 1996, 285, 56-61. (g) Rietz, R.; Rettig, W.; Brezesinski, G.; Bouwman, W. G.; Kjaer, K.; Mo¨hwald, H. Thin Solid Films 1996, 285, 211-215. (h) Vollhardt, D.; Emrich, G.; Gutberlet, T.; Fuhrhop, J. H. Langmuir 1996, 12, 5659-5663. (i) Parazak, D. P.; Uang, J. Y.-J.; Turner, B.; Stine, K. J. Langmuir 1994, 10, 3787-3793. (j) Selinger, J. V.; Wang, Z.-G.; Bruinsma, R. F.; Knobler, C. M. Phys. Rev. Lett. 1993, 70, 1139-1142. (k) Rietz, R.; Brezesinski; Mo¨hwald, H. Ber. Bunsen-Ges. Phys. Chem. 1993, 97, 1394-1399. (l) Andelman, D.; Orland, H. J. Am. Chem. Soc. 1993, 115, 12322-12329. (m) Dvolaitsky, M.; Guedeau-Boudeville, M. A. Langmuir 1989, 5, 1200-1205. (n) Arnett, A. M.; Harvey, N. G.; Rose, P. L. Acc. Chem. Res. 1989, 22, 131-138. (o) Tachibana, T.; Yoshizumi, T.; Hori, K. Bull. Chem. Soc. Jpn. 1979, 52, 34-41. (p) Nassoy, P.; Goldman, M.; Bouloussa, O.; Rondelez, F. Phys. Rev. Lett. 1995, 75, 457-460. (q) Weissbuch, I.; Berfeld, M.; Bouwman, W.; Kjaer, K.; Als-Nielsen, J.; Lahav, M.; Leiserowitz, L. J. Am. Chem. Soc. 1997, 199, 933-942. (r) Kuzmenko, I.; Weissbuch, I.; Gurovich, E.; Leiserowitz, L.; Lahav, M. Chirality 1998, 10, 415-424. (s) Lahav, M.; Leiserowitz, L. Angew. Chem., Int. Ed. 1999, 38, 2533-2536. (t) Rubinstein, I.; Bolbach, G.; Weygand, M. J.; Kjaer, K.; Weissbuch, I.; Lahav, M. Helv. Chim. Acta 2003, 86, 3851-3866. (u) Weissbuch, I.; Rubinstein, I.; Weygand, M. J., Kjaer, K.; Leiserowitz, L.; Lahav, M. Helv. Chim. Acta 2003, 86, 3867-3874. (3) (a) Sheldon, R. A. Chirotechnology; Marcel Dekker: New York, 1993. (b) Blaser, H. U. Tetrahedron: Asymmetry 1991, 2, 843-866.

10.1021/la049333q CCC: $27.50 © 2004 American Chemical Society Published on Web 07/27/2004

Ordering of Monodendrons

very interesting one as it combines chemical functionality with chirality. The presence of a substrate makes it actually easier to form extended chiral patterns.8 Even achiral molecules often assemble in a chiral 2D space group.6 Naturally, both mirror forms of the chiral pattern are present to the same extent. If a chiral molecule self-assembles in a chiral space group, enantiomers form exclusively one of the mirror forms. The question arises what happens when a racemic mixture is to self-assemble on a substrate. Conglomerate formation is one possibility: both enantiomers are physically separated into mirror-type domains which are identical to those of the pure enantiomers. Or alternatively, a racemate can be formed when both enantiomers coassemble.9 In addition to the concept of chirality at surfaces, also 2D supramolecular self-assembly has recently received a lot of attention.10 The goal is to get insight and control of the 2D pattern formation of molecules or mixtures of (4) (a) Sholl, D.; Asthagiri, A.; Power, T. D. J. Phys. Chem. B 2001, 105, 4771-4782. (b) Attard, G. A.; Ahmadi, A.; Jenkins, D. J.; Hazzazi, O. A.; Wells, P. B.; Griffin, K. G.; Johnston, P.; Gillies, J. E. Chem. Phys. Chem. 2003, 4, 123-130. (5) (a) Stevens, F.; Dyer, D. J.; Walba, D. M. Angew. Chem., Int. Ed. Engl. 1996, 35, 900-901. (b) Stevens, F.; Walba, D. M.; Clark, N. A.; Parks, D. C. Acc. Chem. Res. 1996, 29, 591-597 and references therein. (c) De Feyter, S.; Grim, P. C. M.; Rucker, M.; Vanoppen, P.; Meiners, C.; Sieffert, M.; Valiyaveettil, S.; Mu¨llen, K.; De Schryver, F. C. Angew. Chem., Int. Ed. Engl. 1998, 37, 1223-1226. (d) De Feyter, S.; Gesquiere, A.; Grim, P. C. M.; De Schryver, F. C.; Valiyaveettil, S.; Meiners, C.; Sieffert, M.; Mu¨llen, K. Langmuir 1999, 15, 2817-2822. (e) De Feyter, S.; Gesquie`re, A.; De Schryver, F. C.; Meiners, C.; Sieffert, M.; Mu¨llen, K. Langmuir 2000, 16, 9887-9894. (f) Giancarlo, L. G.; Flynn, G. W. Acc. Chem. Res. 2000, 33, 491-501 and references therein. (g) Gottarelli, G.; Masiero, S.; Mezzina, E.; Pieraccini, S.; Rabe, J. P.; Samorı´, P.; Spada, G. P. Chem. Eur. J. 2000, 6, 3242-3248. (h) Qian, P.; Nanjo, H.; Yokoyama, T.; Suzuki, T. M. Chem. Lett. 1998, 1133-1134. (i) Fang, H. B.; Giancarlo, L. C.; Flynn, G. W. J. Phys. Chem. B 1998, 102, 73117315. (j) Qian, P.; Nanjo, H.; Yokoyama, T.; Suzuki, T. M.; Akasaka, K.; Orhui, H. Chem. Commun. 2000, 2021-2022. (k) Ohtani, B.; Shintani, A.; Uosaki, K. J. Am. Chem. Soc. 1999, 121, 6515-6516. (l) Lorenzo, M. O.; Baddelley, C. J.; Muryn, C.; Raval, R. Nature 2000, 404, 376379. (m) Gesquie`re, A.; Jonkheijm, P.; Schenning, A. P. H. J.; MenaOsteritz, E.; Ba¨uerle, P.; De Feyter, S.; De Schryver, F. C.; Meijer, E. W. J. Mater. Chem. 2003, 13, 2164-2167. (n) Chen, Q.; Frankel, D. J.; Richardson, N. V. Surf. Sci. 2002, 497, 37-46. (o) Xu, Q.-M.; Wang, D.; Wan, L.-J.; Wang, C.; Bai, C.-L.; Feng, G.-Q.; Wang, M.-X. Angew. Chem. 2002, 113, 3558-3561. (p) Fasel, R.; Parschau, M.; Ernst, K.-H. Angew. Chem., Int. Ed. 2003, 42, 5178-5181. (6) (a) Smith, D. P. E. J. Vac. Sci. Technol., B 1991, 9, 1119-1125. (b) Rabe, J. P.; Buchholz, S. Phys. Rev. Lett. 1991, 66, 2096-2099. (c) Sowerby, S. J.; Heckl, W. M.; Petersen, G. B. J. Mol. Evol. 1996, 43, 419-424. (d) Charra, F.; Cousty, J. Phys. Rev. Lett. 1998, 80, 16821685. (e) Claypool, C. L.; Faglioni, F.; Matzger, A. J.; Goddard, W. A., III; Lewis, N. S. J. Phys. Chem. B 1999, 103, 9690-9699. (f) Patrick, D. L.; Cee, V. J.; Morse, M. D.; Beebe, T. P. J. Phys. Chem. B 1999, 103, 8328-8336. (g) Yablon, D. G.; Giancarlo, L. C.; Flynn, G. W. J. Phys. Chem. B 2000, 104, 7627-7635. (h) Lim, R.; Li, J.; Li, S. F. Y.; Feng, Z.; Valiyaveettil, S. Langmuir 2000, 16, 7023-7030. (i) Brian France, C.; Parkinson, B. A. J. Am. Chem. Soc. 2003, 125, 5559-5571. (j) Berner, S.; de Wild, M.; Ramoino, L.; Ivan, S.; Baratoff, A.; Gu¨ntherodt, H.-J.; Suzuki, H.; Schlettwein, D.; Jung, T. A. Phys. Rev. B 2003, 68, 115410. (7) (a) Ku¨hnle, A.; Linderoth, T. R.; Besenbacher, F. J. Am. Chem. Soc. 2003, 125, 14680. (b) Spillmann, H.; Dmitriev, A.; Lin, N.; Messina, P.; Barth, J. V.; Kern, K. J. Am. Chem. Soc. 2003, 125, 10725-10728. (c) Chen, Q.; Richardson, N. V. Nat. Mater. 2003, 2, 324-328. (d) Messina, P.; Dmitriev, A.; Lin, N.; Spillmann, H.; Abel, M.; Barth, J. V.; Kern, K. J. Am. Chem. Soc. 2002, 124, 14000-14001. (e) Barth, J. V.; Weckesser, J.; Trimarchi, G.; Vladimirova, M.; De Vita, A.; Cai, C.; Brune, H.; Gu¨nter, P.; Kern, K. J. Am. Chem. Soc. 2002, 124, 7991. (f) Yokoyama, T.; Yokoyama, S.; Kamikado, T.; Okuno, Y.; Mashiko, S. Nature 2001, 413, 619-621. (g) Weckesser, J.; De Vita, A.; Barth, J. V.; Cai, C.; Kern, K. Phys. Rev. Lett. 2001, 87, 096101. (h) Barth, J. V.; Weckesser, J.; Cai, C.; Gu¨nter, P.; Bu¨rgi, L.; Jeandupeux, O.; Kern, K. Angew. Chem., Int. Ed. Engl. 2000, 39, 1230. (i) Bo¨hringer, M.; Morgenstern, K.; Schneider, W.-D.; Berndt, R.; Mauri, F.; De Vita, A.; Car, R. Phys. Rev. Lett. 1999, 83, 324. (j) Bo¨hringer, M.; Morgenstern, K.; Schneider, W.-D.; Berndt, R. Angew. Chem., Int. Ed. 1999, 38, 821. (8) Barlow, S. M.; Raval, R. Surf. Sci. Rep. 2003, 50, 201-341. (9) (a) De Feyter, S.; Gesquie`re, A.; Wurst, K.; Amabilino, D. B.; Veciana, J.; De Schryver F. C. Angew. Chem. 2001, 40, 17, 3217-3320. (b) Cai, Y.; Bernasek, S. L. J. Am. Chem. Soc. 2003, 125, 1655-1659.

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molecules at surfaces, by exploiting noncovalent interactions such as hydrogen bonding11 and metal-coordination12 chemistry. Few reports deal with the influence of solvent on the pattern formation at the liquid-solid interface, though this is potentially an important approach for controlling the 2D supramolecular stucture.13 The three-dimensional self-assembly of supramolecular dendrimers from monodendritic building blocks, followed by self-organization in lattices, has provided rapid access to the construction of giant (functional) architectures.14 The adsorption of some achiral monodendrons on highly oriented pyrolytic graphite (HOPG) has been reported previously.15 We report on the synthesis and the selfassembly of the enantiomers, racemic mixture, and achiral analogue of an alkylated monodendron (Scheme 4) at the liquid-solid interface. The reason of the choice of this compound is 3-fold: (1) The relatively small size of the stereogenic center with respect to the dendron part might influence the degree of molecular chirality transfer into two-dimensional chirality. (2) The high degree of alkylation gives good hope that these bulky types of molecules can self-assemble into an ordered fashion. (3) If imaging is proven successful, higher generation dendrons can be targeted. The effect of the stereogenic center on the 2D pattern formation has been investigated with scanning tunneling microscopy (STM) showing that despite the relatively small size of the stereogenic center with respect to the overall molecular size, 2D chirality is expressed. The racemic compound undergoes spontaneous segregation at the liquid-solid interface. The achiral analogue forms completely different 2D patterns. Moreover, the effect of solvent has been explored where in the case of the achiral (10) De Feyter, Steven; De Schryver, Frans C. Chem. Soc. Rev. 2003, 32, 3, 139-150. (11) (a) De Feyter, S.; Gesquie`re, A.; Abdel-Mottaleb, M. M.; Grim, P. C. M.; De Schryver, F. C.; Meiners, C.; Sieffert, M.; Valiyaveettil, S.; Mu¨llen, K. Acc. Chem. Res. 2000, 33, 8, 520-531. (b) Lei, S. B.; Wang, C.; Yin, S. X.; Wang, H. N.; Xi, F.; Liu, H. W.; Xu, B.; Wan, L. J.; Bai, C.-L. J. Phys. Chem. B 2001, 105, 10838-10841. (c) Giancarlo, L. C.; Flynn, G. W. Acc. Chem. Res. 2000, 33, 491-501. (d) Xie, Z. X.; Charlier, J.; Cousty, J. Surf. Sci. 2000, 448, 201-211. (e) Barth, J. V.; Weckesser, J.; Cai, C. Z.; Gu¨nter, P.; Burgi, L.; Jeandupeur, O.; Kern, K. Angew. Chem. 2000, 112, 1285-1288. (f) Barth, J. V.; Weckesser, J.; Cai, C.; Gu¨nter, P.; Bu¨rgi, L.; Jeandupeux, O.; Kern, K. Angew. Chem., Int. Ed. 2000, 39, 1230-1234. (g) Griessl, S.; Lackinger, M.; Edelwirth, M.; Hietschold, M.; Heckl, W. M. Single Mol. 2002, 3, 25-31. (h) Kim, Y. G.; Yau, S. L.; Itaya, K. Langmuir 1999, 15, 7810-7815. (i) Theobald, J. A.; Oxtoby, N. S.; Philips, M. A.; Champness, N. R.; Beton, P. H. Nature 2003, 424, 1029-1031. (j) De Feyter, S.; Gesquie`re, A.; Klapper, M.; Mu¨llen, K.; De Schryver, F. C. Nano Lett., in press. (k) Keeling, D. L.; Oxtoby, N. S.; Wilson, C.; Humphry, M. J.; Champness, N. R.; Beton, P. H. Nano Lett. 2003, 3, 9-12. (12) Semenov, A.; Spatz, J. P.; Mo¨ller, M.; Lehn, J.-M.; Sell, B.; Schubert, D.; Weidl, C. H.; Schubert, U. S. Angew. Chem., Int. Ed. Engl. 1999, 38, 2547. (13) (a) Vanoppen, P.; Grim, P. C. M.; Ru¨cker, M.; De Feyter, S.; Moessner, G.; Valiyaveetil, S.; Mu¨llen, K.; De Schryver, F. J. Phys. Chem. 1996, 100, 19636-19641. (b) Li, C.-J.; Zeng, Q.-D.; Wang, C.; Wan, L.-J.; Xu, S.-L.; Wang, C.-R.; Bai, C.-L. J. Phys. Chem. B 2003, 107, 747-750. (c) Yablon, D. G.; Wintgens, D.; Flynn, G. W. J. Phys. Chem. B 2002, 106, 5470-5475. (14) (a) Hudson, S. D.; Jung, H.-T.; Percec, V.; Cho, W.-D.; Johansson, G.; Ungar, G.; Balagurusamy, V. S. K. Science 1997, 278, 449-452. (b) Percec, V.; Ahn, C.-H.; Ungar, G.; Yeardley, D. J. P.; Mo¨ller, M.; Sheiko, S. S. Nature 1998, 391, 161-164. (c) Percec, V.; Cho, W.-D.; Ungar, G.; Yeardley, D. J. P. J. Am. Chem. Soc. 2001, 123, 1302-1315. (d) Percec, V.; Glodde, M.; Bera, T. K.; Miura, Y.; Shiyanovskaya, I.; Singer, K. D.; Balagurasamy, V. S. K.; Heiney, P. A.; Schnell, I.; Rapp, A.; Spiess, H.-W.; Hudson, S. D.; Duan, H. Nature 2002, 419, 384-387. (e) Ungar, G.; Liu, Y.; Zeng, X.; Percec, V.; Cho, W.-D. Science 2003, 299, 12081211. (f) Zeng, X.; Ungar, G. Liu, Y.; Percec, V.; Dulcey, A. E., Hobbs, J. K. Nature 2004, 428, 157-160. (15) (a) Prokhorova, S. A.; Sheiko, S. S.; Mourran, A.; Azumi, R.; Beginn, U.; Zipp, G.; Ahn, C. H.; Holecera, M. N.; Percec, V.; Mo¨ller, M. Langmuir 2000, 16, 6862-6867. (b) Gong, J. R.; Lei, S. B.; Wan, L. J.; Deng, G. J.; Fan, Q. H.; Bai, C.-L. Chem. Mater. 2003, 15, 30983104.

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Mamdouh et al. Scheme 1

Scheme 2

derivative, its 2D assembly is solvent dependent, leading to complex 2D structures and providing control for the distance between “supramolecular containers”. 2. Experimental Section 2.1. Materials. Methyl 4-hydroxybenzoate (99%), ethyl Llactate (98%), diethylene glycol (99%), sodium hydride (60% suspension in mineral oil) (all from Lancaster Synthesis), thionyl chloride (99.5%), LiAlH4 (LAH) (95%), anhydrous K2CO3 (all from Aldrich), methyl 3,4,5-trihydroxybenzoate (99%), 1-bromododecane (98%), 4-N,N-dimethylaminopyridine (DMAP) (99%), 3,4-dihydro-2H-pyran (DHP) (99%) (all form Acros Organics), N,N-dimethylformamide, methanol, tetrahydrofuran, dichloromethane, MgSO4, acetone, ethyl acetate, ethylene glycol (all from Fisher, ACS reagents), and silica gel (Sorbent Technology) were used as received. Tetrahydrofuran (Fisher, ACS reagent grade) was refluxed over sodium/benzophenone and freshly distilled before use; dichloromethane (Fisher, ACS reagent grade) was refluxed over CaH2 and freshly distilled before use. All other chemicals were commercially available and were used as received. 2.2. Techniques. 1H NMR (500 MHz) and 13C NMR (125 MHz) spectra were recorded on a Bruker DRX 500 instrument. The purity of the products was determined by a combination of thinlayer chromatography (TLC) on silica gel coated aluminum plates (with F254 indicator; layer thickness, 200 µm; particle size, 2-25 µm; pore size, 60 Å; SIGMA-Aldrich) and high-pressure liquid chromatography (HPLC) using a Perkin-Elmer series 10 highpressure liquid chromatograph equipped with a LC-100 column oven, Nelson Analytical 900 series integrator data station, and two Perkin-Elmer PL gel columns of 5 × 102 and 1 × 104 Å. Tetrahydrofuran (THF) was used as solvent at the oven temperature of 40 °C. Detection was by UV absorbance at 254 nm. Thermal transitions were measured on a TA Instrument 2920 modulated differential scanning calorimeter (DSC). In all cases, the heating and the cooling rates were 10 °C min-1. The transition

temperatures were measured as the maxima and minima of their endothermic and exothermic peaks. Indium was used as calibration standard. An Olympus BX-40 polarized optical microscope (10×/50× magnification) equipped with a Mettler FP 82HT hot stage and a Mettler FP 80 central processor was used to verify thermal transitions and examine the textures in various phases. Optical activity was measured using a Jasco P1010 polarimeter with a Na (536 nm) lamp. Matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry was carried out on a PerSeptive Biosystems-Voyager-DE (Framingham, MA) mass spectrometer operating in linear mode. The spectrometer equipped with a nitrogen laser (337 nm) was calibrated using Angiotensin II and Bombesin as standards. The laser steps and voltages applied were adjusted as a function of the molecular weight and the nature of the compound. The matrix used in MALDI-TOF mass spectrometry was 3,5-dimethoxy-4-hydroxy-trans-cinnamic acid. The solvent used for both matrix and sample was THF. A typical procedure used for sample preparation was as follows. The matrix (10 mg) was dissolved in 1 mL of THF. The sample concentration was 5-10 mg/mL. The matrix solution (25 µL) and the sample solution (5 µL) were mixed well, and then 0.5 µL of the resulting solution was loaded into the MALDI-plate and dried before inserting into the vacuum chamber of the MALDI machine. STM experiments were performed using a Discoverer scanning tunneling microscope (Topometrix Inc., Santa Barbara, CA) along with an external pulse/function generator (model HP 8111 A), with negative sample bias. Tips were electrochemically etched from Pt/Ir wire (80%/20%, diameter 0.2 mm) in 2 N KOH/6 N NaCN solution in water. Prior to imaging, all compounds under investigation were dissolved in 1-phenyloctane (Aldrich 99%) or 1-octanol (Aldrich 99%) at a concentration of approximately 1 mg/g, and a drop of the solution was applied onto a freshly cleaved surface of highly oriented pyrolytic graphite (HOPG, grade ZYB, Advanced Ceramics Inc., Cleveland, OH). The solution of the racemic mixture was prepared by mixing equimolar amounts of

Ordering of Monodendrons

Langmuir, Vol. 20, No. 18, 2004 7681 Scheme 3

Scheme 4

the pure enantiomers. Then, the STM tip was immersed in the solution and images were recorded at the liquid-solid interface. The STM images were acquired in the variable-current mode (constant height). The measured tunneling currents are converted into a gray scale: black (white) refers to a low (high) measured tunneling current. The experiments were repeated in several sessions using several tips to check for reproducibility and to avoid artifacts. For analysis purposes, recording of a monolayer image was followed by imaging the graphite substrate underneath under the same experimental conditions, except for the lowered bias voltage. Before image analysis, the images were corrected for drift, via scanning probe image processor (SPIP) software (Image Metrology ApS), using the recorded graphite images for calibration purposes, allowing a more accurate unit cell determination. However, for display purposes, the images shown are not corrected for scanner drift, except those in panels a and b of Figure 1. The imaging parameters are indicated in the figure captions: tunneling current (It), and sample bias (Vbias).

3. Results and Discussion 3.1. Synthesis. The synthesis of the tapered monodendrons started from the commercially available methyl 4-hydroxybenzoate (4) (Scheme 1), which was alkylated with bromododecane in the presence of K2CO3 in N,Ndimethylformamide (DMF). Reduction of the resulting ester with lithium aluminum hydride (LAH), followed by chlorination with thionyl chloride (SOCl2) provided the benzyl chloride (5) in 85% yield over three steps. The benzyl chloride (5) was used to alkylate methyl 3,4,5trihydroxybenzoate (6), the product of which was saponified in the presence of KOH in a refluxing THF/EtOH mixture to give the acid (7) in 78% yield over two steps. The “S” chiral core was synthesized starting form commercially available ethyl L-lactate (8) (Scheme 2). The free alcohol was transformed into the tetrahydropyranil ether in the presence of 3,4-dihydro-2H-pyran (DHP) and

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Mamdouh et al. Table 1. Unit Cell Vectors and Relation with the Symmetry of the Graphite Substrate compd

a (nm)

b (nm)

γ (deg)

φg (deg)

99 ( 2 +(2.8 ( 1.2) 99 ( 2 -(3.2 ( 1.3) 99 ( 2 +(2.2 ( 1.3)a -(2.4 ( 1.3)b 4.69 ( 0.07 4.61 ( 0.13 120 ( 2 3.9 ( 1.7 3.80 ( 0.06 8.73 ( 0.45 87 ( 1 0(2

no. of molecules

(R)-1 3.14 ( 0.12 2.71 ( 0.06 (S)-1 3.12 ( 0.09 2.69 ( 0.09 (RS)-1 3.15 ( 0.09 2.65 ( 0.11

2 2 2

2c 2d

4 8

a Average of the positive values. b Average of the negative values. 2 at the 1-phenyloctane-HOPG interface. d 2 at the 1-octanolHOPG interface.

c

Figure 1. STM images of the chiral monodendrons physisorbed at the liquid-solid interface. (a) (R)-1. Image size is 10.3 nm × 10.3 nm. It ) 0.65 nA. Vbias ) -0.528 V. (b) (S)-1. Image size is 11.2 nm × 11.2 nm. It ) 0.4 nA. Vbias ) -0.872 V. These images have been rotated to illustrate the mirror-image relationship between the ordering in a and b, respectively. (c) (S)-1. Image size is 9.2 nm × 9.2 nm. It ) 0.6 nA. Vbias ) -0.350 V, with a superimposed molecular model. Both molecules are colored differently for clarity. (d) Tentative molecular model. (e) (RS)-1. Image size is 10.5 nm × 10.5 nm. It ) 0.8 nA. Vbias ) -0.698 V. The ordering is similar to (R)-1. (f) (RS)-1. Image size is 9.4 nm × 9.4 nm. It ) 0.8 nA. Vbias ) -0.896 V. The ordering is similar to (S)-1. Unit cells are indicated in yellow.

catalytic acid. The ethyl ester was then reduced with LAH, and the resulting alcohol was converted to the benzyl ether with benzyl bromide and NaH in THF. The tetrahydropyranil ether was then cleaved in the presence of acid to give the benzyl monoprotected chiral propanediol (9) in 73% yield over four steps. Ethylene glycol (10) was monoprotected with DHP and catalytic acid, and the free alcohol was transformed into its p-tolunesulfonyl ester (11) in 44% yield over two steps. Compound 11 was reacted with the chiral propanediol (9) in the presence of NaH in refluxing THF to yield the “S” chiral diprotected diethylene glycol unit 12 in 75% yield. The benzyl protecting group in 12 was removed by hydrogenation over Pd/C, and the resulting alcohol was transformed into the p-toluenesulfonyl ester. The tetrahydropyranil ether was removed in the presence of acid to give the “S” chiral diethylene glycol unit 13 in 90% yield over three steps. The “R” chiral core was also synthesized starting from the commercially available ethyl L-lactate (8) (Scheme 3). In this case the ethyl ester was reduced with LAH to give the chiral 1,2-propanediol. Then, in a one-pot procedure

the primary alcohol was selectively protected as the triphenyl methyl ether with triphenylmethyl chloride and triethylamine in dichloromethane and the secondary alcohol was transformed into its methanesulfonyl ester with methanesulfonyl chloride and triethylamine in dicholormethane to give the chiral unit 14 in 75% yield over two steps. Ethylene glycol (10) was monoprotected with benzyl bromide and NaH in THF to give 15 in 44% yield, which was treated with NaH and then chiral unit 14 in refluxing THF to give the “R” chiral diprotected diethylene glycol unit 16 in 75% yield after inversion of configuration. The triphenylmethyl protecting group was removed by treating with p-tolunesulfonic acid, and the resulting alcohol was converted to its p-toluenesulfonyl ester with p-tolunesulfonyl chloride. The benzyl protecting group was then removed by hydrogenation over Pd/C to give the “R” chiral diethylene glycol unit 17 in 81% yield over three steps. The monodendritic esters (Scheme 4) were synthesized by reacting the carboxylic acid (7) with the chiral diethylene glycol unit (12) and (17) in the presence of K2CO3 in DMF. The chiral monodendrons (S)-1 and (R)-1 were obtained in 53% and 56% yield, respectively. The achiral monodendron 2 was obtained in 60% yield by reacting acid 7 with diethylene glycol in the presence of p-tolunesulfonyl chloride, K2CO3, and catalytic amounts of 4-(N,N-dimethylamino)pyridine (DMAP) and tetrabutylammonium hydrogensulfate (TBAH) in THF. The synthesis and structural analysis of all compounds are detailed as Supporting Information. 3.2. STM Imaging. Scheme 4 shows the chemical structure of the chiral monodendrons ((R)-1 and (S)-1) and achiral monodendron (2) of the first generation under investigation. The presence of alkyl chains should aid in reducing the mobility of the molecules when physisorbed on graphite.15 3.2.1. Enantiomers and Racemic Mixture. Figure 1 presents typical submolecularly resolved STM images of the 2D ordering formed by the enantiopure compounds (R)-1, (S)-1, and the racemic mixture (RS)-1 of the monodendrons at the 1-phenyloctane-graphite interface. Aromatic moieties are often observed to show a higher tunneling efficiency and therefore they show up brighter than the alkyl chains.16 The alkyl chains are running along one of the main graphite axes. In panels a and b of Figure 1, individual phenyl groups can clearly be identified while the orientation of the dodecyloxy chains is unclear. In the other images, the orientation of the alkyl chains can clearly be distinguished. Those images where individual phenyl groups can be distinguished suggest that the enantiopure compounds and the racemic mixture arrange on the graphite surface as “dimers”. Not all of the alkyl chains are lying along the (16) Lazzaroni, R.; Calderone A.; Lambin, G.; Rabe, J. P.; Bre´das, J. L. Synth. Met. 1991, 41-43, 525.

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Figure 2. (a) STM image of graphite and indication of the main symmetry axes. (b) Model of the upper layer of HOPG. A main graphite symmetry axis is indicated in red. Possible orientation of the unit cell vector a indicated in yellow with respect to the symmetry axes, defining the angle φg.

same direction: two alkyl chains of every monodendron are lying parallel to one of the main symmetry axes of graphite. The third one takes another direction and makes an angle of 75° with the former ones (see Figure 1c). The alkyl chains are interdigitated, and the distance between two adjacent alkyl chains is approximately 0.46 nm. Unit cells are indicated in panels a and b of Figure 1, for the R and S enantiomer, respectively. The values of the unit cell vectors which are the average of tens of different domains probed are indicated in Table 1. Within experimental error, those values are identical for the enantiomers. A tentative model reflecting the 2D ordering of the S-enantiomer is given in Figure 1d. Despite the high resolution, no detailed information can be extracted on the mode of interaction between the molecules, e.g., interaction between chiral branches and the role of the chiral center, on the atomic level, and on the selective formation of enantiomorphous domains. Nevertheless, it is clear that molecular chirality is expressed at the liquid-solid interface. The original recorded STM images in panels a and b of Figure 1 have been rotated in such a way as to clearly show the mirror image type relation between the 2D packings of both enantiomers, which is also expressed by the respective unit cells. Both enantiomers form exclusive enantiomorphous 2D structures. The expression of molecular chirality is also clear when the orientation of the unit cell vector a is compared to the main symmetry of the graphite substrate (see Figure 2). For the R enantiomer, the unit cell vector a forms an angle φg of +2.8° ( 1.2° with respect to the symmetry axis while for the S enantiomer this angle has the same absolute value but differs in sign (-3.2° ( 1.3°). Thus, molecular chirality is expressed at two levels: at the level of the molecular ordering and at the level of the orientation of unit cell vectors with respect to the substrate symmetry. It is not a priori to expect that molecular chirality will be expressed on the surface. Sure, when upon adsorption the stereogenic center is in contact with the substrate or its presence results in substantial adsorbate-adsorbate interactions, which is often the case in this type of 2D crystals formed, the exclusive formation of enantiomorphous structures can be anticipated. However, one has to keep in mind that the relative contribution of the stereogenic center to the adsorbate-substrate and adsorbate-adsorbate interactions is expected to decrease if the overall size of the molecule increases. With respect to the molecular mass, the relative contribution of the

stereogenic center is rather small compared to other systems investigated.5,7 Despite this, the expression of 2D chirality indicates the importance of the stereogenic center and its influence on the 2D molecular packing of the monodendrons. Compared to the pure enantiomers, the racemic mixture (RS)-1 forms similar 2D patterns. The unit cell parameters are identical to those of the pure enantiomers. Therefore, we can conclude that both S- and R-type patterns are formed, as shown in panels e and f of Figure 1, respectively. This is confirmed by the observed angles of the unit cell vector a with respect to the graphite axis (Table 1). These observations indicate that the racemic mixture undergoes spontaneous segregation into enantiopure domains as observed for most of the chiral molecules when applied as a racemic mixture on a substrate.5 3.2.2. Achiral Compound. The stereogenic center not only determines the chirality of the 2D patterns but also has a strong influence on the 2D supramolecular ordering. This was proven by comparing the self-assembly of its achiral analogue 2 (Scheme 4). The packing of the achiral derivative on the graphite surface was completely different from that of the chiral analogues 1. Figure 3 shows some representative STM images of physisorbed monolayers of the achiral monodendron 2 at the 1-phenyloctanegraphite interface. Panels a and b of Figure 3 show large scale STM images illustrating the extent and stability of monolayer formation. Again, bright and dark structures can be observed. The bright structures (spots) correspond to the phenyl rings in the monodendrons. The graylike striped features are the alkyl chains. In contrast to the chiral monodendrons 1, the achiral analogue 2 forms “cyclic” tetramers with a “dark” center. A unit cell is indicated in Figure 3b. A unit cell is composed of four monodendrons. High-resolution images such as those in panels c and e of Figure 3 give detailed insight into the structure of the tetramers. Tentatively, each bright spot can be assigned to the location of a phenyl ring. Not all phenyl groups appear with the same brightness. This indicates that not all of them have the same orientation with respect to the graphite substrate, because different tilt angles of the phenyl rings with respect to the substrate will result in differences in contrast. The formation of tetramers might be due to hydrogen bond formation via the carboxylic group of one monodendron and the hydroxyl group of another monodendron. The phenyl rings are surrounded by alkyl chains. Those alkyl chains are interdigitated and sepa-

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Figure 3. STM images of achiral monodendron 2 physisorbed at the 1-phenyloctane-HOPG interface. (a) Large scale image. Image size is 160 nm × 160 nm. It ) 0.45 nA. Vbias ) -0.614 V. (b) Image size is 23 nm × 23 nm. It ) 0.45 nA. Vbias ) -0.652 V. Unit cell is indicated in yellow. (c) Image size is 12.8 nm × 12.8 nm. It ) 0.5 nA. Vbias ) -0.518 V. The location of the coadsorbed 1-phenyloctane solvent molecules is indicated in yellow. (d) Tentative molecular model. The 1-phenyloctane molecules are indicated in green, and the location of the phenyl groups is indicated by yellow arrows. (e) High-resolution image of one tetramer. Image size is 6.4 nm × 6.4 nm. It ) 0.45 nA. Vbias ) -0.766 V. (f) Tentative model of one tetramer including the two coadsorbed 1-phenyloctane molecules (green) per tetramer.

rated by approximately 0.46 nm and can be divided into two subsets, depending on their contrast. Those two subsets are oriented perpendicular to each other. From every monodendron, two alkyl chains are oriented almost perpendicular to the unit cell vector a, and those alkyl chains are oriented along one of the main symmetry axes of graphite. The third alkyl chain which appears with a higher contrast in the images is oriented along the unit cell vector a. As a result, between two tetramers, there are four of the latter type of alkyl chains. So, this difference in contrast between the alkyl chains can be attributed to their different orientation with respect to the graphite lattice. Upon the basis of our accurate analysis, in the direction perpendicular to the unit cell vector a, there is an excess of two alkyl chains for every tetramer. Most likely, two solvent molecules of 1-phenyloctane are coadsorbed per

Mamdouh et al.

tetramer unit in the monolayer as illustrated in Figure 3c-f. The bright spots indicated by yellow arrows in panels c and e of Figure 3 correspond to the phenyl groups of the coadsorbed solvent molecules. These phenyl rings often appear rather fuzzy which suggests some motional freedom. This coadsorption leads to a densely packed 2D monolayer. It is interesting to note that 1-phenyloctane molecules alone do not show a great tendency to form immobilized monolayers on graphite. After all, this is one of the reasons why 1-phenyloctane is used as solvent. To the best of our knowledge, this is the first time that it is clearly shown that they can be trapped in a matrix. Coadsorption of other small molecules at the organic liquid-solid interface has been demonstrated only on a few occasions.13,17 3.2.3. Solvent Dependence. These experiments were also carried out at the 1-octanol-graphite interface. The ordering and behavior of the chiral molecules 1 was identical to the situation at the 1-phenyloctane-graphite interface. However, the ordering of the achiral molecule 2 in 1-octanol is strikingly different. Typical STM images are shown in Figure 4. Common to the pattern formed in 1-phenyloctane is the formation of tetramers. However, the tetramers appear distorted, and from row to row, the tetramers are tilted with respect to each other as indicated by arrows in Figure 4a. A unit cell is indicated in Figure 4b, which contains eight molecules. Figure 4c is a high-resolution STM image. The distance between two adjacent alkyl chains is approximately 0.46 nm. The distance between the tetramers is smaller along the vector a compared to the pattern formed in 1-phenyloctane (3.80 ( 0.06 nm versus 4.69 ( 0.07 nm) which corresponds perfectly with the observation that the number of alkyl chains oriented perpendicular to unit cell vector a has been reduced by two units: no solvent molecules are coadsorbed. Due to the reduced distance between adjacent tetramers along unit cell vector a, the alkyl chains between the tetramers along this direction cannot be aligned parallel to this vector a. Most likely, these alkyl chains are directed to the solution. Although 1-phenyloctane and 1-octanol molecules are comparable in size, only the former ones are coadsorbed. This might be related to their difference in chemical composition.18 This presents a clear example of the effect of solvent on the 2D ordering at the liquid-solid interface. The center of the tetramers is “empty”, in the sense that no structures could be visualized inside. This opens the opportunity to use these tetramers as baskets and templates. In this case, the solvent provides an easy approach to control the distance between tetramers. 4. Conclusions Chiral monodendrons 1 of the type investigated in this study express their chirality at the liquid-solid interface despite the fact that the chiral center is relatively small with respect to the size of the molecule. Chiral 2D patterns belonging to the p2 space group are formed. The enantiomers form enantiomorphous 2D structures. The chirality is also expressed with respect to the graphite substrate. The racemic mixture forms identical 2D patterns as the pure enantiomers; however, both enantiomorphous struc(17) (a) Eichhorst-Gerner, K.; Stabel, A.; Moessner, G.; Declerq, D.; Valiyaveettil, S.; Enkelmann, V.; Mu¨llen, K.; Rabe, J. P. Angew. Chem., Int. Ed. Engl. 1996, 35, 1492. (b) Qian, P.; Nanjo, H.; Yokoyama, T.; Suzuki, T. M.; Akasaka, K.; Orhui, H. Chem. Commun. 2000, 20212022. (c) Uji-i, H.; Yoshidome, M.; Hobley, J.; Hatanaka, K.; Fukumura, H. Phys. Chem. Chem. Phys. 2003, 5, 4231. (18) Mamdouh W.; et al. In preparation.

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Figure 4. STM images of achiral monodendron 2 physisorbed at the 1-octanol-HOPG interface. (a) Large scale image. Image size is 46.5 nm × 46.5 nm. It ) 0.85 nA. Vbias ) -0.732 V. The differences in symmetry between the tetramers in adjacent rows is indicated by red arrows. (b) Image size is 23.5 nm × 23.5 nm. It ) 0.85 nA. Vbias ) -0.606 V. Unit cell is indicated in yellow. (c) Image size is 8.4 nm × 8.4 nm. It ) 0.85 nA. Vbias ) -0.358 V. (d) Tentative molecular model. The alkyl chains along the unit cell vector a, and the hydroxyl group containing alkyl parts within the tetramers have been omitted.

tures are formed. Apparently, the racemic mixture undergoes spontaneous segregation. Surprisingly, the achiral analogue 2 forms a completely different type of packing showing the formation of tetramers. This 2D supramolecular self-assembly is however solvent dependent. In 1-phenyloctane, solvent molecules are codeposited in a well-defined fashion. In 1-octanol, on the other hand, no solvent molecules are coadsorbed, affecting the 2D ordering substantially. The solvent dependence of the 2D packing provides a straightforward approach to control the distance between the tetramers, which might eventually be exploited for 2D templates. Studies on the effect of increasing monodendron size on the expression of 2D chirality are in progress.

Acknowledgment. The authors thank the Federal Science Policy, through IUAP-V-03, and the Fund for Scientific Research-Flanders (FWO). V.P. and A.D. thank the National Science Foundation (USA) for financial support. W.M. and H.U. are grateful to KULeuven (Interdisciplinary Research Program) for financial support. S.D.F. is a Postdoctoral Fellow of FWO. Supporting Information Available: The synthesis and structural analysis of all compounds. This material is available free of charge via the Internet at http://pubs.acs.org. LA049333Q