Surface Orientation of 3,4,5-Tris-Substituted Benzoic Acid Amphiphiles

Langmuir , 2004, 20 (3), pp 673–679. DOI: 10.1021/la0346200. Publication Date (Web): December 30, 2003. Copyright © 2004 American Chemical Society ...
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Langmuir 2004, 20, 673-679

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Surface Orientation of 3,4,5-Tris-Substituted Benzoic Acid Amphiphiles Ahmed Mourran,*,† Uwe Beginn,*,† Gabriela Zipp,‡,§ and Martin Mo¨ller*,† ITMC/DWI, RWTH Aachen, Worringerweg 1, D-52056 Aachen, Germany, and Department of Organic and Macromolecular Chemistry, OC-III, University of Ulm, Albert-Einstein-Allee 11, D-89069 Ulm, Germany Received April 11, 2003. In Final Form: November 25, 2003 Regarding the molecular orientation on flat substrates, thin films have been studied of a series of wedge-shaped molecules (3,4,5-tris-substituted benzoate-benzo crown ether compounds) consisting of a hydrophobic outer rim and a polar group at the thin end which form columnar mesomorphic and crystalline structures. For most substrates studied here, autophobic dewetting is demonstrated to be caused by the formation of a monomolecular adlayer in which the molecules are oriented normal to the substrate surface with the hydrophobic tails directed away from the substrate. For thick films, this adlayer is shown to cause an “in-plane” orientation of the axis of the columnar state. An ordered in-plane oriented adlayer is observed only for highly ordered pyrolytic graphite as the substrate. In this case, specific interactions with the substrate cause formation of a well-ordered 2D pattern that might favor homeotropic orientation of the columnar structures but has to be optimized by further structural variation. The structure of the adsorbed monolayer is elucidated by combining contact angle measurements, plasmon resonance spectroscopy, and optical and scanning tunneling microscopy.

Introduction The self-assembly of wedge- or cone-shaped molecules to supramolecular objects of rodlike, cylindrical, or spherical shapes in solid and molten states is fairly understood,1 but it is still an open question how these aggregates are affected by the presence of a solid boundary. The introduction of a surface can lead to mesoscopic alignment of either the molecular units or their aggregates. The spontaneous homeotropic and planar alignment of molecular liquid crystalline assemblies by surface anchoring effects has been described in thin films.2 However, the origin of the mechanism controlling the anchoring of such aggregates is not well understood. The ability of the surface to align molecular superstructures is of interest due not only to their great technological importance but also to the fundamental interest in the underlying mechanisms themselves. Recently we have demonstrated that the mesomorphic polymerizable wedge-shaped crown ether amphiphile 2-hydroxymethyl-[1,4,7,10,13-pentaoxacyclopentadecane3,4,5-tris[4-(11-methacryloyl-undecyl-1-oxy)benzyloxy]benzoate (1, Chart 1) self-assembles to rodlike fibers in methacrylate solvents.3-6 Furthermore, it was established * To whom correspondence should be addressed. Dr. Uwe Beginn: RWTH Aachen, ITMC/TexMC, Worringerweg 1, D-52056 Aachen, Germany; tel, +49-241-80-28137; fax, +49-241-80-22185; e-mail, [email protected]. Dr. Ahmed Mourran: RWTH Aachen, ITMC/TexMC, Worringerweg 1, D-52056 Aachen, Germany; tel, +49-241-80-26913; fax, +49-241-80-28139; e-mail, [email protected]. Prof. Dr. Martin Mo¨ller: ITMC/ DWI, Veltmanplatz 8, D-52056 Aachen, Germany; tel, +49-2414469-0; fax, +49-241-4469-100; e-mail, [email protected]. † ITMC/DWI, RWTH Aachen. ‡ Department of Organic and Macromolecular Chemistry, OCIII, University of Ulm. § Present address: Dr. Gabriela Bopp, IVOCLAR Vivadent AG, FL-9494 Schaan, Lichtenstein. (1) Israelachvili, J. Intermolecular and Surface Forces, 2nd ed.: Academic Press: London, 1992. (2) Hudson, S. D.; Jung, H.-T.; Percec, V.; Cho, W.-D.; Johansson, G.; Ungar, G.; Belagurusamy, V. S. K. Science 1997, 278, 449-452.

Chart 1. Structure Formula of 2-Hydroxymethyl[1,4,7,10,13-pentaoxabenzocyclopentadecane]3,4,5-tris[4-(11-methacryloyl-undecyl-1oxy)benzyloxy]benzoate (1), 2-Hydroxymethyl[1,4,7,10,13-pentaoxabenzocyclopentadecane]3,4,5-tris[4-(dodecyl-1-oxy)benzyloxy]benzoate (2), 3,4,5-Tris[4-(dodecyl-1-oxy)benzyloxy]benzoic Acid (3), and 3,4,5-Tris(dodecyl-1-oxy)benzoic Acid (4)

that the supramolecular fibers containing crown ether moieties along the fiber axis act as ion transport channels.7 (3) Percec, V.; Johansson, G.; Zipp, G.; Beginn, U.; Mo¨ller, M. Macromol. Chem. Phys. 1997, 198, 265-277. (4) Zipp, G.; Beginn, U.; Mo¨ller, M. Macromol. Chem. Phys. 1997, 198, 2839-2852. (5) Beginn, U.; Zipp, G.; Mo¨ller, M. Chem.sEur. J. 2000, 6, 20162023. (6) Beginn, U.; Zipp, G.; Mo¨ller, M. J. Polym. Sci., Part A: Polym. Chem. 2000, 38, 631-640.

10.1021/la0346200 CCC: $27.50 © 2004 American Chemical Society Published on Web 12/30/2003

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Figure 1. Differential interference optical (a,b) and scanning force (c,d) micrographs of thin “films” of 1 on glass, cast from a 0.5 wt % hexane solution: (a) spin coating at 2000 rpm; (b) slow evaporation of the solvent; (c) spin coating at 6500 rpm, height image; (d) amplitude image.

Large cylindrical pores (diameter (L) ) 400 nm) of tracketched membranes were used as a template to align supramolecular fibers parallel to the pore wall. We have previously shown that membranes containing such poreconfined supramolecular fiber channels exhibit transport constants that are 1 order of magnitude higher than for nonoriented fibers of the same compound.7,8 In this work, we explore the origin of the surface-induced orientation of 1 and related compounds, depending on the substrates and on the molecular structure of the adsorbate. Combining different experimental techniques including polarized light microscopy, surface plasmon resonance, and scanning probe microscopy (scanning force microscopy (SFM), scanning tunneling microscopy (STM)), we show that the molecular structure of 1 forces in-plane alignment of the columnar stacks on both hydrophobic and hydrophilic substrates. The elucidation of the molecular structure of the adsorbed layer on a hydrophobic model substrate (highly oriented pyrolytic graphite (HOPG)) allows us to propose a strategy to self-assemble supramolecular building blocks in homeotropic orientations. Results and Discussion Compounds 1-4 (Chart 1) were synthesized according to literature procedures; their purity was checked by thin(7) Beginn, U.; Zipp, G.; Mo¨ller, M. Adv. Mater. 2000, 12, 510-513. (8) Beginn, U.; Zipp, G.; Mourran, A.; Mo¨ller, M. Adv. Mater. 2000, 12, 513-516.

layer chromatography (TLC) and NMR and was found to exceed 99% (see Experimental Section). Thick films (d > 100 nm) prepared from melts of 1 (cf. Chart 1) on either nonpolar (e.g., silylated or carboncovered glass) or polar surfaces (e.g., mica, glass, or water) showed typical spherulitic textures of a columnar mesophase.5 In-plane orientation of the columns has always been observed. Smooth and stable films could be obtained only when the film thickness exceeded 1 µm. Identical results have been observed for compound 2.9 Thin films (d < 100 nm) have been prepared by spin coating of a diluted solution (0.5 wt %) of 1 on the substrate with rotation speeds of 2000 and 6500 rpm, respectively. Figure 1a depicts an interference contrast optical micrograph of 1 on a glass substrate (prepared at 2000 rpm). The surface morphology consisted of an array of droplets with an average diameter of 5 µm. Annealing the sample at 60 °C for 12 h led to the partial coarsening of the droplets. The higher rotation speed of ca. 6500 rpm resulted in reduced droplet diameters. The surface morphology was investigated by SFM (Figure 1c). The micrographs showed that the surface was covered with droplets of average heights between 40 and 80 nm. The diameters ranged from 100 to 500 nm. Each droplet exhibited a small but finite contact angle of ca. 8°. Films of the same compound obtained by slow evaporation of the solvent exhibited similar morphologies (cf. (9) Percec, V.; Johansson, G.; Heck, J.; Ungar, G.; Batty, S. V. J. Chem. Soc., Perkin Trans. 1 1993, 1411-1420.

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Table 1. Correlation of the Observed Surface Morphology of the Spin-Coated Films of 1 (0.5 wt % 1, 2000 rpm) and the Preparation, i.e., Solvent, Solvency Parameters, and Substratesa observed morphology of 1 on substrate

solvent

droplets droplets droplets/LC droplets droplets droplets droplets/LC droplets droplets/LC droplets droplets/LC droplets

diethyl ether n-hexane n-hexane n-hexane n-hexane acetone acetone toluene toluene chloroform chloroform 1,2,4-TCB(c)

surface tension [mN/m] [ref 10]

solubility parameter [(J/cm3)1/2] [ref 11]

substrate

surface energy [mN/m] [refs 1, 12, 13]

17.0 17.9 17.9 17.9 17.9 22.7 23.7 27.8 27.8 26.6 26.6

15.8 14.9 14.9 14.9 14.9 21.8 21.8 18.2 18.2 19.0 19.0 23.1(a)

glass(P) glass(P) silyl glass(NP) mica(P) SiOx(b) (P) glass(P) silyl glass(NP) glass(P) silyl glass(NP) glass(P) silyl glass(NP) HOPG(NP)

33-45 33-45 22-24 300 33-45 33-45 22-24 33-45 22-24 33-45 22-24 38-70

a LC ) droplets with mesophase texture; silyl glass ) glass, treated with trimethylchlorosilane; (a) ) estimated from increments; (b) ) surface of the PRS prism; (c) ) trichlorobenzene; HOPG ) highly ordered pyrolytic graphite; (P) ) polar surface; (NP) ) nonpolar surface.

Figure 2. Reflectance of the surface plasmon mode versus angle of incidence for the uncoated prism (b), the spin-coated prism with compound 1 (O), and the prism after removing the droplet pattern by flushing the surface with n-hexane (9). The remaining angular shift of 0.4° was assigned to the presence of a 4.1 nm thin layer of 1. The arrows labeled (1) point out the angular shift due to the presence of the droplet pattern; the arrows labeled (2) point out the angular shift after flushing the sample with n-hexane. The solid curve and the dashed curve are the theoretical fits to obtain the respective layer thickness.

Figure 1b). An initially smooth liquid film of 1 coagulated upon cooling into droplets with less defined shapes and sizes than in the case of spin-coating experiments. When the sample was annealed for 1 h at room temperature, some of the droplets changed their shape from spherical to an irregular shape with rough surfaces. Crossed polarized optical microscopy showed that the droplets exhibit birefringent textures typical for a columnar mesophase. Apparently, for thin films, the surface morphology depends neither on the substrates nor on the preparation methods (see Table 1). Surface plasmon resonance (SPR) spectroscopy14 was used to probe whether an ultrathin eventually monomolecular film may cover the area between the droplets. In these measurements (pictured in Figure 2), the reflectivity of a laser beam is monitored as a function of the (10) Wohlfarth, Ch.; Wohlfarth, B. Landolt-Bo¨ rnstein: Physical Chemistry; Springer: New York, 1997; Vol. IV/16, pp 1 ff. (11) Polymer Handbook; Brandrup, J., Immergut, E. H., Eds.; WileyInterscience: New York, 1989; Chapter VII, p 540 f. (12) Hare, E. F.; Zisman, W. A. J. Phys. Chem. 1955, 59, 335-340. (13) Zisman, W. A. Contact Angle, Wettability, and Adhesion; Good, R. F., Ed.; Advances in Chemistry Series No. 43; American Chemical Society: Washington, DC, 1964. (14) Kretschmann, E. Z. Phys. 1971, 241, 313-324.

incident angle (θ) for a prism/Ag/SiO2/film/air interface. As the angle of incidence is varied, just beyond the critical angle of total reflection the reflectivity sharply decreases to a minimum point designated as the surface plasmon angle, θsp. At this angle, the laser light is coupled into surface plasmon modes at the Ag/SiO2/film/air interface. The angle θsp changes as the film thickness varies, and this shift in θsp can provide a quantitative measure of the film thickness. The SPR curves were fitted according to Fresnel theory to get the corresponding film thickness.15,16 To exclude any effect of the solvent on the angular shift, SPR curves were recorded on the bare Ag/SiOx surface before and after flushing the surface with pure n-hexane. An angular shift of 0.05° was measured which is in the order of the experimental error, ca. 0.03°. A thin layer of 1 was prepared by casting 0.5 wt % hexane solution on the substrate at 2000 rpm. In this condition, the film consists of an array of droplets, similar to the pattern depicted in Figure 1a, with an average diameter comparable to the wavelength of the surface plasmon polaritons. Consequently, the SPR curve becomes broad. An angular shift of +4.5° relative to the bare surface was measured (Figure 2, open circles). To estimate the average coverage of the surface with compound 1, the SPR curve (Figure 2, open circles) was fitted by assuming the presence of a homogeneous flat film. The result is displayed in Figure 2 (dashed line); the theoretical fit reproduces well the angular shift, which corresponds to a 32 nm thick layer. However, a clear misfit of the calculated line shape appears which is due to the film roughness (Figure 2, dashed line). When, however, the originally rough film was extensively flushed with pure n-hexane, until an optically smooth surface was observed, the SPR still indicated a significant angular shift of ca. 0.4 ( 0.05° relative to the bare Ag/ SiO2 surface (Figure 2, filled squares). From this angular shift, the corresponding thickness was found to be 4.1 ( 0.5 nm. We conclude that this film consists of an adhering monolayer of 1 and that the “as-cast” layer can be described as an ultrathin homogeneous film (4.1 nm) on top of which an array of droplets are formed. Flushing the sample with the n-hexane selectively removes the droplets and leaves the stable thin layer. The wetting properties of the flushed SiOx glass prism, that is, of the remaining 4 nm thick adhering monolayer of 1, were studied by measuring the contact angles of water, dodecane, and hexadecane. The oils spontaneously spread (15) Knoll, W. Annu. Rev. Phys. Chem. 1998, 49, 569. (16) Raether, H. Surface Plasmons on Smooth and Rough Surfaces and on Gratings; Springer Tracts in Modern Physics; Springer: Berlin, 1988; Vol. 111.

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Mourran et al. Chart 2. Schematic Depiction of (a) the Autophobic Dewetting of 1 on Its Own Monolayera and (b) the Structure of the Monolayer of 1 on Highly Ordered Pyrolytic Graphite

a γChd ) surface energy of the columnar mesophase; γad ) surface energy of the adsorbed monolayer.

Figure 3. Simulated arrangement of 1 on adsorption on the SiO2/Si-OH surface. Molecular modeling was done with MacroModel 7.0 (ref 17), applying the AMBER force field.

on the surface, while a contact angle of 88° was found against water. The latter value is comparable to contact angles of water on alkene surfaces.1 Thus, the contact angle is consistent with a monolayer of adhering molecules of 1 with the ethane units oriented toward the surface. The structure of the adsorbed monolayer can be illustrated by modeling the geometry of one molecule of 1 on a SiO2/Si-OH surface using force field based molecular modeling calculations (cf. Experimental Section). One single molecule of 1 was placed in different orientations relative to a sheet of SiO2 with a dimension of ca. 1.7 × 2.8 × 0.8 nm3 and minimized with respect to the force field energy. The final configuration of a single molecule resulted in a “head-down” adsorption of 1 with the polar crown ether adsorbed to the polar surface and the alkyl chains directed toward the free surface (cf. Figure 3). Additionally, the space-filling models of 1 showed that the stretched conformation of the molecule exhibits a length between 4.2 and 4.4 nm, which is slightly larger than the measured thickness of the ultrathin film. The experimental data suggest that the specific interaction of 1 with surfaces induces an arrangement of the molecules at the solid boundary which subsequently influences the bulk organization. Consequently, the selective anchoring of the crown ether at the SiO2 surface and the segregation of the alkane tails toward the free surface (low surface energy) favor the planar alignment of the columnar mesophase in a thick film. However, in thin films the interfacial energy between solid/film and film/ air becomes dominant and affects the stability of the layer. As a result, the films always break down in an array of droplets that exhibit a planar alignment of the columnar mesophase and that coexist with the monomolecular film. This effect is well-known as autophobic dewetting, denoting the phenomenon that a liquid dewets its own surface.13 Since the adsorbed molecules align along the planar surface, they form a smectic-like monolayer which is incompatible with the curved surfaces of columnar mesomorphic structures. As a result, the monolayer exhibits

a lower surface energy (γad) than that of the bulk material (γCh) in its columnar mesophase (see Chart 2a). Although the exact value of the layer surface energy could not be measured by conventional surface techniques, the formation of the macroscopic droplets on the monolayer surface indicated that the layer and the macroscopic phase have different surface energies (see Table 1). The data in Table 1 demonstrate droplet formation also in the case when nonpolar surfaces (e.g., carbon-coated glass) were used as the substrate. Apparently the orientation of the first adlayer molecules is similar in these cases, although the experimental evidence is not yet sufficient to give a detailed picture of the structures of adsorbed monolayers of 1 on nonpolar substrates in general. The question arises of how to create surface structures that favor homeotropic orientations of the columnar mesophase; in other words, how to self-assemble the molecule on the surface and conserve the integrity of the bulk self-assembly. To address this question, we studied the molecular structure of monolayers adsorbed on HOPG by STM. Only the in situ adsorption of 2 was investigated because 1 was less stable due to the presence of the polymerizable methacrylate groups. It is however assumed that 1 adsorbs alike. Figure 4 shows a set of STM images obtained at the interface between HOPG and a saturated solution of 2 in 1,2,4-trichlorobenzene. The micrographs show domains separated by a sharp boundary (Figure 4a). Within the domains, stripes of equal width with a periodicity of 3.03 nm were measured. At the domain boundary, the rows were displaced and their orientation was rotated by 160°. Figure 4b depicts that the stripes are rows of single molecules and the bright contrasts in the micrograph are due to the aromatic and crown ether parts (Figure 4). The angle between the main axes of the alkyl side chains and the rows was measured to be 60°. As a result, the underlying substrate influences the orientation of the adsorbed molecules. Since the length of a single molecule (4.4 nm) corrected for the inclination exceeds the width of the stripes, we deduce that the molecules adsorb to the surface via the alkyl tails while the crown ether moiety is slightly turned out from the adsorbed plane. Within each stripe, two types of periodicities were found. The larger one of 0.92 ( 0.06 nm originated from the

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Figure 4. STM micrograph of molecules of 2 adsorbed at the interface of HOPG/trichlorobenzene (a-c). The positions of the aromatic and the crown ether groups of 2 are represented by the white spots in the micrographs.

periodicity of the crown ether/aromatic part of the molecule, while the smaller one (0.45 ( 0.05 nm) arose from the dense, parallel packing of the alkyl chains. With this periodicity, only two alkyl chains (Lalkane chain ≈ 0.4 nm1) can be placed on the surface. The STM observation is in accordance with estimations of the surface area per molecule within this pattern: One molecule of 2 covers 2.8 ( 0.2 nm2, and 1.4 ( 0.2 nm2 is required to place the aromatic part of the molecule. The remaining 1.3-1.4 nm2 can accommodate not more than two alkyl chains with 0.6-0.7 nm2 per chain. Hence, the third chain is not adsorbed and cannot be visualized because of its large conformational mobility. To estimate the molecular dimensions of the adsorbed molecules, molecular modeling calculations involving one molecule of 1 or 2 and a triple molecular layer of graphite (38 × 30 atoms per layer) were performed to simulate a rigid flat surface. After the molecules were placed in different orientations relative to the graphite layers, the system was minimized with respect to the force field energy. In all cases, the final configuration consisted of 1 or 2 lying flat on the substrate, the alkyl chains being attached to the graphite while the crown ether moiety was slightly turned away from the nonpolar sublayer. Since the in-plane orientation of the columnar mesophase was observed also for 2 on polar substrates as well as on HOPG, the bulk mesophase alignment was not affected by the different orientations on the surface layer of 2 in contact with HOPG. The striped surface structure did not result in a reorientation of the bulk structure. This might be explained by the observation that only two alkyl tails were adsorbed while the third remains dangling into the phase above, and it might be worthwhile to search for a compound that forms fully adsorbed layers. To get an insight on the correlation between the molecular shape of the adsorbate and the surface assembly, the molecular structure of the adsorbate molecules was systematically altered. Removing the benzo crown ether moiety from compound 2 yielded the 3,4,5-tris[4′-(dodecyloxy)benzyloxy]benzoic acid 3, which also exhibited regular stripe patterns at the HOPG surface (Figure 5). The lateral distance of the stripes was measured to be (3.2 ( 0.1 nm). Each stripe was separated in two regions containing aromatic (bright) and aliphatic groups (gray). The lateral extension of the aromatic and the aliphatic part was measured to be 2.1 and 1.1 nm, respectively. In each stripe, the aromatic cores of the 3 molecules have been resolved, showing the formation of hydrogen bond cyclic dimers. The long axis of the dimers was tilted by 45-47° with respect to the stripe direction (cf. Figure 5).

Figure 5. (a,b) STM micrograph of molecules of 3 adsorbed at the interface of HOPG/trichlorobenzene. The upper right corner of panel a depicts the micrograph of the underlying HOPG lattice. The positions of aromatic groups of 3 are marked by white spots. (c) Schematic depiction of the molecular arrangement of 3 within its two-dimensional elementary cell.

The length of these dimeric cores was measured to be (1.9 ( 0.3 nm), which is in agreement with molecular modeling calculations of the dimer in the gas phase (2.2 nm). The main periodicity along the stripes was measured to be 1.9 nm/adsorbed molecule. Within this distance, only four interdigitated alkyl tails can be arranged tightly adsorbed on the surface with an average distance between the chains of at least 0.46 nm. It must be concluded again that only two alkyl chains per molecule are adsorbed along the main direction of the graphite surface, while the third alkyl chain dangles into the solution above the adsorbed layer. The two-dimensional elementary cell is a parallelogram with side lengths of 3.8 ( 0.1 nm and 1.6 ( 0.1 nm, enclosing an angle of 62 ( 2°. Without considering the alkyl chains, the plane crystallographic group was identified as p2. The measured covered area per molecule of 3 (2.6 ( 0.1 nm2) falls below the calculated surface area of 3 (3.2 ( 0.05 nm2) by 0.6 ( 0.1 nm2. Since the latter value is comparable to the surface area of a single dodecyl chain, it must be concluded that only two chains per molecule became adsorbed, while the third alkyl tail did not. Formally taking away the three benzyloxy groups from compound 3 yields the 3,4,5-tris(dodecyloxy)benzoic acid

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molecule can adsorb at the HOPG surface, while the other two chains either become placed on the adsorbed chains or may freely dangle toward the free solid/air interface. This hypothesis was tested by scanning the compound 4 under conditions that allow visualization of both the aromatic core and the alkyl tails. The micrograph in Figure 6c depicts the alkyl chains, assigned from the herringbone contrast, that delimit the aromatic core (row of three dimers) and form a crisscrossing network. Since only a fraction of alkyl chains are lying on top of the substrateadsorbed chains, this may explain the encountered difficulties in assigning individual alkyl chains in STM micrographs. Whether this type of surface structure is suitable as a surface template to predetermine the orientation of the columnar phases of 1 will be a matter of future work. Conclusion

Figure 6. (a) STM micrograph of compound 4 adsorbed at the interface of HOPG/trichlorobenzene. The positions of aromatic groups of 4 are marked by white spots. (b) Schematic depiction of the molecular arrangement of 4 within its two-dimensional elementary cell. (c) STM micrograph of compound 4; the contrast in the image is due to both the aromatic core and the alkyl side chains.

4. On HOPG, the adsorbed monolayer of 4 consisted of an assembly of parallelepiped-shaped units (see Figure 6a). The lateral extension of the parallelograms was (2.3 ( 0.1 nm) × (1.7 ( 0.1 nm), the sides enclosing angles of 70° and 110°, respectively. Each unit consisted of six molecules of 4, arranged in rows of three dimers. The hexameric assemblies were arranged in two different sets of lines. Along the first type of lines, the aggregates exhibited alternating orientation (“herringbone”), while along the second type pairs of parallelepipeds had the same alignment. The lines intersected at an angle of 55° (see Figure 6a). The twodimensional elementary cell was a rectangle with side lengths of (a ) 6.0 ( 0.1 nm) and (b ) 4.8 ( 0.1 nm), containing four parallelepipeds (cf. Figure 6a). The plane symmetry group was P1. From these data, the surface area per molecule is 1.2 ( 0.1 nm2, while 0.6 ( 0.1 nm2 is filled by the aromatic part of the molecules. Hence, only 0.6 ( 0.1 nm2 is left to place the alkyl tails. Since one C12H25- chain covers a surface of 0.6-0.7 nm2, it must be concluded that on average only one alkyl chain per 4

By means of systematic variation of the molecular structure of the adsorbate on HOPG, a transition from parallel stripe patterns into regular arrangements of smaller aggregates was achieved. Since it is shown that monolayers with suitable superstructures act as a surface template selecting one orientation of columnar bulk mesophases, this study demonstrates a possible general methodology of how to control bulk phase orientations by means of surface fields. The molecules of 1 and 2 adsorbed epitaxially on HOPG, creating a stripe pattern that affected the bulk orientation of 1 in a manner similar to that of the head-down structured monolayers. Due to packing requirements within the pattern, one of the three alkyl chains did not adsorb. The importance of the free dangling alkyl tails must be elucidated in future work. The structure of the adlayer is decisive for the orientation of the bulk in thick films as was shown for the methacrylated crown ether amphiphile 1. On polar substrates, 1 adsorbs in a “head-on” mode, creating smectic-analogous structures and directing the columnar bulk phase to be oriented parallel to the substrate surface. On the nonpolar HOPG where the adlayer molecules are oriented in-plane perpendicular orientation of the columns was also not observed. We explained this by the observation that a fraction of the side chains, 1 or 2 per molecule, are dangling away from the surface. Such dangling molecular segments were often overlooked because they are not depicted directly by STM. It is a difficult task to predict the superstructure within the monolayer from the molecular structure of the adsorbate. Group theory suggests that the symmetry of an epitaxially grown structure should be a subgroup of the substrate plane group. With HOPG this statement is of limited help, since the plane group of the graphite 100 plane (p6m) contains 12 two-dimensional plane symmetry subgroups out of 17 (all except the five rectangular groups). The structure of the self-assembled surface layer results from a delicate balance between intramolecular interactions (i.e., conformational flexibility), matching of the adsorbate structure with the surface structure, and the in-plane packing of the adsorbed molecules. Experimental Section Materials. n-Hexane, diethyl ether, acetone, chloroform, toluene, and 1,2,4-trichlorobenzene were purified by standard procedures. Hexadecane and dodecane (water free, 99%+, Aldrich) were used as received. 2-Methyl-(1,4,7,10,13-pentaoxabenzocyclopentadecane)-3,4,5tris[4-(11-methacryloyl-undecyl-1-oxy)benzyloxy]benzoate (1) was synthesized following the procedure described in the literature.5

Surface Orientation of Benzoic Acid Amphiphiles Yield: 1.1 g = 90% of theory, oily-waxy material. TLC (silica gel 60/cyclohexane-ethyl acetate ) 1:1): one spot, Rf ) 0.87, Tm ) 17 °C, Tc ) 28 °C. 2-Methyl-(1,4,7,10,13-pentaoxabenzocyclopentadecane)-3,4,5tris[4-(dodecyl-1-oxy)benzyloxy]benzoate (2) was prepared as described in the literature.9 Yield: 1.5 g = 83% of theory, white powder. TLC (silica gel 60/cyclohexane-ethyl acetate ) 1:1): one spot, Rf ) 0.76, Tm ) 92 °C. 3,4,5-Tris(dodecyloxy)benzoic acid (3) was prepared as detailed in the literature.16 Yield: 39.3 g = 97% of theory, white crystalline powder. TLC (silica gel/cyclohexane-ethyl acetate ) 4:1): one spot, Rf ) 0.45, Tm ) 53.5 °C. Film Preparation. Films of 1 or 2 were prepared from 0.5 wt % solutions in pure n-hexane cast on different substrates (e.g., glass, silylated glass, silicon wafer, mica, graphite). Thick films (>100 nm) were prepared by slow evaporation of the solvents over several days at 20 °C. Thin films (