NANO LETTERS
Large-Periodicity Two-Dimensional Crystals by Cocrystallization
2006 Vol. 6, No. 6 1178-1183
Katherine E. Plass, Keary M. Engle, Katie A. Cychosz, and Adam J. Matzger* Chemistry Department and Macromolecular Science and Engineering Program, UniVersity of Michigan, 930 North UniVersity, Ann Arbor, Michigan 48109 Received March 5, 2006; Revised Manuscript Received April 24, 2006
ABSTRACT Patterning surfaces with features on the low end of the nanoscale can efficiently be accomplished with physisorbed monolayers. Here, cocrystallization is revealed as a powerful approach toward dramatically increasing the periodicity of surface features and expanding the length scale on which these patterns can form. By variation of the ratio of adsorbates in solution, surface composition can be controlled such that features on the length scale of several molecules are obtained, offering a facile approach to surface nanopatterning.
Physisorbed monolayers offer a means of patterning surfaces with periodicities that can be controlled by design of the adsorbed molecules. A wide variety of elaborate patterns have been observed through tailoring hydrogen bonding and geometry.1 Seemingly small changes in molecular functionality and geometry can induce disproportionate increases in the complexity of the monolayer.2,3 This ability to generate such a wide variety of patterns will prove useful in the development of technology requiring spatial control on the nanoscale. For most chemisorbed monolayers, the substratedictated bonding sites on the surface and the vertical orientation of adsorbed molecules restrict features to the order of the molecular width.4-6 In contrast, during physisorption molecules lay flat on the surface and the distances between features can be increased with the length of the molecule.7 There is a practical limit to the attainable periodicity, however, because order has been observed to decrease for very large molecules. The problem with using molecules of increasing length to control periodicity is illustrated by the n-alkanes. Hexatriacontane (C36H72) molecules, for example, are ordered with a 4.7 nm periodicity,8 C50H102 displays both ordered and disordered phases due to impaired kinetics of monolayer equilibration, and C192H386 exhibits only nematic order, where the ends of the molecules no longer align and the molecular encoding of periodicity information is lost.9 An alternative means of expanding periodicity involves increasing the number of molecules that constitute a repeating unit. Two packing phenomena may induce this: crystallization in some plane groups with symmetry elements in addition to translation or the existence of symmetry independent molecules. For example, carboxylic acids can be related by a 2-fold rotation such that the periodicity is twice * To whom correspondence may be addressed. E-mail: matzger@ umich.edu. 10.1021/nl0605061 CCC: $33.50 Published on Web 05/05/2006
© 2006 American Chemical Society
the molecular length.10 The periodicity of monolayers of 1,3dinonadecanoylbenzene is expanded compared to the analogous ester because of the multiple molecules that make up its asymmetric unit.2 Addition of more molecular species to the ordered structuresformation of a cocrystal (a crystal consisting of multiple components in a regularly repeating pattern)scan also increase the length between identical molecular features. A large expansion in the length scale of the periodicity using this approach has not yet been observed, perhaps in part because two-dimensional cocrystals examined thus far generally result from incorporation of small molecules. Here we report a cocrystal that does not conform to either of these cases and that achieves a large periodicity through coadsorption of similarly sized components into a crystallographically complex monolayer. Formation of this cocrystal is both unexpected in light of the structural properties of the components and unusual in the expanded length scale of the periodicity with respect to the constituent molecules. Separately, the isomers dioctadecyl isophthalate and dioctadecyl phthalate each form a rather simple physisorbed monolayer at the liquid-graphite interface. When combined in solution in a narrow range of ratios, these two compounds generate a cocrystal on the surface, revealing that variation of the ratio of components present in solution is an important source of supramolecular complexity. The competitive adsorption between dioctadecyl isophthalate (18-meta) and dioctadecyl phthalate (18-ortho) (Chart 1) at the phenyloctane/graphite interface in ambient conditions was monitored by scanning tunneling microscopy (STM) (see Supporting Information for experimental details). The ratio of 18-ortho and 18-meta in solution was varied, and the resultant two-dimensional crystalline phase or phases were identified. Across all experiments, three unique phases were distinguishable due to characteristic features in the STM
the hydrogens is apparent, whereas in others they smear into a bright line. This variation likely arises from a moire´ pattern, in which the registry of the monolayer with the graphite changes, modulating the tunneling current.12 The geometry of the 18-ortho molecule prevents the completely flat adsorption that was seen in the 18-meta phase. Instead, a “hairpin” conformation, akin to that observed for other orthosubstituted benzenes,13 is adopted due to the close intramolecular contact of the alkyl chains, as is shown in the model (Figure 1d). Columns are made up of alternating molecules in a hairpin conformation, offset such that the angle seen in the images is induced. In this phase, both the unit cell length (Table 1) and periodicity of functional groups correspond to the length of one molecule (∼3.1 nm). When the 18-ortho and 18-meta isomers are present in solution in equal amounts, a two-dimensional crystal is formed that is indistinguishable from the one observed in pure 18-meta solution. This preferential adsorption of 18meta indicates that it is more stable on the surface than is 18-ortho. Even as the relative amount of 18-ortho is increased, only the single-component 18-meta phase forms. The ratio of 18-ortho:18-meta in solution must be greater than 24:1 before any 18-ortho is adsorbed (see Supporting Information). The 18-meta monolayer is not completely displaced unless an even larger amount of 18-ortho is added, and 18-ortho is not the sole surface species until the solution ratio is 60:1 18-ortho:18-meta. An estimate of the difference in free energy of adsorption between the two-dimensional crystals can be obtained as previously derived14,15 and is found to be 2.2 ( 0.3 kcal/mol. Several contributing factors make the 18-ortho monolayer less stable than that of 18meta. The molecule-substrate interaction is greater for 18meta because its flat conformation allows greater contact area with the graphite than does the partially desorbed hairpin conformation of 18-ortho. The extended conformation of 18meta also gives it a larger perimeter compared to 18-ortho, increasing the amount of van der Waals contact with neighboring molecules thereby strengthening the assembly. A third, previously unobserved, phase formed from solutions where the ratios of 18-ortho:18-meta were 40:1 and 50:1 (Figure 2a). This new phase, which exhibits an extraordinarily large periodicity, is marked by groupings of three bright columns of aromatic rings, which are separated by a dim trough indicating space between column boundaries. The distance between these troughs corresponds to a periodicity of 12.3 ( 0.1 nm: more than twice the length of the outstretched 18-meta isomer (calculated 5.42 nm), and four times the length of the 18-ortho isomer in a hairpin
Chart 1. Structures of the Three Molecules for Which the Physisorbed Monolayers Formed at the Phenyloctane/Graphite Interface, Both from Pure Solutions and from Binary Mixtures of Ortho and Metaa
a The names describe the number of carbons in the alkyl chains and the substitution pattern of the benzene ring.
images (Table 1 gives structural parameters). The two singlecomponent phases were identified by comparison with the monolayers obtained from pure solutions. The STM image of an 18-meta monolayer3 is shown in Figure 1a. The alkyl chains, which lie parallel to the surface, are indicated by a “zigzag” pattern of small bright spots corresponding to the hydrogen atoms facing away from the substrate. These chains are at 90° angles to the unbroken lines of large bright spots due to the aromatic rings. The length of one alkyl chain separates the aromatic rings, and these alkyl chains are interdigitated. The 18-meta molecules have a flat conformation in which the alkyl chains make 180° angles to one another. A subtle difference in contrast between adjacent columns of aromatic rings is due to an “up-down” alternation in molecular orientation.3,11 An energy-minimized molecular mechanics model (see Supporting Information for details of molecular modeling) that reproduces the observed metrics and symmetry in the STM image is shown in Figure 1b. The unit cell is rectangular with a long edge of 6.7 nm (Table 1), which is the approximate length of the molecule, in the p2mg plane group. This is twice the distance between columns of aromatic rings because the orientation difference between columns makes these features nontranslationally related. The length of one alkyl chain, 3.4 nm, better describes the distance between chemically identical functional groups and therefore the periodicity of functionality. The 18-ortho monolayers (Figure 1c) are structurally distinct from the 18-meta monolayers. Columns of benzene rings are separated by the length of one alkyl chain, similar to those in the 18-meta monolayer. These aromatic rings, however, appear in pairs, angled 24 ( 6° away from the column propagation direction. The alkyl chains exhibit varying resolutions. In some cases, the “zigzag” pattern of
Table 1. Unit Cell Parameters for All Observed Two-Dimensional Crystals35
18-meta 18-ortho 20-meta 18-meta/18-ortho 20-meta/18-ortho
measured unit cell constants
calculated unit cell constants
plane group
no. of molecules in the asymmetric unit
a (nm)
b (nm)
R (deg)
a (nm)
b (nm)
R (deg)
p2mg p2 p2mg p1 p2
0.5 1.0 0.5 8 (4 18-meta and 4 18-ortho) 3 (4 20-meta and 2 18-ortho)
6.7 ( 0.2 3.1 ( 0.2 7.0 ( 0.2 12.3 ( 0.1 9.3 ( 0.3
0.80 ( 0.05 2.0 ( 0.1 0.74 ( 0.02 1.5 ( 0.1 2.0 ( 0.2
87 ( 3 98 ( 5 92 ( 2 95 ( 3 87 ( 6
6.25 2.95 6.75 11.9 9.50
0.88 1.77 0.88 1.79 1.77
90.0 98.2 90.3 91.5 89.8
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Figure 1. The two-dimensional crystalline packing at the phenyloctane/graphite interface from single component solutions of 18-meta (green) (A, B) and 18-ortho (blue) (C, D). (A) STM image of the18-meta monolayer (20 nm × 20 nm, 20.3 Hz scan rate, 800.0 mV, 300 pA) with overlaid molecular model. (B) Calculated packing model of 18-meta minimized on the graphite substrate with the unit cell depicted. (C) STM image of the 18-ortho monolayer (20 nm × 20 nm, 15.2 Hz scan rate, 608.1 mV, 300 pA) with overlaid molecular model. (D) Calculated packing model of 18-ortho minimized on the graphite substrate with the unit cell depicted.
conformation (calculated 2.94 nm). The length of the 18carbon alkyl chain separates the outer columns of benzene rings from the one in the middle and from the trough between methyl groups. From the STM image, it is apparent that alkyl chains have the same “zigzag” pattern and 90° angle to the column of aromatic rings that the 18-meta monolayer exhibits. This new phase is a cocrystal as depicted in the model (Figure 2b). The outer columns of alkyl chains and the adjacent aromatic rings consist of two different 18-ortho molecules in the hairpin conformation. This accounts for the methyl-methyl contacts observed and the larger spacing between identical aromatic rings in this column. The uninterrupted column of aromatic rings in the center consists of 18-meta molecules. The conformation of these molecules is slightly different than that in the pure 18-meta phase, to accommodate a denser, noninterdigitated column of molecules. The 18-meta molecules deform by desorption and overlap of the benzene rings to allow alkyl chain contacts. This nonoptimal conformation predicted by the model prevents the alkyl chains of 18-meta molecules from packing as closely as those in the exterior column. Indeed, the STM image shows that these alkyl chains have lower resolution than those in the outer columns consistent with increased mobility due to loose packing.16,17 These two isomers, which separately have simple packing motifs, combine in an elaborate manner when present in appropriate ratios to create surface features on a length scale greater than the sum of the individual components. Such a phenomenon offers a 1180
unique way of directing monolayer structure so as to construct nanoscale templates. During the transition from any pure two-dimensional crystalline phase to another, three phenomena may occur: cocrystallization, random mixing, or phase segregation. Cocrystals are usually the result of strong intercomponent interactions inducing two materials to combine in an ordered array. Previous examples of ordered cocrystals often involve hydrogen bonding between different species, such as in the multitude of alcohol solvates18-22 or 4,4′-bipyridine23-27 complexes formed with carboxylic acids. Another important case results from the filling of empty space in an open lattice, such as that formed by trimesic acid, which can be filled by coronene.28,29 Random mixing occurs when one component is inserted into the pure-phase structure of the other. For example, dialkylselenide molecules will adsorb in a monolayer of dialkyl sulfide molecules of the same size without disruption of the original monolayer structure, as is the case with mixtures of dialkyl sulfides, dialkyl ethers, and alkanes.30,31 Such behavior is observed when geometry and functionalities are sufficiently compatible that one component causes little disturbance to the structure of the other. Phase segregation occurs when two components adsorb in the same structures that they would in pure solution, but each component is confined to a distinct domain.14 For example, the geometrically and chemically dissimilar hexatriacontane and 4,4′-octadecyl-cyanobiphenyl phase segregate.32 This may be seen as the default mixing behavior, because it occurs Nano Lett., Vol. 6, No. 6, 2006
Figure 2. The two-dimensional cocrystalline packing formed at the liquid/graphite interface from binary component solutions of 18-ortho (blue) and 18-meta (green) (A, B) and 18-ortho and 20-meta (red) (C, D). (A) STM image of the monolayer formed from a 50:1 mixture of 18-ortho:18-meta (20 nm × 20 nm, 10.2 Hz, 700.0 mV, 300 pA) with overlaid molecular model. (B) Calculated packing model minimized on the graphite substrate with the unit cell depicted. (C) STM image of the monolayer formed from a 100:1 mixture of 18-ortho:20-meta (20 nm × 20 nm, 8.72 Hz, 800.0 mV, 300 pA) with overlaid molecular model. (D) Calculated packing model minimized on the graphite substrate with the unit cell depicted.
when conditions of compatibility are not met. It is not obvious from these limiting cases why a mixture of 18-meta and 18-ortho forms a cocrystal. From these previous examples, it could be predicted that the mixture of 18-meta and 18-ortho examined here would exhibit phase segregation. The geometric differences make random mixing unlikely, because insertion of one component into the pure phase of the other would induce significant structural deformation. No clear driving force toward cocrystallization such as satisfaction of strong hydrogen bonds33 or dipole interactions34 is present. Neither of the pure phases has an open structure which would facilitate cocrystallization through a type of host-guest relationship. In the 18-ortho/18-meta cocrystal, a column of 18-meta molecules in a perturbed conformation is inserted between columns of 18-ortho molecules (Figure 2b). This destabilization of 18-meta molecules suggests that there may be a corresponding stabilization of the 18-ortho molecules to make the overall structure stable. If this were the case, then molecules that pack similarly to 18-meta should induce the same structure. To test this hypothesis, mixtures of 18-ortho with 20-meta were examined. 20-meta (Chart 1) has the same packing structure as 18-meta on graphite (see Supporting Information), with an expanded unit cell to accommodate the longer alkyl chains.3 A two-dimensional cocrystalline phase was observed at solution molar ratios of 100:1 and 120:1 18-ortho:20-meta (Figure 2c). A much greater amount of 18-ortho is required to displace 20-meta than was necessary for 18-meta, because the larger size of the former Nano Lett., Vol. 6, No. 6, 2006
induces stronger substrate-molecule and moleculemolecule interactions. The structure of the 18-ortho/20-meta cocrystal is not isostructural with that formed by the 18-ortho/18-meta cocrystal, although several features are shared. Both structures have a repeating pattern where columns of benzene rings and troughs between methyl groups are separated by the length of the alkyl chains. The 18-ortho/20-meta cocrystal has only two lines of benzene rings between troughs, and hence a smaller periodicity of 9.3 ( 0.3 nm, whereas the 18-ortho/18-meta cocrystal has three benzene ring containing columns. The presence of the trough of methyl groups is an indication that 18-ortho is still present in a hairpin conformation in the columns on either side of the trough. The rings do not follow a straight line, but instead waver in a repeating fashion, suggesting that the outside columns consist of both octadecyl and eicosyl chains. A model is proposed (Figure 2d) wherein alkyl chains from 20-meta molecules are interdigitated in the center column, and 18-ortho molecules are inserted into the outer columns between every second 20-meta. Such an arrangement reproduces the STM image well, particularly the periodic wavering of the column of benzene rings, and is reminiscent of the packing motif of a structurally related compound, 1,3-dinonadecanoylbenzene, where this arrangement enables C-H‚‚‚O hydrogen bonding.2,3 Thus, the 18-ortho/20-meta structure can be rationalized in terms of the hydrogen bonding enabled by the proximity of 20-meta molecules (see Supporting Information). This cannot be a factor in the formation of the 181181
ortho/18-meta cocrystal, however, because there is no such contact. This structural variation between the cocrystals of 18-ortho/18-meta and 18-ortho/20-meta is peculiar in light of the isomorphous packing of 18-meta and 20-meta. However, if the 18-meta molecules were substituted for 20meta molecules in the 18-ortho/20-meta structure, the alkyl chain contact decreases (see Supporting Information) because the 18-meta molecules are not long enough to envelop the 18-ortho molecules as the longer chain ester does. Thus, the 20-meta molecules can adopt a hydrogen-bond-stabilized cocrystalline structure because they are long enough to enable alkyl chain contact with the ortho molecules whereas the 18-meta molecules cannot. Examination of the packing motifs of these two different cocrystalline phases reveals different patterns of intermolecular interactions, signaling that no single intermolecular interaction is responsible for inducing cocrystallization in this system. In three-dimensional crystals, cocrystal formation has been associated with formation of denser structures.29 However, inspection of the packing coefficients and densities of the pure and cocrystalline phases reveals no correlation (see Supporting Information). When the solution conditions are 1:1 ortho:meta where the concentration is sufficient for either pure meta or pure ortho to form a monolayer, the meta monolayer outcompetes both the cocrystal and pure ortho phases. Thus the meta monolayer is most stable over the largest range of solution conditions. As the proportion of ortho in solution is increased, it is the cocrystal that outcompetes both pure ortho and meta phases. Thus, as judged by their ability to displace the other forms, the cocrystals are less stable than the pure meta phases but more stable than the ortho phase. As the solution concentration of ortho increases to the point where both isomers coexist on the surface, these cocrystals provide an intermediate step before the high-energy ortho phase forms. Facilitating the discovery of cocrystal phases in this system is the large energy window that allows many concentrations to be explored. Further enabling formation of these phases is the structural flexibility of the meta ester components. The similarity of the packing of the meta components within both of the cocrystals to that of other meta-disubstituted benzenes is notable. This class of compounds has an irregular geometry that tolerates many different packing motifs,3 increasing the ease with which alternative crystalline phases may be adopted. Generation of monolayers at the liquid-solid interface is a simple means of producing ever-more-complex surface patterns, and the ratio of components in a solution is an important, but little explored, variable that can be manipulated to obtain unique two-dimensional crystals. Here periodicity on the scale of several molecules was generated by cocrystallization, a feature conducive to creation of monolayers that may then serve as templates for patterning on the low end of the nanoscale. Cocrystals can be designed through understanding of two-dimensional packing rules, utilizing strong intermolecular interactions and open lattices, but these limiting cases do not explain the elaborate structures reported here that result instead from energetic compromises 1182
to accommodate the changing surface composition. The cocrystals were of intermediate stability to the two pure phases and incorporated a structurally flexible class of compounds, the meta esters. These results demonstrate that examination of multicomponent solution mixtures over a range of solute ratios offers a hitherto untapped means of controlling two-dimensional crystalline packing that can greatly increase the length scale of surface features. Acknowledgment. This work was supported by the National Science Foundation (CHE-0316250). Supporting Information Available: (1) Synthetic procedures, (2) experimental details, (3) list of ratios at which competitive adsorption experiments were performed with the resulting surface phase, (4) additional STM images of 18ortho/18-meta cocrystal, (5) STM image and packing model of 20-meta, (6) additional STM images of 18-ortho/20-meta cocrystal, (7) model of the 18-ortho/20-meta cocrystal showing close contacts, (8) hypothetical model of a 18-ortho/ 18-meta cocrystal in the 18-ortho/20-meta packing motif, (9) packing coefficient and density calculations. This material is available free of charge via the Internet at http:// pubs.acs.org. References (1) Notable examples include: (a) De Feyter, S.; Gesquie`re, A.; Klapper, M.; Mu¨llen, K.; De Schryver, F. C. Nano Lett. 2003, 3, 1485-1488. (b) Qian, P.; Nanjo, H.; Yokoyama, T.; Suzuki, T. M. Chem. Lett. 1999, 1229-1230. (c) Meier, C.; Ziener, U.; Landfester, K.; Weihrich, P. J. Phys. Chem. B 2005, 109, 21015-21027. (d) AbdelMottaleb, M. M. S.; Gomar-Nadal, E.; Surin, M.; Uji-i, H.; Mamdouh, W.; Veciana, J.; Lemaur, V.; Rovira, C.; Cornil, J.; Lazzaroni, R.; Amabilino, D. B.; De Feyter, S.; De Schryver, F. C. J. Mater. Chem. 2005, 15, 4601-4615. (e) Lackinger, M.; Griessl, S.; Heckl, W. A.; Hietschold, M.; Flynn, G. W. Langmuir 2005, 21, 4984-4988. (2) Kim, K.; Matzger, A. J. J. Am. Chem. Soc. 2002, 124, 8772-8773. (3) Plass, K. E.; Kim, K.; Matzger, A. J. J. Am. Chem. Soc. 2004, 126, 9042-9053. (4) Hamers, R. J.; Coulter, S. K.; Ellison, M. D.; Hovis, J. S.; Padowitz, D. F.; Schwartz, M. P.; Greenlief, C. M.; Russell, J. N., Jr. Acc. Chem. Res. 2000, 33, 617-624. (5) Donhauser, Z. J.; Price, D. W.; Tour, J. M.; Weiss, P. S. J. Am. Chem. Soc. 2003, 125, 11462-11463. (6) Lopinski, G. P.; Wayner, D. D. M.; Wolkow, R. A. Nature 2000, 406, 48-51. (7) For example, alkanethiols can adsorb either laterally or perpendicularly to gold depending on the adsorption mechanism. Different chain lengths result in the same structure perpendicularly but have different periodicities when lying lengthwise. See: Darling, S. B.; Rosenbaum, A. W.; Wang, Y.; Sibener, S. J. Langmuir 2002, 18, 7462-7468. (8) Watel, G.; Thibaudau, F.; Cousty, J. Surf. Sci. 1993, 281, L297L302. (9) Askadskaya, L.; Rabe, J. P. Phys. ReV. Lett. 1992, 69, 1395-1398. (10) The n-carboxylic acids are pseudopolymorphic at the liquid/graphite interface. One motif consists of interdigitated dimers, in which the periodicity is only one molecular length. In the other motif dimers are not interdigitated and the periodicity is doubled. See: Hoeppener, S.; Chi, L.; Fuchs, H. ChemPhysChem 2003, 4, 494-498. Takajo, D.; Nemoto, T.; Isoda, S. Jpn. J. Appl. Phys., Part 1 2004, 43, 46674670. (11) De Feyter, S.; Grim, P. C. M.; van Esch, J.; Kellogg, R. M.; Feringa, B. L.; De Schryver, F. C. J. Phys. Chem. B 1998, 102, 8981-8987. (12) The Epicalc program finds that the monolayer is coincident with a supercell of 12.4 nm × 8.0 nm, coming into registry with the HOPG underlayer every four unit cells along the b axis. This matches the repeating pattern observed in Figure 2c. See: Hillier, A. C.; Ward, M. D. Phys. ReV. B 1996, 54, 14037-14051. (13) Schuurmans, N.; Uji-i, H.; Mamdouh, W.; De Schryver, F. C.; Feringa, B. L.; Van Esch, J.; De Feyter, S. J. Am. Chem. Soc. 2004, 126, 13884-13885.
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(14) Kim, K.; Plass, K. E.; Matzger, A. J. J. Am. Chem. Soc. 2005, 127, 4879-4887. (15) Venkataraman, B.; Breen, J. J.; Flynn, G. W. J. Phys. Chem. 1995, 99, 9, 6608-6619. (16) Kim, K.; Plass, K. E.; Matzger, A. J. Langmuir 2003, 19, 71497152. (17) This decrease in resolution cannot be attributed to a moire´ pattern because it is consistent within a column of alkyl chains while a moire´ pattern varies along this direction, as in Figure 2c. (18) Wintgens, D.; Yablon, D. G.; Flynn, G. W. J. Phys. Chem. B 2003, 107, 173-179. (19) Gesquie`re, A.; Abdel-Mottaleb, M. M.; De Feyter, S.; De Schryver, F. C.; Sieffert, M.; Mu¨llen, K.; Calderone, A.; Lazzaroni, R.; Bre´das, J. L. Chem.-Eur. J. 2000, 6, 3739-3746. (20) Vanoppen, P.; Grim, P. C. M.; Ru¨cker, M.; De Feyter, S.; Moessner, G.; Valiyaveettil, S.; Mu¨llen, K.; De Schryver, F. C. J. Phys. Chem. 1996, 100, 19636-19641. (21) Grim, P. C. M.; De Feyter, S.; Gesquie`re, A.; Vanoppen, P.; Ru¨cker, M.; Valiyaveettil, S.; Moessner, G.; Mu¨llen, K.; De Schryver, F. C. Angew. Chem., Int. Ed. 1997, 36, 2601-2603. (22) De Feyter, S.; Grim, P. C. M.; Ru¨cker, M.; Vanoppen, P.; Meiners, C.; Sieffert, M.; Valiyaveettil, S.; Mu¨llen, K.; De Schryver, F. C. Angew. Chem., Int. Ed. 1998, 37, 1223-1226. (23) Kampschulte, L.; Griessl, S.; Heckl, W. M.; Lackinger, M. J. Phys. Chem. B 2005, 109, 14074-14078. (24) De Feyter, S.; Larsson, M.; Gesquie`re, A.; Verheyen, H.; Louwet, F.; Groenendaal, B.; van Esch, J.; Feringa, B. L.; De Schryver, F. ChemPhysChem 2002, 3, 966-969. (25) Qian, P.; Nanjo, H.; Yokoyama, T.; Suzuki, T. M.; Akasaka, K.; Orhui, H. Chem. Commun. 2000, 2021-2022.
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(26) Uji-i, H.; Yoshidome, M.; Hobley, J.; Hatanaka, K.; Fukumura, H. Phys. Chem. Chem. Phys. 2003, 5, 4231-4235. (27) Qian, P.; Nanjo, H.; Yokoyama, T.; Suzuki, T. M. Chem. Commun. 1999, 1197-1198. (28) Griessl, S. J. H.; Lackinger, M.; Jamitzky, F.; Markert, T.; Hietschold, M.; Heckl, W. A. Langmuir 2004, 20, 9403-9407. (29) More subtle filling of void spaces distributed widely throughout a structure has recently been implicated in the promiscuous solvation behavior of a benzodiazepine: Price, C. P.; Glick, G. D.; Matzger, A. J. Angew. Chem. 2006, 45, 2062-2066. (30) Padowitz, D. F.; Messmore, B. W. J. Phys. Chem. B 2000, 104, 9943-9946. (31) Padowitz, D. F.; Sada, D. M.; Kemer, E. L.; Dougan, M. L.; Xue, W. A. J. Phys. Chem. B 2002, 106, 593-598. (32) Baker, R. T.; Mougous, J. D.; Brackley, A.; Patrick, D. L. Langmuir 1999, 15, 4884-4891. (33) Fan, E.; Vicent, C.; Geib, S. J.; Hamilton, A. D. Chem. Mater. 1994, 6, 1113-1117. (34) Goroff, N. S.; Curtis, S. M.; Webb, J. A.; Fowler, F. W.; Lauher, J. W. Org. Lett. 2005, 7, 1891-1893. (35) The structural assignment of the cocrystal of 18-meta/18-ortho is based on the assumption that all 18-meta columns have the same orientation. An assignment of p2, containing four 18-meta and four 18-ortho molecules in the unit cell, with a doubled a-axis, would result from the alternating orientation of the 18-meta columns. The specific arrangement adopted cannot be unambiguously determined from the STM images.
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