Carboxylic Acids: Versatile Building Blocks and Mediators for Two

pubs.acs.org/Langmuir © 2009 American Chemical Society

Carboxylic Acids: Versatile Building Blocks and Mediators for Two-Dimensional Supramolecular Self-Assembly Markus Lackinger*,† and Wolfgang M. Heckl†,‡ †

Ludwig-Maximilians-University and Center for NanoScience (CeNS), Department for Earth and Environmental Sciences, Theresienstrasse 41, DE-80333 Mu¨nchen, Germany, and ‡Deutsches Museum, Museumsinsel 1, DE-80538 Mu¨nchen, Germany Received March 4, 2009. Revised Manuscript Received April 10, 2009 Two-dimensional (2D) supramolecular self-assembly of various organic molecules at the liquid-solid interface is presented and discussed with a focus on compounds that are primarily functionalized by carboxylic groups. The main analytical tool utilized is scanning tunneling microscopy (STM), a high-resolution real-space technique capable of readily providing full crystallographic information (i.e., not only lattice parameters but also number, type, and orientation of molecules within the unit cell). Carboxylic groups are of particular interest because their combined donor and acceptor character with regard to hydrogen bonds provides reliable intermolecular cross-linking, thereby facilitating the self-assembly of well-ordered, stable monolayers. By means of various homomeric (monomolecular) and heteromeric (here, bimolecular) examples, this feature article illustrates the influence of both molecular structure and external conditions (type of solvent, concentration, etc.) on monolayer self-assembly at the liquid-solid interface. A very intriguing aspect of interfacial self-assembly is that many systems are thermodynamically controlled (i.e., adsorbed molecules at the surface are in equilibrium with molecules dissolved in the supernatant liquid phase). This offers the unique possibility not only to steer the system reliably by intensive thermodynamic parameters such as temperature and concentration but also to gain fundamental knowledge about decisive processes and steps in supramolecular self-assembly.

Introduction Self-assembly driven by hydrogen bonds under the participation of carboxylic groups is abundant and extremely important both in nature and supramolecular chemistry. For instance, amino acids are a class of essential biomolecules endowed with carboxylic groups. Hydrogen bonds between carboxylic and amino functionalities prearrange amino acids for peptide bonding, an important step in protein synthesis. Likewise, in man-made supramolecular systems carboxylic groups are very inspiring and widely exploited synthons for hydrogen bonds. Because of the broad interest accompanied by both conceptual and analytical advantages, the main topic of this article is surfacesupported self-assembly. On one hand, the lower dimensionality allows us to efficiently create and study comparatively simple model systems, even though, concededly, the surface itself can have a huge impact. On the other hand, on solid surfaces scanning probe microscopy, in particular, scanning tunneling microscopy (STM), as a high-resolution real-space technique offer the potential to unveil supramolecular ordering with submolecular resolution, albeit the precision of experimentally determined lattice parameters is typically inferior to that of (surface) diffraction techniques such as low-energy electron diffraction (LEED). Self-assembly as a parallel, and hence inherently quick, bottom-up fabrication technique inspires and facilitates many promising applications in surface functionalization, sensors, and organic electronics. Here we will focus on the self-assembly of crystalline monolayer structures. The great majority of surfacesupported systems investigated today exhibits translational symmetry in either one (chains) or two dimensions (monolayers), and strategies for the self-assembly of more complex structures as demanded by molecular electronics are underdeveloped. *Corresponding author. E-mail: [email protected].

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Nevertheless, experiments on crystalline supramolecular monolayers that explore the strength, configuration, and formation probability of intermolecular hydrogen bonds, the influence of the molecular structure, and thermodynamics may also provide the grounds for the design of more complex supramolecular assemblies. In this feature article, we will describe and discuss the selfassembly of carboxylic acids on crystalline substrates into ordered monolayer structures. To be able to provide more detailed insight, we will focus on systems at the liquid-solid interface without external control of the electrochemical potential. The latter would be a topic of its own, and the interested reader is referred to more specialized reviews.1 Despite the astonishing ease of experiments, instructive and profound insights into the thermodynamics of self-assembly, the formation of hydrogen bonding patterns, and cooperation versus competition of different intermolecular interactions can be gained. This feature article is organized as follows: After a brief general summary on the thermodynamic aspects of self-assembly and hydrogen bonding between carboxylic groups, experimental details and specifics are presented. Then various monomolecular examples are introduced with a focus on nanoporous and one-dimensional (1D) hydrogen-bonded systems. For nanoporous systems that exhibit a periodic arrangement of cavity voids, a strategy to increase the pore size is presented, and also two selected examples of guest inclusion are given. The next section is devoted to multicomponent self-assembly, a topic of increasing interest. Three conceptually different examples are discussed: combined solvent-solute structures, a combination of molecules without carboxylic groups with carboxylic acids, and mixing of two different carboxylic acids. Finally, a short paragraph (1) Ye, S.; Kondo, T.; Hoshi, N.; Inukai, J.; Yoshimoto, S.; Osawa, M.; Itaya, K. Electrochemistry 2009, 77, 2.

Published on Web 05/19/2009

DOI: 10.1021/la900785f

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describes theoretical efforts, and the outlook addresses open questions and identifies future challenges.

Thermodynamic Aspects and Hydrogen Bonding Patterns for Carboxylic Acids In contrast to self-organization (dynamic self-assembly), (static) self-assembly drives a system toward its thermodynamic equilibrium (i.e., minimizes the Gibbs free energy), provided that there are no kinetic limitations.2 Because during the course of self-assembly the degree of order increases, it is entropically disfavored. Consequently, the decrease in internal entropy must be compensated for by a respective gain in enthalpy in order to observe the formation of a monolayer at the liquid-solid interface. Upon adsorption of solute molecules, their enthalpy of mixing is converted into a binding enthalpy of molecules on the surface. Commonly, a subdivision into two contributions is made (molecule-molecule and molecule-substrate interactions), although for some systems those two contributors might actually not be independent of each other. For instance, long-range adsorbateadsorbate interactions can be mediated by surface states.3 Because only the overall enthalpy gain matters, different strategies promote monolayer formation: (1) Large, preferentially planar molecules with high adsorption energy.4 Especially on graphite, because of their good registry, alkane tails are widely exploited anchor groups.5 (2) Strongly interacting substrates (e.g., metal surfaces). (3) Enhanced intermolecular interaction, for example, by the interdigitation of alkane tails6 or functionalization with groups for intermolecular hydrogen bonds. In supramolecular systems, hydrogen bonds are the most important interaction because they exhibit cooperativity, directionality, and selectivity.7 In contrast to alkane chain interdigitation, which is not necessarily fully taking effect in the self-assembly of molecules with alkane tails,8 hydrogen bonds reliably provide intermolecular linkage. Another intriguing characteristic of hydrogen bonds is their variability in strength: weak hydrogen bonds have binding energies comparable to van-der-Waals interactions whereas strong hydrogen bonds reach the range of weak covalent bonds. Normally, at room temperature their intermediary bond strength facilitates error correction during self-assembly while still providing sufficient stability. Various contributions also have to be considered and balanced for the entropy.9 The adsorption of a monolayer reduces the number of molecules in solution and thus annihilates translational entropy. Furthermore, upon aggregation molecules lose their rotational entropy and, depending on the specific molecule and structure, part of their conformational entropy. Commonly, vibrational entropy is hardly affected upon self-assembly because of the relatively high energy of normal modes as compared to kT. In conclusion, the adsorption of solute molecules and subsequent monolayer formation can only be observed if all contributions to the Gibbs free energy are balanced. This subtle balance is also (2) Whitesides, G. M.; Mathias, J. P.; Seto, C. T. Science 1991, 254, 1312. (3) Repp, J.; Moresco, F.; Meyer, G.; Rieder, K. H.; Hyldgaard, P.; Persson, M. Phys. Rev. Lett. 2000, 85, 2981. (4) Tahara, K.; Lei, S.; Mossinger, D.; Kozuma, H.; Inukai, K.; Van der Auweraer, M.; De Schryver, F. C.; Hoger, S.; Tobe, Y.; De Feyter, S. Chem. Commun. 2008, 3897. (5) De Feyter, S.; De Schryver, F. C. J. Phys. Chem. B 2005, 109, 4290. (6) Furukawa, S.; Uji-i, H.; Tahara, K.; Ichikawa, T.; Sonoda, M.; De Schryver, F. C.; Tobe, Y.; De Feyter, S. J. Am. Chem. Soc. 2006, 128, 3502. (7) Steiner, T. Angew. Chem., Int. Ed. 2002, 41, 48. (8) Kleiner-Shuhler, L.; Brittain, R.; Johnston, M. R.; Hipps, K. W. J. Phys. Chem. C 2008, 112, 14907. (9) Mammen, M.; Shakhnovich, E. I.; Deutch, J. M.; Whitesides, G. M. J. Org. Chem. 1998, 63, 3821.

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essential for polymorphism (i.e., the preference of a particular monolayer structure over another depending on the conditions and environment). For hydrogen bonds, carboxylic groups are very interesting functional units for many reasons. First, they exhibit dual character because the oxygen atom of the carboxy group can act as an acceptor for hydrogen bonds whereas the hydroxyl group can act as a donor. Thus, with regard to hydrogen bonds carboxylic groups are self-complementary (i.e., two carboxylic groups can form a cyclic dimer interconnected by two equivalent hydrogen bonds). The simplest model system for this configuration is the formic acid dimer with a dissociation energy of ∼0.5-0.6 eV.10 To deduce typical binding configurations, lengths, and angles, statistical analysis of 3D organic crystal structures is a valuable tool. Common binding arrangements for carboxylic acids both on surfaces and in molecular crystals11 are cyclic dimers, trimers, and catemeric motifs as depicted in Figure 1. Moreover, the hydroxyl oxygen atom in the catemeric motif in Figure 1c is sterically accessible as an acceptor for additional hydrogen bonds. The pronounced preference for either cyclic or infinite chain configurations can be understood as a cooperative phenomenon mainly caused by resonance effects and termed resonance-assisted hydrogen bonding (RAHB).12 Two molecules where each is endowed with at least one carboxylic group can form intermolecular hydrogen bonds. Yet, to obtain homomeric networks of hydrogen-bonded molecules on surfaces, two carboxylic groups per molecule are necessary to promote the formation of 1D structures, and at least three carboxylic groups are required for the formation of 2D hydrogen-bonded networks. However, when carboxylic functionalities are too close to each other, steric hindrance and the formation of intramolecular hydrogen bonds can hamper self-assembly into networks.13 The ideal bonding angle of an isolated dimer (i.e., the energetic minimum in the absence of other constrains) corresponds to 180°. Quite frequently, molecule-substrate interactions, other inter- and intramolecular hydrogen bonds, and steric hindrance impose constraints; consequently, deviations from the ideal straight dimer motif can occur. However, carboxylic groups are acidic functionalities in the sense of proton donors and are commonly in equilibrium with their deprotonated state, the anionic carboxylate. This chemical activity of the carboxylic groups can also influence self-assembly. In particular, on more reactive metal substrates or in combination with solvents that stabilize the free proton, carboxylic acids can also be adsorbed in a deprotonated state, which gives rise to strong interactions with the substrate. When in addition free metal atoms are supplied either by deliberate deposition or inherently from the surface (e.g., diffusing free adatoms on Cu(111)), carboxylic groups can form stronger metal coordination bonds that likewise yield versatile stable networks,14 but without the possibility of forming coordination bonds, full or partial deprotonation results in modified intermolecular interactions and new structures emerge as compared to the same fully protonated compound. An example is given by the tricarboxylic acid 1,3,5-benzenetribenzoic acid (BTB, cf. Figure 3b for structure and Figure 5b for the STM topograph of a monolayer on (10) Neuheuser, T.; Hess, B. A.; Reutel, C.; Weber, E. J. Phys. Chem. 1994, 98, 6459. (11) Leiserowitz, L. Acta Crystallogr. 1976, B32, 775. (12) Gilli, P.; Bertolasi, V.; Ferretti, V.; Gilli, G. J. Am. Chem. Soc. 1994, 116, 909. (13) Lackinger, M.; Griessl, S.; Markert, T.; Jamitzky, F.; Heckl, W. M. J. Phys. Chem. B 2004, 108, 13652. (14) Stepanow, S.; Lingenfelder, M.; Dmitriev, A.; Spillmann, H.; Delvigne, E.; Lin, N.; Deng, X. B.; Cai, C. Z.; Barth, J. V.; Kern, K. Nat. Mater. 2004, 3, 229.

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Figure 1. Three different commonly encountered binding motifs for hydrogen bonds between carboxylic functionalities both in the solid state and in surface-supported monolayers: (a) cyclic dimer, (b) catemeric, and (c) cyclic trimer motifs. For the catemeric motifs, carboxylic groups can either be in the (d) syn planar or (e) antiplanar configuration. The antiplanar configuration is slightly higher in energy and observed only when stabilized by additional intra- or intermolecular hydrogen bonds.

graphite). On graphite, self-assembly results in a hexagonal chickenwire structure, where each BTB molecule forms six intermolecular hydrogen bonds,15 and on Ag(111), annealing up to 420 K induces structural phase transitions driven by the partial deprotonation of carboxylic groups.16 Apparently, the deprotonation of carboxylic groups can be controlled by the choice of substrate and the amount of thermal energy supplied, but so far the decisive factors are mainly unexplored. Here, only hydrogen bond-assisted self-assembly will be discussed where the carboxylic groups are fully protonated.

Experimental Details In this section, we will first describe the advantages and disadvantages of various methods for monolayer preparation, introduce the homologous series of fatty acids as suitable solvents for experiments at the liquid-solid interface, and eventually elaborate on the split-image technique for a precise STM-based determination of lattice parameters. To prepare adsorbed molecular monolayers, three main methods are established and commonly applied. The purest and most well defined method with the highest degree of external control is thermal evaporation under ultrahigh vacuum (UHV) conditions onto atomically clean, flat single-crystal surfaces, commonly referred to as organic molecular beam epitaxy (OMBE).17 Although it is the most versatile method, these advantages come at a price: high instrumental cost and effort, time-consuming preparation, and targeted compounds that have to be thermally stable (i.e., must not decompose at the sublimation temperature). The second method is the deposition of molecules from solutions with rather volatile solvents such as toluene.8,18-20 After a droplet of solution has been deposited, the solvent evaporates and leaves the much heavier solute molecules behind on the surface. Depending on the solvent’s vapor pressure, the substrate can either be held at room temperature or heated to higher temperature in order to (15) Kampschulte, L.; Lackinger, M.; Maier, A. K.; Kishore, R. S. K.; Griessl, S.; Schmittel, M.; Heckl, W. M. J. Phys. Chem. B 2006, 110, 10829. (16) Ruben, M.; Payer, D.; Landa, A.; Comisso, A.; Gattinoni, C.; Lin, N.; Collin, J. P.; Sauvage, J. P.; De Vita, A.; Kern, K. J. Am. Chem. Soc. 2006, 128, 15644. (17) Forrest, S. R. Chem. Rev. 1997, 97, 1793. (18) 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. (19) Yan, H.-J.; Lu, J.; Wan, L.-J.; Bai, C.-L. J. Phys. Chem. B 2004, 38, 11251. (20) Lu, J.; Lei, S. B.; Zeng, Q. D.; Kang, S. Z.; Wang, C.; Wan, L. J.; Bai, C. L. J. Phys. Chem. B 2004, 108, 5161.

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promote solvent evaporation. This method is experimentally straightforward with a wide range of applicability, but sample coverage is rather inhomogeneous, ranging from submonolayer to micrometers. Nevertheless, for various reasons, the most interesting and appealing preparation method to us is to use nonvolatile solvents and investigate the monolayers in situ at the liquid-solid interface with the STM tip immersed in the solution. In contrast to aforementioned vacuum deposition and solvent evaporation, this method does not necessarily enforce the adsorption of solute molecules. Rather, it has the potential to investigate and reveal monolayer thermodynamics at the liquid-solid interface.21-23 We will consider systems only where the layer thickness is self-limited to one monolayer. This is because the pristine substrate provides sufficient adsorption energy only for the first monolayer but this necessary stabilization is lacking for a second layer, which would have to grow on top of the first layer. Yet, in light of applications that have to access the thin organic film this method might be disadvantageous because for many systems it is difficult if not impossible to remove the protective liquid layer without destroying the order or even desorbing the monolayer. However, for selected systems rinsing the sample with a “nonsolvent” can leave the monolayer intact.24 A more practical problem and probably the most crucial task for monolayer experiments at the liquid-solid interface is finding the right solvent. Certain requirements apply. First, the vapor pressure should be small enough to minimize solvent evaporation. For ease of experiment, the solvent should be electrically nonconducting, thus STM tips do not have to be insulated. For this purpose, nonpolar solvents are better suited presumably because polar solvents accommodate ions that give rise to Faraday currents. However, the solvent has to match the solute (i.e., the solubility should be large enough). However, one has to state here that solvents for experiments at the liquid-solid interface do not have to be classical solvents and solubilities below 1 mmol/L are already sufficient. In fact, monolayer formation has been observed from solutions with concentrations in the micromolar range.25 In this special case, all formerly dissolved molecules are adsorbed onto the surface, whereas for the systems discussed in (21) Venkataraman, B.; Breen, J. J.; Flynn, G. W. J. Phys. Chem. 1995, 99, 6608. (22) Lei, S.; Tahara, K.; De Schryver, F. C.; Van der Auweraer, M.; Tobe, Y.; De Feyter, S. Angew. Chem., Int. Ed. 2008, 47, 2964 2964. (23) Kampschulte, L.; Werblowsky, T. L.; Kishore, R. S. K.; Schmittel, M.; Heckl, W. M.; Lackinger, M. J. Am. Chem. Soc. 2008, 130, 8502. (24) Unpublished observation. (25) Palma, C. A.; Bonini, M.; Llanes-Pallas, A.; Breiner, T.; Prato, M.; Bonifazi, D.; Samori, P. Chem. Commun. 2008, 5289.

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Table 1. Physical and Chemical Properties of the Homologous Series of Fatty Acids Commonly Applied as Solvents for Self-Assembly Experiments at the Liquid-Solid Interfacea acid

formula

butanoic

C4H8O2

molecular weight density dielectric (g/mol) bp (°C) mp (°C) (g/cm3) constant

viscosity (mPa s)

vapor pressure (mmHg)

pKa

TMA solubility (mmol/L)

88.1

164

-8

0.9577

2.97

1.591 (18 °C)

316

4.81 (20 °C)

3.2

pentanoic C5H10O2

102.1

186

-19

0.9391

2.68

1.8127 (30 °C)

241

4.83 (18 °C)

1.3

C6H12O2

116.2

204

-3

0.9274

2.59

2.5727 (29 °C)

68.5

4.88 (25 °C)

1.0

heptanoic C7H14O2

130.2

223

-7.5

0.9200

3.04

4.3371 (20 °C)

39.0

4.89 (25 °C)

0.75

C8H16O2

144.2

237

16.5

0.9088

2.83

5.105 (25 °C)

17.3

4.98 (25 °C)

0.69

4.96 (25 °C)

0.65

hexanoic

octanoic

158.2 254 10 0.9057 2.49 7.25 (24 °C) nonanoic C9H18O2 a Dielectric constants refer to 20°C, and vapor pressure values refer to 410 K.

the following text, typically the solution contains about 1 to 2 orders of magnitude more molecules than incorporated in the monolayer. The chemical rule of thumb “like dissolves like” in the sense that solute and solvent should exhibit similar functional groups is particularly applicable to carboxylic acid solutes. The homologous series of fatty acids has been proven to be a very lucky choice for STM experiments at the liquid-solid interface: many important physical properties such as molecular weight, boiling and melting points, density, dielectric constant, and viscosity vary more or less monotonically with aliphatic chain length. In principle, this variability offers the potential to gain deeper insight through systematic studies but is both a blessing and a curse. On one hand, the homologous series of fatty acids allows one to tune important physical properties of the solvent in fine increments. On the other hand, a specific physical parameter cannot be varied independently, though it can be difficult and ambiguous to interpret chain-length-dependent effects. Table 1 summarizes general physical properties that depend on the solvent chain length. The vapor pressure of fatty acids, density, and solubility for widely investigated trimesic acid (TMA) decrease monotonically with increasing chain length. Equally, the static dielectric constant decreases with chain length whereas viscosity, boiling point, and melting point (apart from an insignificant odd-even effect) increase with chain length (molecular weight). All fatty acids are weak acids, and their acidity slightly decreases with increasing chain length. Also, the affinity of fatty acids to the substrate (i.e., their adsorption energy on graphite) increases linearly with chain length.26 For STM experiments at the liquid-solid interface, the homologous series from butanoic to nonanoic acid is feasible. As obvious from the vapor pressure, solvent evaporation becomes less problematic for longer-chain-length acids. At room temperature, experiments with nonanoic acid can be pursued for several hours without any problems related to solvent evaporation. This is particularly useful when the temporal evolution of a system will be studied by video STM without the interference by concentration changes. Fatty acids with longer aliphatic chains from decanoic acid are solids at room temperature. Nevertheless, with a heatable sample stage, STM experiments can be conducted at elevated temperatures.27 With a maximum operating temperature of ∼60 °C, the range of suitable solvents can be extended up to pentadecanoic acid. Moreover, temperature-dependent STM experiments allow one to explore interesting physics in monolayers, for instance, diffusion,28 desorption, and phase transitions, which were, at least at the liquid-solid interface, until now mostly terra incognita. Only recently, Hipps et al. reported studies at elevated temperatures on the thermal stability of binary coronene + (26) Mu¨ller, T.; Flynn, G. W.; Mathauser, A. T.; Teplyakov, A. V. Langmuir 2003, 19, 2812. (27) Walch, H.; Maier, A. K.; Heckl, W. M.; Lackinger, M. J. Phys. Chem. C 2009, 113, 1014. (28) Schull, G.; Douillard, L.; Fiorini-Debuisschert, C.; Charra, F.; Mathevet, F.; Kreher, D.; Attias, A. J. Nano Lett. 2006, 6, 1360.

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9.90

heptanoic acid monolayers.29 Coronene on Au(111) is also a nice example of how fatty acid solvent molecules are incorporated into the interfacial monolayer with distinct differences between hexanoic, heptanoic, and octanoic acid.30 For STM experiments at the liquid-solid interface, it is most convenient and straightforward to work with or start from saturated solutions. Preparation is easy but also reproducible. Moreover, adding known amounts of pure solvent allows for quick, easy preparation of dilute solutions. Even so, as long as the solubility is unknown, absolute concentrations remain unknown. Nevertheless, reliable preparation protocols for dilute or even binary solutions can be established. According to our experience, it is highly advisible to sonicate and centrifuge solutions, in particular, for fatty acid solvents and when concentration-dependent effects will be studied. As monitored by dynamic light scattering, some putative solutions are actually dispersions that still contain small crystallites of solute material. The dilution of such dispersions will not lead to reproducible concentrations. However, after time spans of months to years we have observed aging effects in solutions. Some aged solutions precipitate a gel or yield monolayer structures that are different from those obtained with freshly prepared equivalents. Without definite experimental proof, these effects are intuitively attributed to slow association processes, possibly assisted by atmospheric water. At least for selected systems, minor additions of H2O can alter self-assembly and produce a different monolayer structure. The minute amounts of solutions prepared, typically between 1 and 5 mL, are disadvantageous. Because the potential influence of solute impurities increases, it is advisible to keep the amount of sediment in saturated solutions as small as possible. For the precise measurement of crystallographic parameters, scanning probe methods are disadvantageous because of image distortions caused by nonideal piezo behavior (mainly creep and hysteresis) and thermal drift. In particular, experiments conducted under ambient conditions are subject to temperature fluctuations of the environment and are particularly prone to thermal drift. Fortunately, at the liquid-solid interface the so-called split-image technique can significantly improve the accuracy of experimentally determined unit cell parameters. Within a single frame both the adsorbate layer and the substrate are recorded with molecular and atomic resolution, respectively; an example is presented in Figure 2a. This is achieved by changing the tunneling parameters during image acquisition. Normally, for low tunneling voltages (1.0 nA), the graphite substrate dominates the contrast. Although utilized by many groups, this method has so far not been described and analyzed in detail. For instance, it is not entirely clear and might also vary from system to system whether electrons tunnel through molecules for small tunneling voltages or molecules are mechanically pushed aside because of the diminished tip-sample distance.

(29) English, W. A.; Hipps, K. W. J. Phys. Chem. C 2008, 112, 2026. (30) Gyarfas, B. J.; Wiggins, B.; Zosel, M.; Hipps, K. W. Langmuir 2005, 21, 919.

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Figure 2. (a) Split STM image of a BTB monolayer on HOPG(0001) where the upper part depicts the ordered adsorbate layer and the lower part depicts the graphite lattice with atomic resolution (19  20 nm2). (b) Fast Fourier transform (FFT) of the split image. In this case, both lattices, adsorbate and substrate, are hexagonal. Such a split image allows the utilization of the graphite substrate whose lattice parameters are well known as an intrinsic ruler. In this way, the influence of thermal drift, the most prominent error source for adsorbate lattice parameters, can be minimized. Furthermore, the epitaxial relationship between the adsorbate and substrate lattice can be directly measured, and a superstructure matrix can be derived. An FFT of such a split image as depicted in Figure 2b allows one to relate the adsorbate lattice vectors to the substrate lattice vectors and to deduce the superstructure matrix. To improve the accuracy, a splitting ratio where equal numbers of adsorbate and substrate unit cells are imaged is advisible (i.e., the part of the image that depicts the adsorbate layer should be larger by a factor given by the ratio of adsorbate to substrate lattice spacing). Otherwise, peak broadening in the FFT due to a small number of adsorbate periods impairs the precision of the method. In general, the method works best when the adsorbate unit cell is not too large (i.e., length of lattice vectors 3 yields the more densely packed polymorph.15 Another possibly determinant parameter is adsorption rate, which is largely defined by the solute concentration and solvent viscosity. In conclusion, the reason for solvent-induced polymorphism in monolayers of aromatic tricarboxylic acid has not unambiguously been identified, and further experiments, ideally complemented by theoretical considerations, are necessary. Most importantly, before further models are discussed and evaluated, thoughtfully designed experiments should verify whether solvent-induced polymorphism is of kinetic or thermodynamic origin. Likewise, for the even larger tricarboxylic acid TCPEB, solvent-induced polymorphism has also been observed.46 However, in this case all fatty acid solvents yield a similar row structure, where molecules stand almost upright on the surface and are stacked along an axis parallel to the substrate. Figure 6a depicts an STM topograph of the row structure with an overlaid model. In contrast, aromatic solvents without functional groups for intermolecular hydrogen bonds, for instance, 1,2,4-trichlorobenzene, facilitate the self-assembly of a hexagonal chickenwire structure. The emergence of the counterintuitive row structure is explained by the preaggregation of solute molecules into small aggregates in solution, driven by π-π stacking of the rather large aromatic system. These preformed stacks adsorb on the (46) Gutzler, G.; Sophie, L.; Mahata, K.; Schmittel, M.; Heckl, W. M.; Lackinger, M. Chem. Commun. 2009, 680.

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surface with their axes parallel and inevitably seed the row structure. In protic solvents (i.e., solvents with synthons for hydrogen bonds such as fatty acids), the solute-solvent interaction is mainly through hydrogen bonds between carboxylic groups and is also feasible when solute molecules remain π-π stacked. On the contrary, in aromatic aprotic solvents, solute-solvent interaction is primarily of the aromatic type and is maximized by breaking up the stacks into single monomers. Those adsorb preferentially planar, and self-assembly results in the hexagonal chickenwire structure, where the contributions of intermolecular hydrogen bonds are maximized. This type of solvent-induced polymorphism is different from that in the TMA and BTB cases where different fatty acids result in different polymorphs. Here, the capability of the solvent to form intermolecular hydrogen bonds determines the degree of aggregation of solute molecules, which steers interfacial self-assembly. It is postulated that the row structure is a consequence of the preaggregation of solute molecules in solution. On the basis of this hypothesis, the inhibition of π-π stacking should also disfavor the row structure and lead to the chickenwire structure. This is realized in the compound TCPETMB (cf., Figure 3d, R = CH3) by imposing sterical hindrance through comparatively bulky methyl groups substituted on the inner benzene ring. Again, inhibited stacking results in both monomeric dissolution and planar adsorption and consequently in the self-assembly of the chickenwire polymorph. This difference in liquid-phase aggregation between substituted and unsubstituted solute molecules is further supported by concentration-dependent UV-vis spectroscopy.46

Nanoporous Networks as Supramolecular Host Systems. Supramolecular host systems as described above are ideally suited to study the interaction between guests and the host network,14,47 the influence of guests on network formation,20,48,49 selective adsorption,50 or even dynamic processes as guest diffusion.28 Under UHV conditions, molecular guests can simply be introduced by vacuum codeposition,14,51 whereas at the liquid-solid (47) Staniec, P. A.; Perdigao, L. M. A.; Saywell, A.; Champness, N. R.; Beton, P. H. ChemPhysChem 2007, 8, 2177. (48) Furukawa, S.; Tahara, K.; De Schryver, F. C.; Van der Auweraer, M.; Tobe, Y.; De Feyter, S. Angew. Chem., Int. Ed. 2007, 46, 2831. (49) Lei, S.; Surin, M.; Tahara, K.; Adisoejoso, J.; Lazzaroni, R.; Tobe, Y.; De Feyter, S. Nano Lett. 2008, 8, 2541. (50) Li, M.; Deng, K.; Lei, S. B.; Yang, Y. L.; Wang, T. S.; Shen, Y. T.; Wang, C. R.; Zeng, Q. D.; Wang, C. Angew. Chem., Int. Ed. 2008, 47, 6717. (51) Theobald, J. A.; Oxtoby, N. S.; Phillips, M. A.; Champness, N. R.; Beton, P. H. Nature 2003, 424, 1029.

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Lackinger and Heckl

Invited Feature Article

Figure 7. Adsorption of molecular guests within the supramolecular TMA network: (a) C60 buckminster fullerenes and (b) coronene (C24H12). The inset depicts the molecular structure. Preparation is done by the initial deposition of TMA-saturated nonanoic acid solution and the subsequent addition of a droplet containing the molecular guest, likewise dissolved in nonanoic acid. For coronene, all cavities are fully occupied, whereas for C60 only single scattered cavities are occupied. C60 appears as a circular protrusion without any internal structure resolved, whereas coronene shows submolecular contrast that is continuously varying from hexagonal to almost rotational symmetry. (a) Tip-induced lateral manipulation of a single C60 molecule from one cavity of the TMA host network to the adjacent cavity. Reprinted and adapted with permission from refs 52 and 53. Copyright 2004 American Chemical Society. interface the addition of a solution containing the guest species can similarly result in coadsorption within the cavities of the host network.52,53 Normally, the occupancy can be controlled by the number of guests deposited or dissolved.28 Because of their size and shape, coronene, a planar hydrocarbon, and C60 buckminster fullerenes are particularly well suited guests for the ∼1.0-nm-wide cavities of the TMA networks.41,52,53 Fullerenes are a principally interesting guest species, not least because of their variability in structure and electronic properties. For instance, selective adsorption of C80 has been shown in a supramolecular host system that exhibits two differently sized and shaped cavities.50 After adding C60/fatty acid solution to a preassembled TMA chickenwire network, the adsorption of a few statistically distributed fullerenes occurs within the cavities of the host network. The relatively low number of occupied cavities (