Self-Assembled Nanostructures of Oligopyridine Molecules - The

14698

J. Phys. Chem. B 2008, 112, 14698–14717

FEATURE ARTICLE Self-Assembled Nanostructures of Oligopyridine Molecules Ulrich Ziener* Institute of Organic Chemistry III, UniVersity of Ulm, Albert-Einstein-Allee 11, D-89081 Ulm, Germany ReceiVed: July 3, 2008

The high potential of self-assembly processes of molecular building blocks is reflected in the vast variety of different functional nanostructures reported in the literature. The constituting units must fulfill several requirements like synthetic accessibility, presence of functional groups for appropriate intermolecular interactions andsdepending on the type of self-assembly processssignificant chemical and thermal stability. It is shown that oligopyridines are versatile building blocks for two- and three-dimensional (2D and 3D) self-assembly. They can be employed for building up different architectures like gridlike metal complexes in solution. By the appropriate tailoring of the heterocycles, further metal coordinating and/or hydrogen bonding capabilities to the heteroaromatic molecules can be added. Thus, the above-mentioned architectures can be extended in one-step processes to larger entities, or in a hierarchical fashion to infinite assemblies in the solid state, respectively. Besides the organizational properties of small molecules in solution, 2D assemblies on surfaces offer certain advantages over 3D arrays. By precise tailoring of the molecular structures, the intermolecular interactions can be fine-tuned expressed by a large variety of resulting 2D patterns. Oligopyridines prove to be ideal candidates for 2D assemblies on graphite and metal sufaces, respectively, expressing highly ordered structures. A slight structural variation in the periphery of the molecules leads to strongly changed 2D packing motifs based on weak hydrogen bonding interactions. Such 2D assemblies can be exploited for building up host-guest networks which are attractive candidates for manipulation experiments on the single-molecule level. Thus, “erasing” and “writing” processes by the scanning tunneling microscopy (STM) tip at the liquid/solid interface are shown. The 2D networks are also employed for performing coordination chemistry experiments at surfaces. 1. Introduction: Molecular Self-Assembly of Programmed Functional Units Nanotechnology has been growing dramatically in the recent past and is regarded as one of the major technologies for the near future. Two fundamental principles for the generation of nanoscale structures are distinguished on the basis of the direction of size development of the implemented materials. Starting from bulk material and subsequent more and more fine structuring, by, e.g., lithographic methods, is called the topdown approach. The reverse process by assembling small particles like atoms or molecules to ordered (nano)structures is regarded as the bottom-up approach (Figure 1). The formation of nanostructures by the bottom-up approach is often connected with self-processes of the constituting units. These processessself-assembly and self-organizationswere mainly studied at first in biology and physics but have become meanwhile powerful tools in chemistry and materials science, as well. The term “self” in this context requires a mutual recognition of the basic units stored as information in the (molecular) structure which associates spontaneously to the ordered array. The programming of the molecular units is accessible by tailoring their chemical structures. Thus, intermolecular van der Waals, Coulomb, π-π, hydrophobic, coordinative (metal-ligand), or hydrogen bonding interactions are * E-mail: [email protected].

Figure 1. Two approaches to nanostructures: top-down (left) and bottom-up (right).

widely employed attractive forces to drive molecules into ordered assemblies by self-processes. An efficient way to selfassemblies often requires reversibility of the intermolecular forces so as to allow finding the final structure. The chemistry of assemblies beyond molecules held together by such noncovalent interactions is called “supramolecular chemistry”, a term ¨ bermoleku¨le”)1 and strongly introduced already 70 years ago (“U 2-4 diffused in the past 20 years. In the meantime, more than 2000 papers a year are published dealing with the concept of supramolecules and supramolecular chemistry. There is a vast variety of molecular building blocks which are appropriate for self-assembly processes in two or three dimensions. In order to obtain functional assemblies, the function might (i) emerge from the intermolecular interactions of the brickstones within the supramolecules, (ii) be already inherently present in the constituting molecular units, or (iii) both be effective. This makes not only structural demands on the

10.1021/jp805846d CCC: $40.75  2008 American Chemical Society Published on Web 11/06/2008

Feature Article

Ulrich Ziener received his doctoral degree from the University of Tuebingen, Germany, in 1995. His work focused on synthesis and physical characterization of metal phthalocyanines as organic conductors. During a postdoctoral stay at the Max-Planck Institute for Polymer Research in Mainz, Germany, he continued organic materials synthesis in the field of phenylene ethynylenes. In a subsequent stay at the Universite´ Louis Pasteur, Strasbourg, France, his interests layed in the self-assembly behavior of ligands for the buildup of metal organic gridlike architectures. After a further postdoctoral stay at the Institute for New Materials, Saarbru¨cken, Germany, in 2001, he received a permanent scientific position at the University of Ulm, Germany. His interests lie in the fields of supramolecular chemistry and self-assembly of small and macromolecular organic systems like oligopyridines and oligothiophenes.

Figure 2. Basic oligopyridine motif employed for two- and threedimensional self-assembly processes.

Figure 3. Principal assembly of [2 × 2] (left) and [3 × 3] (right) grid complexes.

employed (functional) molecules but poses also questions regarding synthetic availability and variability, chemical and eventually thermal stability, etc., of the molecules used. In the following, it will be shown that oligopyridines mainly based on the structural motif given in Figure 2 are highly versatile compounds which fulfill all of the mentioned requirements. They can be synthesized in a straightforward manner and thus can easily be varied structurally, exhibit high chemical and thermal stability, act as metal binding ligands and as hydrogen bonding donor and acceptor molecules, and pack very well in twodimensional assemblies on surfaces based on the molecular planarity and C2V symmetry. 2. Three-Dimensional Self-Assemblies of Oligopyridine Molecules: Gridlike Complexes 2.1. Single Gridlike Complexes and Their Three-Dimensional Extension. Among the above-mentioned intermolecular driving forces toward self-assembled supramolecular nanostructures, hydrogen and coordinative bonds are the most favorable ones because of their (i) directionality, (ii) absolute value which is high

J. Phys. Chem. B, Vol. 112, No. 47, 2008 14699

Figure 4. Formation of [2 × 2] gridlike complexes with (a) bis(bipyridine)-like23 and (b) bis(terpyridine)-like34 ligand systems.

Figure 5. Examples of ligands for the formation of [2 × 2] gridlike complexes with different binding motifs based on (a, b) terpyridine, (c) bipyridine, or (d, e) a combination of bis- and terdentate binding motifs.

enough for efficient binding in the assembly andsespecially for hydrogen bondsswell below the strength of covalent bonds (1-50 kJ · mol-1)5 in order to facilitate effectively the buildup of the self-assembly due to reversibility (see above), and (iii) flexibility in strength and geometry. The synthetic tools in chemistry give access to the design of variable and sophisticated architectures by tailoring the organic ligands or hydrogen bond donors and acceptors, respectively, and thus adjusting the intermolecular interactions. In the past 20 years, synthetic chemists have put much effort in the development of such architectures6-9 based on discrete coordination compounds ranging from “simple” polygons8,10-13 like squares or rectangles to more intricate structures like ladders, racks,14-16 cages,17,18 boxes,19 helices,15,20,21 or grids.22-32 The grid-type complexes consist of at least four orthogonally arranged ligand molecules complexing four ([2 × 2] grid) or more metal ions (Figure 3). In order to bind the central metal ions, the ligands have to be equipped with appropriate multidentate binding sites depending on the preferred coordination geometry of the ions. Predestinated coordinating moieties for a tetrahedral coordination sphere are orthogonally arranged 2,2′-bipyridine (sub)units and for an octahedral coordination 2,2′:6′,2′′-terpyridine (sub)units. Thus, the first gridlike complex was reported in 1992, built up from a bispyridyl-pyridazyl ligand with Cu(I) ions in a tetrahedral surrounding (Figure 4a).23 Five years later, a corresponding complex was described with Co(II) centers in

14700 J. Phys. Chem. B, Vol. 112, No. 47, 2008

Ziener

Figure 6. (a) Infinite one-dimensional stacks and (b) two-dimensional arrays of gridlike complexes by additional metal coordination or hydrogen bonding.

Figure 7. Synthesis of bis(dimethylpyrimidyl)tetrazine 13 as ligand for tetrahedral coordination.

Figure 8. Synthesis of the pyrazine containing mono- and bisterpyridine-like ligands 16 and 19a-c.

Figure 9. Synthesis of ligands 24a and b via Kro¨hnke’s method.

octahedral coordination and a bis(terpyridine)-like ligand containing pyrimidine units (Figures 2 and 4b).24,33,34 Basically, both principles are maintained in most other gridlike complexes reported later on.35,36 The coordinating nitrogen donor moieties were varied by pyrazol(at)e,29,37,38 naphthyridine,26 or acyclic functional groups like Schiff bases,22,28,39,40 hydrazones,15,41,42 amidines,25,27,30-32,43,44 and carboxamidates25,27,30,32,43-45 (Figure 5).

In order to use the highly ordered arrangement of the metal ions in the grids as addressable units in nanotechnology, the grids have to be enlarged or further two- and/or threedimensional assembly has to follow (Figure 6). Big effort has been undertaken for extending the [2 × 2] to the corresponding [3 × 3],44,46-48 [4 × 4],25,45,48,49 and even [5 × 5] or [4 × 5],50 respectively, gridlike complexes. The drawback of poor solubility in the synthesis of the corresponding large ligands and the

Feature Article


Figure 10. (a) Schematic representation of the one-dimensional stack of gridlike complexes 31b interconnected by La3+ ions; (b) X-ray singlecrystal structure of ([-FeII4(24a)4]-AgI4)n12+ (26a) displaying the wall-like 2D interconnection of the [-FeII4(24a)4]8+ [2 × 2] grid-type building modules by the Ag(I) ions. Reprinted from ref 64 with permission. Copyright 2005 Wiley-VCH Verlag GmbH & Co. KGaA.

Figure 11. Synthesis of halogen substituted [2 × 2] gridlike complexes.

Figure 12. Bis(bipyridyl)-pyrimidine ligands with varying substituents investigated by magnetic measurements.

Figure 13. Ligands for [2 × 2] gridlike complexes furnished with complementary hydrogen bonding functionalities.

structural constraints in coordinating the metal ions by the ligands due to the relative rigidity of the aromatic backbone hampered further extension of this concept. Alternatively, secondary interactions between the complexes like hydrogen bonding or coordination of additional metal ions in a sequential hierarchical self-assembly process are highly desirable. Two- or three-dimensional polymeric (metal-organic) networks have been in the focus of coordination chemistry crystal engineering research for several years. Most often, these structures are prepared in a one-step synthesis from metal ions and tailor-made ligands and thus can hardly be controlled.51-54 To gain better control over the network formation process, we developed new ligands equipped with additional coordination

sites to separate the step for the formation of single mono- or tetranuclear complexes from the “polymerization”. Approaches to the network structures via the formal extension of the bis(bipyridyl) ligand system to pyrimidyltetrazines55 (Figure 7) or substitution of the middle pyridine rings in the bis(terpyridine)-like ligands by pyrazine rings (Figure 8), respectively, failed because of the low complexation strength of those ligands. A larger distance of the binding sites for both hierarchically controlled coordination processes shall support a stronger metal-ligand interaction for the second complexation step. Consequently, we have designed a new bis(terpyridine)-like ligand 24 based on the parent system 2 with extra-pyridine moieties (Figure 9). Standard procedures for the synthesis of oligopyridines are based either on metal catalyzed crosscoupling56,57 or on ring-closure reactions.58-60 Attempts to synthesize oligopyridine 24 by well established protocols for cross-coupling reactions of heterocyclic systems, especially Stille type, failed completely, arising from the poor solubility of the required intermediate compounds. We could circumvent the problems by employing Kro¨hnke’s strategy61 by a ring-closure reaction of the pyridinium activated bisacetylated pyrimidine 22 with the literature-known62 unsaturated ketone 23a, resulting in 24a in almost quantitative yield.63 The corresponding derivative 24b with the terminal pyridine moieties in the meta position could be afforded via the same strategy. The reaction of both ligands 24a and b with Zn(BF4)2 · 6H2O or Zn(OTf)2 and Co(BF4)2 · 6H20, respectively, led to the formation of the [2 × 2] gridlike complexes which showed a much better stability in solution than with the pyrazine derived ligands 19.63 The existence of the tetranuclear compounds could be proven by mass spectrometry and 1H NMR spectroscopy. It is striking that for the Zn complexes the phenyl protons become inequivalent showing up as five new signals and undergo a dramatic high-field shift. These effects are caused by the freezing of the phenyl rings between two neighboring parallel ligands which thus experience a strong shielding effect of the ring current of the parallel ligands. Even more pronounced is the effect of complexation in the case of the Co complexes. The 1H NMR spectrum exhibits 14 singlets covering a range of almost 350 ppm, which is related to the paramagnetism of Co(II).63 The magnetism causes such a strong line broadening that no proton-proton coupling can be detected, but the number of the signals reflects the maintenance of the C2V symmetry of the free ligand in the coordinated state. We also obtained corresponding tetranuclear iron complexes by the reaction of [Fe(BF4)2] · 6H2O or [Fe(ClO4)2] · 6H2O with 24a or 24b, respectively.64 These complexes can undergo a


Ziener

Figure 14. Synthesis of the terpyridine-like ligand 35 and the structure of pyrazine containing 37 and the therefrom derived complexes 36 and 38 for two-dimensional hydrogen bonding.

second self-assembly process with appropriate metal ions coordinating to the outer pyridine moieties. Thus, further complexation of [FeII4(24b)4](ClO4)8 (25b) with La(ClO4)3 led to (-[FeII4(24b)4]-LaIII)n(ClO4)(11)n (26b) as a one-dimensional infinite stack of gridlike complexes interconnected by 8-fold coordinated lanthanum ions (Figure 10a). On the contrary, the corresponding reaction of [FeII4(24a)4](BF4)8 (25a) with Ag(I) ions did not result in a one-dimensional structure but singlecrystal analysis revealed the formation of a 2D wall-like morphology of (-[FeII4(24a)4]-AgI4)n(BF4)(12)n (26a) (Figure 10b). These differences can be understood by a closer look at the precise solid-state geometry of the gridlike complex [FeII4(24a)4](BF4)8 (25a). In this case the outer pyridine units are bent outward. Further complexation by Ag(I) ions leads to an almost perfect linear coordination of the metal ions by two pyridine units which can be achieved only if neighboring gridlike complexes are displaced, resulting in the wall-like fashion built up by molecular brickstones. The presence of the Fe ions is interesting especially with respect to magnetic properties. It could be shown for similar tetranuclear complexes that there is a temperature-, pressure-, and light-dependent spin-transition behavior of the iron ions.65 Correspondingly, 25b shows at room temperature a value of χMT close to the spin-only value expected for four high-spin (HS) Fe(II) ions. Lowering the temperature to 30 K causes a transition to two to three low-spin (LS) Fe(II) ions per molecule, whereas in the case of 25a at room temperature only three HS Fe(II) ions are present and at low temperature even only about two. In the one-dimensional stack of 26b, a similar behavior as for the nonaligned metal grid complexes 25b is found but with an incomplete spin transition at room temperature. Interestingly, in the 2D wall-like structure of 26a, the spin transition is inhibited over the whole temperature range studied, possibly due to steric hindrance in the interconnected 2D network. Since the conversion of Fe(II)(LS) to Fe(II)(HS) is accompanied by a volume increase, each spin transition would have to expand against thesmore rigids2D network of 26a. Only minor coupling is found between the [2 × 2] gridlike units.64 It could be demonstrated that Co(II)-[2 × 2] grids based on bis(bipyridyl)-pyrimidine ligands exhibit significant intramo-

lecular antiferromagnetic interactions, while intermolecular magnetic interactions are negligibly small.66 The antiferromagnetic coupling constant within an entity of four Co(II) ions in the gridlike complexes depends on the substituent R1 on the ligand (see Figure 12) and can be tuned by a controlled chemical variation of the ligands. We prepared a homologous series of halogen substituted ligands with R1 ) Cl, Br, and I, respectively, but only from the chlorinated and brominated ligands 29a and 29b, stable Co complexes could be obtained (Figure 11).67 Finally, magnetic measurements were undertaken from the brominated complex 29b and two other related complexes (Figure 12). It could be revealed by magnetic measurements on single crystals of 32a that the magnetic properties of the gridlike complexes resemble metamagnetic-like behavior; i.e., they can be described as single-molecule metamagnets.67 In the above series (Figures 11 and 12), both the maximum in the susceptibility and the field position of the magnetization step increase in the order 32a, 30b, and 32b which is indicative of an increase in the coupling constant in this series due to the different substituents R1. 2.2. Two-Dimensional Infinite Self-Assemblies of Gridlike Complexes in Bulk. Besides hierarchical self-assembly processes toward one-dimensional columnar stacks or wall-like structures with the grid complexes organized perpendicular to the plane spanned by the metal ions, 2D (infinite) arrays of these functional units are of great interest. To ensure stepwise hierarchical self-assembly, the known oligopyridine systems were equipped with hydrogen donor and acceptor functions. Janiak et al. have described already 5,5′diamino-2,2′-bipyridine metal complexes which assemble into multidimensional networks via hydrogen bonding and π-π stacking interactions.68 A low control over the solid-state structure of these complexes is attributed to the fact that the amino group might bear a double function as a hydrogen donor and acceptor as well as a complexing unit. Thus, complementary hydrogen bonding moieties, still based on amino groups (Figure 13), shall be more specific for hydrogen bonding and would even allow the formation of chess-board-type patterns with different metal complexes (see Figure 6b).

Feature Article


Figure 16. Synthesis of aminopyrazine containing ligand 44 and the corresponding gridlike metal complexes.

Figure 15. Single-crystal X-ray structures of complexes 36 · (PF6)8 (top), 36 · (BF4)8 (middle), and 38 · (OTf)8 (bottom). Reprinted from ref 69 with permission. Copyright 2000 Wiley-VCH Verlag GmbH & Co. KGaA.

We investigated corresponding terpyridine derived ligands for mononuclear complexes as the lower homologue system (Figure 14).69 The pyrimidine containing ligand 39 was obtained by an acylation reaction of the amino group with hexanoic acid chloride and subsequent Stille-type coupling with 2,6-dibromopyridine. The acyl group was introduced for solubility reasons. Interestingly, the reaction of 39 with cobalt acetate deprotected the amino function completely presumably due to the weakening of the amide bond by the electron withdrawing effect of the nitrogen atom in the para position. Different stabilities and electronic properties of the pyrazine comprising ligand 41 prohibited the usage of the corresponding synthetic strategy as for 39. In accordance with the literature,70,71 an alkyl group was introduced at the pyridine ring, supplying sufficient

solubility for the complexation reaction with zinc triflate, resulting in mononuclear complex 41 (Figure 14).69 The complementarity of the hydrogen bonding functionalities of the aminoheterocycles should allow the formation of 2D alternating patterns of metal complexes 40 and 42, as proposed in Figure 13 for the [2 × 2] gridlike structures. Several attempts to obtain single crystals from a 1:1 mixture of 40 and 42 suitable for X-ray diffraction failed. We assume that the presence and absence, respectively, of the pentyl substituent hampers ordering of the two metal complexes in the solid state. On the other hand, single crystals of sufficient quality could be obtained from each of both metal complexes with varying counterions after ion exchange. Thus, the X-ray structures of 36 · (PF6)8, 36 · (BF4)8, and 38 · (OTf)8 revealed that double hydrogen bonding of the aminoheterocyclic moieties dominates the solid-state ordering of these complexes (Figure 15). Expectedly, Co complex 36 · (PF6)8 forms in the solid state an infinite interwoven network with all hydrogen bonds saturated (Figure 15, top). Changing the counterion to the smaller BF4ion leads to a partial breakup of the hydrogen bonds, resulting in a still two-dimensional network with only partially saturated H-bonds but additional interdigitation of heteroaromatic rings (Figure 15, middle). The latter is often found for terpyridine complexes because of π-π-stacking intermolecular interactions.72 We assume that the larger PF6- ions stabilize the highly symmetric network structure of 36 · (PF6)8, whereas the BF4ions in 36 · (BF4)8 form competing (B)-F-H-(N) bonds, showing a subtle interplay between direct intermolecular forces and crystal packing effects. Switching to the pyrazine containing compound 38 · (OTf)8 finally leads to an array of noninterdigitated one-dimensional columns (Figure 15, bottom). Thus, there is a lower connecting efficiency of the self-complementary hydrogen bonds of 38 · (OTf)8 relative to those of the upper mentioned complexes caused by the different space requirement of the triflate counterion which is nonspherical and bigger than PF6- and BF4-. Additionally, triflate competes as the better hydrogen bonding acceptor more strongly with the N-(H)-N bonds. The lengths of the hydrogen bonds for all complexes lie between 2.92 and 3.21 Å and thus in the typical range for these compounds.73-75 We could perform an extension of the above-described ligands (Figure 14) to the bis(terpyridine)-like system, as shown in Figures 16 and 17.76 Compound 44 was obtained analytically pure by a sequence of Stille-type coupling reactions (Figure 16). In the course of the synthesis of pyrimidine containing


Ziener

Figure 17. Synthesis of aminopyrimidine containing ligand 53 and the corresponding gridlike metal complex.

Figure 18. Single-crystal X-ray structure of compound 54. Reprinted from ref 76 with permission. Copyright 2001 Wiley-VCH Verlag GmbH & Co. KGaA.

ligand 53, the amino group had to be protected to avoid interference during the stannylation reactions. In the final step, the Boc group could easily be removed by treatment with HCl (Figure 17) and both ligands could be transformed to the corresponding gridlike [2 × 2] metal complexes. X-ray structure analysis from single crystals which were obtained from Co complex 54 reveal that similarly to 36 · (BF4)8 and 38 · (OTf)8 (Figure 15) a competition between crystal packing and hydrogen bonding leads only to a partial saturation of the expected double hydrogen bonds between neighboring complexes, resulting in a parallel alignment of one-dimensional infinite columns (Figure 18).76 3. Two-Dimensional Self-Assemblies of Oligopyridine Molecules on Surfaces 3.1. Weak Hydrogen Bonds as Structure Directing Motif. All of the investigations mentioned above were mainly aimed on the self-assembly of ligands and metal ions in the homogeneous (solution) phase. For the elucidation of the molecular structures by X-ray analysis, single crystals of sufficient quality are required, which is often a limiting factor. In contrast, 2D self-assembly processes on solid supports77 offer the opportunity of investigating molecular monolayers by scanning tunneling microscopy (STM). Furthermore, the influence of the substrate not only on the ordering but also on the reactivity of the molecules and single-molecule manipulation are central items

of research in the field of surface chemistry with applications in catalysis, sensing, molecular electronics, etc. For the control over 2D self-assembly, hydrogen bonds are a versatile tool because of their high capability of tailoring the strength of the intermolecular interactions and their directionality, depending on the alignment (see above). Heteroatom X-H · · · Y hydrogen bonding directed self-assembly with a strong hydrogen donor is especially useful due to its variety of donor and acceptor units and the amenability of its components. Apart from biological molecules, like, e.g., nucleobases,78,79 synthetic molecules are utilized for self-assembly on surfaces investigated by STM. Among these compounds are urea derivatives,80 alcohols,81 aromatic thiols,82,83 ureido functionalized molecules,84 2-pyrrolidone,85 imines,86 imides,87,88 carbamates,89,90 hydrazides,91 and carboxylic acids.92-100 Besides those well-known relatively strong heteroatom X-H · · · Y hydrogen bonds, only a few examples of C-H · · · X hydrogen bonds with weaker hydrogen donors as structure directing units in 2D assemblies are described,87,92,93,101-106 although in 3D crystal engineering they are discussed already since the 1980s.107-110 Molecular assemblies of pyridyl-vinylbenzoic acid (PVBA) on Ag(111) and on Cu(100), respectively, are attributed to (weak) C-H · · · O and C-H · · · N hydrogen bonds besides (strong) O-H · · · N interactions.93,103 Terephthalic acid molecules and iron atoms on a Cu(100) surface under ultrahigh vacuum conditions form different metallo-supramolecular assemblies which show partially C-H · · · O hydrogen bonds between ortho-protons of the benzene moieties and the carboxylic groups.92,101 The high potential of weak hydrogen bonding networks in surface structuring was shown with cyanobenzene substituted phorphyrines, providing a C-H hydrogen donor and an sp-hybridized N hydrogen acceptor.102 These chemically functionalized porphyrins form different aggregates on Au(111) stabilized by C-H · · · NC hydrogen bonds but are limited to linear chains and small tri- or tetrameric aggregates. Distinct differences of the morphologies between perylene-3,4,9,10-tetracarboxylic-3,4,9,-10-dianhydride (PTCDA) and the corresponding bisimide PTCDI on the Ag-Si(111)3 × 3R30° surface are attributed to C-H · · · O hydrogen bonds in PTCDA.87 The adsorption of 6-nitrospiropyran molecules on a Au(111) surface leads to the expression of 1D homochiral chains which is attributed to the formation of C-H · · · O weak hydrogen bonds and π-stacking of the two moieties of each molecule.104 2,6-Dimethylpyridine forms temperature-dependent dimeric and larger structures on Cu(110) which are explained

Feature Article


Figure 19. (a) STM image of oligopyridine 24a adsorbed on HOPG from TCB, Ub ) -1 V, It ) 84 pA; (b) magnified molecular model area for 24a physisorbed on HOPG; (c) molecular sketch of the molecular structure of 24a physisorbed on HOPG revealing the hydrogen bonding interactions; (d) calculated molecular aggregations of two coplanar pyridine dimers: A, double bound dimer; B, single bound dimer. Reprinted from ref 111.

Figure 20. Three isomers of oligopyridine 24a which vary only in the position of the nitrogen atoms in the outer pyridine rings A and B by keeping the same overall shape.

by the presence of weak hydrogen bonds between methyl group hydrogen atoms and N moieties on adjacent molecules and

single head-to-tail weak hydrogen bonds between ring C-H bonds and N moieties, respectively.105 Most of the systems


Ziener

Figure 21. (a) STM images of 55 (left, 1,2,4-trichlorobenzene, Ub ) -961 mV, It ) 19.4 pA), 56 (middle, 1,2,4-trichlorobenzene, Ub ) -862 mV, It ) 13.4 pA), and 24b (right, 1,3-dichlorobenzene, Ub ) -470 mV, It ) 19 pA) at the HOPG/liquid interface; (b) model of a magnified area in part a for each compound; (c) molecular sketch revealing the hydrogen bonding interactions. Reprinted from ref 111.

Figure 22. Bis(terpyridine)-like long-chain ligand 57.

involving weak hydrogen bonds as structure directing interactions were investigated under UHV conditions in the absence of solvent. In the following, it is described how we used conceptually weak hydrogen bonds of interacting pyridine moieties for the design of nanostructured surfaces at the solid-liquid interface.63,111 Besides the self-assembly properties of oligopyridine 24a with metal ions in solution (see above), it could be revealed that the heterocyclic molecule forms highly ordered 2D patterns at the HOPG/liquid interface (Figure 19).63 1,2,4-Trichlorobenzene (TCB) as solvent leads to a square pattern with directionally alternating voids with distances a ) b ) 3.1 ( 0.2 nm. The submolecular resolution of the STM image allows the unambiguous assignment of a model to the 2D assembly, as shown in Figure 19b. The unit cell is of square symmetry, containing four coplanar molecules. The arrangement of the molecules is rather surprising, since the coverage is with 0.42 molecules/

nm2 significantly lower than the close-packed structure with 0.46 molecules/nm2 (see below). Hence, there must be an additional driving force for the specific assembly of 24a on HOPG. A closer look to the molecular arrangement reveals that certain C-H moieties come in proximity of N atoms of neighboring molecules (Figure 19c). A rough estimation of the H · · · N distance shows that it lies below the van der Waals distance of 2.74 Å and thus can be assumed attractive, pointing to the presence of C-H · · · N hydrogen bonds. To further support the existence of these H-bonds, theoretical calculations on the interaction forces were performed. We calculated the stabilizing energy and the optimal geometry of pyridine dimers in the gas phase at the MP2/6-31G(d,p) level of theory as a model for the interactions of the more complex oligopyridine 24a. Figure 19d gives an illustration of the observed interactions of the pyridine dimers. The interaction energy was calculated to be 13.4 kJ · mol-1 for the double bound dimer and to be 9.7 kJ · mol-1 for the single bound dimer. These energies are comparable to the experimentally determined energy of heteroatom X-H · · · Y hydrogen bonding interactions. The N · · · H interatomic distance of the double bound dimer was calculated to be 2.53 Å, and the N · · · H interatomic distance of the single bound dimer was calculated to be 2.51 Å. These results are a further support for the assumption that “weak” hydrogen bonds play a crucial role in the 2D arrange-

Feature Article

Figure 23. STM image of 57 on HOPG (Ub ) -362 mV, It ) 24 pA). Reprinted from ref 116 with permission. Copyright 2005 WileyVCH Verlag GmbH & Co. KGaA.

ment of the oligopyridine molecules 24a on HOPG. Hence, we varied the position of the nitrogen atoms in the oligopyridine molecule by maintaining the overall shape (Figure 20) to prove the influence of the H-bonds on the packing of the molecules on the surface. The synthesis of isomer 24b was already presented in Figure 9. The same synthetic strategy was followed for the two other compounds 55 and 56. The investigation of the self-assembly properties of compounds 55, 56, and 24b at the HOPG/liquid interface by STM reveals a wealth of different 2D structures depending on concentration, solvent, and molecular structure of the adsorbate.111 Three representative patterns are shown in Figure 21. Obviously these structures are fundamentally different from the one found for compound 24a (Figure 19a). From a careful analysis of the STM images, models can be derived (Figure 19b) which are governed by subtle hydrogen bonding interactions (Figure 19c). Looking at the constitution of the oligopyridines, a simple concept based on geometric considerations leading to either linear or cyclic arrangements of the molecules can be developed. Such a concept gives access to predictions on 2D crystal engineering.111 A more precise study with varying solvents reveals that some of the oligopyridines show polymorphism still based on weak intermolecular hydrogen bonding interactions but clarifying that the mentioned geometric approach oversimplifies the real situation.111 Thus, predictions on the 2D pattern from the molecular geometry should be supported by theoretical calculations. It shall be mentioned that some of the 2D structures of the oligopyridines show homochiral domains111 although the constituting molecular building blocks are nonchiral and nonprochiral.112 The chirality originates exclusively from the packing pattern governed by intermolecular interactions and can be regarded as supramolecular chirality. The overall monolayer is still racemic, as it is composed of statistically distributed enantiomorphic domains. A future goal would be to create macroscopically chiral monolayers by the use of, e.g., chiral solvents as an external trigger.

J. Phys. Chem. B, Vol. 112, No. 47, 2008 14707 In the ongoing discussion, only molecule-molecule interactions were regarded as structure-determining factors for the selfassembly of the oligopyridines on HOPG. In the following, some light shall be shed on the role of molecule-substrate interactions. Thus, investigations on self-assembly processes of compound 24a on metallic substrates (Ag on Ru(0001), Au(111)) were undertaken and compared with the aforementioned results at the HOPG/liquid interface. Futhermore, the experiments were performed under ultrahigh vacuum conditions (UHV) in order to exclude ill-controlled solvent-molecule interactions and elucidate from the resulting higher resolution more precisely the adlayer pattern with the substrate lattice (epitaxial relation).113 Interestingly, for 24a, we found essentially the main characteristics of the preferred pattern formed from TCB solution on HOPG (Figure 19) on the metal substrates as well. Substrate-adsorbate interactions do not control the structural characteristics of the adlayer, but they determine the registry to the substrate lattice. This results in higher-order commensurate, coincident adlayer structures on all substrates. The more pronounced substrate-adsorbate interactions on the metal surfaces compared to HOPG lead to a compression by 1.4% relative to the fully relaxed adlayer structure. On HOPG, the resulting adlayer lattice constant is closer to the fully relaxed value with a compression of only 0.5%. Furthermore, on the metal substrates, the 24a molecules assume orientations that are very close to those of the ideal configurations based on the minimization of the average lateral displacement of the N atoms with respect to the underlying metal surface lattice, thus underlining the importance of the metal-N interactions for the configuration of the adsorbed oligopyridine molecule 24a. This study supports the hypothesis that structure formation is dominated by intermolecular interactions, specifically by the aforementioned C-H · · · N-type hydrogen bonds and additionally, as shown by calculations, by C-H · · · H-C interactions between adjacent molecules.113 3.2. van der Waals Interactions as a Structure-Determining Factor. Besides the directional intermolecular hydrogen bonding forces governing the self-assembly pattern on HOPG, nondirectional interactions such as van der Waals forces and hydrophobic interactions may by employed as additional subtle forces to control molecule-molecule and molecule-substrate (HOPG) interactions. It is known that adsorption of long-chain alkanes and related molecules on HOPG substrates often leads to very well ordered layers that can be readily imaged at high resolution in the STM investigation.114 This can be attributed to surprisingly high values of van der Waals adsorption and two-dimensional crystallization energies of 10.4-12.1 and 4.6-6.4 kJ · mol-1 per CH2 group in alkanes, respectively.115 To take advantage of these substantial interaction energies, we attached long alkyl chains to a bis(terpyridine)-like ligand, resulting in compound 57 with CH2OC16H33 substituents in the periphery (Figure 22).116 Self-assembly of 57 on HOPG results in a highly ordered structure; the representative STM image is shown in Figure 23. Brighter and darker lamellae alternate within the image, with the bright color representing higher electron density. The periodicity of the lamellae is 55 Å. The brighter moiety exhibits a width of 25 Å and the darker a width of 30 Å. The bright stripe shows a structured pattern with numerous small bright spots and is attributed to the aromatic rings. The darker stripe presents an additional pattern of linear arrays of spots, which form an angle of 50° with the ribbons. These linear arrays are assigned to the alkyl chains in 57. The image can be interpreted as a flat arrangement of the aromatic rings on the surface which


Ziener

Figure 24. Proposed model for the conformation and ordering of 57 on HOPG. Reprinted from ref 116 with permission. Copyright 2005 WileyVCH Verlag GmbH & Co. KGaA.

Figure 25. Cyclic current-potential curves for bare ( · · · · ) and oligopyridine 24a covered (s) Au(111) in 0.1 M H2SO4. Scan rate: 5 mV s-1. Reprinted from ref 130.

is expected for optimal contact between the monolayer and the basal plane of graphite. The alkyl chains in 57 are ordered commensurately with the graphite lattice.116 A plausible model for the arrangement of 57 on the surface is shown in Figure 24. In this model, the bright stripes are formed by the aromatic part of two rows of molecules of 57. The alkyl chains form an angle of 50° with this stripe. From geometrical estimations, the expected widths of the ribbons fit well with the experimental values. The tilt angle of the alkyl chains with the main axis of the lamellae allows the optimal contact of about 4.5 Å between neighboring chains. The comparison of the length of the alkyl chains in 57, and the observed width of the darker stripes in the image suggests a noninterdigitated assembly of the alkyl chains.116 Although weak intramolecular C-H · · · N interactions between pyridine moieties of neighboring molecules may stabilize the structure,63,111 the pattern is dominated by the 2D crystallization of the alkyl chains which is understandable against the background of the theoretical values for pyridine dimers (13.4 and 9.7 kJ · mol-1, respectively, for the double and single bound dimer; see above)111 and the van der Waals energies which are expected in the range of 80 kJ · mol-1 for a C16H33 chain.115 As the van der Waals forces are nondirectional, they are less versatile in controlling 2D assemblies compared to the above-described weak hydrogen bonding interactions.

3.3. Electrochemical Control of Adlayer Morphology. A further possibility to steer the morphology of self-assembled monolayers (SAMs) on (metal) surfaces is given by an external electrochemical control. Organothiols and dithiols are among the best investigated compounds, which are employed for selfassembly processes on metals because of the high affinity of sulfur to metal surfaces.117-122 Only little research has focused on SAMs of non-sulfur-containing organic molecules in an electrochemical environment. Such monolayers were, e.g., Schiff bases,123 uracil,124,125 cytosine,126 or pyridineand2,2′-bipyridine.127,128 The Schiff base molecules adsorb on Au(111) with a flat-lying orientation and form an adlayer with well-defined order.123 Very recently, the adsorption of charged organic guest molecules (pyrene tetrasulfonic acid and tetrapyridylporphine) into a lamellar template structure of an alkoxy isophthalic acid on Au(111) was controlled by the electrochemical potential.129 2,2′Bipyridine offers a conformatively higher flexibility than the monocyclic nucleobases. In solution, the transoid conformation N-C-C-N is preferred, whereas, on Au(111) at positive charge densities, it forms an ordered monolayer of molecular chains, determined by the coordination of the lone electron pairs of both ring nitrogen atoms with the substrate surface and lateral π-stacking interactions.127,128 At negative charge densities under acidic conditions, 2,2′-bipyridine is monoprotonated and adopts the transoid conformation with the protonated nitrogen atom pointing to the gold surface and the other to the solution.128 Besides the self-assembling properties of oligopyridine 24a on HOPG,63,111 we were interested how its ordering on metal surfaces is influenced by the electrochemical potential.130 An Au(111) surface modified with 24a was investigated by cyclic voltammetry in H2SO4, as shown in Figure 25. The two broad peaks at around +0.5 and +0.6 V vs SCE, dominating the curve, signal a structure transition within the SAM in the sulfate electrolyte. In order to prove a structural change of the adlayer, STM images at selected potentials were recorded (see exemplarily Figure 26a). Here, the individual molecules are seen as bright dots, resembling a close-packed structure, albeit without much long-range order. The average intermolecular distance amounts to 2.0 ( 0.2 nm, which is in good agreement with 24a nextneighbor distances on HOPG in 1,2,4-trichlorobenzene (see Figure 19) and in 1-phenyloctane.111 Stepping the electrode potential between +0.25 and +0.45 V vs SCE while scanning the surface shows an almost instantaneous and fully reversible

Feature Article


Figure 26. (a) STM image of 24a covered Au(111) in 0.1 M H2SO4 at E ) +0.55 V vs SCE. (b) STM image of 24a covered Au(111) in 0.1 M H2SO4. The electrode potential was stepped from +0.25 to +0.45 V vs SCE and back (see arrows). Reprinted from ref 130.

Figure 27. Molecular brickstones for the buildup of two-dimensional porous networks (a) with strong intermolecular hydrogen bonding or (b) van der Waals forces.

structure change (Figure 26b). A characteristic feature of the structure at +0.25 V compared to +0.45 V vs SCE is an apparently higher mobility of the adsorbed molecules, expressed in the more diffuse contrast in the +0.25 V image parts in Figure 26b. The origin of the increase in mobility refers to a decrease in molecule-substrate interactions with a more negative electrode potential.130 The state of the SAM at positive potentials as a densely packed adlayer seems reasonable, but there is no clear information about the adlayer at potentials negative of +0.4 V vs SCE. Although apparently invisible for the STM, the oligopyridine molecules must be on the surface, as they cannot escape into the aqueous solution. It is suggested (i) that they are too mobile to be imaged by STM due to a less pronounced adherence of the positively charged (protonated) molecules on the SO42adlayer or (ii) that they cluster upright in islands which are found only occasionally by STM.

The potential-driven structure change in the oligopyridine 24a SAM on Au(111) in 0.1 M H2SO4, which is between flat-lying molecules spread over the whole surface and upright standing molecules clustered together, may have interesting consequences for a potential-controlled and reversibly switchable template structure.130 3.4. Template Function and Manipulation of TwoDimensional Self-Assemblies. 3.4.1. 2D Host-Guest Systems and Single-Molecule Manipulation. One main purpose for the construction and control of the 2D patterns described in the previous paragraphs is the use of these monolayers as functional templates for small guests (atoms, clusters, molecules). It is especially intriguing to incorporate single molecules in 2D networks and thus to immobilize them for further investigations.131 Two-dimensional porous networks were built from brickstones capable of hydrogen bonding or van der Waals interactions mediated by long alkyl substituents. Thus, carboxylic acids like trimesic acid (TMA)99 and size-increased derivatives with linkers132-134 were employed for this purpose as well as imides,135,136 imines,86,137 and alkylated or alkoxylated derivatives of an annulene,138 a porphyrin,139 a tristyrylpyridine,140 and a phthalocyanine (Figure 27).141 If TMA 58 is adsorbed onto graphite under UHV conditions, hexagonal porous networks are formed which bear partially single TMA molecules in the cavities of the network.99 The voids of a honeycomb network of 1,3,5-benzenetribenzoic acid (60)swhich is formally TMA extended by three phenylene unitssformed on Ag(111) at room temperature have a suitable size for the uptake of single molecules of a macrocyclic terpyridine and phenantroline containing compound. After annealing, the network is transformed to a close-packed phase and the guest molecules are released.132 The coadsorption of 1,3,5-tris(10-carboxydecyloxy)benzene (TCDB) (59)swhich can be considered as TMA extended by three decyloxy chainsswith different metallophthalocyanines133 or coronene,134 respectively, on HOPG from solution leads to the formation of a host-guest network with cavities of TCDB dimers containing the macrocyclic molecules. The combination of linear perylene tetracarboxylic di-imide (PTCDI) (61) molecules with trifunctional melamine on HOPG yields a hexagonal network with pores large enough to trap compact heptamers of C60 within the pores.135 Corresponding results are found on Au(111) as substrate.136 A hexagonal honeycomb network of 9-diaminoperylene-quinone-3,10-diimine


Ziener

Figure 28. Submolecularly resolved STM picture of the hexagonal network structure of 55 deposited on HOPG from a diluted TCB solution. The inset in the upper right corner shows the orientation of the underlying graphite surface; the contour of one gearwheel-like hexamer composed of six oligopyridine molecules is drawn below. In the upper left corner, one unit cell is shown with a ) b ) 4.40 ( 0.05 nm, enclosing an angle of 60 ( 0.1°. Reprinted from ref 147 with permission. Copyright 2008 Wiley-VCH Verlag GmbH & Co. KGaA.

Figure 29. Calculated molecular surface structure of 55 from diluted solution with the UFF force field. A single gearwheel is highlighted with a black circle The host molecule 55 is contoured black. CuPc in its three energetically equivalent adsorption configurations is highlighted by broken line circles. At the solid/liquid interface, most of the cavities are occupied with coadsorbed oligopyridine molecules. Reprinted from ref 147 with permission. Copyright 2008 Wiley-VCH Verlag GmbH & Co. KGaA.

(DPDI) (62) formed under UHV conditions on Cu(111) is capable of self-trapping single DPDI molecules within the hexagonal cavities.86 The same network can take up zinc octaethylporphyrin (OEP) molecules displaying thermally activated hindered rotation which can be manipulated locally by the STM tip.137 The self-assembly of a cyanophenyl- and ditertbutylphenyl-substituted porphyrin molecule 64 is presumably based on weak CN · · · H-C hydrogen bonds leading on Cu(111) surfaces to the formation of a regular nanoporous network that hosts single porphyrin guests nested on top of the pores by self-

trapping. Switching between different rotational positions of single guest molecules can be triggered by the STM tip.139 The addition of planar π-conjugated guest molecules like coronene to the assembly of alkoxylated annulenes 66 at the HOPG/liquid interface transforms the linear 2D host patterns driven by van der Waals interactions of the long alkyl chains to honeycomb host-guest networks.138 It has to be mentioned that a phase transition is also obtained by variation of the concentration of the annulene.142 An alkoxy substituted 2,4,6-tristyrylpyridine (65) forms a polymer-like assembly at the HOPG/liquid interface. The addition of hexabenzocoronene molecules (HBC) induces the transition to a hexagonal porous network with the HBC molecules trapped inside the cavities.140 Individual phthalocyanine, unsubstituted and substituted porphyrin, as well as calix[8]arene molecules were trapped in 2D networks of octaalkoxy-substituted phthalocyanine 63 at the HOPG/liquid interface by codeposition from solution.141 A chessboard structure of bimolecular ZnPc and zinc(II) octaethylporphyrin molecules (ZnOEP) was formed on Au(111) under electrochemical control with nanocavities serving as traps for C60 molecules.143 Highly periodic nanostripes of sexithiophene 6T on Ag(111) act as a template for the deposition of C60 molecules which arrange along the row direction of the stripes of underlying 6T based on donor-acceptor interactions between 6T and C60. Depending on coverage and annealing temperature, different superstructures can be generated.144 Similarly, a loosely packed 2D structure of pentacene on Ag(111) can act as a trap for C60 molecules, forming a nanomesh. The driving force for the pattern is a subtle interplay between strong molecule-metal and pentacene-C60 donor-acceptor interactions.145 Corannulene (COR) on Cu(110) acts as a trap for C60 molecules. It is concluded that the COR-C60 host-guest binding is mainly due to π-π interaction between the almost perfectly complementary convex and concave faces of C60 and COR, with possibly a further but weaker contribution resulting from CH-π interaction between the rim of COR and C60.146 As shown above, the deposition of oligopyridine 55 from a saturated 1,2,4-trichlorobenzene (TCB) solution (1.5 × 10-3 mol · L-1) onto HOPG leads to a densely packed linear structure, stabilized through weak hydrogen bonds between the terminal

Feature Article


Figure 30. (a) High-resolution STM image of the host-guest network recorded after subsequent addition of CuPc to the network of 55. The host network is imaged with inverse image contrast due to the tunneling conditions used. (b) Host-guest network after a second addition of CuPc solution to the network in part a. The inset (11 nm × 11 nm) shows one occupied cavity and the rhombic unit cell. Reprinted from ref 147 with permission. Copyright 2008 Wiley-VCH Verlag GmbH & Co. KGaA.

Figure 31. Subsequently recorded STM images of the hierarchical network of 55 with CuPc and statistical analysis demonstrating the two manipulation experiments. The arrows indicate the tip position during application of the voltage pulse; the circles indicate the manipulated region. Reprinted from ref 147 with permission. Copyright 2008 WileyVCH Verlag GmbH & Co. KGaA.

pyridine rings (Figure 21, left).111 When we deposit from a diluted solution (3 × 10-5 mol · L-1), the oligopyridine molecules self-assemble into a porous 2D long-range ordered nanostructure with homochiral domains (Figure 28).147 The suggested model of a hexagonal arrangement of the molecules into cyclic hexameric aggregates is further supported by theoretical calculations with the universal force field (UFF)148 (Figure 29).149 The thus generated voids are partially filled with single oligopyridine molecules. These self-trapped molecules are only weakly bound and can be replaced by other guest molecules. The addition of a drop of copper phthalocyanine CuPc in TCB solution (1.7 × 10-5 mol · L-1) to the preformed network of 55 leads to the appearance of several bright spots within the cavities of the network which are attributed to single CuPc molecules (Figure 30a). Doubling the amount of CuPc in the supernatant by adding a further drop of solution approximately doubles the number of filled voids (Figure 30b). This points to an equilibrium between adsorbed and dissolved species. Assuming a Langmuir adsorption isotherm allows us to determine the equilibrium constants for 55 and CuPc on HOPG to Kads ) (55.9

( 2.6) × 104 and (21.2 ( 0.6) × 104 L · mol-1, respectively.150 The corresponding adsorption enthalpies amount to -30.4 ( 0.1 and -32.8 ( 0.1 kJ · mol-1, respectively, both in the expected range of physisorption. A further increase of the surface coverage is restricted by the solubility of CuPc in TCB. From a statistical analysis, the mean resident time of a CuPc molecule in a host cavity can be determined to be 435 ( 20 s. The rather round shape of the bright spots in Figure 30 instead of the expected 4-fold symmetry of the phthalocyanine molecules is attributed to a fast rotation of the molecules in the cavities. From theoretical calculations, it is estimated that the guest molecules change their orientation ca. 2000 times per second at room temperature.147 The low and adjustable occupation of the cavities makes the host-guest network an ideal candidate for single-molecule manipulation. The selective tip controlled desorption of individual CuPc molecules by voltage pulses (+2 V, 10 µs) is selective to the CuPc molecules to which the tip is focused and successful within 76 ( 13% of the manipulation events (Figure 31a).147 We were also able to induce the selective adsorption of single CuPc molecules into network cavities with defined voltage pulses (-2 V, 10 µs) (Figure 31b). Presumably, trapping a CuPc molecule in the dielectric between the tip and the substrate and/or disturbing the equilibrium between immobilized 55 molecules and solvated CuPc molecules are the responsible factors for the selective adsorption processes rather than CuPc molecules which are adsorbed to the tip. It has to be mentioned that this is the first time that single molecules can be “written” on a surface from an almost infinite “ink” reservoir, the supernatant. Comparable single-molecule writing experiments known in the literature were performed by lateral displacement of previously adsorbed molecules or atoms.151-155 Further writing techniques at the liquid/solid interface, e.g., dip-pen nanolithography by AFM156 or STM-based replacement lithography on SAMs,157 usually do not deliver single-molecule resolution. A very recent paper describes the cut-and-paste surface assembly of specifically functionalized DNA oligomers by the AFM technique creating basic geometrical structures.158 3.4.2. Template Function: Incorporation of Metals at the Solid/Gas Interface. The manipulation experiments described above do not change the structure of the host network at all. In further investigations the template function of the oligopyridine 24a was explored by the deposition of Cu atoms on the quasiquadratic network of 24a on HOPG under UHV conditions


Ziener

Figure 32. (a) Large scale image of the coexisting quasi-quadratic (left and bottom side, cf. Figure 19) and hexagonal phases of 24a on HOPG under UHV conditions. Image size 75 nm × 75 nm, Ub ) -2.05 V, It ) 5.62 pA. (b) High-resolution image of the hexagonal phase. Image size 20 nm × 20 nm, Ub ) 2.4 V, It ) 5.62 pA. Reprinted from ref 159 with permission. Copyright 2007 Elsevier.

Figure 33. Gridlike [2 × 2] complex [Co4(28a)4](BF4)8 (67a).

which is corresponding to the pattern formed at the HOPG/ TCB interface (cf. Figure 19).159 After annealing, a rearrangement of the molecules from the quasi-quadratic structure to a hexagonal pattern was found because of the strong interaction between the guest Cu atoms and the host oligopyridine molecules (Figure 32). Comparable experiments were performed by codeposition of more simple building blocks like dicarboxylated stilbene160 or biphenol161 and metal atoms onto Cu(100) or Ag(111) under UHV conditions, resulting in coordination networks by surface-assisted coordination chemistry. Linear chains162 as well as dumbbell shaped coordination supramolecules163 of bipyridyl derivatives complexed with Cu adatoms which diffuse from substrate step edges were obtained directly from the deposition of the organic molecules onto Cu(100). A locally precise deposition of single metal atoms could be achieved by exposing an adlayer of tetraphenylporphyrin molecules on Ag(111) to a beam of cerium atoms under UHV conditions. The metal atoms are selectively adsorbed by the macrocyclic molecules.164 The direct metalation of phthalocyanine H2Pc by Fe atoms on Ag(111) leads to defined FePc complexes.165 The model for the new packing of 24a assumes hexagons of six oligopyridine molecules with C-H · · · N hydrogen bonds interconnected by dimers of Cu atoms (see the inset of Figure 32). The voids in the hexagons are partially filled with selftrapped oligopyridine molecules. 3.5. Gridlike Metal Complexes at the Solid/Liquid Interface. Besides surface-assisted coordination chemistry under UHV conditions, in situ complexation experiments at the liquid/

HOPG interface were shown for alkylated 2,2′-bipyridine derivatives with Pd2+ and Cu+, respectively, provoking a dramatic change of the 2D ordering of the molecules.166,167 An alternative way of generating metal containing hybrid structures on substrates is the formation of desired metal complexes in the first step which is followed by the deposition of these compounds on appropriate surfaces. Although this is an intriguing way of programming the surface directly with a broad variety of different functions, e.g., for catalysis, former investigations concentrated only on a few classes of compounds. The main focus was oriented toward metal containing phthalocyanine and porphyrin complexes like octaalkyl-substituted copper phthalocyanine LB films deposited on graphite,168 CuPc and CoPc on Au(111),169 two-component mixtures of CoPc, CuTPP, CuOEP, CoTPP on Au(111) and Au(100),170 or ZnTPP and ZnPc molecules on Au(111).171 The structures of these monolayers could often be controlled by the electrochemical potential. STM images of molecular monolayers of 15-crown-5-ether substituted cobalt(II) phthalocyanine on Au(111) or Au(100),172 and CoF16Pc, NiTPP CoPc, and mixtures thereof are also described.173 In the case of CuTPP, NiTPP, and CoTPP supported on gold, an easy differentiation between NiTPP and CoTPP, even when mixed monolayer structures are studied, was possible.174 A dipyridyldibenzotetraaza[14]annulene combined with square planar palladium(II) and platinum(II) complexes is known.175 Further 2D arrays of metal complexes investigated by STM comprise iron176 and osmium compounds177 on Cu(111) and Pt(111), respectively. So far, almost exclusively mononuclear complexes were deposited on substrates and there is hardly any SAM of multinuclear metal compounds known. A dinuclear ruthenium complex trans-[Cl(dppe)2Ru(CC)6Ru(dppe)2Cl] was adsorbed on an Au(111) surface; this molecule is a potential candidate for use in molecular quantum-dot cellular automata (QCA) devices.178 A thermotropic liquid-crystalline europium(III) complex of 2-arylimidazo[4,5-f]-1,10-phenanthroline equipped with long alkyl chains shows good affinity for HOPG and could be investigated by STM, revealing a strongly solvent-dependent ordering.179 Tetranuclear gridlike [2 × 2] Co complexes were deposited onto HOPG, forming highly regular patterns with the molecular C2-axis perpendicular or parallel to the surface, respectively, depending on the substituents on the bis(terpyridine) ligand system (see also Figures 3

Feature Article


Figure 34. (a) Fourier filtered STM image of the 2D self-assembly of 67a at the air/HOPG interface (Ub ) -1 V, It ) 8 pA) and (b) top view of a tentative model for the pattern in part a with the C2-axis of the tetranuclear complexes lying parallel to the substrate. Reprinted from ref 63 with permission. Copyright 2002 Wiley-VCH Verlag GmbH & Co. KGaA.

Figure 35. STM images of complex 68 at the TCB/HOPG interface with two different domains: (a) A; (b) B. Reprinted from ref 116 with permission. Copyright 2005 Wiley-VCH Verlag GmbH & Co. KGaA.

and 4).180 Highly regular 2D self-assemblies of such multinuclear complexes display programmed and in principle addressable units which may function, e.g., as memory devices. In order to enhance the regularity by specific adsorbate-substrate interactions, it would be advantageous to equip the aforementioned gridlike complexes with additional binding units like pyridine moieties. Thus, we deposited the gridlike complex [Co4(24a)4](BF4)8 (67a) (Figure 33) from acetonitrile solution onto HOPG and dried the system in air. The STM investigation was performed at the air/HOPG interface (Figure 34a).63 No stable images could be obtained in the presence of any solvent, which we attribute to the fact that there is only a weak binding of the complex molecules to the substrate. According to the results reported in the literature,180 it is assumed that the complexes are “lying” parallel with the C2-axis to the substrate, although the relatively poor resolution does not allow a straightforward model of the packing (Figure 34b). This model is fairly consistent with the

experimental values of periodicity and is further stabilized by π-π-interactions of phenyl rings of neighboring complexes. Apparently, interactions of the outer pyridine units with the substrate do not contribute significantly to the packing but shall gain more influence on metallic substrates which has to be proven.63 In order to stabilize the monolayers of such [2 × 2] gridlike complexes on HOPG, we reacted the long alkyl chain ligand 57 (see Figure 22) with Fe(BF4)2 · 6H2O, resulting in the complex [Fe4(57)4](BF4)8 (68).116 STM investigations at the 1,2,4trichlorobenzene (TCB)/HOPG interface reveal that indeed the stability of the monolayers is strongly increased compared to compound 67a (Figure 35). Two different domains A and B with alternating bright and dark stripes are found exhibiting different morphologies. Models which fit the experimental results nicely are shown in Figure 36 with the complexes “sitting” (C2-axis perpendicular to the substrate) and “lying” on the substrate (C2-axis parallel


Ziener

Figure 36. Tentative models for the self-assembly of 68 at the TCB/HOPG interface for (a) domain A and (b) domain B. Reprinted from ref 116 with permission. Copyright 2005 Wiley-VCH Verlag GmbH & Co. KGaA.

to the substrate), respectively. The fact that the stability of the SAMs of 68 increased significantly compared to compound 67a and that besides the already known side-on arrangement of the complexes (see Figure 34) the one with the C2-axis oriented perpendicularly toward the substrate are found, emphasizes the importance of the van der Waals interactions of the alkyl chains with the graphite substrate for the self-assembly.116 4. Conclusion The self-assembly process as a fundamental principle of bottom-up approaches is based on subtle interactions of tailored molecular brickstones which display a program inherent to the chemical structure. From the pool of chemical intermolecular interactions, mainly coordinative and hydrogen bonds and van der Waals forces can be employed as structure-determining interactions. Bis(terpyridine)-like oligopyridine molecules are easily accessible and are perfect candidates for fulfilling the requirements for self-assembling low-molecular-weight compounds in two- and three-dimensional systems. They form gridlike complexes in solution with octahedrally coordinating metal ions which can be extended to infinite assemblies of grids in a hierarchical process. At liquid/solid and gas/solid interfaces, the presented oligopyridines show also highly ordered arrays. Those 2D patterns can be fine-tuned by a subtle tailoring of the structures of the molecular brickstones. Some of the heterocyclic molecules offer two-dimensional porous network structures which can be further exploited for the buildup of host-guest networks and for single-molecule manipulation experiments by the STM tip. The class of the bis(terpyridine)-like oligopyridines has proven high versatility with respect to accessiblity, variability,

and functionality. We believe that especially the 2D assemblies of these compounds are promising candidates in the field of nanoscience for electronic, optical, and magnetic applications. By the appropriate choice of further components like magnetically interesting systems (e.g., CoPc), fluorophores as guest molecules in the networks, highly conjugated oligomers bound perpendicularly to guest molecules, etc., further functions are added to the assemblies as template systems. The guest molecules are interesting objects by themselves as molecular rotors (ratchets) controlled by tailored interactions with the rim of the network cavities. References and Notes (1) Wolf, K. L.; Frahm, H.; Harms, H. Z. Phys. Chem. B 1937, 36, 237–287. (2) Lehn, J.-M. Supramolecular Chemistry: Concepts and PerspectiVes; VCH: Weinheim, Germany, 1995. (3) Lehn, J.-M. Angew. Chem., Int. Ed. 1990, 29, 1304–1319. (4) Lehn, J.-M. Angew. Chem., Int. Ed. 1988, 27, 89–112. (5) Wasserstoff-Bru¨ckenbindung. In Ro¨mpp Chemie Lexikon, 9th ed.; Falbe, J., Regitz, M., Eds.; Thieme: Stuttgart, Germany, 1995; Vol. 6. (6) Blake, A. J.; Champness, N. R.; Hubberstey, P.; Li, W. S.; Withersby, M. A.; Schro¨der, M. Coord. Chem. ReV. 1999, 183, 117–138. (7) Caulder, D. L.; Raymond, K. N. Acc. Chem. Res. 1999, 32, 975– 982. (8) Stang, P. J.; Olenyuk, B. Acc. Chem. Res. 1997, 30, 502–518. (9) Schmittel, M.; Kalsani, V. Functional, discrete, nanoscale supramolecular assemblies Functional Molecular Nanostructures; Springer: Berlin, 2005; Vol. 245, pp 1-53. (10) Olenyuk, B.; Fechtenko¨tter, A.; Stang, P. J. J. Chem. Soc., Dalton Trans. 1998, 1707–1728. (11) Leininger, S.; Olenyuk, B.; Stang, P. J. Chem. ReV. 2000, 100, 853–908. (12) Wu¨rthner, F.; You, C.-C.; Saha-Mo¨ller, C. R. Chem. Soc. ReV. 2004, 33, 133–146.

Feature Article (13) Derossi, S.; Casanova, M.; Iengo, E.; Zangrando, E.; Stener, M.; Alessio, E. Inorg. Chem. 2007, 46, 11243–11253. (14) Hanan, G. S.; Arana, C. R.; Lehn, J.-M.; Fenske, D. Angew. Chem., Int. Ed. 1995, 34, 1122–1124. (15) Stadler, A. M.; Kyritsakas, N.; Graff, R.; Lehn, J.-M. Chem.sEur. J. 2006, 12, 4503–4522. (16) Sleiman, H.; Baxter, P. N. W.; Lehn, J.-M.; Airola, K.; Rissanen, K. Inorg. Chem. 1997, 36, 4734–4742. (17) Ronson, T. K.; Fisher, J.; Harding, L. P.; Hardie, M. J. Angew. Chem. 2007, 119, 9244–9246. (18) Dalgarno, S. J.; Power, N. P.; Warren, J. E.; Atwood, J. L. Chem. Commun. 2008, 1539–1541. (19) Lee, S. J.; Mulfort, K. L.; Zuo, X.; Goshe, A. J.; Wesson, P. J.; Nguyen, S. T.; Hupp, J. T.; Tiede, D. M. J. Am. Chem. Soc. 2008, 130, 836–838. (20) Stadler, A. M.; Kyritsakas, N.; Vaughan, G.; Lehn, J.-M. Chem.sEur. J. 2007, 13, 59–68. (21) Piguet, C.; Bernardinelli, G.; Hopfgartner, G. Chem. ReV. 1997, 97, 2005–2062. (22) Nitschke, J. R.; Hutin, M.; Bernardinelli, G. Angew. Chem. 2004, 116, 6892–6895. (23) Youinou, M.-T.; Rahmouni, N.; Fischer, J.; Osborn, J. A. Angew. Chem. 1992, 104, 771–773. (24) Ruben, M.; Rojo, J.; Romero-Salguero, F. J.; Uppadine, L. H.; Lehn, J.-M. Angew. Chem., Int. Ed. 2004, 43, 3644–3662. (25) Matthews, C. J.; Onions, S. T.; Morata, G.; Salvia, M. B.; Elsegood, M. R. J.; Price, D. J. Chem. Commun. 2003, 320–321. (26) Plante, J. P.; Jones, P. D.; Powell, D. R.; Glass, T. E. Chem. Commun. 2003, 336–337. (27) Milway, V. A.; Abedin, S. M. T.; Niel, V.; Kelly, T. L.; Dawe, L. N.; Dey, S. K.; Thompson, D. W.; Miller, D. O.; Alam, M. S.; Mu¨ller, P.; Thompson, L. K. Dalton Trans. 2006, 2835–2851. (28) Lan, Y. H.; Kennepohl, D. K.; Moubaraki, B.; Murray, K. S.; Cashion, J. D.; Jameson, G. B.; Brooker, S. Chem.sEur. J. 2003, 9, 3772– 3784. (29) Klingele, J.; Prikhod’ko, A. I.; Leibeling, G.; Demeshko, S.; Dechert, S.; Meyer, F. Dalton Trans. 2007, 2003–2013. (30) Roy, S.; Mandal, T. N.; Barik, A. K.; Pal, S.; Butcher, R. J.; El Fallah, M. S.; Tercero, J.; Kar, S. K. Dalton Trans. 2007, 1229–1234. (31) Cheng, L.; Zhang, W. X.; Ye, B. H.; Lin, J. B.; Chen, X. M. Inorg. Chem. 2007, 46, 1135–1143. (32) Xu, Z. Q.; Thompson, L. K.; Miller, D. O. J. Chem. Soc., Dalton Trans. 2002, 2462–2466. (33) Weissbuch, I.; Baxter, P. N. W.; Cohen, S.; Cohen, H.; Kjaer, K.; Howes, P. B.; Als-Nielsen, J.; Hanan, G. S.; Schubert, U. S.; Lehn, J.-M.; Leiserowitz, L.; Lahav, M. J. Am. Chem. Soc. 1998, 120, 4850– 4860. (34) Hanan, G. S.; Volkmer, D.; Schubert, U. S.; Lehn, J.-M.; Baum, G.; Fenske, D. Angew. Chem., Int. Ed. 1997, 36, 1842–1844. (35) Rojo, J.; Romero-Salguero, F. J.; Lehn, J.-M.; Baum, G.; Fenske, D. Eur. J. Inorg. Chem. 1999, 142, 1–1428. (36) Dawe, L. N.; Abedin, T. S. M.; Thompson, L. K. Dalton Trans. 2008, 1661–1675. (37) Manzano, B. R.; Jalon, F. A.; Ortiz, I. M.; Soriano, M. L.; Gomez de laTorre, F.; Elguero, J.; Maestro, M. A.; Mereiter, K.; Claridge, T. D. W. Inorg. Chem. 2008, 47, 413–428. (38) van der Vlugt, J. I.; Demeshko, S.; Dechert, S.; Meyer, F. Inorg. Chem. 2008, 47, 1576–1585. (39) Nitschke, J. R.; Lehn, J.-M. Proc. Natl. Acad. Sci. U.S.A. 2003, 100, 11970–11974. (40) Price, J. R.; Lan, Y.; Brooker, S. Dalton Trans. 2007, 1807–1820. (41) Ramirez, J.; Stadler, A.-M.; Kyritsakas, N.; Lehn, J.-M. Chem. Commun. 2007, 237–239. (42) Uppadine, L. H.; Gisselbrecht, J. P.; Kyritsakas, N.; Nattinen, K.; Rissanen, K.; Lehn, J.-M. Chem.sEur. J. 2005, 11, 2549–2565. (43) Onions, S. T.; Frankin, A. M.; Horton, P. N.; Hursthouse, M. B.; Matthews, C. J. Chem. Commun. 2003, 2864–2865. (44) Zhao, L.; Xu, Z. Q.; Thompson, L. K.; Miller, D. O. Polyhedron 2001, 20, 1359–1364. (45) Dawe, L. N.; Thompson, L. K. Angew. Chem. 2007, 119, 7584– 7588. (46) Baxter, P. N. W.; Lehn, J.-M.; Fischer, J.; Youinou, M. T. Angew. Chem., Int. Ed. 1994, 33, 2284–2287. (47) Breuning, E.; Hanan, G. S.; Romero-Salguero, F. J.; Garcia, A. M.; Baxter, P. N. W.; Lehn, J.-M.; Wegelius, E.; Rissanen, K.; Nierengarten, H.; van Dorsselaer, A. Chem.sEur. J. 2002, 8, 3458–3466. (48) Garcia, A. M.; Romero-Salguero, F. J.; Bassani, D. M.; Lehn, J.-M.; Baum, G.; Fenske, D. Chem.sEur. J. 1999, 5, 1803–1808. (49) Barboiu, M.; Vaughan, G.; Graff, R.; Lehn, J.-M. J. Am. Chem. Soc. 2003, 125, 10257–10265. (50) Baxter, P. N. W.; Lehn, J.-M.; Baum, G.; Fenske, D. Chem.sEur. J. 2000, 6, 4510–4517. (51) Beatty, A. M. Coord. Chem. ReV. 2003, 246, 131–143.

J. Phys. Chem. B, Vol. 112, No. 47, 2008 14715 (52) Blake, A. J.; Champness, N. R.; Hubberstey, P.; Li, W. S.; Withersby, M. A.; Schroder, M. Coord. Chem. ReV. 1999, 183, 117–138. (53) Hill, R. J.; Long, D. L.; Champness, N. R.; Hubberstey, P.; Schro¨der, M. Acc. Chem. Res. 2005, 38, 335–348. (54) Hosseini, M. W. Coord. Chem. ReV. 2003, 240, 157–166. (55) Kaim, W.; Fees, J. Z. Naturforsch., B: Chem. Sci. 1995, 50, 123– 127. (56) Stille, J. K. Angew. Chem., Int. Ed. 1986, 25, 508–524. (57) Stanforth, S. P. Tetrahedron 1998, 54, 263–303. (58) Adrian, J. C.; Hassib, L.; De Kimpe, N.; Keppens, M. Tetrahedron 1998, 54, 2365–2370. (59) Constable, E. C. The Coordination Chemistry of 2,2′-6′,2′′Terpyridine and Higher Oligopyridines In AdVances in Inorganic Chemistry and Radiochemistry; Emeleus, H. J., Sharpe, A. G., Eds.; Academic Press: New York, 1986; Vol. 30, pp 69-121. (60) Fallahpour, R. A. Curr. Org. Synth. 2006, 3, 19–39. (61) Kro¨hnke, F. Synthesis 1976, 1–24. (62) Durinda, J.; Szucs, L.; Krasnec, L.; Heger, J.; Springer, V.; Kolena, J.; Keleti, J. Acta Fac. Pharm. Bohem. 1966, 12, 89–129. (63) Ziener, U.; Lehn, J.-M.; Mourran, A.; Mo¨ller, M. Chem.sEur. J. 2002, 8, 951–957. (64) Ruben, M.; Ziener, U.; Lehn, J.-M.; Ksenofontov, V.; Gu¨tlich, P.; Vaughan, G. B. M. Chem.sEur. J. 2005, 11, 94–100. (65) Breuning, E.; Ruben, M.; Lehn, J.-M.; Renz, F.; Garcia, Y.; Ksenofontov, V.; Gu¨tlich, P.; Wegelius, E.; Rissanen, K. Angew. Chem. 2000, 112, 2563–2566. (66) Waldmann, O.; Hassmann, J.; Mu¨ller, P.; Hanan, G. S.; Volkmer, D.; Schubert, U. S.; Lehn, J.-M. Phys. ReV. Lett. 1997, 78, 3390–3393. (67) Waldmann, O.; Ruben, M.; Ziener, U.; Mu¨ller, P.; Lehn, J.-M. Inorg. Chem. 2006, 45, 6535–6540. (68) Janiak, C.; Deblon, S.; Wu, H. P.; Kolm, M. J.; Klufers, P.; Piotrowski, H.; Mayer, P. Eur. J. Inorg. Chem. 1999, 1507–1521. (69) Ziener, U.; Breuning, E.; Lehn, J.-M.; Wegelius, E.; Rissanen, K.; Baum, G.; Fenske, D.; Vaughan, G. Chem.sEur. J. 2000, 6, 4132– 4139. (70) Lounasmaa, M.; Johansson, C. J. Tetrahedron 1977, 33, 113– 117. (71) Isler, O.; Gutmann, H.; Straub, O.; Fust, B.; Bohni, E.; Studer, A. HelV. Chim. Acta 1955, 38, 1033–1046. (72) Scudder, M. L.; Goodwin, H. A.; Dance, I. G. New J. Chem. 1999, 23, 695–705. (73) Farag, I. S. A.; Gad, A. M.; Elshabiny, A. M.; Rybakov, V. B. Cryst. Res. Technol. 1993, 28, 1127–1133. (74) Chao, M.; Schempp, E.; Rosenstein, R. D. Acta Crystallogr., Sect. B 1976, 32, 288–290. (75) Clews, C. J. B.; Cochran, W. Acta Crystallogr. 1949, 2, 46–57. (76) Breuning, E.; Ziener, U.; Lehn, J.-M.; Wegelius, E.; Rissanen, K. Eur. J. Inorg. Chem. 2001, 151, 5–1521. (77) Gomar-Nadal, E.; Puigmarti-Luis, J.; Amabilino, D. B. Chem. Soc. ReV. 2008, 37, 490–504. (78) Gottarelli, G.; Masiero, S.; Mezzina, E.; Pieraccini, S.; Rabe, J. P.; Samori, P.; Spada, G. P. Chem.sEur. J. 2000, 6, 3242–3248. (79) Nakagawa, T.; Tanaka, H.; Kawai, T. Surf. Sci. 1997, 370, L144L148. (80) Gesquiere, A.; Abdel-Mottaleb, M. M. S.; Feyter, S. D.; Schryver, F. C. D.; Schoonbeek, F.; Esch, J. v.; Kellogg, R. M.; Feringa, B. L.; Calderone, A.; Lazzaroni, R.; Bredas, J. L. Langmuir 2000, 16, 10385– 10391. (81) Le Poulennec, C.; Cousty, J.; Xie, Z. X.; Mioskowski, C. Surf. Sci. 2000, 448, 93–100. (82) Pinheiro, L. S.; Temperini, M. L. A. Surf. Sci. 1999, 441, 45–52. (83) Pinheiro, L. S.; Temperini, M. L. A. Surf. Sci. 1999, 441, 53–64. (84) Gesquiere, A.; Jonkheijm, P.; Hoeben, F. J. M.; Schenning, A. P. H. J.; Feyter, S. D.; Schryver, F. C. D.; Meijer, E. W. Nano Lett. 2004, 4, 1175–1179. (85) Xie, Z. X.; Charlier, J.; Cousty, J. Surf. Sci. 2000, 448, 201–211. (86) Sto¨hr, M.; Wahl, M.; Galka, C. H.; Riehm, T.; Jung, T. A.; Gade, L. H. Angew. Chem. 2005, 117, 7560-7564. (87) Swarbrick, J. C.; Ma, J.; Theobald, J. A.; Oxtoby, N. S.; O’Shea, J. N.; Champness, N. R.; Beton, P. H. J. Phys. Chem. B 2005, 109, 12167– 12174. (88) Thalacker, C.; Miura, A.; Feyter, S. D.; De Schryver, F. C.; Wu¨rthner, F. Org. Biol. Chem. 2005, 3, 414–422. (89) Kim, K.; Plass, K. E.; Matzger, A. J. Langmuir 2005, 21, 647– 655. (90) Kim, K.; Plass, K. E.; Matzger, A. J. J. Am. Chem. Soc. 2005, 127, 4879–4887. (91) Mourran, A.; Ziener, U.; Mo¨ller, M.; Suarez, M.; Lehn, J.-M. Langmuir 2006, 22, 7579–7586. (92) Stepanow, S.; Lin, N.; Vidal, F.; Landa, A.; Ruben, M.; Barth, J. V.; Kern, K. Nano Lett. 2005, 5, 901–904. (93) Barth, J. V.; Weckesser, J.; Cai, C.; Gu¨nter, P.; Bu¨rgi, L.; Jeandupeux, O.; Kern, K. Angew. Chem. 2000, 112, 1285–1288.

14716 J. Phys. Chem. B, Vol. 112, No. 47, 2008 (94) Clair, S.; Pons, S.; Seitsonen, A. P.; Brune, H.; Kern, K.; Barth, J. V. J. Phys. Chem. B 2004, 108, 14585–14590. (95) Lackinger, M.; Griessl, S.; Markert, T.; Jamitzky, F.; Heckl, W. M. J. Phys. Chem. B 2004, 108, 13652–13655. (96) Li, Z.; Han, B.; Wan, L. J.; Wandlowski, T. Langmuir 2005, 21, 6915–6928. (97) Stepanow, S.; Strunskus, T.; Lingenfelder, M.; Dmitriev, A.; Spillmann, H.; Lin, N.; Barth, J. V.; Wo¨ll, C.; Kern, K. J. Phys. Chem. B 2004, 108, 19392–19397. (98) Wu, D.; Deng, K.; Zeng, Q.; Wang, C. J. Phys. Chem. B 2005, 109, 22296–22300. (99) Griessl, S.; Lackinger, M.; Edelwirth, M.; Hietschold, M.; Heckl, W. M. Single Mol. 2002, 3, 25–31. (100) Otsuki, J.; Nagamine, E.; Kondo, T.; Iwasaki, K.; Asakawa, M.; Miyake, K. J. Am. Chem. Soc. 2005, 127, 10400–10405. (101) Lingenfelder, M. A.; Spillmann, H.; Dmitriev, A.; Stepanow, S.; Lin, N.; Barth, J. V.; Kern, K. Chem.sEur. J. 2004, 10, 1913–1919. (102) Yokoyama, T.; Yokoyama, H.; Kamikado, T.; Okuno, Y.; Mashiko, S. Nature 2001, 413, 619–621. (103) Vidal, F.; Delvigne, E.; Stepanow, S.; Lin, N.; Barth, J. V.; Kern, K. J. Am. Chem. Soc. 2005, 127, 10101–10106. (104) Huang, T.; Hu, Z.; Zhao, A.; Wang, H.; Wang, B.; Yang, J.; Hou, J. G. J. Am. Chem. Soc. 2007, 129, 3857–3862. (105) Lee, J.; Dougherty, D. B.; Yates, J. T. Surf. Sci. 2007, 601, L91L94. (106) Otsuki, J.; Arai, Y.; Amano, M.; Sawai, H.; Ohkita, M.; Hayashi, T.; Hara, M. Langmuir 2008, 24, 5650–5653. (107) Sarma, J.; Desiraju, G. R. J. Chem. Soc., Perkin Trans. 2 1987, 1195–1202. (108) Desiraju, G. R. Chem. Commun. 2005, 2995–3001. (109) Desiraju, G. R. Acc. Chem. Res. 2002, 35, 565–573. (110) Thalladi, V. R.; Weiss, H. C.; Blaser, D.; Boese, R.; Nangia, A.; Desiraju, G. R. J. Am. Chem. Soc. 1998, 120, 8702–8710. (111) Meier, C.; Ziener, U.; Landfester, K.; Weihrich, P. J. Phys. Chem. B 2005, 109, 21015–21027. (112) Katsonis, N.; Lacaze, E.; Feringa, B. L. J. Mater. Chem. 2008, 2065–2073. (113) Hoster, H. E.; Roos, M.; Breitruck, A.; Meier, C.; Tonigold, K.; Waldmann, T.; Ziener, U.; Landfester, K.; Behm, R. J. Langmuir 2007, 23, 11570–11579. (114) McGonigal, G. C.; Bernhardt, R. H.; Thompson, D. J. Appl. Phys. Lett. 1990, 57, 28–30. (115) Yin, S.; Wang, C.; Qiu, X.; Xu, B.; Bai, C. Surf. Interface Anal. 2001, 32, 248–252. (116) Mourran, A.; Ziener, U.; Mo¨ller, M.; Breuning, E.; Ohkita, M.; Lehn, J.-M. Eur. J. Inorg. Chem. 2005, 2641–2647. (117) Ulman, A. Chem. ReV. 1996, 96, 1533–1554. (118) Poirier, G. E. Chem. ReV. 1997, 97, 1117–1128. (119) Hagenstro¨m, H.; Schneeweiss, M. A.; Kolb, D. M. Langmuir 1999, 10, 2435–2443. (120) Schreiber, F. Prog. Surf. Sci. 2000, 65, 151–256. (121) Schweizer, M.; Hagenstro¨m, H.; Kolb, D. M. Surf. Sci. 2001, 490, L627–L636. (122) Esplandiu, M. J.; Hagenstro¨m, H.; Kolb, D. M. Langmuir 2001, 17, 828–838. (123) Kong, D. S.; Yu, Z. Y.; Yuan, S. L. J. Solid State Electrochem. 2005, 9, 174–180. (124) Ho¨lzle, M. H.; Wandlowski, T.; Kolb, D. M. J. Electroanal. Chem. 1995, 394, 271–275. (125) Ho¨lzle, M. H.; Wandlowski, T.; Kolb, D. M. Surf. Sci. 1995, 335, 281–290. (126) Wandlowski, T.; Lampner, D.; Lindsay, S. M. J. Electroanal. Chem. 1996, 404, 215–226. (127) Cunha, F.; Tao, N. J. Phys. ReV. Lett. 1995, 75, 2376–2379. (128) Dretschkow, T.; Lampner, D.; Wandlowski, T. J. Electroanal. Chem. 1998, 458, 121–138. (129) Klymchenko, A. S.; Furukawa, S.; Van der Auweraer, M.; Mu¨llen, K.; De Feyter, S. Nano Lett. 2008, 8, 1163–1168. (130) Dai, Y.; Meier, C.; Ziener, U.; Landfester, K.; Ta¨ubert, C.; Kolb, D. M. Langmuir 2007, 23, 11058–11062. (131) Ma, X.; Yang, Y.; Deng, K.; Zeng, Q.; Zhao, K.; Wang, C.; Bai, C. J. Mater. Chem. 2008, 2074–2081. (132) 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, 15644–15651. (133) Kong, X. H.; Deng, K.; Yang, Y. L.; Zeng, Q. D.; Wang, C. J. Phys. Chem. C 2007, 111, 17382–17387. (134) 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, 5161–5165. (135) Theobald, J. A.; Oxtoby, N. S.; Phillips, M. A.; Champness, N. R.; Beton, P. H. Nature 2003, 424, 1029–1031.

Ziener (136) Perdigao, L. M. A.; Perkins, E. W.; Ma, J.; Staniec, P. A.; Rogers, B. L.; Champness, N. R.; Beton, P. H. J. Phys. Chem. B 2006, 110, 12539– 12542. (137) Wahl, M.; Sto¨hr, M.; Spillmann, H.; Jung, T. A.; Gade, L. H. Chem. Commun. 2007, 1349–1351. (138) Furukawa, S.; Tahara, K.; De Schryver, F. C.; Van der Auweraer, M.; Tobe, Y.; De Feyter, S. Angew. Chem. 2007, 119, 2889-2892. Angew. Chem., Int. Ed. 2007, 46, 2831-2834. (139) Wintjes, N.; Bonifazi, D.; Cheng, F.; Kiebele, A.; Sto¨hr, M.; Jung, T.; Spillmann, H.; Diederich, F. Angew. Chem. 2007, 119, 4167–4170. (140) Bleger, D.; Kreher, D.; Mathevet, F.; Attias, A.-J.; Schull, G.; Huard, A.; Douillard, L.; Fiorini-Debuischert, C.; Charra, F. Angew. Chem. 2007, 119, 7548–7551. (141) Liu, Y. H.; Lei, S. B.; Yin, S. X.; Xu, S. L.; Zheng, Q. Y.; Zeng, Q. D.; Wang, C.; Wan, L. J.; Bai, C. L. J. Phys. Chem. B 2002, 106, 12569– 12574. (142) Lei, S.; Tahara, K.; De Schryver, F. C.; Van der Auweraer, M.; Tobe, Y.; De Feyter, S. Angew. Chem. 2008, 120, 3006–3010. (143) Yoshimoto, S.; Honda, Y.; Ito, O.; Itaya, K. J. Am. Chem. Soc. 2008, 130, 1085–1092. (144) Chen, L.; Chen, W.; Huang, H.; Zhang, H. L.; Yuhara, J.; Wee, A. T. S. AdV. Mater. 2008, 20, 484–488. (145) Zhang, H. L.; Chen, W.; Huang, H.; Chen, L.; Wee, A. T. S. J. Am. Chem. Soc. 2008, 130, 2720–2721. (146) Xiao, W.; Passerone, D.; Ruffieux, P.; Ait-Mansour, K.; Groning, O.; Tosatti, E.; Siegel, J. S.; Fasel, R. J. Am. Chem. Soc. 2008, 130, 4767– 4771. (147) Meier, C.; Landfester, K.; Ku¨nzel, D.; Markert, T.; Groβ, A.; Ziener, U. Angew. Chem. 2008, 120, 3881-3885. Angew. Chem., Int. Ed. 2008, 3847, 3821-3825. (148) Rappe, A. K.; Casewit, C. J.; Colwell, K. S.; Goddard, W. A., Jr.; Skid, W. M. J. Am. Chem. Soc. 1992, 114, 10024–10039. (149) Ku¨nzel, D. Theoretische Untersuchungen zu organischen Oberfla¨chen-Templat-Strukturen. Diploma, University of Ulm, Ulm, Germany, 2007. (150) Kuwabara, T.; Yagi, M. J. Phys. Chem. B 2006, 110, 14673– 14677. (151) Eigler, D. M.; Schweizer, E. K. Nature 1990, 344, 524–526. (152) Zeppenfeld, P.; Lutz, C. P.; Eigler, D. M. Ultramicroscopy 1992, 42-44, 128–133. (153) Griessl, S. J. H.; Lackinger, M.; Jamitzky, F.; Markert, T.; Hietschold, M.; Heckl, W. M. J. Phys. Chem. B 2004, 108, 11556–11560. (154) Sto¨hr, M.; Wahl, M.; Spillmann, H.; Gade, L. H.; Jung, T. A. Small 2007, 3, 1336–1340. (155) Matena, M.; Riehm, T.; Sto¨hr, M.; Jung, T. A.; Gade, L. H. Angew. Chem. 2008, 120, 2448–2451. (156) Mirkin, C. A.; Hong, S. H.; Demers, L. ChemPhysChem 2001, 2, 37–39. (157) Williams, J. A.; Gorman, C. B. Langmuir 2007, 23, 3103–3105. (158) Kufer, S. K.; Puchner, E. M.; Gumpp, H.; Liedl, T.; Gaub, H. E. Science 2008, 319, 594–596. (159) Breitruck, A.; Hoster, H. E.; Meier, C.; Ziener, U.; Behm, R. J. Surf. Sci. 2007, 601, 4200–4205. (160) Lin, N.; Stepanow, S.; Vidal, F.; Kern, K.; Alam, M. S.; Stro¨msdo¨rfer, S.; Dremov, V.; Mu¨ller, P.; Landa, A.; Ruben, M. Dalton Trans. 2006, 2794-2800. (161) Stepanow, S.; Lin, N.; Payer, D.; Schlickum, U.; Klappenberger, F.; Zoppellaro, G.; Ruben, M.; Brune, H.; Barth, J. V.; Kern, K. Angew. Chem. 2007, 119, 724–727. (162) Tait, S. L.; Langner, A.; Lin, N.; Stepanow, S.; Rajadurai, C.; Ruben, M.; Kern, K. J. Phys. Chem. C 2007, 111, 10982–10987. (163) Lin, N.; Langner, A.; Tait, S. L.; Rajadurai, C.; Ruben, M.; Kern, K. Chem. Commun. 2007, 4860–4862. (164) Weber-Bargioni, A.; Reichert, J.; Seitsonen, A. P.; Auwa¨rter, W.; Schiffrin, A.; Barth, J. V. J. Phys. Chem. C 2008, 112, 3453–3455. (165) Bai, Y.; Buchner, F.; Wendahl, M. T.; Kellner, I.; Bayer, A.; Steinruck, H. P.; Marbach, H.; Gottfried, J. M. J. Phys. Chem. C 2008, 112, 6087–6092. (166) Abdel-Mottaleb, M. M. S.; Schuurmans, N.; De Feyter, S.; Van Esch, J.; Feringa, B. L.; De Schryver, F. C. Chem. Commun. 2002, 1894– 1895. (167) De Feyter, S.; Abdel-Mottaleb, M. M. S.; Schuurmans, N.; Verkuijl, B. J. V.; Esch, J. H. v.; Feringa, B. L.; Schryver, F. C. D. Chem.sEur. J. 2004, 10, 1124–1132. (168) Zlatkin, A.; Yudin, S.; Simon, J.; Hanack, M.; Lehman, H. AdV. Mater. Opt. Electron. 1995, 5, 259–263. (169) Yoshimoto, S.; Tada, A.; Suto, K.; Itaya, K. J. Phys. Chem. B 2003, 107, 5836–5843. (170) Suto, K.; Yoshimoto, S.; Itaya, K. Langmuir 2006, 22, 10766– 10776. (171) Yoshimoto, S.; Tsutsumi, E.; Suto, K.; Honda, Y.; Itaya, K. Chem. Phys. 2005, 319, 147–158.

Feature Article (172) Yoshimoto, S.; Suto, K.; Tada, A.; Kobayashi, N.; Itaya, K. J. Am. Chem. Soc. 2004, 126, 8020–8027. (173) Scudiero, L.; Hipps, K. W.; Barlow, D. E. J. Phys. Chem. B 2003, 107, 2903–2909. (174) Scudiero, L.; Barlow, D. E.; Hipps, K. W. J. Phys. Chem. B 2000, 104, 11899–11905. (175) Beves, J. E.; Chapman, B. E.; Kuchel, P. W.; Lindoy, L. F.; McMurtrie, J.; McPartlin, M.; Thordarson, P.; Wei, G. Dalton Trans. 2006, 744–750. (176) Xu, Q. M.; Zhang, B.; Wan, L. J.; Wang, C.; Bai, C. L.; Zhu, D. B. Surf. Sci. 2002, 517, 52–58. (177) Hudson, J. E.; Abruna, H. D. J. Phys. Chem. 1996, 100, 1036– 1042.

J. Phys. Chem. B, Vol. 112, No. 47, 2008 14717 (178) Wei, Z.; Guo, S.; Kandel, S. A. J. Phys. Chem. B 2006, 110, 21846–21849. (179) Cardinaels, T.; Ramaekers, J.; Nockemann, P.; Driesen, K.; Van Hecke, K.; Van Meervelt, L.; Lei, S.; De Feyter, S.; Guillon, D.; Donnio, B.; Binnemans, K. Chem. Mater. 2008, 20, 1278–1291. (180) Semenov, A.; Spatz, J. P.; Mo¨ller, M.; Lehn, J.-M.; Sell, B.; Schubert, D.; Weidl, C. H.; Schubert, U. S. Angew. Chem., Int. Ed. 1999, 38, 2547–2550.

JP805846D

Self-Assembled Nanostructures of Oligopyridine Molecules - The

Recommend Documents