Mesoscopic correlation of supramolecular chirality in one-dimensional hydrogen-bonded assemblies

Two-dimensional supramolecular arrangements of enantiomers and racemic modification of 1,1'-binaphthyl-2,2'-dicarboxylic acid

Formation of adlayers of the optically active compound 1,1'-binaphthyl-2, 2'-dicarboxylic acid (BINAC) on iodine-modified Au (111) surfaces in perchloric acid was investigated by in situ scanning tunneling microscopy (STM). Highly ordered arrays formed on the surfaces via simple spontaneous adsorption from a solution of enantiomers or the racemic BINAC, in spite of the fact that BINAC has a three-dimensionally complex stereochemical structure. Adlayers of both enantiomers essentially shared the same enantiomorphous structure. Observed parameters of the rectangular unit cell lattice for arrays of both enantiomers of BINAC were a = 2.3 +/- 0.2 nm and b = 0.7 +/- 0.2 nm. On the other hand, racemic modification formed an entirely different adlayer, which consisted of an alternate alignment of the two enantiomers, with an oblique unit cell lattice with parameters of a = 1.2 +/- 0.2 nm, b = 0.8 +/- 0.1 nm, and 74 +/- 3 degrees . No domain composed of a single enantiomer was observed. The stronger hetero-intermolecular interactions of enantiomer couples led to the formation of an alternate arrangement in the array prepared by racemic modification.

Tailoring the Mobility of a 3D Molecule Adsorbed on a Metal Surface

Because of its tetrahedral structure, spirobifluorene is an innovative molecule for molecular mechanics studies by means of scanning tunneling microscopy. On Cu(100), it was observed only anchored at defects because of its mobility at room temperature. To frustrate its diffusion, it was functionalized with phenyl and thiophene groups. Tetraphenylspirobifluorene is also mobile on Cu(100), whereas tetrathienylspirobifluorene is fixed in the middle of the terraces. This very different behavior is an original and unexpected result because both benzene and thiophene are reported to be weakly bound to Cu(100) with almost the same adsorption energy.

Losing the expression of molecular chirality in self-assembled physisorbed monolayers

STM imaging on graphite of the S-enantiomer of a chiral diacetylene isophthalic acid derivative reveals that molecular chirality is not expressed in the monolayer due to a specific molecular conformation preventing the stereogenic center to transfer its chiral information.

Chiral Phase Transition in Two-Dimensional Supramolecular Assemblies of Prochiral Molecules

The self-assembly of the rodlike two-dimensional chiral molecule 4-[trans-2-(pyrid-4-yl-vinyl)] benzoic acid on the Cu(100) surface has been investigated by scanning tunneling microscopy. Upon adsorption at T>or=300 K, the molecules are deprotonated and assemble in parquet patterns when the coverage remains below a critical value. Corresponding high-resolution data reveal that the ordering implies mesoscopic chiral resolution as a result of chiroselective interactions (i.e., two domains comprise exclusively one enantiomer). When the critical coverage is exceeded, an abrupt transition to a single racemic phase is observed with a different lateral molecular coupling scheme. The shifting of the subtle balance between the weak lateral coupling, substrate bonding, and the packing requirements encountered with the increased molecular coverage is suggested to be the driving force for this homochiral-to-heterochiral phase transition.

Programming supramolecular assembly and chirality in two-dimensional dicarboxylate networks on a Cu(100) surface

We report a comparative study on the 2D self-assembly of two related ditopic benzoic acid species, which have similar shape and endgroups but different backbone symmetry. High-resolution scanning tunneling microscopy data reveal how the symmetry information of molecular building blocks is readily expressed in the resulting chiral or nonchiral supramolecular networks. The underlying square Cu(100) surface steers network orientation and accounts for carboxylate formation, resulting in an unusual intermolecular hydrogen bond motif. Our results demonstrate that symmetry and chiral resolution in 2D supramolecular assembly can be controlled via the design of functional molecules and choice of substrate.

Self-Assembly at the Liquid/Solid Interface: STM Reveals

The liquid/solid interface provides an ideal environment to investigate self-assembly phenomena, and scanning tunneling microscopy (STM) is the preferred methodology to probe the structure and the properties of physisorbed monolayers on the nanoscale. Physisorbed monolayers are of relevance in areas such as lubrication, patterning of surfaces on the nanoscale, and thin film based organic electronic devices, to name a few. It's important to gain insight in the factors which control the ordering of molecules at the liquid/solid interface in view of the targeted properties. STM provides detailed insight into the importance of molecule-substrate (epitaxy) and molecule-molecule interactions (hydrogen bonding, metal complexation, and fluorophobic/fluorophilic interactions) to direct the ordering of both achiral and chiral molecules on the atomically flat surface. By controlling the location and orientation of functional groups, chemical reactions can be induced at the liquid/solid interface, via external stimuli, such as light, or by controlled manipulation with the STM tip. The electronic properties of the self-assembled physisorbed molecules can be probed by taking advantage of the operation principle of STM, revealing spatially resolved intramolecular differences within these physisorbed molecules.

Evolution of multilevel order in supramolecular assemblies

The process of self-assembly at multiple length scales of bis-urea substituted toluene on a Au(111) surface was studied by low temperature scanning tunneling microscopy. Pattern formation is controlled by specific hydrogen bonds between these molecules but also by significantly weaker lateral coupling between the resulting supramolecular polymers and a quasiepitaxial interlocking with the substrate. The ordered assemblies exhibit a tunnel transparency. Our experiments indicate the necessity of multiple interactions of different strengths for obtaining ordered structures with hierarchical levels of organization.

Octyl-Decorated Fréchet-Type Dendrons: A General Motif for Visualisation of Static and Dynamic Behaviour Using Scanning Tunnelling Microscopy?

A detailed STM study of monolayers of 3,5-bis[(3,5-bisoctyloxyphenyl)methyloxy]benzaldehyde and 3,5-bis[(3,5-bisoctyloxyphenyl)methyloxy]benzyl alcohol adsorbed on graphite is presented. Very highly resolved scanning tunnelling microscopy images are observed at room temperature in air allowing the analysis of the conformation of the adsorbed molecules. These long-chain alkyl-decorated Fréchet-type dendrons are a powerful assembly motif and initially form a pattern based on trimeric units, assembled into hexagonal host structures with a pseudo-unit cell of seven molecules, one of which remains highly mobile. Over time, the supramolecular ordering changes from a trimeric into a dimeric pattern. The chirality arising from the adsorption onto a surface of the dendrons is discussed.

Conformation selective assembly of carboxyphenyl substituted porphyrins on Au (111)

The selective assembly of carboxyphenyl substituted porphyrins on the Au (111) surface has been studied using scanning tunneling microscopy. We find that conformational isomers of the porphyrins are induced upon adsorption and are selectively assembled into hydrogen-bonded supramolecular clusters or wires on the surface. The conformation selective assembly is attributed to the coplanar intermolecular interactions between hydrogen-bonded carboxyphenyl groups.

Adsorption-induced asymmetric assembly from an achiral adsorbate

Symmetry breaking in the self-assembled monolayer (SAM) structure of 1-octadecanol on highly ordered pyrolytic graphite (HOPG) is observed. Due to the slight mismatch of the octadecanol molecule with the graphite lattice, the alkane chain undergoes distortion upon adsorption on the surface. The asymmetric distortion of the octadecanol SAM unit cell pair is observed by scanning tunneling microscopy at the liquid/solid interface. Asymmetric distortion is due to the requirement for planarity of the hydrogen bond connecting the two octadecanol molecules in the chevron-shaped unit cell. This very simple structure provides the first example of an adsorption-induced distortion to form a supramolecular asymmetric structure, which is formed by achiral molecules adsorbed on an achiral surface. What makes this system interesting and different from other examples of adsorption-induced chirality is that the adsorbate itself undergoes asymmetric distortion due to the existence of the substrate and the adsorbate conformation is different from the molecule in solution.

Structures Formed by the Chiral Assembly of Racemic Mixtures of Enantiomers: Iodination Products of Elaidic and Oleic Acids

The self-assembled monolayer structure of the products of elaidic acid iodination (the racemic mixture of 9,10-(9S,10R)-diiodooctadecanoic acid and 9,10-(9R,10S)-diiodooctadecanoic acid) and the products of oleic acid iodination (the racemic mixture of 9,10-(9R,10R)-diiodooctadecanoic acid and 9,10-(9S,10S)-diiodooctadecanoic acid) are studied by high-resolution scanning tunneling microscopy. For the iodination products of elaidic acid, the separation of enantiomers into distinct chiral domains during the formation of the 2-D crystal on the highly ordered pyrolytic graphite (HOPG) surface is not observed. Instead, within the diiodooctadecanoic acid SAM, each row of molecules is composed of opposite racemates. The two opposite racemates pack alternately inside a row, using different faces to adsorb on the surface. The unit cell is composed of a pair of opposite racemates, forming a heterochiral structure. For the iodination products of oleic acid, the racemic mixture is observed to exhibit quasi-phase separation during the formation of the 2-D crystal on the HOPG surface. Each row is composed of homochiral acid molecules, either the 9,10-(9R,10R)-diiodooctadecanoic acid (R) or the 9,10-(9S,10S)-diiodooctadecanoic acid (S). The R row and the S row pack alternately, with a unit cell composed of four molecules. Two of the molecules in the unit cell are the 9,10-(9R,10R)-diiodooctadecanoic acid (R) molecules; two are the 9,10-(9S,10S)-diiodooctadecanoic acid (S) molecules. In the unit cell, the two molecules that have the same chirality pack antiparallel inside the homochiral row, using different faces to adsorb on the surface. These results suggest that several different types of chiral assembly are possible. Enantiomers with opposite chirality exhibit many chiral assembly patterns, forming heterochiral structures on the surface in addition to separation to form macroscopic chiral domains. By using different conformations, similar enantiomers with opposite chirality will display many chiral assembly patterns to form heterochiral structures on the surface.

Nanoparticle dispersion on reconstructed carbon nanomeshes

A nanoporous template which can be used for the preparation of monodispersed metal nanoparticles can have wide-ranging applications in the catalyzed growth of single-walled nanotubes, as well as the preparation of energetic, nanostructured ferromagnetic particle arrays. Here, we found that a honeycomb-like carbon nanomesh with periodically arranged pores of approximately 2-nm dimension could be fabricated on the reconstructed 6H-SiC(0001) surface. The carbon nanomesh arises from the periodic arrangement of segregated carbon clusters on the 6H-SiC surface to form a highly regular, nanoporous film. The carbon nanomesh can be dynamically structured to control the periodicity and depth of the pores by annealing in a vacuum. We evaporated cobalt on the surface of the nanomesh and investigated the diffusion and agglomeration behavior of cobalt clusters using in situ scanning tunneling microscopy. It is found that monodispersed Co nanoclusters that resist aggregation up to a temperature of 500 degrees C can be fabricated on this template.

Expression of molecular chirality and two-dimensional supramolecular self-assembly of chiral, racemic, and achiral monodendrons at the liquid-solid interface

We have investigated the two-dimensional ordering of chiral and achiral monodendrons at the liquid-solid interface. The chiral molecules self-assemble into extended arrays of dimers. As expected, the R enantiomer forms the mirror image type pattern of the chiral two-dimensional structure formed by the S enantiomer. A racemic mixture applied from solution onto the substrate undergoes spontaneous segregation: the enantiomers separate on the surface and appear in different domains. In contrast to the chiral molecules, the achiral analogue self-assembles into cyclic tetramers. Moreover, the pattern formed by the achiral molecule strongly depends on the solvent used. In the case of 1-phenyloctane, solvent molecules are coadsorbed in a 2:1 (dendron:solvent) ratio whereas in 1-octanol, no solvent molecules are coadsorbed. By the appropriate solvent choice, the distance between the potential "supramolecular containers" can be influenced.

Mesoscopic chiral reshaping of the Ag(110) surface induced by the organic molecule PVBA

We report scanning tunneling microscopy observations on the restructuring of a Ag(110) surface induced by the molecule 4-[trans-2-(pyrid-4-yl-vinyl)]benzoic acid (PVBA). Our data reveal that the surface undergoes a mesoscopic step faceting following exposure to submonolayer coverages and thermal activation. A sawtooth arrangement evolves implying long-range mass transport of substrate atoms and forming a regular arrangement of kink sites. Its formation is associated with the molecules' functional headgroups forming carboxylates with [100] Ag microfacets at step edges, and eventually operating to reshape the surface morphology. Interestingly, the resulting microfacets act as chiral templates for the growth of supramolecular PVBA structures. Theoretical modeling based on ab initio results indicates that chiral recognition processes discriminating between the two enantiomers of adsorbed PVBA molecules occur in this process.

Design of extended surface-supported chiral metal-organic arrays comprising mononuclear iron centers

A design strategy for fabricating a surface-supported chiral metal-organic system comprising a regular arrangement of mononuclear iron centers and nanocavities is presented. By sequential deposition of 1,2,4-benzenetricarboxylic acid (tmla) molecules and iron atoms on a Cu(100) surface under ultrahigh vacuum conditions, chiral square-planar Fe(tmla)4 metal-organic complexes are generated, which order in extended homochiral arrays. Structure formation and envisioned functionality of such metal-organic architectures are discussed.

Absolute Configuration of Monodentate Phosphine Ligand Enantiomers on Cu(111)

Scanning tunneling microscopy (STM) has been employed to investigate the chirality of monophosphine compounds that are highly efficient chiral ligands in transition-metal-catalyzed organic transformations. The absolute configuration of 1-(2-diphenyphosphino-1-naphthyl)isoquinoline enantiomers with axial chirality was discriminated directly by the "marker" group, PPh(2) substitutes. Although the two enantiomer molecules adsorb on a Cu(111) surface and form well-defined (4 x 4) structures, the positions of PPh(2) substitutes in the molecular adlayers are different. The mirror symmetry between two adlayers is demonstrated. On the basis of STM results, structural models are proposed to interpret the chiral adsorption. The results presented here supply a straightforward method for axial chirality analysis in adsorbed adlayers by STM.

Hierarchical assembly of two-dimensional homochiral nanocavity arrays

We demonstrate the rational design of nanoporous two-dimensional supramolecular structures by the hierarchical assembly of organic molecules and transition metal atoms at surfaces. Single-molecule level observations with scanning tunneling microscopy monitor the successive aufbau of structures with increasing complexity. From the primary components secondary mononuclear chiral complexes are formed, which represent antecedents for tertiary polynuclear metal-organic nanogrids. These nanogrids represent the constituents of the eventually evolving two-dimensional networks comprising homochiral nanocavity arrays. Our findings visualize the evolution of complex matter in an exemplary way: from per se achiral species via chiral intermediates to mesoscale dissymmetric structures.

Comparison of nanotube structures constructed from alpha-, beta-, and gamma-cyclodextrins by potential-controlled adsorption

"Nanotube" structures of the alpha-, beta-, and gamma-cyclodextrins (CyD's), which are similar to that of CyD-polyrotaxane, were constructed by potential-controlled adsorption onto Au(111) surfaces in sodium perchlorate solution without a threaded polymer. CyD molecules adsorbed randomly on bare Au(111) surfaces without potential control and the desorption of CyD's from Au surfaces was observed at a negative potential of less than -0.60 V versus SCE. On the other hand, in the specific range between these potentials, ordered molecular arrays with "nanotube" structures of the CyD's (alpha-, beta-, and gamma-CyD) were observed on Au(111). The range of potentials for formation of the "nanotube" structures of alpha-, beta-, and gamma-CyD was from -0.15 to -0.20 V, from -0.25 to -0.45 V, and from -0.22 to -0.45 V, respectively. beta- and gamma-CyD require a more negative potential for adsorption-induced self-organization (AISO) than alpha-CyD in order to weaken adsorption and induce self-organization. Furthermore, we have succeeded in the visualization of the dynamic process in solution, such as the self-ordering, and the destruction of the nanotube structure. These results indicate that control of the electrode potential facilitates management of the delicate balance of various interactions, resulting in the formation of two-dimensional supramolecular structures on the substrates.

Density functional theory study of enantiospecific adsorption at chiral surfaces

Density functional theory calculations are carried out for the adsorption of a chiral molecule, (S)- and (R)-HSCH(2)CHNH(2)CH(2)P(CH(3))(2), on a chiral surface, Au(17 11 9)(S)(). The S-enantiomer is found to bind more strongly than the R-enantiomer by 8.8 kJ/mol, evidencing that the chiral nature of the kink sites at the Au(17 11 9) surface leads to enantiospecific binding. The adsorption of two related chiral molecules, HSCH(2)CHNH(2)COOH ("cysteine") and HSCH(2)CHNH(2)CH(2)NH(2), does not, however, lead to enantiospecific binding. The results of the density functional calculations are broken down into a local binding model in which each of the chiral molecule's three contact points with the surface provides a contribution to the overall adsorption bond strength. The enantiospecific binding is demonstrated to originate from the simultaneous optimization of these three local bonds. In the model, the deformation energy costs of both the molecule and the surface are further included. The model reveals that the molecule may undergo large deformations in the attempt to optimize the three bonds, while the surface deforms to a lesser extent. The most favorable binding configurations of each enantiomer are, however, characterized by small deformation energies only, justifying a local binding picture.

Direct observation of chiral metal-organic complexes assembled on a Cu100 surface

We report single-molecule level STM observations of chiral complexes generated by the assembly of achiral components at a metal surface. Following co-deposition of iron atoms and 1,3,5-tricarboxylic benzoic acid (trimesic acid, TMA) on Cu(100) in ultrahigh vaccum, TMA molecules react with the metal centers, and metal-ligand interactions stabilize R and S chiral complexes which are clearly distinguished by STM.

Stereochemical effects in supramolecular self-assembly at surfaces: 1-D versus 2-D enantiomorphic ordering for PVBA and PEBA on Ag(111)

We present investigations on noncovalent bonding and supramolecular self-assembly of two related molecular building blocks at a noble metal surface: 4-[trans-2-(pyrid-4-yl-vinyl)]benzoic acid (PVBA) and 4-[(pyrid-4-yl-ethynyl)]benzoic acid (PEBA). These rigid, rodlike molecules comprising the same complementary moieties for hydrogen bond formation are comparable in shape and size. For PVBA, the ethenylene moiety accounts for two-dimensional (2-D) chirality upon confinement to a surface; PEBA is linear and thus 2-D achiral. Molecular films were deposited on a Ag(111) surface by organic molecular beam epitaxy and characterized by scanning tunneling microscopy. At low temperatures (around 150 K), both species form irregular networks of flat lying molecules linked via their endgroups in a diffusion-limited aggregation process. In the absence of kinetic limitations (adsorption or annealing at room temperature), hydrogen-bonded supramolecular assemblies form which are markedly different. With PVBA, enantiomorphic twin chains in two mirror-symmetric species running along a high-symmetry direction of the substrate lattice form by diastereoselective self-assembly of one enantiomer. The chirality signature is strictly correlated between neighboring twin chains. Enantiopure one-dimensional (1-D) supramolecular nanogratings with tunable periodicity evolve at intermediate coverages, reflecting chiral resolution in micrometer domains. In contrast, PEBA assembles in 2-D hydrogen-bonded islands, which are enantiomorphic because of the orientation of the supramolecular arrangements along low-symmetry directions of the substrate. Thus, for PVBA, chiral molecules form 1-D enantiomorphic supramolecular structures because of mesoscopic resolution of a 2-D chiral species, whereas with PEBA, the packing of an achiral species causes 2-D enantiomorphic arrangements. Model simulations of supramolecular ordering provide a deeper understanding of the stability of these systems.

Organic molecules acting as templates on metal surfaces

The electronic connection of single molecules to nanoelectrodes on a surface is a basic, unsolved problem in the emerging field of molecular nanoelectronics. By means of variable temperature scanning tunneling microscopy, we show that an organic molecule (C90H98), known as the Lander, can cause the rearrangement of atoms on a Cu(110) surface. These molecules act as templates accommodating metal atoms at the step edges of the copper substrate, forming metallic nanostructures (0.75 nanometers wide and 1.85 nanometers long) that are adapted to the dimensions of the molecule.