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

Steering organizational and conformational surface chirality by controlling molecular chemical functionality

Molecular chirality on surfaces has been widely explored, both for intrinsically chiral molecules and for prochiral molecules that become chiral upon adsorption due to the reduced symmetry which follows from surface confinement. However, little attention has been devoted to chiral effects that originate from conformational degrees of freedom for adsorbed molecules. Here we have used scanning tunneling microscopy to investigate the self-assembled structures formed when a class of six linear, organic molecules (oligo-phenylene-ethynylenes) are adsorbed on a Au(111) surface under ultrahigh vacuum conditions. All of the investigated compounds are intrinsically achiral, but most display conformational chirality in the sense that the molecules can adsorb on the surface in different conformations giving rise to either one of two chiral surface enantiomers or a mirror-symmetric achiral meso form. A total of eleven observed adsorption structures are systematically investigated with respect to conformational chirality as well as point chirality (arising where molecular adsorption locally breaks the substrate symmetry) and organizational chirality (arising from the tiling pattern of the molecular backbones). A number of interesting correlations are identified between these different levels of chirality. Most importantly, we demonstrate that it is possible through control of the terminal group functionalization to steer the oligo(phenylene-ethynylene) molecular backbones into surface assemblies that either display pronounced organizational chirality or have mirror symmetric tiling patterns, and that it is furthermore possible to control the conformational surface chirality so the compounds preferentially assume either chiral or achiral surface conformers.

Tunable two-dimensional binary molecular networks

A novel approach to constructing tunable and robust 2D binary molecular nanostructures on an inert graphite surface is presented. The guest molecules are embedded into a host molecular matrix and constrained via the formation of multiple intermolecular hydrogen bonds. By varying the binary molecular ratio and the molecular geometry, various molecular arrays with tunable intermolecular distances are fabricated. The results suggest a promising route for the fabrication of ordered and stable molecular nanostructure arrays for molecular sensors, molecular spintronic devices, and molecular p-n nanojunctions.

Carboxylic acids: versatile building blocks and mediators for two-dimensional supramolecular self-assembly

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

Geometry and nanolength scales versus interface interactions: water dynamics in AOT lamellar structures and reverse micelles

To determine the relative importance of the confining geometry and nanoscopic length scale versus water/interface interactions, the dynamic interactions between water and interfaces are studied with ultrafast infrared spectroscopy. Aerosol OT (AOT) is a surfactant that can form two-dimensional lamellar structures with known water layer thickness as well as well-defined monodispersed spherical reverse micelles of known water nanopool diameter. Lamellar structures and reverse micelles are compared based on two criteria: surface-to-surface dimensions to study the effect of confining length scales, and water-to-surfactant ratio to study water/interface interactions. We show that the water-to-surfactant ratio is the dominant factor governing the nature of water interacting with an interface, not the characteristic nanoscopic distance. The detailed structure of the interface and the specific interactions between water and the interface also play a critical role in the fraction of water molecules influenced by the surface. A two-component model in which water is separated into bulk-like water in the center of the lamellar structure or reverse micelle and interfacial water is used to quantitatively extract the interfacial dynamics. A greater number of perturbed water molecules are present in the lamellar structures as compared to the reverse micelles due to the larger surface area per AOT molecule and the greater penetration of water molecules past the sulfonate head groups in the lamellar structures.

Self-assembly of meta-aminobenzoate on Cu(110)

A variety of structures of meta-aminobenzoate molecules adsorbed on the Cu(110) surface have been characterized by scanning tunneling microscopy (STM) at a wide range of surface coverages, from a single molecule to saturated phases. At the start of molecular domain formation, individual molecules thermally diffuse to form chain structures via intermolecular hydrogen bonding. At higher surface coverages, there coexist three well-ordered phases, namely [Formula: see text] and chiral [Formula: see text] phases. The molecular orientation on the surface also varies with surface coverage. Flat-lying molecules are mainly observed at low surface coverage, while upright molecules start to appear as the surface becomes more highly covered. Our experimental findings and structural analysis are well supported by high-resolution STM images measured at 4.7 K and by molecular packing models with precise lattice parameters.

Molecular wires self-assembled on a graphite surface

We report a scanning tunneling microscopy study of the amino acid l-methionine on highly ordered pyrolytic graphite deposited under ambient conditions. Our experiments demonstrate the ability of l-methionine to form highly regular structures on the surface of the graphite template. By means of self-assembly, the amino acid arranges itself into an array of molecular wires, i.e., well-ordered stripes of uniform width and separation. The spacing of these wires can be controlled with the deposition amount of the amino acid, whereas the width stays constant. The width of the wires is determined by two methionine molecules arranged with their carboxyl group facing each other. The regular separation of individual wires suggest a long-range interaction among them. Molecular mechanics calculations are used to compare the experimental results with a basic model for the methionine configuration on the surface. A model for the adsorption geometry of methionine on graphite is presented.

Effect of monomer structure and solvent on the growth of supramolecular nanoassemblies on a graphite surface

The self-assembly of high aspect ratio hierarchical surface assemblies, as observed by fluid tapping mode AFM, can be achieved through careful design of the supramolecular interactions between low-molecular-weight adsorbates. Needlelike assemblies of monotopic guanine end-capped alkanes grow on a graphite surface when deposited from a water/DMSO solution. The growth of these assemblies can be monitored by AFM in real time, and the growth rate along the two different axes can be understood (through molecular modeling) in terms of the specific adsorbate-adsorbate interactions along those axes. Additionally, through judicious solvent selection (e.g., use of non-H-bonding solvents such as o-dichlorobenzene), which allows the formation of hydrogen-bonding aggregates in solution and influences the surface-adsorbate interactions, dramatically different surface assemblies of these guanine derivatives are obtained.

Distinct differences in self-assembly of aromatic linear dicarboxylic acids

Self-assembly into two-dimensionally ordered supramolecular structures of three aromatic dicarboxylic acids-2,6-naphthalenedicarboxylic acid (NDA), 4,4'-biphenyldicarboxylic acid (BPDA), and 4,4'-stilbenedicarboxylic acid (SDA)-is studied at the liquid-solid interface by scanning tunneling microscopy. All compounds possess structural similarities, namely, two interconnected aromatic moieties and functionalization through two carboxylic groups in linear configuration. For all molecules, ordered monolayers were observed on a graphite substrate, and the resulting structures can be described as a dense packing of one-dimensionally hydrogen-bonded rows. However, concerning the stability of the adsorbate layers, the average domain size, and the degree of order, distinct differences were noticed. Supported by density functional theory (DFT) calculations, these differences are analyzed and explained as a consequence of molecular structure, adsorption geometry, and adsorption energy.

Coexistence of racemic and homochiral two-dimensional lattices formed by a prochiral molecule: dicarboxystilbene on Cu(110)

Dicarboxystilbene, a molecule that becomes chiral in the adsorbed state through the loss of its improper axis of rotation, forms long-range "handed" structures when adsorbed on Cu(110) as revealed by scanning tunnelling microscopy. We show that these structures are created from chiral "adsorption complex" building blocks, giving rise to a complete set of racemic and enantiomerically pure structural assemblies. We interpret the formation of these structures in terms of a balance between hydrogen bond mediated intermolecular interactions and the adsorbate-surface structural relationship and discuss the reasons for temperature-induced conversion from the metastable enantiomerically pure to the racemic structure.

Molecular self-assembly from building blocks synthesized on a surface in ultrahigh vacuum: kinetic control and topo-chemical reactions

Self-assembly of organic molecules on solid surfaces under ultrahigh vacuum conditions has been the focus of intense study, in particular utilizing the technique of scanning tunneling microscopy. The size and complexity of the organic compounds used in such studies are in general limited by thermal decomposition in the necessary vacuum sublimation step. An interesting alternative approach is to deposit smaller molecular precursors, which react with each other on the surface and form the building blocks for the subsequent self-assembly. This has however hitherto not been explored to any significant extent. Here, we perform a condensation reaction between aldehyde and amine precursors codeposited on a Au(111) surface. The reaction product consists of a three-spoke oligo-phenylene-ethynylene backbone with alkyl chains attached through imine coupling. We characterize the self-assembled structures and molecular conformations of the complex reaction product and find that the combined reaction and self-assembly process exhibits pronounced kinetic effects leading to formation of qualitatively different molecular structures depending on the reaction/assembly conditions. At high amine flux/low substrate temperature, compact triimine structures of high conformational order are formed, which inherit organizational motifs from structures formed from one of the reactants. This suggests a topochemical reaction. At low amine flux/high substrate temperature, open porous networks with a high degree of conformational disorder are formed. Both structures are entirely different from that obtained when the triimine product synthesized ex-situ is deposited onto the surface. This demonstrates that the approach of combined self-assembly and on-surface synthesis may allow formation of unique structures that are not obtainable through self-assembly from conventionally deposited building blocks.

Extended one-dimensional supramolecular assembly on a stepped surface

2,6-naphthalene-dicarboxylic acid was adsorbed on a Ag110 surface with an average terrace width of only some tens of a nm. Scanning tunneling microscopy shows that the adsorbates self-assemble into one-dimensional mesoscale length chains. These extend over several hundred nanometers and thus the structure exhibits an unprecedented tolerance to monatomic surface steps. Density functional theory and x-ray photoelectron spectroscopy explain the behavior by a strong intermolecular hydrogen bond plus a distinct template-mediated directionality and a high degree of molecular backbone flexibility.

Stabilization of exotic minority phases in a multicomponent self-assembled molecular network

Trimesic acid (TMA) and alcohols were recently shown to self-assemble into a stable, two-component linear pattern at the solution/highly oriented pyrolytic graphite (HOPG) interface. Away from equilibrium, the TMA/alcohol self-assembled molecular network (SAMN) can coexist with pure-TMA networks. Here, we report on some novel characteristics of these non-equilibrium TMA structures, investigated by scanning tunneling microscopy (STM). We observe that both the chicken-wire and flower-structure TMA phases can host 'guest' C(60) molecules within their pores, whereas the TMA/alcohol SAMN does not offer any stable adsorption sites for the C(60) molecules. The presence of the C(60) molecules at the solution/solid interface was found to improve the STM image quality. We have taken advantage of the high-quality imaging conditions to observe unusual TMA bonding geometries at domain boundaries in the TMA/alcohol SAMN. Boundaries between aligned TMA/alcohol domains can give rise to doubled TMA dimer rows in two different configurations, as well as a tripled-TMA row. The boundaries created between non-aligned domains can create geometries that stabilize TMA bonding configurations not observed on surfaces without TMA/alcohol SAMNs, including small regions of the previously predicted 'super flower' TMA bonding geometry and a tertiary structure related to the known TMA phases. These structures are identified as part of a homologic class of TMA bonding motifs, and we explore some of the reasons for the stabilization of these phases in our multicomponent system.

Coexistence of homochiral and heterochiral adenine domains at the liquid/solid interface

In this work, the self-assembly of the DNA base molecule adenine (A) is imaged with high-resolution scanning tunneling microscopy (STM) at the liquid (1-octanol)/solid (HOPG) interface at room temperature. Rather surprisingly, the STM results reveal, for the first time, the spontaneous formation of two coexisting distinct (homo- and heterochiral) domains of adenine, which are formed at the liquid/solid interface without changing any experimental conditions. Ab initio density functional theory (DFT) calculations support our STM findings and suggest the existence of various A networks of nearly similar stability that all are constructed from the most stable A dimer.

Molecular architectonic on metal surfaces

The engineering of highly organized systems from instructed molecular building blocks opens up new vistas for the control of matter and the exploration of nanodevice concepts. Recent investigations demonstrate that well-defined surfaces provide versatile platforms for steering and monitoring the assembly of molecular nanoarchitectures in exquisite detail. This review delineates the principles of noncovalent synthesis on metal substrates under ultrahigh vacuum conditions and briefly assesses the pertaining terminology-self-assembly, self-organization, and self-organized growth. It presents exemplary scanning-tunneling-microscopy observations, providing atomistic insight into the self-assembly of organic clusters, chains, and superlattices, and the metal-directed assembly of low-dimensional coordination architectures. This review also describes hierarchic-assembly protocols leading to intricate multilevel order. Molecular architectonic on metal surfaces represents a versatile rationale to realize structurally complex nanosystems with specific shape, composition, and functional properties, which bear promise for technological applications.

Quantitative analysis of long-range interactions between adsorbed dipolar molecules on Cu(111)

Highly dispersed superstructures of a dipolar iridium complex are formed on a Cu(111) surface. We show that the dilute superstructures with density-controlled intermolecular separations are stabilized by the strong and long-range repulsive intermolecular interactions. The repulsive intermolecular interactions are quantitatively evaluated by using low-temperature scanning tunneling microscopy, which are characterized by the surface-enhanced dipole-dipole interactions.

Chiral Ordering and Conformational Dynamics for a Class of Oligo-phenylene-ethynylenes on Au(111)

Adsorption structures formed from a class of planar organic molecules on the Au(111) surface under ultrahigh vacuum conditions have been characterized using scanning tunneling microscopy (STM). The molecules have different geometries, linear, bent, or three-spoke, but all consist of a conjugated aromatic backbone formed from three or four benzene rings connected by ethynylene spokes and functionalized at all ends with an aldehyde, a hydroxyl, and a bulky tert-butyl group. Upon adsorption, the molecules adopt different surface conformations some of which are chiral. For the majority of the observed adsorption structures, chirality is expressed also in the molecular tiling pattern, and the two levels of chirality display a high degree of correlation. The formation and chiral ordering of the self-assembled structures are shown to result from dynamic interchanges between a diffusing lattice gas and the nucleated islands, as well as from a chiral switching process in which molecules alter their conformation by an intramolecular rotation around a molecular spoke, enabling them to accommodate to the tiling pattern of the surrounding molecular structures. The kinetics of the conformational switching is investigated from time-resolved, variable temperature STM, showing the process to involve an activation energy of approximately 0.3 eV depending on the local molecular environment. The molecule-molecule interactions appear primarily to be of van der Waals character, despite the investigated compounds having functional moieties capable of forming intermolecular hydrogen bonds.

Ionic Hydrogen Bonds Controlling Two-Dimensional Supramolecular Systems at a Metal Surface

Hydrogen-bond formation between ionic adsorbates on an Ag(111) surface under ultrahigh vacuum was studied by scanning tunneling microscopy/spectroscopy (STM/STS), X-ray photoelectron spectroscopy (XPS), near-edge X-ray absorption fine structure (NEXAFS), and molecular dynamics calculations. The adsorbate, 1,3,5-benzenetricarboxylic acid (trimesic acid, TMA), self-assembles at low temperatures (250-300 K) into the known open honeycomb motif through neutral hydrogen bonds formed between carboxyl groups, whereas annealing at 420 K leads to a densely packed quartet structure consisting of flat-lying molecules with one deprotonated carboxyl group per molecule. The resulting charged carboxylate groups form intermolecular ionic hydrogen bonds with enhanced strength compared to the neutral hydrogen bonds; this represents an alternative supramolecular bonding motif in 2D supramolecular organization.

Zwitterionic self-assembly of L-methionine nanogratings on the Ag(111) surface

The engineering of complex architectures from functional molecules on surfaces provides new pathways to control matter at the nanoscale. In this article, we present a combined study addressing the self-assembly of the amino acid L-methionine on Ag(111). Scanning tunneling microscopy data reveal spontaneous ordering in extended molecular chains oriented along high-symmetry substrate directions. At intermediate coverages, regular biomolecular gratings evolve whose periodicity can be tuned at the nanometer scale by varying the methionine surface concentration. Their characteristics and stability were confirmed by helium atomic scattering. X-ray photoemission spectroscopy and high-resolution scanning tunneling microscopy data reveal that the L-methionine chaining is mediated by zwitterionic coupling, accounting for both lateral links and molecular dimerization. This methionine molecular recognition scheme is reminiscent of sheet structures in amino acid crystals and was corroborated by molecular mechanics calculations. Our findings suggest that zwitterionic assembly of amino acids represents a general construction motif to achieve biomolecular nanoarchitectures on surfaces.

Molecular flexibility as a factor affecting the surface ordering of organic adsorbates on metal substrates

The effect of molecular flexibility on the surface ordering of complex organic adsorbates is explored, using alpha,omega-dihexylquaterthiophene (DH4T) and mixed DH4T|tetracene phases on Ag(111) as model systems. The structure of DH4T/Ag(111) interfaces is determined by the flexibility of the hexyl chains at either end of the quaterthiophene backbone: Above 273 K, DH4T forms a nematic liquid crystalline phase with a director close to the [112] direction of the silver substrate. At 273 K, a reversible phase transition to a long-range ordered, point-on-line coincident phase is observed. However, this ordered state is still affected substantially by the flexible nature of DH4T, which materializes in a large number of local structural defects. If traces of DH4T are coevaporated with tetracene, inclusions of a 1:1 stoichiometric DH4T|tetracene phase are found in a tetracene/Ag(111) matrix (alpha-phase). In this mixed phase, the two surface enantiomers of pro-chiral DH4T on one hand and tetracene on the other form a complex stripe structure. The mixed phase shows a higher degree of order than present at the pure DH4T/Ag(111) interface, which also lacks chiral organization. The addition of tetracene molecules as structural templates stabilizes certain conformations of DH4T and thus, by balancing its structural flexibility, allows the surface-induced chirality of DH4T to become a decisive factor in determining the structure of the mixed phase.

Chiral recognition of PVBA on Pd(111) and Ag(111) surfaces

An asymmetric planar molecule, 4-trans-2-(pyrid-4-yl-vinyl) benzoic acid (PVBA), has been used to establish the organic chiral recognition on fcc(111) metal surfaces. The strong correlation between the orientation and chiral recognition of PVBA on both Ag(111) and Pd(111) guides the choice of a model potential, which determines the relative binding energy of PVBA on fcc(111). An angle-dependent calculation of relative binding energy reproduces the experimental observation of the chiral recognition of PVBA on Ag(111) but not on Pd(111).

Asymmetry Induction by Cooperative Intermolecular Hydrogen Bonds in Surface-Anchored Layers of Achiral Molecules

The mesoscale induction of two-dimensional supramolecular chirality (formation of 2D organic domains with a single handedness) was achieved by self-assembly of 1,2,4-benzenetricarboxylic (trimellitic) acid on a Cu(100) surface at elevated temperatures. The combination of spectroscopic [X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure (NEXAFS)], real-space-probe [scanning tunneling microscopy (STM)], and computational [density functional theory (DFT)] methods allows a comprehensive characterization of the obtained organic adlayers, where details of molecular adsorption geometry, intermolecular coupling, and surface chemical bonding are elucidated. The trimellitic acid species, comprising three functional carboxylic groups, form distinct stable mirror-symmetric hydrogen-bonded domains. The chiral ordering is associated with conformational restriction in the domains: molecules anchor to the substrate with an ortho carboxylate group, providing two para carboxylic acid moieties for collective lateral interweaving through H bonding, which induces a specific tilt of the molecular plane. The ease of molecular symmetry switching in domain formation makes homochiral-signature propagation solely limited by the terrace width. The molecular layer modifies the morphology of the underlying copper substrate and induces mum-sized strictly homochiral terraces.

C60 on SiC Nanomesh

A SiC nanomesh is used as a nanotemplate to direct the epitaxy of C60 molecules. The epitaxial growth of C60 molecules on SiC nanomesh at room temperature is investigated by in situ scanning tunneling microscopy, revealing a typical Stranski-Krastanov mode (i.e., for the first one or two monolayers, it is a layer-by-layer growth or 2-D nucleation mode; at higher thicknesses, it changes to island growth or a 3-D nucleation mode). At submonolayer (0.04 and 0.2 ML) coverage, C60 molecules tend to aggregate to form single-layer C60 islands that mainly decorate terrace edges, leaving the uncovered SiC nanomesh almost free of C60 molecules. At 1 ML C60 coverage, a complete wetting layer of hexagonally close-packed C60 molecules forms on top of the SiC nanomesh. At higher coverage from 4.5 ML onward, the C60 stacking adopts a (111) oriented face-centered-cubic (fcc) structure. Strong bright and dim molecular contrasts have been observed on the first layer of C60 molecules, which are proposed to originate from electronic effects in a single-layer C60 island or the different coupling of C60 molecules to SiC nanomesh. These STM molecular contrast patterns completely disappear on the second and all the subsequent C60 layers. It is also found that the nanomesh can be fully recovered by annealing the C60/SiC nanomesh sample at 200 degrees C for 20 min.

Chiral Close-Packing of Achiral Star-Shaped Molecules on Solid Surfaces

From the interplay of scanning tunneling microscopy and theoretical calculations, we study the chiral self-assembly of achiral HtB-HBC molecules upon adsorption on the Cu(110) surface. We find that chirality is expressed at two different levels: a +/-5 degrees rotation of the molecular axis with respect to the close-packed direction of the Cu(110) substrate and a chiral close-packed arrangement expected for star-shaped molecules in 2D. Out of the four possible chiral expressions, only two are found to exist due the effect of van der Waals (vdW) interactions forcing the molecules to simultaneously adjust to the atomic template of the substrate geometry and self-assemble in a close-packed geometry.

SCANNING TUNNELING MICROSCOPY MANIPULATION OF COMPLEX ORGANIC MOLECULES ON SOLID SURFACES

Organic molecules adsorbed on solid surfaces display a fascinating variety of new physical and chemical phenomena ranging from self-assembly and molecular recognition to nonlinear optical properties and current rectification. Both the fundamental interest in these systems and the promise of technological applications have motivated a strong research effort in understanding and controlling these properties. Scanning tunneling microscopy (STM) and, in particular, its ability to manipulate individual adsorbed molecules, has become a powerful tool for studying the adsorption geometry and the conformation and dynamics of single molecules and molecular aggregates. Here we review selected case studies demonstrating the enormous capabilities of STM manipulations to explore basic physiochemical properties of adsorbed molecules. In particular, we emphasize the role of STM manipulations in studying the coupling between the multiple degrees of freedom of adsorbed molecules, the phenomenon of molecular molding, and the possibility of creating and breaking individual chemical bonds in a controlled manner, i.e., the concept of single-molecule chemistry.

Enantiospecific adsorption of cysteine at chiral kink sites on Au(110)-(1 x 2)

Enantiospecific adsorption of cysteine molecules onto chiral kink sites on the Au(110)-(1x2) surface was observed by scanning tunneling microscopy. l- and d-cysteine dimers were found to adopt distinctly different adsorption geometries at S kinks, which can be understood from the need to reach specific, optimum molecule-substrate interaction points. Extended, homochiral domains of l/d-cysteine were furthermore observed to grow preferentially from R/S kinks. The results constitute the first direct, microscopic observation of enantiospecific molecular interaction with chiral sites on a metal single-crystal surface.

Hydrogen-Bonded PTCDA−Melamine Networks and Mixed Phases

A stable hydrogen-bonding junction is formed between 3,4,9,10-perylene-3,4,9,10-tetracarboxylic-dianhydride (PTCDA) and 1,3,5-triazine-2,4,6-triamine (melamine). This bimolecular system was studied on the Ag-Si(111) square root 3 x square root R 30 degrees surface at sub-monolayer coverage, and two distinct phases are observed. A hexagonal lattice is formed that is stabilized by hydrogen bonding between PTCDA and melamine. This phase, in which melamine acts as a 3-fold vertex, is a close analogue to the 3,4,9,10-perylene-3,4,9,10-tetracarboxylic-diimide-melamine network reported recently. To our knowledge this hydrogen-bonding junction has not been previously observed and might not be expected due to lone pair repulsion. However we confirm that this combination is stable using ab initio methods. In the second intermixed phase parallel rows of PTCDA molecules coexist with an array of melamine molecules, and we propose a model for this structure.

Chiral switching by spontaneous conformational change in adsorbed organic molecules

Self-assembly of adsorbed organic molecules is a promising route towards functional surface nano-architectures, and our understanding of associated dynamic processes has been significantly advanced by several scanning tunnelling microscopy (STM) investigations. Intramolecular degrees of freedom are widely accepted to influence ordering of complex adsorbates, but although molecular conformation has been identified and even manipulated by STM, the detailed dynamics of spontaneous conformational change in adsorbed molecules has hitherto not been addressed. Molecular surface structures often show important stereochemical effects as, aside from truly chiral molecules, a large class of so-called prochiral molecules become chiral once confined on a surface with an associated loss of symmetry. Here, we investigate a model system in which adsorbed molecules surprisingly switch between enantiomeric forms as they undergo thermally induced conformational changes. The associated kinetic parameters are quantified from time-resolved STM data whereas mechanistic insight is obtained from theoretical modelling. The chiral switching is demonstrated to enable an efficient channel towards formation of extended homochiral surface domains. Our results imply that appropriate prochiral molecules may be induced (for example, by seeding) to assume only one enantiomeric form in surface assemblies, which is of relevance for chiral amplification and asymmetric heterogenous catalysis.

Engineering atomic and molecular nanostructures at surfaces

The fabrication methods of the microelectronics industry have been refined to produce ever smaller devices, but will soon reach their fundamental limits. A promising alternative route to even smaller functional systems with nanometre dimensions is the autonomous ordering and assembly of atoms and molecules on atomically well-defined surfaces. This approach combines ease of fabrication with exquisite control over the shape, composition and mesoscale organization of the surface structures formed. Once the mechanisms controlling the self-ordering phenomena are fully understood, the self-assembly and growth processes can be steered to create a wide range of surface nanostructures from metallic, semiconducting and molecular materials.