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.

Molecular nanostructures formed by bottom-up self-organization [1] are model systems for advanced functional surfaces with a broad range of applications, such as sensors or coatings, molecular electronics, and heterogeneous catalysis. Supramolecular structures formed on surfaces under ultrahighvacuum (UHV) conditions through exploitation of noncovalent interactions, such as van der Waals forces, [2] dipole-dipole interactions, [3] hydrogen bonding, [4] or metal complexation, [5] have been studied extensively with scanning tunneling microscopy (STM). Structures stabilized by stronger covalent bonds between the molecular building blocks are anticipated to have an improved thermal and chemical stability, and are thus likely to be more useful for practical applications. However, investigations into covalently interlinked molecular structures on surfaces under UHV conditions are only just emerging.[6]Thin films produced by vapor-deposition polymerization [7] have been studied by STM, as has photoinduced or STM-tipinduced polymerization of diacetylene.[8] Macromolecules have been deposited at surfaces using the pulse injection technique [9,10] and characterized at modest resolution, and polymer architecture and folding have been studied upon electropolymerization [11] or drop-casting.[12] Although polymers deposited or formed in UHV [6,8,13] and at the liquid/solid interface [11,12] have been observed, no detailed high-resolution STM studies of connectivity and branching exist.Herein, we demonstrate the formation of two-component polymeric nanostructures on a Au(111) surface under UHV conditions. The branched surface polymer, which contains pores about 3-10 nm in dimension, is characterized by highresolution STM and it is shown that its connectivity can be controlled by varying the kinetic parameters of the preparation procedure.Figure 1 a shows the investigated condensation polymerization reaction between an aromatic trisalicylaldehyde [14,15] (trialdehyde) and 1,6-diaminohexane (diamine), which results in a polymer connected by imine bonds. In solution the trialdehyde is known to form a cross-linked polymer by reaction with ethylenediamine.[15] Covalent interlinking of similar two-spoke salicylaldehydes and octylamine on Au-(111) under UHV conditions was recently demonstrated by STM and synchrotron-based X-ray spectroscopy. [16] STM images of the reactants adsorbed individually on the Au(111)-(22 ffiffi ffi 3 p ) surface are shown in Figure 1 b and c. Upon co-deposition followed by annealing above 400 K, open filamentous structures are formed (Figure 2 a). The local bonding pattern is revealed from high-resolution STM images 2 ) of b) a close-packed island of trialdehydes [14] (I t = 0.60 nA, V t = 1.05 V) and c) the lamellar structure of 1,6-diaminohexanes (I t = 0.34 nA, V t = À1.9 V). Molecular models are superimposed. I t = tunneling current, V t = tunneling voltage.

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.

Getting a reaction: A condensation reaction occurs between a dialdehyde and an amine coadsorbed on a Au(111) surface in an ultrahigh vacuum. The self‐assembled structures formed by the diimine reaction product on the surface have been investigated by scanning tunneling microscopy (see image). A solvent‐free reaction path is proposed from DFT calculations.

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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.

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.

The adsorption of molecules on surfaces plays an important role in bottom-up nanofabrication. 1,2 Azobenzene and derivatives thereof are particularly interesting since these molecules are considered as model systems for molecular switches, 3-6 based on light-induced reversible trans-cis isomerizations. Consequently, they play an important role for optically active materials and devices. Switching of azobenzene derivatives at the liquid-solid interface has been demonstrated, [7][8][9][10] while investigations into the adsorption and switching behavior of azobenzene at surfaces under well-defined ultrahigh vacuum (UHV) conditions are only just emerging. 11,12 In this Communication, we investigate the adsorption geometries of azobenzene on the Cu(110) surface at low coverage and saturation limits. We show that only the trans-isomer is observed, and we investigate its diffusion behavior in both preferred and energetically metastable adsorption states.Scanning tunneling microscopy (STM) measurements were acquired with the variable-temperature Aarhus STM under UHV conditions. 13,14 Azobenzene (Sigma-Aldrich, 99.5% purity) was held in a transparent glass vial at room temperature and admitted into the UHV system via a leak valve. The Cu(110) surface was cleaned by repeated cycles of 1.5 keV Ar + ion bombardment followed by annealing to 820 K. All STM measurements were obtained in a temperature range of 120-170 K.Upon deposition at room temperature, individual molecules appear as two bright protrusions, attributed to flat lying phenyl rings, separated by a darker line associated with the NdN bond ( Figure 1a). This molecular signature is qualitatively as expected for the trans-isomer. The molecules adsorb with their axis at a slight angle to the close-packed [1 -1 0] direction and with their NdN bond at a bridge site as determined from images showing the Cu lattice at atomic resolution ( Figure 1c). Loss of symmetry upon adsorption results in two different surface enantiomers, distinguishable by the direction of the central dark line, as indicated by arrows in Figure 1a. A possible model of the molecular adsorption geometry is depicted in Figure 1b (the enantiomer with the NdN bond along the dark line is tentatively chosen to correspond to the shown direction of rotation).As the coverage is increased, the molecules do not show a strong tendency to cluster together, even if annealed to 500 K. Near half of saturation coverage (Figure 1d), the molecules begin to order by stacking sideways into columns running along the substrate [001] direction. The molecular head-to-head interaction appears to be weak or even repulsive as no ordering is observed along this direction. At saturation coverage (Figure 1e), the molecules form columns along the [001] direction with a periodicity of 7.9 ( 0.8 Å (dimension a in Figure 1e), consistent with two lattice spacings of the Cu substrate. At this separation, intermolecular H-N hydrogen bonding can occur with a reasonable bond length of less than 4 Å. The column-column distance along the ...