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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