Structural Transformation of 2,7-Dibromopyrene on Au(111) Mediated by Halogen-Bonding Motifs

Fundamental understanding of the bonding motifs that elaborately mediate the formation of supramolecular nanostructures is essential for the rational design of stable artificial organic architectures. Herein, the structural transformation of the adsorption complex of 2, 7-dibromopyrene (Br₂Py) on the Au(111) surface has been investigated by scanning tunnelling microscopy combined with X-ray photoemission spectroscopy and density function theory calculations. In the initial stage of self-assembly, well ordered patterns are formed in the manner of extended supramolecular structures balanced by intermolecular halogen bonding motifs, whilst the Au(111) reconstruction is still fairly visible. Subsequent thermal annealing promotes the dehalogenation and on-surface Ullmann coupling, and polymerized oligomers are consequently constructed. Interestingly, such polymerized chains are still stably mediated by the halogen bonding motif via dissociated Br atoms which are revealed to be absorbed on the bridge site of Au(111), while the number of halogen bonds increases significantly from self-assembly to Ullmann coupling polymerization, indicating that the halogen bonding motif contributes significantly to the extended one-dimensional polymers.     

Chiral induction by seeding surface assemblies of chiral switches

It is demonstrated by scanning tunneling microscopy that coadsorption of a molecular chiral switch with a complementary, intrinsically chiral induction seed on the Au(111) surface leads to the formation of globally homochiral molecular assemblies.     

Influence of molecular geometry on the adsorption orientation for oligophenylene-ethynylenes on Au(111)

The adsorption structures formed from a class of oligophenylene-ethynylenes on Au(111) under ultrahigh vacuum conditions is compared based on high-resolution scanning tunneling microscopy (STM) measurements. The molecules consist of three or four benzene rings connected by ethynylene spokes and are all functionalized identically with an aldehyde, a hydroxyl, and a bulky tert-butyl group. Compounds with the conjugated spokes placed in the para, meta, and threefold configurations were previously found to exclusively form molecular layers with flat-lying adsorption geometries. In contrast, the associated compound with spokes in the ortho configuration surprisingly differs in its adsorption by forming only structures with an upright adsorption orientation. The packing density for the structures formed by the compound with the ortho configuration is less dense than that in conventional self-assembled monolayers while still keeping the conducting backbone in an upright orientation. These structures are thus interesting from the perspective of performing single-molecule conduction measurements on the oligophenylene-ethynylene backbones.     

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.     

Characterization of enantiospecific chemisorption on chiral Cu surfaces vicinal to Cu(111) and Cu(100) using density functional theory

Surfaces of simple fcc metals such as Cu with nonzero and unequal Miller indices are intrinsically chiral. Density functional theory (DFT) calculations are a useful way to study the enantiospecific adsorption of small chiral molecules on these chiral metal surfaces. We report DFT calculations of seven chiral molecules on several structurally distinct chiral Cu surfaces. These surfaces include two surfaces with (111)-oriented terraces and one with (100)-oriented terraces. Calculations are also described on a surface that was modified to mimic the surface structures that typically appear on real metal surfaces following thermally driven fluctuations in step edges. Our results provide initial information on how variation in the surface structure of intrinsically chiral metal surfaces can affect the enantiospecific adsorption of small molecules on these surfaces.     

Chiral structures of 6,12-dibromochrysene on Au(111) and Cu(111) surfaces

Nanoscale low-dimensional chiral architectures are increasingly receiving scientific interest, because of their potential applications in many fields such as chiral recognition, separation and transformation. Using 6,12-dibromochrysene (DBCh), we successfully constructed and characterized the large-area two-dimensional chiral networks on Au(111) and one-dimensional metal-liganded chiral chains on Cu(111) respectively. The reasons and processes of chiral transformation of chiral networks on Au(111) were analyzed. We used scanning tunneling spectroscopy (STS) to analyze the electronic state information of this chiral structure. This work combines scanning tunneling microscopy (STM) with non-contact atomic force microscopy (nc-AFM) techniques to achieve ultra-high-resolution characterization of chiral structures on low-dimensional surfaces, which may be applied to the bond analysis of functional nanofilms. Density functional theory (DFT) was used to simulate the adsorption behavior of the molecular and energy analysis in order to verify the experimental results.     

Conformation Diversity of a Fused‐Ring Pyrazine Derivative on Au(111) and Highly Ordered Pyrolytic Graphite

Heterocyclic aromatic compounds have attracted considerable attention because of their high carrier mobility that can be exploited in organic field‐effect transistors. This contribution presents a comparative study of the packing structure of 3,6‐didodecyl‐12‐(3,6‐didodecylphenanthro[9,10‐b]phenazin‐13‐yl)phenanthro[9,10‐b]phenazine (DP), an N‐heterocyclic aromatic compound, on Au(111) and highly ordered pyrolytic graphite (HOPG). High‐resolution scanning tunneling microscopy (STM) combined with atomistic simulations provide a picture of the interface of this organic semiconductor on an electrode that can have an impact on the field‐effect transistor (FET) performance. DP molecules adsorb with different conformational isomers (R/S: trans isomers; C: cis isomer) on HOPG and Au(111) substrates. All three isomers are found in the long‐range disordered lamella domains on Au(111). In contrast, only the R/S trans isomers self‐assemble into stable chiral domains on the HOPG surface. The substrate‐dependent adsorption configuration selectivity is supported by theoretical calculations. The van der Waals interaction between the molecules and the substrate dominates the adsorption binding energy of the DP molecules on the solid surface. The results provide molecular evidence of the interface structures of organic semiconductors on electrode surfaces.     

Rastertunnelmikroskopie an chiralen Molekülen: Nanochemie

The imaging and manipulation capabilities of the scanning tunnelling microscope (STM) render possible a novel nanoscale chemistry based on experiments with single molecules. Herein, we address several aspects of a nanoscale stereochemistry using the STM. As an example, we investigate 1‐nitronaphthalene on Au(111). 1‐Nitronaphthalene becomes chiral upon planar adsorption on the metal surface. High‐resolution STM images reflect the asymmetric electronic structure of the molecules and allow for the determination of the absolute configuration of any individual molecule within complex molecular structures. At medium coverage, spontaneous breaking of the chiral symmetry results in the formation of homochiral conglomerates, while at high coverage racemic structures prevail. Finally, the tip of the STM is used to separate “supramolecule‐by‐supramolecule” a racemic mixture of chiral 1‐nitronaphthalene aggregates into the enantiopure compounds.     

Highly Enantioselective Adsorption of Small Prochiral Molecules on a Chiral Intermetallic Compound

Intrinsically chiral surfaces of intermetallic compounds are shown to be novel materials for enantioselective processes. Their advantage is the significantly higher thermal and chemical stability, and therefore their extended application range for catalyzed chiral reactions compared to surfaces templated with chiral molecular modifiers or auxiliaries. On the Pd₁‐terminated PdGa(111) surface, room‐temperature adsorption of a small prochiral molecule (9‐ethynylphenanthrene) leads to exceptionally high enantiomeric excess ratios of up to 98 %. Our findings highlight the great potential of intrinsically chiral intermetallic compounds for the development of novel, enantioselective catalysts that can be operated at high temperatures and potentially also in harsh chemical environments.     

Two-dimensional Molecular Phase Transition of Alkylated-TDPB on Au(111) and Cu(111) Surfaces

The derivatives of aromatic cores bearing alkyl chains with different lengths are of potential interest in on-surface chemistry, and thus have been widely investigated both at liquid-solid interfaces and in vacuum. Here, we report on the structural evaluation of self-assembled 1,3,5-tri(4-dodecylphenyl)benzene(TDPB) molecules with increased molecular coverages on both Au(111) and Cu(111) surfaces. As observed on Au(111), rhombic and herringbone structures emerge successively depending on surface coverage. In the case of Cu(111), the same process of phase conversion is also observed, but with two distinct structures. In comparison, the self-assembled structures on Au(111) surface are packed more densely than that on Cu(111) surface under the same preparation conditions. This may fundamentally result from the higher adsorption energy of TDPB molecules on Cu(111), restricting their adjustment to optimize a thermodynamically favorable molecular packing.     

Observation of hierarchical chiral structures in 8-nitrospiropyran monolayers

The adsorption and self-organization of racemic mixture of 8-nitrospiropyran (SP8) molecules on Au(111) surfaces was studied by scanning tunneling microscopy (STM) in ultrahigh vacuum (UHV). The SP8 enantiomers, in spite of their low-symmetric and nonplanar molecular structures, formed well-ordered monolayers on Au(111). In the monolayers, we found two types of enantiomorphous, i.e., mirror-imaged, 2D chiral domains, denoted as lambda and delta phases. Both phases consist of periodically packed chiral quatrefoils. In the lambda domain, the quatrefoils are counterclockwise folded, while in the delta domain, the quatrefoils are clockwise folded. High-resolution STM images revealed that each chiral quatrefoil contains four heterochiral dimers and that each dimer is composed of two antiparallelly packed homochiral SP8 molecules. Therefore both of the two mirror-imaged 2D chiral structures are not chirally pure but racemic 2D crystals. A domain boundary, which serves as the glide reflection line between a lambda domain and a delta domain, was also observed along the [11] direction of the Au(111) substrate.     

The influence of molecular structure on helical twisting power of chiral azobenzene compounds

Photochromic chiral azobenzene compounds with different molecular structures were synthesized, and a cholesteric phase was induced by mixing each chiral azobenzene compound with a non-photochromic chiral compound in a host nematic liquid crystal, E44. Helical pitch and, thus, helical twisting powers (HTP) of the chiral azobenzene compounds and the non-photochromic chiral compound were determined by Cano's wedge method. Molecular structures of the chiral azobenzene compounds were predicted by means of determining their molecular aspect ratio (L/D) with semiempirical molecular calculations (MOPAC at PM3 level). The effects of molecular structure on HTP of the chiral azobenzene compounds are studied in detail. Molecular structures of chiral azobenzene compounds significantly influence their HTPs.     

Au单晶表面氟表面活性剂FSN自组装膜的电化学行为

研究Au(111)和Au(100)表面非离子型氟表面活性剂FSN自组装膜的电化学行为.电化学扫描隧道显微术和循环伏安法测试表明,在0～0.8 V电位区间,FSN自组装膜未发生氧化还原,均一性好,可稳定地存在于电极表面,并显著抑制硫酸根离子在电极表面的吸附和Au单晶表面的重构.在FSN自组装膜Au单晶电极的初始氧化阶段,Au(111)表面有少量突起,而Au(100)表面呈现台阶剧烈变化,但FSN自组装膜的吸附结构没有改变.与Au(100)表面相比,Au(111)表面形成的FSN自组装膜可更有效地抑制Au表面的氧化.     

Site- and orientation-selective anchoring of a prototypical molecular building block

The controlled anchoring of molecular building blocks on appropriate templates is a major prerequisite for the rational design and fabrication of supramolecular architectures on surfaces. We report on a particularly selective adsorption process of hexa-peri-hexabenzocoronene on Au(111), which leads to well-controlled adsorption position and orientation of the polycyclic aromatic hydrocarbons. Scanning tunneling microscopy reveals selective adsorption on monatomic steps in the fcc stacking regions with a specific orientation of 18 degrees between the molecular axis and the step normal. Ab initio calculations for various adsorption sites reveal the lowest total energy for adsorption on a kink site. Energy considerations and the excellent agreement between experimental and simulated images show that adsorption on kink sites is responsible for the specific adsorption angle.     

Fullerene adlayers on various single-crystal metal surfaces prepared by transfer from L films

Fullerene adlayers prepared by the simple Langmuir-Blodgett (LB) method onto various well-defined single-crystal metal surfaces were investigated by in situ scanning tunneling microscopy (STM). The surface morphologies of fullerene adsorbed onto metal surfaces depended largely on the adsorbate-substrate interactions, which are governed by the types of surfaces. Too weak adsorption of C60 molecules onto iodine-modified Au(111) (I/Au(111)) allows surface migration of the molecules, and then, STM cannot visualize the C60 molecules. Stronger and appropriate adsorption onto bare Au(111) leads to highly ordered arrays relatively easily due to the limited surface migration of C60. On iodine-modified Pt(111) (I/Pt(111)) and bare Pt(111) surfaces, which have stronger adsorption, randomly adsorbed molecular adlayers were observed. Although C60 molecules on Au(111) were visualized as a featureless ball due to the maintenance of the rapid rotational motion (perturbation) of C60 on the surface at room temperature, those on I/Pt(111) revealed the intramolecular structures, thus indicating that the perturbation motion of molecules on the surface was prohibited.     

Self-assembly of organic molecules at metal surfaces

Self-assembly represents a promising strategy for surface functionalisation as well as creating nanostructures with well-controlled, tailor-made properties and functionality. Molecular self-assembly at solid surfaces is governed by the subtle interplay between molecule–molecule and molecule–substrate interactions that can be tuned by varying molecular building blocks, surface chemistry and structure as well as substrate temperature.In this review, basic principles behind molecular self-assembly of organic molecules on metal surfaces will be discussed. Controlling these formation principles allows for creating a wide variety of different molecular surface structures ranging from well-defined clusters, quasi one-dimensional rows to ordered, two-dimensional overlayers. An impressive number of studies exist, demonstrating the ability of molecular self-assembly to create these different structural motifs in a predictable manner by tuning the molecular building blocks as well as the metallic substrate.Here, the multitude of different surface structures of the natural amino acid cysteine on two different gold surfaces observed with scanning tunnelling microscopy will be reviewed. Cysteine on Au(110)-(1×2) represents a model system illustrating the formation of all the above mentioned structural motifs without changing the molecular building blocks or the substrate surface. The only parameters in this system are substrate temperature and molecular coverage, controlling both the molecular adsorption state (physisorption versus chemisorption) and molecular surface mobility. By tuning the adsorption state and the molecular mobility, distinctly different molecular structures are formed, exemplifying the variety of structural motifs that can be achieved by molecular self-assembly.     

Bridging the pressure gap in model systems for heterogeneous catalysis with high-pressure scanning tunneling microscopy

A high-pressure scanning tunneling microscope (HP-STM) enabling imaging with atomic resolution over the entire pressure range from ultrahigh vacuum (UHV) to one bar has been developed. By means of this HP-STM we have studied the adsorption of hydrogen on Cu(110), CO on Pt(110) and Pt(111), and NO on Pd(111) at high pressures. For all of these adsorption systems we find that the adsorption structures formed at high pressures are identical to high-coverage structures formed at lower pressures and temperatures. We thus conclude that for these systems the so-called pressure gap can be bridged, i.e. the results obtained under conventional surface science conditions can be extrapolated to higher pressures. Finally, we use the HP-STM to image the CO-induced phase separation of a Au/Ni(111) surface alloy in real time, whereby demonstrating the importance of catalyst stability in the study of bimetallic systems.     

Self‐Assembly of Decoupled Borazines on Metal Surfaces: The Role of the Peripheral Groups

Two borazine derivatives have been synthesised to investigate their self‐assembly behaviour on Au(111) and Cu(111) surfaces by scanning tunnelling microscopy (STM) and theoretical simulations. Both borazines form extended 2D networks upon adsorption on both substrates at room temperature. Whereas the more compact triphenyl borazine 1 arranges into close‐packed ordered molecular islands with an extremely low density of defects on both substrates, the tris(phenyl‐4‐phenylethynyl) derivative 2 assembles into porous molecular networks due to its longer lateral substituents. For both species, the steric hindrance between the phenyl and mesityl substituents results in an effective decoupling of the central borazine core from the surface. For borazine 1 , this is enough to weaken the molecule–substrate interaction, so that the assemblies are only driven by attractive van der Waals intermolecular forces. For the longer and more flexible borazine 2 , a stronger molecule–substrate interaction becomes possible through its peripheral substituents on the more reactive copper surface.     

A two-dimensional molecular network structure of trimesic acid prepared by adsorption-induced self-organization

A two-dimensional molecular network of trimesic acid on Au(111) was visualized by in situ scanning tunneling microscopy with submolecular resolution. The supramolecular structures including an 'order to order' phase transition were constructed by precise potential-controlled adsorption based on adsorption-induced self-organization.     

Chiral morphologies and interfacial electronic structure of naphtho[2,3-a]pyrene on Au(111)

The adsorption of the two-dimensionally chiral naphtho[2,3-a]pyrene molecule has been studied on Au(111). Both structural and electronic properties of the naphtho[2,3-a]pyrene (NP)/Au(111) interface have been measured. Ultraviolet and X-ray photoelectron spectroscopy have been employed to measure the energies of the molecular orbitals of the NP film with respect to the gold Fermi level. A Schottky junction with a large interface dipole (0.99 eV) is formed between Au(111) and NP. Temperature-programmed desorption was used to determine that adsorbed NP has a binding energy of 102.2 kJ/mol. Chiral domains have been observed with scanning tunneling microscopy due to the spontaneous phase separation of the 2-D enantiomers. Two distinct structural polymorphs have been observed, one of which has homochiral paired molecular rows. Models of the 2D structure are proposed that are in excellent agreement with experimental measurements.