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

Procrystalline Self-Assembly of Desymmetrized Pentaphenylcyclopentadiene

The interplay between the molecular shape and the intermolecular interaction plays a decisive role in self-assembled structures. Recently, inherent randomness of low ordered assemblies, resulting from lack of short- and long-range periodicities, has attracted significant attention due to the unique structural, electronic, and mechanical properties. Here, we present procrystalline self-assemblies of pentaphenyl cyclopentadienyl derivatives on Ag(111) and Au(111) with scanning tunneling microscopy, operating at 4.3 K under ultrahigh vacuum conditions. Two examples, using 5-fold symmetric molecules substituted with methyl or fluorine groups, show that weak interactions, such as π-π stacking, CH-π interactions, and CH···F hydrogen bonding, play a pivotal role in formation of the procrystalline assembly. Our results may give insights into the intricate relationship between the molecular shape and the intermolecular interaction in the formation of non-crystalline assemblies.

Two-Dimensional Crystal Transition from Radialene to Cumulene on Ag(111) via Retro-[2 + 1] Cycloaddition

Two-dimensional (2D) crystal-to-crystal transition is an important method in crystal engineering because of its ability to directly create diverse crystal materials from one crystal. However, steering a 2D single-layer crystal-to-crystal transition on surfaces with high chemo- and stereoselectivity under ultra-high vacuum conditions is a great challenge because the transition is a complex dynamic process. Here, we report a highly chemoselective 2D crystal transition from radialene to cumulene with retention of stereoselectivity on Ag(111) via retro-[2 + 1] cycloaddition of three-membered carbon rings and directly visualize the transition process involving a stepwise epitaxial growth mechanism by the combination of scanning tunneling microscopy and non-contact atomic force microscopy. Using progression annealing, we found that isocyanides on Ag(111) at a low annealing temperature underwent sequential [1 + 1 + 1] cycloaddition and enantioselective molecular recognition based on C-H···Cl hydrogen bonding interactions to form 2D triaza[3]radialene crystals. In contrast, a higher annealing temperature induced the transformation of triaza[3]radialenes to generate trans-diaza[3]cumulenes, which were further assembled into 2D cumulene-based crystals through twofold N-Ag-N coordination and C-H···Cl hydrogen bonding interactions. By combining the observed distinct transient intermediates and density functional theory calculations, we demonstrate that the retro-[2 + 1] cycloaddition reaction proceeds via the ring opening of a three-membered carbon ring, sequential dechlorination/hydrogen passivation, and deisocyanation. Our findings provide new insights into the growth mechanism and dynamics of 2D crystals and have implications for controllable crystal engineering.

An intermolecular hydrogen bond plays a determining role in product selection of a surface confined Schiff-base reaction

Herein, we illustrate how the cooperation of intermolecular hydrogen bonds and conformation flexibility leads to the formation of diverse complex covalent nanostructures on the surface, while the relative abundance of the final products can be further tuned by adjusting the molar ratio and concentration of monomers.

On-surface synthesis of gold–coronene molecular wires

Perchlorocoronene undergoes selective dehalogenation under the one-dimensional constraint by intermolecular interactions, resulting in the formation of gold–coronene wires on Au(111) surfaces.

On-surface synthesis of 2D COFs on Cu(111) via the formation of thermodynamically stable organometallic networks as the template

Template-directed polymerization is an effective approach used to afford regular 2D covalent organic frameworks (COFs), thus the regularity of the template is crucial for the quality of the resulting 2D COFs. For the Ullmann reactions on Cu(111), aryl iodides and bromides are activated at low temperature to form organometallic C-Cu-C structures, which lead to kinetic trapping and irregular organometallic networks. Therefore, the subsequent annealing step can only afford irregular 2D COFs. In this manuscript, the molecule 4,4''-dibromo-5'-(4-chlorophenyl)-1,1':3',1''-terphenyl incorporated two Br terminals and one Cl terminal has been used to demonstrate different reactivities of a C-Cl bond and a C-Br bond via the hierarchical activation of the C-Br bond and the C-Cl bond on Cu(111). At room temperature, zigzag, armchair, and ring-like organometallic chains formed due to the activation of the C-Br bond to generate a C-Cu-C structure while C-Cl remained intact, illustrating that the C-Cl bond is more stable than C-Br. Further annealing at 433 K activated the C-Cl bond to produce regular organometallic networks as the thermodynamic product. Using the simpler molecule 1,3,5-tris(4-chlorophenyl)benzene as the precursor, the self-assembly of the intact molecules was observed on Cu(111) at 300 K without activation of the C-Cl bond. After annealing at 433 K, similar thermodynamically stable organometallic networks formed directly, which were used as a template to generate regular 2D COFs upon further annealing at 510 K.

Confined Complexation Reaction of Metal Ions with a Lipophilic Surfactant at the Water/Air Interface: A New Understanding Based on Surface Experiments and Molecular Dynamics Simulations

Understanding the fundamentals of confined chemical reaction was an important subject in various heterogeneous physicochemical processes. Here, we investigated the orientation behavior of an amphiphilic ligand, the tri- n-octylphosphine oxide (TOPO), in a compressed monolayer at the air/water interface and its impact on the complexation reactivity of TOPO molecules with chromate ions at the interface. The analysis of sum frequency generation and polarization modulation infrared reflection absorption spectroscopy experiments combined with surface pressure measurements reveals a significant dependence of the adsorption rate and saturated concentration of chromate ions on the orientation of TOPO molecules during the increase of the surface pressure. In parallel, the analysis of molecular dynamics simulations indicates that the interaction energy between TOPO molecules and chromate ions is strongly sensitive to the orientation of TOPO molecules confined at the water/air interface. The present work provides a novel insight into the unique chemical reactivity of molecules in a confined microenvironment, and it provides a basis for further progresses in improving chemical reactivity and selectivity of molecules in a confined environment by regulating confinement of molecules.

Reversibility and intermediate steps as key tools for the growth of extended ordered polymers via on-surface synthesis

Surface-confined polymerization is a bottom-up strategy to create one- and two-dimensional covalent organic nanostructures with a π-conjugated backbone, which are suitable to be employed in real-life electronic devices, due to their high mechanical resistance and electronic charge transport efficiency. This strategy makes it possible to change the properties of the final nanostructures by a careful choice of the monomer architecture (i.e. of its constituent atoms and their spatial arrangement). Several chemical reactions have been proven to form low-dimensional polymers on surfaces, exploiting a variety of precursors in combination with metal (e.g. Cu, Ag, Au) and insulating (e.g. NaCl, CaCO₃) surfaces. One of the main challenges of such an approach is to obtain nanostructures with long-range order, to boost the conductance performances of these materials. Most of the exploited chemical reactions use irreversible coupling between the monomers and, as a consequence, the resulting structures often suffer from poor order and high defect density. This review focuses on the state-of-the-art surface-confined polymerization reactions, with particular attention paid to reversible coupling pathways and irreversible processes including intermediate states, which are key aspects to control to increase the order of the final nanostructure.

Ullmann coupling reaction of aryl chlorides on Au(111) using dosed Cu as a catalyst and the programmed growth of 2D covalent organic frameworks

The efficiency of Ullmann reaction of aryl chlorides on an Au(111) surface has been substantially increased by using dosed Cu as a catalyst. The different reactivity of aryl bromides and aryl chlorides has been exploited to design a programmed, on-surface synthesis to form 2D covalent organic frameworks.

Non-intuitive clustering of 9,10-phenanthrenequinone on Au(111)

The direct injection of a 9,10-phenanthrenequinone in tetrahydrofuran solution on a Au(111) substrate in high vacuum results in the formation of metastable clusters with a non-intuitive structure. Metastable, rectangular tetramers of this molecule form in which the net molecular dipoles all orient toward the center of the cluster. This structure does not allow for additional hydrogen bonding and thus the origin of its metastability is not clear. We compare this feature to other structures observed on this surface, as well as those formed during the deposition of 9-fluorenone, which does not exhibit this anomalous clustering behavior.

Chemistry at the square nanometer: reactivity at liquid/solid interfaces revealed with an STM

An overview is given of single molecule reactivity at a liquid/solid interface employing a scanning tunneling microscope.

Room-Temperature Synthesis of Covalent Organic Frameworks with a Boronic Ester Linkage at the Liquid/Solid Interface

With various prospected applications in the field of nanoelectronics and catalysis, on-surface synthesis of single-layer covalent organic frameworks (surface COFs) with designable structures and properties have attracted enormous interest. Herein, we report on a scanning tunneling microscopic investigation of the surface-confined synthesis of a covalently bonded boronic ester network directly at the octanoic acid/ highly oriented pyrolytic graphite(HOPG) interface under room temperature. The dynamic reaction process was investigated in detail. STM results indicate that the surface networks undergo structural evolution from a hybrid covalent/noncovalent multiwall porous network to single-wall hexagonal COF with the decrease of monomer concentration. Further experimental observation disclosed that the boronic ester-linked system is sensitive to instantaneous voltage pulses and the stimulation of the STM tip. In addition, the ¹ H NMR spectra has further confirmed that the surface and octanoic acid may play important roles in promoting the reaction between 4,4'-phenylazobenzoyl diboronic acid (ABBA) and 2,3,6,7,10,11-hexahydroxytriphenylene (HHTP) building units.

Ullmann Reaction of Aryl Chlorides on Various Surfaces and the Application in Stepwise Growth of 2D Covalent Organic Frameworks

On-surface Ullmann coupling reaction of aryl chlorides has been achieved on Cu(111), Ag(111), and Au(111), and the mechanism has been investigated on the single molecule level using scanning tunneling microscopy and density functional theory. The different reactivity of the aryl halides was utilized to design a stepwise on-surface synthesis, which affords a zigzag template and then converts to 2D porous networks.

Multi-Component Organic Layers on Metal Substrates

Increasingly high hopes are being placed on organic semiconductors for a variety of applications. Progress along these lines, however, requires the design and growth of increasingly complex systems with well-defined structural and electronic properties. These issues have been studied and reviewed extensively in single-component layers, but the focus is gradually shifting towards more complex and functional multi-component assemblies such as donor-acceptor networks. These blends show different properties from those of the corresponding single-component layers, and the understanding on how these properties depend on the different supramolecular environment of multi-component assemblies is crucial for the advancement of organic devices. Here, our understanding of two-dimensional multi-component layers on solid substrates is reviewed. Regarding the structure, the driving forces behind the self-assembly of these systems are described. Regarding the electronic properties, recent insights into how these are affected as the molecule's supramolecular environment changes are explained. Key information for the design and controlled growth of complex, functional multicomponent structures by self-assembly is summarized.

Surface-Activated Coupling Reactions Confined on a Surface

Chemical reactions may take place in a pure phase of gas or liquid or at the interface of two phases (gas-solid or liquid-solid). Recently, the emerging field of "surface-confined coupling reactions" has attracted intensive attention. In this process, reactants, intermediates, and products of a coupling reaction are adsorbed on a solid-vacuum or a solid-liquid interface. The solid surface restricts all reaction steps on the interface, in other words, the reaction takes place within a lower-dimensional, for example, two-dimensional, space. Surface atoms that are fixed in the surface and adatoms that move on the surface often activate the surface-confined coupling reactions. The synergy of surface morphology and activity allow some reactions that are inefficient or prohibited in the gas or liquid phase to proceed efficiently when the reactions are confined on a surface. Over the past decade, dozens of well-known "textbook" coupling reactions have been shown to proceed as surface-confined coupling reactions. In most cases, the surface-confined coupling reactions were discovered by trial and error, and the reaction pathways are largely unknown. It is thus highly desirable to unravel the mechanisms, mechanisms of surface activation in particular, of the surface-confined coupling reactions. Because the reactions take place on surfaces, advanced surface science techniques can be applied to study the surface-confined coupling reactions. Among them, scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS) are the two most extensively used experimental tools. The former resolves submolecular structures of individual reactants, intermediates, and products in real space, while the latter monitors the chemical states during the reactions in real time. Combination of the two methods provides unprecedented spatial and temporal information on the reaction pathways. The experimental findings are complemented by theoretical modeling. In particular, density-functional theory (DFT) transition-state calculations have been used to shed light on reaction mechanisms and to unravel the trends of different surface materials. In this Account, we discuss recent progress made in two widely studied surface-confined coupling reactions, aryl-aryl (Ullmann-type) coupling and alkyne-alkyne (Glaser-type) coupling, and focus on surface activation effects. Combined experimental and theoretical studies on the same reactions taking place on different metal surfaces have clearly demonstrated that different surfaces not only reduce the reaction barrier differently and render different reaction pathways but also control the morphology of the reaction products and, to some degree, select the reaction products. We end the Account with a list of questions to be addressed in the future. Satisfactorily answering these questions may lead to using the surface-confined coupling reactions to synthesize predefined products with high yield.

Formation of C═C bond via knoevenagel reaction between aromatic aldehyde and barbituric acid at liquid/HOPG and vapor/HOPG interfaces

Controlling chemical reactions on surface is of great importance to constructing self-assembled covalent nanostructures. Herein, Knoevenagel reaction between aromatic aldehyde compound 2,5-di(5-aldehyde-2-thienyl)-1,4-dioctyloxybenzene (PT2) and barbituric acid (BA) has been successfully performed for the first time at liquid/HOPG interface and vapor/HOPG interface. The resulting surface nanostructures and the formation of C═C bond are recorded through scanning tunneling microscopy (STM), and confirmed by attenuated total reflectance Fourier-transform infrared (ATR/FT-IR) spectrometer and UV-vis absorption. The obtained results reveal that Knoevenagel condensation reaction can efficiently occur at both interfaces. This surface reaction would be an important step toward further reaction to produce innovative conjugated nanomaterial on the surface.

Adsorption-geometry induced transformation of self-assembled nanostructures of an aldehyde molecule on Cu(110)

From an interplay of high-resolution STM imaging/manipulation and DFT calculations, we have revealed that different self-assembled nanostructures of BA molecules on Cu(110) are attributable to specific molecular adsorption geometries, and thus the corresponding intermolecular hydrogen bonding patterns. The STM manipulations demonstrate the feasibility of switching such weak-hydrogen-bonding patterns.

Substrate templating guides the photoinduced reaction of C60 on calcite

A substrate-guided photochemical reaction of C60 fullerenes on calcite, a bulk insulator, investigated by non-contact atomic force microscopy is presented. The success of the covalent linkage is evident from a shortening of the intermolecular distances, which is clearly expressed by the disappearance of the moiré pattern. Furthermore, UV/Vis spectroscopy and mass spectrometry measurements carried out on thick films demonstrate the ability of our setup for initiating the photoinduced reaction. The irradiation of C60 results in well-oriented covalently linked domains. The orientation of these domains is dictated by the lattice dimensions of the underlying calcite substrate. Using the lattice mismatch to deliberately steer the direction of the chemical reaction is expected to constitute a general design principle for on-surface synthesis. This work thus provides a strategy for controlled fabrication of oriented, covalent networks on bulk insulators.

On-surface azide-alkyne cycloaddition on Au(111)

We present [3 + 2] cycloaddition reactions between azides and alkynes on a Au(111) surface at room temperature and under ultrahigh vacuum conditions. High-resolution scanning tunneling microscopy images reveal that these on-surface cycloadditions occur highly regioselectively to form the corresponding 1,4-triazoles. Density functional theory simulations confirm that the reactions can occur at room temperature, where the Au(111) surface does not participate as a catalytic agent in alkyne C-H activation but acts solely as a two-dimensional constraint for the positioning of the two reaction partners. The on-surface azide-alkyne cycloaddition offers great potential toward the development and fabrication of functional organic nanomaterials on surfaces.

Insight into organometallic intermediate and its evolution to covalent bonding in surface-confined ullmann polymerization

We provide insight into surface-catalyzed dehalogenative polymerization, analyzing the organometallic intermediate and its evolution into planar polymeric structures. A combined study using scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), low energy electron diffraction (LEED), near-edge X-ray absorption fine structure (NEXAFS) spectroscopy and first-principles calculations unveils the structural conformation of substrate-bound phenylene intermediates generated from 1,4-dibromobenzene precursors on Cu(110), showing the stabilizing role of the halogen. The appearance of covalently bonded conjugated structures is followed in real time by fast-XPS measurements (with an acquisition time of 2 s per spectrum and heating rate of 2 K/s), showing that the detaching of phenylene units from the copper substrate and subsequent polymerization occur upon annealing above 460 ± 10 K.

Sequential and site-specific on-surface synthesis on a bulk insulator

The bottom-up construction of functional devices from molecular building blocks offers great potential in tailoring materials properties and functionality with utmost control. An important step toward exploiting bottom-up construction for real-life applications is the creation of covalently bonded structures that provide sufficient stability as well as superior charge transport properties over reversibly linked self-assembled structures. On-surface synthesis has emerged as a promising strategy for fabricating stable, covalently bound molecular structure on surfaces. So far, a majority of the structures created by this method have been obtained from a rather simple one-step processing approach. But the on-surface preparation of complex structures will require the possibility to carry out various reaction steps in a sequential manner as done in solution chemistry. Only one example exists in literature in which a hierarchical strategy is followed to enhance structural complexity and reliability on a metallic surface. Future molecular electronic application will, however, require transferring these strategies to nonconducting surfaces. Bulk insulating substrates are known to pose significant challenges to on-surface synthesis due to the absence of a metal catalyst and their low surface energy, frequently resulting in molecule desorption rather than reaction activation. By carefully selecting a suitable precursor molecule, we succeeded in performing a two-step linking reaction on a bulk insulating surface. Besides a firm anchoring toward the substrate surface, the reaction sites and sequential order are encoded in the molecular structure, providing so far unmatched reaction control in on-surface synthesis on a bulk insulating substrate.

Steering on-surface polymerization with metal-directed template

On-surface polymerization represents a novel bottom-up approach for producing macromolecular structures. To date, however, most of the structures formed using this method exhibit a broad size distribution and are disorderly adsorbed on the surface. Here we demonstrate a strategy of using metal-directed template to control the on-surface polymerization process. We chose a bifunctional compound which contains pyridyl and bromine end groups as the precursor. Linear template afforded by pyridyl-Cu-pyridyl coordination effectively promoted Ullmann coupling of the monomers on a Au(111) surface. Taking advantage of efficient topochemical enhancement owing to the conformation flexibility of the Cu-pyridyl bonds, macromolecular porphyrin structures that exhibit a narrow size distribution were synthesized. We used scanning tunneling microscopy and kinetic Monte Carlo simulation to gain insights into the metal-directed polymerization at the single molecule level. The results reveal that the polymerization process profited from the rich chemistry of Cu which catalyzed the C-C bond formation, controlled the size of the macromolecular products, and organized the macromolecules in a highly ordered manner on the surface.

Formation of imine oligomers on Au under ambient conditions investigated by scanning tunneling microscopy

Scanning tunneling microscopy has been used to investigate the nucleophilic substitution reaction between melamine (1,3,5-triazine-2,4,6-triamine) and terephthalaldehyde on Au/mica following deposition from solution in ambient conditions. The reaction is observed to proceed at room temperature over a time scale of days with the formation of imine oligomers intermixed with melamine islands on the Au surface. The oligomers ultimately self-assemble into a porous arrangement. The mechanism and extent of the surface confined reactions are discussed.

In situ STM investigation of aromatic poly(azomethine) arrays constructed by "on-site" equilibrium polymerization

Two-dimensional (2D) arrays of π-conjugated aromatic polymers produced by surface-selective Schiff base coupling reactions between an aromatic diamine and an aromatic dialdehyde were investigated in detail using in situ scanning tunneling microscopy. Surface-selective coupling was achieved for almost all diamine/dialdehyde combinations attempted, although several combinations did not proceed even in homogeneous aqueous alkaline solution. Most of the combinations of an aromatic diamine and a dialdehyde, except the combinations of 4,4'-azodianiline with mono/bithiophenedicarboxaldehyde, formed highly ordered π-conjugated polymer arrays on an iodine-modified Au(111) surface in aqueous solution at a suitable pH. The simplest polymer of the various combinations tested, obtained from the combination of 1,4-diaminobenzene with terephthaldicarboxaldehyde, gave a 2D array consisting of linearly connected benzene units. Poly(azomethine) adlayers caused a positive shift in the electrochemical potential of the butterfly shaped oxidative adsorption and reductive desorption of iodine. The acceleration of the reductive desorption of iodine suggests the existence of a weak interaction between the polymer layer and iodine. Not only the first polymer adlayers but also partially adsorbed secondary adlayers with "on-top" epitaxial behavior were frequently observed for all polymer systems. The alignment of the polymer chains in the adlayers possessed a certain regularity in terms of a regular interval between polymer chains because of repulsive interpolymer interactions.

Isoreticular two-dimensional covalent organic frameworks synthesized by on-surface condensation of diboronic acids

On-surface self-condensation of 1,4-benzenediboronic acid was previously shown to yield extended surface-supported, long-range-ordered two-dimensional covalent organic frameworks (2D COFs). The most important prerequisite for obtaining high structural quality is that the polycondensation (dehydration) reaction is carried out under slightly reversible reaction conditions, i.e., in the presence of water. Only then can the subtle balance between kinetic and thermodynamic control of the polycondensation be favorably influenced, and defects that are unavoidable during growth can be corrected. In the present study we extend the previously developed straightforward preparation protocol to a variety of para-diboronic acid building blocks with the aim to tune lattice parameters and pore sizes of 2D COFs. Scanning tunneling microscopy is employed for structural characterization of the covalent networks and of noncovalently self-assembled structures that form on the surface prior to the thermally activated polycondensation reaction.

Combined SPM investigation on the interfacial structure of a phthalocyanine/conjugated polymer composite film

The morphology of the composite film of organic semiconductors determines the properties and performances of devices to a large extent. In this work, we present a combined AFM and STM study on the interfacial structures of CuPcOC8 and CuPcOC8/PmPV composite films on graphite surface. For CuPcOC8 thin films, the face-on epitaxial growth of CuPcOC8 could persist within 3 to 5 monolayers and the formation of π-π stacked columns will occur with edge-on configuration when the film thickness further increases. For the CuPcOC8/PmPV composite film with 1:1 weight ratio, STM results reveal a preferential adsorption of PmPV on graphite surface, while AFM results indicate the phase segregation in the upper layer. STM also reveals in the molecular scale good compatibility of CuPcOC8 with PmPV.

Fullerene/porphyrin multicomponent nanostructures on Ag(110): from supramolecular self-assembly to extended copolymers

A novel two-step bottom-up approach to construct a 2D long-range ordered, covalently bonded fullerene/porphyrin binary nanostructure is presented: in the first place, reversible supramolecular interactions between C60 and 5,15-bis(4-aminophenyl)-10,20-diphenylporphyrin are exploited to obtain large domains of an ordered binary network, subsequently a reaction between fullerene molecules and the amino-groups residing on porphyrin units, triggered by thermal treatment, is used to freeze the supramolecular nanostructure with covalent bonds. The resulting nanostructure resists high temperature treatments as expected for an extended covalent network, whereas very similar fullerene/porphyrin nanostructures held together only by weak interactions are disrupted upon annealing at the same or at lower temperatures.

Templating molecular adsorption using a covalent organic framework

A two-dimensional nanoporous covalent organic framework can be prepared on a Au(111) substrate with near complete surface coverage and can be used to control the organisation of a sublimed layer of C(60).

Self-assembly of enantiopure domains: the case of indigo on Cu(111)

The adsorption of indigo molecules on Cu(111) was investigated by low temperature (5 K) scanning tunneling microscopy from the isolated single molecule regime to one monolayer. Structural optimization and image calculations demonstrate that the molecules are in a physisorbed state. Because of the reduced symmetry at the surface, single molecules acquire a chiral character upon adsorption leading to a two-dimensional (2D) chirality. They adopt two adsorption configurations, related by a mirror symmetry of the substrate, each with a distinct molecular orientation. Consequently, the 2D chirality is expressed by the orientation of the molecule. For higher coverage, molecules self-assemble by hydrogen bonding in nearly homochiral molecular chains, whose orientation is determined by the orientation taken by the isolated molecules. When the coverage approaches one monolayer, these chains pack into domains. Finally, the completion of the monolayer induces the expulsion of the molecules of the wrong chirality that are still in these domains, leading to perfect resolution in enantiopure domains.

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.

Supramolecular chemistry at interfaces: molecular recognition on nanopatterned porous surfaces

Through the illustration of key examples that have recently appeared in the literature, the intention of this review is to provide a perspective of current advances on the molecular recognition at the interfaces aimed at the engineering of multifunctional organic-based materials. The great interest in such systems has been motivated by the need to fabricate smaller and smaller components in order to improve, for example, the information storage capabilities of classical silicon-based devices. Although great progress has been achieved on the exploitation of "top-down" approaches, strong hope is now put on the development of hybrid devices in which the elementary components are replaced with single organic molecules. Nevertheless, the drive towards such devices is restricted by both their stability and difficulties to precisely control and manipulate the structural organisation at the molecular level. To overcome these restrictions, the use of nanotemplated surfaces featuring porous domains in which responsive functional molecules can be precisely accommodated at the single-molecule level is one of the most promising approaches. In the first part of this manuscript, we therefore illustrate the main engineering strategies [1) through non-covalent interactions, 2) surface-confined covalent reactions and 3) assembly of pre-organised cavities such as synthetic macrocycles] currently in use to create two-dimensional (2D) patterned surfaces displaying porous structures at the nanoscale level. Such networks, featuring periodic hollow domains (controllable both in shape and size), are of particular significance as their cavities can be used as receptors for the recognition of remotely controllable functional molecules. In the second part, the confinement of molecular guests within the cavities is discussed, emphasising the selectivity and dynamics of key assemblies, with a particular focus on the biomolecular recognition and post-assembly covalent functionalisation, which could provide the opportunity to fabricate devices currently beyond our reach on an unprecedented precision and efficiency. All the examples will be discussed in terms of structural organisation as studied by scanning tunnelling microscopy (STM) techniques.