A rack-and-pinion device at the molecular scale

Regulating Rotational Dynamics of Co-Adsorbed Guest Molecules via Halogen Bonds in Functionalized Pores of Self-Assembled Molecular Networks at the Liquid-Solid Interface

Understanding and controlling molecular rotation on surfaces is crucial for the development of molecular-scale artificial motors that operate at interfaces. Herein, it is reported the successful co-adsorption of guest molecules within the functionalized 2D pores of self-assembled molecular networks (SAMNs) through directional halogen bonding, as confirmed by scanning tunneling microscopy. Specifically, the porous SAMN formed by dehydrobenzo[12]annulene derivative DBA-Py with a pyridyl group at the termini of its three alkoxy chains, hosts an iodinated trigonal guest molecule, tris(4-iodophenyl)benzene (TIB), through a halogen bond between the nitrogen and iodine atoms. Within the pores, the TIB molecule exhibits rotational motion, preferentially residing at two locations. In contrast, within the pores formed by a mixture of DBA-Py and DBA-Ph, where DBA-Ph features three phenyl groups instead of pyridyl groups, the guest molecule preferentially resides in a single location. This behavior is attributed to the reduced number of energy minima within the pores owing to the decreased number of pyridyl units. Statistical analysis of the guest orientation suggests that the on-surface arrangement of DBA-Py and DBA-Ph is influenced by the guest molecule. This modular approach using functionalized pores in SAMNs provides an effective strategy for controlling molecular rotational behavior.

Repetitive Rolling of Triptycene-Based Molecules on Cu Surfaces

The metal surface-supported rotation of artificial molecular structures is technologically important for developing molecular-level devices. The key factors leading to the practical applications of these molecular machines on metal surfaces are the atomic-scale control of the rotation and the counterbalance of the temperature-driven instability of the molecules. In this work, we present a means by which triptycene-based molecular wheels can roll repetitively on a metal surface. Our results show that regularly stepped surfaces are the perfect candidate not only for stabilizing the molecule on the metal surface but also for providing the pivot points needed for repetitive vertical rotation of the molecule at higher temperatures. In addition to the geometrical compatibility of the substrate and the molecule, intermittent application of the external electric field is needed for rolling the molecule on a metal-stepped surface in a controllable manner.

Synthesis of Ce(IV) Heteroleptic Double-Decker Complex with a New Helical Naphthalocyanine as a Potential Gearing Subunit

This paper describes the synthesis of a cerium(IV)-based molecular gear composed of a thioether functionalized phthalocyanine anchoring ligand, and a helical naphthalocyanine rotating cogwheel functionalized with four carbazoles. The naphthalocyanine ligand 9 was obtained after eleven steps (overall yield of 0.2 %) as a mixture of three geometrical isomers, two of which are chiral and exhibit high levels of steric hindrance, as shown by DFT calculations. Their attributions have been made using 1H-NMR based on their different symmetry groups. The ratio of isomers was also determined and the prochiral C4h naphthalocyanine shown to be the major compound (55 %). Its heteroleptic complexation with cerium (IV) and the anchoring phthalocyanine ligand 10 gave the targeted molecular gear in a 16 % yield.

Stereoselective Solid-State Synthesis of Biologically Active Cyclobutane and Dicyclobutane Isomers via Conformation Blocking and Transference

Conformations in the solid state are typically fixed during crystallization. Transference of "frozen" C=C conformations in 3,5-bis((E)-2-(pyridin-4-yl)vinyl)methylbenzene (CH3-3,5-bpeb) by photodimerization selectively yielded cyclobutane and dicyclobutane isomers, one of which (Isomer 2) exhibited excellent in vitro anti-cancer activity towards T-24, 7402, MGC803, HepG-2, and HeLa cells.

Terminal Group Effect on Two-Dimensional Self-Assembly of Fluorenone-Based Liquid Crystals at the Solid/Liquid Interface

Self-assemblies of two fluorenone-based derivatives (FE and FEC) consisting of a central 2,7-diphenyl-9-fluorenone polar moiety but differing in the flexible terminal groups were investigated by scanning tunneling microscopy (STM) at the 1-octanoic acid/HOPG interface under different concentrations and density functional theory calculation (DFT). STM results reveal a concentration-dependent polymorphic self-assembly behavior for FE, but without the presence of co-adsorbed solvents. As the concentration decreases, the dimer, bracket-like, and ribbon-like self-assembled structures were observed. On the contrary, FEC molecules assemble into only a type of oval-shaped morphology by the intermolecular N···H-O hydrogen bonds with the solvent molecules. Combined with DFT calculations, it can be deduced that the intermolecular van der Waals forces, dipole-dipole interactions, and hydrogen bonding are the main driving forces to stabilize the molecular packing of fluorenone-based polycatenars with strong polarity. Our work is of significance at the molecular level to further clarify the intermolecular interactions and conformational effects on the formation of molecular packing structures with liquid crystal property.

Revealing the Orientation Selectivity of Tetrapyridyl-Substituted Porphyrins Constrained in Molecular "Klotski Puzzles"

Understanding and controlling molecular orientations in self-assembled organic nanostructures are crucial to the development of advanced functional nanodevices. Scanning tunneling microscopy (STM) provides a powerful toolbox to recognize molecular orientations and to induce orientation changes on surfaces at the single-molecule level. Enormous effort has been devoted to directly controlling the molecular orientations of isolated single molecules in free space. However, revealing and further controlling molecular orientation selectivity in constrained environments remain elusive. In this study, by a combination of STM imaging/manipulations and density functional theory calculations, we report the orientation selectivity of tetrapyridyl-substituted porphyrins in response to various local molecular environments in artificially constructed molecular "Klotski puzzles" on Au(111). With the assistance of STM lateral manipulations, "sliding-block" molecules were able to enter predefined positions, and specific molecular orientations were adopted to fit the local molecular environments, in which the intermolecular interaction was revealed to be the key to achieving the eventual molecular orientation selectivity. Our results demonstrate the essential role of local molecular environments in directing single-molecule orientations, which would shed light on the design of molecular structures to control preferred orientations for further applications in molecular nanodevices.

Extended Tripodal Hydrotris(indazol-1-yl)borate Ligands as Ruthenium-Supported Cogwheels for On-Surface Gearing Motions

This paper reports the synthesis of ruthenium-based molecular gear prototypes composed of a brominated or non-brominated pentaphenylcyclopentadienyl ligand as an anchoring unit and a tripodal ligand with aryl-functionalized indazoles as a rotating cogwheel. Single crystal structures of the ruthenium complexes revealed that the appended aryl groups increase the apparent diameter of the cogwheel rendering them larger than the diameter of the anchoring units and consequently making them suitable for intermolecular gearing motions once the complexes will be adsorbed on a surface.

Construction and physical properties of low-dimensional structures for nanoscale electronic devices

Over the past decades, construction of nanoscale electronic devices with novel functionalities based on low-dimensional structures, such as single molecules and two-dimensional (2D) materials, has been rapidly developed. To investigate their intrinsic properties for versatile functionalities of nanoscale electronic devices, it is crucial to precisely control the structures and understand the physical properties of low-dimensional structures at the single atomic level. In this review, we provide a comprehensive overview of the construction of nanoelectronic devices based on single molecules and 2D materials and the investigation of their physical properties. For single molecules, we focus on the construction of single-molecule devices, such as molecular motors and molecular switches, by precisely controlling their self-assembled structures on metal substrates and charge transport properties. For 2D materials, we emphasize their spin-related electrical transport properties for spintronic device applications and the role that interfaces among 2D semiconductors, contact electrodes, and dielectric substrates play in the electrical performance of electronic, optoelectronic, and memory devices. Finally, we discuss the future research direction in this field, where we can expect a scientific breakthrough.

Rotation in an Enantiospecific Self-Assembled Array of Molecular Raffle Wheels

Tailored nano-spaces can control enantioselective adsorption and molecular motion. We report on the spontaneous assembly of a dynamic system-a rigid kagome network with each pore occupied by a guest molecule-employing solely 2,6-bis(1H-pyrazol-1-yl)pyridine-4-carboxylic acid on Ag(111). The network cavity snugly hosts the chemically modified guest, bestows enantiomorphic adsorption and allows selective rotational motions. Temperature-dependent scanning tunnelling microscopy studies revealed distinct anchoring orientations of the guest unit switching with a 0.95 eV thermal barrier. H-bonding between the guest and the host transiently stabilises the rotating guest, as the flapper on a raffle wheel. Density functional theory investigations unravel the detailed molecular pirouette of the guest and how the energy landscape is determined by H-bond formation and breakage. The origin of the guest's enantiodirected, dynamic anchoring lies in the specific interplay of the kagome network and the silver surface.

Molecular Gears: From Solution to Surfaces

This review highlights the major efforts devoted to the development of molecular gears over the past 40 years, from pioneering covalent bis-triptycyl systems undergoing intramolecular correlated rotation in solution, to the most recent examples of gearing systems anchored on a surface, which allow intermolecular transmission of mechanical power. Emphasis is laid on the different strategies devised progressively to control the architectures of molecular bevel and spur gears, as intramolecular systems in solution or intermolecular systems on surfaces, while aiming at increased efficiency, complexity and functionality.

Current-induced forces in single-resonance systems

In recent years, there has been an increasing interest in nanoelectromechanical devices, current-driven quantum machines, and the mechanical effects of electric currents on nanoscale conductors. Here, we carry out a thorough study of the current-induced forces and the electronic friction of systems whose electronic effective Hamiltonian can be described by an archetypal model, a single energy level coupled to two reservoirs. Our results can help better understand the general conditions that maximize the performance of different devices modeled as a quantum dot coupled to two electronic reservoirs. Additionally, they can be useful to rationalize the role of current-induced forces in the mechanical deformation of one-dimensional conductors.

Desymmetrised pentaporphyrinic gears mounted on metallo-organic anchors

Mastering intermolecular gearing is crucial for the emergence of complex functional nanoscale machineries. However, achieving correlated motion within trains of molecular gears remains highly challenging, due to the multiple degrees of freedom of each cogwheel. In this context, we designed and synthesised a series of star-shaped organometallic molecular gears incorporating a hydrotris(indazolyl)borate anchor to prevent diffusion on the surface, a central ruthenium atom as a fixed rotation axis, and an azimuthal pentaporphyrinic cyclopentadienyl cogwheel specifically labelled to monitor its motion by non-time-resolved Scanning Tunneling Microscopy (STM). Desymmetrisation of the cogwheels was first achieved sterically, i.e. by introducing one tooth longer than the other four. For optimal mechanical interactions, chemical labelling was also investigated as a preferential way to induce local contrast in STM images, and the electronic properties of one single paddle were modulated by varying the porphyrinic scaffold or the nature of the central metal. To reach such a structural diversity, our modular synthetic approach relied on sequential cross-coupling reactions on a penta(p-halogenophenyl)cyclopentadienyl ruthenium(ii) key building block, bearing a single pre-activated p-iodophenyl group. Chemoselective Sonogashira or more challenging Suzuki-Miyaura reactions allowed the controlled introduction of the tagged porphyrinic tooth, and the subsequent four-fold cross-couplings yielded the prototypes of pentaporphyrinic molecular gears for on-surface studies, incorporating desymmetrised cogwheels over 5 nm in diameter.

Surface self-assembly involving the interaction between S and N atoms

Regulation of the self-assembly nanostructures by recruiting the electrostatic interaction between S and N atoms.

Transmitting Stepwise Rotation among Three Molecule-Gear on the Au(111) Surface

The realization of a train of molecule-gears working under the tip of a scanning tunneling microscope (STM) requires a stable anchor of each molecule to the metal surface. Such an anchor can be promoted by a radical state of the molecule induced by a dissociation reaction. Our results, rationalized by density functional theory calculations, reveal that such an open radical state at the core of star-shaped pentaphenylcyclopentadiene (PPCP) favors anchoring. Furthermore, to allow the transmission of motion by STM manipulation, the molecule-gears should be equipped with specific groups facilitating the tip-molecule interactions. In our case, a tert-butyl group positioned at one tooth end of the gear benefits both the tip-induced manipulation and the monitoring of rotation. With this optimized molecule, we achieve reproducible and stepwise rotations of the single gears and transmit rotations for up to three interlocked units.

Photochromic Diarylethenes Designed for Surface Deposition: From Self-Assembled Monolayers to Single Molecules

The efficient switching that can occur between two stable isomers of diarylethenes makes them particularly promising targets for opto- and molecular electronics. To examine these classes of molecules for electronics applications, they have been subjected to a series of scanning tunneling microscopy (STM) experiments, which are the focus of this Review. A brief introduction to the chemical design of diarylethenes in terms of their switching capabilities along with the basics of STM are presented. Next, initial STM studies on these compounds under ambient conditions are discussed. An overview of how molecular design affects the isomerization and self-assembly of diarylethenes at the solid-liquid interface as investigated by STM is then presented, as well as single-molecule studies under ultrahigh vacuum. The last section presents further prospects for molecular design in the field.

Synthesis and structural characterisation of bulky heptaaromatic (hetero)aryl o-substituted s-aryltetrazines

Hexaphenylbenzene analogs with an electron-poor tetrazine core are synthesized in two high-yield steps from diphenyl-s-tetrazine. Crystal packing of these unique non-planar heptaaromatics is analyzed in details.

Design of Collective Motions from Synthetic Molecular Switches, Rotors, and Motors

Precise control over molecular movement is of fundamental and practical importance in physics, biology, and chemistry. At nanoscale, the peculiar functioning principles and the synthesis of individual molecular actuators and machines has been the subject of intense investigations and debates over the past 60 years. In this review, we focus on the design of collective motions that are achieved by integrating, in space and time, several or many of these individual mechanical units together. In particular, we provide an in-depth look at the intermolecular couplings used to physically connect a number of artificial mechanically active molecular units such as photochromic molecular switches, nanomachines based on mechanical bonds, molecular rotors, and light-powered rotary motors. We highlight the various functioning principles that can lead to their collective motion at various length scales. We also emphasize how their synchronized, or desynchronized, mechanical behavior can lead to emerging functional properties and to their implementation into new active devices and materials.

Star-Shaped Ruthenium Complexes as Prototypes of Molecular Gears

The design and synthesis of two families of molecular-gear prototypes is reported, with the aim of assembling them into trains of gears on a surface and ultimately achieving controlled intermolecular gearing motion. These piano-stool ruthenium complexes incorporate a hydrotris(indazolyl)borate moiety as tripodal rotation axle and a pentaarylcyclopentadienyl ligand as star-shaped cogwheel, equipped with five teeth ranging from pseudo-1D aryl groups to large planar 2D paddles. A divergent synthetic approach was followed, starting from a pentakis(p-bromophenyl)cyclopentadienyl ruthenium(II) complex as key precursor or from its iodinated counterpart, obtained by copper-catalyzed aromatic Br/I exchange. Subsequent fivefold cross-coupling reactions with various partners allowed high structural diversity to be reached and yielded molecular-gear prototypes with aryl-, carbazole-, BODIPY- and porphyrin-derived teeth of increasing size and length.

Train of Single Molecule-Gears

Two molecule-gears, 1.2 nm in diameter with six teeth, are mounted each on a single copper adatom separated exactly by 1.9 nm on a lead surface using a low-temperature scanning tunneling microscope (LT-STM). A functioning train of two molecule-gears is constructed complete with a molecule-handle. Not mounted on a Cu adatom axle, this ancillary molecule-gear is mechanically engaged with the first molecule-gear of the train to stabilize its step-by-step rotation. Centered on its Cu adatom axle, the rotation of the first gear of the train step by step rotates the second similar to a train of macroscopic gears. From the handle to the first and to this second molecule-gear, the exact positioning of the two Cu adatom axles on the lead surface ensures that the molecular teeth-to-teeth mechanics is fully reversible.

How to control single-molecule rotation

The orientation of molecules is crucial in many chemical processes. Here, we report how single dipolar molecules can be oriented with maximum precision using the electric field of a scanning tunneling microscope. Rotation is found to occur around a fixed pivot point that is caused by the specific interaction of an oxygen atom in the molecule with the Ag(111) surface. Both directions of rotation are realized at will with 100% directionality. Consequently, the internal dipole moment of an individual molecule can be spatially mapped via its behavior in an applied electric field. The importance of the oxygen-surface interaction is demonstrated by the addition of a silver atom between a single molecule and the surface and the consequent loss of the pivot point.

Thermodynamics and Steady State of Quantum Motors and Pumps Far from Equilibrium

In this article, we briefly review the dynamical and thermodynamical aspects of different forms of quantum motors and quantum pumps. We then extend previous results to provide new theoretical tools for a systematic study of those phenomena at far-from-equilibrium conditions. We mainly focus on two key topics: (1) The steady-state regime of quantum motors and pumps, paying particular attention to the role of higher order terms in the nonadiabatic expansion of the current-induced forces. (2) The thermodynamical properties of such systems, emphasizing systematic ways of studying the relationship between different energy fluxes (charge and heat currents and mechanical power) passing through the system when beyond-first-order expansions are required. We derive a general order-by-order scheme based on energy conservation to rationalize how every order of the expansion of one form of energy flux is connected with the others. We use this approach to give a physical interpretation of the leading terms of the expansion. Finally, we illustrate the above-discussed topics in a double quantum dot within the Coulomb-blockade regime and capacitively coupled to a mechanical rotor. We find many exciting features of this system for arbitrary nonequilibrium conditions: a definite parity of the expansion coefficients with respect to the voltage or temperature biases; negative friction coefficients; and the fact that, under fixed parameters, the device can exhibit multiple steady states where it may operate as a quantum motor or as a quantum pump, depending on the initial conditions.

Cyclopentadienyl Ruthenium(II) Complex-Mediated Oxidation of Benzylic and Allylic Alcohols to Corresponding Aldehydes

This work reports an efficient method for the oxidation reaction of aliphatic, aromatic allylic, and benzylic alcohols into aldehydes catalyzed by the cyclopentadienyl ruthenium(II) complex (RuCpCl(PPh3)2) with bubbled O2. Through further optimizing controlled studies, the tendency order of oxidation reactivity was determined as follows: benzylic alcohols > aromatic allylic alcohols >> aliphatic alcohols. In addition, this method has several advantages, including a small amount of catalyst (0.5 mol%) and selective application of high discrimination activity of aliphatic, aromatic allylic, and benzylic alcohols.

Modification of the Potential Landscape of Molecular Rotors on Au(111) by the Presence of an STM Tip

Molecular rotors on solid surfaces are fundamental components of molecular machines. No matter whether the rotation is activated by heat, electric field or light, it is determined by the intrinsic rotational potential landscape. Therefore, tuning the potential landscape is of great importance for future applications of controlled molecular rotors. Here, using scanning tunneling microscopy (STM), we demonstrate that both tip-molecule distance and sample bias can modify the rotational potential of molecular rotors. We achieve the potential energy difference variations of ∼0.3 meV/pm and ∼18 meV/V between two configurations of a molecular rotor, a tetra- tert-butyl nickel phthalocyanine molecule on Au(111) substrate. Further analysis indicates that the mechanism of modifying the rotational potential is a combination of the van der Waals interaction and the interaction between the molecular dipole and an electric field. This work provides insight into the methods used to modify the effective rotational potential energy of molecular rotors.

Directed assembly of fullerene on modified Au(111) electrodes

Here we show a conceptual approach to realize the scanning tunneling microscopy based induced-assembly of fullerene (C60) molecules on top of a buffer organic adlayer at room temperature in a solution environment. The realization of spatially-defined C60 assembly is attributed to the modulation of substrate-molecular interactions with the assistance of a buffer layer.

Same building block, but diverse surface-confined self-assemblies: solvent and concentration effects-induced structural diversity towards chirality and achirality

Fabricating complex nano-networks on solid substrates is a research area that has attracted much attention in the field of molecular self-assembly. By designing a fluorenone derivative of 2-heptyloxy-7-pentadecyloxy-9-fluorenone (HPF), we obtained a surface-confined system that presented diverse nanostructures. The assembled networks for HPF were highly dependent on the solvent and concentration. At the liquid/solid interface, chiral tetramer-S, hexamer-S, and tetramer-linear structures as well as achiral irregular-linear and random structures were recorded. On the dry surface, we observed chiral octamer-S and achiral alternate configurations. During the self-assembly process, the short and long alkyl chains of HPF showed selective identification, which contributed to the formation of S-like or anti-S-like tetramers, hexamers and octamers, resulting in chiral structures. The nanopatterns were stabilized under the driving forces of dipolar interactions, hydrogen bonds and van der Waals interactions. Moreover, we performed forcefield calculations in order to further understand the underlying mechanisms from the viewpoints of their force strengths and binding energies. In general, the present work provides a significant impetus to induce polymorphous structures, and we believe that it will promote the study of chirality and achirality in the field of molecular self-assembly.

Electronic effects and fundamental physics studied in molecular interfaces

Scanning probe instruments in conjunction with a very low temperature environment have revolutionized the ability of building, functionalizing, and analysing two dimensional interfaces in the last twenty years. In addition, the availability of fast, reliable, and increasingly sophisticated methods to simulate the structure and dynamics of these interfaces allow us to capture even very small effects at the atomic and molecular level. In this review we shall focus largely on metal surfaces and organic molecular compounds and show that building systems from the bottom up and controlling the physical properties of such systems is no longer within the realm of the desirable, but has become day to day reality in our best laboratories.

Correlated rotational switching in two-dimensional self-assembled molecular rotor arrays

Molecular devices are capable of performing a number of functions from mechanical motion to simple computation. Their utility is somewhat limited, however, by difficulties associated with coupling them with either each other or with interfaces such as electrodes. Self-assembly of coupled molecular devices provides an option for the construction of larger entities that can more easily integrate with existing technologies. Here we demonstrate that ordered organometallic arrays can be formed spontaneously by reaction of precursor molecular rotor molecules with a metal surface. Scanning tunnelling microscopy enables individual rotors in the arrays to be switched and the resultant switches in neighbouring rotors imaged. The structure and dimensions of the ordered molecular rotor arrays dictate the correlated switching properties of the internal submolecular rotor units. Our results indicate that self-assembly of two-dimensional rotor crystals produces systems with correlated dynamics that would not have been predicted a priori.

Single molecular machines are capable of a variety of functions, but methods to couple motion between them are still lacking. Here, Wasio et al. report the emergent behaviour of spontaneously formed two-dimensional crystals, which display correlated switching of their sub-molecular rotor units.

Effects of alkyl chain number and position on 2D self-assemblies

Alkyl chain number and position effects are explored via the fabrication and regulation of 2D self-assemblies at liquid/HOPG interfaces.

Monitoring and manipulating single molecule rotors on the Bi(111) surface by the scanning tunneling microscopy

MnPc rotors were started and stopped by controlling the intermolecular spacing with the STM tip.

Direct imaging of rotating molecules anchored on graphene

There has been significant research interest in controlling and imaging molecular dynamics, such as translational and rotational motions, especially at a single molecular level. Here we applied aberration-corrected transmission electron microscopy (ACTEM) to actuate and directly image the rotational motions of molecules anchored on a single-layer-graphene sheet. Nanometer-sized carbonaceous molecules anchored on graphene provide ideal systems for monitoring rotational motions via ACTEM. We observed the preferential registry of longer molecular axis along graphene zigzag or armchair lattice directions due to the stacking-dependent molecule-graphene energy landscape. The calculated cross section from elastic scattering theory was used to experimentally estimate the rotational energy barriers of molecules on graphene. The observed energy barrier was within the range of 1.5-12 meV per atom, which is in good agreement with previous calculation results. We also performed molecular dynamics simulations, which revealed that the edge atoms of the molecule form stably bonds to graphene defects and can serve as a pivot point for rotational dynamics. Our study demonstrates the versatility of ACTEM for the investigation of molecular dynamics and configuration-dependent energetics at a single molecular level.

Π Band Dispersion along Conjugated Organic Nanowires Synthesized on a Metal Oxide Semiconductor

Surface-confined dehalogenation reactions are versatile bottom-up approaches for the synthesis of carbon-based nanostructures with predefined chemical properties. However, for devices generally requiring low-conductivity substrates, potential applications are so far severely hampered by the necessity of a metallic surface to catalyze the reactions. In this work we report the synthesis of ordered arrays of poly(p-phenylene) chains on the surface of semiconducting TiO₂(110) via a dehalogenative homocoupling of 4,4″-dibromoterphenyl precursors. The supramolecular phase is clearly distinguished from the polymeric one using low-energy electron diffraction and scanning tunneling microscopy as the substrate temperature used for deposition is varied. X-ray photoelectron spectroscopy of C 1s and Br 3d core levels traces the temperature of the onset of dehalogenation to around 475 K. Moreover, angle-resolved photoemission spectroscopy and tight-binding calculations identify a highly dispersive band characteristic of a substantial overlap between the precursor’s π states along the polymer, considered as the fingerprint of a successful polymerization. Thus, these results establish the first spectroscopic evidence that atomically precise carbon-based nanostructures can readily be synthesized on top of a transition-metal oxide surface, opening the prospect for the bottom-up production of novel molecule–semiconductor devices.

Synthesis of Functionalized Mono-, Bis-, and Trisethynyltriptycenes for One-Dimensional Self-Assembly on Surfaces

This paper describes the synthesis of triptycene-based building blocks that are able to interact through hydrogen bonds to form one-dimensional self-assembled motifs on surfaces. We designed 9,10-diethynyltriptycene derivatives functionalized at the ethynyl end groups by a variety of hydrogen-bonding groups for homomolecular recognition and complementary building blocks for heteromolecular recognition. We also present the synthesis of bis- and trisethynyltriptycenes with terminal alkyne functional groups available for on-surface azide-alkyne cycloaddition reaction to expand the potential of the triptycene building block.

Controllable Scission and Seamless Stitching of Metal-Organic Clusters by STM Manipulation

Scanning tunneling microscopy (STM) manipulation techniques have proven to be a powerful method for advanced nanofabrication of artificial molecular architectures on surfaces. With increasing complexity of the studied systems, STM manipulations are then extended to more complicated structural motifs. Previously, the dissociation and construction of various motifs have been achieved, but only in a single direction. In this report, the controllable scission and seamless stitching of metal-organic clusters have been successfully achieved through STM manipulations. The system presented here includes two sorts of hierarchical interactions where coordination bonds hold the metal-organic elementary motifs while hydrogen bonds among elementary motifs are directly involved in bond breakage and re-formation. The key to making this reversible switching successful is the hydrogen bonding, which is comparatively facile to be broken for controllable scission, and, on the other hand, the directional characteristic of hydrogen bonding makes precise stitching feasible.

Light-induced contraction and extension of single macromolecules on a modified graphite surface

Synthetic rigid-rod polymers incorporating multiple azobenzene photoswitches in the backbone were deposited from solution onto a monolayer of octadecylamine covering the basal plane of graphite. Large contractions and extensions of the single macromolecules on the surface were induced by irradiation with UV and visible light, respectively, as visualized by scanning force microscopy. Upon contraction, the single polymer chains form more compact nanostructures and also may move across the surface, resembling a crawling movement. We attribute the efficiency of these processes to the low mechanical and electronic coupling between the surface and polymers, the high density of azobenzenes in their backbones, and their rigidity, allowing for maximized photodeformations. The visualization of on-surface motions of single macromolecules directly induced by light, as reported herein, could help promote the development of optomechanical nanosystems.

Single rotating molecule-machines: nanovehicles and molecular motors

In the last decade many molecular machines with controlled molecular motions have been synthesized. In the present review chapter we will present and discuss our contribution to the field, in particular through some examples of rotating molecular machines that have been designed, synthesized, and studied in our group. After starting by explaining why it is so important to study such machines as single molecules, we will focus on two families of molecular machines, nanovehicles and molecular motors. The first members of the nanovehicle family are molecules with two triptycenes as wheels: the axle and the wheelbarrow. Then come the four-wheel nanocars. Since triptycene wheels are not very mobile on metallic surfaces, alternative wheels with a bowl-shape structure have also been synthesized and studied on surfaces. The molecular motors are built around ruthenium organometallic centers and have a piano-stool geometry with peripheric ferrocenyl groups.