Force-Activated Isomerization of a Single Molecule

Recent advances in organic molecule reactions on metal surfaces

Chemical reactions of organic molecules on metal surfaces have been intensively investigated in the past decades, where metals play the role of catalysts in many cases. In this review, first, we summarize recent works on spatial molecules, small H2O, O2, CO, CO2 molecules, and the molecules carrying silicon groups as the new trends of molecular candidates for on-surface chemistry applications. Then, we introduce spectroscopy and DFT study advances in on-surface reactions. Especially, in situ spectroscopy technologies, such as electron spectroscopy, force spectroscopy, X-ray photoemission spectroscopy, STM-induced luminescence, tip-enhanced Raman spectroscopy, temperature-programmed desorption spectroscopy, and infrared reflection adsorption spectroscopy, are important to confirm the occurrence of organic reactions and analyze the products. To understand the underlying mechanism, the DFT study provides detailed information about reaction pathways, conformational evolution, and organometallic intermediates. Usually, STM/nc-AFM topological images, in situ spectroscopy data, and DFT studies are combined to describe the mechanism behind on-surface organic reactions.

Discovery and construction of surface kagome electronic states induced by p-d electronic hybridization in Co3Sn2S2

Kagome-lattice materials possess attractive properties for quantum computing applications, but their synthesis remains challenging. Herein, based on the compelling identification of the two cleavable surfaces of Co3Sn2S2, we show surface kagome electronic states (SKESs) on a Sn-terminated triangular Co3Sn2S2 surface. Such SKESs are imprinted by vertical p-d electronic hybridization between the surface Sn (subsurface S) atoms and the buried Co kagome-lattice network in the Co3Sn layer under the surface. Owing to the subsequent lateral hybridization of the Sn and S atoms in a corner-sharing manner, the kagome symmetry and topological electronic properties of the Co3Sn layer is proximate to the Sn surface. The SKESs and both hybridizations were verified via qPlus non-contact atomic force microscopy (nc-AFM) and density functional theory calculations. The construction of SKESs with tunable properties can be achieved by the atomic substitution of surface Sn (subsurface S) with other group III-V elements (Se or Te), which was demonstrated theoretically. This work exhibits the powerful capacity of nc-AFM in characterizing localized topological states and reveals the strategy for synthesis of large-area transition-metal-based kagome-lattice materials using conventional surface deposition techniques.

Chirality control of a single carbene molecule by tip-induced van der Waals interactions

Non-covalent interactions such as van der Waals interactions and hydrogen bonds are crucial for the chiral induction and control of molecules, but it remains difficult to study them at the single-molecule level. Here, we report a carbene molecule on a copper surface as a prototype of an anchored molecule with a facile chirality change. We examine the influence of the attractive van der Waals interactions on the chirality change by regulating the tip-molecule distance, resulting in an excess of a carbene enantiomer. Our model study provides insight into the change of molecular chirality controlled by van der Waals interactions, which is fundamental for understanding the mechanisms of chiral induction and amplification.

Oxide-Metal Interaction Probed by Scanning Tunneling Microscope Manipulation of Cr2O7 Clusters on Au(111)

Interfacial interaction plays a crucial rule in catalysis over supported catalysts, and the catalyst-support interaction needs to be explored at microscopic scale. Here, we use the scanning tunneling microscope (STM) tip to manipulate Cr₂O₇ dinuclear clusters on Au(111) and find that the Cr₂O₇-Au interaction can be weakened by an electric field in the STM junction, facilitating rotation and translation of the individual clusters at the imaging temperature (78 K). Surface alloying with Cu makes the manipulation of the Cr₂O₇ clusters hard due to the enhanced Cr₂O₇-substrate interaction. Density functional theory calculations reveal that barrier for translation of a Cr₂O₇ cluster on the surface can be increased by surface alloying, influencing the tip manipulation. Our study demonstrates that the oxide-metal interfacial interaction can be probed by STM tip manipulation of supported oxide clusters, which provides a new method to investigate the interfacial interaction.

Single-Molecule Interconversion between Chiral Configurations of Boronate Esters Observed in a Nanoreactor

Isomers of some chemical compounds may be dynamically interconvertible. Due to a lack of sensing methods with a sufficient resolution, however, direct monitoring of such processes can be difficult. Engineered Mycobacterium smegmatis porin A (MspA) nanopores can be applied as nanoreactors so that chemical reactions can be directly monitored. Here, an MspA modified with a phenylboronic acid (PBA) adapter was prepared and was used to observe dynamic interconversion between chiral configurations of boronate esters, which appears as telegraphic switching on top of nanopore events. The mechanism of this behavior was further confirmed by trials with different halogenated catechols, dopamine, adenosine, 1,2-propanediol, and (2R,3R)-2,3-butanediol, and its generality has been demonstrated. These results suggest that an engineered MspA possesses an exceptional resolution in its monitoring of chemical reaction processes and may inspire the future design of nanopore small-molecule sensors.

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.

A DFT study on the isomerization mechanism of azobenzene derivatives on silicon substrates

The cis–trans isomerization mechanism of azobenzenesulfonamide derivatives on silicon substrates was investigated using DFT. The most favorable cooperative mode of the N2 inversion of the L followed by the N1 inversion of the R was proposed.

Light-Induced Quantitative and Electrical-Field-Induced Barrierless Switching of Spiropyran Derivative on Graphite Surface

A new class of pyridine-based spiropyran (SP) shows photoinduced reversible switching between the closed SP and ring-opened merocyanine (MC). We show that a condensed crystalline monolayer of SP on graphite can be quantitatively converted to MC upon UV irradiation. In solution only ∼10% of SP can be transformed to MC because of the establishment of a photostationary state. Using an electrical field applied by a scanning tunneling microscopy (STM) tip, single molecules are reversibly switched between SP and MC forms in their condensed phases without any threshold voltage at ambient conditions. The microscopic structure of submonolayer films of SP and MC are investigated using atomic force microscopy (AFM) and STM.

Tuning Single-Molecule Conductance by Controlled Electric Field-Induced trans-to-cis Isomerisation

External electric fields (EEFs) have proven to be very efficient in catalysing chemical reactions, even those inaccessible via wet-chemical synthesis. At the single-molecule level, oriented EEFs have been successfully used to promote in situ single-molecule reactions in the absence of chemical catalysts. Here, we elucidate the effect of an EEFs on the structure and conductance of a molecular junction. Employing scanning tunnelling microscopy break junction (STM-BJ) experiments, we form and electrically characterize single-molecule junctions of two tetramethyl carotene isomers. Two discrete conductance signatures show up more prominently at low and high applied voltages which are univocally ascribed to the trans and cis isomers of the carotenoid, respectively. The difference in conductance between both cis-/trans- isomers is in concordance with previous predictions considering π-quantum interference due to the presence of a single gauche defect in the trans isomer. Electronic structure calculations suggest that the electric field polarizes the molecule and mixes the excited states. The mixed states have a (spectroscopically) allowed transition and, therefore, can both promote the cis-isomerization of the molecule and participate in electron transport. Our work opens new routes for the in situ control of isomerisation reactions in single-molecule contacts.

The many flavours of mechanochemistry and its plausible conceptual underpinnings

Mechanochemistry describes diverse phenomena in which mechanical load affects chemical reactivity. The fuzziness of this definition means that it includes processes as seemingly disparate as motor protein function, organic synthesis in a ball mill, reactions at a propagating crack, chemical actuation, and polymer fragmentation in fast solvent flows and in mastication. In chemistry, the rate of a reaction in a flask does not depend on how fast the flask moves in space. In mechanochemistry, the rate at which a material is deformed affects which and how many bonds break. In other words, in some manifestations of mechanochemistry, macroscopic motion powers otherwise endergonic reactions. In others, spontaneous chemical reactions drive mechanical motion. Neither requires thermal or electrostatic gradients. Distinct manifestations of mechanochemistry are conventionally treated as being conceptually independent, which slows the field in its transformation from being a collection of observations to a rigorous discipline. In this Review, we highlight observations suggesting that the unifying feature of mechanochemical phenomena may be the coupling between inertial motion at the microscale to macroscale and changes in chemical bonding enabled by transient build-up and relaxation of strains, from macroscopic to molecular. This dynamic coupling across multiple length scales and timescales also greatly complicates the conceptual understanding of mechanochemistry.