Surface-Isomerization Dynamics of trans-Stilbene Molecules Adsorbed on Si(100)-2×1

Two phases of trans-stilbene in a polystyrene matrix

Variability in the spectral properties of solid conformations of stilbene under various external conditions still remains obscure. The photophysical properties of trans-stilbene solution in solid polystyrene glass have been studied by absorption and time-resolved fluorescence. Concentration-induced quenching has been observed for small concentrations of stilbene. At large concentrations, the spectroscopic characteristics become split between the two phases of the sample: single-molecule properties are responsible for absorption, while the micro-crystalline phase dominates in fluorescence. Ab initio and molecular dynamics analyses suggest permanent twisting of the stilbene molecular structure upon crystallization, which supports spectroscopic phase separation.

Force-Activated Isomerization of a Single Molecule

Understanding and controlling isomerization at the single molecular level should provide new insight into the molecular dynamics and design guidelines of functional devices. Scanning tunneling microscopy (STM) has been demonstrated to be a powerful tool to study isomerization of single molecules on a substrate, by either electric field or inelastic electron tunneling mechanisms. A similar molecular isomerization process can in principle be induced by mechanical force; however, relevant study has remained elusive. Here, we demonstrate that isomerization of a N,N-dimethylamino-dianthryl-benzene molecule on Ag(100) can be mechanically driven by the STM tip. The existence of an out-of-plane dimethylamino group in the molecule is found to play a pivotal role in the isomerization process by providing a steric hindrance effect for asymmetric interaction between the STM tip and the molecule. This underlying mechanism is further confirmed by performing molecular dynamics simulations, which show agreement with experimental results. Our work opens the opportunity to manipulate the molecular configuration on the basis of mechanical force.

Multi-functional switches of ditopic ligands with azobenzene central bridges at a molecular scale

Ligands are designed to have ditopic bipyridine terminal groups linked through photochromic azobenzene central units, which exhibit multi-switchable properties by different external stimuli. The molecule can switch between cis-and trans-conformations at their bipyridine terminal groups upon protonation and at their central azobenzene units upon irradiation of photons. As a result, the system shows four different isomeric states: cis-TRANS, trans-TRANS, cis-CIS and trans-CIS. The four conformers are switched and are visualized by scanning tunneling microscopy at the solid-liquid interface, which gives a direct demonstration of the multi-functional switches at a molecular level.

Probing charge transfer dynamics in a single iron tetraphenylporphyrin dyad adsorbed on an insulating surface

Although the dynamics of charge transfer (CT) processes can be probed with ultimate lifetime resolution, the inability to control CT at the nanoscale is one of the most important roadblocks to revealing some of its deep fundamental aspects. In this work, we present an investigation of CT dynamics in a single iron tetraphenylporphyrin (Fe-TPP) donor/acceptor dyad adsorbed on a CaF2/Si(100) insulating surface. The tip of a scanning tunneling microscope (STM) is used to create local ionic states in one fragment of the dyad. The CT process is monitored by imaging subsequent changes in the neighbor acceptor molecule and its efficiency is mapped, revealing the influence of the initial excited state in the donor molecule. In the validation of the experiments, simulations based on density functional theory show that holes have a higher donor-acceptor CT rate compared to electrons and highlight a noticeable initial state dependence on the CT process. We leverage the unprecedented spatial resolution achieved in our experiments to show that the CT process in the dyad is governed via molecule-molecule coherent tunneling with negligible surface-mediated character.

Exploiting a single intramolecular conformational switching Ni-TPP molecule to probe charge transfer dynamics at the nanoscale on bare Si(100)-2 × 1

Acquiring quantitative information on charge transfer (CT) dynamics at the nanoscale remains an important scientific challenge. In particular, CT processes in single molecules at surfaces need to be investigated to be properly controlled in various devices. To address this issue, the dynamics of switching molecules can be exploited. Here, nickel-tetraphenylporphyrin adsorbed on the Si(100) surface is used to study the CT process ruling the reversible activation of two chiral molecular conformations. Via the electrons of a scanning tunneling microscope (STM), a statistical study of molecular switching reveals two specific locations of the molecule for which their efficiency is optimized. The CT mechanism is shown to propagate from two lateral aryl groups towards the porphyrin macrocycle inducing an intramolecular movement of two symmetric pyrroles. The measured switching efficiencies can thus be related to a Markus-Jordner model to estimate relevant parameters that describe the dynamics of the CT process. Numerical simulations provide a precise description of the molecular conformations and unveil the molecular energy levels that are involved in the CT process. This quantitative method opens a completely original approach to study CT at the nanoscale.

Isomerism control of diethylstilbestrol by metal surface induced O–H cleavage

Thetransorcisconformation of a stilbene is found to be critically dependent on the supporting metal surface.

Light-Induced Contraction/Expansion of 1D Photoswitchable Metallopolymer Monitored at the Solid-Liquid Interface

The use of a bottom-up approach to the fabrication of nanopatterned functional surfaces, which are capable to respond to external stimuli, is of great current interest. Herein, the preparation of light-responsive, linear supramolecular metallopolymers constituted by the ideally infinite repetition of a ditopic ligand bearing an azoaryl moiety and Co(II) coordination nodes is described. The supramolecular polymerization process is followed by optical spectroscopy in dimethylformamide solution. Noteworthy, a submolecularly resolved scanning tunneling microscopy (STM) study of the in situ reversible trans-to-cis photoisomerization of a photoswitchable metallopolymer that self-assembles into 2D crystalline patterns onto a highly oriented pyrolytic graphite surface is achieved for the first time. The STM analysis of the nanopatterned surfaces is corroborated by modeling the physisorbed species onto a graphene slab before and after irradiation by means of density functional theory calculation. Significantly, switching of the monolayers consisting of supramolecular Co(II) metallopolymer bearing trans-azoaryl units to a novel pattern based on cis isomers can be triggered by UV light and reversed back to the trans conformer by using visible light, thereby restoring the trans-based supramolecular 2D packing. These findings represent a step forward toward the design and preparation of photoresponsive "smart" surfaces organized with an atomic precision.

Mechanistic interpretation of selective catalytic hydrogenation and isomerization of alkenes and dienes by ligand deactivated Pd nanoparticles

Unsupported thiolate-capped palladium nanoparticle catalysts are found to be highly substrate-selective for alkene hydrogenation and isomerization. Steric and poisoning effects from thiolate ligands on the nanoparticle surface control reactivity and selectivity by influencing alkene adsorption and directing either di-σ or mono-σ bond formation. The presence of overlapping p orbitals and α protons in alkenes greatly influences the catalytic properties of deactivated palladium nanoparticles leading to easily predictable hydrogenation or isomerization products.

Atomic scale study of corrugating and anticorrugating states on the bare Si(1 0 0) surface

In this article, we study the origin of the corrugating and anticorrugating states through the electronic properties of the Si(1 0 0) surface via a low-temperature (9 K) scanning tunneling microscope (STM). Our study is based on the analysis of the STM topographies corrugation variations when related to the shift of the local density of states (LDOS) maximum in the [Formula: see text] direction. Our experimental results are correlated with numerical simulations using the density-functional theory with hybrid Heyd-Scuseria-Ernzerhof (HSE06) functional to simulate the STM topographies, the projected density of states variations at different depths in the silicon surface as well as the three dimensional partial charge density distributions in real-space. This work reveals that the Si(1 0 0) surface exhibits two anticorrugating states at +0.8 and +2.8 V that are associated with a phase shift of the LDOS maximum in the unoccupied states STM topographies. By comparing the calculated data with our experimental results, we have been able to identify the link between the variations of the STM topographies corrugation and the shift of the LDOS maximum observed experimentally. Each surface voltage at which the STM topographies corrugation drops is defined as anticorrugating states. In addition, we have evidenced a sharp jump in the tunnel current when the second LDOS maximum shift is probed, whose origin is discussed and associated with the presence of Van Hove singularities.

Mechanism of a molecular photo-switch adsorbed on Si(100)

We present a combined scanning tunneling microscopy and density functional theory study of a compact molecular photoswitch on a Si(100) surface.

Surface-assisted cis–trans isomerization of an alkene molecule on Cu(110)

Interplay of STM imaging and DFT calculations demonstrates the isomerization of an alkene molecule on Cu(110) under ultrahigh vacuum conditions. We show that the on-surfacecis–transisomerization could efficiently occur well below room temperature, in which the surface is speculated to play a key role in assisting this isomerization process.

Reversible electron-induced cis-trans isomerization mediated by intermolecular interactions

Reversible isomerization processes are rarely found when organic molecular switches are adsorbed on metal surfaces. One obstacle is the large energy difference of the isomeric forms, since usually the most planar conformer has the largest adsorption energy. In the example of an imine derivative, we show a strategy for also stabilizing the non-planar isomer by intermolecular bonding to its neighbors. Tunneling electrons from the tip of a scanning tunneling microscope can then be used to induce reversible switching between the trans and cis-like state. Supported by model force-field calculations, we illustrate that the most probable cause of the enhanced stability of the three-dimensional cis state at specific adsorption sites is the electrostatic interaction with N sites of the neighboring molecule.

Single molecule logical devices

After almost 40 years of development, molecular electronics has given birth to many exciting ideas that range from molecular wires to molecular qubit-based quantum computers. This chapter reviews our efforts to answer a simple question: how smart can a single molecule be? In our case a molecule able to perform a simple Boolean function is a child prodigy. Following the Aviram and Ratner approach, these molecules are inserted between several conducting electrodes. The electronic conduction of the resulting molecular junction is extremely sensitive to the chemical nature of the molecule. Therefore designing this latter correctly allows the implementation of a given function inside the molecular junction. Throughout the chapter different approaches are reviewed, from hybrid devices to quantum molecular logic gates. We particularly stress that one can implement an entire logic circuit in a single molecule, using either classical-like intramolecular connections, or a deformation of the molecular orbitals induced by a conformational change of the molecule. These approaches are radically different from the hybrid-device approach, where several molecules are connected together to build the circuit.

Single molecule manipulation at low temperature and laser scanning tunnelling photo-induced processes analysis through time-resolved studies

This paper describes, firstly, the statistical analysis used to determine the processes that occur during the manipulation of a single molecule through electronically induced excitations with a low temperature (5 K) scanning tunnelling microscope (STM). Various molecular operation examples are described and the ability to probe the ensuing molecular manipulation dynamics is discussed within the excitation context. It is, in particular, shown that such studies can reveal reversible manipulation for tuning dynamics through variation of the excitation energy. Secondly, the photo-induced process arising from the irradiation of the STM junction is also studied through feedback loop dynamics analysis, allowing us to distinguish between photo-thermally and photo-electronically induced signals.

Current versus temperature-induced switching in a single-molecule tunnel junction: 1,5 cyclooctadiene on Si(001)

The biconformational switching of single cyclooctadiene molecules chemisorbed on a Si(001) surface was explored by quantum chemical and quantum dynamical calculations and low-temperature scanning tunneling microscopy experiments. The calculations rationalize the experimentally observed switching driven by inelastic electron tunneling (IET) at 5 K. At higher temperatures, they predict a controllable crossover behavior between IET-driven and thermally activated switching, which is fully confirmed by experiment.