Reversible Photoswitching and Isomer-Dependent Diffusion of Single Azobenzene Tetramers on a Metal Surface

Azobenzene is a prototypical molecular switch that can be reversibly photoisomerized between the nearly planar and apolar trans form, and the distorted, polar cis form. Thus far, most studies related to azobenzene derivatives have focused on planar adsorbed molecules. We present here the study of a three-dimensional shape-persistent molecular architecture consisting of four tetrahedrally arranged azobenzene units and adsorbed on a Ag(111) surface. While the azobenzenes of the tripod in contact with the surface lose their switching ability, different isomers of the upright standing arm of the tetramer are obtained reversibly and efficiently by illumination at different wavelengths, revealing time constants of only a few minutes. Diffusion on the surface turns out to be dependent on the isomeric state - trans or cis - of the upright oriented azobenzene group. Hence, molecular mobility can be modulated via their isomeric state, which suggests that for instance molecular growth processes could be controlled by external stimuli

Toward printing molecular nanostructures from microstructured samples in ultrahigh vacuum

Transferring molecular nanostructures from one surface to another in ultrahigh vacuum (UHV) by mechanical contact might be a possible route to avoid the severe limitations of in situ molecular synthesis on technologically relevant template surfaces. Here, transfer printing in UHV of molecular structures between metal surfaces is investigated by a combination of scanning tunneling microscopy and scanning electron microscopy/energy dispersive x-ray spectroscopy. The authors present the complete procedure of the printing and characterization process. Microstructured Au-coated MoS₂ samples exhibiting a periodic pillar structure are used as stamp surfaces with Au(111) single crystals as target surface. Polymers of 1,3,5-tris(4-bromophenyl)benzene molecules and graphene nanoribbons with an armchair edge structure are grown on the pillars of the stamp surface. After bringing the two surfaces in mechanical contact, the transferred material is found on the target while decapping occurs on the stamp surface. Polymer structures are probably buried under the transferred stamp material, and in rare cases, evidence for molecular structures is found in their vicinity

Gating a single-molecule transistor with individual atoms

Transistors, regardless of their size, rely on electrical gates to control the conductance between source and drain contacts. In atomic-scale transistors, this conductance is sensitive to single electrons hopping via individual orbitals1, 2. Single-electron transport in molecular transistors has been previously studied using top-down approaches to gating, such as lithography and break junctions1, 3, 4, 5, 6, 7, 8, 9, 10, 11. But atomically precise control of the gate—which is crucial to transistor action at the smallest size scales—is not possible with these approaches. Here, we used individual charged atoms, manipulated by a scanning tunnelling microscope12, to create the electrical gates for a single-molecule transistor. This degree of control allowed us to tune the molecule into the regime of sequential single-electron tunnelling, albeit with a conductance gap more than one order of magnitude larger than observed previously8, 11, 13, 14. This unexpected behaviour arises from the existence of two different orientational conformations of the molecule, depending on its charge state. Our results show that strong coupling between these charge and conformational degrees of freedom leads to new behaviour beyond the established picture of single-electron transport in atomic-scale transistors

Reversible Photoswitching and Isomer-Dependent Diffusion of Single Azobenzene Tetramers on a Metal Surface

Azobenzene is a prototypical molecular switch that can be reversibly photoisomerized between the nearly planar and apolar trans form, and the distorted, polar cis form. Most studies related to azobenzene derivatives have focused on planar adsorbed molecules. We present herein the study of a threedimensional shapepersistent molecular architecture consisting of four tetrahedrally arranged azobenzene units that is adsorbed on a Ag(111) surface. While the azobenzenes of the tripod in contact with the surface lost their switching ability, different isomers of the upright standing arm of the tetramer were obtained reversibly and efficiently by illumination at different wavelengths, revealing time constants of only a few minutes. Diffusion on the surface was dependent on the isomeric statetrans or cisof the upright oriented azobenzene group. Hence, molecular mobility can be modulated by its isomeric state, which suggests that molecular growth processes could be controlled by external stimuli.(VLID)295142

Inside Back Cover: Reversible Photoswitching and Isomer‐Dependent Diffusion of Single Azobenzene Tetramers on a Metal Surface (Angew. Chem. Int. Ed. 46/2018)

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Conductance of a single flexible molecular wire composed of alternating donor and acceptor units

cited By 27International audienceMolecular-scale electronics is mainly concerned by understanding charge transport through individual molecules. A key issue here is the charge transport capability through a single—typically linear—molecule, characterized by the current decay with increasing length. To improve the conductance of individual polymers, molecular design often either involves the use of rigid ribbon/ladder-type structures, thereby sacrificing for flexibility of the molecular wire, or a zero band gap, typically associated with chemical instability. Here we show that a conjugated polymer composed of alternating donor and acceptor repeat units, synthesized directly by an on-surface polymerization, exhibits a very high conductance while maintaining both its flexible structure and a finite band gap. Importantly, electronic delocalization along the wire does not seem to be necessary as proven by spatial mapping of the electronic states along individual molecular wires. Our approach should facilitate the realization of flexible ‘soft’ molecular-scale circuitry, for example, on bendable substrates

Publisher's Note: “Toward printing molecular nanostructures from microstructured samples in ultrahigh vacuum” [J. Vac. Sci. Technol. B 34 , 011801 (2016)]

International audienc

Probing individual weakly-coupled π-conjugated molecules on semiconductor surfaces

A weak perturbation of a single molecule by the supporting substrate is a key ingredient to molecular electronics. Here, we show that individual phthalocyanine molecules adsorbed on GaAs(110) and InAs(111)A surfaces represent prototypes for weakly coupled single-molecule/semiconductor hybrid systems. This is demonstrated by scanning tunneling spectroscopy and bias-dependent images that closely resemble orbital densities of the free molecule. This is in analogy to results for molecules decoupled from a metal substrate by an ultrathin insulating layer and proves a weak electronic molecule-substrate coupling. Therefore, such systems will allow single-molecule functionality to be combined with the versatility of semiconductor physics

Reversible and Efficient Light-Induced Molecular Switching on an Insulator Surface

Prototypical molecular switches such as azobenzenes exhibit two states, <i>i.e.</i>, <i>trans</i> and <i>cis</i>, with different characteristic physical properties. In recent years various derivatives were investigated on metallic surfaces. However, bulk insulators as supporting substrate reveal important advantages since they allow electronic decoupling from the environment, which is key to control the switching properties. Here, we report on the light-induced isomerization of an azobenzene derivative on a bulk insulator surface, in this case calcite (101̅4), studied by atomic force microscopy with submolecular resolution. Surprisingly, <i>cis</i> isomers appear on the surface already directly after preparation, indicating kinetic trapping. The photoisomerization process is reversible, as the use of different light sources results in specific molecular assemblies of each isomer. The process turns out to be very efficient and even comparable to molecules in solution, which we assign to the rather weak molecular interaction with the insulator surface, in contrast to metals