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A key challenge in the field of nanotechnology, in particular in the design of molecular machines, novel materials or molecular electronics, is the bottom-up construction of covalently bound molecular architectures in a well-defined arrangement. To date, only rather simple structures have been obtained because of the limitation of one-step connection processes. Indeed, for the formation of sophisticated structures, step-by-step connection of molecules is required. Here, we present a strategy for the covalent connection of molecules in a hierarchical manner by the selective and sequential activation of specific sites, thereby generating species with a programmed reactivity. This approach leads to improved network quality and enables the fabrication of heterogeneous architectures with high selectivity. Furthermore, substrate-directed growth and a preferred orientation of the molecular nanostructures are achieved on an anisotropic surface. The demonstrated control over reactivity and diffusion during covalent bond formation constitutes a promising route towards the creation of sophisticated multi-component molecular nanostructures. [ABSTRACT FROM AUTHOR]

The design of a single-molecule machine consisting of functional components requires a detailed understanding of its mechanical motion. The scanning tunnelling microscope (STM) is the only available tool for driving and imaging such a nanoscale machine on a surface. Both lateral hopping motions and conformational changes of single molecules can be induced using the STM tip. However, no rolling of a wheel has been demonstrated so far at the nanoscale, even though this is a very useful motion at the macroscopic scale. Here we show how the rolling of a single molecule equipped with two wheels (0.8 nm in diameter) can be induced by the STM tip. The characteristics of the rolling are recorded in the STM feedback loop manipulation signal and in real time. We capture unambiguous signatures of the conformational change happening during the rolling. Our approach of controlling the intramolecular mechanics provides a path towards the bottom-up assembly of more complex molecular machines. [ABSTRACT FROM AUTHOR]

Abstract: The sublimation of molecules under ultra-high vacuum conditions becomes critical with increasing molecular complexity as thermal fragmentation comes into play. In this case, scanning tunneling microscopy experiments of large molecules commonly focus on the small portion of the surface displaying intact molecules. In this work, we shed light on the major part of the surface where molecular fragments are observed. Upon the deposition of nanometer-sized wheelbarrows on a Cu(100) surface, only a few intact molecules, but several highly symmetric objects were observed. Using a full range of techniques the chemical structure and a possible formation pathway of these molecules were proposed. [Copyright &y& Elsevier]

The Lander molecule (C90H98) consists of a long polyaromatic molecular wire and four lateral di-tert-butyl-phenyl spacer groups, designed to maintain the molecular wire parallel above the substrate. It represents a model system for investigating the electronic contacts of a molecular wire to a nanoscale metallic electrode. In this article, some recent manipulation experiments of single Lander molecules by low temperature scanning tunneling microscopy (LT-STM) are presented. The selective adsorption of the molecule, the molecule-induced reconstruction of copper substrates, and their application to the investigation of contacts between molecules and nanostructures or between molecules are discussed. Manipulation experiments are reported, where the molecular wire part of a Lander molecule is contacted to a monoatomic step and to a two-atom-wide metallic nanostructure. The contact is characterized by the apparent height of the contact point in STM images and, in case of the Cu(111) substrate, by the perturbation observed in the electronic standing wave patterns. [ABSTRACT FROM AUTHOR]

Molecular switches are of fundamental importance in nature, and light is an important stimulus to selectively drive the switching process. However, the local dynamics of a conformational change in these molecules remain far from being completely understood at the single-molecule level. Here, we report the direct observation of photoinduced tautomerization in single porphycene molecules on a Cu(111) surface by using a combination of low-temperature scanning tunneling microscopy and laser excitation in the near-infrared to ultraviolet regime. It is found that the thermodynamically stable trans configuration of porphycene can be converted to the metastable cis configuration in a unidirectional fashion by photoirradiation. The wavelength dependence of the tautomerization cross section exhibits a steep increase around 2 eV and demonstrates that excitation of the Cu d-band electrons and the resulting hot carriers play a dominant role in the photochemical process. Additionally, a pronounced isotope effect in the cross section (~100) is observed when the transferred hydrogen atoms are substituted with deuterium, indicating a significant contribution of zero-point energy in the reaction. Combined with the study of inelastic tunneling electron-induced tautomerization with the STM, we propose that tautomerization occurs via excitation of molecular vibrations after photoexcitation. Interestingly, the observed cross section of ~10-19 cm² in the visible-ultraviolet region is much higher than that of previously studied molecular switches on a metal surface, for example, azobenzene derivatives (10-23-10-22 cm²). Furthermore, we examined a local environmental impact on the photoinduced tautomerization by varying molecular density on the surface and find substantial changes in the cross section and quenching of the process due to the intermolecular interaction at high density. [ABSTRACT FROM AUTHOR]