Direct Visualization of Molecule Deprotonation on an Insulating Surface

Kittelmann M, Rahe P, Gourdon A, Kühnle A. Direct Visualization of Molecule Deprotonation on an Insulating Surface. ACS Nano. 2012;6(8):7406-7411.Elucidating molecular-scale details of basic reaction steps on surfaces is decisive for a fundamental understanding of molecular reactivity within many fields, including catalysis and on-surface synthesis. Here, the deprotonation of 2,5-dihydroxybenzoic acid (DHBA) deposited onto calcite (101;4) held at room temperature is followed in situ by noncontact atomic force microscopy. After deposition, the molecules form two coexisting phases, a transient striped phase and a stable dense phase. A detailed analysis of high-resolution noncontact atomic force microscopy images indicates the transient striped phase being a bulk-like phase, which requires hydrogen bonds between the carboxylic acid moieties to be formed. With time, the striped phase transforms into the dense phase, which is explained by the deprotonation of the molecules. In the deprotonated state, the molecules can no longer form hydrogen bonds, but anchor to the surface calcium cations with their negatively charged carboxylate group. The deprotonation step is directly confirmed by Kelvin probe force microscopy images that unravel the change in the molecular charge

Sequential and Site-Specific On-Surface Synthesis on a Bulk Insulator

Kittelmann M, Nimmrich M, Lindner R, Gourdon A, Kühnle A. Sequential and Site-Specific On-Surface Synthesis on a Bulk Insulator. ACS Nano. 2013;7(6):5614-5620.The bottom-up construction of functional devices from molecular building blocks offers great potential in tailoring materials properties and functionality with utmost control. An important step toward exploiting bottom-up construction for real-life applications is the creation of covalently bonded structures that provide sufficient stability as well as superior charge transport properties over reversibly linked self-assembled structures. On-surface synthesis has emerged as a promising strategy for fabricating stable, covalently bound molecular structure on surfaces. So far, a majority of the structures created by this method have been obtained from a rather simple one-step processing approach. But the on-surface preparation of complex structures will require the possibility to carry out various reaction steps in a sequential manner as done In solution chemistry. Only one example exists in literature in which a hierarchical strategy is followed to enhance structural complexity and reliability on a metallic surface. Future molecular electronic application will, however, require transferring these strategies to nonconducting surfaces. Bulk insulating substrates are known to pose significant challenges to on-surface synthesis due to the absence of a metal catalyst and their low surface energy, frequently resulting In molecule desorption rather than reaction activation. By carefully selecting a suitable precursor molecule, we succeeded in performing a two-step linking reaction on a bulk Insulating surface. Besides a firm anchoring toward the substrate surface, the reaction sites and sequential order are encoded In the molecular structure, providing so far unmatched reaction control in on-surface synthesis on a bulk insulating substrate

Substrate Templating Guides the Photoinduced Reaction of C-60 on Calcite

Lindner R, Rahe P, Kittelmann M, Gourdon A, Bechstein R, Kühnle A. Substrate Templating Guides the Photoinduced Reaction of C-60 on Calcite. Angewandte Chemie. International Edition. 2014;53(30):7952-7955.A substrate-guided photochemical reaction of C-60 fullerenes on calcite, a bulk insulator, investigated by non-contact atomic force microscopy is presented. The success of the covalent linkage is evident from a shortening of the intermolecular distances, which is clearly expressed by the disappearance of the moire pattern. Furthermore, UV/Vis spectroscopy and mass spectrometry measurements carried out on thick films demonstrate the ability of our setup for initiating the photoinduced reaction. The irradiation of C-60 results in well-oriented covalently linked domains. The orientation of these domains is dictated by the lattice dimensions of the underlying calcite substrate. Using the lattice mismatch to deliberately steer the direction of the chemical reaction is expected to constitute a general design principle for on-surface synthesis. This work thus provides a strategy for controlled fabrication of oriented, covalent networks on bulk insulators

On-Surface Covalent Linking of Organic Building Blocks on a Bulk Insulator

Kittelmann M, Rahe P, Nimmrich M, Hauke CM, Gourdon A, Kühnle A. On-Surface Covalent Linking of Organic Building Blocks on a Bulk Insulator. ACS Nano. 2011;5(10):8420-8425.On-surface synthesis in ultrahigh vacuum provides a promising strategy for creating thermally and chemically stable molecular structures at surfaces. The two-dimensional confinement of the educts, the possibility of working at higher (or lower) temperatures in the absence of solvent, and the templating effect of the surface bear the potential of preparing compounds that cannot be obtained in solution. Moreover, covalently linked conjugated molecules allow for efficient electron transport and are, thus, particularly interesting for future molecular electronics applications. When having these applications in mind, electrically insulating substrates are mandatory to provide sufficient decoupling of the molecular structure from the substrate surface. So far, however, on-surface synthesis has been achieved only on metallic substrates. Here we demonstrate the covalent linking of organic molecules on a bulk insulator, namely, calcite. We deliberately employ the strong electrostatic interaction between the carboxylate groups of halide-substituted benzoic adds and the surface calcium cations to prevent molecular desorption and to reach homolytic cleavage temperatures. This allows for the formation of aryl radicals and intermolecular coupling. By varying the number and position of the halide substitution, we rationally design the resulting structures, revealing straight lines, zigzag structures, and dimers, thus providing clear evidence for the covalent linking. Our results constitute an important step toward exploiting on-surface synthesis for molecular electronics and optics applications, which require electrically insulating rather than metallic supporting substrates

Controlled Activation of Substrate Templating in Molecular Self-Assembly by Deprotonation

Kittelmann M, Nimmrich M, Neff JL, et al. Controlled Activation of Substrate Templating in Molecular Self-Assembly by Deprotonation. Journal of Physical Chemistry C. 2013;117(45):23868-23874.Templated assembly of organic molecules constitutes a promising approach for fabricating functional nanostructures at surfaces with molecular-scale control. Using the substrate template for steering the adsorbate growth enables creating a rich variety of molecular structures by tuning the subtle balance of intermolecular and molecule-surface interactions. On insulating surfaces, however, surface templating is largely absent due to the comparatively weak molecule-surface interactions compared to metallic substrates. Here, we demonstrate the activation of substrate templating in molecular self-assembly on a bulk insulator by controlled deprotonation of the adsorbed molecules upon annealing. Upon deposition of 4-iodobenzoic acid onto the natural cleavage plane of calcite held at room temperature, high molecular mobility is observed, indicating a small diffusion barrier. Molecular islands only nucleate at step edges. These islands show no commensurability with the underlying substrate, clearly indicating the absence of surface templating. Upon annealing the substrate, the molecules undergo a transition from the protonated to the deprotonated state. In the deprotonated state, the molecules adopt a well-defined adsorption position, resulting in a distinctly different, substrate-templated molecular structure that is stable at room temperature. Our work, thus, demonstrates the controlled activation of substrate templating by changing the molecule-surface interaction upon annealing

Synthesis of polyaromatic hydrocarbons with a central rotor

A series of molecular landers comprising a central mobile part has been synthesised. The two rigid main boards consist of acenaphtho[1,2-k]fluoranthene groups, each substituted in positions 7 and 14 by 3,5-di-tert-butylphenyl groups. The boards are linked by a diethynylanthracene, a phenyl, or a biphenyl rotor

Preparative-scale synthesis of nonacene

International audienceAbstractDuring the last years we have witnessed progressive evolution of preparation of acenes with length up to dodecacene by on-surface synthesis in ultra-high vacuum or generation of acenes up to decacene in solid matrices at low temperatures. While these protocols with very specific conditions produce the acenes in amount of few molecules, the strategies leading to the acenes in large quantities dawdle behind. Only recently and after 70 years of synthetic attempts, heptacene has been prepared in bulk phase. However, the preparative scale synthesis of higher homologues still remains a formidable challenge. Here we report the preparation and characterisation of nonacene and show its excellent thermal and in-time stability

Novel magneto-optic layers based on semiconductor nanostructures for Kerr microscopy.

Symposium on Magneto-Optical Materials for Photonics and Recording held at the 2004 MRS Fall Meeting, Boston, MA, NOV 29-DEC 29, 2004International audienceA novel type of magneto-optic layers based on CdMnTe quantum wells is used to image the magnetic flux pattern at the surface of type I superconductors. The magneto-optic layer is designed as an anti-reflecting optical cavity. The quantum wells are arranged in a Bragg structure and placed at maxima of the electric field in the cavity in order to enhance Faraday rotation