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

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

Long-Range Order Induced by Intrinsic Repulsion on an Insulating Substrate

An ordered arrangement of molecular stripes with equidistant appearance is formed upon the adsorption of 3-hydroxybenzoic acid onto calcite (10.4) held at room temperature. In a detailed analysis of the next-neighbor stripe distances measured in noncontact atomic force microscopy images at various molecular coverages, we compare the observed stripe arrangement with a random arrangement of noninteracting stripes. The experimentally obtained distance distribution deviates substantially from what is expected for a random distribution of noninteracting stripes, providing direct evidence for the existence of a repulsive interaction between the stripes. At low molecular coverage, where the average stripe distance is as large as 16 nm, the stripes are significantly ordered, demonstrating the long-range nature of the involved repulsive interaction. The experimental results can be modeled with a potential having a 1/<i>d</i>² distance dependence, indicating that the observed long-range repulsion mechanism originates from electrostatic repulsion of adsorption-induced dipoles solely. This effect is particularly pronounced when local charges remain unscreened on the surface, which is characteristic of nonmetallic substrates. Consequently, the observed generic repulsion mechanism is expected to play a dominant role in molecular self-assembly on electrically insulating substrates

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

Controlling Molecular Self-Assembly on an Insulating Surface by Rationally Designing an Efficient Anchor Functionality That Maintains Structural Flexibility

Molecular self-assembly on surfaces is dictated by the delicate balance between intermolecular and molecule–surface interactions. For many insulating surfaces, however, the molecule–surface interactions are weak and rather unspecific. Enhancing these interactions, on the other hand, often puts a severe limit on the achievable structural variety. To grasp the full potential of molecular self-assembly on these application-relevant substrates, therefore, requires strategies for anchoring the molecular building blocks toward the surface in a way that maintains flexibility in terms of intermolecular interaction and relative molecule orientation. Here, we report the design of a site-specific anchor functionality that provides strong anchoring toward the surface, resulting in a well-defined adsorption position. At the same time, the anchor does not significantly interfere with the intermolecular interaction, ensuring structural flexibility. We demonstrate the success of this approach with three molecules from the class of shape-persistent oligo(<i>p</i>-benzamide)s adsorbed onto the calcite(10.4) surface. These molecules have the same aromatic backbone with iodine substituents, providing the same basic adsorption mechanism to the surface calcium cations. The backbone is equipped with different functional groups. These have a negligible influence on the molecular adsorption on the surface but significantly change the intermolecular interaction. We show that distinctly different molecular structures are obtained that wet the surface due to the strong linker while maintaining variability in the relative molecular orientation. With this study, we thus provide a versatile strategy for increasing the structural richness in molecular self-assembly on insulating substrates

One-Pot Synthesis and AFM Imaging of a Triangular Aramide Macrocycle

Macrocyclizations in exceptionally good yields were observed during the self-condensation of <i>N</i>-benzylated phenyl <i>p</i>-aminobenzoates in the presence of LiHMDS to yield three-membered cyclic aramides that adopt a triangular shape. An <i>ortho</i>-alkyloxy side chain on the <i>N</i>-benzyl protecting group is necessary for the macrocyclization to occur. Linear polymers are formed exclusively in the absence of this Li-chelating group. A model that explains the lack of formation of other cyclic congeners and the demand for an <i>N-</i>(<i>o</i>-alkoxybenzyl) protecting group is provided on the basis of DFT calculations. High-resolution AFM imaging of the prepared molecular triangles on a calcite(10.4) surface shows individual molecules arranged in groups of four due to strong surface templating effects and hydrogen bonding between the molecular triangles

Chemical Identification at the Solid–Liquid Interface

Solid–liquid interfaces are decisive for a wide range of natural and technological processes, including fields as diverse as geochemistry and environmental science as well as catalysis and corrosion protection. Dynamic atomic force microscopy nowadays provides unparalleled structural insights into solid–liquid interfaces, including the solvation structure above the surface. In contrast, chemical identification of individual interfacial atoms still remains a considerable challenge. So far, an identification of chemically alike atoms in a surface alloy has only been demonstrated under well-controlled ultrahigh vacuum conditions. In liquids, the recent advent of three-dimensional force mapping has opened the potential to discriminate between anionic and cationic surface species. However, a full chemical identification will also include the far more challenging situation of alike interfacial atoms (i.e., with the same net charge). Here we demonstrate the chemical identification capabilities of dynamic atomic force microscopy at solid–liquid interfaces by identifying Ca and Mg cations at the dolomite–water interface. Analyzing site-specific vertical positions of hydration layers and comparing them with molecular dynamics simulations unambiguously unravels the minute but decisive difference in ion hydration and provides a clear means for telling calcium and magnesium ions apart. Our work, thus, demonstrates the chemical identification capabilities of dynamic AFM at the solid–liquid interface

PAA-PAMPS Copolymers as an Efficient Tool to Control CaCO₃ Scale Formation

Scale formation, the deposition of certain minerals such as CaCO₃, MgCO₃, and CaSO₄·2H₂O in industrial facilities and household devices, leads to reduced efficiency or severe damage. Therefore, incrustation is a major problem in everyday life. In recent years, double hydrophilic block copolymers (DHBCs) have been the focus of interest in academia with regard to their antiscaling potential. In this work, we synthesized well-defined blocklike PAA-PAMPS copolymers consisting of acrylic acid (AA) and 2-acrylamido-2-methyl-propane sulfonate (AMPS) units in a one-step reaction by RAFT polymerization. The derived copolymers had dispersities of 1.3 and below. The copolymers have then been investigated in detail regarding their impact on the different stages of the crystallization process of CaCO₃. Ca²⁺ complexation, the first step of a precipitation process, and polyelectrolyte stability in aqueous solution have been investigated by potentiometric measurements, isothermal titration calorimetry (ITC), and dynamic light scattering (DLS). A weak Ca²⁺ induced copolymer aggregation without concomitant precipitation was observed. Nucleation, early particle growth, and colloidal stability have been monitored in situ with DLS. The copolymers retard or even completely suppress nucleation, most probably by complexation of solution aggregates. In addition, they stabilize existing CaCO₃ particles in the nanometer regime. In situ AFM was used as a tool to verify the coordination of the copolymer to the calcite (104) crystal surface and to estimate its potential as a growth inhibitor in a supersaturated CaCO₃ environment. All investigated copolymers instantly stopped further crystal growth. The carboxylate richest copolymer as the most promising antiscaling candidate proved its enormous potential in scale inhibition as well in an industrial-filming test (Fresenius standard method)