Unravelling the localization of counter-ions in ionic self-assemblies on a HOPG surface

International audienceAdsorption of supramolecular networks is considered a useful technique to tune the electronic properties of the underlying surfaces. Organic molecules with the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) at suitable positions with respect to the Fermi level of underlying surfaces can be used to dope surfaces. The degree of doping is strongly driven by the on-surface organization of adsorbed molecules. Among all π-conjugated molecules, redoxactive viologens (or 4,4’-bipyridiniums) with low-lying LUMOs have emerged as promising candidates to address this major challenge.<br&gtAtomic force microscopy (AFM) is a commonly used method to investigate the self-organization of molecules on surfaces under ambient conditions at the sub-molecular level. However, in the case of organic salts, such as viologens, the position of the organic cations is well determined whereas the position of the corresponding anions has been less investigated mainly because these anions are often smaller than organic cations. The role of electrostatic interactions between anions and cations is essential to the understanding of on-surface organization. These electrostatic interactions strongly depend on the relative positions of the charged species and require the precise determination of the positions of both the cations and the anions. Whereas attempts to combine solid state NMR (ssNMR) with optical microscopy have been successfully reported for liquid crystalline viologen derivatives, the complementarity of ssNMR and AFM was neither established nor reported. This work describes an original strategy, based on a combination of AFM images and ssNMR, to unravel the position of anions and cations in organic salts deposited on a highly-oriented pyrolytic graphite (HOPG) surface. This simple strategy shows that ssNMR can quickly indicate the tendency of building blocks to organize efficiently prior to AFM studies.<br&gtWe have combined ss-NMR spectroscopy and AFM topography images to obtain a complete picture of the supramolecular self-assemblies of organic salts on an HOPG surface. This original, simple and efficient method paves the way to the construction of supramolecular self-assembled nanostructures with promising electronic properties

Functionalized 4,4'-bipyridines: synthesis and 2D-organization on HOPG

International audienceCommercial 4,4'-bipyridine is a popular scaffold which is primarily employed as a linker in 3D self-assembled architectures such as metallo-organic frameworks or as connector in 2D networks. The introduction of alkyl substituents on the bipyridine skeleton is instrumental when 4,4'-bipyridines are used as linkers to form 2D self-assembled patterns on surfaces. Here, various synthetic strategies to access 4,4'-bipyridines functionalized at various positions are described. These easily scalable reactions have been used to introduce a range of alkyl substituents at positions 2 and 2', or 3 and 3' and at positions 2,2' and 6,6' in the case of tetra-functionalization. Scanning tunneling microscopy studies of molecular monolayers physisorbed at the graphite-solution interface revealed different supramolecular patterns whose motifs are primarily dictated by the nature and position of the alkyl chains

Going Up the Ladder: Stacking Four 4,4′-Bipyridine Moieties within a Ti(IV)-Based Tetranuclear Architecture

Biphenol-based ligands have proven their ability to bind titanium(IV) centers and generate sophisticated self-assembled structures in which auxiliary nitrogen ligands often complete the coordination sphere of the metal and improve stability. Here, a central 4,4′-bipyridine, which acts as both a spacer and a source of monodentate nitrogen to complete the coordination sphere of the Ti(IV) complex, was incorporated within two bis-2,2′-biphenol strands, 3H4 and 4H4. Both proligands possess structural features that are well adapted to form self-assembled structures built from titanium–oxygen–nitrogen units; however, their different degrees of torsional freedom strongly influenced the nuclearity of the complexes formed. The presence of a phenyl spacer between the bipyridine and the biphenol moieties of 3H4 provided enough flexibility for the ligand to wrap around one titanium(IV) center to form a mononuclear complex Ti(3)(DMF)2 in the presence of dimethylformamide (DMF). Assembly of the more rigid ligand 4H4 with Ti(OiPr)4 afforded a tetranuclear complex Ti4(4)2(4H)2(OEt)2 containing four stacked 4,4′-bipyridine units as shown by the X-ray structure of the complex. Density functional theory studies suggested that the assembly of this tetrametallic complex involves a dimetallic intermediate with TiO6 nodes that is converted to the thermodynamically stable tetranuclear complex with two TiO6 nodes and two TiO5N units with enhanced covalent character