Directing the assembly of charged organic molecules by a hydrophilic-hydrophobic nanostructured monolayer at electrified interfaces

Selective anion-induced crystal switching and binding in surface monolayers modulated by electric fields from scanning probes

Anion-selective (Br(-) and I(-)) and voltage-driven crystal switching between two differently packed phases (α ⇆ β) was observed in 2D crystalline monolayers of aryl-triazole receptors ordered at solution-graphite interfaces. Addition of Br(-) and I(-) was found to stimulate the α → β phase transformation and to produce ion binding to the β phase assembly, while Cl(-) and BF4(-) addition retained the α phase. Unlike all other surface assemblies of either charged molecules or ion-templated 2D crystallization of metal-ligand or receptor-based adsorbates, the polarity of the electric field between the localized scanning tip and the graphite substrate was found to correlate with phase switching: β → α is driven at -1.5 V, while α → β occurs at +1.1 V. Ion-pairing between the countercations and the guest anions was also observed. These observations are supported by control studies including variation of anion species, relative anion concentration, surface temperature, tip voltage, and scanning time.

Template-assisted assembly: scanning tunneling microscopy study of solvent-dependent adlattices of alkyl-derivatized tetrathiafulvalene

The self-assembly of an adsorbate as a function of the strength of solvent-substrate adsorption is an important yet relatively unexplored subject. In this study, how the strength of solvent-substrate adsorption and solvent-solvent attraction affects the assembly of tetrakis(octadecylthio)tetrathiafulvalene (1) is scrutinized by scanning tunneling microscopy (STM). For solvents with strong intermolecular interactions and adsorption onto graphite, such as long n-alkanes (C(n)H(2n+2), n ≥ 13), STM reveals that the solvent molecules form lamellae which become a template to direct the assembly of 1 into one-dimensional arrays. The lengths of one of the unit cell vectors for the assemblies are increased and well correlated with the solvent sizes. In situ STM monitoring of 1 introduced onto graphite with preadsorbed n-tetradecane adlattices shows that the developed assemblies of 1 have striped features aligned parallel to the underlying template. In contrast, for solvents with weak adsorption, such as short n-alkanes (C(n)H(2n+2), n ≤ 12), toluene, and 1,2,4-trichlorobenzene, the adlattice structures of 1 are solvent-independent.

Supramolecular assembly/reassembly processes: molecular motors and dynamers operating at surfaces

Among the many significant advances within the field of supramolecular chemistry over the past decades, the development of the so-called "dynamers" features a direct relevance to materials science. Defined as "combinatorial dynamic polymers", dynamers are constitutional dynamic systems and materials resulting from the application of the principles of supramolecular chemistry to polymer science. Like supramolecular materials in general, dynamers are reversible dynamic multifunctional architectures, capable of modifying their constitution by exchanging, recombining, incorporating components. They may exhibit a variety of novel properties and behave as adaptive materials. In this review we focus on the design of responsive switchable monolayers, i.e. monolayers capable to undergo significant changes in their physical or chemical properties as a result of external stimuli. Scanning tunneling microscopy studies provide direct evidence with a sub-nanometre resolution, on the formation and dynamic response of these self-assembled systems featuring controlled geometries and properties.

2D analogues of the inverted hexagonal phase self-assembled from 4,6-dialkoxylated isophthalic acids at solid-liquid interfaces

Self-assembly of organic molecules at solid-liquid interfaces is a route for developing novel functional materials on surfaces and modeling assembly phenomena in 3D. 5-Alkoxylated isophthalic acids (ISA) are known to self-assemble into two-dimensional (2D) lamellae at the interface between a surface of Au(111) or HOPG (highly oriented pyrolytic graphite) and a solvent. Presently, the self-assembly of 4,6-dialkoxylated isophthalic acid derivatives with variable alkyl chain length is investigated at Au(111)-water, Au(111)-tetradecane and HOPG-tetradecane interfaces with a particular focus on the first one. The main aspect of this study is to evaluate the role of the molecular geometry and different interactions in the 2D assembly of amphiphilic molecules. In contrast to 5-alkoxylated ISA, 4,6-dialkoxylated ISA derivatives self-assemble preferentially into arrays of cyclic pentameric/hexameric structures, which appear as 2D analogues of the inverted hexagonal phase of lipids. As a general trend, the derivatives bearing shorter alkyl chains show a higher level of ordering at Au(111)-liquid interfaces. In particular, at the Au(111)-water interface, the 4,6-diheptyloxy ISA derivative forms exclusively pentamers, which are arranged in a quasi-hexagonal lattice. Moreover, the cyclic pentameric features are not empty but host a single isophthalic acid residue which is found to be dynamic. Finally, the packing of the diheptyloxy derivative shows a distinct potential dependence: while at more negative potentials the pentameric arrangement is converted into lamellae, at more positive potentials a loosely packed zig-zag pattern is formed. The present results show that at different solid-liquid interfaces 4,6-dialkoxylated ISA derivatives tend to form cyclic structures that are 2D analogues of an inverted hexagonal phase, akin to lipids having two hydrophobic alkyl chains and a small polar head group. Moreover, the substrate potential at the Au(111)-water interface can tune the 2D molecular arrangement.