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

Ion-Pair Oligomerization of Chromogenic Triangulenium Cations with Cyanostar-Modified Anions That Controls Emission in Hierarchical Materials

The hierarchical assembly of colored cationic molecules with receptor-modified counteranions can be used to control optical properties in materials. However, our knowledge of when the optical properties emerge in the hierarchical organization and the variety of cation–anion salts that are available to create these materials is limited. In this work, we extend the salts from small halides to large inorganic anions and determine how the structure coevolves with the emission properties using solution assemblies. We study the chromogenic trioxatriangulenium (<b>TOTA</b>⁺) cation and its coassembly with cyanostar (<b>CS</b>) macrocycles selected to modify tetrafluoroborate (BF₄^–) counteranions through formation of 2:1 sandwich complexes. In the solid state, the <b>TOTA</b>⁺ cation stacks in an alternating manner with the sandwich complexes producing new red-shifted emission and absorption bands. Critical to assigning the structural origin of the new optical features across the four levels of organization (1° → 4°) is the selection of specific solvents to produce and characterize different assemblies present in the hierarchical structure. A key species is the electrostatically stabilized ion pair between the <b>TOTA</b>⁺ cation and sandwich complex. The red-shifted features only emerge when the ion pairs oligomerize together into larger (<b>TOTA</b>·[<b>CS</b>₂BF₄])_<i>n</i> assemblies. New electronic states emerge as a result of multiple copies of the <b>TOTA</b>⁺ making π-contact with cyanostar–anion complexes. Our findings and the ease with which the materials can be prepared as crystals and films by mixing the salt with a receptor provide a strong platform for the <i>de novo</i> design of new optical materials

Self-Assembled Polystyrene Beads for Templated Covalent Functionalization of Graphitic Substrates Using Diazonium Chemistry

A network of self-assembled polystyrene beads was employed as a lithographic mask during covalent functionalization reactions on graphitic surfaces to create nanocorrals for confined molecular self-assembly studies. The beads were initially assembled into hexagonal arrays at the air–liquid interface and then transferred to the substrate surface. Subsequent electrochemical grafting reactions involving aryl diazonium molecules created covalently bound molecular units that were localized in the void space between the nanospheres. Removal of the bead template exposed hexagonally arranged circular nanocorrals separated by regions of chemisorbed molecules. Small molecule self-assembly was then investigated inside the resultant nanocorrals using scanning tunneling microscopy to highlight localized confinement effects. Overall, this work illustrates the utility of self-assembly principles to transcend length scale gaps in the development of hierarchically patterned molecular materials