STM Study of the Self-Assembly of Biphenyl-3,3',5,5'-Tetracarboxylic Acid and Its Mixing Behavior with Coronene at the Liquid-Solid Interface.

We report a scanning tunneling microscopy (STM) study of the molecular self-assembly of biphenyl-3,3',5,5'-tetracarboxylic acid (BPTC) at the octanoic acid/graphite interface. STM revealed that the BPTC molecules generated stable bilayers and monolayers under high and low sample concentrations, respectively. Besides hydrogen bonds, the bilayers were stabilized by molecular π-stacking, whereas the monolayers were maintained by solvent co-adsorption. A thermodynamically stable Kagomé structure was obtained upon mixing BPTC with coronene (COR), while kinetic trapping of COR in the co-crystal structure was found by the subsequent deposition of COR onto a preformed BPTC bilayer on the surface. Force field calculation was conducted to compare the binding energies of different phases, which helped to provide plausible explanations for the structural stability formed via kinetic and thermodynamic pathways.

Flow-induced-crystallization: tailoring host-guest supramolecular co-assemblies at the liquid-solid interface.

Here, we report that using the method of simply contacting a sample solution droplet with a piece of tissue paper can create a solvent flow (capillary force). During this process, the dynamics and solvent removal can promote the formation and stabilization of a meta-stable linear quasi-crystal composed of p-terphenyl-3,5,3',5'-tetracarboxylic acid (TPTC) molecules, which would otherwise pack into thermodynamically favored random tiling. The tailored quasi-crystal (linear) template allows atop it higher-efficiency accommodation of fullerene molecules (C60) from 40.1% to 97.5%, compared with that obtained in the random-tiling (porous) case. Overall, the result of this study presents an unusual yet remarkably simple strategy for tailoring complex host-guest supramolecular systems at the liquid-solid interface.

Monolayer and Bilayer Formation of Molecular 2D Networks Assembled at the Liquid/Solid Interfaces by Solution-Based Drop-Cast Method.

In recent years, extending self-assembled structures from two-dimensions (2D) to three-dimensions (3D) has been a paradigm in surface supramolecular chemistry and contemporary nanotechnology. Using organic molecules of p-terphenyl-3,5,3',5'-tetracarboxylic acid (TPTC), and scanning tunneling microscopy (STM), we present a simple route, that is the control of the solute solubility in a sample solution, to achieve the vertical growth of supramolecular self-assemblies, which would otherwise form monolayers at the organic solvent/graphite interface. Presumably, the bilayer formations were based on π-conjugated overlapped molecular dimers that worked as nuclei to induce the yielding of the second layer. We also tested other molecules, including trimesic acid (TMA) and 1,3,5-tris(4-carboxyphenyl)-benzene (BTB), as well as the further application of our methodology, demonstrating the facile preparation of layered assemblies.

Electric-field-induced supramolecular phase transitions at the liquid/solid interface: cat-assembly from solvent additives.

We demonstrate by using scanning tunneling microscopy that a series of trace organic solvent additives can efficiently promote the electrically triggered phase transition of trimesic acid (TMA), which would otherwise occur rather sporadically. DFT simulations taking into account the electric field effect elucidate such tailored phase transformations, based on the Gibbs activation and free energies of the deprotonation reactions of TMA.

Nanoscale tailoring of supramolecular crystals via an oriented external electric field.

The oriented external electric field of a scanning tunneling microscope (STM) has recently been adapted for controlling the chemical reaction and supramolecular phase transition at surfaces with molecular precision. However, to date, advance controls using such electric-fields for crystal engineering have not been achieved yet. Here, we present how the directional electric-field of an STM can be utilized to harness supramolecular crystallization on a solid surface. We show that a glass-like random-tiling assembly composed of p-terphenyl-3,5,3',5'-tetracarboxylic acid can transform into close-packed periodic assemblies under positive substrate bias conditions at the liquid/solid interface. Importantly, the nucleation and subsequent crystal growth for such field-induced products can be artificially tailored at the early stage in a real-time fashion. Through this method, we were able to produce a two-dimensional supramolecular single crystal. The as-prepared crystals with apparent brightness are ascribed to a spectroscopic feature linked to the electron density of states, which is thus strongly STM bias dependent.