We investigated the self-assembly of trimesic acid (TMA) at the solution–graphite interface by using scanning tunneling microscopy (STM). We show that the polymorphism of the adsorbate structures of TMA can be controlled by the substrate temperature during the deposition of the molecules out of the solution for various solvents of different polarity. TMA was dissolved in phenyloctane, octanoic acid, and undecanol. At elevated substrate temperatures, various periodic assemblies of TMA could be obtained. By increasing the temperature of the preheated substrate, the specific 2D supramolecular network structure and the corresponding packing density can be precisely tuned in each kind of the solvents studied. The results found by STM are explained by the increased concentration of the solution at the preheated substrate as well as the higher mobility of the solute molecules increasing the opportunity of interactions between the molecules, in particular different hydrogen bonding motifs. Our interpretation is supported by simulations for each structure using the semiempirical quantum-chemical method PM6-DH+.

A coupling-limited approach for the Ullmann reaction-like on-surface synthesis of a two-dimensional covalent organic network starting from a halogenated metallo-porphyrin is demonstrated. Copper-octabromo-tetraphenylporphyrin molecules can diffuse and self-assemble when adsorbed on the inert Au(111) surface. Splitting-off of bromine atoms bonded at the macrocyclic core of the porphyrin starts at room temperature after the deposition and is monitored by X-ray photoelectron spectroscopy for different annealing steps. Direct coupling between the reactive carbon sites of the molecules is, however, hindered by the molecular shape. This leads initially to an ordered non-covalently interconnected supramolecular structure. Further heating to 300 °C and an additional hydrogen dissociation step is required to link the molecular macrocycles via a phenyl group and form large ordered polymeric networks. This approach leads to a close-packed covalently bonded network of overall good quality. The structures are characterized using scanning tunneling microscopy. Different kinds of lattice defects and, furthermore, the impact of polymerization on the HOMO-LUMO gap are discussed. Density functional theory calculations corroborate the interpretations and give further insight into the adsorption of the debrominated molecule on the surface and the geometry and coupling reaction of the polymeric structure.

The controlled and reversible interconversion between the free-base and the doubly dehydrogenated form of a 5,10,15,20-tetra(p-hydroxyphenyl)porphyrin molecule in an ordered array is demonstrated. This is achieved through voltage pulses by hydrogen transfer between the center of the porphyrin and the tip of a scanning tunneling microscope (STM). The local dehydrogenation leads to significant shifts in the energetic positions of the molecular orbitals. Density functional theory (DFT) calculations corroborate our conclusions and allow to gain more insight into the different energy level alignment before and after dehydrogenation. Due to the different conductance at a given voltage a clear distinction of both molecular species is possible, which also enables the application as a single-molecular switch.