Adsorption and Self-Assembly of the 2,3,5,6-Tetra(2′-pyridyl)pyrazine Nonplanar Molecule on a Au(111) Surface

Abstract

We report our investigation of adsorption and self-assembly of a nonplanar molecule 2,3,5,6-tetra(2′-pyridyl)pyrazine (TPPZ) on a Au(111) surface using ultrahigh vacuum low-temperature scanning tunneling microscopy joint with density functional theory (DFT) calculations. We find that the nonplanar TPPZ molecules exhibit various adsorption configurations depending on the coverage of molecules. The molecules mainly adsorb at step edges with a flat-lying configuration at low coverages and gather into chiral trimers almost equidistantly separated from each other in the fcc domains accompanied by diffusive molecules in the hcp domains of the herringbone reconstructed Au(111) surface at a coverage of about 0.2 monolayer (ML) and then form two dominant types of ordered domains, i.e., stripe-like (S-phase) and honeycomb-like (H-phase) superstructures, which may reflect the chiral separation characteristics at a coverage of about 1 ML. In the trimers and ordered domains, the adsorption configurations of molecules become declining or almost erect, i.e., an “edge-on” configuration, quite different from the flat-lying configuration at low coverages. After annealing to 380 K the S-phase transfers to the H-phase, and the H-phase may persist after annealing up to 410 K, which can be attributed to the existence of C–H···N hydrogen bonds between the TPPZ molecules with the same chirality. Our observations can be energetically interpreted by considering the interplay of molecule–substrate interaction and intermolecular interaction including van der Waals interaction and hydrogen bonds on the basis of the DFT calculations, where the hydrogen bonds should be a key factor for the formation of the stable ordered H-phase with chiral separation.