Pyridyl-functionalized tripod molecules on Au(111): Interplay between H-bonding and metal coordination

TL;DR

This study explores how pyridyl-functionalized molecules self-assemble on gold surfaces, revealing the roles of hydrogen bonding, metal coordination, and temperature in structural stability and transformation.

Contribution

It combines STM experiments with DFT calculations to elucidate the interplay of hydrogen bonding and metal coordination in molecular self-assembly on Au(111).

Findings

Molecules form a well-ordered, close-packed structure at room temperature.

Post-deposition annealing stabilizes assemblies via Au-ligand bonds.

High-temperature annealing leads to bond breaking and formation of covalent dimers.

Abstract

The self-assembly of pyridyl-functionalized triazine (T4PT) was studied on Au(111) using low-temperature scanning tunneling microscopy (STM) under ultra-high vacuum conditions combined with density functional theory (DFT) calculations. In particular, we investigated the effect of temperature on the intermolecular interactions within the assemblies. STM measurements revealed that T4PT molecules form a well-ordered, close-packed structure, with the molecules adopting a planar conformation parallel to the Au surface for coverages $\leq1$ monolayer upon room temperature deposition. The intermolecular interactions stabilizing the self-assembled arrangement is based on a combination of hydrogen bonding and weak van der Waals forces. Upon post-deposition annealing, the assemblies are additionally stabilized by metal-ligand bonding between the pyridyl ligands and native Au adatoms. Further post-deposition annealing at temperatures above $200^{\circ}$C led to the breaking of the N-Au bonds with the molecular assemblies transforming into a second close-packed hydrogen bonded structure. For temperatures exceeding $230^{\circ}$C, few covalently linked dimers formed, most likely as a result of CH-bond activation. We rationalize the kinetically-driven structure formation by unveiling the interaction strengths of the bonding motifs using DFT and compare the molecular conformation to the structurally similar pyridyl-functionalized benzene (T4PB).