Where do the counterions go? Tip-induced dissociation of self-assembled triazatriangulenium-based molecules on Au(111)

Abstract

Chemical coupling of functional molecules on top of the so-called platform molecules allows the formation of functional self-assembled monolayers (SAMs). An often-used example of such a platform is triazatriangulenium (TATA), which features an extended aromatic core providing good electronic contact to the underlying metal surface. Here, we present a study of the SAM formation of a TATA platform on Au(111) employing scanning tunneling microscopy (STM) under ambient atmospheric conditions. In solution, the TATA platform is stabilized by BF₄ counterions, while after deposition on a gold substrate, the localization of the BF₄ counterions remains unknown. We used 1,2,4-trichlorobenzene as a solvent of TATA–BF₄ to induce SAM formation on a heated (∼50 °C) Au substrate. We show by STM how to detect and distinguish TATA–BF₄ from TATA platforms, which lost their BF₄ counterions. Finally, we observe a change of the counterion position on the SAM during the STM scanning, which we explain by an electric-field-induced decrease of the electrostatic interaction in TATA–BF₄ on the surface. We applied DFT calculations to reveal the influence of the gold lattice and the electric field of the STM tip on the stability of TATA–BF₄ physisorbed on the surface.