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

• Published in: Physical chemistry chemical physics : PCCP Vol. 23; no. 16; pp. 993 - 9937
• Main Authors: Snegir, S, Dappe, Y. J, Sysoiev, D, Pluchery, O, Huhn, T, Scheer, E
• Format: Journal Article
• Language: English
• Published: England Royal Society of Chemistry 28.04.2021
• Subjects: Chemical Sciences; Coupling (molecular); Electric contacts; Electric fields; Gold; Metal surfaces; or physical chemistry; Platforms; Scanning tunneling microscopy; Self-assembled monolayers; Self-assembly; Substrates; Theoretical and
• ISSN: 1463-9076; 1463-9084
• DOI: 10.1039/d1cp00221j

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 BF 4 counterions, while after deposition on a gold substrate, the localization of the BF 4 counterions remains unknown. We used 1,2,4-trichlorobenzene as a solvent of TATA-BF 4 to induce SAM formation on a heated (∼50 °C) Au substrate. We show by STM how to detect and distinguish TATA-BF 4 from TATA platforms, which lost their BF 4 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-BF 4 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-BF 4 physisorbed on the surface. The interaction of the Au(111) surface with the molecules that have ionic bonds can lead to dissociation of the latter. This effect can be reinforced by the electric field of the STM tip, which requires the use of delicate STM tunneling parameters to visualize both the parent ion and its counterion.