On-Surface Synthesis and Characterization of a High-Spin Aza-[5]-Triangulene

TL;DR

This study synthesizes a nitrogen-doped triangulene on a gold surface, confirming its high-spin ground state (S=2) through advanced microscopy and spectroscopy, and explores how N-substitution influences its electronic and aromatic properties.

Contribution

It provides the first direct evidence of a high-spin (S=2) aza-[5]-triangulene on a surface, combining synthesis, microscopy, spectroscopy, and simulations to understand its electronic structure.

Findings

Confirmed high-spin S=2 ground state on Au(111) surface.

Detected radical edge states via STM and spectroscopy.

Explained radical distribution with charge and aromaticity changes.

Abstract

Triangulenes are open-shell triangular graphene flakes with total spin increasing with their size. In the last years, on-surface-synthesis strategies have permitted fabricating and engineering triangulenes of various sizes and structures with atomic precision. However, direct proof of the increasing total spin with their size remains elusive. In this work, we report the combined in-solution and on-surface synthesis of a large nitrogen-doped triangulene (aza-[5]-triangulene) and the detection of its high spin ground state on a Au(111) surface. Bond-resolved scanning tunneling microscopy images uncovered radical states distributed along the zigzag edges, which were detected as weak zero-bias resonances in scanning tunneling spectra. These spectral features reveal the partial Kondo screening of a high spin state. Through a combination of several simulation tools, we find that the observed distribution of radical states is explained by a quintet ground state (S = 2), instead of the expected quartet state (S = 3/2), confirming the positively charged state of the molecule on the surface. We further provide a qualitative description of the change of (anti)aromaticity introduced by N-substitution, and its role in the charge stabilization on a surface, resulting in a S = 2 aza-[5]-triangulene on Au(111).