Affiliation: | 1. Centro Singular de Investigación en Química Biolóxica e Materiais Moleculares (CiQUS) and Departamento de Química Orgánica, Universidade de Santiago de Compostela, 15782- Santiago de Compostela, Spain;2. CIC nanoGUNE-BRTA, 20018 Donostia-San Sebastián, Spain;3. Centro de Física de Materiales CSIC/UPV-EHU-Materials Physics Center, 20018 Donostia-San Sebastián, Spain Donostia International Physics Center (DIPC), 20018 Donostia-San Sebastián, Spain;4. School of Physical and Chemical Sciences, Queen Mary University of London, UK;5. Donostia International Physics Center (DIPC), 20018 Donostia-San Sebastián, Spain Ikerbasque, Basque Foundation for Science, 48009 Bilbao, Spain;6. Centro de Física de Materiales CSIC/UPV-EHU-Materials Physics Center, 20018 Donostia-San Sebastián, Spain |
Abstract: | Triangulenes are a class of 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) on a Au(111) surface, and the detection of its high-spin ground state. 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 quartet state (S=3/2) expected for the neutral species. This confirms that electron transfer to the metal substrate raises the spin of the ground state. 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 an S=2 aza-triangulene on Au(111). |