  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).
